Closing the water cycle with biomass: Waste-to-water solar evaporator featuring wood-supported henna photothermal layer for dual-source purification

Abstract

Addressing the critical global challenge of freshwater scarcity, this study presents an innovative solar-driven desalination system leveraging sustainable biomass-derived porous carbon to overcome the energy-intensive limitations of conventional methods like reverse osmosis. Herein, a cost-effective bilayer solar evaporator is fabricated through potassium carbonate-assisted microwave pyrolysis of henna leaves, yielding hierarchically porous carbon (HLPAC) with an ultrahigh specific surface area (1101.7 m² g⁻¹) and exceptional light absorption. The HLPAC is integrated with a natural wood substrate (HLPACW), synergizing the wood's inherent hydrophilicity, low thermal conductivity, and microchannels with the carbon layer's superior photothermal properties. This design optimizes solar energy utilization by enhancing broadband light absorption (97 % across 250–2500 nm), minimizing heat loss, and enabling rapid water transport. The optimized HLPACW 1:1 (HLPACW-1) evaporator achieves an evaporation rate of 2.38 kg m⁻² h⁻¹ under 1 sun irradiation (88 % efficiency), surpassing pure water by 47.6-fold and outperforming state-of-the-art solar evaporators. The composite's hierarchical porosity reduces vaporization enthalpy to 1427 J g⁻¹, enhancing energy efficiency. Crucially, the system demonstrates outstanding durability, maintaining performance over 15 cycles with negligible salt accumulation and achieving 99.9 % ion rejection in seawater desalination. It also exhibits exceptional heavy metal removal (e.g., 99.5 % for Pb²⁺), enabling dual-functionality for desalination and wastewater remediation. By transforming biomass waste into a scalable, eco-friendly material, this work pioneers a circular economy approach to water purification. The evaporator's low-cost fabrication, resilience in high-salinity environments, and dual purification capabilities position it as a transformative solution for sustainable freshwater generation in resource-limited regions, aligning with global decarbonization and sustainability goals.