Interactions of soil components and their effects on speciation of chromium in soils

Abstract

For evaluation of real risks of soil contaminated with chromium it is important to identify and understand the reactions of Cr species with main soils components. In this research reactions of Cr(III) and Cr(VI) with Fe(OH) 3, MnO 2, CaCO 3, kaolinite and natural organic matter (NOM) were examined in batch experiments. Since the batches were compositions of soil component mixtures the experimental design was carried out by applying Simplex algorithm and the results of the experimental domain were interpreted by partial least squares (PLS). The research confirmed the significance of studying all the soil components simultaneously instead of studying only the effects of one component at a time. The ability of pure components to change the speciation of Cr(III) and Cr(VI) in liquid phase was different to that in mixtures, which indicated that the soil components interacted with each other. The order of pure components to convert Cr(III) by adsorption, precipitation or oxidation was Fe(OH) 3=kaolinite=CaCO 3>NOM>MnO 2, while in mixtures the order was: Fe(OH) 3>CaCO 3>kaolinite>MnO 2>NOM. This means that in mixtures with the highest amount of Fe(OH) 3 the conversion of Cr(III) was highest. In mixtures with the highest amount of NOM the changes in speciation of dissolved Cr(III) were the lowest. The order for the pure components to change the speciation of Cr(VI) was: Fe(OH) 3>NOM>kaolinite>CaCO 3>MnO 2 and in mixtures the order was: NOM>Fe(OH) 3>kaolinite>MnO 2>CaCO 3. The reactions between different soil components also affected the oxidation/reduction ability of soils. As separate component MnO 2 oxidized Cr(III) to Cr(VI). However, in mixtures with NOM or Fe(OH) 3 the oxidation was hindered. The effect of pH on speciation of Cr in mixtures was also examined and is discussed.