Proposed Mechanism of Dissolution

The possible mechanisms of dissolution are (1) Ligand-Promoted Dissolution and/or (2) Redox-controlled Dissolution

Ligand-Promoted Dissolution

Ligand-Promoted Dissolution generally involves a fast ligand adsorption step, a slow detachment of activated surface complexes, and a regeneration of initial surface sites. Rapid adsorption of organic matter was observed in Electrophoretic mobility (Zeta Potential) measurements. If dissolution occurs by the formation of surface complexes, the binding between organic matter and Hg must be very strong, much stronger than the model compounds studied here, to overcome the Hg-S covalent bonds on the surface.

Redox-Controlled Dissolution

Humic substances have been implicated in the abiotic reduction of Hg(II) to Hg(0) in aqueous solutions. No evidence exists for humic-mediated, abiotic reduction of Hg from soils or other solid surfaces. Reduction of Hg on cinnabar surface is unlikely because generation of Hg(0) could not be detected when cinnabar was reacted with organic matter.

Oxidation of sulfide on cinnabar surface would weaken the sulfur-mercury bonds, facilitating surface complexation and subsequent release of Hg into solution. When cinnabar was reacted with organic matter (F1-HPoA) in nitrogen-sparged solutions (O₂<20 ppb), the measured sulfide concentrations were only about 5% of total Hg concentrations, while sulfate constituted the remaining major fraction. This would indicate that sulfide on the surface is being oxidized by organic matter.

Oxidative dissolution of cinnabar by Fe³⁺ has been proposed to take place at low pH (pH < 2). While quinone moieties in humic molecules have been shown to be potential electron acceptors in microbial reactions, humic substances are not considered to be important oxidants in abiotic reactions. When oxygen-free solutions of organic matter were reacted with 10^-3 M sulfide, sulfate was not produced in solution.