Arsenic removal with iron(II) and iron(III) in waters with high silicate and phosphate concentrations

Arsenic removal by passive treatment, in which naturally present Fe(II) is oxidized by aeration and the forming iron(III) (hydr)oxides precipitate with adsorbed arsenic, is the simplest conceivable water treatment option. However, competing anions and low iron concentrations often require additional iron. Application of Fe(II) instead of the usually applied Fe(III) is shown to be advantageous, as oxidation of Fe(II) by dissolved oxygen causes partial oxidation of As(III) and iron(III) (hydr)oxides formed from Fe(II) have higher sorption capacities. In simulated groundwater (8.2 mM HCO3(-), 2.5 mM Ca2+, 1.6 mM Mg2+, 30 mg/L Si, 3 mg/L P, 500 ppb As(III), or As(V), pH 7.0 +/- 0.1), addition of Fe(II) clearly leads to better As removal than Fe(III). Multiple additions of Fe(II) further improved the removal of As(II). A competitive coprecipitation model that considers As(III) oxidation explains the observed results and allows the estimation of arsenic removal under different conditions. Lowering 500 microg/L As(III) to below 50 microg/L As(tot) in filtered water required > 80 mg/L Fe(III), 50-55 mg/L Fe(II) in one single addition, and 20-25 mg/L in multiple additions. With As(V), 10-12 mg/L Fe(II) and 15-18 mg/L Fe(III) was required. In the absence of Si and P, removal efficiencies for Fe(II) and Fe(III) were similar: 30-40 mg/L was required for As(II), and 2.0-2.5 mg/L was required for As(V). In a field study with 22 tubewells in Bangladesh, passive treatment efficiently removed phosphate, but iron contents were generally too low for efficient arsenic removal.     

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.     

Evaluation of mixed valent iron oxides as reactive adsorbents for arsenic removal

The objective of this research was to determine if Fe(II)-bearing iron oxides generate ferric hydroxides at sufficient rates for removing low levels of arsenic in packed-bed reactors, while at the same time avoiding excessive oxide production that contributes to bed clogging in oxygenated waters. Column experiments were performed to determine the effectiveness of three media for arsenic removal over a range in empty bed contact times, influent arsenic concentrations, dissolved oxygen (DO) levels, and solution pH values. Corrosion rates of the media as a function of the water composition were determined using batch and electrochemical methods. Rates of arsenic removal were first order in the As(V) concentration and were greater for media with higher corrosion rates. As(V) removal increased with increasing DO levels primarily due to faster oxidation of the Fe2+ released by media corrosion. To obtain measurable amounts of arsenic removal in 15 mM NaCl electrolyte solutions containing 50 microg/L As(V), the rate of Fe2+ released by the media needed to be at least 15 times greater than the As(V) feed rate into the column. In waters containing 30 mg/L of silica and 50 microg/L of As(V), measurable amounts of arsenic removal were obtained only for Fe2+ release rates that were at least 200 times greater than the As(V) feed rate. Although all columns showed losses in hydraulic conductivity overthe course of 90 days of operation, the conductivity values remained high, and the losses could be reversed by backwashing the media. The reaction products produced by the media in domestic tap water had average As-to-Fe ratios that were approximately 25% higher than those for a commercially available adsorbent.     

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.     

Bacterial formation of tooeleite and mixed arsenic(III) or arsenic(V)-iron(III) gels in the Carnoulès acid mine drainage,France. A XANES,XRD, and SEM study

The oxidation of Fe(II) in acid mine drainage (AMD) leads to the precipitation of Fe(III) compounds which may incorporate toxic elements, such as arsenic (As), within their structure or adsorb them at their surface, thus limiting their mobility. The present work provides evidence for spatial and seasonal variations of microbial activity that influence arsenite oxidation and As immobilization in the heavily contaminated AMD from the Carnoulès mine, Gard, France ([As III] = 80 to 280 mg x L(-1) in the acidic spring draining the waste-pile). In the first tens of meters of the AMD, the rapid oxidation of Fe(II) leads to the coprecipitation of large amounts of As with Fe(III) in bacterial mats. XRD, XANES, and SEM analyses of sediments and stromatolite samples revealed the unusual formation of As(III)-rich compounds, especially nanocrystalline tooeleite, Fe6(AsO3)4(SO4)(OH)4 x 4H2O, a rare ferric arsenite sulfate oxy-hydroxide mineral, together with XRD-amorphous mixed As(III)/As(V)-Fe(III) oxy-hydroxide compounds. In the wet season, the suspended sediments of the upstream zone essentially consist of tooeleite associated with am-As(III)-Fe(III) oxy-hydroxides, while am-As(V)-Fe(III) oxy-hydroxides, having As:Fe molar ratios as high as 0.6-0.8, dominate in the dry season. Comparing natural and bioassay samples revealed that the formation of As(III)-rich compounds in the wet season may be related to the metabolic activity of bacterial strains able to oxidize Fe(II) but not As(III). One of these strains, having an Acidithiobacillus ferrooxidans genotype, has been isolated from the Carnoulès AMD. In contrast, the formation of As(V)-rich compounds in the dry season can be related to both biotic and abiotic oxidation of As(III) to As(V). Some Thiomonas strains isolated from the Carnoulès AMD were shown to be able to catalyze the oxidation of As(III) to As(V) in solution. Therefore, they can promote the formation of mixed As(V)-Fe(III) oxy-hydroxides, provided enough Fe(II) oxidizes. These results yield a better understanding of natural processes at this site and may help in designing efficient As-removal processes.     

Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters

An estimated 30-50 million people in Bangladesh consume groundwater with arsenic contents far above accepted limits. A better understanding of arsenic redox kinetics and simple water treatment procedures are urgently needed. We have studied thermal and photochemical As(III) oxidation in the laboratory, on a time scale of hours, in water containing 500 micrograms/L As(III), 0.06-5 mg/L Fe(II,III), and 4-6 mM bicarbonate at pH 6.5-8.0. As(V) was measured colorimetrically, and As(III) and As(tot) were measured by As(III)/As(tot)-specific hydride-generation AAS. Dissolved oxygen and micromolar hydrogen peroxide did not oxidize As(III) on a time scale of hours. As(III) was partly oxidized in the dark by addition of Fe(II) to aerated water, presumably by reactive intermediates formed in the reduction of oxygen by Fe(II). In solutions containing 0.06-5 mg/L Fe(II,III), over 90% of As(III) could be oxidized photochemically within 2-3 h by illumination with 90 W/m2 UV-A light. Citrate, by forming Fe(III) citrate complexes that are photolyzed with high quantum yields, strongly accelerated As(III) oxidation. The photoproduct of citrate (3-oxoglutaric acid) induced rapid flocculation and precipitation of Fe(III). In laboratory tests, 80-90% of total arsenic was removed after addition of 50 microM citrate or 100-200 microL (4-8 drops) of lemon juice/L, illumination for 2-3 h, and precipitation. The same procedure was able to remove 45-78% of total arsenic in first field trials in Bangladesh.     

Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation

We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.     

pH dependence of Fenton reagent generation and As(III) oxidation and removal by corrosion of zero valent iron in aerated water

Corrosion of zerovalent iron (ZVI) in oxygen-containing water produces reactive intermediates that can oxidize various organic and inorganic compounds. We investigated the kinetics and mechanism of Fenton reagent generation and As(III) oxidation and removal by ZVI (0.1m2/g) from pH 3-11 in aerated water. Observed half-lives for the oxidation of initially 500 microg/L As(III) by 150 mg Fe(0)/L were 26-80 min at pH 3-9. At pH 11, no As(III) oxidation was observed during the first two hours. Dissolved Fe(III) reached 325, 140, and 6 microM at pH 3, 5, and 7. H2O2 concentrations peaked within 10 min at 1.2, 0.4, and < 0.1 microM at pH 3, 5, and 7, and then decreased to undetectable levels. Addition of 2,2'-bipyridine (1-3 mM), prevented Fe(II) oxidation by O2 and H2O2 and inhibited As(III)oxidation. 2-propanol (14 mM), scavenging OH-radicals, quenched the As(III) oxidation at pH 3, but had almost no effect at pH 5 and 7. Experimental data and kinetic modeling suggest that As(III) was oxidized mainly in solution by the Fenton reaction and removed by sorption on newly formed hydrous ferric oxides. OH-radials are the main oxidant for As(III) at low pH, whereas a more selective oxidant oxidizes As(III) at circumneutral pH.     

Electrochemical study of arsenate and water reduction on iron media used for arsenic removal from potable water

Zerovalent iron filings have been proposed as a filter medium for removing As(III) and As(V) compounds from potable water. The removal mechanism involves complex formation of arsenite and arsenate with the iron surface and with iron oxides produced from iron corrosion. There is conflicting evidence in the literature on whether As(V) can be reduced to As(III) by iron filter media. This research uses electrochemical methods to investigate the redox reactions that occur on the surface of zerovalent iron in arsenic solutions. The effect of arsenic on the corrosion rate of zerovalent iron was investigated by analysis of Tafel diagrams for iron wire electrodes in anaerobic solutions with As(V) concentrations between 100 and 20,000 microg/L. As(V) reduction in the absence of surface oxides was investigated by analysis of chronoamperometry profiles for iron wire electrodes in solutions with As(V) concentrations ranging from 10000 to 106 microg/L. The effect of pH on As(V) reduction was investigated by analyses of chronopotentiometry profiles for iron wire electrodes at pH values of 2, 6.5, and 11. For freely corroding iron, the presence of As(III) and As(V) decreased the iron corrosion rate by a factor of 5 as compared to that in a 3 mM CaSO4 blank electrolyte solution. The decrease in corrosion rate was independent of the arsenic concentration and was due to the blocking of cathodic sites for water reduction by arsenic compounds chemisorbed to the iron surface. The chronoamperometry and chronopotentiometry experiments showed that elevated pH and increased As(III) to As(V) ratios near the iron surface decreased the thermodynamic favorability for As(V) reduction. Therefore, reduction of As(V) occurred only at potentials that were significantly below the apparent equilibrium potentials based on bulk solution pH values and As(III) to As(V) ratios. The potentials required to reduce more than 1% of the As(V) to As(III) were below those that are obtainable in freely corroding iron media. This indicates that there will be minimal or no reduction of As(V) in iron media filters under conditions relevant to potable water treatment.     

Understanding soluble arsenate removal kinetics by zerovalent iron media

Zerovalent iron filings have been proposed as a filter medium for removing arsenic compounds from potable water supplies. This research investigated the kinetics of arsenate removal from aqueous solutions by zerovalent iron media. Batch experiments were performed to determine the effect of the iron corrosion rate on the rate of As(V) removal. Tafel analyses were used to determine the effect of the As(V) concentration on the rate of iron corrosion in anaerobic solutions. As(V) removal in column reactors packed with iron filings was measured over a 1-year period of continuous operation. Comparison of As(V) removal by freely corroding and cathodically protected iron showed that rates of arsenate removal were dependent on the continuous generation of iron oxide adsorption sites. In addition to adsorption site availability, rates of arsenate removal were also limited by mass transfer associated with As(V) diffusion through iron corrosion products. Steady-state removal rates in the column reactor were up to 10 times faster between the inlet-end and the first sampling port than between the first sampling port and the effluent-end of the column. Faster removal near the influent-end of the column was due to a faster rate of iron oxidation in that region. The presence of 100 microg/L As(V) decreased the iron corrosion rate by up to a factor of 5 compared to a blank electrolyte solution. However, increasing the As(V) concentration from 100 to 20,000 microg/L resulted in no further decrease in the iron corrosion rate. The kinetics of arsenate removal ranged between zeroth- and first-order with respect to the aqueous As(V) concentration. The apparent reaction order was dependent on the availability of adsorption sites and on the aqueous As(V) concentration. X-ray absorption spectroscopy analyses showed the presence of iron metal, magnetite (Fe3O4), an Fe(III) oxide phase, and possibly an Fe(II,III) hydroxide phase in the reacted iron filings. These mixed valent oxide phases are not passivating and permit sustained iron corrosion and continuous generation of new sites for As(V) adsorption.     

Removal of arsenic from groundwater by zerovalent iron and the role of sulfide

Several recent investigations have shown encouraging potential for the removal of arsenic (As) from groundwater by granular zerovalent iron (Fe0). In contrast to previous studies conducted, we have investigated the applicability of this method and the nature of As bonding under conditions with dissolved sulfide. Three column tests were performed over the period of 1 year using solutions with either As(V) or As(II) (2-200 mg/L) in the input solution. Arsenic outflow concentrations decreased from initially 30-100 microg/L to concentrations of below 1 microg/L with time. XANES (X-ray absorptions near edge structure) and EXAFS (expanded X-ray absorption fine structure) spectra indicated that As in the solid phase is not only directly coordinated with oxygen, as is the case in adsorbed or coprecipitated arsenite and arsenate. Samples with high sulfur content showed additional bonding, for which Fourier transformations of EXAFS data exhibited a peak between 2.2 and 2.4 A. This bonding most likely originated from the direct coordination of sulfur or iron with As, which was incorporated in iron sulfides orfrom adsorbed thioarsenites. The formation of this sulfide bonding supports the removal of As by Fe0 because sulfide production by microbial sulfate reduction is ubiquitous in permeable reactive barriers composed of Fe0.     

In situ remediation of arsenic in simulated groundwater using zerovalent iron: laboratory column tests on combined effects of phosphate and silicate

We performed three column tests to study the behavior of permeable reactive barrier (PRB) materials to remove arsenic under dynamic flow conditions in the absence as well as in the presence of added phosphate and silicate. The column consisted of a 10.3 cm depth of 50:50 (w:w, Peerless iron:sand) in the middle and a 10.3 cm depth of a sediment from Elizabeth City, NC, in both upper and lower portions of the 31-cm-long glass column (2.5 cm in diameter) with three side sampling ports. The flow velocity (upflow mode) was maintained at 4.3 m d(-1) during the 3-4-month experiments. As expected, dissolved As concentrations in different positions of the column generally followed the order: column influent > bottom port effluent > middle port effluent > top port effluent > column effluent. The steady-state As removal in the middle Peerless iron and sand mixture zone might be attributed to the continuous supply of corroded iron in the form of iron oxides and hydroxides that served as the sorbents for both As(V) and As(III). Consistent with previous batch study findings, dissolved phosphate (0.5 or 1 mg of P L(-1)) and silicate (10 or 20 mg of Si L(-1)) showed strong inhibition for As(V) and As(III) (1 mg of As(V) L(-1) + 1 mg of As(III) L(-1) in 7 mM NaCl + 0.86 mM CaSO4) removal by Peerless iron in the column tests. The presence of combined phosphate and silicate resulted in earlier breakthrough (C = 0.5C0) and earlier complete breakthrough of dissolved arsenic relative to absence of added phosphate and silicate in the bottom port effluent. Competition between As(V)/As(III) and phosphate/silicate forthe sorption sites on the corrosion products of Peerless iron seems to be the cause of the observations. This effect is especially important in the case of silicate for designing a PRB of zerovalent iron for field use because dissolved silicate is ubiquitous in terrestrial waters.     

Biogenic Fe(III) minerals lower the efficiency of iron-mineral-based commercial filter systems for arsenic removal

Millions of people worldwide are affected by As (arsenic) contaminated groundwater. Fe(III) (oxy)hydroxides sorb As efficiently and are therefore used in water purification filters. Commercial filters containing abiogenic Fe(III) (oxy)hydroxides (GEH) showed varying As removal, and it was unclear whether Fe(II)-oxidizing bacteria influenced filter efficiency. We found up to 10(7) Fe(II)-oxidizing bacteria/g dry-weight in GEH-filters and determined the performance of filter material in the presence and absence of Fe(II)-oxidizing bacteria. GEH-material sorbed 1.7 mmol As(V)/g Fe and was ~8 times more efficient than biogenic Fe(III) minerals that sorbed only 208.3 μmol As(V)/g Fe. This was also ~5 times more efficient than a 10:1-mixture of GEH-material and biogenic Fe(III) minerals that bound 322.6 μmol As(V)/g Fe. Coprecipitation of As(V) with biogenic Fe(III) minerals removed 343.0 μmol As(V)/g Fe, while As removal by coprecipitation with biogenic minerals in the presence of GEH-material was slightly less efficient as GEH-material only and yielded 1.5 mmol As(V)/g Fe. The present study thus suggests that the formation of biogenic Fe(III) minerals lowers rather than increases As removal efficiency of the filters probably due to the repulsion of the negatively charged arsenate by the negatively charged biogenic minerals. For this reason we recommend excluding microorganisms from filters (e.g., by activated carbon filters) to maintain their high As removal capacity.     

Arsenic adsorption by Fe(III)-loaded open-celled cellulose sponge. Thermodynamic and selectivity aspects

Nowadays there is a great concern on the study of new adsorbent materials for either the removal or fixation of arsenic species because of their high toxicity and the health problems associated to such substances. The present paper reports a basic study of the adsorption of arsenic inorganic species from aqueous solutions using an open-celled cellulose sponge with anion-exchange and chelating properties (Forager Sponge). Consequences of preloading the adsorbentwith Fe(III) to enhance the adsorption selectivity are discussed and compared with the nonloaded adsorbent properties. The interactions of arsenic species with the Fe(III)-loaded adsorbent are accurately determined to clarify the feasibility of an effective remediation of contaminated waters. Arsenate is effectively adsorbed by the nonloaded and the Fe(III)-loaded sponge in the pH range 2-9 (maximum at pH 7), whereas arsenite is only slightly adsorbed by the Fe(III)-loaded sponge in the pH range 5-10 (maximum at pH 9), being that the nonloaded sponge is unable to adsorb As(III). The maximum sorption capacities are 1.83 mmol As(V)/g (pH approximately 4.5) and 0.24 mmol As(lII)/g (pH approximately 9.0) for the Fe(III)-loaded adsorbent. This difference is explained in terms of the different acidic behavior of both arsenic species. The interaction of the arsenic species with the Fe(III) loaded in the sponge is satisfactorily modeled. A 1:1 Fe:As complex is found to be formed for both species. H2AsO4- and H3AsO3 are determined to be adsorbed on Fe(III) with a thermodynamic affinity defined by log K = 2.5 +/- 0.3 and log K = 0.53 +/- 0.07, respectively. As(V) is, thus, found to be more strongly adsorbed than As(III) on the Fe(III) loaded in the sponge. A significant enhancement on As(V) adsorption selectivity by loading Fe(III) in the sponge is observed, and the effectiveness of the Fe(III)-loaded sponge for the As(V) adsorption is demonstrated, even in the presence of high concentrations of interfering anions (chloride, nitrate, sulfate, and phosphate).     

Treatment of mine drainage using permeable reactive barrers: column experiments

Permeable reactive barriers designed to enhance bacterial sulfate reduction and metal sulfide precipitation have the potential to prevent acid mine drainage and the associated release of dissolved metals. Two column experiments were conducted using simulated mine-drainage water to assess the performance of organic carbon-based reactive mixtures under controlled groundwater flow conditions. The simulated mine drainage is typical of mine-drainage waterthat has undergone acid neutralization within aquifers. This water is near neutral in pH and contains elevated concentrations of Fe(II) and SO4. Minimum rates of SO4 removal averaged between 500 and 800 mmol d(-1) m(-3) over a 14-month period. Iron concentrations decreased from between 300 and 1200 mg/L in the influent to between <0.01 and 220 mg/L in the columns. Concentrations of Zn decreased from 0.6-1.2 mg/L in the input to between 0.01 and 0.15 mg/L in the effluent, and Ni concentrations decreased from between 0.8 and 12.8 mg/L to <0.01 mg/L. The pH increased slightly from typical input values of 5.5-6.0 to effluent values of 6.5-7.0. Alkalinity, generally <50 mg/L (as CaCO3) in the influent, increased to between 300 and 1,300 mg/L (as CaCO3) in the effluent from the columns. As a result of decreased Fe(II) concentrations and increased alkalinity, the acid-generating potential of the simulated mine-drainage water was removed, and a net acid-consuming potential was observed in the effluent water.     

Arsenate and arsenite removal by zerovalent iron: kinetics, redox transformation, and implications for in situ groundwater remediation

Batch tests were performed utilizing four zerovalent iron (Fe0) filings (Fisher, Peerless, Master Builders, and Aldrich) to remove As(V) and As(III) from water. One gram of metal was reacted headspace-free at 23 degrees C for up to 5 days in the dark with 41.5 mL of 2 mg L(-1) As(V), or As(III) or As(V) + As(III) (1:1) in 0.01 M NaCl. Arsenic removal on a mass basis followed the order: Fisher > Peerless Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L(-1) in 4 days with the exception of Aldrich Fe0. More As(III) was sorbed than As(V) by Peerless Fe0 in the initial As concentration range between 2 and 100 mg L(-1). No As(III) was detected by X-ray photoelectron spectroscopy (XPS) on Peerless Fe0 at 5 days when As(V) was the initial arsenic species in the solution. As(III) was detected by XPS at 30 and 60 days present on Peerless Fe0, when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe0 when As(II) was added to the solution. A steady distribution of As(V) (73-76%) and As(III) (22-25%) was achieved at 30 and 60 days on the Peerless Fe0 when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe0 and Fe2+) and an oxidizing species (MnO2) in Peerless Fe0 is probably responsible for the coexistence of both As(V) and As(III) on Fe0 surfaces. The desorption of As(V) and As(III) by phosphate extraction decreased as the residence time of interaction between the sorbents and arsenic increased from 1 to 60 days. The results suggest that both As(V) and As(III) formed stronger surface complexes or migrated further inside the interior of the sorbent with increasing time.     

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.     

Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction

The oxidation kinetics of As(III) with natural and technical oxidants is still notwell understood, despite its importance in understanding the behavior of arsenic in the environment and in arsenic removal procedures. We have studied the oxidation of 6.6 microM As(II) by dissolved oxygen and hydrogen peroxide in the presence of Fe(II,III) at pH 3.5-7.5, on a time scale of hours. As(III) was not measurably oxidized by O2, 20-100 microM H2O2, dissolved Fe(III), or iron(III) (hydr)-oxides as single oxidants, respectively. In contrast, As(III) was partially or completely oxidized in parallel to the oxidation of 20-90 microM Fe(II) by oxygen and by 20 microM H2O2 in aerated solutions. Addition of 2-propanol as an *OH-radical scavenger quenched the As(III) oxidation at low pH but had little effect at neutral pH. High bicarbonate concentrations (100 mM) lead to increased oxidation of As-(III). On the basis of these results, a reaction scheme is proposed in which H2O2 and Fe(II) form *OH radicals at low pH but a different oxidant, possibly an Fe(IV) species, at higher pH. With bicarbonate present, carbonate radicals might also be produced. The oxidant formed at neutral pH oxidizes As(III) and Fe(II) but does not react competitively with 2-propanol. Kinetic modeling of all data simultaneously explains the results quantitatively and provides estimates for reaction rate constants. The observation that As(III) is oxidized in parallel to the oxidation of Fe(II) by O2 and by H2O2 and that the As(III) oxidation is not inhibited by *OH-radical scavengers at neutral pH is significant for the understanding of arsenic redox reactions in the environment and in arsenic removal processes as well as for the understanding of Fenton reactions in general.     

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.     

Arsenic sequestration by ferric iron plaque on cattail roots

Typha latifolia (cattail) sequesters arsenic within predominantlyferric iron root coatings, thus decreasing mobility of this toxic element in wetland sediments. Element-specific XRF microtomographic imaging illustrated a high spatial correlation between iron and arsenic in root plaques, with little arsenic in the interior of the roots. XANES analyses demonstrated that the plaque was predominantly ferric iron and contained approximately 20% As(III) and 80% As(V), which is significant because the two oxidation states form species that differ in toxicity and mobility. For the first time, spatial distribution maps of As oxidation states were developed, indicating that As(III) and As(V) are both fairly heterogeneous throughoutthe plaque. Chemical extractions showed that As was strongly adsorbed in the plaque rather than coprecipitated. Iron and arsenic concentrations ranged from 0.03 to 0.8 g Fe g(-1) wet plaque and 30 to 1200 microg As g(-1) wet plaque, consistent with a mechanism of As adsorption onto Fe(III) oxyhydroxide plaque. Because this mechanism decreases the concentrations of both As(III) and As(V) in groundwater, we propose that disruption of vegetation could increase the concentrations of mobile arsenic.