Study On Degradation Of Organic Pollutants In Polvphenol-Fe₃O₄ Fenton-like System

Fenton reagent(Fe²⁺ and H₂O₂)can degrade refractory organics with ·OH produced by H₂O₂ and Fe²⁺.It has been received widely attention in recent years.The engineering practice of organic wastewater treatment shows some disadvantages of Fenton reactions,which include the difficult regeneration of Fe???,the secondary pollution of iron sludge,the low utilization efficiency of H₂O₂,and the narrow range of working pH.Therefore,it is urgent to explore a low cost,high efficiency of Fenton oxidation technology.In order to improve the circulation ability of iron ion,EDTA,NTA,catechol and other organic matter are always introduced into Fenton system,which can promote the Fenton reaction and implement the efficient mineralization of pollutants in a wide pH range.However,the addition of organic ligands will increase the total organic carbon?TOC?of Fenton system and consume a certain amount of hydroxyl radicals?·OH?.Fe₃O₄ possesses peroxidase activity and could catalyze H₂O₂ decomposition to produce-OH.Moreover,Fe₃O₄ is magnetic,and it is easy to be separated and recycled.Polyphenol organics containing catechol group have been demonstrated to bind tightly with iron oxide,enhancing the dispersibility and catalytic activity of iron oxides.In this study,Fe₃O₄ is modified by polycatechol and gallic acid making it functionalization,which benefits the application of magnetic nanoparticles in water treatment.The adsorptive property and the catalytic mechanism of the prepared polyphenol-Fe₃O₄ magnetic nanocomposites,are all researched deeply.Specific work content is divided into the following several parts:?1?The influences of the addition of catechol and gallic acid?GA?on the degradation of methylene blue?MB?by heterogeneous Fenton reaction with Fe₃O₄ as a catalyst were investigated.The results indicated that the addition of catechol and GA not only promoted the adsorption of MB but also enhanced the catalytic ability of Fe₃O₄,thereby promoting the degradation of MB by Fe₃O₄/Fenton.When Fe₃O₄ 1.0 g L"1,MB 0.1 mM,H₂O₂ 160 mM,the removal efficiency of MB was enhanced from 63%to 79%and 90%with the addition of 0.1 mM catechol and 0.1 mM GA,respectively.The result of electron spin resonance?ESR?showed that the yield of ·OH in Fe₃O₄/Fenton with the addition of polyphenol was higher than that of Fe₃O₄/Fenton.The addition of polyphenol enhanced the catalytic activity of Fe₃O₄ for the decomposition of H₂O₂,increased the yield of ·OH,and promoting the degradation of MB.?2?The formation and characteristics of PCC were investigated.In the mixture solution of catechol and Fe?III?,there are coordination reaction,redox reaction and the polymerization reaction of catechol.Catechol polymerized to form polycatechol?PCC?catalyzed by Fe???,which was a kind of black precipitation.In the meantime,a little of Fe?III?was combined onto the surface of PCC through coordination.In the processes of polymerization reaction,catechol monomers combined with c-o-c bond.In order to aviod increasing the TOC of Fenton system due to addition of polyphonel,as well as enhancing the catalytic activity of Fe₃O₄,the polyphonel modified Fe₃O₄ MNPs?polyphonel-Fe₃O₄ MNPs?were prepared.The features of Fe₃O₄/PCC MNPs were characterized by TEM,XRD,FTIR and XPS and so on.The results indicated that the Fe₃O₄/PCC MNPs were prepared sucsessfully verified by the C-O-C,C-O based on FTIR analysis,and the binding beteen Fe₃O₄ and polycatechol was through the complexation of phenolic hydroxyl and iron oxide confirmed by the Fe-O-C based on XPS analysis.Furthermore,the dispersity of Fe₃O₄/PCC MNPs was much better than that of Fe₃O₄,indicating that the modification of polycatechol effectively avoiding the agglomeration of nanoparticles,which made the Fe₃O₄/PCC MNPs small particle size?6.32 nm?and large specific surface area?111.24 m2 g-1?.The Fe₃O₄/PCC MNPs were negatively charged in the range of pH 2.0?10.0 due to the presence of phenolic hydroxyl groups on the surface of the MNPs.?3?Five cationic dyes including methylene blue?MB?,cationic turquoise blue GB?GB?,malachite green?MG?,crystal violet?CV?and cationic pink FG?FG?were chosen as model compounds to study the adsorption and degradation behavior of Fe₃O₄/PCC MNPs.The results indicated that the adsorption of the five cationic dyes were all fit better with Langmuir equation than Freundlich equation according to the correlation coefficients.And the adsorption kinetics of these five cationic dyes on Fe₃O₄/PCC MNPs were well described by pseudo-second-order kinetic model.The saturated adsorption capacities of MB,GB,MQ Cv and FG on the Fe₃O₄/PCC MNPs were 60.06 mg g-1,50.03 mg g-1,63.21 mg g-1,43.72 mg g-1 and 52.97 mg g-1,respectively.Moreover,the higher the solution pH was,the more negative charges the Fe₃O₄/PCC behaved,and the more favorable for the MB adsorption.The coexisting cations could easily occupy the negative charge on the surface of the MNPs,which inhibited the adsorption of MB.Furthermore,Fe₃O₄/PCC MNPs were used as Fenton catalyst to decompose H₂O₂ to generate ·OH radicals to degrade MB and the catalytic ability of the MNPs was investigated.The removal effeciency of MB by Fe₃O₄/PCC?99.7%?was higher than that of Fe₃O₄?10%?,and the yield of · OH radicals in the Fe₃O₄/PCC MNPs Fenton system was higher than that of Fe₃O₄ MNPs Fenton system.The removal efficiency of TOC reached as much as 48.3%using 1.0 g L-1 Fe₃O₄/PCC MNPs and 40 mM H₂O₂ at pH 6.0 and 30? within 120 min.MB could be removed efficiently by Fe₃O₄/PCC MNPs in a wide pH range from 3.0 to 10.0,indicating the pH range was expanded greatly.The content of Fe?II?on the surface of Fe₃O₄/PCC changed from 32.6%to 34.8%after oxide reaction,and it was not decreased due to the modification of polycatechol,which was conducive to the regeneration of Fe?II?.The advantages of Fe₃O₄/PCC were as shown as follows:on one hand,the surface nagetive charged of Fe₃O₄/PCC made it adsorb cation dyes,facilitating to the attack of ·OH.On the other hand,the addition of polycatechol enhanced the regeneration of Fe?II?.Moreover,the surface defects of the MNPs caused by polycatechol modification are beneficial to the catalytic activity.All in all,Fe₃O₄/PCC MNPs behaved a strong stability,easy separation to recycle,inexpensive and facil preparetion processes.?4?Gallic acid?GA?modified Fe₃O₄ and supported by graphite oxide?GO?synthesized according to Hummers' method,Fe₃O₄/GA/GO MNPs,were prepared and charactered.The MNPs were used as Fenton catalyst to catalyze H₂O₂ decomposition to generate ·OH and degrade MB.The machenisms of synthesis and catalysis of Fe₃O₄/GA/GO MNPs were also discussed.Fe₃O₄/GA/GO MNPs had a strong adsorption ability due to the negative charge on the surface of the MNPs,and also because of the large specific surface area.The target pollutants adsorbed onto the surface of the catalyst,was conducive to the attack of·OH radicals,thus promoting the Fenton reaction.In addition,the modification of GA could reduce the reduction potential of Fe?III?/Fe?II?on the surface of Fe₃O₄/GA/GO MNPs,promoting Fe?III?/Fe?II?cycle and enhancing Fe?II?regenerate ability.The defect on the surface of Fe₃O₄/GA/GO MNPs caused by the modification of GA and GO also enhanced the catalytic ability of the Fe₃O₄/GA/GO MNPs.