Influence of Organic Ligands on Chromium(VI) Reduction by Iron(II)

Abstract

Iron(II) is one of the most important reductants that transforms toxic chromium(VI) to essentially nontoxic chromium(III), but the effect of iron speciation on this redox reaction is not well-understood. We determined rate constants for Cr(VI) reduction by a series of Fe(II)−organic complexes, using UV−vis spectroscopy and kinetic fitting. The experiments with 1−20 μM Cr(VI), 1−60 μM Fe(II), and 5−1000 μM organic ligand at pH 4.0−5.5 can be described with the following rate law: −d[Cr(VI)]/dt = ∑LkL[Fe(II)L][Cr(VI)], where kL is pH-dependent. Fe(III)-stabilizing ligands such as bi- and multidentate carboxylates and phenolates generally accelerate the reaction, whereas Fe(II)-stabilizing ligands such as phenanthroline essentially stop the reaction. The rate coefficients increase with decreasing electron reduction potential of the Fe(III)L−Fe(II)L redox couples. The relationship of log kL versus EH°(Fe(III)L) is quite linear over 10 orders of magnitude. Dissolved organic matter extracted from the organic horizon of a forested spodosol shows qualitatively the same behavior as the investigated carboxylates. The presence of organic ligands leads to soluble Cr(III) and Fe(III) complexes. These results are important in DOC-rich soils and natural waters with respect to Cr(VI) reduction rates, the mobility of the products, and the reoxidation probability of newly reduced Cr(III), e.g., by naturally occurring manganese oxides.