Highly durable and sustainable heterogeneous fabric using in-and-out fluorinated urethane coating for elimination of bacteria and oil-water separation

Industrial oily wastewater discharge and oil spill accidents are major sources of water pollution with severe ecological and economic impacts. Membrane filtration is attractive for oil-water separation due to convenience, low cost, and environmental friendliness, and many studies functionalize membranes via superwetting coatings to enhance separation. Real-world oily wastewaters are complex, containing diverse oils, lubricants, metals, organic/inorganic components, and pathogens, and can include solids that damage membranes; thus, robust and multifunctional separation media are needed. Electrospun nanofibrous membranes offer high porosity and tunable pores but suffer from poor mechanical strength and limited long-term durability. Antimicrobial additives can impart bactericidal properties but risk generating resistant bacteria and can degrade membranes upon continuous use. This work proposes a highly durable porous fiber membrane for oil-water separation and bacterial penetration blocking by applying an in-and-out fluorinated polyurethane (F-PU) coating to activated carbon fabric (ACF). F-PU, synthesized from perfluoroalkyl alcohol, EG, and IPDI, reacts with ACF surface hydroxyls to form urethane bonds, permeating inner pores and outer surfaces. The resulting heterogeneous fabric is designed to be highly hydrophobic/oleophilic, mechanically robust, effective in oil-water separation, and able to block bacterial penetration while maintaining performance after scratching.

The paper situates its contribution within prior work on membrane-based oil-water separation and superwetting surface design. It references: (i) membrane filtration as a practical, chemical-free approach for oily wastewater; (ii) superhydrophobic/superoleophobic coatings to tailor membrane wettability for improved separation; (iii) electrospun nanofibrous membranes offering high porosity but limited by poor mechanical properties and durability; and (iv) antimicrobial coatings that, while effective, may cause super-bacteria and membrane defects with continuous use. These gaps motivate a membrane that combines tailored wettability, durability under harsh conditions, and bacteria blocking without relying on consumable antibiotics.

Materials: Activated carbon fabric (ACF, ACC-5092-20; 135 g/m², 0.55 mm thickness, 1800 m²/g surface area) from Kynol. IPDI, EG, DMSO, DCM, toluene, hexane, olive oil, nitromethane (NM), trichloroethylene (TCE), chlorobenzene (CB), 1,2-dichloroethane (DCE), chloroform (CF) from Sigma-Aldrich. 1H,1H,2H,2H-tridecafluoro-1-n-octanol (TDFO) from BLD Pharmatech. Synthesis of F-PU coating material (TDFO-IPDI-EG-IPDI): Two-step addition. Step 1: Prepolymer IPDI-EG-IPDI prepared by reacting 4.32 mL IPDI and 0.58 mL EG in 85.22 mL DMF at 80 °C for 12 h under stirring (molar ratio IPDI:EG:TDFO = 2:1:1 overall). Step 2: 4.66 mL TDFO added to the prepolymer, reacted at 80 °C for 12 h under stirring in an oil bath. FT-IR used to confirm formation (disappearance of TDFO OH, reduced NCO, appearance of urethane C=O and N–H bands). Fabrication of in-and-out coated ACF: ACF contains –OH, ketone, and carboxylic acid groups. Ketones were reduced using sodium borohydride to generate more –OH groups. Treated ACF was immersed in the TDFO-IPDI-EG-IPDI solution; –OH of ACF reacted with terminal NCO of the F-PU at 80 °C for 72 h (oil bath), enabling coating penetration into inner pores and outer surfaces. Characterization: Surface morphology and elemental analysis by XPS (Thermo Fisher K-alpha), SEM (JEOL IT-500), and FE-SEM/EDS (Zeiss SUPRA 55VP). Nitrogen adsorption by BET (Micromeritics ASAP 2020). Static contact angles measured by goniometer (Smart Drop Standard). Mechanical tensile properties measured by UTM (Instron 3366; rate 5 mm/min). Oil/water absorption and separation tests: Absorption measured with DI water, DCM, toluene, hexane, olive oil, CF. 1 cm² samples were immersed until saturation; absorption capacity computed as (Wa − Wo)/Area, where Wa is saturated weight and Wo initial weight. For gravity-driven separation, coated or bare ACF was placed between two glass containers; 1:1 v/v oil/water mixtures (various oils) were poured. Separation efficiency (%) = Va/Vo × 100, where Va is volume permeated and Vo initial oil volume. Flux F = V/(S·t), with V permeate volume, S effective area, t time. Each test repeated three times. Bacteria penetration blocking tests: Gram-positive Staphylococcus aureus (ATCC 25923) and gram-negative Escherichia coli (ATCC 25922) cultured in Tryptic soy broth at 37 °C aerobically; suspensions diluted to OD600 = 0.1. Sterilized samples (bare ACF as control; coated ACF as test) were placed in 24-well transwell inserts (6.5 mm insert, 8.0 µm pore). Each well received 0.8 mL fresh medium; 80 µL bacterial suspension added in inserts. After 24 h incubation, inserts removed; OD600 of bottom media measured to assess bacterial permeation. Scratch durability assessed by sandpaper scratching up to 50 cycles prior to wettability and bacterial tests. Cell compatibility (C2C12, 3×10^4 cells/well, 48 h, CCK-8) referenced to verify coating non-toxicity.

- Successful synthesis of F-PU (TDFO-IPDI-EG-IPDI) confirmed by FT-IR: decreased NCO peak (~2270 cm−1), disappearance of TDFO OH (~3500 cm−1), emergence of urethane C=O (1705 cm−1) and N–H (1535 cm−1) bands, and broad N–H stretching (~3340–3320 cm−1), indicating urethane linkage and hydrogen bonding. - In-and-out coating verified by FESEM/EDS and XPS: fluorine and nitrogen detected inside and outside coated ACF; bare ACF showed only carbon and oxygen. - Wettability: Bare ACF hydrophilic (water contact angle 0°, absorbs droplet). Coated ACF highly hydrophobic with uniform water contact angle 133.3 ± 2.7°. Under oil, coated ACF exhibited superhydrophobic behavior toward water; in water, oil rapidly spread and was absorbed, evidencing oleophilicity and water repellency. - Selective absorption: Coated ACF showed greatly reduced water absorption while maintaining oil absorption similar to bare ACF, indicating high selectivity for oil in oil/water mixtures. - Oil/water separation: Gravity-driven separation efficiencies above 95% for CF/water, NM/water, TCE/water, DCM/water, CB/water, and DCE/water. For CF/water over 20 consecutive cycles, flux exceeded 2 × 10^4 L m−2 h−1 and separation efficiency remained ~95% each cycle, demonstrating reusability and stability. - Mechanical durability: After up to 50 scratch cycles (sandpaper), coated ACF retained high water contact angle with only slight decrease; in-and-out coating within pores preserved wettability after surface damage. Tensile strength increased from 322.41 MPa (bare) to 575.2 MPa (coated), a 1.78× improvement. - Bacteria blocking: Coated ACF blocked permeation of S. aureus and E. coli by ≥99% based on OD600 measurements. Performance maintained after 30 and 50 scratch cycles, evidencing durability. Coating materials showed no toxicity in C2C12 cell viability assay (viability higher after coating than before).

The in-and-out F-PU coating strategy addresses the need for robust, multifunctional membranes for complex oily wastewater. The perfluoroalkyl segments lower surface energy to impart hydrophobicity and underwater water-repellency, while urethane linkages enable strong bonding to ACF and hydrogen-bonded hard segments that enhance mechanical strength. Coating both inner pores and outer surfaces ensures maintained wettability and function even after surface abrasion, which is critical in real environments with solid particulates and mechanical impact. The coated ACF demonstrates efficient, gravity-driven oil selectivity and high flux across various oil/water mixtures with sustained performance over multiple cycles. Crucially, without incorporating consumable antibiotics, the membrane blocks bacterial penetration by physical exclusion and surface properties, mitigating risks of resistance development and preserving separation integrity. Collectively, the results show that tailoring both chemistry and coating topology yields a durable, reusable fabric suitable for oily wastewater treatment where both oil separation and bacterial control are required.

An in-and-out fluorinated polyurethane coating was synthesized (from EG, IPDI, and TDFO) and covalently applied to activated carbon fabric, forming a heterogeneous structure that confers high hydrophobicity/oleophilicity, enhanced mechanical strength, scratch resistance, and excellent oil/water separation with high flux and >95% efficiency across diverse oils. The coated fabric effectively blocked bacterial permeation (≥99% for S. aureus and E. coli) and maintained performance after abrasion, indicating high durability. This strategy demonstrates a scalable route to robust separation fabrics for oily wastewater with simultaneous bacterial blocking without antibiotics. Future work may involve validating long-term antifouling and performance in real industrial effluents, broader microbial spectra, and scale-up and module integration for practical deployment.

The study is conducted at lab scale with model oil/water mixtures and two bacterial species; performance in real, complex industrial effluents (with surfactants, particulates, variable pH/salinity) is not reported. Long-term fouling resistance, chemical stability under harsh conditions, and extended cycling beyond 20 cycles are not detailed. Antimicrobial assessments focus on penetration blocking rather than broader biofouling over prolonged operation.