Cobalt sulfide nanosheets synthesized by anion exchange method for efficient Hg0 removal from flue gas

Abstract

Transition metal sulfide adsorption has been recognized as a potential method for Hg0 removal, and the physicochemical properties of the adsorbent can significantly affect the Hg0 removal efficiency. The nanosheet structure with abundant unsaturated surface atoms can result in lower mass transfer resistance and give more adsorption sites. Thus, in the present work, cobalt sulfide nanosheet adsorbents with different Co/S ratio were synthesized by anion exchange method. Characterization results show that CS-10 has the lowest crystallinity and the highest Co3+/Co2+ and S22−/S2− ratio. These results give CS-10 the best Hg0 removal efficiency of nearly 100 % at a high GHSV of 180 000 h−1 at 60 °C. In addition, CS-10 has a good resistance to SO2 whose Hg0 removal efficiency decrease after SO2 introduction but recover to original level after SO2 cutting off. After being impregnated in Na2S solution, the adsorption efficiency of the used CS-10 can be easily regenerated. XPS results show that HgS can resolve in Na2S solution via HgS(ad) + S2− → [HgS2]2−. Furthermore, Co3+ and S22− as the main active sites for Hg0 removal can be replenished during the impregnation in Na2S solution. Kinetic study points that Hg0 adsorption characteristics on CS-10 obey pseudo-first-order and pseudo-second-order kinetic models, suggesting that the Hg0 diffusion on the external surface and the reaction at the adsorption site can both influence the Hg0 removal process.