Oxidation of elemental mercury by aqueous chlorine (HOCl/OCl-): Implications for tropospheric mercury chemistry

Abstract

The stoichiometry and kinetics of elemental mercury (Hg0) oxidation by aqueous chlorine (HOCl/OCl-) have been investigated. The stoichiometric ratio of Hg0 to HOCl/OCl- is found to be 1:1, the same as the electron transfer ratio. The rate constants of the oxidation are measured in a npvel fashion by using chloramine (NH2Cl) as the free chlorine reservoir. The rate constants at room temperature (23° ∼ 25°C) for the Hg0-HOCl and Hg°-OCl- are measured to be (2.09±0.06) × 106 and (1.99±0.05) × 106 M-1s-1, respectively. Based on the solubility data of chlorine, the intrinsic Henry's law constant of chlorine is calculated to be 7.61 × 10-2 M atm-1 at 25°C. Model study using the kinetic data in this investigation shows that the oxidation of Hg0 by aqueous chlorine is an important pathway contributing dissolved divalent mercury (Hg(II)) in atmospheric water, especially at higher cloud water pH when solubility of chlorine is greatly increased and before sunrise when chlorine reaches its peak concentrations in the marine troposphere. Copyright 1998 by the American Geophysical Union.