Efficient Removal of Hg⁰ from Flue Gas Using Sulfur Site-Rich CuIn₅S₈-Modified Graphitic Carbon Nitride Adsorbents: Performance, Kinetics, and Mechanism Study

Mercury emissions from coal combustion flue gas pose significant risks to ecosystems and human health. In this work, a novel adsorbent was developed for efficient removal of Hg0 from coal-fired flue gas. The adsorbent consisted of bimetallic sulfide CuIn5S8 with multiple active sulfur (S) sites incorporated into graphitic carbon nitride(g-C3N4). The resulting 20CuIn5S8/g-C3N4 (20CIS/CN) composite exhibited outstanding Hg0 removal capacity, achieving over 99% adsorption efficiency at temperatures of 80 and 120 °C. Even after continuous adsorption for more than 10 h, the adsorption efficiency remained at approximately 89%. At higher temperatures of 160 and 200 °C, the efficiency remained high at 97.56 and 89.11%, respectively. Importantly, the inhibitory effect of SO2 and NO in the flue gas on the Hg0 adsorption efficiency of 20CIS/CN was minimal. Adsorption capacity at adsorption equilibrium of 20CIS/CN (15.088 mg/g) was 14.2 times higher than that of a commercial activated carbon adsorbent (1.06 mg/g). The adsorption mechanism was investigated by density functional theory (DFT), revealing the stability of the In–S–Hg six-coordinated octahedral structure on the CuIn5S8(0 0 1) surface. Experimental, XPS, and Hg-TPD analyses supported the proposed mechanism, demonstrating the transformation of Hg0 into stable HgS through interaction with abundant reactive S sites on the CuIn5S8 surface. These findings highlight the exceptional adsorption capacity of 20CIS/CN composites and their potential as a replacement for traditional commercial adsorbents. The development of modified adsorbents with abundant S-active sites for efficient Hg0 adsorption in flue gas can contribute to effective environmental remediation strategies.