Hofmeister effect-based 3D hydrogel sponge via non-contact localized crystallization for achieving zero liquid discharge desalination of high-salinity brine

Abstract

Due to lower evaporation enthalpy and controllable water management,- 3D hydrogel-based evaporators has attracted significant attention in solar desalination. However, challenges such as the closed pore structure, the strong adhesive force between salt deposition and photothermal materials, and the byproducts of high-salinity wastewater have limited long-term application in high-salinity brine. To address these limitations, a 3D zero liquid discharge evaporator has been innovatively constructed. It is composed of a high porosity 3D CNT/PDA@PVA sponge-like hydrogel in optimal ionic conditions (Hofmeister effect) and pearl-patterned cotton towels. The assembled evaporator has achieved excellent water evaporation performance of 3.0 kg m⁻² h⁻¹ (1.08 kg m⁻² h⁻¹, in the dark) through side area-assistance and environmental energy. The non-contact salt deposition design can avoid direct salt deposition on the surface of the photothermal material. Thus, due to the diffusion-convection effect and Marangoni flow in the highly porous material and pearl-patterned cotton towels, the formed zero liquid discharge evaporator has remained a stable water evaporation rate of 2.65 ± 0.1 kg m⁻² h⁻¹ and recovered salt (59.16 g m⁻² h⁻¹) in a 10 wt% NaCl solution for over 100 h. Most importantly, replacing the cotton towel with a new one allows the assembled evaporator to continuously operate in a 20 wt% NaCl solution while protecting it from damage. Significantly, the collected water can be directly used for drinking and wheat cultivation. This work offers new insights for designing a 3D-zero liquid discharge evaporator that can operate long-term in high-salinity brine, thereby expanding its practical applications.