Abstract:

Objective Efficient treatment of oily wastewater is of great significance for environmental and economic needs. The conventional separation technology can effectively separate oil from oil-water emulsion, but its separation efficiency is relatively low. Membrane separation technology can achieve the separation of oil-water emulsion by adjusting the pore size and changing the wettability of membrane materials. Nanofiber membrane prepared by electrospinning technology has been widely used to separate oil-water emulsion due to its high porosity, high connectivity, large specific surface area, adjustable wettability and other comprehensive characteristics. This research aims to enhance the oil-water emulsion separation performance of nanofiber membrane and to explore the influence mechanism of membrane materials on the hydrophilicity of fiber membrane.

Method Amidoxime (PAO) was synthesized from polyacrylonitrile (PAN), and then the PAO nanofiber membrane was prepared by electrospinning method. The surface morphology, fiber diameter, average pore diameter, surface roughness, surface wettability, mechanical properties, functional group composition of fiber membrane and oil-water emulsion separation performance were tested and analyzed.

Results The mass fraction of PAO directly was found to affect the surface morphology of the PAO membrane. The results showed that when the mass fraction of PAO in the spinning solution was 10%, the average fiber diameter was 0.21 μm and the diameter distribution of PAO membrane was relatively uniform with an average pore size of 0.88 μm (Fig. 2, Fig. 3). It was demonstrated that the surface roughness played an important role in influencing the wettability of the membrane surface. The surface roughness of the nanofiber membrane prepared after amidoximation was significantly improved, attributing to the increase in the diameter of the electrospun fiber (Fig. 4). The fabricated PAO nanofiber membranes were found to have abundant —NH₂ and —OH hydrophilic groups, the 10%PAO nanofiber membrane exhibited excellent surface wettability with an initial water contact angle of 15.6° and an underwater oil contact angle of 157° (Fig. 5, Fig. 6). The stress and strain of the original PAN nanofiber membrane was 2.25 MPa and 25.15%, respectively. After amidoximation, the mechanical properties of the 10%PAO membrane did not decrease, remaining at 2.47 MPa and 26.64%, respectively, proving that direct amidoximation of the spinning solution can maintain various properties after amidoximation treatment and keep the flexibility and strength (Fig. 7). The permeation fluxes of the 10%PAO nanofiber membrane for silicone oil water, n-hexane water, petroleum ether water and edible oil water emulsion were 1 362.9, 1 658.9, 1 614.2 and 1 425.9 L/(m²·h), respectively, and the corresponding oil rejection rates were 99.1%, 98.5%, 99.3% and 98.6%, respectively (Fig. 12). This shows that PAO nanofiber membrane has good oil-water emulsion separation performance. It can separate various oil water emulsion only under gravity conditions, and shows excellent separation efficiency and precision, showing a good prospect for oil-water separation.

Conclusion When the content of the PAO spinning solution was 10%, the PAO nanofiber membrane showed good surface wettability and oil-water emulsion separation performance. The initial water contact angle is 15.6° and the underwater oil contact angle is 157°, and its permeation flux of silicone oil emulsion is 1 362.9 L/(m²·h), and the rejection rate is 99.1%. The surface hydrophilicity and oil-water emulsion separation performance of the fiber membrane are significantly improved compared with the PAN fiber membrane without amidoxime treatment which shows good prospects for the separation of oily wastewater.

Key words: polyacrylonitrile, nanofibrous membrane, amidoxime, electrospinning, oil-water separation