Water vapor mass transfer in alginate–graphite bio-based hydrogel for atmospheric water harvesting

This study presents experimental and theoretical investigations on water vapor mass transfer of a novel hydrogel compound based on alginate and graphite. This hydrogel enables rapid, reproducible, and thermally driven cycles for the adsorption and desorption of water vapor from ambient air for atmospheric water harvesting applications. We study the impacts of hydrogel composition on sorption capacity and kinetics using sorption/regeneration experiments under various environmental conditions. Theoretical models based on Fick's law of diffusion and Linear Driving Force are developed and validated with experiments to optimize thermal cycling conditions within the temperature range of 20–100 °C. The bio-based hydrogel exhibited remarkable water uptake, ranging from 0.5 to 0.9 g/g, with RH below 30 and 50 %, respectively. This low-humidity setting enables a water production rate of 1.6–2.9 L/kg of sorbent per day with a low-grade thermal regeneration (60–100 °C). Natural graphite microparticles improve water vapor release kinetics during regeneration, with an effective diffusivity coefficient of around 10⁻¹¹ m²/s.