Engineering anti-scaling superhydrophobic membranes for photothermal membrane distillation

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.

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.

Influence of photothermal nanomaterials localization within the electrospun membrane structure on purification of saline oily wastewater based on photothermal vacuum membrane distillation

Today, synergistic combination of special nanomaterials (NMs) and electrospinning technique has emerged as a promising strategy to address both water scarcity and energy concerns through the development of photothermal membranes for wastewater purification and desalination. This work was organized to provide a new perspective on membrane design for photothermal vacuum membrane distillation (PVMD) through optimizing membrane performance by varying the localization of photothermal NMs. Poly(vinylidene fluoride) omniphobic photothermal membranes were prepared by localizing graphene oxide nanosheets (GO NSh) (1) on the surface (0.2 wt%), (2) within the nanofibers structure (10 wt%) or (3) in both positions. Considering the case 1, after 7 min exposure to the 1 sun intensity light, the highest temperature (∼93.5 °C) was recorded, which is assigned to the accessibility of GO NSh upon light exposure. The case 3 yielded to a small reduction in surface temperature (∼90.4 °C) compared to the case 1, indicating no need to localize NMs within the nanofibers structure when they are localized on the surface. The other extreme belonged to the case 2 with the lowest temperature of ∼71.3 °C, which is consistent with the less accessibility of GO NSh during irradiation. It was demonstrated that the accessibility of photothermal NMs plays more pronounced role in the membrane surface temperature compared to the light trapping. However, benefiting from higher surface temperature during PVMD due to enhanced accessibility of photothermal NMs is balanced out by decrease in the permeate flux (case 1: 1.51 kg/m2 h and case 2: 1.83 kg/m2 h) due to blocking some membrane surface pores by the binder. A trend similar to that for flux was also followed by the efficiency. Additionally, no change in rejection was observed for different GO NSh localizations.

Substrate-Independent Superhydrophobic Coating Capable of Photothermal-Induced Repairability for Multiple Damages Fabricated via Simple Blade Coating

Repairable superhydrophobic surfaces have promising application potential in many fields. However, so far, it is still a challenge to develop a superhydrophobic surface with repairability for multiple types of damage through a simple method. In this paper, a repairable superhydrophobic coating was obtained on various substrates by blade-coating mixtures of polydimethylsiloxane (PDMS), polyvinylidene fluoride (PVDF), and multiwalled carbon nanotubes (MWCNTs) modified with dopamine (PDA) and octadecylamine (ODA). The obtained coating has a good liquid-repellent property with a water contact angle above 150° and a water sliding angle of ∼6° and possesses an excellent absorbance (∼97%) in the wavelength range of 250-2500 nm. Due to its high absorbance, the coating displays an outstanding photothermal effect with a temperature rise of ∼65 °C under irradiation by 1.0 kW/m2 of simulated sunlight. Furthermore, after being degraded by multiple stimuli, including plasma treatment, acid/alkali/oil immersion, sand impact, and the icing-thawing cycle, the coating can recover superhydrophobicity via sunlight irradiation, demonstrating the good photothermal-induced repairability of the coating. It can be expected that the good water-repellent property, photothermal effect, and repairability give this coating a promising prospect in practical applications.

Nacre-inspired composite paper of PVA crosslinked basalt scale and nanocellulose with enhanced mechanical, electrical insulating and ultraviolet-resistant aging performance

Silicate scales are commonly incorporated into cellulose nanofiber (CNF) as functional fillers to enhance electrical insulation and UV-shielding properties. Nevertheless, the addition of substantial quantities of silicate scales in the quest for enhanced functional properties results in reduced interface bonding capability and compromised mechanical properties, thereby restricting their application. Here, inspired from nacre, layered composite paper with excellent mechanical strength, electrical insulation and UV-resistance properties was fabricated through vacuum assisted self-assembly using CNF, PVA and basalt scales (BS). Unlike the conventional blending strategy, the pre-mixed PVA and BS suspension facilitates the formation of Al-O-C bond, thereby enhancing the interfacial bonding between BS and CNF. Consequently, the composite paper (BS@PVA/PVA/CNF) containing 60 wt% BS demonstrates higher mechanical strength-approximately 140 % higher than that of BS/CNF composite paper, achieving a strength of 33.5 MPa. Additionally, it demonstrates enhanced dielectric properties, surpassing those of CNF paper by up to 107 %. Moreover, it exhibits robust ultraviolet-resistant aging performance, retaining ~87 % of its tensile strength after undergoing a simulated two-year aging period. As a result, this work presents a simple and innovative design strategy for enhancing interfacial bonding and optimizing layer structure, providing essential guidelines for large-scale production of high-performance insulation and aging-resistant composite paper.

Challenges and strategies for commercialization and widespread practical applications of superhydrophobic surfaces

Superhydrophobic (SH) surfaces have progressed rapidly in fundamental research over the past 20 years, but their practical applications lag far behind. In this perspective, we first present the findings of a survey on the current state of SH surfaces including fundamental research, patenting, and commercialization. On the basis of the survey and our experience, this perspective explores the challenges and strategies for commercialization and widespread practical applications of SH surfaces. The comprehensive performances, preparation methods, and application scenarios of SH surfaces are the major constraints. These challenges should be addressed simultaneously, and the actionable strategies are provided. We then highlight the standard test methods of the comprehensive performances including mechanical stability, impalement resistance, and weather resistance. Last, the prospects of SH surfaces in the future are discussed. We anticipate that SH surfaces may be widely commercialized and used in practical applications around the year 2035 through combination of the suggested strategies and input from both academia and industry.

Which Surface Is More Scaling Resistant? A Closer Look at Nucleation Theories for Heterogeneous Gypsum Nucleation in Aqueous Solutions

Developing engineered surfaces with scaling resistance is an effective means to inhibit surface-mediated mineral scaling in various industries including desalination. However, contrasting results have been reported on the relationship between scaling potential and surface hydrophilicity. In this study, we combine a theoretical analysis with experimental investigation to clarify the effect of surface wetting property on heterogeneous gypsum (CaSO₄·2H₂O) formation on surfaces immersed in aqueous solutions. Theoretical prediction derived from classical nucleation theory (CNT) indicates that an increase of surface hydrophobicity reduces scaling potential, which contrasts our experimental results that more hydrophilic surfaces are less prone to gypsum scaling. We further consider the possibility of nonclassical pathway of gypsum nucleation, which proceeds by the aggregation of precursor clusters of CaSO₄. Accordingly, we investigate the affinity of CaSO₄ to substrate surfaces of varied wetting properties via calculating the total free energy of interaction, with the results perfectly predicting experimental observations of surface scaling propensity. This indicates that the interactions between precursor clusters of CaSO₄ and substrate surfaces might play an important role in regulating heterogeneous gypsum formation. Our findings provide evidence that CNT might not be applicable to describing gypsum scaling in aqueous solutions. The fundamental insights we reveal on gypsum scaling mechanisms have the potential to guide rational design of scaling-resistant engineered surfaces.

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.