Additive-free and brine-discharge-free solar-thermal desalination with simultaneous complete mineral mining from ocean water.

Abstract

Solar-thermal interfacial desalination is a sustainable solution to meet the ever-increasing global freshwater demand. However, when treating actual ocean water, salt accumulation on the evaporator surfaces and brine discharge are major issues limiting the performance and posing environmental concerns. By utilizing a femtosecond laser surface processing technique, we create a multi-functional superwicking black metal (SWBM) panel that can pull a thin water film uphill across its surface, absorb nearly all solar radiation, and most importantly, automatically move the crystalized salts from the active regions to the passive regions for self-cleaning and salt collection. This SWBM serves as an energy-efficient, self-maintained, and additive-free and brine-discharge-free solar-thermal interfacial crystallizer (ABF-STIC) that simultaneously produces fresh water and harnesses nearly all salts directly from ocean water. This self-cleaning effect is attributed to the coffee ring effect and salt creeping, which can be enhanced by deeper and wider grooves, enabling self-cleaning even when treating real ocean water. Our ABF-STIC tracks the sun and operates continuously for a week to purify actual ocean water, achieving an average evaporation rate of 1.76±0.04 kg m^-2 h^-1 and salt harvesting rate of 61.74 ± 2.46 g m^-2 h^-1 under one sun, corresponding to ~ 74% solar to vapor conversion efficiency and nearly 100% salt extraction.