Tuesday, November 6, 2007
186-2
Coprecipitation of Arsenate and Arsenite by Carbonate/Sulfate Green Rusts.
Chunming Su, USEPA, National Risk Management Research Lab, 919 Kerr Research Drive, Ada, OK 74820 and Charity D. Holder, East Central University, Department of Biology, Ada, OK 74820.
Green rust minerals are iron corrosion products identified in permeable reactive barriers of zerovalent iron that is being used to intercept and degrade groundwater contaminants in the subsurface. Green rusts also occur naturally in soils and sediments where mild reducing conditions persist. The formation of green rusts may be important in the fate and transformation of arsenic in groundwater. The objectives of this study were to evaluate the extent and nature of arsenic coprecipitation with green rusts and to examine the influence of arsenic incorporation on the mineralogy of formed solid phases. Stoichiometric green rusts were obtained by coprecipitation of ferrous and ferric ions in both carbonate and sulfate-containing solutions spiked with arsenic (ranging from 0.0005 to 0.1 M arsenate or arsenite) and without arsenic. X-ray diffraction was used to identify the solid phases. Solution phase arsenic speciation was determined using IC-ICP-MS. Both pH and redox potential were important factors in the formation of green rusts. The batch tests showed that As removal ranged from 90 -100% by carbonate green rust with As(V) and As(III). Partial oxidation of initially added As(III) to As(V) occurred. The oxidation of As(III) is advantageous because As(V) is less toxic and less mobile than As(III) in the environment. Sulfate green rust formed at 0.05 M As(III) at a pH of 6.34. Other iron oxides also removed both As(V) and As(III) efficiently. This study has important implications for engineered systems and natural attenuation approaches to remediate arsenic-laden groundwater.