Reaction Mechanism Of Nitrobenzene Reduction Mediated By Different Fractions Of Black Carbon

Black carbon is the carbon-rich residue formed in the incomplete combustion of fossil fuel and biomass.It widely presents in the soil,sediment and other natural environment.Because of its special structural properties,black carbon has strong sorption affinity to organic pollutants,and is the key environmental medium to control the ecological effects of these organic pollutants.In recent years,the impact of black carbon on the environmental transformation process of organic pollutants has become a hotspot in related research.However,there is still no clear and consistent understanding of the mechanisms of black carbon-mediated pollutant reactions.In the rain erosion and water disturbance events,black carbon can be fractionated into different size fractions.The structures of these size fractions are significantly different,influencing the environmental transformation behavior of organic pollutants.The research on this front is very limited.Therefore,it is of great significance to study the effects of different size fractions of black carbon on the environmental transformation process of organic pollutants,as well as to explore the underlying mechanisms.In this investigation,black carbon was fractionated by ultrasonic water washing method to obtain four size fractions:bulk BC,rest BC,colloidal BC,and dissolved BC.The particle size,element composition,surface functional group and persistent free radical content of these size fractions were studied by laser particle size analysis,elemental analysis,X-ray electron spectroscopy,Fourier transform infrared spectroscopy and electron spin resonance spectroscopy.The widely found and highly toxicity nitrobenzene was selected as the model organic pollutants.The impact of black carbon size fractions on the reductive conversion of nitrobenzene was studied in the presence of sodium sulfide in anaerobic environment.The structural changes of black carbon size fractions before and after the reaction was analyzed and discussed.The reaction products and the effect of the products on the conversion of nitrobenzene were also studied.Based on these data,the mechanisms of the black carbon mediated nitrobenzene conversion by sulfide was discussed.The main conclusions are as follows:(1)The size of bulk BC,rest BC,colloidal BC and dissolved BC were 27.90?m,42.80?m,1.33?m,<0.45?m,respectively.Dissolved BC contains the highest oxygen content and polar oxygen functional groups,but the least aromatic structure.Colloidal BC has lower oxygen content than dissolved BC,but higher than that of bulk BC.Bulk BC contains the highest free radical,while dissolved BC contains the lowest free radical.(2)The major reaction products of nitrobenzene reduction by sodium sulfide are N-phenylhydroxylamine and aniline;black carbon can significantly promote the reduction kinetics of nitrobenzene.The pseudo first-order kinetics constant increased for of 20 to 110 times.Different size fractions have different effects on promoting the reduction of nitrobenzene.The carbon-normalized pseudo first-order kinetics follows colloidal BC>dissolved BC>bulk BC>rest BC.(3)Sodium sulfide is mainly converted to sodium polysulfide and elemental sulfur in the reaction,in which sodium polysulfide is mainly present in the solution and the elemental sulfur is present both in the solution and on the black carbon surface.The direct reaction of black carbon and sulfide is an important source of polysulfide.The Fourier transform infrared spectroscopy revealed significant changes of black carbon after the reaction.The polysulfide can be detected by UV-vis spectra in the supernatant after sodium sulfide reacting with black carbon.The fourier transform infrared spectra suggest that the oxygen-containing functional groups on the surface of the black carbon may be involved in the oxidation of sodium sulfide.Tert-butyl alcohol inhibited the formation of polysulfides,indicating that polysulfides may also formed by the combination of sulfur atom radicals.(4)The sodium polysulfide formation by the reaction between black carbon and sodium sulfide is an important mechanism to promote the reduction of nitrobenzene.The reaction between sodium sulfide and black carbon yielded sodium polysulfide.Nitrobenzene can be reduced in the supernatant containing polysulfide,while it can not be reduced on the black carbon particles without supernatant.In addition,the reaction efficiency of the size fractions of black carbon is positively correlated with their ability to produce sodium polysulfide.These results indicate that the sodium sulfide plays a decisive role in the reduction process of nitrobenzene mediated by black carbon,owing to its strong nucleophilicity.