Agricultural and food wastes derived cellulose hydrogels for water reservoirs and dye sorption

Abstract

Hydrogels derived from agricultural and food waste exhibit significant potential across a wide range of applications, addressing both environmental challenges and industrial demands. In this study, cellulose was isolated from various food and agricultural waste sources, including rice straw, mosambi (sweet lemon) peel, and lychee peel, and subsequently used to synthesize hydrogels. The synthesized cellulose and hydrogel were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and thermogravimetric techniques (TGA). SEM images showed rice straw cellulose fibres were oriented, crystalline, and bundles with well-defined fibrous strands, and mosambi peel cellulose fibres were flake-like shapes, and lychee peel cellulose fibres were interlocking, porous, and coiled in shape. The prepared cellulose hydrogel showed swelling 1360% − 1592% with water retention 12 days in open air. The sorption of cationic methylene blue and anionic methyl orange dyes into the neutral hydrogel matrix was further investigated. It was observed that the pseudo-second-order model best fits for all the dye-hydrogel systems, regardless of the primary cellulose source. These observations suggest that the sorption mechanism of cationic or anionic dyes into the neutral hydrogel matrix is complex, combining physisorption and chemisorption. Adsorption kinetic models showed that the Freundlich and Temkin models fit better than the Langmuir model, indicating that the adsorption process is governed by multilayer adsorption on heterogeneous surfaces, dominated by physical interactions with possible contributions from weak H-bonding. The primary affinity is hydrogen bond (H-bond) formation between dye molecules and the hydrogel, which was confirmed by disrupting the H-bond using HCl and eluting the dye molecules from the hydrogel matrix. This observation suggests that the contribution of the neutral segments of partially functionalized (cationic or anionic) hydrogels should be considered when quantifying total dye adsorption.