Application Of The Kapok Fiber Derived 2e~--ORR Catalysts In The Degradation Of Sulfamethoxazole Through Electro-Fenton Process

Electro-Fenton technology is a new advanced oxidation technology developed from Fenton technology.In electro-Fenton system,H₂O₂ was in-situ generated on the surface of the cathode through two-electron oxygen reduction reaction（2e^--ORR）.Therefore,developing cathode catalyst with high ORR catalytic efficiency and H₂O₂selectivity is very significant for the improvement of pollutant degradation efficiency.Biomass is a renewable,environment-friendly,cost effective,and oxygen rich natural carbonaceous raw material,and they can be transferred to excellent 2e^--ORR catalysts after carbonization at high temperature,and surface functionalization treatment.In this thesis,natural kapok fiber（KF）was used as precursor to prepare biochar（KFBCs）which were used as the 2e^--ORR catalysts to fabricate air diffusion cathode for the construction of electro-Fenton system for the degradation of sulfamethoxazole（SMX）.The main research content and conclusion are as follows:（1）Natural KF was fibrous with a diameter of about 19.8μm,and its surface is very smooth.After carbonization,the obtained biochar,namely KFBC,was composed by irregular fiber fragments with several microns in length.The influence of carbonization temperature on the defect structures and surface functional groups of the KFBCs was significant.The surface defects and graphite nitrogen（graphite-N）ratios gradually increased when the carbonization temperature was increased from 600 ^oC to 800 ^oC.In addition,the KFBC obtained by carbonization at 700 ^oC for 1 h,namely KFBC-700-1,exhibited the highest percentages of carboxyl（-COOH）and pyridine-nitrogen（pyridine-N）,10.0%and 22.9%.Based on the electrochemical experiments,as the ORR catalytic activity of the KFBCs gradually increased with the increase of both the carbonization temperature and time,whereas their H₂O₂ selectivity decreased in turn.Finally,the KFBC-700-1 achieved the highest H₂O₂electro-generation efficiency.The epicuticular wax played important role toward the ORR catalytic activity of the KFBCs,since the H₂O₂ electro-generation performance deteriorated after the surface processing with chloroform,sodium chlorite（NaClO₂）,and NaOH.（2）The electro-Fenton system constructed with the KFBC-700-1 air-diffusion cathode exhibited excellent H₂O₂ in situ generation performance in a wide pH range of 2.0～11.0;The accumulated H₂O₂ concentration within 30 min reached at the maximum concentration,namely108.5 mg·L^-1.（3）The homogeneous electro-Fenton system catalyzed by Fe²⁺exhibited excellent degradation performance toward the SMX with the initial concentration of 10 mg·L^-1.The removal of SMX reached at the maximum value,namely 100%removal and 29.4%of mineralization within 60 min when the initial pH was 3.0 and the Fe²⁺concentration was 1.0m M.Moreover,after five running cycles,the degradation of SMX still reached at 80.7%.Compared with the homogeneous system,the FeOCl mediated heterogeneous electro-Fenton system showed a wide pH applicable range,since the SMX removal within 40 min maintained above 99.0%in the entire tested pH range of 2.0～11.0;And SMX can be completely removed when the reaction time was prolonged to 60 min.Amid,the SMX degradation efficiency reached at the maximum value of 0.132 min^-1 at the initial pH of 5.0 and FeOCl of 0.8 g·L^-1.After five running cycles,the degradation of SMX only slightly decreased to 99.6%within 60min,suggesting the outstanding stability of the reaction system.Finally,there are three possible degradation pathway for the SMX in the electro-Fenton system,based on the thirteen detected degradation intermediates.