Treatment Of High Concentration Nitrobenzene Wastewater By Extraction-homogeneous Catalytic Ozonation Enhanced By High Gravity

Nitrobenzene?NB?is an important chemical raw material.A large amount of nitrobenzene wastewater will be produced in the production process,which has strong teratogenic,carcinogenic and mutagenic effects on human body,animals and plants.At the same time,nitro is electron withdrawing group,which makes the?electron density on the benzene ring greatly decrease.The chemical property of NB is relatively stable,and hard to be degraded.High concentration of nitrobenzene in the wastewater is difficult to be treated,and the COD is generally above 2000mg/L,with the properties of high color,peculiar smell,strong acid and strong alkalinity.The single treatment method often cannot achieve the desired effect and the treatment cost is high.Thus,the combination of different processes is a common method for treating this kind of wastewater at present.The extraction process has the characteristics of large wastewater treatment capacity and low energy consumption.This is suitable for the pretreatment stage for the wastewater containing nitrobenzene of high concentration.The homogeneous catalytic ozonation technology has a fast reaction rate and obvious treatment effect,and it can be used in the further treatment of the nitrobenzene wastewater at low concentration.However,in the extraction process,the extraction efficiency is limited by the extractant and the liquid-liquid mass transfer,and the insufficiency of the gas-liquid mass transfer in the process of homogeneous catalytic ozonation also affects the degradation effect.In view of the problems existing in the extraction process and the homogeneous catalytic ozonation technology,the unique advantages of the high gravity technology in enhancing liquid-liquid and gas-liquid mass transfer need to be used.The idea of enhanced extraction-homogeneous catalytic ozonation for the treatment of nitrobenzene wastewater at high concentration by high gravity technology is put forward:firstly,the extractants suitable for nitrobenzene wastewater are screened in the pretreatment stage,using impinging stream-rotating packed bed?IS-RPB?as extraction equipment for the nitrobenzene wastewater at high concentration.Then the RPB-O₃/Fe²⁺and RPB-O₃/H₂O₂ processes were used to further treat the acidic and alkaline nitrobenzene wastewater after extraction with rotating packed bed?RPB?as reaction equipment.Finally,the degradation products of nitrobenzene in different processes were detected by high performance liquid chromatography mass spectrometry?HPLC-MS?,and the degradation pathway of nitrobenzene was further speculated.First of all,the extractant is preferably selected.The previous results showed that cyclohexane was with less dosage requirement and had better extraction effect than fatty acid methyl ester.The removal efficiency of nitrobenzene reached 97%under the conditions of nitrobenzene concentration of 1500mg/L,phase ratio of 1:1,pH=6.4,extraction time of 20min,temperature of 35?and rotating speed of 350 r/min,It was discovered that the extraction efficiency was not affected by the pH.Then,the pretreatment of the nitrobenzene wastewater at high concentration was processed with IS-RPB as extractor and cyclohexane as extractant.The result showed that the removal efficiency of NB reached 94.93%,and extraction stage efficiency was 99.99%respectively.The nitrobenzene concentration was finally reduced to 76.05mg/L,under the following experimental conditions including:a nitrobenzene concentration of 1500mg/L,a temperature of 25?,a pH of 6.4,a liquid flow rate of 50 L/h?initial impact velocity v₀=7.88m/s?,a phase ratio of 1:1 and a rotating speed for IS-RPB of 400 r/min,which laid the foundation for subsequent oxidation treatment.Under similar experimental conditions,compared with the single equipment of IS and RPB,respectively,the removal efficiency of nitrobenzene increased by 1.9%to 17.05%and the extraction stage efficiency increased by 2%to 15.96%.The advantages of the new mechanism of liquid-liquid contact and mixing enhanced by high gravity are demonstrated.Secondly,the nitrobenzene wastewater with low concentration acid after extraction was further treated by RPB-O₃/Fe²⁺process.The results were as followed:when the concentration of nitrobenzene was 76.05mg/L,COD was 139.9mg/L,reaction temperature was 25?,mass concentration of ozone was 36mg/L with 40L/h of gas-flow,the initial pH=3.5,?=80,liquid rate at 120L/h,0.4mmol/L of Fe²⁺dose and 30 minutes of recycling time.The removal efficiency of nitrobenzene and COD finally reached 99.63%and 79.56%,respectively.The concentration of nitrobenzene in wastewater was 0.28 mg/L,which reached the national first class emission standards?GB8978-1996?.With this understanding,compared with conventional RPB-O₃ equipment,the removal efficiency of nitrobenzene increased by 11.2%and the COD removal rate increased by 29.23%;compared with BR-O₃/Fe²⁺process,the removal efficiency of nitrobenzene and COD increased by 25.3%and 31.34%,respectively.These results showed that the supergravity technology could effectively strengthen the ozone mass transfer and realize the high efficiency degradation of nitrobenzene.After the calculation,RPB-O₃/Fe²⁺?RPB-O₃ and BR-O₃/Fe²⁺were used to degrade 0.16gCOD/1gO₃?0.1gCOD/1gO₃?0.097gCOD/1gO₃,respectively.Thirdly,the nitrobenzene wastewater with low concentration alkaline after extraction was further treated by RPB-O₃/H₂O₂ process.The result were as followed:when the concentration of nitrobenzene was 76.05mg/L,COD was 139.9mg/L,reaction temperature was 25?with the initial pH=10,?=80,40L/h of gas-flow,mass concentration of ozone was40mg/L,liquid rate was 120L/h,the total dosage of H₂O₂ was 5 mmol/L,reagent dosing was for 3 times and 30 minutes of recycling time.The removal efficiency of nitrobenzene and COD finally reached 99.2%and 80.01%,respectively.With this understanding,compared with the RPB-O₃,the removal efficiency of nitrobenzene increased by 10.2%and the COD removal rate increased by 22.88%;compared with BR-O₃/H₂O₂ process,the removal efficiency of nitrobenzene and COD increased by 18.3%and 23.54%,respectively.After the calculation,RPB-O₃/H₂O₂?RPB-O₃ and BR-O₃/H₂O₂ were used to degrade 0.14gCOD/1gO₃?0.1gCOD/1gO₃?0.099gCOD/1gO₃,respectively.Finally,the degradation intermediates of RPB-O₃/Fe²⁺and RPB-O₃/H₂O₂ were detected by HPLC-MS.The HPLC-MS spectrogram showed that the degradation intermediates of the two processes were the same,including nitrophenol,benzoquinone,butenic acid,butylenedioic acid.The oxidation degradation pathway of nitrobenzene was finally deduced.