Dark oxidation of dissolved and liquid elemental mercury in aquatic environments

Elemental mercury (Hg⁰) can be found in liquid or dissolved forms in aquatic systems. Whereas dissolved Hg⁰ is measured in virtually all aquatic systems, liquid Hg⁰ droplets are mainly observed at poorly lit sediment/water interfaces of ecosystems with local point sources such as hydrothermal vents, gold extraction sites, and near industrial facilities. Here, we report that, in the dark, liquid and dissolved forms of Hg behave differently with respect to their oxidation. Liquid Hg⁰ is rapidly oxidized in oxygenated solution in the presence of chloride. Liquid Hg⁰ oxidation rates are positively correlated with chloride concentrations and droplet surface area. When liquid Hg is removed from solution, the oxidation stops even though the solution is still saturated with dissolved Hg⁰. Liquid Hg⁰ droplets in oxygenated marine or brackish environments should be oxidized and release Hg²⁺ to solution. In freshwaters or anoxic seawater, liquid Hg will dissolve releasing Hg_aq⁰ which, itself, will slowly oxidize.