Photocatalytic oxidation removal of elemental mercury from flue gas. A review

Air pollution caused by mercury emissions from combustion and pyrolysis of fossil fuels, biomass, and solid wastes is a major health issue. Elemental mercury (Hg-0), the dominant species in flue gas, is poorly captured by actual control equipments because Hg-0 has a high volatility and is insoluble in water. Alternatively, photocatalytic processes can oxidize Hg-0 into Hg compounds that are easier to remove. Here we review the recent research on oxidative removal of Hg-0 by photocatalysts, with focus on titanium dioxide, bismuthides, silver compounds and hybrid photocatalysts. We discuss the Hg-0 removal performances and mechanisms of catalytic oxidation. The TiO2 photocatalyst often suffers from low absorption of solar energy and easy recombination of photoinduced electron-hole couples. BiOIO3 appears as a promising photocatalyst due to its high Hg-0 oxidation activity under visible light irradiation. Silver carbonates and silver halides are newly developed visible-light-responsive photocatalysts for Hg-0 removal. Fast separation and transformation of photogenerated electrons and holes control the performance of Hg-0 removal by photocatalysts.