Effect of Fe3+ concentration and pH on arsenic removal, migration and speciation transformation in lab-scale wet flue gas desulfurization system

Abstract

Wet flue gas desulfurization (WFGD) system of coal-fired power station can synergistically remove arsenic from flue gas. However, the detailed mechanism of arsenic removal by desulfurization slurry and its further migration and transformation from gas to desulfurization products still remains unclear. In this study, two important factors, the Fe3+ concentration and the pH value, were investigated for their effects on arsenic removal, redistribution and transformation in a lab-scale reactor. The results show low Fe3+ concentration (≤1 mg L-1) in slurry may lead to the decrease of particle size and the increase of Zeta potential of gypsum particles, thus promoting arsenic removal from flue gas, while more Fe3+ (10–500 mg L-1) could reduce its removal due to the increase of mass transfer resistance. Arsenic removal efficiency decreases with pH value rising. The slurry with Fe3+ is easier to remove As(III) from flue gas than As(V) due to the strong polarity of AsCl3(g). Either increasing Fe3+ concentration (pH = 5.5) or rising pH value (Fe3+ concentration = 50 mg L-1) can promote arsenic migration from desulfurization wastewater into gypsum. At any pH value, Fe-gypsum has a stronger ability to adsorb As(V) than As(III). Acid leaching model test indicates the arsenate incorporated into gypsum lattice can reach 12% without Fe3+, but the presence of Fe3+ hinders the arsenate doped into gypsum crystal, which is less than 1%. This raises the potential environmental risk of further utilization of gypsum due to the unstable arsenic speciation. The main arsenates substituting SO42- doped into gypsum lattice are HAsO42- and AsO43-.