Dark oxidation of mercury droplet: Mercurous [Hg(I)] species controls transformation kinetics

Abstract

Liquid elemental mercury droplet (Hg(0)l) is an important species in heavy Hg-contaminated environments. The oxidation processes of Hg(0)l and its related mechanisms are still poorly understood. Herein, for the first time, it was verified that mercurous species [Hg(I)] was an important species in natural water contaminated by Hg(0)l as well as in the simulated dark oxidation of Hg(0)l. The formation and further transformation of Hg(I) controlled the overall oxidation process of Hg(0)l and were affected by different environmental factors. Through kinetic modeling using ACUCHEM program, oxidation of Hg(0) to Hg(I) (Hg(0) → Hg(I)) was determined to be the rate-limiting step in Hg(0)l oxidation because its k value ((8.7 ± 0.21) × 10−11s−1) is seven orders of magnitude lower than that of Hg(I) oxidation (Hg(I) → Hg(II), (4.7 ± 0.15) × 10−4s−1). Ligands like OH−, Cl−, and natural organic matter enhanced the formation of Hg(I) via promoting the constants of comproportionation (up to (9.5 ± 0.78) × 10−4s−1). These findings highlight the importance of Hg(I) in Hg(0)l oxidation process by controlling the transformation kinetics of Hg species, facilitating an improved understanding of the environmental redox cycles of Hg.