Sustainable interfacial solar steam generation with a biochar-alginate bilayer for RO brine treatment

Interfacial solar steam generation presents a green and energy-efficient strategy for water purification and brine treatment, moving closer to zero liquid discharge (ZLD) by localizing heat at the air-water interface using advanced photothermal materials. In this work, we developed an all-in-one photothermal bilayer desalination system utilizing biochar derived from waste macadamia nut shells via pyrolysis as a sustainable and cost-effective photothermal material. The bilayer foam design features a top photothermal layer with varying biochar concentrations (80, 160, and 300 mg) embedded within an alginate matrix to maximize light absorption and heat generation. Beneath this lies a porous alginate network that acts as a thermal insulator and facilitates water transport, optimizing interfacial solar steam generation. Among the tested samples, biofoam 2 delivered an impressive water evaporation rate of 2.33 kg m^-2 h^-1 (99.38% efficiency) under 1 kW m^-2 solar irradiation, 2.9 times greater than that of pure water. Under practical conditions, biofoam 2 treated real reverse osmosis (RO) brine (Total Dissolved Solids (TDS): ∼60 000 ppm), achieving a high evaporation rate of 1.34 kg m^-2 h^-1 and a freshwater production rate of 2 L m^-2 day^-1. The photothermal desalination process reduced the TDS to 19 ppm, corresponding to a 2880-fold decrease, alongside significant removal of key ions. Long-term stability and salt resistance were demonstrated through five continuous hours of RO brine interfacial solar steam generation, underscoring the system's robustness. This integrated photothermal bilayer system offers a promising low-cost and high-performance solution for pure water production and sustainable ZLD from highly saline RO brine.