Fe Flocs Research Articles

Exploring the influence of aquatic phosphate on Fe floc dynamics in water treatment

The formation of flocs is crucial in the coagulation process of water treatment. However, the nature of ligand exchange on the surface of primary nanoparticles (PNPs) during floc formation requires further investigation to enhance our understanding of the coagulation mechanism. Phosphate (P) is a ubiquitous nutrient ion in aquatic surface water, in this study, the impact of P on floc growth under different pH conditions were investigated. The results revealed that floc growth patterns depended on both P dosage and pH. The mode of ligand exchange between P and in-situ formed ferric hydroxide within a pH range of 5 to 10 was further explored, and remarkable disparities in pH changes induced by P addition were observed. At lower pH levels, OH− release occurred relatively slowly, stabilizing with continued P addition. At neutral pH, OH− release was comparatively higher with P addition, while under alkaline conditions, both the quantity of OH− and its release rate decreased. It was deduced that Fe–OH21/2+ sites function as "active sites," while Fe–OH1/2− sites act as "inert sites" on the surface of PNPs formed during flocculation. These sites are crucial in the interconnections between flocs formed during coagulation and in floc growth. Analyses of Fe PNPs by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), with and without P addition, revealed that the introduction of P inhibits or interferes with the self-crystallization of Fe PNPs through chemical coordination reactions. The results offer deeper insights into the coagulation mechanism and the transformation of Fe flocs in raw waters containing P during water treatment practices.

Ferrate(VI) assists in reducing cytotoxicity and genotoxicity to mammalian cells and organic bromine formation in ozonated wastewater

Ozonation of wastewater containing bromide (Br−) forms highly toxic organic bromine. The effectiveness of ozonation in mitigating wastewater toxicity is minimal. Simultaneous application of ozone (O3) (5 mg/L) and ferrate(VI) (Fe(VI)) (10 mg-Fe/L) reduced cytotoxicity and genotoxicity towards mammalian cells by 39.8% and 71.1% (pH 7.0), respectively, when the wastewater has low levels of Br−. This enhanced reduction in toxicity can be attributed to increased production of reactive iron species Fe(IV)/Fe(V) and reactive oxygen species (•OH) that possess higher oxidizing ability. When wastewater contains 2 mg/L Br−, ozonation increased cytotoxicity and genotoxicity by 168%-180% and 150%-155%, respectively, primarily due to the formation of organic bromine. However, O3/Fe(VI) significantly (p < 0.05) suppressed both total organic bromine (TOBr), BrO3−, as well as their associated toxicity. Electron donating capacity (EDC) measurement and precursor inference using Orbitrap ultra-high resolution mass spectrometry found that Fe(IV)/Fe(V) and •OH enhanced EDC removal from precursors present in wastewater, inhibiting electrophilic substitution and electrophilic addition reactions that lead to organic bromine formation. Additionally, HOBr quenched by self-decomposition-produced H2O2 from Fe(VI) also inhibits TOBr formation along with its associated toxicity. The adsorption of Fe(III) flocs resulting from Fe(VI) decomposition contributes only minimally to reducing toxicity. Compared to ozonation alone, integration of Fe(VI) with O3 offers improved safety for treating wastewater with varying concentrations of Br−.

Enhanced purification of hospital wastewater by a novel ferrate(VI) flocs recycling process

Traditional Fe(VI) treatment process does not fully utilize the fresh iron flocs. This study proposed a novel Fe(VI) flocs recycling process. The flocs were recycled into the front-end reactor and the flocculation effluent further reacted with Fe(VI). Compared with Fe(VI) alone, the CODCr of the secondary effluent of hospital wastewater decreased from 74.9 to 59.4 mg/L and TP decreased from 0.36 to 0.10 mg/L at the dosage of 20 mg/L Fe(VI). However, the enhanced removal of NH3-N and TOC by the flocs recycling process is limited. The Fe(VI) flocs recycling process also strengthened the removal of luminescent bacterial acute toxicity, from 0.52 ± 0.01 in secondary effluent to 0.17 ± 0.01 mg-ZnSO4/L. Compared with polymeric ferric chloride and ferric chloride (FeCl3), Fe(VI) showed the best performance on purifying wastewater in the flocs recycling process. Compared with FeCl3, the CO bond contents in flocs from Fe(VI) changed more obviously. Flocs from Fe(VI) have a smaller average particle size than those from FeCl3, which may result in better adsorption capacity of Fe(VI) flocs. In conclusion, the flocs recycling process not only taken full use of the Fe(III) flocs and reduced the iron sludge, but also strengthened the purification of the wastewater without additional Fe(VI) dosing.

Destabilization of polystyrene nanoplastics with different surface charge and particle size by Fe electrocoagulation

Nanoplastics (NPs) discharged from wastewater could pose a major threat to organisms in aquatic environments. Effective removal of NPs by the current conventional coagulation-sedimentation process is not yet satisfactory. This study aimed to investigate the destabilization mechanism of polystyrene NPs (PS-NPs) with different surface properties and sizes (i.e., 90 nm, 200 nm, and 500 nm) by Fe electrocoagulation (EC). Two types of PS-NPs were prepared by a nanoprecipitation method using sodium dodecyl sulfate and cetrimonium bromide solutions to produce negatively-charged SDS-NPs and positively-charged CTAB-NPs. For both NPs, obvious floc aggregation from 7 μm to 14 μm was observed only at pH 7 with particulate Fe accounted for >90 %. At pH 7, Fe EC removed 85.3 %, 82.8 %, and 74.7 % of the negatively-charged SDS-NPs at small-, mid-, and large-sizes from 90 nm, 200 nm, to 500 nm, respectively. Small-size SDS-NPs(90 nm) were destabilized through physical adsorption on the surface of Fe flocs, while the main removal mechanism of mid- and large-SDS-NPs(200 nm and 500 nm) involved the enmeshment of large Fe flocs. Compared to SDS-NPs(200 nm and 500 nm), Fe EC performed similar destabilization behavior to two CTAB-NPs(200 nm and 500 nm), but it resulted in much lower removal rates of 54.8 % - 77.9 %. The Fe EC also exhibited no removal (<1 %) ability toward the small-size and positively-charged CTAB-NPs(90 nm) due to insufficient formation of effective Fe flocs. Our results provide insight into the destabilization of PS in nano-scale with different sizes and surface properties, which clarifies the behavior of complex NPs in a Fe EC-system.

Coagulation characteristic and mechanism of Fe(III) salts toward typical Cr(III) complexes in wastewater treatment.

Cr(III) complexes are typical pollutants in various industrial wastewater and pose a serious threat to the ecosystem and humans. The coagulation process is commonly used in water treatment plants, yet its removal characteristic and mechanism toward Cr(III) complexes have been rarely reported. In this study, the Fe(III) coagulation process was adopted for the evaluation of Cr(III) complex removal in terms of Cr residual concentration as well as floc size. The results showed that Fe(III) with a dose of 0.8mM removed more than 80% of total Cr for Cr3+ and Cr(III)-acetate, whereas poor removal rate (~ 50%) was obtained for Cr(III)-citrate under the same conditions. Neutral and alkaline conditions facilitated Cr(III)-acetate removal by Fe(III) coagulation, while limited influence was observed for Cr(III)-citrate with various pH. The main removal mechanism of Cr(III)-acetate was precipitation. Cr(III)-citrate elimination largely relied on the adsorption property and sweeping effect of Fe floc. Moreover, Cr(III)-acetate was easier to be separated from a solution since the generated floc sizes were 270μm. Flocs that formed in the Cr(III)-citrate treatment were only 0.3μm, resulting in separation difficulties during the coagulation process. The presence of Cr(III)-acetate and Cr(III)-citrate caused a significant decline in membrane flux. This study provided fundamental knowledge of Fe coagulation treatment in Cr(III) complex-containing wastewater.

Effective removal of Sb(V) from aqueous solutions by electrocoagulation with composite scrap iron-manganese as an anode

Improving the removal rate of pentavalent antimony (Sb(V)) by electrocoagulation (EC) is of great significance to the environment. In this paper, the EC with composite scrap iron and manganese filings as an anode (Fe-Mn EC) was investigated for the high-efficiency elimination of Sb(V). The results showed that Fe-Mn EC can enhance the removal of Sb(V) by 11.18-17.36% compared with the traditional iron electrocoagulation (Fe EC). Meanwhile, Sb(V) removal increased with the growth of current concentration as well as Mn content in the anode. However, the Sb(V) removal rate was inhibited when Mn content exceeded 20%. Moreover, the flocs generated during the Fe and Fe-Mn EC (Fe flocs and Fe-Mn flocs) were analyzed both structurally and theoretically using XRD, SEM, BET, and adsorption experiment. The results indicated that the components of Fe-Mn flocs were mostly Mn-substituted FeOOH, which appeared as the structure of nanometer flakes and large internal surface areas. Meanwhile, the Fe-Mn flocs had the ability of much faster Sb(V) adsorption rate; its Sb(V) adsorption capacity was 2.5 times more than that of the Fe flocs. The thermodynamics constants of both Fe and Fe-Mn flocs proved that adsorption was associated with monolayer physical adsorption. To the best of our knowledge, this is the first report of the electrocoagulation with composite scrap iron-manganese as an anode to remove Sb, which provide a new idea and potential technical support for the removal of Sb(V).

Sequential Fe2+ oxidation to mitigate the inhibiting effect of phosphate and silicate on arsenic removal

Sequential iron (as Fe2+) oxidation has been found to yield improved arsenic (as As(III)) uptake than the single-step oxidation. The objective of this study was to gain a better understanding of interactions with phosphate (PO43−) and silicate (SiO42−) during sequential Fe2+ and As(III) oxidation and removal, as these are typically found in groundwater and known to interfere with As removal. The laboratory experiments were performed using single and multi-step jar tests with an initial As(III/V), Fe2+, PO43−, SiO42− concentrations, and pH of 200 μg/L, 2.5 mg/L, 2 mg/L, 16 mg/L and 7.0, respectively representing the targeted natural groundwater in Rajshahi district, Bangladesh. The sequential Fe2+ and As(III) oxidation in the multi-step jar tests indicated that the PO43− hindrance on As removal in the first Fe2+ oxidation step was compensated for in the second. Moreover, smaller Fe flocs (<0.45 μm) were observed in the presence of SiO42−, potentially providing more surface area during the second Fe2+ oxidation step leading to better overall As removal. Altogether it may be concluded that controlling the As(III) and Fe2+ oxidation sequence is beneficial for As removal compared to single-step Fe2+ oxidation, both in the presence and absence of PO43− and/or SiO42−.

Coagulation Behavior of Antimony Oxyanions in Water: Influence of pH, Inorganic and Organic Matter on the Physicochemical Characteristics of Iron Precipitates.

The presence of inorganic and organic substances may alter the physicochemical properties of iron (Fe) salt precipitates, thereby stabilizing the antimony (Sb) oxyanions in potable water during the chemical treatment process. Therefore, the present study aimed to examine the surface characteristics, size of Fe flocs and coagulation performance of Sb oxyanions under different aqueous matrices. The results showed that surface properties of Fe flocs significantly varies with pH in both Sb(III, V) suspensions, thereby increasing the mobility of Sb(V) ions in alkaline conditions. The negligible change in surface characteristics of Fe flocs was observed in pure water and Sb(III, V) suspension at pH 7. The key role of Van der Waals forces of attraction as well as hydration force in the aggregation of early formed flocs were found, with greater agglomeration capability at higher more ferric chloride dosage. The higher Sb(V) loading decreased the size of Fe flocs and reversed the surface charge of precipitates, resulting in a significant reduction in Sb(V) removal efficiency. The competitive inhibition effect on Sb(III, V) removal was noticed in the presence of phosphate anions, owing to lowering of ζ-potential values towards more negative trajectory. The presence of hydrophobic organic matter (humic acid) significantly altered the surface characteristics of Fe flocs, thereby affecting the coagulation behavior of Sb in water as compared to the hydrophilic (salicylic acid). Overall, the findings of this research may provide a new insight into the variation in physicochemical characteristics of Fe flocs and Sb removal behavior in the presence of inorganic and organic compounds during the drinking water treatment process.

Mechanism insight into the role of clay particles on enhancing phosphate removal by ferrate compared with ferric salt.

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.

Membrane fouling reduction through electrochemically regulating flocs aggregation in an electro-coagulation membrane reactor

Coupling coagulation and applied electric field is an efficient method to regulate cake layer porosity and hydrophilicity for alleviating ultrafiltration membrane (UF) fouling. However, the Al/Fe flocs aggregation behavior are induced from electric field and determine the cake layer structure, which has not been studied comparatively yet. Herein, the anti-fouling performance in an efficient electro-coagulation membrane reactor (ECMR, in which UF membrane modules are placed between electrodes) was investigated with Al/Fe anode and various electrochemical parameters from the viewpoint of regulating flocs aggregation. Both the cake layers formed from Al and Fe flocs under an electric field were more porous and hydrophilic in comparison with that formed without electric fields, resulting in an enhanced water flux under higher electric field strength. Comparing with Fe flocs, Al flocs had a faster growth rate and larger size, facilitating membrane pore block resistant, which was more pronounced in a higher current density. Furthermore, the cake layer formed from Al flocs was more porous than that formed from Fe flocs. Therefore, the anti-fouling performance of ECMR with Al anode was superior to that of ECMR with Fe anode. When the electric field strength increased from 0 to 10 V/cm, the normalized specific flux was improved from 71.2% to 89.4% for ECMR (Al) and from 48.1% to 70.1% for ECMR (Fe) at 30 min.

Comparative performance of green rusts generated in Fe0–electrocoagulation for Cd2+ removal from high salinity wastewater: Mechanisms and optimization

The treatment of wastewater containing high concentration of inorganic salts has always been one of the focuses of environmental researchers. In this work, the effect of Cl− and SO42− on the removal of Cd2+ from wastewater using Fe0-electrocoagulation (Fe0-EC) were investigated by evaluating the transformation of Fe mineral. The experimental results indicated that the removal of Cd2+ from wastewater was depended on the property of Fe minerals. The generation of sulfate green rust (GRSO4) produced in the presence of SO42− showed stronger adsorption than the chloride green rust (GRCl) for Cd2+, and GRSO4 was obtained even in the mixture Cl− and SO42− solutions, because Fe(II)–Fe(III) GRs (layered double hydroxides, LDHs) showed stronger affinity for divalent SO42− than monovalent Cl−. High concentration of inorganic anions in wastewater resulted in the negative charged Fe flocs. High concentration of Cl− promoted the oxidation of Fe(II) to Fe(III) by chlorine–containing oxidants, and increased the proportion of Fe(III)/Fe(II) in Fe flocs, secondary Fe mineral magnetite (Fe3O4) was formed because of the increase of pH. Therefore, the presence of GRSO4 intermediate increased the Cd2+ removal by adsorption (coagulation and coprecipitation), and then the generated GRSO4 were gradually transformed into lepidocrocite (γ–FeOOH) by oxygen from air. Finally, the parameter optimization were conducted by adjusting the ratio of Cl− and SO42− (RC:S), current density (j), initial pH (pHi), initial Cd2+ concentration (C0), and temperature (T0). The removal efficiency of Cd2+ reached 99.5% after 10 min Fe0-EC under the optimal parameters: RC:S = 25:50 mmoL/mmol, j = 6 mA/cm2, pHi = 7–9, and T0 = 40 °C.

Fabricating biogenic Fe(III) flocs from municipal sewage sludge using NAFO processes: Characterization and arsenic removal ability

This study involved fabricating biogenic Fe(III) flocs enriched from municipal sludge using microbial nitrate-dependent anaerobic Fe(II)-oxidizing (NAFO) processes. The research focused on bacterial community compositions and physicochemical properties of the biogenic Fe(III) flocs and their ability to adsorb arsenic (As). High-throughput sequencing analysis showed that significant microbial succession occurs in the raw sludge after the NAFO processes. The predominant bacterial communities in the biogenic Fe(III) flocs included Rhodanobacter, Parvibaculum, Gemmatimonas and Segetibacter genera. Microscopic and spectroscopic analyses included scanning electron microscopy - energy disperse spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. These tests indicated that biogenic Fe(III) flocs were a mixture of NAFO bacteria and nanosized, poorly crystalline Fe(III) oxide particles. Batch experiments showed that after 120 min of reaction time, more than 95% of As(III) and As(V) (at an initial concentrations of 0.25 mg/L) were effectively removed with 120 ppm biogenic Fe(III) flocs. In addition, biogenic Fe(III) flocs removed As more effectively than abiotic Fe(III) flocs. These findings indicated that biogenic Fe(III) flocs produced from municipal sludge using NAFO processes performed well in removing As.

Influence of HPO42-, H4SiO4, Ca2+, Mg2+ on Fe floc growth and As(III) removal in aerated, natural groundwater

Our aim was to systematically investigate the influence of anions (HPO42−), cations (Ca2+, Mg2+) and neutral H4SiO4 on Fe flocculation and As(III) removal in the complex natural water matrix. For this purpose, three different anaerobic groundwaters were selected and manipulated by dosing of Ca2+, Mg2+, HPO42−, or by their removal by cation – and anion exchange. The change in Fe floc volume and of dissolved Fe and As were followed in aerated jar experiments. Fe floc growth was improved by addition of Ca2+ or Mg2+, and hindered by their removal. This hindered floc growth was more severe for groundwaters with higher P:Fe ratios, where Fe flocs carry a larger net negative surface charge, and rely stronger on Ca2+ or Mg2+ for charge neutralisation. When expressing the charge balance of the different groundwaters as the molar ratio (Ca2+ + Mg2+)/P, a linear relationship was found with the cumulative Fe floc volume, with a plateau at molar ratios >500. At environmentally relevant concentrations, H4SiO4 was found more likely to compete with As(III) for adsorption capacity than HPO42−. As(III) removal was strongly related to Fe removal - independent of Ca2+ or Mg2+ presence - indicating that As(III) is primarily adsorbed at an early stage in the flocculation process.

Role of Calcium in the Coagulation of NOM with Ferric Chloride.

Natural organic matter (NOM) is capable of interfering with Fe hydrolysis and influencing the size, morphology, and identity of Fe precipitates. Conversely, Ca2+ raises surface potential and increases the size and aggregation of Fe precipitates, leading to more effective coagulation and widening the pH range of water treatment. Experiments and modeling were conducted to investigate the significance of the Fe/NOM ratio and the presence of Ca2+ in coagulation. At the high Fe/NOM ratio, sufficient or excess Fe was available for NOM removal, and coagulation proceeded according to expectations based upon the literature. At the low Fe/NOM ratio, however, NOM inhibited Fe hydrolysis, reduced zeta potential, and suppressed the formation of filterable Fe flocs, thereby interfering with NOM removal. In these dose-limited systems without Ca2+, complexation of Fe species by NOM appears to be the mechanism by which coagulation is disrupted. Equilibrating NOM with 1 mM Ca2+ in dose-limited systems prior to dosing with FeCl3 increased Fe hydrolysis and zeta potential, decreased the fraction of colloidal Fe, and improved NOM removal. In systems with Ca2+, data and modeling indicate that Ca2+ complexation by NOM neutralizes some of the negative organic charge and minimizes Fe complexation, making Fe species available for hydrolysis and effective coagulation. This finding represents an important advance in understanding not only how Ca2+ may improve coagulation outcomes, but also in predicting the conditions under which Ca2+ may prove beneficial.

Efficient removal of perfluoroalkyl acids (PFAAs) from aqueous solution by electrocoagulation using iron electrode

Electrocoagulation (EC) technique was used to investigate the removal performance of aqueous perfluoroalkyl acids (PFAAs) with relatively high concentration as simulating the wastewater from organic fluorine industry. A comparison between iron and aluminum electrode was made to remove PFAAs with the similar electrolysis condition. Effective removal of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) was obtained by EC process, especially for Fe anode. Several key factors were studied to optimize the EC process using Fe electrode. At the optimal operating parameters including 25.0mA/cm2 of current density, 180rpm of stirring speed, and 2g/L NaCl as supporting electrolyte, more than 99% of PFOS could be removed with 0.25mM of initial concentration after 50-min electrolysis. Additionally, the adsorptive removal of aqueous PFOS on iron hydroxide flocs during EC was also verified by Fourier transform infrared spectra. The investigation of removal efficiency for PFAAs with different carbon chain length after EC and the characteristic of zeta potential on Fe flocs indicated that the electrostatic adsorption was primarily responsible for the PFAAs sorption on the iron hydroxide flocs.

Highly efficient removal of perfluorooctanoic acid from aqueous solution by H 2 O 2 -enhanced electrocoagulation-electroflotation technique

Abstract Electrocoagulation (EC) technique was used to investigate the removal performance of aqueous perfluorooctanoic acid (PFOA) with relatively high concentration as simulating the wastewater from organic fluorine industry. A comparison was done with the similar amount of coagulant between EC and chemical coagulation process. PFOA removal obtained was higher with EC process, especially for Fe anode. Several factors were studied to optimize the EC process. At the optimal operating parameters including 37.5 mA/cm 2 of current density, initial pH 3.77, and 180 rpm of mixing speed, 93% of PFOA could be removed with 100 mg/L of initial concentration after 90-min electrolysis. Furthermore, the remove efficiency could be obviously improved by H 2 O 2 intermittent addition, which removed more than 99% of PFOA within 40-min EC. It could be attributed to that H 2 O 2 facilitated the oxidative transformation from ferrous to ferric ion. In addition, the adsorptive removal of aqueous PFOA on Fe flocs during EC was also verified by fourier transform infrared spectra.

Legacy impacts of acid sulfate soil runoff on mangrove sediments: Reactive iron accumulation, altered sulfur cycling and trace metal enrichment

The off-site impacts of iron-rich, acidic runoff from drained acid sulfate soils (ASS) on the geochemistry of estuarine sediments are relatively poorly constrained. In this study, we examine the geochemistry of intertidal zone mangrove sediments and porewaters from an estuarine tributary that received ~15–20years of acute ASS drainage, compared to adjacent control sites. Mixing of acidic, iron-rich ASS drainage waters with circumneutral pH estuarine waters drove precipitation of Fe(III) floc and has led to significant, localized accumulation of reactive-Fe species (FeR; Σ1M HCl and citrate-buffered dithionite extracts; up to 4200μmolg−1) as well as significantly elevated porewater Fe2+ (>1000μM) in ASS impacted sediments. X-ray diffraction indicates that goethite, lepidocrocite and ferrihydrite are important Fe(III) minerals in surficial sediments at ASS impacted sites. The abundant FeR is shifting the equilibrium of reduced inorganic sulfur (RIS) mineral-formation pathways towards accumulation of S(0) and acid volatile sulfide (AVS) species and is buffering porewater S(-II) to low concentrations. In contrast, RIS speciation at the control sites is dominated by pyrite (SCR) and porewater S(-II) is present. Some porewater metals (i.e. Al, Mn, Cu, Cr) are significantly elevated at sites proximal to the source of ASS drainage compared to control sites. Abundant FeR also provides a substrate for sorption and accumulation of metals and correspondingly there is significant relative enrichment of reactive metal fractions (1M HCl extractable) in sediments at the ASS impacted sites, with Al≈Mn≈U>Cu>Cr>Ni. This study demonstrates that ASS drainage, which causes accumulation of reactive Fe in mangrove environments, can substantially influence sediment biogeochemistry by altering reduced inorganic sulfur cycling and enhancing the availability of some trace metals.

Regrowth of Broken Hydroxide Flocs: Effect of Added Fluoride.

Hydrous oxides of Al(III) and Fe(III) play a large part in environmental processes and in the action of coagulants used in water and wastewater treatment. Aggregates (flocs) of hydroxide precipitates can be rather weak and are easily broken by applied shear. It is usually found that broken flocs do not fully regrow under low-shear conditions, and this could be a serious disadvantage in practical applications. The irreversible nature of floc breakage suggests that some form of specific, chemical interaction between precipitate particles must be at least partly responsible. On the basis of experiments reported here and elsewhere, we propose that hydroxyl bridges between particles play a part. When these are broken, there is a reduction in the number of "active" surface groups that are able to form new bridges. When small amounts of fluoride are added during breakage of Al flocs, there can be significant improvement in floc regrowth, although this depends on a number of factors, especially pH. With Fe flocs, fluoride has no noticeable effect. These results can be explained by the formation of soluble Al-F complexes and some dissolution of the Al(OH)3 precipitate. This creates a new surface with more "active" groups that can form new hydroxyl bridges.

Groundwater arsenic removal by coagulation using ferric(III) sulfate and polyferric sulfate: A comparative and mechanistic study

Elevated arsenic (As) in groundwater poses a great threat to human health. Coagulation using mono- and poly-Fe salts is becoming one of the most cost-effective processes for groundwater As removal. However, a limitation comes from insufficient understanding of the As removal mechanism from groundwater matrices in the coagulation process, which is critical for groundwater treatment and residual solid disposal. Here, we overcame this hurdle by utilizing microscopic techniques to explore molecular As surface complexes on the freshly formed Fe flocs and compared ferric(III) sulfate (FS) and polyferric sulfate (PFS) performance, and finally provided a practical solution in As-geogenic areas. FS and PFS exhibited a similar As removal efficiency in coagulation and coagulation/filtration in a two-bucket system using 5mg/L Ca(ClO)2. By using the two-bucket system combining coagulation and sand filtration, 500L of As-safe water (<10μg/L) was achieved during five treatment cycles by washing the sand layer after each cycle. Fe k-edge X-ray absorption near-edge structure (XANES) and As k-edge extended X-ray absorption fine structure (EXAFS) analysis of the solid residue indicated that As formed a bidentate binuclear complex on ferrihydrite, with no observation of scorodite or poorly-crystalline ferric arsenate. Such a stable surface complex is beneficial for As immobilization in the solid residue, as confirmed by the achievement of much lower leachate As (0.9μg/L–0.487mg/L) than the US EPA regulatory limit (5mg/L). Finally, PFS is superior to FS because of its lower dose, much lower solid residue, and lower cost for As-safe drinking water.

Membrane fouling by Fe-Humic cake layers in nano-scale: Effect of in-situ formed Fe(III) coagulant

The application of in-situ Fe(III) coagulant, formed by reacting Fe(II) and KMnO4 (Fe/Mn), before membrane ultrafiltration (Fe/Mn-CUF) has been compared with conventional dosing of an Fe(III) coagulant (Fe-CUF). The performance of the alternative coagulants has been evaluated in terms of membrane fouling using model waters containing humic acid. A lower development of trans-membrane pressure (TMP) suggested that in-situ formed Fe(III) caused a lower rate of fouling compared to the dosed Fe(III). The lower TMP development with in-situ formed Fe(III), compared to dosed Fe(III), was associated with smaller, fewer and less compact coagulant flocs and differences in the membrane cake layer; in both cases influent flocs were aggregates of nanometer-scale primary particles.Interestingly, TEM and SEM images showed that the size of nanometer-scale particles formed by in-situ Fe(III) was slightly larger than dosed Fe (III), and this contributed to a more porous cake layer. In addition, the Fe/Mn flocs formed a thinner cake layer than that formed by Fe flocs. Overall, the effect of decreasing the thickness and density of the cake layer and increasing the size of the nanometer-sized primary particles was significantly beneficial in reducing membrane fouling. Also NOM analysis by size exclusion chromatography (SEC) indicated that the in-situ Fe(III) coagulant achieved greater NOM removal than the dosed Fe(III), both in the UF influent and filtrate waters. In summary, the results have shown that in-situ Fe(III) is a superior pretreatment to conventional dosed Fe(III), in terms of significantly improving membrane performance (reduced fouling) with a little higher improvement in water quality.