Removal of gaseous elemental mercury by modified diatomite

Novel adsorbents with low cost and high efficiency that do not produce secondary pollutants are vital for removing gaseous elemental mercury (Hg⁰) from coal-fired power plants. In this study, eight diatomite-based adsorbents were developed and used to remove Hg⁰ in a bench-scale fixed-bed reactor. The effects of active substances, reaction temperature, and gas components on the Hg⁰ removal performance of diatomite (Dia) and the mechanisms were investigated. After modification, the specific surface area of diatomite increased by 2-to-12 fold, and the Hg⁰ removal performance was greatly improved. The Hg⁰ removal efficiencies of the adsorbents decreased in the following order: I-Dia > Br-Dia > Cl-Dia. The Hg⁰ removal efficiency of CuBr₂-Dia reached 91% in the simulated flue gas at the optimal reaction temperature (140 °C). The simultaneous presence of O₂ and HCl promoted the Hg⁰ removal by CuBr₂-Dia. NO alone also played a significant role in Hg⁰ removal. However, SO₂ exhibited clear inhibitory effect. The average Hg⁰ removal efficiencies of CuBr₂-Dia were 60% under 1200 ppm SO₂, 87% under 1200 ppm SO₂ + 300 ppm NO, and 93% under 4% O₂ + 1200 ppm SO₂ + 300 ppm NO. The changes in the active adsorption sites caused by NO, and those caused by NO + SO₂ were different and irreversible. During the Hg⁰ removal process, Hg⁰ was oxidized to the Hg²⁺ or Hg⁺ species, while Cu²⁺ and Br radicals were reduced to Cu⁺ and Br^-, respectively.