Hydrophilic/hydrophobic Composite Membranes Based On The Dopamine Modification And Its Membrane Distillation Performance

Membrane distillation(MD)is a membrane separation process using a porous hydrophobic membrane as a separation medium with a vapor pressure difference on both sides of the membrane as a mass transfer driving force.It can simultaneously achieved heat and mass transfer and can effectively remove the volatile components of the solution,while theoretically 100%cut off for non-volatile components.However,the membrane distillation technology has not yet been able to realize large-scale industrial applications,The main reasons are attributed to to the lower membrane flux,the membrane easy to be contaminated or even wetted.In this paper,a single dopamine coating modification method was proposed to construct a special surface which ultra-thin,porous and in-air hydrophilic/underwater oleophobic on the commercial polytetrafluoroethylene(PTFE)microporous membrane.The vacuum membrane distillation(VMD)flux and oil pollution resistance of the original PTFE membrane are simultaneously improved.Inspired by dopamine can be coated on the surface of any material under oxygen and weak alkaline conditions,the dopamine modified hydrophilic/hydrophobic membrane was prepared and the preparation process was be controlled.In the vacuum membrane distillation test,the flux value of the dopamine modified membrane(0.5DA@PTFE-4)was 89.6 kg/m2·h as compared with the flux value of the pristine PTFE membrane of 28.8 kg/m2·h,the flux increased by 210%(the feed temperature is 70?).The pristine membrane and dopamine modified membrane were operated under vacuum membrane distillation for 35 hours,and the results showed that the dopamine modified membrane possessed better stability.Finally,through systematic studies on whether the membrane pores are wetted and the membrane distillation performance of different hydrophilic material modified membranes,we speculated that the introducing polar groups on the membrane surface can promote the membrane interact with polar water molecules and accelerate the vaporized rate.Then the flux of the dopamine modified membrane was improved.Based on the unique characteristic of dopamine modified membrane that is hydrophilic in air and oleophobic under water,the separation performance of the pristine membrane and the modified membrane in the vacuum membrane distillation for treating oily brine(3.5%NaCl brine contained 500mg/L mineral oil)were studied.The pristine membrane was easily fouled and wetted due to its hydrophobic feature while the hydrophilic modified membrane exhibited a certain degree of resistance to oil pollution and has operated more than 30 hours in oily brine.