Sustainable Interfacial Solar Steam Generation With Biochar-Alginate Bilayer For RO Brine Treatment

Abstract

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 nutshells 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/m² solar irradiation, 2.9 times greater than 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 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.