Study On The Preparation Of Superhydrophobic UHMWPE Composite Membrane For Membrane Distillation

Membrane distillation（MD）is a new and efficient membrane separation technology.Under the action of water vapor pressure difference,the water vapor on the hot side enters the cold side through the membrane hole and then condenses on the cold side.It has the advantages of mild operating conditions,the ability to use low-grade heat source to deal with high concentration of brine,and theoretically has a 100%desalting rate.However,in the process of MD,it is inevitable that the MD film will be wet or polluted by surfactants,which makes the application of MD technology in industry severely limited.The superhydrophobic modification of MD membrane is a feasible approach at present.In this paper,the ultrahigh molecular weight polyethylene（UHMWPE）membrane was prepared first,and then the membrane was superhydrophobic modification.In this paper,the ultra-thin nylon（PA）net was used as the support layer,and the dissolved UHMWPE/decalin was hot pressed together with PA net.The UHMWPE composite membrane was prepared by thermoinduced phase separation（TIPS）.In order to reduce the film thickness and increase the distillation flux of the composite film,the influence of hot pressing conditions on the structure and properties of the composite film was studied,and the optimal dosage of UHMWPE/decalin gel（6.0g/100 cm²）and hot pressing pressure（7.5MPa）were determined.Furthermore,the membrane pore was further enlarged by hot cleaning with decalin at 85℃for12h.The prepared UHMWPE composite membrane had a distillation flux of 38.54 kg/（m²h）and stable cold side conductivity in the MD test of 3.5wt%Na Cl solution.However,when the3.5wt%Na Cl solution containing 0.1m M SDS was treated,the composite membrane was wetted in about 10 min.Resulting in a decrease in the distillation flux and a sharp increase in the cold side conductivity.In addition,in the treatment of 20 m M Na₂SO₄ and 20 m M Ca Cl₂saturated solution,the membrane pore of UHMWPE composite membrane was blocked by Ca SO₄ crystal about 2 h,resulting in the decrease of membrane distillation flux and the increase of cold side conductivity.The rGO/CNTs layer was constructed on the surface of UHMWPE composite membrane by spray deposition.The wettability of reduced graphene oxide（rGO）and the hydrophobicity of carbon nanotubes（CNTs）were used to construct micro-nanostructures on the surface of the membrane to achieve the superhydrophobicity of the membrane and enhance the scaling resistance of the membrane.The optimum preparation conditions of modified membrane were studied and established:The ratio of rGO/CNTs in the modified liquid was 1:1,the load of rGO/CNTs in the modified membrane was 3 mg/12cm²,the reduction time of hydrogen iodate was 60min,and the water contact Angle（WCA）of the modified membrane was 153.4°,and the distillation flux was 32.45 kg/（m²h）.Compared with unmodified UHMWPE/rGO/CNTs composite membrane M3,its surfactant wettability and scaling resistance were significantly improved.When treating 3.5wt%Na Cl solution containing 0.2 m M SDS,the distillation time of stable distillation performance was increased from 10min to 11h.In the case of saturated feed solution composed of 20 m M Na₂SO₄ and 20 m M Ca Cl₂,the distillation time of stable distillation performance can be increased from 2 h to 10 h in the case of unmodified membrane.The results showed that the modified membrane of UHMWPE/rGO/CNTs had excellent anti-wetting and scaling properties.By in situ mineralization of tannic acid（TA）and FeCl₃·6H₂O on the surface of UHMWPE membrane,FeOOH nanocrystal rods are formed,resulting in micro-nano classification rough structure and a large number of active groups on the surface of the membrane,which solves the problem that the lack of functional groups of polyethylene molecules leads to the difficult modification of UHMWPE membrane.Then fluorosilane was selected to treat UHMWPE membrane to achieve superhydrophobicity and improve the anti-wetting and anti-scaling properties of the modified membrane.The optimum preparation conditions of modified membrane were determined:UHMWPE-TA-FeOOH modified film was prepared by heating UHMWPE-TA-Fe（III）modified film in 0.4wt%FeCl₃·6H₂O solution for 16 h.Then,the modified films were fluorinated for 12 h with 100μL/12cm² FDTS in a-0.1MPa vacuum oven at 60℃.The optimal membrane has a WCA of 159.7°and a MD flux of 26.73 kg/（m²h）.Compared with the unmodified UHMWPE composite membrane,the prepared UHMWPE-TA-FeOOH fluorosilane modified membrane M0.4 has significantly improved the surfactant wettability and scaling resistance.The distillation time of stable distillation performance can be increased from 10 min to 6 h when the 3.5wt%Na Cl solution containing 0.1m M SDS is treated.In the case of saturated feed solution consisting of 20 m M Na₂SO₄ and 20 m M Ca Cl₂,the distillation time of stable distillation performance can be increased from 2 h to 12 h in the case of unmodified membrane.The results show that UHMWPE-TA-FeOOH fluorosilane modified film has excellent anti-wetting and scaling properties.