The indiscriminate discharge of oily industrial wastewater and frequent offshore oil spill accidents contribute to water pollution, posing a serious threat to the ecological balance crucial for human survival. Addressing the efficient and rapid separation of oil-water mixtures has become a global priority. Inspired by the "lotus leaf effect," researchers have leveraged biomimicry to develop superhydrophobic materials with unique structures, offering a straightforward and effective approach to wastewater separation (Yan et al., 2020; Xue et al., 2023; Zheng et al., 2022). Typically, superhydrophobic materials exhibit varying wettability towards different liquids due to the significant surface tension disparity between oil and water at the material interface, enabling selective repulsion or wetting of water and oil to achieve efficient separation of oil-water mixtures (Sam et al., 2019; Zhang et al., 2019; Xu et al., 2020).
Cotton fabric, as a natural material, boasts advantages such as affordability, accessibility, environmental friendliness, ease of modification, good mechanical properties, and natural degradation. In light of environmental concerns, the development of fluorine-free superhydrophobic fabrics using CFs as the substrate has garnered considerable attention (Xu et al., 2022; Meng et al., 2020). Typically, finishing agents are applied to textile surfaces through dipping or spraying, incorporating inorganic nanoparticles and specialized coatings onto cotton fibers to establish micro-nanostructured roughness and low surface energy, thus achieving superhydrophobicity (Han et al., 2020; Ahmad et al., 2020; Xiong et al., 2019; Wang et al., 2013; Wang et al., 2023; Li et al., 2023; Cheng et al., 2018; Xu et al., 2023; Li et al., 2022; Song et al., 2017; Xu et al., 2023; Mohamed et al., 2021; Li et al., 2021; Ji et al., 2019). However, nanoparticle detachment compromises their efficacy and lifespan, while shed nanoparticles also pose environmental concerns, limiting their application scope.
To ensure the enduring presence of nanoscale roughness on superhydrophobic CFs, employing physical or chemical techniques to micro-etch the cotton fibers' surfaces and subsequently introducing low-surface-energy substances can confer upon CFs both durability and remarkable superhydrophobicity (Xu et al., 2023; Xu et al., 2022; Li et al., 2022; Xu et al., 2020; Yang et al., 2022; Shanmugavelayutham et al., 2021). Cheng et al. (2019) treated cotton fibers with a 7.5% mass fraction of sulfuric acid, followed by the introduction of a crosslinker, epoxy soybean oil resin, combined with stearic acid to create superhydrophobic cotton fibers devoid of nanoparticles. These fibers exhibited outstanding mechanical stability and chemical properties, maintaining excellent superhydrophobicity even after undergoing tape stripping, ultrasonic treatment, solvent corrosion, and exposure to low/high temperatures. Additionally, using cellulase to etch cotton fiber surfaces yielded similarly impressive results in mechanical properties and resistance to various treatments (Cheng et al., 2019). However, such treatments may lead to a decrease in CFs' mechanical properties. Yao et al. (2021) utilized oxygen plasma to etch cellulose fiber surfaces, enhancing their roughness and activating hydroxyl groups to undergo condensation reactions with trisilanol butyl polyhedral oligosiloxane (TS-POSS), thereby grafting it onto the fiber surfaces. The resulting superhydrophobic cellulose/TS-POSS material exhibited a static water contact angle of 152.9°. Nguyen-Trig et al. (2019) achieved superhydrophobic CFs via dip-coating with NaOH and plasma etching treatments, followed by deposition of silica nanoparticles and tetraethyl orthosilicate (TEOS), yielding a water contact angle of 172°. Nonetheless, the use of plasma equipment proves costly and impractical for large-scale application.
Cai et al. successfully dissolved low-polymerization cellulose in an aqueous NaOH/urea solvent system at low temperatures (-10℃), enhancing the stability of the cellulose solution (Cai et al., 2005; Lue et al., 2009). Alkali/urea chemical etching treatment resulted in the formation of concave-convex structures and grooves on the cellulose material surface, altering the fibers' mechanical properties (Zhang et al., 2016; Lu et al., 2018). In this study, we present a green, nanoparticle-free, fluorine-free, and sustainable approach to fabricating superhydrophobic CFs. CFs were etched using a NaOH/urea solution under low-temperature conditions to create micro-nano rough surfaces. Subsequently, the modified CFs were impregnated with a solution of hexadecyltrimethoxysilane (HDTMS) and butane tetracarboxylic acid (BTCA) to reduce the fibers' surface energy and enhance superhydrophobic durability. These superhydrophobic CFs exhibited exceptional mechanical robustness, durability, and self-cleaning properties. Most importantly, these sustainable superhydrophobic CFs demonstrated outstanding efficiency and reusability in oil/water separation applications.