Solvents mediated-synthesis of 3D-BiOX (X = Cl, Br, I) microspheres for photocatalytic removal of gaseous Hg0 from the zinc smelting flue gas

Abstract

Three-dimensional (3D-BiOX (X = Cl, Br, I)) microspheres were fabricated by a facile solvothermal method with ethylene glycol (EG) as the solvent. The as-prepared microspheres compounds BiOX were characterized by XRD, SEM, BET, UV–vis DRS, FT-IR and XPS techniques and employed as a photocatalyst to photocatalytic oxidize elemental mercury (Hg0) from a smelting flue gas. Results showed that the band gaps values of 3.12, 2.69 and 1.77 eV were obtained for the direct electronic transition of BiOCl, BiOBr and BiOI samples, respectively. The specific surface area and surface hydroxyl groups are the two crucial factors affecting the photocatalytic oxidation of Hg0 BiOI microsphere possesses the maximum of the surface hydroxyl groups, largest BET specific surface area (35.8964 m2/g), strongest optical absorption properties, narrowest band gap width (1.77 eV) and the highest photocatalytic activity (ηoxi = 96.83%) under UV–vis irradiation. The photogenerated holes (h+) produced in the Valence Band (VB) can react with surface-bonded H2O (ad) or OH−, I− to produce hydroxyl radicals (OH) and I2, respectively. The photogenerated electrons (e−) produced in the Conduction Bands (CB) can react with adsorbed dissolved oxygen to form super-oxide radicals (O2−). In addition, for the BiOBr reaction system, the OH, h+ and O2− could play key roles in the formation of HgO, while in BiOI photocatalytic system, except for OH, O2− and h+, I2 could also be an important species for the generation of HgO and HgI2 species.