Limitations of the removal of cyanide from coking wastewater by ozonation and by the hydrogen peroxide-ozone process

The purpose of this study is to compare the efficiency of ozonation and the hydrogen peroxide-ozone process for the removal of cyanide from coking wastewater. The most efficient oxidation process is combined with coagulation-flocculation-decantation and lime-soda ash softening pretreatments. The oxidation in aqueous solution and industrial wastewater (at pH 9.5-12.3) by O-3 was carried out using a range of concentration of consumed O-3 from 10 to 290 mg/L. A molar ratio of H2O2/O-3 from 0.1 to 5.2 with different concentrations of O-3 constants was used for the H2O2-O-3 process. The maximum cyanide removal obtained in coking wastewater was 90% using a mass ratio of O-3/CN -of 9.5. Using lower concentrations of O-3, cyanide is not removed and can even be generated due to the presence of other cyanide precursor organic micropollutants in the industrial matrix. The concentration of O-3 is reduced to half for the same cyanide removal efficiency if the pretreatments are applied to reduce the carbonate and bicarbonate ions. The cyanide removal efficiency in coking wastewater is not improved if the O-3 is combined with the H2O2. However, the preliminary cyanide removal treatment in aqueous solution showed an increase in the cyanide removal efficiency for the H2O2-O-3 process.