Kinetics of Homogeneous and Surface-Catalyzed Mercury(Ⅱ) Reduction by Iron(Ⅱ)

摘要

Production of elemental mercury, Hg(0), via Hg(Ⅱ) reduction is an important pathway that should be considered when studying Hg fate in environment. We conducted a kinetic study of abiotic homogeneous and surface-catalyzed Hg(0) production by Fe(Ⅱ) under dark anoxic conditions. Hg(0) production rate, from initial 50 pM Hg(Ⅱ) concentration, increased with increasing pH (5.5-8.1) and aqueous Fe(Ⅱ) concentration (0.1-1 mM), The homogeneous rate was best described by the expression, r_(hom)=k_(hom) [FeOH~+] [Hg(OH)_2]; k_(hom) = 7.19 × 10~(+3) L (mol min)~(-1). Compared to the homogeneous case, goethite (α-FeOOH) and hematite (α-Fe_2O_3) increased and γ-alumina (γ-Al_2O_3) decreased the Hg(0) production rate. Heterogeneous Hg(0) production rates were well described by a model incorporating equilibrium Fe(Ⅱ) adsorption, rate-limited Hg(Ⅱ) reduction by dissolved and adsorbed Fe(Ⅱ), and rate-limited Hg(Ⅱ) adsorption. Equilibrium Fe(Ⅱ) adsorption was described using a surface complexation model calibrated with previously published experimental data. The Hg(0) production rate was well described by the expression r_(het) = k_(het) [>SOFe_(Ⅱ)] [Hg(OH)_2], where >SOFe_(Ⅱ) is the total adsorbed Fe(Ⅱ) concentration; k_(het) values were 5.36 × 10~(+3), 4.69 × 10~(+3), and 1.08 × 10~(+2) L (mol min)~(-1) for hematite, goethite, and γ-alumina, respectively. Hg(0) production coupled to reduction by Fe(Ⅱ) may be an important process to consider in ecosystem Hg studies.