Kinetics of homogeneous and surface-catalyzed mercury(II) reduction by iron(II)

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-Fe₂O₃) increased and γ-alumina (γ-Al₂O₃) 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.