| Wastewater inclusive of nitric acid is extensively produced in many industries,such as mining,fertilizer production,and sewage irrigation,which has caused remarkable eutrophication of water and has severely damaged the environment and ecosystems.Despite much effort on the general nitrate-based wastewater treatment,studies on the wastewater containing low concentration of nitric acid remain very limited.For simulated wastewater with diluted nitric acid,solvent extraction method and supporting liquid membrane extraction technology were used to recycle nitric acid nitrogen,so as to achieve harmless and resource disposal of industrial nitric acid wastewater.First of all,solvent extraction was employed to extract nitric acid from mimetic wastewater.A comparison of various types of extractants was made on the performance of extracting nitric acid.The results show that the extractant Alamine 336 has a superior performance as compared to other extractants.The influence of the extraction time,concentration of Alamine 336,initial concentration of nitric acid and oil/water ratio were investigated in the Alamine 336-related solvent extraction system.It is found that the extraction of nitric acid has a positive correlation with the aforementioned factors.Particularly,the amount and the extraction percentage of the extracted nitric acid are increased with increasing the initial concentration of nitric acid.The mechanism underlying this peculiar phenomenon is speculated that various structures of the extracted complex TOA-n HNO3?n is not less than 1?can be formed in the presence of different concentrations of nitric acid.Furthermore,back-extraction of nitric acid from loaded organic phase can be achieved by using sodium hydroxide.With the conditions in the present study,i.e.,oil/water ratio of 1:2,10.0 vol%of Alamine 336,and 2.0 g·L-1 of nitric acid could be reduced to 15 mg·L-1 after two theoretical grades of extraction.On the basis of solvent extraction,a supporting liquid membrane extraction system was proposed.Three-chamber continuous solvent extraction was used to support the liquid film.The effects of membrane type,extraction time,material and liquid concentration,material phase volume and two-phase flow rate on mass transfer performance of membrane extraction were investigated.The results show that the mass transfer performance is better with the increase of the film porosity and the decrease of the film thickness.Results show:with hydrophilic PVDF membrane pore size of 0.464microns,extractant Alamine 336 concentration is 10.0 vol%,material initial nitrate concentration is 1.0 g·L-1,using reverse extraction phase concentration is 0.01 mol/L sodium hydroxide solution,liquid phase and organic phase and the extraction phase flow are all under the condition of 150.0 ml·min-1,after about 270.0 min nitric acid in the waste water can be reduced to 14.4 mg·L-1,and reach the standard level of emissions.The mass transfer coefficient KA of the complex membrane extraction system is 1.06×10-5 m·s-1,and the mass transfer coefficient KA of the single extraction system is 5.62×10-6 m·s-1,that is,the complex membrane extraction system has a high mass transfer rate.The mass transfer model of supported liquid membrane extraction was established.The mass transfer process was divided into mass transfer steps of material phase diffusion,mass transfer steps of transmembrane transport,experimental reaction process and mass transfer steps of organic phase diffusion,and the total mass transfer coefficient was estimated according to the classical mass transfer resistance model.By comparing the measured value of total mass transfer coefficient with the simulated value,it is proved that the model can effectively fit the measured total mass transfer coefficient,and the error between the measured mass transfer coefficient and the theoretical value is within10%.The mass transfer coefficient of each mass transfer step is compared,and the mass transfer step of transmembrane transport is determined as the mass transfer control rate step.Furthermore,sodium hydroxide has been experimentally indicated to promisingly back-extract the nitric acid from loaded organic phase.Our results provide useful information on the facile recovery of diluted nitric acid,thus shedding light on the highly efficient treatment of the nitrate-based wastewater in a myriad of industries. |
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