Stability of dimethylmercury and related mercury-containing compounds with respect to selected chemical species found in aqueous environment

Abstract

Dimethylmercury (CH3-Hg-CH3) and other Hg-containing compounds can be found in atmospheric and aqueous environments. These substances are highly toxic and pose a serious environmental and health hazard. Therefore, the understanding of chemical processes that affect the stability of these substances is of great interest. The mercury-containing compounds can be detected in atmosphere, as well as soil and aqueous environments where, in addition to water molecules, numerous ionic species are abundant. In this study we explore the stability of several small, Hg-containing compounds with respect to water molecules, hydronium (H3O+) ions as well as other small molecules/ions using density functional theory and wave function quantum chemistry methods. It is found that the stability of such molecules, most notably of dimethylmercury, can be strongly affected by the presence of the hydronium H3O+ ions. Although the present theoretical study represents gas phase results, it implies that pH level of a solution should be a major factor in determining the degree of abundance for dimethylmercury in aqueous environment. In particular, it is found that CH 3-Hg-CH3 reacts readily with the H3O + ion producing CH3-Hg-OH2+ and methane indicating that low-pH levels favor the decomposition of dimethylmercury. On the other hand, our study suggests that high-pH levels in aqueous environment would favor stronglybound complexes of [CH 3-Hg-CH3•OH]- species. Overall, the theoretical evidence presented in this study offers an explanation for the available experimental data concerning the stability of dimethylmercury and other mercury-containing compounds having the general structure X-Hg-Y (X,Y = CH3 and Cl) with respect to various ligands L (L = H2O, NH3, H3O+, OH-, Cl- and NH4+).