Co, N@C Activated Persulfate For Catalytic Removal Of Perfluorooctanoic Acid In Water

Perfluorooctanoic acid（PFOA）is one of the most frequently detected perfluorinated compounds（PFCs）in the environment.Due to its special physical and chemical properties,PFOA has been widely used in the production and manufacturing of fluorine chemical industry and daily consumer goods,and will eventually be released in large quantities and accumulate into the environment,thus bringing immeasurable negative impacts on the ecological environment and human health.The existence of C-F bond makes PFOA extremely stable and difficult to biodegrade in the environment.At present,the persulfate-based advanced oxidation technology is featured with strong oxidation,low selectivity and mild reaction conditions,and has shown great development potential in the advanced treatment of persistent organic pollutants.In this thesis,carbon-based materials were selected as the substrate,and after functional modification,two composite catalytic materials were prepared.By combination with persulfate,the advanced oxidation systems were developed for highly efficient removal of PFOA.The specific contents included:1)Using graphitic carbon nitride（g-C₃N₄）prepared by calcination of nitrogen-rich precursor melamine as a substrate,a composite photocatalyst Co/TM/g-C₃N₄ with more active sites was obtained by doping tourmaline with spontaneous polarization effect,and further coupling with trace Co.Supported by synergised peroxymonosulfate（PMS）and visible light irradiation,the advanced oxidation system Co/TM/g-C₃N₄/PMS/Vis was constructed for the catalytic degradation of PFOA in water.The results indicated that the Co/TM/g-C₃N₄/PMS/Vis system could achieve 81.1%removal of PFOA and 31.1%defluorination at 4 h under the optimal conditions.By the combination of free radical quenching,electron paramagnetic resonance（EPR）and quantification of free radicals,the active species involved in PFOA degradation in the Co/TM/g-C₃N₄/PMS/Vis system were free radicals(SO₄^·-,·OH,O₂^·-)and non-free radicals（¹O₂ and hole h⁺）,withO₂^·-and h⁺predominant in the system.PFOA and intermediates were detected by solid-phase extraction and liquid-mass combination technology,and in the degradation solution,the short-chain perfluorinated carboxylic acids of C3-C7 were found,with predominant C4 and C5.The toxicity of intermediates was estimated,and a decreasing toxicity was found.In addition,the influencing factors of the Co/TM/g-C₃N₄/PMS/Vis system（such as coexisting anions and humic acid）,the reusability of Co/TM/g-C₃N₄ and the application in actual water were investigated,with the findings that Co/TM/g-C₃N₄has good catalytic performance and application prospects.2)Using Calotropis gigantea fiber（CGF）with thin-walled large hollow structure as the raw material,ZIF-67 was self-assembled into the hollow lumen via in-situ growth,by then the magnetic Co,N co-doped biochar catalyst Co/C@C was constructed by pyrolysis under N₂ atmosphere.Assisted by PMS,the advanced oxidation system Co/C@C/PMS was constructed for the catalytic degradation of PFOA in water.The results showed that the adsorption of PFOA by Co/C@C was 38.5%,and under the the optimal condition,the removal percentage was 72.2%and the defluorination rate was 56.7%by Co/C@C/PMS system.Identification of active species and electrochemical tests demonstrated that the main active species involved in PFOA degradation in the Co/C@C/PMS system were SO₄^·-,followed byO₂^·-,¹O₂ and electron transfer.By using solid phase extraction combined with liquid-mass technique,the intermediates of PFOA degradation were analyzed and the degradation path was proposed to reveal the interfacial processes（adsorption and catalysis）on Co/C@C surface.Combined with economic benefits and universality,the application potential of Co/C@C was investigated via the reusability and the application performance in the actual water body.In comparison,it is found that the catalyst Co/C@C is easy to recover,consumes less energy,and achieves the goal of efficient removal of PFOA with less PMS consumption than Co/TM/g-C₃N₄.