Sulfate Radical Induced Degradation Of Methyl Violet Azo Dye With Cufe Layered Doubled Hydroxide As Heterogeneous Photoactivator Of Persulfate

In recent years,sulfate radicals?SO₄·^-?based advanced oxidation process for persistent organic pollutants removal has been getting more and more attention.Compared with OH·,SO₄·^-has higher oxidation-reduction potential,strong ability to mineralize organic matters,the mild reaction conditions,suitable for a wild range pH.Meanwhile,a small number of the precipitate after treatment,and convenient operation makes SO₄·^-has ananadvantage over hydroxyl radicals?OH·?in the process of degradation of organic matters.In general,SO₄·^-can be produced by activation of persulfate?PS?through photoactivation,thermalactivation,transition metalactivation,andalkaline activation.Among thses methods,the transition metal ion Fe²⁺is an excellent catalyst for PS activation.Moreover,ultravi olet?UV?light radiation has been found to significantly promote the conversion of Fe²⁺/Fe³⁺,thereby further enhancing the activation efficiency.However,Fe²⁺in solution is potentially biotoxic and difficult to recover.Furthermore,UV has a 4%occupancy ratio in solar spectroscopy and is much less than visible light?Vis??45%?.Therefore,the development of heterogeneous catalytic reaction systems based on visible light will be far-reaching significant and valuable.In this study,a highly efficient a nd stable heterogeneous catalyst-copper-iron layered doubled hydroxide?CuFe-LDH?was synthesized by double drop co-precipitation method.It combined with visible light can activate PS to produce SO₄·^-for achieving rapid and efficient degradation of methyl violet?MV?in dye wastewater.Meanwhile,this subject also investigated the different experimental parameters?PS concentration,CuFe-LDH dose,pH,initial MV concentration and common coexisting anions in water?on MV removal efficiencyin CuFe-LDH/PS/Vis catalytic oxidation system.Simultaneously,the main active group,and the MV degradation mechanisms also were reliminarily discussed.?1?Synthesis of highly efficient heterogeneous catalyst CuFe-LDHIn this experiment,CuFe-LDH with different Cu/Fe ratios was prepared by double drop co-precipitation method.The obtained CuFe-LDH was characterized by XRD,SEM,EDX,PL,UV-Vis-DRS and other techniques.According to the analysis results,the best crystallinity and a layered hyd rotalcite structure of CuFe-LDH was obtained under the Cu/Fe molar ratio of 3:1.EDX spectrum showed the Cu/Fe molar ratio in the catalyst material was about 3.18,very close to the original molar ratio.All the results indicated that a heterogeneous catal yst CuFe-LDH with good crystallinity had been synthesized successfully.Simultaneously,UV-Vis-DRS also confirmed that CuFe-LDH had excellent light absorption characteristics,which further confirmed the viability of the experiment.?2?High efficiency of CuFe-LDH/PS/Vis system for the degradation of methyl violet in dye wastewaterThe degradation of MV in dye wastewater was carried out in CuFe-LDH/PS/Vis system.CuFe-LDH showed high catalytic activity.The optimal reaction conditions were:the initial MV concentration 20 mg/L,CuFe-LDH dosage 0.2 g/L,PS dosage0.2 g/L,and no need to adjust pH.Under the condition,MV could be completely degraded within 18 min,and the degradation process conform to pseudo first-order kinetics,where the rate constant k=0.202 min^-1.?3?Reaction mechanism of CuFe-LDH/PS/Vis system and stability of CuFe-LDHIn the CuFe-LDH/PS/Vis system,it was confirmed by free radical trapping experiments and ESR that SO₄·^-,OH·existed in the MV degradation process,and SO₄·^-dominated the degradation.On the one hand,Cu and Fe in CuFe-LDH activatedPS to generate SO₄·^-by transforming their valence state;on the other hand,the photogenerated electrons by visible light excitation of CuFe-LDH could accelerate the valence state transition of metal ions,and electrons can also be captured by PSto produce SO₄·^-.The generated free radicals attacked the MV molecules for fast and efficient MV degradation.After used for 4 times,it still showed hig h relatively catalytic activity.It confirmed its good stability of CuFe-LDH.The above results showed that the CuFe-LDH/PS/Vis system was a new advanced oxidation technology with high efficiency,environmental protection and wide application prospects.