ABSTRACT

Remediation of Chromium in the Vadose Zone

Chromium is a common metal found at contaminated sites, where it is normally found in the +III and +VI oxidation states. Cr(III) is less toxic and less mobile than Cr(VI) so it results in less environmental risk. Typically, groundwaters become contaminated when chromium that has been released at the surface passes through a vadose zone and reaches the saturated zone. Immobilization of chromium in the vadose zone by conversion to Cr(III) would provide an effective and low cost method of protecting groundwater. The existence of an effective gaseous reductant (sulfur dioxide) provides the opportunity for developing such a low cost remediation technology. Initial experiments have demonstrated the effectiveness of sulfur dioxide for this purpose. However, data is needed to define the kinetics and stoichiometry of the reduction reactions so that remedial technologies can be developed.

The goal of this project is to develop fundamental information on the reduction of Cr(VI) in soils by sulfur dioxide that can be used to develop remedial technologies. To achieve this goal three specific objectives will be pursued: 1) Develop experimental and analytical procedures; 2) Characterize the kinetics and stoichiometry of redox reactions among Cr(VI), SO₂ and O₂ in the presence of different soil types; 3) Apply characterization data to develop a model for gas transport and reaction in the soil system; 4) Test the simulation model in soil columns.

An experimental plan will be carried out with each task related to one of the specific objectives. Laboratory experiments will be conducted in four stages to characterize the kinetics and stoichiometry of the redox reactions (Task 2). First, the kinetics and stoichiometry of the redox reaction in aqueous solution will be investigated. Results of these experiments will be used to interpret results in soil systems. Second, the reduction reactions will be investigated in batch soil-gas reactors to determine effects of water content, and SO₂ concentration on kinetics and stoichiometry of Cr(VI) reduction. Results of these experiments will be used to develop a kinetic model to describe changes of Cr(VI) concentration in soils. Third, the ability of soils to be reduced by SO₂ and then to reduce Cr(VI) in the absence of SO₂ will be investigated in batch soil-gas reactor systems. Results of these experiments will be used to predict the residual ability of soils to retain reducing ability after dissipation of SO₂. Fourth, the oxidation of SO₂ by O₂ will be investigated is soil-gas reactors. Results of these experiments will be used to predict the fate of SO₂ after reaction with Cr(VI) is complete. A one-dimensional transport/reaction model will be developed using data obtained in Task 2 for use in predicting effectiveness of a vadose zone remedial technology applied to conditions at different sites (Task 3). The ability of the model to predict SO₂ and Cr(VI) concentrations in column experiments will be determined in the final part of the experimental plan (Task 4).