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Photodegradation is an important pathway for monomethylmercury (MeHg) degradation in aquatic ecosystems. In this process, dissolved organic matter (DOM) plays an essential role. However, little information is available regarding the photo-transformation of MeHg in shallow aquatic environments, where a significant portion of MeHg is associated with soil suspensions. In this study, 14 soils sampled from different sites in China were used to simulate these conditions. Our results clearly demonstrated that soil organic matter (SOM) was the most important factor controlling the MeHg photodegradation in suspension. Degradation in this heterogeneous aqueous system was shown to be mediated by the 1O 2 produced by organic matter on the surface of the soil particles rather than by DOM. This was confirmed by the strong correlation between the kinetics rate constant of MeHg degradation and steady state concentrations of 1O 2 (R 2 = 0.81). Our results propose a new pathway of MeHg induced by sediment soils under sunlight irradiation. Identification of this pathway may improve the estimates of potential ecological risk of Hg in shallow field ecosystems. Image 1 • MeHg can be photo-degraded with simulated sunlight when soil particles are existed in natural shallow. • Soil organic matter rather than dissolved organic matter plays a dominant role in MeHg photodegradation. • Singlet oxygen is dominating active species for MeHg photodegradation on the surface of soil particles. [ABSTRACT FROM AUTHOR]

The aim of this study was to identify how hydrologic factors (e.g., rainfall, maximum depth, reservoir and catchment area, and water residence time) and water chemistry factors (e.g., conductivity, pH, suspended particulate matter, chlorophyll-a, dissolved organic carbon, and sulfate) interact to affect the spatial variance in monomethylmercury (MMHg) concentration in nine artificial reservoirs. We hypothesized that the MMHg concentration of reservoir water would be higher in eutrophic than in oligotrophic reservoirs because increased dissolved organic matter and sulfate in eutrophic reservoirs can promote in situ production of MMHg. Multiple tools, including Pearson correlation, a self-organizing map, and principal component analysis, were applied in the statistical modeling of Hg species. The results showed that rainfall amount and hydraulic residence time best explained the variance of dissolved Hg and dissolved MMHg in reservoir water. High precipitation events and residence time may mobilize Hg and MMHg in the catchment and reservoir sediment, respectively. On the contrary, algal biomass was a key predictor of the variance of the percentage fraction of unfiltered MMHg over unfiltered Hg (%MMHg). The creation of suboxic conditions and the supply of sulfate subsequent to the algal decomposition seemed to support enhanced %MMHg in the bloom reservoirs. Thus, the nutrient supply should be carefully managed to limit increases in the %MMHg/Hg of temperate reservoirs. [ABSTRACT FROM AUTHOR]

The 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 ( 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 ( O(Hg)C(CH 3 ) 3 ), 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. [ABSTRACT FROM AUTHOR]

We examined Hg biogeochemistry in Baihua Reservoir, a system affected by industrial wastewater containing mercury (Hg). As expected, we found high levels of total Hg (THg, 664–7421 ng g−1) and monomethylmercury (MMHg, 3–21 ng g−1) in the surface sediments (0–10 cm). In the water column, both THg and MMHg showed strong vertical variations with higher concentrations in the anoxic layer (>4m) than in the oxic layer (0–4 m), which was most pronounced for the dissolved MMHg (p < 0.001). However, mercury levels in biota samples (mostly cyprinid fish) were one order of magnitude lower than common regulatory values (i.e. 0.3–0.5 mg kg−1) for human consumption. We identified three main reasons to explain the low fish Hg bioaccumulation: disconnection of the aquatic food web from the high MMHg zone, simple food web structures, and biodilution effect at the base of the food chain in this eutrophic reservoir. [Copyright &y& Elsevier]

Surface sediments (0–2cm) were collected at 40 sites along the Tagus Estuary in July and December 2004. The sediments were analysed for total mercury, monomethylmercury (MMHg) and interpretative parameters (e.g. redox potential, pH, Corg). No significant differences in total Hg, pH, Al, Fe, Mn and Corg were found between sediments collected in the two periods, but MMHg concentrations were higher in July. On average sediments were warmer and more reducing in summer. On the basis of these results, an increase of 7kg of MMHg (+37%) in surface sediments of the Tagus Estuary was estimated. Presumably higher temperatures in summer promote the increase of microbial activity and higher methylation rates. The alterations observed in this study point to the potential importance of seasonal changes in MMHg production at surface sediments with eventual changes in the MMHg uptake by benthic invertebrates and other organisms in the food web. [Copyright &y& Elsevier]

Deception Island is an active volcano in the Antarctica being volcanism a source of mercury. To improve the understanding of the Hg cycle in this remote ecosystem, pyroclastic gravel and water samples were collected and total (THg) and monomethylmercury (MMHg) concentrations were measured as well as the potential for Hg methylation. Gravel samples collected close to active fumaroles showed the highest THg levels (72 ng/g) while in water samples the highest concentrations of THg (1.2 ng/L) and MMHg (0.45 ng/L) where found. Methylation activity was barely observable in gravel samples. Biotic methylation rates in water were up to 13 times higher compared to those recorded previously in other polar waters. Abiotic methylation processes also play an important role, with up to 0.54 ± 0.43% of added Hg converted instantaneously to MMHg. These results suggest that Deception Island presents favourable conditions for MMHg explaining the elevated concentrations of both THg and MMHg in this ecosystem. • Deception Island (DI) is an active volcano in the South Shetland Archipelago. • Active volcanism in DI is the main source of Hg in DI. • Hg methylation rates in DI waters are higher than in other polar aquatic systems. • Methylmercury proportions (THg) are up to 450% higher than other marine systems. • DI is a natural Hg contaminated system with direct impacts on living organisms. [ABSTRACT FROM AUTHOR]