Reaction Mechanism of Selenite, S_2O_8~(2-) and Mn~(2+) in a Limestone-gypsum FGD Liquor

摘要

Our previous work revealed that Mn~(2+) prevented the oxidation of selenite (Se(IV)O_3~(2-)) to selenate (Se(VI)O_4~(2-)) by peroxodisulfate ion (S_2O_8~(2-)), which is the dominant oxidizing agent in flue gas desulfrization (FGD) liquor. In this work, the mechanism of reaction between selenite, S2O_8~(2-) and Mn~(2+) as a inhibitor of selenite oxidation was studied using aqueous solutions composed of commercial reagents and limestone-gypsum FGD liquors sampled from coal fired power plants. The oxidation of selenite to selenate is promoted by the sulfate ion radical (SO_4~-) which results from decomposition of S_2O_8~(2-). In the presence of Mn~(2+), selenite oxidation was prevented due to the difference in rates of reaction with SO_4~-. The ratio of the oxidation rate constants of selenite and Mn~(2+) with SO_4~- were determined at FGD temperature (50 °C), and were found to be little influenced by the various coexisting components in FGD liquors. An appropriate concentration of Mn~(2+) for the prevention of selenite oxidation can be obtained via calculation based on the reaction mechanism for varying reagent concentrations.