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 ¹H 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 μmol 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 ¹H NMR spectroscopy and ¹³C isotope labeled Cys. The log <i>K</i> ± 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 ¹H 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