Fouling in reverse osmosis membranes: monitoring, characterization, mitigation strategies and future directions

Deciphering Mechanisms of Silica-Metal Scaling on RO Membranes via 3D Structural and Compositional Analysis

Deciphering the structure and composition of the scaling layer is crucial for understanding its formation mechanisms in the reverse osmosis (RO) process. However, conventional characterization techniques face challenges in providing high three-dimensional resolution and precise compositional analysis of mixed scales, which hinders in-depth elucidation of the underlying mechanisms. In this study, we combined the exceptional depth resolution of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the superior mixed-scale discrimination capability of thermogravimetry-infrared spectroscopy (TG-IR) to analyze Si/Al scaling, a common issue in industrial RO systems. Under acid conditions, ToF-SIMS measurements revealed Al species enrichment on the membrane surface, attributed to the strong affinity between Al3+ and the membrane. The preferential deposition of Al3+ further facilitated the heterogeneous nucleation of polymerized silica through the electrostatic shielding effect, leading to the rapid formation of a thin and dense scaling layer. In contrast, neutral and alkaline conditions produced a slower-developing, uniform, thicker, and loosely structured scaling layer through physical deposition of supersaturated Si/Al complex scales. TG-IR analysis revealed that neutral conditions favored coprecipitated adsorption-bound Si/Al species (6-coordinate Al) and Si/Al polymers (4-coordinate Al), whereas alkaline conditions primarily produced coprecipitated silica and Al(OH)4-. These findings advance the mechanistic understanding of Si/Al scaling and provide a foundation for targeted control strategies in silica-metal combined scaling in RO systems.

Zinc-based metal-organic frameworks for sustainable water desalination and anti-scaling solutions

Water scarcity and pollution pose significant challenges worldwide, necessitating innovative solutions for sustainable water supply. Traditional desalination methods have limitations in terms of energy consumption, fouling, and environmental impact. This study focuses on the synthesis and characterization of zinc-based metal-organic frameworks (Zn-MOFs) as advanced fillers for desalination techniques. Zn-MOFs were synthesized using a simple precipitation technique and characterized using techniques such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy. The performance of Zn-MOFs was evaluated in terms of scale deformation experiments. The findings revealed that Zn-MOFs not only significantly reduce the concentration of Ca2⁺ ions responsible for scale (e.g., calcium carbonate scale) formation but also exhibit superior fouling resistance and high salt rejection capabilities. At a dosage of 3000 mg/L and pH 7.5, a remarkable 99% removal efficiency was achieved for half-scale concentration (synthetic water was prepared by the following scale concentrations: 3665 mg/L CaCl2, 685 mg/L NaHCO3, and 12,000 mg/L NaCl), while a 91.6% efficiency was obtained at normal scale concentrations (synthetic water was prepared by the following scale concentrations: 7330 mg/L CaCl2, 1370 mg/L NaHCO3, and 24,000 mg/L NaCl). These results highlight the Zn-MOFs' advantages over conventional fillers and traditional techniques by offering improved stability, superior adsorption capacity, and enhanced scale management for desalination applications. This work contributes to advancing water treatment technologies by providing a more sustainable and effective approach for mitigating fouling and enhancing desalination efficiency.

Reverse Osmosis Membrane Engineering: Multidirectional Analysis Using Bibliometric, Machine Learning, Data, and Text Mining Approaches

Membrane engineering is a complex field involving the development of the most suitable membrane process for specific purposes and dealing with the design and operation of membrane technologies. This study analyzed 1424 articles on reverse osmosis (RO) membrane engineering from the Scopus database to provide guidance for future studies. The results show that since the first article was published in 1964, the domain has gained popularity, especially since 2009. Thin-film composite (TFC) polymeric material has been the primary focus of RO membrane experts, with 550 articles published on this topic. The use of nanomaterials and polymers in membrane engineering is also high, with 821 articles. Common problems such as fouling, biofouling, and scaling have been the center of work dedication, with 324 articles published on these issues. Wang J. is the leader in the number of published articles (73), while Gao C. is the leader in other metrics. Journal of Membrane Science is the most preferred source for the publication of RO membrane engineering and related technologies. Author social networks analysis shows that there are five core clusters, and the dominant cluster have 4 researchers. The analysis of sentiment, subjectivity, and emotion indicates that abstracts are positively perceived, objectively written, and emotionally neutral.

Combined Effects of Surface Roughness, Solubility Parameters, and Hydrophilicity on Biofouling of Reverse Osmosis Membranes

The fouling of protein on the surface of reverse osmosis (RO) membranes is a surface phenomenon strongly dependent on the physical and chemical characteristics of both the membrane surface and the foulant molecule. Much of the focus on fouling mitigation is on the synthesis of more hydrophilic membrane materials. However, hydrophilicity is only one of several factors affecting foulant attachment. A more systematic and rationalized methodology is needed to screen the membrane materials for the synthesis of fouling-resistant materials, which will ensure the prevention of the accumulation of foulants on the membrane surfaces, avoiding the trial and error methodology used in most membrane synthesis in the literature. If a clear correlation is found between various membrane surface properties, in combination or singly, and the amount of fouling, this will facilitate the establishment of a systematic strategy of screening materials and enhance the selection of membrane materials and therefore will reflect on the efficiency of the membrane process. In this work, eight commercial reverse osmosis membranes were tested for bovine serum albumin (BSA) protein fouling. The work here focused on three surface membrane properties: the surface roughness, the water contact angle (hydrophilicity), and finally the Hansen solubility parameter (HSP) distance between the foulant understudy (BSA protein) and the membrane surface. The HSP distance was investigated as it represented the affinities of materials to each other, and therefore, it was believed to have an important contribution to the tendency of foulant to stick to the surface of the membrane. The results showed that the surface roughness and the HSP distance contributed to membrane fouling more than the hydrophilicity. We recommend taking into account the HSP distance between the membrane material and foulants when selecting membrane materials.

Current and emerging trends of inorganic, organic and eco-friendly corrosion inhibitors

Effective corrosion control strategies are highly desired to reduce the fate of corrosion. One widely adopted approach is the use of corrosion inhibitors, which can significantly mitigate the detrimental effects of corrosion. This systematic review provides a thorough analysis of corrosion inhibitors, including both inorganic and organic compounds. It explores the inhibition mechanisms, highlighting the remarkable inhibitive efficiency of organic compounds attributed to the presence of heteroatoms and conjugated π-electron systems. The review presents case studies and investigations of corrosion inhibitors, shedding light on their performance and application potential. Moreover, it compares the efficacy, compatibility, and sustainability of emerging environmentally friendly corrosion inhibitors, including biopolymers from natural resources as promising candidates. The review also highlights the potential of synergistic impacts between mixed corrosion inhibitors, particularly organic/organic systems, as a viable and advantageous choice for applications in challenging processing environments. The evaluation of inhibitors is discussed, encompassing weight loss (WL) analysis, electrochemical analysis, surface analysis, and quantum mechanical calculations. The review also discusses the thermodynamics and isotherms related to corrosion inhibition, further improving the understanding of inhibitor's behavior and mechanisms. This review serves as a valuable resource for researchers, engineers, and practitioners involved in corrosion control, offering insights and future directions for effective and environmentally friendly corrosion inhibition strategies.

Removal of Heavy Metals from Wastewaters and Other Aqueous Streams by Pressure-Driven Membrane Technologies: An Outlook on Reverse Osmosis, Nanofiltration, Ultrafiltration and Microfiltration Potential from a Bibliometric Analysis

A bibliometric study to analyze the scientific documents released until 2024 in the database Scopus related to the use of pressure-driven membrane technologies (microfiltration, ultrafiltration, nanofiltration and reverse osmosis) for heavy metal removal was conducted. The work aimed to assess the primary quantitative attributes of the research in this field during the specified period. A total of 2205 documents were identified, and the corresponding analysis indicated an exponential growth in the number of publications over time. The contribution of the three most productive countries (China, India and USA) accounts for more than 47.1% of the total number of publications, with Chinese institutions appearing as the most productive ones. Environmental Science was the most frequent knowledge category (51.9% contribution), followed by Chemistry and Chemical Engineering. The relative frequency of the keywords and a complete bibliometric network analysis allowed the conclusion that the low-pressure technologies (microfiltration and ultrafiltration) have been more deeply investigated than the high-pressure technologies (nanofiltration and reverse osmosis). Although porous low-pressure membranes are not adequate for the removal of dissolved heavy metals in ionic forms, the incorporation of embedded adsorbents within the membrane structure and the use of auxiliary chemicals to form metallic complexes or micelles that can be retained by this type of membrane are promising approaches. High-pressure membranes can achieve rejection percentages above 90% (99% in the case of reverse osmosis), but they imply lower permeate productivity and higher costs due to the required pressure gradients.

Unveiling the photocatalytic potential of graphitic carbon nitride (g-C3N4): a state-of-the-art review

Graphitic carbon nitride (g-C3N4)-based materials have emerged as promising photocatalysts due to their unique band structure, excellent stability, and environmental friendliness. This review provides a comprehensive and in-depth analysis of the current state of research on g-C3N4-based photocatalysts. The review summarizes several strategies to improve the photocatalytic performance of pristine g-C3N4, e.g., by creating heterojunctions, doping with non-metallic and metallic materials, co-catalyst loading, tuning catalyst morphology, metal deposition, and nitrogen-defect engineering. The review also highlights the various characterization techniques employed to elucidate the structural and physicochemical features of g-C3N4-based catalysts, as well as their applications of in photocatalytic degradation and hydrogen production, emphasizing their remarkable performance in pollutants' removal and clean energy generation. Furthermore, this review article investigates the effect of operational parameters on the catalytic activity and efficiency of g-C3N4-based catalysts, shedding light on the key factors that influence their performance. The review also provides insights into the photocatalytic pathways and reaction mechanisms involving g-C3N4 based photocatalysts. The review also identifies the research gaps and challenges in the field and presents prospects for the development and utilization of g-C3N4-based photocatalysts. Overall, this comprehensive review provides valuable insights into the synthesis, characterization, applications, and prospects of g-C3N4-based photocatalysts, offering guidance for future research and technological advancements in this rapidly growing field.

Nanomaterials-modified reverse osmosis membranes: a comprehensive review

Because of its great efficiency and widespread application, reverse osmosis (RO) is a popular tool for water desalination and purification. However, traditional RO membranes have a short lifespan due to membrane fouling, deterioration, decreased salt rejection rate, and the low water flux with aging. As a result, membrane modification has received a lot of attention recently, with nanomaterials being extensively researched to improve membrane efficacy and lifespan. Herein, we present an in-depth analysis of recent advances of RO membranes modification utilizing nanomaterials. An overview of the various nanomaterials used for membrane modification, including metal oxides, zeolites, and carbon nanomaterials, is provided. The synthesis techniques and methods of integrating these nanomaterials into RO membranes are also discussed. The impacts of nanomaterial change on the performance of RO membranes are addressed. The underlying mechanisms responsible for RO membrane enhancements by nanomaterials, such as improved surface hydrophilicity, reduced membrane fouling via surface repulsion and anti-adhesion properties, and enhanced structural stability, are discussed. Furthermore, the review provides a critical analysis of the challenges and limitations associated with the use of nanomaterials to modify RO membranes. Overall, this review provides valuable insights into the modification of RO membranes with nanomaterials, providing a full grasp of the benefits, challenges, and future prospects of this challenging topic.

Scaling and cleaning of silica scales on reverse osmosis membrane: Effective removal and degradation mechanisms utilizing gallic acid

Silica scaling on membranes represents one of the most important issues in industrial water systems because of its complex composition and difficulty in removal. However, there is a lack of understanding of the mechanisms for cleaning silica scales from reverse osmosis (RO) membranes. To address this research gap, this study investigated the scaling and cleaning behavior of silica on RO membrane processes, with a specific focus on the silica scale cleaning mechanism using gallic acid (GA). The membrane flux continuously decreased with operation time, even at the lowest initial silicic acid concentration, owing to silica scale blockage. The GA solution exhibited a strong efficacy in cleaning silica-scaling RO membranes. The membrane flux returned to 89.7% of the initial value by removing 81.87% of the silica scale within the first 30 min of the study period. The cleaning mechanism of GA involved its adsorption onto the surface of silica scale particles to form a surface complex and subsequently transition into a water-soluble 1:3 complex within the solution. This complex interaction facilitated the gradual decomposition of the silica scales that adhered to the membrane surface. This study has valuable implications for the development of efficient and effective silica scale cleaning solutions, providing insights into the complex interplay between GA and silica scaling mechanisms.

State-of-the-Art Advances and Current Applications of Gel-Based Membranes

Gel-based membranes, a fusion of polymer networks and liquid components, have emerged as versatile tools in a variety of technological domains thanks to their unique structural and functional attributes. Historically rooted in basic filtration tasks, recent advancements in synthetic strategies have increased the mechanical strength, selectivity, and longevity of these membranes. This review summarizes their evolution, emphasizing breakthroughs that have positioned them at the forefront of cutting-edge applications. They have the potential for desalination and pollutant removal in water treatment processes, delivering efficiency that often surpasses conventional counterparts. The biomedical field has embraced them for drug delivery and tissue engineering, capitalizing on their biocompatibility and tunable properties. Additionally, their pivotal role in energy storage as gel electrolytes in batteries and fuel cells underscores their adaptability. However, despite monumental progress in gel-based membrane research, challenges persist, particularly in scalability and long-term stability. This synthesis provides an overview of the state-of-the-art applications of gel-based membranes and discusses potential strategies to overcome current limitations, laying the foundation for future innovations in this dynamic field.

Self-cleaning cellulose acetate/crystalline nanocellulose/polyvinylidene fluoride/Mg0.975Ni0.025SiO3membrane for removal of diclofenac sodium and methylene blue dye in water

Recalcitrant pollutants present in wastewater, without an effective treatment, have several effects on aquatic ecosystems and human health due to their chemical structure and persistence. Therefore, it is crucial the development of efficient technologies to eliminate such pollutants in water. Nano-photocatalysts are considered a promising technology for water remediation; however, one common drawback is the difficulty of recovering it after water processing. One effective strategy to overcome such problem is its immobilization into substrates such as polymeric membranes. In this study, a polymeric membrane with embedded Mg0.975Ni0.025SiO3is proposed to remove model pollutants diclofenac sodium and methylene blue dye by synergetic adsorption and photocatalytic processes. Mg0.975Ni0.025SiO3was synthesized by the combustion method. The matrix polymeric blend consisting of a blend of cellulose acetate, crystalline nanocellulose and polyvinylidene fluoride was obtained by the phase inversion method. The composite membranes were characterized by FTIR, x-ray diffraction, and scanning electron microscopy. With pollutant solutions at pH 7, the pollutant adsorption capacity of the membranes reached up to 30% and 45% removal efficiencies for diclofenac sodium and methylene blue, respectively. Under simulated solar irradiation photocatalytic removal performances of 70% for diclofenac sodium pH 7, and of 97% for methylene blue dye at pH 13, were reached. The membrane photocatalytic activity allows the membrane to avoid pollutant accumulation on its surface, given a self-cleaning property that allows the reuse of at least three cycles under sunlight simulator irradiation. These results suggest the high potential of photocatalytic membranes using suitable and economical materials such as cellulosic compounds and magnesium silicates for water remediation.

Surface Treatment of Polymer Membranes for Effective Biofouling Control

Membrane biofouling is the consequence of the deposition of microorganisms on polymer membrane surfaces. Polymeric membranes have garnered more attention for filtering and purifying water because of their ease of handling, low cost, effortless surface modification, and mechanical, chemical, and thermal properties. The sizes of the pores in the membranes enable micro- and nanofiltration, ultrafiltration, and reverse osmosis. Commonly used polymers for water filter membranes are polyvinyl chloride (PVA), polyvinylidene fluoride (PVDF), polyamide (PA), polyethylene glycol (PEG), polyethersulfone (PES), polyimide (PI), polyacrylonitrile (PAN), polyvinyl alcohol (PA), poly (methacrylic acid) (PMAA), polyaniline nanoparticles (PANI), poly (arylene ether ketone) (PAEK), polyvinylidene fluoride polysulfone (PSF), poly (ether imide) (PEI), etc. However, these polymer membranes are often susceptible to biofouling because of inorganic, organic, and microbial fouling, which deteriorates the membranes and minimizes their lives, and increases operating costs. Biofouling infection on polymer membranes is responsible for many chronic diseases in humans. This contamination cannot be eliminated by periodic pre- or post-treatment processes using biocides and other chemicals. For this reason, it is imperative to modify polymer membranes by surface treatments to enhance their efficiency and longevity. The main objective of this manuscript is to discuss application-oriented approaches to control biofouling on polymer membranes using various surface treatment methods, including nanomaterials and fouling characterizations utilizing advanced microscopy and spectroscopy techniques.

In-depth insight on microbial electrochemical systems coupled with membrane bioreactors for performance enhancement: a review

A conventional activated sludge (CAS) system has traditionally been used for secondary treatment in wastewater treatment plants. Due to the high cost of aeration and the problem of sludge treatment, researchers are developing alternatives to the CAS system. A membrane bioreactor (MBR) is a technology with higher solid-liquid separation efficiency. However, the use of MBR is limited due to inevitable membrane fouling and high energy consumption. Membrane fouling requires frequent cleaning, and MBR components must be replaced, which reduces membrane lifetime and operating costs. To overcome the limitations of the MBR system, a microbial fuel cell-membrane bioreactor (MFC-MBR) coupling system has attracted the interest of researchers. The design of the novel bioelectrochemical membrane reactor (BEMR) can effectively couple microbial degradation in the microbial electrochemical system (MES) and generate a microelectric field to reduce and alleviate membrane fouling in the MBR system. In addition, the coupling system combining an MES and an MBR can improve the efficiency of COD and ammonium removal while generating electricity to balance the energy consumption of the system. However, several obstacles must be overcome before the MFC-MBR coupling system can be commercialised. The aim of this study is to provide critical studies of the MBR, MES and MFC-MBR coupling system for wastewater treatment. This paper begins with a critical discussion of the unresolved MBR fouling problem. There are detailed past and current studies of the MES-MBR coupling system with comparison of performances of the system. Finally, the challenges faced in developing the coupling system on a large scale were discussed.

Production and Bioconversion Efficiency of Enzyme Membrane Bioreactors in the Synthesis of Valuable Products

The demand for bioactive molecules with nutritional benefits and pharmaceutically important properties is increasing, leading researchers to develop modified production strategies with low-cost purification processes. Recent developments in bioreactor technology can aid in the production of valuable products. Enzyme membrane bioreactors (EMRs) are emerging as sustainable synthesis processes in various agro-food industries, biofuel applications, and waste management processes. EMRs are modified reactors used for chemical reactions and product separation, particularly large-molecule hydrolysis and the conversion of macromolecules. EMRs generally produce low-molecular-weight carbohydrates, such as oligosaccharides, fructooligosaccharides, and gentiooligosaccharides. In this review, we provide a comprehensive overview of the use of EMRs for the production of valuable products, such as oligosaccharides and oligodextrans, and we discuss their application in the bioconversion of inulin, lignin, and sugars. Furthermore, we critically summarize the application and limitations of EMRs. This review provides important insights that can aid in the production of valuable products by food and pharmaceutical industries, and it is intended to assist scientists in developing improved quality and environmentally friendly prebiotics using EMRs.