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Title: 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

Author
item JIANG, TAO - Southwest University
item SKYLLBERG, ULF - Swedish University Of Agricultural Sciences
item WEI, SHIQIANG - Southwest University
item WANG, DINGYONG - Southwest University
item LU, SONG - Southwest University
item JIANG, ZHENMAO - Southwest University
item Flanagan, Dennis

Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2015
Publication Date: 1/28/2015
Citation: Jiang, T., Skyllberg, U., Wei, S., Wang, D., Lu, S., Jiang, Z., Flanagan, D.C. 2015. 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. Geochimica et Cosmochimica Acta. 154:151-167. DOI:10.1016/j.gca.2015.01.011.

Interpretive Summary: Mercury (Hg) is a particularly toxic and dangerous chemical that is naturally occurring in the environment. Chemically, mercury exists in several different oxidation states, and it can be transformed from an ionic form to an elemental form, and vice versa through various reactions. Most commonly, mercury occurs as ionic Hg(II) which can be reduced to Hg(I) and further reduced to elemental mercury Hg(0). In streams and lakes, natural light controls how rapidly mercury is reduced while in soils and sediments dark reduction occurs in the presence of different soil compounds. This study examined various factors controlling how rapidly mercury was reduced under dark conditions when mixed in liquid solutions containing soil humic acids (HA) that came from coal, peat, and organic soil. This was meant to mimic mercury contaminated soils and reactions occurring in the soil solution. The experimental equipment was able to measure the amount of elemental mercury produced in a reaction chamber, generated from an initial known amount of Hg2+ (added as mercury nitrate) in contact with a known amount of a humic acid, at a constant temperature, through varying amounts of time. We were also able to successfully model this chemical reduction reaction with equations that took into account various weak and strong components of the humic acids in solution. This research impacts other scientists studying the fate of mercury in the environment and expands knowledge in this area. Further, by knowing the composition of soil and/or sediment composition in areas where mercury pollution has occurred, this research can allow for prediction of what may happen to this dangerous chemical, and levels of remediation that may be necessary.

Technical Abstract: Redox transformations involving electron transfer from natural organic matter (NOM) are important for the mercury (Hg) biogeochemical cycle. In the water column light drives the reduction of Hg(II) to Hg(0), whereas in soils and sediments dark reduction of Hg(II) is of greater importance. The objective of this study was three-fold: 1) to compare the dark Hg(II) reduction rates among three different types of organic materials; HA extracted from coal, peat and organic soil, 2) to quantify the effect of strong and weak complexation of Hg(II) to HA functional groups on Hg(II) reduction, and 3) to determine if the concentration of electron donating groups could limit the reduction. The kinetics of Hg(II) reduction during a 53 hour experiment was modeled by two parallel pseudo-first order reactions, reflecting the weak and strong complexation of Hg(II) to O/N and RSH groups, respectively. The fraction of Hg(II) bonded to strong (74-78%) and weak (22-26%) sites were similar for the coal, peat and soil HA. Based on the kinetic model, the concentration of strong complexing groups (RSH) varied between 0.6 and 0.8×10-3 per C atoms in the three HAs. For coal and peat HA, these estimates were in agreement with sulfur X-ray absorption near-edge spectroscopy (S XANES) determinations, whereas XANES determined RSH concentrations were about half (0.3×10-3) for soil HA. Experiments conducted for 20 hours demonstrated a limitation of the Hg(II) reduction by the concentration of organic redox groups at a Hg(II)/DOC molar ratio exceeding 0.8×10-3. At this ratio more than 60% of Hg(II) was weakly bonded with two O/N ligands. Our experiments confirm previous suggestions that the complexation of Hg(II) by O/N and RSH sites associated to NOM controls the reduction of Hg(II). We also demonstrate that the concentration of redox active groups (presumably quinones) may limit the reaction in highly contaminated environments.