Preloading hydrous ferric oxide into granular activated carbon for arsenic removal

Arsenic is of concern in water treatment because of its health effects. This research focused on incorporating hydrous ferric oxide (HFO) into granular activated carbon (GAC) for the purpose of arsenic removal. Iron was incorporated into GAC via incipient wetness impregnation and cured at temperatures ranging from 60 to 90 degrees C. X-ray diffractions and arsenic sorption as a function of pH were conducted to investigate the effect of temperature on final iron oxide (hydroxide) and their arsenic removal capabilities. Results revealed that when curing at 60 degrees C, the procedure successfully created HFO in the pores of GAC, whereas at temperatures of 80 and 90 degrees C, the impregnated iron oxide manifested a more crystalline form. In the column tests using synthetic water, the HFO-loaded GAC prepared at 60 degrees C also showed higher sorption capacities than media cured at higher temperatures. These results indicated that the adsorption capacity for arsenic was closely related to the form of iron (hydr)oxide for a given iron content For the column test using a natural groundwater, HFO-loaded GAC (Fe, 11.7%) showed an arsenic sorption capacity of 26 mg As/g when the influent contained 300 microg/L As. Thus, the preloading of HFO into a stable GAC media offered the opportunity to employ fixed carbon bed reactors in water treatment plants or point-of-use filters for arsenic removal.     

Sugarcane bagasse treated with hydrous ferric oxide as a potential adsorbent for the removal of As(V) from aqueous solutions

The mechanism of As(V) removal from aqueous solutions by means of hydrated ferric oxide (HFO)-treated sugarcane bagasse (SCB-HFO) (Saccharum officinarum L.) was investigated. Effects of different parameters, such as pH value, initial arsenic concentration, adsorbent dosage, contact time and ionic strength, on the As(V) adsorption were studied. The adsorption capacity of SCB-HFO for As(V) was found to be 22.1 mg/g under optimum conditions of pH 4, contact time 3 h and temperature 22 °C. Initial As(V) concentration influenced the removal efficiency of SCB-HFO. The desorption of As(V) from the adsorbent was 17% when using 30% HCl and 85% with 1 M NaOH solution. FTIR analyses evidenced two potential binding sites associated with carboxyl and hydroxyl groups which are responsible for As(V) removal. Adsorption, surface precipitation, ion exchange and complexation can be suggested as mechanisms for the As(V) removal from the solution phase onto the surface of SCB-HFO.     

Removal of arsenite and arsenate using hydrous ferric oxide incorporated into naturally occurring porous diatomite

In this study, a simplified and effective method was tried to immobilize iron oxide onto a naturally occurring porous diatomite. Experimental resultsfor several physicochemical properties and arsenic edges revealed that iron oxide incorporated into diatomite was amorphous hydrous ferric oxide (HFO). Sorption trends of Fe (25%)-diatomite for both arsenite and arsenate were similar to those of HFO, reported by Dixit and Hering (Environ. Sci. Technol. 2003, 37, 4182-4189). The pH at which arsenite and arsenate are equally sorbed was 7.5, which corresponds to the value reported for HFO. Judging from the number of moles of iron incorporated into diatomite, the arsenic sorption capacities of Fe (25%)-diatomite were comparable to or higher than those of the reference HFO. Furthermore, the surface complexation modeling showed that the constants of [triple bond]SHAsO4- or [triple bond]SAsO4(2-) species for Fe (25%)-diatomite were larger than those reference values for HFO or goethite. Larger differences in constants of arsenate surface species might be attributed to aluminum hydroxyl ([triple bond]Al-OH) groups that can work better for arsenate removal. The pH-controlled differential column batch reactor (DCBR) and small-scale column tests demonstrated that Fe (25%)-diatomite had high sorption speeds and high sorption capacities compared to those of a conventional sorbent (AAFS-50) that is known to be the first preference for arsenic removal performance in Bangladesh. These results could be explained by the fact that Fe (25%)-diatomite contained well-dispersed HFO having a great affinity for arsenic species and well-developed macropores as shown by scanning electron microscopy (SEM) and pore size distribution (PSD) analyses.     

Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility

Arsenic derived from natural sources occurs in groundwater in many countries, affecting the health of millions of people. The combined effects of As(V) reduction and diagenesis of iron oxide minerals on arsenic mobility are investigated in this study by comparing As(V) and As(III) sorption onto amorphous iron oxide (HFO), goethite, and magnetite at varying solution compositions. Experimental data are modeled with a diffuse double layer surface complexation model, and the extracted model parameters are used to examine the consistency of our results with those previously reported. Sorption of As(V) onto HFO and goethite is more favorable than that of As(III) below pH 5-6, whereas, above pH 7-8, As(II) has a higher affinity for the solids. The pH at which As(V) and As(III) are equally sorbed depends on the solid-to-solution ratio and type and specific surface area of the minerals and is shifted to lower pH values in the presence of phosphate, which competes for sorption sites. The sorption data indicate that, under most of the chemical conditions investigated in this study, reduction of As(V) in the presence of HFO or goethite would have only minor effects on or even decrease its mobility in the environment at near-neutral pH conditions. As(V) and As(III) sorption isotherms indicate similar surface site densities on the three oxides. Intrinsic surface complexation constants for As(V) are higher for goethite than HFO, whereas As(III) binding is similar for both of these oxides and also for magnetite. However, decrease in specific surface area and hence sorption site density that accompanies transformation of amorphous iron oxides to more crystalline phases could increase arsenic mobility.     

Use of hydrochloric acid for determinining solid-phase arsenic partitioning in sulfidic sediments

We examined the use of room-temperature hydrochloric acid (1-6 M) and salt solutions of magnesium chloride, sodium carbonate, and sodium sulfide for the removal of arsenic from synthetic iron monosulfides and contaminated sediments containing acid-volatile sulfides (AVS). Results indicate that acid-soluble arsenic reacts with H2S released from AVS phases and precipitates at low pH as disordered orpiment or alacranite. Arsenic sulfide precipitation is consistent with geochemical modeling in that conditions during acid extraction are predicted to be oversaturated with respect to orpiment, realgar, or both. Binding of arsenic with sulfide at low pH is sufficiently strong that 6 M HCl will not keep spiked arsenic in the dissolved fraction. Over a wide range of AVS concentrations and molar [As]/[AVS] ratios, acid extraction of arsenic from sulfide-bearing sediments will give biased results that overestimate the stability or underestimate the bioavailability of sediment-bound arsenic. Alkaline solutions of sodium sulfide and sodium carbonate are efficient in removing arsenic from arsenic sulfides and mixed iron-arsenic sulfides because of the high solubility of arsenic at alkaline pH, the formation of stable arsenic complexes with sulfide or carbonate, or both.     

A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: I. Laboratory development

A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (+/-1%), and As was completely recovered from HFO-doped gels (+/-4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment.     

Mechanisms of selenate adsorption on iron oxides and hydroxides

Selenate (SeO4(2-)) is an oxyanion of environmental importance because of its toxicity to animals and its mobility in the soil environment. It is known that iron(III) oxides and hydroxides are important sorbents for SeO4(2-) in soils and sediments, but the mechanism of selenate adsorption on iron oxides has been the subject of intense debate. Our research employed Extended X-ray absorption fine structure and attenuated total reflectance-Fourier transform infrared spectroscopies to determine SeO4(2-) bonding mechanisms on hematite, goethite, and hydrous ferric oxide (HFO). It was learned that selenate forms only inner-sphere surface complexes on hematite but forms a mixture of outer- and inner-sphere surface complexes on goethite and HFO. This continuum of adsorption mechanisms is strongly affected by both pH and ionic strength. These results suggest that adsorption experiments should be conducted on several different iron oxides and over a wide range of reaction conditions to accurately assess the reactivity of oxyanions on iron oxides.     

Arsenic removal from Bangladesh tube well water with filter columns containing zerovalent iron filings and sand

Arsenic removal is often challenging due to high As(III), phosphate, and silicate concentrations and low natural iron concentrations. Application of zerovalent iron is promising, as metallic iron is widely available. However, removal mechanisms remained unclear and currently used removal units with iron have not been tested systematically, partly due to their large size and long operation time. This study investigated smaller filter columns with 3-4 filters, each containing 2.5 g of iron filings and 100-150 g of sand. At a flow rate of 1 L/h, these columns were able to treat 75-90 L of well water with 440 microg/L As, 1.8 mg/L P, 4.7 mg/L Fe, 19 mg/L Si, and 6 mg/L dissolved organic carbon (DOC) to below 50 microg/L As(tot), without addition of an oxidant. As(III) was oxidized in parallel to oxidation of corrosion-released Fe(II) by dissolved oxygen and sorbed on the forming hydrous ferric oxides (HFO). The open filter columns prevented anoxic conditions. DOC did not appear to interfere with arsenic removal. Manganese was reduced after a slight initial increase from 0.3 mg/L to below 0.1 mg/L. About 100 mg of Fe(0)/L of water was required, 3-5 times less than that for larger units with sand and iron turnings.     

Iron(III) modification of Bacillus subtilis membranes provides record sorption capacity for arsenic and endows unusual selectivity for As(V)

Bacillus subtilis is a spore forming bacterium that takes up both inorganic As(III) and As(V). Incubating the bacteria with Fe(III) causes iron uptake (up to ～0.5% w/w), and some of the iron attaches to the cell membrane as hydrous ferric oxide (HFO) with additional HFO as a separate phase. Remarkably, 30% of the Bacillus subtilis cells remain viable after treatment by 8 mM Fe(III). At pH 3, upon metalation, As(III) binding capacity becomes ～0, while that for As(V) increases more than three times, offering an unusual high selectivity for As(V) against As(III). At pH 10 both arsenic forms are sorbed, the As(V) sorption capacity of the ferrated Bacillus subtilis is at least of 11 times higher than that of the native bacteria. At pH 8 (close to pH of most natural water), the arsenic binding capacity per mole iron for the ferrated bacteria is greater than those reported for any iron containing sorbent. A sensitive arsenic speciation approach is thus developed based on the binding of inorganic arsenic species by the ferrated bacteria and its unusual high selectivity toward As(V) at low pH.     

Simultaneous microbial reduction of iron(III) and arsenic(V) in suspensions of hydrous ferric oxide

Bacterial reduction of arsenic(V) and iron(III) oxides influences the redox cycling and partitioning of arsenic (As) between solid and aqueous phases in sediment-porewater systems. Two types of anaerobic bacterial incubations were designed to probe the relative order of As(V) and Fe(III) oxide reduction and to measure the effect of adsorbed As species on the rate of iron reduction, using hydrous ferric oxide (HFO) as the iron substrate. In one set of experiments, HFO was pre-equilibrated with As(V) and inoculated with fresh sediment from Haiwee Reservoir (Olancha, CA), an As-impacted field site. The second set of incubations consisted of HFO (without As) and As(III)- and As(V)- equilibrated HFO incubated with Shewanella sp. ANA-3 wild-type (WT) and ANA-3deltaarrA, a mutant unable to produce the respiratory As(V) reductase. Of the two pathways for microbial As(V) reduction (respiration and detoxification), the respiratory pathway was dominant under these experimental conditions. In addition, As(III) adsorbed onto the surface of HFO enhanced the rate of microbial Fe(III) reduction. In the sediment and ANA-3 incubations, As(V) was reduced simultaneously or prior to Fe(III), consistent with thermodynamic calculations based on the chemical conditions of the ANA-3 WT incubations.     

Arsenic removal using polymer-supported hydrated iron(III) oxide nanoparticles: role of donnan membrane effect

The conditions leading to the Donnan membrane equilibrium arise from the inability of ions to diffuse out from one phase in a heterogeneous system. In a polymeric cation exchanger, negatively charged sulfonic acid groups are covalently attached to the polymer chains, and thus, they cannot permeate out of the polymer phase. Conversely, a polymeric anion exchanger contains a high concentration of non-diffusible positively charged quaternary ammonium functional groups. It is well-established that submicron or nanoscale hydrated iron(III) oxide (HFO) particles exhibit high sorption affinity toward both arsenates and arsenites. In this study, commercially available cation and anion exchangers were used as host materials for dispersing HFO nanoparticles within the polymer phase using a technique previously developed. The resulting polymeric/inorganic hybrid sorbent particles were subsequently used for arsenic removal in the laboratory. The most significant finding of the study is that the anion exchanger as a substrate containing dispersed HFO offered substantially higher arsenate removal capacity as compared to the cation exchanger, all other conditions remaining identical. In fact, HFO nanoparticles dispersed within the gel-type cation exchanger were unable to remove arsenic. The Donnan membrane effect resulting from the nondiffusible negatively charged sulfonic acid groups in the cation exchanger did not allow permeation of arsenate into the polymer phase (i.e., arsenate was excluded from the spherical beads dispersed with HFO nanoparticles). On the contrary, anion-exchanger-supported HFO particles or HAIX offered very high arsenic removal capacity; less than 10% of influent arsenic broke through after 10 000 bed vol. HAIX was also amenable to efficient regeneration with 2% NaOH and 3% NaCl and capable of simultaneously removing both perchlorate and arsenic selectively. According to the information in the open literature, HAIX is the first hybrid sorbent that utilizes the Donnan membrane effect of the host material for sorption enhancement. From a generic viewpoint, other metal oxide/metal nanoparticles may also be judiciously embedded in appropriate support materials that would reject or enhance permeation of targeted ionic solutes.     

Rates of hydrous ferric oxide crystallization and the influence on coprecipitated arsenate

Arsenate coprecipitated with hydrous ferric oxide (HFO) was stabilized against dissolution during transformation of HFO to more crystalline iron (hydr)oxides. The rate of arsenate stabilization approximately coincided with the rate of HFO transformation at pH 6 and 40 degrees C. Comparison of extraction data and X-ray diffraction results confirmed that hematite and goethite were the primary transformation products. HFO transformation was significantly retarded at or above an arsenate solid loading of 29 455 mg As/kg HFO. However, HFO transformation proceeded at a significant rate for arsenate solid loadings of 4208 and 8416 mg As/kg HFO. At a solid loading of 8416 mg As/kg HFO, XRD results suggested arsenate primarily partitioned to hematite. Comparison of HFO transformation rates observed in this research to rates obtained from the literature at pH 6 and temperatures ranging from 24 to 70 degrees C suggests that arsenate stabilization could be realized in oxic environments with a significantfraction of iron (hydr)oxides. While this process has not been documented in natural systems, the predicted half-life for transformation of an arsenic-bearing HFO is approximately 300 days at 25 degrees C at solid loading < 8415 mg As/kg HFO. The projected time frame for arsenate stabilization indicates this process should be considered during development of conceptual and analytical models describing arsenic fate and transport in oxic systems containing reactive iron (hydr)oxides. The likelihood of this process would depend on the chemical dynamics of the soil or sediment system relative to iron (hydr)oxide precipitation-dissolution reactions and the potential retarding/competing influence of ions such as silicate and organic matter.     

Coadsorption of arsenic(III) and arsenic(V) onto hydrous ferric oxide: effects on abiotic oxidation of arsenic(III), extraction efficiency, and model accuracy

Single solute adsorption and coadsorption of As(III) and As(V) onto hydrous ferric oxide (HFO), oxidation of As(III), and extraction efficiencies were measured in 0.2 atm O2. Oxidation was negligible for single-adsorbate experiments, but significant oxidation was observed in the presence of As(V) and HFO. Single-adsorbate As(III) or As(V) were incompletely extracted (0.5 M NaOH for 20 min), but all As was recovered in coadsorbate experiments. Single-adsorbate data were well-simulated using published surface complexation models, but those models (calibrated for single-adsorbate results) provided poor fits for coadsorbate experiments. An amended model accurately simulated single- and coadsorbate results. Model predictions of significant change in As(III) surface complex speciation in coadsorbate experiments was confirmed using zeta potential measurements. Our results demonstrate that mobility of arsenic in groundwater and removal in engineered treatment systems are more complicated when both As(III) and As(V) are present than anticipated based on single-adsorbate experimental results.     

Solubility of hematite revisited: effects of hydration

Measured pH and dissolved ferric iron concentration ([Fe(III)diss]) in contact with well-characterized hematite indicated an equilibrium with hematite immediately after synthesis, but [Fe(III)diss] increased with hydration time to be consistent with the predicted solubility of goethite or hydrous ferric oxide (HFO), hydrated analogues of hematite. X-ray diffraction did not detect structural modification of hematite after 190 days of hydration, but M?ssbauer spectroscopy detected hydration that penetrated several crystalline layers. When the hematite suspension was diluted with water, solids were invariably identified as hematite, but [Fe(III)diss] and pH indicated an equilibrium with goethite or HFO. This is the first experimental confirmation that the interfacial hydration of anhydrous hematite results in higher solubility than predicted by bulk thermodynamic properties of hematite. Correspondence of the results with previously published measurements and implications for environmental chemistry of ferric oxides are also discussed.     

Processes at the sediment water interface after addition of organic matter and lime to an Acid Mine Pit Lake mesocosm

A strategy to neutralize acidic pit lakes was tested in a field mesocosm of 4500 m(3) volume in the Acidic Pit Mine Lake 111 in Germany. Carbokalk, a byproduct from sugar production, and wheat straw was applied near to the sediment surface to stimulate in lake microbial alkalinity generation by sulfate and iron reduction. The biogeochemical processes at the sediment-water interface were studied over 3 years by geochemical monitoring and an in situ microprofiler. Substrate addition generated a reactive zone at the sediment surface where sulfate and iron reduction proceeded. Gross sulfate reduction reached values up to 10 mmol m(-2) d(-1). The neutralization rates between 27 and 0 meq m(-2) d(-1) were considerably lower than in previous laboratory experiments. The precipitation of ferric iron minerals resulted in a growing acidic sediment layer on top of the neutral sediment. In this layer sulfate reduction was observed but iron sulfides could not precipitate. In the anoxic sediment H2S was oxidized by ferric iron minerals. H2S partly diffused to the water column where it was oxidized. As a result the net formation of iron sulfides decreased after 1 year although gross sulfate reduction rates continued to be high. The rate of iron reduction exceeded the sulfate reduction rate, which resulted in high fluxes of ferrous iron out of the sediment.     

Observation of surface precipitation of arsenate on ferrihydrite

X-ray diffraction and Raman spectroscopy were used in this study to characterize arsenate phases in the arsenate-ferrihydrite sorption system. Evidence has been obtained for surface precipitation of ferric arsenate on synthetic ferrihydrite at acidic pH (3-5) underthe following experimental conditions: sorption density of As/Fe approximately 0.125-0.49 and arsenic equilibrium concentration of <0.02-440 mg/L. Surface precipitation occurred under apparently undersaturated (in the bulk solution phase) conditions, and probably involved initial uptake of arsenate by surface complexation followed by transition to ferric arsenate formation on the surface as indicated by XRD analysis. At basic pH (i.e., pH 8), however, no ferric arsenate was observed in arsenate-ferrihydrite samples at a sorption density of As/Fe approximately 0.125-0.30 and an arsenic equilibrium concentration of 2.0-1100 mg/ L. At pH 8, arsenate is sorbed on ferrihydrite predominantly via surface adsorption, and the XRD patterns resemble basically that of ferrihydrite.     

X-ray absorption spectroscopy study of the effects of pH and ionic strength on Pb(II) sorption to amorphous silica

Pb(III) sorption to hydrous amorphous SiO2 was studied as a function of pH and ionic strength using XAS to characterize the sorption products formed. Pb sorption increased with increasing pH and decreasing ionic strength. The XAS data indicated that the mechanism of Pb(II) sorption to the SiO2 surface was pH-dependent. At pH < 4.5, a mononuclear inner-sphere Pb sorption complex with ionic character dominated the Pb surface speciation. Between pH 4.5 and pH 5.6, sorption increasingly occurred via the formation of surface-attached covalent polynuclear Pb species, possibly Pb-Pb dimers, and these were the dominant Pb complexes at pH > or = 6.3. Decreasing ionic strength from I = 0.1 to I = 0.005 M NaClO4 significantly increased Pb sorption but did not strongly influence the average local coordination environment of sorbed Pb at given pH, suggesting that the formation of mononuclear and polynuclear Pb complexes at the surface were coupled; possibly, Pb monomers control the formation of Pb polynuclear species by diffusion along the surface, or they act as nucleation centers for additional Pb uptake from solution. This study shows that the effectiveness of SiO2 in retaining Pb(II) is strongly dependent on solution conditions. At low pH, Pb(II) may be effectively remobilized by competition with other cations, whereas sorbed Pb is expected to become less susceptible to desorption with increasing pH. However, unlike for Ni(II) and Co(II), no lead phyllosilicates are formed at these higher pH values; therefore, SiO2 is expected to be a less effective sink for Pb immobilization than for these other metals.     

Effect of added salt and increase in ionic strength on skim milk electroacidification performances

Bipolar-memibrane electroacidification (BMEA) technology which uses the property of bipolar membranes to split water and the demineralization action of cation-exchange membranes (CEM), was tested for the production of acid casein. BMEA has numerous advantages in comparison with conventional isoelectric precipitation processes of proteins used in the dairy industry. BMEA uses electricity to generate the desired ionic species to acidify the treated solutions. The process can be precisely controlled, as electro-acidification rate is regulated by the effective current density in the cell. Water dissociation at the bipolar membrane interface is continuous and avoids local excess of acid. In-situ generation of dangerous chemicals (acids and bases) reduces the risks associated with the handling, transportation, use and elimination of these products. The aim of this study was to evaluate the performance of BMEA in different conditions of added ionic strength (p(added) = 0, 0.25, 0.5 and 1.0 M) and added salt (CaCl2, NaCl and KCl). The combination of KCl and p(added) = 0.5 M gave the best results with a 45% decrease in energy consumption. The increased energy efficiency was the result of a decrease in the anode/cathode voltage difference. This was due to an increase of conductivity, produced by addition of salt, necessary to compensate for the lack of sufficiently mobile ions in the skim milk. However, the addition of salts, irrespective of type or ionic strength, increased the required operation time. The protein profile of isolates were similar under all experimental conditions, except at 1.0 M-CaCl2.     

Preparation and evaluation of GAC-based iron-containing adsorbents for arsenic removal

Granular activated carbon-based, iron-containing adsorbents (As-GAC) were developed for effective removal of arsenic from drinking water. Granular activated carbon (GAC) was used primarily as a supporting medium for ferric iron that was impregnated by ferrous chloride (FeCl2) treatment, followed by chemical oxidation. Sodium hypochlorite (NaClO) was the most effective oxidant, and carbons produced from steam activation of lignite were most suitable for iron impregnation and arsenic removal. Two As-GAC materials prepared by FeCl2 treatment (0.025 -0.40 M) of Dacro 20 x 50 and Dacro 20 x 40LI resulted in a maximum impregnated iron of 7.89% for Dacro 20 x 50 and 7.65% for Dacro 20 x 40Ll. Nitrogen adsorption-desorption analyses showed the BET specific surface area, total pore volume, porosity, and average mesoporous diameter all decreased with iron impregnation, indicating that some micropores were blocked. SEM studies with associated EDS indicated that the distribution of iron in the adsorbents was mainly on the edge of As-GAC in the low iron content (approximately 1% Fe) sample but extended to the center at the higher iron content (approximately 6% Fe). When the iron content was > approximately 7%, an iron ring formed at the edge of the GAC particles. No difference in X-ray diffraction patterns was observed between untreated GAC and the one with 4.12% iron, suggesting that the impregnated iron was predominantly in amorphous form. As-GAC could remove arsenic most efficiently when the iron content was approximately 6%; further increases of iron decreased arsenic adsorption. The removal of arsenate occurred in a wide range of pH as examined from 4.4 to 11, but efficiency was decreased when pH was higher than 9.0. The presence of phosphate and silicate could significantly decrease arsenate removal at pH > 8.5, while the effects of sulfate, chloride, and fluoride were minimal. Column studies showed that both As(V) and As(III) could be removed to below 10 microg/L within 6000 empty bed volume when the groundwater containing approximately 50 microg/L of arsenic was treated.     

Arsenic removal from groundwater by household sand filters: comparative field study, model calculations, and health benefits

Arsenic removal efficiencies of 43 household sand filters were studied in rural areas of the Red River Delta in Vietnam. Simultaneously, raw groundwater from the same households and additional 31 tubewells was sampled to investigate arsenic coprecipitation with hydrous ferric iron from solution, i.e., without contact to sand surfaces. From the groundwaters containing 10-382 microg/L As, < 0.1-48 mg/L Fe, < 0.01-3.7 mg/L P, and 0.05-3.3 mg/L Mn, similar average removal rates of 80% and 76% were found for the sand filter and coprecipitation experiments, respectively. The filtering process requires only a few minutes. Removal efficiencies of Fe, phosphate, and Mn were > 99%, 90%, and 71%, respectively. The concentration of dissolved iron in groundwater was the decisive factor for the removal of arsenic. Residual arsenic levels below 50 microg/L were achieved by 90% of the studied sand filters, and 40% were even below 10 microg/L. Fe/As ratios of > or = 50 or > or = 250 were required to ensure arsenic removal to levels below 50 or 10 microg/L, respectively. Phosphate concentrations > 2.5 mg P/L slightly hampered the sand filter and coprecipitation efficiencies. Interestingly, the overall arsenic elimination was higher than predicted from model calculations based on sorption constants determined from coprecipitation experiments with artificial groundwater. This observation is assumed to result from As(lll) oxidation involving Mn, microorganisms, and possibly dissolved organic matter present in the natural groundwaters. Clear evidence of lowered arsenic burden for people consuming sand-filtered water is demonstrated from hair analyses. The investigated sand filters proved to operate fast and robust for a broad range of groundwater composition and are thus also a viable option for mitigation in other arsenic affected regions. An estimation conducted for Bangladesh indicates that a median residual level of 25 microg/L arsenic could be reached in 84% of the polluted groundwater. The easily observable removal of iron from the pumped water makes the effect of a sand filter immediately recognizable even to people who are not aware of the arsenic problem.