Identification of mercury methylation product by tert-butyl compounds in aqueous solution under light irradiation

Abstract

a b s t r a c tThe methylation of mercury (Hg) is of great concern as methylmercury (MeHg), the most toxic species, is produced. This study examined the possibilities of tert-butyl compounds (tert-butyl alcohol (TBA) and tert-butyl hydroperoxide (TBH)) and other alcohols serving as methyl donors for Hg photo-methylation under light irradiation. The yield of MeHg varied among the methyl donors, and it was also significantly influenced by salinity and pH. MeHg could be generated in the presence of TBH under visible light irradiation. The hydroxyl radical ( Å OH) was found to promote MeHg production at low levels, but degrade MeHg in excess. The photo-production of MeHg was tentatively proposed via the complexation of Hg and methyl donors, the formation of an intermediate (, and the intramolecular methyl transfer from methyl donors to Hg. This study implicates photoreactions between Hg and organic pollutants in understanding the fate and transformation of Hg in the aquatic environment.

Recent advances in the study of mercury methylation in aquatic systems

“…, and concentrations declined as salinity rose (Chen et al 2015). No correlations were observed for salinity and MeHg concentrations in natural mangrove ecosystems (de Oliveira et al 2015) or hypersaline lakes (Johnson et al 2015).…”

“…However, abiotic methylation is generally thought to be of little importance in most aquatic systems (Ullrich et al 2001). Another possible avenue of abiotic methylation is photomethylation under UV radiation-a process that has been identified in vitro (Akagi et al 1974;Hayashi et al 1977), but the exact mechanism of which remains unclear (Chen et al 2015). The noted simultaneous production and degradation of MeHg may result in MeHg in sediments being in steady-state (Pak and Bartha 1998); however, levels in biota may be much higher due to bioaccumulation (Ullrich et al 2001).…”

“…In sewage treatment plants that use hydrated ferric oxide for water purification, the highest levels of methylation were found in areas rich in dissolved Fe 2+ and organic matter (Bravo et al 2015). The presence of compounds containing hydroxyl radicals has been found to enhance methylation at low concentrations due to their ability to function as methyl donors during photomethylation, but these compounds degrade MeHg when present in excess (Chen et al 2015). A study of a variety of lagoon systems found that regardless of Hg source, Hg was influenced by the transformations of other elements, particularly redox-sensitive and microbially important elements like sulfur, iron, and manganese, and by the interactions of those elements with organic material (Faganeli et al 2012).…”

Distribution and bioavailability of mercury in the surface sediments of the Baltic Sea

“…Salinity can also be an important factor conditioning Hg Fig. 1 Map of the sub-basins of the Baltic Sea with the location of sampling stations in and the industrial hot spots in the Baltic Sea catchment area (HELCOM 2010) transformations on the water-sediment interface it can affect the methylation potential of Hg (Braaten et al 2014;Chen et al 2015;de Oliveira et al 2015;Jonsson et al 2014). Salinity can also impair the Hg adsorption onto clay minerals (Green-Ruiz 2009), thus affecting the release of Hg into the near-bottom water.…”

“…The form of Hg in marine sediments is influenced by several physicochemical processes, such as adsorption onto clay minerals and organic matter, the formation of complexes with organic and inorganic ligands, precipitation and co-precipitation (mainly mercury sulphide (HgS)), oxidation and reduction reactions and the formation of the most dangerous metal-organic compounds (mainly MeHg) (Jackson 1998 ; Bełdowski and Pempkowiak 2007 ; Bełdowski et al 2014 ). Salinity can also be an important factor conditioning Hg transformations on the water-sediment interface it can affect the methylation potential of Hg (Braaten et al 2014 ; Chen et al 2015 ; de Oliveira et al 2015 ; Jonsson et al 2014 ). Salinity can also impair the Hg adsorption onto clay minerals (Green-Ruiz 2009 ), thus affecting the release of Hg into the near-bottom water.…”