The Performance,mechanisms And Active Sites Of Carbon-based Materials Activated Persulfate For Organic Matter Degradation

The remediation of organic pollution in water body is one of the major topics in the field of environment.In recent years,sulfate radical-based advanced oxidation processes have shown great potential for the efficient degradation of organics due to its high redox capacity,low cost and easy operation condition.The selection of catalysts is the key to improve the efficiency of this technology,in which carbon-based materials have attracted widespread attention.However,the activation mechanisms of persulfate(PS)by carbon-based materials are still controversial,such as activation pathways and active sites,which hinders the further development of this technology.Besides,the catalytic performance and reusability of carbon materials still need to be further improved.Thus,the exploration of novel carbon-based materials with high catalytic activity is of great significance to the development of this technology and the removal of organic pollution in water.Herein,in this paper,we conducted a series of researches on the d evelopment of noval carbon-based materials for PS activation and explored the related mechanisms.Comprehensive investigations were carried out on three kinds of materials:pure carbon materials,nitrogen-doped carbon materials and the composite of carbon materials and metal oxides.The active sites and interfacial interactions in different systems were further deeply discussed and analyzed by density functional theory(DFT)calculation.This study can provide new methods and ideas for the preparation and a pplication of noval carbon-based functional materials,supply theoretical support for the activation of PS,and give reference and basis for the practical application of PS advanced oxidation processes in the future.The main research results are as follow s:(1)Ordered mesoporous carbon CMK-3 can efficiently activate PS,as 90%of2,4-dichlorophenol(2,4-D,200 mg/L)could be removed in 20 min.High efficiency was also achieved in actual pharmaceutical wastewater treatment.Due to its high specific surface area,CMK-3 could fully contact with PS and pollutants,accelerating electron transfer.The large adsorption capacity was benefit to the activation of PS and the subsequent degradation of pollutants.Defect sites and functional groups(especially C-OH)on CMK-3 surface might act as active sites for the reaction.The coverage of intermediates,decrease of defect sites and changes of functional groups were the main reasons resulting in the partial passivation of CMK-3 after use.Different from the traditional PS activation system,radical and nonradical oxidation worked together in the CMK-3/PS system,in which nonradical pathway by direct electron transfer played a dominant role in the degradation of pollutants,while radical pathway was critical in accelerating the reaction.SO₄^·-,O₂^·-and·OH were all involved in the radical pathway,but the contribution of·OH was dominant.CMK-3could act as an electronic conductor in the reaction process.(2)Among the series of biochar derived from different parts of sesame straw,biochar prepared with sesame stalk core and calcined at 800?(SC-800)owned the highest specific surface area(617.53 m²/g),rich pore structure and oxygen-containing functional groups compared with those prepared with sesame stalk and sesame ear.So it showed the best catalytic activity,and the removal efficiency of phenol(15mg/L)could reach 93%in 180 min.The anti-interference ability of SC-800/PS system was strong as anions in solution had almost no effect,and it could effectively work in a very wide range of pH(pH 3?11).The reusability of SC-800 was not very good but it could be partly restored by annealing.The adsorption of intermediates was the main reason of deactivation.Oxygen-containing functional groups,especially C-OH,showed a good linear relationship with the adsorption and degradation rate of phenol,so it was considered to be the main active site for SC-800 to activate PS.During the reaction,SO₄^·-,·OH,O₂^·-and ¹O₂ were generated,in which O₂^·-was the intermediate for the formation of ¹O₂.The nonradical pathway dominated by ¹O₂ played a leading role in the degradation of phenol,and the radical oxidation process dominated by·OH was also involved.In addition,SC-800 also showed high removal efficiency in natural water for phenol and many kinds of other organic compounds.(3)Taking full advantage of the capillarity of recycled paper,nitrogen-doped biochar with low cost and high PS catalytic activity c ould be successfully synthesized by using HCO₃^-as linker and urea as nitrogen source.The corrosion and complexation of HCO₃^-played a crucial role in improving the specific surface area and electrical conductivity of biochar,contributing to the successful doping of nitrogen and improving the catalytic performance of materials.The utilization of capillarity could avoide excessive blockage of pores in recycled paper during nitrogen doping process,thus improving the catalytic activity.Calcination temperature of biochar had little effect on the catalytic performance,while reaction parameters had significant impacts.The removal efficiency of pollutants increased with the increase of biochar and PS dosage.High reaction rate could be achieved under neutral conditions,but excessive acid or alkali might inhibite reaction.The prepared nitrogen-doped biochar showed good removal effect on many kinds of pollutants and its catalytic performance was stronger than that of multi-walled carbon nanotubes and graphene oxide,so it had good application prospect.For the removal of pollutants,carbon skeleton of biochar played a major catalytic role,and the defect sites with C-OH functional groups and pyridine nitrogen on the edge of the carbon skeleton could act as active sites for activating PS.Radical pathway dominated by·OH played a leading role in the degradation of 2,4-D,and ¹O₂ and SO₄^·-were also involved but the contribution was less.(4)Carbon-coated Mn₃O₄ composites(Mn₃O₄/C)with nanocube structure were synthesized by hydrothermal process and calcination,which can effectively activate PS for 2,4-D degradation.The composition and catalytic performance of Mn₃O₄/C could be regulated by altering the calcination temperature and precursor ratio.The composite prepared at 400°C with a precursor ratio(glucose/KMnO ₄)of 0.5exhibited the best catalytic performance,and 95%of 2,4-D(100 mg/L)could be removed in 140 min.Because externally coated carbon layer protect ed the internal Mn₃O₄ from being corroded,the prepared materials showed commendable reusability and the removal efficiency could still reach 92%after four cycles.In addition to PS,Mn₃O₄/C could also effectively activate permonosulfate(PMS),and could degrade a variety of organic pollutants.Mn₃O₄/C composites not only acted as catalysts for activating PS,but also served as electronic conductor to accelerate electron transfer.Defect edge containing C-OH group in carbon layer was the possible active site of PS activation.The degradation process of 2,4-D mainly involved radical pathway(SO₄^·-and·OH)and nonradical pathway(¹O₂),in which·OH played a dominating role.(5)With the help of DFT calculation,it was found that in the process of PS activation by carbon-based materials,PS would gradually approach the corresponding sites on the surface of catalyst,and obtained electrons from carbon layer to weake the O-O bond until it strided the energy barrier to reach the transition state and achieved complete activation.Then the energy of the system would decrease rapidly.The activation process of PS was an exothermic reaction.The adsorption between PS and carbon layer was mainly realized by van der Waals interaction.Oxygen-containing functional groups could effectively weaken the O-O bond,enhance the interaction and electron transfer between PS and carbon layer,and significantly reduce the activation energy of the reaction.Among them,C-OH functional group owned the highest activity with activation energy of 9.478 kcal/mol,and the hydrogen bond between C-OH and PS was beneficial to strengthen the interaction between PS and carbon layer.With nitrogen doping,the activation energy of reaction could be reduced to half compared with that of undoped carbon materials,which made the activation of PS easier.Among the three N-doped sites(pyrrole N,pyridine N and graphite N),the activity of pyridine N was the highest,and the corresponding activation energy was4.593 kcal/mol.The most active site in Mn₃O₄/C composites was the C-OH functional group at the defect site of carbon layer.It played a major role in weakening O-O bond to realize the activation of PS,while Mn₃O₄ could enhance this process.Mn₃O₄ could also enhance the interaction between C-OH group and PS as well as the electron transfer between PS and carbon layer,significantly promoting the catalytic activity of the composites.