Study Of Quaternary Ammonium Modified Polyethyleneimine Composite Nanofiltration Membrane And The Performance Of Magnesium-lithium Separation

The rapid development of new energy industries such as lithium-ion batterie in China has led to a rapid increase in the demand for lithium.China is rich in lithium resources,more than 80%of which is stored in salt lake brines.However,the ratio of magnesium to lithium ions in these brines is as high as tens or even to hundreds,and the cost of lithium extraction using traditional technology is high and the efficiency is low.Positively charged nanofiltration membranes are able to separate divalent magnesium ions and monovalent lithium ions in salt lake brines,but the permeability of existing nanofiltration membranes is low.To address this problem,strong electrolyte molecules containing 1-3 quaternary ammonium sites were designed and synthesized in this work,and they were used to modify the Polyethyleneimine（PEI）-Trimesoyl Chloride（TMC）polyamide nanofiltration membranes.The results showed that the mean effective pore size and positive charge density of the modified membranes simultaneously increased,which improved the permeability of membranes while maintaining the salt rejection and enhanced the selectivity of the nanofiltration membranes for magnesium and lithium.The effect of quaternary ammonium monomer structure on the modified membrane microstructure and separation properties was further investigated to deepen the understanding of the modification mechanism.The specific works are as follows.（1）Quaternary ammonium molecule N,N’-bis（2-aminoethyl）-N,N,N’,N’-tetra（2-hydroxypropyl）-1,2-dibromoethylammonium（QEDTP）with hydroxyl and amino groups were designed and synthesized.The initial polyamide（PA）composite nanofiltration membrane was prepared on the polysulfone（PSF）substrate membrane by interfacial polymerization reaction between PEI and TMC.The aqueous QEDTP solution was coated on the surface of the PEI-TMC composite membrane,and the QEDTP molecules were grafted into the PEI-TMC composite membrane through the condensation reaction of the hydroxyl and amino groups in the QEDTP molecule with the residual acid chloride group in the membrane.The mean effective pore size of the membrane increased from2.7?to 5.3?,and the pure water flux increased from 25.8 L·m^-2·h^-1to 127.2 L·m^-2·h^-1under 6 bar after the modification of QEDTP molecule.The Mg-Li separation performance of PEI-TMC composite membrane and QEDTP modified membrane was tested,and the Mg-Li separation factor of QEDTP modified membrane could reach 15.6,which is more than 2.5 times higher than that of PEI-TMC composite membrane（6.5）.In addition,the QEDTP-modified membrane is extremely stable in 180 h’s continuous operation at 6 bar,and the flux recovery rate was 74.5%after contamination treatment with bovine serum albumin aqueous solution for up to 40 h and cleaning with deionized water,which has good anti-pollution ability.（2）Three strong electrolyte monomers containing single quaternary ammonium（M1）,double quaternary ammonium（M2）and triple quaternary ammonium（M3）sites were designed and synthesized.The surface roughness and separation layer thickness of the M1,M2,M3 modified membranes were reduced in order,and the water contact angle was increased in order,and all of them were larger than that of the PEI-TMC composite membrane.The order of positive charge density on the membrane surface is M2>M1>M3.The mean effective pore size of the three modified membranes were 5.0?,5.2?,and 5.5?,respectively,and the permeance of the modified membranes were increased by more than 2.5 times compared with the PEI-TMC composite membrane.In addition,the modified membranes showed good heavy metal ions removal ability with>85%rejection of Cu²⁺,Cr³⁺,Zn²⁺,Co²⁺,Ni²⁺,which is potential to be applied to the removal of heavy metal ions in waste water.