Zero-valent Iron Assisted Photocatalytic Degradation Of Typical Polychlorinated Naphthalene In Aqueous Systems:Synergistic Effect And Transformation Products

PCNs have been demonstrated to be of dioxin-like toxicity and bioaccumulation.PCNs also have some harmful effects on organisms.Therefore,it is necessary to study the effective degradation method of PCNs.This study selected 2-chloronaphthalene and octachloronaphthalene as typical contaminants of PCNs congeners.The photocatalytic degradation of 2-chloronaphthalene and octachloronaphthalene by zero-valent iron(ZVI)assist titanium dioxide(Ti02)were investigated.The effects of the main factors on the degradation of 2-chloronaphthalene and octachloronaphthalene were studied.The photodegradation mechanism of 2-chloronaphthalene was inferred from the degradation products of 2-chloronaphthalene.Then,the octachloronaphthalene was degraded under the optimal photodegradation conditions of 2-chloronaphthalene.The purpose of this study was to find an effective degradation method and optimal degradation conditions of PCNs.The experimental results show that 2-chloronaphthalene can be completely degraded in the UV/ZVI/TiO2 system,and the degradation rate is fast.The degradation efficiency of octachloronaphthalene can reach 79%,and the degradation rate is obviously lower than that of 2-chloronaphthalene.The degradation rate of 2-chloronaphthalene and octachloronaphthalene gradually decreased with the increase of initial concentration.The acidic conditions are more contuibute to photodegradation of 2-chloronaphthalene and octachloronaphthalene.At different initial concentrations,the photodegradation process of 2-chloronaphthalene and octachloronaphthalene followed to the first-order kinetic reaction.The degradation of 2-chloronaphthalene was mainly indirect photodegradation,and the degradation of octachloronaphthalene was mainly direct photodegradation.The anions in the water can react with hydroxyl radicals to inhibit the photodegradation of 2-chloronaphthalene.The order of inhibition is NO3-<HCO3-<Cl-<CO32-.The radical quenching experiment found that hydroxyl radicals play a major role.In the octachloronaphthalene degradation process,with the increase of chloride ion traps,the degradation rate increased from 79%to 89%.The photodegradation products of 2-chloronaphthalene was detected by LC-MS.there are mainly four kinds of photodegradation products of 2-chloronaphthalene:naphthol,naphthalene,phenethyl alcohol and ethylbenzene.The photodegradation process of 2-chloronaphthalene was speculated based on the intermediate products of 2-chloronaphthalene.The chlorine of 2-chloronaphthalene was replaced by hydroxy to form naphthol,and then hydroxyl radical oxidized naphthol,naphthalene ring was opened to produce phenylethyl alcohol,phenylethyl alcohol Continued dehydroxylation to ethylbenzene,the final ethylbenzene continues to be oxidized by hydroxyl radicals to open the ring,and finally oxidized to inorganic molecules such as CO2 and H2O.