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

A novel advanced oxidation process (AOP) of ultraviolet/chlorite-ammonia (UV/NaClO2-NH4OH) was developed to remove Hg-0 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, O-2, Br-, Cl-, and HCO3- suppressed Hg-0 oxidation. In the presence of UV, rising temperature accelerated the release of Hg2+ from NaClO2-NH4OH; while SO2, Br- and HCO3- facilitated Hg-0 oxidation. In the absence and presence of UV, Hg-0 oxidation was controlled by ClO2- and by ClO center dot/Cl2O2/HO center dot/ClO2, respectively. The formations of ClO center dot/HO center dot/ClO2 were confirmed by electron spin resonance (ESR). X-ray photoelectron spectroscopy (XPS) revealed that the products of Hg-0 and ClO2- were HgCl2, and ClO2, Cl-, ClO3-, Cl-2, 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 Hg-0 oxidation.