Elemental mercury removal by a novel advanced oxidation process of ultraviolet/chlorite-ammonia: Mechanism and kinetics

Highlights: • A novel UV/NaClO₂-NH₄OH process for Hg⁰ removal was proposed. • ClO/Cl₂O₂/HO/ClO₂ were determined to control Hg⁰ oxidation. • The Hatta numbers are 23–133 and 69–305 corresponding to without and with UV. • HgCl₂ is the final product of Hg⁰ identified by XPS. -- Abstract: A novel advanced oxidation process (AOP) of ultraviolet/chlorite-ammonia (UV/NaClO₂-NH₄OH) was developed to remove Hg⁰ from flue gas. The distribution of mercury concentration in three solutions of NaClO₂-NH₄OH, KCl, and H₂SO₄-KMnO₄ was determined by cold atom fluorescence spectrometry (AFS). The role of NH₄OH was to help NaClO₂ preserving and/or stabilizing Hg²⁺ meanwhile inhibiting the photo-production of ClO₂. In the absence of UV, decreasing pH promoted the release of Hg²⁺ from NaClO₂-NH₄OH; introducing NO, SO₂, O₂, Br⁻, Cl⁻, and HCO₃⁻ suppressed Hg⁰ oxidation. In the presence of UV, rising temperature accelerated the release of Hg²⁺ from NaClO₂-NH₄OH; while SO₂, Br⁻ and HCO₃⁻ facilitated Hg⁰ oxidation. In the absence and presence of UV, Hg⁰ oxidation was controlled by ClO₂⁻ and by ClO/Cl₂O₂/HO/ClO₂, respectively. The formations of ClO/HO/ClO₂ were confirmed by electron spin resonance (ESR). X-ray photoelectron spectroscopy (XPS) revealed that the products of Hg⁰ and ClO₂⁻ were HgCl₂, and ClO₂, Cl-, ClO₃-, Cl₂, and ClO₄-, respectively. Analysis of kinetics showed that the Hatta numbers were 23–133 and 69–305 without and with UV, respectively, thus, the gas-film mass transfer was the rate-determining step. This paper gives a new insight in radical behavior in Hg⁰ oxidation.