AFM在膜污染中的应用: 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. CharaModificterization of Probes for Membrane Fouling Characterization

TFrom the interaction between the membranes and foulants is related itial 10-nanometer resolution 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, charactercurrent sub-angstrom level, researchers have enhanced the resolution of microscopes. This was achieved by optimizing three-dimensional presentation of membrane surface information, allowing for an exhaustive detailed expression of the surface characteristics of the membrane. 

1.1. Characterization of Membrane Morphology

Vie scanning probes and increasualization of the membrane surface morphology aids in understanding the relevant properties of the membrane. Atomic force microscopy (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 polymerization 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]. Furthng the AFM scanning speeds by enlarging the scanning head sizes and using higher resonance frequencies. Continuous advancements have also renderer,d 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]. Using AFMmore sensitive for mechanical detection, facit is also possible to observe the ion transport channels of membranes. Exalitating the determination of modified 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]. By the local mechanical properties of materials at a nanoscale. These enhancementstudying the surfaces of nanofiltration membranes using AFM in different imaging modes, various AFM imaging mode characteristics can be obtained [4]rely mainly on cutting-edge AFM probes. The tapping mode in 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].
Evidentlyprobe tip is a critical component, AFM can 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 d its performance directly influences the precision and reliability of AFM measurement 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 softs. Three primary methods are used for preparing colloidal probes to investigate membrane surfaces. 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

Tfouling. These are attaching modified contaminant particles to the probe tip for force measurement, directly modifying the surface 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, sontaminants, and using an adhesive to adsorb the contaminants for measurement. The research team utilized bioadhesives such as cation 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 dopamine to directly attach contaminants to the AFM probe tip, resulting in a colloidal process of membrane 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 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,e for AFM force measurements. The foulant colloidal probes were prepared using AFM 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), and then filled uniform, and smooth. 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 membrane and strengthenly into microplates (pore size of 5 μm). The photosensitive glue (A332) was filled evenly on the other side of the microplate. Then, the membrane channels, significantly enhancing the permeation evaporation performance of the membrane [7]. Simicroplate was fixed on the AFM platform. The cantilaeverly, analysis of multi-walled carbon nanotube (MWCNT) dispersed PS nanofiltration membranes [8] using AFM shows t was lowered to adhat 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 performancere the glue, then lifted it to the other side to adhere the organic foulants. The adhered needle tip was irradiated with a UV lamp for [9].
However,30 studies show that for rough surfaces, nanoscalto achieve quantitative modified 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 AFcation with organic foulants. This method decreases the contact area to approximately 5 μm, leading to more accurate and reliable measurement results.
M can measure roughness changes during the membrane modification process in situ. When characterizing hydrophilic polymer-functionalized polysulfone (PSF) blend membranes using AFM [10], ry researchers have made efforts in modifying colloidal probes. Flesearchers found that the addition of 4VP side chains enhancedschmann the[1] surface roughness of the modified membrane. In another study, PSF membranes modified with titanium oxide compounds [11] had highirst employed AFM to quantitatively der 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 sulfonateine the 3D shape of atomic probe tips, opening new possibilities for [12] can achieve ansti-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.
Precidying the physical mechanisms in (laser-assisted) atomic probes. Owing to the complexity of the se measurement of membrane mple surface roughness with AFM can be used to explore which roughness is more resistant to contamination on the morphology and composition, modified membrane surface, thus achieving membrane performance adjustment. Although many studies have shown that increased roughness may lead to increased tendencies for membrane fouling [6][10] AFM probe tips with varying surface chemical affinities could enhance selectivity, other research findings suggest that adding micrometer to nanometer-sized particles to increase surface roughness (similar to lotus leaf biomimetic structures) can reduce membrane foulingnsuring more accurate and precise measurements in [8][9]. This discrpepancy is mainly because a single roughness parameter is insufficient to summarize the complexity ofcific applications, utilizing AFM in conjunction with custom-modified membrane surface fouling. Establishing a relationship between roughness R measured with AFM and the roughness index H can more quickly and accurately evaluate-coated and HA-coated probes to assess the adhesion forces between membrane surface roughness-HA and HA-HA [132]. In addition, an overall assessment should combine AFM with various other techniques to carefully examine membrane surface characteristics, such as surfacally, this demonstrated the potential, hydrophilicity/hydrophobicity, functional groups, and foulant properties. Through this comprehensive judgment of characterization results, the relationship between surface roughness and application of modified AFM colloidal probe microinterface force measurements for UF membrane fouling can be thoroughly analyzed. Establishing the relationship between membrane surface roughness measured by AFM and membrane surface potential,behavior. Additionally, the AFM tip can be modified by incorporating different representative organic functional groups, etc.[3], cnan 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

Membranemely benzyl, hexyl, propionic acid, ethylamine hydrochloride, and propionic channels 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 cid propyl ester. These authors measured the adhesion forces between the modified membranes, including the structure and pore size ofAFM tips and reverse osmosis membrane channels. These researchers [14][15][16][17][18][19] characs to determized the surfaces of modified membranes using 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] acne the potential scaling tendency of each functional group category on the membrane. To enhance thieved atomic-level surface functionalization of nanofiltration membranes using graphene oxide (GO) combined with plasma-enhanced atomic layer deposition accuracy of AFM force measurement data, Nguyen et al. (ALD)[4] technology. A novel data analysis method [14] mployed four distinctegrating AFM with “pore reconstruction technology” was used to assess membrane channel structures, including size, shape, and interlayer distances. The obtained membrane channel information is vital for the permeation 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], identi AFM probes to gauge the nanomechanical properties of three different samples, providing valuable insights for probe selection for better interpretation ofy the membrane channel structures of superhydrophilic copper mesh membranes coated with zinc oxide nanostructures (ZnO NW) used for oil–water separationforce indentation data. [17], acqFuire information about the shapthermore, the use of membrane channels in modified seawater desalination nanofiltration membranes created using molecular layer deposition (MLD) techniquestips broadens the applicability of AFM measurements. [18],By and obttain 3D shape information of membrane channels in chitosan and polystyrene sulfonate-modified polyamide microfiltration membranes prepared by layer-by-layer (LBL) deposition methods [19].
Combching a mineral particle to a tipless AFM cantilever, a mining 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 reducral probe for AFM measurements can be created and, afterward, applied an atomic force microscope equipped with pyrite or chalcopyrite tips to investigate the risk of fouling. The systematic distributiadhesion of pores in uniform membranes can enhance the interception capacity for foulants [20]. Notably, ththermophilic thiosulfate ge-ometric shape of membrane channels greatly influences membranexidizing bacteria fouling[5]. Typically, the fouling intensity caused by slit-shaped pores is lower than that caused by circular pores [21]. Additmodified AFM tips signifionally, AFM can accurately present the degree of membrane channel clogging antly enhanced the state of membrane fouling in real liquid-phase environments. In contrast, SEM requires accuracy and reliability of the AFM measurements 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 , reduced the probe replacement frequency, and rendered them suitable for a more extensive range of apprevliously mentioned, employing AFM to characterize different membranes allows for a more comprehensive observation of three-dimensional surface morphology, membrane roughnesscations. Modified colloidal probes could achieve in situ measurements, membrane channel assessments, and an overall evaluation o of the material characteristics and application performance ofinteraction forces between membranes and modified membranes. As depicted in Figure 1,foulants under varying this section categorizes and summarizes the different modes, characteristics, and outcomes of modified membrane characterization via 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], uniquon concentration conditions. This ability provides a valuable resely shaped modified block copolymer microfiltration membranes [23], zrch meolite-filled polyethersulfone membranes [24], modihod fied Carbosep M5 ceramic membranes [25], innovative positively char measurged nanofiltration membranes [26], ornganic membranes for oil–water separation [27], and composi–foulant inte ceramic microfiltration membranes for greywater treatment [28]. Thictions in was technique (atomic force microscopy) has become a powerful tool in the design and fabrication of functional membraneswater treatment.
Membranes 14 00035 g005
Figure 1. Different aspects and results of AFM characterization of modified membranes in different modes.

2. CharInvestigacterization of Contaminantting Membrane Fouling Process by Coupling AFM with Other Functional Modules

DiffWherent types ofn studying membrane foulants 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 observationing processes, the liquid module of AFM could be used to change the solution environment and conduct in situ AFM measurements of the contaminantmembrane morphology and structure on membrane surfaces. Accordingly, AFM could be used to monitor the adsorption and adhesion of contaminants in real-time under various environmental conditions. Morphological 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 pollutant interaction forces. Moreover, AFM encompasses various functional modules that could be employed to investigate membrane fouling mechanisms. Characterizing contaminants facilitates superior understanding of mphenomena under diverse conditions.
Genembrane fouling principles, and offers essential guidance for the design, operation, and maintenance of membrane filtration systems. This is an important factor in the investigation of lly, researchers use an open module to examine the membrane fouling mechanisms.

2.1. Organic Contaminants

Naturalprocess in organic matter (NOM) is the primary contaminant in wastewater. It is a complex heterogeneous system comprising diverse organic molecules [29], such as hum air environment. However, given the complexic substances, polysaccharides, and proteins, which can all affect the y of membrane performance. Observations using AFM in aquatic −fouling environments have revealed that natural polysaccharide sodium alginate (SA) predominantly exists as single helical chains, with diameters of approximately 0.2–0.3 nm [30]. S, AFM could be integrated with multiple funcannting humic acid sodium (HA)-contaminated mica surfaces with AFM has uncovered spherical particles and aggregates, featuring colloidal diameters under 100 nm and heights from 0.5 to 7 nm [31]. Ional modules to conduct research under various environmen studies on protein membrane fouling, most protein molecules have been observed as monomers on mica surfaces [32]. Exal conditions. The chemical environment of a solutracellular iorganic matter (EOM) can lead to severe ultrafiltration is crucial in membrane fouling. AFM enables the observation of the aggregation and blockage behaviors of pollutants on the membrane surface [33], and Coupling AFM with a liquid cell module enables resevaluates the effects of cleaning/pre-treatmentrchers to perform [34]. Utilizing AFM tmechnology 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 asurements in water and other solvents, which is important for studying membrane fouling.

2.2. Biological Contaminants

In addition to typical organic contaminants, biological contaminants can impair membrane treatment efficiency in water treatment n actual treatment processes. Escherichia coli Usis a common pathogenicg a microorganism that compromises the safety of water resources and drinking water. Researchers have used AFM to investigate the morphological changes in E. coli pump, chemical solutions with altered condition membrane surfaces under varying pH conditions [35] and (e.g., pH, concorrelate it with membrane filntration and cleaning [36]. In addit, and ion, AFM has been used to examine changes in the morphology of antibiotic-resistantconcentration) can E. coli on membrane surfaces during photocatalytic Fenton water treatment [37]. Recently, owintroduced ing 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 system, allowing real-time AFM monitoring of the membrane surfaces. High-speed atomic force microscopy (HS-AFM) has been employed to analyze Chlorella vulgaris treate morphology and with electrocoagulation flotation (ECF) [38]. Anothughner study used AFM to determine the energy required to disrupt individual microalgae cells [39]. Guidance cs changes as the soluld be offered for alleviating biological fouling caused by microalgae. For the living microbial cells, AFM-based single-cell force spectroscoption conditions vary (AFM-SCFS)[6]. Thas significant value for characterizing the structure, mechanical properties, and molecular activity of individual living microbial cells [40]. The technis approach facilique can measure the 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] ates simulation of actual water treatment employed AFM to explore the dynamic effects of various envivironmental factors on microorganisms and membrane surface interactions at a molecular scale. This provides a research basis for the effective inhibitis, enabling dynamic observation of biological foulants on membrane surfaces.
AFM-SFCS allcows sensitive measurements of the mechanical ditions as the properties of individual molecules. This allows researchers to gain insight into the mechanical properties of individual molecules such as stretching, deformationthe chemical solution change, and fracture, 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 the group’s research on AFM-SFCS in the environmental field, researchers 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 immobilizationprovides more precise in situ observations of contaminant adsorption and attachment processes on the membrane surface.
Additionally, assessing the viability 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 celadd-on FAST module can independently acquire probe signals is[7] anot consistent; it encompasses rod or spherical shapes and other irregular shapes. Durd can be installed without modifying the adhesion 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 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 of membrane materials and superior biofouling control strategies.

2.3. Emerging Contaminants

Emergi. By incorporating contaminants in wastewater treatment processes, such as microplastics,n electrochemical module antibiotics[8], and endocrine-disrupting compounds (EDCs) have been attracting increasing academic attention at national and international levels. As a high-resolution tool, AFM enables a more detailed examination of the physesearchers can observe electrochemical properties of microplasticcesses [42]. For instance, Melo-Agustín et al. the [43] employed AFM for morphological analysis of microplastic smbrane surfaces, 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] , which is invaluable for examining chemical reactions and ion mintgroduced a 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 contaminants during water treatation processes during membrane fouling. Temperature significantly affects the contaminant adsorption and attachment processes, which exacerbates pollution. For instance, Zhang et al. [45] e on the mployed AFM to determine the interaction forces between NPs (hematite and corundum) and Escherichia coli mbrane surfacells, gaining further understanding of the membrane with distinct fouling mechanism of microplastics.
Addicharactionally, antibiotics are 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 onristics and interaction mechanisms between contaminants and the membrane surface, thereby enhancing the efficiency of membranes in intercepting them. For instance, Liu et al. [46] used AFM to investi at different temperatures. Integrating higate the adsorption of EDCs on nanofiltration membrane surfaces, subsequently enhancing the EDC removal rate by preparing modified nanofiltration membranes. Wu et al. [47] attache- and low-temperature modules with AFM enables measurements und sulfamethoxazole (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 er various temperature conditions. These AFM modules offer numerous experimental and theoretical viewpoints. Rmethods and conditions for researchers have also examined the impact of microplastics on antibiotic transport during sand filtration [48] on membrane fouling, contriby grafuting 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.to a deeper understanding of the occurrence, development, and impact of membrane fouling. By applying these modules, Table 1 resummearizes 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 foulantchers can provide robust support for the improvement and development of membrane filtration technologies.
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. MicrPoscopic Identification of Membrane Fouling Processes under Changing Factortential of AFM Coupled with Other Techniques

The previCous text, respectively, introduced the membrane and contaminants observed by AFM. However, membrane fouling is a complex fouling process influenced by multiple factors. Therefore, the research team conducted abundant research on the effects of changes in the ionic concentration, pH, and time onmbining AFM with other imaging and spectroscopic techniques could provide comprehensive information regarding membrane fouling using AFM. By employing. Typically, as shown in Table 1, AFM, researchers can monitor thprovides high-resolution surface morphological changes in contaminants under various 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. 
Iy information and, when integrated with scanning electron microscopy (SEM) or transmission electron microscopy (TEM), SEM and TEM offer structural and elemental conic cmponditions: the research team [49][50][51][52] used AFM to study the efsition infects of different valence ions on membrane fouling of NOMs. Using AFM force measurements, morphology rmation, resulting in more comprehensive characterization, and[9][10]. Cother technical methods, the effect of monovalent ions such as Na+ and K+ mbined with fluon organic compounds was found to be based on their charge and structure. However, the effect of divalent ions such as Ca2+ aescence spectroscopy, chemical composition ind Mg2+ fon organic compounds also inclumation is provided complexation.[11], Among twhem, it is closely related to the special functional groups, types, and structurich is useful for studying the properties of NOMs. Miao et al. [55][56] emplbioloyed AFM to ginvestigate the effects of Nca+,l Mg2+, and Ca2+ on HA fouling through HA er organic membrane fouling experiments. These authors observed that membrane fouling intensified at lowers. Integration with Fourier-transform infrared spectroscopy (FTIR) Ca2+[12] opr Mg2+ conducentrations and significantly decreased at substantially higher Ca2+ or Mg2+s information on chemical concentratimpons, albeit with the two ions having different mechanisms.
pH:sition, which Researchers investigated changes in membrane fouling under different pH conditions using AFM [51]. The renables in situ analysisults 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 deposiof the molecular structure, bonding, and distribution on the membrane surface and alleviated adsorptive fouling and irreversibility. For the –NH2 fun, and further reveals the cthemional group, an increase in pH led to more severe polysaccharide fouling owing to a lower degree of protonation, and the resulting foucal characteristics and mechanisms of membrane fouling. Coupling was highly irreversible. Modification usingith X-ray diffraction (XRD) GO[13] alleproviated the adsorptive fouling of these two polysaccharides on PVDF; however, the extent of alleviation depended des information on the abundance of functional groups on the polysaccharides.
Time: Intecrystalline properestingly, researchers found that time changes could affectes of inorganic membrane fos.
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]
Culing [41]. Researchers studied the pollution behavior of three selected model foulants at different adsorption times. For the SA-Ca2+ systeently, although electrochem, ica longer adsorption time slightly increased the adsorption capacity of SA but significantly reduced its reversibility. With regards to BSA-Ca2+, the extendl atomic force microscopy (EC-AFM) is widely used time did not change the amount of BSA deposited on the membrane surface but led to more residualn the field of materials BSA after cleaning. Similarly[14], in the HA-Ca2+ system, the adsorption time had almost no effect on the adsorption 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 factpotential in the field of studying membrane contamination should not be overlooked. EC-AFM can initiate electrochemical reactions by applying an external potential to the scanning prors: Rbesearchers also used , allowing AFM to study the effects of voltage on the fouling of a novel polypyrrole (PPy) and stainless steel mesh conductive composite membrane [54]. Reobserve electrochemically active regionsearchers found 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 intermolecular arrangement of the PPy molecules, resulting in the expansion of surface morphology and, thereby, decreasing the roughness.
Inon the surface and collect scanning images to study the local chemical reaction behavior, polarization phenomena, and impurity deposition processes addition to the these he membrane fouling investigations, Arkhangelsky et alsurface. [57] employed AFM to Thinvestigate 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 contaminant ability offers an intuitive understanding of the morphological changes and the membrane, leading to partial organic matter destruction and a modified meevolution of impurities on the membrane surface. In[8], contrast, sodium hydroxide (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 mem well as a highly effective means of exploring membrane fouling, whereas pre-ozonation oxidation exacerbated it [53]. The mechanise mstudies leveraged 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 . Moreover, the technique offers important guidance and a basis for the mechanism of converting irreversible fouling into reversible fouling, and effectively informs membrane fodesigning novel anti-fouling control strategies.

4. Measurement of Interactions in Membrane Fouling

In the membranes treatment process, the micro-interaction between membranes and foulants significantly affects the formation of and technologies for membrane fouling. The AFM technology offers valuable insights into the characteristics of foulants and membrane–foulant interactions, which could be leveraged to develop moe cleaning. Both AFM and Raman spectroscopy are effective strategies for preventing and controlling membrane fouling. Such strategies include optimizing membrane mafor characterizing materials and surface modifications, enhancing pre-treatment processes,properties and[16]. creating Whinnovative 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. Nanomle AFM provides information on surface morphology, roughness, and nanomechanical measurements using AFM and the quantification of interfacial interaction forces during membrane foulingproperties, Raman spectroscopy provide essential information on the nanomechanical properties of foulants and membrane surfaces. Such s chemical composition and structural information is critical for understanding membrane fouling.
The type of . Using AFM facolloidal probe fabricated plays a key role for the interaction force measurements. Combining AFM with BSA-adsorbed SiO2 microsphere colloidilitates the observation and al probes to investigate membrane surface fouling in the presence of BSA [58]. Talyses of these authors observed that the adhesion force between PVDF-BSA were −1.5 nN, whereas the adsorption and adhesion force 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 micprocesses of surface contaminants, as well as underospheres coated with PP/PA are utilized in AFM to investigate the mechanism of membrane fouling caused by HA [59]. Force mtanding of the morphological feasturements showed that the interaction between the membrane and foulants was the primary factor contributing tos of the contaminants on the membrane fouling behavior. In a study of membrane fouling involving HA and SA [60], indsurfacention. and retraction curves obtained from force sRaman spectroscopy measurements 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 toenables obtaining chemical information about the contaminants on the membrane surface.
To, furthider understand and clarify the fouling behavior of HA and SA on membranes, Miao et al. [61] ntifying the types and strused AFM in conjunction with PVDF and foulant-coated probes to investigate the intermolecular forces between the membrane and tures of contaminants (SA[15], HA, or HA/SA mixtfures), as well as the forces between the contaminants themselves. Owing to the strong thers understanding of the interaction between the hydroxyl groups in SA and PVDF, the adhesion forcemechanism between PVDF and SA was more than double that of PVDF-HA. The formation of organic fouling oncontaminants and membranes can be studied by adsorbing the corresponding EfOM components onto the surface of PVDF microspheres sintered on cantilevers prepared to form EfOM-coated colloidal prob surfaces, and provides [62]. Using AFM, these authors 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 memidance for the optimization of membrane fouling under theiltration systems. A combined acation of BSA and HA [63]. They creAFM and Rated colloidmal probes with BSA directly attachedn techniques to[17] the probe tip 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 in revealing the cvides more comprehensive and accurate informatiomplex mechanisms of related for membrane fouling.
Analyzing the AFM results for the interaction forces between individual and multiple organic contaminants with membranes has led to the following conclusions. Generally, the interaction between earch, helping researchers delve deeper into membranes 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 fouling mechanisms, and offers robust support for the improvement and development of membrane fouling. Owing to the strong interaction between the hydroxyl groups iiltration technologies.
In SA summand PVDF, SA, rather than HA, has been identified as 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 tecry, AFM can be combined with other imaging and spectroscopic techniques to provide more comprehensive data and deeper understanding. These improvements in AFM technologies in wastewater treatment and desalination.
Recenty and analy, AFM has tincreasingly 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]. Scholacal methods have further refined AFM technology, presenting new possibilities and ideas for its application. In the futurs have used AFM’s single force spectroscopy curves to elucidate the mechanism of scaling in electrodialysis induced by anionic polyacrylamide (APAM) in anion exchang, AFM technology will be applied more widely in the membranes (AEM) [66]. AFM can also characterize the interaction forces between various coatings and other substances, apart from membranes, such as the interactions between bubbles in different solutions [67], ling and water treatment fields, providing dissoleved organics [68], and slophmerical 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 membrannt support and assurance, and facilitating further scientific research.

4. High-Speed Scanning Atomic Force Microscopy Technology

Thes change under stress conditions. Using AFM to measure these forces can more accurately reflect the biological fouling on membrane surfaces. Yumiyama and others [69] dhigh-speed version of AFM (HS-AFM) is an innovative imaging technirectly measqured the interaction forces between individual yeast cells. Scholars have also studied the adhesion forces between yeast cells ane that surpasses traditional AFM in speed microbubbles (MB) [7018]. These studies 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 s technique employs a non-resonant probe, and the distance between the probe and sample can be adjunderstanding 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 impuritied in real-time, enabling ultrafast scanning and imaging, with scanning rates exceeding a thousand pixels per second.
Res could interact more strongly with specific membrane materials, potentially leading to severearch on HS-AFM related to membrane fouling. For example, positively charged impurities could be strongly adsorbed onto negatively charged membrane materials, forming a fouling layer. Conversely, if repulsive forces are generated between the membrane material and the impurities be is advancing progressively. Because of their charges, the degree of fouling could decrease. Ionic composition can also have a significant impact on foulant–foulant interactions [71]. Therefore, the modelinits high-speed scanning and analyshis of such interactions could provide key insights for predicting and optimizing the performance of membrane processes.
By comgh spatial resolution capabinling the results of AFM force measurements with certain existing theories or models, such as the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory [72][73] ties, HS-AFM can swiftly and accurand the Hermia modelely [74][75], it is pobssible to predict the manner in which forces act as particles approach the erve and image membrane surface, as well as their impact on particle adsorption behavior. This allows for the prediction ofs, providing new tools and platforms for exploring membrane fouling based on molecular characteristics. Wang et al. [49] employemechanisms and sthe XDLVO model to calculate the interaction energy between PVDF membranes and organic matter under different ionic strengths, finding that as the Na+ udying anti-fouling technoncentration increased, the Lewis acid–base (AB) force values gradually decreased. The AB forces are related to the chemical functional groups of the particles and the membrane [76]. Thlogies. Further, HS-AFM can be used to track the adhesis result shows that an increase 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 then behavior and evolution of contaminants on a membrane surface as 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, these authors [78][79] found that the resue in real-time. By employing high-speed scanning technolts 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 gy, HS-AFM can record dynamic changes in membrane and foulants. Integrating AFM force measurement techniques to analyze blocking mechanisms during membrane filtration [74] aidsurface contaminants win better understanding the phenomenon of membrane fouling and in developing effective fouling prevention strategies.
The Hermia modeth high temporal resolution (millisecond level [75]), by fittncluding the relationship between apparent fouling resistance and membrane filtration time, identifies the types of fouling caused by different blocking mechanisms [80]. Huang et al. [81] demorphology, size, and density of the contaminants. Thereby, improveloped the Unified Membrane Fouling Index (UMFI) based on the Hermia model. By directly testing commercial membranes, UMFI can quantify the likelihood of membraneunderstanding is facilitated of the physical behavior and fouling, 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 sh mechanisms of contaminants. In addition, HS-AFM could be notused that 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 o study the adhesion, diffusion, and reaction processes are conventionally confronted by more complex, dynamic, and changing conditions. New research seeks to integrate experimental and theoreticaof contaminants at a molecular level approa[19], suches for a more comprehensive understanding and prediction of s measuring the changes in the interaction forces between impurities and memmembrane materials. For example, AFM and other nanoscale characterization techniques are used for direct observation and measurement of the interactions between impurities andsurface contaminants and anti-fouling membranes, 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. This information is important for designing more efficient anti-fouling membrane materials is a key factor ins and membrane science and engineering. Integration of various experimental and theoretical methods is required to gain a comprehensive and in-depth understanding. In this process, AFM coul-cleaning technologies. In conclusion, as an emerging high-speed imaging technique, HS-AFM is being developed and improved continuously [20], and predict the adsorption tendency of pollutants by measuring the interaction forces between the pollutants and the s anticipated to uncover new avenues for investigating membrane surface. Further, AFM could also monitor the fouling process, such as adsorption, diffusion,fouling mechanisms and aggregation 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 ofnti-fouling technologies in the future, which, ultimately are crucial for advancing membrane fouling trendscleaning.

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