Behavior of Chromium in Soils. VI. Interactions Between Oxidation‐Reduction and Organic Complexation

Abstract

Initial oxidation rates of different forms of Cr(III) added to an Aquic Udorthent soil, and the maximum Cr(VI) levels observed, decreased in the order: freshly‐precipitated Cr(OH)₃ > Cr‐citrate > aged Cr(OH)₃ in citrate > aged Cr(OH)₃. The oxidation of Cr(III) in tannery sewage sludge and in tannery effluent added to a Typic Eutrochrept soil also was studied as a function of time and loading rate. Amounts of Cr(VI) formed increased with Cr(III) added from 0.025 to 6.2 mmol/100 g soil (29 to 7,200 kg/ha) for sewage sludge Cr(III), and from 0.1 to 1.8 mmol/100 g soil (103 to 2,100 kg/ha) for the tannery effluent Cr(III). Levels of water‐soluble Cr(VI) increased for a month in most of the treatments, despite the presence of reducing agents in the soil and in the wastes. Adding citric acid to the soil along with the tannery wastes increased the amount of Cr(VI) formed at each treatment level, and decreased the Cr(III) loading rate required for observation of oxidation. Mutual facilitation of Cr(III) oxidation by chelation and of further chelation by oxidation was postulated as a possible mechanism for the continuous formation of Cr(VI) in a soil.The reduction of soluble Cr(VI) in unlimed (pH 5.3) and limed (pH 6.5) samples of an Ultic Hapludalf resulted in the formation of soluble organic complexes of Cr(III) if citric acid was added with the Cr(VI). During a 9‐d incubation period, more reduction of Cr(VI) occurred in unlimed than in limed samples, but higher levels of soluble Cr(III)‐chelate were measured in the limed treatments. Increasing the ratio of citrate to Cr(VI) increased the rate of reduction of the Cr(VI) in the Aquic Udorthent. Soluble Cr(III)‐chelate formed, and the length of time the Cr(III) remained soluble also increased with the citrate/Cr ratio.