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Effect of sulfite on divalent mercury reduction and re-emission in a simulated desulfurization aqueous solution

Chang, Lin, Zhao, Yongchun, Li, Hailong, Tian, Chong, Zhang, Yi, Yu, Xuehai, Zhang, Junying
Fuel processing technology 2017 v.165 pp. 138-144
aqueous solutions, flue gas desulfurization, mercury, oxygen, pH, pollution control, slurries, sulfates, sulfites, sulfur dioxide, temperature
Wet flue gas desulfurization (WFGD) systems have several benefits, including SO2 removal and Hg pollution control. However, the absorbed ionic mercury may be transformed into insoluble elemental mercury because of the chemical interaction with the aqueous scrubbing solution, which causes mercury re-emission and reduces the mercury capture efficiency of the scrubber. This study investigates the effects of operating temperatures, pH, and O2 and SO4²⁻ concentrations on the reduction of Hg²⁺ in the presence of SO3²⁻ in a simulated desulfurization aqueous solution. The results indicate that excess SO3²⁻ inhibits Hg²⁺ reduction due to the formation of the stable Hg(SO3)2²⁻ complex, whereas a low SO3²⁻ concentration (<2mM) leads to enhanced mercury re-emission due to the formation of more redox-unstable HgSO3. The Hg⁰ release increases from 18.6% to 59.6% when the operating temperature increases from 45°C to 55°C. A pH decrease in the slurry from 5 to 3 causes a significant increase in the Hg⁰ emission from 6.9 to 127.9μg/m³ in a simulated desulfurization slurry. In addition, O2 caused a secondary emission of Hg⁰ by damaging the stable redox complexes of Hg²⁺ and SO3²⁻. In the presence of oxygen, the Hg⁰ emission decreases; subsequently, a HgSO3SO4²⁻ complex is formed. Sulfate (HgSO4 and HgSO3SO4²⁻) inhibit the emission of Hg⁰.