The ferrous ion reduction/co-precipitation process is applied to the removal of the oxyanions selenate and selenite from FGD scrubber purge waters typical of coal-fired electric power utilities. The study determined that treatment performance was inhibited by the presence of thiosulfate,
selenium-thionate complexes, and the sulfur-nitrogen compound hydroxylamine disulfonate (HADS) in the scrubber liquor. HADS was found to severely inhibit ferrous ion reduction of the oxy-anions of selenium in non-forced oxidation FGD scrubber systems, resulting in incomplete treatment and
extremely high ferrous reagent doses. HADS removal using the free hydroxyl radical was found to be an effective pretreatment for overcoming some of the retardant effects of this constituent. Inhibition of selenium removal using the ferrous ion reduction/co-precipitation process is effectively
lessened when HADS concentrations are less than 0.5 mM/L in limestonebased FGD scrubber purge waters. The ferrous ion reduction/co-precipitation process was most optimal when the ORP of the scrubber liquor was between −650 and −750 mV and the pH was maintained between 8.8
and 9.2. The high doses of ferrous reagent (> 40 mM/L) used in the treatment appear to guarantee that the elemental selenium coprecipitated with the ferrous hydroxide solids in the process will remain with the sludge wasted from the process.
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