Stability of Arsenic and Selenium Immobilized by in Situ Microbial Reduction

A promising in situ remediation alternative for metal oxyanions such as arsenic, chromium, and selenium is microbial reduction to insoluble oxide, sulfide, or elemental phases. An important question that must be addressed before this technology can be implemented is the stability of these phases. This paper describes an investigation of the stability of reduced arsenic and sulfide precipitates produced by Desulfovibrio desulfuricans. The precipitates were first leached by the toxicity characteristic leaching procedure (TCLP), which demonstrated that the metal concentrations in the leachate are below the standards established by RCRA. A series of long-term leaching tests showed moderate release of the immobilized metals over a period of approximately 100 pore volumes, followed by very low leachability for at least 200 additional pore volumes. These results suggest that, although the metals may be slightly leachable, in an in situ immobilization process, their concentration would be below federal drinking water standards. INTRODUCTION Current alternatives for remediating contamination of soils and groundwater by metals, metalloids, and radionuclides are limited and thus present a major challenge to environmental managers. There is much current interest in in situ stabilization technologies involving the use of microbial systems to immobilize contaminants, which are present as anions under oxidizing conditions as these are often the most subject to transport through soil and groundwater. These contaminants include metals such as Cr, Mo, and V; the metalloids As and Se; and many radionuclides including Tc, U, Pu, and Am. Much of the work on biological immobilization strategies has focused on the use of microbially mediated reduction in which anaerobic organisms reduce the metals to insoluble precipitates including oxides (e.g. Cr2O3, UO2), sulfides (e.g. FeS, MnS, FeAsS, AsS2, MoS2), and possibly elemental forms (Se). These metals are expected to remain insoluble as long as reducing conditions are maintained in the subsurface formation. However, to date no investigation has considered the stability of these phases over long time periods (i.e., decades to centuries) as geochemical conditions in the formation and groundwater change. Microbial reduction of metals has been demonstrated for a wide variety of metals and metalloids. Important reviews on this topic have been provided by Lovley and Coates (1997), Chen and Hao (1998), and Nealson and Stahl (1997). Reduction has been demonstrated both for sulfate-reducing bacteria (SRB) (Lovley, 1993a, 1994; El Bayoumy et al., 1999; Tucker et al., 1998) and for dissimilatory metal-reducing bacteria (DMRB) (Frederickson et al., 2000; Lovley 1993a, 1995).