Interfacial Reduction - Oxidation Mechanisms Governing Fate and Transport of Contaminants in the Vadose Zone

Many soil contamination sites at Department of Energy installations contain radionuclides and toxic metals such as technetium (Tc), uranium (U) and chromium (Cr). In Situ Gaseous Reduction (ISGR) using dilute hydrogen sulfide (H2S) as reductant is a technology uniquely suitable for the vadose zone soil remediation to reduce and immobilize these contaminants. It is conceivable that the ISGR approach can be applied either to immobilize pre-existing contaminants or to create a reductive permeable reactive barrier (PRB) through hydrogen sulfide gas treatment of soils for contaminant interception. This project aims to improve our understanding of the complex interactions among the contaminants (U and Tc), H2S, and various soil constituents. The collaborative effort involving the University of Missouri-Columbia, Pacific Northwest National Laboratory, and Illinois Institute of Technology will provide the knowledge needed to further develop and optimize the ISGR technology. Specific research tasks include: (a) examine the reduction kinetics of Tc(VII) and U(VI) by H2S; (b) measure the reduction kinetics of Tc(VII) and U(VI) by iron sulfides; (c) characterize the speciation of immobilized Tc and U and investigate the immobilization mechanisms; (d) assess the long-term stability of the contaminants immobilized by the ISGR treatment; and (e) validate the pure phase experimental results under natural soil conditions