Coupling solar-driven interfacial evaporation with forward osmosis for continuous water treatment

Dual Cross-Linking Coal Tar-Derived Phenolic Resin Porous Carbon-Based Hydrogel Solar Evaporators for Efficient Wastewater Purification

Porous carbon-based hydrogel evaporators show extensive application potential in the field of solar-driven water evaporation due to their wide availability, excellent hydrophilicity, and abundant porous structure. However, the closed pore structure of the single-molecule cross-linked hydrogel and the high production cost of the nanoporous carbon materials not only affect the salt resistance of the carbon-based hydrogel but also restrict its large-scale application. Herein, we developed a uniform porous APRC-PVA/PEG hydrogel evaporator by integrating poly(vinyl alcohol) (PVA)/polyethylene glycol (PEG) dual-network hydrogels with broadband solar-absorbing, cost-effective porous carbon nanosheets (APRC) derived from coal tar-based phenolic resin. The rich pores and three-dimensional double-network structure of the evaporator ensured excellent water transport performance and a high light absorption rate (≈98%). Meanwhile, the low thermal conductivity of the evaporator (dry: 0.09 W m-1 K-1; wet: 0.29 W m-1 K-1) reduces thermal loss to the bulk water, enabling a water evaporation rate of 1.45 kg m-2 h-1 under 1 sun irradiation and a low enthalpy of evaporation of 1614.605 J g-1. The evaporator also shows good potential in the field of brine, industrial wastewater, and organic dye wastewater.

Solar Evaporators for Saline Water: Sustainable Clean Water Harvesting and Critical Mineral Resources Extraction

Solar-driven interfacial water evaporation (SIWE) can efficiently utilize solar energy to separate or extract various ions from saline water, providing an environmentally friendly, economical, and sustainable approach to clean water and critical mineral resources harvesting. However, for ongoing practical implementation, solid salt accumulation at the interface will inevitably impair the SIWE performance, while direct disposal of residual concentrated brine poses significant environmental risks. As such, advancing solar evaporators for sustainable clean water harvesting and critical mineral resources extraction is pivotal in the resources-energy-environment nexus. Critically, this review spotlights the latest research progress in engineering nonselective salt-rejecting solar evaporators (NS-SRSEs) for sustainable desalination, emphasizing interfacial structural design and surface modification. We then delineate our endeavors aimed at the construction strategies of selective salt-extraction solar evaporators (S-SESEs) for getting access to critical mineral resources such as uranium and lithium. Finally, current challenges and opportunities are outlined in the high-value saline water utilization of NS-SRSE and S-SESE for real-world applications that balance high efficiency, durability, and adaptability with a low environmental impact. Looking ahead, we anticipate ongoing advancements in promoting solar evaporators from laboratory research to practical applications, contributing to global efforts in sustainable water management and critical mineral resources recovery.

High-Performance Silicone Sponge Evaporators with Low Thermal Conductivity for Long-Term Solar Interfacial Evaporation and Freshwater Harvesting

Solar interfacial evaporation (SIE) has emerged as a highly promising approach for sustainable freshwater harvesting. However, maintaining a stable evaporation rate and achieving a high freshwater yield in high-salinity brines remain a significant challenge. In this study, we present the development of silicone sponge-based evaporators with a "free-salt" structure, designed to enhance the efficiency of SIE and freshwater collection. These evaporators, designated as PSS@Fe3O4/CNTs, were fabricated by grafting durable silicone onto a silicone sponge framework, followed by the incorporation of Fe3O4 nanoparticles and carbon nanotubes. The unique combination of exceptional photothermal properties and a controlled yolk-shell structure with low thermal conductivity enabled the PSS@Fe3O4/CNT evaporators to sustain a stable evaporation rate of 1.87 kg m-2 h-1 in real seawater over 200 h of continuous operation under 1 sun illumination. Importantly, no salt accumulation was observed on the evaporator surfaces, even when exposed to highly concentrated brines. In a closed system equipped with a condenser, these evaporators achieved freshwater production rates of 14.5 and 11.8 kg m-2 over 10 h from 10 and 20 wt % NaCl solutions, respectively, under 1 sun illumination. These values correspond to normalized production rates of 1.45 and 1.18 kg m-2 h-1, showcasing the consistent and efficient performance of the evaporators across varying salinity levels. Beyond salt rejection, the PSS@Fe3O4/CNT evaporators also demonstrated the ability to effectively remove various heavy metal ions (e.g., Cu2+ and Zn2+) and organic pollutants from contaminated water. This work provides valuable insights into innovative evaporator designs for efficient freshwater production from seawater and wastewater.

Bifunctional Portable Powder for Freshwater Production With Moisture Harvesting and Undrinkable Water Purification

Freshwater scarcity threatens human survival, particularly in extreme environments like deserts, oceans, and space. Compatible atmospheric water harvesting and undrinkable water purification offer an affordable approach to solving freshwater scarcity in these extreme environments. Nonetheless, developing composite sorbent to attain efficient atmospheric water harvesting and undrinkable water purification remains challenging. Hence, a portable hybrid hygroscopic powder (HLC powder) consisting of hydroxypropyl chitosan, dibenzaldehyde-functional poly(ethylene glycol), lithium chloride (LiCl), and nano carbon black is proposed. The HLC powder with optimized LiCl load can capture moisture from the air, showing a high water uptake of 1.76 g g-1 at 34% relative humidity (RH) and appropriate over a wide humidity from 34% to 75% RH. pH-responsive sol-gel transition induced by Schiff base bonds transforms the HLC solution into hydrogel, inhibiting hydrated salt leakage. Meanwhile, to achieve efficient undrinkable water purification, the LiCl-free hybrid powder is utilized to convert the undrinkable water, including seawater, dye water, and human urine, to photothermal hydrogel evaporators with low evaporation enthalpies and high evaporation rates ranging from 1.81 to 2.05 kg m-2 h-1 under one sun. This strategy establishes a new path to conveniently obtaining freshwater, breaking hydrological restrictions.

Scalable Asymmetric Fabric Evaporator for Solar Desalination and Thermoelectricity Generation

The integration of solar interfacial evaporation and power generation offers a sustainable solution to address water and electricity scarcity. Although water-power cogeneration schemes are proposed, the existing schemes lack scalability, flexibility, convenience, and stability. These limitations severely limit their future industrial applications. In this study, we prepared a hybrid fabric composed of basalt fibers and cotton yarns with asymmetric structure using textile weaving technology. The cotton yarn in lower layer of fabric facilitates water transport, while the basalt fibers in upper layer enable thermal localization and water supply balancing. The carbon black is deposited on top layer by flame burning to facilitate photothermal conversion. The fabric exhibits a high evaporation rate of 1.52 kg m-2 h-1, which is 3.6 times that of pure water, and an efficiency of 88.06% under 1 kW m-2 light intensity. After assembly with a thermoelectric module, the hybrid system achieves a maximum output power density of 66.73 mW m-2. By exploiting the scalability of fabric, large-scale desalination and power production can be achieved in outdoor environments. This study demonstrates the seamless integration of fabric-based solar evaporation and waste heat-to-energy technologies, thereby providing new avenues for the development of scalable and stable water-power cogeneration systems.

A lignin-derived carbon dot-upgraded bacterial cellulose membrane as an all-in-one interfacial evaporator for solar-driven water purification

Solar-driven interfacial evaporation has emerged as an efficient approach for wastewater treatment and seawater desalination. New trends demand adaptive technology to develop photothermal membranes with multifunctional features. Herein, we report a robust multi-purpose near-infrared (NIR)-active hydrogel composite (c-BC@N-LCD) from broad-spectrum active nitrogen-doped lignin-derived carbon dots (N-LCDs) covalently cross-linked with a bacterial cellulose (BC) matrix. BC provides adequate porosity and hydrophilicity required for easy water transport while managing heat loss. A commendable evaporation rate (ER) of 2.2 kg m-2 h-1 under one sun (1 kW m-2) is achieved by c-BC@N-LCD. The developed hydrogel system is also found to be efficient for desalination (∼2.1 kg m-2 h-1) and for remediating various pollutants (heavy metal ions, dyes, and pharmaceuticals) from feed water. The efficacy of the membrane remains unaltered by different grades of water, and hence can be adoptable for economically stressed communities living in water-polluted regions as well as those residing in coastal areas.

Monolith floatable dual-function solar photothermal evaporator: efficient clean water regeneration synergizing with pollutant degradation

Meeting the growing demands of attaining clean water regeneration from wastewater and simultaneous pollutant degradation has been highly sought after. In this study, nanometric CuFe2O4 and plasmonic Cu were in situ confined into graphitic porous carbon nanofibers (CNF) through electrospinning and controlled graphitization, which were integrated onto a melamine sponge (S-FeCu/CNF) as a monolithic evaporator via a calcium ion-triggered network crosslinking method using sodium alginate (SA). This monolithic evaporator serves a dual purpose: harnessing solar-driven photothermal energy for water regeneration and facilitating synchronous contaminant mineralization through advanced oxidation processes (AOPs). The metal-modified FeCu/CNF graphitic porous carbon exhibited an enhanced light absorption property (≥95%) and was further securely anchored on the sponge by a calcium ion-triggered SA crosslinking technique, thereby efficiently restraining salt deposition. The FeCu/CNF evaporator demonstrated a solar-vapor conversion efficiency of 105.85% with an evaporation rate of 1.61 kg m-2 h-1 under one sun irradiation. The evaporation rate of the monolithic S-FeCu/CNF evaporator is close to 1.76 kg m-2 h-1, and an evaporation rate of 1.54 kg m-2 h-1 can be achieved even in 20% NaCl solution, with resistance to salt deposition and cycling stability. Synchronously, the monolithic D-S-FeCu/CNF evaporator also acts as a heterogeneous catalyst to activate peroxymonosulfate (PMS) and trigger rapid pollutant degradation, which also shows excellent catalytic cycling stability, producing clean water that satisfies the World Health Organization (WHO) standards. This work provides a potentially valuable solution for addressing desalination and wastewater treatment.

Renewable biomass reinvigorates sustainable water-energy nexus

The water-energy nexus has garnered worldwide interest. Current dual-functional research aimed at co-producing freshwater and electricity faces significant challenges, including sub-optimal capacities ("1 + 1 < 2"), poor inter-functional coordination, high carbon footprints, and large costs. Mainstream water-to-electricity conversions are often compromised owing to functionality separation and erratic gradients. Herein, we present a sustainable strategy based on renewable biomass that addresses these issues by jointly achieving competitive solar-evaporative desalination and robust clean electricity generation. Using hydrothermally activated basswood, our solar desalination exceeded the 100% efficiency bottleneck even under reduced solar illumination. Through simple size-tuning, we achieved a high evaporation rate of 3.56 kg h-1 m-2 and an efficiency of 149.1%, representing 128%-251% of recent values without sophisticated surface engineering. By incorporating an electron-ion nexus with interfacial Faradaic electron circulation and co-ion-predominated micro-tunnel hydrodynamic flow, we leveraged free energy from evaporation to generate long-term electricity (0.38 W m-3 for over 14d), approximately 322% of peer performance levels. This inter-functional nexus strengthened dual functionalities and validated general engineering practices. Our presented strategy holds significant promise for global human-society-environment sustainability.

Intercalation of V2O5 and Polypyrrole into a Graphene Oxide Layer: A Hybrid Multifunctional Photothermal Structure for Efficient Solar Evaporation, Water Purification, Disinfection, and Power Production

Development of a hybrid multifunctional photothermal structure with multifunctional capabilities is deliberated as an effective approach for harvesting abundant solar energy for sustainable environmental applications. Achieving enhanced solar to thermal conversion efficiency utilizing a suitably designed, environmentally compatible thermal management structure however remains a significant challenge. Herein, we report the intercalation of V2O5 and polypyrrole into a graphene oxide layer to design a hybrid photothermal assembly (PPy-V2O5-GO) and its multifunctional proficiencies. The hybrid photothermal structure demonstrated synergistic photothermal conversion, buoyant porous structure sustaining water transmission, and efficient steam release. V2O5 and polypyrrole-intercalated optimized graphene oxide structure attained an evaporation rate of 1.9 kg m-2 h-1 with a conversion efficiency of 92% under 1 sun solar radiation. At maximum, the assembly's surface temperature hit 64 ± 2 °C, suggesting its suitability as a solar water purifier. Outdoor experiments suggest the evaporator assembly's capability to accumulate a total output of 15 kg m-2 over a single day. Cell viability investigations revealed strong antimicrobial properties of PPy-V2O5-GO against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria, eliminating nearly all under 1 sun, making it a potential candidate for photothermal therapy. Furthermore, when combined with a commercial thermoelectric module, the framework displayed exceptional photothermal conversion efficiency, hinting at its potential for electrical power generation. The integration of PPy-V2O5-GO with a Bi2Te3-based thermoelectric module significantly boosted the thermoelectric generator's performance, offering an enhanced power output of 2.8 mW and a high power density of 1.24 mW/cm2, making them suitable for off-grid or remote-area application. Overall, the PPy-V2O5-GO photothermal assembly's stability, lack of leaching, effectiveness in producing pure water from seawater, antimicrobial efficacies, and recyclability make it an excellent choice for sustainable water treatment and power generation.

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.

Photocatalytic Co-Reduction of N2 and CO2 with CeO2 Catalyst for Urea Synthesis

The effective conversion of carbon dioxide (CO2 ) and nitrogen (N2 ) into urea by photocatalytic reaction under mild conditions is considered to be a more environmentally friendly and promising alternative strategies. However, the weak adsorption and activation ability of inert gas on photocatalysts has become the main challenge that hinder the advancement of this technique. Herein, we have successfully established mesoporous CeO2-x nanorods with adjustable oxygen vacancy concentration by heat treatment in Ar/H2 (90 % : 10 %) atmosphere, enhancing the targeted adsorption and activation of N2 and CO2 by introducing oxygen vacancies. Particularly, CeO2 -500 (CeO2 nanorods heated treatment at 500 °C) revealed high photocatalytic activity toward the C-N coupling reaction for urea synthesis with a remarkable urea yield rate of 15.5 μg/h. Besides, both aberration corrected transmission electron microscopy (AC-TEM) and Fourier transform infrared (FT-IR) spectroscopy were used to research the atomic surface structure of CeO2 -500 at high resolution and to monitor the key intermediate precursors generated. The reaction mechanism of photocatalytic C-N coupling was studied in detail by combining Density Functional Theory (DFT) with specific experiments. We hope this work provides important inspiration and guiding significance towards highly efficient photocatalytic synthesis of urea.

A hierarchical porous aerohydrogel for enhanced water evaporation

Natural solar-powered steam generation provides a promising strategy to deal with deteriorating water resources. However, the practical applications of this strategy are limited by the tedious manufacturing of structures at micro-nano levels to concentrate heat and transport water to heat-localized regions. Herein, this work reports the fabrication of hierarchically porous aerohydrogel with enhanced light absorption and thermal localization at the air-solid interface. This aerohydrogel steam generator is fabricated by a simple yet controllable micropore generation approach to assemble air and hydrogel into hierarchically porous gas-solid hybrids. The tunable micropore size in a wide range from 99±49µm to 316±58μm not only enables contrasting sunlight absorptance (0.2 - 2.5µm) by reducing the reflection of solar light but also harnesses water transportation to the heating region via a capillary force-driven liquid flow. Therefore, a solar-vapor conversion efficiency of 91.3% under one sun irradiation was achieved using this aerohydrogel evaporator, reaching a ready evaporation rate of 2.76kg m-2 h-1 and 3.71kg m-2 h-1 under one and two sun irradiations, respectively. Our work provides a versatile and scalable approach to engineering porous hydrogels for highly efficient steam generation and opens an avenue for other potential practical applications based on this aerohydrogel.