| Lithium is the lightest metal element which has been widely utilized in batteries,ceramics,glass and other fields.Over 70%of lithium compounds are produced from salt lake brines.Therefore,lithium extraction from brines is the most significant way to obtain lithium resource.Salt lakes in Western China are rich in lithium resources,but the mass ratio of magnesium to lithium is extremely high so that lithium ions need to be purified before they have commercial value.Although the methods such as precipitation,solvent extraction,salting out and adsorption have been attempted to applied in lithium extraction,highly selective,low-cost and environmentally friendly techniques have not been realized.Nanofiltration(NF)is an emerging pressure driven membrane separation technology,which selectively separation behavior is primarily determined by Donnan exclusion theory.Commercial polyamide NF membranes are generally negatively charged because of hydrolysis of acyl chloride,thus they have good separation performance to multivalent anions and monovalent anions.However,magnesium and lithium belong to divalent cations and monovalent cations,so the selectivity of commercial NF membranes is relatively poor.Therefore,the fabrication of positively charged NF membranes is expected to promote the separation of magnesium and lithium from brines,which has important practical value.In this paper,polyamide NF membrane was firstly aminated by secondary interfacial polymerization.Positively charged amine layer was successfully constructed on the surface of polyamide nanofiltration membranes,and separation selectivity of modified membranes for magnesium and lithium was improved.On this basis,aiming at the problem of hydrolysis of acyl chloride during secondary interfacial polymerization,EDC/NHS amidation was further proposed to prepare NF membranes with higher positive charge and better separation performance for Mg2+/Li+.The contents and conclusions of this paper are as follow:(1)Preparation of positively charged NF membrane by secondary interfacial polymerization:Using residual acyl chloride groups on the surface of the pristine membrane(PA)as reaction sites,secondary interfacial polymerization reaction was carried out on the surface of PA with polyethyleneimine(PEI)solution to construct positive amine layer.Taking water contact angle,Zeta potential and flux of membrane as criteria,the optimal conditions for secondary interfacial polymerization were as follow:PEI 10000 concentration was 0.2 wt%and secondary interfacial polymerization reaction time was 2 min.At this point,water contact angle of the membrane surface decreased from 74° to 66°,Zeta potential rosed from-28.0 mV to-9.2 mV,flux of membrane slightly decreased,and the surface structure and morphology of membrane did not change significantly.The modified membrane(SP-PA)was used for the separation of MgCl2/LiCl(Mg2+/Li+=150/1)binary mixed solution with a total salinity of 2000 ppm.The results showed that the separation factor S of SP-PA was 8.79,higher than 4.78 of PA.In order to solve the problem of hydrolysis of acyl chloride and steric hindrance during secondary interfacial polymerization,ethanol was selected as solvent and PEI600 was selected as positive charge modification reagent.The results showed that under the same test conditions,the separation factor of resultant membrane for Mg2+/Li+reached 12.37.(2)Preparation of positively charged nanofiltration membrane by EDC/NHS amidation:Since acyl chloride groups on the surface of PA membranes after interfacial polymerization are hydrolyzed to carboxyl groups,the carboxyl groups are activated by EDC/NHS,then amidated and grafted with PEI to construct charged positive amine layer.Similarly,water contact angle and Zeta potential of membrane were used as criteria,and the optimal conditions for EDC/NHS amidation were as follow:PEI concentration was 3wt%,amidation temperature was 39? and reaction time was 120 min.At this point,water contact angle of the membrane surface decreased from 75° to 40°,Zeta potential rosed from-28.0 mV to 6.4 mV,flux of membrane slightly decresed,and the surface structure and morphology of the membrane did not change significantly.The modified membrane(EDC-PA)was used for the separation of MgCl2/LiCl(Mg2+/Li+=150/1)binary mixed solution with a total salinity of 2000 ppm.The results showed that the separation factor S of EDC-PA membrane was 13.19,higher than 4.78 of PA.In order to solve the problem of steric hindrance during EDC/NHS amidation,PEI600 was selected as positive charge modification reagent.The results showed that under the same test conditions,the separation factor of resultant membrane for Mg2+/Li+reached 16.30.(3)Study on the application performance of surface-charged positive-charged nanofiltration membrane:The similar simulated brine was prepared as feeding solution for Qinghai Dachaidan and Dongtai Jinnar salt lake brine to test the Mg2+/Li+ separation performance of EDC-PA600.The results showed that the rejection of Mg2+ in the simulated brine of Qinghai Dachaidan Salt Lake is 81.1%,the rejection of Li+is-131.4%;and the rejection of Mg2+ in the simulated brine of Dongtai Jinnar Salt Lake is 76.8%,the rejection of Li+is-105.8%.The above results proved that the EDC-PA600 membrane can effectively separate magnesium and lithium in the brines.In order to further investigate the effect of hardness in brine on the stability of membrane application,anti-scaling performance of EDC-PA600 membrane was evaluated by using high concentration Ca2+and SO42-solutions as feeding solution.The results show that the flux of EDC-PA600 membrane can still maintain 80%of the original flux after reaching steady state,and it has excellent anti-scaling performance.In summary,based on Donnan exclusion of nanofiltration membranes,positively charged amine layer was constructed on the surface of polyamide nanofiltration membrane simply and mildly by secondary interfacial polymerization and EDC/NHS amidation,which not only improved the selectivity of Mg2+/Li+,but also improved the anti-scaling performance.This work provides ideas for the development of novel Mg2+/Li+ separation nanofiltration membranes. |
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