Iron Modified <i>Opuntia ficus-indica</i> Cladode Powder as a Novel Adsorbent for Dyes Molecules

In this study, Opuntia ficus-indica cladode powder (OFIC), locally sourced from Rabta in Tunis, was utilized as a novel, eco-friendly adsorbent in both raw and iron(III) chloride-modified forms. The presence of iron in the modified material was confirmed by X-ray fluorescence spectroscopy (XRF). The neat and modified biomass were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM), and their usefulness as adsorbent for cationic Neutral Red (NR) and anionic Congo Red (CR) dyes were explored under batch conditions. Equilibrium studies revealed that the iron-modified Fe(OH)x@Cellulose adsorbent exhibited superior adsorption capabilities for both dyes compared to the raw material. Moreover, CR dye was more effectively adsorbed by Fe(OH)x@Cellulose than NR. The adsorption isotherms for both dyes were fitted. The results demonstrated that the adsorption of both NR and CR dyes onto the biosorbent Fe(OH)x@Cellulose was closely followed by the Langmuir model, with R2 values of 0.980 and 0.973 for NR and CR, respectively, and the pseudo-second-order kinetic model better depicted the adsorption kinetic. Thermodynamic analysis revealed a negative enthalpy value (−67.15 kJ/mol) for NR adsorption, suggesting an exothermic process, while a positive enthalpy value (3.99 kJ/mol) was observed for CR adsorption, indicating an endothermic process.