Construction Of A Recyclable Photo-assisted Sono-fenton System And Study On The Mechanism Of Endocrine Disrupting Compounds Degradation In Water

In recently years,Endocrine Disrupting Compounds?EDCs?in the aquatic environment have attracted widespread attention throughout the world due to their universality and harmfulness.EDCs are often detected in surface water and ground water in China and the polluting situation is serious.Due to the properties of stable chemical structure and resistence to biodegradation of most EDCs,it is difficult to effectively remove them using traditional sewage treatment and drinking water treatment processes.In the past few decades,various highly efficient Advanced Oxidation Technologies?AOTs?have emerged.Owing to the remarkable advantages of strong oxidizing free radicals in degrading refactory organic pollutants,researchers have conducted a lot of work investigating various types of AOTs.Some mature AOTs have even been widely used in practical application?such as Fenton technology?.However,the sustainability and environmental friendliness of AOTs have became the key factors that need to be considered apart from high efficiency.Ultrasonic oxidation?US?is a rare "green" technology among various AOTs,owning its unique advantages and inherent problems.According to previous reports,introducing Fenton process into US system can improve the degradation efficiency of US system,but a large amount of iron input not only increases the cost,but also causes secondary pollution.Based on these,the present study proposes to construct a novel and efficient photo-assisted sono-Fenton system,which intends to use solar energy to improve the efficiency of sono-Fenton process with the aim of reducing iron consumption.This technology was mainly investigated to remove EDCs in water.Additionally,the role and mechanism of applying the most widely investigated photocatalyst Ti O2 to promote iron cycling in sono-Fenton process were further discussed,which will provide a promising approach and theoretical basis for the application of sono-Fenton technology.This paper can be divided into four parts to systematically investigate the photo-assisted sono-Fenton system:?1?study of the efficiency and mechanism of EDCs degradation by US alone;?2?construction of sono-Fenton system and study on the EDCs degradation mechanism;?3?construction of P25-mediated photo-Fenton process and mechanism analysis;?4?construction of P25-mediated photo-assisted sono-Fenton system and analysis of EDCs degradation efficiency.Firstly,two typical EDCs,Bisphenol A?BPA?and dimethyl phthalate?DMP?,were selected to systematically study their degradation in US alone.The results showed that,under the same power input,the degradation efficiency of BPA and DMP in high frequency ultrasound was significantly higher than that in low frequency ultrasound,and the degradation kinetics agreed with pseudo first-order kinetics.Particularly,high frequency US system had faster H₂O₂ accumulation rate.The primary oxidizing species in the US system was found to be ·OH.The EDCs degradation mechanism in US process was found to be mainly caused by ·OH oxidation at the gas-liquid interfaces,so US process was more suitable for degrading BPA with stronger hydrophobicity.In addition,BPA was chosen as the target compound to investigate the effect of various water quality parameters?i.e.,BPA concentration,p H,temperature,background ions,etc.?on its degradation by 400 k Hz ultrasound.The results showed that,with the increase of BPA concentration,BPA degradation rate was decreased,while the amount of degraded BPA molecules was increased.More acidic condition and the bulk temperature of about 40 °C favored BPA degradation.The presence of CO2-3,HCO-3 and NO-2 showed obvious inhibition while NO-3,SO2-4,and Cl-demonstrated minor influence on degradation BPA.Analysis of BPA degradation products by LC/MS revealed that the mechanism of ultrasonic degradation of BPA were the attack the benzene ring and breakup the C-C bond by ·OH radical.However,BPA mineralization efficiency was weak by using US alone.Furthermore,in order to make full use of the in situ generated H₂O₂ in the ultrasonic system,the sono-Fenton process was constructed by adding Fe²⁺,thereby improving the degradation ability of EDCs.The results showed that,increasing Fe²⁺ dosage could increase the degradation rate of BPA or sulfadiazine?SDZ?,while the rate improvement of more hydrophilic SDZ was more significant.However,when Fe²⁺ was excessive,the quenching reaction between Fe²⁺ and ·OH radical led to decline of EDCs degradation.Analysis of degradation kinetics showed that,the degradation of hydrophobic BPA agreed with pseudo first-order kinetics,while the degradation of hydrophilic SDZ agreed better with Behnajady kinetics model.Moreover,the change of Fe²⁺ dosing modes had little effect on SDZ degradation,which indicated that the in-situ supply of H₂O₂ by ultrasonic process played a full role in making full use of free radicals.Therefore,in order to further improve the efficiency of sono-Fenton system,the most important thing was to effectively realize the regeneration of Fe²⁺.It was found that photolysis of ferric iron under visible light to generate Fe²⁺ could improve the degradation of SDZ in sono-Fenton process,but the Fe²⁺ regeneration rate was relatively low under visible light,and the promotion under low iron dose was not remarkable.Then,P25 was selected as a photocatalyst to enhance the cycling of Fe³⁺/Fe²⁺.Merits of adding P25 to homogeneous photo-Fenton-like process?ph-F?were evaluated under visible light using Bisphenol A?BPA?as a model pollutant.Interactions between P25 and Fe³⁺/H₂O₂ were emphasized.Results show that adsorption of Fe?III?on P25 produced redshift of light absorption,and interactions between P25 and H₂O₂ promoted photoelectron generation,effectively introducing visible light into ph-F.The visible-light-driven ph-F demonstrated adequate performance at high Fe³⁺/H₂O₂ dosage,while P25 addition showed significant acceleration of BPA degradation with saving amount of Fe³⁺/H₂O₂.The mechanism was confirmed to be enhanced Fe³⁺/Fe²⁺ cycling by photo-electrons,particularly pronounced at low [Fe?III?]0.Additionally,H₂O₂ was utilized more efficiently in P25-ph-F than that in ph-F by diminishing the radical scavenging role of H₂O₂ at lower [Fe?III?]0.Kinetics and ESR analysis supported this mechanism.Compared to ph-F,the P25-ph-F process also demonstrated stronger potentials in degrading BPA at high concentrations and better mineralization capability with reduced Fe³⁺/H₂O₂ reagents.The sustainability of P25-ph-F was also examined in regard to its advantage under sunlight and the strong recyclable and reusable capability.BPA decomposition was dominated by ·OH attack at both the aromatic ring and the connecting carbon,and P25-ph-F was more competent in transforming the primary intermediates than ph-F.Finally,under visible light irradiation,the P25-mediated photo-assisted sono-Fenton system was successfully constructed with highly efficient cycling of Fe³⁺/Fe²⁺.Firstly,the effect of P25 on the degradation of BPA,DMP and DEP in ultrasound alone and sono-Fenton system was studied by changing the dosage of P25.Results showed that,the optimum dosage of P25 was 0.5 g/L,and excessive P25 was adverse to subsequent solid-liquid separation and the optical shielding effect caused by P25 could also influence the cycling of Fe³⁺/Fe²⁺.LED light was used to simulate solar irradiation,and the reduction of Fe³⁺ could be accelerated with the involvement of P25 regardless of the initial concentration of iron ions.Compared with the photo-assisted sono-Fenton process?P25 free?,the addition of P25 could significantly improve the degradation of DMP and DEP.The synergistic index?SI?by combining P25 with photo-assisted sono-Fenton process was calculated based on degradation rate constant?kobs?,and the SI obtained with low iron ion concentration?0.02 mmol/L?was the highest?SI = 1.24?.Overall,this study showed that P25-mediated photo-asssisted sono-Fenton could achieve efficient degradation and good mineralization of various EDCs,which offers a promising idea for the development of ultrasound and sono-Fenton related processes in the future.