Application of AFM in Membrane Fouling: Comparison
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Membrane separation technology has emerged as the preferred method for producing clean water during wastewater treatment and desalination. This preference is attributed to the high separation accuracy, energy efficiency, lack of secondary pollution, and ease of operation of the technology. Membrane fouling is a key obstacle in membrane applications, including ultrafiltration (UF), microfiltration (MF), nanofiltration (NF), and reverse osmosis (RO). Membrane fouling is a particularly serious problem in the pre-treatment processes of industrial wastewater, leading to poor water quality and increased operating costs. A thorough understanding of fouling formation and properties is required in wastewater treatment using membranes and contributes to slowing down membrane fouling and implementing appropriate control measures. In response, extensive foundational investigations of membrane fouling have been conducted, with researchers seeking to clarify primary foulants, membrane–foulant interactions, and potential fouling mitigation techniques.

  • membrane fouling
  • morphology
  • roughness
  • interactions

1. ModCharacterificzation of Probes for Membrane Fouling Characterization

FThe interom the initial 10-nanometer resolution to the current sub-angstrom level, researchers have enhanced the resoluaction between the membranes and foulants is related to the membrane surface properties, such as hydrophilicity, roughness, and charge. The development of high-quality antifouling membranes could reduce the interaction forces between foulants and membrane surfaces and decelerate membrane fouling. The technique facilitates the visualization of membrane surfaces at high resolution, characterizing three-dimensional presentation of microscopes. This was achieved by optimembrane surface information, allowing for an exhaustive detailed expression of the surface characteristics of the membrane. 

1.1. Characterization of Membrane Morphology

Visualizing the scanning probes and increasing the AFM scanning speeds by enlargation of the membrane surface morphology aids in understanding the relevant properties of the membrane. AFM excels in visualizing morphological features, allowing for the in situ characterization of morphological changes occurring at the interface of functional layers on membrane surfaces during the scanning head sizes and using higher resonance frequencies. Continuous advancements have alsopolymerization process. During the phase inversion process, utilizing AFM to scan nanofiltration or reverse osmosis membranes prepared under various parameters in a liquid environment enables the in situ observation of more detailed and systematic changes in the surface functional layers [1]. Furendered AFM more sensitive for mechanical detectther, AFM can be used to investigate the degradation effects of soil microbial communities on polyethylene membranes by observing changes in the microstructure of the membrane [2]. Usion, facilitating the deterg AFM, it is also possible to observe the ion transport channels of membranes. Examination of the local mechanical properties of materials at a nanoscale. Thesemodified anion exchange membrane (AEM) materials with densely grafted ionic clusters through atomic force microscopy can reveal distinct ion conduction pathways and demonstrate that the modified AEMs exhibit excellent nano-phase separation [3]. eBy studyinhancements rely mainly on cutting-edg the surfaces of nanofiltration membranes using AFM in different imaging modes, various AFM imaging mode characteristics can be obtained [4]. The tapping mode AFM probes. The AFM probe tip is a critical componentin AFM allows for precise measurement of the 3D structure of soft and delicate surfaces without damaging their morphology. This technique is particularly useful for studying the interfacial polymerization process of active functional layers on nanofiltration membrane surfaces [5].
Evidently, AFM cand its performance directly influences the precision and reliability of AFM be used to understand membrane layer surface smoothness and uniformity; capture membrane surface microscopic morphology, including surface defects, nanoscale protrusions, or depressions; and provide intuitive images of membrane surface morphology. Thus, AFM is not only capable of in situ measurements. Three primary methods are used for preparing colloidal probes to investigate of surface morphology changes induced by hydrochemical conditions, but also enables the understanding and discovery of ion transport channels and nanoscale morphologies through 3D visualization images. Particularly, the tapping mode is almost non-destructive to thin and soft membrane fouling. These are attaching modified contaminant particles to the probe tip for force measurfaces. Additionally, AFM can be used to study the surface potential signals of membrane materials, overlaying them with the physicochemical properties of specific areas. These aspects are crucial for characterizing membrane performance and understanding membrane fabrication.

1.2. Characterization of Roughness

The surfacement, directly modifying the contaminants, and using an adhesive to adsorb the contaminants for measurement. The research team utilized bioadhesives roughness of a membrane is a key factor influencing interfacial performance and fouling processes. Using AFM not only allows for the observation of a membrane’s surface morphology, but due to its three-dimensional measuring capabilities along the x, y, and z axes, AFM can also precisely characterize in situ the roughness of the membrane surface/functional layer and provide detailed 3D surface topography maps. AFM can be used to understand the roughness of various types of membranes, such as dopamine to directly attach contaminants to the AFM probe tip, resulting in a colloidalcation exchange membranes. During long-term operation, AFM can precisely measure changes in roughness, thereby establishing the relationship between roughness and ion exchange performance. This enhances the monitoring of the ion exchange effectiveness and contamination level of cation exchange membranes [6].
In the process of membe for AFM force measurements. The foulant colloidal probes were prepared using AFMrane modification, the incorporation of specific active components, such as surfactants or polymer monomers, in the adsorption crosslinking process can increase surface roughness and alter the structure of the membrane’s functional layer. Similarly, modifications with the Cypher ES. The organic foulants microspheres came in powder form, were ground using a ball mill (leading to a particle size of approximately 0.2 μm)carbon nanotubes, metal oxides, and other substances can change the membrane’s morphology, increase roughness, and expand membrane channels, thereby altering the membrane’s performance. Using AFM, changes in membrane surfaces and channels can be observed, and the roughness can be accurately measured. Roughness serves as an important parameter in membrane fabrication. The results generally show that the surface of the original membrane is low in roughness, uniform, and then filled uniformly into microplates (pore size of 5 μm). The photosensitive glue (A332) was filled evenly on the other sidesmooth. When modifiers or carbon nanotubes are added, nanoscale modified structures are formed, increasing the roughness. This enhancement in roughness can improve the membrane’s anti-fouling properties, permeation evaporation performance, ion selectivity, and regulate the membrane’s hydrophilicity or hydrophobicity. Asymmetric polystyrene membranes manufactured using the wet phase inversion method with the addition of surfactant (Pluronic F127) increase the surface roughness of the microplate. Then,embrane and strengthen the microplate was fixed on the AFM platform. The cembrane channels, significantly enhancing the permeation evaporation performance of the membrane [7]. Similarly, analysis of multilever was lowered-walled carbon nanotube (MWCNT) dispersed PS nanofiltration membranes [8] using AFM tsho adhere the glue, then lifted it to the other side to adhere the organic foulants. The adhered needle tip was irradiated with a UV lamp forws that the surface of the original PS membrane is smooth and uniform, while the addition of MWCNTs increases the surface roughness and makes the structure more evident, thus improving the hydrogen permeability of the PS membrane. Roughness measurements using AFM reveal that plasma treatment and surface acidification also increase the roughness of ion exchange membranes, enabling more ion exchange and facilitating the preparation of ion exchange membranes with excellent performance 30[9].
However, studies show to achieve quantitativhat for rough surfaces, nanoscale modification with organic foulants. This method decreases the contact area to approximately 5 μm, leading to more accurate and reliable measurement results.
ed structures have better tendencies to prevent membrane fouling, but when the modified structures are too large, they can exacerbate membrane fouling. These related insights can be obtained through the precise measurement of roughness using AFM. In addition, modifying the membrane surface through copolymerization and grafting methods can increase membrane roughness, and AFM cany researchers have made efforts in modifying colloidal probes. Fl measure roughness changes during the membrane modification process in situ. When characterizing hydrophilic polymer-functionalized polysulfone (PSF) blend membranes using AFM [10], researchers found that the addischmanntion of 4VP side chains enhanced [1]the surfirst employed AFM to quantitativelyace roughness of the modified membrane. In another study, PSF membranes modified with titanium oxide compounds [11] hadefine the 3D shape of atomic probe tips, opening new possibilities for higher surface roughness, and these modified membranes exhibited excellent hydrophilicity and anti-fouling properties. Simple coding of the surfactants polydopamine and 3-(N, N-dimethyl myristoyl) propane sulfonate [12] can achieve santudying the physical mechanisms in (laser-assisted) atomic probes. Owing to the complexity of the i-fouling properties of polyethersulfone (PES) ultrafiltration membranes. Scanning these modified membranes with AFM can obtain roughness parameters Rq and Ra, indicating that the modified membranes significantly mitigate flux decline and enhance anti-fouling performance.
Precise meamplsurement of membrane surface morphology and composition,roughness with AFM can be used to explore which roughness is more resistant to contamination on the modified AFM probe tips with varying surface chemical affinities could enhance selmembrane surface, thus achieving membrane performance adjustment. Although many studies have shown that increased roughness may lead to increased tendencies for membrane fouling [6][10], other researctivity, ensuring more accurate and precise measurements inh findings suggest that adding micrometer to nanometer-sized particles to increase surface roughness (similar to lotus leaf biomimetic structures) can reduce membrane fouling [8][9]. This discrepecific applications, utilizing AFM in conjunction with custom-modifiedancy is mainly because a single roughness parameter is insufficient to summarize the complexity of membrane-coated and HA-coated probes to assess the adhesion forces between surface fouling. Establishing a relationship between roughness R measured with AFM and the roughness index H can more quickly and accurately evaluate membrane-HA and surface roughness HA-HA [213]. In additially, this demonstrated thon, an overall assessment should combine AFM with various other techniques to carefully examine membrane surface characteristics, such as surface potential application of modified AFM colloidal probe microinterface force measurements for UF, hydrophilicity/hydrophobicity, functional groups, and foulant properties. Through this comprehensive judgment of characterization results, the relationship between surface roughness and membrane fouling behavior. Additionally, the AFM tip can be modifican be thoroughly analyzed. Establishing the relationship between membrane surface roughness measured by incorporating different representative organicAFM and membrane surface potential, functional groups, [3]etc., canamely benzyl, hexyl, propionic acid, ethylamine hydrochloride, and propionic better help optimize membrane hydrophilicity/hydrophobicity, permeation selectivity, ion selectivity, and anti-fouling properties, providing guidance for the design and optimization of membrane interfaces.

1.3. Measurement of Membrane Channels

Membrane chacid propyl ester. These authors measured the adhesion forces between the nnels are crucial for the filtration performance of membranes, as the size and structure of these channels directly influence the membrane’s selectivity and permeation flux, which relates to the trade-off effect of the membrane. AFM has been utilized to detect various surface parameters of modified AFM tips and revmembranes, including the structure and pore size of membrane channels. These researchers [14][15][16][17][18][19] characterized the se osmosisurfaces of modified membranes to determine the potential scaling tendency of eausing AFM and found that the deposition process of modifiers made the membrane surfaces smoother, eliminating small-scale rough features, reducing pore sizes, and decreasing sensitivity to foulants. For instance, Kim et al. [15] achieved fatomic-level surface functional group category on the membrane. To ization of nanofiltration membranes using graphene oxide (GO) combined with plasma-enhance the accuracy ofd atomic layer deposition (ALD) technology. A novel data analysis method [14] integrating AFM fwith “pore recorce measurement data, Nguyen et al.nstruction technology” was used to assess membrane channel structures, including size, shape, and interlayer distances. The obtained membrane channel information is vital for [4]the pemployed four distinct AFM probes to gauge the nanomechanical properties of three different samplesrmeation selection process in membrane desalination and can also serve as a crucial factor in assessing the propensity for membrane fouling. AFM can precisely measure the interlayer distances of zinc oxide-coated aluminum membrane channels modified by deposition methods [16], providing valuable insights for probe selection for better interpretentify the membrane channel structures of superhydrophilic copper mesh membranes coated with zinc oxide nanostructures (ZnO NW) used for oil–water separation of[17], foacquirce indente information data. Furthermore, the use ofabout the shape of membrane channels in modified tips broadens the applicability of AFM measurements.seawater desalination nanofiltration membranes created using molecular layer deposition (MLD) techniques By[18], atnd obtaching a mineral particle to a tipless AFM cantilever, ain 3D shape information of membrane channels in chitosan and polystyrene sulfonate-modified polyamide microfiltration membranes prepared by layer-by-layer (LBL) deposition methods [19].
Combineral probe for AFM measurements can be created and, afterward, applied an atomic force microscope equipped with pyrite or chalcopyrite tips to investigate the adhesiing the surface and cross-sectional images obtained from AFM can construct three-dimensional images of membrane channels, providing detailed information on the size, shape, and arrangement of channels, and more accurately predicting the degree of membrane channel clogging and membrane fouling. Studies indicate that uniformly distributed pore structures, as opposed to uneven distributions, are likely to reduce the risk of fouling. The systematic distribution of thermophilic thiosulfpores in uniform membranes can enhance the interception capacity for foulants [20]. Notably, te-oxidizing bacteriahe geometric shape of membrane channels greatly influences membrane [5]fouling. Typically, the modified AFM tips signfouling intensity caused by slit-shaped pores is lower than that caused by circular pores [21]. Addifticantly enhanceonally, AFM can accurately present the degree of membrane channel clogging and the accuracy and reliability of the AFM state of membrane fouling in real liquid-phase environments. In contrast, SEM requires measurements, reduced the probe replacement frequency, and rendered them suitable for a more extensive range under dry and vacuum conditions, which may lead to distorted results and inaccurate pore information. Therefore, membrane channel data obtained through AFM are crucial for tracking membrane fouling trajectories and assessing and improving membrane performance.
As previof applications. Modified colloidal probes could achieve in situ measurements ously mentioned, employing AFM to characterize different membranes allows for a more comprehensive observation of three-dimensional surface morphology, membrane roughness measurements, membrane channel assessments, and an overall evaluation of the interaction forces betweenmaterial characteristics and application performance of membranes and foulantmodified membranes. As depicted in Figure 1, this usectionder varying ion concentr categorizes and summarizes the different modes, characteristics, and outcomes of modified membrane characterization conditions. This ability provides a valuable resvia AFM as referenced in the mentioned literature. AFM has also been extensively applied in characterizing modified membranes of various materials, such as modified Langmuir–Blodgett (LB) thin films [22], uniquely sharch meped modified block copolymer microfiltration membranes [23], zeolite-filled polyethersulfone membranes [24], modified for measuCarbosep M5 ceramic membranes [25], innovative positively charged nanofiltratinon membranes [26], organic membrane–foulant ins for oil–water separation [27], and composite ceractions in wamic microfiltration membranes for greywater treatment [28]. This tewater treatmentchnique (atomic force microscopy) has become a powerful tool in the design and fabrication of functional membranes.
Membranes 14 00035 g005
Figure 1. Different aspects and results of AFM characterization of modified membranes in different modes.

2. InvCharactestigating Membrane Fouling Process by Coupling AFM with Other Functional Modulerization of Contaminants

WhDifferen studyingt types of membrane fouling processes, the liquid module of AFM could be used to change the solution environment and conduct in situ AFM measurementsants are encountered in membrane-based technologies and in other techniques; therefore, employing AFM for scrutinizing the characteristics of contaminants at a microscopic level is vital. The nanometer-scale resolution of AFM enables direct observation of the membranecontaminant morphology and structure on membrane pollutant interaction forcesurfaces. Accordingly, AFM could be used to monitor the adsorption and adhesion of contaminants in real-time under various environmental conditions. Moreover, AFM encompasses various functional modules that could be employed to investigatephological changes in living microorganisms during metabolism could be recorded in tapping or non-contact modes, which is a challenge for other techniques. Employing the capabilities of AFM in this way enhances understanding of membrane fouling phenomena under diverse conditions.
Gemechanisms. Characterizing contaminants facilitates superior understanding of membrane fouling principles, and offerally, researchers use an open module to examine the s essential guidance for the design, operation, and maintenance of membrane filtration systems. This is an important factor in the investigation of membrane fouling processmechanisms.

2.1. Organic Contaminants

Natural organin an air environment. However, given the complexc matter (NOM) is the primary contaminant in wastewater. It is a complex heterogeneous system comprising diverse organic molecules [29], such as humic substy ofances, polysaccharides, and proteins, which can all affect the membrane−fouling performance. Observations using AFM in aquatic environments, AFM could be integrated have revealed that natural polysaccharide sodium alginate (SA) predominantly exists as single helical chains, with multiple fundiameters of approximately 0.2–0.3 nm [30]. Sctannional modules to conduct researchng humic acid sodium (HA)-contaminated mica surfaces with AFM has uncovered spherical particles and aggregates, featuring colloidal diameters under vario100 nm and heights from 0.5 to 7 nm [31]. In studies environmental conditions. The chemical envion protein membrane fouling, most protein molecules have been observed as monomers on mica surfaces [32]. Extracellular onment of a solution is crucial irganic matter (EOM) can lead to severe ultrafiltration membrane fouling. Coupling AFM with a liquid cell modulAFM enables the observation of the aggregation and blockage behaviors of pollutants on the membrane surface [33], and envables researchers to performluates the effects of cleaning/pre-treatment [34]. Utilizing AFM mteasurements in water and other solvents, which is important for studyingchnology aids in further understanding the impact of natural organic matter (NOM) on membrane performance during water treatment processes, thereby laying the foundation for mitigating organic membrane fouling.

2.2. Biological Contaminants

In addin actual treatment tion to typical organic contaminants, biological contaminants can impair membrane treatment efficiency in water treatment processes. UEscherichia coli isi a commong a micropump, chemical s pathogenic microorganism that compromises the safety of water resources and drinking water. Researchers have used AFM to investigate the morphological changes in E. coli oln membrane surfaces under varying pH conditions [35] and correlate it with membraltered cone filtration and cleaning [36]. In aditions (e.g., pH, concentration,dition, AFM has been used to examine changes in the morphology of antibiotic-resistant E. coli on membrand ion concentration)e surfaces during photocatalytic Fenton water treatment [37]. Recan be introduced into the system, allowing real-time AFM monitoring of the ently, owing to the potential problem of microalgae in water treatment processes, particularly in membrane treatment, AFM has been applied extensively to study microalgal cell morphology and nanomechanical properties on membrane surface morphology ans. High-speed atomic force microscopy (HS-AFM) has been employed to analyze Chlorella vulgaris treated with electrougocoagulation flotation (ECF) [38]. Anothness changes ar study used AFM to determine the energy required to disrupt individual microalgae cells th[39]. Guidance could be offered solution conditions varyfor alleviating biological fouling caused by microalgae. For the living microbial cells, AFM-based single-cell force spectroscopy (AFM-SCFS) has significant value for [6].characterizing Tthis approach facile structure, mechanical properties, and molecular activity of individual living microbial cells [40]. The technique can measure tates simulation of actual water treatmenthe mechanical properties of a single microorganism, quantify individual microorganism adhesion forces, and perform structural imaging of microbial behavior while simultaneously sensing microbial activity in real-time. Wang et al. [41] employed AFM to explore the dynviamic effects of various environments, enabling dynamic observatial factors on microorganisms and membrane surface interactions at a molecular scale. This provides a research basis for the effective inhibition of biological foulants on membrane csurfaces.
AFM-SFCS allows senditions as the sitive measurements of the mechanical properties of the chemical solution change individual molecules. This allows researchers to gain insight into the mechanical properties of individual molecules such as stretching, deformation, and provides more precise in situ observations of contaminant adsorption and attachment processes on the membrane surfacefracture, which is important for understanding the properties of biomolecules, polymers, and other materials. Nevertheless, AFM-SCFS has not reached maturity yet and still presents several technical challenges. Based on our group’s research on AFM-SFCS in the environmental field, we found that this technology faces the following problems in its application. First, the adhesion of live single cells to the probe tip is difficult and requires the selection of suitable adhesives for cell immobilization.
Additionally, assessing the add-on FAST module can independently acquire probe signaviability of single cells on the probe tip after attachment is challenging, prompting researchers to explore more advanced methods for examining post-adhesion cell viability. The morphology of single cells [7]is and can be installed without modifyiot consistent; it encompasses rod or spherical shapes and other irregular shapes. During the existing scanner hardware or electronic equipment. This module facilitates seamless switching between fast and slow scanning modes, contributing to a clearer and more comprehensive understanding ofadhesion process, it is crucial to consider different adhesion positions and variations in contact areas with the measurement surface to prevent inconsistencies in the recorded force magnitudes. Furthermore, even when live cells successfully adhere, it is difficult to maintain consistent single-cell activity at the probe tip (considering the different activity levels of young and aged cells at various stages). Ongoing investigation and refinement of AFM-SCFS techniques is anticipated to address these issues in the near future. Researchers could, therefore, gain better understanding of the characteristics of biological contaminants using AFM technology, further elucidate the membrane fouling process, guide biofouling removal, and offer theoretical support and practical guidance for the development of long-lasting antifouling membrane materials and superior biofouling. By incorporati control strategies.

2.3. Emerging Contaminants

Emerging contan electrochemical moduleminants in wastewater treatment processes, such as microplastics, antibiotics, and endocrine-disrupting compounds (EDCs) have been [8],attracting incresearchers can observe electrochemasing academic attention at national and international levels. As a high-resolution tool, AFM enables a more detailed examination of the physical processeperties of microplastics [42]. For in the stance, Melo-Agustín et al. [43] employembraned AFM for morphological analysis of microplastic surface, which is invaluable for examining chemical reactions as, discovering that polyethylene (PE) microplastic surfaces exhibit higher levels of roughness than polypropylene (PP) microplastic surfaces. This observation suggests that PE is more susceptible to degradation than PP, potentially leading to greater contaminant adsorption. Chen et al. [44] introduced ion migration processes during membrane fouling. Temperature significantly affects thea method that combines AFM with infrared spectroscopy (AFM-IR) to characterize nanoplastics (NPs). This hybrid AFM technique can identify and image the chemical composition of nanoplastics at a high spatial resolution (20–100 nm), thereby offering a novel approach to NP characterization. However, the large specific surface area of microplastics often causes them to function as ‘carriers’ of other contaminant adsorption and attachs during water treatment processes on the , which exacerbates pollution. For instance, Zhang et al. [45] employembrane surfad AFM to determine the interaction forces between NPs (hematite and corundum) and Escherichia coli cells, with distinct gaining further understanding of the membrane fouling characmechanism of microplastics.
Additionally, antibiotics areristics and interaction mechanisms between contaminants and frequently occurring emerging pollutants in aquatic environments, and even at trace concentrations, antibiotics in wastewater can adversely affect human health. AFM can effectively characterize the morphology and interaction forces of antibiotics on the membrane surface at different temperatures. Integrating hi, thereby enhancing the efficiency of membranes in intercepting them. For instance, Liu et al. [46] used AFM to investigate th- and low-temperature modules with AFM enables measurements une adsorption of EDCs on nanofiltration membrane surfaces, subsequently enhancing the EDC removal rate by preparing modified nanofiltration membranes. Wu et al. [47] attached sulfamer various temperature conditions. These AFM modules offer numerousthoxazole (SMX), a representative antibiotic, to an AFM tip to measure the SMX adhesion force distribution. Their study revealed the adhesion mechanism of SMX and, potentially, that of other sulfonamide antibiotics at a molecular level from both experimental methods and conditions for rand theoretical viewpoints. Research on membrane fouling, contriers have also examined the impact of microplastics on antibiotic transport during sand filtration [48] buy grafting to a deeper understanding of the occurrence, development, and impact of membrane fouling. By applying these modules,ciprofloxacin (CIP) and sulfamethoxazole (SMX) onto AFM probes to determine the adhesion forces between representative microplastics (PS and PE) and quartz sand. Their study explored the mechanism of microplastics that enhances antibiotic transport in sand filtration systems from the perspective of molecular interactions. Table 1 summaresearchers can provide robust support for the improvement and development of membrane filtration technologieizes the entire literature on different aspects of AFM studies of different pollutants in this section. In summary, employing AFM to investigate contaminant morphology under various water treatment conditions contributes to a deeper understanding of the characteristics of the contaminants, which, in turn, could inform the removal of attendant membrane foulants.
Table 1. Summary of research on AFM applications in contaminants.
Research ContentAFM ModelCharacterization PropertiesResultsUsage PatternsReference
Sodium alginate (SA)Bruker AXS Multi-mode 8, Madison, WI, USAMorphologyAlginates exist in single coiled chainsContact mode[30]
Effect of Na+ on organic foulingCypher ES, Oxford Instruments Asylum Research, Abingdon, UKMorphology and interaction force//[49]
Effect of carboxyl and hydroxyl groups on adsorptive polysaccharide foulingCypher ES, Oxford Instruments Asylum Research, Abingdon, UKMorphologyTransformation from ‘egg box’ model to formation of network gel/[50]
Effects of –COOH and –NH2 on adsorptive polysaccharide foulingCypher ES, Oxford Instruments Asylum Research, Abingdon, UKMorphology and interaction forceIn pH range 4–6, adherence of polysaccharide fouling and its reversibility

depend on the functional groups		Tapping mode[51]
Effect of sodium and potassium on polysaccharide fouling on PVDF and graphene-oxide-modified PVDF membrane surfacesCypher ES, Oxford Instruments Asylum Research, Abingdon, UKInteraction forceSA fouling in Na+ condition more severe than that in K+ owing to higher attraction forces under identical ion strengthsTapping mode[52]
Humic acid (HA)Nanoscope IIIa SPM, Digital Instruments, Goleta, CA, USAMorphologySpherical particles and aggregates are found with apparent colloidal diameters < 100 nm and heights ranging from ~0.5 to ~7 nmTapping mode[31]
Effect of Na+ and Mg2+ on adsorptive humic acid foulingMultiMode 8.0 AFM (Bruker, Ettlingen, Germany)Interaction forceCations mainly affect HA fouling by controlling electrostatic and hydration forces of membrane–HA and HA–HAContact mode[53]
Effect of Ca2+ and Mg2+ on adsorptive humic acid foulingMultiMode 8.0 AFM (Bruker, Ettlingen, Germany)Interaction forceMitigation mechanisms differed for both ions/[54]
Bovine serum albumin (BSA)/MorphologyMost protein molecules are spread onto mica surface as monomersTapping mode[32]
Effect of chlorination and ozonation on adsorptive protein foulingMultiMode 8.0 atomic force microscope (AFM, Bruker, Ettlingen, Germany)Interaction forceBSA fouling definitively mitigated by pre-chlorination but enhanced by pre-ozonationContact mode[53]
Flagellar morphology of E. coli cultured at different pH conditionsNanowizard AFM (JPK Instrument, Berlin, Germany)MorphologyDifferences in flagellar morphology at different pH valuesContact mode[35]
E. coli under action of different disinfectantsDigital Instruments Veeco Metrology Group, Santa Barbara, CA, USAMorphologyDifferences in cell morphology under action of different disinfectantsTapping mode[36]
Changes in cell morphology of antibiotic-resistant E. coliAsylum Research Cypher AFM (Oxford Instruments, Abingdon, UK)MorphologyDamage to E. coli cells eventually leads to cell lysis/[37]
Different types of MPsAFM diMultiMode V (Veeco, San Jose, CA, USA)Morphology and roughnessDifferent types of MPs have different characteristics/[43]
Combined AFM and infrared spectroscopy IR (AFM-IR) characterization of MPs/Morphology and roughness//[44]
Forces between two NPs and E. coliAgilent 5500 AFM (Molecular Imaging, Phoenix, AZ, USA)Interaction forceParticle sizes of both hematite (α-FeO) and corundum (α-AlO) NPs significantly affected the strength of the adhesion forceContact mode[45]
Changes in hydrogel occurring when algae are present in the cultureHS-AFM, Bristol Nano Dynamics Ltd., Bristol, UKRoughnessRoughness on the algal flocs significantly more pronounced than in the hydrogel layerContact mode[38]
Clostridium perfringens treated by electrocoagulation floatation (ECF) methodAFM (Ntegra with Solaris platform, manufactured by NT MDT, Moscow, Russia)Interaction forceInefficiency of mechanical cell crushing processTapping mode[39]

3. PMicroscotential of AFM Coupled with Other Techniquepic Identification of Membrane Fouling Processes under Changing Factors

CThe previous text, respectively, intrombining AFM with other imaging and spectroscopic techniques could provide comprehensive information regardingduced the membrane and contaminants observed by AFM. However, membrane fouling is a complex fouling process influenced by multiple factors. Therefore, our research team conducted abundant research on the effects of changes in the ionic concentration, pH, and time on membrane fouling. Typically, as shown using AFM. By employing AFM, researchers can monitor the morphological changes in Table 1,contaminants AFMunder provides high-resolution surface morphology information and, when integrated with scanning electron microscopy (SEM) or transmissivarious environmental conditions, facilitating real-time observation of the adsorption process on membrane surfaces. Once contaminants are adsorbed, alterations in environmental factors (ionic conditions, pH, membrane surface properties, and time) could cause varying fouling morphologies and characteristics compared with their initial states. 
Ionic electron microscopy (TEM),conditions: Our research team SEM[49][50][51][52] anused TEM offer structural and elemental composition information, resulting in more comprehensiveAFM to study the effects of different valence ions on membrane fouling of NOMs. Using AFM force measurements, morphology characterization, [9][10].and Combined with fluther technical methods, the effect of monovalent ions such as Na+ and K+ on orescence spectroscopy, chemganic compounds was found to be based on their charge and structure. However, the effect of divalent ions such as Ca2+ and Mg2+ on organical composition information is provided [11]unds also included complexation. Among them, which is useful for studying the propertit is closely related to the special functional groups, types, and structures of biolNOMs. Miao et al. [55][56] employed AFM to investigicate the effects of Nal+, Mg2+, and Ca2+ on HA fouling ther organicrough HA membranes. Integration with Fourier-transform infr fouling experiments. These authors observed that membrane fouling intensified at lower Ca2+ or Mg2+ concentrations and spectroscopy (FTIR)ignificantly decreased at substantially higher [12]Ca2+ por Mg2+ codunces information on chemical comntrations, albeit with the two ions having different mechanisms.
poH: We invesition, which enables in situ anatigated changes in membrane fouling under different pH conditions using AFM [51]. The resulytsis of the molecular structure, bonding, and distribu showed that at a pH range of 4–6, the adherence of polysaccharide fouling, and its reversibility, depended on the functional groups. When the organics were rich in –COOH, an increase in pH reduced their deposition on the membrane surface, and further reveals the and alleviated adsorptive fouling and irreversibility. For the –NH2 funchemtical characteristics and mechanisms of membrane fouling. Couponal group, an increase in pH led to more severe polysaccharide fouling owing to a lower degree of protonation, and the resulting fouling with X-ray diffraction (XRD)as highly irreversible. Modification using [13]GO proallevides informated the adsorptive fouling of these two polysaccharides on PVDF; however, the extent of alleviation depended on the crystalline propabundance of functional groups on the polysaccharides.
Time: Interesties of inorganicngly, we found that time changes could affect membrane fouling [41]. We s.
Table 1. Potential of AFM coupled with other techniques.
Atomic force microscope (AFM)withOther TechniquesResultsReferences
Scanning electron microscopy (SEM)/

Transmission electron microscope (TEM)		Provides high-resolution surface morphology information with structural and elemental composition information[9][10]
Fluorescence spectroscopyProvides high-resolution surface morphology information with chemical composition information[11]
Fourier-transform infrared spectroscopyProvides high-resolution surface morphology information with chemical composition, which enables in situ analysis of the molecular structure, bonding, and distribution on the membrane surface[12]
X-ray diffractionProvides information on the crystalline properties of inorganic membranes[13]
ElectrochemistryAllows AFM to observe electrochemically active regions on the surface and collect scanning images to study the local chemical reaction behavior, polarization phenomena, and impurity deposition processes on the membrane surface[14]
Raman spectroscopyProvides chemical composition and structural information[15]
Cturrdiently, although electroched the pollution behavior of three selected model foulants at different adsorption times. For the SA-Ca2+ systemic, al atomic force microscopy (EC-AFM) longer adsorption time slightly increased the adsorption capacity of SA but significantly reduced its reversibility. With regards to BSA-Ca2+, the extended tis widely used ime did not change the amount of BSA deposited on the field of materials [14]membrane surface but led to more residual BSA after cleaning. Similarly, in the HA-Ca2+ system, potential ithe adsorption time had almost no effect on the field of studying membrane contamination should not be overlooked. EC-AFM can initiate electrochemical reactions by applying an external potential to the scanning padsorption amount of HA but reduced its reversibility. Duration had a significant effect on the quantity and reversibility of membrane fouling, depending on the chemical properties of the membrane. Therefore, the AFM measurement results indicate that the longer the adsorption time, the denser the fouling layer and the stronger the interaction force between the fouling membranes.
Other factobe, allowingrs: We also used AFM to observe electrochemically active registudy the effects of voltage on the fouling of a novel polypyrrole (PPy) and stainless steel mesh conductive composite membrane [54]. We founs on the surface and collect scanning images to study the local chemical reaction behavior, polarization phenomena,d that the PPy ‘cauliflower’ structure expanded as the applied voltage increased, and the corresponding roughness of the feature area gradually decreased from 5.91 to 4.34 nm. This result could probably be ascribed to the delocalized conjugated electron carrier in the conducting polymer moving along the polymer chain under an external electric field, which changed the dipole moment of the PPy molecules. Such change caused changes in the conformation and impurity deposition processesntermolecular arrangement of the PPy molecules, resulting in the expansion of surface morphology and, thereby, decreasing the roughness.
In addition the o the these membrane surfacefouling investigations, Arkhangelsky et al. Th[57] employed AFM to inves ability offers an intuitive understanding of the morphological changetigate the membrane-cleaning process and examined the influence of different cleaning agents on membrane surfaces. Analysis employing AFM revealed that the sodium hypochlorite (NaOCl) cleaning agent affected the contaminants and evolution of impurities on thethe membrane, leading to partial organic matter destruction and a modified membrane surface. [8]In contrast, sodium hydroxide as well as a highly effective means of exploring mem(NaOH) treatment completely destroyed the proteins, yielding a smooth surface with minimal residual matter. Similarly, using AFM to examine the fouling behavior of BSA on the membrane, it was found that pre-chlorination significantly mitigated membrane fouling mec, whereas pre-ozonation oxidation exacerbated it [53]. These studies leveranisms. Moreover, the technique offers important guidance and aged AFM technology to characterize the morphology of common contaminants on membrane surfaces and to elucidate the alterations and characteristics of the membrane fouling surface morphology under various conditions, such as time and pH. This information provides a theoretical basis for designing novel anti-the mechanism of converting irreversible fouling into reversible fouling, and effectively informs membrane fouling control strategies.

4. Measurement of Interactions in Membrane Fouling

In the membrane treatment process and technologies for , the micro-interaction between membranes and foulants significantly affects the formation of membrane cleaning. Both AFM and Raman spectroscopy afouling. The AFM technology offers valuable insights into the characteristics of foulants and membrane–foulant interactions, which could be leveraged to develop more effective for characterstrategies for preventing and controlling membrane fouling. Such strategies include optimizing maembrane material s and surface propertiesmodifications, enhancing pre-treatment processes, [16].and creating Whinnovatile AFM provides information on surface morphology, roughness, and nave cleaning and regeneration technologies, which could reduce operational costs and prolong the lifespan of the membranes. The interaction force between foulants and the membrane is crucial for determining the efficiency of membrane fouling removal. Nanomechanical properties, Raman spectroscopymeasurements using AFM and the quantification of interfacial interaction forces during membrane fouling provides chemical composition and structural essential information on the nanomechanical properties of foulants and membrane surfaces. Such information. Using is critical for understanding membrane fouling.
The type of AFM colloidal probe facilitates the bricated plays a key role for the interaction force measurements. Combining AFM with BSA-adsorbed SiO2 microbservation and analyphere colloidal probes to investigate membrane surface fouling in the presence of BSA [58]. These authors ofbserved that the adsorption andhesion force between PVDF-BSA were −1.5 nN, whereas the adhesion processes of surface contaminants, as well as undeforce between BSA-BSA were nearly zero, suggesting that BSA fouling behavior was predominantly influenced by the physicochemical interaction between the membrane polymer and BSA. Membrane-coated colloidal probes made of SiO2 microstanding ofpheres coated with PP/PA are utilized in AFM to investigate the morphologicalechanism of membrane fouling caused by HA [59]. Force fmeatures of the contaminants onsurements showed that the interaction between the membrane and foulants was the primary factor contributing to the membrane surfafouling behavior. In a study of membrane fouling involving HA and SA [60], indention and retraction curve. Ramans obtained from force spectroscopy enables obtaining chemical information about the contaminants onmeasurements using an AFM probe modified with silicon nitride were used to characterize the surface stiffness and adhesive properties of fouled and clean membranes. These authors discovered that bacterial cells neither adhered to nor penetrated the organic fouling layer but, instead, traversed the thin foulant layer and directly adhered to the membrane surface,.
To further iundentifying the typrstand and clarify the fouling behavior of HA and SA on membranes, Miao et al. [61] usesd AFM in conjunction with PVDF and and structures offoulant-coated probes to investigate the intermolecular forces between the membrane and contaminants [15](SA, HA, for HA/SA mixturthers understanding of the es), as well as the forces between the contaminants themselves. Owing to the strong interaction mechanismbetween the hydroxyl groups in SA and PVDF, the adhesion force between contaminants andPVDF and SA was more than double that of PVDF-HA. The formation of organic fouling on membrane surfaces, and provids can be studied by adsorbing the corresponding EfOM components onto the surface of PVDF microspheres sintered on cantilevers prepared to form EfOM-coated colloidal probes [62]. Using AFM, these auidance for the optimization ofthors demonstrated that the adhesion force between PVDF and different parts of the EfOM follow the order PVDF-TPI (affinitive) < PVDF-HPO (hydrophobic) < PVDF-HPI (hydrophilic). Several researchers have examined membrane filtration systems. A ouling under the combinaed action of AFM and RBSA and HA [63]. They creamted colloidan techniquesl probes with BSA directly attached to [17]the provbe tides more comprehensive and accuratp and employed AFM-based chemical force spectroscopy for adhesion force measurements. Furthermore, employing AFM to examine the interaction energy between polyvinyl chloride (PVC) membranes and three water contaminants, namely HA, BSA, and dextran (DEX) [64], helps information for revealing the complex mechanisms of related membrane fouling.
Analyzing the AFM research, helping researchers delve deeper into ults for the interaction forces between individual and multiple organic contaminants with membranes has led to the following conclusions. Generally, the interaction between membrane fouling mechanisms, and offers robust support for the improvement and development ofs and foulants is stronger than the interaction forces among the foulants themselves. HA adsorption significantly decreases the BSA adhesion force on hydrophobic surfaces. The fouling rate of PVC membranes follows the order of DEX > BSA > HA, demonstrating that selecting suitable pretreatment processes to remove specific foulants can effectively control polyvinyl chloride membrane filtration technologies.
Iouling. Owing to the strong interaction between the hydroxyl groups in SA and PVDF, SA, rather than HA, has been identified asummary, AFM can be combined with other imaging and spectroscopic techniques to provide more comprehensive data and deeper understanding. These improvements in AFM the primary cause of PVDF membrane fouling. This implies that the pretreatment process for removing SA is crucial in controlling PVDF membrane fouling. It suggests that employing appropriate methods, such as pretreatment, membrane modification, or cleaning, to reduce the hydrogen bonding interactions between PVDF and foulants is an effective strategy for reducing adhesion forces. Choosing pretreatments that convert HPI and HPO fractions into TPI fractions is essential for controlling PVDF membrane fouling during secondary effluent filtration. Therefore, AFM force measurements provide valuable information for selecting membrane modifications, feedwater pretreatment, and cleaning technology and anaies in wastewater treatment and desalination.
Recentlyt, AFM has ical methods have further refined AFM technology, presenting new possibilities and ideas for its application. In the futncreasingly been employed to investigate the interaction forces between various foulants and membranes. Single force spectroscopy curves from AFM are used to assess the interactions between membranes and foulants, serving as a crucial parameter for adjusting the properties of modified membranes. This technique can measure not only the interaction forces between hard objects but also those involving softer entities, such as in the interaction force measurements between pretreated modified membranes and related membrane foulants [65]. Scholars have urse, AFM technology will be applied more widely in thd AFM’s single force spectroscopy curves to elucidate the mechanism of scaling in electrodialysis induced by anionic polyacrylamide (APAM) in anion exchange membranes (AEM) [66]. AFM can also characterize the interaction fouling and water treatment fieldsrces between various coatings and other substances, apart from membranes, such as the interactions between bubbles in different solutions [67], prdissovidilved organics [68], angd development support and assurance, and facilitating further scientific research.

4. High-Speed Scanning Atomic Force Microscopy Technology

Thspherical particles of asphalt coating. It can also measure the interactions between living microorganisms and membranes, which is vital since living cells, being alive, secrete exosomes under external forces and their interaction forces with membranes high-speed version of AFM (HS-AFM) is an innovative imaging technchange under stress conditions. Using AFM to measure these forces can more accurately reflect the biological fouling on membrane surfaces. Yumiyama and others [69] diqrectly measue that surpasses traditional AFM in speedred the interaction forces between individual yeast cells. Scholars have also studied the adhesion forces between yeast cells and microbubbles (MB) [1870]. These studis technique employs a non-resonant probe, and the distance between the probe and sample can be adjes demonstrate the utility of AFM’s single force spectroscopy curves in measuring the interaction forces between foulants and membranes, which can be used to evaluate the characteristics and interactions at the membrane–foulant interface. This aids in developing high-performance modified membranes and more effective membrane cleaning methods.

5. Modeling or Analysis of the Interaction in Membrane Fouling

In understed in real-time, enabling ultrafast scanning and imaging, with scanning rates exceeding a thousand pixels per secondanding membrane fouling processes, the characteristics of impurities (such as size, shape, charge properties, and chemical stability) and the attributes of membrane materials (such as pore size, surface roughness, chemical stability, and charge properties) significantly influence the interaction modes between impurities and the membrane. Certain impurities could interact more strongly with specific membrane materials, potentially leading to severe membrane fouling.
R For example, posearch on HS-AFM related to itively charged impurities could be strongly adsorbed onto negatively charged membrane fomaterials, forming a fouling is advancing progressively. Blayer. Conversely, if repulsive forces are generated between the membrane material and the impurities because of its high-speed scannintheir charges, the degree of fouling could decrease. Ionic composition can also have a significant impact on foulant–foulant interactions [71]. Therefore, the modeling and hanalysigh spatial resolution capas of such interactions could provide key insights for predicting and optimizing the performance of membrane processes.
By combilnities, HS-AFM can swiftly and accurng the results of AFM force measurements with certain existing theories or models, such as the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory [72][73] and telyhe Hermia model [74][75], it is pobserve and imagssible to predict the manner in which forces act as particles approach the membrane surfaces, providing new tools and platform, as well as their impact on particle adsorption behavior. This allows for exploringthe prediction of membrane fouling mechanisms an based on molecular characteristics. Wang et al. [49] employed sthe XDLVO model tudying anti-fouling teo calculate the interaction energy between PVDF membranes and organic matter under different ionic strengths, finding that as the Na+ choncenologies. Further, HS-AFM can be usetration increased, the Lewis acid–base (AB) force values gradually decreased. The AB forces are related to track the adhe chemical functional groups of the particles and the membrane [76]. This result shows that an incresion behavior and evolution of contaminants on aase in ionic strength enhances the AB interaction between the membrane and organic matter, which is consistent with the total amount of organic matter adsorbed on the membrane surface in real-time. By employing high-speeas the ionic strength increases. Not only that, XDLVO interactions and surface roughness may collectively influence the transport and fate of emerging multifunctional nanohybrids in the environment [77]. In addition, scathese authors [78][79] founningd that technology, HS-AFM can record dynamic changes in he results calculated by the XDLVO theory aligned with the AFM analysis results, suggesting that AFM force–distance curves could effectively validate the calculated results and that AFM is highly reliable for measuring the interactions between the membrane surface contaminantand foulants. Integrating AFM force measurement techniques to analyze blocking mechanisms during membrane filtration [74] aids with high temporal resolution (millisecond leven better understanding the phenomenon of membrane fouling and in developing effective fouling prevention strategies.
The Hermia model) [75], by fincludtting the morphology, size, and density of the contaminants. Thereby, imrelationship between apparent fouling resistance and membrane filtration time, identifies the types of fouling caused by different blocking mechanisms [80]. Huang et al. [81] developroved understanding is facilitated of the physical behavior andthe Unified Membrane Fouling Index (UMFI) based on the Hermia model. By directly testing commercial membranes, UMFI can quantify the likelihood of membrane fouling mechanisms of contaminants. In addition, HS-AFM c, which is very useful for evaluating fouling observed in low-pressure membranes (LPMs) across different water treatment scales. AFM force measurement technology can be used to validate these established models, helping to deepen the understanding of membrane blocking mechanisms.
It should be usnoted to study the adhesion, diffusion, and reactionhat although the XDLVO theory and Hermia model provide useful insights, they do not encompass all types of fouling behavior. These theories and models are more suitable for predictions under steady conditions, whereas actual water treatment processes of contaminants at a molecular leveare conventionally confronted by more complex, dynamic, and changing conditions. New research seeks to integrate experimental and theoretical [19], suapproaches as measuring the changes infor a more comprehensive understanding and prediction of the interaction forces between memimpurities and membrane surface contaminants and anti-foulingmaterials. For example, AFM and other nanoscale characterization techniques are used for direct observation and measurement of the interactions between impurities and membranes. This information is important for designing more efficient anti-fouling, whereas molecular dynamics simulations and quantum chemical calculations are used to understand these processes at the atomic scale. Analyzing and modeling the potential interactions between different impurities and membranes and materials is a key factor in membrane-cleaning technologies. In conclusion, as an emerging high-speed imaging technique, HS-AFM is being developed and improved continuously science and engineering. Integration of various experimental and theoretical methods is required to gain a comprehensive and in-depth understanding. In this process, AFM could predict [20],the and is anticipated to uncover new avenues for investigatingsorption tendency of pollutants by measuring the interaction forces between the pollutants and the membrane fouling mechanismssurface. Further, AFM could also monitor the fouling process, such as adsorption, diffusion, and anti-fouling technologies in the future, which, ultimately are crucial for advancingggregation of pollutants on the membrane surface in real-time. Combining AFM with relevant theories and models helps to further the exploration of the membrane fouling process and the prediction of membrane cleaningfouling trends.

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