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

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.