Reduction of Cr(VI) by Soil Humic Fractions: Elucidation of Reaction Mechanisms by Spectroscopic Characterization

Abstract

As a heavy metal element with great harm and heavy pollution in soil pollution, chromium (Cr) has posed a great threat to the soil environment and human living environment. Chromium mainly exists in the form of Cr(III) and Cr(VI) in the soil. It is of great significance to reduce Cr(VI) with strong toxicity and easy migration to Cr(III) with low toxicity and low migration. There are a large number of active groups in soil humus, which have a strong affinity with heavy metal ions and play an important role in the reduction of chromium. Humic substances are classified into three categories; however, fewer studies have been carried out simultaneously to investigate the functional groups involved in the reactions of humic acid (HA), fulvic acid (FA), and humic matter (HM) with Cr. In the present study, HA, FA, and HM were used to interact with Cr to investigate the mechanism of humus reduction of Cr(VI). The results showed the reducing ability of humic substances: HA > FA > HM. The functional groups that play a reducing role in humic substances are mainly carboxyl groups, ester groups, phenolic hydroxyl groups, and polysaccharides. The C=O and C—O bonds play a major role in the reduction of humus, but not all C=O and C—O bonds have changed during the reaction. The adsorption–reduction process was mainly carried out on the surface of humic substances, and Cr(III) was mainly adsorbed on humic substances with trace amounts of Cr(VI).