Kinetics And Mechanism Of Direct Photodegradation Of Halogenated Parabens Under Simulated Sunlight

Parabens are a classic of pharmaceutical and personal care products,which they have stronger antimicrobial properties than benzoic acid and sorbic acid because of the phenol hydroxyl.Since the mid-80s,parabens were extensively used in cosmetics,care products,medicine even food industry,they have been exposed to natural water environment frequently.Chlorine disinfection of traditional urban wastewater treatment technology have obvious removal effect on parabens,which attributed to that phenolic hydroxyl structure of parabens is easy to react with chlorine then produce chlorinated derivatives,In addition,the presence of trace bromide in sewage can also lead to the production of brominated derivatives.Once these halogenated derivatives have been generated in the water,it will be difficult to further conversion.And studies have proved that these halogenated derivatives have stronger biological toxicity than parabens,which is highly likely to cause the secondary water pollution and endanger human body health.From the perspective of environmental safety,the transition and transformation process of disinfection by-products of parabens in the water environment deserve to be detailed researched.This paper chose seven halogenatedparaben as target subjects to study their photochemical behavior in natural water environment under simulated sunlight.The fundamental factors:initial concentration of HPB,pH,coexisting substances on photochemical degradation kinetics and mechanism were discussed.The main research contents of this experiment and main conclusion are as follows:（1）Compared to parabens,the UV-vis absorption spectra of halogenated parabens performed obvious red-shift,maximum absorption peak from 254 nm of parabens red shift to around300 nm.（2）Concentration of parabens and their halogenated derivatives did not change in the dark,however,photodegradation of halogenated parabens followed the pseudo first-order reaction kinetics equation under simulated sunlight.As the decrease in initial concentration,the degradation rate constants increased.（3）Reaction rates of monochlorinated and monobrominated parabens increased with the pH from 6.0 to 8.0,and the reaction rate remained stable from pH 8.0 to 10.0.In contrast,reaction rates of dichlorinated and dibrominated parabens almost stayed unchanged in the range of pH 6.0to 10.0.（4）The existence of fulvic acid,humic acid greatly inhibited the direct degradation of halogenated parabens,chlorine ion concentration have little effect on photodegradation.Due to fulvic acid and humic acid played the role of light shielding rather than photosensitization.（5）In order of photolysis rates of halogenated parabens,they are as followed:3-BrEtP>3-BrMeP>3,5-diBrMeP>>3-ClEtP>3-ClMeP>3,5-diClEtP>3,5-diClMeP.The differences of photolysis rates were computed by QSAR model:three parameters C-X bond dissociation energy（BE）,the electronic effect of halogen atoms constant（σ）and the space effect of constant（Es）decided that brominated parabens own stronger light reaction activity than chlorinated parabens,and dibrominated（dichlorinated）parabens react faster than monobrominated（monochlorinated）parabens.（6）Halogenated parabens degraded directly under the simulated sunlight,mainly due to itsself triplet ³HPB^*and the role of oxygen O₂ instead of hydroxyl free radicals^?OH and singlet oxygen ¹O₂.Through a series of reactions such as dehalogenation,oxidation,alkylation,molecule state and deproton state of halogenated methylparabens produced different products.（7）Parabens and monohalogenated derivatives were detected obvious estrogen activity,and monochlorinated derivatives have stronger estrogen activity than the corresponding monobrominated derivatives,conversely the estrogen activity of dihalogenated derivative in the same concentration was undetected.