The Mechanism of Abiotic Methylation and Demethylation of Mercury in Aquatic Environment

Abstract

Methylmercury (MeHg) is among the most widespread contaminants that pose severe health risks to humans and wildlife. In determination the levels of MeHg in aquatic environments, methylation of inorganic mercury (Hg(II)) to MeHg and demethylation of MeHg are the two most important processes in the cycling of MeHg. So, the knowledge of the efficiency of these different pathways of Hg methylation and demethylation is one of the key steps to predict MeHg concentrations in the different environmental compartments and to estimate the Hg bio-accessibility to the organisms. However, the factors that influence the competing methylation and demethylation reactions are yet insufficiently understood and little to no attempt has been made to determine end products, especially abiotic processes. The relative importance of each reaction and the resulting net effect will probably depend on the environmental conditions. Therefore, this study investigated the possible photochemical processes and mechanism of Hg demethylation and methylation in water with simulating various environmental conditions. The main objectives of this study were (1) to investigate the influence of several environmental factors and other water constituents on photo-decomposition of MeHg (Study 1), (2) to understand the mechanism of MeHg demethylation process in seawater by assessing the production of dissolved gaseous mercury (DGM) generated from MeHg photo-degradation (Study 2), and (3) to assess the possibility of various methyl donors such as acetate, malonate, dimethylsulfoxide, and litter-derived DOM for photochemical methylation of Hg(II) in aquatic systems (Study 3). For Study 1, photo-initiated decomposition of MeHg was investigated under UVA irradiation in the presence of natural water constituents including nitrate (NO3−), ferric (Fe3+), and bicarbonate (HCO3−) ions, and DOM such as humic and fulvic acid (HA and FA). MeHg degradation followed the pseudo-first-order kinetics

the rate constant increased with increasing UVA intensity ranged from 0.3 to 3.0 mW cm-2. In the presence of NO3−, Fe3+, and FA, the decomposition rate of MeHg increased significantly due to photosensitization by reactive species such as hydroxyl radical (OH•). However, the presence of HA and HCO3− ions lowered the degradation rate through a radical scavenging effect. Increasing the pH of the solution increased the degradation rate constant by enhancing the generation of OH•. Therefore, OH• play an important role in the photo-decomposition of MeHg in water, and natural constituents in water can affect the photo-decomposition of MeHg by changing radical production and inhibition. For Study 2, the photo-induced formation of dissolved gaseous mercury (DGM, Hg0) from MeHg removal was investigated. This study examined the effect of various environmental factors (i.e., light wavelength and intensity and MeHg concentration), and primary water constituents on the abiotic photo-degradation of MeHg, especially under different salinity. Photo-degradation rates of MeHg were positively correlated with the UV light intensity, implying that the attenuation of UV radiation had a significant effect on MeHg photo-degradation in water. However, a high dissolved organic carbon (DOC) concentration and salinity inhibited MeHg photo-degradation. DGM was always produced during the photo-degradation of MeHg. Photo-degradation rates of MeHg and DGM production decreased with increasing salinity, suggesting that the presence of chloride ions inhibited MeHg photo-degradation. Therefore, this study imply that MeHg in freshwater could be more rapidly demethylated than that in seawater and MeHg flowing into the lake or river would be almost removed by photo-demethylation. However, MeHg flowing to seawater would be hardly removed, which could have more chance for bioaccumulation in seawater. For Study 3, the photochemical methylation of Hg(II) using various methyl donors such as acetate, malonate, dimethylsulfoxide (DMSO), and litter-derived dissolved organic matter (LDOM) was examined. The methylation reaction via acetate was followed the pseudo-first-order kinetics for Hg(II), and the methylation ability of acetate decreased with the solution pH increased. In the Hg(II) methylation by LDOM, LDOM leaded to the production of new MeHg under not only UV irradiation but dark condition. Especially, from the results of the production new MeHg by LDOM in the microbial free and dark condition, this work suggests the possibility that the abiotic chemical reaction such as a non-dependence upon light occurs in the natural aquatic environment. In addition, for the MeHg formation of Hg(II) by DMSO in seawater, abiotic methylation reaction appeared to be promoted via Hg-DMSO complexes, and limited when the reactant is a chloro complex (i.e., seawater) due to its inhibitory effect probably because of higher stability 0of the Hg-Cl bond. Therefore, this study emphasized the importance of possible abiotic methylation by a non-dependence upon light in aquatic systems, while the abiotic chemical reactions for methylation are mostly caused by a dependence upon light up to date. In conclusion, this thesis achieved MeHg methylation and demethylation through photochemical reaction in aquatic systems. From the results of this thesis, the site-specific environmental factors i.e. environmental conditions of spatial and temporal variations can be effect on the relative importance of each reaction and the resulting net effect in the aquatic environment. In other words, the reduction of MeHg accumulation possibility in aquatic food chain will be mainly affected by the enhancement of demethylation processes with increasing of UV radiation at the surface waters. Ultimately, the results of this thesis could be a significant contribution to understand the possible photochemical processes and mechanism of Hg demethylation and methylation in water and to estimate the factors that influence the competing methylation and demethylation reactions.