Mechanistic Insights Into The Nonradical Oxidation In Non-Metal Modified Biochar/Persulfate Systems

The development and progress of social civilization has spawned many environmental problems,which seriously threaten the well-being of mankind and the future of the nation.Faced with increasingly severe water pollution prevention and control issues,combined with the ever-growing demand for the harmless disposal and resource utilization of waste biomass,this research aims to implement the conversion and utilization of waste biomass for the production of high-value functional materials,develop biochar-based environmental catalysts to drive the persulfate-based advanced oxidation processes,and finaly achieve the effective elimination of emerging pollutants,which is in line with the green and sustainable development goals of‘turning waste into treasure’and‘using waste to treat waste’.This research focuses on the two scientific problems,i.e.,the lack of non-metal modification strategies in the application of biochar-derived materials for the persulfate-based advanced oxidation technology,and the fuzzy mechanisms of persulfate activation.Three non-metal modification methods were investigated to reveal the feasibilities of these methods to enhance the removal efficiency of the target emerging contaminants and the specific mechanisms of persulfate activation over biochar-based materials.Main contents and results are listed as follows:The popular nitrogen doping was confirmed to be an effective strategy to improve the efficiency of peroxydisulfate（PDS）activation by biochar in sulfadiazine（SDZ）degradation.Nitrogen-doped biochar（NBC）with different doping levels were successfully prepared by high-temperature co-pyrolysis with urea as the nitrogen source and corncob as the biomass source.The pyrolysis process increased the porosity of the pristine biochar and at the same time allowed nitrogen atoms to be inserted into the structure of the biochar layer.The larger specific surface area resulted in the exposure of more sites for the contact between PDS and SDZ,and the introduction of nitrogen species,especially edge pyridinic nitrogen and pyrrolic nitrogen,induced the electronic structure change of the pristine biochar.Extreme points of electrostatic potential were thereby generated and provided chemical reaction power for NBC to activate PDS and remove SDZ.The oxidation process was confirmed to follow a novel nonradical electron transfer mechanism.Specifically,PDS would first contact with NBC to form a metastable surface complex.When the electron donor,namely SDZ,was present,the electron transfer path would be triggered,and electrons conducted from SDZ to PDS via the electron mediator NBC,resulting in the oxidative removal of SDZ and the reductive decomposition of PDS.Owing to the high selectivity of nonradical oxidation,the NBC/PDS system maintained excellent SDZ removal efficiency in water containing various inorganic anions.In addition,under the electron transfer mechanism,when there was no electron donor in the system,the decomposition rate of PDS would slow down,which could reduce the unnecessary consumption of oxidizing agents.Based on the above results,we further selected nitrogen-rich municipal sludge as the biomass precursor to produce nitrogen-doped sludge biochar（SB）,and then implemented the sulfurization of SB to obtain sulfurized biochar（SSB）to further improve the activation capacity and the removal efficiency of contaminants.Taking in to account the important factor that SDZ can be directly oxidized by peroxymonosulfate（PMS）,which will hinder the mechanistic insights into the oxidation mechanisms,the endocrine disruptor bisphenol A（BPA）which is chemically inert to both PMS and PDS was selected as the target contaminant in chapter 4 to reveal the different nonradical mechanisms in SSB/PMS and SSB/PDS systems.Results showed that SB was successfully sulfurized by a calcination process using sublimed sulfur as the sulfur source.The increase in the specific surface area of the biochar caused by the evaporation of sulfur during the treatment process and the integration of sulfur atoms into the edge structure of the biochar were the keys to the improvement of BPA removal efficiency.Sulfur atoms cooperated with the inherent nitrogen species in SB to induce further rearrangement of the electronic structure of biochar,resulting in the creation of more electron-deficient and electron-rich sites,namely Lewis acidic sites and Lewis basic sites,which endowed SSB with stronger PMS and PDS activation ability.The process of using SSB to activate PMS and PDS to remove BPA followed disparate nonradical mechanisms.The activation of PMS occurred at the Lewis acidic sites,and the main reactive oxygen species produced was ¹O2,while the activation of PDS occurred near the Lewis basic sites and still followed the electron transfer mechanism.Although the oxidation mechanisms of the two systems were different,the degradation pathways of BPA were basically the same,which could be summarized as direct oxidation,β-position cleavage of isopropyl group,and subsequent series of ring opening and mineralization reactions.The above studied non-metal atom modification methods can be assigned to covalent bonding methods.Despite the capacity of biochar for persulfate activation was successfully improved,the loss and conversion of reaction sites occurred during the reutilization,resulting in the decrease of contaminant removal efficiency.Therefore,in the next chapter,we employed polymeric carbon nitride（PCN）with stable structure and high nitrogen content to prepare composite materials.By fine-tuning the mass ratio of corncob to melamine in the co-thermal polymerization,a composite material,CNBC mainly composed of non-covalent bonding of biochar and PCN was obtained.As a result,the robust structure of the original PCN was retained to a great extent.Due to the inherent energy level difference between biochar and PCN,and theπ-πinteraction between the molecular layers,the aromaticπ-conjugated system of PCN underwent electron delocalization.The coupling effect was enhanced,forming an interlayer electron transport path from PCN to biochar,breaking the electron migration inertia of PCN,and endowing CNBC with excellent PMS activation ability and stability.The process followed a nonradical oxidation mechanism with ¹O2 as the main reactive oxygen species.Combining the identification of active species,theoretical calculation simulation,and in-situ detection experiments,the key role of O2^·-as the intermediate of other reactive oxygen species was revealed.PMS would first contact with CNBC surface,lose electrons,and break the S-O bond.SO3 and HO2^·were then generated and the later would be transformed into O2^·-through acid-base reaction immediately.O2^·-would further lose electrons on the surface of CNBC or undergo a recombination reaction to generate nonradical species,i.e.,¹O2,or directly react with PMS to generate radical species,i.e.,^·OH and SO₄^·-.Based on the sustainable biochar material platform,this research established simple and effective development methods to strengthen the redox reaction of persulfate at the interface of biochar and meanwhile revealed the specific nonradical mechanisms in the biochar/persulfate systems.The obtained results can provide guidance for the development of green and efficient biochar-derived non-metal environmental functional materials and provide theoretical support for the practical application of biochar-driven persulfate-based oxidation technology.