Research On The Removal Of Perfluoroalkyl And Polyfluoroalkyl In Water By Vacuum-Ultraviolet/Ultraviolet (VUV/UV) Process

Perfluorooctanoic acid（PFOA）is a perfluoroalkyl and polyfluoroalkyl substance that is frequently found in water and is difficult to remove using conventional water treatment processes.However,the ultraviolet/persulfate（UV/PDS）process has proven effective in treating PFOA.In recent years,vacuum ultraviolet/ultraviolet（VUV/UV）technology has gained attention as an advanced oxidation technology for water treatment.Compared to traditional low-pressure mercury lamps,the new VUV/UV mercury lamps can degrade pollutants by providing additional 185 nm VUV photons without increasing energy input.Consequently,VUV/UV technology combined with VUV/UV mercury lamps has the potential to efficiently degrade PFOA when used to drive the UV/PDS process.However,the mechanisms behind this approach for removing PFOA from rural drinking water are not well-understood.Therefore,the main objective of this article is to investigate the characteristics and mechanisms of the VUV/UV/PDS process for enhancing PFOA degradation in water.The results of this study could provide valuable insights for effectively removing PFOA from water using VUV/UV/PDS technology.Firstly,a novel method has been developed to measure the intensity of VUV radiation using the mini-fluidic photoreaction system.The method relies on the decomposition of water into H₂O₂ using 185 nm VUV light,with the production rate of H₂O₂ used to calculate the VUV irradiation intensity.The generation rate of H₂O₂ during the first 6 minutes follows pseudo-zero-order reaction kinetics,with a positive correlation to the VUV radiation intensity.The production rate of H₂O₂decreases gradually as the p H of the solution or the concentration of the buffer solution increases.Conversely,the production rate of H₂O₂ increases with water temperature and stabilizes as the temperature increases.The circulation flow rate affects the production rate of H₂O₂ irregularly and sharply,especially when the Reynolds number reaches 2000～4000.This method has been tested in different VUV reactors,with results showing that a Reynolds number greater than 4000leads to a stable production rate of H₂O₂ and an error rate of less than 12%.Compared to traditional VUV radiation intensity measurement methods,this new method has the advantages of being highly sensitive,easy to operate,and applicable in a wide range of scenarios.Secondly,the study also examined the degradation kinetics of PFOA under VUV/UV radiation using the mini-fluidic photoreaction system,and found that the method is highly efficient at degrading the chemical.The VUV and UV irradiation intensities in the mini-fluidic photoreaction system were measured using the trace H₂O₂ generation method and the chemical photosensitizer uridine,respectively,and were determined to be 1.16×10^-4 and 1.39×10^-3 einstein·m^-2·s^-1,respectively.At a UV irradiation dose of 21.2×10^-2 einstein·m^-2,the removal rate of PFOA was over 52%and followed pseudo-first-order kinetics.Compared to degradation by UV alone,PFOA removal by VUV/UV degradation was much higher due to VUV ability to generate the strong oxidizing agent HO^·and directly photolyze pollutants.The p H had no significant impact on PFOA degradation,and the degradation rate at p H 6.0 was slightly higher than at p H 7.5 and p H 9.0,while the PFOA concentration had no significant effect on its degradation.However,inorganic ions such as Cl^-and NO₃^-,as well as NOM,could suppress the VUV/UV degradation of PFOA by scavenging HO^·and competing for VUV photons,while SO₄^2-had little effect.Finally,this article selected the VUV/UV/PDS process among multiple degradation methods,which showed the best degradation effect on PFOA.Therefore,we studied the enhancement mechanism of PFOA degradation by VUV/UV/PDS.The experimental results showed that with the increase of initial p H,the transformation of SO₄^·-and the absorption of photons by OH^-were limited to a certain range,resulting in a slight decrease in the degradation effect of PFOA.After in-depth discussion of the process mechanism,it was found that when the PDS concentration was 0 to 0.9 mmol·L^-1,the collaborative factor R was below 1and decreased slightly（from 1 to 0.88）.This indicates that although the VUV/UV mercury lamp can significantly enhance the UV/PDS process for removing PFOA,the combined use of PDS and VUV/UV does not have a significant synergistic effect.The main mechanism is that the increase of PDS concentration will increase the generation of SO₄^·-,but at the same time,PDS competes with VUV photons for absorption,leading to a weakening of the direct VUV degradation effect of PFOA and a slight decrease in the collaborative factor for PFOA degradation.Compared with the UV/PDS process,the VUV/UV/PDS process can enhance the degradation of PFOA through direct VUV photolysis and SO₄^·-generated by VUV photolysis of PDS without increasing energy consumption.In summary,the development of a new VUV irradiation dose measurement method has good reference significance for the design of VUV reactors,monitoring the operation of disinfection systems,and ensuring the safety of disinfection.In terms of the kinetic degradation of PFOA by VUV/UV process,influencing factors,and the enhancement mechanism of VUV/UV/PDS process,overall,compared with the UV/PDS process,the VUV/UV/PDS process can enhance PFOA degradation through VUV direct photolysis and SO₄^·-generated by VUV photolysis of PDS without increasing energy consumption.This study provides theoretical and application support for the determination of VUV irradiation dose and the application of a new VUV/UV mercury lamp for the removal of PFASs.