Sustainable Atmospheric Water Harvesting Using Biomass-Derived Hydrogels: Effects of Cross-Linker Concentration and Sorption Kinetics

Clean water scarcity represents a significant global challenge, driven by the degradation of surface water resources due to pollution and the impacts of climate change. Atmospheric water harvesting strategies using sorbents offer an available and sustainable solution. Most atmospheric water harvesting studies have focused on hydrogel designs utilizing conventional polymer desiccants derived from fossil fuels. These synthetic polymers are unsustainable and nonbiodegradable, causing negative ecological and public health impacts during degradation, which raises concerns about the direction of eco-friendly material science and technology. Here, biohydrogels (SCG0, SCG3, SCG5, and SCG7) based on chitosan and carboxymethyl cellulose from biomass were synthesized through a simple process. The effect of the cross-linking content on the properties of hydrogels and their water sorption performance were studied through FTIR, TGA, and FESEM analyses, mechanical strength, and water sorption experiments. A SCG5 hydrogel containing 5% w/w glutaraldehyde exhibits the most effective cross-linking formation, leading to superior thermal stability, good compressive strength, and a better water absorption performance compared to the SCG0, SCG3, and SCG7 hydrogels. The SCG5 hydrogel showed strong hydrophilicity when a drop wetted in its surface within 0.26 s. The mass change of SCG5 in water gained 2158% with a maximum sorption rate of 84.7 g g^-1 h^-1, and the water vapor sorption capacity of SCG5 at 90% RH reached 28.03% with a maximum sorption rate of 0.55 g g^-1 h^-1. Additionally, it exhibited rapid vapor desorption with a rate of 0.39 g g^-1 h^-1, releasing over 98% of the absorbed water within 20 min, and remarkable stability after multiple sorption-desorption cycles. Studies on different sorption kinetic models of biohydrogels based on chitosan and carboxymethyl cellulose were carried out, and the experimental data best fitted the Elovich model the most. It means that activated site sorption is the rate-limiting process; the sorption mechanism occurs on a nonuniform surface of biohydrogels or nonconstant active sites.