Study On Preparation And Performance Of Polyvinyl Alcohol Pervaporation Desalination Composite Membranes

The global freshwater crisis is growing for the population increase and speedy economy growth.Obtaining freshwater resources from unconventional water sources such as seawater,brackish water or salt-containing wastewater is a valid way to deal with the freshwater crisis.At present,thermal desalination technology(such as multi-effect distillation and multi-stage flash distillation)has been gradually replaced by reverse osmosis(RO)desalination technology due to the much lower energy consumption and less corrosion problem.RO has been widely used in desalination with a market share of more than 60%.However,RO cannot handle highly concentrated brine,but also cause the production of plenty of concentrated brine due to the limitation of its desalination principle.Pervaporation(PV)desalination technology is a thermal-membrane desalination technology developed in recent years.The driving force of PV desalination process is the water vapor pressure difference across the PV membrane.PV is particularly suitable for the treatment of highly concentrated brine because its separation selectivity is not affected by the salt concentration.PV desalination technology has received extensive attention and expectation from academia and industry in recent years.Since salt cannot be vaporized,the salt rejection rate of the PV desalination process is unquestionable,and the core constraint of its application is the efficiency of water passing through the membrane.Multilayer composite is the most efficient form of membrane structure.High-efficiency PV desalination membrane usually consists of a dense hydrophilic layer(separation layer)and a porous support layer with low resistance.The dense hydrophilic layer makes PV composite membrane has excellent anti-fouling performance.However,the water flux of the current PV desalination membrane is relatively low,which hinders the development of PV desalination technology.In this paper,with polyvinyl alcohol(PVA)as the separation layer,the structure of the PV composite membrane was adjusted to improve the desalination performance.There are three aspects of research:1.The interfacial compatibility between hydrophilic PVA and hydrophobic polytetrafluoroethylene(PTFE)films was solved by transition layer method.To solve the problem of interface combination between the support layer and the separation layer,researchers usually choose hydrophilic ultrafiltration membrane as the support layer of PV composite membrane.However,hydrophilic ultrafiltration membrane usually has less surface pore sizes and low surface porosities compared with hydrophobic polytetrafluoroethylene(PTFE)microfiltration membrane.Using PTFE microfiltration membrane as the substrate shall provide much lower sublayer resistance to water transport.To avoid the delamination between the PTFE substate and the hydrophilic dense selective layer,a gutter layer was introduced.Specifically,a layer of PVDF transition layer was prepared on the surface of PTFE by nonsolvent-induced phase conversion(NIP)method.Then the PVDF layer was coated with a polyvinyl alcohol(PVA)dense layer which acts as a hydrophilic separation layer.The PVA/PVDF/PTFE composite membrane can not only perform traditional vacuum PV desalination(vacuum is pumped from the bottom of the membrane,and then the water vapor was collected by condensation),but also perform direct contact pervaporation(DCPV)desalination(the bottom of the membrane contacts the circulating coolant to collect water vapor directly)for its hydrophobic PTFE supporting layer.Under PV and DCPV modes,the water flux reached 83.4±6.5 kg·m-2·h-1 and 44.5±3.0 kg·m-2·h-1,respectively,when the membrane was treated with 3.5 wt.%sodium chloride(NaCl)solution at 75?.The salt rejection was>99.9%in both modes.During 24 hours of continuous DCPV testing,the salt rejection and water flux of the PVA/PVDF/PTFE composite membrane remained unchanged when the 3.5 wt.%NaCl solution containing tween 20(tween 20,10 mg·L-1,pollutant)was used as feed at 75?,which proved that the PVA/PVDF/PTFE composite membrane had good stability and anti-fouling property.2.A method of spray coating was used to prepare PVA/PTFE composite membrane.PVA solution was directly coated on the surface of PTFE hydrophobic microfiltration membrane by spray coating to remove the PVDF transition layer.In this way,the mass transfer resistance of PV composite membrane was reduced.It was found that tiny PVA droplets(micron size or smaller)could enter the PTFE surface pores under spraying conditions,and the PVA droplets could dry quickly in a few seconds.Thus,the PVA can fill the holes on the surface of the PTFE film and wrap the filaments on the surface of the PTFE film,thus constructing a mechanical interlocking structure.The results show that the adhesive force(0.15 MPa)between layers of PVA/PTFE composite membranes prepared by spray-coating is 1.5 times higher than that(0.09 MPa)of PVA/PVDF/PTFE composite films prepared by roller-coating.When treated 3.5 wt.%NaCl solution at 75?,the water flux of PVA/PTFE composite membrane reached 143.4±8.9 kg·m-2·h-1,and 64.2±2.9 kg·m-2·h-1 in PV and DCPV modes,respectively.And the salt rejection of both modes was higher than 99.9%.The water flux of PVA/PTFE composite membrane in PV and DCPV could still reach 120.6±4.2 kg-m-2·-h-1and 56.6±1.9 kg·m-2·h-1 respectively when treated 10wt.%NaCl solution at 75?.3.Application of nanofiber composite membranes with low mass transfer resistance in PV membranes.In order to prepare PVASBQ/NC-PAN pervaporation composite membrane,we choose electrospun polyacrylonitrile(PAN)membrane as support layer,and adjusted PAN surface aperture by nanocellulose(NC).The ultraviolet light(UV)crosslinked poly(vinyl alcohol)-styrene pyridinium salt(PVA-SBQ)was coated on the surface of NC layer.The PAN membrane has high porosity and large aperture.PVA-SBQ was used as functional separation layer which can be rapidly crosslinked within 1 minute under ultraviolet light.In the whole process of composite membrane preparation,water was used as the solvent.The composite membrane is easy to be prepared on an industrial scale because it is prepared by a simple scrap-coating method.The PV water flux of PVA-SBQ/NC-PAN composite membrane reached 122.6±10.8 kg·m-2·h-1 when treated 3.5 wt.%NaCl solution at 75?.The water fluxes reached 114.6±13.8 kg·m-2·h-1 and 79.9±13.3 kg-m-2·h-1,when treated 10wt.%and 20 wt.%sodium chloride aqueous solution at 75?respectively.