Atomic force microscopy's (AFM) nanoscale imaging and measurement capabilities allow researchers to gain molecular-level insights into membrane morphology, fouling, surface interactions, and performance under simulated water treatment conditions. These unique capabilities make AFM instrumental in understanding and optimizing the membranes and barriers central to water purification technologies.
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Enhancing Water Treatment Efficiency: The Significance of Membrane Analysis and the Role of AFM
Membranes are widely used in water treatment facilities for desalination and purification through reverse osmosis and membrane separation. However, membrane fouling, characterized by contaminant accumulation on the membrane surface, presents substantial challenges, resulting in elevated pressure requirements, frequent cleaning, and compromised water quality.
A significant contributor to this fouling is the formation of microbial biofilms, which accelerate membrane deterioration. Traditionally, chemicals and backwashing have been used to tackle membrane fouling in water treatment plants, but their increased use can reduce the lifespan of membranes.
Atomic force microscopy (AFM) has become indispensable for membrane experts aiming to enhance separation processes by gaining deeper insights into fouling mechanisms. This versatile tool captures high-resolution images of membrane surfaces in relevant process conditions and quantifies the adhesion forces involved in fouling.
The adhesion forces are measured by moving a probe attached to the AFM cantilever toward and away from the sample, tracking cantilever deflection, and employing Hooke's Law, spring constant calibration, and deflection sensitivity to convert deflection into force values.
The resulting force-distance curve offers valuable information on interaction forces and local material properties such as elasticity, adhesion, hardness, and surface charge densities. These properties indicate how resilient a membrane or fouling layer is to physical and chemical processes, with harder polymer surfaces often exhibiting reduced wear during operation and cleaning cycles.
Potential Applications of AFM in Water Treatment Research
Disinfectants Effects on Biofilm Properties in Drinking Water Distribution Systems
Biofilms in drinking water distribution systems can support the persistence and transmission of pathogens, and their structural and mechanical properties significantly affect the accumulation and release of pathogens. Therefore, it's essential to understand how disinfectant exposure affects biofilm properties to predict, assess, and control pathogen risks in DWDS.
A study published in Environmental Science & Technology used AFM indentation tests with a spherical probe to estimate the biofilm's elastic modulus. AFM analyses demonstrated that one month of disinfectant exposure effectively controlled biofilms by increasing stiffness and resistance to detachment, reducing pathogen release. In addition, the study emphasized that shear stress in drinking water distribution systems could reduce pathogen accumulation.
These insights are crucial in predicting and managing pathogen transmission associated with biofilms in water distribution systems.
Characterizing Barrier Layers in Desalination Membranes
In desalination, AFM helps understand and enhance the performance of polyamide thin-film composite membranes, the key salt-rejection barrier layer.
High-resolution AFM imaging provides a detailed view of polyamide membranes' "ridge-and-valley" structure, helping researchers understand their surface morphology. In addition, AFM nanoindentation enables the mapping of local variations in important mechanical properties like hardness, elastic modulus, and adhesion.
This information allows researchers to optimize membrane design and composition, leading to more efficient and long-lasting water treatment processes. Additionally, monitoring polyamide degradation and damage during disinfectant exposure helps ensure the reliability and effectiveness of desalination pretreatment systems, contributing to cleaner and safer drinking water production.
Measuring Colloidal Particle-Membrane Adhesion
AFM allows for studying material-membrane interactions, particularly when using a colloidal probe with microspheres to mimic foulants. This technique helps assess a membrane's fouling resistance and bond strength between foulants and the membrane, aiding in membrane modification for fouling resistance.
Surface-modified microspheres enable the creation of probes that mimic foulant interactions with membranes, offering valuable insights into these interactions. For example, carboxylated latex particle probes replicate bacterial-cell interactions with fouled membranes, while calcium ions induce attractive forces in the presence of extracellular polymeric substances (EPS) fouling. These probes also evaluate the fouling resistance of modified membranes, uncovering intricate electrostatic and hydrophobic interactions.
These modified probes aid in understanding membrane fouling at a molecular level, enabling the development of fouling-resistant membranes, optimized cleaning procedures, and tailored pretreatment processes in water treatment. This leads to improved water quality and more sustainable water treatment practices.
Swansea University's AFM-Enhanced Membrane Research: Innovating Sustainable Water Treatment and Industry Solutions
Swansea University researchers were pioneers in applying AFM to membrane separation in process engineering, leading to significant advancements in membrane optimization processes with applications in industries like food processing, water treatment, and medicine. Their work, underpinned by rigorous research, has not only improved membrane processes and quality but has also delivered economic, societal, environmental, and health benefits on an international scale.
The university's research has been centered on harnessing and enhancing novel membrane technologies via AFM to optimize water treatment processes, making them more appealing than alternative methods like evaporation.
Axium Process, a Welsh SME, is a significant global player in membrane and filtration technologies. Specializing in hygienic engineering design and fabrication, the company has employed Swansea University's optimization research to develop and implement membrane systems for water treatment. This collaboration has contributed £1 million to Axium Process and created well-paying job opportunities in local manufacturing sectors.
Another notable impact was on First Milk, which saved over £500,000 annually and improved whey quality by investing £3 million in reverse osmosis membrane technology based on Swansea's research. The impact extends internationally as First Milk supplies cheese, milk, and ingredients globally.
In addition to commercial benefits, this research positively impacts society and the environment by enhancing water and wastewater treatment processes. For example, their research on membrane fouling and innovative membrane modification techniques has led to improvements in nanofiltration for clean water production. This includes developing membrane systems for desalination and household water in the Gulf region through UV-initiated graft polymerization-based membrane modification by Water Nano.
The Way Forward
AFM has demonstrated immense value for water treatment membrane research by enabling nanoscale characterization under physiochemically relevant conditions. Ongoing advancements, including high-speed AFM, multiparametric imaging, tip functionalization, and fluid cells, promise deeper insights into membrane-foulant interactions.
Combining AFM with other techniques and computer modeling will provide a comprehensive understanding of membrane interfaces, driving next-gen water treatment membranes.
More from AZoOptics: Surface Topography Imaging with AFM
References and Further Reading
Shen, Y., Huang, C., Monroy, G. L., Janjaroen, D., Derlon, N., Lin, J., ... & Nguyen, T. H. (2016). Response of simulated drinking water biofilm mechanical and structural properties to long-term disinfectant exposure. Environmental science & technology, 50(4), 1779-1787. https://doi.org/10.1021/acs.est.5b04653
Johnson, D. J., & Hilal, N. (2013). Membrane Characterization by Atomic Force Microscopy. Encyclopedia of Membrane Science and Technology, 1-20. https://doi.org/10.1002/9781118522318.emst142
Powell, L. C., Hilal, N., & Wright, C. J. (2017). Atomic force microscopy study of the biofouling and mechanical properties of virgin and industrially fouled reverse osmosis membranes. Desalination, 404, 313-321. https://doi.org/10.1016/j.desal.2016.11.010
Cheng, S., Oatley, D. L., Williams, P. M., & Wright, C. J. (2011). Positively charged nanofiltration membranes: review of current fabrication methods and introduction of a novel approach. Advances in Colloid and Interface Science, 164(1-2), 12-20. https://doi.org/10.1016/j.cis.2010.12.010
James, S. A., Hilal, N., & Wright, C. J. (2017). Atomic force microscopy studies of bioprocess engineering surfaces–imaging, interactions and mechanical properties mediating bacterial adhesion. Biotechnology Journal, 12(7), 1600698. https://doi.org/10.1002/biot.201600698
Zhu, R., Tan, D. T., & Shuai, D. (2016). Research highlights: applications of atomic force microscopy in natural and engineered water systems. Environmental Science: Water Research & Technology, 2(3), 415-420. https://doi.org/10.1039/C6EW90012G
Swansea University. (2023). Optimisation of membrane systems and its benefit to water treatment, food processing and medicine - from characterisation and fabrication to control. [Online]. Available at: https://www.swansea.ac.uk/research/research-impact/optimisation-of-membrane-systems-and-its-benefit-to-water-treatment-food-processing-and-medicine/
REF 2021. The commercial, production and environmental benefits of improved membrane design for industry and healthcare. [Online]. Impact Case Study (REF3). Available at: https://results2021.ref.ac.uk/impact/96e9f591-d560-4882-bb22-5a05d6e1a929/pdf