Article

Preparation and hydration characteristics of carbodiimide crosslinked lignite humic acids

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Abstract

Polar and apolar moieties of humic acids are spatially separated forming domains of different polarity. In this work, we tested the procedures to crosslink functional groups in polar domains of humic acids by using carbodiimide coupling and analyzed to which extent influenced the modification their hydration properties and stability. For this reason, we prepared eight derivatives of lignite humic acids using either water-soluble N-Ethyl-N′-(3-dimethylaminopropyl)carbodiimide (EDC) or water-insoluble N,N′-dicyclohexylcarbodiimide (DCC) under various conditions. Characterization of prepared derivatives showed that both methods lead to formation of crosslinked humic structures. Using of EDC resulted in lower degree of crosslinking, but better hydration properties. Higher moisture uptake and water holding capacity were observed in humic acids, which were pre-wetted prior to crosslinking for at least 24 h. Although the EDC derivatives of humic acids contained only between 60 and 85% of original free carboxylic groups, they showed similar moisture uptake as parental humic acids by equilibration at relative humidities of both 94% and 76%. Under water-saturated conditions, the EDC derivatives showed faster swelling kinetics and reached almost the same water holding capacity as the original sample after 18 days. However, both the EDC and DCC derivatives began to degrade already after 3–9 days during swelling tests, which subsequently decreased their hydration. The results suggested that water holding capacity, swelling kinetics and moisture uptake of humic acids were not influenced significantly by the amount of polar groups, but also by their spatial arrangement and distribution.

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Qualitative and quantitative aspects of hydra- tion of four humic acids (HA) and three fulvic acids (FA) originating from different sources were investigated. DSC experiments at subambient temperatures were carried out in order to monitor differences in ice behavior originating from freezable water surrounding humic molecules. It was found that kinetic effects play a significant role in hydra- tion processes of both HA and FA. In fact, the hydration took part over 21 days which was detected as a progressive decrease in ice melting enthalpy. Simultaneously, the peak shapes and positions changed indicating structural changes in the physical structure of the humic substances. In case of FA, the dependency of melting enthalpy on water con- centration showed a linear trend resembling a complete hydration previously observed for water-soluble hydro- philic polymers. In contrast, the melting enthalpy of some HA increased in a step-like way with increasing water content, suggesting preservation of original hydrophobic scaffold during the hydration. The differences between the rather young FA and the rather old HA lead to the con- clusion that water can play a significant role in processes of humification. We assume that separation of hydrophobic and hydrophilic domains and thus increase in nanoscale heterogeneity represents an important physical contribution to the overall humification process. It was also demon- strated that the higher content of oxygen in humic mole- cules is not the only indicator of higher water holding capacity. Instead the porosity of humic matrix seems to contribute as additional parameter into these processes.
Article
Humic acids originating from South-Moravian lignite were subjected to a comparative study with the aim to assess the alteration of their physico-chemical properties after various lignite pre-treatments. Physical modification was achieved with two organic acids, such as acetic acid and citric acid and chemical modification by nitric acid and hydrogene peroxide in various concentrations. Elemental analysis, solid-state NMR, GC–MS analysis of polyols and size exclusion chromatography were carried out for chemical–physical characterization of obtained humic acids. Their biological effect, in form of potassium and ammonium humates, was tested on maize (Zea mays) seedlings. In these tests, potassium humates achieved far better overall results than ammonium humates. Results were inter-correlated in order to appraise the influence of humic acids physical and chemical properties on biological activity. Surprisingly, fractions with the lowest molecular size (0–35 kDa) showed no correlation with bioactivity results (Pearson coefficient from 0.05 to −0.4). On the contrary, middle-sized fractions (35–175 kDa) showed highly significant positive correlation (Pearson coefficient up to 0.92) and the highest molecular-size-fractions (275–350 kDa) showed negative correlation (Pearson coefficient up to −0.75). These findings were identical for both potassium and ammonium humates. No connection was found between bioactivity of humates and polyols content which was remarkably high; it reached 150 mg per g of humic acids in the most extreme case of 5% hydrogene peroxide pre-treatment. In the final analysis, the preparation mode bore pivotal responsibility for the control of humic acids biological effect and showed the best results for potassium humates obtained from lignite pre-treated by acetic acid and by 2% hydrogen peroxide.
Article
Purpose This article reviews our current understanding about how organic chemicals and water interact dynamically with, and therefore coevolve with, soil and sediment natural organic matter (NOM). NOM can be regarded as a polymer-like phase that responds to the input of organic compounds in ways analogous to synthetic polymers. Methods Sorption selectivity of organic compounds is shown to result in part from the three-dimensional microstructure of NOM related to its glassy character. Sorption to NOM conforms to polymer theory by exhibiting isotherm shape and irreversible behaviors characteristic of the glassy organic physical state. The glassy state is a metastable state characterized by the presence of excess free volume (holes). Results In polymers and NOM, incoming molecules preferentially occupy holes due to the absence of a cavitation penalty. Incoming molecules can enlarge existing holes and create new holes that do not relax completely when the molecules leave. The physical changes in NOM induced by sorption result in hysteresis in the isotherm that persists indefinitely at ambient temperature. Conclusions Sorption selectivity and hysteresis have important implications for the fate and bioavailability of contaminants.
Article
Cross-linking of humic substances with organic bridging groups is hypothesized to contribute to the humification of soil organic matter. Model cross-linked humic substances were prepared by cross-linking Amherst soil humic acid by a diepoxide and a polycarboxylic acid in the solid state, applying procedures established for cross-linking of polymers and textile fabrics. Products of the crosslinking reactions were analyzed by FTIR and 13C CPMAS NMR. Physicochemical properties of the products were determined by solubility experiments and thermal analysis. The incorporation of the cross-linker into the matrix of the humic acid by covalent linkages was confirmed by both the disappearance of bands of the reactive functional groups of the cross-linker in the FTIR spectrum and the increase of signals related to the incorporation of the cross-linker into the matrix of the humic acid in the FTIR and 13C-CPMAS-NMR spectra. The formation of covalent ester and ether linkages by the cross-linking reaction was indicated. Water solubilities at pH 6.2 of the cross-linked samples as determined by UV/Vis spectrometry were reduced compared to controls. Fewer water molecule bridges were formed in the cross-linked samples, which was attributed to a lower number of available functional groups and increased distances between humic acid strands caused by the cross-linking molecules. Reduced reactivities of humic acid strands in the cross-linked samples further indicated successful cross-linking. The reactions investigated in this study can be regarded as models for reactions occurring in natural soils to test the significance of cross-linking reactions in the humification process of soil organic matter.
Article
Two principles are presented that define the molecular nature and ecological role of humic substances (HS). The First Principle (i) accounts for and organizes an extensive body of apparently disparate data relating to the inability to purify and establish a molecular structure for HS; (ii) offers a conceptual framework for dealing with HS and for evaluating the applicability and limitations of various experimental methods; and (iii) identifies molecular heterogeneity, in combination with pronounced chemical reactivity, as constituting the essence of HS. Five corollaries to the First Principle spell out its consequences in more specific detail. New definitions of HS that offer greater insight into the molecular nature of these materials arise from the First Principle. The inapplicability of the molecular structure concept to HS is explained. The concept of hypothetical pseudostructures is introduced to help visualize the chemical reactions and interactions of HS without the unjustified assignment of specific structures to the material as a whole. Constraints in the design of experiments and in the interpretation of experimental data caused by the heterogeneous nature of HS are discussed. The Second Principle makes a connection between the molecularly heterogeneous and chemically reactive nature of HS and the ecological need for a reactive and persistent medium for plant growth. Concepts presented herein have broad implications in many fields, including chemistry, geochemistry, environmental and soil sciences, and ecology.
Article
A multistep alkaline hydrolysis under phase transfer catalysis conditions (PTC) was carried out on humic acid and humin fractions from an acid soil sample (anmoor soil) from the Plateau de Millevaches (France). A large part (87%) of the initially insoluble humin became soluble after a first series of hydrolysis. The main fraction obtained, soluble high molecular-weight products, was still reactive when submitted again to a second series of hydrolysis. Humic acid showed a lower reactivity. Linear dicarboxylic acids were produced in significant amounts from humin, they were short-chained (mainly C9) in the first series and long-chained (C16–C28) with an even carbon number preference indicating a plant origin in the second series. Linear C12–C32 monocarboxylic acids and alkanols were freed from humin and humic acids. The short linear fatty acids and alkanols, accompanied with iso- and anteiso-C15 and C17 members, denote a bacterial input while long even components, as well as various aromatic acids, originate from plants. The results indicate that ester groups are involved in the cross-linking of macromolecular chains forming the matrix of humin. Aliphatic dicarboxylic acids act as bridges between alkyl chains in the matrix, while fatty acids, alcohols and aromatic acids correspond mainly to monosubstituents of the matrix (bound by ester groups). It is interesting to note that this model resembles the kerogen of many immature sediments.
Article
A nonlinear method for fitting acid–base potentiometric titration data was applied to commercial and vermicompost humic acids in order to determine the concentration and conditional pKa's of the ionizable sites. Data was also treated by a linear method based on modified Gran functions. In both cases the discrete site model was adopted, which means that the humic acid was treated as a mixture of monoprotic acids. Electrostatic effects were evaluated by performing titrations in media of 0.010, 0.10, and 1.0 mol l−1 NaCl. Results were compared with those obtained by conductimetric titration with NaOH and Ba(OH)2, infrared spectra of humic acid with different grades of protonation, and classical Ca-acetate and Ba(OH)2 exchange methods. Models with five and six binding sites were the best to represent experimental data of potentiometric titration using both the linear and nonlinear techniques.
Article
The composition of humic acids (HAs) isolated from a South Moravian lignite deposit was examined via thermochemolysis and RuO4 oxidation. The combination of thermochemolysis with TMAH and TEAAc shows that fatty acids were mostly in a “free” form, tightly trapped within the humic acid organic network. Humic acids contain also trapped hydrocarbons. The relative distributions of these compounds confirm their higher plant origin. RuO4 oxidation resulted in a lipophilic fraction containing mainly fatty acids. The hydrophilic fractions released upon RuO4 oxidation comprised mostly aliphatic C2–C9 dicarboxylic acids and hydroxy acids related to short methylene bridges, to bridges between aromatic and/or oxygenated moieties such as aliphatic chains of ligneous units.
Article
The combination of TMAH thermochemolysis and TEAAc treatment makes it possible to discriminate between the different forms of mono- and dicarboxylic acids present in the structure of humin and humic acids, that is, “free” uncombined acids, methyl or ethyl esters present as tightly trapped molecules within the matrix, or acids chemically linked to the matrix by ester groups. The results confirm that ester groups are involved in the structure of humin and humic acids. The cross-linking of moieties originating from microbial metabolism or inherited from higher plants is partly ensured by these chemical groups. On the other hand, significant amounts of fatty monocarboxylic acids and linear dicarboxylic acids are present as free acids in the humin of the studied sample. Humin contains also fatty acid methyl esters. Free, uncombined α,ω-dicarboxylic acids were only found in humin.
Article
Differential scanning calorimetry (DSC) was used to determine the number of water molecules in the hydration shell of hyaluronan of different molecular weights and counterions. First, traditional experiments including freezing/thawing of free water in semi-diluted solutions were carried out leading to the determination of melting enthalpy of freezable water. Non-freezing water was determined using extrapolation to zero enthalpy. For sodium hyaluronan within the molecular weight range between 100 and 740 kDa the hydration shell was determined as 0.74 g g−1 HYA. A larger hydration shell containing 0.84 and 0.82 g g−1 HYA was determined for hyaluronan of 1390 kDa in its sodium and protonized form, respectively. Second, melting enthalpy of freezing water was further studied applying water evaporation experiments. Resulted plot of enthalpy vs concentration indicated an additional heat evolution process which occurs at specific concentration and decreases the measured evaporation enthalpy. The heat evolution was attributed to the mutual approaching of hyaluronan molecular chains, their mutual interactions and formation of the ordered hyaluronan structure which starts immediately when the hydration water is desorbed from the hyaluronan surface. The concentration at which the process occurred was related to “non-evaporable water” which was determined as 0.31–0.38 g g−1 for sodium hyaluronan and 0.84 g g−1 for its protonized form. The second approach provides additional information enabling a deeper insight into the problem of hyaluronan hydration.
Article
The stabilizing effect of water molecule bridges on polar regions in humic substances (HSs) has been investigated by means of molecular dynamics (MD) simulations. The purpose of these investigations was to show the effect of water molecular bridges (WAMB) for cross-linking distant locations of hydrophilic groups. For this purpose, a tetramer of undecanoid fatty acids connected to a network of water molecules has been constructed, which serve as a model for spatially fixed aliphatic chains in HSs terminated by a polar (carboxyl) group. The effect of environmental polarity has been investigated by using solvents of low and medium polarity in force-field MD. A nonpolar environment simulated by n-hexane was chosen to mimic the stability of WAMB in a hydrophilic hotspot surrounded by a nonpolar environment, while the more polar acetonitrile environment was chosen to simulate a more even distribution of polarity around the carboxylic groups and the water molecules. The dynamics simulations show that the rigidity of the oligomer chains is significantly enhanced as soon as the water cluster is large enough to comprise all four carboxyl groups. Increasing the temperature leads to evaporization processes which destabilize the rigidity of the tetramer-water cluster. Embedding it into the nonpolar environment introduces a pronounced cage effect which significantly impedes removal of water molecules from the cluster region. On the other hand, a polar environment facilitates their diffusion from the polar region. One important consequence of these simulations is that although the local water network is the stabilizing factor for the organic matter matrix, the degree of stabilization is additionally affected by the presence of nonpolar surroundings.
Article
We propose a mild stepwise fractionation of molecular components of a humic acid (HA) suprastructure and their structural identification by advanced analytical methods. This procedure may be the basis of a "Humeomics" approach to characterize natural humic molecules and clarify their relations with ecosystems functions. Sequential fractionation included: (1) organic solvent extraction, (2) transesterification with boron trifluoride in methanol (BF(3)-CH(3)OH), (3) methanolic alkaline hydrolysis (KOH-CH(3)OH), and (4) cleavage of ether and glycosidic bonds with HI. Structural identification of initial and final material, separated organo-soluble and hydrosoluble fractions, and subfractions was conducted by GC-MS, HPSEC-ESI-MS (high-resolution, Orbitrap), and solid- and liquid-state NMR. GC-MS revealed in organosoluble unbound fractions the presence of both saturated and unsaturated, linear and branched, alkanoic, hydroxyalkanoic and alkandioic acids, n-alkanes, and n-alkanols. These components decreased progressively in fractions obtained after weak and strong ester cleavage. Unsubstituted alkanoic acids with variable chain length were ubiquitously detected in all fractions, thereby suggesting their fundamental function in the architecture of humic suprastructures. An important role in differentiating supramolecular associations should also be attributed to substituted alkanoic acids that were detected in variable amounts in different fractions. The content of aromatic acids and steroids was only noticed in the latter fractions. HPSEC-ESI-MS of initial and final solid fractions showed similar compounds, as indicated by GC-MS, whereas the hydrosoluble fraction after transesterification revealed fewer of these compounds but noticeable nitrogen-containing acids. A large amount of "cyclic" acids were identified by MS empirical formula in initial HA, and, to a lesser extent, in the final fractionation residue as well as in the hydrosoluble fraction. The predominant alkyl NMR signals in organosoluble extracts and those of CH-N, CH-O, and O-CH-O groups in hydrosoluble fraction confirmed mass spectrometry results. Homo- and heterocorrelated liquid-state NMR spectra indicated spin systems interactions varying with separated fractions. Solid-state and dipolar-dephasing NMR spectra of final residue showed predominance of sp(2) carbons, 66% of which were quaternary carbons, and a significant increase in conformational rigidity with respect to initial HA. Separated fractions accounted for 60% of initial HA weight, and losses were attributed to hydration water, liberated volatile compounds, and decarboxylation. Quantization of analytes showed that the sum of compound classes in separated fractions was greater than that for the initial HA, thereby showing that stepwise fractionation increased significantly the analytical identification of humic molecules. Our results suggest this "Humeomics" approach as a valid path for mapping humic molecular composition and assess humus origin and formation.
Article
The impact of kidney stone disease is significant worldwide, yet methods for quantifying stone components remain limited. A new approach requiring minimal sample preparation for the quantitative analysis of kidney stone components has been investigated utilizing attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FT-IR). Calcium oxalate monohydrate (COM) and hydroxylapatite (HAP), two of the most common constituents of urinary stones, were used for quantitative analysis. Calibration curves were constructed using integrated band intensities of four infrared absorptions versus concentration (weight %). The correlation coefficients of the calibration curves range from 0.997 to 0.93. The limits of detection range from 0.07 +/- 0.02% COM/HAP where COM is the analyte and HAP is the matrix, to 0.26 +/- 0.07% HAP/COM where HAP is the analyte and COM is the matrix. This study shows that linear calibration curves can be generated for the quantitative analysis of stone mixtures provided the system is well understood especially with respect to particle size.
Article
Molecular formulas have been assigned for 4626 individual Suwannee River fulvic acids based on accurate mass measurements from ions generated by electrospray ionization and observed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). Formula assignments were possible because of the mass accuracy of FTICR MS at high field (9.4 T) and the regular mass spacing patterns found in fulvic acid mixtures. Sorting the 4626 individually observed ions according to Kendrick mass defect and nominal mass series (z* score) revealed that all could be assigned to 1 of 266 distinct homologous series that differ in oxygen content and double bond equivalence. Tandem mass spectrometry based on infrared multiphoton dissociation identified labile fragments of fulvic acid molecules, whose chemical formulas led to plausible structures consistent with degraded lignin as a source of Suwannee River fulvic acids.
Article
The primary goal of this work was to develop quinonoid-enriched humic materials with enhanced redox properties that could be used as potentially effective redox mediators and reducing agents for in situ remediation of soil and aquatic environments. Two different strategies were formulated and tested to derive these materials. The first strategy called for the oxidation of phenolic fragments associated with the humic aromatic core. In a second strategy, polycondensation of these phenolic fragments was carried out with hydroquinone and catechol. The oxidized derivatives and copolymers obtained were characterized using elemental and functional group analyses, and capillary zone electrophoresis. The redox properties were evaluated using ESR spectrometry and reducing capacity determinations. The reducing capacities of copolymers ranged between 1 and 4 mmol/g, which were much higher than the parent material and the oxidized derivatives. Hence, preference should be given to the copolycondensation approach. The quinonoid-enriched humics are nontoxic, water soluble, and resistant to biodegradation; thus, they could be applied as soil amendments to reduce highly mobile oxoanions of heavy metals and radionuclides, or as redox mediators to enhance in situ bioremediation. Otherwise, cross-linked copolymers could be created to serve as inexpensive reductants in permeable reactive barriers designed to remove highly oxidized contaminants from polluted groundwaters.
Article
Three humic phenolic monomers, catechol (CAT), caffeic acid (CAFF), and p-coumaric acid (COUM), were subjected to oxidative coupling catalyzed by biomimetic water-soluble iron-porphyrin (Fe(TDCPPS)CI) in either separate or mixed solution, and the reaction products were characterized by gas chromatography-mass spectrometry (GC-MS) and electrospray-mass spectrometry (ESI-MS). The GC-MS analysis proved the formation of C-C and C-O dimers, whereas the ESI-MS/MS analysis also suggested trimerization for all the monomers and tetramerization for CAT. On the basis of mass spectra, molecular structures were assigned to the observed oligomers. In the phenolic separate solutions, dimers represented about 65%, 44%, and 30% of reaction products for CAT, CAFF, and COUM, respectively, whereas trimers were from 4 to 5%. A relevant part of the products were unidentified oligomers and several degradation compounds, mostly aromatic aldehydes and alcohols and aromatic or aliphatic carboxylic acids. When all three humic phenolic monomers underwent the catalyzed coupling reaction in one mixed solution, 14% of the reaction products were identified as C-C dimers of CAT. Although no other C-O dimers of CAT, nor any dimers of COUM and CAFF, could be identified, some other structurally unknown oligomers were present among the reaction products of the mixed solution. However, no oligomers larger than tetramers were formed in either separate or mixed solutions. This work indicates the essential role of biomimetic metal-porphyrins in catalyzing the oxidative coupling of humic phenolic monomers in aqueous media, thereby promoting the polymerization of natural organic matter.
Synthesis of cross-linked humic substances using phenol–formaldehyde polycondensation
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Environmental particles: structure and surface reactions of soil particle Analytical Pyrolysis and Computer Modelling of Humic and Soil Particles
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Spectrometric Identification of Organic Compounds
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Chemistry of Protein Conjugation and Cross-linking
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Wong, S.S., 1991. Chemistry of Protein Conjugation and Cross-linking. CRC Press, Taylor and Francis Group, Boca Raton, FL, USA.
Collagen-grafted ultra-high molecular weight polyethylene for biomedical applications
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Environmental particles: structure and surface reactions of soil particle
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