Durability and Regeneration of Functional Membrane Coatings for Wastewater Treatment

Abstract

Functional membrane coatings enhance wastewater treatment by regulating interfacial transport, suppressing fouling, and introducing selective or reactive sites. However, their practical effectiveness depends on maintaining these functions during prolonged operation and repeated regeneration. This review examines how coating chemistry, morphology, and attachment to porous supports govern treatment performance, deterioration, and recovery. The reviewed studies were published during the period 2015–2026. Hydration-mediated fouling resistance, selective transport, adsorption, and catalytic conversion are discussed in relation to their distinct failure mechanisms and regeneration requirements. Particular attention is given to the distinction between hydraulic recovery and restoration of pollutant-removal function: recovered water flux does not necessarily indicate renewed adsorption capacity or catalytic activity, while high rejection can persist despite substantial throughput loss. Physical cleaning, chemical desorption, assisted cleaning, and layer renewal are critically compared in terms of functional recovery, coating retention, cumulative damage, and resource consumption. The analysis identifies paired hydraulic and pollutant-specific measurements, together with useful treated volume, as a basis for evaluating regeneration effectiveness. Finally, priorities are outlined for validating coating durability in complex wastewaters, quantifying material release and secondary wastes, and establishing reliable lifecycle and cost comparisons.

1. Introduction

Functional membrane coatings provide a versatile means of improving wastewater treatment by tailoring the chemistry and structure of the water-facing interface. Hydrated groups can reduce foulant attachment, selective layers regulate solute passage, and immobilized adsorbents or catalysts introduce functions that porous supports alone cannot provide. These benefits depend on preserving accessible flow pathways and active sites while maintaining sufficient coating–support adhesion. During operation, however, foulant deposition, adsorption-site exhaustion, catalyst deactivation, and coating loss can progressively reduce treatment performance. Cleaning and regeneration introduce additional hydraulic, chemical, and thermal stresses. The central challenge is therefore to sustain water production and pollutant removal through repeated treatment and recovery, rather than to maximize initial performance alone.

Recent studies illustrate both the benefits and the limitations of interfacial functionalization. A tannic-acid/Fe(III) coating on nylon increased ciprofloxacin hydrochloride removal from 24.3% to 96.7% at a feed concentration of 10 mg L⁻¹ while achieving a permeance of 3815 L m⁻² h⁻1 bar⁻¹ [1]. Co-hydrolyzed silane/polyphenol coatings and electrostatically cross-linked nanocomposite layers instead improve surface hydration and resistance to foulant attachment [2,3]. Nevertheless, effective separation does not necessarily imply complete recovery. A polyaniline-coated alumina membrane maintained greater than 99% crude-oil-emulsion rejection but recovered approximately 70% of its initial flux after cleaning [4]. Similarly, a caffeic-acid/ε-polylysine coating retained 99.89% of oil and 96.99% of 100-nm polystyrene particles during a 300-min mixed-feed challenge, yet its water-rinse flux recovery ratio was 72.63% [5]. These results demonstrate that high pollutant retention can coexist with persistent hydraulic resistance. They also show why incomplete flux recovery must be distinguished from intrinsic coating degradation, which requires additional structural or chemical evidence.

Practical wastewater treatment further requires coatings to withstand variable feed composition and repeated maintenance. A cyclodextrin/polydimethylsiloxane interface was evaluated over 72 h, with hydraulic cleaning every 4 h and chemical cleaning after 48 h [6]. Pilot ultrafiltration with a powder-activated-carbon precoating and catalytic ceramic membranes examined with hospital wastewater provide additional application-relevant examples [7,8]. Although these architectures differ in how they retain or renew their functional phases, both highlight the importance of evaluating useful treatment output alongside maintenance requirements. Broader wastewater-to-energy studies likewise identify component stability, fouling, auxiliary inputs, and economic constraints as barriers to practical implementation [9,10]. For functional coatings, these considerations translate into specific questions: whether regeneration remains effective in complex feeds, whether active material is retained, and whether the recovered treatment interval justifies the associated energy, chemical consumption, and secondary-waste generation.

Existing reviews have established the importance of surface chemistry in antifouling membrane construction [11] and compared techniques for immobilizing catalytic phases [12]. Building on these contributions, there is a critical need to connect coating design with the diagnosis of performance loss and the selection of an appropriate recovery strategy. Antifouling describes resistance to foulant accumulation, whereas self-cleaning concerns the removal or transformation of deposited material. Regeneration additionally requires verification that the relevant treatment function is restored during subsequent operation. These outcomes are related but not interchangeable: recovered water flux does not establish renewed adsorption capacity or catalytic activity, and restored pollutant removal after recoating may conceal an accumulating transport penalty. A useful comparison must therefore consider hydraulic performance, pollutant-specific removal, and coating retention over the same operating history.

Figure 1 summarizes this relationship between fabrication, interfacial function, operational failure, and regeneration. Coating chemistry, morphology, and attachment determine both the accessible treatment pathways and their susceptibility to deterioration. Failure diagnosis then guides the choice between physical cleaning, chemical desorption, assisted cleaning, and layer renewal. Verification during the next treatment run closes this sequence by establishing which functions have recovered and whether the coating or support has sustained additional damage.

Figure 1. Relationships among coating fabrication, interfacial function, operational failure, and regeneration. Coating structure and attachment govern transport and active-site accessibility, while the identified failure mechanism guides recovery selection. Hydraulic performance, pollutant-specific removal, and coating retention are evaluated together to verify regeneration.

This review focuses on functional coatings on porous filtration supports, linking their structural characteristics to treatment mechanisms, deterioration, and recovery. Membrane distillation interfaces, pore-confined catalysts, and self-healing membranes are included as complementary examples, with their distinct transport and recovery mechanisms explicitly distinguished. This narrative review considers studies published during the period 2015–2026, selected by coating architecture, treatment mechanism, and interpretable performance or reuse measurements, with targeted topic searches and citation cross-checking. Comparisons prioritize within-study evidence and account for differences in feed composition, driving force, and cycle definitions. Particular attention is given to whether regeneration restores both water production and pollutant-removal function, providing a basis for assessing coating durability, application suitability, and the requirements for practical implementation.

2. Fabrication and Structural Characteristics of Functional Membrane Coatings

2.1. Coating Formation and Morphological Control

Coating formation sets the spatial distribution of functional groups and the fraction of the support accessible to flow. A caffeic-acid/ε-polylysine deposition series provides a quantitative example: apparent surface porosity decreased from 40.8 ± 2.4% to 3.3 ± 0.9% after 100 min, while gravity-driven clean-water flux, measured at approximately 10 cm water head, declined from 1262.6 ± 26.4 to 590.4 ± 22.2 L m⁻² h⁻¹ as deposition time increased from 20 to 100 min [5]. The selected 80-min coating retained 682.3 ± 27.7 L m⁻² h⁻¹. Figure 2a illustrates a related use of marine-derived chitosan, obtained from shrimp waste and combined with trimesoyl chloride to form a water-facing layer on a ceramic support [13]. The significance of this route is the conversion of a biomass-derived polymer into an exposed hydrophilic interface; its precursor origin alone does not determine separation durability. In the caffeic-acid/ε-polylysine system, Figure 2b shows E. coli inactivation exceeding 93.4%, whereas Figure 2c shows S. aureus inactivation exceeding 97.0%, with high antibacterial activity retained after 30 days of water immersion [5]. Figure 2d connects these results to the proposed oxidative-stress and membrane-disruption actions of caffeic acid and electrostatic contact killing by ε-polylysine. The cross-linked coating also presents hydrated groups to water. Antibacterial activity, surface hydration and pore narrowing are distinct contributions, so persistent bacterial inactivation does not alone establish resistance to repeated fouling and cleaning.

Figure 2. Formation and spatial uniformity of hydrophilic coatings. (a) Shrimp-derived chitosan coating on a ceramic membrane [13]. (b,c) Antibacterial activity after different immersion times and (d) proposed antibacterial and hydration mechanisms of a caffeic-acid/ε-polylysine coating. (e) Proposed retention of oil–microplastic co-contaminants, (f) a 33.0 cm × 20.0 cm membrane and (g) location-dependent flux and rejection measurements [5]. Immersion duration, fabrication scale and regeneration cycles denote different experimental challenges.

The separation and scale-up panels extend this chemistry from bacterial contact to mixed-pollutant transport [5]. Figure 2e depicts size exclusion, hydration-mediated resistance and electrostatic interactions in an oil–microplastic feed, alongside possible adsorption competition and dispersant redistribution. The proposed contributions are consistent with simultaneous retention, although the separation measurements do not isolate their individual effects: the 300-min experiment retained 99.89% of oil and 96.99% of 100-nm polystyrene. Figure 2f shows a 33.0 cm × 20.0 cm membrane and nine sampling positions, while Figure 2g demonstrates comparable local transport and rejection, with a reported overall flux of 447.2 ± 25.8 L m⁻² h⁻¹, 93.95% polystyrene rejection and 99.80% oil rejection. These panels demonstrate retained antibacterial activity during immersion, mixed-feed separation and comparable performance across the tested membrane area. The decrease in surface porosity and clean-water flux with increasing deposition time shows that greater coating coverage can restrict water transport. Coating duration should be selected to provide sufficient coverage for the intended interfacial function while preserving accessible flow pathways. Flux and rejection measurements at multiple positions further assess whether the selected fabrication conditions produce spatially uniform separation performance.

Apparent surface porosity denotes the projected open-area fraction measured by image segmentation; it is distinct from bulk void fraction, effective pore-throat size and connected water-accessible porosity. SEM, gravimetry, capillary-flow porometry and solute-cutoff tests therefore cannot be compared without their measurement definitions and sample states. In the Darcy regime, $J={\displaystyle \frac{\Delta P}{\mu R_{\mathrm{total}}}}$, where μ is viscosity and R_total includes support, coating and foulant resistance. Tortuosity, wetting and pore connectivity determine whether visible openings contribute to flow; otherwise, porosity comparisons should remain within a study.

The hydraulic consequence of a coating also depends on support architecture. An ultrathin conformal hydrogel filter reported a gravity-driven flux of up to 14.8 m h⁻¹ and greater than 99.5% oil/water separation [14], whereas a surface-segregated PVDF composite reported a permeance of 571.52 L m⁻² h⁻¹ bar⁻¹ and a 14.18% flux decline in its emulsion test [15]. The gravity throughput and pressure-normalized permeance describe different driving forces and cannot be ranked as equivalent transport coefficients. Chitosan-coated ceramics, CuO-coated meshes and hydrogel-functionalized PVDF similarly distribute deposited material over different pore entrances and flow networks [13,16,17]. A cellulose-acetate-coated PVDF-HFP distillation fiber operates through vapor transport and liquid-entry control [18], while a lysozyme–polymer brush on cotton changes the chemistry of an open textile network [19]. The transferable fabrication criterion is consequently the preservation of the relevant transport pathway, assessed using liquid permeance, gravity throughput or vapor flux as appropriate to the support.

Polymer deposition and coordination-mediated assembly offer different controls over the location and chemistry of the functional layer. Aminomalononitrile immobilizes pseudo-zwitterionic compounds [20], cellulose-hydrogel deposition introduces a hydrated polymer network [21], and PDA anchors Ce-MOF to PES [22]. Sulfonated PDA supplies hydrophilic ionic groups through a related adhesive platform [23]. These routes modify wetting and foulant interactions, but connected pores and accessible active sites remain necessary for useful treatment. Interlayer/mineral and hydrogel/framework architectures separate anchoring from the exposed functional phase [24,25,26], allowing adhesion and loading to be adjusted through different components. This separation is useful when the required anchoring chemistry would otherwise obstruct or deactivate the outer surface. Polymeric–inorganic hybrids [27], oxide-coated meshes [28], sulfonate-coated meshes [29] and MIL-101(Cr)-coated meshes [30] illustrate how the support aperture and deposited phase jointly determine transport. Rapid tannic-acid/APTES deposition [31] and metal–phenolic assembly on PES and PVDF [32,33] offer coordination-rich alternatives. Their common fabrication requirement is controlled coverage with accessible flow pathways; a lower contact angle alone does not establish that this balance has been achieved.

Spatially controlled deposition can reduce unnecessary loading within the flow network. Successive inkjet delivery of Fe³⁺ and oxidant directs tannic-acid assembly, while MOF-nanosheet growth controls the morphology of an exposed active phase [34,35]. Corona-plasma grafting of zwitterionic monomers on PES modifies surface chemistry without relying on the same mineral-growth process [36]. Sulfonic nanohydrogels, PDA-mediated amphiphilic grafting, polyvinyl-acetate-coated nylon/SiO₂ fibers and mist-based lignin modification provide related routes with different support geometries [37,38,39,40]. These methods are most informative when deposition location is linked to a matched pressure–flux response and pollutant-removal measurement. For a renewable interface, reconstruction introduces an additional structural constraint. A supramolecular coating on a thin-film composite membrane illustrates intentional layer regeneration [41]; reconstruction of the antibiotic-binding tannic-acid/Fe(III) film supplies a complementary example of rebuilding an adsorptive interface [1]. High post-renewal removal does not by itself establish restoration of pore accessibility, making baseline permeance an essential companion to functional recovery. Table 1 compares deposition routes by processing complexity, chemical demand, scalability and uniformity. These trade-offs are route-dependent: rapid deposition is useful only when coverage, adhesion and function remain reproducible after washing and scale-up.

Table 1. Processing trade-offs for coating fabrication. Qualitative entries are route-dependent considerations; they do not substitute for matched measurements of production time, chemical consumption, coating yield or environmental impact.

Route and Examples	Processing and Chemical Demand	Scale-Up and Uniformity	Durability and Environmental Trade-Off	Ref
Dip/coordination assembly	Simple bath equipment; dwell, rinsing and bath renewal control time and reagent use.	Accessible for sheets and fibers; bath aging, uptake gradients and drainage affect coverage.	Anchoring and pore filling must be balanced; spent metal/ligand baths and wash water need accounting.	[1,5,24]
Layer-by-layer assembly	Repeated adsorption/rinse steps give thickness control but multiply processing time and solution handling.	Conformal treatment is possible; delivery inside long fibers and step-to-step consistency require validation.	Charge and pH sensitivity can enable renewal or cause loss; stripping creates contaminant-rich waste.	[42,43,44]
Spray/inkjet/mist deposition	Metered delivery can reduce bulk-bath use; nozzle compatibility and drying add controls.	Area-selective deposition; overlap, edge effects and nozzle fouling can create nonuniform loading.	Reduced solution use is conditional on transfer efficiency; aerosols, solvents and drying energy remain relevant.	[34,40]
Grafting/plasma activation	Activation, monomer handling and washing add equipment or reagent steps.	Surface modification can limit pore filling; dose and treatment uniformity must be controlled.	Covalent attachment can improve retention, but chain damage and residual monomers require assessment.	[20,36]
Mineral growth/sputtering/ALD	Growth time, precursor use, vacuum or thermal requirements depend on the route.	Precise coatings are possible; shadowing and precursor access complicate porous/large-area supports.	Robust inorganic films can add resistance; precursor waste, energy and metal release cannot be inferred from wettability.	[24,45,46]

2.2. Coating–Support Interactions and Structural Integrity

An anchoring interlayer must retain the active phase without isolating it from water and reactants. PDA with amino-functionalized SiO₂ on PES, mineral–hydrogel hybrids on PVDF and NH₂-MIL-88B on quartz fibers illustrate distinct interlayer geometries [47,48,49]. Figure 3a compares conventional multistep mineralization with one-pot metal–half-Salen assembly: the latter forms an initial coordination layer and then a mineralized β-FeOOH layer under a compatible reaction environment [24]. The reported reductions of approximately 50% in processing time, 75% in reagent use and 90% in waste liquid arise from this integrated preparation. Figure 3b explains the chemical requirement through competition between ligand coordination and metal-ion hydrolysis. Sufficiently strong coordination maintains the supporting network while additional metal participates in mineral growth; weak coordination can instead allow separation of the mineral phase from the substrate. Figure 3g locates uranium capture, dye transformation, antibacterial activity and oil repellency within the resulting architecture, and Figure 3h tests uranium removal under dark, light, H₂O₂ and combined light/H₂O₂ conditions. The combined treatment reached 98.3% removal after 720 min, supporting reaction-assisted uranium immobilization beyond passive uptake. The source identifies an insoluble uranyl-peroxide product, so removal should not be described as adsorption alone. Separate dye and oil experiments reported 99.8% degradation and greater than 99.3% rejection, respectively, while six uranium catalysis–desorption cycles retained removal above 83.2% and desorption above 88.2%.

Figure 3. Coordination-controlled mineralization and hierarchical functional interfaces. (a) Multistep versus one-pot metal–half-Salen mineralization and (b) competition between coordination and metal hydrolysis. (g) Proposed functions and (h) uranium-removal kinetics of β-FeOOH under different reaction conditions [24]. (c) Lotus-leaf antifouling geometry, (d) its underwater analog on Co-HHTP-mediated MnO₂, (e) cross-sectional Co/Mn distributions and (f) antifouling/permeance comparison [25]. The panels connect interlayer chemistry and morphology to the corresponding treatment responses.

Figure 3c–f provides a separate coordination/mineralization example and shows how an interlayer organizes the exposed antifouling surface [25]. Figure 3c depicts the air-pocket barriers of a lotus leaf, while Figure 3d translates the hierarchical geometry into water-filled pockets on a superhydrophilic Co-HHTP-mediated MnO₂ interface [25]. The functional analogy concerns limited pollutant–solid contact under the relevant surrounding liquid, rather than identical wettability in air. Figure 3e maps Co and Mn across the membrane cross-section, connecting the coordination interlayer to the mineralized outer phase. Figure 3f then compares hydraulic permeance and the source-defined antifouling figure of merit: the engineered membrane showed a 24.8-fold antifouling enhancement and an approximately 186% increase in the emulsion-separation permeance shown in Figure 3f relative to its plotted comparator. This comparison is distinct from the source’s clean-water permeance measurement. The same study reported 99.9% oil rejection and near-complete permeance recovery in the specified crude-oil protocol. A separate triple-defensive coating combines attachment, hydration and exposed low-surface-energy domains, with water permeance above 4200 L m⁻² h⁻¹ bar⁻¹ and FRR above 95% [50]. These examples converge on a structural principle: the interlayer must control mineral placement and attachment, while the water-facing topography preserves reactant access and reduces foulant contact. Increasing mineral loading without preserving this arrangement does not by itself ensure better transport or recovery.

Structural integrity is expressed differently on an open mesh and within a finer porous membrane. MnCo₂O₄-, chitosan/cellulose-nanocrystal- and TiO₂-coated meshes can retain high gravity throughput despite a locally imperfect surface, whereas pore occlusion on a filtration skin can markedly reduce permeance [51,52,53]. Hydrated manganese–phosphate interfaces challenged by crude oil and zwitterionic membrane-bioreactor surfaces experience different combinations of adhesion, shear and biological loading [54,55]. Post-exposure coverage and composition, measured alongside transport and rejection, help distinguish an intact interface from a layer whose function persists despite partial material loss. TEPA–SiO₂@PDA on copper mesh and polyacrylamide/tannic-acid hydrogel on PVDF further demonstrate that attachment must be evaluated under the solvents, oxidants and shear expected during cleaning [56,57]. A multilayer SiO₂/aminomalononitrile/TiO₂/lysine interface achieved 95.5 ± 4.1% UV-assisted flux recovery in an emulsion protocol [58]. That value establishes hydraulic recovery under irradiation, while retained catalyst mass and post-cleaning activity are separate measures of the junction’s survival.

Adhesion requires wet-state mechanical evidence, such as scratch/progressive-load, peel/pull-off where geometry permits, or abrasion/shear tests, with loading conditions and failure location specified. Cohesive coating rupture must be distinguished from separation at the support, and responses on meshes or fibers are geometry-dependent. Cross-sectional microscopy, surface composition and dissolved/particulate release into permeate, concentrate and cleaning liquid provide complementary evidence of coverage, chemical change and material loss. Hydraulic FRR is not bond strength. Reduced activity with retained mass suggests masking or deactivation; reversible thickness changes suggest swelling; persistent resistance with retained chemistry suggests blockage.

A hydrogel/ZIF-8 coating combines a hydrated, oil-repellent matrix with accessible dye-binding regions [26]. It delivered 1313.4 L m⁻² h⁻¹ flux and greater than 99.77% emulsion separation, with model emulsified/dyed wastewater separation reported up to 99.7%. Ten-cycle oil separation, filtration recovery and repeated dye uptake used different protocols, so they establish individual functions rather than simultaneous recovery in a shared mixed-feed cycle. The coating must preserve both access to active sites and attachment under the intended maintenance conditions.

Across these architectures, chemistry governs hydration, binding and trigger sensitivity; thickness balances active inventory against transport resistance; morphology controls exposure and stress; and support stiffness, pores and surface chemistry determine infiltration and adhesion. Stronger anchoring can improve retention while limiting site accessibility or intentional stripping. Useful designs therefore retain the required function with minimal obstruction under a compatible recovery protocol.

Application suitability depends on the limiting function. Hydrated coatings reduce oil adhesion, but coarse coated meshes cannot be assumed to retain surfactant-stabilized emulsions from immiscible-mixture tests [5,14,59]. Selective/adsorptive layers for dissolved solutes require breakthrough and regenerant balances [1,60]; catalytic layers additionally require reactant access, oxidant accounting, transformation-product analysis and release measurements [61,62]. Variable industrial feeds favor chemically compatible supports and accessible maintenance [63,64]. Distillation coatings instead control wetting under thermal driving forces [65,66]. These requirements preclude a universal ranking by initial flux or removal.

3. Interfacial Transport and Pollutant-Removal Mechanisms

Coating structure and surface chemistry determine how water and pollutants encounter the functional interface. Dispersed droplets are affected by wetting, adhesion and pore-entry conditions, whereas dissolved solutes must reach selective, adsorptive or catalytic regions. A coating can combine these functions, but their controlling variables differ: surface hydration influences foulant attachment, the availability of binding sites limits adsorption capacity, and access to catalytic sites and reactants governs conversion. Each mechanism is assessed through the measurements needed to establish its contribution to treatment, providing a basis for understanding how and why membrane performance declines during operation.

3.1. Interfacial Hydration, Water Transport, and Dispersed-Pollutant Rejection

For oil-in-water emulsions, a hydrated pore entrance can inhibit direct droplet contact while maintaining water access. Metal–tannin and peptide coatings on PVDF implement this principle through different chemistries [67,68]. Figure 4a distinguishes direct oil adhesion on a bare membrane, passive resistance from smooth hydrophilic particles and the stronger contact barrier proposed for wrinkled particles [69]. Water retained around the hierarchical surface limits the approach and lateral migration of viscous droplets, making hydration continuity and local contact geometry central to antifouling. Figure 4b follows smooth caffeic-acid/APTES particles of approximately 0.35, 0.6 and 1.0 µm through Fe³⁺-induced surface restructuring; elemental maps establish the distribution of the coating constituents. Figure 4c shows increasing clean-water permeance through the modified series, with the most structured interface displaying a reported 750% increase over the unmodified membrane. Figure 4d connects that accessible flow network to emulsion separation, reaching 7247 L m⁻² h⁻¹ bar⁻¹ with 99.6% crude-oil rejection under the reported 1-bar dead-end test. The structural and transport evidence is consistent with reduced oil contact while retaining open flow pathways; the reported permeance gain depends on the specific comparator and feed. Zwitterionic grafting on cellulose fibers and amphiphilic graphene-oxide modification provide related routes to this balance [70,71].

Surface hydration and low oil adhesion depend on the arrangement of both polar and nonpolar groups. Nonionic brushes and sulfonate coatings modify foulant interactions through hydrated segments or ionic groups [29,72], while plasma-grafted zwitterionic PES supplies a charge-balanced interface [36]. Figure 4e illustrates a denser transport regime: oppositely charged titania nanosheets provide a hydrated water-facing layer on a reverse-osmosis membrane while the underlying selective skin controls salt passage [73]. Figure 4f identifies the oil–surface contact area and intermolecular attraction governing droplet detachment on graphene oxide [71]. Figure 4g compares underwater oil contact angles with normalized contact area, showing that interfaces with similarly large angles can still differ in their actual contact with oil. Figure 4h relates the experimental and calculated angles to surface energy and hydrophobic-chain architecture. These panels explain why static wettability is an incomplete descriptor of fouling resistance: chain arrangement and retained interfacial water control how strongly a droplet remains attached. Feed-dependent cleaning makes this distinction measurable. Water rinsing recovered 93.95% of the flux after petroleum-ether emulsion filtration, but only 58.95%, 68.29% and 81.82% after filtration of 100-, 200- and 500-nm particles [5]. An interface that releases droplets can still retain small rigid particles within its pore network, as supported by the source’s post-rinse microscopy.

Figure 4. Relationships among surface architecture, hydration and oil adhesion. (a–d) Proposed fouling-resistance mechanism, surface morphology and elemental maps, clean-water permeance, and emulsion transport/rejection of hierarchically structured coatings [69]. (e) Hydration and selective transport associated with titania-nanosheet modification of a reverse-osmosis membrane [73]. (f–h) Oil-contact/detachment schematic and experimental/calculated wetting relationships for hydrophobic-chain-engineered graphene oxide [71]. These panels illustrate different transport regimes and do not constitute a common performance comparison.

Figure 4. Relationships among surface architecture, hydration and oil adhesion. (a–d) Proposed fouling-resistance mechanism, surface morphology and elemental maps, clean-water permeance, and emulsion transport/rejection of hierarchically structured coatings [69]. (e) Hydration and selective transport associated with titania-nanosheet modification of a reverse-osmosis membrane [73]. (f–h) Oil-contact/detachment schematic and experimental/calculated wetting relationships for hydrophobic-chain-engineered graphene oxide [71]. These panels illustrate different transport regimes and do not constitute a common performance comparison.

Mixed feeds complicate the relationship between repellency and retention because droplets, surfactants and rigid particles can aggregate or compete for access to the surface. The caffeic-acid/ε-polylysine membrane retained 99.89% of oil and 96.99% of 100-nm particles during its mixed-feed test [5]. Its subsequent 72.63% rinse FRR shows that simultaneous retention did not provide complete hydraulic recovery. GO-coated and oriented-MOF-nanosheet meshes supply open pathways whose selectivity depends on coating coverage and liquid-entry conditions [74,75]. A reduced-GO/PDA/fluorinated membrane uses a different wetting preference and must be distinguished from a hydrated, water-permeating interface [76]. Tween-20-modified PVDF changes the surface environment of a porous polymer support [77]. Across these architectures, a high separation percentage can arise from different combinations of interfacial selectivity and geometric retention. Comparing coated-area integrity, permeate composition and operating flux after a specified washing or chemical challenge establishes which of those contributions persists.

Droplet-size distributions identify the small-droplet fraction able to enter pores. Surfactant identity and concentration relative to the critical micelle concentration govern stabilization, adsorption and wetting; oil loading promotes collisions and cake growth. Viscosity lowers flux at fixed resistance, while ionic strength modifies aggregation, electrostatic screening, ligand competition and swelling. Higher pressure can compact deposits or deform droplets through openings. Comparisons therefore require oil identity, droplet distribution, surfactant/solid/salt concentrations, viscosity, temperature and driving force; missing descriptors prevent attribution of differences to coating chemistry alone.

Fiber networks introduce an additional coverage constraint because a small, unprotected opening can dominate oil breakthrough. TiO₂-coated PAN, plasma-functionalized fibers and coated electrospun membranes illustrate how interfacial chemistry is distributed over interconnected gaps [78,79,80]; CuO- and photoactive TiO₂-coated meshes extend the same problem to larger apertures [81,82]. Evidence of reduced deposition should connect surface coverage to both oil passage and hydraulic response. A pressure-responsive piezoelectric membrane achieved 98.85% emulsion separation, less than 14% flux decline over 2 h and nearly complete flux recovery in the prescribed cleaning test [83]. These three endpoints establish rejection, in-operation stability and subsequent recovery, respectively. The greater than 99% crude-oil rejection but approximately 70% cleaned-flux recovery of polyaniline/alumina provides the complementary case of an effective barrier with persistent resistance [4]. A polyionic nanogel coating reported greater than 99% emulsion separation and permeance above 8014.70 ± 298.86 L m⁻² h⁻¹ bar⁻¹ over five dead-end separation cycles; its 96.13% Cr(VI) removal was assessed separately [84]. Hydrogel/ZIF-8, NiAlFe-LDH and electrically assisted textile interfaces likewise combine droplet separation with a dissolved-pollutant function [26,85,86]. Each function requires its own treatment and regeneration endpoint, even when both originate from the same coated surface.

Dispersed-pollutant rejection requires a persistent aqueous interface with connected flow pathways. Its benefit remains feed-dependent: surfactants, viscosity and particle size alter attachment and pore entry, while a retained cake can strengthen rejection but restrict flow. Flux, permeate composition and post-cleaning structure must therefore be evaluated together before attributing residual resistance to fouling, swelling or coating loss. Dissolved pollutants introduce the additional capacity and reaction limits discussed below.

3.2. Interfacial Adsorption and Catalytic Removal of Dissolved Pollutants

Dissolved contaminants encounter a different service limit from dispersed droplets. Selective exclusion depends on the effective pore and charge environment, adsorption consumes a finite inventory of accessible sites, and catalysis depends on continuing access to reactants and active centers. Both zwitterionic PVDF designed for oil separation and metal capture and multifunctional hydrophilic coatings targeting oil, dyes and metals bring several of these interactions together at one interface [87,88]. Their first-use multifunctionality is informative but does not establish simultaneous recovery after regeneration. A conductive textile membrane achieved 99.9% oil and 97.6% methylene-blue removal during one mixed-feed pass at 6.5 kPa and a DC current of 10 mA [86]. By comparison, a hydrogel/ZIF-8 coating reported 1313.4 L m⁻² h⁻¹ flux and greater than 99.77% emulsion separation, with dye uptake investigated in a distinct protocol [26]. Both studies demonstrate multifunctionality, including model mixed-feed treatment, while the extent of paired recovery must be judged from their respective reuse protocols. Following each analyte against the treated volume on the same membrane is necessary to determine whether the dissolved-pollutant function remains effective after the droplet-separation surface has been cleaned.

Ion-binding interfaces must account for competition as well as initial affinity. MOF-grafted membranes targeting selenium in saline produced water and nitrate-selective multilayers face different co-ion environments [89,90]. A PEDOT:PSS/Fe–Mn-oxide/PVDF system treated 2.70 L of 100 µg L⁻¹ As(V) solution below a 10 µg L⁻¹ outlet limit, but only 1.70 L after alkaline regeneration [60]. Figure 5a shows the proposed metal-removal pathways of Fe/ZSM-5-coated clay, including adsorption, ion exchange, surface complexation and precipitation [91]. These pathways depend on ion identity and solution chemistry and should not all be assigned to every metal. Figure 5c compares pH-drift curves: the reported point of zero charge changes from approximately 8.86 for the support to 6.63 after zeolite modification. This shift indicates a changed surface-charge environment, which can influence metal uptake. A point-of-zero-charge measurement does not by itself establish ion exchange, surface complexation or precipitation. Pb-binding metal–phenolic films exploit coordination-responsive capture and release [92,93], whereas tannic-acid/CNT/PVDF, gradient hydrogel fibers and GO/chitosan/PVDF address dissolved-organic uptake [94,95,96]. Across these cases, binding-site accessibility and speciation in the feed determine how much of the nominal chemical functionality participates during flow.

Figure 5. Chemical capture and selective transport of dissolved pollutants. (a) Proposed metal-removal pathways and (c) pH-drift determination of the point of zero charge for Fe/ZSM-5-coated clay [91]. (b) PFOA/PFOS retention, molecular-weight cutoff and surface charge before/after PAA/PAH coating; (e) concentration-dependent comparison of ultrafiltration, coated ultrafiltration, nanofiltration and reverse-osmosis membranes [42]. (d) PFBA effluent concentration versus throughput before/after regeneration of amine-functionalized PVDF [97].

Size exclusion depends on pore dimensions, whereas electrostatic exclusion reflects charge-dependent partitioning. Adsorption may involve ion exchange, as demonstrated by counter-ion displacement, or complexation, which is supported by coordination evidence. Precipitation requires evidence of a new solid and appropriate pH/speciation controls. Catalytic conversion changes chemical form; retention or adsorption alone does not demonstrate it. Lower permeate concentration can thus represent transfer to a coating, concentrate or precipitate rather than destruction. For metals, oxidation-state conversion and sequestration redistribute elemental mass without eliminating it.

Figure 5b compares a coated transport barrier with pristine ultrafiltration, nanofiltration and reverse-osmosis membranes [42]. PAA/PAH coating reduced the nominal cutoff of the 20-kDa ultrafiltration membrane to 12 kDa and shifted its zeta potential from −13.21 to −30.07 mV. In the displayed comparison, PFOA removal increased from 29.90% to 73.30% and PFOS removal increased from 34.40% to 77.80%. Figure 5e shows that these values depend on the concentration interval: coated UP020 gave 64.5–65.1% PFOA and 74.1–75.9% PFOS removal in the lower-concentration series, compared with 71.9–73.3% and 75.6–77.8%, respectively, in the higher-concentration series. Coating the more open 150-kDa support yielded lower retention. The concurrent changes in cutoff and charge are consistent with contributions from pore restriction and electrostatic exclusion; adsorption must still be distinguished from sustained rejection through loading-dependent measurements. Figure 5d provides such an adsorption-oriented comparison. Primary- and quaternary-amine-functionalized PVDF were challenged with a mixed feed containing approximately 120 µg L⁻¹ PFOA and 140 µg L⁻¹ PFBA, followed by regeneration [97]. The displayed PFBA outlet trajectories reveal feed-dependent capture and subsequent reuse rather than complete recovery inferred from an initial removal value. Acid-resistant polyamine nanofiltration illustrates an additional selective-transport architecture [98]. These examples separate modifications that restrict solute passage from binding interfaces whose accessible capacity is progressively consumed.

Ligand selection determines which dissolved contaminants benefit from accessible interfacial area. PDA-supported β-FeOOH provides phosphorus-binding sites [99], cyclodextrin-functionalized nanofibers target steroid hormones [100], and thiolated titania/PVC or cellulose nanofibers provide metal-binding groups [101,102]. Coated polyurethane fibers for ethinylestradiol and PDA–chitosan/PLA interfaces further illustrate the relationship between target chemistry and ligand accessibility [103,104]. Dye-interacting hydrogel coatings and cross-linked CNT/chitosan layers can combine sorption with changes in pore transport [105,106], whereas annealed and weak-polyelectrolyte multilayers form selective nanofiltration skins [43,107]. Carbon-functionalized thin-film composites for PFAS, Fe-MOF-containing skins for urea and amino-MOF/GO or PDA-functionalized nanofibers add further internal area or binding chemistry [108,109,110,111]. That area becomes useful only when solutes can reach it during flow and the active phase survives regeneration. For example, a 25-mm-diameter La/Fe/PAN filter treated 535 mL of 50 µg L⁻¹ As(III) below a 10 µg L⁻¹ outlet limit at 0.5 mL min⁻¹ and pH 7, while its reported ten-cycle sorbent result arose from a separate test [112]. This embedded-adsorbent comparison illustrates why dynamic membrane capacity and separate sorbent reuse cannot be treated as the same regeneration result.

Catalytic interfaces can extend removal beyond a finite adsorption inventory, but sustained conversion requires retained activity and continuing delivery of reactants and, where applicable, oxidant, light or electrical energy. FeOOH/CuO arrays combined at least 1400 L m⁻² h⁻¹ emulsion flux and at least 99.2% separation with electroflotation-assisted cleaning within 2 min, while electro-Fenton dye treatment was investigated as an additional function [113]. A β-FeOOH coating achieved 99.8% dye degradation in a specified acidic H₂O₂ challenge and greater than 99.3% oil rejection in a separate emulsion experiment [24]. The active phase contributes to both separation and reaction, but evidence for one function does not establish recovery of the other. Parent-solute disappearance, mineralization, catalyst retention and water-pathway recovery probe different parts of the mechanism. Their joint evolution determines whether later decline originates from deposited matter, inaccessible active sites, intrinsic deactivation or loss of the layer.

For catalytic coatings, parent-compound disappearance or decolorization alone does not establish mineralization. Dark-adsorption, catalyst-free and energy/oxidant controls, transformation-product analysis, TOC or DOC reduction, and an organic-carbon balance are needed to distinguish retention from conversion. Dissolved-metal and particulate-release measurements assess secondary contamination; residual oxidant and product toxicity should also be considered. COD reduction alone is insufficient evidence of complete or harmless destruction.

The mechanisms imply different recovery decisions: masked hydrated surfaces require deposit removal; altered selective pores or charges require integrity/rejection checks; occupied adsorption sites require desorption and repeated breakthrough tests; and masked catalysts may benefit from cleaning, whereas lost or irreversibly altered active phases require renewal. Multifunctional coatings should be assessed against the earliest limiting function.

4. Performance Decline of Coated Membranes During Operation

Performance loss can appear as restricted flow, earlier solute breakthrough, reduced conversion or coating detachment. Paired hydraulic and effluent measurements identify the limiting response; microscopy, pore characterization, composition and material-release measurements then distinguish deposits, occupied sites, chemical deactivation and material loss.

Reversible fouling is resistance removed by a specified compatible cleaning step; residual fouling is irreversible only relative to that cleaning window. Coating degradation denotes persistent alteration or loss of the functional layer, whereas support deterioration involves substrate compaction or damage. These processes can coexist and cannot be diagnosed from post-rinse flux alone.

4.1. Fouling-Induced Decline in Hydraulic Performance

Hydraulic decline can originate from surface-cake growth, pore invasion, coating swelling or reduced wetted area. Interfacial energy analysis, nonionic brush tests and sulfonated PDA interfaces examine different contributions to this resistance [23,72,114]. The mixed oil–microplastic experiment provides a direct example: after 300 min, oil and 100-nm-particle retention remained 99.89% and 96.99%, while operating flux decreased from 456.9 ± 14.4 to 390.6 ± 19.1 L m⁻² h⁻¹ and subsequent rinse FRR was 72.63% [5]. The separation barrier remained effective, but water pathways were not fully restored. Additional biological or adsorptive functions on lysozyme-brush cotton, nanocellulose/PES and GO/chitosan/PVDF alter the interactions responsible for deposit formation [19,96,115]. Their interpretation requires a clean-water baseline, the operating trajectory and a post-cleaning baseline on the same membrane. These measurements distinguish reversible deposition from residual resistance; microscopy or resistance analysis then identifies whether the latter is associated with internal blockage or the external layer.

Feed composition determines how much of that resistance is recoverable. Hydrogel coatings on mesh or PVDF differ in aperture, swelling and the likelihood of pore invasion [21,26,57,116]. Salt-tolerant cellulose, chitosan–alginate, sulfonate and MIL-101(Cr) meshes address additional wetting and stability constraints [29,30,117,118], but static salt or pH exposure does not reproduce repeated fouling and cleaning. The MXene/MWCNT study isolates the effect of oil type within one membrane system [119]. Across five water-washed canola-oil cycles, emulsion flux decreased from 14.8 to 10.26 L m⁻² h⁻¹ and rejection decreased from 94.4% to 89.3%. With viscous motor oil, flux decreased from 10.2 to 0.7 L m⁻² h⁻¹ while rejection remained near 99% and rinse-only FRR was approximately 7%. High retained rejection in the latter case is compatible with a strongly resistive oil deposit. Absolute throughput, oil properties and cleaned baseline determine whether maintained selectivity represents useful hydraulic service.

High rejection during severe flux decline does not ensure useful capacity. Fixed-pressure operation loses output; fixed-flux operation requires greater pressure and pumping work. Shorter runs increase cleaning, waste and downtime. Service limits should therefore include effluent quality, minimum production, allowable pressure and recoverability. The motor-oil example [119] demonstrates a hydraulic limitation despite retained rejection, not an economically validated lifetime.

4.2. Decline in Pollutant-Removal Function and Coating Integrity

A decline in pollutant-removal function can occur before a pronounced hydraulic change. Arsenate-binding Fe–Mn oxide and selenium-binding MOF interfaces can lose accessible capacity [60,89], whereas polyelectrolyte and polyamine selective skins can change charge or solute passage [42,97,98]. In the As(V) example, the reduction from 2.70 to 1.70 L after regeneration corresponds to 62.9% recovery of threshold-defined treated volume, rather than a measurement of water-flux recovery [60]. Site occupation, ligand alteration and active-phase loss are possible causes of earlier breakthrough, but they cannot be distinguished from the volume-recovery result alone. Phosphorus-binding β-FeOOH, thiolated cellulose and amino-MOF/GO interfaces illustrate the relevant range of chemistries [99,102,110]. Pb-binding metal–phenolic films retained approximately 95% removal over ten regeneration cycles, and another coordination-responsive membrane reported high Pb removal over a defined electroplating-water throughput [92,93]. These observations support retained treatment function under the reported challenges. Distinguishing renewed sites from partial active-phase loss additionally requires the full outlet trajectory and post-cycle layer composition.

Catalytic function can decline through masking, chemical changes or active-phase loss; visible coating retention alone does not establish retained activity. Fe–Cu-LDH/PVDF, photo-Fenton coatings, FeOCl-coated ceramics, cobalt-oxide-containing pores and CoMn-MOF-74/ozonation interfaces expose active centers to different oxidative environments [61,120,121,122,123,124]. TiO₂/PVDF, MnO₂ and FeOOH/CuO also couple reaction and separation through different geometries [25,113,125]. Figure 6a illustrates this chemistry–geometry coupling for carbon fibers coated with Cu–TiO₂ and subsequently modified with octadecyltrimethoxysilane (OTMS) [126]. Figure 6b shows oil uptake of 5.5–6.7 g g⁻¹ and retention of 87.5–89.7% for four liquids, characterizing the oil-affine silanized surface. Figure 6c follows water contact angle through 20 cycles, with the surface remaining above 150° despite gradual decline. Figure 6d follows underwater oil contact angle over a longer operating history and shows its recovery after UV-assisted treatment within the source’s maintenance protocol. Figure 6e displays relatively stable, oil-dependent mixture-separation flux over 24 h. These panels characterize wetting and transport stability rather than directly measuring catalytic deactivation. They separate initial affinity, repeated wetting behavior, maintenance response and sustained throughput. Restoration of an underwater contact angle supports renewed wetting selectivity, while retention of catalytic activity or active-phase mass requires the corresponding chemical measurements.

Figure 6. Wetting function and operational response of two coated interfaces. (a) Cu–TiO₂/carbon-fiber preparation and OTMS modification, (b) oil uptake/retention, (c) cyclic water contact angle, (d) underwater oil contact angle and UV-assisted recovery of unsilanized Cu–TiO₂/carbon fibers, and (e) 24-h mixture-separation flux of the silanized oil-permeating variant [126]. (f) Nine-oil separation efficiencies and underwater droplet profiles, (g) flux for six mixtures, (h) water-entry schematic and (i) underwater oil-entry schematic of bamboo-cellulose-hydrogel-coated copper mesh [116]. Oil uptake, wettability, sustained filtration and regeneration probe distinct interfacial properties.

Figure 6f–i supplies a complementary water-affine interface: bamboo-cellulose hydrogel coats a copper mesh [116]. Figure 6f shows 98.12–99.96% separation across nine oils together with underwater oil-droplet profiles. Figure 6g shows gravity-driven water flux varying from 4326 to 9168 L m⁻² h⁻¹ across six immiscible oil/water mixtures, demonstrating that nearly constant high separation efficiency can coexist with more than a twofold throughput difference. Figure 6h represents capillary entry of water into a hydrophilic opening; Figure 6i represents the positive pressure barrier encountered by oil beneath water. The signs of the indicated pressure differences depend on the schematic’s convention, but the physical distinction is spontaneous water access versus inhibited underwater oil entry. Comparing these panels with the silanized carbon-fiber system shows that selective wetting can favor different permeating phases. A high contact angle acquires meaning only after the surrounding liquid, permeating phase and pore-entry conditions are specified. Interfacial survival is best judged by whether that same transport preference and useful throughput persist after the prescribed maintenance step.

Deliberate layer removal provides a useful contrast to unintended degradation. The antibiotic-binding tannic-acid/Fe(III) film was stripped using 0.1 M HCl for 1 h and reconstructed after use [1]. Ciprofloxacin removal reached 99.4% in cycle ten, while permeance decreased from 3815 to 2615 L m⁻² h⁻¹ bar⁻¹. Residual nanoparticles were proposed as a source of added resistance, but the removal–permeance divergence itself is the direct observation. For unsilanized Cu–TiO₂/carbon fibers, 10 min of UV treatment restored underwater oil repellency. The silanized, oil-permeating variant required UV treatment followed by renewed OTMS modification to restore its original wetting function; both variants were examined over 100 immiscible-mixture separation cycles [126]. These examples distinguish rebuilding active chemistry from retaining the original surface. Table 2 translates the resulting failure signatures into discriminating measurements and initial interventions, providing the basis for the regeneration routes examined next.

Table 2. Diagnostic signatures and conditional first interventions for coated interfaces. Each intervention requires compatibility with coating chemistry, selective-layer integrity and support geometry. Risks are stated explicitly; failure thresholds remain application-specific.

Failed Element/ Examples	Paired Observable Signature	Discriminating Measurement	Conditional First Intervention	Principal Risks	Ref
External deposit	Flux falls; rejection can remain high.	Matched clean-water permeance before/after rinse; microscopy locates deposits.	Rinse/backwash if coating and geometry tolerate shear.	Delamination; incomplete internal/biofilm removal; contaminated wash water.	[5,119]
Occupied adsorption sites	Earlier breakthrough despite retained water flow.	Repeat threshold-defined treated volume and mass uptake under matched feed/flow.	Desorb only if ligands and support tolerate pH or electrical switching.	Binding-site loss; metal release; concentrated regenerant and neutralization waste.	[60]
Catalytic sites	Conversion or cleaning rate declines.	Activity with retained catalyst mass, speciation and leachate measurements.	Remove masking deposits; renew an irreversibly altered or lost phase.	Reactive attack on binder/support; transformation products; catalyst leaching.	[25,61]
Coating/modifier	Coverage or chemistry changes; flow and removal diverge.	Post-cycle coverage, composition, mass and hydraulic/effluent response.	Strip/recoat only if the underlying support remains intact.	Residual resistance; reagent and material demand; spent-layer waste.	[1,126]
Support/selective barrier	Persistent defect, compaction or altered rejection after coating removal.	Support-only control, integrity test and cross-sectional morphology.	Repair if validated; otherwise replace the support.	Recoating may mask damage and cannot guarantee restored selectivity.	/

Diagnosis begins with matched clean-water permeance and target-specific effluent measurements, followed by a mild compatible rinse, microscopy of residual deposits, and assessment of coating chemistry/coverage, release and support integrity. Recovered flow without recovered breakthrough or conversion points to occupied or altered active sites; declining rejection with coating loss prompts checks for delamination or barrier defects. A damaged support requires validated repair or replacement. Table 2 links these decisions to compatibility and intervention risks.

Combined pressure/shear, pH, oxidants and temperature can cause edge delamination, swelling, binder attack and interfacial strain. Single-stressor controls should precede representative feed–loading–cleaning sequences, with paired permeance, effluent, coating-state and release measurements. The 30-day immersion [5], 2-h filtration [83] and 72-h maintenance records [6] represent different limited exposures, not long-term combined-stress validation. Dose-matched cleaning-only and support controls distinguish foulant removal from maintenance-induced damage.

5. Performance Recovery and Reuse of Coated Membranes

Regeneration should restore the limiting function while preserving useful coating and support components. Hydraulic cleaning removes detachable deposits, desorption releases occupied sites, assisted cleaning promotes foulant release or transformation, and recoating replaces a spent layer. Success is verified in the next run through paired water production, effluent quality and coating-state measurements.

5.1. Physical Removal of Fouling Deposits

Water rinsing and backwashing can restore flow when removable external deposits dominate resistance and the active layer withstands the imposed shear. Hydrogel-coated PVDF, cellulose-hydrogel/PVDF, zwitterionic cellulose fibers and carbide-derived-carbon/ceramic interfaces provide different combinations of foulant adhesion and support stiffness [21,57,70,127]. Their recoverable fraction depends strongly on the feed: rinsing restored 72.63% of the baseline flux after the mixed oil–microplastic challenge, but only approximately 7% after viscous motor-oil loading of MXene/MWCNT [5,119]. These values identify different cleaning limitations rather than a cross-study material ranking. Real-feed operation provides a complementary challenge. Cyclodextrin/PDMS membranes were tested for 72 h with backwashing and forward flushing every 4 h, including a chemical intervention after 48 h [6]. Polymer-brush and PDA/CNT-coated membranes modify deposition through related surface interactions [128,129], while the co-hydrolyzed silane/polyphenol coating reported greater than 90% FRR after its oily/biofoulant challenge [2]. Persistent resistance after a specified rinse calls for localization of the residual deposit. Deteriorating effluent quality without comparable flux loss instead indicates that repeated hydraulic cleaning is unlikely to restore the limiting chemical function.

Rinsing is least effective for strongly adsorbed viscous oils, internal deposits and persistent biofilm matrices. Backwashing reaches different locations but requires a reverse-pressure-tolerant support and selective skin. Greater shear may improve removal while delaminating coatings or damaging fibers. Flow direction, pressure/flow, duration, rinse volume and material release should therefore be reported; cleaning intensity is not transferable between meshes, asymmetric skins, ceramic tubes and hollow fibers from FRR alone.

5.2. Desorption and Regeneration of Adsorption Sites

When an attached coating has occupied binding sites, desorption must release captured solutes while preserving the chemistry needed for the next run. PEDOT:PSS/Fe–Mn oxide used alkaline regeneration, whereas Pb-binding metal–phenolic films used dilute acid [60,92,93]. Electrical affinity switching provides a different route in Figure 7a–f [130]. Figure 7a distinguishes membrane construction and eventual support/solvent recycling: a ceramic-supported carbon-paper assembly is functionalized through spin coating and polypyrrole electropolymerization, while end-of-life disassembly uses solvent immersion. Figure 7b shows the filtration configuration in which organic molecules and cations encounter the AOT-doped polypyrrole interface before permeation. Figure 7c compares removal across six organic compounds and six metal ions, establishing broad initial affinity under the tested conditions. Figure 7d then follows Pb²⁺ and 1-NP removal during five consecutive filtration/regeneration cycles. Brief stimulation at the source-specified positive or negative 1.5-V potential changes interfacial affinity and releases the respective adsorbates; filtration resumes without a continuously applied field. This sequence directly tests whether occupied sites can be reactivated on the same membrane, rather than inferring regeneration from the initial multi-solute removal panel.

Figure 7. Affinity regeneration and the hydraulic constraints of coated membranes. (a) Janus-membrane construction and end-of-life support/solvent recycling, (b) filtration configuration, (c) initial organic/metal removal, (d) electrically regenerated Pb²⁺/1-NP removal, (e) spiked-tap-water comparison and (f) source-specific performance accounting [130]. (g) Pressure–flux responses, (h) concentration-dependent emulsion flux and (i) separation efficiency of MXene/MWCNT membranes with different deposited masses [119]. Electrical site regeneration and solvent-based material recycling are separate operations.

The remaining panels connect affinity switching to water output and hydraulic constraints. Figure 7e compares Pb²⁺ and 1-NP removal in spiked tap water with NF90 and NF270, showing renewed Janus-membrane performance after the indicated intervention [130]. At 227 L m⁻² h⁻¹, specific treatment capacity exceeded 1100 L m⁻², defined in the source by greater than 90% removal of both monitored solutes. The source estimates an energy demand of 0.01 kWh m⁻³ from its operating and regeneration assumptions; this is not a measured whole-process energy or cost saving. Figure 7f summarizes its source-specific removal, flux, energy and cost comparison; those axes depend on the selected analytes and accounting assumptions. Figure 7g shows approximately linear pressure–flux responses of MXene/MWCNT membranes with different deposited masses [119]. Figure 7h shows decreasing flux as emulsion concentration rises from 67.5 to 250 mg L⁻¹, while Figure 7i retains approximately 98–100% separation across the same challenges. These lower panels establish a hydraulic burden that affinity switching alone does not address. The two source systems illustrate distinct limitations: electrochemical affinity switching renews capture, whereas hydraulic cleaning must address restricted water pathways. Their juxtaposition does not demonstrate joint recovery on a single membrane. The maintenance route should be selected according to the function limiting the next run, rather than according to conductivity or initial removal alone.

Desorption transfers contaminants to a receiving stream. Report regenerant concentration/volume, contact time, exported contaminant concentration, rinsing and neutralization; reuse is limited by accumulated salts, competition and declining next-run capacity. Electrical switching also requires a receiving-stream balance. Extreme pH or polarization can hydrolyze ligands, dissolve oxides or alter polymer doping. Initial removal recovery, equilibrium capacity recovery and dynamic breakthrough-volume recovery are distinct: the As(V) membrane recovered only approximately 63% of useful volume after pH-11 treatment [60]. Repeat capacity measurements under matched feed, flow, area and outlet criteria.

5.3. Catalytic, Electrical and Photothermal Cleaning

Assisted cleaning is an option when deposits persist after hydraulic cleaning and the interface or an external treatment can promote their release or transformation. Its effectiveness depends on placing heat or reactive species near the blocked pathways while limiting damage to the polymer support, binder and functional phase. Lignin-coated PVDF and chitosan/lignin variants illustrate this balance for photoelectrocatalytic cleaning [131,132]. With TiO₂-modified electrodes, a lignin-coated PVDF hollow-fiber membrane achieved 94.3% FRR and 98.9% oil rejection after the prescribed treatment; FRR remained above 91.8% and oil rejection above 98.0% over ten consecutive cycles [61]. This paired series supplies evidence for oil-separation recovery on the reused membrane. It does not require assuming that hydraulic recovery measures every chemical function. Visible-light-responsive membranes, NiFe-LDH composites and TiO₂/PAN interfaces use different irradiation geometries and active phases [78,133,134]. Their dose, photon access and catalyst retention govern whether organic-foulant transformation produces a reproducible recovery benefit.

Figure 8a–c connects catalytic loading to the accessibility of the deposited phase [135]. Figure 8a shows electrospun PAN followed by alternating tannic-acid and Fe(III) deposition; the subscripts identify assembly passes, not reuse cycles. Figure 8b follows RhB concentration through a dark interval and subsequent visible-light irradiation. Five coating passes gave 98% reported RhB removal after 360 min illumination, whereas seven passes gave approximately 72%, indicating that additional material did not translate into greater usable activity. Figure 8c compares apparent kinetic responses and confirms the lower effective reaction rate of the heavily deposited interface. Aggregation and reduced exposure of reaction sites provide a plausible explanation, consistent with the loading-dependent response. A separate photocatalytic reuse experiment retained degradation above 70% and showed fewer attached grains after cycling, connecting activity decline to changes in the deposited phase. Hydrogel catalytic separators, Al₂O₃@ZnO, ZnO-coated ceramics and photocatalytic double-network hydrogels offer further support–catalyst combinations [136,137,138,139]. These results establish loading-dependent dye disappearance and reuse stability. Absorbance-based removal does not establish complete mineralization or hydraulic self-cleaning without organic-carbon and post-fouling transport measurements. The design requirement is exposed catalytic area at a loading that remains attached and accessible during reaction and reuse.

Electrical and oxidative interventions also differ in the location of reaction and current transport. Laser-induced-graphene nanofiltration skins, CuCo₂O₄- and CuO-coated meshes, and FeOOH/CuO arrays provide distinct conductive interfaces [113,140,141,142]. The FeOOH/CuO system achieved at least 99.2% oil separation and electroflotation-assisted cleaning within 2 min under its protocol [113]. PVA/PES–TiO₂ for pretreated rubber wastewater and photocatalytic nanofiltration place reaction zones in different separation layers [143,144]. Electrical cleaning must distinguish bubble-assisted deposit release from electrochemical transformation, while photocatalytic cleaning must record the irradiation and oxidant conditions. Delivered charge or irradiation dose, next-cycle effluent quality and active-phase release are complementary measurements. In the oil-fouling example, a tannic-acid-based photo-Fenton coating recovered 96.96% of original flux after five cycles [121]. By comparison, the ten-cycle lignin/PVDF series reported both hydraulic and oil-rejection recovery [61]. The distinction is the evidence supplied for the next treatment run, rather than the nominal use of an assisted-cleaning mechanism.

Figure 8. Catalytic accessibility, wetting-mediated separation and layer renewal. (a–c) Assembly, RhB concentration evolution and kinetic analysis of tannic-acid/Fe(III)-coated PAN; subscripts denote coating passes [135]. (d,g,h) Gravity-driven mixture-separation setup, wettability and separation efficiency of WO₃- and ZnO-coated meshes [59]. (e,f) Rejection and chemical-oxygen-demand removal under dark/UV conditions for PDA- or polyaniline-stabilized TiO₂/PVDF interfaces [62]. (i,j) Jute-derived cellulose/ceramic skin replacement and separation efficiency across the stated replacement cycles [145]. Catalytic fabrication, photocatalytic reuse, filtration and skin replacement are different cycle definitions.

Figure 8. Catalytic accessibility, wetting-mediated separation and layer renewal. (a–c) Assembly, RhB concentration evolution and kinetic analysis of tannic-acid/Fe(III)-coated PAN; subscripts denote coating passes [135]. (d,g,h) Gravity-driven mixture-separation setup, wettability and separation efficiency of WO₃- and ZnO-coated meshes [59]. (e,f) Rejection and chemical-oxygen-demand removal under dark/UV conditions for PDA- or polyaniline-stabilized TiO₂/PVDF interfaces [62]. (i,j) Jute-derived cellulose/ceramic skin replacement and separation efficiency across the stated replacement cycles [145]. Catalytic fabrication, photocatalytic reuse, filtration and skin replacement are different cycle definitions.

The other Figure 8 panels distinguish selective wetting from photocatalytic contribution. WO₃-coated ceramics, ZnO/alumina and CeO₂/Co₃O₄ meshes combine oxide chemistry with different supports [45,146,147]. Figure 8d shows gravity-driven separation of hexadecane, octane and olive-oil mixtures using oxide-coated meshes [59]. Figure 8g compares water-in-air, oil-in-air, water-in-oil and oil-in-water contact angles, with underwater oil angles close to 170°. Figure 8h then tests separation across coating materials and mesh openings, retaining approximately 97–100% efficiency; this is evidence of wetting-mediated mixture separation rather than of pollutant degradation. Figure 8e compares dye rejection under dark/UV conditions for PDA- or polyaniline-stabilized TiO₂/PVDF [62]. Figure 8f supplies the corresponding COD measurements: PDA/PANI coatings alone remained near 33% COD removal, whereas TiO₂-containing coatings reached 68.4% and 57.6%, respectively, after 3 h. High dye rejection and greater COD reduction describe different effluent-quality responses. The latter is consistent with an additional photocatalytic contribution under the stated test conditions, but permeate COD alone cannot separate retention from mineralization or establish that the transformation products are harmless. The analytical implication is to assess both selective retention and organic-load reduction when assigning a catalytic benefit.

Photothermal assistance has a limited dose window. For MXene/MWCNT, motor-oil viscosity decreased from 280 to 78.5 mPa s at 60 °C, but 10-min cleaning gave approximately 92.8% FRR versus approximately 60% after 20 min [119]. Later cycles retained 4.2 L m⁻² h⁻¹ flux and approximately 69% recovery. The source associates poorer extended-heating recovery with oil hardening and adhesion. More heating or catalyst loading therefore need not improve regeneration; next-run transport, removal and layer retention must be checked.

Assisted-cleaning comparisons require wavelength, irradiance, illuminated area and time, or cell voltage/current, configuration and polarization time, together with temperature and oxidant dose. Optical dose is time-integrated irradiance. Electrical energy is ∫UI dt, while wall-plug consumption also includes lamps, heating and auxiliaries. Electrode potential is not necessarily cell voltage. Normalize recovery energy by the useful treated volume subsequently, or report event/area inputs when that volume is unavailable. The ±1.5-V switching [130], 2-min electroflotation [113] and 10-min UV recovery [126] cannot be ranked by duration. Permeance must be remeasured at the reference temperature to separate cleaning from viscosity reduction; altered rejection or chemistry requires a damage assessment.

The MXene/MWCNT experiment used an 808-nm laser at 1 kW m⁻², reaching approximately 38.5 °C after 10 min before a subsequent 1-h separation test [119]. The calculated incident doses are 0.60 and 1.20 MJ m⁻² for 10 and 20 min, respectively; these are not wall-plug energy per useful volume. Conductivity decreased from 64.52 ± 1.57 to 58.64 ± 0.97 S m⁻¹ after cyclic use, providing an electrical proxy rather than a coating-mass measurement.

5.4. Coating Renewal and Repair

Layer renewal becomes appropriate when cleaning the existing film no longer restores its intended function. Figure 8i shows a jute-derived cellulose skin formed on a ceramic support through a urea/NaOH processing route [145]. The hydrated cellulose layer receives the oil-rich deposit and can be removed and reconstructed, allowing the mechanically durable support to serve successive surface layers. Figure 8j reports separation efficiency remaining approximately 98% across the displayed SR-0 to SR-5 replacement sequence; the source additionally reports greater than 99% original-flux recovery after 16 fouling cycles. The two records establish retained separation during replacement and hydraulic restoration under their respective protocols. Starch–amine, carboxymethyl-chitosan, pH-responsive chitosan and disassemblable polyelectrolyte layers use different triggers for the same separation of surface and support lifetimes [44,148,149,150,151]. PDA-, TiO₂-, GO- and silica-containing interfaces are intended to retain coverage and chemistry during maintenance [46,152,153,154,155]; this intention does not establish permanent attachment. Fe–Mn-oxide-assisted oxidative cleaning restores function in place [156], while renewed amines [157], hydrophilic PVDF modification [158] and renewable polyelectrolyte interfaces [159,160] illustrate different combinations of retention and reconstruction. The advantage of replacement is a reproducible water-facing interface; its constraint is the added material use and any resistance left by each reconstruction.

The contrast between retention, replacement and physical repair becomes clear from their recovery records. TiO₂-biomineralized mesh and PDMS-NH₂/PFONa–SiO₂-coated mesh depend on maintaining an exposed wetting interface [161,162]. A carboxymethyl-chitosan layer retained greater than 88.8% FRR through four coating/decoating/recoating cycles under natural organic matter fouling [149]. A pH-responsive chitosan coating gave 95% recovery compared with 60.3% for its control during natural-surface-water ultrafiltration [150]. The starch–amine layer exceeded 96.2% FRR after three coat–foul–clean–recoat sequences [148]. These results establish hydraulic benefits of renewal. Antibiotic-film reconstruction additionally preserved 99.4% removal in cycle ten but reduced initial permeance by 31.5%, revealing a cumulative transport penalty [1]. Sacrificial polyelectrolytes gave 97 ± 3% water-flux recovery after replenishment in a reverse-osmosis test, compared with 83 ± 3% for the unmodified control [163]. As a comparison beyond surface-coating renewal, microcapsule-assisted repair restored approximately 103% of the original flux and 90% of the original particle rejection after induced membrane damage [164]. Recovery above 100% should not be interpreted as complete restoration of the original selective structure. Protective-layer renewal, adsorbent rebuilding and repair after induced damage consequently support different durability claims.

Sacrificial-layer replacement removes a fouled protective film, while functional-layer renewal restores active material. Neither repairs a damaged selective barrier. Their lifecycle value depends on equivalent effluent quality and useful volume relative to replacement, accounting for new coating mass, stripping/recoating chemicals, rinse water, residual resistance, spent solvent/solids and support lifetime. The 31.5% permeance loss during antibiotic-film renewal [1] exposes a cumulative penalty; retained separation after cellulose-skin replacement [145] does not alone demonstrate a lower lifecycle cost or impact. Table 3 compares the routes on this functional basis.

Table 3. Recovery routes and source-specific endpoints. Percentages and cycle counts cannot be ranked across studies with different feeds, driving forces, temperatures, loading/cleaning histories, baselines or cycle definitions. Cycle counts describe source protocols, not equivalent service lifetimes. Missing common-basis measurements do not imply zero inputs or material loss. Permeance in the replacement example is in L m⁻² h⁻¹ bar⁻¹.

Recovery Route/Evidence	Source-Defined Endpoints	Exposure and Layer Evidence	Inputs and Comparison Limits	Ref
Physical rinse	Mixed oil/PS [5]: FRR 72.63%; motor oil [119]: about 7%.	Different feeds and loading/cleaning histories; these FRRs do not establish a material ranking or coating-mass retention.	Water, pressure/flow and wash volume required; no matched energy-per-useful-volume comparison.	[5,119]
Alkaline desorption	Useful As(V)-treated volume 2.70 → 1.70 L; recovery 62.9%. This endpoint is not FRR.	One reported before/after dynamic comparison; hydraulic and active-mass recovery are separate.	pH 11; regenerant quantity, contaminant export and neutralization needed for full accounting.	[60]
Electrical affinity switching	Pb2+/1-NP removal followed over five filtration/regeneration cycles.	Source-specific ±1.5-V stimulation; post-cycle active-layer mass not supplied by removal alone.	Source-estimated 0.01 kWh m−3 is assumption-dependent; whole-process cost is not established.	[130]
Photoelectrocatalytic cleaning	FRR > 91.8% and oil rejection > 98% over ten reported cycles.	Paired hydraulic/oil endpoints; dissolved-pollutant recovery is a separate question.	Irradiance, electrical input, cleaning time, and retained catalyst are required to match efficiency.	[61]
Photothermal cleaning	Approximately 92.8% FRR at 10 min versus 60% at 20 min.	Later motor-oil cycles show incomplete recovery; duration alone does not establish coating survival.	Temperature/viscosity controls and measured irradiation energy are needed.	[119]
Film replacement	TA–Fe/nylon [1]: antibiotic removal 99.4% in cycle ten; permeance 3815 → 2615.	Cellulose [145]: SR-0–SR-5 skin replacements and a separate 16-fouling-cycle record. These counts are not equivalent.	Include stripping, replacement mass, resistance, waste and downtime; no common lifecycle ranking.	[1,145]

6. Verifying Recovery of Water Production and Pollutant Removal

6.1. Paired Performance Measurements Across Treatment Cycles

Table 4 separates recovered endpoints: paired oil rejection/FRR [61], antibiotic removal after reconstruction with declining permeance [1], threshold-defined arsenate breakthrough [60], photocatalytic activity after reuse [135], and post-PMS hydraulic recovery [165]. Only measurements obtained through the same loading–intervention sequence establish paired functional recovery; initial activity or water-flux recovery alone is insufficient.

Table 4. Joint assessment of treatment, regeneration and coating durability. Cycle counts retain the source definitions. Quantitative coating-mass retention is not established by the endpoints extracted here; structural observations are identified separately. NE denotes information not established in this extracted comparison; it does not assert that the original publication omitted the measurement. Different feeds, baselines and transport modes preclude a pooled performance ranking.

Coating and Defined Challenge	Treatment Endpoint Established	Recovery Endpoint Established	Coating Durability and Resistance Evidence	Ref
Caffeic-acid/ε-polylysine; 100-nm PS + oil, 300 min Pressure/head: NE; effective wetted area: NE; test temperature: NE.	Oil 99.89% and PS 96.99% retained	Water-rinse FRR 72.63% after the stated 300-min mixed-feed challenge; not a complete long-term regeneration series.	Oil/PS retention and FRR are distinct endpoints. Quantitative coating retention and a resistance decomposition are unresolved here.	[5]
TA–Fe/nylon; ciprofloxacin; reconstruction through cycle ten. Pressure/head, effective area and temperature: NE.	Cycle-ten antibiotic removal 99.4% after recoating	Permeance 3815 → 2615 L m−2 h−1 bar−1	Calculated permeance retention 68.5%; conditional total-resistance rise 45.9% at matched viscosity/driving force. Coating mass retention is not established in this comparison.	[1]
PEDOT: PSS/Fe–Mn/PVDF; 100 µg L−1 As(V) Pressure/head: NE; effective wetted area: NE; test temperature: NE.	2.70 L before, 1.70 L after alkaline treatment to 10 µg L−1 effluent limit	Source-reported 62.9% recovery of threshold-defined treated volume (approximately 63.0% from rounded volumes)	One before/after desorption comparison; hydraulic resistance and retained active mass are unresolved here. Useful-volume recovery is not FRR.	[60]
PEC-cleaned lignin/PVDF; oil emulsion Pressure/head: NE; effective wetted area: NE; test temperature: NE.	Oil rejection > 98% across ten cycles	FRR > 91.8% across the same reported series	Paired hydraulic/oil reuse evidence. Quantitative coating retention and resistance decomposition are unresolved here; dissolved-contaminant recovery is not established.	[61]
TA/Fe(III)/PAN; RhB under visible light Pressure/head: NE; effective wetted area: NE; test temperature: NE.	98% reported RhB removal after 360 min illumination for five coating passes	Separate photocatalytic reuse: degradation retained above 70%; fewer attached grains after cycling	Fewer attached grains after reuse support material-loss concerns. Quantitative coating retention, hydraulic recovery and resistance are unresolved here.	[135]
Cu–TiO2/carbon fiber; immiscible oil/water Pressure/head: NE; effective wetted area: NE; test temperature: NE.	>97.3% separation in specified mixture experiments	Unsilanized surface: 10 min UV restores underwater oil repellency; silanized oil-permeating surface: UV plus OTMS re-modification	Recovery of wetting differs from recovery of flux. Re-modification replaces organic surface chemistry; quantitative retained mass and hydraulic resistance are unresolved here.	[126]

The baseline is the conditioned clean coated membrane before first use. At matched transmembrane pressure ΔP and reference temperature, J_w,0 denotes its original clean-water flux. For cycle n, $J_{\mathrm{w},n}^{-}$ and $J_{\mathrm{w},n}^{+}$ denote clean-water flux immediately before feed loading and after recovery and reconditioning, respectively. The superscripts identify measurement stages rather than algebraic signs. Original-baseline and within-cycle flux recovery are defined separately as

$${\mathrm{FRR}}_{0,n}={\displaystyle \frac{J_{\mathrm{w},n}^{+}}{J_{\mathrm{w},0}}}\times 100\%$$

(1)

$${\mathrm{FRR}}_{\mathrm{cyc},n}={\displaystyle \frac{J_{\mathrm{w},n}^{+}}{J_{\mathrm{w},n}^{-}}}\times 100\%$$

(2)

Equation (1) tracks recovery relative to the original membrane. Equation (2) describes recovery within the current cycle and can conceal cumulative decline when the preloading baseline has already fallen. Clean-water permeance L_p and its original-baseline retention PR_n are defined as

$$L_{\mathrm{p}}={\displaystyle \frac{J_{\mathrm{w}}}{\Delta P}},{\mathrm{PR}}_{\mathrm{n}}={\displaystyle \frac{L_{\mathrm{p},n}^{+}}{L_{\mathrm{p},0}}}\times 100\%$$

(3)

The original and post-recovery permeances use matched reference conditions. Pressure normalization alone does not correct viscosity, osmotic-pressure or concentration-polarization differences between tests. Apparent pollutant removal is

$${\eta }_{\mathrm{app}}=\left(1-{\displaystyle \frac{C_{\mathrm{p}}}{C_{\mathrm{f}}}}\right)\times 100\%$$

(4)

Here, C_f and C_p are contemporaneous feed and permeate concentrations. Integrated removal uses flow-weighted analyte masses and does not by itself establish destruction.

Useful treated volume, V_use, integrates permeate flow while all effluent limits and minimum production requirements are met; ${\displaystyle \frac{V_{\mathrm{use}}}{A}}$ normalizes by area. Breakthrough volume ends at the defined outlet-limit crossing. Dynamic adsorbed mass is the inlet–outlet analyte balance corrected for storage, concentrate and verified reaction, normalized by stated adsorbent/coating mass. For an unreplaced coating, mass retention is the remaining coating or active mass divided by its initial mass and multiplied by 100%, measured directly or through a validated marker balance. XPS intensity and image coverage are proxies, not mass-retention percentages. Source-defined metrics are retained when their baselines cannot be reconstructed. Useful volume is an experimental performance measure only when obtained from measured permeate flow over a verified compliance interval; values extrapolated beyond that interval must be identified as projections.

Deposition cycles are fabrication passes; filtration cycles are loading intervals; cleaning cycles are specified interventions; complete regeneration cycles include loading, intervention, reconditioning and next-run verification. Replacement adds stripping/redeposition. Counts require durations, processed volumes and checkpoints: five TA/Fe(III) deposition passes [135], five adsorption/regeneration cycles [130] and SR-0–SR-5 replacements [145] are different exposures.

Recovery ratios require replicate/batch numbers, numerator and baseline uncertainties, and controls for temperature, conditioning and feed/instrument drift. For independent, nonzero X and Y, let $r={\displaystyle \frac{X}{Y}}$. Its relative standard uncertainty is approximately ${\displaystyle \frac{u_{\mathrm{r}}}{\left|r\right|}}\approx \sqrt{{\left({\displaystyle \frac{u_{\mathrm{X}}}{X}}\right)}^2+{\left({\displaystyle \frac{u_{\mathrm{Y}}}{Y}}\right)}^2}$, where u denotes standard uncertainty; correlated baselines require covariance. Values above 100% may reflect uncertainty or structural change. Reported FRR variation, such as 95.5 ± 4.1% [58] and 97 ± 3% [163], is retained. Uncertainty is unavailable for the rounded 2.70/1.70-L comparison [60], so calculated 63.0% is not more precise than reported 62.9%. Activity-only reuse [135] cannot establish joint recovery.

Table 4 operating conditions. NE means that a condition could not be established from the extracted information. A statement that the original publication did not report it would require source verification; no such omission is inferred from NE. The 10-cm gravity head in clean-water characterization [5] is not assigned to the mixed-feed recovery run. Feeds, exposure times and cycle definitions are retained, but incomplete pressure/head, area and temperature records prevent matched permeance, area-normalized capacity or energy-per-useful-volume rankings.

Joint assessment exposes trade-offs concealed by a single recovery value. Mixed-feed retention with incomplete rinse FRR [5] differs from paired oil/FRR recovery over ten assisted cycles [61]. Antibiotic-film renewal retains removal but reduces permeance from 3815 to 2615 L m⁻² h⁻¹ bar⁻¹: 68.5% retention and, conditionally at matched viscosity/driving force, 45.9% higher total resistance [1]. This calculated resistance change does not identify its structural cause. Arsenate desorption restores 62.9% of useful volume [60], while fewer attached grains after photocatalytic reuse suggest unquantified material loss [135]. Different feeds, baselines and missing retained-mass data prevent an overall durability ranking.

Complex feeds challenge the selectivity and accessibility established with model solutions. Structured antifouling coatings provide controlled multicomponent challenges [50]. Titania-nanoflower/calcium-bentonite hollow fibers reported greater than 90% FRR during the first five UV-assisted cycles with real refinery-produced water [63], while BiOI-coated ceramic nanofiltration was examined with semiconductor wastewater [64]. MoS₂/intercalant channels provide a distinct selective architecture for comparison [166]. These tests must retain the identity and pretreatment of each matrix when used to support real-feed durability. Figure 9a shows Cr(VI) capture during vacuum filtration through chitosan-coated iron-oxide nanoparticles immobilized on PVDF [167]. Figure 9b follows removal against solution pH in the same material system, reaching 90.45% at pH 4 and decreasing at both higher pH and pH 2. The nonmonotonic response reflects changes in protonation, oxide surface charge and Cr(VI) speciation; stronger acidity does not simply guarantee better capture. Figure 9c tests competing anions at 1 and 10 mg L⁻¹, with phosphate causing a particularly strong reduction in removal relative to the control. Figure 9d shows removal above approximately 95% in the tested tap, drinking and river waters. These results demonstrate capture from the tested spiked water matrices while identifying a competing-ion vulnerability; they do not establish long-term operation with untreated industrial wastewater. The high values in Figure 9d do not contradict Figure 9c: a water sample challenges the coating only with the constituents and concentrations present in that sample. Feed robustness is established by combining representative waters with controlled tests of the solutes most likely to disrupt the binding mechanism.

Figure 9. Feed-dependent capture and protection against pore wetting. (a) Vacuum-filtration capture of Cr(VI) on chitosan-coated iron-oxide/PVDF, (b) solution-pH dependence of Cr(VI) removal, (c) interference by competing anions and (d) removal from the tested spiked water matrices [167]. (e) Surfactant-induced wetting of hydrophobic PTFE and protection by a hydrophilic polyurethane coating during membrane distillation [65]. Adsorption-site competition and vapor-pore wetting represent different feed-dependent failure mechanisms.

Figure 9e addresses a different vulnerability: surfactants can wet the gas-filled pores of a hydrophobic PTFE membrane during distillation and allow salt-bearing liquid to enter the recovered stream [65]. The study proposes that a hydrophilic polyurethane coating reduces surfactant-induced wetting of the underlying hydrophobic PTFE while retaining a vapor-transport architecture. This protective function must be judged through vapor flux and distillate contamination, rather than liquid-filtration FRR. The design implication differs from Figure 9a–d: liquid-filtration coatings require water access to reactive sites, whereas distillation coatings must preserve the barrier against liquid entry. PDA/MOF ultrafiltration challenged by algal water, titania/zirconia-coated mullite, PDA/PEI/MOF-coated PVC for shale-gas wastewater and substrate-independent coatings add further feed–support combinations [168,169,170,171]. Activated-carbon precoating supplies a process-level sorption/protection function [7], while slippery polypropylene and electrospun TiO₂ address distillation wetting and micropollutant conversion, respectively [66,172]. Across these processes, durability is the persistence of the intended transport and removal mechanism under the relevant matrix. Matched measurements before loading, after loading and after intervention then separate feed-induced performance changes from their reversibility. Post-cycle permeance against the original baseline detects cumulative hydraulic drift; repeated breakthrough or retained conversion diagnoses chemical-function recovery.

Cumulative damage requires a shared record of operating time, useful volume, cleaning events and chemical/irradiation exposure. At common checkpoints, pair reference-temperature permeance and effluent trajectories with coating/active-mass or validated-marker balances, chemistry and release; include cleaning-only controls and contaminants exported in regenerants/solids. Antibiotic reconstruction quantifies hydraulic decline despite preserved removal [1], whereas TA/Fe(III)/PAN records activity loss without paired hydraulic recovery [135]. Neither supplies a complete lifetime mass balance.

6.2. Useful Treated Volume and the Cost of Regeneration

Cumulative useful water production relates these cycle measurements to treatment service. A pilot catalytic ceramic membrane processed 1200 L of hospital wastewater at a reported permeance of 150 L m⁻² h⁻¹ bar⁻¹, and separately reported greater than 97% removal of monitored emerging contaminants over a 168-h stability test [8]. The pilot throughput and stability record should retain their respective operating conditions rather than be combined into an assumed single run. Its catalyst was confined within ceramic nanopores rather than deposited only as a water-facing film; this is evidence of continuous real-feed performance rather than of repeated coating reconstruction. Upcycled membrane-distillation treatment of anaerobic-membrane-bioreactor effluent provides another process example with vapor transport [173]. NiFe-LDH membranes reported 93–100% FRR over six recovery cycles under a visible-light/PMS protocol, while a PEDOT:PSS/bimetallic-MOF/PVDF interface reported As(III) uptake and repeated rejection after alkaline regeneration [134,174]. These records become comparable within their intended application when useful volume is defined by an explicit effluent limit and paired with recovery inputs. Figure 10 illustrates how such a definition changes the interpretation of an adsorption membrane: the outlet trajectory and post-desorption run length establish how much treated water remains acceptable before the next intervention [60].

Figure 10. Structure, binding and functional recovery of PEDOT: PSS/Fe–Mn-oxide/PVDF [60]. (a) Compared formulations and proposed binding pathway, (b) concentration-dependent As(V) breakthrough, (c) alkaline desorption and re-adsorption, (d) surface topography, (e) equilibrium isotherms, (f) pseudo-first-order and (g) pseudo-second-order kinetic representations. The 10 µg L⁻¹ outlet criterion defines useful treated volume; equilibrium capacity and kinetic fits provide complementary material characterization.

Quantitative comparisons require transport mode, pressure/gravity head, wetted area, temperature, feed/pretreatment and duration or volume. Pressure-driven tests additionally require configuration and water recovery; distillation requires both stream temperatures and vapor-pressure driving force. The 14.8 m h⁻¹ gravity flux [14], 571.52 L m⁻² h⁻¹ bar⁻¹ permeance [15] and distillation response [65] are therefore different process measures, not a common ranking.

Figure 10a compares PVDF, PEDOT: PSS/PVDF, Fe–Mn-oxide/PVDF and their combined formulation, relating dispersion/wetting to metal-hydroxyl sites [60]. Surface topography (Figure 10d) and 27.33 m²g⁻¹ specific area support increased area, not binding chemistry alone. Figure 10e reports a Langmuir maximum of 30.43 mg g⁻¹ versus 0.21 mg g⁻¹ for PVDF. Figure 10f,g compares pseudo-first- and pseudo-second-order fits, with uptake approaching equilibrium after approximately 12 h. These empirical fits describe kinetics; separate XPS/EXAFS evidence supports ligand exchange and inner-sphere Fe/Mn–O–As complexation.

The dynamic panels establish the regeneration conclusion more directly than equilibrium capacity. Figure 10b shows volumes of approximately 6.00, 2.70 and 1.70 L below a 10 µg L⁻¹ outlet limit at feed concentrations of 50, 100 and 150 µg L⁻¹, respectively [60]. Increasing the feed concentration shortens the acceptable run because the available sites receive contaminant mass more rapidly. Figure 10c shows adsorption, alkaline desorption and re-adsorption at the same 100 µg L⁻¹ feed: after pH-11 treatment, useful volume falls from 2.70 to 1.70 L. The source reports 62.9% recovery, consistent with the rounded volume ratio of 1.70/2.70 ≈ 63.0%. This is partial recovery of threshold-defined treated volume, not clean-water flux recovery or a direct measurement of equilibrium adsorption-capacity retention. A single high removal percentage immediately after regeneration would obscure this reduction. Embedded La/Fe/PAN comparator studies treated 535 mL at 0.5 mL min⁻¹ and 1375 mL at 2 mL min⁻¹ from 50 µg L⁻¹ As(III) feeds below the same nominal outlet limit using 25-mm-diameter membranes [112,175]. These As(III) tests differ in target speciation from the As(V) coating example. Their differing loading, preparation and residence time preclude direct capacity ranking. Figure 10 establishes the appropriate comparison within a membrane system: repeat the same feed, area, flow conditions and outlet criterion, then measure the usable interval restored by regeneration. These breakthrough volumes are experimental treatment outcomes; they do not measure cost savings or an economically validated service lifetime.

Regeneration cost per cumulative useful volume should include pumping/backwash, light, electrical and thermal energy; cleaning, neutralization and recoating reagents; rinse water; labor, downtime, coating/support replacement, concentrate handling and disposal. Use equivalent effluent criteria and time horizons, annualize equipment/replacement consistently, and avoid double-counting lost production. Experimentally measured flows, acceptable run volumes, regeneration inputs and downtime should be reported separately from assumed tariffs, labor requirements, lifetimes and scale factors. Economic or environmental benefits calculated from these assumptions remain scenario projections until supported by matched process measurements, including secondary-waste management. Sensitivity analysis should identify which assumptions control the projected benefit. The Janus estimate of 0.01 kWh m⁻³ [130] is source-specific; without long-duration useful-volume and waste-treatment data, only scenario-based cost comparisons are justified.

Real wastewater changes both useful output and maintenance demand. Competing ions occupy sites; organic matter/solids mask catalysts and pores; biofilms and variable pH, salinity or oil/surfactant loading alter recovery. Pilot records should include pretreatment, feed variability, reject/regenerant quality and downtime. The hospital [8] and refinery [63] studies improve application relevance but do not establish a common cost basis. Desorption yields concentrated liquids, oxidation can release products/metals or residual oxidant, and stripping creates contaminated solids/solvent. Quantify these streams and their treatment per useful volume before claiming environmental benefit.

Scale-up must preserve access to the recovery mechanism throughout modules. Turbidity attenuates light; current, backwash delivery and coating thickness may be nonuniform. Relate spatial coverage and release to pressure loss, effluent quality and intervention frequency, and assess required pretreatment or module redesign. Wastewater-to-energy studies [9,10] reinforce system-level accounting of net output and maintenance, but their numerical lifetimes and costs cannot be transferred to functional coatings.

7. Conclusions

Functional coatings improve treatment through hydration-mediated fouling resistance, selective pore/charge environments, adsorption and catalytic conversion. Their durability depends on accessible pathways and active sites, with attachment compatible with recovery. The reviewed evidence links these structural choices to distinct failure signatures and shows why water production, pollutant-specific removal and retained coating function must be evaluated over the same history.

The integrated design–diagnosis–recovery framework translates these relationships into a practical maintenance sequence. It links the limiting hydraulic or pollutant-removal function and the coating/support condition to a compatible intervention, then requires verification of water quality and useful throughput in the next run. This sequence provides a basis for application-specific cleaning or renewal triggers and identifies the evidence needed to justify maintenance choices. Its effects on service lifetime and lifecycle cost require validation in prospective operating studies.

Physical cleaning is appropriate for detachable deposits; desorption for occupied, chemically intact sites; assisted cleaning for persistent foulants within a compatible dose window; and renewal for an unrecoverable surface film on a sound support. Renewal cannot repair a damaged selective barrier. Different feeds, transport modes, baselines and cycle definitions, together with incomplete retained-mass, product, uncertainty and long-duration records, limit cross-study ranking. Useful volume at a common effluent target provides a functional basis for comparison.

Near-term priorities are matched coated/support/cleaned controls, independent replicates and paired permeance–removal measurements, followed by material balances that distinguish masking, exhaustion, chemical change and release. Combined real-feed and maintenance challenges should establish operating windows and intervention triggers while quantifying products and spent regenerants. Longer-term priorities are uniform deposition and recovery access inside modules, extended variable-feed pilots, and measured downtime and support integrity. Lifecycle comparisons should then combine energy, chemicals, rinse water, replacement layers and waste management per useful volume against equivalent treatment alternatives.