Reductive capacity of soils for chromium

Abstract

A standard batch procedure was developed to measure soil reductive capacity (SRC) based on the ability of soil to reduce Cr(VI). Parameters that were investigated and optimized included extractant (sulfate) concentration, reaction temperature. types of reductants, reductant contact time and Cr(VI) contact time. 0.1 M sulfate was used as the extracting solution as a measure of safety. Reaction temperature did not have a significant influence on the reduction kinetics of Cr(VI). Dithionite was selected as a standard reductant due to its efficiency in increasing SRC by reducing labile soil components in the soil itself. A time period of 2 days was optimum contact time to achieve full reduction and oxidation. Soil treated with Fe(II) and the combination of Fe(II) and dithionite showed a constant SRC regardless of the frequency of reduction/oxidation cycle, whereas the intrinsic reductive capacity and total reductive capacity of the soil treated with dithionite were exhausted within several cycles. Column tests were conducted using soil with measured intrinsic and total reductive capacities. Differences observed between Cr(VI) breakthrough and that predicted from batch studies are likely to have benn caused by slow Cr(VI) reduction kinetics and heterogeneity of the soil. The relative behavior of the column tests agreed with observations of the reactants in the batch experiments. The tests demonstrate that soils can be used to develop an in situ treatment process in which soils are reduced by the addition of reductants and groundwater contaminated with Cr(VI) is passed through the reduced soil zone.