Mercury speciation and sorption processes in mining environments

Mercury contamination as a result of mercury mining, gold mining, and the associated weathering and transport processes related to mining wastes is a significant concern in mercury-bearing, mine-impacted environments and may pose serious hazards to human health. The speciation of Hg is a critical factor influencing these processes, because the types of Hg species present and their relative proportions relate directly to the solubility and reactivity of Hg in mine wastes. Additionally, the sorption of Hg to particle surfaces, which aids in inhibiting its mobility in aquatic systems, is an important process controlling the distribution of Hg and its exposure to living organisms. A combination of analytical techniques was therefore applied to study the speciation and sorption of Hg in mine environments, including EXAFS spectroscopy, ICP-AES, batch uptake experiments, SEM, TEM, and XRD.; Hg speciation in Hg mine wastes as determined using EXAFS spectroscopy was found to consist primarily of insoluble Hg-sulfides (cinnabar and metacinnabar), although minor proportions of more soluble Hg phases (Hg-oxides, (oxy)chlorides, and sulfates) were also identified in certain samples. EXAFS-based Hg speciation results correlated relatively well with Hg speciation determined by sequential extractions, particularly in quantifying the proportion of Hg-sulfides present. Several factors influence Hg speciation including the geological origin of the Hg ores, the ore roasting process, and particle size.; Studies of Hg(II) sorption to goethite, γ-alumina, and bayerite using EXAFS spectroscopy determined that Hg(II) dominantly forms monodentate and bidentate inner-sphere complexes at the substrate surfaces. The degrees of uptake and modes of sorption were relatively constant for each substrate from pH 4–8. Hg(II) sorption was inhibited by the presence of chloride due to the formation of stable, nonsorbing aqueous HgCl₂ complexes in solution, limiting the amount of free Hg(II) to sorb. The presence of sulfate, in contrast, caused enhanced Hg(II) uptake due to the sorption of sulfate ions at the substrate-water interface, thus reducing the electrostatic barrier to Hg(II) sorption. Such studies of Hg speciation and sorption are critical in predicting its transport and potential bioavailability in natural systems and have implications for the prioritization and remediation of Hg-contaminated regions worldwide.