Zero liquid discharge and salt recovery from heavy metal wastewater via hollow-umbrella hydrogel solar evaporation.

The sustainable treatment of heavy metal-laden industrial wastewater toward zero liquid discharge (ZLD) requires simultaneous water purification, heavy metal immobilization, and salt recovery. Here, we demonstrate a hollow-umbrella hydrogel evaporator addressing all these needs within a single platform. Fabricated from a chitosan/PEI/phytic acid matrix with MXene as the photothermal material, it features a hierarchically porous structure with a central hollow channel and radially aligned pores achieved by sequential vertical and radial directional freezing. This design enables three synergistic functions: hollow-geometry-enhanced natural convection accelerates vapor diffusion; unidirectional water transport directs ions to the hydrophobic umbrella edge for directional crystallization while heavy metals are selectively adsorbed by abundant amino and hydroxyl groups during migration; and hydrophobic edge modification spatially separates evaporation and crystallization zones, preventing salt accumulation. Consequently, the CPPMO-3 evaporator achieves an evaporation rate of 4.97 kg m⁻² h⁻¹ with 97.9% energy efficiency under 1 kW·m⁻² solar irradiation, and adsorption capacities of 234.6 mg g⁻¹ for Cr(VI) and 155.6 mg g⁻¹ for Pb(II). By harvesting the edge-crystallized salt before adsorption saturation, clean salt can be recovered without heavy metal contamination. This work provides a viable pathway toward ZLD treatment and salt resource recovery from high-salinity heavy metal wastewater.