Degradation Of Nitrobenzene Wastewater Under Iron-carbon Micro-electrolysis Enhanced By Ultrasound Coupled With Fenton

The iron-carbon micro-electrolysis method as an effective treatment method in nitrobenzene?NB?wastewater treatment,which can make it easier oxidized or biodegraded by transforming the nitro group on benzene ring to an amine group to activate the benzene ring.However,the iron-carbon micro-electrolysis bed often exists problem of easily passivation and poorly continuous usability in the long-term operation of organic waste water treatment process,which have to clean cyclically for the packing bed regeneration.Therefore,the cost of wastewater treatment is increased and iron-carbon micro-electrolysis method is difficult to run continuously and efficiently in the long-term.Based on the problem of iron-carbon micro-electrolysis,the ultrasound?US?was used to enhance the iron-carbon micro-electrolysis to degrade NB wastewater with high efficiency and continuity.In addition,aiming at the poor mineralization of nitrobenzene wastewater by the ultrasound/iron-carbon micro-electrolysis method,the ultrasound/iron-carbon micro-electrolysis was proposed to degrade NB wastewater with Fenton method,and the process condition of organic wastewater treatment was optimized.Firstly,the effects of zero valent iron?Fe⁰?dosage,granular activated carbon?GAC?dosage,initial p H value of wastewater and ultrasound power on degradation of NB with ultrasound/iron-carbon micro-electrolysis?US/Fe⁰/GAC?method were investigated.The results showed that,when the packings were not changed,the removal efficiency of four batches of same NB wastewater treated by US/Fe⁰/GAC is over 90%,whereas under iron-carbon micro-electrolysis?Fe⁰/GAC?treatment,the NB removal efficiencies for 4 times are 48%,36%,25% and 17%,respectively.The effect of ultrasound on the surface morphology of iron and carbon were investigated by SEM and EDS detection.The results showed that the reactive sites of iron and carbon can be regenerated continuously during the reaction process under ultrasound,and this process makes Fe⁰/GAC run continuously and efficiently.The suitable operating parameters of degrading NB obtained are Fe⁰ dosage 20 g·L^-1,GAC dosage 10 g·L^-1,initial p H=4 for wastewater.Under these conditions,after 80 min,the NB removal efficiency reaches 93%.Secondly,based on the single factor experiments of US/Fe⁰/GAC,the mathematical model of the process parameters for the removal of NB by US/Fe⁰/GAC was established by using the response surface method.The results showed that the model was highly accurate.The contribution rate of the experimental factors is Fe⁰ dosage ? the initial p H value of the wastewater> ultrasound power> GAC dosage,and the interaction between the Fe⁰ dosage and the initial pH value of the wastewater,the initial p H value of the wastewater and the ultrasound power are obviously.Thirdly,Fe⁰/GAC and Fenton method were used to degrade NB wastewater.The results showed that the final removal efficiency of N B for 4 batches of same NB wastewater treated by Fe⁰/GAC-Fenton drops from 69.4% to 31.66%,and TOC removal efficiency is down to 19.2% from 48.11%;whereas under US/Fe⁰/GAC-Fenton,N B removal efficiency of 4 times are close to 100%,and TOC removal efficiency are above 60%.Compared with the simple Fe⁰/GAC-Fenton method,the ultrasound not only enhances the efficiency of Fe⁰/GAC-Fenton for NB wastewater degradation,but also improves the mineralization efficiency of NB wastewater.The optimum conditions of US/Fe⁰/GAC-Fenton in NB wastewater degradation were as follows: the H2O2 total dosage 4 mL,adding time=5,US/Fe⁰/GAC effluent p H=4.Under these conditions,after 30 min,the N B removal efficiency reaches 100%,TOC removal efficiency reaches 75%.Finally,the degradation pathway of NB under US/Fe⁰/GAC-Fenton was studied by high performance liquid chromatography and gas chromatography-mass spectrometry.Under the reduction of US/Fe⁰/GAC,the NB is hydrogenated to nitrosobenzene,and then reduced to the phenylhydroxylamine,and converted to aniline finally.Then,under the degradation of Fenton method,aniline is oxidized to the p-imine quinone firstly,and then oxidized to the p-benzoquinone.In addition,the residual NB in the wastewater after US/Fe⁰/GAC treatment is oxidized to the p-nitrophenol by the oxidization of Fenton method,and then it is converted to the p-benzoquinone by denitrification.Then,the p-benzoquinone is oxidized and decomposed to the oxalic acid,malonic acid,succinic acid,trans-butenedioic acid and other small molecules,and oxidized into carbon dioxide,water finally.