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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
Umeå University, Faculty of Science and Technology, Department of Chemistry.
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2018 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 52, no 15, p. 8292-8301Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018. Vol. 52, no 15, p. 8292-8301
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-151396DOI: 10.1021/acs.est.8b00919ISI: 000441477600027PubMedID: 29983050OAI: oai:DiVA.org:umu-151396DiVA, id: diva2:1245372
Funder
Swedish Research Council, 2013-6978Swedish Research Council, 621-2014-5370The Kempe Foundations, JCK-1501The Kempe Foundations, SMK-2745The Kempe Foundations, SMK-1243Available from: 2018-09-05 Created: 2018-09-05 Last updated: 2018-09-05Bibliographically approved

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Sparrman, TobiasBjörn, Erik

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