Reduction of hexavalent chromium by compost-derived dissolved organic matter

Compost-derived dissolved organic matter (DOM_C) is a heterogeneous assemblage of different redox-active organic molecules. We hypothesize that DOM_C can interact with Cr(VI) and reduce it to Cr(III), thereby influencing the dynamics of Cr(VI) in soil and aquatic environments. Here, DOM_C, along with soil humic substances isolated from red soil and black soil, were fractionated into humic acid fractions (i.e., HA_C, HA_B, and HA_R) and fulvic acid fractions (i.e., FA_C, FA_B, and FA_R), respectively. The reduction and interaction between Cr(VI) and the six organic matter fractions were investigated. The results showed that the total Cr(VI) reduction capacity (TRC) of the six organic matter fractions was 26.77-49.34 μM Cr(VI) per mg OM. The TRC of HA fractions was 35.54-49.34 μM Cr(VI) per mg OM, which exceeded that of FA fractions (26.77-31.29 μM Cr(VI) per mg OM). DOM_C had a HA/FA ratio of 0.64, which was higher than that of black soil humic substance (0.59) and red soil humic substance (0.20). The sum of the TRC of DOM_C was 35.57 μM Cr(VI) per mg OM, which was larger than that of black soil humic substance (32.87 μM Cr(VI) per mg OM) and red soil humic substance (33.01 μM Cr(VI) per mg OM). The TRC was positively correlated with TOC, TN, phenol C, alkyl C, and aromatic C contents and negatively correlated with E₂/E₃, O-alkyl C, and carboxyl C contents. The reduction of Cr(VI) at pH 6 was negligible, whereas 32-67% Cr(VI) was reduced at pH 2. The Cr(VI) reduction capacities (RC2, RC2, and RC6) at pH 2-6 were positively correlated (R² > 0.71) with phenol C. Spectral analysis showed that there was no obvious complexation between Cr(VI) and the six organic matter fractions at pH 6, and thus the reduction of Cr(VI) was negligible, but solution pH could affect the accessibility of organic molecules to Cr(VI) and thus influence Cr(VI) reduction.