Behavior of Chromium in Soils: II. Hexavalent Forms

Abstract

Adsorption and reduction of added Cr(VI) were characterized in soils with contrasting pH's, organic matter contents, and chemical and mineralogical properties.Presence of soil organic matter brought about spontaneous reduction of Cr(VI) to Cr(III), even at pH's above neutrality. Reduction did not occur in soils very low in organic matter unless an energy source was added. Cow manure added to practically organic‐free Cecil B2 reduced Cr(IV) only after the pH had been lowered below 3 with HCl.The solubility of Cr(VI) in the presence of excess Al changed in a pattern remindful of orthophosphate. All of the soils, except a pH 7.8 Cca horizon material, adsorbed Cr(VI). Presence of orthophosphate prevented the adsorption of Cr(VI), presumably by competition for the adsorption sites. Consistent with this finding, KH₂PO₄ was found to be the best extracting agent for Cr(VI).It was concluded that behavior of Cr(VI), if it remains in soils, is similar to that of orthophosphate. However, unlike phosphate, Cr(VI) is quickly reduced by soil organic matter. Thus, Cr(VI) added to a soil will remain mobile only if its concentration exceeds both the adsorbing and the reducing capacities of the soil.