Photo-assisted Selective Removal Efficiency And Mechanism Of Polybrominated Diphenyl Ethers In Surfactant Solutions

Polybrominated diphenyl ethers(PBDEs),as a widely used brominated flame retardant,are often detected in the environment.Because of their high toxicity and persistence in the environment,they pose a serious threat to human health and ecological safety.Surfactant aided washing technology has great advantages in the treatment of high level PBDEs in soil and sediment.However,the treatment of PBDEs washing wastes has become the main factor restricting the further application of this technology.Photodegradation is an important way to remove PBDEs in the environment,is used to treat the surfactant washing wastes of PBDEs.Based on the comprehensive analysis of present situation of pollution and treat technology,the photo-treatment technology is applied to the treatment of PBDEs in surfactant washing wastes.The photodegradation pathway and kinetics of PBDEs were analyzed,and the reaction kinetics model of PBDEs in surfactant solution was established.The mechanism of photodegradation to remove PBDEs from surfactant under different environmental conditions was explored,and the optimal method for the PBDEs photodegradation process was optimized.The optimal method of the fast degradation rate of PBDEs,less accumulation of intermediate products,and high recovery of washing waste was proposed.The main achievements of this study are as follows:(1)The photo-treatment can selectively remove PBDEs from surfactants through the photodegradation of PBDEs in different surfactant solutions and recover the surfactant.Among the cationic(CTAB),anionic(SDBS),and non-ionic(TX-100)surfactants,the nonionic surfactant of TX-100 showed better efficiency for PBDEs in both washing and photodegradation processes.Among the TX series surfactants with different hydrophilic chain length(TX-100,TX-165,TX-305,and TX-405),TX-100 with shorter hydrophilic chain has better efficiency.In the process of photodegradation,the degradation of PBDEs mainly underwent the process of step-by-step reductive debromination.In addition,hydroxy-biphenyl products may be generated by cleavage of ether bonds and polybrominated dibenzofurans may be produced by dehydrogenation and cyclization.The accumulative amount of dibenzofuran in TX-100 is least.As the selectivity of micelle dissolution,PBDEs photo treated in surfactants is a spontaneous continuous mineralization system,which first reduces and then oxidizes.E.coli test indicated that the photo-treatment process reduces the toxicity of the surfactant eluate.In the process of photo treatment,the loss of surfactant is less than 15%after the complete degradation of PBDEs,and it can be reused.The surfactant in the solution is mainly oxidized and degraded by reactive oxygen radicals produced in the process of UV irradiation.The photo-treated surfactant solutions have better washing efficiency because of the produced small molecules of organic matter,which promotes the solubilization of PBDEs.(2)The effects of NO₃^-and Fe³⁺which are rich in soil and have high photoactivity on the photodegradation of PBDEs in surfactant solution were investigated.The reactive oxygen radicals produced by NO₃^-accelerated the degradation of BDE-15 and TX-100.·OH and·NO₂radicals produced by NO₃^-led to the formation of nitro and hydroxyl products of BDE-15 and TX-100 during UV treatment.In the presence of NO₃^-,the produced nitro and hydroxyl compounds increase the toxicity of the washing wastes and lead to the high ecological risk of the photo-treatment process.Under the combined effects of free radicals,complexation effect,electron transfer,photosensitization,light shielding,light competition,and halogen bond,Fe³⁺promoted the degradation of BDE-47 in Brij35 solution at low concentration but inhibited it at high concentration.In the presence of Fe³⁺,BDE-47 mainly underwent reduced debromination but with a little hydroxyl oxidation.The existence of Fe³⁺can accelerate the consumption of Brij35.Brij35 is mainly degraded by the oxidation of reactive oxygen radicals and Fe³⁺,and the degradation products mainly includes hydrophilic chain breaking-,hydroxylation-,and carboxylation-products.The redox reaction caused by the ferric cycle process of Fe³⁺in Brij35solution is the main factor affecting the BDE-47 and Brij35 degradation.Therefore,the effect of coexisting NO₃^-and Fe³⁺should be considered in the photodegradation of PBDEs containing eluent wastewater.(3)The application of TiO₂ catalyst in photocatalytic treatment of PBDEs in surfactant solution was investigated.Under anaerobic conditions,Ag/TiO₂ can promote the photodegradation of BDE-47 in TX-100 solution.The monolayer formed by the adsorption of TX-100 on the surface of the catalyst can accelerate the adsorption of BDE-47 on Ag/TiO₂,which is the main reason of TX-100 promoting the BDE-47 photocatalytic degradation.The photocatalytic degradation of BDE-47 in TX-100 solution mainly underwent the reduction and debromination process caused by the electron enrichment on Ag⁰.The loss of TX-100 can be caused by the reactive oxygen radicals produced by valence band hole and hole oxidation during UV irradiation.Compared with direct photodegradation,the accumulation of products in photocatalytic degradation is less,which reduces the ecological risk in photo-treatment process.The addition of 1%methanol accelerated the BDE-47 degradation and reduced the loss of TX-100.