Recovery of phosphorus and metallic nickel along with HCl production from electroless nickel plating effluents: The key role of three-compartment photoelectrocatalytic cell system

A three-compartment photoelectrocatalytic (PEC) cell system combined with ion exchange and chemical precipitation was proposed to recover phosphorus and nickel from electroless nickel plating effluents containing hypophosphite (H2PO2−) and nickel ions (Ni2+). Ion exchange was used to concentrate and separate Ni2+ and H2PO2−. As a key unit, the established PEC system consisted of TiO2/Ni-Sb-SnO2 photoanode and Ti cathode. With 25.8 mM NaH2PO2 and 500 mM NiCl2, 100 % H2PO2− was oxidized to PO43− in the anode cell, 78 % Ni2+ was recovered as metallic Ni in the cathode cell, and 900 mM HCl was obtained in the middle cell within 24 h at 3.0 V. Based on quenching experiments and ESR technique, OH radicals were mainly responsible for H2PO2− oxidation. In situ Raman spectroscopy indicated that Ni2+ initially reacted with OH− to form α-Ni(OH)2, which was gradually reduced to metallic Ni. Fortunately, a slight pH decrease in the cathode cell in the three-compartment cell system was beneficial for Ni2+ reduction to Ni°. The obtained PO43− was recovered by chemical precipitation. Finally, recovery of phosphorus and metallic nickel along with HCl production from an actual electroless nickel plating effluents in terms of efficiency, cost-benefit, and stability assessment were demonstrated.