Thermodynamics of Hg(ll) Bonding to Thiol Groups in Suwannee River Natural Organic Matter Resolved by Competitive Ligand Exchange, Hg L_(III)-Edge EXAFS and ~1H NMR Spectroscopy

摘要

A molecular level understanding of the thermodynamics and kinetics of the chemical bonding between mercury, Hg(II), and natural organic matter (NOM) associated thiol functional groups (NOM-RSH) is required if bioavailability and transformation processes of Hg in the environment are to be fully understood. This study provides the thermodynamic stability of the Hg(NOM-RS)(2) structure using a robust method in which cysteine (Cys) served as a competing ligand to NOM (Suwannee River 2R101N sample) associated RSH groups. The concentration of the latter was quantified to be 7.5 +/- 0.4 mu mol g(-1) NOM by Hg L-III-edge EXAFS spectroscopy. The Hg(Cys)(2) molecule concentration in chemical equilibrium with the Hg(II)-NOM complexes was directly determined by HPLC-ICPMS and losses of free Cys due to secondary reactions with NOM was accounted for in experiments using H-1 NMR spectroscopy and C-13 isotope labeled Cys. The log K +/- SD for the formation of the Hg(NOM-RS)(2) molecular structure, Hg2+ + 2NOM-RS- = Hg(NOM-RS)(2), and for the Hg(Cys)(NOM-RS) mixed complex, Hg2+ + Cys(-) + NOM-RS- = Hg(Cys)(NOM-RS), were determined to be 40.0 +/- 0.2 and 38.5 +/- 0.2, respectively, at pH 3.0. The magnitude of these constants was further confirmed by H-1 NMR spectroscopy and the Hg(NOM-RS)(2) structure was verified by Hg L-III-edge EXAFS spectroscopy. An important finding is that the thermodynamic stabilities of the complexes Hg(NOM-RS)(2), Hg(Cys)(NOM-RS) and Hg(Cys)(2) are very similar in magnitude at pH values 7, when all thiol groups are protonated. Together with data on 15 low molecular mass (LMM) thiols, as determined by the same method (Liem-Ngyuen et al. Thermodynamic stability of mercury(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory. Environ. Chem. 2017, 14, (4), 243-253.), the constants for Hg(NOM-RS)(2) and Hg(Cys)(NOM-RS) represent an internally consistent thermodynamic data set that we recommend is used in studies where the chemical speciation of Hg(II) is determined in the presence of NOM and LMM thiols.