Mercury in Water

Abstract

Mercury (Hg) is a global pollutant threatening our water resources and human health. Every one of us contains some levels of Hg or methylmercury (MeHg) in our body from consumption of contaminated food such as fish and rice in some countries. Our understanding of the behavior of Hg in natural aquatic environments is limited because the key factors controlling microbial uptake and conversion of inorganic Hg to neurotoxic MeHg remain obscure. MeHg concentrations in water can bioaccumulate up to eight orders of magnitude in the food chain. Metallic or elemental Hg can evaporate and be transported around the globe, plus it readily undergoes chemical, photochemical, and biological transformations (i.e. oxidation, reduction, methylation, and demethylation) in water and sediments. Natural dissolved organic matter (DOM) plays a critical role in mediating these transformations because of its exceptionally strong binding affinity for Hg and its dual functional roles in reducing and oxidizing Hg. Recent discovery of Hg‐methylation genes has significantly advanced our understanding of MeHg biosynthesis, although the simultaneous degradation of MeHg at low concentrations (i.e. picomolar to nanomolar) by anaerobic and methanotrophic bacteria is also recognized now as a potentially important process for controlling net MeHg production, a key factor that determines Hg concentrations and bioaccumulation in biota. Treatment technologies for Hg removal from water are available, but they are mostly applicable to industrial waste streams and at contamination sites with high Hg concentrations (i.e. micromolar to millimolar). Reductions in Hg human exposure might be achievable by decreasing anthropogenic releases, such as coal combustion, over the long‐term since Hg methylation responds to Hg inputs, particularly to freshly deposited Hg in natural aquatic systems.

Demethylation─The Other Side of the Mercury Methylation Coin: A Critical Review

“…MeHg can be degraded either directly or indirectly by light through photolysis or photochemical reactions, a mechanism so-called photodemethylation (Figure ). Photodemethylation is obviously only significant in surface water due to rapid attenuation of light in the water column, especially for ultraviolet (UV) radiation. ,, Additionally, the rate and efficiency of photodemethylation strongly depend on the radiation intensity and wavelength; shorter waveband UV-B light (280–320 nm) is far more effective in degrading MeHg than longer waveband UV-A light (320–400 nm) and the visible or photosynthetic active radiation (PAR) (400–700 nm). ,,,− For example, in a study of photodemethylation along a lake-wetland gradient in a boreal coniferous forest, the relative ratio of demethylation rate constants with UV-B, UV-A, and PAR was found to be 3100:43:1 in surface waters . Similarly, Black et al reported that photodemethylation rate constants were at least 400-fold greater for UV-B and 37-fold greater for UV-A than PAR in water.…”

“…Proposed photodemethylation pathways of methylmercury (MeHg) predominantly in the form of either MeHg–DOM or MeHgCl complexes in freshwater or seawater. , Direct photolysis of the C–Hg bond in MeHg (left), photolysis by photoexcited DOM-MeHg (i.e., organic ligand-MeHg) complexes resulting in direct energy transfer and breakdown of the C–Hg bond (middle), and indirect photolysis by photochemically produced free radicals and reactive oxygen species (right). UV-B irradiation (280–320 nm) is the most effective in causing direct photolysis of MeHg, although it is a minor component of solar irradiation reaching the earth’s surface. , …”

“…MeHg in natural water is unlikely present as the free CH 3 Hg + ion, but rather it is complexed with DOM, especially in freshwater, where DOM concentrations are usually many orders of magnitude higher than MeHg concentrations. Therefore, different rate constants observed with UV-B, UV-A, and PAR under different environmental conditions ,,,, may be explained partially by the variations in water chemistry, such as the quantity and quality of DOM. DOM can influence photodegradation rates of MeHg in multiple ways, either by increasing the rates of demethylation through the formation of MeHg–DOM complexes and photochemically produced free radicals (discussed below) or by decreasing the rates by quenching free radicals or through the attenuation of solar radiation that causes photodemethylation.…”

“…The result suggests that, in purified water, only UV-B may play a role in direct photolysis of MeHg. Rose et al also found that UV-B radiation under the solar spectrum was mostly responsible for the MIF observed during photodemethylation, although UV-B makes up only less than 0.25% of the total solar radiation that reaches Earth’s surface. ,, While UV-A makes up about 5% of the total solar radiation reaching Earth’s surface, ,, it seems ineffective in causing direct photolysis of MeHg in purified water. , However, natural water or even purified water always contains some levels of DOM (e.g., >0.2 mg L –1 ), which are usually in a large excess compared to typical picomolar concentrations of MeHg in water. Therefore, MeHg in natural water is mostly complexed with DOM via the Hg–S bond , and light absorption by the MeHg–DOM complexes leads to direct or indirect energy transfer to and breakdown of the C–Hg bond.…”

“…While several recent review articles have addressed Hg biogeochemical processes, − there is a scarcity of comprehensive reviews solely focused on MeHg degradation and none focuses on mechanistic aspects. This Review focuses on the mechanisms of MeHg degradation or demethylation, as new processes and mechanistic details have recently emerged for both biotic and abiotic pathways.…”