Study On Synergistic Heterogeneous Fenton-photocatalytic Oxidation System Based On Sulfate Radicals

Recent years, with the gradual establishment and improvement of environmentaland energy-related policies and regulations in China, the industrial water has beenrequired to have a higher recycling rate. Although the increasing of the water reusesaves the water resources and reduce production costs effectively, but also causes theaccumulation of organic matter in the water. So, the high concentration of varioustoxicity and refractory organic pollutants were usually rich in the industrial wastewater,and the traditional biological waste water treatment process is difficult to remove themeffectively. But, the advanced oxidation processes （AOPs） are considered to be aneffective way to deal with them, it can completely decompose the organic pollutantsinto harmless CO2and H2O. Among different AOPs, the Fenton reaction （Fe2+/H2O2）and TiO2photocatalytic oxidation technology are best suited to the industrial organicwastewater treatment as they can react at room temperature and atmospheric pressure,furthermore, their reaction rates are fast and the operation conditions are simple,theoretically. Actually, Fenton reaction requires excessive dose of Fe2+to catalyze H2O2decomposition to ensure the degradation of pollutants, because the Fe2+ions will beoxidized and precipitated in the form of Fe（OH）3at pH>3.7. The large quantities ofchemical ferric sludge will cause secondary pollution, in spite of Fe（OH）3has someflocculating effect. On the other hand, the TiO2photocatalysis is required sufficientintensity UV as light source. In the case of some dark color wastewater, the lightpenetration is weak, the treatment effect is poor. Furthermore, a large number ofnano-TiO2powders are not react with pollutant molecules effectively as reunited. Also,the suspended nano powders are difficult to remove from the water by filtration or othermethods, and it also lead to secondary pollution, easily. Therefore, in practicalengineering Fenton and TiO2photocatalysis need auxiliary facilities to adapt to differentwater quality and ensure the effective operation of the processes, and the treatmenteffect, so resulting in higher technology costs.The main contents and conclusions of the paper are as follows:①In order to overcome these technological bottlenecks, the authors innovativelydesigned and synthesized a “bifunctional catalyst”-Co-TiO2by using sol-gel method.“Bifunctional” means: When pH=6.9, the Co-TiO2catalyst can initiate SR-Fentonreaction via catalyzed PMS （KHSO5） to produce sulfate radicals （SO4） in dark （Co-TiO2/PMS, SR-Fenton） or exited by light （Photo/Co-TiO2, Photo） as photocatalyticcatalyst, respectively. Therefore, Co-TiO2catalyst is able to be applied in both dark andbright environments to removal organic pollutants in water flexibly. The results showthat: within2h reaction time, the degradation rates of50mg/L of rhodamine B （RhB）and phenol （PhOH） were77.5%and64.9%, respectively, in Co-TiO2/PMS process, atpH=6.9. At the same time, under visible （Vis） irradiation, the RhB and PhOHphotocatalytic degradation rate were45.6%and52.3%by Vis/Co-TiO2process.When the light, Co-TiO2and PMS coexist in the reaction system, SR-Fenton andphotocatalysis formed a synergistic oxidation system. Based on this, the authors hasdeveloped a novel “SR-Fenton/photocatalytic synergistic oxidation technology（Vis/Co-TiO2/PMS, SR-Fenton/Photo）” which integrated SR-Fenton and TiO2photocatalytic process in a heterogeneous catalytic system. Tests showed that under thesame conditions, degradation rate of RhB and PhOH （ηD） both increased to100%inVis/Co-TiO2/PMS system, and the reaction time reduced to80min. Calculation ofsynergistic coefficient （ηSyn） of SR-Fenton and TiO2photocatalysis based on the ηDof100mg/L RhB and PhOH degradation in Vis/Co-TiO2/PMS process were15.7%and12.6%. In addition, Co-TiO2showed good reusability and the Co2+ions leaching rate（ηLCo2+） was only1.4%. Magnetic test indicated that the Co-TiO2catalysts hasferromagnetic, so it has potential to recovery by magnetic force.②To analyze the catalytic mechanism of Co-TiO2in the SR-Fenton/Photosynergistic system, this paper used Co-TiO2/PMS, Vis/Co-TiO2and Vis/Co-TiO2/PMSas comparison system:1) In the study of the Co-TiO2/PMS process, the UV-DRS and XPS spectra showedthe Co3O4particles loaded on the surface of Co-TiO2catalyst. The radical quenchingtest showed theSO₄·^-was the main radical specie of Co-TiO2/PMS reaction, and thein-situ ATR-FTIR analysis determined the Co3O4particles were the SR-Fenton activesites of Co-TiO2catalyst. Using oxalate to quantify the radicals in the reaction madesure that1.0g Co-TiO2can catalyze1mol PMS to produce1molSO₄·^-. In the study ofVis/Co-TiO2process, the XRD spectra showed that a part of the Co2+ions has implantedinto the TiO2lattice.Based on first-principles calculation and UV-DRS, VBXPS tests,the Eg, EVBT, and ECBBwere2.30,2.0, and-0.3eV. So, it is a narrow-band semiconductor,can utilize visible light effectively. The visible light photocatalytic activity of Co-TiO2might be attributed to the Co2+ions doping. Meanwhile, EEM tests showed that theCo2+-doping also reduced the combination of photo-generated electron-hole pairs （hvb+/ecb）, and enhanced photocatalytic activity of Co-TiO2.2) From the Co XPS spectra of the fresh and spent Co-TiO2catalyst ofVis/Co-TiO2/PMS reaction, the Co（II） proportion of Co3O4particles, loaded on thesurface of Co-TiO2catalyst, rose from33%to46%; while it dropped from33%to12%in the Co-TiO2/PMS reaction. This results told us the strong reductive photo generatedelectrons from photocatalytic reaction reduced a part of Co（III） in Co3O4to Co（II）, andthe higher Co（II） amount improved the SR-Fenton catalytic efficiency. Meanwhile, thetrial also showed that PMS can effectively improve the apparent quantum yield （ΦApp）of TiO2photocatalytic process. After adding PMS to the UV/TiO2process, ΦAppofUV/TiO2reaction increases from0.0074to0.073μmol/J, and it can be inferred that thePMS can capture the photo-excited electrons. The enhancement of SR-Fenton andphotocatalysis of synergistic system improved the oxidizing ability of Vis/Co-TiO2/PMSreaction obviously. Using the radical quenching method to recognize the radicals inVis/Co-TiO2/PMS process, it can be found that when pH=6.9, OH andSO₄·^-both existin the hybrid system, and the radical total amount was34%more than the Co-TiO2/PMSreaction.3)Based on the results of GC/MS, LC/MS and quantity of major intermediates ofRhB degradation, we can found that the amount of by-products in the Vis/Co-TiO2/PMSprocess was greater than individual Co-TiO2/PMS or Vis/Co-TiO2reaction and theirgeneration rate was faster. The RhB degradation kinetics of Co-TiO2/PMS, Vis/Co-TiO2and Vis/Co-TiO2/PMS all fitted quasi-first-order kinetics well. The synergistic effectreflected in the reaction kinetics was that the apparent rate constants （kobs） ofVis/Co-TiO2/PMS （0.01399min-1） was one magnitude bigger than Vis/Co-TiO2（0.00161min-1） and Co-TiO2/PMS reaction （0.0699min-1）. Meanwhile, comparing withCo-TiO2/PMS the apparent activation energy （EAppa） was decreased from51.3kJ/mol to39.5kJ/mol. The synergistic coefficient calculated based kobs（Sc） was1.8.③Finally, the author using Vis/Co-TiO2/PMS synergistic system to remove thetrace POPs/EDCs substance atrazine and PAHs substance naphthalene and phenanthrene.Results indicated that the atrazine can be degrade effectively and some toxicintermediates are also completely removed. The reaction conditions were as follows:visible light source,[ATZ]0=0.1mM, mcata=0.01g, PMS/[ATZ]0=1:2, pH=6.9,50minreaction time. The synergistic efficiency （ηSyn） was37%and the synergistic coefficient（Sc） was5.30, respectively. Furthermore, the trace naphthalene and phenanthrene alsocan be rapid removal by the synergistic process, and their removal rates were not affected by NOM in water obviously. Compared with the similar studies, theVis/Co-TiO2/PMS synergistic system effectively saves the pharmaceutical dosageeffectively, furthermore, it also shorten the reaction time and reduce technology costs.Consequently, the Co-TiO2bifunctional catalyst and the synergisticSR-Fenton/Photo system build in this study partially overcome the drawbacks of acidicpH environment, large pharmaceutical dosage, and chemical sludge of traditionalFenton, as well as, ultraviolet light source dependence and catalyst recovery problem ofTiO2photocatalysis.