The Study On Citric Acid Promotes Zero-valent Iron To Activate Molecular Oxygen And Produce Free Radicals

Zero valent iron（ZVI,Fe⁰）has been widely used for environmental remediation because of its inexpensive,abundant reserves in earth and environmental friendliness.Nanoscale zero-valent iron（nZVI,Fe⁰）has better degradation efficiency to contaminant due to its bigger specific surface area and more active sites.In recently years,more and more attentions have been paid to oxidative degradation over nZVI by activating molecular oxygen to produce reactive oxygen species（ROS）which are able to oxidize pollutants such as halogenated compounds,phenols.The modification on the surface of nZVI by transition metals to form bimetallic nZVI is benefit to improve the reactivity of it,and the dehalogenation of halogenated compounds.On the other hand,the addition of ligand in the process of pollutants degradation using nZVI will enhance the generation of ROS and reduce the deposition of iron.In this study,Fe⁰ and silver doped bimetallic material（Ag/Fe⁰）were synthesized using aqueous phase reduction method.Citric acid（CA）was chosen to mix with nZVI when degrading pollutants to enhance the generation of ROS,reduce the deposition of iron（hydrogen）oxide and promote the degradation of pollutants.The chief research includes the following:（1）Nanoscaled Fe⁰ was synthesized by NaBH₄ reduction method and used to degrade4-chlorophenol（4-CP）in the presence of CA,and the process of degradation as well as the mechanism of oxidation were investigated systematically.The highest degradation efficiency of 4-CP was achieved at 2m M of CA,and 56.2%of 4-CP was removed after 1 h under confined dark condition without any ventilation of oxygen in the presence of Fe⁰ and CA.The addition of CA promoted the release of ferrous ions as well as the generation of free radicals,and reduced the deposition of iron.The iron citrate complexes formed in Fe⁰-CA system promoted the degradation of 4-CP.（2）In the absence of 4-CP,there were no influence to the release of Fe（II）and Fe（III）in aqueous solution,as well as the generation of H₂O₂,which indicated that CA accelerated the activation of molecular oxygen by Fe⁰ to produce free radicals.The control experiments show that the iron citrate complexes itself could not degrade 4-CP directly,and the complexes would facilitate the generation of ROS reduced by H₂O₂ to degrade 4-CP.MINEQL+4.6 soft was utilized to calculated the existent species in Fe⁰-CA system,and Fe（II）[Cit]^-was found to be the key species playing an important role on the.Furthermore,the possible mechanism of degradation of 4-CP by Fe⁰-CA system was put forward.（3）Bimetallic Ag/Fe⁰was synthesized and used to degrade pentabromophenyl ether（BDE209）in the presence of CA,and the degradation pathway of BDE209 were deeply evaluated by analyzing the intermediate products.The highest degradation efficiency of BDE209 was achieved when atom ratio of Ag/Fe is 0.2%.The degradation efficiency of BDE209 by Fe⁰-CA system and Ag/Fe⁰-CA system after 24 h were 68.7%and 72.7%,respectively.But the mineralization of the two systems was obviously different,scince the removal rate of total organic carbon（TOC）by Fe⁰-CA system and Ag/Fe⁰-CA system were9.1%and 18.2%,respectively.The degradation pathway of BDE209 by Fe⁰-CA system and Ag/Fe⁰-CA system was proposed as follows:Firstly,the ether bond was not destroyed,and the bromine atoms were replaced by hydroxyl groups and oxidized to open the benzene ring subsequently;Secondly,the ether bond and the structure of the benzene ring were destroyed,and oxidized to bromobenzene quinone with hydroxyl groups and penta cyclic compounds;Thirdly,either bromophenol or alcohol products were formed after debromination of BDE209.Furthermore,Ag/Fe⁰-CA system showed higher oxidation efficiency of BDE209than Fe⁰-CA system.