Strategies For Removal Of Phosphite And Ni In Electroless Ni-plating Wastewater: Establishment Of Oxidation Systems And Exploring Underlying Mechanisms

Nickel(Ni)complexes with organic ligands and low-valence phosphorus are the major pollutants abundant in the electroless Ni-plating wastewater.Generally,the effective separation and removal of these pollutants is achieved by a sequential two-step strategy that integrates an advanced oxidation process(AOP)and a flocculation and sedimentation process.This method has inherent technical shortcomings with respect to the extensive consumptions of chemicals(e.g.,Fenton reagents,flocculation reagents,and alkali)and energy,and the possibility of inducing secondary pollution.For example,the frequently existed phosphite,a reduced form of P,is difficult to be removed owing to its resistance to biotransformation and chemical precipitation;oxidation of phosphite to phosphate is considered to be a prerequisite,followed by the precipitation and flocculation step.The reluctant complexes(like ethylenediaminetetraacetic acid(EDTA-Ni(?))is also generally treated by the high-cost AOPs that degrades EDTA,and the subsequent separation process that involves the use of alkali and flocculants for transforming aqueous Ni to solid Ni.The devastating mineralization of EDTA or oxidation of phosphite requires large amounts of oxidants,catalytic materials,and energy.Therefore,it is highly desirable to develop simple,economical,and green methods for manipulating these pollutants in the electroless Ni-plating wastewater.Our strategy is to establish integrated electrochemical or chemical systems that enable one-step treatment of phosphite or EDTA-Ni(?),lessening the need for external chemicals such as oxidants,iron salts,oralkali.Our efforts are also devoted to exploring the underlying reaction mechanisms and identifying the key species responsible for pollutant oxidation.The main findings are summarized as follows:(1)Elimination of HPO₃^2-from water is more difficult than elimination of phosphate(PO₄^3-)owing to its higher solubility and resistance to biotransformation.We report an efficient,facile electrochemical method integrating electrooxdiation(EO)and electrocoagulation(EC)to treat phosphite-laden wastewater.The mechanistic studies demonstrate that in situ-generated Fe²⁺at an Fe anode can react with in situ-generated O₂at a mixed metal oxide(MMO)anode,leading to formation of·O₂^-,a reactive species predominantly responsible for oxidation of HPO₃^2-to PO₄^3-.The PO₄^3-was immediately coagulated by Fe hydroxides that are formed due to the production of OH^-at a stainless-steel cathode.The integrated EO/EC system enabled HPO₃^2-removal efficiency of 74.25%(MMO anode,100 m A;Fe anode,100 m A;and reaction time:60 min),a significantly higher efficiency rate than that obtained from the control experiments in the absence of an MMO anode(<23.41%)and that obtained from the chemical coagulation process(<5.03%).The quenching experiments with scavengers and electron spin resonance tests verified the pivotal role of·O₂^-in transformation of HPO₃^2-.Testing with nickel-plating wastewater further demonstrated the superiority of this integrated system,as evidenced by efficient removal of HPO₃^2-and nickel from the solution.(2)Taking into considerations the availability of Cl^-in the wastewater and the ineffectiveness of the above system for complete removal of highly concentrated HPO₃^2-at low p H,we establish an active chlorine-mediated Electrochemical AOP system based on the dual-anode reactor,which is utilized to strengthen the oxidation of HPO₃^2-for the purpose of pretreating the acidic wastewater.The active hypochlorous acid(HCl O)was produced from the electrolysis of Cl^-at the MMO anode,and the application of a small current at the Fe anode significantly improved the oxidation rate of HPO₃^2-((16)(15)m M HPO₃^2-can be oxidized by(23)(15)(4)-(16)(15)(15)(4)within(16)(20)(15)minutes).The P-rich precipitates were obtained,which might be recovered as chemical products.After the pre-oxidation treatment,the EO/EC system can be constructed to quickly remove the oxidized PO₄^3-by adjusting the current applied to the circuit.The scavenging experiments and the EPR tests confirmed the occurrence of the Fenton-like reaction in the three-electrode system.It was speculated that the Fe²⁺/HCl O reaction produces Fe(IV)in the pre-oxidation process that promotes the oxidation of HPO₃^2-.(3)The reaction between Fe²⁺and HCl O constitutes a promising AOP for removing pollutants from wastewater,and·OH has been considered the dominant reactive oxidant despite limited evidence for this.We demonstrate that the Fe²⁺/HCl O reaction enables the production of Fe^IVO²⁺rather than·OH in acid medium,a finding that is strongly supported by multiple lines of evidence.Both X-ray absorption near-edge structure(XANES)spectroscopic tests and M(?)ssbauer spectroscopic tests confirmed the appearance of Fe ^IVO²⁺as the reactive intermediate in the reaction between Fe²⁺and HCl O.The determination of Fe^IVO²⁺generation was also derived from the methyl phenyl sulfoxide(PMSO)-based probe experiments with respect to the formation of PMSO₂ without·OH adducts and the density functional theory(DFT)studies according to the lower energy barrier for producing Fe^IVO²⁺compared with·OH.All the results and evidences of the comprehensive invesigations demonstrated that Fe^IVO²⁺is the direct product of the Fe²⁺/HCl O reaction,which is unlike the Fe²⁺/H₂O₂ reaction,presenting a non-radical process mechanism.(4)Decomplexation by AOPs is known to be the key step in treatingheavy-metal-complexed wastewater,and extensive investigations of the mechanism of the dissociation pathway indicate that the oxidative destruction of chelating compounds and their intermediates is necessary prior to the hydroxide precipitation of heavy metals.We show that the decomplexation of EDTA-Ni(?)and the simultaneous formation of Ni hydroxide precipitates can be successfully achieved with the addition of peroxomonosulfate(PMS)under alkaline conditions,a simple and convenient oxidation process that does not rely on external energy or chemicals for radical generation in AOPs.At alkaline p H,the aqueous Ni concentration can be significantly reduced to below 0.1 mg L^-1 when PMS was introduced to the EDTA-Ni(?)solution.More importantly,a mechanistic insight into the active intermediate responsible for observed reactivity was gained by conducting in-situ Raman,quenching,electron paramagnetic resonance(EPR)and ultra-performance liquid chromatograph-mass spectra(UPLC-MS)tests,and density functional theory(DFT)calculations.Based on the combination of experimental and theoretical evidence,we propose an alternative reaction pathway in which Ni(?)(heavy metal)rather than EDTA(ligand)is attacked by PMS,which leads to the production of high-valent Ni species(in the form of EDTA-Ni^IV(28)O).This is a key and highly oxidative species being further self-decomposed to facilitate generation of Ni(III)hydroxides that can be easily separated from the aqueous phase to the solid phase.Overall,our findings suggest that the involvement of Ni(IV)species might be a viable redox pathway that plays an important role in promoting decomplexation.