Study Of Wastewater Oil/Water Separation Using Underwater Superoleophobic Mesh Membrane With TiO₂ Coating

The discharge of oil-containing wastewater will cause serious oil pollution in aqueous environment,thus efficient oil/water separation technologies have to be aodpted to remove oils from wastewater to mitigate the negative effects of oil pollution.In comparison to traditional oil/water separation methods,oil/water separation with superhydrophilic/underwater superoleophobic materials has advantages of high efficiency,low energy cost,simple operation and little risk of secondary pollution.However,there still exist several prob lems to be solved for these materials currently,such as lacking researches on real wastewater separation,high risk of oil breaking through and being hard to recover oils directly.In these regards,the present work fabricated three kinds of underwater su peroleophobic mesh membranes with TiO₂ coating by serving stainless steel mesh as supporting structure,and investigated the performances of oil/water separation of these membranes for both ideal oil/water mixture and real oil-containing wastewater,the effects of coating structure on pressure-bearing capability and water transportation features,as well as the membrane washing when blocked by particles and the membrane photocatalytic regeneration when polluted by organic compounds.The TiO₂-coated mesh membrane is superhydrophilic in the air with water contact angle of 0°,and superoleophobic under water with oil contact angles of above 150°.The membrane can separate several ideal oil/water mixtures（like hexadecane/water,1,2-dichloroethane/water,n-hexane/water,methylbenzene/water and engine oil/water mixtures）with efficiencies of more than 99.3% and water flux of 7.24 × 10³ L/m²/h.When the membrane is employed to separate coal chemical wastewater,the initial water flux reaches 1.23 × 10³ L/m²/h and separation efficiency is around 99%.After separation,the oil content left in wastewater is 47.6 mg/L,which is lower than the maximun tolerable content of oil（30-50 mg/L）for common biochemical treatment processes.In the initial stage of coal chemical wastewater separation,the membrane can reject particles in wastewater with hydrodynamic diameter of more than 1 μm.With the proceding of separation,particles with hydrodynamic diameter of more than 0.3 μm are rejected and accumulate in the pores and on the surface of the membrane,thus causing serious pore blocking,making flux decrease to 86.4 L/m²/h and enhancing the risk of membrane fouling by oils.The particles retained on the surface and in the pores of the membrane can be effectively cleaned through back washing under pressure of 0.02 MPa,making the membrane recover its initial water flux.Specially,the membrane can photocatalytically degrade its surface-residual organic pollutants from wastewater under ultraviolet illumination to achieve membrane regeneration.The TiO₂-deposited mesh membrane with high pressure-bearing capability was developed through introducing TiO₂ deposition layer onto membrane surface.The membrane is also superhydrophilic in the air and superoleophobic under water.Under low applied pressure,the membrane keeps its original surface morphology and achieves fast oil/water separation with water flux of 4.90 × 10³ L/m²/h and separation efficiency of above 99.4%.When high applied pressure（≥ 9.8 k Pa）or pressure surge occurs,the TiO₂ deposition layer will be compacted to reduce pore diameter,thus improving the pressure-bearing capability of the membrane.For the tested oils like hexadecane,1,2-dichloroethane,cyclohexane and methylbenzene,the membrane can bearing pressure of more than 12.6 k Pa,greatly reducing the risk of oil breaking through during oil/water separation process.After the TiO₂ deposition layer is compacted,the membrane can keep fast oil/water separation driven by high applied pressure,pressure surge or oil pressure resulting from oil accumulation.Moreover,once compactness occurs,the underwater superoleophobicity,high oil/water separation efficiency and performance of photocatalytically regeneration are kept undamaged.To achieve simultaneous oil/water separation,the TiO₂ asymmetrically coated mesh membrane was developed via sol-gel and dip coating methods.The membrane has oil and water contact angles of 0° in the air,oil contact angles of around 150° under water and water contact angles of around 150° under oil,showing superamphilic in the air as well as strongly oleophobic under water and hydrophobic under oil.When prewetted by water,the membrane rejects n-hexane and allows water to pass through its pores with flux of 1.35 ×10⁴ L/m²/h.If prewetted by n-hexane,the mesh rejects water and allowing n-hexane to penetrate with flux of 7.28 ×10³ L/m²/h.Both the two prewetting-driven oil/water separation modes can realize separtaion efficiency of more than 99.9%.Specially,simultaneous oil/water separation can be achieved to realize oil removing from water and oil recovery if two membranes are installed in an elaborately-designed double-channel device and prewetted by oil and water respectively.In this process,the membrane can achieve separation efficiencies of more than 99.8% for mixtures of hexadecane/water,n-hexane/water,1,2-dichloroethane/water and methylbenzene/water.The water contents in the recovered oils are lower than 1151.2 mg/L,which keeps on a low level.Furthermore,in the double-channel device,the membrane can separate coal chemical wastewater with efficiency of 98.9% and the water content in the recovered oil is 327.4 mg/L.The present work: verifies the feasibility and validity of underwater superoleophobic mesh membrane for real wasterwater oil/water separation by taking coal chemical wastewater as an example,demonstrates the siginificant roles of TiO₂ with photocatalytic activity in membrane regeneration after organic polluting and stainless steel mesh with good mechanical strength in membrane washing after particle blocking;mitigating the tradeoff of high pressure-bearing capability and low water flux efficiently by introducing compactable structures into membrane surface;achieving simultaneous oil/water separation with one kind of membranes in slef-constructed double-channel device driven by prewetting.This work promotes the application of underwater superoleophobic membranes in the field of oil-containing wastewater treatment.