Detection of fouling on electrically conductive membranes by electrical impedance spectroscopy

Solar interfacial evaporation devices for desalination and water treatment: Perspective and future

Water is an essential commodity for society, and alternate resources such as seawater and wastewater are vital for the future. There are various desalination technologies that can provide sufficient and sustainable water sources. Renewable energy-based desalination technologies like solar-based interfacial evaporation are very efficient and sustainable desalination methods. Solar-based interfacial evaporation has been a focus due to its efficient and easy-to-use methods. Still, research is needed for fouling resistance, scalable and low-cost materials, and devices for solar interfacial evaporation. Recent research focuses on the materials for evaporation devices, but various other aspects of device design and fabrication methods are also necessary to improve device performance. In this article, all the evaporator device configurations and strategies for efficient evaporator devices are compiled and summarized. The evaporator devices have been classified into eight main categories: monolayer, bilayer, tree-like design, low-temperature designs, 3D-Origami-based designs, latent heat recovery design, design with storage/batch process, and contactless design. It was found that a good absorber, well-engineered air-water interface, and bottom-layer insulation are necessary for the best systems. The current research focuses on the vapor production output of the devices but not on the water production from devices. So, the focus on device-based water production and the associated cost of the water produced is essential. This article articulates the strategies and various scalable and efficient devices for evaporation-based solar-driven desalination. This article will be helpful for the researchers in improving devices output and coming up with a sustainable desalination and water treatment.

Design and characterization of an electrochemically-modulated membrane chromatography device

Membrane separations offer a compelling alternative to traditional chromatographic methods by overcoming mass transport limitations. We introduce an additional degree of freedom in modulating membrane chromatography by using metalized membranes in a potential-driven process. Investigating the impact of a gold coating on membrane characteristics, the sputtered gold layer enhances the surface conductivity with stable electrochemical behavior. However, this comes at the expense of reduced permeability, wettability, and static binding capacity (∼ 474 µg g-1 of maleic acid). The designed device displayed a homogenous flow distribution, and the membrane electrodes exhibit predominantly capacitive behavior during potential application. Modulating the electrical potential during the adsorption and desorption phase strongly influenced the binding and elution behavior of anion-exchange membranes. Switching potentials between ±1.0 V vs. Ag/AgCl induces desorption, confirming the process principle. Elution efficiency reaches up to 58 % at -1.0 V vs. Ag/AgCl in the desorption phase without any alteration of the mobile phase. Increasing the potential perturbation ranging from +1.0 V to -1.0 V vs. Ag/AgCl resulted in reduced peak width and improved elution behavior, demonstrating the feasibility of electrochemically-modulated membrane chromatography. The developed process has great potential as a gentle and sustainable separation step in the biotechnological and chemical industry.

Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring

Membrane distillation (MD) is a thermally driven separation process that is driven by phase change. The core of this technology is the hydrophobic microporous membrane that prevents mass transfer of the liquid while allowing the vapor phase to pass through the membrane's pores. Currently, MD is challenged by its high energy consumption and membrane degradation due to fouling, scaling and wetting. The use of electrically conductive membranes (ECMs) is a promising alternative method to overcome these challenges by inducing localized Joule heating, as well as mitigating and monitoring membrane fouling/scaling/wetting. The objective of this review is to consolidate recent advances in ECMs from the standpoint of conductive materials, membrane fabrication methodologies, and applications in MD processes. First, the mechanisms of ECMs-based MD processes are reviewed. Then the current trends in conductive materials and membrane fabrication methods are discussed. Thereafter, a comprehensive review of ECMs in MD applications is presented in terms of the different processes using Joule heating and various works related to membrane fouling, scaling, and wetting control and monitoring. Key insights in terms of energy consumption, economic viability and scalability are furnished to provide readers with a holistic perspective of the ECMs potential to achieve better performances and higher efficiencies in MD. Finally, we illustrate our perspectives on the innovative methods to address current challenges and provide insights for advancing new ECMs designs. Overall, this review sums up the current status of ECMs, looking at the wide range of conductive materials and array of fabrication methods used thus far, and putting into perspective strategies to deliver a more competitive ECMs-based MD process in water treatment.

Designing sustainable membrane-based water treatment via fouling control through membrane interface engineering and process developments

Membrane-based water treatment processes have been established as a powerful approach for clean water production. However, despite the significant advances made in terms of rejection and flux, provision of sustainable and energy-efficient water production is restricted by the inevitable issue of membrane fouling, known to be the major contributor to the elevated operating costs due to frequent chemical cleaning, increased transmembrane resistance, and deterioration of permeate flux. This review provides an overview of fouling control strategies in different membrane processes, such as microfiltration, ultrafiltration, membrane bioreactors, and desalination via reverse osmosis and forward osmosis. Insights into the recent advancements are discussed and efforts made in terms of membrane development, modules arrangement, process optimization, feed pretreatment, and fouling monitoring are highlighted to evaluate their overall impact in energy- and cost-effective water treatment. Major findings in four key aspects are presented, including membrane surface modification, modules design, process integration, and fouling monitoring. Among the above mentioned anti-fouling strategies, a large part of research has been focused on membrane surface modifications using a number of anti-fouling materials whereas much less research has been devoted to membrane module advancements and in-situ fouling monitoring and control. At the end, a critical analysis is provided for each anti-fouling strategy and a rationale framework is provided for design of efficient membranes and process for water treatment.

Electrochemical impedance spectroscopy (EIS) reveals the role of microbial fuel cell-ceramic membrane bioreactor (MFC-CMBR): Electricity utilization and membrane fouling

Ceramic membrane has become a major concern due to creasing cost and competitive efficiency. Microbial fuel cell-ceramic membrane bioreactor (MFC-CMBR) is considered alternative technology for larger-scale industrial application because of its advantages of convenient detecting and control of membrane fouling. However, MFC-CMBR are highly susceptible to membrane fouling and harsh operating requirements in these wastewaters of different compositions. This research critically discusses that electrochemical response in different types of MFC-CMBRs and control of electricity utilization on ceramic membrane fouling. The experimental results indicated that the application of sludge acclimated in coupling system with higher external resistance could ensure that lower costs (electricity utilization and membrane cleaning) provided enough membrane fouling control. The improved performance of MFC-CMBR-1 could be attributed to its enhanced nitrification/denitrification activity and capacity of electrons migration between electrode and sludge mixture. The coupling system alleviated membrane fouling and impedance increasing by improving the characteristics of sludge (increased particle size, decreased adsorption adhesion free energy), EPS (decreased hydrophobicity, molecular weight distribution regulation). And filtration tests showed that roughness and contact angle for the MFC-CMBR tend for better development compared to CMBR, dependent on the changes in the chemical surface groups as a result of electric distribution ratio. In addition, correlation analysis and filtration experiments showed that the extracellular polymer substances (EPS) enhanced the charge transfer resistance (R_ct), and the protein substance in EPS was the main fouling substance when external resistance was close to the internal resistance of MFC. In summary, the low internal resistance of ceramic membrane lead to obvious better fouling control and electricity utilization than organic membrane, and the paper provides insight into the MFC-CMBR systems for a wide range of detecting membrane fouling and applications of membrane fouling mitigation.

Trapping and detecting nanoplastics by MXene-derived oxide microrobots

Nanoplastic pollution, the final product of plastic waste fragmentation in the environment, represents an increasing concern for the scientific community due to the easier diffusion and higher hazard associated with their small sizes. Therefore, there is a pressing demand for effective strategies to quantify and remove nanoplastics in wastewater. This work presents the “on-the-fly” capture of nanoplastics in the three-dimensional (3D) space by multifunctional MXene-derived oxide microrobots and their further detection. A thermal annealing process is used to convert Ti₃C₂T_x MXene into photocatalytic multi-layered TiO₂, followed by the deposition of a Pt layer and the decoration with magnetic γ-Fe₂O₃ nanoparticles. The MXene-derived γ-Fe₂O₃/Pt/TiO₂ microrobots show negative photogravitaxis, resulting in a powerful fuel-free motion with six degrees of freedom under light irradiation. Owing to the unique combination of self-propulsion and programmable Zeta potential, the microrobots can quickly attract and trap nanoplastics on their surface, including the slits between multi-layer stacks, allowing their magnetic collection. Utilized as self-motile preconcentration platforms, they enable nanoplastics’ electrochemical detection using low-cost and portable electrodes. This proof-of-concept study paves the way toward the “on-site” screening of nanoplastics in water and its successive remediation.

Nanoplastic water pollution represents an increasing concern. Here, photogravitactic MXene-derived microrobots are programmed to trap nanoplastics in the layered structure and magnetically transfer them to low-cost electrodes for further detection.

Membrane Distillation Hybrid Peroxydisulfate Activation toward Mitigating the Membrane Wetting by Sodium Dodecyl Sulfate

The fouling/wetting of hydrophobic membrane caused by organic substances with low-surface energy substantially limits the development of the membrane distillation (MD) process. The sulfate radical (SO4 ·−)-based advanced oxidation process (AOP) has been a promising technology to degrade organics in wastewater treatment, and peroxydisulfate (PDS) could be efficiently activated by heat. Thus, a hybrid process of MD-AOP via PDS activated by a hot feed was hypothesized to mitigate membrane fouling/wetting. Experiments dealing with sodium dodecyl sulfate (SDS) containing a salty solution via two commercial membranes (PVDF and PTFE) were performed, and varying membrane wetting extents in the coupling process were discussed at different PDS concentrations and feed temperatures. Our results demonstrated permeate flux decline and a rise in conductivity due to membrane wetting by SDS, which was efficiently alleviated in the hybrid process rather than the standalone MD process. Moreover, such a mitigation was enhanced by a higher PDS concentration up to 5 mM and higher feed temperature. In addition, qualitative characterization on membrane coupons wetted by SDS was successfully performed using electrochemical impedance spectroscopy (EIS). The EIS results implied both types of hydrophobic membranes were protected from losing their hydrophobicity in the presence of PDS activation, agreeing with our initial hypothesis. This work could provide insight into future fouling/wetting control strategies for hydrophobic membranes and facilitate the development of an MD process.

Mixed metal oxide anodes used for the electrochemical degradation of a real mixed industrial wastewater

Mixed industrial wastewaters are often highly contaminated with heavy metals and organic pollutants. Treating these mixed wastewaters requires many stagewise unit operations. Our work investigates using an electrochemical oxidation-in-situ coagulation (ECO-IC) process as a pre-treatment step toward the efficient treatment of real mixed industrial wastewater rich with heavy metals and organic contaminants. The process degraded organic contaminants in the wastewater via anodic electrochemical oxidation. Simultaneously, heavy metals were precipitated in the solution by coagulants (iron hydroxides) formed in-situ by cathode-generated hydroxyl ions reacting with the significant amounts of dissolved iron in the wastewater. IrO₂-RuO₂ mixed metal oxide anodes were identified as the best electrodes for organic compound degradation demonstrating 97% degradation of methyl orange (MO) as a model compound within 15 min. These anodes were used to treat real industrial wastewater produced from the industrial cleaning of train tanker cars transporting industrial solvents. The electrochemical treatment experiments resulted in a treated solution with a lower heavy metal content, achieving 96% reduction in Fe and 30% reduction in As content. Only moderate decreases in organic content were observed up to a maximum of 13% reduction in total organic carbon after 1 h of treatment. Electrochemical treatment of the mixed industrial wastewater produced greater effective diameter of the suspended particles and distinct sediment, liquid, and suspended foam phases that could be easily separated for further treatment. ECO-IC shows promise as an efficient and chemical-free method to coagulate heavy metals in real industrial wastewaters and could be an effective pre-treatment in their separation.

Recent Progress in One- and Two-Dimensional Nanomaterial-Based Electro-Responsive Membranes: Versatile and Smart Applications from Fouling Mitigation to Tuning Mass Transport

Membrane technologies are playing an ever-important role in the field of water treatment since water reuse and desalination were put in place as alternative water resources to alleviate the global water crisis. Recently, membranes are becoming more versatile and powerful with upgraded electroconductive capabilities, owing to the development of novel materials (e.g., carbon nanotubes and graphene) with dual properties for assembling into membranes and exerting electrochemical activities. Novel nanomaterial-based electrically responsive membranes have been employed with promising results for mitigating membrane fouling, enhancing membrane separation performance and self-cleaning ability, controlling membrane wettability, etc. In this article, recent progress in novel-nanomaterial-based electrically responsive membranes for application in the field of water purification are provided. Thereafter, several critical drawbacks and future outlooks are discussed.

Property Characterization and Mechanism Analysis of Polyoxometalates-Functionalized PVDF Membranes by Electrochemical Impedance Spectroscopy

Polyoxometalates (POMs) has proved its advantage in constructing high-performance nanocomposite membranes such as catalytic membranes, adsorptive membranes, and forward osmosis membranes. However, it is challenging or tedious to characterize its distribution and effect on the membrane structures due to the equipment resolution limitation, discrete nano-scaled structures of POMs, and limited doping amount compared to the polymeric membrane matrix. In this paper, POMs-functionalized polyvinylidene fluoride (PVDF) membranes were fabricated by phase inversion combined with the sol-gel method, and electrochemical impedance spectroscopy (EIS) was utilized to analyze the nanocomposite membrane intrinsic properties. Through adjusting the additives in the sol-forming process, a set of membranes with varied intrinsic properties were developed accordingly. The wetting degree of the membranes related to the hydrophilic nature of the membrane surfaces had a crucial influence on the impedance measurements at the early stage. Through EIS analysis, it was demonstrated that the amination of the membrane matrix through (3-aminopropyl)trimethoxysilane (APTMS) treatment and the immobilization of POMs through electrostatic attraction would not generate new pore structures into the membrane and only alter the membrane surface roughness and composition. To my knowledge, it is the first time that EIS was utilized to characterize the hydrophilicity of the membranes and pore structures of the POMs-modified membranes. Our findings indicate that EIS can provide valuable information for probing the structures of other nano-materials-incorporated membranes.

A Conductive Microfiltration Membrane for In Situ Fouling Detection: Proof-of-Concept Using Model Wine Solutions

Cross-flow microfiltration, using a microporous membrane, is a well-established technique for wine clarification in oenology because of its cost-effectiveness and high-throughput. However, membrane fouling remains a significant issue for wine filtration in high-throughput systems. Herein, an approach for in situ real-time monitoring of fouling in filtration systems using a conductive filtration membrane and a model fluid for filtration is reported. The membrane is fabricated by embedding poly(3,4-ethylenedioxythiophene) into an electrospun sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene microporous membrane, producing a conductive microfiltration membrane. Measurement of the resistance of the conductive membrane during filtration with the fouling solutions containing pectin, as one of the major foulants in unfiltered wine and pre-fermentation grape juice, shows a time- and concentration-dependent response. This work opens a door to new methodology for in situ monitoring of fouling processes in wine and juice filtration systems.