Kinetics of divalent mercury reduction by zero-valent iron: the effects of pH, chloride, and dissolved organic carbon

Abstract

The effects of pH, Cl-, and dissolved organic carbon (DOC) on the kinetics of Hg2+ reduction to Hg-0 were investigated in anoxic Fe-0-H2O system. A reaction kinetic model was developed to describe separately the surface-mediated and aqueous phase Hg reduction processes. The Hg2+ was rapidly reduced to Hg-0 in the presence of 2 g L-1 Fe-0 (0.42 m(2) L-1) within 3 h although the presence of DOC significantly, about an order of magnitude, reduced the reduction kinetics. The zero-valent iron (ZVI) surface area normalized Hg2+ reduction rates varied between 0.5 and 2.74 m(-2) min(-1). In the presence of 0.1-10 mg/L DOC, the rates varied between 0.026 and 0.1 m(-2) min(-1). Modeling studies showed that the increase of pH and NaCl concentrations and the decrease of DOC levels increased surface-mediated Hg2+ reduction rate. Higher pH seemed to increase the reduction rates and this was attributed to the enhanced adsorption of Hg2+ to ZVI surface at higher pH. The Cl- undergoes strong complexation with Hg2+ (i.e., HgCl+, HgCl2, and HgCl3-) and prevent the adsorption of Hg2+ to Fe-0 surface and subsequent reduction. However, the enhanced corrosion and greater release of Fe2+ by the pitting corrosion process in the presence of Cl- affected the overall Hg2+ reduction far more significantly, hence, increased Hg2+ reduction was observed in the presence of Cl- in solution. The DOC seemed not only to decrease the reactivity of Hg2+ by rendering strong complexation but also to prevent the adsorption of Hg2+ to the Fe-0 surfaces thus inhibiting surface reduction.