Preparation And Properties Of Water-softening Nanofiltration Membrane Based On Azlactone Doping

Semi-aromatic polyamide membranes are widely used in water treatment.Due to their inherent structural characteristics,such membranes have better anti-fouling performance than all-aromatic polyamide membranes（MPD-PIP type polyamide membranes）,but lower retention rates of Ca²⁺and Mg²⁺.Among them,steric hindrance and electrostatic repulsion are two main effects that affect salt interception of semi-aromatic polyamide nanofiltration membrane.Most of the reported water softening nanofiltration membranes only focus on one effect to improve the salt retention performance of the membrane,ignoring the synergistic effect of the both.As a result,the retention rate of the obtained water softening nanofiltration membrane is not significantly improved.Herein,a novel organic molecule triformyl-[4,4-dimethyl-5（4H）-acridine]（TMDMA）was firstly synthesized,which can be directly used as a secondary reaction monomer or additive to participate in the interfacial polymerization,by improving the steric resistance effect and electrostatic repulsion effect of the polyamide nanofiltration membrane.With the introduction of TMDMA,the mean effective pore size of the polyamide membrane is decreased from 0.23nm to 0.18 nm.XPS test results showed that the addition of TMDMA reduced the carboxyl group content on the membrane surface,because TMDMA inhibited the diffusion of amine monomer,which prolonged the reaction time of interfacial polymerization.More oil-phase monomer TMC reacted,so the carboxyl group content on the membrane surface decreased.Moreover,the significantly decreased negative surface charge of the fabricated polyamide membrane is observed since the carboxyl from hydrolysis of the azlactone exhibits a higher p Ka（acidity coefficient）than that of the benzoic acid.The desalination experiments reveal that the Ca Cl₂ and Mg Cl₂ rejection of the resulting membranes are increased by 26%（from 71.0%to 97%）and 14.0%（from 83%to 97%）,while the water fluxes are maintained at 192 and 167kg m^-2 h^-1 MPa^-1,demonstrating a satisfactory water softening performance that exceeds commercial nanofiltration membranes.Additionally,the fabricated polyamide membranes can significantly resist the flux decline during fouling experiment that normally results from the deposition of Ca²⁺-foulant.In addition,we studied the the application of TMDMA molecular in all the fat polyamide membrane system（monemer-PIP and monemer-TAEA type polyamide film）.It proves that the TMDMA molecules involved in interfacial polymerization process can adjust the surface charge and the mean effective pore size of membrane to improve the desalination performance of water softening nanofiltration membranes.TMDMA molecular have been widely commercialized and used in water treatment processes.