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The variation of flocs activity during floc breakage and aging, adsorbing phosphate, humic acid and clay particles
Abstract
The mechanism of removal of humic acid, phosphate and kaolin particles by coagulation with alum and PACl or adsorption by their pre-formed precipitates was investigated, and it was found that the coagulation mechanisms for monomeric Al at neutral pH and polymeric Al 13 at alkaline pH were very similar. The removal of phosphate and humic acid by coagulation with alum or PACl did not change with stirring time (between 1 min and 15 min), independent of the dose and species of coagulants. However, for adsorption of these impurities by pre-formed precipitates, the results were significantly different. Both Al ³⁺ and nano-sized Al 13 could precipitate and form aggregates at pH 7 and pH 9, respectively, and their precipitates became less active (fewer binding sites on the surface of precipitate) with the increase of shear time or shear rates before adsorbing pollutants. Thus, although the total surface area increased (the average size of flocs became smaller) at higher applied shear rates or longer shear time, the removal efficiency of humic acid and phosphate decreased. Also, from the MW distributions, it was confirmed that less humic acid was removed by the adsorption on alum precipitate pre-formed with longer shear time. Chemical groups (OH 2 and OH) on the surface of precipitate determined the removal efficiency of phosphate and humic acid, and the activity of precipitate become lower as a result of higher applied shear and longer shear time. This is confirmed be due to some crystallization of the amorphous precipitate, forming inactivated hydroxyl. When kaolin was added 10 min after the alum or PACl precipitate formed, the precipitates captured kaolin particles only on their surface, whereas when alum was added to kaolin suspensions particles were trapped within the growing flocs. When alum/kaolin flocs were broken at high shear rate re-growth of flocs decreased with increasing shear time, but after a short breakage period, long aging of broken flocs had little effect on floc regrowth.
... Flocculation of cohesive particles is not only limited to cohesive sediment science. In other research areas, such as sanitary engineering, water treatment works, and colloidal science, particle flocculation is an essential element in some dynamic processes, and related studies regarding its mechanism are always a research focus [18][19][20][21][22][23][24][25][26][27][28][29]. Especially, the temporal evolution of the floc size distribution of cohesive particles in a turbulent fluid has been investigated by many researchers via theoretical formulation [2,9,[30][31][32], numerical simulation [33][34][35][36][37][38][39], and experimental observation [6,23,26,40] in many disciplines. ...
... Kaolin clay was collected as flocculation material in a rectangular tank reactor (150 mm×150 mm) stirred with an R1342-type impeller (50 mm in diameter), and the coagulant was Polyaluminum chloride (PACl) with a basicity value of 75%. In this study, three shear procedures were provided: (1) G = 7.7, 12. [29]. The adopted equipment was Flocculator ZR4-2, and the floc size was measured timely using a laser diffraction instrument and a PDA 3000 in the jar test. ...
... Comparisons of the proposed entropic model (Equation (11) [29] are presented in Figures 8-15, respectively. Table 2 lists these comparison results. ...
In this study, an extended model for describing the temporal evolution of a characteristic floc size of cohesive sediment particles when the flocculation system is subject to a piecewise varied turbulent shear rate was derived by the probability methods based on the Shannon entropy theory following Zhu (2018). This model only contained three important parameters: initial and steady-state values of floc size, and a parameter characterizing the maximum capacity for floc size increase (or decay), and it can be adopted to capture well a monotonic pattern in which floc size increases (or decays) with flocculation time. Comparison with 13 literature experimental data sets regarding floc size variation to a varied shear rate showed the validity of the entropic model with a high correlation coefficient and few errors. Furthermore, for the case of tapered shear flocculation, it was found that there was a power decay of the capacity parameter with the shear rate, which is similar to the dependence of the steady-state floc size on the shear rate. The entropic model was further parameterized by introducing these two empirical relations into it, and the finally obtained model was found to be more sensitive to two empirical coefficients that have been incorporated into the capacity parameter than those in the steady-state floc size. The proposed entropic model could have the potential, as an addition to existing flocculation models, to be coupled into present mature hy-drodynamic models to model the cohesive sediment transport in estuarine and coastal regions.
... Pollution of the aquatic ecosystems poses a serious threat to global water quality due to the presence of a wide range of contaminants, including, microplastics, organic matter and other various particulates (Cunha et al., 2019;Du et al., 2020;Jones et al., 2002). Additionally, the presence of dissolved organic matter (DOM) in surface waters leads to the formation of a wide range of disinfection by-products (DBPs), such as Trihalomethanes (THMs), Haloacetic acids (HAAs), Bromate and Chlorite (Richardson et al., 2007), during water treatment (Richardson et al., 2007;Su et al., 2017;Wu et al., 2019). The occurrence of DBPs in water supplies has been reported to significantly increase the potential incidence of cancerous diseases in humans (Wu et al., 2019). ...
... Additionally, the presence of dissolved organic matter (DOM) in surface waters leads to the formation of a wide range of disinfection by-products (DBPs), such as Trihalomethanes (THMs), Haloacetic acids (HAAs), Bromate and Chlorite (Richardson et al., 2007), during water treatment (Richardson et al., 2007;Su et al., 2017;Wu et al., 2019). The occurrence of DBPs in water supplies has been reported to significantly increase the potential incidence of cancerous diseases in humans (Wu et al., 2019). Hence, the removal of DOM during water treatment before disinfection is an important approach for improving drinking water quality (Wu et al., 2019). ...
... The occurrence of DBPs in water supplies has been reported to significantly increase the potential incidence of cancerous diseases in humans (Wu et al., 2019). Hence, the removal of DOM during water treatment before disinfection is an important approach for improving drinking water quality (Wu et al., 2019). Although various methods have been shown to achieve an adequate degree of DOM removal, such as coagulationflocculation and membrane filtration, these processes have a substantial cost and a wide range of potential risks to health and safety (Berthon, 2002;Yang et al., 2012). ...
Naturally present aquatic microorganisms play an important role in water purification systems, such as the self-purification of surface waters. In this study, two water sources representing polluted surface water (Olympic Green; OG) and unpolluted surface water (Jingmi river; JM), were used to explore the self-purification of surface water by bacteria under different environmental conditions. The dominant bacterial community of OG and JM waters (both are Firmicutes and Proteobacteria) were isolated, cultured, and then used to carry out flocculation tests. Results showed that the flocculation ability of the dominant bacteria and extracellular polymeric substances (EPS) obtained from OG isolation was significantly greater than that from JM. Further examination illustrated that the main components of EPS were polysaccharides, which played an important role in improving the flocculation ability of bacteria. EPS from dominant cultural bacteria strains (OG1 and JM3) isolated from the two different sources lacked hydrophilic groups (e.g. COOH) and had a networked structure which are the main reasons to enhance the flocculation ability. The bacterial diversity and redundancy analysis (RDA) results also showed that microbial community composition is determined by water quality (SS, TOC, and NH4⁺), and different Bacteroidetes, Actinobacteria and Proteobacteria community structures can improve the water body's ability to remove environmental pollutants (such as SS, humic acid and fulvic acid). These findings provide new information showing how bacterial communities change with environmental factors while maintaining the purity of surface water.
... Coagulation is a common process in conventional water treatment, and Fe salts, as well as aluminium salts, are widely usedas Wu et al., 2019 ), and that with aging some crystallization of nanoparticles occurs and changes in their structures occur ( Yu et al., 2016a ;Yu et al., 2018 ). In addition, it was found that organic matter, such as humic-like substances which exist ubiquitously in surface water, can affect the aggregation between nanoparticles ( Bian et al., 2011 ;Jarvis et al., 2005 ), and the crystallization and transformation of Fe (hydr)oxides ( Xing et al., 2020 ). ...
... When Fe(II) was used as the coagulant, the maximum FI value (size of flocs) was much greater and the lag time (from 0 s to the time when the FI value began to increase) was much longer than with Fe(III) as the coagulant ( Fig. 1 a and b). The existence of the active oxygen-containing groups (-OH 2 and -OH) in the inner coordination spheres of the Fe ions, is the main cause of their tendency to attract each another through forming μ-OH bonds, and to form nanoparticles which aggregate to form flocs ( Wu et al., 2019 ;Yu et al., 2016b ). We believe that Fe(III) may have more active oxygen-containing groups (active points) than Fe(II) at the beginning, resulting in the formation of nanoparticles and flocs more rapidly than Fe(II). ...
... Our previous studies have shown that crystallization is aprocess of dehydration resulting in the decrease of active hydroxyl group ( Wu et al., 2019 ). The subsequent slight decrease of peak intensity corresponding to Fe-O-H (2700 s) might be caused by the weakening of functional group activity when the nanoparticles aggregated or were broken during coagulation ( Yu et al., 2010 ;Yu et al., 2016b ). ...
Although Fe(II) salts have been widely used as coagulants in water treatment for many years, the underlying mechanisms of their performance remain unclear. Here, we present a detailed study of the coagulation behavior of Fe(II) salts and crystallization of flocs, and investigate the effect of humic acid (HA) under different DO concentrations and pH conditions. The behavior of Fe(II) and Fe(III) coagulants was found to be markedly different with the flocs from Fe(II) consisting of planar-like crystalline γ-FeOOH in contrast to the small amorphous spherical-like flocs from Fe(III) dosing. The effect of HA on Fe(II) coagulation was different under different DO concentrations and pH, whereby the growth of γ-FeOOH was inhibited by the presence of HA, but independent of DO concentration and pH. It was found that Fe(II) was present within the internal structure of γ-FeOOH, and a plausible formation mechanism is proposed. Firstly, planar nanoparticles of Fe(OH)2 were formed via Fe(II) ion hydrolysis which then served as the nucleus for subsequent crystal growth. With oxidation, Fe(II) on the surface of nanoparticles transformed to Fe(III). Finally, the formation of γ-FeOOH in Fe(II) coagulation was accompanied by a change in solution colour to yellow.
... The SVI results implied that longer mixing time with softeners only with higher coagulant dose allowed particles were bridged and rapidly settled; however longer flocculation time would hinder this positive interaction due to floc breakage. Wu et al. [54] also found floc breakage as flocculation time increased. Fig. 7c shows that reduction of SVI with increasing coagulation speed was observed at longer flocculation time (>12 min). ...
... Fig. 7c shows that reduction of SVI with increasing coagulation speed was observed at longer flocculation time (>12 min). Previous studies [54,63] suggested that the size of flocs decreased with increase of mixing speed, resulting in an increase of the total surface areas of flocs and hence more polymers could adsorb. Under this situation, longer flocculation time would allow more bridging to occur and denser flocs could be settled. ...
Effective removal of colloidal impurities from steam-assisted gravity drainage (SAGD) produced water is essential for enhancing water recycling in industry. Response surface methodology (RSM) was employed to optimize the thermal softening-coagulation-flocculation-sedimentation process with softeners (Ca(OH)2, MgO and Na2CO3), poly-DADMAC as coagulant, and cationic polyacrylamide (PAM) as flocculant, and assess the interaction effects of operational variables for sludge volume index (SVI) and the removal efficiency of turbidity (TSS), particulate hardness, silica, total organic carbon (TOC) and total inorganic carbon (TIC) in synthetic SAGD produced water. The optimal conditions at 0.93 desirability were 67 mg/L poly-DADMAC dose, 14 min mixing time with softeners only, 200 rpm coagulation speed, and 16 min flocculation time. At this condition, the predicted maximum removal of turbidity, TSS, particulate hardness, silica, TOC and TIC were 99.2 %, 99.1 %, 99.4 %, 27.0 %, 69.0 %, and 30.3 %, respectively, and the value of SVI was 38.1 mL/g. Our results indicated that poly-DADMAC dose and mixing time with softeners only were the most influential factors for the treatment process. Furthermore, the temperature effect on removal mechanisms was investigated at optimal conditions under room temperature and high temperature (80 °C). The results of floc characterization and surface force measurement indicated that temperature could facilitate the removal of colloidal impurities via forming larger and denser flocs and changing their surface composition. The results also provided confirmation that adsorption and subsequent bridging are the main removal mechanisms in the coagulation-flocculation process. This study illuminates the importance of the interactions among operational variables and provides in-depth insights into the mechanism for the removal of colloidal impurities under high temperature.
... At pH 6, there might be a significant excess of adsorption sites. 37 Therefore, the removal rate of HA showed little decrease even after several breakages. ...
... 43,44 Previous studies believed that the water and hydroxyl groups within the inner coordination spheres of the metal ions played an important role in the aggregation of nanoscale primary particles. 37,45 These types of groups can form OH·OH 2 − (written as μ-H 3 O 2 ) bridges between Al (III) sites, which further convert to μ-OH bridges via a reaction that releases water With the aggregation of the primary nanoparticles, large flocs are gradually formed during the first several minutes of coagulation. Eventually, the flocs reached an equilibrium of energy-driven structure and size, 15 which depends on the properties of the water (e.g., pH, ionic strength, and alkalinity) and the coagulant dose. ...
... Molecules 2020, 25, 5100 2 of 18 Therefore, several methods have been developed and employed for the removal and/or degradation of hazardous organic dyes from contaminated wastewater, such as photodegradation, extraction, adsorption, membrane separation, coagulation, flocculation, chemical oxidation, ion exchange and biological treatment [3][4][5][6][7][8][9]. Among all the techniques, adsorption by natural and synthetic adsorbents has been widely used because of its simplicity and efficiency for toxic dye removal in wastewater [10][11][12][13][14][15]. ...
... In fact, the hydroxyl group (OH − ) in the solution at pH 11 favors the positive charge of the MB, which has a pKa equal to 3.8 [40]. However, at acidic values, the lower removal efficiency could be linked to the excess of proton ions in the solution competing with the basic dye cations on the removal sites of Fe 2 (MoO 4 ) 3 . Similar findings were reported by Kooli et al. [41] in a study of waste bricks applied as a promising removal agent for basic blue 41 from aqueous solutions. ...
... 21−23 For example, the crystallization of the precipitate during aging decreased the adsorption capacity of phosphate, which is due to the decrease of surface areas. 24,25 Phosphate mobilization is often associated with Fe precipitates in natural and engineered systems, owing to its high affinity for Fe. 26−28 Phosphate can be adsorbed on the surface of Fe (hydr)oxides via a reaction with surface hydroxyl groups. ...
... Phosphate could be adsorbed rapidly by Fe or Al coagulants via a reaction with hydroxyl groups (−OH and −OH 2 ) on the nanoprecipitate surface. 25 However, these groups were consumed during the aggregation process of nanoprecipitates, and the formation of larger agglomerates with increasing reaction time reduced the surface area available on the nanoparticles for sorption of phosphate; 27 thus, the extent of phosphate retention decreased with increasing Fe(II) preprecipitation time. In order to better understand the different extents of phosphate retention under different Fe(II) preprecipitation times, the TEM technique was used to characterize the corresponding formed precipitates. ...
... The tridecamer Al 13 is more stable in resisting pH variation, allowing it to be adsorbed on particle surface following the Langmuir isotherm model (Wu et al., 2007). Although alum and Al 13 can spontaneously generate tenuous precipitates in water, large and compact flocs can be formed only in the presence of colloidal particles (Wu et al., 2019). Therefore, the adsorption of coagulant or its hydrolysates on the particle surface is the first step in the coagulation process. ...
... Therefore, the existence of particles can weaken the influence of pH on aggregation of Al 13 at neutral to alkaline range. This can be confirmed by coagulation experiments applying two PACl added modes (adding kaolin particles with no delay and 10 min delay after PACl dosed) (Wu et al., 2019;Yu et al., 2015). ...
Unraveling the transformation of coagulants and their interaction with contaminants at the micro-level is vital to advancing our understanding of the coagulation mechanism. To the best of our knowledge, the coagulation effectiveness of [AlO4Al12(OH)24(H2O)12]⁷⁺ (Al13), regarded as the dominant species in polyaluminum chloride (PACl), is highly related to its aggregation characteristic, but the detailed process of Al13 aggregation in coagulation time scale was not well studies. Here we systematically studied the deprotonation and aggregation processes of Al13 by alkaline titration to simulate the reaction in coagulation case. By reacting with OH⁻, Al13 can continuously lose protons regardless of pH until its positive charge was well neutralized. The initial Al13 aggregates (Al13agg) appeared at B of 2.70 and large Al13agg was generated by coalescence of small initial Al13agg. Most Al13 polycations kept their main structure unchanged during aggregation and part was decomposed into monomers or oligomers. Density functional theory (DFT) results reveal that Al13 becomes unstable after deprotonation, but the aggregation of Al13 bridged by Al monomers can stabilize the polycations. Al13 needs to be hydrolyzed before interacting with colloidal particles, but particles can promote the aggregation of Al13 by weakening the repulsion force between the polymers. Strong and compact flocs can be generated induced by in-situ aggregation of Al13 in neutral and alkaline conditions. This study can provide a deep understanding about the role of Al13agg in removing particles and instruct the development of new efficient coagulants against the various water qualities.
... The tridecamer Al 13 is more stable in resisting pH variation, allowing it to be adsorbed on particle surface following the Langmuir isotherm model (Wu et al., 2007). Although alum and Al 13 can spontaneously generate tenuous precipitates in water, large and compact flocs can be formed only in the presence of colloidal particles (Wu et al., 2019). Therefore, the adsorption of coagulant or its hydrolysates on the particle surface is the first step in the coagulation process. ...
... Therefore, the existence of particles can weaken the influence of pH on aggregation of Al 13 at neutral to alkaline range. This can be confirmed by coagulation experiments applying two PACl added modes (adding kaolin particles with no delay and 10 min delay after PACl dosed) (Wu et al., 2019;Yu et al., 2015). ...
The complexity of natural water made it difficult to remove fluoride. Based on the environmental problems found in the investigation, the fluoride removal research in the water containing algal cells was carried out. In this study, AlCl3 and [AlO4Al12(OH)24(H2O)12]⁷⁺ (Al13) were used to remove fluoride. Additionally, the role of aluminum speciation in fluoride removal and the effect of Microcystis aeruginosa on the fluoride removal by different aluminum species coagulants were elucidated. The results showed that AlCl3 mainly removed fluoride by physical interactions, surface adsorption and enmeshment. When algal cells were added to the system, the fluoride removal rate increased from 22.75 % to 72.99 % at a dosage of 40.0 mg/L. This was because algal cells greatly increased the distribution of Al(OH)3 in the flocs. In particular, the specific surface area of the flocs containing algal cells reached 160.77 m²/g, which allowed more fluoride to be adsorbed. However, excessive Al³⁺ led to serious damage to algal cells and release of intracellular organic matter (IOM), worsening the effect of defluoridation. F⁻ and Al³⁺ formed AlF²⁺ and AlF2⁺ via complexation in water. These compounds were not conducive to defluoridation. Al13 removed fluorine mainly through ion exchange, substitution and hydrogen bonding. Algal cells had an inhibitory effect on defluorination, which was observed in the process of coagulation by different Al dosages. Al13 achieved agglomeration of algal cells and generated small and dense flocs through charge neutralization and electrostatic patch mechanism. Once Al13 combined with algal cells and algae organic matter (AOM), the reaction between Al13 and fluoride would be weakened. Al13 not only maintained the defluoridation performance, but also did not damage the integrity of algal cells, even at high dosages.
... Molecules 2020, 25, 5100 2 of 18 Therefore, several methods have been developed and employed for the removal and/or degradation of hazardous organic dyes from contaminated wastewater, such as photodegradation, extraction, adsorption, membrane separation, coagulation, flocculation, chemical oxidation, ion exchange and biological treatment [3][4][5][6][7][8][9]. Among all the techniques, adsorption by natural and synthetic adsorbents has been widely used because of its simplicity and efficiency for toxic dye removal in wastewater [10][11][12][13][14][15]. ...
... In fact, the hydroxyl group (OH − ) in the solution at pH 11 favors the positive charge of the MB, which has a pKa equal to 3.8 [40]. However, at acidic values, the lower removal efficiency could be linked to the excess of proton ions in the solution competing with the basic dye cations on the removal sites of Fe 2 (MoO 4 ) 3 . Similar findings were reported by Kooli et al. [41] in a study of waste bricks applied as a promising removal agent for basic blue 41 from aqueous solutions. ...
The present study investigated iron molybdate (Fe 2 (MoO 4) 3), synthesized via a simple method, as a nanosorbent for methylene blue (MB) dye removal from aqueous solutions. Investigations of the effects of several parameters like contact time, adsorbent dose, initial dye concentration, temperature and pH were carried out. The results showed that MB removal was affected, significantly, by adsorbent dose and pH. Interestingly, lower values of adsorbent dose resulted in the removal of higher amounts of MB. At the optimum pH, the removal efficiency of 99% was gained with an initial MB concentration of ≤60 ppm. The kinetic study specified an excellent correlation of the experimental results with the pseudo-second-order kinetics model. Thermodynamic studies proved a spontaneous, favorable and endothermic removal. The maximum amount of removal capacity of MB dye was 6173 mg/g, which was determined from the Langmuir model. The removal efficiency was shown to be retained after three cycles of reuse, as proven by thermal regeneration tests. The presence and adsorption of the dye onto the Fe 2 (MoO 4) 3 nanoparticle surface, as well as the regeneration of the latter, was ascertained by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). These findings are indicative that the investigated nanosorbent is an excellent candidate for the removal of MB in wastewater.
... Although, the flocs formed in the citric acid system exhibited poor recovery ability, with the regrowth FI in the equilibrium state reaching only about half of the initial value. In contrast, the flocs formed in the three amino acid systems displayed stronger elastic recovery capabilities, indicating that the formed flocs had more active sites and were effectively exposed upon breaking, allowing the recovery of nanoparticles to a higher level [29]. ...
... In the formula, D2f is two-dimensional fractal dimension. R is the characteristic length, and the numerical value is the perimeter P [46]. Three kinds of flocs treated by (a) PAC, (b) PAC+PAM and (c) magnetic flocculation were diluted, and images were taken by computer permeable polarizing microscope. ...
In order to improve the removal efficiencies of SS and Fe3+ in anaerobic digestion reject water for the subsequent biological treatment process, on the basis of the single factor test in the early stage, the response surface method was used, and the structure of the formed floc was analyzed by magnetic flocculation. The optimum amounts of magnetic powder, polyaluminum chloride (PAC) and polyacrylamide (PAM) were 40.51 mg/L, 31.31 mg/L and 4.05 mg/L, respectively. At this time, the removal efficiencies of SS and Fe3+ were 97.84% and 98.35%. The effects of floc particle size, scanning electron microscopy, infrared spectroscopy and two-dimensional fractal dimension of flocs on the flocculation ability showed that: compared with conventional coagulation, the average particle size of flocs treated by magnetic flocculation was 76.56 μm, the Fe-O-Al absorption peak appeared at 984 cm−1, the flocculation ability was significantly improved, the surface of the floc was rough and porous, and the structure was dense, and the sedimentation performance was significantly improved also.
... Floc formation kinetics was monitored and investigated by employing a "turbidity fluctuation" technique with a PDA (3000 series, Rank Brothers Ltd., UK). The settings of this monitoring system, including the PDA 3000, a peristaltic pump and a computer, were similar to that employed in a previous study (Wu et al., 2019). Specifically, 3 mm (inner diameter) tube was installed and this monitor system was operated at a flow rate of 16 mL/min. ...
Factors responsible for undesirable coagulation performance in winter have not been well understood by far. The aim of this work is to identify the critical factor(s) and to provide applicable suggestions for drinking water production under low temperature. Specifically, water samples were collected from the river Jingmi (JM), a major drinking water source of Beijing City (China), for a series of coagulation tests. Coagulation processes influenced by factors, including particles and organic matter (natural biopolymers and humics) which commonly present in surface waters, were investigated. A photometric dispersion analyzer (PDA) was employed to reveal flocs formation and their properties impacted by water temperature, ranging from 5 °C to 25 °C. Results showed that the absence of natural biopolymers (40–90 kDa, identified as mixtures of proteins and polysaccharides) in surface water in winter significantly hindered flocs growth at low temperature; flocs size and growth rate continuously improved with artificially increasing concentration of biopolymers with/without particulates (kaolin particles in this study), demonstrating positive roles of biopolymers in coagulating kaolinite/organic water. Nanoscale observation on flocs using scanning electron microscope (SEM) demonstrated a bridging capability of biopolymers in aggregating primary particles into larger clusters, and this facilitated flocs growth at low temperature. The findings of this work are expected to improve the understanding of coagulation mechanisms under low temperature, also contributing to better management of coagulation when purifying surface water during winter months.
... achieve effluents less than 0.1 mg/L phosphate, the molar ratio can surpass 6.0. Therefore, the amount of Me : P needed to precipitate phosphate will decrease as the effluent concentration increases and will approach a ratio of 1.0 as the effluent phosphate exceeds 1.0 mg/L (Bai et al., 2010;Ge et al., 2018;Narasiah et al., 1994;Toor and Kim, 2019;M. M. Wu et al., 2019;Yang et al., 2010). ...
Owing to their strong bond with anions, rare earth elements (REEs) are prime contenders in wastewater treatment to meet the stringent phosphorus (P) effluent quality requirements. REEs outcompete traditional metals to abate phosphorus. The application of lanthanides in wastewater treatment is mainly through adsorption, where REEs are incorporated into a carrier matrix to improve the adsorption capacity. As coagulants, information on the performance of lanthanides is lacking. In this review, the performance of major water coagulants (iron and aluminum) is discussed and compared to two lanthanides: cerium and lanthanum. The use of lanthanides as adsorbents and as coagulants is elucidated during P treatment. The recovery of P and REEs is also discussed. Where details were lacking in the literature, experiments were conducted to fill these research gaps. Using REEs as adsorbents limits their P precipitation potential; as coagulants, REE capacity is 520.79 mg P/g La³⁺ and 469.96 mg P/g Ce³⁺. In addition, as coagulants, they are not affected by pH (3.0 < pH < 10.0); however, carbonates and sulfate are the major species that can reduce the performance of REEs during P treatment. REE-P precipitation is orchestrated through the formation of an REE-PO4 bond. Unfortunately, this strong bond between lanthanides and phosphate makes phosphate recovery almost impractical. If the goal is to recover REEs and reuse P in other applications like fertilizers, REEs are not the best candidates. We recommend additional research dedicated to understanding lanthanide coagulants in typical wastewater treatment facilities and their release from phosphate precipitates under different environmental conditions.
... The addition of DOM content into freshwater ecosystems occurs mainly during the raining seasons (Chiu et al., 2020;D.J et al., 1992;Dalva and Moore, 1991), and changes in precipitation can also affect the input of terrestrial dissolved organic matter (tDOM) to freshwater ecosystems by modifying their hydrology, temperature profiles, light regimes, and biochemical processes (Houser, 2011;Paerl and Huisman, 2008;Sowerby et al., 2010). It has been shown that DOM in surface source waters can react with disinfectants during drinking water treatment to form a range of disinfection by-products (DBPs) (Wu et al., 2019), which potentially represent a risk to the water ecology and human health (Su et al., 2017). Therefore, exploring the migration, transfer, and removal of DOM in the soils of riparian zones can make a significant contribution to understanding their role in water purification. ...
Riparian zones are important natural means of water purification, by decreasing the aqueous concentration of terrestrial organic matter (OM) through adsorption and microbial degradation of the organic matter within the aquatic ecosystem. Limited studies have been reported so far concerning the migration of dissolved organic matter (DOM) in the horizontal and vertical planes of riparian zones. In this study, the migration of DOM in riparian zones, from forest soil to wetland soil, and with soil depth, were explored, based on a case study reservoir. Results showed that riparian wetlands can absorb the OM from the forest soils and adjacent reservoir, and act as a major OM sink through microbial action. Methylomirabilota and GAL15 bacteria increased with soil depth for the two soil systems, and the wetland soil system also contained microbial sulfates, nitrates and carbonates. These microorganisms successfully utilize the Fe³⁺, SO4⁻, and CO3⁻ as electron acceptors in the wetland system, resulting in enhanced OM removal. Although the variation of soil DOM in the vertical direction was the same for both forest and wetland soils, the Chemical structure of the DOM was found to be significantly different. Furthermore, the soil was found to be the main source of DOM in the forest ecosystem, with lignin as the main ingredient. The lignin structure was gradually oxidized and decomposed, with an increase in carboxyl groups, as the lignin diffused down into the soil and the adjacent reservoir. PLS-PM analysis showed that the soil physicochemical properties were the main factors affecting DOM transformation. However, microbial metabolism was still the goes deeper affecting factor. This study will contribute to the analysis that migration and transform of soil organic matter in riparian zone.
... The flocs were sedimented for around 24 hrs, followed by supernatant discarded, and then the sediments were added to the sewage for the removing of PO 4 3--P. It was reported that the flocculation index significantly decreased when re-grow the flocs after stop mixing for chemical coagulation with Fe and Al, especially with the co-existance of humic acid [39,51]. Therefore, the flocs during the EC process was probably experience the similar breakage and re-growth period, resulting in the lower removal efficiency of phosphate by the flocs after sedimentation. ...
Domestic wastewater quality conditions are complex, which will affect the types of flocs in Fe-electrocoagulation (EC). However, the influence of different types of flocs has not been considered for the removal of phosphate in real wastewater. Herein, the removal of phosphate in real domestic wastewater secondary effluent was investigated during Fe-EC process under low and high dissolved oxygen (DO) concentrations. The removal of phosphate was also investigated during Al-EC process for comparison. The results showed that phosphate (1.3 mg/L) could be efficiently removed with an efficiency of 98% ± 2% both during Fe-EC process with low and high DO concentration, as well as during Al-EC process, with a slight lower removal rate during Al-EC (in 5 min) compared with Fe-EC (in 2 min), at 10 mA/cm². The flocs composition was mainly green rust during Fe-EC with low DO, amorphous trivalent iron oxide/hydroxide during Fe-EC with high DO, and amorphous alum hydroxide during Al-EC. The removal mechanism was also deduced that the removal of phosphate by Fe-EC with high DO and Al-EC was mainly by coagulation, while by Fe-EC with low DO concentration was mainly by ion-exchange adsorption.
... The structure of two-line ferrihydrite (δ-Keggin) is very similar to Al 13 (ε-Keggin) [54], which can provide a number of adsorption sites on its surface. Therefore, under neutral or alkaline conditions, SPP had a strong ability to adsorb phosphate. ...
Fe(III)-(hydr)oxides can be formed by the oxidation-hydrolysis of Fe(II) minerals, its initial state is usually amorphous, which can form dense flocs quickly with a high specific surface area, resulting in widely used in the field of water treatment. However, the rate and path of oxidation-hydrolysis lead to the difference in the crystal structure of the precursor, which are directly determined by the crystallinity and adsorption activity of the final oxidized hydrolysate. Therefore, this study investigated the phase transition of siderite under different oxidation-hydrolysis paths. The results suggested KMnO4 could first oxidize the surface layer Fe(II) of siderite; and then Mn(II), hidden in the crystal lattice of siderite, was continuously exposed to the surface of siderite; after that, Mn(II) was oxidized by KMnO4 to form MnO2, which acts as an ion channel to allow internal Fe(II) of siderite further hydrolysis to form crystalline Fe(OH)2 and then further oxidation to form crystalline two-line ferrihydrite (δ-Keggin). Although Fe(OH)2 as transient precursor will disappear with the continuation of oxidation, its presence will greatly reduce the nucleation barrier of two-line ferrihydrite. These mineral phase transitions resulted in the low concentration of KMnO4 (0.03 mmol/L) could substantially enhance the ability of siderite to remove phosphate, with the maximum adsorption capacity (13.04 mg/g, Langmuir). However, H2O2 could only oxidize Fe(II) on the surface of siderite to form amorphous Fe(OH)3, while Mn(II) in the siderite lattice could not be oxidized. The surface coverage of amorphous Fe(OH)3 and exposed Mn(II) formed a dense passive film, resulting in the termination of the oxidation and showed a low adsorption activity (3.28 mg/g).
... Food products such as beverages contain phosphorus although only about 16% of it is retained by humans in their daily diet, while the use of industrial products such as detergents and fertilizers lead to large losses of phosphorus to the environment through farmland runoff, animal manure, crop straw and other carriers [3]. Presence of excess phosphorus in water bodies causes serious ecological damage in ecosystems [4][5][6][7], while it is well known that phosphorus is limited in the earth and is a non-renewable resource. With increasing demands for phosphorus, there is the threat of its exhaustion within a hundred years [8]. ...
Structured MgO-Co adsorbents synthesized with a template-free method were developed for removing phosphorus from aqueous solutions. The adsorbents had petal-like folds, specific surface areas of more than 200 m 2 /g, and phosphorus adsorption capacities of ca. 240 mg/g with removal efficiencies of 99% after 5 cycles of reuse. Adsorption of phosphorus onto the material occurs by ligand exchange, Lewis acid-base interaction and elec-trostatic attraction with mechanistic features varying according to solution pH. In the adsorption mechanism, electrostatic attraction is important for pH values from 1 up to the zero point charge (pH = 12.8), while ligand exchange is dominant for solution pH values from 1 to 11, while Lewis acid-base interactions become dominant at solution pH values greater than pHzpc. Mg(H 2 PO 4) 2 and MgHPO 4 are the main adsorption products. Struc-tured MgO-Co materials have high efficiency for phosphorus adsorption, require few chemical steps for synthesis, are readily immobilized or recovered with magnetic force and are applicable to recovery of phosphorus from actual wastewater streams.
... The current chemical methods for phosphate removal in freshwater are mainly divided into two categories, as following: sorption and chemical precipitation (Hu et al. 2020;Yang et al. 2018;Wu et al. 2019). Many materials, used as adsorbents to remove the phosphate by metal-phosphate complex (i.e. ...
The application of ferrate (Fe(VI)) and ferric chloride as coagulants for treating phosphate wastewater in the presence of kaolin clay particles was comparatively studied. The phosphate removal processes by ferrate and ferric chloride assisted with kaolin clay particles were investigated under different Fe/P molar ratios. At neutral pH, complete removal of phosphates by ferrate and ferric chloride was observed at 2:1 and 6:1 of Fe/P molar ratio, respectively. The effect of kaolin clay particles on the phosphate removal process was discussed by zeta potential, size particle distribution, FTIR and XPS. We showed that with the increase of Fe/P molar ratio, the interaction intensity of kaolin clay particles with Fe flocs was decreased by ferric chloride coagulation while firstly increased and then decreased by ferrate. This depends on the Fe species with positive charge from ferric chloride hydrolysis and ferrate decomposition. Phosphate can inhibit the formation of FeOH2+ and Fe(OH)2+ in the ferric chloride hydrolysis but promote the formation of FeOOH and Fe(OH)2+ in the ferrate decomposition. Kaolin clay particles can more remarkably promote phosphate removal by ferrate than by ferric chloride.
... Meanwhile, considering its numerous merits, lots of emerging integrated purification technologies Recently, some research works focused on the influence of additional dosage of coagulant and its dosing strategy on the coagulation process, through which the efficiency of coagulation can be enhanced ( Yu et al., 2010a ) or the membrane fouling can be alleviated ( Liu et al., 2011 ). Yu did lots of work in this field ( Yu et al., 2015( Yu et al., , 2010bYu et al., 2016 ) and claimed that the additional dosage of coagulant will introduce the fresh precipitant, which can improve the binding ability (via the 'active point') of the flocs formed during coagulation ( Wu et al., 2019 ;Yu et al., 2018 ). However, no comparison has been done between the additional coagulant or additives added at the beginning and steady period of the coagulation process. ...
Structure properties of flocs (size, fractal dimension (Df), etc.) have a high impact on coagulation efficiency. In this work, the influences of three different additives (ferric salt (Fe), phosphate (P), and citric acid (CA)) on coagulation process/efficiency were investigated. Results showed that a small amount of extra Fe can facilitate the growth of Al flocs by providing more ‘active sites’. Although zeta potential and Df showed a limited change, the average floc size increased apparently and the increment was more obvious when Fe was added after the formation of the flocs. In contrast, P addition during the rapid mixing period will decrease the final average floc size, while the influence is less significant when P was added after the growth of the flocs. In terms of CA, a more striking negative effect on the growth ability of the flocs was observed compared to P. The strong complexing/coordination interactions between CA and aluminum hydroxide is the main reason behind the influence. CA also significantly decreased the Df value of the flocs compared to P, and Df showed a comparatively higher decrease when P or CA was added during the rapid mixing stage compared to the addition after the flocs formation. These results indicated that the addition of CA or P during the rapid mixing stage ‘inactivated’ or occupied more ‘active sites’ on the preliminarily formed Al NPs during the hydrolysis process, and therefore presented stronger impact on the morphology/size of the formed flocs.
... Most of the above-mentioned pollutants are toxic, carcinogenic and mutagenic, thus their control and removal techniques are of great urgency. As of now, numerous methods including adsorption (Salleh et al., 2011), coagulation (Wu et al., 2019a), biodegradation (Piai et al., 2020), membrane filtration and advanced oxidation processes (AOPs) (Wu et al., , 2018aJiang et al., 2019) have been proposed for the removal of organic pollutants from water. Amongst them, adsorption being the one of the most simple and effective method, is highly recommended for the extensive environmental practices. ...
Iron-nickel bimetallic organic frameworks (FeNiX-BDC, H2BDC: terephthalic acid) were developed as bifunctional materials for adsorption and photo-Fenton degradation of organic dyes with different charge properties. Significantly enhanced adsorption capacity of FeNi1/15-BDC towards methylene blue (MB) and methyl orange (MO) was achieved, 5.3 and 2.6 times higher than that of pristine Fe-BDC, which was attributed to enlarged specific surface area and pore volume and the decreased surface charges induced by Ni doping. The adsorption kinetics demonstrated that chemisorption was dominant and intra-particle diffusion was the rate-controlling step. Two-stage degradation including slow induction stage and rapid oxidation stage fitted with pseudo-zero-order kinetics well. The increased rate constants (2.472 vs. 1.188 min⁻¹ for MB; 0.616 vs. 0.421 min⁻¹ for MO) in the induction stage as well as the superior removal capability by asynchronism relative to synchronism jointly corroborating the improved adsorption performance was favor for subsequent degradation. Notably, this heterogeneous system not only exhibited obvious advantages like wider pH working range (3-9), better stability and reusability of catalysts, but also achieved the dual objectives of in-situ decontamination and adsorbent regeneration. The coupling of adsorption and degradation along with synergism between photocatalysis and Fenton-like process are responsible for the reinforced removal of organic contaminants.
... As the solids gradually crystallize, a portion of the B is released as some of the AleOeB bonds are slowly transformed to AleOeAl bonds. Wu et al. (2019) also reported observing structural changes in Al(OH) 3 (s), evidenced by similar XRD and FTIR analyses, indicating a shift from amorphous to more crystalline structure after 24 h. Based on their study, given enough time (which can be affected by stirring or other processes), Al solid would form boehmite structure. ...
Boron (B), normally present in ground water and sea water, is a vital micronutrient for plants, but is also toxic in excessive amounts. Under typical conditions, aqueous boron is present as boric acid (H3BO3), which is uncharged, making B particularly challenging to remove by mechanisms commonly applicable to removal of trace constituents. Adsorption of B onto aluminum hydroxide solids (Al(OH)3(s)) generated using aluminum-based electrocoagulation (EC) is a promising strategy for B removal. Infrared spectroscopy analysis indicated complexation of B(OH)3 with aluminum hydroxide solids via surface hydroxyl groups, while X-ray and infrared spectroscopy results indicated that the structure of the Al(OH)3(s) was influenced both by EC operating conditions and by water quality. A linear adsorption model predicted B removal well when initial concentrations were lower than 50 mg/L, but fit the experimental data poorly at higher initial B concentrations. The Langmuir adsorption model provided a good fit for a broader range of initial B concentrations (5–1000 mg/L). Factors affecting B adsorption during the EC process, including current intensity, Al dissolution rate, boron concentration, pH, and total dissolved solid (TDS), were investigated. Increasing current intensity initially led to a higher Al dissolution rate, and therefore higher B adsorption, but there was a limit, as further increases in current intensity caused rapid formation of Al(OH)3(s) having a large particle size and a low capacity to complex B. Boron removal decreased as its concentration increased. The best removal of B occurred at pH 8, corresponding to a slightly positive zeta potential for aluminum hydroxide and a small but significant fraction of negatively charged B species. Higher TDS concentrations facilitated the use of higher current intensities, i.e., the limit on the effective Al dissolution rate increased with increasing TDS. Two real water samples (river water and oilfield produced water) spiked with B were treated using EC, resulting in up to 50% B removal from river water (C0 = 10 mg/L, current = 0.2 A) in 2 h, and 80% B removal from produced water (C0 = 50 mg/L, current = 1.0 A) in 2 h.
Brewery—a water-intensive industry—produces a significant amount of wastewater, requiring effective treatment before discharge to the terrestrial or aquatic ecosystem. The treatment efficacy of the single/two-stage membrane aerated biofilm bioreactor (MABR)...
Flocculation is a traditional and effective method to remove Cr from wastewater, but the addition of flocculants inevitably leads to secondary pollution. In this study, Cr flocculation was induced using hydroxyl radical (•OH) (•OH flocculation) generated in an electro-Fenton-like system, achieving total Cr removal of 98.68% at initial pH = 8 within 40 min. The obtained Cr flocs showed significantly higher Cr content, lower sludge yield, and good settling properties compared to alkali precipitation and polyaluminum chloride flocculation. •OH flocculation behaved like a typical flocculant, introducing electrostatic neutralization and bridging. The mechanism proposed that •OH could overcome the steric hindrance of Cr(H2O)63+ and combine with it as an additional ligand. Then Cr(III) was proved to undergo multi-step oxidation to form Cr(IV) and Cr(V). After these oxidation reactions, •OH flocculation took precedence over Cr(VI) generation. As a result, Cr(VI) didn't accumulate in solution until •OH flocculation was completed. This work provided a clean and eco-friendly strategy for Cr flocculation instead of flocculants and extended the application of advanced oxidation processes (AOPs), which is expected to enrich existing strategies of AOPs towards Cr removal.
It is important to regulate the in-situ generated iron(III) (hydr)oxides with the function of adsorption/coagulation during ferrate (Fe(VI)) oxidation and hydrolysis which was beneficial for removal of natural organic matter (NOM). This study investigated the effect of UV activation on the Fe(VI) hydrolysis and Fe(III) flocculation performance. Results indicated UV activation can effectively improve the protein and humic acid(HA) removal in natural surface water. In situ UV differential absorption spectroscopy showed that UV effectively activated the hydrolysis of Fe(VI), which converted Fe(VI) into intermediate high-valence iron with higher reactivity, and promoted polynuclear Fe-Hydroxide and iron(III) (hydr)oxide (PnFe-H), which improved the oxidation and flocculation of pollutants. With the radical species analysis, UV activation promoted the generation of Fe(IV) and H2O2, which greatly increased the generation rate of superoxide radicals (O2•-) and hydroxyl radicals (•OH). Furthermore, the structure analysis of the hydrolysate proved that UV enhanced the hydrogen bond and produced a large number of polar hydroxyl groups combined with iron ions, which further triggered the conversion of Fe(III) into amorphous Fe-hydroxide and ferrihydrite with large specific surface area and high surface activity. Meanwhile, the aggregation behavior and interaction energy of Fe-(oxy)hydroxide indicated that the lower free energy obtained in UV-activation system and increased the aggregation. This study clarified new insights on UV-activated ferrate, which can expand our understanding on ferrate applications in water purification processes.
Number of studies have explored the mechanism of humic acid (HA) removal by coagulation, while studies on the formation and growth mechanism of HA-floc are still limited. In this study, a series of dynamically monitored experiments were carried out and floc characterization techniques were applied to investigate HA-floc growth mechanisms and properties under different stirring conditions. First, HA was removed in the early stages and varied insignificantly during floc growth, and the stirring condition did not affect the removal of HA. Moreover, HA hindered the effective collision, resulting in a slower floc growth rate and a lower fractal dimension. Although stirring condition did not markedly affect the HA removal performance, it had a remarkable effect on floc growth and structure. For example, gentle stirring resulted in larger floc d50 and pore size. Intense stirring accelerated the breakage or aging of flocs, resulting in a reduction in the final floc size at equilibrium. In particular, fast stirring in the early stage of coagulation advanced floc growth, but decreased the zeta potential and active sites of flocs, and reduced the capacity of the primary aggregates to grow into larger flocs. This study provides information on the floc formation and growth mechanism and the properties of flocs formed under different stirring conditions, which are helpful for optimizing the coagulation process on organic matter removal and guide the processes utilizing flocs.
For the in-situ treatment process of landscape waters, ponds, reservoirs and other water bodies, there is a significant deviation between the experiment results and the actual treatment effect due to volume amplification effect. Therefore, this study systematically explored the influence of hydraulic conditions, dosing frequency, dosing intervals of coagulant, and coagulant dilution multiples on the removal efficiency of particulate and organic matter on a large scale to precisely simulate the practical treatment situation. Results showed that dosing intervals and dosing frequency played an important role in the aggregation and settling process of particulate matter by influencing the energy barrier between particles, while hydraulic conditions affected the structure and settling characteristics of flocs. The overall trend of the total organic matter was consistent with that of particulate matter. However, only the freshly formed aluminum hydroxide precipitates had adsorption activity to humic substances. The adsorption could be completed rapidly so that hydraulic conditions had no influence on the aggregation process of humic substances. While during stirring and aging process, humic substances was found to be released due to the polymerization and crystallization of aluminum precipitates. From the results of pilot tests, a treatment strategy was implemented for a reservoir in southern China and the ideal treatment effect was achieved. This study provided theoretical foundation and design guidance for the in-situ coagulation treatment of landscape waters, ponds and reservoirs.
In this study, waste corn cobs were oxidized and grafted with diethylenetriamine (DETA), which chelates a large amount of Fe³⁺. Then, nitrogen-doped zero-valent iron/biochar (ZVI/BC-N) was prepared using a simple carbothermal reduction method. ZVI/BC-N had a larger specific surface area and porous structure due to the irregular folding phenomenon on its surface caused by Fe and N codoping. At the same time, ZVI/BC-N contained more ZVI reactive crystals due to DETA chelation. The pseudo-second-order and Elovich models better describe the behavior of ZVI/BC and ZVI/BC-N in phosphorus adsorption, where the adsorption process was controlled by a combination of the reaction rate and diffusion. The isotherm and thermodynamic fitting results showed that the adsorption of phosphorus by ZVI/BC was a single-molecular-layer physical adsorption process, while ZVI/BC-N exhibited multimolecular layer chemisorption, and the maximum amounts of phosphorus adsorbed by ZVI/BC and ZVI/BC-N were 17.93 and 82.78 mg·g⁻¹, respectively. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy showed that both CN and CH on biochar were involved in the adsorption process, while the coordination between OH, CO and phosphate, the adsorption of ZVI and chemical processes were the main mechanisms of phosphorus removal. The stronger adsorption performance of ZVI/BC-N compared to ZVI/BC might be related to its high ZVI content and rich surface structure. This study aims to provide a method for the preparation of biochar composites with a high ZVI content through simple modifications and provide a new approach for the efficient removal of phosphorus from water.
It was important to regulate the formation of Fe-hydroxyl during ferrate (Fe(VI)) oxidation and hydrolysis which was beneficial for interfacial adsorption of natural organic matter (NOM). Based on the influence of weak magnetic field (WMF) on the physical and chemical characteristics of particles in chemistry. This study investigated the effect of WMF on Fe(VI) oxidation and Fe(III) flocculation performance by regulating iron species during hydrolysis, for NOM removal. Results indicated WMF efficiently accelerate the removal of NOM that the reactions rate constants in magnetization system was twice as much as the control group. With the structure and electrochemical analysis, WMF enhanced Hydrogen-bond and caused much polar hydroxyl groups combined with iron ions, further triggered Fe(III) transformed to amorphous Fe-hydroxide and ferrihydrite with large specific surface area and high surface activity which removed the pollutants by adsorption and co-precipitation, instead of crystalloid Fe2O3 and Fe3O4. In addition, the nucleation aggregation behavior and interaction energy of Fe-(oxy)hydroxide revealed that the lower free energy obtained in magnetization system, could lead to higher nucleation rate, and promoted the aggregation. WMF increased hydrophobicity of Fe-(oxy)hydroxides, further more easily adsorbed with humic acid (HA) and bovine serum albumin (BSA) with lower interaction energies than in control group. The selective removal mechanism of Fe-(oxy)hydroxide hardly to aggregate with pollutants which caused by the difference of electrostatic interaction, was illustrated that electronegativity HA and SA were liable to electrostatically attract with Fe-(oxy)hydroxide and removed while the low electronegativity BSA was difficult to remove which its attraction was weakened.
Microorganic pollutants (MOPs) in aquatic environment with low levels but high toxicity are harmful to ecosystem and human health. Fe(VI) has a dual-functional role in oxidation and coagulation, and can effectively remove MOPs, heavy metal, phosphate, particulates and colloids. Moreover, Fe(VI) can combine with traditional coagulants, or use as a pretreatment for membrane treatment because of its characters to generate nanoparticles by degradation in water. Based on the relevant toxicity experiments, Fe(VI) had been proved to be safe for the efficient treatment of MOPs. For better utilization of Fe(VI), its oxidation and coagulation mechanisms are summarized, and the knowledge about the control parameters, utilization methods, and toxicity effect for Fe(VI) application are reviewed in this paper. pH, different valences of iron, environmental substances, and other parameters are summarized in this study to clarify the important factors in the treatment of MOPs with Fe(VI). In the future study, aiming at cost reduction in Fe(VI) preparation, transportation and storage, enhancement of oxidation in the intermediate state, and better understanding the mechanism between interface and Fe(VI) oxidation will help promote the application of Fe(VI) in the removal of MOPs. This study offers guidelines for the application and development of Fe(VI) for the treatment of MOPs in aquatic environment.
Most studies on the interaction between coagulation and NOM (natural organic matter) currently focus on pollutant removal and coagulant species distribution, while studies on floc aging are lacking. Investigation onto the effects of floc aging could guide further processes that utilize flocs, such as densadeg sludge recirculation, floc predeposition for ultrafiltration, sludge condensation, and other traditional sludge reflux processes. In this study, flocs generated by Al13 and AlCl3 in microparticle- and nanoparticle-containing water were investigated, and the effect of floc aging on NOM was quantified based on several organic matter characterization techniques. Flocs absorb and release organics during aging. The flocs generated from micro-SiO2 have a significant absorbing effect for LWM-N (low-molecular-weight neutral substances) and protein-like substances, while the absorption of NOM by flocs generated from nano-SiO2 is insignificant. HS (humic substances) with high aromaticity are released during floc aging. From the molecular perspective, the molecules released during floc aging are those with higher double bond equivalents and higher aromaticity, while the absorbed molecules are those with lower double bond equivalents and lower aromaticity. 2D-COS (two-dimensional correlation spectroscopy) demonstrated that the flocs generated by Al13 and AlCl3 had the same organic release patterns but different intensities, while the flocs generated in the micro-SiO2 and nano-SiO2 systems had different organics release patterns. Abundant aluminum hydrolysates with low polymerization and amorphous Al(OH)3 would be produced from AlCl3 during the coagulation process and then undergo hydroxyl-bridging reaction and crystallization during floc aging, thus releasing more HS with high aromaticity into the supernatant; in comparison, prehydrolyzed Al13 produces a more stable floc and releases less HS during aging. The flocs produced by nano-SiO2 and Al-based coagulants release higher aromaticity HS into the water than those produced by micro-SiO2, which may be related to the formation of more highly polymerized degree hydrolysates and nanocrystalline Al(OH)3 in the nano-SiO2 system. The flocs generated in water with micro-SiO2 may contain a large amount of Al-OH and have a loose structure, thus further absorbing NOM, such as protein-like substances and LWM-N. In contrast, the flocs generated from nano-SiO2 possess abundant adsorbed water and a denser structure; thus, organic matter cannot be absorbed stably.
Microplastics (MPs) have attracted much attention worldwide as one kind of emerging pollutant, with the social demand for plastic products surging and microplastic pollution increasingly serious. In this experiment, magnesium hydroxide formed under alkaline conditions was combined with anionic polyacrylamide (PAM) as a dual-coagulant to deal with the simulated natural water containing polyethylene (PE). And the whole process was monitored by intelligent Photometric Dispersion Analyzer (iPDA), the Flocculation Index (FI) value of flocs and MPs removal efficiency were used as indicators to evaluate coagulation-flocculation performance. The results suggested that the flocs formed were not sufficient to remove PE particles only in the case of dosing magnesium ion. When PAM was added after magnesium hydroxide formation, the maximum FI value reached up to 7.2 and the PE removal efficiency was greatly improved, reaching 84.9% ± 3%. Additionally, flocs settling rate with average size of 57.19 μm reached to 4.8×10⁻³ m/s. At the same time, the types, dosing time and dosage of anionic PAM also had a great influence on the removal of MPs. Based on investigations of zeta potential and floc properties, adsorption bridging and sweeping were the main coagulation mechanisms. The MPs removal behaviors exhibited during coagulation processes have potential application in water treatment.
Dissolved organic matter is one of the difficult problems in the treatment of textile dyeing wastewater, among which humic substances are the most harmful to the environment and human health. Herein, cationic starch‐based flocculants are prepared by free radical polymerization of corn starch (St) and methacryloyloxyethyl trimethyl ammonium chloride (DMC) in a redox‐initiated system of ammonium persulfate and sodium bisulfite. Corn starch‐graft‐poly(methacryloyloxyethyl trimethyl ammonium chloride) is denoted as St‐g‐PDMC. The St‐g‐PDMC is comprehensively characterized in terms of structural (element analysis, Fourier transform infrared, X‐ray diffraction, proton nuclear magnetic resonance spectroscopy), morphological (scanning electron microscopy), and thermal (differential scanning calorimetry) properties. Humic acid (HA) solution is selected to simulate negatively charged dissolved organic matter. The flocculation performance and mechanism of St‐g‐PDMC, cationic polyacrylamide (CPAM), and polymerized aluminum chloride (PAC) are systematically investigated. The experimental results show that St‐g‐PDMC functions mainly through the electric neutralization and bridging effect as the flocculation mechanism, CPAM using its own structure and the colloid formed by PAC through hydrolysis is more in line with the bonded bridging and net roll sweep trapping. The combined formulation of St‐g‐PDMC and PAC shows advantages over a single flocculant in the treatment of textile dyeing wastewater, and its cost is evaluated. In this work, the performance of three different types of flocculants on the removal of humic substances from textile dyeing wastewater is investigated. The starch‐based flocculant St‐g‐PDMC shows excellent flocculation ability for HA removal with the addition of a single flocculant. Among them, the combination formulation of St‐g‐PDMC and polymerized aluminum chloride is the most effective, and they play synergistic roles.
Ensuring the microbiological safety of drinking water is critical to protect public health. This study aimed to evaluate the reliability of real-time bacteriological counter coupled with an online dialysis membrane-based pre-treatment system for continuously monitoring bacterial cell counts in sand filter effluents of a full-scale drinking water treatment plant. The pre-treatment system, which included anion exchange resins (porous polymeric microbeads that trap ions for releasing other ions) for dialysate regeneration, successfully achieved the stable attenuation of background interfering substances (humic acids) during the 19-d test. The real-time bacteriological counter equipped with the pre-treatment system provided a continuous profile of bacterial cell counts in the sand filter effluent (0.2–2.5 × 10⁴ counts/mL). The online analysis identified different timing of concentration peaks between particle and bacterial cell counts after backwashing. Bacterial community analysis revealed that Proteobacteria, Planctomycetes, and Cyanobacteria were the dominating phyla. Further, total bacterial cell counts determined by fluorescence microscopy and SYBR® Green I staining, a commonly accepted parameter, was found to be an indicator of online-monitored bacterial cell counts. The results indicated the potential of monitoring the bacterial cell counts in a sand filter process for providing an early warning of filter failures, which can allow plant operators to diagnose the overall system and provide countermeasures.
Efficient and space-saving technologies for on-site treatment of stormwater runoff are required to control water pollution in the urban surface. The intermittent nature of stormwater runoff and extremely limited land available greatly hindered the application of current wastewater treatment technologies, and thus synchronous removal of multiple contaminants (especially for nutrient) efficiently was failed by current processes. In this study, a new compact CFFA treatment system, consisting of coagulation, flocculation, filtration and ammonium ion exchange units, was constructed for on-site treatment of stormwater runoff based on batch test optimization and pilot-scale test verification. The coagulation process effectively aggregated particles and precipitated phosphorus by dosing Al2(SO4)3, while flocculation using anionic polyacrylamide further enlarged particle size for efficient micromesh filtration. The dynamic micromesh filtration obtained turbidity and phosphorus removal efficiencies comparable to 30 min gravity settling with greatly smaller footprint. Ion exchange by zeolite showed higher exchange capacity owing to lower initial ammonium nitrogen concentration in the stormwater runoff. The pilot-scale experiments with treatment capacity of 1 L/s showed that the CFFA treatment system achieved synchronous removal of particles (97.2%), nitrogen (79.7%), phosphorus (95.0%) and organic matters (83.3%) efficiently within short hydraulic retention time of 0.35 h, yielding effluent with chemical oxygen demand, suspended solids, total phosphorus and total nitrogen of 38.7, 7.80, 0.22 and 2.80 mg/L, respectively. The CFFA treatment system had the highest pollutant removal loads compared to reported runoff treatment processes in literatures, and was well suited to on-site treatment of stormwater runoff with high space utilization efficiency.
Photocatalysis is highly efficient for the treatment of dyeing wastewater. However, the wastewater still contains a large amount of nano-TiO2 after photocatalysis. In order to reduce the cost of nano-TiO2, different coagulants were used to improve the solid/liquid separation efficiency. At the same time, the reuse of flocs has raised increasing attention after coagulation. In this study, we report on the recycling of flocs for the production of AlCl3 and TiO2 formed flocs (AT1) and Al13 and TiO2 formed flocs (AT2) through a simple calcination process. Results indicated the flocs formation processes under different concentration of CO3²⁻ were Al species distribution and coagulant dosage dependent. Without HA molecules, the reaction between Al-based coagulants and CO3²⁻ are the controlling steps in the coagulation process, and Ala and Alb species had different reaction processes. The influence of HA molecules was also studied, and the HA-Al aggregates which could bridge the TiO2 nanoparticles, and thus induce sweep flocculation. Consequently, highly enhanced aggregation rate, due to the heteroaggregation between these aggregates and TiO2 was observed. After calcination, the AT2-10 and AT2-20 (CO3²⁻: 10, 20 mmol/L) with petal-like structures (mesoporous), relatively high specific surface areas, i.e. 83.98 m²/g and 78.38 m²/g and high photocatalytic degradation, i.e. 72.79%, 67.78% (compared to nano-TiO2, AT1-10 and AT1-20), were obtained. Additionally, oxygen vacancy (OV) was generated on the surface of the recycled AT2-10 and AT2-20, which display strong photocatalytic performance for the degradation of MO > 65%, under ultraviolet (UV) irradiation and without any sacrificial reagents. This study proposes a new perspective on recycling coagulated flocs for practical application in wastewater treatment.
Coagulation is well-established for controlling regulated disinfection by-products (DBPs), but its effectiveness for controlling unregulated DBPs remains unclear. The efficiency of coagulation in controlling unregulated DBPs requires clarification owing to their relatively high toxicity. In this study, three Al-based coagulants, aluminum sulfate (Alum), polyaluminum chloride (PAC), and a novel type of covalently bond hybrid coagulant (CBC, synthesized using AlCl3) were selected, and the coagulation performance of these Al-based coagulants in controlling DBPs and DBP-associated toxicity was compared over 5 classes of DBPs, including trihalomethanes, haloacetic acids, haloacetaldehydes, haloacetonitriles, and halonitromethanes. The results showed that Alum was the least efficient in removing DBP precursors among the three coagulants. The effectiveness of CBC and PAC for DBP control varied with the characteristics of source waters. CBC had an advantage in water with a low content of humic acids, and reduced DBP concentration and DBP-associated toxicity by 47% and 25%, respectively. For water rich in aromatic organics, CBC might serve as DBP precursors at a high-required dosage, suggesting that a trade-off between enhanced DBP control and serving as DBP precursors should be considered for CBC coagulation; PAC achieved the most reduction in DBP concentration and DBP-associated toxicity by 50% and 34%, respectively.
A new class of catalytically self‐propelled nanomotors was fabricated by modifying natural clay tubes, halloysite, with randomly distributed particles of MnO2 and Fe3O4. The prepared MnO2‐Fe3O4/HNTs composites were validated to be efficient Fenton catalysts in degradation of rhodamine B (RhB). Compared to the previous preparation of rolled‐up microtubes or other template‐assisted syntheses, this strategy has its merits in utilizing clay minerals of abundance, cheap‐price, and no complex instruments needed. The nanomotors were able to be prepared in a large scale. The MnO2‐Fe3O4/HNTs motors displayed powerful autonomous movement, and a high velocity of up to 380 µm s‐1 was achieved in 5.0 wt. % H2O2 solution. For Fenton catalysis of RhB, the removal ratio of 94% dye molecules was obtained within 30 min, which was triply higher than other samples of the non‐propelling sample Fe3O4/HNTs. The autonomous movement provided adsorptive bubble separation, and the adsorption capacity was greatly enhanced by halloysite. These synergistic effects boosted removal efficiency of dye molecules. The presence of magnetic MnO2‐Fe3O4 made these motors move directionally in external magnetic fields, and provided a facile recovery for collecting heterogeneous catalysts.
Hazardous metal pollution became a severe environmental issue in China. An efficient precipitation-flotation process was developed to achieve fast removal for metal-ions from wastewater. Structure and strength of precipitate particles/flocs significantly influence the flotation removal of metal-ions. Formation and growth-evolution of precipitate flocs in precipitate flotation were studied by stage analysis of precipitate particles-formation, flocs-regulation and flotation separation. The results demonstrate that early formed precipitates MHA(humics-metal complexing particles) have small particle size, high fractal dimension, low strength and recovery factor. The addition of Fe³⁺ and CTAB(cetyl trimethyl ammonium bromide) reagents make the precipitate particles aggregated to flocs(MHA-Fe, MHA-Fe-CTAB) much more large, loose, coarse, and small-density. The final generated MHA-Fe-CTAB flocs are hard to be broken up, easy to be recovered and efficient to be separated by flotation process. The flotation removal of MHA-Fe-CTAB flocs is clearly higher than that of MHA or MHA-Fe. The flotation results of MHA-Fe-CTAB are as follows: flotation removal of 98.7 ± 0.40%–99.9 ± 0.10%, residual TOC of 0.96 ± 0.38–1.35 ± 0.41 mg/L and turbidity of 0.44 ± 0.09–0.63 ± 0.16 NTU. Introducing Fe³⁺ and CTAB reagents into flotation solution contributes to the growth-evolution of precipitate flocs, which could intensify the metal-ions removal via precipitate flotation process and result in more ideal purification indexes for metal-containing wastewater.
This study establishes a new understanding of the contributions of Al residue in a megalopolitan drinking water supply system with mixed water sources. The different influences and contributions of foreign water source, resident migration and season changing to Al residue in drinking water were investigated. Especially, the role of Southern water transferred over 1200 km via the South-to-North Water Diversion Project in the Al residue of drinking water supply system of a northern megalopolitan were revealed for the first time. Comparisons of big data on Al residue in the water supply system with sole and mixed water sources showed that the introduction of Southern water enhanced the Al residue in drinking water by over 35%. The world's largest annual residents’ migration during Chinese Lunar New Year and the changes of season affect the water pipework hydrodynamics, which were embodied as the periodic changes of particulate aluminium and the relations with resident's temporal-spatial distribution in the megalopolitan. Because of the differences in water quality, Southern water promotes the release of historically deposited Al and facilitates the cleaning of old pipes.
Increasing attention has been focused on the removal of micropollutants from contaminated drinking source water. However, low rejection efficiency and membrane fouling still inhibit further application of nanofiltration membrane in this field. Interesting results were found that the residual hydrolyzed-aluminum nanoparticles from supernatant after coagulation and sedimentation strongly improved the nanofiltration performance for micropollutant removal. A simulated raw water containing humic acids, micropollutants and kaolinite clay was employed to investigate the factors of water matrix affecting the nanoparticle-enhanced nanofiltration for micropollutant removal. Results of experiments showed that these hydrolyzed-aluminum nanoparticles easily induced the aggregation of bisphenol-A (BPA) and humic acids in the supernatant. The enhancement of BPA removal was mainly attributed to the repelling interaction between the Al-BPA-DOC complexity and in situ-modified membrane surface during nanofiltration.
‘This in situ surface modification by the hydrolyzed-aluminum nanoparticles improved membrane hydrophilicity, roughness and positively-charging capacity. For the treatment of River Songhua water spiked with micropollutant, the percentage removal of BPA was improved to be 88.5%, much more than the case of single nanofiltration without coagulation (60.7%). Meanwhile, the membrane fouling was reduced by 2.13 times than the case of single nanofiltration without the dynamically deposited-layer of nanoparticles. This in situ modification of nanofiltration membrane by hydrolyzed-aluminum nanoparticles achieved excellent removal efficiency for micropollutants from River Songhua water background.
A series of hierarchical micro/nano Ce-based composites were derived from Ce-MOF via thermal treatment in N2 atmosphere. Different from conventional complete decomposed materials, forming cerium oxide in air, Ce-MOF that calcinated in N2 at lower temperatures (400 °C or 500 °C) showed a partial thermal decomposition with high percent content of Ce(III). Even though the complete decomposed products held higher surface areas, the partial decomposed samples exhibited extremely higher phosphate uptake, with working capacity 2–4 times higher than that of ceria. The results implied a predominant effect of different valence states on phosphate removal by Ce-based materials, in which Ce(III) species were demonstrated playing the major role to form binding with phosphate. The maximum adsorption capacity (189.4 mg/g) was achieved by Ce-MOF-500(S) with wide applicable scope of pH ranging from 2 to 12 and great selectivity for phosphate in the presence of competing anions. Remarkably, Ce-MOF-500(S) described obvious enhanced phosphate adsorption ability under alkaline condition. This was due to the fact that the hydrolyzed Ce(III) species brought more active sites in the form of hydroxyl groups for ligand exchange with phosphate. Furthermore, based on the analysis of FTIR, XPS, XRD and zeta potential, electrostatic attraction, ligand exchange and surface precipitation were confirmed as the main adsorption mechanisms for partial decomposed samples, while electrostatic attraction was the main mechanism for complete decomposed samples.
As widely used Al-based coagulants, poly aluminum chloride (PAC) and aluminum sulfate (AS) were adopted in a short term at the start-up stage (from 10th to 16th) to enhance the formation of aerobic granules, and their effects on aerobic granulation were elucidated. The results suggested that both PAC and AS facilitated the granulation by improving the physicochemical properties of sludge. The reactor performance in pollutant removal was also enhanced. Specifically, in terms of extracellular polymeric substances (EPS), PAC dosing mainly stimulated the production of loosely bound EPS (LB-EPS), whereas more tightly bound EPS (TB-EPS) were secreted with the presence of AS. Based on the elemental analysis, polymeric Al hydrolyzed from PAC mainly worked on the exterior of microbial aggregates, and thus the attached aluminum in granules was gradually eliminated by ion exchange and hydraulic shear force. In contrast, the aluminum species in AS hydrolyzed into monomeric and oligomeric Al, and thus could diffuse into the interior of microbial aggregates and eventually created an “Al-core” in the granules. Overall, the present study describes the AGS formation with Al-based coagulants and the mechanisms of PAC- and AS-enhanced aerobic granulation.
Heteroaggregation with clay mineral particles (CMPs) is significant to the environmental application and fate of increasingly produced nanoparticulate zero-valent iron (nZVI). Co-settling, kinetic aggregation, calculation of the classical Derjaguin-Landau-Verwey-Overbeek interaction energy, and electron microscopic observation were carried out to investigate the interaction between nZVIs (three naked nZVIs of different sizes and one carboxymethyl cellulose (CMC) coated nZVI) and CMPs (kaolinite and montmorillonite). Under pH 6.5 and 9.5 conditions, Lewis acid-base interaction contributed to the attachment between nZVIs and CMPs, while electrostatic attraction was involved in the nZVIs-CMPs attachment under pH 3.5. Compared with heteroaggregates formed by nZVIs attaching to CMPs edges and faces under pH 6.5 and 3.5 conditions, the heteroaggregates were smaller with nZVIs mainly connecting to CMPs edges under pH 9.5. Small nZVI homoaggregates were bound to CMP edges at low nZVI concentrations (nZVI/CMPs mass ratio at 0.015) with CMPs concentrations at 330 mg/L and large nZVIs-CMPs heteroaggregates formed by nZVI bridging with increasing nZVI concentrations. The smallest nZVI exhibited the strongest heteroaggregation with CMPs; CMC coating inhibited the interfacial interaction and heteroaggregation between nZVIs and CMPs; kaolinite had higher potential to interact with nZVIs at neutral condition. These findings are helpful for understanding the interaction between nZVIs and minerals and of significance to the environmental remediation using nZVIs.
Interest in thermoresponsive materials are generating significant interest on account of the sharp and tunable temperature de-swelling transition of the polymer chain. Such materials have shown promise in drug delivery devices, sensing systems and in self-assembly. Incorporation of nanoparticles (NPs), typically through covalent attachment of the polymer chains to the NP surface, can add additional functionality and tunability to such hybrid materials. The versatility of these thermoresponsive polymer/nanoparticle materials has been shown previously, however, significant and important differences exist in the published literature between virtually identical materials. Here we use PNIPAm-AuNPs as a model system to understand the aggregation behavior of thermoresponsive polymer-coated nanoparticles made by either grafting-to or grafting-from methods. We show that, contrary to popular belief, the aggregation of PNIPAm-coated AuNPs, and likely other such materials, relies on the size and concentration of unbound ‘free’ PNIPAm in solution. It is this unbound polymer that also leads to an increase in solution turbidity, a characteristic that is typically used to prove nanoparticle aggregation. The size of PNIPAm used to coat the AuNPs, as well as the concentration of the resultant polymer-AuNP composites, is shown to have little effect on aggregation. Without free PNIPAm, contraction of the polymer corona in response to increasing temperature is observed, instead of nanoparticle aggregation and is accompanied by no change in solution turbidity or color. We develop an alternative method for removing all traces of excess free polymer and develop a new approach for analysing the aggregation behavior of such materials, that truly allows for heat triggered aggregation to be studied.
The human lens is comprised largely of crystallin proteins assembled into a highly ordered, interactive macro-structure essential for lens transparency and refractive index. Any disruption of intra- or inter-protein interactions will alter this delicate structure, exposing hydrophobic surfaces, with consequent protein aggregation and cataract formation. Cataracts are the most common cause of blindness worldwide, affecting tens of millions of people, and currently the only treatment is surgical removal of cataractous lenses. The precise mechanisms by which lens proteins both prevent aggregation and maintain lens transparency are largely unknown. Lanosterol is an amphipathic molecule enriched in the lens. It is synthesized by lanosterol synthase (LSS) in a key cyclization reaction of a cholesterol synthesis pathway. Here we identify two distinct homozygous LSS missense mutations (W581R and G588S) in two families with extensive congenital cataracts. Both of these mutations affect highly conserved amino acid residues and impair key catalytic functions of LSS. Engineered expression of wild-type, but not mutant, LSS prevents intracellular protein aggregation of various cataract-causing mutant crystallins. Treatment by lanosterol, but not cholesterol, significantly decreased preformed protein aggregates both in vitro and in cell-transfection experiments. We further show that lanosterol treatment could reduce cataract severity and increase transparency in dissected rabbit cataractous lenses in vitro and cataract severity in vivo in dogs. Our study identifies lanosterol as a key molecule in the prevention of lens protein aggregation and points to a novel strategy for cataract prevention and treatment.
White light-emitting diodes (WLEDs) are candidates to revolutionize the lighting industry towards energy efficient and environmental friendly lighting and displays. The current challenges in WLEDs encompass high luminous efficiency, chromatic stability, high colour-rending index and price competitiveness. Recently, the development of efficient and low-cost downconverting photoluminescent phosphors for ultraviolet/blue to white light conversion was highly investigated. Here we report a simple route to design high-efficient WLEDs by combining a commercial ultraviolet LED chip (InGaAsN, 390 nm) and boehmite (γ-AlOOH) hybrid nanoplates. Unusually high quantum yields (ηyield=38-58%) result from a synergic energy transfer between the boehmite-related states and the triplet states of the benzoate ligands bound to the surface of the nanoplates. The nanoplates with ηyield=38% are able to emit white light with Commission International de l'Eclairage coordinates, colour-rendering index and correlated colour temperature values of (0.32, 0.33), 85.5 and 6,111 K, respectively; overwhelming state-of-the-art single-phase ultraviolet-pumped WLEDs phosphors.
Lewatit FO36 resin was covered with Fe (III) nano-particles, and it was used as a new way to eliminate phosphate. Column experiments were carried out in 11 stages in fixed bed columns with constant flow rate of 9 ml/min and the empty bed contact time (EBCT) of 2.1 min. The adsorption capacity was calculated for different concentration of phosphate solutions. After resin was regenerated by using NaOH and NaCl solutions, the adsorption capacity of resin was computed for 6mg/L of phosphate, typically. The adsorption capacity of resin was checked again a typical concentration of phosphate. The adsorption capacity measurements of regenerated resin show that the concentration of phosphate reached to 1.6mg/g after an 8.5% decrease when the initial concentration of phosphate is 6 mg/L. Competition of anions with phosphate was analyzed using chloride, sulfate, bicarbonate and a combination of these anions. Finally the effect of resin in phosphate removal was studied for a typical real sample, and the data was analyzed using statistical software (SPSS 13). The statistical results indicated that Cl−, SO
43−, HCO
3− and combined competing anions did not have a strong influence on the phosphate removal efficiency.
Several polyaluminum chloride (PACl) coagulants were prepared, with different OH/Al ratios (B values), and characterized by Ferron assay. These were used in studies of floc formation, breakage and re-growth with kaolin suspensions under controlled shear conditions, using a continuous optical monitoring method. Particular attention was paid to the effect of small additional coagulant dosages, added during the floc breakage period, on the re-growth of broken flocs. The results showed that the re-growth ability was greatly dependent on the nature of the PACl species added as second coagulant. The re-growth ability of broken flocs was greatest when the second coagulant was PACl(0) (i.e. AlCl(3), with B = 0) and least with PACl(25) (B = 2.5). In the latter case there was no effect on floc re-growth, irrespective of the initial coagulant used. PACls with intermediate B values gave some improvement in floc re-growth, but less than that with PACl(0). Additional dosage of PACl(0) gave re-grown flocs about the same size or even larger than those before breakage. The re-growth of broken flocs is significantly correlated with the species Al(a) (monomeric) and Al(b) (polymeric), as determined by Ferron assay. The amorphous hydroxide precipitate formed from PACl(0), (mainly Al(a)) can greatly improve the adhesion between broken flocs and give complete re-growth. However, for PACl(25), mostly composed of Al(b), the nature of the precipitate is different and there is no effect on floc re-growth.
The aim of this paper was to investigate the feasibility of the removal of phosphate from aqueous solution by electro-coagulation (EC). The current density (CD) between 2.5 and 10 mA cm(-2) and duration in the limits of 5-20 min were tried for different concentrations. In order to determine optimal operating conditions, the EC process used for the phosphate removal was examined in dependence with the CD, initial concentrations and time. The results of the experimental batch processing showed high effectiveness of the EC method in removing phosphate from aqueous solutions.
When aluminum salts are added to water at around neutral pH, a precipitate of Al hydroxide is formed very rapidly. Initially the precipitate is in the form of nano-scale primary particles, which then aggregate to form flocs. The nature of the flocs depends greatly on the solution composition, for instance on the presence of humic acid (HA), which not only increases the size of the primary nanoparticles, but also decreases the connection points between them. The nanoparticles become smaller with aging, both with and without HA, as a result of crystallization. The aggregated amorphous nanoparticles (settled flocs) undergo a room temperature structural modification best characterized as a disorder-to-order transition, following elimination of water. During this process, the apparent Al concentration in the supernatant of water increases with age. The "dissolved Al" concentration in the supernatant becomes higher with increasing pH and, to some extent, in the presence of HA. However, it can be shown that the "dissolved Al" in the supernatant exists in the form of crystalline nano-particles or larger clusters, which are detached from the settled flocs. TEM results confirmed that HA only adsorbed on the surface of nano-particles during the coagulation process, which shows precipitate nanoparticles formed firstly during sweep coagulation before the adsorption of HA or complexed Al3+-HA. However, the adsorbed outer layer of HA does not change the crystallization process for the inner part of nano-particles. This laboratory study may have implications for the release of Al from sediments into lake water, following addition of coagulants to lower phosphorus concentrations.
A migration electric-field assisted electrocoagulation (MEAEC) system was developed to increase phosphate removal from domestic wastewater, with reduced energy consumption, using a titanium charging (inert) electrode and a sacrificial iron anode. In the MEAEC, an electric field was applied between the inert electrode (titanium) and an air cathode to drive migration of phosphate anions towards the sacrificial anode. Current was then applied between the sacrificial anode (Fe or Al mesh) and the air cathode to drive electrocoagulation of phosphate. A MEAEC with the Fe electrode using primary clarifier effluent achieved 98% phosphate removal, producing water with a total phosphorus of 0.3 mg/L with <6 min total treatment time (five cycles; each 10 s inert electrode charging, and 1 min electrocoagulation), at a constant current density of 1 mA/cm2. In the absence of the 10 s charging time, electrocoagulation required 15 min for the same removal. With an aluminum anode and the same phosphorus removal, the MEAEC required 7 cycles (7 min total treatment, 1 min 10 s total charging), while conventional electrocoagulation required 20 min. The energy demand of Fe-MEAEC was only 0.039 kWh/m3for 98% phosphate removal, which was 35% less than with the Al-MEAEC of 0.06 kWh/m3, and 28% less than that previously obtained using an inert graphite electrode. Analysis of the precipitate showed that a less porous precipitate was obtained with the Al anode than with the Fe anode. The phosphorus in precipitate of Fe-MEAEC was identified as PO43-and HPO42-, while the Fe was present as both Fe2+and Fe3+. Only HPO42-and Al3+were identified in the precipitate of the Al-MEAEC. These results indicated that the MEAEC with a titanium inert charging electrode and iron anode could achieve the most efficient phosphate removal with very low energy demands, compared to previous electrochemical approaches.
Citrate (Cit) and polyethylenimine (BPEI)-coated silver nanoparticles (AgNPs) were used to understand how the type of capping agents and surface charge affect their colloidal stability, dissolution, and ecotoxicity in the absence/presence of Pony Lake Fulvic Acid (PLFA). In the presence of PLFA, Cit-AgNPs were stabilized, while BPEI-AgNPs were aggregated. The aggregation of BPEI-AgNPs decreased with the time and their stabilizing effect increased at high PLFA concentration. The dissolution also differed between both AgNPs and was influenced by the PLFA concentration. Generally, BPEI-AgNPs showed a lower amount of dissolved Ag than Cit-AgNPs. The dissolved Ag concentration decreased for both AgNPs at low PLFA concentration (5 mg/L). In contrast, the extent of nanoparticle dissolution increased at high PLFA concentration (30 mg/L) but only for BPEI-AgNPs. In the absence of PLFA, the ecotoxicity of Cit-AgNPs to Daphnia magna was higher than that of BPEI-AgNPs. However, the ecotoxicity of AgNPs in the presence of PLFA was up to 70% lower than in their absence. We demonstrated that the differences in colloidal stability, dissolution, and ecotoxicity may be attributed to the different capping agents, surface charge and concentration of natural organic matter (NOM) as well as to the formation of dissolved Ag complexes with NOM.
Lanthanum-based materials are effective for sequestering phosphate in water, however their removal mechanisms remain unclear, and the effects of environmentally relevant factors have not yet been studied. Hereby, this study explored the mechanisms of phosphate removal using La(OH)3 by employing extended X-ray absorption spectroscopy (EXAFS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), density functional theory (DFT) and chemical equilibrium modeling. The results showed that surface complexation was the primary mechanism for phosphate removal and in binary phosphate configurations, namely diprotonated bidentate mononuclear (BM-H2) and bidentate binuclear (BB-H2), coexisting on La(OH)3 in acidic conditions. By increasing the pH to 7, BM-H1 and BB-H2 were the two major configurations governing phosphate adsorption on La(OH)3, while BB-H1 was the dominant configuration of phosphate adsorption at pH 9. With increasing phosphate loading, the phosphate configuration of on La(OH)3 transforms from binary BM-H1 and BB-H2 to BB-H1. Amorphous Ca3(PO4)2 forms in the presence of Ca, leading to enhanced phosphate removal at alkaline conditions. The contributions of different mechanisms to the overall phosphate removal were successfully simulated by a chemical equilibrium model that was consistent with the spectroscopic results. This study provides new insights into the molecular-level mechanism of phosphate removal by La(OH)3.
The use of lanthanum (La)-based materials for phosphate removal from water and wastewater has received increasing attention. However, challenges remain to enhance phosphate sorption capacities and recover La-based sorbents. In this study, magnetic La(OH) 3 /Fe 3 O 4 nanocomposites with varied La-to-Fe mass ratios were synthesized through a precipitation and hydrothermal method. Based upon preliminary screening of synthesized La(OH) 3 /Fe 3 O 4 nanocomposites in terms of phosphate sorption capacity and La content, La(OH) 3 /Fe 3 O 4 nanocomposite with a La-to-Fe mass ratio of 4:1 was chosen for further characterization and evaluation. Specifically, for these materials, magnetic separation efficiency, phosphate sorption kinetics and isotherm behavior, and solution matrix effects (e.g., coexisting ions, solution pH, and ionic strength) are reported. The developed La(OH) 3 /Fe 3 O 4 (4:1) nanocomposite has an excellent magnetic separation efficiency of >98%, fast sorption kinetics of 30 min, high sorption capacity of 83.5 mg P/g, and strong selectivity for phosphate in presence of competing ions. Phosphate uptake by La(OH) 3 /Fe 3 O 4 (4:1) was pH-dependent with the highest sorption capacities observed over a pH range of 4–6. The ionic strength of the solution had little interference with phosphate sorption. Sorption-desorption cyclic experiments demonstrated the good reusability of the La(OH) 3 /Fe 3 O 4 (4:1) nanocomposite. In a real treated wastewater effluent with phosphate concentration of 1.1 mg P/L, 0.1 g/L of La(OH) 3 /Fe 3 O 4 (4:1) efficiently reduced the phosphate concentration to below 0.05 mg P/L. Electrostatic attraction and inner-sphere complexation between La(OH) 3 and P via ligand exchange were identified as the sorption mechanisms of phosphate by La(OH) 3 /Fe 3 O 4 (4:1).
A major cause of ultrafiltration (UF) membrane fouling in surface water treatment is natural organic matter (NOM). Some studies have reported that heated aluminum oxide particles (HAOPs), prepared by boiling a suspension containing precipitates of the common coagulant alum, can remove substantial amounts of NOM and reduce fouling when they were pre-deposited on UF membranes. However, the influence of the size and structure of the HAOPs in mitigating NOM membrane fouling has not been fully explored so far. This work has investigated the change in microstructure of the HAOPs during the heating process and the subsequent effect on the performance of the membrane process, and especially on the mitigation of fouling. As the heating time increased, the structure of the HAOPs transformed gradually from an amorphous nature to a semi-crystal, and then to a microcrystalline phase. It was found that this micro-crystallization process played a key role in affecting the structural properties of the nano-scale particles and the membrane filtration performance. During the crystalline transition, a change of particle size distribution occurred and the average particle size was found to decrease gradually owing to a dehydration reaction. The smaller particle size of the HAOPs provides a denser pre-filtration layer for NOM separation, and their more rigid structure reduces layer compression and hydraulic resistance during operation. Optimization of the pre-heating condition and surface loading can effectively enhance the performance of the HAOPs layer in reducing NOM fouling in the UF membrane system.
The application of ozone pre-treatment for ultrafiltration (UF) in drinking water treatment has been studied for more than 10 years, but its performance in mitigating or exacerbating membrane fouling has been inconclusive, and sometimes contradictory. To help explain this, our study considers the significance of the influent organic matter and its interaction with ozone on membrane fouling, using solutions of two representative types of extracellular polymeric substances (EPS), alginate and bovine serum albumin (BSA), and samples of surface water. The results show that at typical ozone doses there is no measurable mineralization of alginate and BSA, but substantial changes in their structure and an increase in the size of nano-particle aggregates (micro-flocculation). The impact of ozonation on membrane fouling, as indicated by the membrane flux, was markedly different for the two types of EPS and found to be related to the size of the nano-particle aggregates formed in comparison with the UF pore size. Thus, for BSA, ozonation created aggregate sizes similar to the UF pore size (100 k Dalton) which led to an increase in fouling. In contrast, ozonation of alginate created the nano-particle aggregates greater than the UF pore size, giving reduced membrane fouling/greater flux. For solutions containing a mixture of the two species of EPS the overall impact of ozonation on UF performance depends on the relative proportion of each, and the ozone dose, and the variable behaviour has been demonstrated by the surface water. These results provide new information about the role of nano-particle aggregate size in explaining the reported ambiguity over the benefits of applying ozone as pre-treatment for ultrafiltration.
The operational significance of energy-intensive rapid mixing processes remains unaddressed in coagulation and flocculation of insoluble precipitates (flocs), which play an important role in the removal of impurities from drinking water supplies. In this study, the influence of rapid mixing and associated mixing energy on floc aggregation was examined for a surface water source characterized by a high fraction of aquatic humic matter. Infrared spectral analyses showed that the colloidal complexes resulting from ligand exchange between iron and dissolved natural organic matter (DOM) were not substantially influenced by the mixing energy input. This signified that DOM removal by coagulation can be achieved at lower mixing intensity, thereby reducing energy consumption. In contrast, macroscopic investigations showed the coagulation mixing energy affected floc size distributions during the slow mixing stage in flocculation and, to some extent, their settling characteristics. The results from analysis of floc properties clearly showed that more mixing energy was expended than necessary in coagulation, which is typically designed at a high mixing intensity range of 600-1000 s-1 in treatment plants. The key findings from this study have practical implications to water utilities to strategically meet water quality goals while reducing energy demands.
For successful aqueous-based applications, it is necessary to fundamentally understand and control nanoparticle dispersivity and stability over a range of dynamic conditions, including variable ionic strengths/types, redox chemistries, and surface ligand reactivity/degradation states (i.e. surface aging). Here, we quantitatively describe the behavior of artificially aged, oleic acid (OA) bilayer coated iron oxide nanoparticles (IONPs) under different scenarios. Hydrogen peroxide (H2O2), used here as a model oxidant under both dark and light ultra-violet (UVA) conditions, was employed to ‘age’ materials, to varying degrees, without increasing ionic strength. Short-term stability experiments indicate that OA-IONPs, while stable in the dark, are effectively de-stabilized when exposed to UVA/H2O2/OH based oxidation processes. Compared to bicarbonate, phosphate (1.0 mM) has a net stabilizing effect on OA-IONPs under oxidative conditions, which can be attributed to (surface-based) functional adsorption. Corresponding aggregation kinetics in the presence of monovalent (Na+) and divalent cations (Ca2+) show that attachment efficiencies (α) are strongly dependent on the cation concentrations/types and degree of surface aging. Taken together, our findings directly highlight the need to understand the critical role of particle surface transformation(s), via oxidative aging, among other routes, with regard to the ultimate stability and environmental fate of surface functionalized engineered nanoparticles.
Despite the importance of phosphorus as a nutrient for humans and its role in ecological sustainability, its high abundance, resulting in large part from human activities, causes eutrophication that negatively affects the environment and public health. Here, we present the use of ferrate(VI) as an alternative agent for removing phosphorus from aqueous media. We address the mechanism of phosphate removal as a function of the Fe/P mass ratio and the pH value of the solution. The isoelectric point of γ-Fe2O3 nanoparticles, formed as dominant Fe(VI) decomposition products, was identified to play a crucial role in predicting their efficiency in removing of phosphates. Importantly, it was found that the removal efficiency dramatically changes if Fe(VI) is added before (ex-situ conditions) or after (in-situ conditions) the introduction of phosphates into water. Removal under in-situ conditions showed remarkable sorption capacity of 143.4 mg P per gram of ferric precipitates due to better accessibility of active surface sites on in-situ formed ferric oxides/oxyhydroxides. At pH = 6.0-7.0, complete removal of phosphates was observed at a relatively low Fe/P mass ratio (5:1). The results show that phosphates are removed from water solely by sorption on the surface of γ-Fe2O3/γ-FeOOH core/shell nanoparticles. The advantages of Fe(VI) utilization include its environmentally friendly nature, the possibility of easy separation of the final product from water by a magnetic field or by natural settling, and the capacity for successful phosphate elimination at pH values near the neutral range and at low Fe/P mass ratios.
Time-dependent aggregation, sedimentation, dissolution and transformation of three copper-based engineered nanomaterials (ENMs) of varied properties were measured in eight natural and artificial waters. Nano-Cu and Cu(OH)2 aggregated rapidly to >103 nm while the aggregate size of nano-CuO averaged between 250 to 400 nm. Aggregate size for both nano-Cu and nano-CuO showed a positive correlation with ionic strength with a few exceptions. Aggregate size did not correlate well with sedimentation rate, suggesting sedimentation was influenced by other factors. Controlling factors in sedimentation rates varied by particle: Cu(OH)2 particles remained stable in all waters but groundwater, nano-Cu was generally unstable except in waters with high organic content, and nano-CuO was stabilized by the presence of phosphate, which reversed surface charge polarity at concentrations as low as 0.1 mg PO4(3-) L(-1). Dissolution generally correlated with pH, although in saline waters dissolved copper formed insoluble complexes. Nano-Cu was rapidly oxidized, resulting in dissolution immediately followed by the formation of precipitates. These results suggest factors including phosphate, carbonate, and ENM oxidation state may be key in determining Cu ENM behavior in natural waters.
Kaolin suspensions were coagulated with AlCl3 and a high-basicity PACl at pH 7, at dosages that gave zeta potentials close to zero. The actions of the two coagulants were completely different. With AlCl3, the formation of an amorphous hydroxide precipitate played a dominant role. When the coagulant was added to the suspension, flocs grew rapidly and incorporated most of the kaolin particles within the hydroxide precipitate. When the suspension was added some time after the coagulant, the clay particles were found to be mainly on the outer floc surfaces, although the floc size was about the same. The light scattering properties of the flocs were very dependent on the number and location of particles in the precipitate. With PACl, delaying the addition of kaolin had no influence on the final floc properties.
In further tests, different suspensions over a range of concentrations were coagulated with alum at pH 7. Monitoring by a ‘turbidity fluctuation’ technique showed an apparent increase in floc size with increasing particle concentration. However, floc sizes determined from microscope images were very nearly constant, independent of particle nature and concentration. With different particle types, the monitoring results were greatly dependent on the light scattering properties of the particles.
Particles incorporated within hydroxide flocs appeared to have no influence on floc properties, such as size and strength.
The ability of flocs growth has a significant effect on the efficiency of solid/liquid separation during coagulation process. The growth and re-growth of flocs by one time or continuous dosage were explored at neutral pH, including one time and continuous additional dosage strategy after flocs breakage. The size of flocs formed by continuous dosage before breakage was much larger than the one formed by one time dosage. There was significant irreversibility of floc breakage when no additional coagulant was added. For the one with one time initial dosage strategy, the size of re-grown flocs by one time additional dosage was nearly the same as that before breakage. The re-grown flocs formed by continuous additional dosage were a little smaller than that formed by one time additional dosage. It definitely showed that the growth or connection of flocs was probably not determined by the flocs size distribution and fractal dimension, but by the surface activity of flocs. Additional dosage could improve flocs re-grown ability because the surface activity of broken flocs was repaired by adding new alum.
Mass spectra obtained using fast atom bombardment provided qualitative information about coagulation's effectiveness under various treatment conditions.
Removal of natural aquatic dissolved organic matter (DOM) by conventional coagulation using ferric chloride was investigated. Reverse osmosis was used to isolate DOM from the Suwannee River in southern Georgia and from Lake Allatoona in northwestern Georgia. The two most significant differences between the source waters are pH and organic carbon concentration. Extensive jar‐testing identified regions of removal based on initial concentration of DOM, coagulant dosage, and pH conditions. Fast atom bombardment mass spectrometry was used to characterize the molecular‐weight distributions of DOM before and after coagulation. Trends in the shape of the mass spectra correlated well with data for DOM removal and suggested that the mechanism for DOM removal varies with the pH and coagulant dosage. At higher pH conditions and lower coagulant dosages, masses up to 1,000 daltons (D) were detected in the mass spectra after coagulation. At lower pH conditions and higher coagulant dosages, no masses above 750 D appeared in the mass spectra.
The investigation of the flocculation of humic and fulvic acids has shown that the sodium salts of these acids, which are anionic polyelectrolytes, react chemically with the flocculants, such as cationic polyelectrolytes, and aluminum sulfate through carboxylate and phenolate groups. A stoichiometric relationship has been found between humic and fulvic acids content in solution and the cationic polyelectrolyte or alum dose required for effective flocculation of the humic substances. As the pH value becomes lower, flocculation is better; the degree of dissociation of the organic molecule is lower, and the flocculant requirement is lower.
Although Al13 polymer (AlO4Al12(OH)24(H2O)127+) has been identified as an effective aluminum polycation in coagulation of natural organic matter (NOM), the coagulation behavior of Al13 polymer largely depends on its preparation conditions and characteristics. The performances of pre-hydrolyzed Al13 and in situ formed Al13 in humic acid (HA) coagulation were comparatively investigated in this study. Floc properties, which were evaluated in terms of floc size, strength and compaction degree, were measured using a laser diffraction particle sizing device. Additionally, variations of these floc properties with coagulation time were also studied. The results showed that, AlCl3 showed the best HA removal efficiency of above 92% at pH 5.5–6.0, where in situ formed Al13 was present. The preformed Al13 species gave rise to better removal efficiency than AlCl3 across the pH range with the exception of pH 5.5–6.0. This indicated that both preformed and in situ formed Al13 polymer could enhance the HA removal efficiency. Flocs formed by the preformed Al13 were stronger, as reflected by the lower γ′ value of 0.53; while the γ′ value for the non-preformed Al13 was 0.72. Additionally, the pre-hydrolyzed Al13 species tended to produce more compact flocs with larger Df values than the in situ formed.
The thermal transformation sequences of boehmite (γ-AlOOH) and two grades of gibbsite [γ-Al(OH)3] upon soak and flash calcination are reported. The techniques used were X-ray diffraction (XRD), differential thermal analysis (DTA), Fourier-transform IR (FTIR) and 27Al magic-angle spinning (MAS) NMR spectroscopies. Boehmite undergoes the dehydroxylation sequence boehmite, γ, δ, θ, α-Al2O3 under both soak and flash calcination. The dehydroxylation sequence of gibbsite, however, depends on the calcination method and the particle size of the feed material. Soak calcination of a fine gibbsite (ca. 0.5 µm) gave the dehydroxylation sequence gibbsite, χ, κ, α-Al2O3; with flash calcination the sequence gibbsite, χ, γ, δ, θ, α-Al2O3 was observed. Soak calcination of coarse gibbsite (ca. 14 µm) gave both the dehydroxylation pathways (a): gibbsite, boehmite, γ, δ, θ, α-Al2O3 and (b): gibbsite, χ, κ, α-Al2O3, and pathway (a) was predominant. Flash-calcined coarse gibbsite experiences a crossover between these routes (χ–γ) without formation of κ-Al2O3. Flash calcines of gibbsite undergo this χ–γ phase change at ca. 800°C.
The acid-base properties of the polynuclear Al13 complex have been studied by alkalimetric titration using three different experimental methods. While the positive charge of 7+ of the Al13 polymer is preserved at pH values below 6, it is lost almost completely between pH 6 and 7, i.e., within 1 pH unit. During the titrations, the Al13 complex tends to aggregate, especially at high concentrations and after long experimental duration. It was found that the titration curve of the aggregated complex is significantly less steep than that of the dissolved form. The acid-base properties indicate that the aggregated Al13 represents an intermediate state between the dissolved polymer and the solid aluminum hydroxides.
The proposed Disinfectants/Disinfection By-Products (D/DBP) Rule includes a requirement that surface water systems using conventional treatment must remove DBP precursors—as measured by total organic carbon (TOC)—in addition to meeting standards for the D/DBPs themselves. TOC removed criteria are based on influent TOC and alkalinity (referred to as step 1); however, systems can demonstrate the need for alternative TOC removal performance criteria based on jar-testing (referred to as step 2). Tests based on various source waters throughout the United States show that some waters will be able to meet step 1 TOC removal criteria and that although step 2 analysis is not required, jar testing can be used to determine means of complying with the DBP precursor removal criteria. In other waters, however, jar tests will be needed to determine a point of diminishing returns (i.e.,
An analysis of the type of rapid mixing necessary in specific regions of the stability diagram suggests that it is possible to tailor the design of the rapid-mix operation to the predominant mode of alum coagulation.
The role of aluminum speciation in coagulation–flocculation of humic acid (HA) by polyaluminum chloride (PACl) with high content of Al13 was investigated using 27Al NMR spectroscopy, solid-state 27Al NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS) and Al-Ferron method. Experimental results show that, in the coagulation process, the Al13 polymers are the dominant species in acidic pH and some of them change to Al precipitates at higher pH. The mechanism of HA removal is directly related to the form of aluminum in solution. The maximum removal of the HA is found to occur in the pH range from 5.0 to 6.0, which means that charge neutralization by Al13 polymer is the main mechanism of coagulation at slightly acidic pH. The results of zeta potential and solid-state 27Al NMR spectroscopy of the flocs also confirm that positive polymers are very effective in charge neutralizing with HA molecules. Adsorption and sweep coagulation of HA by amorphous Al(OH)3(s) play an important role at pH>6.0 or over dosing.
Hydroxyaluminosilicates (HASA and HASB) are important secondary mineral phases in the biogeochemical cycle of aluminium. HASA is formed by the reaction of silicic acid (Si(OH)4) with an aluminium hydroxide (Al(OH)3(s)) template with further substitution of Si(OH)4 into HASA resulting in HASB. Recently, fluoride and phosphate-substituted forms of HAS have been synthesised and characterised. Thermogravimetric analysis incorporating differential scanning calorimetry (TGA–DSC) is an effective method for studying the structure of mineral phases and was used herein as a possible tool to discriminate between different forms of HAS as well as to elucidate further upon their mechanism of formation. All of the HAS studied exhibited distinctive thermal behaviour dependent upon their Si:Al ratio and the inclusion of fluoride or phosphate in their structure. The observed thermal characteristics were sufficient to allow different HAS to be identified though they did not offer any structural information in addition to that which was obtained previously by solid-state NMR and microprobe analysis. We have identified an hitherto unrecognised form of a purely amorphous Al(OH)3(s) the thermal signature of which was a sharp exothermic peak (ca. 282°C) which though present in HASA-like structures (260–275°C) was absent from HASB-like structures. Solid-state NMR of heated HAS and Al(OH)3(s) could not identify any significant changes in the coordination of aluminium which might be associated with this characteristic exotherm. However, NMR was successful in identifying Al(V) as a potential intermediate in the transformation of HASA to HASB.The identification of a novel form of purely amorphous Al(OH)3(s) may be of interest to chemists and materials scientists alike.
The present study provides an electrocoagulation process for the removal of phosphate from drinking water using mild steel as the anode and stainless steel as the cathode. The studies were carried out as a function of pH, temperature, current density, and so forth, and the adsorption capacity was evaluated using both Langmuir and Freundlich isotherm models. The results showed that the maximum removal efficiency of 98% was achieved at a current density of 0.05 A·dm-2 at a pH of 6.5. The adsorption of phosphate preferably fitting the Langmuir adsorption isotherm suggests monolayer coverage of adsorbed molecules. The adsorption process follows second-order kinetics. Temperature studies showed that adsorption was endothermic and spontaneous in nature.
This work described the preparation, characterization, and electrochemical behavior toward heavy metal ions of the AlOOH-reduced graphene oxide nanocomposites. This new material was synthesized through a green one-pot hydrothermal method. The morphologic and structure of the nanocomposites were characterized using atomic force microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoemission spectroscopy, Fourier transform-infrared spectroscopy, and transmission electron microscopy. Electrochemical properties were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The chemical and electrochemical parameters that have influence on deposition and stripping of metal ions, such as pH value, deposition potential, and deposition time, were also studied. Due to the strong affinity of AlOOH to heavy metal ions and the fast electron-transfer kinetics of graphene, the combination of solid-phase extraction and stripping voltammetric analysis allowed fast and sensitive determination of Cd(II) and Pb(II) in drinking water, making these new nanocomposites promising candidates for practical applications in the fields of detecting heavy metal ions. Most importantly, these new nanocomposites may possess many unknown properties waiting to be explored.
Aluminium and iron salts are widely used as coagulants in water and wastewater treatment and in some other applications. They are effective in removing a broad range of impurities from water, including colloidal particles and dissolved organic substances. Their mode of action is generally explained in terms of two distinct mechanisms: charge neutralisation of negatively charged colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate (‘sweep flocculation’). The relative importance of these mechanisms depends on factors such as pH and coagulant dosage. Alternative coagulants, based on prehydrolysed forms of aluminium and iron, are more effective than the traditional additives in many cases, but their mode of action is not completely understood, especially with regard to the role of charge neutralisation and hydroxide precipitation. Some basic features of metal hydrolysis and precipitate formation are briefly reviewed and the action of hydrolysing coagulants is then discussed, with examples from the older literature and from some recent studies on model systems. Dynamic monitoring of floc formation and breakage can give useful insights into the underlying mechanisms. Although the results can be reasonably well explained in terms of established ideas, a detailed understanding of the ‘sweep flocculation’ mechanism is not yet available. There are also still some uncertainties regarding the action of pre-hydrolysed coagulants.
Dissolved organic compounds in a Swiss lake were fractionated into three molecular size classes by gel exclusion chromatography, and adsorption of each fraction on colloidal alumina was studied as a function of pH. Organic compounds with molecular weight (mr) greater than 1000 formed strong complexes with the alumina surface, but low molecular weight compounds were weakly adsorbed. Electrophoretic mobility measurements indicated that alumina particles suspended in the original lake water were highly negatively charged because of adsorbed organic matter. Most of the adsorbed organic compounds were in the mr range 1000 < mr < 3000. Adsorption of these compounds during the treatment of drinking water by alum coagulation may be responsible for the preferential removal of trihalomethane precursors. Adsorption may also influence the molecular-weight distribution of dissolved organic material in lakes.