Degradation Of Nitrobenzene-containing Wastewater By Persulfate Advanced Oxidation Process Enhanced By High Gravity Technology

As an important chemical intermediate,nitrobenzene is widely used in the petrochemical,pharmaceutical and explosive industries.With the rising demand for nitrobenzene in industry,the treatment of nitrobenzene-containing wastewater aroused more and more attention.Persulfate advanced oxidation process has strong oxidizing ability.However,due to the stability of persulfate,the SO₄^·-generated after persulfate catalytic activation can degrade pollutants better.O₃/Na₂S₂O₈ and NZVI-Na₂S₂O₈ oxidation processes can be used to degrade alkaline and acidic nitrobenzene-containing wastewater,respectively.However,they have some disadvantages,such as low ozone mass transfer efficiency,easy agglomeration of nano-zero-valent iron and low catalytic activity.The contact area between the gas-liquid or liquid-liquid is increased,and the renewal rate of the phase interface is accelerated under the high gravity environment created by rotating packed bed?RPB?.Based on the characteristics that high gravity technology can enhance mass transfer and micro mixing,we put forward the idea of high gravity enhanced persulfate advanced oxidation process to degrade nitrobenzene-containing wastewater,aiming at solving the problems of low mass transfer efficiency of ozone and easy agglomeration of nano zero valent iron.In this paper,high gravity technology was employed to intensify the persulfate advanced oxidation process,and applied to O₃ activated persulfate system?RPB-O₃/Na₂S₂O₈ system?and the NZVI pre-reduction and persulfate oxidation system?sequential NZVI-Na₂S₂O₈system?to degrade alkaline and acid nitrobenzene-containing wastewater,respectively.The influencing factors,degradation mechanism and degradation way in its decomposition were investigated.The conclusions are shown as follows:RPB-O₃/Na₂S₂O₈ system was used for the degradation of alkaline nitrobenzene-containing wastewater,and the effects of operating parameters on degradation efficiency of nitrobenzene was studied.The results show that direct and indirect reactions of ozone were deeply affected by initial pH of wastewater,and the increase of pH was beneficial to the decomposition of ozone and the activation of persulfate.The increase of the high gravity factor was conducive to the nitrobenzene degradation,and the content of free radicals was affected by Na₂S₂O₈concentration.Nitrobenzene degradation efficiency increased with the increase of gas phase O₃ concentration,but ozone utilization efficiency decreased.The optimum operational conditions were obtained at the initial pH value of 10,high gravity factor of 40,Na₂S₂O₈concentration of 2.5 mmol/L,gas phase O₃ concentration of 60 mg/L and initial nitrobenzene concentration of 200 mg/L.Under the optimized conditions,the degradation efficiency of nitrobenzene and removal efficiency of TOC were 90.59%and 35.68%,respectively,at 30min.However,nitrobenzene degradation efficiencies by ozone oxidation alone and persulfate oxidation treatment were 69.44%and 27.14%,respectively.Through the contrast experiment with the bubbling reaction system?BR-O₃/Na₂S₂O₈?,it was found that a synergistic effect between the RPB and O₃/Na₂S₂O₈ oxidation processes for the degradation of nitrobenzene.NZVI was prepared by high gravity method,and then used in sequential NZVI-Na₂S₂O₈system to degrade acidic nitrobenzene-containing wastewater.The effects of operating parameters on degradation efficiency of nitrobenzene and the removal efficiency of total organic carbon?TOC?was studied.The results show that the increase of NZVI concentration was beneficial to the reduction of nitrobenzene,but excess iron sludge would be produced.The increase of Na₂S₂O₈ concentration was conducive to the mineralization of wastewater,and the existing forms of iron were affected by initial pH value.The optimum operational conditions were obtained at the NZVI concentration of 0.75 g/L,Na₂S₂O₈ concentration of26.8 mmol/L,initial pH of 5 and initial nitrobenzene concentration of 200 mg/L for sequential NZVI-Na₂S₂O₈ system.Under the optimized conditions,the nitrobenzene degradation efficiency and the TOC removal efficiency were 100%and 49.25%,respectively,at 30 min.Compared with the Fe²⁺/Na₂S₂O₈ system,it was proved that the slow release of Fe²⁺by using NZVI as iron source is beneficial to the degradation of nitrobenzene.The results indicated that sequential reduction-oxidation process was better for the degradation of nitrobenzene by compared with NZVI/Na₂S₂O₈ system.Compared with the RPB-O₃/Na₂S₂O₈ system,in addition to treating different acid and alkaline wastewater,not only nitrobenzene is all removed in the sequential NZVI-Na₂S₂O₈ system,but the TOC removal efficiency was also higher.The degradation mechanism of nitrobenzene in RPB-O₃/Na₂S₂O₈ and sequential NZVI-Na₂S₂O₈ system was studied by electron paramagnetic resonance?EPR?tests and capturing tests.It was found that there were three ways of oxidation in RPB-O₃/Na₂S₂O₈system including direct oxidation by O₃,indirect oxidation by·OH and SO₄^·-,within which·OH played the major role.The two main active radicals(SO₄^·-and·OH)were determined in the sequential NZVI-Na₂S₂O₈ system,within which SO₄^·-played the major role.The intermediate products of nitrobenzene degradation in the two systems were obtained by GC-MS,and possible degradation pathways were deduced.