Removal of Methylated Arsenic in Groundwater with Iron Filings ZHONGQI CHENG,* ALEXANDER VAN GEEN, AND ROSELINE LOUIS Lamont Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964 NIKOLAOS NIKOLAIDIS Department of Environmental Engineering, Technical University of Crete, Chania, Greece RICHARD BAILEY Sevensons Environmental Services, Vineland, New Jersey 08360 Field and laboratory experiments were conducted to study the potential use of Fe filings to remove monomethyl arsenate (MMA) and dimethyl arsenate (DMA) from contaminated waters. Batch experiments show that the affinity of MMA for Fe filings is comparable to that of inorganic arsenate, but lower than that for arsenite. In contrast, very little DMA was retained by Fe filings or their corrosion products. The wide range of affinities of different As species for Fe filings was confirmed by a series of retention and selective-leaching experiments using small columns and As-spiked groundwater. The effectiveness of Fe filings was also demonstrated with a field deployment at a U.S. Superfund site where groundwater is highly contaminated with both organic and inorganic As species. Over the course of 4 months, a 3 L cartridge of Fe filings removed >85% of As contained in 16000 L of groundwater containing 1-1.5 mg/L total dissolved As, ∼30% of which was organic As. The results indicate that Fe filings could be used to treat groundwater contaminated with MMA, which is the main organic form of As at most contaminated sites. Indications of partial demethylation in the column of Fe filings suggest conditions might be optimized further to enhance the removal of DMA. Introduction Methylated As species, monomethyl arsenate (MMA) and dimethyl arsenate (DMA), occur as important pollutants in groundwater at sites with a history of pesticide manufacturing and improper disposal. A query of the United States Superfund Site Database (http://www.epa.gov/superfund/ sites/npl/basic.htm, accessed April 10, 2005) showed that about 42% of 1236 sites that are currently on the National Priority List contain As, with at least 107 sites reporting MMA contamination. Although DMA is not included in the database, this species is often produced along with MMA and therefore probably present at many of the sites as well. At the Vineland Superfund site in New Jersey, for example, high levels of both inorganic and organic forms of As are present in the dissolved and the solid phases. Total As concentrations reach thousands of parts per million in contaminated soils and are as high as tens of milligrams per liter in groundwater extracted from wells tapping the main contaminant plume (1). An elaborate groundwater pump and treat system that processes 2 million gallons of con- taminated groundwater per day relies on the oxidation of organic As with hydrogen peroxide, followed by coagulation with ferric chloride, polymer flotation, and subsequent sand filtration (1). The process is effective in the sense that concentrations of As in the effluent discharged to a nearby stream are typically <30 μg/L. Complete oxidation of organic As species is notoriously difficult and costly to achieve, however, even under UV radiation (1-3). The current operation to remediate the situation is expected to last several decades at a projected cost of ∼$100 000 000. The present study explores an alternative method to treat either ground- water or sediment washings contaminated with both inor- ganic and organic As species that could potentially reduce the cost of mitigation. Effective removal of inorganic As(V) and As(III) from groundwater by zerovalent Fe has been demonstrated for practical use with flow-through cartridge systems as well as permeable reactive barrier systems (4-7). The mechanism of As removal by zerovalent Fe is not fully understood but is generally believed to involve adsorption onto iron(II) and/ or iron(III) oxyhydroxides that are continuously generated by corrosion of the Fe filings (4, 8-13). Whereas oxidation of As(III) on corroded Fe surfaces has been observed, reduction of As(V) has not (9, 11). Electrochemical and spectroscopic studies have confirmed strong binding of both inorganic As(V) and As(III) to the corroded surfaces of Fe filings (10, 13). Since the molecular structure of MMA and DMA is similar to that of arsenate (Supporting Information, Table S.1), these organic species could therefore conceivably also be removed by Fe filings. Laboratory experiments have indeed shown that MMA and DMA adsorbs onto freshly precipitated iron(III) oxyhydroxides as well as alumina at pH < 7(14, 15). In the present study, the removal of MMA and DMA by Fe filings is compared to the removal of inorganic As(V) and As(III) under similar conditions. The removal of As species by Fe filings and the dependence on groundwater pH and phosphate concentrations are first quantified in a set of parallel batch experiments. Small flow-through columns of Fe filings were then used to evaluate the removal efficiency of As species in different groundwater matrices. Columns loaded with different As species were also subjected to three- step sequential leaching while the As levels in the effluent were monitored to study the various retention mechanisms. Finally, the performance of a cartridge system containing Fe filings and sand was evaluated for a mixture of inorganic and organic As species contained in contaminated groundwater from the Vineland Chemical Superfund site. Materials and Methods Materials. Iron filings were used as received from Connelly- GPM, Inc. of Chicago, IL (stock no. CC-1004, ∼12-18 mesh). Silica sand was no. 4 Q-Rok mined by U.S. Silica in Berkley Springs, WV (16-20 mesh). Small 20 mL columns used for bench-scale studies and the larger 3 L cartridges deployed in the field were packed with a 1:1 (by volume) mixture of Fe filings and sand. The resulting porosity of the columns was ∼35%. The small columns were made with the barrel of 20 mL polypropylene syringes (9.5 cm length, 1.8 cm i.d.), * Corresponding author phone: (845) 365-8649; fax: (845) 365- 8154; e-mail: czhongqi@ldeo.columbia.edu. Environ. Sci. Technol. 2005, 39, 7662-7666 7662 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 19, 2005 10.1021/es050429w CCC: $30.25  2005 American Chemical Society Published on Web 08/27/2005