Latest development in salt removal from solar-driven interfacial saline water evaporators: Advanced strategies and challenges

Developing Salt-Rejecting Evaporators for Solar Desalination: A Critical Review

Solar desalination, a green, low-cost, and sustainable technology, offers a promising way to get clean water from seawater without relying on electricity and complex infrastructures. However, the main challenge faced in solar desalination is salt accumulation, either on the surface of or inside the solar evaporator, which can impair solar-to-vapor efficiency and even lead to the failure of the evaporator itself. While many ideas have been tried to address this ″salt accumulation″, scientists have not had a clear system for understanding what works best for the enhancement of salt-rejecting ability. Therein, for the first time, we classified the state-of-the-art salt-rejecting designs into isolation strategy (isolating the solar evaporator from brine), dilution strategy (diluting the concentrated brine), and crystallization strategy (regulating the crystallization site into a tiny area). Through the specific equations presented, we have identified key parameters for each strategy and highlighted the corresponding improvements in the solar desalination performance. This Review provides a semiquantitative perspective on salt-rejecting designs and critical parameters for enhancing the salt-rejecting ability of dilution-based, isolation-based, and crystallization-based solar evaporators. Ultimately, this knowledge can help us create reliable solar desalination solutions to provide clean water from even the saltiest sources.

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.

Harnessing Synchronous Photothermal and Photocatalytic Effects of Substoichiometric MoO3-x Nanoparticle-Decorated Membranes for Clean Water Generation

Solar-driven interfacial evaporation provides a promising pathway for sustainable freshwater and energy generation. However, developing highly efficient photothermal and photocatalytic nanomaterials is challenging. Herein, substoichiometric molybdenum oxide (MoO3-x) nanoparticles are synthesized via step-by-step reduction treatment of l-cysteine under mild conditions for simultaneous photothermal conversion and photocatalytic reactions. The MoO3-x nanoparticles of low reduction degree are decorated on hydrophilic cotton cloth to prepare a MCML evaporator toward rapid water production, pollutant degradation, as well as electricity generation. The obtained MCML evaporator has a strong local light-to-heat effect, which can be attributed to excellent photothermal conversion via the local surface plasmon resonance effect in MoO3-x nanoparticles and the low heat loss of the evaporator. Meanwhile, the rich surface area of MoO3-x nanoparticles and the localized photothermal effect together effectively accelerate the photocatalytic degradation reaction of the antibiotic tetracycline. With the benefit of these advantages, the MCML evaporator attains a superior evaporation rate of 4.14 kg m-2 h-1, admirable conversion efficiency of 90.7%, and adequate degradation efficiency of 96.2% under 1 sun irradiation. Furthermore, after being rationally assembled with a thermoelectric module, the hybrid device can be employed to generate 1.0 W m-2 of electric power density. This work presents an effective complementary strategy for freshwater production and sewage treatment as well as electricity generation in remote and off-grid regions.

In situ MIL-101 growth on cotton cloth to fabricate multifunctional phase change composites driven by solar and magneto-thermal for all-day desalination

It is certainly one of the most feasible ways to extract fresh water from seawater in the face of the current depletion of fresh water resources. Although solar energy as a heat source for desalination is the cleanest and most abundant way, its intermittent and seasonal also poses an obstacle to its practical application. In order to solve the above-mentioned issues, we prepared a series of phase change composites (PCCs) with excellent light-absorbing and magnetic properties by growing MIL-101(Fe) in situ on cotton fabric. All-day desalination through the synergistic action of phase change material (PCM) and magnetic particles. The evaporation rate of PCC can reach 2.76 kg m-2h-1 with an evaporation efficiency of 90.19 % under one sunlight condition. The evaporation rate of sea water under the synergistic effect of magnetic particles and PCM reached 4.53 kg m-2h-1 in the absence of sunlight. This paper provides a new approach to all-day desalination without contact heating.

An all-in-one FeOx-rGO sponge fabricated by solid-phase microwave thermal shock for water evaporation and purification

Developing high-efficiency photothermal seawater desalination devices is of significant importance in addressing the shortage of freshwater. Despite much effort made into photothermal materials, there is an urgent need to design a rapidly synthesized photothermal evaporator for the comprehensive purification of complex seawater. Therefore, we report on all-in-one FeOx-rGO photothermal sponges synthesized via solid-phase microwave thermal shock. The narrow band gap of the semiconductor material Fe3O4 greatly reduces the recombination of electron-hole pairs, enhancing non-radiative relaxation light absorption. The abundant π orbitals in rGO promote electron excitation and thermal vibration between the lattices. Control of the surface hydrophilicity and hydrophobicity promotes salt resistance while simultaneously achieving the purification of various complex polluted waters. The optimized GFM-3 sponge exhibitedan enhanced photothermal conversion rate of 97.3% and a water evaporation rate of 2.04 kg/(m2·hr), showing promising synergistic water purification properties. These findings provide a highly efficient photothermal sponge for practical applicationsof seawater desalination and purification,as well as develop a super-rapid processing methodology for evaporation devices.

A Transpiration-Driven Electrokinetic Power Generator with a Salt Pathway for Extended Service Life in Saltwater

To ensure prolonged functionality of transpiration-driven electrokinetic power generators (TEPGs) in saltwater environments, it is imperative to mitigate salt accumulation. This study presents a salt pathway transpiration-driven electrokinetic power generator (SPTEPG), incorporating MXene, graphene oxide (GO), and carbon nanotubes (CNTs) as active materials, along with cellulose nanofibers (CNF) and poly(vinyl alcohol) (PVA) as aqueous binders and nonwoven fabrics. This unique combination confers exceptional hydrophilicity and enhances the energy generation performance. When tested with deionized water, the SPTEPG achieved a maximum voltage of 0.6 V and a current of 4.2 μA. In simulated seawater conditions, the presence of conductive ions in the solution boosted these values to 0.64 V and 42 μA. The incorporation of the salt pathway mechanism facilitates the return of excess salt deposits to the bulk solution, thus extending the SPTEPG's service life in saltwater environments. This research offers a straightforward yet effective strategy for designing transpiration-driven power generators suitable for saline water applications.

Carbonaceous Materials-Based Photothermal Process in Water Treatment: From Originals to Frontier Applications

The photothermal process has attracted considerable attention in water treatment due to its advantages of low energy consumption and high efficiency. In this respect, photothermal materials play a crucial role in the photothermal process. Particularly, carbonaceous materials have emerged as promising candidates for this process because of exceptional photothermal performance. While previous research on carbonaceous materials has primarily focused on photothermal evaporation and sterilization, there is now a growing interest in exploring the potential of photothermal effect-assisted advanced oxidation processes (AOPs). However, the underlying mechanism of the photothermal effect assisted by carbonaceous materials remains unclear. This review aims to provide a comprehensive review of the photothermal process of carbonaceous materials in water treatment. It begins by introducing the photothermal properties of carbonaceous materials, followed by a discussion on strategies for enhancing these properties. Then, the application of carbonaceous materials-based photothermal process for water treatment is summarized. This includes both direct photothermal processes such as photothermal evaporation and sterilization, as well as indirect photothermal processes that assisted AOPs. Meanwhile, various mechanisms assisted by the photothermal effect are summarized. Finally, the challenges and opportunities of using carbonaceous materials-based photothermal processes for water treatment are proposed.

3D Cellular Solar Crystallizer for Stable and Ultra-Efficient High-Salinity Wastewater Treatment

Recent developed interfacial solar brine crystallizers, which employ solar-driven water evaporation for salts crystallization from the near-saturation brine to achieve zero liquid discharge (ZLD) brine treatment, are promising due to their excellent energy efficiency and sustainability. However, most existing interfacial solar crystallizers are only tested using NaCl solution and failed to maintain high evaporation capability when treating real seawater due to the scaling problem caused by the crystallization of high-valent cations. Herein, an artificial tree solar crystallizer (ATSC) with a multi-branched and interconnected open-cell cellular structure that significantly increased evaporation surface is rationally designed, achieving an ultra-high evaporation rate (2.30 kg m-2  h-1 during 2 h exposure) and high energy efficiency (128%) in concentrated real seawater. The unit cell design of ATSC promoted salt crystallization on the outer frame rather than the inner voids, ensuring that salt crystallization does not affect the continuous transport of brine through the pores inside the unit cell, thus ATSC can maintain a stable evaporation rate of 1.94 kg m-2  h-1 on average in concentrated seawater for 80 h continuous exposure. The design concept of ATSC represents a major step forward toward ZLD treatment of high-salinity brine in many industrial processes is believed.

Constructing A Solar Evaporator by Stacking Exhausted Wood Sponges for Freshwater Generation and Fertilizer Recovery

Solar water evaporation is an efficient and sustainable technology. To reduce energy consumption and improve cost efficiency, the surface modification of wood sponge by polypyrrole-glutathione (PGWS) was achieved using an in-situ synthetic method. The PGWS exhibits excellent adsorption efficiency for Hg(II) ions with adsorption capacity of 330.8 mg g-1 at 25 °C. Following Hg(II) absorption, the PGWS could be upcycled for solar steam generation. A stackable device was constructed by placing two wood sponges under a Hg(II) saturated PGWS [PGWS-Hg(II)], this system exhibited the highest water evaporation rate of 2.14 kg m-2  h-1 under 1 kW m-2 . Moreover, collecting paper was inserted between the stacked PGWS-Hg(II) and wood sponge for the collection of salts. As such salt can be successfully collected from simulated fertilizer plant effluent and then used as a nutrient for growing plants using a hydroponic system. The facile design of stackable evaporation provides an opportunity for wastewater utilization by harvesting solar energy.

A Salt-Resistant and Antibacterial Cu2 ZnSnS4 -Based Hydrogel for High Efficient Photothermal Distillation in Seawater Desalination and Sewage Purification

Solar steam generation technology (SSGT) using unlimited solar energy is regarded as one of the most promising sustainable technologies to produce clean water. However, most of studies on SSGT simply focus on how to improve salt resistance as well as exclude inorganic and organic pollutants in targeted water, and only very limited studies pay attention to the micro-organisms in the collected water. Herein, one porous Cu2 ZnSnS4 -based photothermal hydrogel (CZTS-PH) with antibacterial properties as well as good salt resistance was successfully prepared. The CZTS-PH was measured with the water evaporation rate as high as 3.249 kg m-2  h-1 and photothermal conversion efficiency of 96.3 % under one sun irradiation. Impressively, owing to the amino groups in the skeleton, CZTS-PH can significantly deteriorate the cell membrane and lead to the death of the Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), which ensures its long-term stability photothermal conversion and the safety of clean water. Overall, the admired photothermal conversion efficiency, and the excellent salt resistance and antibacterial performance suggest that CZTS-PH could be a promising full-scale device applied in seawater desalination and water purification.

Thermal Management of the Solar Evaporation Process

Solar-driven interfacial evaporation has caught wide attention for water purification due to its green and environment-friendly properties. The key issue is how to effectively utilize solar radiation for evaporation. To fully understand the thermal management of the solar evaporation process, a multiphysics model has been built by the finite element method to clarify the heat transfer process for the improvement of solar evaporation. Simulation results indicate that the evaporation performance can be improved by tuning the thermal loss, local heating, convective mass transfer, and evaporation area. The thermal radiation loss of the evaporation interface and thermal convection loss to the bottom water should be avoided, and local heating is good for evaporation. Convection above the interface can improve evaporation performance, although it would enhance the thermal convective loss. In addition, evaporation also can be improved by increasing the evaporation area from 2D to 3D structures. Experimental results confirm that the solar evaporation ratio can be improved from 0.795 kg m^-2 h^-1 to 1.122 kg m^-2 h^-1 at 1 sun with a 3D interface and thermal insulation between the interface and bottom water. These results can provide a design principle for the solar evaporation system based on thermal management.

Potential use of saline resources for biofuel production using halophytes and marine algae: prospects and pitfalls

There exists a global challenge of feeding the growing human population of the world and supplying its energy needs without exhausting global resources. This challenge includes the competition for biomass between food and fuel production. The aim of this paper is to review to what extent the biomass of plants growing under hostile conditions and on marginal lands could ease that competition. Biomass from salt-tolerant algae and halophytes has shown potential for bioenergy production on salt-affected soils. Halophytes and algae could provide a bio-based source for lignoceelusic biomass and fatty acids or an alternative for edible biomass currently produced using fresh water and agricultural lands. The present paper provides an overview of the opportunities and challenges in the development of alternative fuels from halophytes and algae. Halophytes grown on marginal and degraded lands using saline water offer an additional material for commercial-scale biofuel production, especially bioethanol. At the same time, suitable strains of microalgae cultured under saline conditions can be a particularly good source of biodiesel, although the efficiency of their mass-scale biomass production is still a concern in relation to environmental protection. This review summaries the pitfalls and precautions for producing biomass in a way that limits environmental hazards and harms for coastal ecosystems. Some new algal and halophytic species with great potential as sources of bioenergy are highlighted.

Self-Supporting Nanoporous Copper Film with High Porosity and Broadband Light Absorption for Efficient Solar Steam Generation

Solar steam generation (SSG) is a potential technology for freshwater production, which is expected to address the global water shortage problem. Some noble metals with good photothermal conversion performance have received wide concerns in SSG, while high cost limits their practical applications for water purification. Herein, a self-supporting nanoporous copper (NP-Cu) film was fabricated by one-step dealloying of a specially designed Al98Cu2 precursor with a dilute solid solution structure. In-situ and ex-situ characterizations were performed to reveal the phase and microstructure evolutions during dealloying. The NP-Cu film shows a unique three-dimensional bicontinuous ligament-channel structure with high porosity (94.8%), multi scale-channels and nanoscale ligaments (24.2 ± 4.4 nm), leading to its strong broadband absorption over the 200-2500 nm wavelength More importantly, the NP-Cu film exhibits excellent SSG performance with high evaporation rate, superior efficiency and good stability. The strong desalination ability of NP-Cu also manifests its potential applications in seawater desalination. The related mechanism has been rationalized based upon the nanoporous network, localized surface plasmon resonance effect and hydrophilicity.

High-Performance Solar Steam Generator Using Low-Cost Biomass Waste Photothermal Material and Engineering of the Structure

In this work, high-performance, low-cost, environmentally friendly multilayered solar steam generation systems are fabricated by engineering the structure and using a biomass photothermal material. Remarkably, the biomass photothermal material is extracted from the pyrolysis waste of linseed (flax) grains. The introduced system desalinates water using solar energy as the renewable source of energy, and its light absorber is from the waste of a renewable source. The biomass waste powder possesses a mesoporous structure, providing high light absorption through photon scattering and its high surface area. Moreover, to harvest the incident light efficiently and manage the thermal energy generated, devices including light absorbers with cone and cubic configurations and different water manager layers are fabricated and compared to each other. To confirm the high performance of the introduced photothermal material, different systems comprising graphite, graphene oxide, and carbon nanotube light absorbers are also fabricated. Using a biomass light absorber combined with harvesting of the light in different directions (cone configuration), the system with a water evaporation rate of 1.59 kg/m²h corresponding to an efficiency of 92.9% is achieved. Furthermore, by depositing a thin layer of the transparent thermal superinsulator silica aerogel on the light absorber layer, the generated heat is localized and the heat losses are prevented, leading to a 7.5% enhancement of the water evaporation rate of the biomass system. The eco-friendly biomass-based system shows no significant change in its performance through operation for 40 desalination cycles of Persian Gulf water.

Plant-Mimetic Vertical-Channel Hydrogels for Synergistic Water Purification and Interfacial Water Evaporation

The integration of renewable solar energy-driven interfacial evaporation and photocatalysis has recently emerged as one of the most promising technologies for simultaneous freshwater production and pollutant removal. However, the construction of an advanced integrated system with the merit of a fast supply of water and pollutant molecules remains challenging for efficient solar-driven evaporation and photocatalytic performance. Herein, inspired by the transpiration of plants, we fabricate a biomimetic, vertically channeled polypyrrole/foam-like carbon nitride/poly(vinyl alcohol) hydrogel (PCH) by directional freeze-drying. We prove that the vertically aligned channels not only reduce heat loss and improve energy conversion efficiency but also facilitate the transport of water and organic pollutants to the air-water interface. Benefiting from the advantages above, the PCH evaporator presents a high solar evaporation efficiency of 92.5%, with the evaporation rate achieving 2.27 kg m^-2 h^-1 under 1 kW m^-2 irradiation, exceeding many advanced interfacial solar-driven evaporators. Meanwhile, PCH reaches a degradation efficiency of 90.6% within 1 h when dealing with tetracycline (a typical antibiotic)-polluted water, remarkably higher than that of the hydrogel without vertically aligned channels (68.6%). Furthermore, the as-formed reactive oxygen species effectively kill Gram-positive and Gram-negative bacterial in the source water, achieving the all-round water purification. In an outdoor experiment, after 11 h of sunlight irradiation, the tetracycline degradation efficiency and freshwater production of the PCH evaporator rise to 99.0% and 6.2 kg m^-2, respectively. This work highlights the novel biomimetic approach to fabricate multifunctional photothermal materials for simultaneous freshwater production and polluted-water remediation.

Reviewing wood-based solar-driven interfacial evaporators for desalination

Solar‒driven interfacial water evaporation is a convenient and efficient strategy for harvesting solar energy and desalinating seawater. However, the design and fabrication of solar evaporators still challenge reliable evaporation and practical applications. Wood-based solar-driven interfacial water evaporation emerge as a promising and environmentally friendly approach for water desalinating as it provides renewable and porous structures. In recent years, surface modifications and innovative structural designs to prepare high performance wood-based evaporators is widely explored. In this review, we firstly describe the superiority of wood for the fabrication of wood-based solar evaporators, including the pore structure, chemical structure and thermal insulation. Secondly, we summarize the recent developments in wood-based evaporators from surface carbonization, decoration with photothermal materials, bulk modification and structural design, and discuss from the aspects of water transportation capacity, thermal conductivity and photothermal efficiency. Finally, based on these previous results and analysis, we highlight the remaining challenges and potential future directions, including the selection of high-efficient photothermal materials, heat and mass transfer mechanism in wood-based evaporators including large-scale production at a low cost.

Microfluidic Salinity Gradient-Induced All-Day Electricity Production in Solar Steam Generation

Synergistic generation of freshwater and electricity using solar light would be an ideal solution for global freshwater challenges and energy demands. Recently, interface solar steam generation has been considered one of the promising cost-effective alternatives for freshwater generation. Here, we have systematically maintained the salinity gradient within two-legged paper-based microfluidic channels to transport wastewater from the reservoir to the evaporator surface and generate electricity all-day-long. Flowing seawater (3.5 wt % NaCl) on one leg and tap water on the other of the water-conducting channels connected to a conical evaporator, we achieved an average open-circuit voltage (V_OC) of 150 mV and a short-circuit current of 6.5 μA across each channel along with a water evaporation efficiency of 88%. As the V_OC depends only on the ion concentration gradient within the channel in the direction perpendicular to the water flow, the electricity generation persists throughout the day and can be tuned by varying the salinity. Increasing the salt concentration of the seawater to 20 wt %, the V_OC increased to 250 mV in a single channel. In an evaporator connected with four such channels, we achieved a maximum output power density of 9.9 mW m^-2 in a series combination without sacrificing the evaporation rate. Furthermore, removing agglomerated salt from the evaporator surface, we harvested salt at a rate of 0.33 kg m^-2 h^-1. Therefore, our approach provides an alternative way of freshwater generation, salt harvesting, and all-day-long electricity production simultaneously.

Self-Regulating Solar Steam Generators Enable Volatile Organic Compound Removal through In Situ H2O2 Generation

Interfacial solar steam generation for clean water production suffers from volatile organic compound (VOC) contamination during solar-to-steam conversion. Here, we present a solar steam generator based on the integration of melamine foam (MF), polydopamine (PDA), and Ag/AgCl particles. Together with the high photothermal conversion efficiency (ca. 87.8%, 1 kW/m²) achieved by the PDA thin film, the Ag/AgCl particles can efficiently activate the localized generation of H₂O₂ and •OH in situ, thus degrading the VOCs during the rapid vapor generation. The generation of H₂O₂ and •OH in situ also facilitates the creation of a buffer zone containing H₂O₂ and •OH for the rapid removal of organic pollutants in the surrounding water attracted to the solar vapor generator, demonstrating a self-cleaning steam generator toward various volatile compounds such as phenol, aniline, 2,4-dichlorophenol, and N,N-dimethylformamide in a wide range of concentrations.

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.

Narrow-Bandgap LaMO3 (M = Ni, Co) nanomaterials for efficient interfacial solar steam generation

Photothermal water evaporation provides a pathway towards a promising solution to global freshwater scarcity. Synergistic integration of functions in a material in diverse directions is a key strategy for designing multifunctional materials. Lanthanum-based perovskite complex oxides LaMO₃ (M = Ni and Co) have narrow band gaps with a high absorption coefficient. These functionalities have not been appropriately explored for photothermal energy conversion. Here, we synthesized nanostructured metallic LaNiO₃ and semiconducting LaCoO₃ and used them to design interfacial solar steam generators. Effective light absorption capability over the entire solar spectrum of these materials leads to a photothermal efficiency of the order of 83% for both materials. Using a cone-shaped 3D interfacial steam generator with a LaNiO₃ absorber, we achieved an evaporation rate of 2.3 kg m^-2 h^-1, corresponding to solar vapor generation efficiency of over 95%. To the best of our knowledge, this evaporation rate is higher than any oxide-based interfacial solar steam generator reported so far. Furthermore, we have also shown an effective way of using such evaporators for long-term seawater desalination.

Towards sustainable saline agriculture: Interfacial solar evaporation for simultaneous seawater desalination and saline soil remediation

Interfacial solar steam generation is an efficient way to produce freshwater from saline water. This technology was further harnessed here for simultaneous saline soil remediation and enhanced agricultural sustainability. An interfacial solar evaporation and planting system was designed that uses treated seawater for saline soil washing and agricultural irrigation. In outdoor experiments the evaporator realized high freshwater production (10.95 kg m^-2 day^-1) with a soil washing efficiency 3 times greater than traditional distillation. Post treatment plant assays showed that initially highly saline soils could be restored to functional agricultural soils with germination rates of 65% after soil washing, where solar evaporation could continuously provide irrigation water for plant growth. This system is fully automated and uses only solar energy and seawater for saline soil remediation and irrigation. The development of this system provides a potentially useful solution to alleviate global problems associated with water scarcity, soil salinization, and desertification.

Synergistic adjustment of water channels and light absorption pathways to co-generate salt collection and clean water production

Solar-driven interface evaporation for clean water production has attracted significant concern due to its energy-saving and environmental protection. However, it is still challenging for the evaporator to continuously and efficiently produce clean water in practical applications because of salt particle deposits and insufficient water supply. Here, an improved and easy-to-manufacture solar evaporator device (Co-NCNT-GO system) enhances water supply and light absorption by introducing a water supply layer (melamine sponge) and bamboo-like structure carbon nanotubes embedded with metal cobalt particles (Co-NCNT). The salt accumulation on the edge of the Co-NCNT-GO film is achieved by controlling the concentration gradient of brine in the center area and the edge area of the film. This paper aims to study the photothermal mechanism of the Co-NCNT-GO system through a series of characterization and theoretical calculations (DFT) and discuss the influence of different water supply areas on the salt recovery capacity. The results show that Co-NCNT-GO significantly reduces the band (0.054 au) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUNO) by graphite nitrogen-doped CNTs, which is beneficial to improve the light-to-heat conversion capability. Furthermore, the Co-NCNT-GO film has good water wettability due to the higher adsorption energy of pyridine nitrogen and water molecules in Co-NCNT (-9.33 kcal/mol). Simultaneously, it is found that the water evaporation capacity and water supply capacity significantly affect whether the salt can be continuously crystallized at the edge of the film. When the ratio of water supply area to light and heat area is 4:2.5, the salt recovery rate is 46.54 g m^-2 h^-1 during 108 h continuous desalination under one sun illumination. This rationally designed structure and adjustable water transport channel can simultaneously meet high-efficiency evaporation and salt recovery, which can have great potential in practical applications.

Integration of water collection and purification on cactus- and beetle-inspired eco-friendly superwettable materials

Freshwater shortage has been a terrible threat for the sustainable progress and development of human society in 21st century. Inspired from natural creatures, harvesting water from atmosphere has been a feasible and effective method to alleviate water shortage crisis. However, the recent works related to water collection just focuses on how to optimize fog-harvesting manners and efficiencies, the safety and availability of collected water are always ignored. In this paper, we proposed a new strategy accessed to freshwater resources through combining water collection and purification together on eco-friendly superwettable material inspired by cactus spines and desert beetles. Six superhydrophilic wedge-shaped patterns prepared by P25 TiO₂ nanoparticles (NPs) were constructed on candle soot@polydimethylsiloxane (CS@PDMS) superhydrophobic coating. The special superhydrophilic regions not only effectively captured water from foggy environment but generated Laplace pressure gradient to faster drive water away. The bioinspired material exhibited an efficient water collection rate (WCR) of 14.9 ± 0.2 mg min^-1 cm^-2, which was 5.3 and 2.5 times larger than that on uniformed superhydrophilic and superhydrophobic surfaces, respectively. Because of the existence of photocatalytic P25 NPs in wetting areas, the harvested wastewater containing nine kinds of pesticides (0.5 mg/L) could be purified in low concentrations (< 5%) under UV light (365 nm, 5.0 ± 0.6 mW cm^-2). Ten zebrafishes were still alive in such purified water for 72 h, as a contrast, the same number of fishes would almost die in untreated harvested wastewater in just 7 h. This work indeed opens up a new sight to freshwater accessibility, aiming to a promising project for alleviating water shortage around the world.

A self-floating electrospun nanofiber mat for continuously high-efficiency solar desalination

Solar desalination is an environment-friendly and sustainable technology to address the shortage of freshwater resources. However, it still faces huge challenges to develop a salt-rejection solar desalination system with continuous high efficiency. Herein, an electrospun nanofiber mat was fabricated for continuously high-efficiency solar desalination with carbon nanotube as a photothermal material, polyvinylidene fluoride as a floating support material and polyvinylpyrrolidone as a pore-forming agent. The porous structure and superhydrophilic surface provide significant water transport channels and thus avoid salt deposition, even in the high-salinity brine (20 wt% NaCl). The integration of strong broadband absorption property, excellent photothermal performance, floatability, durability and stability endows the solar desalination system with continuously high evaporation efficiency. The evaporation rate and solar conversion efficiency reached up to 1.372 kg m^-2 h^-1 and 86.1%, respectively, in simulated seawater under one sun irradiation and lasted for 11 h with little fluctuation. This work opens a new avenue for the rational design and fabrication of solar desalination systems to promote practical application.

Reed Leaves Inspired Silica Nanofibrous Aerogels with Parallel-Arranged Vessels for Salt-Resistant Solar Desalination

Sufficient and clean freshwater is still out of reach for billions of people around the world. Solar desalination from brine is regarded as one of the most promising proposals to solve this severe crisis. However, most of the reported evaporators to date still suffer from the decreasing evaporation rate caused by salt crystallization accumulated on their surface. Here, inspired by the vascular tissue structure, transpiration, and antifouling function of reed leaves, we design biomimetic hierarchical nanofibrous aerogels with parallel-arranged vessels and hydrophobic surfaces for highly efficient and salt-resistant solar desalination. Foldable vessel walls and flexible silica nanofibers give the reed leaf-inspired nanofiber aerogels (R-NFAs) excellent mechanical properties and enable them to withstand repeated compression. Besides, the R-NFAs can efficiently absorb sunlight (light absorption efficiency: 94.8%) and evaporate the brine to vapor, similar to reed leaves (evaporation rate: 1.25 kg m^-2 h^-1 under 1 sun). More importantly, enabled by the hydrophobic surfaces and parallel-arranged vessels, the R-NFAs can work stably in high-concentration brine (saturated, 26.3 wt %) under high-intensity light (up to 6 sun), demonstrating potent salt resistance. It is expected that R-NFAs with combined antisalt pore and surface structures will provide a designed concept for salt-resistant solar desalination.

Anisotropic Evaporator with a T-Shape Design for High-Performance Solar-Driven Zero-Liquid Discharge

Solar-driven evaporation is regarded as a sustainable wastewater treatment strategy for clean water recovery and salt condensation. However, achieving both high evaporation rate and long-term stability remain challenging due to poor thermal management and rapid salt accumulation and blocking. Here, a T-shape solar-driven evaporator, composed of a surface-carbonized longitudinal wood membrane (C-L-wood) is demonstrated as the top "" for solar harvesting/vapor generation/salt collection and another piece of natural L-wood as the support "" for brine transporting and thermally insulating. The horizontally aligned micro-channels of C-L-wood have a low perpendicular thermal conductivity and can effectively localize the thermal energy for rapid evaporation. Meanwhile, the brine is guided to transport from the support L-wood ("") to the centerline of the top evaporator and then toward the double edge (""), during which clean water is evaporated and salt is crystallized at the edge. The T-shape evaporator demonstrates a high evaporation rate of 2.43 kg m^-2 h^-1 under 1 sun irradiation, and is stable for 7 days of the outdoor operation, which simultaneously realizes clean water evaporation and salt collection (including Cu²⁺ , CrO₄^2- , Co²⁺ ), and achieves zero-liquid discharge. Therefore, the T-shape design provides an effective strategy for high performance wastewater treatment.

Self-contained Janus Aerogel with Antifouling and Salt-Rejecting Properties for Stable Solar Evaporation

Janus structural interfacial vaporization by separating the solar absorber from the bulk working fluid has attracted tremendous attention due to its low heat losses and high solar conversion efficiency for desalination, water purification, energy generation, etc. However, a totally separated double-deck structure with a discontinuous interfacial transition and inefficient photothermic materials undermines its long-term use and large-scale practical exploitation. Herein, a low-cost Janus monolithic chitosan aerogel with continuous aligned run-through microchannels has been demonstrated to have a highly efficient photothermic effect on seawater desalination and wastewater purification. The top solar absorber layer enhances broadband light absorption and photothermal conversion efficiency via the multiple internal reflection of incident light in the aligned microchannels. Moreover, the insulating/hydrophilic bottom layer promotes water transportation and heat localization, and simultaneously prevents salt/fouling accumulation. As a result, a long-term solar vaporization rate of ∼1.76 kg m^-2 h^-1 is achieved, corresponding to a light-to-vapor efficiency of ∼91% under 1 sun irradiation. Notably, the large-vessel microchannels throughout the aerogel and favorable swelling property provide sufficient water replenishment and storage for completely isolating self-contained evaporation, illustrating an enhanced and time-extended self-contained solar steam generation. Such a low-cost bilayer aerogel with remarkable cycling stability in various fluids offers potential opportunities for freshwater production in remote rural areas.

Facile and low-cost ceramic fiber-based carbon-carbon composite for solar evaporation

Solar-driven interfacial evaporation aiming at producing clean water without conventional energy consumption, has attracted worldwide research interest. Nevertheless, complex preparation processes and costly absorber materials might be the challenges for the practical application of this technology. Herein, a ceramic fiber was preferably selected as the supporting matrix, and a composite of activated carbon and carbon black was used as the photothermal material. Different evaporation system configurations containing the as-synthesized solar absorber were constructed and compared. It was found that, due to an improved heat insulation and water transportation, the one-dimensional configuration exhibited a maximum evaporation rate of 1.70 kg m^-2 h^-1 and the highest solar-to-vapor energy conversion efficiency of 91.8% under one sun. Furthermore, material cost and preparation complexity were also incorporated to assess the comprehensive performance of this solar absorber. The ceramic fiber-based activated carbon‑carbon black composite (CF-ACB) solar absorber proposed in this contribution, featuring cost-effectiveness, easiness-to-manufacture and great evaporation performance, illuminated its application potential of future solar desalination to provide clean water for people who live in remote and less developed areas with limited and insufficient access to fresh water.

A graphene assembled porous fiber-based Janus membrane for highly effective solar steam generation

Owing to the shortage of clean water as the global problem, the exploration of photothermal substances with high performance solar steam generation for sustainable water purification is essential and urgent. Herein, we demonstrate the assembly of two-dimensional graphene into one-dimensional rough, loose, and porous fibers and further use the assembled fibers to fabricate Janus membrane evaporator. The specific configuration guarantees an enhanced light harvesting property through multiple reflections, and improves the vapor transport ability through the constructed interlaced network. As a result, the as-obtained evaporator exhibits high solar absorbance, superior photothermal property and energy conversion efficiency, which is much higher than those of other reported Janus membrane evaporators and also better than the fabricated carbon nanotube-, and graphene sheet-based Janus membrane evaporator. The water purification results indicate that the fabricated graphene fiber-based Janus membrane is highly effective in seawater desalination without obvious salt accumulation and heavy metal wastewater purification. This study proposes a neotype graphene assembly for the fabrication of Janus membrane evaporator, which has potential applications in desalination and wastewater decontamination.

Sustainable Solar Evaporation from Solute Surface via Energy Downconversion

Solar-powered interfacial evaporation, a cost-effective and ecofriendly way to obtain freshwater from contaminated water, provides a promising path to ease the global water crisis. However, solute accumulation has severely impacted efficient light-to-heat-to-vapor generation in conventional solar evaporators. Here, it is demonstrated that an interfacial solar thermal photo-vapor generator is an efficient light-to-heat photo-vapor generator that can evaporate water stably in the presence of solute accumulation. An energy downconversion strategy which shifts sunlight energy from visible-near infrared to mid infrared-far infrared bands turns water from transparent to its own absorber, thus changing the fixed evaporation surface (black absorber) in a traditional solar evaporator to a dynamic front (solute surface). Light reflected from the solute can be recycled to drive evaporation. The prototype evaporator can evaporate at a high speed of 1.94 kg m-2 h-1 during a persistent solute accumulation process for 32 h. Such an ability to produce purified water while recycle valuable heavy metals from waste water containing heavy metal ions can inspire more advanced solar-driven water treatment devices.