A scalable fish-school inspired self-assembled particle system for solar-powered water-solute separation

Research progress of industrial wastewater treatment technology based on solar interfacial adsorption coupled evaporation process

Solar interface evaporation is an effective method for the treatment of water that has low energy consumption. Adsorption is recognized to be one of the most stable wastewater treatment methods and is widely used. Combining solar interface evaporation with adsorption provides a novel and low-cost approach for the efficient removal of heavy metals and organic pollutants from industrial wastewater. This paper reviews the characteristics and application of some common wastewater treatment methods. The photothermal conversion and the conceptual design of interface evaporation combined with adsorption are introduced and the photo-thermal conversion and adsorption methods are discussed. The study provides a summary of recent studies and advancements in interfacial evaporation-coupled adsorption materials, which include hydrogels, aerogels, and biomass materials for adsorption, and carbon materials for photothermal conversion. Finally, the current challenges encountered in industrial wastewater treatment are outlined and its prospects are discussed. The aim of this review is to explore a wide range of possibilities with the interfacial evaporation-coupled adsorption method and propose a new low-cost and high-efficiency method for industrial wastewater treatment.

Solar-enhanced lithium extraction with self-sustaining water recycling from salt-lake brines

Lithium is an emerging strategic resource for modern energy transformation toward electrification and decarbonization. However, current mainstream direct lithium extraction technology via adsorption suffers from sluggish kinetics and intensive water usage, especially in arid/semiarid and cold salt-lake regions (natural land brines). Herein, an efficient proof-of-concept integrated solar microevaporator system is developed to realize synergetic solar-enhanced lithium recovery and water footprint management from hypersaline salt-lake brines. The 98% solar energy harvesting efficiency of the solar microevaporator system, elevating its local temperature, greatly promotes the endothermic Li+ extraction process and solar steam generation. Benefiting from the photothermal effect, enhanced water flux, and enriched local Li+ supply in nanoconfined space, a double-enhanced Li+ recovery capacity was delivered (increase from 12.4 to 28.7 mg g-1) under one sun, and adsorption kinetics rate (saturated within 6 h) also reached twice of that at 280 K (salt-lake temperature). Additionally, the self-assembly rotation feature endows the microevaporator system with distinct self-cleaning desalination ability, achieving near 100% water recovery from hypersaline brines for further self-sufficient Li+ elution. Outdoor comprehensive solar-powered experiment verified the feasibility of basically stable lithium recovery ability (>8 mg g-1) directly from natural hypersaline salt-lake brines with self-sustaining water recycling for Li+ elution (440 m3 water recovery per ton Li2CO3). This work offers an integrated solution for sustainable lithium recovery with near zero water/carbon consumption toward carbon neutrality.

Quasi-waffle solar distiller for durable desalination of seawater

Water purification via interfacial solar steam generation exhibits promising potential. However, salt crystallization on evaporators reduces solar absorption and obstructs water supply. To address it, a waffle-shaped solar evaporator (WSE) has been designed. WSE is fabricated via a zinc-assisted pyrolysis route, combining low-cost biomass carbon sources, recyclable zinc, and die-stamping process. This route enables cost-effective production without the need of sophisticated processing. As compared to conventional plane-shaped evaporators, WSE is featured by extra sidewalls for triggering the convection with the synergistic solute and thermal Marangoni effects. Consequently, WSE achieves spontaneous salt rejection and durable evaporation stability. It has demonstrated continuous operation for more than 60 days in brine without fouling.

Cogeneration of Clean Water and Valuable Energy/Resources via Interfacial Solar Evaporation

Water, covering over two-thirds of the Earth's surface, holds immense potential for generating clean water, sustainable energy, and metal resources, which are the cornerstones of modern society and future development. It is highly desired to produce these crucial elements through eco-friendly processes with minimal carbon footprints. Interfacial solar evaporation, which utilizes solar energy at the air-liquid interface to facilitate water vaporization and solute separation, offers a promising solution. In this review, we systematically report the recent progress of the cogeneration of clean water and energy/resources including electricity, hydrogen, and metal resources via interfacial solar evaporation. We first gain insight into the energy and mass transport for a typical interfacial solar evaporation system and reveal the residual energy and resources for achieving the cogeneration goal. Then, we summarize the recent advances in materials/device designs for efficient cogeneration. Finally, we discuss the existing challenges and potential opportunities for the further development of this field.

Making Nanofiber Membrane Stand on End to Construct Vertically Interfacial Evaporators for Efficient Solar Evaporation, Omnidirectional Solar Absorption, and Ultrahigh-Salinity Brine Desalination

Solar-driven interfacial desalination is widely considered to be a promising technology to address the global water crisis. This study proposes a novel electrospun nanofiber-based all-in-one vertically interfacial solar evaporator endowed with a high steam generation rate, steady omnidirectional evaporation, and enduring ultrahigh-salinity brine desalination. In particular, the electrospun nanofiber is collected into the tubular structure, followed by spraying with a dense crosslinked poly(vinyl alcohol) film, which renders them sufficiently strong for the preparation of a vertically array evaporator. The integrated evaporator made an individual capillary as a unit to form multiple thermal localization interfaces and steam dissipation channels, realizing zone heating of water. Thus a high steam generation rate exceeding 4.0 kg m-2 h-1 in pure water is demonstrated even under omnidirectional sunlight, and outperforms existing evaporators. Moreover, salt ions in the photothermal layer can be effectively transported to the water in capillaries and subsequently exchanged with the bulk water due to the strong action of capillary force, which ensures an ultrahigh desalination rate (≈12.5 kg m-2 h-1 under 3 sun) in 25 wt% concentration brine over 300 min. As such, this work provides a meaningful roadmap for the development of state-of-the-art solar-driven interfacial desalination.

Interfacial Solar Evaporation: From Fundamental Research to Applications

In the last decade, interfacial solar steam generation (ISSG), powered by natural sunlight garnered significant attention due to its great potential for low-cost and environmentally friendly clean water production in alignment with the global decarbonization efforts. This review aims to share the knowledge and engage with a broader readership about the current progress of ISSG technology and the facing challenges to promote further advancements toward practical applications. The first part of this review assesses the current strategies for enhancing the energy efficiency of ISSG systems, including optimizing light absorption, reducing energy losses, harvesting additional energy, and lowering evaporation enthalpy. Subsequently, the current challenges faced by ISSG technologies, notably salt accumulation and bio-fouling issues in practical applications, are elucidated and contemporary methods are discussed to overcome these challenges. In the end, potential applications of ISSG, ranging from initial seawater desalination and industrial wastewater purification to power generation, sterilization, soil remediation, and innovative concept of solar sea farm, are introduced, highlighting the promising potential of ISSG technology in contributing to sustainable and environmentally conscious practices. Based on the review and in-depth understanding of these aspects, the future research focuses are proposed to address potential issues in both fundamental research and practical applications.

All-in-one self-floating porous foams as robust heat-blocking layers for efficient photothermal conversion and solar desalination

Solar-driven interfacial evaporation is a highly efficient and ecofriendly technology for producing freshwater. Herein, self-floating plasmon Ag/black TiO2/carbon porous layered foams (Ag-BTCFs) were demonstrated as efficient solar-thermal convectors using freeze-drying cast-molding and high-temperature surface hydrogenation strategies. This all-in-one three-dimensional (3D) cross-linked self-floating porous layered foam material with full-spectrum absorption can fully harvest sunlight (∼95.45%) and effectively block heat transfer to its sublayer. The synergy of sufficient utilization of absorbed ultraviolet radiation by black TiO2 (b-TiO2), visible light absorption by Ag nanoparticles (Ag NPs) via localized surface plasmon resonance, and near-infrared absorption by layered-amorphous carbon can achieve full-solar-spectrum absorption to concentrate thermal energy. In addition to their synergistic effect, they are conducive to the relaxation of hot electrons when utilizing photogenerated holes to degrade pollutants in domestic wastewater. The steam generation efficiency of Ag-BTCFs is up to 1.79 kg m-2h-1 due to their solar energy conversion efficiency of 81.74% under 1 sun irradiation, which is five times higher than the evaporation rate of pure water. Notably, the material's efficient ion removal rate of 99.80% for solar desalination indicates its high potential for various applications. This strategy provides new insights for fabricating recyclable heat-blocking layer systems against thermal loss to enhance solar steam generation.

A smart thermal-gated bilayer membrane for temperature-adaptive radiative cooling and solar heating

Due to the huge energy consumption of traditional cooling- and heating-based electricity, passive radiative cooling and solar heating with a minimum carbon footprint using the outer space and Sun as natural thermodynamic resources have attracted much attention. However, most passive devices are static and monofunctional, and cannot meet the practical requirements of dynamic cooling and heating under various conditions. Here, we demonstrate a smart thermal-gated (STG) bilayer membrane that enables fully automatic and temperature-adaptive radiative cooling and solar heating. Specifically, this device can switch from reflective to absorptive (white to black) in the solar wavelength with the reduction in optical scattering upon ambient temperature, corresponding to a sunlight reflectivity change from 0.962 to 0.059 when the temperature drops below ∼30 °C, whereas its mid-infrared emissivity remains at ∼0.95. Consequently, this STG membrane achieves a temperature of ∼5 °C below ambient (a key signature of radiative cooling) under direct sunlight (peak solar irradiance >900 W m-2) in summer and a solar heating power of ∼550 W m-2 in winter. Theoretical analysis reveals the substantial advantage of this switchable cooling/heating device in potential energy saving compared with cooling-only and heating-only strategies when widely used in different climates. It is expected that this work will pave a new pathway for designing temperature-adaptive devices with zero energy consumption and provide an innovative way to achieve sustainable energy.

Waterwheel-inspired rotating evaporator for efficient and stable solar desalination even in saturated brine

Solar desalination is one of the most promising technologies to address global freshwater shortages. However, traditional evaporators encounter the bottleneck of reduced evaporation rate or even failure due to salt accumulation in high-salinity water. Inspired by ancient waterwheels, we have developed an adaptively rotating evaporator that enables long-term and efficient solar desalination in brines of any concentration. The evaporator is a sulphide-loaded drum-type biochar. Our experiments and numerical simulations show that this evaporator, thanks to its low density and unique hydrophilic property, rotates periodically under the center-of-gravity shift generated by salt accumulation, achieving self-removal of salt. This allows it to maintain a high evaporation rate of 2.80 kg m-2 h-1 within 24 h even in saturated brine (26.47%), which was not achieved previously. This proof-of-concept work therefore demonstrates a concentration- and time-independent, self-rotation-induced solar evaporator.

Highly Salt-Resistant interfacial solar evaporators based on Melamine@Silicone nanoparticles for stable Long-Term desalination and water harvesting

Interfacial solar-driven evaporation (ISE) is one of the most promising solutions for collecting fresh water, however, poor salt-resistance severely limits the long-term stability of solar evaporators. Here, highly salt-resistant solar evaporators for stable long-term desalination and water harvesting were fabricated by depositing silicone nanoparticles onto melamine sponge, and then modifying the hybrid sponge sequentially with polypyrrole and Au nanoparticles. The solar evaporators have a superhydrophilic hull for water transport and solar desalination, and a superhydrophobic nucleus for reducing heat loss. Spontaneous rapid salt exchange and reduction in salt concentration gradient were achieved due to ultrafast water transport and replenishment in the superhydrophilic hull with a hierachical micro-/nanostructure, which effectively prevents salt deposition during ISE. Consequently, the solar evaporators have long-term stable evaporation performance of 1.65 kg m^-2h^-1 for 3.5 wt% NaCl solution under 1 sun illumination. Moreover, 12.87 kg m^-2 fresh water was collected during consecutive 10 h ISE of 20 wt% brine under 1 sun without any salt precipitation. We believe that this strategy will shed a new light on the design of long-term stable solar evaporators for fresh water harvesting.

Natural Cellulose Fiber-Derived Photothermal Aerogel for Efficient and Sustainable Solar Desalination

Aerogels are becoming a promising platform to fabricate photothermal materials for use in solar steam generation (SSG), which have remarkable application potential in solar desalination, due to their excellent thermal management, salt resistance, and considerable water evaporation rate. In this work, a novel photothermal material is fabricated by forming a suspension between sugarcane bagasse fibers (SBF) and poly(vinyl alcohol), tannic acid (TA), and Fe³⁺ solutions via hydrogen bonds of hydroxyl groups. After freeze drying, the fabricated SBF aerogel-based photothermal (SBFAP) material possesses a 3D interconnected porous microstructure, which could enhance water transportation ability, reduce thermal conductivity, and quickly dissolve salt crystals on the SBFAP surface. Thanks to the formation of micro/nanosized complexes between TA and Fe³⁺ ions on the SBFAP material, the SBFAP exhibits high light capture and water evaporation rate (2.28 kg m^-2 h^-1). In particular, due to strong hydrogen bonding and the SBF, the SBFAP material is reinforced, thereby exhibiting excellent structural stability in seawater. Moreover, the high salt tolerance of SBFAP favors its high desalination performance for at least 76 days of continuous evaporation under actual conditions. This research paves the way for the fabrication of natural cellulose fiber-based photothermal materials for application in solar desalination.

Fibrous Aerogels with Tunable Superwettability for High-Performance Solar-Driven Interfacial Evaporation

Solar-driven interfacial evaporation is an emerging technology for water desalination. Generally, double-layered structure with separate surface wettability properties is usually employed for evaporator construction. However, creating materials with tunable properties is a great challenge because the wettability of existing materials is usually monotonous. Herein, we report vinyltrimethoxysilane as a single molecular unit to hybrid with bacterial cellulose (BC) fibrous network, which can be built into robust aerogel with entirely distinct wettability through controlling assembly pathways. Siloxane groups or carbon atoms are exposed on the surface of BC nanofibers, resulting in either superhydrophilic or superhydrophobic aerogels. With this special property, single component-modified aerogels could be integrated into a double-layered evaporator for water desalination. Under 1 sun, our evaporator achieves high water evaporation rates of 1.91 and 4.20 kg m-2 h-1 under laboratory and outdoor solar conditions, respectively. Moreover, this aerogel evaporator shows unprecedented lightweight, structural robustness, long-term stability under extreme conditions, and excellent salt-resistance, highlighting the advantages in synthesis of aerogel materials from the single molecular unit.

Three-dimensional open architecture enabling salt-rejection solar evaporators with boosted water production efficiency

Direct solar desalination exhibits considerable potential for alleviating the global freshwater crisis. However, the prevention of salt accumulation while maintaining high water production remains an important challenge that limits its practical applications because the methods currently employed for achieving rapid salt backflow usually result in considerable heat loss. Herein, we fabricate a solar evaporator featuring vertically aligned mass transfer bridges for water transport and salt backflow. The 3D open architecture constructed using mass transfer bridges enables the evaporator to efficiently utilize the conductive heat that would otherwise be lost, significantly improving the water evaporation efficiency without compromising on salt rejection. The fabricated evaporator can treat salt water with more than 10% salinity. Moreover, it can continuously and steadily work in a real environment under natural sunlight with a practical solar-to-water collection efficiency of >40%. Using the discharged water from reverse osmosis plants and sea water from the Red Sea, the evaporator demonstrates a daily freshwater generation rate of ~5 L/m², which is sufficient to satisfy individual drinking water requirements. With strong salt rejection, high energy efficiency, and simple scalability, the 3D evaporator has considerable promise for freshwater supply for water-stressed and off-grid communities.

Conventional salt-rejection evaporators typically exhibit low evaporation rate due to large heat loss. Here, authors demonstrate a solar evaporator featuring vertically aligned mass transfer bridges that takes advantage of the conductive heat to enable optimized water transport and salt backflow.

Photothermal Aerogel Beads Based on Polysaccharides: Controlled Fabrication and Hybrid Applications in Solar-Powered Interfacial Evaporation, Water Remediation, and Soil Enrichment

Solar-powered interfacial evaporation has emerged as an innovative and sustainable technology for clean water production. However, the rapid, mass and shape-controlled fabrication of three-dimensional (3D) steam generators (SGs) for versatile hybrid applications remains challenging. Herein, composite aerogel beads with self-contained properties (i.e., hydrophilic, porous, photothermal, and durable) are developed and demonstrated for threefold hybrid applications including efficient solar-powered interfacial evaporation, water remediation, and controlled soil enrichment. The rational incorporation of selected polysaccharides enables us to fabricate bead-like aerogels with rapid gelation, continuous processing, and enhanced ion adsorption. The composite beads can attain a high water evaporation rate of 1.62 kg m-2 h-1 under 1 sun. Meanwhile, high phosphate adsorption capacity of over 120 mg g-1 is achieved in broad pH (2.5-12.4) and concentration (200-1000 mg L-1) ranges of phosphate solutions. Gratifyingly, we demonstrate the first example of recycling biomaterials from interfacial SGs for controlled nutrient release, soil enrichment, and sustainable agriculture. The phosphate-saturated beads can be gradually broken down in the soil. Macronutrients (N, P, and K) can be slowly released in 50 days, sustaining the plant germination and growth in a whole growth stage. This work shines light on the mass and controlled fabrication of aerogel beads based on double-network biopolymers, not merely scaling up solar-powered interfacial evaporation but also considering water remediation, waste material disposal, and value-added conversion.

Critical aspects to enable viable solar-driven evaporative technologies for water treatment

While passive solar-driven evaporative systems promise higher economic and environmental sustainability in water treatment, many challenges remain for their effective adoption. Here, the author identifies three main pillars and corresponding issues which future research should focus on to bring these technologies to the next maturity level.

Enhanced Contactless Salt-Collecting Solar Desalination

Solar desalination is expected to solve the problem of global water shortage. Yet its stability is plagued by salt accumulation. Here, a paper-based thermal radiation-enabled evaporation system (TREES) is demonstrated to achieve sustainable and highly efficient salt-collecting desalination, featuring a dynamic evaporation front based on the accumulated salt layer where water serves as its own absorber via energy down-conversion. When processing 7 wt % brine, it continuously evaporates water at a high rate─2.25 L m-2 h-1 under 1 sun illumination─which is well beyond the input solar energy limit for over 366 h. It is revealed that such enhanced evaporation arises from the unique vertical evaporation wall of the paper-TREES, which captures the thermal energy from the heated bottom efficiently and gains extra energy from the warmer environment. These findings provide novel insights into the design of next-generation salt-harvesting solar evaporators and take a step further to advance their applications in green desalination.

Adjustable object floating states based on three-segment three-phase contact line evolution

Adjusting the floating states when objects float on water shows great potential for assembly, mineral flotation, nanostructured construction, and floating robot design, but the real-time regulation of floating states is challenging. Inspired by the different floating states of a falling fruit, we propose a facile strategy to transform the object between different floating states based on a three-segment three-phase contact line evolution. In addition, the potential of floating state transformation in solar-powered water evaporation, interface catalysis, and drug delivery is demonstrated. These findings provide insights into floating regulation and show great potential for floating-related applications.

Objects floating on water are ubiquitous in nature and daily life. The floating states of objects are significant for a wide range of fields, including assembly, mineral flotation, nanostructured construction, and floating robot design. Generally, an object exhibits a unique and fixed floating state. The real-time regulation of floating states by a simple method is attractive but challenging. Based on in-depth analysis of the different floating states of fruits falling on water, we reveal that the mutable floating states are caused by the three-segment three-phase contact line dynamics. Accordingly, we propose a “buoyancy hysteresis loop” for the transformation of objects between different floating states. More importantly, we demonstrate the potential applications of floating state transformation in solar-powered water evaporation and interface catalysis. The evaporation and catalytic efficiencies can be changed several times by switching the floating state. These findings deepen the understanding of the interfacial effect to the floating of micro-objects and show great potential for floating-related fields.

Upscaling 3D Engineered Trees for Off-Grid Desalination

More than 70% of the population without access to safe drinking water lives in remote and off-grid areas. Inspired by natural plant transpiration, we designed and tested in this study an array of scalable three-dimensional (3D) engineered trees made of natural wood for continuous water desalination to provide affordable and clean drinking water. The trees took advantage of capillary action in the wood xylems and lifted water more than 1 foot off the ground with or without solar irradiation. This process overcame some major challenges of popular solar-driven water evaporation and water harvesting, such as intermittent operation, low water production rate, and system scaling. The trade-off between energy transfer and system footprint was tackled by optimizing the interspacing between the trees. The scaled system has a ratio of surface area (vapor generation) to project area (water transport) up to 118, significantly higher than the prevailing flat-sheet design. The extensive surface area evaporated water at a temperature cooler than the surrounding air, drawing on multiple environmental energy sources including solar, wind, or ambient heat in the air and realized continuous operation. The total energy for evaporation reached over 300% of the one-sun irradiance, enabling a freshwater production rate of 4.8 L m^-2 h^-1 from an array of 16 trees in an enclosed room and 14 L m^-2 h^-1 under a 3 m/s airflow. Furthermore, we found that the ambient heat in the air contributed 60%-70% of the total latent heat of vaporization when energy sources were decoupled. During long-term desalination tests, the engineered trees demonstrated a self-cleaning mechanism with daily cycles of salt accumulation and dissolution. Combining the quantification from an evaporation model and meteorology data covering the globe, we also demonstrated that the 3D engineered trees can be of particular interest for sustainable desalination in the Middle East and North Africa (MENA) regions.