Fabrication And Oil/water Separation Performance Of Porous PVDF-based Monoliths

The continuous and rapid development of the global industry has led to an increase in the demand for petrochemical products year by year.In recent years,the occurrence of marine oil spill,organic chemical leakage and direct discharge of industrial oily sewage has shown a high trend,which has caused serious damage to the ecological environment and water resources on which human beings rely.How to restore polluted water bodies in an environmentally friendly and efficient manner has become a worldwide problem that has attracted much attention.At present,the common methods of oily sewage treatment mainly include air flotation,gravity sedimentation,adsorption,chemical flocculation,in-situ burning,microbial degradation,etc.,but most of the above methods are expensive,inefficient,cumbersome to operate,and even cause secondary pollution.Physical adsorption materials with special wettability have become one of the research hotspots in the field of oil-water separation due to their low cost,high separation efficiency,environmental friendliness,simple operation,and oil recyclability.Among them,polymer-based porous monoliths exhibit excellent oil-water separation ability as adsorbents by virtue of their controllable three-dimensional interconnected porous structure,high porosity,large specific surface area,and hydrophobicity and oleophilicity.However,the poor environmental suitability and the inability to efficiently adsorb high-viscosity crude oil under harsh conditions greatly restrict the application of porous monoliths in oily sewage treatment.In view of this situation,this paper aims to develop polymer-based porous monolith with excellent environmental suitability and overcome its performance limitations in practical oil-water separation applications by endowing the monolith with photo-thermal/electro-thermal functions.Firstly,polyvinylidene fluoride（PVDF）was used as the main raw material,and a superhydrophobic porous PVDF monolith was successfully prepared by thermally impacted non-solvent induced phase separation（TINIPS）method.The microstructure,oil-water separation performance and environmental suitability of the monoliths were characterized in detail by scanning electron microscope,automatic mercury injection apparatus,specific surface area and pore size analyzer,contact angle tester and other experimental instruments.The research results show that:（1）The morphology of porous PVDF monoliths presents a novel formicary-like three-dimensional interconnected pore structure.When the PVDF solution concentration is 13%（w/v）,the uniformity and connectivity of the porous structure of the monolith is the best,and the porosity is up to 90.7%;（2）By virtue of its unique three-dimensional interconnected pore structure and low surface energy,porous PVDF monolith can possess superhydrophobic/superoleophilic properties（water contact angle=154°,oil contact angle=0°）without any modification,and therefore can selectively and efficiently adsorb various low viscosity oil/organic solvents from water,with high saturated adsorption capacity(9.0-21.3 g g^-1)and fast adsorption rate（reaching saturation adsorption state within 15 s）;（3）Under harsh conditions such as intense ultraviolet radiation（72 h）,rapidly alternating temperature（-20-65 ^oC）,corrosive solutions,oil-water emulsion,suspended solids and severe liquid disturbance,porous PVDF monolith was able to maintain stable oil/water separation capability,showing outstanding environmental suitability;（4）Combined with the pump-assisted adsorption device,the porous PVDF monolith can continuously and selectively separate the floating oil and organic solvent on the water surface,with a separation efficiency of up to 99.5%and an oil flux of 3520 L m^-2 h^-1.Then,in order to endow the porous monolith with the ability to rapidly adsorb high-viscosity crude oil,pure PVDF monolith was used as the porous matrix,and the two-dimensional MXene（Ti₃C₂T_x）nanosheets with preeminent photo-thermal/electro-thermal conversion performance were used as the surface functional layer material,the porous PVDF/MXene monolith was prepared by simple dip-coating method.The microstructure,photo-thermal/electro-thermal conversion property and crude oil/water separation preformance of the composite were investigated in depth.The research results are as follows:（1）Compared with pure porous PVDF monolith,the microscopic morphology of porous PVDF/MXene monolith has changed significantly due to the attachment of MXene nanosheets to the porous framework,showing tremella-like three-dimensional interconnected pore structure as a whole,the porosity decreased slightly（83.0%）.Based on its high hydrophobic/superoleophilic properties（water contact angle=146°,oil contact angle=0°）,the PVDF/MXene monolith exhibits prominent oil-water separation selectivity;（2）Porous PVDF/MXene monolith has outstanding full solar spectral absorption capacity（wavelength of 200-2500 nm）,with an average absorption rate of 94.7%.Under the simulated solar irradiation condition（1.0 k W/m²）,the surface of the monolith heats up rapidly and reaches 76.0℃within 1 min,indicating excellent photo-thermal conversion performance;（3）Under a voltage of 4 V,the surface of porous PVDF/MXene monolith can be heated to 117.3°C within 42 s,and the highest temperature can reach 130.3°C（5 min）,demonstrating remarkable electro-thermal conversion performance;（4）Under the combined action of weak solar irradiation（0.5k W/m²）and low voltage（4 V）,porous PVDF/MXene monolith can continuously and rapidly separate high-viscosity crude oil assisted by a pump adsorption device,and the crude oil flux can reach 506 kg m^-2 h^-1,which is 36 times higher than that at room temperature.The results show that porous PVDF/MXene monolith with both photo-thermal and electro-thermal conversion functions can not only be applied to the treatment of oil-polluted water,but also have the ability of all-weather operation.