Activation Efficiency And Mechanism Of Sulfide-modified Nanoscale Zero-valent Iron-carbon-based Catalysts For Sodium Persulfate

Advanced oxidation technology based on persulfate has higher selectivity,adaptability,longer half-life of sulfate radicals and higher redox potential,making it a more promising technology for deeper treatment of antibiotic wastewater.Fe-carbon-based catalysts combine the advantages of carbon carriers and iron active centers to catalyst the activation of persulfate with high efficiency activity and mineralization,triggering widespread interest.However,the component functions and competing synergies in the design of catalyst-based conversion radical/non-radical activation pathways remain to be explored.In order to clarify the interaction between carbon carriers and iron active centers,biochar and carbon nanotubes with differential structure-function were selected,and the degradation efficiency of carbon materials loaded with zero-valent iron sulfide nanoparticles on sodium persulphate（PDS）was investigated to explore the activation mechanism in depth.The main results are as follows:（1）Biochar/carbon nanotubes loaded with sulfide-modified nanoscale zero-valent iron（S-n ZVI@BC and S-n ZVI@CNTs）were prepared by high-temperature pyrolysis and a modified two-step liquid-phase reduction method with the optimal S/Fe molar ratio and Fe/C mass ratio are 0.25 and 1:5 respectively.Morphological characterization revealed a lamellar iron sulphide layer on the surface of the sulfide-modified nanoscale zero-valent iron.The carbon carriers have defective sites,sp~2 hybridized carbon and an abundance of surface functional groups（-COH,-COOH and C=O）,providing an abundance of active sites for the activation of PDS.（2）Sulfide-modified nanoscale zero-valent iron-carbon-based catalysts were able to significantly improve their efficacy in activating PDS for the degradation of sulfamethoxazole（SMX）.When the compound catalyst dosage was 200 mg/L,the PDS dosage was 0.2-1.5 m M and the initial p H was 5.0,it was able to completely degrade 10 mg/L SMX within 30-60minutes with a high degree of mineralisation.In addition,five possible degradation pathways for SMX under the non-radical pathway were analytically identified,including hydroxylation,δ-bond breaking,amine oxidation,and aromatization by electrophilic substitution.The PDS consumption of the S-n ZVI@CNTs/PDS system was much lower than that of the S-n ZVI@BC/PDS system,attributed to the different activation mechanisms.The pure nonradical pathway had low oxidant consumption and high resistance to environmental interference.（3）The identification of the active substances and the mode of production showed that the S-n ZVI@BC/PDS system was a system of multiple radical/nonradical coexistence（hydroxyl radicals,sulphate radicals,superoxide radicals and singlet oxygen）.The S-n ZVI@CNTs/PDS system was a completely non-radical system（singlet oxygen and electron transfer）.The different functions of the carbon carriers,mainly in terms of structural dominance and differences in surface functional groups,have a dominant influence on the activation mechanism.（4）The-COH and-COOH functional groups and sp~2 hybrid carbon defect sites on the surface of biochar contributed to the generation of reactive radicals.The abundant off-domain electron,C=O functional groups of carbon nanotubes mediate the production of singlet oxygen.As a bridge,the carbon nanotubes bind to the PDS to form the complexes for electron transfer between the pollutant and the PDS.（5）Sulfide-modified nanoscale zero-valent iron（S-n ZVI）functions differently in the radical/non-radical pathway.In free radical generation,S-n ZVI promotes iron recycling between Fe（II）/Fe（III）in the S-n ZVI@BC/PDS system for reuse.In the pure nonradical pathway,S-n ZVI could accelerate the electron transfer between SMX and PDS in the S-n ZVI@CNTs/PDS system.