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Instruments / Technical Area	Publication Details	Published	Actions
I20-Scanning-X-ray spectroscopy (XAS/XES)	Toward an internally consistent model for Hg(II) chemical speciation calculations in bacterium–natural organic matter–low molecular mass thiol systems Yu Song , Gbotemi A. Adediran , Tao Jiang , Shusaku Hayama , Erik Bjorn , Ulf Skyllberg Environmental Science & Technology DOI: 10.1021/acs.est.0c01751 Diamond Proposal Number(s): [9157] Open Access Show Abstract ▼ Abstract: To advance the scientific understanding of bacteria-driven mercury (Hg) transformation processes in natural environments, thermodynamics and kinetics of divalent mercury Hg(II) chemical speciation need to be understood. Based on Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopic information, combined with competitive ligand exchange (CLE) experiments, we determined Hg(II) structures and thermodynamic constants for Hg(II) complexes formed with thiol functional groups in bacterial cell-membranes of two extensively studied Hg(II) methylating bacteria: Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. The Hg EXAFS data suggest 5% of the total number of membrane thiol functionalities (Mem-RStot = 380 ± 50 µmol g−1 C) are situated closely enough to be involved in a 2-coordinated Hg(Mem-RS)2 structure in Geobacter. The remaining 95% of Mem-RSH are involved in mixed-ligation Hg(II)-complexes, either with low molecular mass (LMM) thiols like Cys, Hg(Cys)(Mem-RS) or with neighboring O/N membrane functionalities, Hg(Mem-RSRO). We report log K values for the formation of the structures Hg(Mem-RS)2, Hg(Cys)(Mem-RS), and Hg(Mem-RSRO) to be 39.1 ± 0.2 , 38.1 ± 0.1 and 25.6 ± 0.1, respectively, for Geobacter; and 39.2 ± 0.2 , 38.2 ± 0.1 and 25.7 ± 0.1, respectively for ND132. Combined with results obtained from previous studies using the same methodology to determine chemical speciation of Hg(II) in presence of natural organic matter (Suwannee River DOM) and 15 LMM thiols, an internally consistent thermodynamic data set is created, which we recommend to be used in studies of Hg transformation processes in Bacterium–NOM–LMM thiols systems.	Jun 2020
I20-Scanning-X-ray spectroscopy (XAS/XES)	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 Yu Song , Tao Jiang , Van Liem-Nguyen , Tobias Sparrman , Erik Bjorn , Ulf Skyllberg Environmental Science & Technology DOI: 10.1021/acs.est.8b00919 Diamond Proposal Number(s): [9157] Open Access Show Abstract ▼ 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.	Jul 2018

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