Oxidation of cyanide in water by singlet oxygen generated by the reaction between hydrogen peroxide and hypochlorite

Abstract

The synergistic combination of hydrogen peroxide and hypochlorite ion in water results in formation of the highly oxidizing intermediate species singlet oxygen (1O2), which was effective in the oxidation of free cyanide (CN−) in water.The present work reports the findings of a study of the kinetics of aqueous cyanide oxidation with singlet oxygen generated by the reaction between hydrogen peroxide and hypochlorite, both added separately to the solution to be treated. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology involved a set of kinetic experiments measuring [CN] versus time, up to 60min, with initial [CN−] equal to 10, 100, 500 or 1000mg/L; oxidant type and dose (H2O2 only; NaClO only; and H2O2+NaClO) for various molar ratios of Oxidant/CN; and pH=9–11. The objective was to evaluate the effect of those factors on the reaction kinetics at room temperature. The results showed the synergy between hydrogen peroxide and hypochlorite on the cyanide oxidation reaction rate.The process was fast and efficient over the studied pH range of 9–11, and up to an initial CN− concentration of 1000mg/L. For an initial [CN−]=100mg/L, pH=9, and molar ratio ([H2O2]+[NaClO])/[CN−]=2:1 it was possible to achieve a final concentration of [CN−]<0.2mg/L (99.8% reduction) in t=20min at 25°C in a batch reaction. By comparison, the same reaction with either of the separate oxidants (H2O2 or NaClO) at the same molar ratio of oxidant/CN−=1:1 resulted in a maximum of 87% breakdown of the cyanide (using NaClO) for the same 20min reaction period.