The Removal Of Methyl Orange And 2,4-DCP By Enhanced Zero-valent-zinc Technology

The removal of azo dyes and chlorophenol compounds for environmental treatment has received increasing attention due to their characteristics of chemical structure stability,high toxicity,poor biodegradablity and easy to be biological enrichment amplification,which not only affect the environmental quality,but also have a serious threat to human health.In this study two different enhanced zero-valent zinc(ZVZ)methods were adopted respectively to degrade methyl orange(MO)and 2,4-dichlorophenol(2,4-DCP).The influencing factors for the degradation of MO by nano zero-valent zinc(nZVZ)assisted with silica gel and the degradation of 2,4-DCP by micro zero-valent zinc with the irradiation of ultraviolet light(Zn0/UV)were investigated and the possible mechanisms were explored.This study consisted of two parts:In Part ?:The degradation of MO by nZVZ alone and nZVZ assisted with silica gel(SG)was investigated under different conditions.The results showed that nZVZ increased the rate of degradation,compared with micro zero-valent zinc.But nZVZ is easy to agglomerate,and the oxide and hydroxide of zinc(Zn(?)(hydr)oxides)was formed on surface due to the rapid corrosion of nZVZ particles,leading to the insufficient contaction with MO.However,the degradation of MO by nZVZ could be markedly enhanced when silica gel was added,which was attributed to the good dispersion and supporter role of silica gel and effectively prevent Zn(?)(hydr)oxides generating,so the decolorization of MO could be complete in a short time,even under neutral and alkaline conditions.And the solution pHs maintained near-neutral after the degradation of MO in different initial pH conditions,which is of great significance to the practical application of wastewater treatment.The efficiency of MO removal was improved with a pH decrease,an increase of the initial amount of silica gel and nZVZ and an anaerobic condition.The degradation of MO by nZVZ alone and by nZVZ assisted with silica gel can both be described by a pseudo-first-order kinetic.And the surface of nZVZ and silica gel before and after the reaction was characterized by the microscopic analysis of morphology and elemental analysis,combined with the full wavelength scanning,revealing the potential mechanism responsible for the enhanced reactivity of silica gel.Futhermore,the environment-friendly material nano hydroxyapatite(nHAP)was used to adsorb Zn2+ produced in the degradation process,which meet the national emission standard of 2 ppm.Finally,the mineralization of MO after degradation by nZVZ and with subsequent UV irradiation was investigated by monitoring total organic carbon(TOC),offering an efficient method to mineralization of MO.In Part ?:The degradation of 2,4-DCP by micro zero-valent zinc(Zn0)enhanced by UV irradation was investigated.The results showed that the Zn0/UV system was superior to the control groups including Zn0 alone,xenon lamp(simulated sunlight)alone,mercury lamp(UV)alone and Zn0/xenon lamp systems.The effects of the initial solution pH,Zn0 dosage,light intensity,temperature,the atmosphere of O2 and N2 on 2,4-DCP degradation by UV alone and Zn0/UV were studied.2,4-DCP(50 mg/L)was almost completely removed by 0.01 g zero-valent zinc,under the irradation of a 300 W medium pressure Hg lamp at pH 5 and 25? within 60 min.Under the optimum conditions,the dechlorination rate and the dissolution Zn2+ were determined and the adsorption of Zn2+ was carried out with nHAP.The Zn0/UV system was also adopted to degrade 2-chlorophenol(2-CP),and 4-chlorophenol(4-CP).Possible reaction mechanism and degradation pathways of 2,4-DCP were speculated through detecting free-radical formation using electron paramagnetic resonance(EPR)spectroscopy.