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Thermodynamics of Hg(II) bonding to thiol groups in Suwanee River natural organic matter resolved by competitive ligand exchange, Hg LIII-edge EXAFS and 1H NMR spectroscopy

DOI: 10.1021/acs.est.8b00919 DOI Help

Authors: Yu Song (Swedish University of Agricultural Sciences) , Tao Jiang (Swedish University of Agricultural Sciences) , Van Liem-nguyen (Umeå University) , Tobias Sparrman (Umeå University) , Erik Bjorn (Umeå University) , Ulf Skyllberg (Swedish University of Agricultural Sciences)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Environmental Science & Technology

State: Published (Approved)
Published: July 2018
Diamond Proposal Number(s): 9157

Abstract: 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 (Suwanee 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 LIII-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 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 1H NMR spectroscopy and the Hg(NOM-RS)2 structure was verified by Hg LIII-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., 2017),1 the constants for Hg(NOM-RS)2 and Hg(Cys)(NOM-RS) represent an inherently consistent thermodynamic data set which we recommend is used in studies where the chemical speciation of Hg(II) is determined in presence of NOM and LMM thiols.

Subject Areas: Chemistry, Environment

Instruments: I20-Scanning-X-ray spectroscopy (XAS/XES)

Other Facilities: Beijing Synchrotron Facility