CHROMATE REDUCTION BY SUBSURFACE SOILS UNDER ACIDIC CONDITIONS

Experimental studies of Cr(VI) reduction by five subsoils (Holston-Cloudland, Kenoma, Cecil-Paculet, Watson-Alvira, and Ocala) were conducted to determine whether the Fe(II) contained in soil minerals could cause Cr(VI) to be reduced to Cr(III) and precipitated as hydroxide solids. All of the subsoils contained small amounts of Fe(II) and organic matter, but only the acidic subsoils (1:2 soil pH = 4.2-4.3) caused Cr(VI) reduction at their natural pH values. Rates of Cr(VI) reduction by four of the subsoils increased with a decrease in pH below 4.0. Consideration of available kinetic information, coincidental timing of Cr(VI) disappearance and Fe(II) appearance in solution, and observed reaction stoichiometries indicate that Cr(VI) reduction by Fe(II) species is of equivalent importance as reduction by organic material under acidic conditions. Chromium(VI) reduction in the acidic subsoils resulted in the precipitation of (Fe,Cr)(OH)3(am), depending on the solution pH. The amount of NH2OH.HCl-extractable Fe in the acidic subsoils was found to be useful for determining the mole fraction of Fe(III) in the (Fe,Cr)(OH)3(am) solid solution, allowing prediction of Cr(III) concentrations in acidic solutions to within a logarithmic unit of those observed in experimental soil-water suspensions.