Degradation Of Nitrobenzene-Containing Wastewater By O₃/H₂O₂Enhanced By High Gravity

As the nitro groups of nitrobenzene compounds is electron-withdrawing groups whichcould increase the stability of the benzene rings and make them difficult to be degraded byoxidation process. Nitrobenzene has been listed as one of the priority control pollutants andthe emission of nitrobenzene-containing wastewater has been limited strictly by manycountries. Ozone/hydrogen peroxide (O3/H2O2) advanced oxidation process is not controlledby composition and concentration of such wastewater and has become a hot research spot inrecent years. However, its oxidation efficiency suffers from controls of ozone mass transferprocess in water and ozonation reaction simultaneously. Due to its poor water solubility, theenhancement of traditional equipment for ozone dissolution rate is not obvious, or existengineering magnification difficulties that limit the application of ozonation. High gravitytechnology, is a kind of process intensification technology, which using Rotating Packed Bed(RPB) to create high gravity filed. RPB can greatly improve the gas-liquid contact area andthe phase interface updating rate in gas-liquid contact reaction system, reduce the filmthickness and the film resistance. Compared with gas-liquid mass transfer rate of theconventional reaction apparatus, it can be increased1~2orders of magnitude, andeffectively enhance the mass transfer of the ozone from the interface to the liquid phase of thedelivery amount, thereby the efficiency of oxidative degradation was improved.In this paper, the high gravity technology coupled with O3/H2O2advanced oxidationprocess was used to the nitrobenzene-containing wastewater treatment. Firstly, ozone masstransfer performance of the high gravity field was studied, which verify the high gravitytechnology can improve the ozone mass transfer rate effectively and increase dissolved ozoneamount per unit time to provide guarantees for the subsequent efficient oxidative degradation of nitrobenzene. Then, the oxidative degradation efficiency of RPB-O3/H2O2for simulatedwastewater nitrobenzene was examined. Finally, in terms of influence of coexisting substancein actual wastewater on mass transfer and decomposition of ozone, which could affect theefficiency of oxidative degradation, the effects of coexisting substances on this process ofdegradation of nitrobenzene was studied. The main results are as follows:The ozone mass transfer ability, using water for ozone absorption in the high gravityfield, was investigated. The results showed that the ozone concentration of single absorptionin deionized water could reach2.57mg/L at the initial pH of3.5, high gravity factor of100,gas flow of75L/h, liquid flow rate of90L/h, gas phase ozone concentration of50mg/L, andit was significantly higher than the aeration reaction equipment in which the ozoneconcentration of water absorption could only reach1.12mg/L. Furthermore, the ozoneconcentration could reach5.1mg/L after cycle absorption for5min in the high gravity field,which is the theoretical equilibrium value of51%under such conditions. Comparing theozone liquid volumetric mass transfer coefficient KLa，it could see that the KLa value inrotating packed bed device (Rotating Packed Bed, RPB) is2.5times that in conventionalaeration reactor.The effects of RPB-O3/H2O2process on degradation efficiency of nitrobenzene wasinvestigated. The experimental results showed that the removal efficiency of nitrobenzeneincreased with the increasing of gravity factors and initial pH, but it showed a decreasingtrend after the first increase with the increase of hydrogen peroxide concentration and liquidflow. Then, the response surface methodology was used to optimize the experimentalconditions to obtain the optimum process conditions: H2O2concentration5.7mmol/L, the gasphase ozone concentration of50mg/L, liquid flow rate of125L/h, high gravity factor of100.And under optimum conditions, the nitrobenzene removal rate was99%, CODCrwas89.1mg/L, the biodegradability coefficient greater was more than0.3after treating60min.In the RPB-O3/H2O2Process of nitrobenzene wastewater treatment, the effects of addingcoexisting substances of real wastewater on oxidative degradation efficiency wereinvestigated. The study found that the addition of sodium carbonate, sodium hydroxide, sodium phosphate, sodium sulfate and sodium nitrate could promote the removal efficiencyof nitrobenzene; and the addition of sodium chloride, sodium bicarbonate, sodium bisulfate,ethanol, acetic acid, formic acid and tert-butanol showed inhibition effects on the removalefficiency of nitrobenzene. In addition, when the sodium carbonate concentration was lessthan15mmol/L, it showed promoting effect in the high gravity field, while it showedinhibiting effect in the gravity field; and when the concentration was higher than15mmol/L,both of them showed inhibition effects on the degradation of nitrobenzene in the O3/H2O2process.