Mercury(II) reduction and sulfite oxidation in aqueous systems: kinetics study and speciation modeling

Abstract

Environmental contextWastewater contains various substances such as sulfur-containing chemicals and heavy metals including mercury ions. Several technologies have been developed to trap mercury ions; however, mercury can undergo reactions with sulfite and change to its vapour form, which easily escapes to the atmosphere. Here, we devised a model to predict the formation of vapour-phase mercury as a function of sulfite concentration, temperature and water acidity based on coal-fired power plant wastewater. AbstractThe re-emission of mercury (Hg) as a consequence of the formation and dissociation of the unstable complex HgSO3 is a problem encountered in flue gas desulfurisation treatment in coal-fired power plants. A model following a pseudo-second-order rate law for Hg2+ reduction was derived as a function of [SO32–], [H+] and temperature and fitted to experimentally obtained data to generate kinetics rate values of 0.120±0.04, 0.847±0.07, 1.35±0.4mM–1 for 40°C, 60°C and 75°C respectively. The rate of reduction of Hg2+ increases with a temperature increase but shows an inverse relationship with proton concentration. Plotting the model-fit kinetics rate constants yields ΔH=61.7±1.82 kJ mol–1, which is in good agreement with literature values for the formation of Hg0 by SO32–. The model could be used to better understand the overall Hg2+ re-emission due to SO32– happening in aquatic systems such as flue gas desulfurisation wastewaters.