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Unraveling the importance of solid and adsorbed phase mercury speciation for methylmercury formation, evasion and bioaccumulation
Umeå University, Faculty of Science and Technology, Department of Chemistry. (EcoChange)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Betydelsen av kvicksilvers speciation i fast och adsorberad fas för bildning, avgång och bioackumulering av metylkvicksilver (Swedish)
Abstract [en]

Monomethylmercury, MeHg, is formed under anoxic conditions in waters, sediments and soils and then bioaccumulated and biomagnified in aquatic food webs, negatively effecting both human and wildlife health. It is generally accepted that precipitation of mercury, Hg, and adsorption of Hg to e.g. organic matter and mineral surfaces are important processes limiting the reactivity of Hg mobilized in the environment by natural and anthropogenic activities. However, knowledge concerning the role of different solid and adsorbed chemical forms of Hg for MeHg formation, evasion and bioaccumulation is missing. Such information is vital for the understanding of environmental processes controlling MeHg formation and bioaccumulation, as well as for predicting how changes in e.g. loading rates of atmospheric Hg and the outcome of climate change scenarios and anthropogenic land use could alter Hg concentrations in biota.

In this thesis, a novel experimental approach, using isotopically enriched solid and adsorbed phases of inorganic Hg, HgII, as tracers, was developed. Using this approach, we successfully determined rates of MeHg formation from solid and adsorbed Hg species in sediment slurries and in mesocosm systems under conditions closely resembling those in field. We conclude that the solid/adsorbed phase speciation of HgII is a major controlling factor for MeHg net formation rates. Microcosm experiments revealed that newly formed MeHg was a major contributor to the evasion of MeHg from the water‒sediment system, emphasizing the importance of MeHg formation rate, rather than MeHg concentration, in the sediment for this process. From mesocosm systems, we provide experimental evidence, as well as quantitate data, for that terrestrial and atmospheric sources of HgII and MeHg are more available for methylation and bioaccumulation processes than HgII and MeHg stored and formed in sediments. This suggests that the contribution from terrestrial and atmospheric sources to the accumulation of Hg in fish may have been underestimated. As a consequence, in regions where climate change is expected to further increase land runoff, terrestrial MeHg sources may have even higher negative effects on biota than previously thought. Data and concepts presented in this thesis lay the basis for unprecedented in-depth modeling of processes in the Hg biogeochemical cycle that will improve our understanding and the predicting power on how aquatic ecosystems may respond to environmental changes or differences in loading rates for atmospheric Hg.

Abstract [sv]

Monometylkvicksilver, MeHg, bildas under anoxiska förhållanden i naturliga vatten, sediment och jordar och bioackumuleras och magnifieras därefter i den akvatiska näringskedjan med negativa effekter på djur och människor som följd. Det är generellt vedertaget att utfällning av Hg och adsorption av Hg till exempelvis organiskt material och mineralytor begränsar tillgängligheten för biogeokemiska reaktioner av Hg som mobiliserats i miljön via naturliga och antropogena processer. Kunskap om betydelsen av speciationen av Hg i fasta och adsorberade faser för bildning, avgång och bioackumulering av MeHg är dock bristfällig. Denna information är kritisk för att förstå vilka processer som kontrollerar bildning och bioackumulering av MeHg samt för att kunna prediktera hur olika ekosystem kan förväntas svara på exempelvis ändrad deposition av atmosfäriskt Hg eller hur klimatförändringar kan påverka koncentrationerna av Hg i fisk.

I denna avhandling har en experimentell metod utvecklades, där isotopanrikade fasta och adsorberade kemiska former av oorganiska tvåvärt Hg, Hg II används som s.k. "tracers". Denna metod användes för att bestämma MeHg bildningshasigheter i homogeniserade sediment prover samt i mesokosmsystem där förhållandena efterliknar de som förväntas i naturliga ekosystem. Från dessa drar vi slutsatsen att speciationen av HgII i fast/adsorberad fas är en viktig kontrollerande faktor som begränsar nettobildningen av MeHg. Mikrokosmexperiment visade att i första hand nyligt bildad MeHg avgick till gasfas vilket understryker betydelsen av MeHg bildningshastighet, snarare än koncentration, i sedimentet för denna process. Från mesokosmexperimenten visar vi, med kvantitativa data, att terrestra och atmosfäriska källor av HgII och MeHg är mer tillgängliga för bildning och bioackumulering av MeHg än HgII och MeHg lagrat eller bildat i sedimenten. Orsaken till detta är framförallt skillnad i speciationen av Hg i fasta/adsorberade faser. Detta innebär att bidraget från MeHg från terrestra och atmosfäriska källor till koncentrationen av Hg i fisk kan ha underskattats, samt att de negativa effekterna på MeHg exponering i områden där exempelvis klimat-förändringar förväntas leda till ökad terrest avrinning kan bli mer allvarliga än vad som tidigare predikterats. Data som presenteras i denna avhandling möjliggör modellering av Hg’s biogeokemiska cykel på en ny detaljnivå samt möjliggör säkrare prediktioner av hur olika ekosystem kan förväntas svara mot miljöförändringar eller ändrad deposition av atmosfäriskt Hg.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2013. , 57 p.
Keyword [en]
mercury, methylmercury, aquatic systems, estuary, Hg, MeHg, bioaccumulation, methylation, demethylation, evasion, sediment, water, terrestrial sources, atmospheric sources, climate change, eutrophication
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-64286ISBN: 978-91-7459-546-8 (print)OAI: oai:DiVA.org:umu-64286DiVA: diva2:600355
Public defence
2013-02-15, KBC-huset, hörsal KB3A9, Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2008-4363Knut and Alice Wallenberg Foundation, 94.160
Note

Till Finansiärer skall också följande läggas till: Kempe stiftelsen (SMK-2942, SMK-2745, JCK-2413).

Available from: 2013-01-25 Created: 2013-01-22 Last updated: 2017-09-01Bibliographically approved
List of papers
1. Substantial emission of gaseous monomethylmercury from contaminated water-sediment microcosms
Open this publication in new window or tab >>Substantial emission of gaseous monomethylmercury from contaminated water-sediment microcosms
2010 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 44, no 1, 278-283 p.Article in journal (Refereed) Published
Abstract [en]

Emission rates of gaseous monomethylmercury (CH(3)Hg(II)), as well as elemental mercury (Hg(0)) and dimethylmercury [(CH(3))(2)Hg(II)], were determined in Hg-contaminated water-sediment microcosms (duplicates of three treatments) by gaseous species-specific isotope dilution analysis (SSIDA). Incubation of approximately 500 g (wet mass) of sediments containing 30 mumol of ambient Hg with an addition of 2.6 mumol of (201)Hg(II) tracer resulted in average (n = 6) gaseous emissions of 84 +/- 26, 100 +/- 37, and 830 +/- 380 pmol of ambient CH(3)Hg(II), CH(3)(201)Hg(II), and (201)Hg(0), respectively, during 108 days of incubation. In contrast to Hg(0), a transient temporal pattern was observed for measured CH(3)Hg(II) emission rates, which peaked at day 12 and decreased to much lower levels by the end of the experiments. At day 12, CH(3)Hg(II) constituted 30-50% of the total emitted gaseous Hg, emphasizing the significance of this species to total Hg emissions from anoxic sediment-water systems. Emission rates of gaseous CH(3)Hg(II) did not reflect the accumulated CH(3)Hg(II) content in the sediment, suggesting that emissions mainly originated from newly methylated Hg(II). Speciation modeling of the pore water suggests that CH(3)Hg(II) was emitted as CH(3)HgSH(0)(g).

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2010
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-30649 (URN)10.1021/es9020348 (DOI)000273267000047 ()19950964 (PubMedID)
Available from: 2010-01-11 Created: 2010-01-11 Last updated: 2017-12-12Bibliographically approved
2. Mercury methylation rates for geochemically relevant HgII species in sediments
Open this publication in new window or tab >>Mercury methylation rates for geochemically relevant HgII species in sediments
Show others...
2012 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 46, no 21, 11653-11659 p.Article in journal (Refereed) Published
Abstract [en]

Monomethylmercury (MeHg) in fish from freshwater, estuarine and marine environments are a major global environmental issue. Mercury levels in biota are mainly controlled by the methylation of inorganic mercuric mercury (HgII) to MeHg in water, sediments and soils. There is, however, a knowledge gap concerning the mechanisms and rates of methylation of specific geochemical HgII species. Such information is crucial for a better understanding of variations in MeHg concentrations among ecosystems and, in particular, for predicting the outcome of currently proposed measures to mitigate mercury emissions and reduce MeHg concentrations in fish. To fill this knowledge gap we propose an experimental approach using HgII isotope tracers, with defined and geochemically important adsorbed and solid HgII forms in sediments, to study MeHg formation. We report HgII methylation rate constants, km, in estuarine sediments which span over two orders of magnitude depending on chemical form of added tracer: metacinnabar (β-201HgS(s)) < cinnabar (α-199HgS(s)) < HgII reacted with mackinawite (≡FeS-202HgII) < HgII bonded to natural organic matter (NOM-196HgII) < a typical aqueous tracer (198Hg(NO3)2(aq)). We conclude that a combination of thermodynamic and kinetic effects of HgII solid-phase dissolution and surface desorption control the HgII methylation rate in sediments and causes the large observed differences in km-values. The selection of relevant solid-phase and surface adsorbed HgII tracers will therefore be crucial to achieving biogeochemically accurate estimates of ambient HgII methylation rates.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2012
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-60708 (URN)10.1021/es3015327 (DOI)23017152 (PubMedID)
Available from: 2012-10-29 Created: 2012-10-23 Last updated: 2017-10-24Bibliographically approved
3. Differentiated reactivity of geochemical mercury pools control methylmercury levels in sediment and biota
Open this publication in new window or tab >>Differentiated reactivity of geochemical mercury pools control methylmercury levels in sediment and biota
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(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:umu:diva-64281 (URN)
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2013-01-30Bibliographically approved
4. Impact of nutrient and humic matter loadings on methylmercury formation and bioaccumulation in estuarine ecosystems
Open this publication in new window or tab >>Impact of nutrient and humic matter loadings on methylmercury formation and bioaccumulation in estuarine ecosystems
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:umu:diva-64285 (URN)
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2013-01-30Bibliographically approved

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