Photochemical Transformation Of Dissolved Black Nitrogen And Its Catalytic Degradation Mechanism For Organic Pollutants

Black carbon（BC）produced by biomass carbonization is widely distributed in soil,and dissolved black carbon（DBC）has strong migration characteristics.It is widely distributed in terrestrial ecosystems and continuously migrates to inland water and oceans through surface runoff.It is an important connection with the two major carbon pools of land and ocean.The composition of BC is complex,also containing a variety of doping elements,besides carbon.Nitrogen is one of the important constituent elements of organisms.After carbonization of organisms,nitrogen come into the carbon layer structure of BC as doping elements,which called black nitrogen.Its soluble part is dissolved black nitrogen（DBN）.However,compared with DBC,the research on the environmental behavior and effects of DBN is still significantly insufficient.Therefore,this paper investigated the photochemical conversion behavior of DBN and its catalytic degradation mechanism for organic matter,and systematically studied the photochemical conversion behavior and reaction mechanism of DBN by means of spectral characterization and electron spin trapping.Also,the reactive oxygen species and reactive nitrogen species contribute to the photochemical transformation of DBN are discussed.on this basis,the degradation mechanism of DBN to organic pollutants under light and non-light conditions is revealed.These results have certain reference value for in-depth understanding of the environmental behavior and potential environmental effects of environmental black nitrogen,the preparation and environmental application of new carbonized materials,and the global carbon and nitrogen cycle.The main conclusions of this paper are as follows:（1）The study found that the nitrogen content of BN produced by different plant sources was roughly 10-16%,which was not obvious relevant between raw materials and preparation temperature,indicating that the raw materials for DBN preparation are extensive.The preparation temperature significantly affected the N content of DBN.DBN had a relatively high nitrogen content at a moderate preparation temperature,and the form of pyridine nitrogen and pyrrole nitrogen accounted for more than 43%of the total nitrogen content,the rest mainly existed in the form of graphitic nitrogen and oxidized nitrogen.The electron transport ability of DBN could be enhanced by the presence of pyridine-nitrogen and pyrrole nitrogen.（2）For the first time,it was found that DBN was more reactive to photochemical transformation than DBC,and can simultaneously produce reactive oxygen species（ROS）and reactive nitrogen species（RNS）,with stronger photocatalysis for organic pollutants degradation.After solar radiation,the oxidized N,pyrrole N and pyridinic N,and the content of polar C of DBN at moderate temperature increased significantly.Due to its heteroaromatic nitrogen structure,DBN was easier to transform than DBC,and the half-life of DBN produced at 500 ^oC（8.6 h）was two times shorter than that of DBC（23 h）.The critical flocculation concentration（from 380 m M to 190 m M）of DBN decreased after illumination,indicating the light-induced aggregation behavior of DBN.The RNS produced by DBN played a more important role than ROS（¹O₂,O₂^-and·OH）in the photoconversion,so the photocatalytic degradation of bisphenol A by DBN was higher.The contributions of·NO and·OH to photodegradation were 17.50%and 15.60%,respectively.The study elucidates the photodynamics of pyrolytic carbon species in aquatic environment,with implications for the significant fate of organic pollutants.（3）Secondly,it was proved that DBN effectively mediated the degradation of 2-chlorophenol（2-CP）participated by S^2-.The removal rate of 2-cp by DBN450 in the presence of S^2-was about 39.1%within 120 h,and the first-order rate constant was 0.0041 h^-1.Compared with DBC(0.0012 h^-1),DBN showed more active properties than DBC in abiotic reductive dehalogenation reactions.The removal efficiency of 2-CP was closely related to the adsorption of S^2-and the yield of persistent free radicals（PFRs）by DBN samples,reflecting the dominant role of sulfide in promoting the catalytic degradation of 2-CP by DBN.ROS,RNS and non-free radicals contributed about 42.64%,29.55%and 27.81%to the removal of 2-CP by DBN450,respectively.The RNS produced by the reaction of ROS and N-oxides in DBN（such as·NO and NO₂·）are"secondary"free radicals.This study contributes to the understanding the storage and cycle of DBN in terrestrial/aquatic system,and would have profound implications for the fate of organic pollutants and metal in water and soil,as well as large-scale N cycle.