Removal of cyanides in coking wastewater by ferrate pre-oxidization followed by photochemical process

UV direct photolysis and TiO2 photocatalytic oxidation with 250W high pressure mercury lamp (lambda >= 365 nm) used as irradiation source was found to be ineffective in the removal of the total. cyanide in coking wastewater, less than 30% total cyanide was removed. Therefore, ferrate pre-oxidization followed by photochemical processes to remove the total cyanide in coking wastewater was investigated in this paper. The effects of pH and ferrate concentrations on the removal efficiency of total cyanide in the pre-oxidation process were studied. The results showed that the concentration of cyanide in the coking wastewater was reduced from 13.5 mg.L-1 to 2.9 mg.L-1 at pH 5 with 10.88 mg.L-1 ferrate (in terms of Fe(VI) ions). UV irradiation with or without TiO2 could further remove the remaining total cyanide from the coking wastewater after ferrate pre-oxidation. Using ferrate as pre-oxidant followed by TiO2 photocatalytic oxidation removed nearly 100% of total cyanide after 4 hours irradiation, which implies that ferrate pre-oxidization followed by photochemical processes is an effective method to remove total cyanide from the coking wastewater.