Physio-chemical effects of freshwaters on the dissolution of elementary mercury

Elemental mercury (Hg-0) is widely used by Artisanal and small-scale gold miners (ASGMs) to extract gold from ore. Due to the unavailability of appropriate waste disposal facilities, Hg-0-rich amalgamation tailings are often discharged into nearby aquatic systems where the Hg-0 droplets settle in bottom sediment and sediment-water interfaces. Hg-0 dissolution and following biogeochemical transformations to methylmercury (MeHg) have been concerned owing to its potential risk to human health and the ecosystem. For reliable estimates of Hg exposure to human bodies using pollutant environmental fate and transport models, knowledge of the Hg-0 dissolution rate is important. However, only limited literature is available. Therefore, it was investigated in this study. Dissolution tests in a 'dark chamber' revealed that an increase in medium pH resulted in a decrease in the dissolution rate, whereas, a large Hg-0 droplet surface area (SA) and high Reynolds number (Re) resulted in a faster dissolution. A multivariate first order dissolution model of the form: (k) over cap = -7.9 x 10(-5)[pH] + 7.0 x 10(-4)[log Re] + 7.9 x 10(-4)[SA] - 2.5 x 10(-3) was proposed (adjusted R-2 = 0.99). The Breusch-Pagan and White heteroscedasticity tests revealed that the model residuals are homoscedastic (p-value = 0.05) at the 5% significance level. Parameter sensitivity analysis suggests that slow mercury dissolution from the Hg-0 droplets to aquatic systems might mask emerging environmental risk of mercury. Even after mercury usage in ASGM is banned, mercury dissolution and following contamination will continue for about 40 years or longer owing to previously discharged Hg-0 droplets. (C) 2019 Elsevier Ltd. All rights reserved.