Photo-electrochemical oxidation of hypophosphite and phosphorous recovery by UV/Fe2+/peroxydisulfate with electrochemical process

Abstract

Hypophosphite, a reducing agent, has been widely used in metallurgical industry, however, the effective treatment of hypophosphite wastewater with phosphorus recovery remains a challenge. In this work, various systems including control (anode oxidation), PDS, UV, Fe2+/PDS, UV/Fe2+/PDS systems were carried out and compared in terms of oxidation efficiency of hypophosphite and recovery efficiency of total phosphorus. The UV/Fe2+/PDS system demonstrated the optimal performance at the optimized conditions ([H2PO2−] = 10 mM; pH0 = 3.0; Current intensity = 0.4 A; [K2S2O8] = 20 mg/L) by analyzing the effects of the operating conditions of initial pH0, current intensity and K2S2O8 concentration. The precipitates of the phosphate recovery products obtained at the optimized conditions were investigated by XRD, SEM-EDX, TEM, FTIR and XPS and the results confirmed that the precipitates were ferric phosphate with a high purity. Radical scavengers and ESR spectroscopy were applied to investigate the generated radicals in the UV/Fe2+/PDS system with the results showing that SO4− and OH radials were contributable to hypophosphite oxidation. The mechanism of hypophosphite oxidation and phosphorus recovery was proposed as three steps. Firstly, SO4− and OH radicals were generated by Fe2+/UV activation and anode, respectively. Secondly, the generated SO4− and OH radicals facilitated the oxidation of hypophosphite to phosphate. Thirdly, the oxidized phosphate was recovered by ferric ions with the formation of ferric phosphate precipitates.