Functionalization of Chitosan and Application in Nanocomposites for Methyl Orange Adsorption

Abstract The production of efficient adsorbents for azo dye removal by synthesizing and evaluating aminosilane-modified chitosan (S-Chi), as well as the formation of nanocomposites containing magnetite (Mgt) and silver nanoparticles (AgNPs), is addressed in this work. The nanocomposites were prepared using a suspension technique and characterized to confirm Chi functionalization and changes in the surface properties. Adsorption of anionic dye methyl orange (MO) from aqueous solution was assessed through kinetic, equilibrium, pH-dependent, thermodynamic, and reuse experiments. The performance was compared with that of the nanocomposites prepared with unmodified chitosan. Overall, silane treatment shifted the pH at the point of zero charge (pHPZC) of the particles from 1.5 to 6.6, thereby enhancing electrostatic interactions with MO and increasing removal efficiency relative to the unmodified composite (from 6.3% to >99%). After modification, the average pore volume increased from 0.014 cm3 g–1 to 0.031 cm3 g–1. The adsorption process followed a pseudo-second-order (PSO) kinetic model, while equilibrium data showed mixed Langmuir–Freundlich behavior, with a maximum adsorption capacity (qmax) of 96 mg g–1. Thermodynamic results indicated a spontaneous process accompanied by decreased entropy, consistent with MO organization on the adsorbent surface. Moreover, the mechanism combined both physisorption and chemisorption, being favored at a pH range of 4.0–6.0 due to protonation of amino groups. Finally, the modified nanocomposites also showed reuse potential, maintaining 61% removal efficiency in a second adsorption cycle.