The global scarcity of clean water is a critical issue, exacerbated by increasing surface water contamination from industrial emissions. Traditional water purification methods, such as thermal distillation and reverse osmosis, suffer from high energy consumption, complex processes, and economic limitations. Solar-powered interfacial systems offer a sustainable alternative, but existing technologies struggle with complex contaminant rejection. This research aims to develop a high-performance solar-powered water purification system capable of overcoming these challenges. The significance of this work lies in its potential to provide a cost-effective, environmentally friendly solution for clean water production in diverse and challenging environments, particularly in areas lacking access to reliable water sources and advanced purification technologies. The growing global water crisis demands innovative and efficient water purification strategies that are both sustainable and scalable. Current technologies fall short in addressing complex contamination issues, highlighting the need for advanced materials and systems with enhanced selectivity and antifouling properties. This study directly addresses this gap by designing a novel multifunctional material that surpasses the limitations of existing solar-powered water purification methods.

Existing solar-powered water production strategies, while promising in their sustainability and decentralized nature, are limited by their inability to effectively manage complex contaminants found in many water sources. Previous studies have shown success in solar-powered interfacial evaporation using materials like graphene, polypyrrole, and metal nanoparticles. However, these systems often exhibit poor selectivity, allowing the passage of volatile organic compounds (VOCs), non-volatile organic compounds (NOCs), ions, and microorganisms into the purified water. The presence of VOCs, such as dichloromethane, toluene, and phenol, is a significant concern due to their toxicity even at low concentrations. NOCs, frequently in the form of water-insoluble oils, can clog the pores of existing solar harvesters, reducing efficiency. Similarly, bacterial contamination renders many materials ineffective. This review highlights the need for a material system that addresses these issues through enhanced selectivity, and multifunctional antifouling properties.

The researchers developed a graphene/alginate hydrogel (GAH) system using a combination of material design and bio-inspired surface engineering. The fabrication process involved mixing sodium alginate and graphene oxide, gelation, chemical reduction, and multiple cycles of aluminum-ion-induced cross-linking to create a dense hydrogel structure. Direct-laser writing was employed to create a fish-scale-inspired micro-nano pattern on the surface. Characterization methods included optical microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and UV-Vis-near-infrared spectroscopy to assess the morphology, structure, and optical properties of the GAH. Water content was determined to analyze the internal water state and its impact on contaminant transport. The performance of the GAH was evaluated through testing the selective water transport, and assessing its antifouling capabilities against various contaminants including volatile organic compounds (VOCs), ions, non-volatile organic compounds (NOCs), bacteria, and salts. VOC rejection was measured by analyzing the concentration of toluene, dichloroethane, dioxane, and phenol in treated and untreated samples. Ion rejection was assessed using K+, Ca2+, Na+, and Mg2+ ions. Bacterial removal efficiency was determined through bacterial regrowth tests. Oil adhesion rejection was evaluated by measuring the underwater contact angle with various organic solvents. Salt fouling prevention was evaluated through long-term testing using seawater under simulated sunlight conditions. The detailed methods for GO suspension and GAH preparation, including specific reagents, concentrations, and procedures, are included in the supplementary information.

The GAH demonstrated exceptional performance in selective water harvesting and multifunctional antifouling. It exhibited a high selectivity for water transport, effectively rejecting a broad range of contaminants. The rejection rates were >99.5% for VOCs (toluene, dichloroethane, dioxane, and phenol), >99.3% for ions (Na+, Mg2+, K+, and Ca2+), and 100% for NOCs (various pigments) and bacteria. The GAH achieved an underwater oil contact angle >140°, demonstrating robust oil adhesion rejection. It also showed a nearly 100% bacteria deactivation rate. Notably, the GAH's salt rejection rate reached ~94.9% in a 3.6 wt.% NaCl solution and remained substantial even in saturated NaCl solution. Long-term tests showed stable performance over 5 days, with no salt fouling observed, maintaining high water extraction rates and ion rejection capabilities. The high efficiency of solar-thermal conversion (>95% sunlight absorption) further enhanced the system's performance. The GAH's ability to simultaneously reject VOCs, ions, NOCs, bacteria, and salts represents a significant advancement compared to existing solar-powered water purification technologies.

The results show that the GAH effectively addresses the limitations of previous solar-powered water harvesting technologies. The high selectivity and multifunctional antifouling properties are attributed to the unique combination of a hyper-dense internal structure, the absence of bulk water within the hydrogel, and the fish-scale-inspired surface engineering. The dense internal structure minimizes the transport of contaminants through the material, while the bound water state promotes the selective transport of water molecules via hydrogen bonding. The micro-nano structured surface enhances light absorption for efficient solar-thermal conversion, while promoting superhydrophilicity and superoleophobicity for oil adhesion prevention. The presence of rGO nanosheets on the surface contributes to the antibacterial properties. The high osmotic pressure within the GAH, driven by the rich ionic environment within the cross-linked rGO/alginate network, prevents salt crystallization and fouling. This combination of features makes the GAH a highly efficient and robust system for producing clean water from various contaminated sources. The findings have significant implications for developing practical, low-carbon water purification strategies to alleviate the global drinking water crisis.

This study successfully demonstrated a novel solar-powered water extraction system based on a graphene/alginate hydrogel (GAH) with superior selectivity and multifunctional antifouling properties. The GAH's unique design, combining a hyper-dense structure, bound water transport, and bio-inspired surface engineering, leads to remarkable performance in rejecting a wide range of contaminants, including VOCs, ions, NOCs, oil, bacteria, and salts. Future research could focus on scaling up the production of GAH, exploring different hydrogel compositions and surface modifications, and testing its performance in real-world environments with diverse contaminant compositions and concentrations. This technology holds immense potential for providing safe drinking water in regions with limited access to clean water resources.

While the GAH demonstrates exceptional performance under laboratory conditions, further studies are needed to evaluate its long-term durability and stability in various real-world applications. The scalability and cost-effectiveness of the fabrication process should also be investigated for broader practical implementation. The influence of variations in sunlight intensity and environmental factors on the GAH's performance warrants further investigation. Finally, comprehensive life cycle assessment is needed to fully assess the environmental impact of the GAH production and deployment.