AFM在膜污染中的应用: History
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Contributor:
Mohan Wei
,
Yaozhong Zhang
,
Yifan Wang
,
Xiaoping Liu
,
Xiaoliang Li
,
Xing Zheng

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. Modification of Probes for Membrane Fouling Characterization

From the initial 10-nanometer resolution to the current sub-angstrom level, researchers have enhanced the resolution of microscopes. This was achieved by optimizing the scanning probes and increasing the AFM scanning speeds by enlarging the scanning head sizes and using higher resonance frequencies. Continuous advancements have also rendered AFM more sensitive for mechanical detection, facilitating the determination of the local mechanical properties of materials at a nanoscale. These enhancements rely mainly on cutting-edge AFM probes. The AFM probe tip is a critical component, and its performance directly influences the precision and reliability of AFM measurements. Three primary methods are used for preparing colloidal probes to investigate membrane fouling. These are attaching modified contaminant particles to the probe tip for force measurement, directly modifying the contaminants, and using an adhesive to adsorb the contaminants for measurement. The research team utilized bioadhesives such as dopamine to directly attach contaminants to the AFM probe tip, resulting in a colloidal probe 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 uniformly into microplates (pore size of 5 μm). The photosensitive glue (A332) was filled evenly on the other side of the microplate. Then, the microplate was fixed on the AFM platform. The cantilever was lowered to 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 for 30 s to achieve quantitative modification with organic foulants. This method decreases the contact area to approximately 5 μm, leading to more accurate and reliable measurement results.
Many researchers have made efforts in modifying colloidal probes. Fleischmann [1] first employed AFM to quantitatively define the 3D shape of atomic probe tips, opening new possibilities for studying the physical mechanisms in (laser-assisted) atomic probes. Owing to the complexity of the sample surface morphology and composition, modified AFM probe tips with varying surface chemical affinities could enhance selectivity, ensuring more accurate and precise measurements in specific applications, utilizing AFM in conjunction with custom-modified membrane-coated and HA-coated probes to assess the adhesion forces between membrane-HA and HA-HA [2]. Initially, this demonstrated the potential application of modified AFM colloidal probe microinterface force measurements for UF membrane fouling behavior. Additionally, the AFM tip can be modified by incorporating different representative organic functional groups [3], namely benzyl, hexyl, propionic acid, ethylamine hydrochloride, and propionic acid propyl ester. These authors measured the adhesion forces between the modified AFM tips and reverse osmosis membranes to determine the potential scaling tendency of each functional group category on the membrane. To enhance the accuracy of AFM force measurement data, Nguyen et al. [4] employed four distinct AFM probes to gauge the nanomechanical properties of three different samples, providing valuable insights for probe selection for better interpretation of force indentation data. Furthermore, the use of modified tips broadens the applicability of AFM measurements. By attaching a mineral particle to a tipless AFM cantilever, a mineral probe for AFM measurements can be created and, afterward, applied an atomic force microscope equipped with pyrite or chalcopyrite tips to investigate the adhesion of thermophilic thiosulfate-oxidizing bacteria [5]. The modified AFM tips significantly enhanced the accuracy and reliability of the AFM measurements, reduced the probe replacement frequency, and rendered them suitable for a more extensive range of applications. Modified colloidal probes could achieve in situ measurements of the interaction forces between membranes and foulants under varying ion concentration conditions. This ability provides a valuable research method for measuring membrane–foulant interactions in wastewater treatment.

2. Investigating Membrane Fouling Process by Coupling AFM with Other Functional Modules

When studying membrane fouling processes, the liquid module of AFM could be used to change the solution environment and conduct in situ AFM measurements of the membrane morphology and membrane pollutant interaction forces. Moreover, AFM encompasses various functional modules that could be employed to investigate membrane fouling phenomena under diverse conditions.
Generally, researchers use an open module to examine the membrane fouling process in an air environment. However, given the complexity of membrane−fouling environments, AFM could be integrated with multiple functional modules to conduct research under various environmental conditions. The chemical environment of a solution is crucial in membrane fouling. Coupling AFM with a liquid cell module enables researchers to perform AFM measurements in water and other solvents, which is important for studying membrane fouling in actual treatment processes. Using a micropump, chemical solutions with altered conditions (e.g., pH, concentration, and ion concentration) can be introduced into the system, allowing real-time AFM monitoring of the membrane surface morphology and roughness changes as the solution conditions vary [6]. This approach facilitates simulation of actual water treatment environments, enabling dynamic observation of membrane conditions as the properties of the chemical solution change, and provides more precise in situ observations of contaminant adsorption and attachment processes on the membrane surface.
Additionally, the add-on FAST module can independently acquire probe signals [7] and can be installed without modifying 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 of membrane fouling. By incorporating an electrochemical module [8], researchers can observe electrochemical processes on the membrane surface, which is invaluable for examining chemical reactions and ion migration processes during membrane fouling. Temperature significantly affects the contaminant adsorption and attachment processes on the membrane surface, with distinct fouling characteristics and interaction mechanisms between contaminants and the membrane surface at different temperatures. Integrating high- and low-temperature modules with AFM enables measurements under various temperature conditions. These AFM modules offer numerous experimental methods and conditions for research on membrane fouling, contributing to a deeper understanding of the occurrence, development, and impact of membrane fouling. By applying these modules, researchers can provide robust support for the improvement and development of membrane filtration technologies.

3. Potential of AFM Coupled with Other Techniques

Combining AFM with other imaging and spectroscopic techniques could provide comprehensive information regarding membrane fouling. Typically, as shown in Table 1, AFM provides high-resolution surface morphology information and, when integrated with scanning electron microscopy (SEM) or transmission electron microscopy (TEM), SEM and TEM offer structural and elemental composition information, resulting in more comprehensive characterization [9][10]. Combined with fluorescence spectroscopy, chemical composition information is provided [11], which is useful for studying the properties of biological and other organic membranes. Integration with Fourier-transform infrared spectroscopy (FTIR) [12] produces information on chemical composition, which enables in situ analysis of the molecular structure, bonding, and distribution on the membrane surface, and further reveals the chemical characteristics and mechanisms of membrane fouling. Coupling with X-ray diffraction (XRD) [13] provides information on the crystalline properties of inorganic membranes.
Table 1. Potential of AFM coupled with other techniques.
Atomic force microscope (AFM) withOther Techniques Results References
Scanning electron microscopy (SEM)/
Transmission electron microscope (TEM)		 Provides high-resolution surface morphology information with structural and elemental composition information [9][10]
Fluorescence spectroscopy Provides high-resolution surface morphology information with chemical composition information [11]
Fourier-transform infrared spectroscopy Provides 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 diffraction Provides information on the crystalline properties of inorganic membranes [13]
Electrochemistry Allows 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 spectroscopy Provides chemical composition and structural information [15]
Currently, although electrochemical atomic force microscopy (EC-AFM) is widely used in the field of materials [14], its potential 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 probe, allowing 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. This ability offers an intuitive understanding of the morphological changes and evolution of impurities on the membrane surface [8], as well as a highly effective means of exploring membrane fouling mechanisms. Moreover, the technique offers important guidance and a basis for designing novel anti-fouling membranes and technologies for membrane cleaning. Both AFM and Raman spectroscopy are effective for characterizing material surface properties [16]. While AFM provides information on surface morphology, roughness, and nanomechanical properties, Raman spectroscopy provides chemical composition and structural information. Using AFM facilitates the observation and analyses of the adsorption and adhesion processes of surface contaminants, as well as understanding of the morphological features of the contaminants on the membrane surface. Raman spectroscopy enables obtaining chemical information about the contaminants on the membrane surface, identifying the types and structures of contaminants [15], furthers understanding of the interaction mechanism between contaminants and membrane surfaces, and provides guidance for the optimization of membrane filtration systems. A combination of AFM and Raman techniques [17] provides more comprehensive and accurate information for membrane fouling research, helping researchers delve deeper into membrane fouling mechanisms, and offers robust support for the improvement and development of membrane filtration technologies.
In summary, AFM can be combined with other imaging and spectroscopic techniques to provide more comprehensive data and deeper understanding. These improvements in AFM technology and analytical methods have further refined AFM technology, presenting new possibilities and ideas for its application. In the future, AFM technology will be applied more widely in the membrane fouling and water treatment fields, providing development support and assurance, and facilitating further scientific research.

4. High-Speed Scanning Atomic Force Microscopy Technology

The high-speed version of AFM (HS-AFM) is an innovative imaging technique that surpasses traditional AFM in speed [18]. This technique employs a non-resonant probe, and the distance between the probe and sample can be adjusted in real-time, enabling ultrafast scanning and imaging, with scanning rates exceeding a thousand pixels per second.
Research on HS-AFM related to membrane fouling is advancing progressively. Because of its high-speed scanning and high spatial resolution capabilities, HS-AFM can swiftly and accurately observe and image membrane surfaces, providing new tools and platforms for exploring membrane fouling mechanisms and studying anti-fouling technologies. Further, HS-AFM can be used to track the adhesion behavior and evolution of contaminants on a membrane surface in real-time. By employing high-speed scanning technology, HS-AFM can record dynamic changes in membrane surface contaminants with high temporal resolution (millisecond level), including the morphology, size, and density of the contaminants. Thereby, improved understanding is facilitated of the physical behavior and fouling mechanisms of contaminants. In addition, HS-AFM could be used to study the adhesion, diffusion, and reaction processes of contaminants at a molecular level [19], such as measuring the changes in the interaction forces between membrane surface contaminants and anti-fouling membranes. This information is important for designing more efficient anti-fouling membranes and membrane-cleaning technologies. In conclusion, as an emerging high-speed imaging technique, HS-AFM is being developed and improved continuously [20], and is anticipated to uncover new avenues for investigating membrane fouling mechanisms and anti-fouling technologies in the future, which, ultimately are crucial for advancing membrane cleaning.

This entry is adapted from the peer-reviewed paper 10.3390/membranes14020035

References

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  6. Wang, Y.; Zheng, X.; Wang, Z.; Shi, Z.; Kong, Z.; Zhong, M.; Xue, J.; Zhang, Y. Effects of –COOH and –NH2 on adsorptive polysaccharide fouling under varying pH conditions: Contributing factors and underlying mechanisms. J. Membr. Sci. 2021, 621, 118933.
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  14. Pura, J.L.; Salvo-Comino, C.; García-Cabezón, C.; Rodríguez-Méndez, M.L. Concurrent study of the electrochemical response and the surface alterations of silver nanowire modified electrodes by means of EC-AFM. The role of electrode/nanomaterial interaction. Surf. Interfaces 2023, 38, 102792.
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  16. De, A.; Malpani, D.; Das, B.; Mitra, D.; Samanta, A. Characterization of an arabinogalactan isolated from gum exudate of Odina wodier Roxb.: Rheology, AFM, Raman and CD spectroscopy. Carbohydr. Polym. 2020, 250, 116950.
  17. Seweryn, S.; Skirlinska-Nosek, K.; Sofinska, K.; Szajna, K.; Kobierski, J.; Awsiuk, K.; Szymonski, M.; Lipiec, E. Optimization of tip-enhanced Raman spectroscopy for probing the chemical structure of DNA. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2022, 281, 121595.
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  19. Flechsig, H.; Ando, T. Protein dynamics by the combination of high-speed AFM and computational modeling. Curr. Opin. Struct. Biol. 2023, 80, 102591.
  20. Lu, H.; Fang, Y.; Ren, X.; Zhang, X. Improved direct inverse tracking control of a piezoelectric tube scanner for high-speed AFM imaging. Mechatronics 2015, 31, 189–195.
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