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

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.