Mechanistic insights into interfacial evaporation selective crystallization for one-step high-purity salt extraction from saline water

Extracting high-purity (>99%) salt resources from saline water through conventional separation processes typically involves multiple steps, high energy consumption, and substantial waste generation, posing significant environmental challenges. While interfacial evaporation crystallization has attracted growing interest for salt extraction due to its structural simplicity, effective renewable energy utilization, and direct solid product generation, its limited crystallization selectivity often results in co-crystallization of multiple salt species, yielding mixed or low-purity products that constrain practical applicability. In this thesis, we propose a diffusion-driven selective crystallization strategy for high-purity salt production directly from source water with mixed salts. The essence of the strategy is to purposefully suppress non-ion-selective transfer processes (e.g., convection) so to make the difference in ion diffusion drive targeted ion to selectively move to the crystallization surface. As a proof concept, a floating porous membrane evaporator was designed, which universally achieved high-purity salt production (>99.10%) from saline water of mixed ions such as Na⁺/K⁺, Ba²⁺/K⁺ and Mg²⁺/Li⁺. Beyond experimental insights, this thesis further established a theoretical model that accurately predicted the boundary conditions essential for achieving species-selective crystallization, providing a foundational framework that will guide precise salt separation. The diffusion-driven selective crystallization strategy opens a new horizon for selective crystallization and enriches the toolbox of high-purity salt production. Building on this theoretical framework, we further developed a solar interfacial evaporation crystallizer designed for practical application scenarios. To enable sustainable extraction of high-purity sodium resources, the system is designed to utilize difference in Na⁺/K⁺ diffusion to drive selective NaCl crystallization from real seawater, with the entire process powered by solar energy. Utilizing this system, we successfully achieved direct, one-step production of NaCl crystals with a purity of 99.36% from real seawater at a rate of 28 g·m⁻²·d⁻¹ under regular solar radiation, all without any pre- or post-treatment. Beyond high-purity NaCl, this system also successfully achieved one-step sustainable extraction of high-purity BaCl₂ and MgCl₂ from mixed source brine. This system provides an eco-friendly and low-cost solution for large-scale one-step resource extraction from saline water. Drawing from insights into ion transfer from our previous findings, this thesis further introduces a novel advancement in interfacial evaporation selective crystallization by leveraging the difference in salt dissolution rate. The essence of the strategy is to purposefully regulate hydrated ion transfer driven by convection from the salt crystal surface to the source water so to utilize dissolution rate difference to achieve complete dissolution of unwanted salt in crystals. As a proof of concept, a solar evaporator was rationally designed, enabling the sustainable selective crystallization of high-purity salt from source brine with mixed ions such as Na⁺/K⁺ and Li⁺/K⁺. Beyond its success in extracting resources from saline water, this strategy can also directly extract high-purity salt crystals from mixed solid salts, such as KCl/NaCl and KCl/LiCl, in a single-step dissolution process. Finally, its real-world applicability was demonstrated by achieving 99% pure KCl production at a rate of 36.60 g·m⁻²·d⁻¹ from real fly ash leachate. This strategy expands the mechanistic foundation of selective crystallization, and offers a versatile, low-energy approach for sustainable and high-purity salt separation. In summary, this thesis addresses and resolves critical challenges in the efficient extraction of high-purity salts from saline water by interfacial evaporation selective crystallization technology. The findings provide a foundation for the development of sustainable, selective crystallization systems and will drive future advancements in resource recovery and industrial-scale applications in salt separation.