Study On The Mechanism Of Photocatalytic Reduction Degradation Of Perfluorooctanoic Acid Under Visible Light

Perfluorooctanoic acid（PFOA）is widely used in industrial and civilian products.It is a new type of refractory environmental pollutants,which has serious toxicity hazards to the liver,nervous system and immune system of humans and animals.Meanwhile,PFOA is widely distributed globally and poses a huge threat to ecosystems and human health.Therefore,there is an urgent need for the development of efficient processing technologies for PFOA.Photocatalytic technology can achieve efficient degradation of organic pollutants under mild conditions.However,its degradation efficiency for PFOA is still relatively low,and the current research is mainly focused on photocatalytic oxidative degradation of PFOA and photocatalytic reduction degradation of PFOA The performance and mechanism are lack of in-depth study.In response to the above problems,this paper takes two typical photocatalysts-dibismuth tetroxide（Bi₂O₄）and cadmium sulfide（CdS）as examples,to study the reduction and degradation performance and mechanism of PFOA in detail,and investigate the Pt loading rate and electron donor,p H and other conditions affect the mechanism of reducing degradation performance,established a new method of reducing PFOA defluorination under visible light（VL）.This work provides an important theoretical and experimental basis for the efficient degradation of perfluorinated compounds.The main conclusions reached are as follows:（1）Bi₂O₄ was prepared with sodium bismuthate as raw material by hydrothermal method,and Pt nanoparticles were deposited by photoreduction method to prepare Pt-Bi₂O₄composite photocatalyst.The degradation experiment of PFOA found that with Pt-Bi₂O₄as the catalyst and under the combined action of nitrogen and KI as the electron donor,the defluorination rate of PFOA after VL light exposure for 6 hours was 44%,and the degradation rate was 45%.The total recovery rate can reach 99%,indicating that the Pt-Bi₂O₄/KI/VL system can achieve photocatalytic reduction degradation of PFOA,and the system has high stability.（2）Use CdS with stronger reducing ability as photocatalyst,synthesize CdS by coprecipitation method using cadmium chloride,sodium hydroxide and sodium sulfide as materials,and load Pt nanoparticles at the same time to construct Pt-CdS composite photocatalyst.In the degradation experiment of PFOA,it was found that when KI was not added as an electron donor under nitrogen,the defluorination rate was 17%after 8 hours;after adding KI,under the CdS catalysis with a Pt loading rate of 0.5%,VL light The 6-hour PFOA defluorination rate reached 49%and the degradation rate reached 79%.The mechanism study shows that CdS has a more suitable band gap width than Bi₂O₄,and the photogenerated electron-hole separation rate is high,which makes it have a higher photocatalytic reduction defluorination efficiency.（3）Use HPLC-MS/MS to detect PFOA degradation intermediate products,ion chromatography to detect the concentration of fluoride ions generated in the solution,and analyze the mechanism of defluorination degradation.The results of the study found that in both the"VL/Pt-Bi₂O₄/KI"and"VL/Pt-CdS/KI"systems,the PFOA molecules are gradually removed by the-CF₂unit to achieve photocatalytic reduction degradation,and the main products are all short chains of perfluorocarboxylic acids such as fluoroheptanoic acid and perfluorohexanoic acid.Studies have confirmed that the defluorination process of PFOA in water has undergone a photocatalytic reduction pathway,in which photogenerated electrons play a leading role.Pt is used as a cocatalyst for capturing photo-generated electrons,while KI can capture photo-generated h+to inhibit photo-generated e^--h⁺recombination,thereby jointly promoting defluorination efficiency.During the degradation process,intermediate products of short-chain perfluorocarboxylic acid were formed,and these intermediates could further achieve mineralization degradation.These results confirm that the two systems"VL/Pt-Bi₂O₄/KI"and"VL/Pt-CdS/KI"show high-efficiency degradation performance of PFOA in water,and the degradation pathway and mechanism are achieved by the reduction of photogenerated electrons,Indicating that PFOAcan achieve reductive degradation under visible light catalysis.