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Associate Professor Cao Ning & Professor Guo Xiaorui & Professor Tang Zhonghua from Northeast Forestry University: Novel Superhydrophilic Multifunctional Oil-Water Separation Membranes with High Flux and Durability
Oil pollution generated by industries such as petrochemicals, textiles, and food, as well as frequent oil spills in offshore oil extraction, severely harms the ecological environment and human health. However, finding suitable green methods to treat oily wastewater, address critical issues like membrane fouling, and remove dyes in wastewater remains challenging. Guided by the philosophy of "utilizing nature, transforming nature, and returning to nature," developing a biodegradable, green, and pollution-free multifunctional membrane holds significant practical importance.
Recently, the team of Associate Professor Cao Ning, Professor Guo Xiaorui, and Professor Tang Zhonghua from Northeast Forestry University, along with Teacher Zhu Zhihao from Anqing Normal University, published their latest research in the journal Separation and Purification Technology, titled "Novel superhydrophilic nanofiber membranes with high flux and durability enable multifunctional oil-water separation". The researchers prepared a high-flux, green, multifunctional ellagic acid (EA)-iron (Fe)-polylactic acid (PLA)/polyvinylpyrrolidone (PVP) nanofiber membrane via electrospinning and layer-by-layer self-assembly.
The novel metal-phenolic networks (MPNs) constructed by this method endow the PLA nanofiber membrane with superhydrophilicity and underwater oleophobicity, achieving separation fluxes of 50,360 L·m⁻²·h⁻¹ for n-hexane/water oil-water mixtures and 2,965 L·m⁻²·h⁻¹ for n-hexane oil-in-water emulsions—far exceeding levels reported in other literatures. Meanwhile, the strong adhesion between the MPNs selective layer and the PLA support layer enhances the stability of the EA-Fe-PLA/PVP nanofiber membrane, maintaining stable separation performance even after 100 cycles. Additionally, as MPNs act as a photocatalyst, the membrane exhibits persistent and efficient photocatalytic activity, holding great potential for degrading dye-contaminated wastewater.
Figure 1: Schematic of design methodology and multifunctionality of EA-Fe-PLA/PVP nanofiber membrane.
Figure 2: Characterization and chemical structure of EA-Fe-PLA/PVP nanofiber membrane.
EDS images detected uniform distribution of C, O, and Fe elements on the surface of the EA-Fe-PLA/PVP nanofiber membrane (Figure 2a). The chemical structure of the EA-Fe-PLA/PVP nanofiber membrane was evaluated by FT-IR spectroscopy, confirming its successful preparation (Figure 2b). SEM images and corresponding AFM images showed that the originally smooth nanofiber membrane surface was successfully coated with rough micro/nano structures, clearly illustrating the membrane structure transformation process (Figures 2c-d). The formation mechanism of the MPNs selective layer was clarified via DFT theoretical calculations (Figure 2e).
Figure 3: Surface wettability of nanofiber membrane.
Water contact angle (WCA) has become a key parameter for evaluating the surface wettability of oil-water separation membranes. As shown in Figure 3a, both EA-Fe-PLA/PVP0-5 and EA-Fe-PLA/PVP0.15-4 exhibited superhydrophilicity, with water droplets penetrating the nanofiber membrane within seconds (Figures 3a-c). Underwater oil contact angle results indicated excellent oleophobicity for these membranes, with UOCA approaching 140° (Figure 3d). Generally, material wettability depends on surface morphology and chemical composition. As shown in Figure 3e, a relevant model was constructed to explain the wettability mechanism of the EA-Fe-PLA/PVP nanofiber membrane.
Figure 4: Oil-water separation performance of EA-Fe-PLA/PVP0.15-4.
Figure 4a visually demonstrates the separation process of oil/water mixtures. To optimize separation performance, parameters including PVP porogen content, Fe³⁺ concentration, and dipping cycles of EA/Fe³⁺ were tuned (Figures 4b-d). Results showed EA-Fe-PLA/PVP0.15-4 exhibited outstanding separation performance, achieving a n-hexane/water mixture flux of 50,360 L·m⁻²·h⁻¹ and 99.5% separation efficiency under gravity. Additionally, to test the general applicability of EA-Fe-PLA/PVP0.15-4, its separation performance for other oils (petroleum ether, dimethyl silicone oil, soybean oil, vacuum pump oil) was evaluated (Figure 4e). Laplace theory was introduced to accurately explain the oil/water separation mechanism (Figure 4f).
Figure 5: Oil-water separation performance of EA-Fe-PLA/PVP0-5.
Oil-water mixtures typically include not only suspended oil but also emulsified oil. Figure 5a shows the separation of sodium dodecyl sulfonate (SDS)-stabilized oil-in-water emulsions driven by gravity alone. As shown in Figures 5b-c, through separation performance optimization, EA-Fe-PLA/PVP0-5 demonstrated the best results, achieving separation fluxes of 4,120, 2,966, 2,209, and 1,811 L·m⁻²·h⁻¹ with 98% efficiency for petroleum ether, n-hexane, dimethyl silicone oil, and xylene oil-in-water emulsions—far surpassing literature reports (Figure 5d). Moreover, EA-Fe-PLA/PVP0-5 effectively separated oil-in-water emulsions stabilized by different surfactants (Figures 5e-g). An oil-in-water emulsion separation schematic was drawn to further explain the mechanism (Figure 5h).
Figure 6: Stability, self-cleaning performance, and large-scale preparation of composite membrane.
Stability is a critical indicator for membrane separation. Figures 6a and d show the cycle stability of EA-Fe-PLA/PVP0-5 and EA-Fe-PLA/PVP0.15-4, maintaining original separation fluxes (50,000 L·m⁻²·h⁻¹ and 2,900 L·m⁻²·h⁻¹) with separation efficiency >99%. The membrane’s stability under extreme conditions (acid, alkali, high-salt solutions, and UV irradiation) was evaluated (Figures 6b and e), showing minimal or no loss in flux and efficiency >98%, indicating excellent environmental durability. Figures 6g and h demonstrate the membrane’s outstanding self-cleaning performance, a prerequisite for long-term stability. Large-scale EA-Fe-PLA/PVP0-5 membranes were easily fabricated via simple layer-by-layer self-assembly (Figure 6i), highlighting great potential for industrial production.
Figure 7: Biodegradation and photocatalytic degradation tests.
As shown in Figures 7a and b, EA-Fe-PLA/PVP0-5 achieved nearly 100% biodegradation within 80 minutes in the presence of proteinase K. To validate its photocatalytic degradation capability, representative dyes (rhodamine B, congo red, methylene blue) were tested. Results showed EA-Fe-PLA/PVP0-5 completely removed dye pollutants within 5 hours under xenon lamp irradiation, with congo red degraded most efficiently (100% removal in 75 minutes, Figures 7c-h). A schematic diagram (Figure 7i) was proposed to explain the possible photocatalytic degradation mechanism.
