Chromate Reduction and Retention Processes within Arid Subsurface Environments MATTHEW GINDER-VOGEL, † THOMAS BORCH, †,§ MELANIE A. MAYES, ‡ PHILLIP M. JARDINE, ‡ AND SCOTT FENDORF* ,† Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Chromate is a widespread contaminant that has deleterious impacts on human health, the mobility and toxicity of which are diminished by reduction to Cr(III). While biological and chemical reduction reactions of Cr(VI) are well resolved, reduction within natural sediments, particularly of arid environments, remains poorly described. Here, we examine chromate reduction within arid sediments from the Hanford, WA site, where Fe(III) (hydr)oxide and carbonate coatings limit mineral reactivity. Chromium(VI) reduction by Hanford sediments is negligible unless pretreated with acid; acidic pretreatment of packed mineral beds having a Cr(VI) feed solution results in Cr(III) associating with the minerals antigorite and lizardite in addition to magnetite and Fe(II)-bearing clay minerals. Highly alkaline conditions (pH > 14), representative of conditions near high-level nuclear waste tanks, result in Fe(II) dissolution and concurrent Cr(VI) reduction. Additionally, Cr(III) and Cr(VI) are found associated with portlandite, suggesting a secondary mechanism for chromium retention at high pH. Thus, mineral reactivity is limited within this arid environment and appreciable reduction of Cr(VI) is restricted to highly alkaline conditions resulting near leaking radioactive waste disposal tanks. Introduction Chromium enters the environment primarily through its widespread use in industrial applications, including tanning, metallurgy, and plating. Once introduced, it persists as either Cr(VI) or Cr(III). Hexavalent Cr exists in groundwater systems as the oxyanion HxCrO4 x-2 (chromate), which exhibits high water solubility and is a mutagen, teratogen, and carcinogen (1). In contrast, Cr(III) is relatively nontoxic and strongly partitions to the solid phase (2). Chromium(VI) can be reduced to Cr(III) by aqueous and sorbed Fe(II) (3, 4), organic matter (5), Fe(II)-bearing minerals (6-8), sulfide compounds (9, 10), and through microbial processes (11). These reductants all ultimately lead to Cr- (III), but the rates and products differ. Enzymatic reduction of chromate may result in the formation of soluble organic Cr(III) complexes (12-14), while chromate reduction by soluble Fe(II) results in the formation of Cr1-xFex(OH)3 precipitates (2). Subsurface chromate contamination is a major environ- mental threat at the U.S. Department of Energy’s Hanford Site (southeastern Washington State, USA), the location of plutonium production starting during World War II. The climate of the Hanford Site is arid, with an average rainfall of 15.9 cm y -1 , resulting in a 10-60 m deep vadose zone (16). In the 100 Area of the Hanford Site, chromate used as a corrosion inhibitor in nuclear reactor cooling water was discharged to unlined surface cribs, resulting in chromate contamination reaching the Columbia River (16), which is not only a primary source of drinking water, but also a spawning-ground for salmon. Additionally, thousands of liters of hot (100 °C), caustic (pH > 14), chromate-containing high- level nuclear waste (HLW) from the reduction-oxidation (REDOX) plutonium recovery process has leaked into the vadose zone, as a result of multiple failures of single-shell tanks in the S-SX tank farm (17). Should chromate from these tanks reach the water table, rapid migration to the Columbia River would result. Analysis of chromium’s redox state in a core obtained from a HLW plume beneath tank SX 108 revealed that 29-75% of the total Cr had been reduced to Cr(III) (18). In aerobic, arid environments, with limited organic carbon (such as the Hanford Site), chromate reductants are es- sentially restricted to Fe(II)-bearing mineral phases of geologic origin. The uppermost principal geologic unit of the Hanford Site consists of material deposited in cataclysmic ice age floods during the past 12 000-700 000 y, termed the Hanford formation. Potential sources of Fe(II) in the Hanford sediments include iron-bearing silicates such as lizardite [MgxFe3-xSi2O5(OH)4] (this work), antigorite [(Mg,Fe)3Si2O5- (OH)4] (this work), biotite (19), magnetite, and ilmenite (19). Chromate reduction by Fe(II)-bearing minerals is controlled by their surface reactivity. Mineral surface coatings such as carbonates (20), silicates, and Fe(III) (hydr)oxides (6, 15) inhibit electron transfer from underlying Fe(II) to aqueous Cr(VI), all of which commonly form in the vadose zone of arid environments. In the Hanford subsurface, chromium contamination has occurred in two distinct geochemical environments, termed the “near-field” and “far-field” environments in this Article. The near-field environment is adjacent to leaking tanks of HLW and is characterized by high pH, salt concentration, and temperature, resulting in extensive mineral dissolution and surface modification. The far-field environment is representative of the background geochemical conditions (circumneutral pH, ambient temperature, etc.). These con- ditions are relevant at locations far from a leaking tank and in arid environments in general. Chromate reduction by Fe(II) (aq), magnetite, ilmenite, and biotite has been studied extensively in model systems at acidic and neutral pH (6, 9, 21, 22), and in the case of magnetite (23) and Fe(II) (aq) (24), at high pH as well. What remains elusive are the minerals serving as chromate reductants and the factors limiting (or promoting) chromate reduction within arid soils and sediments. In the present work, we examine the reduction of chromate by sediment obtained from the Hanford formation beneath the Interim Disposal Facility (IDF) at the Hanford Site. The objectives of this study are to determine what geochemical conditions are required for chromate reduction by Hanford formation sediments and which minerals are a source of chromate reduction. In addition to providing information about chromate reduction specific to the Hanford Site, these * Corresponding author phone: (650)723-5238; fax: (650)723-2199; e-mail: fendorf@stanford.edu. † Stanford University. ‡ Oak Ridge National Laboratory. § Present address: Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523. Environ. Sci. Technol. 2005, 39, 7833-7839 10.1021/es050535y CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7833 Published on Web 09/17/2005