Application Of Hollow Fiber Nanofiltration Membrane In The Treatment Of Low Concentration Uranium-containing Wastewater

Uranium is regarded as the foundation of the nuclear power industry.However,a large amount of low-level radioactive wastewater are generated during the mining,processing,and utilization process of uranium,which endangers human health and the environment.Current LLRW treatment technology is still facing several challenges such as the ineffective deeppurification and the generation of huge amount of secondary waste.Different types of hollow fiber nanofiltration(NF)membranes were fabricated in this study targeting on the treatment of several low-concentration uranium-containing wastewater such as the fluoro-uranium wastewater,the workshop greywater,and the desorption solution of ion-exchange resin,The surface modification method of polyelectrolyte self-assembly(LBL))+ GA cross-linking +polydopamine/polyethyleneimine(PDA/PEI)co-deposition was adopted to achieve the effective control of active layer structure and the stable uranium removal under low pressure operation.The surface properties of the NF membranes were also investigated using field emission scanning electron microscopy(FSSEM),Fourier transform infrared spectroscopy(FTIR),static contact angle to reveal the mechanism of uranium removal by the novel lowpressure hollow fiber NF membranes.The major conculsions of the study were summerized as below to provide theoretical guidance and technical support for the treatment of lowconcentration uranium-containing wastewater using NF membrane:(1)A new type of low-pressure NF membrane was fabricated by the self-assembly of poly(styrene sulfonate)(PSS)/poly(allylamine)(PAH)layers with glutaraldehyde(GA)crosslinking to treat the acidic fluoro-uranium wastewater with high fluorine concentration generated from the pressurized water reactor fuel element production line.The membrane preparation process(number of coating layers,GA cross-linking)was optimized to improve the uranium and fluorine removal rate of the NF membrane.The cross-linked LBL membrane could maintain high uranium removal rate above 90% at low pressure of less than 4 bars even at high fluorine concentration of 15 g/L.The simulation analysis using water chemistry software confirmed that the size sieving effect plays the most important part to determine the uranium removal process of the LBL nanofiltration membrane.Meanwhile,the LBL membrane possessed good acid-base stability to ensure the stable separation performance in the pH range of 2-10.On the other hand,the negatively charged commercial polyamide NF3 membrane was shown to be ineffective in the treatment of acidic fluoro-uranium wastewater in the comparative study.With the aid of the ammonium precipitation method adopted in the current wastewater treament line,the uranium concentration in the effluent was less than 0.10mg/L after the NF process,which proves the potential application of LBL membrane in uranium fluoride wastewater.(2)The co-depostion modification of polydopamine/polyethyleneimine(PDA/PEI)was further appiled on the surface of 2.5GA cross-linked membrane to improve the structural stability of the active layer in order to treat the high-salt containing desorption solution of ionexchange resin and surfactant-containing workshop greywater.The raffinose retention test confirmed that GA cross-linking and PDA/PEI coating modification can effectively prevent membrane swelling in high salt environment.And thus the stability of the selective layer was improved so as the membrane retention performance.The uranium removal rate of PDA membrane decreased by only 4.6% in the 8-hr continuous test with the desorption solution.While the decrease rate for NF3 membrane was up to 23%.According to the XPS analysis,some uranium were adsorbed on the membrane surface in the forms of U(Ⅳ)and U(Ⅵ).It was speculated that the uranyl ions were chelated and coordinated by the amino and hydroxyl groups on the membrane surface which further improved the uranium removal rate of the2.5GA membrane.In addition,the PDA/PEI surface deposition could also improve the hydrophilicity and anti-fouling property of the NF membrane in the treatment of the surfactantcontaining workshop greywater.The PDA membrane could acheive better uranium and COD removal rate than the existing pre-treatment process using commercial microfiltration and nanofiltration membrane during the 8-hr test.This study demonstrated that the modified PDA membrane had stable separation performance in the treatment of workshop greywater at low operating pressure.It provides an important basis for a more systematic study on the application of LBL membrane in other uranium-containing wastewater treatment.