Influence of the composition and character of dissolved organic matter (DOM) on the removal of mercury from surface water in metal based-coagulation systems
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According to The United Nations Environment Programme (UNEP), mercury pollution still represents a major threat to human health and the environment, mainly in regions where metallic mercury (Hg⁰) is used to extract gold or where coal burning increases the emissions of mercury in the atmosphere. In the particular case of the metallic mercury used in artisanal and small-scale gold mining (ASGM), there is substantial evidence about the direct impact of mercury on human health, mainly due to the inhalation of vapor mercury and the intake of fish with high levels of methylmercury (MeHg). Once metallic mercury is dumped into rivers and soils, this pollutant can react with sulfur reduced ligands in sediments or can be converted to Hg(II) and it will interact with dissolved organic matter (DOM) in aquatic environments. Such interaction will control the fate of this pollutant in water, as well as its toxicity and mobility downstream rivers. Accordingly, these Hg-DOM interactions will also control the ability to remove this pollutant during water treatment. This research shows that out, of the different functional groups present in dissolved organic matter, reduced sulfur ligands (S [subscript red] ) play a key role in removing mercury from waters with low Hg/DOM ratio. This trend is particularly evident in waters with low in aromatic content (which translate to low DOC removal) and high concentrations of S [subscript red] ligands. At much higher mercury concentrations, once the S [subscript red] ligands are saturated with mercury, the carboxylic ligands control the removal of mercury from solution. Under these conditions, the removal of mercury is proportional to the removal of carbon (∼1:1 ratio) as all the Hg(II) ions are bound to the all the functional groups available for Hg(II) complexation, even binding weak ligands such as carboxylic acids. In all cases, it is necessary to provide optimal coagulation conditions (pH and coagulant dose) to effectively remove dissolved organic matter, and therefore associated mercury, from solution.