Highly hydrophilic and self-floatable halloysite-based inorganic geopolymer with double-layer structure toward efficient solar interfacial evaporation and wastewater treatment

Solar interfacial evaporation has been regarded as one of the most sustainable technologies for mitigating freshwater crisis. Constructing low-cost evaporators with high hydrophilicity, strong light absorption, good salt resistance, excellent mechanical strength and flexible adaptability is of great significance for solar interfacial evaporation. To this end, a self-floatable double-layer inorganic evaporator (CPDA@HG) was rationally designed using natural halloysite nanotube (HNT). The abundant vertical channels and excellent hydrophilicity of the geopolymer evaporator substrate (HG) endows it with rapid water transfer and outstanding salt dissolution performance, while the carbonized polydopamine modified meta-halloysite (MHNT) as photothermal layer is conducive to enhancing light absorption. Besides, the HNT dispersed in HG can further enhance the hydrophilicity of the evaporator, and the inorganic material characteristics of the geopolymer imparts it remarkable structural stability. As expected, CPDA@HG evaporator demonstrates high evaporation rates of 2.57/4.51 kg m^-2 h^-1 under 1000/2000 W m^-2, exceeding the currently reported geopolymer-based evaporators. Besides, CPDA@HG exhibits eminent salt resistance and high evaporation rate of 2.36 kg m^-2 h^-1 under a high salt concentration of 10%. Notably, it also displays excellent evaporation performance in dye, acid and alkali wastewater, which can meet the standards of pure water after evaporation. Encouragingly, the evaporation rate is greater than 2.5 kg m^-2 h^-1 in cycling with neglectable fluctuation, indicating the evaporator has distinguished long-term durability. Outdoor tests further substantiates the enormous practical application potential of CPDA@HG. This study provides a versatile and low-cost strategy for development of efficient evaporators for solar seawater desalination and wastewater treatment.