Study On Anti-fouling And Antiwetting Properties Of Hydrophobic And Hydrophilic Composite Membranes In Membrane Distillation

A large amount of hypersaline brine is produced during oil and gas extraction and zero liquid discharge（ZLD）,which is difficult to handle with conventional pressure-driven membranes.Membrane distillation（MD）is less affected by high salinity and is expected to play a role in the brine reduction.However,MD faces the challenges of membrane fouling and wetting,and it is urgent to prepare MD membranes with good performance.In this study,the flat commercial polytetrafluoroethylene（PTFE）and polyvinylidene fluoride（PVDF）membranes were separately modified,and hydrophilic/hydrophobic PVA-PAA/PTFE and hydrophobic PVDF composite membranes were prepared.The membrane morphology and chemical composition were characterized by electron microscopy,atomic force microscopy and infrared,and the membrane pore size and wettability were characterized by pore size analysis,LEP and contact angle tests.The two composite membranes were applied in direct contact membrane distillation（DCMD）to treat salt solutions containing calcium sulfate,kerosene,humic acid and anionic surfactant（SDS）,respectively,to analyze the fouling and wetting mechanism of the substrate on the composite membrane.The actual landfill leachate was concentrated with composite membranes to explore their potential application in actual wastewater.Firstly,flat PTFE membrane was coated with a hydrophilic layer by spin coating-thermal crosslinking to prepare Janus composite membrane（PVA-PAA/PTFE）.The coating was the mixture of polyvinyl alcohol（PVA）and polyacrylic acid（PAA）,which was cross-linked via high temperature dehydration and condensation,and was stable in microwave-assisted hot water.The surface of composite membrane was dense and the roughness was reduced from 117 nm to 5.3 nm.PVA-PAA/PTFE membrane showed an asymmetric wettability,was hydrophilic in air,with a water contact angle of 59.1°,and was super-oleophobic under water.When concentrating a kerosene emulsion with 3.5%Na Cl,PVA-PAA/PTFE membrane exhibited excellent anti-fouling performance,the permeate flux remained at 24.1 kg/m²·h,and the conductivity was below 6μS/cm after the concentration factor reached 3.Concentrating the 500 mg/L HA solution with 3.5%Na Cl and 14.7 m M Ca SO₄ solution,when the concentration factor reached 3,the flux decay rates of PTFE membrane were 40.7%and 48.4%,respectively,which were 33.8%and 16.5%,respectively for PVA-PAA/PTFE membrane,showing fouling and wetting resistance to HA and Ca SO₄.When SDS solution was tested at a constant concentration for 12 hours,the flux of composite membrane was stable at 16.0 kg/m²·h and the conductivity was 12.5μS/cm.The PVA-PAA coating delayed the contact of SDS with the substrate,slowing down membrane fouling and wetting.When the actual landfill leachate was treated and the concentration ratio reached 3,the flux of composite membrane was 17.6 kg/m²·h,which was more stable than that of PTFE membrane,and the conductivity was lower than 3μS/cm,indicating that the prepared Janus composite membrane had application prospects in the treatment of landfill leachate.Secondly,PVDF membrane was soaked in a PTFE aqueous emulsion to prepare a hydrophobic PVDF composite membrane with slip characteristics.With the increase of emulsion concentration,the pore size and surface porosity of composite membranes gradually decreased,but the LEP value increased.The optimized modification conditions were 100%emulsion and soaking for 10 min.When the 500 mg/L HA solution with 3.5%Na Cl was concentrated,the flux of PVDF composite membrane was maintained at 18.7 kg/m²·h,and the conductivity was below 2μS/cm.When 14.7 m M Ca SO₄ solution was concentrated by 3 times,the flux of composite membrane decreased by only 0.8%,showing an anti-scaling property.When concentrating the actual landfill leachate,the flux of PVDF pristine membrane decreased by 32.7%,which was only 7.1%for the composite membrane,and the conductivity was maintained at 3μS/cm.The previous experiment results proved that the PVDF composite membrane prepared in this study could not only resist the fouling of HA and Ca SO₄,but also had certain anti-fouling and anti-wetting capabilities in the treatment of landfill permeate.Finally,for the two composite membranes of PVA-PAA/PTFE and hydrophobic PVDF,a comparative analysis was carried out from three aspects:the maturity and cost of modification,flux decay rate,and anti-fouling and anti-wetting capabilities.