Gaseous mercury removal by graphene-like carbon nitride impregnated with ammonium bromide

Graphitic carbon nitride (g-C 3 N 4 ) has attracted increasing interest owing to its unique electronic property, facile synthesis, chemical and thermal stability. In this work, a graphene-like g-C 3 N 4 is synthesized by direct thermal polymerization of environmentally benign guanidine thiocyanate. The graphene-like g-C 3 N 4 is further modified with NH 4 Br via incipient-wetness impregnation approach and applied to remove Hg 0 from coal-fired flue gas. The results indicate that the optimal polymerization temperature is 700 °C. The Hg 0 removal performance of g-C 3 N 4 is enhanced by NH 4 Br modification. The optimal NH 4 Br content and reaction temperature range are 3 wt% and 120–180 °C, respectively. NO exerts a promotional effect on mercury removal, while SO 2 displays a dual impact on Hg 0 adsorption. Low concentration of SO 2 is beneficial for mercury removal, whereas high concentration of SO 2 restrains Hg 0 removal to some extent. The edge carbons of g-C 3 N 4 may function as the principal reactive sites for Hg 0 removal. The Hg 0 might be captured on the armchair edge carbons of the g-C 3 N 4 by forming covalent carbon-mercury bonds via Lewis acid-base conjugation. Loading NH 4 Br onto g-C 3 N 4 may produce carbenium bromine ion pairs at the zigzag edge carbons that represent as oxidation sites, which convert Hg 0 into HgBr 2 . Besides, the adsorbed Hg 0 may also be oxidized into HgO by the chemisorbed oxygen.