Recovery of Phosphorus from Hypophosphite-Laden Wastewater: A Single-Compartment Photoelectrocatalytic Cell System...

Recovery of phosphorus through precipitation from hypophosphite-laden wastewater is more difficult than from orthophosphate-laden wastewater because of the higher solubility of hypophosphite (H₂PO₂^-). Herein, a single-compartment photoelectrocatalytic (PEC) cell system consisting of a TiO₂/Ni-Sb-SnO₂ bifunctional photoanode and an activated carbon fiber (ACF) cathode with dosing Fe²⁺ ions was developed for recovery of phosphorus in the form of FePO₄ from hypophosphite-laden wastewater. In the PEC process, H₂PO₂^- with an initial concentration of 1.0 mM was completely oxidized and recovered within 30 min at 3.0 V, and the pseudo-first-order rate constant of H₂PO₂^- oxidation was ∼4 times than that in the electrocatalytic process and even ∼89 times than that in the photocatalytic process. The bifunctional photoanode can simultaneously generate ^•OH radicals and O₃; the ACF cathode can electrogenerate H₂O₂; H₂O₂, O₃, and the added Fe²⁺ can interact with each other to produce ^•OH radicals and Fe³⁺ ions. ^•OH radicals mainly from the Fenton process were responsible for oxidation of H₂PO₂^- to PO₄^3-, which immediately combined with Fe³⁺ to form FePO₄ at the optimized conditions to realize recovery of phosphorus. The long-term stability of this system was demonstrated. The efficiency for actual electroless nickel plating effluents was exhibited.