Degradation Of Chlorophenols In Water By Advanced Oxidation System Excited By Zero Valent Iron

2,4,6-Trichlorophenol（2,4,6-TCP）is one of the priority pollutants in the environment because of its stable structure and strong toxicity due to the unique position relationship between chlorine substituents and hydroxyl groups,for this kind of refractory materials,advanced oxidation technology is preferred.In recent years,advanced oxidation technology based on zero valent iron（ZVI）has attracted the attention of researchers in the field of water treatment due to its advantages of environmental friendliness,high efficiency and no secondary pollution.The advanced oxidation technology based on zero valent iron includes two systems with hydroxyl radicals（^·OH）and sulfate radicals(SO₄^·-)as oxidants.The two systems have different oxidation mechanisms for the target compounds.Therefore,in this study,2,4,6-trichlorophenol was used as the target pollutant,and two systems of zero valent iron/peroxydisulfate（PDS）and zero valent iron/organic ligand nitrilotriacetic acid（NTA）were used to treat 2,4,6-trichlorophenol in water,and the following works were carried out:（1）The ZVI/PDS system was established,and the effects of initial pH,sodium persulfate dosage,zero valent iron dosage and initial concentration of target substance on the degradation of 2,4,6-trichlorophenol were studied.The results showed that the best molar ratio of reactant 2,4,6-TCP/ZVI/PDS was 1/30/20.With the decrease of initial pH of the solution,the better performance of ZVI/PDS system on 2,4,6-trichlorophenol treatment was.The removal rate reached 85% at neutral pH（pH=7）and 95% at acid pH（pH=3）.The degradation effect of 2,4,6-trichlorophenol by ZVI/PDS system is better than that by ZVI and PDS alone.Through kinetic analysis,it is found that the degradation of 2,4,6-trichlorophenol by ZVI/PDS system accords with the pseudo first-order kinetic model.Scanning electron microscopy（SEM）was used to characterize the surface morphology of zero valent iron before and after the reaction.It can be seen that a large number of oxide clusters attached to the surface of zero valent iron after the reaction,which reduced the rate of ferrous ion produced by zero valent iron,also decreased the rate of degradation of 2,4,6-trichlorophenol in the later stage of ZVI/PDS system.It was concluded that the degradation of 2,4,6-trichlorophenol by ZVI/PDS system was mainly caused by sulfate radical.（2）ZVI/NTA system was established to investigate the effects of initial pH,concentration of triacetic acid,zero valent iron dosage and initial concentration of target substance on the degradation of 2,4,6-trichlorophenol.The results showed that the best reaction conditions of ZVI/NTA system were:the initial concentration of2,4,6-trichlorophenol is 20 mg/L,the dosage of zero valent ironwas 2 g/L,and the concentration of aminotriacetic acid was 0.3 mmol/L.Under this ratio,the degradation rate of 2,4,6-trichlorophenol can reach 100%in the range of initial pH=3～9.Through the kinetic analysis of 2,4,6-trichlorophenol degradation,it is found that the degradation of 2,4,6-trichlorophenol by ZVI/NTA system conforms to the pseudo first-order kinetic model.SEM was used to characterize the surface morphology of zero valent iron.It was found that the particle diameter of zero valent iron decreased after the reaction,and there was no passivation layer or oxide cluster on the surface,which indicated that the existence of aminotriacetic acid inhibited the formation of passivation layer on the surface of zero valent iron.Free radical scavenger tert-butanol can inhibit the degradation of 2,4,6-trichlorophenol in ZVI/NTA system.Therefore,it was concluded that the degradation of 2,4,6-trichlorophenol in ZVI/NTA system was mainly through the hydroxyl radicals.（3）Small organic acids such as dichlorohydroquinone,dichloro-p-benzoquinone and acetic acid were detected by GC-MS,it was concluded that the degradation of2,4,6-trichlorophenol by ZVI/PDS system and ZVI/NTA system can be three stages:oxidation/reduction dechlorination,opening of intermediate products and mineralization and decomposition of small molecular organic acids.In comparison,the dechlorination rate and mineralization rate of ZVI/NTA system are higher,reaching 90% and 48%,respectively.