Research On The Formation Mechanisms Of Typical Bromine Byproducts Under Ultrasonic Irradiation

Using ultrasonic to degrade the chemical pollutants,especially the refractory organic pollutants,is a new type of water treatment technology which has been being developed in recent years.It is a technology of high temperature pyrolysis,advanced oxidation and supercritical oxidation,and has the advantages of high efficiency,wide range of applications,and can be combined with other water treatment technologies.The reports on lab research level and application of degradation of organic pollutants in water by ultrasonic are also increasing and many valuable results were obtained.However,there are not reports about brominated by-products generating in the process of using ultrasonic cavitation oxidation technology to degrade organic matter.So the mechanism of degradation of phenols by ultrasonic and brominated by-products generating in this process will be studied in this paper.The study of inorganic brominated byproducts generated in the process of ultrasonic cavitation oxidation found that,in the presence of Br-,BrO3-can be generated during ultrasonic cavitation oxidation reaction.BrO3-in the ultrasonic cavitation are resulted in the pyrolysis of water molecules to produce hydroxyl radicals(·OH),then ·OH reacte with Br-to produce Br2·-,followed by the reaction between Br2·-and ·OH to produce BrO3-finally.The the yields of BrO3-decrease with the energy density of ultrasonic,indicating that a part of BrO3-are oxidized to BrO4-.The research targets are phenol(P)and 4-nitrophenol(4-NP)in the study of ultrasonic degradation of phenols.P and4-NP can be degraded in the absence of water,and the subsequent research found that the ultrasonic degradation of both of them were promoted by Br-,and the degradation reaction were fitting in the pseudo first order kinetics model.In the process of ultrasonic degradation of phenols,phenols can not only be degraded by ·OH,but also by Br2·-.4-NP's structural properties are similar to P,but 4-NP is more hydrophobic,making 4-NP more easily to get close to the gas-solution interface to compete ·OH.The degradation rates of P and4-NP increase with the increase of power intensity of ultrasonic degradation.The maximum degradation rate of P occurred when initial concentration of Br-was 600 times of initial concentration of P,while the maximum degradation rate of 4-NP occurred when initial concentration of Brwas 400 times of initial concentration of 4-NP.And the ratio between the ideal time of P and 4-NP with Br-was 3:2.The research targets are 2-bromophenol,4-bromophenol and 2,4-dibromophenol as the byproducts in the process of ultrasonic degradation of P.The research targets are 2-bromo-4-nitrophenol and 2,6-dibromo-4-nitrophenol as the byproducts in the process of ultrasonic degradation of 4-NP.Experimental results show that,in the presence of Br-,2-bromophenol,4-bromophenol and 2,4-dibromophenol can be generated during ultrasonic degradation of P,and 2-bromo-4-nitrophenol and 2,6-dibromo-4-nitrophenol can be generated during ultrasonic degradation of 4-NP.Monobromophenols can be further degraded by cavitation oxidation to other byproducts,while dibromophenols will be accumulated.The initial generation rates of 5 brominated phenols increase at first and then decrease with the increase of initial concentration of phenols.The initial generation rates of 5 brominated phenols increase with the increase of power intensity of ultrasonic degradation.The generation mechanism of brominated phenols is: Br2·-react with phenols to produce phenoxyl radical(HPhO·),then Br2·-further oxidize HPhO·to produce monobromophenols;by such analogy,monobromophenols also react with Br2·-to produce appropriate HPhO·,then Br2·-further oxidize these HPhO·to produce dibromophenols;finally dibromophenols are oxidized to produce tribromophenols.Moreover,brominated phenols and phenols can be further oxidized to be OH-PBDEs.