Progress of photothermal membrane distillation for decentralized desalination: A review

Interfacial Heating in Membrane Distillation: Advances, Optimization Strategies, and Industrial Applications for Desalination

Interfacial heating (IH) membrane distillation (MD) is a promising MD variation with significant potential for freshwater production from brine and seawater. Unlike conventional MD, IH-MD heats locally between the hydrophobic membrane and saline water to enhance the vapor flux and minimize heat loss. However, a unified understanding of the performance of various IH-MD systems remains lacking. Stability challenges such as membrane wetting, scaling, fouling, and corrosion caused by the introduction of heating materials pose significant obstacles to industrial application. This review critically examined recent advances in interfacial heating methods, including photothermal, Joule, conduction, and induction heating. Photothermal approaches offer sustainability and improved energy efficiency but are limited by sunlight exposure and adsorption, while electrothermal methods provide stable interfacial heating flux at the cost of higher energy use and potential material degradation. Strategies to enhance energy performance and durability are discussed in detail, such as combining multiple heating methods, refining module and configuration designs, optimizing membrane properties, and adjusting operating conditions. We also assessed the economic viability of IH-MD for industrial applications. While IH-MD faces challenges related to material durability, system complexity, and scale-up, its ability to eliminate thermal polarization, reduce energy consumption, and enable integration with renewable energy sources positions it as a transformative approach for future sustainable desalination and water purification technologies. This review aims to bridge the knowledge gaps between scientific innovation and real-world applications of various IH-MD technologies.

Monstera adansonii-inspired lignocellulose-derived bionic solar evaporator for solar steam generation

A novel three-dimensional solar evaporator (PPy@CC) inspired by monstera adansonii was developed by the carbonization and polypyrrole decoration of waste lignocellulose (corncob). The original microscopic porous structure was retained by the carbonization of corncob, and the bionic monstera adansonii leaves were formed by loading polypyrrole on its evaporation interface, which enhanced the absorption capacity of sunlight and provided sufficient diffusion channels for water through the porous interconnect structure. The results showed that the PPy@CC has the average evaporation rates of 2.417 kg m-2 h-1 and 2.048 kg m-2 h-1 respectively in pure water and 3.5 wt% seawater, and has better salt resistance and stability. In addition, the PPy@CC evaporator can also be used to efficiently generate water steam in alkaline and acidic wastewater, the pH values of the desalinated water obtained in both scenarios were neutral, showing excellent wastewater treatment effect. Outdoor experiments further confirmed its potential for practical applications, an average evaporation rate of 2.822 kg m-2 h-1 in 3.5 wt% seawater can be obtained, and stable performance over long periods of operation with no salt crystallization observed. Therefore, PPy@CC was a solar evaporator with practical application potential, which was suitable for seawater desalination and industrial wastewater treatment.

Regulating gypsum scaling-induced wetting in membrane distillation by heterogeneous crystallization: Role of filter media

Mineral scaling and scaling-induced wetting are critical issues in membrane distillation (MD) during treatment of saline wastewaters. Gypsum scaling and scaling-induced wetting in MD were successfully regulated by heterogeneous crystallization with in-line granular filtration in this study. Stable water recovery increased from 32.5 % to more than 52.5 % in one-cycle operation, depending on filter media properties. Because a large mass of crystals were retained or/and adsorbed in the granular filter, the scaling mass on membrane surface was reduced by 41.2 %, 23.1 %, 54.7 % and 78.1 % by filter charged with activated carbon, sand, fiber and activated alumina, respectively. When activated carbon, sand, fiber and activated alumina were used, the final MD fluxes were 1.58, 1.04, 1.96 and 3.43 times that without filter, and permeate conductivity decreased by 43.0 %, 46.8 %, 83.2 % and 81.3 %, respectively. The multi-cycle tests showed that heterogeneous crystallization gradually occurred in the granular filter, thereby promoting seeding-induced crystallization that reduced gypsum scaling and scaling-induced wetting in MD. Excellent anti-scaling and anti-wetting performance of in-line granular filtration was also confirmed for synthetic and real industrial wastewater. The results of this study provide guidance for mineral scaling control in MD to allow resource utilization for saline wastewater.

Ceramic-carbon Janus membrane for robust solar-thermal desalination

The desalination performance of conventional distillation membranes is limited by insufficient stability and energy efficiency, impeding their application in sustainable water production. Herein, we report a ceramic-carbon Janus membrane with solar-thermal functionality for enhanced desalination performance, energy efficiency, and stability for hypersaline water treatment. The feed and permeate sides of this Janus membrane are designed with different properties such as wettability, conductivity, and solar-thermal conversion to enhance performance. We demonstrate that this membrane exhibits higher solar-thermal efficiency (66.8-68.8%) and water flux (3.3-5.1 L m-2 h-1) than most existing polymeric solar-thermal distillation membranes. Simulation results ascribe enhanced performance to an increased membrane surface temperature, which mitigates temperature polarization and attenuation, thus enhancing the desalination driving force. The nano-carbon membrane surface accelerates water evaporation by inducing a transition from free water to intermediate water with decreased hydrogen bonding and a lower evaporation energy barrier. Water vapor molecules transport through the membrane pores by a combined mechanism of Knudsen diffusion and viscous flow. Even for seawater and hypersaline water, the membrane exhibits stable water flux and salt rejection due to its scaling-resistant surface and stable interfacial temperature. This work provides a strategy for rationally designing next-generation Janus membranes for sustainable water purification.

Minimizing the Use of Per- and Polyfluoroalkyl Substances for Textured Wetting-Resistant Surfaces

Per- and polyfluoroalkyl substances (PFAS) have been used as synthetic chemicals to create textured wetting-resistant surfaces, which have a broad range of applications including omniphobic membranes, self-cleaning textiles, and anticorrosion coatings. However, the high persistence, toxicity, and bioaccumulation potential of PFAS have led to rising public concerns and stringent regulations, especially after the U.S. Environmental Protection Agency (USEPA) announced legally enforceable maximum contamination levels for six PFAS species in April 2024. In this paper, we provide our perspective that the use of PFAS can be avoided in the fabrication of textured omniphobic and superomniphobic surfaces, which display high wetting resistance against not only high surface tension liquids but also more importantly low surface tension liquids. We first discuss the role of PFAS in the design of conventional wetting-resistant surfaces. We then discuss the state-of-the-art strategies for creating PFAS-free textured omniphobic and superomniphobic surfaces with high wetting resistance while elucidating the underlying mechanism. Further, we emphasize that PFAS are indeed not always needed for textured surfaces with a sufficiently high wetting resistance in specific environmental applications such as desalination and wastewater treatment. We envision that this paper will motivate the scientific community to rethink and revolutionize the design framework toward more sustainable wetting-resistant surfaces, thereby circumventing the use of PFAS and the consequent health and environmental risks.

Hierarchically Promoted Light Harvesting and Management in Photothermal Solar Steam Generation

Solar steam generation (SSG) presents a promising approach to addressing the global water crisis. Central to SSG is solar photothermal conversion that requires efficient light harvesting and management. Hierarchical structures with multi-scale light management are therefore crucial for SSG. At the molecular and sub-nanoscale levels, materials are fine-tuned for broadband light absorption. Advancing to the nano- and microscale, structures are tailored to enhance light harvesting through internal reflections, scattering, and diverse confinement effects. At the macroscopic level, light capture is optimized through rationally designed device geometries, configurations, and arrangements of solar absorber materials. While the performance of SSG relies on various factors including heat transport, physicochemical interactions at the water/air and material/water interfaces, salt dynamics, etc., efficient light capture and utilization holds a predominant role because sunlight is the sole energy source. This review focuses on the critical, yet often underestimated, role of hierarchical light harvesting/management at different dimensional scales in SSG. By correlating light management with the structure-property relationships, the recent advances in SSG are discussed, shedding light on the current challenges and possible future trends and opportunities in this domain.

Preparation of sulfur-doped porous carbon from polyphenylene sulfide waste for photothermal conversion materials to achieve solar-driven water evaporation

In recent years, solar-driven photothermal water evaporation technology for seawater desalination and wastewater treatment has developed rapidly, which is of great significance for addressing the issue of freshwater scarcity. However, due to the high costs associated with the manufacturing, maintenance, and operation of such devices, their application remains challenging in remote and resource-scarce regions. Due to its excellent light absorption capability in the near-infrared region, high hydrophilicity, and stable chemical properties, coupled with the low cost of recycling waste carbonized polyphenylene sulfide, this material is an excellent choice as a photothermal material for solar-driven water evaporation devices. Ordinary wood in nature usually has a highly regenerative porous structure, which is a natural water transport channel that facilitates the transport of water from the bottom to the top, allowing it to be rapidly converted into vapor. Based on this characteristic, this article innovatively proposes to prepare waste polyphenylene sulfide from porous carbonized materials (KCP) as the photothermal conversion material for novel photothermal water evaporation devices, achieving solar-driven water evaporation. This material efficiently facilitates the conversion between solar and thermal energies and exhibits excellent hydrophilicity, thereby enabling the rapid utilization of absorbed solar energy for water evaporation on the surface of the evaporator. In this study, a porous carbonized polyphenylene sulfide photothermal water evaporator (KCP-wood) was fabricated by using freeze-drying and in situ coating to load the photothermal conversion material onto a wood substrate. Under simulated one-sun irradiation, this evaporator achieved a water evaporation rate of 2.41 kg m-2 h-1 and a photothermal conversion efficiency of 91.3%. Additionally, a systematic study was conducted on the photothermal performance of various light-water evaporators, encompassing photothermal conversion efficiency, stability, thermal conductivity, and anti-fouling capabilities. Finally, the practical performance of the light-water evaporator under various environmental conditions was validated, demonstrating its excellent stability and durability. It is capable of effectively applying to high-efficiency water resource utilization and solar energy conversion fields.

Refractory plasmonic material based floating solar still for simultaneous desalination and electricity generation

Floating interfacial solar evaporation offers a land-saving, eco-friendly, and low-infrastructure alternative for freshwater production. However, challenges include maximizing heat localization, preventing salt accumulation, and operating under harsh environmental conditions. This work demonstrates a plasmonic titanium carbide (TiC) nanoparticle (NP)-based floating solar desalination system that produces clean water using sunlight on saline water sources. The components of the floating still were carefully chosen to optimize freshwater output, with TiC produced by upcycling tire waste. Outdoor experiments in Halifax, Canada, where solar insolation reached around 6 kW m-2 day-1, resulted in daily water yields of up to 3.67 L m-2, corresponding to a solar-to-vapor conversion efficiency of 40%. Water can be produced at a cost of $0.0086 L-1, and the still can be modified to generate thermoelectricity, enabling small onboard devices to test water quality without external electricity. This study contributes to the development of scalable floating solar desalination systems, providing potable water for water-stressed communities.

Solar-driven water evaporation using a collaborative photothermal conversion material system based on carbonized waste polyphenylene sulfide and copper sulfide

Recently, water resources have become scarce due to the growing global population and human impact on the environment, coupled with the effects of climate change. For solving the problem of global freshwater shortage and increasing the value of discarded polyphenylene sulfide (PPS) filter bags, in this study, balsa wood was used as the base of a photothermal solar evaporator, chitosan solution was used as the binder, and the main photothermal conversion materials used were polyphenylene sulfide (CP) carbide and copper sulfide. In order to create synergistic photothermal conversion materials, freeze-drying and in situ precipitation were used to deposit the photothermal conversion materials on top of the balsa wood. The prepared CP/CuS-wood evaporator has excellent water evaporation performance and light conversion capability, with a water evaporation rate of 2.68 kg m-2 h-1 and a photothermal conversion efficiency of 93.2% under simulated one solar intensity irradiation. In addition, the evaporator can effectively remove organic dyes such as methylene blue and methyl orange. The evaporator's durability and seawater desalination capability have also been confirmed through seawater desalination experiments and outdoor tests. Studies have shown that solar interface photothermal evaporators are a viable solution for desalination and wastewater treatment. This eco-friendly, economically viable and stable photothermal evaporator mentioned in this paper has pioneering features and will be a new paradigm for desalination and wastewater treatment.

Carbon-Based Nanocomposite Membranes for Membrane Distillation: Progress, Problems and Future Prospects

The development of an ideal membrane for membrane distillation (MD) is of the utmost importance. Enhancing the efficiency of MD by adding nanoparticles to or onto a membrane's surface has drawn considerable attention from the scientific community. It is crucial to thoroughly examine state-of-the-art nanomaterials-enabled MD membranes with desirable properties, as they greatly enhance the efficiency and reliability of the MD process. This, in turn, opens up opportunities for achieving a sustainable water-energy-environment nexus. By introducing carbon-based nanomaterials into the membrane's structure, the membrane gains excellent separation abilities, resistance to various feed waters, and a longer lifespan. Additionally, the use of carbon-based nanomaterials in MD has led to improved membrane performance characteristics such as increased permeability and a reduced fouling propensity. These nanomaterials have also enabled novel membrane capabilities like in situ foulant degradation and localized heat generation. Therefore, this review offers an overview of how the utilization of different carbon-based nanomaterials in membrane synthesis impacts the membrane characteristics, particularly the liquid entry pressure (LEP), hydrophobicity, porosity, and membrane permeability, as well as reduced fouling, thereby advancing the MD technology for water treatment processes. Furthermore, this review also discusses the development, challenges, and research opportunities that arise from these findings.

Recent Advances in High-Rate Solar-Driven Interfacial Evaporation

Recent advances in solar-driven interfacial evaporation (SDIE) have led to high evaporation rates that open promising avenues for practical utilization in freshwater production and industrial application for pollutant and nutrient concentration, and resource recovery. Breakthroughs in overcoming the theoretical limitation of 2D interfacial evaporation have allowed for developing systems with high evaporation rates. This study presents a comprehensive review of various evaporator designs that have achieved pure evaporation rates beyond 4 kg m-2 h-1, including structural and material designs allowing for rapid evaporation, passive 3D designs, and systems coupled with alternative energy sources of wind and joule heating. The operational mechanisms for each design are outlined together with discussion on the current benefits and areas for improvement. The overarching challenges encountered by SDIE concerning the feasibility of direct integration into contemporary practical settings are assessed, and issues relating to sustaining elevated evaporation rates under diverse environmental conditions are addressed.

Progress in membrane distillation processes for dye wastewater treatment: A review

Textile and cosmetic industries generate large amounts of dye effluents requiring treatment before discharge. This wastewater contains high levels of reactive dyes, low to none-biodegradable materials and chemical residues. Technically, dye wastewater is characterised by high chemical and biological oxygen demand. Biological, physical and pressure-driven membrane processes have been extensively used in textile wastewater treatment plants. However, these technologies are characterised by process complexity and are often costly. Also, process efficiency is not achieved in cost-effective biochemical and physical treatment processes. Membrane distillation (MD) emerged as a promising technology harnessing challenges faced by pressure-driven membrane processes. To ensure high cost-effectiveness, the MD can be operated by solar energy or low-grade waste heat. Herein, the MD purification of dye wastewater is comprehensively and yet concisely discussed. This involved research advancement in MD processes towards removal of dyes from industrial effluents. Also, challenges faced by this process with a specific focus on fouling are reviewed. Current literature mainly tested MD setups in the laboratory scale suggesting a deep need of further optimization of membrane and module designs in near future, especially for textile wastewater treatment. There is a need to deliver customized high-porosity hydrophobic membrane design with the appropriate thickness and module configuration to reduce concentration and temperature polarization (CP and TP). Also, energy loss should be minimized while increasing dye rejection and permeate flux. Although laboratory experiments remain pivotal in optimizing the MD process for treating dye wastewater, the nature of their time intensity poses a challenge. Given the multitude of parameters involved in MD process optimization, artificial intelligence (AI) methodologies present a promising avenue for assistance. Thus, AI-driven algorithms have the potential to enhance overall process efficiency, cutting down on time, fine-tuning parameters, and driving cost reductions. However, achieving an optimal balance between efficiency enhancements and financial outlays is a complex process. Finally, this paper suggests a research direction for the development of effective synthetic and natural dye removal from industrially discharged wastewater.

Versatile GO/ANFs Aerogel for Highly Efficient Solar-Powered Water Purification in Wide Environments

Solar-driven interfacial evaporation is a very promising choice for producing clean water. Despite the considerable investigation of pure NaCl brine purification, solar-driven complex water purification, such as real-world seawater desalination as well as domestic and industrial wastewater treatment, has rarely been investigated, mainly due to its compositions being much more complicated than NaCl brine. Herein, we developed a graphene oxide/aramid nanofiber (GO/ANFs) aerogel by a freeze-drying process. The GO/ANFs aerogel combined opened porous microchannels, superhydrophilicity, anti-oil-fouling capacity, enhanced broad-spectrum light absorption (more than 92%), and good solar/heat management. These integrated properties enabled the GO/ANFs aerogel to be an advanced solar interfacial evaporator for efficient freshwater production with the characteristics of localized heat conversion, quick water transport, and salt crystallization inhibition, and the rate of steam production rate was as high as 2.25 kg m-2 h-1 upon exposure to 1 solar irradiation. Importantly, the high-water-vapor generation rate was maintained even under complicated conditions, including real-world seawater, dye water, emulsions, and corrosive liquid environments. Considering its promising adaptability to a wide range of environments, this work hopes to inspire the development of brine desalination, wastewater purification, clean water production, and solar energy utilization.

Investigation of direct contact membrane distillation (DCMD) performance using CFD and machine learning approaches

Direct Contact Membrane Distillation (DCMD) is emerging as an effective method for water desalination, known for its efficiency and adaptability. This study delves into the performance of DCMD by integrating two powerful analytical tools: Computational Fluid Dynamics (CFD) and Artificial Neural Networks (ANN). The research thoroughly examines the impact of various factors, such as inlet temperatures, velocities, channel heights, salt concentration, and membrane characteristics, on the process's efficiency, specifically calculating the water vapor flux. A rigorous validation of the CFD model aligns well with established studies, ensuring reliability. Subsequently, over 1000 data points reflecting variations in input factors are utilized to train and validate the ANN. The training phase demonstrated high accuracy, with near-zero mean squared errors and R2 values close to one, indicating a strong predictive capability. Further analysis post-ANN training shed light on key relationships: higher membrane porosity boosts water vapor flux, whereas thicker membranes reduce it. Additionally, it was detailed how salt concentration, channel dimensions, inlet temperatures, and velocities significantly influence the distillation process. Finally, a mathematical model was proposed for water vapor flux as a function of key input factors. The results highlighted that salt mole fraction and hot water inlet temperature have the most effect on the water vapor flux. This comprehensive investigation contributes to the understanding of DCMD and emphasizes the potential of combining CFD and ANN for optimizing and innovating water desalination technology.

Superhydrophobic sand evaporator with core-shell structure for long-term salt-resistant solar desalination

Solar-driven water evaporation, as an environmentally benign pathway, provides an opportunity for alleviating global clean water scarcity. However, the rapidly generated interfacial steam and localized heating could cause increased salt concentration and accumulation, deteriorating the evaporation performance and long-term stability. Herein, a novel superhydrophobic sand solar (FPPSD) evaporator with a core-shell structure was proposed through interface functionalization for continuous photothermal desalination. The collective behavior essence of the sand aggregate gave itself micron-scale self-organized pores and configurable shapes, generating desirable capillary force and supplying effective water-pumping channels. More importantly, combining the dopamine, polypyrrole (PPy), and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) through π-π conjugation and multiple hydrogen bonding effects gave the FPPSD evaporator with stable superhydrophobic property and highly efficient photothermal conversion capability. Therefore, the FPPSD evaporator showed a continuous and stable photothermal performance even after 96 h continuous evaporation under 3-sun irradiation for 10 wt% saline solution, among the best values in the reported works of literature, demonstrating its excellent salt-resistance stability. Furthermore, this novel FPPSD evaporator displayed outstanding environmental stability that kept its initial water transport capacity even after being treated under harsh conditions for 30 days. With excellent salt-resistance ability and stable environmental stability, the FPPSD evaporator will provide an attractive platform for sustainable solar-driven water management.

Advancements in Nanoenabled Membrane Distillation for a Sustainable Water-Energy-Environment Nexus

The emergence of nano innovations in membrane distillation (MD) has garnered increasing scientific interest. This enables the exploration of state-of-the-art nano-enabled MD membranes with desirable properties, which significantly improve the efficiency and reliability of the MD process and open up opportunities for achieving a sustainable water-energy-environment (WEE) nexus. This comprehensive review provides broad coverage and in-depth analysis of recent innovations in nano-enabled MD membranes, focusing on their role in achieving desirable properties, such as strong liquid-repellence, high resistance to scaling, fouling, and wetting, as well as efficient self-heating and self-cleaning functionalities. The recent developments in nano-enhanced photothermal-catalytic applications for water-energy co-generation within a single MD system are also discussed. Furthermore, the bottlenecks are identified that impede the scale-up of nanoenhanced MD membranes and a future roadmap is proposed for their sustainable commercialiation. This holistic overview is expected to inspire future research and development efforts to fully harness the potential of nano-enabled MD membranes to achieve sustainable integration of water, energy, and the environment.

Recent Advances in Dopamine-Based Membrane Surface Modification and Its Membrane Distillation Applications

Surface modification of membranes is essential for improving flux and resistance to contamination for membranes. This is of great significance for membrane distillation, which relies on the vapor pressure difference across the membrane as the driving force. In recent years, biomimetic mussel-inspired substances have become the research hotspots. Among them, dopamine serves as surface modifiers that would achieve highly desirable and effective membrane applications owing to their unique physicochemical properties, such as universal adhesion, enhanced hydrophilicity, tunable reducibility, and excellent thermal conductivity. The incorporation of a hydrophilic layer, along with the utilization of photothermal properties and post-functionalization capabilities in modified membranes, effectively addresses challenges such as low flux, contamination susceptibility, and temperature polarization during membrane distillation. However, to the best of our knowledge, there is still a lack of comprehensive and in-depth discussions. Therefore, this paper systematically compiles the modification method of dopamine on the membrane surface and summarizes its application and mechanism in membrane distillation for the first time. It is believed that this paper would provide a reference for dopamine-assisted membrane separation during production, and further promote its practical application.

Synergistic Effect of Ti3C2Tx MXene Nanosheets and Tannic Acid-Fe3+ Network in Constructing High-Performance Hydrogel Composite Membrane for Photothermal Membrane Distillation

Photothermal membrane distillation (PMD) has emerged as a promising and sustainable approach for seawater desalination and wastewater purification. However, the wide application of the technique is severely impeded by low freshwater production and membrane fouling/wetting issues. Herein, we developed an advanced hydrogel-engineered membrane with simultaneously enhanced photothermal conversion capacity and desired fouling and wetting resistance for PMD. By the synergies of photothermal Ti3C2Tx MXene nanosheets and the tannic acid-Fe3+ network in the hydrogel, the membrane was endowed with excellent surface self-heating ability, yielding the highest freshwater production rate (1.71 kg m-2 h-1) and photothermal efficiency among the fabricated hydrogel composite membranes under 1 sun irradiation. Meanwhile, the PMD membrane could robustly resist oil-induced fouling and surfactant-induced wetting, significantly extending the membrane lifespan in treating contaminated saline water. Furthermore, when desalinating real seawater, the membrane exhibited superior durability with a stable vapor flux and excellent ion rejection (e.g., 99.24% for boron) for 100 h.

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.

Wetting-Based Comparison of Ag, Carbon Black, and MoS2 Composite Membranes for Photothermal Membrane Distillation

Photothermal membrane distillation is a new-generation desalination process that can take advantage of the ability of specific materials to convert solar energy to heat at the membrane surface and thus to overcome temperature polarization. The development of appropriate photothermal membranes is challenging because many criteria need to be considered, including light to heat conversion, permeability and low wetting, and fouling, as well as cost. Based on our experience with wetting characterization, this study compares photothermal membranes prepared using different well-known or promising materials, i.e., silver nanoparticles (Ag NPs), carbon black, and molybdenum disulfide (MoS2), in terms of their structural properties, permeability, wettability, and wetting. Accordingly, membranes with different proportions of photothermal NPs are prepared and fully characterized in this study. Wetting is investigated using the detection of dissolved tracer intrusion (DDTI) method following membrane distillation operations with saline solutions. The advantages of MoS2 and carbon black-based photothermal membranes in comparison with polyvinylidene difluoride (PVDF) membranes include both a permeability increase and a less severe wetting mechanism, with lower wetting indicators in the short term. These materials are also much cheaper than Ag NPs, having higher permeabilities and presenting less severe wetting mechanisms.

Plasmonic Phenomena in Membrane Distillation

Water scarcity raises important concerns with respect to human sustainability and the preservation of important ecosystem functions. To satisfy water requirements, seawater desalination represents one of the most sustainable solutions. In recent decades, membrane distillation has emerged as a promising thermal desalination process that may help to overcome the drawbacks of traditional desalination processes. Nevertheless, in membrane distillation, the temperature at the feed membrane interface is significantly lower than that of the bulk feed water, due to the latent heat flux associated with water evaporation. This phenomenon, known as temperature polarization, in membrane distillation is a crucial issue that could be responsible for a decay of about 50% in the initial transmembrane water flux. The use of plasmonic nanostructures, acting as thermal hotspots in the conventional membranes, may improve the performance of membrane distillation units by reducing or eliminating the temperature polarization problem. Furthermore, an efficient conversion of light into heat offers new opportunities for the use of solar energy in membrane distillation. This work summarizes recent developments in the field of plasmonic-enhanced solar evaporation with a particular focus on solar-driven membrane distillation applications and its potential prospects.

Polydopamine/SiO2 Hybrid Structured Superamphiphobic Fabrics with Good Photothermal Behavior

In recent years, photothermal materials that can convert light into heat energy have attracted extensive attention. In this work, we report a simple and effective approach to construct a self-cleaning photothermal superamphiphobic fabric. Dopamine (DA) can self-polymerize into polydopamine (PDA) and adhere to the surface of cotton fabric as a secondary reaction platform. Then, SiO₂ nanoparticles were in situ grown on the PDA@fabric surface by the sol-gel method. The PDA clusters can not only provide good photothermal conversion performance but also be integrated with SiO₂ to create micro-nano rough structures. Finally, the surface of SiO₂ was modified by the long chain of fluorosilane to decrease the fabric surface energy, resulting in superamphiphobicity. The contact angles of water, ethylene glycol, and pump oil on the modified fabric surface could reach 161.1, 158.1, and 142.2°, respectively, making the fabric resistant to contamination by water, common beverages, and oil. Due to the adhesion of the PDA layer, the strong binding force between the fabric and SiO₂ particles enabled the modified fabric to withstand various chemical and mechanical attacks, showing excellent mechanical robustness and harsh environmental stability. More importantly, the surface temperature of the modified fabric could be increased from 19.6 to 37.0 °C, which is close to the human body temperature, under the irradiation of simulated sunlight (I = 15 A, 300 s). The photothermal superamphiphobic fabrics with self-cleaning properties show great promise in the photothermal conversion field.

NiSe and CoSe Topological Nodal-Line Semimetals: A Sustainable Platform for Efficient Thermoplasmonics and Solar-Driven Photothermal Membrane Distillation

The control of heat at the nanoscale via the excitation of localized surface plasmons in nanoparticles (NPs) irradiated with light holds great potential in several fields (cancer therapy, catalysis, desalination). To date, most thermoplasmonic applications are based on Ag and Au NPs, whose cost of raw materials inevitably limits the scalability for industrial applications requiring large amounts of photothermal NPs, as in the case of desalination plants. On the other hand, alternative nanomaterials proposed so far exhibit severe restrictions associated with the insufficient photothermal efficacy in the visible, the poor chemical stability, and the challenging scalability. Here, it is demonstrated the outstanding potential of NiSe and CoSe topological nodal-line semimetals for thermoplasmonics. The anisotropic dielectric properties of NiSe and CoSe activate additional plasmonic resonances. Specifically, NiSe and CoSe NPs support multiple localized surface plasmons in the optical range, resulting in a broadband matching with sunlight radiation spectrum. Finally, it is validated the proposed NiSe and CoSe-based thermoplasmonic platform by implementing solar-driven membrane distillation by adopting NiSe and CoSe nanofillers embedded in a polymeric membrane for seawater desalination. Remarkably, replacing Ag with NiSe and CoSe for solar membrane distillation increases the transmembrane flux by 330% and 690%, respectively. Correspondingly, costs of raw materials are also reduced by 24 and 11 times, respectively. The results pave the way for the advent of NiSe and CoSe for efficient and sustainable thermoplasmonics and related applications exploiting sunlight within the paradigm of the circular blue economy.

The advent of thermoplasmonic membrane distillation

Freshwater scarcity is a vital societal challenge related to climate change, population pressure, and agricultural and industrial demands. Therefore, sustainable desalination/purification of salty/contaminated water for human uses is particularly relevant. Membrane distillation is an emerging hybrid thermal-membrane technology with the potential to overcome the drawbacks of conventional desalination by a synergic exploitation of the water-energy nexus. Although membrane distillation is considered a green technology, efficient heat management remains a critical concern affecting the cost of the process and hindering its viability at large scale. A multidisciplinary approach that involves materials chemistry, physical chemistry, chemical engineering, and materials and polymer science is required to solve this problem. The combination of solar energy with membrane distillation is considered a potentially feasible low-cost approach for providing high-quality freshwater with a low carbon footprint. In particular, recent discoveries about efficient light-to-heat conversion in nanomaterials have opened unprecedented perspectives for the implementation of sunlight-based renewable energy in membrane distillation. The integration of nanofillers enabling photothermal effects into membranes has been demonstrated to be able to significantly enhance the energy efficiency without impacting on economic costs. Here, we provide a comprehensive overview on the state of the art, the opportunities, open challenges and pitfalls of the emerging field of solar-driven membrane distillation. We also assess the peculiar physicochemical properties and synthesis scalability of photothermal materials, as well as the strategies for their integration into polymeric nanocomposite membranes enabling efficient light-to-heat conversion and freshwater.

Transforming Ti3C2Tx MXene's intrinsic hydrophilicity into superhydrophobicity for efficient photothermal membrane desalination

Owing to its 100% theoretical salt rejection capability, membrane distillation (MD) has emerged as a promising seawater desalination approach to address freshwater scarcity. Ideal MD requires high vapor permeate flux established by cross-membrane temperature gradient (∆T) and excellent membrane durability. However, it’s difficult to maintain constant ∆T owing to inherent heat loss at feedwater side resulting from continuous water-to-vapor transition and prevent wetting transition-induced membrane fouling and scaling. Here, we develop a Ti₃C₂T_x MXene-engineered membrane that imparts efficient localized photothermal effect and strong water-repellency, achieving significant boost in freshwater production rate and stability. In addition to photothermal effect that circumvents heat loss, high electrically conductive Ti₃C₂T_x MXene also allows for self-assembly of uniform hierarchical polymeric nanospheres on its surface via electrostatic spraying, transforming intrinsic hydrophilicity into superhydrophobicity. This interfacial engineering renders energy-efficient and hypersaline-stable photothermal membrane distillation with a high water production rate under one sun irradiation.

Membrane distillation is susceptible to thermal inefficiency and membrane wetting issues during seawater desalination. Here, authors design a MXene-engineered membrane that imparts efficient localized photothermal effect and strong water repellency, achieving sustainable freshwater production.

Accessing Highly Efficient Photothermal Conversion with Stable Open-Shell Aromatic Nitric Acid Radicals

It is very challenging to prepare stable radicals as they are usually thermodynamically or kinetically unstable in air. Herein, a series of star-shaped aromatic nitric acid radicals were prepared via facile demethylation and consequent oxidation. As phenol radicals without steric hindrance group protection, they exhibit high electrochemical and thermal stability due to their rich resonance structures including closed-shell nitro-like and open-shell nitroxide structure with unpaired electrons delocalized in conjugated backbones. Among them, TPA-TPA-O₆ powder exhibited extremely wide absorption from 300 to 2000 nm covering the whole solar spectral irradiance, high photothermal conversion efficiency, and negligible photobleaching effect in seawater desalination. Under the irradiation of one sunlight, the water evaporation efficiency of TPA-TPA-O₆ is recorded to be as high as 89.41 % and the water evaporation rate is 1.293 kg m^-2  h^-1 , which represents the top performance in pure organic small molecule photothermal materials.

Recent Progress in the Membrane Distillation and Impact of Track-Etched Membranes

Membrane distillation (MD) is a rapidly developing field of research and finds applications in desalination of water, purification from nonvolatile substances, and concentration of various solutions. This review presents data from recent studies on the MD process, MD configuration, the type of membranes and membrane hydrophobization. Particular importance has been placed on the methods of hydrophobization and the use of track-etched membranes (TeMs) in the MD process. Hydrophobic TeMs based on poly(ethylene terephthalate) (PET), poly(vinylidene fluoride) (PVDF) and polycarbonate (PC) have been applied in the purification of water from salts and pesticides, as well as in the concentration of low-level liquid radioactive waste (LLLRW). Such membranes are characterized by a narrow pore size distribution, precise values of the number of pores per unit area and narrow thickness. These properties of membranes allow them to be used for more accurate water purification and as model membranes used to test theoretical models (for instance LEP prediction).