Hierarchical Superwetting ZOMO-PAA@CuC2O4 Nanorod-Coated Copper Mesh for Robust and Efficient Oily Wastewater Treatment

TL;DR

A new nanorod-coated copper mesh membrane is developed for efficient and durable oily wastewater treatment with high separation efficiency and self-cleaning properties.

Contribution

A hierarchical ZOMO-PAA@CuC2O4 nanorod-coated membrane is introduced for robust oil-water separation with superwettability and recyclability.

Findings

The membrane achieves ultrafast water spreading and underwater oil repellence with contact angles above 150°.

It separates various oil emulsions with high fluxes (1695–2675 L·m−2·h−1) and over 99.1% efficiency.

The membrane maintains performance under acidic, alkaline, and saline conditions across multiple cycles.

Abstract

Efficient oil-water separation remains a major challenge in oily wastewater treatment, highlighting the need for advanced materials that combine superwettability, structural durability, and long-term recyclability. Here, we develop a hierarchical ZOMO-PAA@CuC2O4 NR@CM membrane via sequential chemical oxidation, oxalic acid etching, and spray-coating of ε-Keggin-type Na-ZnM ZOMO nanoparticles within a polyacrylic acid (PAA) matrix. The resulting architecture couples CuC2O4 nanorods with hydrophilic ZOMO-PAA coatings to achieve superhydrophilicity and underwater superoleophobicity. Structural characterization confirmed uniform nanoparticle dispersion, high crystallinity, and robust framework integrity. The membrane exhibits ultrafast water spreading (0°), underwater oil contact angles above 150°, and sliding angles as low as 4°, enabling broad-spectrum oil repellence, antifouling, and self-cleaning. The as-prepared membrane efficiently separates both surfactant-free and surfactant-stabilized emulsions, including aliphatic and aromatic oils stabilized by cationic, anionic, and non-ionic surfactants, with high water fluxes (1695–2675 L·m−2·h−1 and ~900 L·m−2·h−1, respectively) and separation efficiencies above 99.1%. The membrane further demonstrates chemical stability under acidic, alkaline, and saline conditions, alongside consistent oil–water separation behavior across multiple cycles. These findings establish ZOMO-PAA@CuC2O4 NR@CM as a robust and scalable platform for advanced oily wastewater treatment.

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Full text

1. Introduction

The treatment of oily wastewater has become an increasingly urgent challenge due to frequent industrial discharges, accidental oil spills, and urban stormwater runoff [1,2]. Conventional strategies such as flotation [3], coagulation [4], biological treatments [5], and membrane-based processes [6,7,8] have been widely employed, yet their application is often restricted by drawbacks including high operational cost, sludge generation, secondary pollution, and instability under variable environmental conditions [9]. In recent years, advanced materials such as oil–water separation membranes and oil-absorbing sponges have shown great promise for efficient and cost-effective oil–water remediation [10,11,12,13], enabling not only the recovery of valuable oil resources but also significant reductions in environmental contamination. Beyond industrial applications, integrating these separation technologies into urban stormwater management [14] offers a sustainable pathway to intercept oil and grease from runoff, mitigate surface water pollution, and enhance water reuse and ecological resilience [15,16]. Such integrated approaches hold the potential to address both industrial oily wastewater and diffuse urban pollution, aligning with broader goals of environmental protection and sustainable urban development.

Over the past decades, membrane-based separation has emerged as one of the most promising strategies for oily wastewater treatment due to its operational simplicity, energy efficiency, and capacity to deliver high fluxes under mild operating conditions [17,18,19]. Among the diverse membrane technologies, superwetting membranes have attracted particular attention because of their tunable surface wettability, which enables selective permeation of either oil or water [20,21,22]. In general, these membranes can be divided into two categories: (i) superhydrophobic/superoleophilic (SHB/SOL) membranes [10,18,23], which allow oil to pass through while rejecting water, and (ii) superhydrophilic/underwater superoleophobic (SHL/UWSOB) membranes [24,25,26], which preferentially permit water permeation while effectively repelling oil droplets. The SHB/SOL membranes, often referred to as “oil-removing” membranes, have demonstrated high efficiency in separating immiscible oil–water mixtures [27,28]. However, their intrinsic oleophilicity makes them highly susceptible to oil fouling, leading to pore blockage, flux decline, and compromised recyclability during long-term operation [29,30]. In contrast, SHL/UWSOB membranes, inspired by natural systems such as fish scales, clam shells, and crustacean carapaces, rely on the formation of a stable hydration layer that acts as a physical and energetic barrier against oil adhesion [12,31]. This underwater oil-repellent property not only minimizes fouling but also enables efficient separation of oil-in-water (O/W) emulsions, even when stabilized by surfactants, which are particularly challenging to separate [25,32]. Recent studies have highlighted the superior performance of SHL/UWSOB membranes in emulsion separation [33]. For instance, Zhang et al. fabricated TiO_2_-coated meshes with it that achieved >99% oil rejection with surfactant-stabilized emulsions [34]. Similarly, Man et al. reported polymer–nanoparticle hybrid coatings that combined mechanical robustness with stable oil-water separation efficiency above 95.4% [35]. Despite these advances, challenges remain in developing membranes that simultaneously offer structural durability, anti-fouling stability, and scalability for practical applications.

To address these limitations, copper-based substrates have recently gained interest owing to their low cost, ease of surface modification, and mechanical robustness [36]. In particular, the in situ growth of copper oxalate (CuC_2_O_4_) nanorods has proven effective in generating hierarchical micro/nanostructures that endow copper meshes with enhanced surface roughness and hydrophilicity, thereby promoting rapid water spreading and underwater oil repellency [20,21]. However, CuC_2_O_4_ nanostructures alone often suffer from instability under prolonged operation and harsh chemical conditions [37,38]. To improve durability and functionality, zeolitic octahedral metal oxides (ZOMOs), porous crystalline inorganic materials constructed from metal–oxygen octahedral units [39,40,41], have recently emerged as promising surface modifiers owing to their intrinsic superhydrophilicity, large surface area, and abundant active sites [18,19,20]. Unlike previously reported oxide-, tungstate-, or MOF-modified CuC_2_O_4_ systems that mainly rely on surface roughness effects, ZOMOs act as a multifunctional interfacial modifier by enhancing surface hydration, increasing water affinity through polar metal-oxygen sites, and improving coating uniformity when integrated with a binding layer [41,42]. These combined effects facilitate the formation of a stable hydration layer under water, which effectively suppresses oil droplet penetration, particularly in surfactant-stabilized O/W emulsions [36]. With a well-defined geometric structure [42,43], ZOMOs provide robust frameworks featuring multiple active sites, which have been successfully exploited in catalysis, environmental remediation, and oil–water separation [44,45,46]. Hierarchical micro-/nanoscale surfaces, critical for achieving special wettability, are often constructed by assembling particles of varying sizes [47]. When integrated with CuC_2_O_4_ nanorods, ZOMOs not only reinforce hierarchical surface roughness but also promote the formation of a dense hydration layer, thereby stabilizing underwater superoleophobicity against diverse oil types, including surfactant-stabilized emulsions. Furthermore, poly(acrylic acid) (PAA), known for its strong hydrophilicity, excellent blending ability, and superior water solubility [48,49], serves as an effective binder that enhances interfacial adhesion between ZOMOs and CuC_2_O_4_ nanorods, yielding a robust and durable hybrid coating. Based on these considerations, we hypothesize that this synergistic architecture combining CuC_2_O_4_-induced roughness with ZOMO–PAA-derived hydrophilicity offers an effective strategy for fabricating superhydrophilic and underwater superoleophobic copper mesh membranes with high separation efficiency, antifouling stability, and recyclability for oil-in-water emulsion separation. To the best of our knowledge, systematic investigations of such integrated ZOMO–PAA@CuC_2_O_4_ nanorod-based membranes remain limited, underscoring the novelty of this approach.

Accordingly, the purpose of this study is to develop a novel hierarchical ZOMO-PAA@CuC_2_O_4_ nanorod-coated copper mesh (defined as ZOMO-PAA@CuC_2_O_4_ NRs@CM) membrane for efficient oil-in-water emulsion separation. As schematically illustrated in Figure 1a, Cu(OH)2 nanowire arrays (NWAs) were first grown on the copper mesh via chemical oxidation, serving as self-sacrificing templates that provided the necessary surface roughness and enabled the subsequent in situ growth of CuC_2_O_4_ nanorods (NRs) through immersion in oxalic acid solution. Thereafter, ε-Keggin-type Na–ZnM zeolitic octahedral metal oxide (ZOMO) nanoparticles, dispersed in PAA, were deposited onto the nanorod arrays using a simple spray-coating process. The synergistic architecture of CuC_2_O_4_ nanorods and ZOMO-PAA coating imparts the copper mesh membrane with superhydrophilicity and underwater superoleophobicity, exhibiting anti-oil adhesion, antifouling and self-cleaning properties. Importantly, comprehensive evaluations confirm that the membrane achieves both high separation efficiency and elevated water permeation flux during oil-in-water emulsion separation, while also maintaining long-term operational stability and consistent performance over multiple cycles. This study not only establishes an effective fabrication route for robust superwetting copper-based membranes, but also highlights the broader significance of integrating hierarchical nanostructures with ZOMO chemistry for scalable, durable, and energy-efficient solutions for industrial oily wastewater treatment and advanced emulsion separation applications.

2. Results and Discussion

2.1. Characterization of the ZOMO-PAA@CuC2O4 NR@CM Membrane

Hierarchical ZOMO-PAA@CuC_2_O_4_ NR@CM membranes were prepared via sequential chemical oxidation, chemical etching, and spray-coating deposition (Figure 1a). In the first step, Cu(OH)2 nanowire arrays (NWAs) were grown in situ on copper mesh through chemical oxidation [50], providing both a structural scaffold and a source of copper ions. Subsequent immersion in oxalic acid converted the Cu(OH)2 NWAs into CuC_2_O_4_ nanorods (NRs) [51], thereby increasing surface roughness and enhancing structural stability. Finally, hydrophilic ε-Keggin-type Na-ZnM ZOMO nanoparticles embedded within a PAA matrix were uniformly deposited onto the CuC_2_O_4_ NRs by spray-coating, yielding a hierarchical membrane architecture that combines superhydrophilicity with underwater superoleophobicity.

Figure 1b–e depict the sequential morphological evolution of copper mesh into the hierarchical ZOMO-PAA@CuC_2_O_4_ NR@CM membrane. The pristine copper mesh (Figure 1(b_1_,b_2_)) is smooth and hydrophilic, with minor fabrication marks and a golden-brown appearance (inset, Figure 1(b_1_)). After alkaline chemical oxidation, the surface turns blue (inset, Figure 1(c_1_)) due to the uniform growth of Cu(OH)2 NWAs, which introduce primary micro/nano-scale roughness (Figure 1(c_2_)). The Cu(OH)2 nanowires have lengths of approximately 5–9 μm and average diameters of 120 ± 30 nm. Subsequent immersion in oxalic acid converts the nanowires into thicker CuC_2_O_4_ nanorods with an average diameter of 240 ± 45 nm (Figure 1(d_2_)), accompanied by a greenish-blue color shift (inset, Figure 1(d_1_)). The morphology of the synthesized ε-Keggin-type Na-ZnM ZOMO nanoparticles were examined by SEM (Figure S1). SEM imaging (Figure S1a) shows densely packed, uniformly dispersed sub-micron particles (50–80 nm) without large voids or agglomerates, indicating structural homogeneity. Finally, spray-coating ε-Keggin-type Na-ZnM ZOMO nanoparticles embedded in a PAA matrix produces a conformal coating that preserves the nanorod architecture (Figure 1(e_1_,e_2_)). The EDS analysis of the as-prepared membrane shows the presence of the elements Cu, Mo, Na, C, Zn, O. As seen on Figure 2, these elements are evenly distributed across the as-prepared membrane, indicating successful growth of CuC_2_O_4_ and coating with ε-Keggin Na-ZnM ZOMOs. The resulting ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane displays a greenish-blue–brown surface (inset, Figure 1(e_1_)) and a robust hierarchical architecture, endowing it with exceptional superhydrophilicity.

The crystalline structure and composition of the Na-ZnM ZOMO nanoparticles were further examined by XRD and FTIR (Figure S1b,c)). The XRD pattern (Figure S1b) exhibits sharp reflections between 5° and 60° (2θ), characteristic of ε-Keggin-type frameworks and confirming high crystallinity [52]. FTIR analysis (Figure S1c) further validates this framework: a broad band near 3400 cm^−1^ corresponds to O–H stretching from adsorbed water or hydroxyl groups; peaks at 1600–1400 cm^−1^ are attributed to H–O–H bending; and strong absorptions at 1200–500 cm^−1^ arise from metal–oxygen and bridging M–O–M vibrations [53], diagnostic of the ε-Keggin polyoxometalate structure [54]. These results confirm the successful synthesis of ε-Keggin Na-ZnM ZOMO with high crystallinity, and preserved framework integrity.

The effect of varying ZOMO loadings (1, 2, 3, and 4 wt%) on the surface morphology and crystalline structure of ZOMO-PAA@CuC_2_O_4_ NR@CM membranes was evaluated using SEM and XRD (Figures S2 and S3). At 1 wt% ZOMO (Figure S2a), the membrane exhibits well-defined, uniformly distributed nanorod structures with clear separation between individual rods. Increasing the loading to 2 wt% (Figure S2b) results in a denser, more interconnected network, with nanorods partially aggregating into entangled structures and reduced voids. Upon increasing ZOMO loading to 3 wt% (Figure S2c), further aggregation occurs, forming a coarser and highly interconnected structure where individual rods became less distinct. At a higher loading content of 4 wt% (Figure S2d), the membrane displays a compact, irregular morphology with fused nanorods, suggesting excessive nanoparticle loading and structural merging. XRD patterns (Figure S3) confirm that all membranes retain similar crystalline phases, with increased diffraction peak intensities corresponding to higher ZOMO loadings, indicating enhanced crystallinity. Despite the morphological changes observed in the SEM images, the consistent peak positions suggest structural stability across different ZOMO loading levels. As a result, 2 wt% ZOMO loading is selected as the optimal composition for further characterization and O/W emulsion separation.

The crystalline evolution of copper mesh during sequential modification was characterized by XRD (Figure 3a). Pristine copper exhibits strong reflections at 2θ of 42.8° and 49.9°, corresponding to the (111) and (200) planes (JSPDC PDF #70-3039) [55]. After chemical oxidation, the Cu(OH)2 NWA@CM membrane displays characteristic peaks at 2θ of 17.2° (020), 24.4° (021), 34.6° (111), 38.6° (022), 43.4° (131), and 53.9° (202), consistent with the Cu(OH)2 phase (JSPDC PDF #80-0656) [56]. Conversion into CuC_2_O_4_ nanorods introduces new reflections at 2θ of 22.9° (110), 36.4° (120), 38.9° (011), 46.7° (220), and 51.7° (121), consistent with the CuC_2_O_4_ facet planes (JCPDS PDF #21-0297) [57]. For the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane, an additional peak at 2θ = 8.22° is observed, in line with the prominent peak found in Na-ZnM nanoparticles (Figure S1b), confirming the successful deposition of Na-ZnM ZOMO nanoparticles. The functional groups of the pristine and modified copper mesh membranes were further analyzed by FTIR (Figure 3b). Compared to the pristine copper mesh, the Cu(OH)2 NWA@CM membrane shows characteristic peaks between 3656–3206 cm^−1^, attributed to O-H stretching vibrations, a minor peak at 936 cm^−1^, and a strong peak at 678 cm^−1^, corresponding to C-O stretching and Cu-O-H bending vibrations [58]. Upon conversion to CuC_2_O_4_ nanorods, strong peaks emerge at 1599 cm^−1^ (C=O asymmetric stretching), 1356 cm^−1^ (C-O asymmetric stretching), 1313 cm^−1^ (C-O symmetric stretching), and 817 cm^−1^ (O-C=O bending) [59], respectively. Similar FTIR peaks are observed for the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane, further confirming the successful deposition of ZOMO-PAA onto the CuC_2_O_4_ NR surface.

2.2. Surface Wettability of ZOMO-PAA@CuC2O4 NR@CM Membrane

Surface wettability plays a critical role in governing the oil–water separation performance of membranes [60]. To evaluate this property, 5 µL droplets of water and oil were placed on the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membranes, in air and underwater, respectively. In air, a water droplet spreads instantly across the membrane surface, yielding a water contact angle (WCA) of 0° (Figure 4a(i)), indicative of superhydrophilicity. In contrast, a Sudan III-dyed oil droplet remains intact when placed underwater, with an underwater oil contact angle (UWOCA) of 157.6° (Figure 4a(ii,iii)), confirming underwater superoleophobicity. The as-prepared membrane exhibits similarly high UWOCA values (>150°) against a range of oils, including 1,2-dichloroethane, cyclohexane, isooctane, kerosene, petroleum ether, and sunflower oil, demonstrating its versatility (Figure 4b). This performance is attributed to the formation of a stable hydration layer, promoted by abundant O=C and –OH functional groups from CuC_2_O_4_, Na-ZnM ZOMO, and PAA, together with the hierarchical surface structures. Notably, water droplets spread across the surface in less than 1 s (Figure 4c), highlighting the strong water affinity of the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane. It has been extensively recognized that materials exhibiting both superhydrophilicity and underwater superoleophobicity are typically characterized by UWOCAs above 150° and sliding angles (SA) below 10° [61,62]. Consistent with this criterion, the ZOMO-PAA-CuC_2_O_4_ NR@CM membrane displays a SA of 4° (Figure 4d), underscoring its excellent underwater superoleophobic behavior.

The antifouling, anti-adhesion, and self-cleaning performance of the superwetting ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane were systematically evaluated (Figure 5a–d). In the anti-oil-adhesion test (Figure 5a), oil droplets are forcefully applied to and withdrawn from the membrane surface without any sign of deformation or residue, confirming its underwater low oil-adhesive behavior. This property is attributed to the superhydrophilic/underwater superoleophobic nature of the as-prepared membrane, where a stable hydration layer within the hierarchical structure prevents direct oil–surface contact [63]. The underwater oil repellence and self-cleaning ability of the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane were further verified using Sudan II-stained heavy oil (1,2-dichloroethane) and light oil (n-hexane) (Figure 5b,c). In both cases, droplets failed to adhere to the surface and rolled off easily. Heavy oil droplets sprayed onto the membrane bounced off without leaving stains (Figure 5b), while light oil droplets detached and floated to the water surface (Figure 5c). The ability to repel oils with contrasting densities confirms the universality of the membrane’s superwetting properties, an important advantage for oily wastewater treatment involving mixed oil contaminants. The anti-pollution capability of the membrane was further demonstrated with crude oil as a representative complex foulant (Figure 5d). After exposure, the pre-wetted membrane resists fouling, and adhered oil droplets detach rapidly upon gentle rinsing. This self-cleaning effect arises from low oil adhesion and the protective hydration layer acting as a barrier against irreversible fouling [64]. Taken together, these results highlight that the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane combines broad-spectrum oil repellence with durable antifouling and self-cleaning performance, making it a strong candidate for efficient and long-term oil–water separation.

2.3. Oil-Water Separation Performance of the ZOMO-PAA@CuC2O4 NR@CM Membrane

The separation efficiency of the ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane was evaluated using both surfactant-free (SFE) and surfactant-stabilized (SSE) oil-in-water (O/W) emulsions of cyclohexane, toluene, isooctane, and n-hexane. The representative digital photographs, microscopy images, and droplet size distribution curves (Figure S4 and Figure 6) demonstrate that the milky emulsions transform into optically clear filtrates after passing through the membrane, with oil droplets effectively retained. Microscopical images confirm that the feed emulsions contain abundant micron-sized droplets, while the filtrates exhibit no visible oil droplets. These observations are corroborated by DLS analysis. For SFEs, the droplet size distributions of cyclohexane-, toluene-, isooctane-, and n-hexane-in-water emulsions range from 1720–3090 nm, 1990–3580 nm, 2305–4145 nm, and 2305–4145 nm, respectively, before separation. After filtration, the droplet sizes decrease dramatically to 190–255 nm, 105–165 nm, 140–340 nm, and 190–295 nm, respectively (Figure S4). A similar trend is observed for SSEs, where the initial droplet sizes are 995–1720 nm (cyclohexane), 710–1280 nm (toluene), 1105–1990 nm (isooctane), and 825–1485 nm (n-hexane). After filtration, these sizes are reduced to 45–195 nm, 140–225 nm, 80–245 nm, and 95–240 nm, respectively (Figure 6). The disappearance of micron-scale peaks in the droplet size distributions indicates significant reduction in droplet size and partial rejection of emulsified oil droplets [24,65]. To further examine surfactant effects, cyclohexane-in-water emulsions stabilized by CTAB (cationic), SDS (anionic), and Tween 80 (non-ionic) were tested (Figure 7). All emulsions initially appear milky with abundant oil droplets, but the filtrates become transparent after separation, displaying markedly reduced droplet sizes. Specifically, the initial droplet size distributions for CTAB-, SDS-, and Tween80-stabilized emulsions are 1485–2660 nm, 825–1485 nm, and 825–1485 nm, respectively. After separation, these values decrease to 255–340 nm, 10–340 nm, and 90–190 nm, respectively. DLS provides qualitative insight into droplet size evolution but cannot be regarded as direct evidence of complete oil removal, particularly in the presence of surfactants. Together, these findings demonstrate that the ZOMO-PAA@CuC_2_O_4_ NR@CM membrane provides excellent separation efficiency across diverse O/W systems, regardless of oil type or surfactant presence. Its superior performance can be attributed to the synergistic effects of the hydrophilic–underwater-oleophobic surface and hierarchical nanostructure, which effectively suppress oil adhesion and fouling while enabling rapid water permeation. The ability to consistently separate both aliphatic (cyclohexane, isooctane, n-hexane) and aromatic (toluene) emulsions, as well as emulsions stabilized by cationic, anionic, and non-ionic surfactants, highlights the robustness and broad applicability of this membrane design.

The water flux and COD values of the filtrates were measured to quantitatively evaluate oil-water separation performance of the as-prepared ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane, and the corresponding separation efficiencies were calculated (Figure 8). For SFE O/W emulsions, the membrane exhibits high water fluxes (1695–2675 L·m^−2^·h^−1^) and low COD values (17–39 mg·L^−1^), with separation efficiencies consistently above 99.7% (Figure 8(a_1_,a_2_)). Specifically, the measured water fluxes and COD values for cyclohexane-, toluene-, isooctane-, and n-hexane-in-water emulsions are 2675 (17), 1695 (39), 1980 (25), and 2140 L·m^−2^·h^−1^ (26 mg·L^−1^), respectively, corresponding to separation efficiencies of 99.98%, 99.95%, 99.96%, and 99.97%. In contrast, for SSE O/W emulsions, the water fluxes are relatively lower (885–1000 L·m^−2^·h^−1^) with higher COD values (45–56 mg·L^−1^) compared to SFE O/W emulsions, reflecting the enhanced stability of surfactant-stabilized oil droplets. Nevertheless, the separation efficiencies remained consistently high, ranging from 99.19% to 99.47% across different oils (Figure 8(b_1_,b_2_)), demonstrating the excellent demulsification capability of the membrane. To further investigate surfactant effects, cyclohexane-in-water emulsions stabilized with CTAB, SDS, and Tween 80 were tested, yielding water fluxes of 975, 855, and 900 L·m^−2^·h^−1^, COD values of 48, 54, and 43 mg·L^−1^, and corresponding separation efficiencies of 99.38%, 99.31%, and 99.45% (Figure 8(c_1_,c_2_)). It is worth noting that although COD was employed as a commonly accepted bulk parameter to evaluate separation performance at the laboratory scale, it does not allow for the unambiguous differentiation of oil droplets, surfactants, and dissolved organic compounds. Complementary oil quantification approaches, such as total organic carbon (TOC) analysis or extraction-based methods, represent important directions for future work. Overall, these results confirm that the ZOMO-PAA@CuC_2_O_4_ NR@CM membrane achieves high water flux, low COD, and nearly complete oil rejection in both surfactant-free and surfactant-stabilized emulsions, underscoring its strong potential for practical oily wastewater purification.

2.4. Postulated Mechanism of Emulsion Separation

To elucidate the O/W emulsion separation process and underlying mechanism of the ZOMO-PAA@CuC_2_O_4_ NR@CM membrane, the liquid wetting and permeation models are illustrated in Figure 9. A photograph of the gravity-driven oil-water separation using custom-made separation device and the separation process are provided in Figure S5. Evidently, water droplets selectively permeate through the membrane, while oil is effectively repelled, leading to efficient demulsification of the O/W emulsions. The theoretical liquid intrusion pressure (∆P_c_) explains the wetting and permeation behaviors of the membrane and is described by the Young–Laplace equation [12]:

$$[eqn]$$

where ∆P_c_ is the intrusion pressure (Pa), γ_L_ is the oil–water interfacial tension (N/m), θ represents the water contact angle on the membrane surface (^o^), and $[eqn]$ is the radius of the curvature of the liquid surface (m).

When water droplets contact the superhydrophilic membrane surface (θ_α_ < 90°), it penetrates spontaneously because the hydrostatic pressure (∆P_c_ < 0) induces a positive capillary effect (Figure 9a). Consequently, the surface cannot support additional pressure, allowing water to spread rapidly and be retained within the membrane’s micro/nano hierarchical structures. This process forms a stable hydration layer that imparts strong oil repellence and underwater superoleophobicity to the membrane. In contrast, when oil contacts the prewetted membrane, it is repelled due to the negative capillary effect (θ_α_ > 90° and ∆P_c_ > 0), indicating that the membrane can resist a certain pressure before the hydration layer deteriorates (Figure 9b). The experimental intrusion pressure can be calculated as the following Equation [8]:

$$[eqn]$$

where ρ is the oil density, g is gravitational acceleration, and h_max_ is the maximum supported oil column height. Thus, a downward intrusion pressure (∆P_c_ < 0) facilitates water permeation, while an upward intrusion pressure (∆P_c_ > 0) prevents oil penetration. During this process, repelled oil droplets coalesce on the membrane surface into larger droplets, which eventually detach and rise to form a floating oil layer. This mechanism ensures efficient oil/water separation, with water collected beneath the membrane and oil retained above.

2.5. Environmental Stability and Recyclability

Environmental stability is a prerequisite for practical separation membranes. The ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane demonstrates remarkable robustness under harsh chemical environments and repeated operation (Figure 10). After immersion in solutions of varying pH for 24 h, the as-prepared membrane retains strong underwater superoleophobicity, with oil contact angles exceeding 150° across most conditions and only a slight decrease to 142.3° at pH 1 (Figure 10a). Similarly, exposure to saline solutions for 24 h causes negligible changes in wettability up to 5 wt% NaCl, with a moderate reduction to 131.0° at 10 wt% NaCl (Figure 10b). Considering that natural seawater contains about 3.5 wt% NaCl, these results highlight the membrane’s strong stability under corrosive conditions. It is worth noting that, given the known water solubility and swelling behavior of PAA, the potential for PAA swelling and gradual dissolution is recognized as a limitation of the ZOMO–PAA@CuC_2_O_4_ NR coatings during prolonged operation.

The as-prepared ZOMO-PAA@CuC_2_O_4_ NR@CM (2 wt%) membrane exhibits functional reusability over successive separation cycles, with separation efficiency remaining high while flux decreases due to progressive fouling. For SFEs (Figure 10c), it delivers a high initial flux (~2200 L·m^−2^·h^−1^) with only gradual decline over multiple cycles due to slight fouling or pore blockage, while the COD values remain consistently below 50 mg·L^−1^, confirming long-term separation efficiency with negligible oil leakage. In SSEs (Figure 10d), the initial flux is relatively lower (~1000 L·m^−2^·h^−1^) and declines more steeply across cycles due to stronger fouling effects. Nevertheless, the COD values remain stable in a range of 100–120 mg·L^−1^, demonstrating effective separation capacity under more challenging conditions. Accordingly, these results establish the ZOMO-PAA@CuC_2_O_4_ NR@CM membrane as an environmentally stable, recyclable, and structurally resilient platform, with superior performance in surfactant-free emulsion systems and stable functionality in surfactant-stabilized emulsions

3. Materials and Methods

3.1. Materials

The standard 400-mesh copper mesh (>99.95% purity, pore size 33.5 μm) was purchased from McKays Wire Mesh Co. Ltd., (Tianjin, China). Zinc chloride (ZnCl_2_, ≥95.0%), molybdenum powder (Mo, ≥99.0%), sodium molybdate dihydrate (Na_2_MoO_4_∙2H_2_O, 85%), oxalic acid dihydrate (H_2_C_2_O_4_∙2H_2_O, ≥99.8%), ammonium persulfate ((NH_4_)2_S_2_O_8, ≥98.0%), sodium hydroxide (NaOH, ≥96.0%), hydrochloric acid (HCl, 37%), and poly(acrylic acid) (PAA, Mw ≈ 4,000,000) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., (Shanghai, China). Organic solvents including ethanol (≥99.7%), isopropyl alcohol (≥99.7%), acetone (≥99.5%), and N,N-dimethylformamide (DMF, ≥99.7%) were supplied by Kelong Chemical Co., Ltd., (Chengdu, China). Oils such as cyclohexane, n-hexane, toluene, isooctane, and 1,2-dichloroethane were purchased from Taitan Chem. Co., Ltd., (Shanghai, China). Surfactants, including cationic hexadecyltrimethylammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS), and nonionic Tween 80, were obtained from Merck Chemicals (Shanghai) Co., Ltd., (Shanghai, China). All reagents were used as received without further purification. The deionized water used in all experiments was supplied by a local reverse osmosis (RO) system.

3.2. Fabrication of ZOMO-PAA@CuC2O4 NR@CM Membranes

The pristine copper meshes (3 cm × 3 cm) were first pretreated with organic solvents and hydrochloric acid to remove surface impurities and oxide layers, following a previously reported protocol [24]. The hierarchical CuC_2_O_4_ nanorods (NRs) were then constructed on the pretreated meshes via a two-step chemical transformation. Initially, Cu(OH)2 nanorod arrays were generated by immersing the pretreated copper meshes in an oxidation solution consisting of 10 g NaOH and 2.97 g (NH_4_)2_S_2_O_8 dissolved in 100 mL of deionized water at room temperature (25 °C) for 30 min. The resulting Cu(OH)2-coated copper meshes were rinsed thoroughly with deionized water and dried under a nitrogen stream. Subsequently, the Cu(OH)2 nanowires were converted into CuC_2_O_4_ nanorods by immersion in a 0.05 M oxalic acid (H_2_C_2_O_4_) solution for 2 h. After the etching reaction, the obtained CuC_2_O_4_ NR@CM membranes were washed with deionized water and dried in a vacuum oven at 60 °C overnight.

To impart superhydrophilicity and underwater superoleophobicity, the CuC_2_O_4_ NR@CM membranes were further modified with ε-Keggin-type Na–ZnM ZOMO nanoparticles dispersed in a poly(acrylic acid) (PAA) matrix via spray coating. The detailed synthesis procedure for ZOMO nanoparticles using a hydrothermal method is provided in the Electronic Supplementary Information (ESI, Section S1.1). For the coating process, 0.5 g of poly(acrylic acid) (PAA) was dissolved in 10 mL of deionized water and ultrasonicated (200 W, 40 kHz) for 10 min to obtain a homogeneous solution. Subsequently, ZOMO nanoparticles were introduced at different loadings (1, 2, 3, and 4 wt%) and uniformly dispersed by ultrasonication for 30 min, forming a stable suspension. This suspension was spray-coated onto the CuC_2_O_4_ NR@CM surface under a spraying pressure of 0.4 MPa at a fixed distance of 15 cm. The as-prepared coated membranes, denoted as ZOMO-PAA@CuC_2_O_4_ NR@CM (x%) (where x = 1, 2, 3, or 4 wt%), were dried in a vacuum oven at 60 °C for 12 h prior to subsequent characterization and performance evaluation.

3.3. Characterization

The surface morphology of the as-prepared copper mesh was examined by scanning electron microscopy (SEM; Regulus 8230, Hitachi, Japan), and the corresponding elemental distribution was analyzed using energy-dispersive X-ray spectroscopy (EDS). Surface functional groups were characterized by attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR; PerkinElmer, Waltham, MA, USA). The crystalline phases of ZOMOs and membrane coatings were identified using X-ray diffraction (XRD; DX-2007X, Dandong Haoyuan Instrument Co., Ltd., Dandong City, China). Wettability was evaluated by static contact angle measurements using a goniometer (JC2000C1, Zhongchen Digital Technology Equipment Co., Ltd., Shanghai, China). The stability and droplet size distributions of oil-in-water (O/W) emulsions were determined via dynamic light scattering (DLS; Zetasizer Nano ZS90, Malvern Instruments, Worcestershire, UK).

3.4. Oil-in-Water (O/W) Emulsion Separation Performance Testing

A custom-designed gravity-driven separation device (Figure S6) was employed to evaluate the oil-water emulsion separation performance of the as-prepared copper mesh membrane. Detailed preparation procedures for the surfactant-free oil-in-water emulsion (SFE O/W) and surfactant-stabilized oil-in-water emulsions (SSE O/W) are provided in ESI, Section S1.2. The device consisted of two vertically aligned glass tubes of equal inner diameter, providing an effective filtration area of 3.14 cm^2^. The as-prepared copper mesh were pre-wetted with deionized water and clamped securely between the two tubes fixed on a retort stand. During the separation process, 100 mL of O/W emulsion was poured into the upper tube while maintaining a liquid column height of 15 ± 0.5 cm, corresponding to a constant gravity-driven pressure of ~0.0015 bar. The filtrate was collected for 2 min, and its volume was recorded for further calculations. The permeation flux (F) and separation efficiency (SE) were determined according to the following equations:

$$[eqn]$$

where F (L·m^−2^·h^−1^) is the permeation flux, V (L) is the filtrate volume, S (m^2^) is the effective filtration area, and t (h) is the filtration time.

$$[eqn]$$

where SE (%) is the separation efficiency, C_f_ is the oil concentration in the filtrate, and C_o_ is the oil concentration in the feed, both of which were used to measure COD. The COD of the emulsion feed and filtrate was analyzed using a 5B-3C (V8) COD analyzer (Beijing Lianhua Yongxing Technology Development Co., Beijing, China). The detailed measurement procedure is provided in the ESI, Section S1.3. Additionally, the emulsion stability and the separation effect before and after filtration were further confirmed by optical microscopy and droplet size distribution analysis.

3.5. Environmental Stability and Recyclability Testing

The environmental stability and durability of the as-prepared membrane was evaluated through a series of salt corrosion and acid/alkaline resistance tests. For the salt corrosion test, membranes were immersed in NaCl solutions of varying concentrations (1–10 wt%) for 24 h, followed by UWOCA measurements. Acid and alkaline resistance were assessed by immersing the membranes in solutions with pH values ranging from 1 to 13 for 24 h, after which, UWOCA measurements were recorded. During the recycling separation experiments, the water permeation flux was recorded at 1 min intervals, while the final COD value was determined at the end of each 5 min separation cycle. After each cycle, the as-prepared copper mesh membrane was thoroughly rinsed with deionized water and ethanol to remove residual contaminants before reuse.

4. Conclusions

In summary, this study reports the successful fabrication of a hierarchical ZOMO-PAA@CuC_2_O_4_ NR@CM membrane with outstanding potential for oily wastewater remediation. The membrane achieved superhydrophilicity and underwater superoleophobicity through the synergistic interplay of CuC_2_O_4_ nanorods and ZOMO-PAA coatings, imparting excellent oil resistance, antifouling capability, and self-cleaning behavior. Systematic separation experiments confirmed its effectiveness in treating diverse oil-in-water emulsions, including both surfactant-free and surfactant-stabilized systems. The membrane consistently exhibited high water flux, low COD levels, and separation efficiencies exceeding 99%, even under demanding conditions. Beyond separation performance, it demonstrated notable durability, chemical stability across acidic, alkaline, and saline environments, and recyclability over repeated cycles, underscoring its robustness and reliability. Overall, this work not only presents a simple and scalable fabrication strategy for superwetting copper mesh-based composite membranes but also establishes a sustainable, efficient, and durable approach to advanced oily wastewater treatment.