EFFECT OF ORGANIC COMPLEXING LIGANDS ON CR(III) OXIDATION BY MnOX

Abstract

Although Cr(III) is considered a Cr species with low mobility and toxicity, the potential oxidation of Cr(III) to Cr(VI) in soils containing Mn requires further scientific attention because of the acute toxicity of Cr(VI). This study evaluates the inhibition or enhancement of Cr(III) oxidation by Mn oxide in the presence of various organic ligands and concentrations. Batch experiments indicate that 200 μM of organic ligands inhibited 20 μM Cr(III) oxidation by Mn oxide at pH 4 because of the occurrence of reductive dissolution of Mn oxide. Although oxalate did not show strong inhibition of Cr(III) oxidation at pH 10, nevertheless, citrate and N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) ligands did retard Cr(III) oxidation. It is possible that the co-precipitation of Cr(III) and organic ligand, which has a low dissociation constant, results in low release of Cr(III) and subsequent oxidation. The current results imply that the management of Cr(III)-containing waste is very important to prevent potential oxidation of Cr(III) by Mn oxides, particularly at low pH. However, the presence of organic ligands in the field may lead to low Cr(III) oxidation where Cr(III) waste is disposed.