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

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 60 min, with initial [CN^-] equal to 10, 100, 500 or 1000 mg/L; oxidant type and dose (H₂O₂ only; NaClO only; and H₂O ₂ + 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 1000 mg/L. For an initial [CN^-] = 100 mg/L, pH = 9, and molar ratio ([H₂O₂] + [NaClO])/[CN ^-] = 2:1 it was possible to achieve a final concentration of [CN ^-] < 0.2 mg/L (99.8% reduction) in t = 20 min 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 20 min reaction period.