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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 (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

Published in

Environmental Science and Technology
2018, volume: 52, number: 15, pages: 8292-8301
Publisher: AMER CHEMICAL SOC

SLU Authors

UKÄ Subject classification

Environmental Sciences

Publication identifier

  • DOI: https://doi.org/10.1021/acs.est.8b00919

Permanent link to this page (URI)

https://res.slu.se/id/publ/96562