Preparation Of Positively Charged Nano-filtration Membranes Functionalized By Quaternization For Enhanced Donnan Exclusion

Polyamide composite nanofiltration membranes（NF）are often used to separate mono-/multi-valent salts.According to the Donnan effect,commercial polyamide nanofiltration membrane with negatively charged shows high separation selectivity for mono-/multi-valent anions,but lower separation selectivity for mono-/multi-valent cation mixed salts.However,there are more demands of separating these cations in the actual industrial application,such as the purification of lithium in the lithium industry,reducing hardness of drinking water in the water supply,the purification of brine and so on.To enhance the application capability of nanofiltration separation technology in the industrial field,based on the Donnan effect,this thesis constructed two methods to modify and strengthen the positive charge on the surface of the commercial polyamide nanofiltration membrane,to improve the separation performance of NF membrane for mono-/multi-valent cations.The main contents are as follows:（1）Michael addition reaction grafting quaternary ammonium groups to enhance the positive charge of nanofiltration membrane surfacePolyamide nanofiltration membrane contains many carboxyl groups on its surface and lacks the reactive groups required for Michael addition reaction.Through"EDC/NHS activation+amination" method firstly grafted with polyethyleneimine（PEI）enriching in amine groups（primary amine,secondary amine,tertiary amine）on the surface of NF membrane,to get polyethyleneimine grafted nanofiltration membrane（NF-g-PEI）,and construct an active layer containing a large number of amine groups.By optimizing the reaction conditions,the NF-g-PEI membrane with the amine group content on the membrane surface as high as 309±21.8 μmol·m-2 was prepared.Compared with NF membrane,the rejection of NF-g-PEI membrane for MgCl₂ was increased from 72.8 ± 10.4%to 83.4 ± 7.9%,and the flux of MgCl₂ solution was reduced from 71.1 ± 0.2 L·m-2-h-1 to 66.8 ± 4.9 L·m-2·h-1.In order to further improve the positive chargeability of the membrane surface,strengthen the Donnan repulsion effect of the membrane against multi-valent cations,through Michael addition reaction,the methacryloxyethyl trimethyl ammonium chloride（DMC）was grafted onto the surface of the NF-g-PEI membrane to construct a positively charged strengthening layer.Compared with NF-g-PEI membrane,the rejection of nanofiltration membrane grafted DMC on its surface（NF-g-PEI/DMC）for MgCl.was further increased to 89.0 ± 2.4%,and the flux of MgCl₂ solution was basically unchanged,which was 67.4 ± 0.8 L·m-2·h-1.The NF-g-PEI/DMC membrane was applied to the separation of Lithium/magnesium chloride mixed salt with a concentration of 1000 ppm,Mg²⁺:Li⁺=10:1,and pH=8～9.Compared with NF membrane(the rejection of Mg²⁺is 83.9 ± 4.5%,the rejection of Li⁺was-5.5 ± 1.7%,and the separation factor S is 6.7 ± 1.8);The rej ection of NF-g-PEI/DMC membrane for Mg²⁺was increased to 93.7± 0.8%,the rejection of Li⁺is-7.3 ± 0.9%,and the separation factor S was 17.0 ± 4.4,improved separation of lithium and magnesium.（2）Nitrogen alkylation reaction quaternized amine group to enhance the positive chargeability of nanofiltration membrane surfaceGrafting DMC had realized the inversion and enhancement of the surface chargeability of NF membrane,but DMC has a long molecular chain,which makes it difficult to achieve high-density grafting,which restricts the increase in the content of positively charged groups on the surface of the nanofiltration membrane.Therefore,based on the nitrogen alkylation-reaction,this article used CH3I to quaternize the amine groups in the PEI active layer to improve the positive chargeability of the positively charged groups on the surface of the NF-g-PEI membrane,so a positively charged nanofiltration membrane（NF-g-PEI/alkyl）with stronger charge on the surface was obtained.The rejection of this membrane to MgCl₂ is 94.0± 1.8%,which was significantly higher than 89.1± 2.4%of NF-g-PEI/DMC membrane,and the flux of MgCl₂ solution was increased to 74.6±1.8 L.m-2·h-1 from 67.4± 0.8 L·m-2·h-1.The NF-g-PEI/alkyl membrane was applied to the separation of Lithium/magnesium chloride mixed salt solution with a total concentration of 1000 ppm,Mg²⁺:Li⁺=10:1,and pH=8～9.Compared with NF-g-PEI/DMC membrane,the rejection of NF-g-PEI/alkyl membrane for Mg²⁺ was increased to 96.1 ± 1.0%,and the rejection of Li⁺was-8.2 ± 1.0%,the separation factor S was 21.9± 6.2,further improve the separation effect of lithium and magnesium.In summary,this thesis was based on the theory of Donnan effect,using Michael addition reaction and nitrogen alkylation reaction,wich have mild conditions and simple operation,to modify the commercial polyamide nanofiltration membrane,realized positive charge modifying and strengthening on the surface of commercial composite NF membrane,which effectively improved the separation selectivity for lithium and magnesium,and provided technical reference for the separation of the mono-/multi-valent cation system of the nanofiltration membrane.