Arsenic removal from aqueous solution via ferrihydrite crystallization control

Removal of arsenate anion from aqueous solution by coprecipitation with ferrihydrite has been studied under conditions in which the Fe/As ratio is maintained at a constant level, while the degree of supersaturation with respect to the iron oxide precipitate is varied. An Fe/As ratio of 12 was chosen, and supersaturation was controlled by varying the iron concentration or the pH. The relationship between supersaturation and arsenic removal was found to follow an exponential curve, with greater arsenic removal occurring at higher supersaturation ratios for each of the pH values tested. Higher supersaturation ratios were required to achieve a given level of arsenic removal at pH 7 than would be required to achieve the same level of removal at pH 3.5. The results provide important guidelines for selection of appropriate concentrations of iron(III) required for arsenic removal under various circumstances. Powder XRD analysis of the arsenate-ferrihydrite precipitates showed an increasing degree of structural order with decreasing levels of supersaturation. TEM images of the precipitates revealed that aggregates with a morphology similar to that of schwertmannite are formed in some samples at low supersaturation levels. The results described in this paper indicate that the overall efficiency of arsenic removal involves a combination of both supersaturation and pH effects, with pH controlling the affinity of arsenate for the ferrihydrite surface, and supersaturation controlling the surface area and physical properties of the ferrihydrite product.     

Removal of arsenic from high ionic strength solutions: effects of ionic strength, pH, and preformed versus in situ formed HFO

Arsenic sorption to hydrous ferric oxide (HFO) is an effective treatment method for removing dissolved arsenic from fresh drinking water sources. However, detailed information is limited regarding arsenic removal from solutions of high ionic strength such as brackish groundwater, seawater, or high-pressure membrane process residuals. Bench-scale treatment experiments were conducted exploring arsenic removal from simple solutions with ionic strengths ranging from 0.008 to 1.5 M by addition of ferric chloride followed by solid/liquid separation (microfiltration or ultrafiltration). Arsenic removal from these solutions during in situ iron precipitation was approximately 90% at Fe:As molar ratios of 10 to 15 and > 95% for Fe:As molar ratios greater than 20. Arsenic removal at iron doses of 10(-6) to 10(-4) mol-Fe/L improved when pH was lowered from 8 to less than 6.5 at ionic strength 0.2 M; this improvement was not as significant at ionic strength 0.7 M. Arsenic removal diminished when alkalinity was increased from 400 to 1,400 mg/L as calcium carbonate; however, arsenic removal at the higher alkalinity improved when pH was lowered from approximately 8 to less than 7. Arsenic removal with preformed HFO solids and subsequent microfiltration was significantly less than that observed with in situ HFO precipitation. Increased removal by in situ precipitation compared to that of preformed solids is explained by an increased number of adsorption sites due to uptake during iron oxy-hydroxide polymerization as well as an increase in surface area resulting in diminished surface charge effects. Model simulations of arsenic uptake by in situ precipitation adequately captured these effect by changing the model parameters used to model arsenic uptake by preformed HFO, specificallythe total number of surface sites and surface area.     

Sorption and desorption of arsenic to ferrihydrite in a sand filter

Elevated arsenic concentrations in drinking water occur in many places around the world. Arsenic is deleterious to humans, and consequently, As water treatment techniques are sought. To optimize arsenic removal, sorption and desorption processes were studied at a drinking water treatment plant with aeration and sand filtration of ferrous iron rich groundwater at Elmevej Water Works, Fensmark, Denmark. Filter sand and pore water were sampled along depth profiles in the filters. The sand was coated with a 100-300 microm thick layer of porous Si-Ca-As-contaning iron oxide (As/Fe = 0.17) with locally some manganese oxide. The iron oxide was identified as a Si-stabilized abiotically formed two-line ferrihydrite with a magnetic hyperfine field of 45.8 T at 5 K. The raw water has an As concentration of 25 microg/L, predominantly as As(II). As the water passes through the filters, As(III) is oxidized to As(V) and the total concentrations drop asymptotically to a approximately 15 microg/L equilibrium concentration. Mn is released to the pore water, indicating the existence of reactive manganese oxides within the oxide coating, which probably play a role for the rapid As(III) oxidation. The As removal in the sand filters appears controlled by sorption equilibrium onto the ferrihydrite. By addition of ferrous chloride (3.65 mg of Fe(II)/L) to the water stream between two serially connected filters, a 3 microg/L As concentration is created in the water that infiltrates into the second sand filter. However, as water flow is reestablished through the second filter, As desorbs from the ferrihydrite and increases until the 15 microg/L equilibrium concentration. Sequential chemical extractions and geometrical estimates of the fraction of surface-associated As suggest that up to 40% of the total As can be remobilized in response to changes in the water chemistry in the sand filter.     

Coprecipitation of arsenate with metal oxides. 2. Nature, mineralogy, and reactivity of iron(III) precipitates

Coprecipitation of arsenic with iron or aluminum occurs in natural environments and is a remediation technology used to remove this toxic metalloid from drinking water and hydrometallurgical solutions. In this work, we studied the nature, mineralogy, and reactivity toward phosphate of iron-arsenate coprecipitates formed at As(V)/Fe(III) molar ratios (R) of 0, 0.01, or 0.1 and at pH 4.0, 7.0, and 10.0 aged for 30 or 210 days at 50 degrees C and studied the desorption of arsenate. At R = 0, goethite and hematite (with ferrihydrite at pH 4.0 and 7.0) crystallized, whereas at R = 0.01, the formation of ferrihydrite increased and hematite crystallization was favored over goethite. In some samples, the morphology of hematite changed from rounded platy crystals to ellipsoids. At R = 0.1, ferrihydrite formed in all the coprecipitates and remained unchanged even after 210 days of aging. The surface area and chemical composition of the precipitates were affected by pH, R, and aging. Chemical dissolution of the samples showed that arsenate was present mainly in ferrihydrite, but at R = 0.01, it was partially incorporated into the structures of crystalline Fe oxides. The sorption of phosphate on to the coprecipitates was affected not only by the mineralogy and surface area of the samples but also by the amounts of arsenate present in the oxides. The samples formed at pH 4.0 and 7.0 and at R = 0.1 sorbed lower amounts of phosphate than the precipitates obtained at R = 0 or 0.01, despite the former having a larger surface area and showing only a presence of short-range ordered materials. This is mainly due to the fact that in the coprecipitates at R = 0.1 arsenate occupied many sorption sites, thus preventing phosphate sorption. Less than 20% of the arsenate present in the coprecipitates formed at R = 0.1 was removed by phosphate and more from the samples synthesized at pH 7.0 or 10.0 than at pH 4.0. Moreover, we found that more arsenate was desorbed by phosphate from a ferrihydrite on which arsenate was added than from an iron-arsenate coprecipitate, attributed to the partial occlusion of some arsenate anions into the framework of the coprecipitate. XPS analyses confirmed these findings.     

Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: a comparison with synthetic analogues

Ochreous precipitates containing 5.5-69.8 g/kg As were isolated from mine drainage in Finland and were composed of schwertmannite, ferrihydrite, and goethite. Schwertmannite formation was favored at pH 3-4, but its structure was degraded at high As levels. A series of coprecipitates were therefore prepared from mixed iron arsenate/sulfate solutions to define the limits of schwertmannite stability. Schwertmannite was replaced as the dominant phase by a poorly crystalline ironIII hydroxy arsenate (FeOHAs) when As/Fe mole ratios exceeded 0.15. The FeOHAs gave an X-ray diffraction pattern similar to that obtained from an "amorphous" ironIII arsenate (As/Fe = 1.0) with broad peaks at 0.30 and 0.16 nm. The FeOHAs possessed a magnetic hyperfinefield of 41.9T at 4.2 K that was intermediate to those of schwertmannite (46.1 T) and the ironIII arsenate (24.8 T). These data indicate a strong disruptive effect of arsenate on magnetic ordering and structure development in schwertmannite. Equilibration of 0.01 M arsenate solutions with freshly prepared schwertmannite and 2-line ferrihydrite at pH 3.0 for up to 60 d gave sorbed As contents of 175 and 210 g/kg, respectively. Arsenate sorption degraded the host schwertmannite and ferrihydrite, perhaps due to the formation of an FeOHAs surface phase.     

Adsorption of arsenate onto ferrihydrite from aqueous solution: influence of media (sulfate vs nitrate), added gypsum, and pH alteration

Mineral processing effluents generated in hydrometallurgical industrial operations are sulfate based; hence it is of interest to investigate the effect sulfate matrix solution ("sulfate media") has on arsenate adsorption onto ferrihydrite. In this work, in particular, the influence of media (SO4(2-) vs NO3-), added gypsum, and pH alteration on the adsorption of arsenate onto ferrihydrite has been studied. The ferrihydrite precipitated from sulfate solution incorporated a significant amount of sulfate ions and showed a much higher adsorption capacityfor arsenate compared to nitrateferrihydrite at pH 3-8 and initial Fe/As molar ratios of 2, 4, and 8. Adsorption of arsenate onto sulfate-ferrihydrite involved ligand exchange with SO4(2-) ions that were found to be more easily exchangeable with increasing pH. Added gypsum to the adsorption system significantly enhanced the uptake of arsenate by ferrihydrite at pH 8. Equilibration treatment at acidic pH and addition of gypsum markedly improved the stability of adsorbed arsenate on ferrihydrite when pH was elevated. Comparison of arsenate adsorption onto ferrihydrite to coprecipitation of arsenate with iron(III) showed the latter process to lead to higher arsenic removal.     

Individual and competitive adsorption of arsenate and phosphate to a high-surface-area iron oxide-based sorbent

Individual and competitive adsorption of arsenate and phosphate were studied on a high-surface-area Fe/Mn-(hydr)oxide sorbent with surface and bulk properties similar to those of two-line ferrihydrite. It has maximum adsorption densities of 0.42 micromol As m(-2) at neutral pH and 1.24 micromol As m(-2) at pH 3. A surface complexation model (SCM) that used the diffuse double layer model was developed that could simulate single and binary sorbate adsorption over pH 4-9. The predominant adsorbed arsenate and phosphate species were modeled as bidentate binuclear surface complexes at low pH and as monodentate complexes at high pH. The model initially overpredicted the inhibition of arsenate adsorption by the presence of phosphate. The overprediction was resolved by separating surface sites into two types: ones to which both arsenate and phosphate bind and a smaller number to which only phosphate binds. The modified model predicted the competitive adsorption of arsenate and phosphate over pH 4-9 at total As concentrations of 6.67 and 80.1 microM and a total P concentration of 129 and 323 microM. The model may be used to predict arsenic adsorption to the sorbent for a given water source based on solution chemistry.     

Iron and arsenic cycling in intertidal surface sediments during wetland remediation

The accumulation and behavior of arsenic at the redox interface of Fe-rich sediments is strongly influenced by Fe(III) precipitate mineralogy, As speciation, and pH. In this study, we examined the behavior of Fe and As during aeration of natural groundwater from the intertidal fringe of a wetland being remediated by tidal inundation. The groundwater was initially rich in Fe(2+) (32 mmol L(-1)) and As (1.81 μmol L(-1)) with a circum-neutral pH (6.05). We explore changes in the solid/solution partitioning, speciation and mineralogy of Fe and As during long-term continuous groundwater aeration using a combination of chemical extractions, SEM, XRD, and synchrotron XAS. Initial rapid Fe(2+) oxidation led to the formation of As(III)-bearing ferrihydrite and sorption of >95% of the As(aq) within the first 4 h of aeration. Ferrihydrite transformed to schwertmannite within 23 days, although sorbed/coprecipitated As(III) remained unoxidized during this period. Schwertmannite subsequently transformed to jarosite at low pH (2-3), accompanied by oxidation of remaining Fe(2+). This coincided with a repartitioning of some sorbed As back into the aqueous phase as well as oxidation of sorbed/coprecipitated As(III) to As(V). Fe(III) precipitates formed via groundwater aeration were highly prone to reductive dissolution, thereby posing a high risk of mobilizing sorbed/coprecipitated As during any future upward migration of redox boundaries. Longer-term investigations are warranted to examine the potential pathways and magnitude of arsenic mobilization into surface waters in tidally reflooded wetlands.     

Thermodynamic stabilization of hydrous ferric oxide by adsorption of phosphate and arsenate

Hydrous ferric oxide (HFO) is an X-ray amorphous compound with a high affinity for anions under strongly or mildly acidic conditions. Because of the usually small particle size of HFO, the adsorption capacity is high and adsorption may significantly impact the thermodynamic properties of such materials. Here we show that adsorption of phosphate and arsenate stabilizes HFO by experimental determination of enthalpies of formation (by acid-solution calorimetry) and estimates of standard entropies for six phosphate- or arsenate-enriched HFO samples. At pH values lower than ～5, the phosphate-doped HFO is not only less soluble than ferrihydrite (anion-free HFO) but also crystalline FeOOH polymorphs feroxyhyte and lepidocrocite. The arsenate-doped HFO is also stabilized with respect to the ferrihydrite. Phosphate availability in soils can be controlled by the phosphate-enriched HFO which is many orders of magnitude less soluble than apatite or crystalline Fe(III) phosphates, for example strengite (FePO(4)·2H(2)O). Thermodynamic dissolution models for scorodite (FeAsO(4)·2H(2)O) and As-enriched HFO show that under mildly acidic or circumneutral conditions, scorodite dissolves, As-HFO precipitates, and a substantial amount of As(V) is released into the aqueous solution (at pH 7, log m(As) ～ -2.5). The data presented in this paper can be used to model the equilibrium concentration of Fe(III), P(V), or As(V) in soil solutions or in natural or anthropogenic sediments polluted by arsenic.     

Removal of arsenic from synthetic acid mine drainage by electrochemical pH adjustment and coprecipitation with iron hydroxide

Acid mine drainage (AMD), which is caused by the biological oxidation of sulfidic materials, frequently contains arsenic in the form of arsenite, As(III), and/or arsenate, As(V), along with much higher concentrations of dissolved iron. The present work is directed toward the removal of arsenic from synthetic AMD by raising the pH of the solution by electrochemical reduction of H+ to elemental hydrogen and coprecipitation of arsenic with iron(III) hydroxide, following aeration of the catholyte. Electrolysis was carried out at constant current using two-compartment cells separated with a cation exchange membrane. Four different AMD model systems were studied: Fe(III)/As(V), Fe(III)/As(III), Fe(II)/As(V), and Fe(II)/As(III) with the initial concentrations for Fe(III) 260 mg/L, Fe(II) 300 mg/L, As(V), and As(III) 8 mg/L. Essentially quantitative removal of arsenic and iron was achieved in all four systems, and the results were independent of whether the pH was adjusted electrochemically or by the addition of NaOH. Current efficiencies were approximately 85% when the pH of the effluent was 4-7. Residual concentrations of arsenic were close to the drinking water standard proposed by the World Health Organization (10 microg/L), far below the mine waste effluent standard (500 microg/L).     

Characterizing and quantilying controls on arsenic solubility over a pH range of 1-11 in a uranium mill-scale experiment

A mill-scale hydrometallurgical experiment (2700 m3 of effluent treated/day) was conducted for three months at the Rabbit Lake uranium mine site located in northern Saskatchewan, Canada, to determine the controls on the solubility of dissolved arsenic over a pH range of 1-11 and to develop a thermodynamic database for the dominant mineralogical controls on arsenic in the mill and the resulting mill tailings. The arsenic concentrations in the mill ranged from 526 mg/L at pH 1.0 (initial) to 1.34 mg/L at pH 10.8 (final discharge). Geochemical modeling of the chemistry data shows that arsenic solubility is controlled by the formation of scorodite (FeAsO4-2H2O) from pH 2.4 to pH 3.1, with 99.8% of dissolved arsenic precipitated as scorodite. Model results show that scorodite is unstable (releasing arsenic back in to solution) above pH 3.1 and arsenic adsorption to the surface of 2-line ferrihydrite is the dominant controlling factor in the solubility of arsenic from pH 3.2 to pH 11.0, with 99.8% of dissolved arsenic removed from solution via this mechanism. Finally, model results show -0.2% of the total dissolved arsenic adsorbs to the surface of amorphous aluminum hydroxide from pH 5.0 to pH 8.0. Minor alterations to the thermodynamic properties of arsenite and arsenate adsorption to 2-line ferrihydrite allowed the fit between measured mill-scale and modeled concentrations for the pH range of 3.2-11.0 to be optimized.     

Modeling silica sorption to iron hydroxide

Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0-9.5 and silica concentrations of 0-200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.     

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.     

Formation of metal-arsenate precipitates at the goethite-water interface

Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO4, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 micromol m(-2), respectively, and Zn surface saturation was absent at pH 4 and approximately 1.0 micromol m(-2) at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 +/- 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO4, gradually forming an adamite-like [Zn2(AsO4)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.     

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.     

Implications of aqueous silica sorption to iron hydroxide: mobilization of iron colloids and interference with sorption of arsenate and humic substances

This work highlighted practical implications of aqueous silica sorption to iron hydroxide in natural and engineered systems. Two types of surfaces were prepared by exposing 10 mg/L preformed Fe(OH)3 to aqueous silica (0-200 mg/L as SiO2) for periods of 1.5 h or 50 days. After 1.5 h, the concentration of iron passing through a 0.45 microm pore size filter at pH 6.0-9.5 was always negligible, but if zeta potential < or =-15 mV as much as 35% of the iron passed through filters after 50 days of aging. When arsenate was added to 10 mg/L iron hydroxide particles equilibrated with aqueous silica for 1.5 h, percentage arsenate removals were high. In contrast, if silica was preequilibrated with iron for 50 days, arsenate removals decreased markedly at higher pH and aqueous silica concentrations. Similar trends were observed for humic substances, although their removal was nearly completely prevented at pH 8.5 at SiO2 concentrations above 50 and 10 mg/L at 1.5 h and 50 days exposure, respectively. The mechanism of interference was hindered sorption to the iron hydroxide surface.     

Removal of arsenic by bead cellulose loaded with iron oxyhydroxide from groundwater

A new adsorbent, bead cellulose loaded with iron oxyhydroxide (BCF), was prepared and applied for the adsorption and removal of arsenate and arsenite from aqueous systems. The continuing loading process of Fe in the cellulose beads was realized through hydrolization of ferric salts when alkaline solution was added dropwise. Spherical BCF had excellent mechanical and hydraulic properties. Akaganeite (beta-FeOOH), the reactive center of BCF that was stably loaded into the cellulose, had a high sensitivity to arsenite as well as arsenate. The maximum content of Fe in BCF reached 50% (w/w). In this study we investigated the adsorption behavior of arsenite and arsenate on BCF, including adsorption isotherms, adsorption kinetics, the influence of pH and competing anions on adsorption, and column experiments. The adsorption data accorded with both Freundlich and Langmuir isotherms. The adsorption capacity for arsenite and arsenate was 99.6 and 33.2 mg/g BCF at pH 7.0 with an Fe content of 220 mg/ mL. Kinetic data fitted well to the pseudo-second-order reaction model. Arsenate elimination was favored at acidic pH, whereas the adsorption of arsenite by BCF was found to be effective in a wide pH range of 5-11. Under the experimental conditions, the addition of sulfate had no effect on arsenic adsorption, whereas phosphate greatly influenced the elimination of both arsenite and aresenate. Silicate moderately decreased the adsorption of arsenite, but not arsenate. Both batch experiments and column experiments indicated that BCF had higher removal efficiency for arsenite than for arsenate. While the influent contaminant concentration was 500 microg/L in groundwater and the empty-bed contact time (EBCT) for arsenite and arsenate was 4.2 and 5.9 min, breakthrough empty-bed volumes at the WHO provisional guideline value of 10 microg/L were 2200 and 5000, respectively. BCF can be effectively regenerated when elution is done with 2 M NaOH solution. The column experiments for four cycles showed that stable and high removal efficiency of arsenic was sustained by BCF after regeneration.     

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.     

Hydrous ferric oxide incorporated diatomite for remediation of arsenic contaminated groundwater

Two reactive media [zerovalent iron (ZVI, Fisher Fe0) and amorphous hydrous ferric oxide (HFO)-incorporated porous, naturally occurring aluminum silicate diatomite [designated as Fe (25%)-diatomite]], were tested for batch kinetic, pH-controlled differential column batch reactors (DCBRs), in small- and large-scale column tests (about 50 and 900 mL of bed volume) with groundwater from a hazardous waste site containing high concentrations of arsenic (both organic and inorganic species), as well as other toxic or carcinogenic volatile and semivolatile organic compounds (VOC/SVOCs). Granular activated carbon (GAC) was also included as a reactive media since a permeable reactive barrier (PRB) at the subject site would need to address the hazardous VOC/SVOC contamination as well as arsenic. The groundwater contained an extremely high arsenic concentration (341 mg L(-1)) and the results of ion chromatography and inductively coupled plasma mass spectrometry (IC-ICP-MS) analysis showed that the dominant arsenic species were arsenite (45.1%) and monomethyl arsenic acid (MMAA, 22.7%), while dimethyl arsenic acid (DMAA) and arsenate were only 2.4 and 1.3%, respectively. Based on these proportions of arsenic species and the initial As-to-Fe molar ratio (0.15 molAs mole(-1)), batch kinetic tests revealed that the sorption density (0.076 molAs molFe(-1)) for Fe (25%)-diatomite seems to be less than the expected value (0.086 molAs molFe(-1) calculated from the sorption density data reported by Lafferty and Loeppert (Environ. Sci. Technol. 2005, 39, 2120-2127), implying that natural organic matters (NOMs) might play a significant role in reducing arsenic removal efficiency. The results of pH-controlled DCBR tests using different synthetic species of arsenic solution showed that the humic acid inhibited the MMAA removal of Fe (25%)-diatomite more than arsenite. The mixed system of GAC and Fe (25%)-diatomite increased the arsenic sorption speed to more than that of either individual media alone. This increase might be deduced by the fact that the addition of GAC could enhance arsenic removal performance of Fe (25%)-diatomite through removing comparably high portions of NOMs. Small- and large-scale column studies demonstrated that the empty bed contact time (EBCT) significantly affected sorpton capacities at breakthrough (C = 0.5 C0) forthe Fe0/sand (50/50, w/w) mixture, but notfor GAC preloaded Fe (25%)-diatomite. In the large-scale column tests with actual groundwater conditions, the GAC preloaded Fe (25%)-diatomite effectively reduced arsenic to below 50 microg L(-1) for 44 days; additionally, most species of VOC/SVOCs were also simultaneously attenuated to levels below detection.     

Coprecipitation of arsenate with metal oxides: nature, mineralogy, and reactivity of aluminum precipitates

Arsenic mobilization in soils is mainly controlled by sorption/desorption processes, but arsenic also may be coprecipitated with aluminum and/or iron in natural environments. Although coprecipitation of arsenic with aluminum and iron oxides is an effective treatment process for arsenic removal from drinking water, the nature and reactivity of aluminum- or iron-arsenic coprecipitates has received little attention. We studied the mineralogy, chemical composition, and surface properties of aluminum-arsenate coprecipitates, as well as the sorption of phosphate on and the loss of arsenate from these precipitates. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0, or 10.0 and As/Al molar ratio (R) of 0, 0.01, or 0.1 and were aged 30 or 210 d at 50 degrees C. In the absence of arsenate, gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate, very poorly crystalline precipitates formed. Short-range ordered materials (mainly poorly crystalline boehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0, and 10.0 (R= 0.1) and did not transform into Al(OH)3 polymorphs even after prolonged aging. The surface properties and chemical composition of the aluminum precipitates were affected by the initial pH, R, and aging. Chemical dissolution of the samples by 6 mol L(-1) HCl and 0.2 mol L(-1) oxalic acid/ oxalate solution indicated that arsenate was present mainly in the short-range ordered precipitates. The sorption of phosphate onto the precipitates was influenced by the nature of the samples and the amounts of arsenate present in the precipitates. Large amounts of phosphate partially replaced arsenate only from the samples formed at R = 0.1. The quantities of arsenate desorbed from these coprecipitates by phosphate increased with increasing phosphate concentration, reaction time, and precipitate age butwere always lessthan 30% of the amounts of arsenate present in the materials and were particularly low (<4%) from the sample prepared at pH 4.0. Arsenate appeared to be occluded within the network of short-range ordered materials and/or sorbed onto the external surfaces of the precipitates, but sorption on the external surfaces seemed to increase by increasing pH of sample preparation and aging. Furthermore, at pH 4.0 more than in neutral or alkaline systems the formation of aluminum arsenate precipitates seemed to be favored. Finally, we have observed that greater amounts of phosphate were sorbed on an aluminum-arsenate coprecipitate than on a preformed aluminum oxide equilibrated with arsenate under the same conditions (R = 0.1, pH 7.0). In contrast, the opposite occurred for arsenate desorption, which was attributed to the larger amounts of arsenate occluded in the coprecipitate.