Hydrogel Biocomposite of Alginate and Mucilage of <i>Opuntia ficus-indica</i> Cactus in the Adsorption of Methylene Blue in Aqueous Solution

This work analyzes the production of a hydrogel composed of mucilage from the cactus <i>Opuntia ficus-indica</i> (OFI) and sodium alginate. In obtaining the new material, green synthesis was used, free of chemical compounds, and applied in the treatment of textile effluent for the adsorption of methylene blue (MB). The hydrogel was characterized by FT-IR, XRD, SEM, and zeta potential. The swelling study showed a maximum value of 262% at pH 6.0. Adsorption studies revealed a maximum adsorptive capacity of 7.21 mg g^-1 in 400 min at 298 K. Furthermore, the experimental data showed better fit to the pseudo-second order and Langmuir models for kinetic and isothermal studies, respectively. The adsorptive process showed spontaneous and exothermic behavior as well as a chemisorptive nature. It is noteworthy that in the studies conducted at a higher concentration of the contaminant, the maximum adsorption was 760 mg g^-1. The reuse of the hydrogel was effective for five cycles, maintaining the adsorption of approximately 50% MB removal. Therefore, the biodegradable hydrogel is a material that contributes to the environment, is low cost, with simple synthesis, and is a promising new material for large-scale applications, considering its sustainable character and high efficiency in the adsorption of MB in aqueous solution.