Solar-Driven Nanofluidic Ion Regulation for Fractional Salt Crystallization and Reutilization

Solar water evaporation (SWE) has emerged as an appealing method for water and salt recovery from hypersaline wastewater. However, different ions usually transfer and accumulate uncontrollably during ion-water separation, making salt fractionalization impractical for conventional SWE, and the resulting mixed salts are hard to use and still require significant costs for disposal. To achieve salt fractionalization and reutilization, achieving ion-water and ion-ion separation simultaneously are crucial in advancing SWE toward sustainability. Here, we present a wood-derived nanofluidic solar-driven fractional crystallizer that regulates the ion transfer processes and extracts nearly pure salt from a mixture of salts. SWE continuously induces capillary flow to propel and concentrate the ions in the wood channels. Meanwhile, engineered functional groups on the channel walls dominate the ion separation process via differentiated interactions with different ions. During ion transfer through channels, SO₄^2- approaches the channel wall to compete for the positive charges and propels Cl^- away, slowing SO₄^2- transport and enlarging the transport energy barrier gap (2.65 to 19.28 kJ mol^-1) between SO₄^2- and Cl^-. Through in situ observation, positive charges on the channel wall make SO₄^2- lag Cl^- 12.4 times that of bare Wood-D, accounting for the enhanced ion separation and the consequent fractional salt crystallization.