Oxidation of Gaseous Elemental Mercury in Acidified Water: Evaluation of Possible Sinking Pathway of Atmospheric Gaseous Mercury in Acid Cloud, Fog, and Rain Droplets

Abstract

This is the first report investigating the transformation of gaseous elemental mercury (GEM), the major form of airborne mercury, into oxidized mercury in bulk liquid, a possible sinking pathway of atmospheric GEM in clouds, fog, rain droplets and ocean spray. A 100–150 ng m−3 GEM standard gas, a 50–150 times higher concentration than the typical atmospheric concentration, was introduced into a 2.5 L rectangular glass vessel, at the bottom of which a 0.5 L uptake solution of pure water (pH 6–7), weakly acidified pure water with sulfuric or nitric acid (pH 3.2–3.6) or seawater (pH 8) was resting. The standard gas was introduced into the space above the solution in the vessel at the rate of 0.82 L min−1 and exited from the opposite end of the vessel, which was open to the room’s pressure. After exposing the solution to the gas for 0.5–4 h, a portion of the uptake solution was sampled, and the dissolved elemental mercury (Hg0aq) and dissolved oxidized mercury (Hg2+aq) in the solution were analyzed by the conventional trapping method, followed by cold vapor atomic fluorescent spectrometer measurements. The results showed that the quantities of total dissolved mercury (THgaq = Hg0aq + Hg2+aq) in the pure water and seawater were compatible, but those were slightly lower than the equilibrated Hg0aq concentrations estimated from Henry’s law, suggesting non-equilibrium throughout the whole solution. In contrast, the quantity of Hg2+aq and THgaq in the acidified pure water with sulfuric acid was significantly enhanced. Over the 4 h exposure, the THgaq concentrations were two times higher than the equilibrated Hg0aq concentration. This was due to the slow oxidation reaction of Hg0aq by the sulfuric acid in the bulk phase. Using the collision rate of GEM with the surface of the solution and the observed uptake, the estimated uptake coefficient of GEM by this uptake was (5.5 ± 1.6) × 10−6. Under the typical atmospheric concentration, this magnitude results in an atmospheric lifetime of 4970 years, negligibly small compared with other atmospheric oxidation processes.