Modeling of the structure-specific kinetics of abiotic, dark reduction of Hg(II) complexed by O/N and S functional groups in humic acids while accounting for time-dependent structural rearrangement

Abstract

Dark reduction of Hg(II) to Hg(0) in deep waters, soils and sediments accounts for a large part of legacy Hg recycling back to the atmosphere. Natural organic matter (NOM) plays a dual role in the process, acting as an electron donor and complexation agent of Hg(II). Experimental determination of rates of dark Hg(II) reduction is complicated by the simultaneously ongoing kinetics of Hg(II) rearrangement from the abundant, relatively weakly bonding RO/N (carboxyl, amino) groups in NOM to the much stronger bonding RSH (thiol) group. In this study, kinetics of the rearrangement are accounted for and we report rates of dark Hg(II) reduction for two molecular structures in presence of humic acids (HA) extracted from three different sources. Values on the pseudo first-order rate constant for the proposed structure Hg(OR)2 (kred Hg(OR)2) were 0.18, 0.22 and 0.35h−1 for Peat, Coal and Soil HA, respectively, and values on the constant for the proposed structure RSHgOR (kred RSHgOR) were 0.003 and 0.006h−1 for Peat and Soil HA, respectively. The Hg(SR)2 structure is the thermodynamically most stable, but the limited time of the experiment (53h) did not allow for a determination of the rate of the very slow reduction of Hg(II) in this structure. For two out of three HA samples the concentration of RSH groups optimized by the kinetic model (0.6×10−3 RSH groups per C atoms) was in good agreement with independent estimates provided by sulfur X-ray absorption near-edge spectroscopy (S XANES). Experiments conducted at varying concentrations of Hg(II) and HA demonstrated a positive relationship between Hg(II) reduction and concentrations of specific Hg(II) structures and electron donor groups, suggesting first order in each of these two components. The limitation of the Hg(II) reduction by electron donating groups of HA, as represented by the native reducing capacity (NRC), was demonstrated for the Coal HA sample. Normalization to NRC resulted in pseudo second-order rate constants (kred Hg(OR)2/NRC=0.012h−1, kred RSHgOR/NRC=1.9−2.1×10−4h−1) that were very similar for Soil and Peat HA.