Promotion Mechanisms For Environmental Functional Carbon Materials Mediating The Reduction Of Organic Pollutants

Nitroaromatics and azo dyes are widely used industrial chemicals,and it pose serious harm to aquatic environments and human beings.These types of toxic organic pollutans can hardly be directly mineralization,while their reduction productsare easy to biodegradable.Thus,chemicaland biological reduction transformation aresignificant technology for the removal of organic pollutants in the water,while the kinetics of reduction technology are usurally slow.Recently,related study showed that functional carbon materials could facilitate the reduction transformation of organic pollutants in the water.However,it is still hard to understand the promotion mechanism of organic pollutants transformation by carbon materials.This dissertation studied the functional carbon materials-mediated organic pollutants reduction,andaimed atelucidating the mechanisms for thecarbon materials-mediated the reductive transformation of organic contaminants.It provide great opportunitiesfor the promising future application of organic pollutants removal andcarbon materials inenvironmental remediation.Main contents and results of this dissertationare as follows:1.In the process of carbon materials-mediated the reduction of azo dyes by sulfide,the reaction mechanismwas studied.The effect of different kinds of carbon materials,including carbon nanotubes,biochar and graphene,on the reduction of azo dyes by sulfide were studied,and the reaction intermediate polysulfidewas also studied,we discovered the promotion mechanism for carbon materials mediated the reduction of azo dyes by sulfide.In this study,it wasobserved for first time that the degradation of azo dyes by sulfide initially underwent a lag phase followed by a fast degradation phase.Interestingly,the presence of all thecarbon materials only reduced the lag phase length of the azo dye decolorization but did not significantly enhance the reaction rate in the fast degradation phase.For example,100 mg L'1 carbon nanotubes reduce the time of stagnate phase time from 64.32±7.04 minto13.31±6.05 min.Related characterization and analysisindicated that carbon materials facilitated the transformation of sulfide into polysulfides including disulfide and trisulfide,resulting in fast azo dye reduction.Moreover,the oxygen functional groups of the carbon materials may play an important role in the transformation of sulfide into poly sulfides by accelerating the electron transfer.2.Effect of different nitrogen species on thenitrogen doped graphene-mediated azo dye reduction by sulfide was studied.The catalytic performance of nitrogen doped graphene for thereduction of azo dye by sulfide wasevaluated,and analyzed the relationship between contents of different nitrogen species in nitrogen doped graphene and their catalytic performance.The promotion mechanism for the nitrogen doped graphene-mediated the reduction of azo dye by sulfide was observed.The results showed that nitrogen doping could improve catalytic performance of graphene for the reduction ofazo dye by sulfide.For example,nitrogen doped graphen annealed at 1000 ? with a stagnate phase time of 0.54±0.56 mincompared with the 23.17±2.16 min for the reduce graphene oxide.Moreover,higher annealing tempreture treatment would further improve catalytic performance of nitrogen doped graphene.Additionally,a good positive correlation between the catalytic performance of nitrogen doped graphene and relative content of graphite N.And results showed that nitrogen doped graphene facilitate the formation and accumulation of polysulfide.DFT calculation results showed sulfide tend to bond with graphite N species compared with other configuration in nitrogen doped graphene,and the excellent electrical conductivity of graphite N would facilitate the electron transfer between combined sulfideand azo dye with the formation of poly sulfide.The formation polysulfide would further facilitate the fast degradation of azo dye till the decolorization process finished.3.The promotion mechanism for the biochar mediatingthe reduction of nitrobenzene by anaerobicmixed culture was evaluated.We initially investigated the effect of the biochar on nitrobenzene removal in short-term experiments and then possible enhancement mechanisms for the biochar were explored.Additionally,the effects of different key parameters on biological nitrobenzene reduction were studied in the presence of biochar.Lastly,the long-term influence of the biochar on biological nitrobenzene removal with the mixed culture was also evaluated.The results showed that the estimated pseudo first-order rate constant of nitrobenzene removal in the presence of 1.0 g L-1 biochar was four times higher than that without biochar addition,with the reaction rate improved from 1.9×10-2 h-1to 6.4×10-2 h-1.The possible enhancement mechanisms may be owing to that biochar could adsorb nitrobenzene and activate the N-O bond ofthemolecules,and theoxygen functional groups on the surface of biochar could promote the electron transfer between microbe and nitrobenzene.Additionally,the improvement of biochar dosage,glucose concentration and temperature would have positive effect on the biochar-mediatednitrobenzene biological reduction,while the improvement of nitrobenzene concentration,sodium ionand ammonium ion concentrationwould have negative effect on the biochar-mediated nitrobenzene biological reduction.Long-term experiments demonstrated that biochar could not only increase nitrobenzene removal rate but also improve the system stability,which may owing to that biochar could adsorb nitrobenzene and reduce their toxicity to the microbes,and biochar couldalso changethe microbial communities withthe relative abundance of nitrobenzene reduction related bacteria as well as electrochemically active bacteria was enriched in biochar addition systems.4.Therelationship betweenchemical structure of azo dye and theirchemical and biotic reduction decolorization ratewith presence of carbon nanotubes were studied.The chemical and biological reduction process of 8 different kinds of azo dyes werestudied.Electrochemical cell experimentswas used to study the azo dye decolorization process and cyclic voltammetry(CV)was used to study the azo dyeelectrochemical characteristics.Finally,the relationship between azo dye decolorization rate and their molecular orbital energy was evaluated.The results showed that a closer position for electron-withdrawing substituent to azo bond may resulted in faster azo dye chemical and bilogicaldecolorization,while a closer position for electron-repulsive substituent to azo bond may resulted in a slower azo dye chemical and bilogical decolorization.And 100mg L-1 carbon nanotubes addition would accelerate the chemical and bilogical reduction rate of azo dye,while it didn't affect the relationship between azo dye chemical structure and their decolorization reaction rate.Azo dye degradation in electrochemical cell experiments indicated that electron transferbetween electron donor and azo dye play an important roles in the azo dye decolorization process.CV results showedthat there existed a good positive linear relationships between wave potential corresponding to the azo bond reduction and average chemical decolorization rate of azo dye.Thus,molecular orbital energy valuewas selected to establish the relationship with azo dye chemical and biotic reductionrate.