Advanced oxidation removal of hypophosphite by O₃/H₂O₂ combined with sequential Fe(II) catalytic process

Elimination of hypophosphite (HP) was studied as an example of nickel plating effluents treatment by O₃/H₂O₂ and sequential Fe(II) catalytic oxidation process. Performance assessment performed with artificial HP solution by varying initial pH and employing various oxidation processes clearly showed that the O₃/H₂O₂─Fe(II) two-step oxidation process possessed the highest removal efficiency when operating under the same conditions. The effects of O₃ dosing, H₂O₂ concentration, Fe(II) addition and Fe(II) feeding time on the removal efficiency of HP were further evaluated in terms of apparent kinetic rate constant. Under improved conditions (initial HP concentration of 50 mg L⁻¹, 75 mg L⁻¹ O₃, 1 mL L⁻¹ H₂O₂, 150 mg L⁻¹ Fe(II) and pH 7.0), standard discharge (<0.5 mg L⁻¹ in China) could be achieved, and the Fe(II) feeding time was found to be the limiting factor for the evolution of apparent kinetic rate constant in the second stage. Characterization studies showed that neutralization process after oxidation treatment favored the improvement of phosphorus removal due to the formation of more metal hydroxides. Moreover, as a comparison with lab-scale Fenton approach, the O₃/H₂O₂─Fe(II) oxidation process had more competitive advantages with respect to applicable pH range, removal efficiency, sludge production as well as economic costs.