Directional salt crystallization system for high-efficiency solar-driven evaporation to achieve zero-liquid discharge

• A novel directional salt crystallization system is designed for desalination and wastewater treatment. • The system exhibits a high evaporation rate of 2.20 kg m −2 h −1 in desalinating 15 wt% brine. • Salt crystals can be collected efficiently through continuous evaporation. Solar-driven interfacial evaporation is an efficient technology for water treatment, enabling the production of freshwater and the removal of impurities. However, achieving a high evaporation rate and a long-term stable operation is challenging due to the salt accumulation on the evaporation surface. In this study, a novel three-dimensional directional salt crystallization system is proposed. The melamine sponge modified with polydopamine and carbon black was used to transport brine, absorb sunlight, and generate steam. The non-photothermal evaporation regions were innovatively constructed by placing a thermally conductive aluminum structure with the deflector paper around the sponge, which increased the effective evaporation area of the system to accelerate the evaporation of brines from the overall system, resulting in an evaporation rate of 2.20 kg m −2 h −1 under one sun irradiation in a 15 wt% NaCl solution. Moreover, the non-photothermal evaporation regions were isolated from the evaporation interface as a salt crystallization region, which completely avoided salt accumulation at the evaporation interface and ensured the long-term stable operation of the system. Salt crystals can be easily collected by replacing the deflector paper regularly, and the salt collection efficiency is over 95 % to achieve zero-liquid discharge (ZLD) treatment. More importantly, the directional salt crystallization system achieves efficient purification of heavy metal ion wastewater and dye wastewater. Consequently, the system presents a highly efficient method for ZLD seawater treatment and wastewater purification.