Ultra-absorbent hybrid hydrogel based on alginate and SiO₂ microspheres: A high-water-content system for removal of methylene blue

This work reports the synthesis of hybrid hydrogels based on alginate and π-bonds coupled silica (SiO₂) microspheres (~250 nm) with high performance for water absorption. For formation of the hybrid hydrogel, the alginate was covalently gelled in the presence of SiO₂ vinylated with vinyltrimethoxysilane using crosslinking chemistry. These hydrogels can absorb until 889.76 ± (12.65) g of water per 1 g of dry polymer without their tridimensional shape being unmade. The chemical nature of the polymer and the surface charges of the microspheres play a major role in creating the highly hydrophilic structure and determining the superabsorption properties. The porosity of the matrix also contributes to increase the effectiveness of the absorption, owing to an interconnected pore network that allows absorbed water to be distributed throughout the polymer matrix. In the buffer solutions, the hydrogels showed lower capacity for absorption, which was attributed to ions from weak acids and their respective salts changing charge distribution between the polymer network and the surrounding liquid layer. The applicability of the hydrogel for removal of methylene blue (MB) was investigated. The removal of MB increases with an increase in the amount of modified alginate and monomers, but reduces with a decrease in the amount of vinylated SiO₂. The experimental data were better correlated with pseudo-second order model and the Langmuir model was better fit for the equilibrium data, indicating that MB was absorbed predominantly via chemisorption owing to electrostatic interactions of the dye molecule with a single absorption site.