Ionotropic metal-doped chitosan hydrogels for Indigo Carmine removal

This study explores the use of ionotropic metal-doped chitosan (CHI) hydrogels as adsorbents for the removal of Indigo Carmine dye from aqueous solutions. CHI hydrogels were synthesized via ionotropic gelation using varying concentrations of aluminum (Al-CHI) and iron (Fe-CHI) ions. Scanning electron microscopy revealed a porous surface while energy-dispersive X-ray spectroscopy confirmed the presence of various elements on the bead surfaces, including metal ions, sulfur originating from the sulfonate groups from the dye, and carbon and oxygen from the CHI backbone. Fourier transform infrared spectroscopy identified interactions between the metal ions and the amine/hydroxyl groups of CHI during the formation of ionotropic hydrogel. These metal/amine, hydroxyl groups interacted with the dye during adsorption. The hydrogel with less metal ions exhibited increased swelling and higher dye adsorption capacity at low pH due to the protonation of the amine group in CHI and the formation of metal cationic species. Al-CHI hydrogels showed increased swelling compared to Fe-CHI due to the latter having a compact structure. The zeta potential measurements indicated colloidal instability across all tested pH levels (5, 7, 9), which favored enhanced dye adsorption due to increased electrostatic interactions. Adsorption performance was strongly pH-dependent, with maximum removal (~80%) achieved at pH 5 for low-doped Al-CHI (3.90 mg/g). The adsorption capacity increased from 2.09 mg/g for native CHI to 3.90 mg/g for low-doped Al-CHI and 2.97 mg/g for low-doped Fe-CHI, demonstrating the enhancement in performance due to ionotropic metal doping. Adsorption kinetics followed a pseudo-second-order model (R² > 0.99), indicating chemisorption. Native CHI showed a strong fit with the Langmuir isotherm (R² = 0.9988), indicating monolayer adsorption, while the metal-doped hydrogels exhibited good agreement with the BET isotherm (R² > 0.98), supporting a multilayer adsorption mechanism. The use of non-toxic metal doping instead of conventional chemical crosslinkers offers a low-cost, environmentally safer strategy for wastewater treatment.