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Insights for size-dependent reactivity of hematite nanomineral surfaces through Cu2+ sorption
Abstract
Macroscopic sorption edges for Cu2+ were measured on hematite nanoparticles with average diameters of 7 nm, 25 nm, and 88 nm in 0.1 M NaNO3. The pH edges for the 7 nm hematite were shifted approximately 0.6 pH units lower than that for the 25 nm and 88 nm samples, demonstrating an affinity sequence of 7 nm > 25 nm = 88 nm. Although, zeta potential data suggest increased proton accumulation at the 7 nm hematite surfaces, changes in surface structure are most likely responsible for the preference of Cu2+ for the smallest particles. As Cu2+ preferentially binds to sites which accommodate the Jahn–Teller distortion of its coordination to oxygen, this indicates the relative importance of distorted binding environments on the 7 nm hematite relative to the 25 nm and 88 nm particles. This work highlights the uniqueness of surface reactivity for crystalline iron oxide particles with decreasing nanoparticle diameter.
... The particle size of hematite crystals has attracted attention as the surface reactions may change with particle size (Madden et al., 2006;Noerpel and Lenhart, 2015). The particle sizes in a wide range from nano to micron scale had been observed in the synthesized hematite over the past few decades (Zhu et al., 2013;Huang et al., 2017). ...
... As higher concentration of Fe 3+ precursors can enhance the amount of nucleation emerges and decrease the particle size of hematite (Schwertmann and Cornell, 2008;Chen et al., 2020). Similarly, Madden et al. fabricated hematite particles with the average diameter from 88 nm to 7 nm with increasing ferric nitrate concentration (Madden et al., 2006). Since foreign additives were absent during the synthesizing, the changed size can be rationally explained by the different level of supersaturation. ...
... The two-site surface complexation model study revealed that the equilibrium constants of U(VI) complexes increased with decreasing particle size of hematite (Zeng et al., 2009). In addition, Madden et al. (2006) determined that the affinity of Cu(II) to hematite changed with particle size as the smaller hematite particles (7 nm) had a stronger adsorption affinity for Cu(II). The larger surface curvature of the smaller particles exhibits a relatively high surface free energy, resulting in a higher potential for stabilizing ions at the electrical double layer (Chernyshova et al., 2010). ...
Hematite is ubiquitous in nature and holds great promise for a wide variety of applications in many frontiers of environmental issues such as heavy metal remediation in environment. Over the past decades, numerous efforts have been made to control and tailor the crystal structures of hematite to improve its adsorption performance for heavy metal ions (HMIs). It is now well established that the adsorption behavior of hematite nanocrystals is strongly affected by their particle sizes, crystal facet contributions, and defective structures. This review examined the size- and facet-dependent hematite, as well as the defective hematite according to their fabrication methods and growth mechanisms. Furthermore, the adsorption performance of various hematite particles for HMIs were introduced and compared to clarify the structure-active relationships of hematite. We also overviewed the advances in charge distribution (CD)-multisite complexation (MUSIC) modeling studies about the HMIs adsorption at the hematite-water interface and the binding parameters. The Present review systematically describes how the formation conditions impact the structural and surface properties of hematite particles, thereby providing new strategies for enhancing the performance of hematite for environmental remediation.
... Also, the minimally adsorbed Mn(II) on {0 0 1}, if any, may be coordinated in different configurations from those on {0 1 2} and {1 1 3} facets, similar to the behavior of Cr(VI) on hematite surfaces (Huang et al., 2016). Different bonding modes are very likely to affect the oxidation processes due to the varied electronic structures of the adsorbed Mn(II) (Luther et al., 1999;Madden et al., 2006;Namgung et al., 2020). Since the adsorption of Mn (II) is a precondition for the oxidative nucleation of MnO x through the adsorption-induced oxidation mechanism (Lan et al., 2017;Inoué et al., 2019), the stronger reactivity of {0 1 2} and {1 1 3} facets in binding Mn 2+ than {0 0 1} facet can be an important cause for the facet-specific oxidation of Mn(II) on hematite particles. ...
... Specifically, the edge {1 1 3} facets on HNP and {0 1 2} facets on HNC act as the substrates for the templated growth of MnO x nanowires, while the basal {0 0 1} facets on the HNP particles do not produce precipitates. Based on the existing knowledge of the oxidative production of MnO x catalyzed by semi-conducting nano-FeO x (Sung and Morgan, 1981;Junta and Hochella., 1994;Madden and Hochella, 2005;Madden et al., 2006;Lan et al., 2017;Inoué et al., 2019), this study takes the next step in the atomic-level understanding of heterogeneous oxidative growth processes, which shows intriguing atomistic complexities. The facet-specificity of the reaction processes should be related to the distribution of reactive sites on {0 0 1}, {0 1 2}, and {1 1 3} facets, as well as the bulk ET between {0 0 1} and {1 1 3} facets on HNP. ...
It is recognized that different facets of minerals vary distinctively in their chemical reactivity with aqueous solutions. However, detailed molecular and atomistic understandings of these phenomena are relatively limited. This study investigated the interaction of aqueous Mn²⁺ and dissolved oxygen on various facets of two morphology-types of iron oxide (hematite) nanocrystals. These interactions result in the oxidation of Mn(II) and heterogeneous growth of Mn(II)/Mn(III) and Mn(III) oxides. The nanoscale morphology and atomic structure of the manganese oxide products were characterized in detail. Our results, for the first time, directly demonstrate the facet-specific oxidation of Mn(II) and nucleation of Mn(II/III) oxides, followed by their epitaxial growth on hematite. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction measurements reveal the growth of MnOx nanowires on {113} of hematite nanoplates (HNP) and {012} facets of hematite nanocubes (HNC), while the basal {001} facets on the HNP particles do not produce precipitates. The average oxidation states of the MnOx on HNP and HNC determined using electron energy-loss spectroscopy (EELS) show that both Mn(II) and Mn(III) are present. The mineral composition and growth mechanisms of MnOx catalyzed by HNP and HNC are similar. High-resolution TEM analysis reveals the presence of both hausmannite and manganite on HNP and HNC. The crystallographic relationship between the heterogeneously formed manganite with hematite, which has not been reported before, proves that hematite provides reaction sites and can function as an atomic template for the formation of MnOx nanowires. These findings advance our understanding of the redox chemistry and heterogeneous growth of minerals as controlled by the surficial structure of the substrate mineral. This has important geochemical implications as the catalytic growth of less common, highly reactive phases like MnOx are known to be consequential in complex natural and anthropogenic environments.
... (b) TEM image of the hematite NPs at pH 6. earlier by Prof. Michael Hochella's group at Virginia Tech., and we have followed the same procedure regarding hematite synthesis. We compared the diffraction peaks of the hematite NPs synthesized by us with the XRD data of size variant hematite NPs having average diameters of 7 nm, 25 nm, and 88 nm, synthesized byMadden et al. (2007).2 The diffractions peaks of 7 nm hematite synthesized by us match with the XRD pattern of the 7nm hematite synthesized byMadden et al. (2007).2 ...
... We compared the diffraction peaks of the hematite NPs synthesized by us with the XRD data of size variant hematite NPs having average diameters of 7 nm, 25 nm, and 88 nm, synthesized byMadden et al. (2007).2 The diffractions peaks of 7 nm hematite synthesized by us match with the XRD pattern of the 7nm hematite synthesized byMadden et al. (2007).2 Except matching of the XRD peaks both the diffraction patterns have a high background. ...
The evaluation of the nanoscale forces between a silica probe and hematite nanoparticles (NPs) thin layer-coated mica (Hm-mica) in the presence of structurally different natural organic matter (NOM) and inorganic and organic guanidinium (Gd+) cations determines the efficiency of cementing agents towards the stable soil aggregate formation. The force-distance curves showed that electrostatic repulsions between the probe and the 5 and 50 mg C/L humic acid (HA)-modified Hm-mica substrates were initiated at higher probe-substrate separations relative to tannic acid (TA)-modified Hm-mica substrates. The abundance of highly ionizable polar carboxylate moieties in HA compared to TA contributed to stronger probe-substrate repulsion. The ion-specific interactions of the weakly hydrated divalent cations with the adsorbed HA diminished probe-substrate electrostatic repulsion. However, possible exposure of hydrophobic domains contributed to weak adhesion with the silica probe. The maximum adhesion of 0.11 ± 0.04 mN.m-1 was recorded in 5 mg C/L HA-modified Hm-mica in the presence of Ca2+. Nevertheless, 50 mg C/L TA modified Hm-mica exhibited a significant rise in adhesion to 1.44 ± 0.5 mN.m-1 in the presence of Ba2+. Porous nanostructures produced due to Ba2+-assisted molecular gelation of TA possibly contributed to the strong adhesion. Unlike inorganic cations, organic Gd+ has increased probe-substrate adhesion considerably to 3.13 ± 0.29 mN.m-1 in 50 mg C/L HA-modified Hm-mica. The porous nanotubular structures produced by HA in the presence of Gd+ possibly maintained the interfacial water content and facilitated adhesion with the silica probe. Thermodynamic parameters derived from the reactions between the NOMs and the cations further supplemented the force data. Therefore, interactions of sesquioxides nanominerals with soil biodegradation products can improve environmental quality and mitigate the effects of destabilizing forces.
... The authors [20] measured the macroscopic edges of Cu 2+ sorption by hematite NPs with average diameters of 7, 25 and 88 nm for 0.1 M NaNO 3 . The surface areas were 188 and 62 m 2 /g for the 7 and 25 nm hematite samples, respectively; the surface area of the 88 nm particles was 9.1 m 2 . ...
Scanning and transmission electron microscopy, powder X-ray diffraction and thermogravimetric analyses were used to study the dynamics of the sorption processes of ligand-free iron nanoparticles produced by highly efficient physical synthesis, namely, the molecular beam method. The structure, chemical and phase composition of Fe-NaCl condensates with different iron contents, crystallite dimensions (nanoparticles) and nanoparticle surface areas depending on the condensation temperature, which characterize the sorption capacity, primarily for moisture and oxygen, were studied. Finally, the gravimetric analysis method was used to investigate the kinetics of the relative change in the weight of porous Fe–NaCl condensates with different iron contents, depending on the condensation temperature. With increasing synthesis temperature, the nanoparticle size increases, and the specific surface area decreases. Therefore, by changing the size of the nanoparticles at the same volume, we can regulate the ratio of the nanoparticle surface to the nanoparticle volume, i.e., change the properties of the reaction surface and, in this way, the contribution of the excess surface energy to the total free energy of the system. The mass fraction of physically adsorbed and bound oxygen (moisture) correlates with the size (area, surface) of the nanoparticles.
Graphical Abstract
Sorption of oxygen and water by EB PVD ligand-free Fe@Fe3O4 nanoparticle in open matrix nanopore
... The authors [ 19 ] measured the macroscopic edges of Cu 2+ sorption by hematite NPs with average diameters of 7 nm, 25 nm and 88 nm for 0.1 М NaNO 3 . The surface areas were 188 m 2 /g and 62 m 2 /g for the 7 nm and 25 nm hematite samples, respectively; the surface area of the 88 nm particles was 9.1 m 2 . ...
Scanning and transmission electron microscopy, X-ray phase and thermogravimetric analyses were used to study the dynamics of the sorption processes of ligand-free iron nanoparticles produced by highly efficient physical synthesis, namely, the molecular beam method. The structure, chemical and phase composition of Fe-NaCl condensates with different iron contents, crystallite dimensions (nanoparticles) and nanoparticle surface areas depending on the condensation temperature, which characterize the sorption capacity, primarily for moisture and oxygen, were studied. Finally, the gravimetric analysis method was used to investigate the kinetics of the relative change in the weight of porous Fe-NaCl condensates with different iron contents, depending on the condensation temperature. With increasing synthesis temperature, the nanoparticle size increases, and the specific surface area decreases. Therefore, by changing the size of the nanoparticles at the same volume, we can regulate the ratio of the nanoparticle surface to the nanoparticle volume, i.e., change the properties of the reaction surface and, in this way, the contribution of the excess surface energy to the total free energy of the system. The mass fraction of physically adsorbed and bound oxygen (moisture) correlates with the size (area, surface) of the nanoparticles.
... For instance, dropping 60 mL of 1 M nitric acid from a syringe into 750 mL of boiling deionized water and then mixing it on a hot plate are needed to synthesize hematite nanoparticles. After that, nano-hematite was prepared from the cooled suspension after dialysis, freeze-drying, and other operations [112]. ...
... Natural nanocolloids (Ncs) represent a specific type of matter, defined as a highly dispersed multiphase heterogeneous system of nanometer magnitude (1-100 nm), also known as nano-colloidal dispersions or mixtures [1][2][3]. Like engineered nanoparticles, natural Ncs have an extensive specific surface area and can absorb other harmful pollutants in a large capacity [2,4]. For example, our previous studies showed that natural Ncs contain dangerous contaminants such as polycyclic aromatic hydrocarbons (PAHs, 14.2-50.5 µg/kg) and toxic heavy metals such as Cr [5]. ...
Nanocolloids (Ncs) are highly dispersed mixtures of nanoscale (1–100 nm) heterogeneous systems, which are ubiquitous in aquatic environments. Ncs are considered a vital pollutant carrier due to their special surface properties and unique hydrodynamic characteristics. They play an essential role in the process of promoting pollutant migration and transformation. In recent years, with the increase in chemicals in the environment and the complexity of environmental pollution, the health threats of Ncs in ecological systems are arousing great concerning. Therefore, recent work to characterize the ecotoxicity of Ncs has focused on the potential environmental health implications, including exploration of toxicity to aquatic organisms from a wide range of the ecosystem food webs. Herein, we summarize the formation, distribution, and characterization of natural Ncs in the marine environments. Moreover, we highlight the adverse impacts of Ncs on representatives of various trophic levels aquatic organisms (e.g., algae, bacteria, invertebrates, and fish). The mechanisms of Ncs ecotoxicity at the cellular level are reviewed, and the remaining unclear points on toxic tools such as oxidative damage and metabolic disorder are presented. We also discuss the research challenges and future developments within the field of ecotoxicity. This study will bridge our knowledge gap on the ecotoxicity of Ncs.
... These organo-mineral interactions can be divided into various mechanisms. Specific adsorption refers to the formation of polar covalent bonds between adsorbed molecules and mineral surface atoms with relative high bond energies (Luo 2007). Nonspecific adsorption is characterized by the long-range Coulomb forces between an adsorbate and a mineral surface, and the electrostatic bond energies are relatively low . ...
The stabilization of soil organic matter is crucial for global carbon cycling processes as soil stores large amounts of organic carbon. The occlusion of SOM within minerals sequesters these organic molecules, rendering them inaccessible to interference from biotic and abiotic factors. However, the microscopic mechanisms of occlusion are lacking. In the past few years, many researchers have focused on the elucidation of the occlusion process, and the results are summarized in this review. The occlusion of representative SOM such as natural extracted or commercial humic substances, sugars, amino acids within minerals including calcite, clay, metal oxides, were observed by various in situ and ex situ methods, such as atomic force microscopy, nano-scale secondary ion mass spectrometry and synchrotronbased infrared micro spectroscopy. These results have shown that minerals can occlude SOM either via organo-mineral aggregation or within growing hillocks, which are classical growth features on crystal surfaces, and the microscopic mechanisms have
been illustrated in this review. The occlusion process is influenced by various factors, including the characteristics of minerals and the composition of SOM and soil solution conditions, which are mediated by the interactions of organo-mineral interfaces. Finally, some new perspectives for future research of occlusion are provided in order to give new possibilities for observing and comparing the detailed occlusion process in soils from different areas. In summary, SOM
can be retained, protected and stabilized by soil minerals via occlusion either by aggregation or within growth hillocks, influenced by various factors. The results have implications for global carbon cycling in soil ecological systems.
... Due to their large specific surface area and high reactivities as compared to other ubiquitously distributed minerals (e.g., quartz, aluminosilicates), FeO x and MnO x nanominerals and mineral nanoparticles have received a great deal of attention in the fields of environmental and earth sciences (Banfield and Zhang, 2001;Waychunas et al., 2005). FeO x and MnO x exert key controls on many geochemical processes across a wide range of time and space scales primarily through playing a significant role in driving element cycling and electron transfer (Martin and Meybeck, 1979;Hochella et al., 1999;Thamdrup, 2000;Weaver and Hochella, 2003;Martin, 2005;Madden et al., 2006;Hai et al., 2020). Firstly, Fe and Mn are widely involved in microorganism activities. ...
The chemistry of Fe and Mn in natural geochemical systems are closely coupled, as Fe and Mn always co-exist in various forms (e.g., hydrated ions, soluble complexes, and particles) in a variety of surface and near-surface environments (e.g., waters, soils, and sediments) and show strong mutual interactions. The redox cycling of Fe and Mn functions as a “pump” for element cycling and energy flow, assuring the significant roles of Fe and Mn species in environmental system dynamics. Specifically, an increasing number of studies have found that the coupled redox between Fe and Mn can bilaterally affect the crystallization and transformation of the (oxyhydr)oxides of Fe and Mn (i.e., FeOx and MnOx), which are among the most consequential nanominerals and mineral nanoparticles in these environments. In this review, we map the complex reaction networks between Fe and Mn by discussing the reaction characteristics and mechanisms of each coupled system with various co-existing Fe and Mn species (i.e., FeOx-Mn(II), MnOx-Fe(II), and Fe(II)-Mn(II) systems). Due to the higher redox potentials of MnOx/Mn(II) compared to those of FeOx/Fe(II), MnOx can trigger the oxidation of Fe(II) through direct electron transfer, with MnOx undergoing reductive dissolution; while in the FeOx-Mn(II) system, where direct redox reaction between FeOx and Mn(II) is thermodynamically unfavorable, surface-catalyzed oxidation of Mn(II) can be induced by FeOx. In the Fe(II)-Mn(II) system, these species can experience complex homo-and heterogeneous oxidation and crystallization in aerobic environments. The coupled redox cycling of Fe and Mn, which involves crystallization of FeOx and MnOx, dissolution of MnOx substrates, production of reactive oxidants, and the blockage of reactive surfaces of substrates, etc., can exert more complex and significant influences on the biogeochemical processes as compared to individual Fe or Mn.
... Hydrothermal synthesis generally produces hematite NPs with an irregular subspherical morphology. The crystals seem be formed by the aggregation of primary grains of the order of several nm, as shown in Fig. 2 (Madden et al., 2006;Das et al., 2011;Colombo et al., 2012a). ...
Goethite, hematite, ferrihydrite, and other iron oxides bind through various sorption reactions with humic substances (HS) in soils creating nano-, micro-, and macro-aggregates with a specific nature and stability. Long residence times of soil organic matter (SOM) have been attributed to iron-humic substance (Fe-HS) complexes due to physical protection and chemical stabilization at the organic–mineral interface. Humic acids (HA) and fulvic acids (FA) contain many acidic functional groups that interact with Fe oxides through different mechanisms. Due to the numerous interactions between mineral Fe and natural SOM, much research has led into a better identification and definition of HS. In this review, we first focus on the surface colloidal properties of Fe oxides and their reactivity toward HS. These minerals can be efficiently identified by usual techniques, such as XRD, FTIR spectroscopy, XAS, Mössbauer, diffuse reflectance spectroscopies (DRS), HRTEM, ATM, NanoSIMS. Second, we present the recent state of art regarding the adsorption/precipitation of HS onto iron mineral surfaces and their effects on binding metalloid and trace elements. Finally, we consider future research directions based on recent scientific literature, with particular focus on the ability of Fe nano-particles to increase Fe bioavailability, improve carbon sequestration, reduce greenhouse gas emissions, and decrease the impact of persistent organic and inorganic pollutants. The methodology in this field has rapidly developed over the last decade. However, new procedures to estimate the nature of Fe-HA bonds will be important contributions in clarifying the role of natural iron oxides in soil for carbon stabilization.
... Pure anatase TiO 2 NPs display higher sorption capacity mixtures relative to the rutile form [118]. The particle dimension is of paramount importance, for instance, it was observed that the sorption of Cu 2+ on hematite is higher than 7 for 88 nm particles and that the affinity is significantly higher at lower pH for smaller particles [119]. In the same way, the affinity of Pb 2+ for TiO 2 NPs of 20-33 nm was much higher than that on 520 nm materials [20]. ...
In the last decade, metal engineered nanomaterials (ENMs) have seen an exponential
use in many critical technologies and products, as well an increasing release into the environment. Coastal ecosystems worldwide may receive ENM-polluted waters and wastes, with a consequent alteration of habitats and contamination of aquatic biota. There is a scarcity of data regarding the fate of these emerging contaminants in such environments. Open issues include the determination of the sources, the quantification of the interactions with marine sediments, the bioaccumulation pathways, the ecotoxicology on marine fauna and the identification of the principal biotic and abiotic factors that may alter metal ENMs toxicity. Little is known about their potential transference into the food web, as well toxicity features and co-stressors of single or multiple ENMs under laboratory and real environmental conditions for various taxonomic phyla. This review reports current knowledge on the ecological impact of ENMs under the complex environmental conditions of estuary systems, identifies gaps in current knowledge and provides directions for future research.
... Recently, nanoparticles have been used for many medical purposes rather than the same bulky materials because nanoparticles have unique physical and chemical properties and also exhibit different activities, due to limited electronic properties (Hochella 2002;Lead and Wilkinson 2006;Madden et al. 2006;Mohammed and Safwat 2013;Mohammed et al. 2020). Nanomaterials exhibited many biological effects with their potential advantages (Rudramurthy et al. 2016;Abdel-Daim et al. 2019;El-Sayed and Kamel 2020a, b;El-Sayed and Kamel 2020a;Bhattacharya et al. 2021). ...
Nicotine is the most abundant ingredient in cigarette smoking and has serious side effects on the lung, heart, reproductive system, and many other human organs. Saponins extracted from many plants exhibit multiple biological actions such as anti-cancer effects. Therefore, the possible protective effect of fenugreek saponin (FS) and nanofenugreek saponin (NFS) against nicotine-induced toxicity in male rats was investigated in this study. Animals were divided into a control group and the nicotine (1.5 mg/kg/day), FS (25, 50, and 100 mg/kg/day), or/and NFS (20, 40, and 80 mg/kg/day) administered groups. Micronucleus assay, histopathological, and sperm abnormality examinations as well as measurement of the acetylcholinesterase (AChE) gene expression were conducted. Our findings revealed that nicotine treatment induced significant increases in the incidence of micronucleus, sperm abnormalities, and expression levels of AChE in addition to inducing histopathological changes in rat testis. On the other hand, administration of FS or NFS with nicotine significantly decreased the incidence of micronuclei and the percentage of sperm abnormalities as well as the expression levels of AChE gene. Moreover, nicotine-induced histological alterations were reduced by given FS or NFS with nicotine. In conclusion, nicotine-induced sperm abnormalities, chromosomal damage, and histological injuries were mitigated by administration of FS or NFS with nicotine, and thus, FS and NFS could be used as ameliorating agents against nicotine toxicity.
Graphical abstract
... Kinetic studies of nanozymes at various particle sizes indicated that smaller particle sizes resulted in a larger kinetic parameter (e.g., K m , V max ) (Asati et al., 2009), which is attributed to the fact that smaller particle sizes result in a larger specific surface area, surface charge and crystal defects and thus better catalytic performances. The catalytic activity of mineral nanozymes has been reported to be proportional to their specific surface area (Madden et al., 2006;Yin et al., 2015;Gilbertson et al., 2016). Moreover, with a decrease in the particle sizes of minerals, these minerals exhibit more reactive sites, such as edges, corners, steps and kinks (Liu et al., 2018). ...
In Earth systems, thousands of terragrams (Tg) (1 Tg = 10 12 g) of mineral nanoparticles move around annually. Some mineral nanoparticles have exhibited unexpected intrinsic enzyme-like characteristics (so called "mineral nanozymes"), and is ubiquitously distributed in natural ecosystems such as the atmosphere, oceans, waters, and soils. Compared with natural enzymes, these mineral nanozymes have several advantages such as tunable catalytic efficiency and robustness to harsh conditions, e.g., heat, acid, and alkaline conditions. As mineral nanozymes are new products of multidisciplinary cross-cutting, they have been widely applied in various fields. This review, for the first time, systematically introduces the species and properties of mineral nanozymes in Earth systems, discusses the critical roles played by nanozymes in environmental biogeo-chemical cycles, compiles the interfacial processes and mechanisms of mineral nanozymes, and provides an overview of the future prospects of mineral nanozymes.
... The increase in zeta potential seen from the larger cMNP (−49.95 mv) to the rMNP (−22.9 mv) can possibly be explained by the increase in proton accumulation along the edge of the smaller particles [42]. The presence of the silica coating increases the charge considerably from −49.95 to −27.3 mv (Si@cMNP) and slightly from −22.9 to −25.4 mv (Si@rMNP), suggesting silica coated MNP will be taken up more readily by cells. ...
Magnetic magnetite nanoparticles (MNP) are heralded as model vehicles for nanomedicine, particularly cancer therapeutics. However, there are many methods of synthesizing different sized and coated MNP, which may affect their performance as nanomedicines. Magnetosomes are naturally occurring, lipid-coated MNP that exhibit exceptional hyperthermic heating, but their properties, cancer cell uptake and toxicity have yet to be compared to other MNP. Magnetosomes can be mimicked by coating MNP in either amphiphilic oleic acid or silica. In this study, magnetosomes are directly compared to control MNP, biomimetic oleic acid and silica coated MNP of varying sizes. MNP are characterized and compared with respect to size, magnetism, and surface properties. Small (8 ± 1.6 nm) and larger (32 ± 9.9 nm) MNP are produced by two different methods and coated with either silica or oleic acid, increasing the size and the size dispersity of the MNP. The coated larger MNP are comparable in size (49 ± 12.5 nm and 61 ± 18.2 nm) to magnetosomes (46 ± 11.8 nm) making good magnetosome mimics. All MNP are assessed and compared for cancer cell uptake in MDA-MB-231 cells and importantly, all are readily taken up with minimal toxic effect. Silica coated MNP show the most uptake with greater than 60% cell uptake at the highest concentration, and magnetosomes showing the least with less than 40% at the highest concentration, while size does not have a significant effect on uptake. Finally, surface functionalization is demonstrated for magnetosomes and silica coated MNP using biotinylation and EDC-NHS, respectively, to conjugate fluorescent probes. The modified particles are visualized in MDA-MB-231 cells and demonstrate how both naturally biosynthesized magnetosomes and biomimetic silica coated MNP can be functionalized and readily up taken by cancer cells for realization as nanomedical vehicles.
... For corundum plates with a surface predominantly consisting of HSNL-covered basal pinacoids, the interaction with adjacent quartz is very likely obstructed. Nanometersized minerals theoretically have a higher relative surface energy (or zeta potential) than their bulk minerals due to a large ratio of surface area/volume and should react more rapidly than the bulk minerals (Madden et al. 2006). Thin corundum plates adjacent to quartz should, therefore, react faster to Al 2 SiO 5 polymorphs than larger corundum grains with smaller surface area/volume ratio. ...
The metastable paragenesis of corundum and quartz is rare in nature but common in laboratory experiments where according to thermodynamic predictions aluminium-silicate polymorphs should form. We demonstrate here that the existence of a hydrous, silicon-bearing, nanometer-thick layer (called “HSNL”) on the corundum surface can explain this metastability in experimental studies without invoking unspecific kinetic inhibition.
We investigated experimentally formed corundum reaction products synthesized during hydrothermal and piston-cylinder experiments at 500 – 800 °C and 0.25 – 1.8 GPa and found that this HSNL formed inside and on the corundum crystals, thereby controlling the growth behavior of its host. The HSNL represents a substitution of Al with Si and H along the basal plane of corundum. Along the interface of corundum and quartz, the HSNL effectively isolates the bulk phases corundum and quartz from each other, thus apparently preventing their reaction to the stable aluminium silicate. High temperatures and prolonged experimental duration lead to recrystallization of corundum including the HSNL and to the formation of quartz + fluid inclusions inside the host crystal. This process reduces the phase boundary area between the bulk phases, thereby providing further opportunity to expand their coexistence. In addition to its small size, its transient nature makes it difficult to detect the HSNL in experiments and even more so in natural samples. Our findings emphasize the potential impact of nanometer-sized phases on geochemical reaction pathways and kinetics under metamorphic conditions in one of the most important chemical systems of the Earth’s crust.
... It can be seen that hem-1 µm had a compacted-irregular shape while hem-80 nm had a rhombohedral shape (Fig. 1c,d) which concurs with previous studies [16,29]. The external SSA of hem-1 µm and hem-80 nm as measured by the N 2 -BET method were 4.838 and 33.1 m 2 /g, respectively [30]. ...
Microorganisms are commonly bonded to various soil minerals, which may influence the redox processes and bacterial metabolism. However, little is known about the impact of particle size of soil minerals on these redox processes in the subsurface environment. In this study, the Cr(VI) bioreduction by Shewanella oneidensis MR-1 (S. oneidensis) was investigated in the presence of various hematite (α-Fe2O3) particles with average diameters of 1.0 μm (hem-1 μm) and 80.0 nm (hem-80 nm) under different pH conditions. Fourier transformed infrared spectroscopy coupled with two-dimensional correlation spectroscopy (FTIR-2D-COS) analysis and isothermal titration calorimetry (ITC) were used to explore the interaction between S. oneidensis and hematite and monitor the bacterial metabolic activity, respectively. X-ray photoelectron spectroscopy (XPS) was used to elucidate the Cr(VI) removal mechanisms. Our results showed that 78% of chromate can be reduced to Cr(III) by S. oneidensis alone. Whereas, chromate reduction rates were 62% and 85% in the presence of hem-1 μm and hem-80 nm, respectively. The enhancement of Cr(VI) reduction by S. oneidensis-hem-80 nm complex may be due to the large surface area as well as the positive charge of hem-80 nm at neutral pH, which influences the physical contact between S. oneidensis and iron oxides. The microcalorimetric results showed that both hem-1 μm and hem-80 nm promoted the normal physiological functions of S. oneidensis. XPS confirmed the gradual FeCr2O4 formation and Fe(II) depletion during the Cr(VI) reduction process. This work expands our understanding of microbial-mineral interaction and its role in Cr(VI) removal mechanisms in the subsurface environment.
... Iron oxides including goethite and hematite are abundant in natural environment including soil and sediment (e.g., the concentrations in the soil and sediment samples are in the range from a few mg/L to several hundred mg/L) (Li et al., 2019b;Wang et al., 2015a. Since iron oxides can interact with various substances, they have been exhibited considerable effect on the fate and transport of metal ions (e.g., lead (Hassellov and von der Kammer, 2008), arsenic (Yean et al., 2005), and copper (Madden et al., 2006;Wang et al., 2011a,b)) and engineered nanoparticles (e.g., silver nanoparticles (El-Badawy et al., 2013) and GO Duster et al., 2016;Qi et al., 2019;Wang et al., 2017)). For example, Yean et al. (2005) found that desorption of arsenic from smaller magnetite nanoparticles exhibited stronger desorption hysteresis due to the formation of inner-sphere surface complexes (i.e., highly stable iron-arsenic complexes). ...
Since iron oxide minerals are ubiquitous in natural environments, the release of graphene oxide (GO) into environmental ecosystems can potentially interact with iron oxide particles and thus alter their surface properties, resulting in the change of their transport behaviors in subsurface systems. Column experiments were performed in this study to investigate the co-transport of GO nanoparticles and hematite colloids (a model representative of iron oxides) in saturated sand. The results demonstrated that the presence of hematite inhibited GO transport in quartz sand columns due to the formation of less negatively charged GO-hematite heteroaggregates and additional deposition sites provided by the adsorbed hematite on sand surfaces. Contrarily, GO co-present in suspensions significantly enhanced the transport of hematite colloids through different mechanisms such as the increase of electrostatic repulsion, decreased physical straining, GO-facilitated transport of hematite (i.e., highly mobile GO nanoparticles served as a mobile carrier for hematite). We also found that the co-transport behaviors of GO and hematite depended on solution chemistry (e.g., pH, ionic strength, and divalent cation (i.e., Ca²⁺)), which affected the electrostatic interaction as well as heteroaggregation behaviors between GO nanoparticles and hematite colloids. The findings provide an insight into the potential fate of carbon nanomaterials affected by mineral colloids existing in natural waters and soils.
... This behavior is attributed to the crystallite size and purity of the particles, which were higher for MF-60 > MF-70 > MF > 80, in line with the adsorption behavior. These material features provide high particle size, surface area, and a larger number of active surface sites (corners, edges, steps), as well as hydroxyl groups, to facilitate and improve the metal adsorption [29,30]. Many magnetic particles based on MFs have been tested for Nd(III) and heavy metal removal from aqueous solutions. ...
Neodymium is a key rare-earth element applied to modern devices. The purpose of this study is the development of a hybrid biomaterial based on chitosan (CS) and manganese ferrite (MF) for the recovery of Nd(III) ions from the aqueous phase. The preparation of the beads was performed in two stages; first, MF particles were obtained by the assessment of three temperatures during the co-precipitation synthesis, and the best nano-MF crystallites were incorporated into CS to obtain the hybrid composite material (CS-MF). The materials were characterized by FTIR, XRD, magnetization measurements, and SEM-EDX. The adsorption experiments included pH study, equilibrium study, kinetics study, and sorption-desorption reusability tests. The results showed that for MF synthesis, 60 °C is an appropriate temperature to obtain MF crystals of ~30 nm with suitable magnetic properties. The final magnetic CS-MF beads perform maximum adsorption at pH 4 with a maximum adsorption capacity of 44.29 mg/g. Moreover, the material can be used for up to four adsorption-desorption cycles. The incorporation of MF improves the sorption capacity of the neat chitosan. Additionally, the magnetic properties enable its easy separation from aqueous solutions for further use. The material obtained represents an enhanced magnetic hybrid adsorbent that can be applied to recover Nd(III) from aqueous solutions.
... Peacock and Sherman [45] described the adsorption of Cu(II) ion by considering surface complexes (Fe−O¯ + Cu²⁺ → Fe−OCu⁺) on the hematite particles. Unpaired bonds at the surface of hematite create a localized electric field and electric potential further increases if the particles approach nanosize (typically < 7 nm) because of a decrease in the symmetry of the bonding environments on the smaller particles relative to the larger particles, resulting in relatively higher uptake of the cation [48]. However, Tombácz et al. [49] found that for lower pH values (pH < ~2), where the activity of the proton is high, protonation (Fe−O¯ + H⁺ → Fe−OH + H⁺ → Fe−OH₂⁺) of the surface sites of the hematite particles takes place. ...
The purification of hydrometallurgical process solutions by Fe(III) precipitation is a common and large-scale industrial operation. This step is notorious for valuable metal loss occurring with the iron precipitation product, which is usually directed to tailings. In this study, factors affecting Fe(III) precipitation and associated copper loss were studied in synthetic process solutions using statistical methods. The variables studied were: Initial acid concentration, retention time, seed addition, and initial Fe(III), Cu(II), and chloride concentrations. The importance of each variable and its interaction effects were studied against two responses, i.e., percent of Fe(III) precipitated as hematite and percent of Cu lost to solids. The results showed that a combination of high acid and moderate seeding was required to simultaneously achieve high proportions of Fe(III) precipitated as hematite and lower copper loss to the precipitates. High acid concentrations create low supersaturation for Fe(III), which minimizes the consequences of homogeneous nucleation and favors particle growth.
... Among the endpoints evaluated in zebrafish exposed to NPs, larval body length proved to be the most sensitive and repeatable biomarker [11]. Spine malformations and pericardial oedema were observed in the early life stages of the zebrafish D. rerio (Fig. 3). ...
... Abbas et al. [172] speculated a shift in the pH pzc of metal oxides, occurring with the decrease in size, particularly when it is less than 5 nm. Madden et al. [173] observed a remarkable shift in pH edge for the 7 nm hematite (Fe 2 O 3 ) when compared with the 25 nm and 88 nm samples. ...
... Under different pH conditions, the adsorption mechanism and power of Cu(II) on HFO were different, leading to the difference in the total Cu removal effect. At pH 2, the main hydrolysis products of Cu(II) ions in solution are Cu 2+ and CuCl + , and the hydroxyl groups on the surface of HFO are protonated; thus, the positive charge of HFO solid-liquid interface is not conducive to the adsorption of Cu 2+ or CuCl + on its surface (Tamez et al., 2016;Madden et al., 2006). As pH increased, the hydroxyl groups on the surface of HFO gradually deprotonated, and the solid-liquid interface of HFO tended to be negative. ...
In this study, a polymer-supported, nanosized, and hydrated Fe(III) oxide (HFOD) was developed as a Fenton-like catalyst for the efficient removal of metal complexes in water. HFOD was prepared through the irreversible impregnation of hydrated iron(III) oxide (HFO) nanoparticles into cation exchange resin and characterized through X-ray photoelectron spectroscopy (XPS) and ion chromatography. The mechanism of Cu(II) ion removal and the degradation pathway of Cu(II)-citrate were analyzed through UV-vis spectrophotometry (UV) and liquid chromatography-mass spectrometry (LC-MS). The optimal removal rate of Cu(II) and TOC by a Fenton-like reaction at pH 4 and 40 mM H2O2 reached 81.6 % and 75.6 %, respectively. The removal efficiency of Cu(II)-citrate was remarkably affected with the addition of humic acid. However, the addition of competitive ions did not significantly reduce the removal rate of Cu(II)-citrate, thereby proving that the Fenton-like reaction by HFOD had a certain salt tolerance. Simultaneously, hydroxyl radical (•OH) was verified as the main free radical for Cu(II)-citrate degradation in a Fenton-like reaction, and citrate degradation was a process decarboxylation. HFOD recycling experiments and stability experiments showed that HFOD had high stability with good acid/alkali resistance and showed remarkable potential in the practical application of fixed-bed as catalysts for Fenton-like reactions.
... But at higher concentrations, these may also harm the health of the soil, plants, and microorganisms as these can cause toxicity and improve generation of reactive oxygen species that result in interruption of cellular metabolism (Siddiqi and Husen 2017). The bioavailability and ecotoxicity of nanoparticles depend upon different phases, i.e., dissolved, colloidal, or particulate phase, size, shape, and surface area (Pal et al. 2007;Madden et al. 2006;Siddiqi and Husen 2016). Michálkov et al. (2014) studied the stabilization of three oxides of Fe and Mn nanoparticles for elimination of Cd, Cu, and Pb from soil. ...
Anthropogenic pollution caused by excessive use of chemicals, metals, radioactive substances, and organic pollutants has deteriorated quality of environmental assets, i.e., air, water, and soil. To restore the quality of all these essential systems, scientists and researchers are trying to stabilize contaminants in-situ rather than in in-vivo conditions. One of such effort is phytoremediation, which utilizes the application of green plants, herbs, and shrubs at contaminated sites to restrict the movement of pollutants and to decontaminate polluted sites. Application of various kinds of plants for bioremediation of polluted soils is an eco-friendly approach, with negligible effect over environment and also without disturbing the soil physicochemical properties. This technology also offers an opportunity to rejuvenate precious metals and utilize left biomass for the production of bioenergy. The present book chapter deals with different aspects of phytoremediation processes for remediation and recovery of contaminated soil and improving its efficiency with augmentation of different organic and inorganic amendments. Soil amendments can lessen up the bioavailability of contaminants in soils and decrease the risk of food chain contamination. These amendments include the application of biochar, vermicomposting, slow-release fertilizers, and nanoparticles to the soil to enhance the phytoremediation process. Role of these amendments on bioavailability of contaminants, their uptake, translocation, bioaccumulation, and its effect on growth and developments of plants has been thoroughly addressed in the present chapter. Further, different constraints like slow growth rate and effect of seasonal variations on development of plants have also been discussed.
... At pH > pH zpc , the IONP surfaces became negatively charged resulting from deprotonation reactions. The decreasing protons concentration at magnetite surface favors cation attraction and allows Cu 2þ to complex onto the FeO À surface sites (Madden et al., 2006;Huang and Chen, 2009). According to ATR-FTIR analyses performed on magnNP after Cu addition (Cu ¼ 0.5 mM) (SI-D), the adsorption of Cu onto magnNP was discernable by the vibration at 1350 cm À1 . ...
The oxidation of magnetite into maghemite and its coating by natural organic constituents are common changes that affect the reactivity of iron oxide nanoparticles (IONP) in aqueous environments. Certain ubiquitous compounds such as humic acids (HA) and phosphatidylcholine (PC), displaying a high affinity for both copper (Cu) and IONP, could play a critical role in the interactions involved between both compounds. The adsorption of Cu onto four different IONP was studied: magnetite nanoparticles (magnNP), maghemite NP (maghNP), HA- and PC-coated magnetite NP (HA-magnNP and PC-magnNP, respectively). According to the results, the percentage of adsorbed Cu increases with increasing pH, irrespective of the IONP. Thus, protonation/deprotonation reactions are likely involved within Cu adsorption mechanism. Contrary to the other studied IONP, HA-magnNP favor Cu adsorption at most of the pH tested including acidic pH (pH = 3), suggesting that part of the active surface sites for Cu2+ were not grabbed by protons. The Freundlich adsorption isotherm of HA-magnNP provides the highest sorption constant KF (bonding energy) and n value which supports a heterogeneous sorption process. The heterogeneous adsorption between HA-magnNP and Cu2+ can be explained by both the diversity of the binding sites HA procured and the formation of multidendate complexes between Cu2+ and some of the HA functional groups. Such favorable adsorption process was neither observed on PC-coated-magnNP nor on maghNP, whose behaviors were comparable to that of magnNP. On another hand, HA and PC coatings considerably reduced iron (Fe) dissolution from magnNP as compared with magnNP. It was suggested that HA and PC coatings either provided efficient shield against Fe leaching or fostered dissolved Fe re-adsorption onto the functional groups at the coated magnNP surfaces. Thus, this study can help to better understand the complex interfacial reactions between cations-organic matter-colloidal surfaces which are relevant in environmental and agricultural contexts.
This work showed that magnetite NP properties can be affected by surface modifications, which drive NP chemical stability and Cu adsorption, thereby affecting the global water chemistry.
Because of numerous applications of nanoparticles, nanotechnology is a rapidly emerging subject for biotechnology field. Environment friendly techniques for developing nanoparticles (NPs) offer various benefits, including more surface area, potent catalytic activity, and the ability to put the metal salt and enzyme into optimal contact. Among many protocols of developing NPs, mycoproteins, enzymes, and reducing agents may be used to synthesize metal NPs from metal salts. In recent years, scientists have investigated the intracellular and extracellular chemistry of fungi which is helpful to produce metal NPs. Metal NPs have garnered considerable attention due to their cytotoxic effect on cancer cell lines and their potential as an antibacterial agent to inhibit the development of dangerous pathogens. Several research studies have examined the possibilities of metal NPs in health care, pharma, and agriculture. To recover contaminated environments, scientists may use bioremediation; however, this is not guaranteed to be successful. Nanoparticles may be effective for bioremediation because they have a lower toxicological impact on microorganisms and can boost the biological activity of the materials. Mycoremediation is often studied because it is a way to clean up polluted areas by using NPs to neutralize dangerous chemicals.
Eutrophication creates multiple environmental problems, threatening the ecological security and sustainability of estuarine and coastal ecosystems worldwide. Key nutrients of concern are nitrogen (N) and phosphorus (P), which are the main controls in eutrophication. Considering that sediments are inseparable sinks of N and P, concern has grown regarding the forms in which N and P occur in the surface sediments of estuaries and coastal areas. Nonetheless, studies on the natural N-bearing or P-bearing nanoparticles in estuarine and coastal sediments have rarely been reported. Herein, the surface sediments (0–5 cm) of the Pearl River Estuary in China were collected and subjected to analysis. Using high-resolution transmission electron microscopy (HR-TEM) analysis, numerous natural N-bearing and P-bearing nanoparticles were observed. The results revealed that there are some differences in the occurrence forms of N and P in nanoparticles, suggesting that N and P could be adsorbed by nanoparticles of minerals such as hematite, goethite, muscovite, anorthite and quartz in estuarine and coastal environments, and further form N-bearing and P-bearing nanoparticles. These nanoparticles contained small amounts of N (1.52–3.73 wt%) and P (0.22–1.12 wt%), and were mainly single crystal or polycrystalline in form, with sizes ranging from 10 nm × 50 nm to 250 nm × 400 nm. In addition, P was shown to exist in the form of Ca and Fe phosphate nanoparticles in the estuarine sediments. The Ca and Fe phosphate nanoparticles had higher phosphorus content (5.02–9.97 wt%), mainly amorphous, with sizes ranging from 50 nm × 120 nm to 250 nm × 400 nm. Moreover, N-bearing and P-bearing nanoparticles could influence the migration, precipitation and release processes of N and P, and play a certain role in the N-cycling and P-cycling of estuarine and coastal ecosystems. Furthermore, we explored the role of N-bearing and P-bearing nanoparticles in the N-cycling and P-cycling in estuarine and coastal ecosystems. Thus, this study could provide new ideas for water environment management and other related research fields.
Pyrite is common in coal gangue formed in a reduced environment and has a significant influence on the recycling utilization of coal gangue. Four types of pyrite with different structures and morphologies, namely octahedral pyrite framboid, pentagonal dodecahedral pyrite framboid, irregular grain pyrite, and euhedral octahedral crystalline pyrite, were selected from coal gangue to study their thermal behavior and phase transformation processes during heat treatments in different atmosphere via high-temperature XRD, TG-DSC, SEM-EDS, and XRF. The results show that pyrite in coal gangue can be decomposed into pyrrhotite when heated in an inert atmosphere. Pyrites with different morphologies and structures exhibit distinct thermal behavior. The higher S/Fe ratio promotes the better crystallinity of pyrite and thus lowers the phase transformation temperature from pyrite to pyrrhotite. The thermal decomposition temperature of the sulfur-rich and well crystalline pyrites was approximately 600 °C, and the transformation temperature of sulfur-deficit pyrites to pyrrhotite was approximately 640 °C. The S release rate from the former two pyrites was also slower than that from the latter two. The pyrites can be oxidized to nano-hematite when heated in an air atmosphere at above 400–500 °C, with the transformation proceeding in the sequence of pyrite–nano-microsphere magnetite–nano-microsphere hematite. The nano-hematite aggregates had a pseudomorph of the original pyrite crystals. This study has great significance for the high-level recycling utilization of coal gangue, such as iron removal and direct material.
The designed novel hybrid laminate membranes have been synthesized with silane surface functionalized Kevlar fabric packed between two layers of cobalt bismuth nanoferrites (CBF)‐loaded cellulose acetate. Primarily, nanoreinforcement; Co‐Bi nano‐ferrites were synthesized using sol–gel technique and functionalization is carried out using 3‐aminpropyl‐tri‐methoxysilane. Secondarily, thermally assisted evaporation methodology coupled with dip‐coating is adopted to synthesize hybrid Kevlar laminate membranes. Finally, structural, thermal, and flux along rejection studies are done on resultant hybrid membranes. The surface morphology, porosity generation, and homogeneous distribution of nanoincorporation with various concentration are revealed from micrographs of SEM of membranes. Noteworthy, impact of nanoreinforcement has been observed to holdup thermal oxidation of synthesized unique formulations of hybrid membranes employing TGA. Presence of CBF in cellulose acetate (CA)/Kevlar laminate membranes are modified glass transition and crystallization temperatures as cleared in DSC. De‐ionized water for flux and heavy metal salts rejection capabilities of each formulated membrane were tested at applied pressures 60, 120, and 180 psi. The important findings are indicated that, increasing concentration of CBF loading in CA remarkably enhanced efficacy of metal‐nitrates rejection with membranes. It is attributed to sole magneto‐physiochemical behavior of nanoferrites that, facilitate enhanced adsorption in designed hybrid membranes for heavy metal rejection from industrial wastes.
Most properties of charged particles’ dispersions are closely related to the mount of charge residing on the surface of particles. Very few works have been done so far to investigate curvature-dependence of surface potential and charge density of dispersed charged particles. Although there are some experimental studies in the literature on this subject, only a few of them have examined the mathematical relationship between particle size and surface charging parameters, resulting in contradictory findings. Unfortunately, the majority of the limited research efforts on surface charging have focused on dispersion of charged particles in aqueous solutions, but there are systems in biological and engineering worlds where an electrolyte solution is encapsulated in the core of charged particles and immersed in a background non-electrolyte medium. In an attempt to fill this scientific gap and gain a better understanding of surface charging, analytical models are developed in the present work for prediction of surface potential, surface charge, and surface charge density for cylindrical and spherical particles in such systems.
First, the problem under investigation is non-dimensionalized to facilitate the modeling process, consider the simultaneous effects of key variables, and generalize the results and behaviors. Then, the mutual relationship between every pair of variables is extracted by employing Gauss's law and combining it with the electro-neutrality condition in an aqueous cavity. Afterwards, the resulting generalized seemingly simple, but inherently complex, integral equations are solved to build the characteristic curves and regressed to derive explicit characteristic equations. Finally, the derived characteristic equations are transformed into a dimensional form to obtain simple formulas for explicit calculations of charging parameters on the surface of electrolyte-encapsulating charged particles in terms of easily measurable properties of the electrolyte solution, the size of the cavity, and thermodynamic conditions. This predictive capability of the new models will significantly reduce the time, costs, and efforts required to simultaneously characterize the size and charging parameters of particles because it removes the need for such a challenging measurement as charging parameters are explicitly related to particle size through thermodynamic conditions, as well as available or easy-to-measure properties of the solution.
Nanoparticles (NPs) in the Earth surface systems have been a global concern due to their abnormal effects and their scientific significance and application value in environmental remediation and resource exploration. By means of comparative and comprehensive studies: (1) A description of the development of techniques for detecting NP characteristics and comparisons between those techniques is summarized; (2) the characters of typical NPs, including minerals at the nanoscale, nano organic matter, natural metal NPs, nanoinclusions, and NPs in soil and water are summarized; (3) the physical, chemical, microbial, and multigeological origins of NPs are revealed; and (4) the abnormal properties of NPs including the size, migration, aggregation, and adsorption effects and oxidation-reduction reactivity, are elaborated upon.
The formation, migration, deposition, and accumulation of NPs in the Earth surface are closely related to the environment evolution and resource enrichment, and therefore NPs have environment and resource effects. On this basis, the uses of NPs in environmental remediation and resource exploration are clarified: (1) uses of NPs and modified NPs and combinations of NPs and other remediation technologies are introduced, and the main factors influencing the remediation effect, such as the soil characteristics, hazardous element species, and NP themselves, are examined; (2) NPs originating from plutonic fluid and weathering can indicate the mineralizing mechanism and instruct the identification of concealed deposits. For gas and oil exploration, organic NPs can reflect hydrocarbon-generating processes and determine gas and oil occurrence states. Organic NPs are also widely involved in the mineralization process of metal elements and thus can indicate the coexistence characters of organic matter and metal.
Bioremediation techniques have become noticeable and valuable tools to reduce, reuse, and recycle different industrial effluents through eco-friendly practices. Industries are well known to release anthropogenic-related chemicals into the environment over the century and consequences are witnessed as contamination of soil, water, and air, respectively. The untreated or impertinently treated wastewater effluents are known to be toxic to plants and animals, including humans that lead to negative impacts on the earth. Remediation has emerged for degrading contaminants using physical, chemical, and biological methods. Bioremediation techniques are used nowadays around the world meticulously. It is technology based along with the combined action of plants and associated microbial communities to degrade, remove, transform, or immobilize toxic compounds in effluents. This chapter discusses the classes of organic effluents, toxicological mechanism, and its environmental impact and also emphasizes the current and advanced eco-friendly techniques in the remediation of organic effluents through microbial, algal bioremediation and phytoremediation. Bioremediation techniques are potential, cost-effective, and in addition to that remains as a solution to the challenge of treating many classes of contaminants, compared to the conventional chemical and physical methods, which are often very expensive and ineffective compared to biological methods.
Protein adsorption onto mineral nanoparticle surfaces is critical to the function and fate of biological compounds in environmental and industrial systems. However, adsorption kinetics, coverage, and conformation of biological macromolecules are poorly understood, particularly in the presence of ubiquitous oxyanions. In this study, the adsorption of two proteins, beta-lactoglobulin (β-LG) and bovine serum albumin (BSA), onto hematite (α-Fe2O3) nanoparticles was investigated in the presence and absence of pre-adsorbed phosphate. Using solution and temporal solid-phase attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, our results show dynamic changes in the secondary structure of both proteins when adsorbed onto nanoscale α-Fe2O3 surfaces, compared to their unbound conformations. However, these differences were attenuated in the presence of adsorbed phosphate. Adsorbed phosphate significantly reduced the protein surface coverage on iron oxide nanoparticle surfaces, and impacted protein adsorption kinetics. The latter was observed to be protein-specific, with β-LG exhibiting a higher adsorption rate and sigmoidal kinetics compared to slower, more Langmuir-type kinetics of BSA adsorption. Our results reveal the importance of phosphate on protein-mineral adsorption kinetics and conformation, a critical driver of protein function, in complex environmental systems.
Metallic oxide nanoparticles (NPs) anchored in biochar provide a promising measure forward into the scaled-up application of these NPs in water treatment, and reducing the size of the dwelled NPs is expected to boost the adsorption performance of biochar-based composites because of the size and surface effect. Nevertheless, it is still of great challenge to regulate the size of the impregnated NPs due to their intrinsic self-agglomeration caused by high surface energy. In this study, we fabricated the charged biochar (C-BC) bearing high-density negatively charged groups (i.e., carboxyl and hydroxyl groups) via HNO3 oxidization to load the model metal oxide FeOOH NPs. The average sizes of anchored FeOOH NPs were ultrasmall, ranging from 19.9 ± 1.5 to 3.1 ± 0.5 nm, and decreased with the increased amount of carboxyl and hydroxyl groups in C-BC. Whether in batch adsorption or fixed-bed column setting, adsorption of Cd(II) onto the as-made composites was greatly enhanced by carboxyl and hydroxyl groups in carrier. The normalized adsorption capacities of Cd(II) by ferric mass of the loaded FeOOH were 499.9 to 724.9 mg/g-Fe, approximately 18.6 to 27.1 and 2.51 to 3.64 folds over the bulky FeOOH and FeOOH-impregnated biochar. Our study results should provide a significant reference on how to acquire highly efficient biochar-based composites for water decontamination.
Nanoparticles are assemblies of atoms in the size range less than 100 nanometers. At these length scales, the properties of particles may deviate significantly from those of the equivalent bulk material indicating that changes in physical and chemical properties of materials depend on the dimensions of the particle. The presence of mineral nanoparticles has been reported in a range of natural environments. Such nanoparticles can arise from a variety of mechanisms, including chemical weathering processes, precipitation from relatively saturated solutions in hydothermal and acid mine drainage environments, evaporation of aqueous solutions in soils, and biological formation by a variety of different microorganisms. Furthermore, recent increased applications of nanoparticles in different types of industries, including construction and building material manufacturing, have caused prevalent occurrences of different types of synthetic nanoparticles in the environment. In this chapter, a comprehensive reviews on occurrences and observations of naturally and anthropogeniccally generated nanoparticles in the environment and their characterization techniques will be discussed along with directions and suggestions for the future research topics and areas for nanomaterials.
Hematite (α-Fe2O3) exerts a strong control over the transport of minor but critical metals in the environment and is used in multiple industrial applications; the photocatalysis community has explored the properties of hematite nanoparticles over a wide range of transition metal dopants. Nonetheless, simplistic assumptions are used to rationalize the local coordination environment of impurities in hematite. Here, we use ab initio molecular dynamics (AIMD)-guided structural analysis to model the extended X-ray absorption fine structure (EXAFS) of Cu2+- and Zn2+-doped hematite nanoparticles. Specific defect-impurity associations were identified, and the local coordination environments of Cu and Zn both displayed considerable configurational disorder that, in aggregate, approached Jahn-Teller-like distortion for Cu but, in contrast, maintained hematite-like symmetry for Zn. This study highlights the role of defects in accommodating impurities in a nominally low-entropy phase and the limits to traditional shell-by-shell fitting of EXAFS for dopants/impurities in unprecedented bonding environments.
地球系统中每年周转的矿物纳米颗粒有数十亿吨. 部分矿物纳米颗粒已被证明具有类酶活性(称为“矿物 纳米酶”), 并广泛分布在大气、海洋、水体、土壤等生态环境中. 与天然酶相比, 矿物纳米酶催化效率可控, 同时 对热、酸、碱具有较大范围的耐受性. 作为多学科交叉融合的新产物, 矿物纳米酶已被广泛应用于多个学科领 域. 本文系统梳理了地球系统中矿物纳米酶的种类与特性, 探讨了矿物纳米酶在环境生物地球化学循环中的关键 作用, 重点阐述了矿物纳米酶的界面过程与机制, 最后展望了地球系统中矿物纳米酶的优先发展方向.
In this work, iron-based nanomaterials (IONs-1 and IONs-2) were synthesized from zero-valent iron nanoparticles (IONs-0) and characterized. IONs-1 were synthesized from IONs-0 by hydrogen peroxide treatment, followed by calcination at 350 °C. IONs-2 were synthesized from IONs-0 by sequential treatment with hydrogen peroxide and sodium hydroxide, respectively. IONs-1 presented the following phases: superparamagnetic Fe³⁺ (7%), α-Fe2O3 (17%), and γ- Fe2O3 (76%). IONs-2 presented the following phases: superparamagnetic Fe³⁺ (5%), γ-Fe2O3 (51%) and Fe3S4 (54%). Through microscopy images, it was possible to verify that the IONs-1 and IONs-2 exhibit, respectively, nanorod and well-defined nanosheets formats. The specific surface areas of IONs-1 and IONs-2 were 85.37 and 223.84 m² g⁻¹, respectively. IONs-1 and IONs-2 were used to remove Direct Red 80 dye, resulting in about 100% removal.
As one of the most important energy resources in the world, coal contributes a great deal to the world economy. Coal mining and processing involve multiple dust generation processes including coal cutting, transport, crushing and milling etc. Coal dust is one of the main sources of health hazard for the coal workers. Exposure of coal dusts can be prevented through administrative controls and engineering controls. Ineffective control of coal dust exposure can harm coal workers’ health. Although many efforts have been made to eliminate these threats, recent years have seen an unexpected increase in coal workers’ pneumoconiosis (CWP) in Appalachian basin in US. To explore the reasons for this phenomenon, in this review, we first reviewed the historical studies on coal mine dust including the regulation and engineering controls. Then, the effects of coal dust on human health was comprehensively reviewed. Next, the effects of nanoparticles on human health were reviewed, with an emphasis on toxicity of nanoparticles such as carbon nanotubes in other industries. From all this information, we hypothesize that nano-sized coal dust has contributed to the increase of CWP prevalence in recent years. As no research has been reported in this area, four directions which may need further investigation and future studies are recommended in this review. They include: 1) Systematic characterization of physicochemical properties of nano-size coal dust; 2) Toxicity and pathogenesis of nano-sized coal dust; 3) Development of real-time monitoring technology and equipment for nano-sized coal dust; 4) Development of exposure control technology and equipment. The intent of this review paper is to demonstrate the variation of coal dust properties and their impact on the mine worker’s health. We suggest that the impact of nano-sized coal mine dust on miner’s health has not yet been understood well and further improvements are necessary.
Surface electric charge of dispersed particles is an essential determinant of physicochemical properties, coagulation and flocculation processes, and stability of colloidal solutions. Size-dependence of surface potential, charge density, and total...
The dispersion and coagulation of soil colloidal particles concern highly with their mobility and activity, as well as the role played in biogeochemical cycle of elements. Particle size is an important factor that affects both the van der Waals potential energy and electrostatic potential energy. However, the size effect of soil particles on surface charge properties and suspension stability has rarely been investigated. Results showed that the zeta potentials (in absolute values) of soil colloidal particles (CP, particle diameter less than 1000 nm) were higher than soil nanoparticles (NP, particle diameter less than 100 nm) for the same solution pH, while the specific surface area of soil NP was 1.6 times of soil CP; taken together, the surface charge density of soil NP was smaller than that of soil CP and the surface charge number of soil NP was slightly higher than soil CP. The stability of soil NP and CP was also different. The critical coagulation concentration (CCC) of soil NP was 1.4 times of soil CP, indicating higher mobility of smaller soil particle in natural conditions. Based on DLVO theory, the Hamaker constants of soil NP and CP were simulated to be 2.06 × 10⁻²⁰ J and 1.86 × 10⁻²⁰ J. It could be concluded that the size effect of soil particle influences suspension stability and particle mobility through its effect on Hamaker constant. The results could deepen our understanding for aggregation mechanisms of soil colloid-sized particles and further help in predicting their environmental behaviors.
Gatifloxacin (GAT) is a new generation fluoroquinolone antibiotic and its adsorption onto iron minerals influenced by coexisting trace elements [e.g., Cu(II)] has not been well investigated. To evaluate the adsorption behavior of GAT and Cu(II) onto goethite and hematite, the complexation constants of GAT with Cu(II) were determined using potentiometric titration, and the effects of Cu(II) concentration and solution pH on GAT adsorption were investigated using batch experiments. It was observed that GAT adsorption was negatively correlated with molar concentration ratio of Cu(II) to GAT. In our experimental pH range (i.e., 3.0–10.8), the calculated main species involved in GAT adsorption were Cu(GAT±)²⁺ and Cu(GAT±)2²⁺ under acidic to neutral conditions, and formation of Cu(GAT⁻)2(s) facilitated the removal of GAT from solution under alkaline condition. The adsorption data were well fitted by the Freundlich model and showed high nonlinearity. In adsorption onto goethite, the primary interactions shifted from electrostatic repulsion to formation of goethite–Cu(II)–GAT ternary surface complexes with increase of GAT concentration. For hematite, electrostatic repulsion was the main inhibiting mechanism and became stronger with increase of Cu(II) concentration. Our findings suggest that it is necessary to consider the complexation between GAT and coexisting metal cations in evaluating its transport in soils rich in different iron minerals.
Nanoparticles can effectively remove organic dyes from wastewater due to their excellent adsorption performances which are closely related to particle size. Herein, the relations between the thermodynamic properties and the particle size were deduced, and the influence mechanisms were also discussed. Then the adsorptions of four dyes on cubic nano-CeO2 were investigated. We found that cubic nano-CeO2 has strong adsorption selectivity for four dyes and the size dependence of the adsorption thermodynamic properties. This strong selectivity is attributed to electrostatic interaction and hydrogen bonding. Furthermore, with the particle size decreasing, the logarithm of standard adsorption equilibrium constant (lnKo) increase, while the standard molar Gibbs energy change of adsorption (ΔadsGmθ), the standard molar adsorption enthalpy change (ΔadsHmθ) and the standard molar adsorption entropy change (ΔadsSmθ) decrease, and there are good linear relationships between this thermodynamic properties and the reciprocal of the particle size. The essences of effect of particle size on adsorption thermodynamics are that lnKθ and ΔadsGmθ are influenced by both the molar surface area and the difference in surface tensions after adsorption, ΔadsHmθ by the molar surface area, the differences in surface tensions and in temperature coefficients of surface tensions, ΔadsSmθ by the molar surface area and the difference in temperature coefficients of the surface tensions.
Soil chemists have long-recognized that knowledge of the elemental composition of soils is generally of little use in assessing the availability of these elements to plants. An obvious illustration of this principle is the common occurrence of Fe and Mn deficiency in plants despite the relatively high levels of Fe and Mn in many soils. For this reason, chemical soil tests have relied on measurement of extractable or “labile” fractions of elements. Such tests are empirical and provide little basis to relate metal extractability to the chemical forms of the metal in the soil. As soils are increasingly used in our society for purposes other than agriculture, the frequency and extent of soil contamination by toxic metals will increase. Empirical relationships may have to be replaced by a more fundamental understanding of the soil processes controlling metal solubility to prevent practices that could have deleterious effects on soil productivity and environmental quality.
Adsorption-desorption studies using copper(II) as the adsorbate and synthetic crystalline iron oxide, goethite (á-FeO.OH), as the adsorbent have been carried out on the acid side of the pH of isoelectric point (7.5) in the presence of a large excess of indifferent electrolyte. Specific adsorption of copper(II) on goethite is accompanied by the release of one to two moles of protons per mole of cation adsorbed. Desorption experiments have revealed two types of adsorption sites for copper(II) on the oxide surface, one of low bonding energy and the other of high bonding energy, corresponding with the 'readily desorbed' and 'less readily desorbed' fractions of copper(II) respectively. It is suggested that the two kinds of bonding are associated with the cation being coordinated to one and two surface-OH moieties respectively. The apparent hysteresis between adsorption and desorption isotherms is attributed to the existence of two types of adsorption sites of differing binding constants corresponding with the 'readily desorbed' and 'less readily desorbed' fractions of the cation respectively. The gradual interchange of some 'readily desorbed' copper(II) into a category that is not readily desorbed after an initial 'time-lag' between adsorption and desorption is attributed to a possible time-dependent reaction involving isomorphous substitution of lattice Fe3+ by Cu2+ of comparable ionic size. Some aspects of the relevance of present work to soil-plant system are indicated.
A variety of evidence suggests that the fluids forming stratiform and red-bed copper deposits derived their Cu, Ag, and other metals from adjacent red sandstones and shales. To examine variability in relative adsorption of 0.5 mg/l Ag, Co, Cu, Ni, Pb, and Zn, adsorption on goethite has been measured as a function of pH, temperature and redox state. The observed range of metal associations in red-bed and stratiform copper deposits seems explainable by variations among districts in pH, Eh, temperature, major element content of pore fluid, and Fe oxide character of the diagenetic environment. Similar adsorption phenomena may account for varying metal ratios in other low-temperature ore deposits and in noneconomic metal enrichments. -from Authors
The difference of the point of zero charge (PZC) and isoelectric point (IEP) values for γ- and α-Fe2O3 were measured and are quantitatively explained by the difference in crystal structure. Great care was taken to assure the purity of the samples and that measurements were taken under identical conditions. The PZC and the IEP for the “unhydrated” samples, were the same for each crystal structure, with the values for γ-Fe2O3 being more acidic, by 1.1 or 1.2 pH units, than for α-Fe2O3. This difference can be quantitatively explained by Parks’ equation, in terms of the difference in the coordinations of the Fe3+ ions in the two crystal structures.
Two types of surface hydroxyl groups on maghemite have been identified and their acidities evaluated. One of these surface groups bonds to the surface iron ions of the octahedral sites and shows a stretching vibration mode of infrared absorption at 3690 cm−1. The other bonds to the tetrahedral sites and vibrates at 3630 cm−1. The acidity of the groups positioned at the octahedral sites was evaluated as 12.5 in the pKa value while that of those on the tetrahedral sites was found to be 7.9. A charge transfer mechanism between these neighboring two types of hydroxyl groups is proposed as an explanation for these acidities.
Scanning tunneling microscope (STM) images and scanning tunneling spectroscopy (STS) spectra of hematite (α-FeâOâ) surfaces were calculated using ab-initio methods, not only to interpret experimentally collected STM data, but also to gain insight into atomic level changes in electronic structure that are associated with heterogeneous surface reactions. The electronic structure and wave functions inside the studied crystal were obtained as a periodic solution of the Schroedinger equation by using the program Crystal92. STM images and STS spectra were calculated by applying a technique similar to the Tersoff and Hamann (1985) method. Experimental STM images of the upper valence band of hematite (001) surfaces, cleaved in air, show a periodic array of bright spots that differs slightly from the O-O separation in the bulk. However, our calculations show that these spots are located at the Fe positions of the surface Fe atoms and above the Fe atoms between the first and second hexagonally close-packed O layers. The calculated STS spectra for tip positions above the three non-equivalent Fe positions show significant differences, in particular because the contribution of O 2p-like and Fe 3d-like states changes with the distance between the tip and the respective Fe atom underneath. Hematite crystals that were used to obtain STM images experimentally in previous studies were cleaved in air, and the presence of adsorbed HâO and Oâ was considered in this study. Calculations that optimize the surface atomic arrangement with respect to total energy of the slab indicate that HâO and Oâ adsorbed to the surface have binding energies too low to withstand the dragging force and the electric potential applied during the scanning process. In addition, only calculations of STM images of fresh hematite surfaces exactly mimic the periodicity of high electronic density spots, as observed in experiments.
The catalytic oxidation of CO has been investigated in the presence of vacuum-activated α-Fe2O3 under various partial pressures of CO and O2 at temperatures from 200 to 350 °C. The oxidation rates have been correlated with 1.5-order kinetics; first order with respect to CO and 0.5 order with respect to O2. CO appears to be adsorbed essentially as an ionic species, while O2 is adsorbed as an ionic as well as a nonionic species. Two surface sites, probably an Fei2+ interstitial and an oxygen vacancy, might be required to adsorb CO and O2. The conductivity data show that the adsorption of CO exceeds the O2 adsorption and indicate that the adsorption process of CO (CO(g) ⇌ CO+(ads) + e-) is the rate-determining step. From the agreement between the kinetic data and conductivity measurements, the oxidation mechanism of CO and the defect structure of vacuum-activated α-Fe2O3 are suggested.
The primary objective of this study is to determine the effect of substrate type on the coordination environments of Cu2+ adsorbed on amorphous SiO2, γ-Al2O3, and anatase at a surface coverage of approximately 1 μmol/m2. We also collected X-ray absorption fine structure (XAFS) data for several Cu2+-containing model compounds, including tenorite (VICuO), spertiniite [VICu(OH)2], dioptase (VICuSiO2·H2O), shattuckite [VICu5(SiO3)4(OH)2], chrysocolla [VI(Cu,Al)2H2Si2O5 (OH)4·nH2O], and Cu2+ acetate monohydrate [VICu(CH3CO2)2·H2O], for comparison with the sorption sample data. Detailed analysis of these model compounds indicates that the bonding of
second neighbors surrounding a central Cu absorber determines whether these second neighbors can be detected by XAFS. The
XAFS results of Cu2+ sorption samples are consistent with the presence of Jahn-Teller distorted Cu2+(O,OH)6 octahedra, with four equatorial Cu-O bonds (1.95 Å) and two longer axial bonds; the axial Cu-O bonds are difficult to characterize
quantitatively by XAFS spectroscopy. Cu2+ sorbed on amorphous SiO2 was found to have Cu second and third neighbors at 2.95 Å, 3.30 Å, and 5.72 Å, but no Cu-Si correlation was detected for
these sorption products associated with amorphous SiO2. Based on XAFS and wet chemical results, it seems likely that a Cu(OH)2 precipitate has formed in the Cu2+/amorphous SiO2 system. Cu2+ sorbed on γ-Al2O3 is present as a mixture of monomeric, dimeric, and perhaps a small number of oligomeric hydroxo-bridged Cu(O,OH)6 species with a Cu-Cu distance of approximately 2.95 Å. Sorbed Cu2+ on anatase is present predominantly as hydroxo-bridged Cu dimers. At similar sorption densities, Cu2+ cluster sizes on amorphous SiO2 are significantly larger than those on γ-Al2O3 or anatase, indicating that the substrate has an important effect on the type of Cu2+ sorption complex or precipitates formed.
The mechanisms of Cu2+ adsorption onto goethite, hematite and kaolinite are different. Goethite and hematite showed a similar adsorption behavior (ionic-strength independent), but kaolinite gave somewhat different result (ionic-strength dependent). These experimental results were successfully simulated using a surface complexation model, TLM, which defines the inner- or outer-sphere complex. The chemical nature of Cu2+ adsorption onto kaolinite was qualitatively identified by EPR spectroscopy.
The interaction of water with the (1×1) and (2×1) surfaces of α-Fe2O3(012) was examined with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS), low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) in the temperature range between 100 and 950 K. The (1×1) surface is fully oxidized and has a bulk-like concentration and structure of cation and anion sites. After vacuum annealing at 950 K a (2×1) pattern is observed in LEED. Although the structure of the (2×1) surface is not fully understood, it possesses a greater surface concentration of cation sites than the (1×1) surface, some of which are probably reduced. H2O adsorbs dissociatively on both surfaces as evidenced by HREELS losses at 3625 and 960 cm−1 due to the stretching and bending modes of terminal hydroxyl groups. These losses shift as expected for D2O. Bridging hydroxyls are also formed by proton transfer to bridging oxygen anion sites from dissociating water molecules, but have a poorly resolved O–H stretch at 3400 cm−1 suggesting they are involved in hydrogen-bonding interactions. Further evidence for water dissociation on both surfaces comes from isotopic scrambling of oxygen between these hydroxyls and the -enriched surfaces. Although both surfaces dissociate water, the structural differences between the (1×1) and (2×1) surfaces result in different ratios of molecular-to-dissociative water. Terminal hydroxyls occupy roughly 6×1014 sites cm−2 on the (1×1) surface, but only about 4.5×1014 sites cm−2 on the (2×1) surface. The balance of available cation sites bind molecular water that evolves in TPD below 300 K from either surface. This molecular water is readily detected in both HREELS and SSIMS. The surface structure also influences the hydroxyl recombinative desorption kinetics. Terminal and bridging hydroxyls recombine to liberate water in TPD at 350 K from the (1×1) surface and at 405 K from the (2×1) surface. The recombinative desorption state of water at 350 K from the (1×1) surface exhibits first-order desorption kinetics with an activation energy of about 120 kJ mol−1 and a pre-exponential of 1×1017 s−1. The first-order behavior and the high pre-exponential suggests that recombinative desorption from the (1×1) surface involves pairing of bridging and terminal hydroxyl groups. In contrast, recombinative desorption from the (2×1) surface is pseudo-zeroth order in appearance suggesting that hydroxyls are bound in one-dimensional arrays with desorption occurring preferentially at the ends of each array.
A solvation and electrostatic model has been developed for estimating electrolyte adsorption from physical and chemical properties of the system, consistent with the triple-layer model. The model is calibrated on experimental surface titration data for ten oxides and hydroxides in ten electrolytes over a range of ionic strengths from 0.001 M-2.9 M (Sahai and Sverjensky, 1997a). The model assumes the presence of a single type of surface site, >SOH. It is proposed that for a 1:1 electrolyte of the type M+L−, the logarithms of the adsorption constants (Ks,M+and Ks,L−) representing the equilibria contain contributions from an ion-intrinsic component and a solvation component. According to Born solvation theory, log Ks,M+ and log Ks, L− can be linearly correlated with inverse dielectric constant of the k-th mineral () resulting in the equations The ion-intrinsic part (log Kii″) is a linear function of the inverse electrostatic radius () of the j-th aqueous ion, where, in general, j = M+ or L−. The interfacial solvation coefficient () associated with the solvation component is linearly related to the inverse effective radius () of the adsorbed ion and to the charge (Zj) on the ion. The model is consistent with surface protonation constants (Ks,1and Ks,2) calculated from experimental points of zero charge and values of ΔpK predicted from the Pauling bond-strength per unit bond-length () of the bulk mineral (Sahai and Sverjensky, 1997a), site-densities (Ns) from isotopic-exchange data, and outer-layer capacitance (C2) equal to 0.2 F m−2. As a first approximation, we also find an empirical trend between capacitance (C1) of the inner-layer and where re,ML is the electrostatic radius and ωML is the solvation coefficient of the aqueous electrolyte. Taken together, these correlations enable the calculation of surface protonation and electrolyte adsorption at equilibrium from the properties of the mineral/ solution system.
Adsorption of divalent metal ions (M{sup 2+}) onto oxide and hydroxide surfaces from solutions of strong electrolytes has typically been inferred to take place without the involvement of the electrolyte anion. Only in situations where M{sup 2+} forms a strong enough aqueous complex with the electrolyte anion (for example, CdCl{sup +} or PbCl{sup +}) has it been frequently suggested that the metal and the electrolyte anion adsorb simultaneously. A review of experimental data for the adsorption of Cd{sup 2+}, Pb{sup 2+}, Co{sup 2+}, UO{sub 2}{sup 2+}, Zn{sup 2+}, Cu{sup 2+}, Ba{sup 2+}, Sr{sup 2+}, and Ca{sup 2+} onto quartz, silica, goethite, hydrous ferric oxide, corundum, {gamma}-alumina, anatase, birnessite, and magnetite, from NaNO{sub 3}, KNO{sub 3}, NaCl, and NaClO{sub 4} solutions over a wide range of ionic strengths (0.0001 M-1.0 M), reveals that transition and heavy metal adsorption behavior with ionic strength is a function of the type of electrolyte. In NaNO{sub 3} solutions, metal adsorption exhibits little or no dependence on the ionic strength of the solution. However, in NaCl solutions, transition and heavy metal adsorption decreases strongly with increasing ionic strength. In NaClO{sub 4} solutions, metal adsorption decreases strongly with increasing ionic strength. In NaClO{sub 4} solutions, metal adsorption exhibits little dependence on ionic strength but is often suggestive of an increase in metal adsorption with increasing ionic strength. Analysis of selected adsorption edges was carried out using the extended triple-layer model and aqueous speciation models that included metal-nitrate, metal-chloride, and metal-hydroxide complexes.
A variety of defects on {100} cleavage surfaces of pyrite (FeS2) are observed directly using ultra high vacuum scanning tunneling microscopy. Step edges are aligned along and surface directions. Atomic scale images indicate that the atomic structure, with a respect to the Fe lattice, and local density of occupied states is unchanged at a step edge, including kink and corner sites. The inferred presence of monosulfides at step edges, based on X-ray photoelectron spectra on similar surfaces elsewhere, does not lead to occupied states higher in energy that dz2 dangling bond states at Fe sites. A sequence of consecutive images at the atomic scale captured evidence of dynamic structural changes at defects on this surface at room temperature. Step edges are seen to be generally stable over the course of the STM observations, whereas vacancies, their surrounding sites, and corner step edge sites are not. Theoretical maps of the attachment energy for an Fe adatom over a {100} surface cell indicate the presence of low energy diffusion channels along the topology of the closest S atoms in the uppermost atomic S monolayer. Calculation of the activation energy barriers for the self-diffusion of an Fe adatom over a {100} terrace predict low 0.1 eV diffusion barriers along channels and 0.24 eV across channels. Subsequently, calculated Fe adatom mobilities over the time scale of the STM observations are very high, ranging from 105-106 ? over the course of one minute, calculated for room temperature and depending on the diffusion direction. The structural changes documented in the STM images are explained as resulting from the natural process of surface self-diffusion.
This book provides a comprehensive account of the fundamental properties of metal-oxide surfaces and their interaction with atoms, molecules, and overlayers. It provides both a general overview of the basic properties of metal oxides and an extensive compilation of the experimental and theoretical work performed on single crystal oxide surfaces. There are seven chapters in all, treating topics such as the geometric structure of metal-oxide surfaces, the electronic structures of both transition- and non-transition-metal-oxide surfaces, and molecular adsorption on oxide surfaces. An extensive list of references, covering nearly one thousand citations, is provided.
X-ray absorption near edge structure (XANES) spectra is used as a probe of surface structure of /alpha-Fe2O3 nanocrystal, prepared by sol-gel method. We present O K-edge XANES of /alpha-Fe2O3 in nanocrystal and bulk by total electron yield at the photoemission station of Beijing Synchrotron Radiation Facility. The spectrum of /alpha-Fe2O3 shows a splitting of the pre-edge structure, which is interpreted as two subsets of Fe 3d t2g and eg orbitals in oxygen octahedral (Oh) crystal field, and is also sensitive to long-range order effects. However, no distinguishable splitting of the pre-edge peak of nanocrystal /alpha-Fe2O3 is observed. This suggests that there exists the distorted octahedral coordination around Fe sites and also the long-range disorder due to the surface as compared with bulk /alpha-Fe2O3.
We measured the adsorption of Cu(II) onto goethite (α-FeOOH), hematite (α-Fe2O3) and lepidocrocite (γ-FeOOH) from pH 2–7. EXAFS spectra show that Cu(II) adsorbs as (CuO4Hn)n−6 and binuclear (Cu2O6Hn)n−8 complexes. These form inner-sphere complexes with the iron (hydr)oxide surfaces by corner-sharing with two or three edge-sharing Fe(O,OH)6 polyhedra. Our interpretation of the EXAFS data is supported by ab initio (density functional theory) geometries of analogue Fe2(OH)2(H2O)8Cu(OH)4and Fe3(OH)4(H2O)10Cu2(OH)6 clusters. We find no evidence for surface complexes resulting from either monodentate corner-sharing or bidentate edge-sharing between (CuO4Hn)n−6 and Fe(O,OH)6 polyhedra. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed even though we are supersaturated with respect to CuO and Cu(OH)2. Having identified the bidentate (FeOH)2Cu(OH)20 and tridentate (Fe3O(OH)2)Cu2(OH)30 surface complexes, we are able to fit the experimental copper(II) adsorption data to the reactions 3(FeOH)+2Cu2++3H2O=(Fe3O(OH)2)Cu2(OH)30+4H+ and 2(FeOH)+Cu2++2H2O=(FeOH)2Cu(OH)20+2H+. The two stability constants are similar for the three iron (hydr)oxide phases investigated.
The Structure and Reactivity of Semiconducting Mineral Surfaces: Convergence of Molecular Modeling and Experiment
Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. These poorly crystalline minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always associated with them. Both two- and six-line ferrihydrite have been identified with selected-area electron diffraction. For the vernadite-like mineral, the two d values observed are approximately between 0.1 and 0.2 Å larger than those reported for vernadite, the Mn hydrous oxide that is thought to have a birnessite-like structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral associated with ferrihydrite is produced abiotically via the heterogeneous oxidation of Mn²⁺aq initially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite-like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.
Abstract– Due to the importance of clay minerals in metal sorption many studies have attempted to derive mechanistic models that describe adsorption processes. These models often include several different types of adsorption sites, including permanent charge sites and silanol and aluminol functional groups on the edges of clay minerals. The edge sites have similar pH-dependent adsorption properties as many oxide minerals. To provide a basis for development of adsorption models it is critical that molecular level studies be done to characterize sorption processes. In this study we conducted XAFS and ESR spectroscopic experiments on copper (II) sorbed on smectite clays using suspension pH and ionic strength as variables. At low ionic strength, results suggest that Cu is sorbing in the interlayers and maintains its hydration sphere. At high ionic strength Cu atoms are excluded from the interlayer and sorb primarily on the silanol and aluminol functional groups of the montmorillonite or beidellite structures. Interpretation of the XAFS and ESR spectroscopy results provides evidence that multinuclear complexes are forming on the edge sites. Fitting of EXAFS spectra revealed that the Cu-Cu atoms in the multinuclear complexes are 2.65 apart, and have coordination numbers near one. This structural information suggests that small Cu dimers are sorbing on the surface. These complexes are consistent with observed sorption on mica and amorphous silicon dioxide, as well as the Cu-bearing silicate minerals plancheite and shattuckite, yet are inconsistent with previous spectroscopic results for Cu sorption on montmorillonite. We hypothesize that the differences in sorption mechanisms on the edges of the montmorillonite are due to loading level. The results reported in this paper provide mechanistic data that will be valuable for modeling surface interactions of Cu with clay minerals, and predicting the geochemical cycling of Cu in the environment.
The partitioning (or sorption) of trace elements from aqueous solutions onto mineral surfaces and natural organic matter (NOM) has played a major role in determining the trace element content of natural waters. This review examines sorption processes on mineral surfaces for nine trace elements (Cr, Co, Ni, Cu, Zn, Sr, Cd, Hg, Pb), focusing on the results of modern x-ray spectroscopic studies. Such studies provide unique information on the structure and composition of sorption products, including their mode of attachment to mineral surfaces or functional groups in NOM under in situ conditions (i.e., with aqueous solution present at 25°C). The types of chemical reactions (acid-base, ligand exchange, redox, dissolution/reprecipitation) that can occur at mineral-aqueous solution interfaces are also reviewed, and some of the factors that affect the reactivity of mineral surfaces are discussed, including changes in the geometric and electronic structures of mineral surfaces when they first react with aqueous solutions and constraints on the bonding of adions to surface functional groups imposed by Pauling bond valence sums. A summary of electrical double layer (EDL) theory is presented, including the results of several recent x-ray spectroscopic and parameter regression studies of the EDL for metal-(oxyhydr) oxide-aqueous solution interfaces. The effects of common inorganic and organic complexants on the sorption of trace metal cations at mineral-solution interfaces are considered, in the context of spectroscopic studies where possible. The results of sorption studies of trace metal cations on NOM, common bacteria, and marine biomass are reviewed, and the effects of coatings of NOM and microbial biofilms on cation uptake on mineral surfaces are discussed, based on macroscopic and spectroscopic data. The objective here is to assess the relative importance of inorganic versus organic sorption processes in aquatic systems. The paper concludes with a discussion of the effects of water composition on trace element removal mechanisms, with the aim of providing an understanding of the effects of the high salinity of seawater on trace element sorption processes. The information presented in this review indicates that sorption processes on mineral, NOM, and microbial and algal surfaces, including true adsorption and precipitation, are highly effective at removing trace elements from natural waters and generally supports Krauskopf's (1956) conclusion that such processes are likely responsible for the present trace element concentrations in seawater.
In this mini-review we present an environmental iron mobility/transport scheme consisting of inter-related controls, whereby the first coordination shell of iron modulates the iron redox potential (E1/2), and the oxidation state of iron controls the chemistry of the first coordination sphere and therefore the immediate chemical environment of the iron. Siderophores (microbially generated iron specific chelators) may be viewed as iron redox mediators. Siderophore chelation of environmental iron in a reduced (Fe(II)) oxidation state results in facile air oxidation of iron due to the negative redox potentials observed for Fe-siderophore complexes. This solubilizes the iron and locks it into a specific coordination environment, thereby preventing hydrolysis and precipitation. The high-spin Fe → Fe electron transfer process may be viewed as a switch that controls the thermodynamic stability and kinetic lability of the first coordination shell. Reduction of iron(III)-siderophore complexes to iron(II)-siderophore complexes decreases thermodynamic stability, increases the rate of siderophore ligand exchange, and increases the ease of siderophore donor atom protonation, thus facilitating a rapid turnover of the first coordination shell. Results are presented for iron-siderophore pH and oxidation state dependent speciation studies that are relevant to environmental and microbial iron mobility and transport.
Periods of transient nonsteady state dissolution can contain much information about dissolution mechanisms. Here, pH-jump-induced dissolution transients are used to explore the kinetics of production, at pH 3 and pH 6, of α-Fe2O3 surface sites active for dissolution at pH 1. We find that such sites are generated in a matter of minutes or less at higher pH. The steady state dissolution rate of hematite at pH 1 is ≤10.7 pmol m−2 s−1, whereas the rate of active site production at pH 6 in the first 30 min. of aging is at least 119 pmol m−2 s−1. Apparently, active sites are produced relatively slowly at low pH and relatively rapidly at circumneutral pH, despite the fact that dissolution rates are near a minimum at circumneutral pH. Using aqueous water exchange rates as a proxy for surface ligand exchange rates, this is qualitatively consistent with relatively slow water exchange by aqueous Fe3+ ions at low pH and relatively rapid water exchange by Fe3+ hydrolysis products (e.g., Fe(OH)2+) at circumneutral pH. Consequently, the highest overall dissolution rates are achieved not at steady state at low pH, but by cycling between neutral and low pH. Our results call into question the assumption that oxide mineral surfaces, particularly those of iron and aluminum oxides, are inert on the time scale of proton or ligand adsorption (e.g., during the acid-base titrations typically used to measure oxide surface charge due to proton adsorption).
We have investigated the binding environments of Cu2+ and Pb2+ complexed by soil humic substances using synchrotron-based X-ray absorption spectroscopy. With the assistance of bond network analysis, analysis of X-ray absorption near edge structure (XANES) and radial structure functions derived from extended X-ray absorption fine structure (EXAFS) spectra of Cu-humate at pH 4, 5, and 6 yielded a tetragonally-distorted octahedral binding environment for Cu with 4 O atoms at an average distance of 1.94 Å, 2 O atoms at an average distance of 2.02 Å, and 4 C atoms at an average distance of 3.13 Å. Analysis of Pb-humate samples at pH 4, 5, and 6 yielded 4 O atoms at average distances between 2.46 Å to 2.32 Å and 2 C atoms at an average distance of 3.26 Å for Pb. We interpret the presence of C atoms in the second atomic shell of the metal binding site as evidence that both Cu2+ and Pb2+ form innersphere complexes with soil humic substances. Within the pH range 4–6, there is no significant change in the structure of the binding sites for either Cu or Pb.
Using spin-density functional theory we investigated various possible structures of the hematite (0001) surface. Depending on the ambient oxygen partial pressure, two geometries are found to be particularly stable under thermal equilibrium: one being terminated by iron and the other by oxygen. Both exhibit huge surface relaxations ( -57% for the Fe and -79% for the O termination) with important consequences for the surface electronic and magnetic properties. With scanning tunneling microscopy we observe two different surface terminations coexisting on single crystalline alpha- Fe2O3 (0001) films, which were prepared in high oxygen pressures.
For dispersed transition metal oxides, the specific catalytic activity in the reactions of CO and hydrocarbons oxidation was compared with the densities of bulk and surface defects that were estimated using a combination of diffraction and spectroscopic methods. On this basis, the oxide systems were classified with respect to the scale and origin of the structural sensitivity manifestation. The results were discussed from the point of view of the atomic structure of the most developed surface faces of these oxides and their stoichiometry ranges. The most active surface sites were found to be associated with the surface extended defects including those located at the outlets of bulk extended defects.
Synchrotron-based photoemission spectroscopy (PES), low energy electron diffraction, X-ray reflectivity, and optical and atomic force microscopy were used to study the reaction of water vapor [at pressures p(H2O)≤2.0×10−5Torr for 3min] with UHV cleaved MgO(100) surfaces with different levels of surface defects. The surfaces studied included flat and stepped single-crystal surfaces, wavy polycrystalline surfaces, and Ar+-sputtered flat single-crystal surfaces. AFM and optical microscopy studies revealed that these surfaces have very different surface step densities. The threshold p(H2O) value for hydroxylation of the flat MgO(100) surface is ≈10−4Torr (independent of exposure time), and reactions below this threshold occur dominantly at surface defect sites, which provides a means of estimating surface defect densities. Our results show that water dissociatively chemisorbs at defect sites on MgO(100) surfaces at the residual gas pressure of the UHV chamber (
Scanning tunnelling microscopy (STM) images of two different reconstructions of an α-Fe2O3(0001) crystal are presented. Annealing the sample to 1000 K creates a selvedge stabilised by a thin film of Fe3O4, with its (111) plane parallel to the basal plane of the underlying substrate. The STM images confirm that this surface is structurally equivalent to that previously reported for the surface of Fe3O4(111) single crystals, in that two coexisting terminations, denoted A and B, are present separated by alternate steps. Termination A has been identified with 14ML of O atoms capping 34ML of Fe atoms, while termination B consists of 12ML of Fe atoms overlaying a close-packed O layer. Some regions of the sample are disordered but contain small triangular islands of termination A. This structure is attributed to Ar ion induced sputter damage. A different termination, created by annealing the sample at 1100 K in 1 × 10−6 mbar O2, has a distinctive hexagonal LEED pattern, with all the main beams floreted, being surrounded by a hexagon of smaller spots. The STM results show that this surface is stabilized by coexisting α-Fe2O3(0001) and Fe1−xO(111) phases, with each phase existing in atomically well ordered islands of mesoscopic dimensions. The islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between the two types of oxygen sub-lattices.
Copper uptake by ferric oxide and silica particles is studied through batch kinetic experiments. Copper uptake rates are found to be strongly dependent on pH and on the sorbate/sorbent molar concentration ratio. Dramatic changes to the zeta potential of both colloids from baseline values are observed. Modeling of copper uptake and zeta potential charge reversals using the surface complexation model (SCM) yields poor descriptions under high surface coverage conditions. The conventional SCM, modified in the recent literature to (i) the surface polymer model (SPM), which additionally incorporates uptake of dimeric copper species; and (ii) the continuum model (CM), which includes formation of surface precipitates, is extended here to model uptake kinetics. Both the SPM and the CM are successful in modeling copper uptake rates as well as zeta potential variations over a wide range of solution conditions. For systems with high surface loadings, copper removal from solution appears to result from the formation of monomeric and dimeric surface complexes, as well as through precipitation mechanisms. It is further concluded that a kinetic model incorporating diffusion through the surface film of sorbed and precipitated copper species as the rate-limiting process, in association with the SPM and CM, successfully describes the effect of pH and colloid concentration on copper uptake and oxide particle zeta potential histories.
The oxygen coverage, structure, and thermodynamic stability of (0001) surfaces of Fe2O3 (hematite) as a function of temperature and oxygen pressure are investigated by ab initio density functional theory with the generalized gradient approximation. Spin-polarized total energy and force calculations are performed using the projector augmented wave method as implemented in the Vienna ab initio simulation package. At high chemical potentials of oxygen (i.e., high pressure or low temperature), the most stable (0001) surface of hematite is completely covered with oxygen atoms. At low chemical potentials, a structure with one surface iron atom per two-dimensional unit cell is found to be the most stable surface termination. Around 800 K at an oxygen partial pressure of 0.2 bar, this reactive surface iron atom can bind and release an oxygen atom, thus switching between formal oxidation states (III) and (V), i.e., between stoichiometric and ferrate-like states. The fully reduced (iron terminated) surface is found to be thermodynamically unstable and dissociates adsorbed oxygen molecules spontaneously.
The surface atoms of solid catalysts generally have lower coordination numbers than the bulk atoms, that is they are "coordinatively unsaturated" (cus). As [KnA¶zinger][1] explains in this Perspective, this has important consequences for their catalytic properties. The surface atom coordination sphere may be completed by adsorbed molecules, and these may be activated for catalytic transformations, in close analogy to processes occurring on metal complexes. Over et al. (page [1474][2]) succeed in verifying this concept of coordinatively unsaturated sites (cus) on the atomic scale.
[1]: http://www.sciencemag.org/cgi/content/full/287/5457/1407
[2]: http://www.sciencemag.org/cgi/content/short/287/5457/1474
The ability of a high surface-area gibbsite to adsorb Cu ²⁺ was studied using a Cu ²⁺ ion-selective electrode, electron spin resonance, infrared spectroscopy, and electron microscopy. The gibbsite chemi-sorbed small amounts of monomelic Cu ²⁺ (<0.5 mmole/100 g) which was oriented with its z-axis perpendicular to the (001) plane of the mineral. The proposed chemisorption sites are at gibbsite crystal “steps” observed by electron microscopy. Although Cu ²⁺ adsorption on gibbsite as a function of pH was largely reversible, exposure of the chemisorbed Cu ²⁺ to NH 3 did not result in desorption from the surface despite the displacement of several OH ⁻ or H 2 O ligands by NH 3 . The results indicate that at least one Cu-O-Al bond is formed in the process of chemisorption.
At pH > 5, the gibbsite appeared to promote the hydrolysis and polymerization of Cu ²⁺ , with further adsorption at the surfaces. Infrared spectroscopy revealed no effect of the adsorption on the (001) surface hydroxyl groups, although the anisotropic diffusion of protons in the gibbsite structure was verified from deuteration studies.
Scanning tunneling microscopy (STM) images of the PbS (100) surface with a step and several kinks were obtained with atomic resolution. These images show an increased tunneling current at step edge sites and an apparent deformation of the lattice near the step. The experimental images are compared with theoretical ab initio calculations for which we developed a hybrid method of constant current and constant height mode STM image simulation. With these calculations, we find that the apparent deformation is mainly an electronic effect rather than relaxation of atoms. In addition, with the help of these calculations we can identify the changes of individual terrace-like and step-like orbitals that are observed using the STM in terms of the energy, density and shape of these states. This detailed knowledge of the electronic behavior of the Pbs surface near a step can be used as a basis for explaining adsorption, acid/base, and redox behavior on PbS terraces and at steps, and the differences between the two.
Size-dependent surface binding forms of the carboxylic group on the surface of TiO2 nanoparticles were investigated by a surface-binding transient probe molecule all-trans-retinoic acid (ATRA), where the excited triplet-state probe molecule generated by a photoinduced interfacial charge recombination for the adsorbed monolayer acts as a reporter for the different surface binding forms. Different TiO2 nanoparticles of varying sizethat is, 6, 0.8−1.4, and 0.7 nmwere prepared, and their physical properties were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS). The transient absorbance difference spectra of the ATRA/TiO2 reveal that, when the size changes from 6 nm to <1.4 nm, the simple adsorption forms (physical adsorption and hydrogen bonding form) decrease from 36% to 3%, whereas chemical binding forms increase from 74% to 97%. Such a size-dependent effect is attributed to the variation in the relative compositions of surface Ti atoms of different coordination states, which, in turn, form undercoordinated surface defect sites when the size varies at nanoscale. Referring to the results of the size-dependent coordination states of the surface Ti atoms from X-ray absorption near-edge spectroscopy (XANES) [T. Rajh et al., J. Phys. Chem. B 1999, 103, 3515−3519], the change of relative compositions for the 6-, 5-, and 4-fold coordinated surface Ti atoms, with respect to the particle size, can be rationalized. The dependency of the coordination state of the surface Ti atoms on the particle size provides a quantitative basis for tuning the compositions of the surface Ti atoms of different coordination state, as well as the surface binding forms by the control of the particle size. Surface modifications utilized in the Grätzel cells were discussed, in view of the surface binding and the coordination state of the surface Ti atoms.
### Structural aspects of natural nanomaterials
A large number of mineral species occur only as micron-sized and smaller crystallites. This includes most of the iron and manganese oxyhydroxide minerals, and other species whose formation processes and growth conditions limit ultimate size. Microscopic investigation of these species generally reveals sub-micron structure down to the nanometer level, including evidence of aggregation, agglomeration and assembly of nanometer units into larger crystals and clots. The bulk of studies in the literature dealing with nanoparticle structure and growth deal with metals, silicon, and other semiconductor materials. A great deal of attention has been given to the electronic properties of such solids, owing to both new commercial applications and new fundamental physics and chemistry tied to this area. Most applicable mineralogical or geochemical studies have not addressed the same issues, instead being more concerned with relatively bulk chemical properties. Very little has been done to understand how natural nanoparticulates (and related types of natural nanomaterials) form, how their microstructure is related to the growth process, and how their structure varies from larger crystallites or bulk material of the same composition. Magnetic and electronic properties of natural nanomaterials are similarly understudied.
In this chapter aspects of nucleation, aggregation and growth processes that give rise to specific microstructures and forms of nanomaterials are considered. Next the way in which the surface structure of nanoparticulates may differ from the interior, and how physical structure may be modified by reduced particle size is examined. The various techniques by which nanoparticle structure, size, microstructure, shape and size distribution are determined are then considered with examples. Finally some of the outstanding problems associated with nanoparticle structure and growth are identified, emphasizing natural processes and compositions.
### Definitions
Naturally occurring nanomaterials exist in a variety of complex forms. In this chapter a short set of definitions will be stated for clarity. …
The competitive sorption of Cu(II) and Pb(II) to colloidal hematite was investigated as a function of pH and total metal concentration. Acid–base titrations of the hematite and single-metal sorption experiments for Cu and Pb at low to medium surface coverages were used to calibrate two surface complexation models, the triple layer model, and a 2-pK basic Stern model with ion-pair formation. The surface site density was systematically varied from 2 to 20 sites/nm2. Three different metal surface complexes were considered: (1) an inner-sphere metal complex; (2) an outer-sphere metal complex; and (3) an outer-sphere complex of singly hydrolyzed metal cations. Both models provided excellent fits to acid–base titration and single-metal sorption data, regardless of the surface site density used. With increasing site density, ΔpK of the stability constants for protonation reactions increased and metal surface complexes decreased steadily. The calibrated models based on different site densities were used to predict competitive sorption effects between Cu and Pb and single-metal sorption at higher total metal concentrations. Precipitation of oversaturated solid phases was included in the calculations. Best predictions of competitive sorption effects were obtained with surface site densities between 5 and 10 sites/nm2. The results demonstrate that surface site density is a key parameter if surface complexation models are exposed to more complex, multicomponent environments. We conclude that competitive metal sorption experiments can be used to obtain additional information about the relevant surface site density of oxide mineral surfaces.
【Summary】 Humic substances (HSs) are by far the most abundant of the organic components of nature, and are present in all soils and natural waters that contain organic matter. The more widely accepted values for organic carbon (OC) in soil organic matter (SOM) are in the range of (14~15)×10 17
Mineral particles with diameters on the scale of nanometers (nanoparticles) are important constituents of natural environments. The small size of such particles has a host of consequences for biogeochemical systems, which we will review in this chapter. We begin by briefly reviewing what is known about how and when nanoparticles form and the ways in which nanoparticles impact natural processes.
Nanoparticles form via a variety of inorganic and biological pathways and may be introduced into the environment as a consequence of human activity. They are widespread in the environment (Banfield and Navrotsky 2001; Penn et al. 2001; Kennedy et al. 2003a,b; van der Zee et al. 2003), although few quantitative studies of their abundance are available. While all crystals begin as very small particles, an important subset retain small size at the Earth’s surface over relatively long time scales, because the combination of low temperature and low solubility inhibits growth. As a consequence, nanoparticles have the potential for a long lifetime in the environment, and widespread transport under certain circumstances.
Processes that result in the removal of nanoparticles from an environment include dissolution, settling from air, transport in solution, and crystal growth. Particle aggregation may be an important component of these processes because it will promote settling, limit dispersal via solution transport, and can lead to aggregation-based crystal growth.
The presence of nanoparticles can profoundly influence biological systems. Because they are frequently formed in environments that are populated by microorganisms, nanoparticles often adhere to cell surfaces or cell-associated polymers (see Fig. 1⇓ for examples). These coatings can have important consequences for metabolic activity, for example, by restricting communication between the cell and its surroundings. They may also provide protection from predators, inhibit desiccation, screen cells from ultraviolet radiation, and alter the cell buoyancy (e.g., …
Adsorption and desorption of water on well‐ordered and sputter‐damaged single crystal MgO(100) surfaces were studied by a combination of molecular beam reflection and temperature programmed desorption techniques. Adsorption exhibits precursor‐mediated kinetics and desorption exhibits a strong dependence on substrate treatment, demonstrating the importance of surface defects. © 1996 American Institute of Physics.
The structures of Fe2O3 nanoparticles with different sizes were investigated using Fe K-edge X-ray absorption near-edge structure (XANES) and the FEFF calculations, as well as surface modification with enediol ligands. The studies not only revealed the existence of under-coordinated Fe sites in the nanoparticles but also confirmed that these under-coordinated sites were located on the surface. Upon binding of enediol ligands, surface sites were restructured to octahedral sites. In particular, the nature of the surface defects and their correlation with the unique properties of the nanoparticles were discussed. Model calculations were conducted for FemOn (m ≥ 1, n ≥ 4) clusters of various sizes centered at Fe sites with octahedral (Oh), distorted octahedral (C3v) and tetrahedral (Td) coordination geometry using FEFF8.10 programs. The main features of the calculated spectra agree with the experimental results and were correlated to the density of states, the Fe coordination geometry, and the long-range order of the lattice.
Surface modification of nanocrystalline metal oxide particles with enediol ligands was found to result in altered optical properties of nanoparticles. The surface modification results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The optical shift is correlated to the dipole moment of the Ti−ligand complexes at the particle surface and decreases with the ligand size. The binding was found to be exclusively characteristic of colloids in the nanocrystalline domain(<20 nm). X-ray near-edge structure measurements at Ti K edge indicate that in this size domain the surface Ti atoms adjust their coordination environment to form undercoordinated sites. These five-coordinated defect sites are the source of novel enhanced and selective reactivity of the nanoparticle toward bidentate ligand binding as observed using IR spectroscopy. Enediol ligands have the optimal geometry for chelating surface Ti atoms, resulting in a five-membered ring coordination complex and restored six-coordinated octahedral geometry of surface Ti atoms. The binding of enediol ligands is enhanced because of the stability gained from adsorption-induced restructuring of the nanoparticle surface. Consistent behavior was found for the three different nanocrystalline metal oxide systems: TiO2, Fe2O3, and ZrO2.
The stereochemistry of hydrated Cu(II) ions on the interlamellar surfaces of microcrystalline layer silicates has been investigated by observing the anisotropic components of the g factor in the esr spectra of oriented film samples at room temperature. When a monolayer of water occupies the interlamellar regions the ion has axial symmetry and the symmetry axis is perpendicular to the silicate layers. The Cu(n) ion most likely is coordinated to four water molecules in the xy plane and to two silicate oxygens along the z axis. Under conditions where two layers of water occupy the interlamellar regions, the ion is in an axially elongated tetragonal field of six water molecules and the symmetry axis is inclined with respect to the silicate layers at an angle near 45°. If several layers of water molecules occupy the interlamellar regions, the Cu(H2O)62+ ion tumbles rapidly and gives rise to a single, isotropic esr signal analogous to that normally observed for the ion at temperatures above 50°K.
Transient, non-steady-state responses of hematite dissolution rate to pH-jumps, from high to low pH, contain information about dissolution mechanisms and can be used to improve our understanding of dissolution processes operating under variable natural conditions. Our data show that, following each downward pH-jump, the hematite dissolution rate jumps up but then decays exponentially to a new steady state over a period of about 36 h. This requires that, after a pH-jump, the nature of the surface Fe sites themselves, and not only surface charge, gradually changes. Our results are consistent with the depletion of a reservoir of Fe sites active for dissolution on the hematite surface after a jump to pH 1, and show that such active sites can be reproducibly regenerated during returns to higher pH. We interpret the data with regard to long-standing crystal growth and dissolution models [e.g., Burton−Cabrera−Frank, BCF (Burton, W. K.; Cabrera, N.; Frank, F. C. Philos. Trans. R. Soc. London Ser. A 1951, 243, 299−358)] that assume the existence of “adsorbed nutrient” that is structurally distinct from metal centers in the solid surface structure. The general concept behind the model should be applicable to other minerals as well as hematite.