Preparation Of Novel Janus Membrane And Its Application In Direct Contact Membrane Distillation

Various production processes such as coal mining,chemical industry,printing and dyeing,leather processing,paper making,thus resulting in a large amount of acidic and alkaline wastewater.Serious ecological environment damage and huge economic losses may be caused if they are not treated effectively before being discharged into the environment.Membrane distillation（MD）is a newly developing thermal distillation technology via membrane.While it presents great prospects for the treatment of acid containing wastewater,the acid and alkane resistance of membrane material becomes the bottleneck problem in its application.Besides,it is also adversely affected by membrane wetting and fouling during prolonged operation.In order to solve this problem,a new Janus membrane with acid and alkane resistant was explored and a new Janus membrane with simultaneous anti-wetting and anti-fouling properties was investigated in this project.Here,we have reported a new acid and alkali resistant Janus membrane for DCMD process,which is prepared by coating aramid polymer onto the single surface of the pristine PVDF membrane.From the characterization of field emission scanning electron microscopy（FESEM）,attenuated total reflection-Fourier transform infrared spectroscopy（ATR-FTIR）and X-ray photoelectron spectroscopy（XPS）,it can be seen that aramid was successfully coated on the upper surface of the pristine PVDF membrane.In addition,the measurement of the contact angle after acid and alkaline treatment verifies that the aramid modified Janus membrane has better acid and alkaline resistance than the pristine PVDF membrane at high temperature.In the DCMD process,when solutions with different p H values were used as the feed solution,the p H on the permeate side did not fluctuate significantly,which proved that the aramid modified Janus membrane had good separation efficiency under acid and alkaline conditions.When Janus membrane concentrated 2.00 mol/L H₂SO₄ and0.10 mol/L Na OH solution,2.00 mol/L H₂SO₄ solution was concentrated to 3.75mol/L and 0.10 mol/L Na OH solution was concentrated to 0.31 mol/L after DCMD running for 24 h.The above results show that the modified Janus membrane has a good ability to concentrate acid and alkali,which shows that the prepared Janus membrane has great potential in the treatment of high concentration acid and alkali wastewater.Therefore,we also prepared a new Janus membrane with special wettability by directly depositing gallic acid（GA）and polyethyleneimine（PEI）on one side of the PVDF/polydimethylsiloxane（PDMS）blend membrane.The characterizations from FESEM,ATR-FTIR and XPS demonstrate the successful preparation of Janus membrane.In the contact angle test,it can be seen that the water contact angles on both sides of the Janus membrane with asymmetric wettability are 141±3°and 8±2°,respectively.The Janus membrane exhibited excellent anti-wetting and antifouling properties when subjected to DCMD experiments to treat high-salt oil-water emulsions containing surfactants.During the 36 h operation of DCMD,a high vapor flux of 15.7±0.2 Kg·m^-2·h^-1 was maintained,and the effluent quality was good.The excellent wetting resistance of the Janus membrane is attributed to the low surface energy of the underlying PVDF/PDMS membrane,which prevents the droplet spreading behavior,while the fouling resistance is due to the presence of the hydrophilic surface,which forms a strong hydration layer that avoids the oil droplets adhere to the membrane surface.Finally,the Janus membrane can remain stable in harsh acid and alkaline environments,from which it can be seen that the prepared Janus membrane has a bright future in challenging wastewater treatment.