Oxidation Of Hypophosphite By Photo Electro-fenton Process For Synchronous Removal Phosphate In Water

In the electroless plating process,sodium hypophosphite is used as a reducing agent.With the large-scale plated parts producing,a amount of unutilized hypophosphite is contained in the cleaning tank and bath of the process,which cause the plating to be waste.Moreover,it will cause water pollution if discharge directly.However,the hypophosphite,possessing a stable chemical structure and a high solubility,is difficult to oxidize by general oxidation process.Therefore,wastewater containing hypophosphite is often treated by chemical precipitation,electrodialysis,microelectrolysis,and advanced oxidation.And the Electro-Fenton process acts on the secondary oxidation for hypophosphite（H₂PO₂^-）to phosphate(PO₄^3-)and the deposition of iron phosphate（FePO₄）,achieving the purpose of hypophosphite oxidation and total phosphorus（TP）removal for iron phosphate recovery.In this paper,sodium hypophosphite solution was used as simulated wastewater,and Electro-Fenton（E-Fenton）,Photo electro-Fenton（UV/E-Fenton）and UV/Fe²⁺activated persulfate(UV/Fe²⁺-K₂S₂O₈)were used to analyse the oxidation efficiency of hypophosphite to remove TP on the influences of H₂O₂ dosage,initial pH,current intensity and feeding modes of H₂O₂.The product（FePO₄）structure characterized by SEM,TEM/EDX,XRD,FT-IR,XPS.The oxidation mechanism of hypophosphite was discussed with active radicals;the oxidation efficiency of hypophosphite and removal efficiency of TP were compared under different experimental conditions.The main research results are as follows:（1）For the E-Fenton oxidation,Fe²⁺was derived from the Fe electrode,H₂O₂ was added externally.The experimental results showed that with the H₂O₂ dosage of 600.0mmol/L,the initial pH of 2.5,and the current intensity of 0.3 ampere（A）,the oxidation efficiency of hypophosphite and the removal efficiency of TP can both reach 99.0%under the reaction of 120 min.The 2-segment feeding model of H₂O₂ had a significant effect on the oxidation efficiency of hypophosphite,and shortened its complete oxidation time to 20 min,which improved the oxidation efficiency.Meanwhile,the Methanol had a strong trapping effect on hydroxyl radicals（·OH）.It was verified that·OH is the active radical for the hypophosphites oxidization in the system.（2）The UV/E-Fenton system was introduced 254.0 nm UV to synergistic Fe²⁺for the photolysis of H₂O₂.The experimental results showed that with the H₂O₂ dosage of300.0 mmol/L,the initial pH of 2.5,and the current intensity of 0.3 A,the hypophosphite oxidation efficiency and TP removal efficiency reached 99.0%for the 60min reaction.Under the same conditions,compared with the oxidation efficiency of hypophosphite in the E-Fenton system,10 min reaction of UV/E-Fenton can complete oxidation for hypophosphite,highlighting the effect of UV for the hypophosphite oxidation.UV/Fe²⁺synergistic effect is more strong than the single UV(Fe²⁺)for the photolysis of H₂O₂ to·OH,which further improved the oxidation efficiency,Meanwhile,the hydroxy hydrated iron was formed during the reaction of hypophosphite oxidation,which adsorbed phosphate and caused the product FePO₄ to adhere to the Fe electrode.The structural characterization of the recovered FePO₄showed that the hydrated hydroxy iron was decomposed by UV.The particles were finer（20.0 nm）and the purity is higher.The analysis of the change of the valence state of Fe with XPS was related to the mechanism explained for oxidation hypophosphite with·OH.（3）For UV/Fe²⁺-K₂S₂O₈ system,the experimental results showed that the K₂S₂O₈dosage of 25.0 mmol/L in one shot,the current intensity of 0.3 A,the the oxidation efficiency of hypophosphite and removal efficiency of TP reacted 95.0%and 85.0%for60 min,no matter what the initial pH in the acid or alkaline environment.During the reaction,the pH environment of the system was acidic,indicating that the initial pH had no significant effect on the persulfate activation system.Moreover,in the alkaline,there was no·OH detected.Therefore,the·SO₄^-generated in acid environment was the active radical for hypophosphites oxidization.Compared with Fe²⁺-activated persulfate,the UV/Fe²⁺caused a higher yield of·SO₄^-radical for per unit time,which was more conducive to the oxidation of hypophosphite and the removal of TP for the recovery of iron phosphate.The FePO₄ was finer under the the acidic.The increased initial pH caused FePO₄ to agglomerate,affecting the purity and crystallinity.Although the initial pH had no effect on the oxidizing ability of the system,the valence state of the Fe changes significantly,indicating that the pH had a great influence on the product formation process,when reacting in the alkaline,especially.The E-Fenton,UV/E-Fenton,and UV/Fe²⁺-K₂S₂O₈ processes provide·OH and·SO₄^-radicals for the hypophosphite oxidation process,while also providing the Fe³⁺to precipitate iron phosphate.The whole reaction system does not generate secondary pollution,which reduces the cost and complexity of pharmaceutical dosing,and will realize the utilization of phosphorus.