Sequential Capture of Gaseous Hg⁰ in Flue Gas and Aqueous Hg²⁺ in Desulfurization Solution by Cu–ZnS: Mechanisms, Kinetics, and Its Application in Hg Recovery as a Supplement of Selective Catalytic Reduction + Wet Flue Gas Desulfurization

The combined technology of selective catalytic reduction and wet flue gas desulfurization (SCR + WFGD) to control Hg0 emissions from coal-fired flue gas is often unsatisfactory due to the undesired Hg0 oxidation efficiency of SCR, the re-emission of Hg0 from WFGD, and the potential secondary pollution. To offset the shortcomings of SCR + WFGD, Cu–ZnS that resulted from Cu2+ activation of natural sphalerite was developed as a difunctional sorbent to sequentially capture gaseous Hg0 in flue gas and aqueous Hg2+ in desulfurization solution. Not only did Cu–ZnS exhibit significant performance in capturing the gaseous Hg0 downstream of an electrostatic precipitator, but it also displayed a superior ability in capturing aqueous Hg2+. Hg0 re-emission from WFGD, which resulted from the reduction of aqueous Hg2+ by SO2, was completely suppressed because Cu–ZnS could adsorb both aqueous Hg2+ and re-emitted Hg0 in desulfurization solution. Spent Cu–ZnS containing Hg can be separated from the desulfurization slurry via flotation for Zn smelting, and adsorbed Hg can be at last recovered as mainly liquid Hg0 in modern smelters for centralized control. Therefore, the sequential capture of gaseous Hg0 and aqueous Hg2+ by Cu–ZnS may be an economically viable and eco-friendly technology to control Hg pollution of coal-fired power plants.