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Determination of mercury chemical speciation in the presence of low molecular mass thiols and its importance for mercury methylation
Umeå University, Faculty of Science and Technology, Department of Chemistry. (Erik Björn)
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Methylmercury (MeHg) is a neurotoxic compound that threatens the well-being of humans and wildlife. It is formed through the methylation of inorganic mercury (HgII) under suboxic/anoxic conditions in soils, sediment and waters. The chemical speciation of HgII, including specific HgII species in aqueous and solid/adsorbed phases, plays a key role in MeHg formation. Chemical forms of HgII which have been reported to be available for uptake in methylating bacteria include neutral HgII–sulfide complexes, HgII complexes with specific low molecular mass (LMM) thiols, and nanoparticulate HgS(s). Accurate determination of the chemical speciation of HgII is thus crucial when elucidating the mechanism of MeHg formation. The concentration of HgII–LMM thiols complexes is predicted to be extremely low (sub fM range). Current analytical methods do not allow direct quantification of HgII complexes due to the very low concentration of these complexes, and therefore determination rely on thermodynamic modeling. Accurate stability constants for HgII–LMM thiols complexes and quantification of LMM thiol ligands in environments are thus required to precisely determine the concentration of such complexes.

In this thesis, a novel analytical method was developed based on online pre-concentration coupled with liquid chromatography tandem mass spectrometry to determine the concentration of 16 LMM thiols (Paper I). This method was successful in detecting 8 LMM thiols in boreal wetland porewaters, with mercaptoacetic acid and cysteine being the most abundant. The total concentration of individual detected LMM thiols ranged from sub nM (LOD=0.1 nM) to 77 nM. Moreover, the stability constant (β2) for HgII complexes with 15 LMM thiols were directly determined for the first time by competing ligand exchange experiments combined with liquid chromatography ICPMS analysis (Paper II). Values of log β2 for the reaction Hg2+ + 2LMM-RS- = Hg(LMM-RS)2 ranged from 34.6 for. Based on the determined constants of Hg(LMM-RS)2 complexes and state-of-the-art constants from literature for other HgII complexes, we established comprehensive thermodynamic speciation models for MeHg and HgII in boreal wetlands (Paper III). The speciation of HgII was coupled with the HgII methylation rate constant (km) determined with different enriched Hg isotope tracers (Paper IV). There was a good correlation (R2=0.88) between the km determined by a HgII(aq) tracer added as Hg(NO3)2 with high bioavailability and a tracer where HgII was bond to thiol groups in natural organic matter (HgII-NOM(ads)) and has a lower bioavailability. The HgII(aq) tracer was consistently methylated at 5 times higher rate than the HgII-NOM(ads) tracer. A good correlation was observed between the concentration of biologically produced LMM thiols and km in the boreal wetlands. In a mesocosm study of estuarine sediment-brackish water systems, increased concentration of phytoplankton chlorophyll α due to macro nutrient additions led to an increase in HgII methylation rate of the HgII(aq) but not of the HgII-NOM(ads) tracer or ambient HgII species (Paper V). Furthermore, simulated newly deposited HgII species from atmospheric and terrestrial sources were exhibited significantly higher HgII methylation rates when compared with simulated aged sediment HgII pools. Through the development and adoption of novel analytical methods, this thesis reveals the significance of LMM thiols in Hg biogeochemistry by precise determination of HgII–LMM thiol complexes in natural environmental systems.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2016. , 45 p.
Keyword [en]
Mercury, methylmercury, LMM thiol, mass spectrometry, biogeochemistry, soil, sediment, ICP, WinSGW, modeling, S XANES, ligand exchange, stability constant, boreal wetlands
National Category
Analytical Chemistry
Research subject
Analytical Chemistry; biology, Environmental Science
Identifiers
URN: urn:nbn:se:umu:diva-119938ISBN: 978-91-7601-469-1 (print)OAI: oai:DiVA.org:umu-119938DiVA: diva2:925704
Public defence
2016-06-08, KBC-huset, KB3A9, Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2016-05-18 Created: 2016-05-03 Last updated: 2016-12-21Bibliographically approved
List of papers
1. Determination of Sub-Nanomolar Levels of Low Molecular Mass Thiols in Natural Waters by Liquid Chromatography Tandem Mass Spectrometry after Derivatization with p‑(Hydroxymercuri) Benzoate and Online Preconcentration
Open this publication in new window or tab >>Determination of Sub-Nanomolar Levels of Low Molecular Mass Thiols in Natural Waters by Liquid Chromatography Tandem Mass Spectrometry after Derivatization with p‑(Hydroxymercuri) Benzoate and Online Preconcentration
2015 (English)In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 87, no 2, 1089-1096 p.Article in journal (Refereed) Published
Abstract [en]

Low molecular mass (LMM) thiols is a diverse group of compounds, which play several important roles in aquatic ecosystems, even though they typically occur at low concentrations. Comprehensive studies of LMM thiols in natural waters have so far been hampered by selectivity and limit of detection constraints of previous analytical methods. Here, we describe a selective and robust method for the quantification of 16 LMM thiols in natural waters. Thiols were derivatized with 4-(hydroxymercuri)benzoate (PHMB) and preconcentrated online by solid-phase extraction (SPE) before separation by liquid chromatography and determination by electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Their quantification was performed by selective reaction monitoring (SRM), while the presence of a product ion atm/z 355, specific for thiols and common for the investigated compounds, also allows to screen samples for unknown thiols by a precursor ion scan approach. The robustness of the method was validated for aqueous matrices with different pH, sulfide, and dissolved organic carbon (DOC) concentrations. The limits of detection for the thiols were in the sub-nanomolar range (0.06–0.5 nM) and the methodology allowed determination of both reduced and total thiol concentrations (using tris(2-carboxyethyl)phosphine (TCEP) as reducing agent). Six thiols (mercaptoacetic acid, cysteine, homocysteine, N-acetyl-cysteine, mercaptoethane-sulfonate, and glutathione) were detected with total concentrations of 7–153 nM in boreal lake or wetland pore waters while four thiols (mercaptoacetic acid, cysteine, homocysteine, and N-acetyl-cysteine) were detected in their reduced form at concentrations of 5–80 nM.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2015
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:umu:diva-100354 (URN)10.1021/ac503679y (DOI)000348332300037 ()
Available from: 2015-03-02 Created: 2015-03-02 Last updated: 2017-12-04Bibliographically approved
2. Stability constants for mercury (II) complexes with low molecular mass thiols as determined by competing ligand exchange and liquid chromatography inductively coupled plasma mass spectrometry
Open this publication in new window or tab >>Stability constants for mercury (II) complexes with low molecular mass thiols as determined by competing ligand exchange and liquid chromatography inductively coupled plasma mass spectrometry
(English)Manuscript (preprint) (Other academic)
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:umu:diva-129242 (URN)
Available from: 2016-12-21 Created: 2016-12-21 Last updated: 2016-12-21
3. Thermodynamic modelling of the chemical speciation of mercury and methylmercury under sulfidic conditions in boreal wetland soils
Open this publication in new window or tab >>Thermodynamic modelling of the chemical speciation of mercury and methylmercury under sulfidic conditions in boreal wetland soils
(English)Manuscript (preprint) (Other academic)
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:umu:diva-129243 (URN)
Available from: 2016-12-21 Created: 2016-12-21 Last updated: 2016-12-21
4. Mercury chemical speciation and biological production of low molecular mass thiols in control of methylmercury formation in boreal wetlands
Open this publication in new window or tab >>Mercury chemical speciation and biological production of low molecular mass thiols in control of methylmercury formation in boreal wetlands
(English)Manuscript (preprint) (Other academic)
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:umu:diva-129244 (URN)
Available from: 2016-12-21 Created: 2016-12-21 Last updated: 2016-12-21
5. Effects of Nutrient Loading and Mercury Chemical Speciation on the Formation and Degradation of Methylmercury in Estuarine Sediment
Open this publication in new window or tab >>Effects of Nutrient Loading and Mercury Chemical Speciation on the Formation and Degradation of Methylmercury in Estuarine Sediment
Show others...
2016 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 50, no 13, 6983-6990 p.Article in journal (Refereed) Published
Abstract [en]

Net formation of methylmercury (MeHg) in sediments is known to be affected by the availability of inorganic divalent mercury (HgII) and by the activities of HgII methylating and MeHg demethylating bacteria. Enhanced autochthonous organic matter deposition to the benthic zone, following increased loading of nutrients to the pelagic zone, has been suggested to increase the activity of HgII methylating bacteria and thus the rate of net methylation. However, the impact of increased nutrient loading on the biogeochemistry of mercury (Hg) is challenging to predict as different geochemical pools of Hg may respond differently to enhanced bacterial activities. Here, we investigate the combined effects of nutrient (N and P) supply to the pelagic zone and the chemical speciation of HgII and of MeHg on MeHg formation and degradation in a brackish sediment-water mesocosm model ecosystem. By use of Hg isotope tracers added in situ to the mesocosms or ex situ in incubation experiments, we show that the MeHg formation rate increased with nutrient loading only for HgII tracers with a high availability for methylation. Tracers with low availability did not respond significantly to nutrient loading. Thus, both microbial activity (stimulated indirectly through plankton biomass production by nutrient loading) and HgII chemical speciation were found to control the MeHg formation rate in marine sediments. 

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-125668 (URN)10.1021/acs.est.6b01567 (DOI)000379366300047 ()
Note

Special issue: Jerald L. Schnoor tribute issue

Available from: 2016-09-14 Created: 2016-09-14 Last updated: 2017-10-23Bibliographically approved

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