Degradation Of 2,4-Dichlorophenoxyacetic Acid By Heterogeneous Fenton-like System And Activated Persulfate Process

In recent years,the organochlorine pesticides 2,4-dichlorophenoxyacetic acid（2,4-D）has been frequently detected in both surface water and groundwater worldwide,and the potential negative effectsof 2,4-Don public health have also attracted increasing concern.Hence,more powerful and efficient methods are required to treat the water and wastewater contaminated with this kind of pollutant.In this study,2,4-D was chosen as the model pollutant.FeS,FeS/Fe₂O₃ and Fe/Fe₂O₃ were prepared,characterized and used as the heterogeneous catalystsin both Fenton-like system and activated persulfate（PS）for 2,4-D degradation.Several impact factors,including initial pH,oxidant（H₂O₂ or PS）concentrations,catalystdosage and reaction temperature,and the reaction kinetics wereinvestigated.The degradation mechanisms of heterogeneous catalysts and possible transformation of 2,4-D were proposed.The main results are listed as follow:（1）FeS,FeS/Fe₂O₃ and Fe/Fe₂O₃ exhibited good performance both in catalyzing H₂O₂ and activating PS for 2,4-D oxidation.Different influences of initial pH values on 2,4-D removal were observed in different reaction systems.Acidic pH would accelerate 2,4-D degradation.Additionally,the degradation efficiency of 2,4-D was enhanced with increasing catalyst dosage,H₂O₂ dosage,PSconcentration and reaction temperature,whereas excessive doses of the oxidants inhibited 2,4-D decomposition to some extent.（2）Under all the test conditions,the degradationof 2,4-Dfitted well with the pseudo-first-order kinetics pattern and the observed reaction rate constants well followed the Arrhenius equation.The lowEa value in our study indicated that the 2,4-Doxidation by both heterogeneous Fenton-likeand activated PS process required a relatively low energy and could be easily achieved.Additionally,the reusability tests revealed thatFeS still maintained satisfactory catalytic activity in five consecutive experiments of 2,4-D degradation,although it had a short time of deactivation after first-run in both heterogeneous Fenton-likeand activated PS process.（3）The scavenging experiments revealed that in heterogeneous Fenton-like system,2,4-D was mainly decomposed by the attack of HO·at pH = 4.5,especially the surface-bounded HO·.On the other hand,both HO·and sulfate radicals（SO4-·）were responsible for 2,4-D degradationin the activated PS process,whileHO·might play a dominant role.（4）Based on the quenching tests and electron paramagnetic resonance（EPR）analysis,the possible mechanisms of 2,4-D oxidation in heterogeneous Fenton-like system and activated PS system were proposed.The transformation productsof 2,4-D during the degradation were identifiedusinghigh performance liquid chromatography（HPLC）,ion chromatograph（IC）and gas chromatography mass spectrometry（GC–MS）,and the possible reaction pathways were proposed based on the measured results.