Remarkable enhancement of elemental mercury removal performance over natural low-grade pyrolusite through acid-etching

Abstract

Using natural low-grade pyrolusite as a novel Hg0 absorbent has economic and environmental benefits, but faces the challenge of restricted adsorption and oxidation capacity. Herein, a HNO3-etching strategy is employed to modify the physicochemical property. Characterizations and relevant theoretical calculations reveal the etching’s role: (i) dissolving impurities and adjusting pore structure, which magnify pre-adsorption capacity of reactants and facilitate mutual mass transfer; (ii) creating more oxygen vacancies and promoting electron transfer, which favor the adsorption-dissociation of O2 and the migration of more O-containing species; (iii) increasing reaction rate and lowering energy barrier. The 12-hour experimental results show that the removal efficiency of Hg0 can reach 92.8 % through etching, and the increment of 30.6 % reflects the huge promotion of etching on mercury removal. Moreover, the etching also improves the stability, sulfur/water resistance and regeneration performance. Furthermore, the desorption temperature of HgO diminishes after etching, as the etching enhances the activity of Oads and Olat. By contrasting the preferential and co-adsorption pathways on pristine/defective surfaces, alterations in adsorption mechanism are revealed, further elucidating the effects of increased defects on the enhanced Hg0 oxidation activity. This work offers a practical strategy to synchronously achieve structural optimization and defect construction of pyrolusite, thereby enhancing its removal performance, which holds potential implications for industrial applications in mitigating mercury emissions.