Mercury transformations in resuspended contaminated sediment controlled by redox conditions, chemical speciation and sources of organic matter. (1st January 2018)

• Record Type: Journal Article
• Authors: Zhu, Wei; Song, Yu; Adediran, Gbotemi A.; Jiang, Tao; Reis, Ana T.; Pereira, Eduarda; Skyllberg, Ulf; Björn, Erik
• Is Part Of: Geochimica et cosmochimica acta. Volume 220(2018)
• Page Start: 158
• Page End: 179
• Publication Date: 2018-01-01
• Subjects: Mercury -- Chemical speciation -- Redox oscillation -- Organic matter -- Methylation -- Demethylation -- Reduction -- Sediment resuspension; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2017.09.045
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Mercury (Hg) contaminated sediments can be significant sources of Hg in aquatic ecosystems and, through re-emission processes, to the atmosphere. Transformation and release of Hg may be enhanced by various sediment perturbation processes, and controlling biogeochemical factors largely remain unclear. We investigated how rates of Hg transformations in pulp-fiber enriched sediment contaminated by Hg from chlor-alkali industry were controlled by (i) transient redox-changes in sulfur and iron chemistry, (ii) the chemical speciation and solubility of Hg, and (iii) the sources and characteristics of organic matter (OM). Sediment-bottom water microcosm systems were exposed to four combinations of air and nitrogen gas for a total time of 24 h. The treatments were: 24 h N2, 0.5 h air + 23.5 h N2, 4 h air + 20 h N2 and 24 h of air exposure. As a result of these treatments, microcosms spanned a wide range of redox potential, as reflected by the dissolved sulfide concentration range of ≤0.3–97 µM. Four different chemical species of inorganic divalent Hg (Hg II ) and methyl mercury (MeHg), enriched in different Hg isotope tracers, were added to the microcosms: 201 Hg(NO3 )2 (aq), 202 Hg II adsorbed to OM ( 202 Hg II -OM(ads)), 198 Hg II as microcrystalline metacinnabar ( β - 198 HgS(s)) and Me 204 HgCl(aq). Microcosm systems were composed of bottom water mixed with sediment taken at 0–2, 0–5 and 0–10 cm depth intervals. The composition of OM varied with sediment depth such that compared to deeper sediment, the 0–2 cm depth-interval had a 2-fold higher contribution of labile OM originating from algal and terrestrial inputs, serving as metabolic electron-donors for microorganisms. The potential methylation rate constant ( k meth ) of Hg tracers and net formation of ambient MeHg (MeHg/THg molar ratio) increased up to 50% and 400%, respectively at intermediate oxidative conditions, likely because of an observed 2-fold increase in sulfate concentration stimulating the activity of sulfate reducing bacteria with a capability of methylating Hg II . Due to differences in Hg II water-sediment partitioning, k meth varied by a factor of 11–70 for the different isotope-enriched Hg tracers. The chemical form of Hg II was a major controlling factor for k meth and its response to the resuspension-oxidation of the system. The β - 198 HgS(s) tracer had the lowest k meth and it was mainly constrained by redox-driven Hg II solubility. The 202 Hg II -OM(ads) tracer showed an intermediate value on k meth . It was controlled by both Hg II solubility and availability of electron donors and acceptors, regulating bacterial activity. The 201 Hg(NO3 )2 (aq) tracer had the highest value on k meth which was limited mainly by bacterial activity. The k meth was up to a factor of 3 higher in the 0–2 cm sediment depth-interval than in 0–5 and 0–10 cm intervals due to a larger contribution of labile OM in the 0–2 cm sediment. Reduction of Hg II to Hg 0 followed by volatilization exclusively occurred at high sulfidic conditions in the top 0–2 cm sediment. Aromatic moieties of terrestrial OM, present mainly in the top sediment, is suggested to control the reduction of Hg II . The Hg 0 volatilization rate constant for the 202 Hg II -OM(ads) tracer exceeded that for β - 198 HgS(s) by one order of magnitude. Our results suggest that contaminated sediments posing a high risk for reactivation of legacy Hg following transient redox resuspension events are characterized by depletion of sulfate in the sediment porewater prior to resuspension, predominance of Hg II species with solubility exceeding that of crystalline β -HgS(s), and conditions promoting in situ formation and/or import of labile OM from algal and terrestrial sources.