Effect of oxidizing agents on selenate formation in a wet FGD

In a coal combustion process, a considerable amount of selenium is captured in the wet FGD, where it is oxidized from selenite (Se(IV)O32-) to selenate (Se(VI)O42-), which is difficult to remove. Diethyl-p-phenylene-diammonium (DPD) absorptiometric analysis and ion chromatography identified peroxodisulfate ion (S2O82-) as the dominant oxidizing agent in the FGD liquor. Selenite was easily oxidized to selenate in the presence of S2O82- and the oxidation was accelerated as the temperature increased. Addition of Mn2+ ion was found to be effective in controlling selenate formation. When Mn2+ ion was added, S2O82- oxidized not selenite to selenate but rather Mn2+ to MnO2, which captured some dissolved selenite.