Degradation Of Typical Organic Pollutants In Water By Persulfate-based Advanced Oxidation Systems With Catalysts Based On Fe/Co

With the deteriorating of population explosion,water shortage,and environmental pollution,water pollution has been becoming an urgent issue and needs to be solved by contemporary society.Sulfate radical(SO4·-)-based advanced oxidation process is a kind of chemical repair technology which can quickly and efficiently purify all kinds of organic pollutants in water.In recent years,it has been attracted much attentions from experts and scholars due to its outstanding oxidation ability,wide application range,environmental friendlinessand other advantages.The persulfate including peroxidisulfate(PDS)and peroxomonosulfate(PMS)is relatively stable in room temperature and hard to decompose and generate SO4·-to degrade organic pollutants.Hence,it needs extra energy or catalysts,the transition metal ionsare usually used as catalysts because they are simple,convenient,and effective.Fe2+ and Co2+ are the optimal activators for PDS and PMS respectively.However,there are still some drawbacks such as radical scavenger reactions and generation of iron sludge after reaction in the Fe2+ activation system.Also,Co2+ would result in secondary pollution due to its toxicity.In order to solve these problems,the stable and effective heterogeneous catalysts have been taken place of the homogeneous catalysts by researchers.Two typical refractory organic pollutants,dibutyl phthalate(DBP)and azo dye acid orange 7(AO7)were used as the target contaminants,different particle sizes of zero valent iron(ZVI),the core-shell Fe0@Fe3O4 complex and the cobalt metal organic frameworks Co3(BTC)2·12H2O(Co-BTC)were employed as the activator of persulfate.Raman spectroscopy,infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS),scanning electron microscopy(SEM),X-ray Diffraction(XRD),electrochemical,high performance liquid chromatography(HPLC)and gas chromatograph-mass spectrometer(GCMS)were used to analysis the characterization of materials,activation mechanism and the pollutants degradation mechanism.The main research contents and conclusions are shown in the following:(1)In order to reduce the scavenging reactions and improve the pollutants degradation efficiency,different particle size of ZVI,including milli-ZVI(1mm),micro-ZVI(150?m)and nano-ZVI(50 nm)were employed to activate PDS for degradation of AO7.The results demonstrated that AO7 removal in micro-ZVI/PDS system significantly improved compared with the Fe2+/PDS system,the production of iron sludge also reduced.The results was due to the slow generation of Fe2+ by ZVI,which avoid the fast quenching reactions between free radicals and Fe2+ due to the plenty of free radicals.The results also showed that different ZVI types exhibited a respective level of ability to activate persulfate to degrade acid orange 7,which ranged as nano-ZVI> micro-ZVI> milli-ZVI.The variation trend of Fe2+,pH and ORP were analysis for clearing the ZVI activation mechanism,as well as the characterization of ZVI after reactions by XRD and Raman spectroscopy,which indicated that homogeneous activation was the primary way to produce free radicals in micro-ZVI/PDS system.In order to further understand the differences between three types of ZVI on the degradation of AO7,the intermediate products of AO7 were identified by GC/MS measurement.In addition,it was found that the intermediates in the milli-ZVI system were the least while they were the most in the micro-ZVI system.(2)In view of the fact that waters or sewages often contain some inorganic anions,organic matter and so on,to clear the influence and the influence mechanism of these substances on the micro-ZVI/PDS system can provide theoretical guidance for improving the treatmentefficiency of this system.It is interesting to find that ClO4-,CH3COO-and humic acid(HA)were found to accelerate AO7 decolorization rates while other inorganic anions retarded AO7 decolorization in the following sequence: NO2-> H2PO4-> HPO42-> EDTA > SO42-> CO32-> HCO3->NO3-> Cl-.The influence mechanisms were through complexation reactions with Fe2+ generated from ZVI,consuming of SO4·-by scavenging reactions,compete for ZVI with PDS,and acting as an electron ?shuttle‘ and facilitating electron transfer from ZVI surface to PDS.In addition,the presence of dissolved SiO2 also decreased the AO7 degradation ratio and prolonged the PDS half-life.But the effect of dissolved SiO2 on the different particle size ZVI was different,and the dissolved SiO2 had the most significant inhibited effect on the milli-ZVI system.The decomposition of PDS combined with the characterization by Raman spectra,FT-IR,and EDS on the ZVI surface indicated that the dissolved SiO2 affected the corrosion of ZVI,altered the surface area with the adsorption of SiO2,and appeared to influence the decomposition of PDS and AO7.(3)For clearing the effect and effect mechanism of pH on the micro-ZVI/PDS system,the effect of initial pH on the degradation of DBP,the decomposition of PDS,and the formation and evolution of micro-ZVI corrosion layer were investigated.The results showed the DBP degradation decreased with the increase of the initial pH.The constituents and the morphology of the iron corrosion coating along different initial pH and over reaction time were conducted with Raman spectra,XPS,and SEM,which revealed that in alkaline and neutral solutions,the inner layer of iron oxides was mainly composed of ?-Fe2O3 with some ?-FeOOH while the outer layer mostly consisted of Fe3O4 and ?-FeOOH.Oppositely,the iron oxides formation in acidic solution mainly consisted of Fe3O4,?-FeOOH and a small amount of ?-Fe2O3 and FeO in the inner layer,the outer layer was mostly composed of ?-Fe2O3 andsome ?-FeOOH.These iron corrosion coatings exhibited an inhibitory effect on the degradation of DBP,which may be due to that the iron oxides hindered the electron transfer from ZVI core to the solid-liquid interface and PDS.(4)In order to make clear the DBP oxidation mechanisms in the micro-ZVI/PDS system,Chemical Probe and Electron Spin Resonance spin-trapping methods were conducted to identify the kinds of free radicals.The results indicated that both SO4·-and hydroxyl radical(·OH)were found to be primary oxidants at pH 3.0 and pH 7.0 while ·OH and superoxide radical(O2·-)were the major species to oxidize DBP at pH 11.0.The transformation of DBP degradation products were analysis by GC-MS,and their degradation mechanisms and pathways in p H 3.0 were proposed according to the intermediate products,which suggested that dealkylation,hydroxylation,decarboxylation,and hydrogen extraction were the dominant degradation mechanisms.(5)In order to improve the catalytic activity of micro-ZVI on activating of PDS for the degradation of DBP,the micro-ZVI was modified with a facile and simple one-step thermal oxidation method that ZVI reacts with water vapor under high purity nitrogen atmosphere and high temperature.The results of XRD,Raman spectra,and XPS characterization showed that the modified material were core-shell Fe0@Fe3O4 composites.Compared to micro-ZVI,the catalytic performance of Fe0@Fe3O4 composites greatly enhanced under neutral conditions and the DBP removal increased with increasing the concentrations of PDS and Fe0@Fe3O4,and the DBP degradation efficiency reached 94.7% under optimal conditions.In addition,Fe0@Fe3O4 exhibited excellent reusability according to six consecutive reaction cycles with no obvious loss of catalytic activity.More interestingly,the DBP degradation rate increased in the first three cycles,which was due to the increase of the homogeneous reactions.The mechanism for activating of PDS by Fe0@Fe3O4 composites involved in their special interphase surface which could readily produce Feoct2+ species by an efficient electron transfer process that initiated the reaction to generate SO4·-by a Haber-Weiss mechanism.(6)In order to reduce the secondary pollution by toxic cobalt ions after reaction in the Co2+/PMS system,two kinds of Metal-Organic Framework(Co3(BTC)2·12H2O,Co-BTC(A)and Co-BTC(B))were synthesized by hydrothermal method with two different organic solvents.The results indicated that although the particle size of Co-BTC(B)was small,CoBTC(A)showed better performance on DBP degradation by activating of PMS,where the highest degradation rate of 100% was obtained within 30 min.The DBP degradation efficiency decreased by 0%,8.26%,10.9%,and 25.6% in the 2nd,3rd,4th,and 5th runs,respectively.The loss of active catalytic sites of Co(II)from Co-BTC(A)was responsible for the activity decay.According to the analysis of XRD,XPS,FT-IR and electrochemical analysis,the possible mechanism for activation of PMS by Co-BTC(A)involved in homogeneous and heterogeneous reactions in the solutions and the surface of Co-BTC(A),respectively,which resulted in decomposition of PMS to produce SO4·-.