Adsorption-Based Mitigation of Azo Dye Toxicity: Removal of Direct Red 23 Using Amberlite XAD-4 Resin

The release of persistent azo dyes into aquatic systems remains a critical environmental and toxicological concern due to their high chemical stability, resistance to biodegradation, and potential to generate carcinogenic aromatic amines. This study evaluates the adsorption of Amberlite XAD-4 (X4), a hydrophobic polystyrene–divinylbenzene resin, for the removal of the toxic azo dye Direct Red 23 (DR 23) from aqueous solutions. Batch experiments were performed to assess the influence of contact time and initial dye concentration, supported by kinetic and equilibrium modeling. Adsorption proceeded through a multistage mechanism involving thin-layer diffusion, intraparticle diffusion, and final equilibrium, which was reached after 48 h. The pseudo-second-order kinetic model (PSO) with R2 = 0.9648 best described the adsorption behavior. Equilibrium data was fitted by the Langmuir isotherm (R2 = 0.9990), yielding a maximum adsorption capacity of 56.8 mg g−1, consistent with the experimentally observed saturation plateau. FTIR spectra revealed characteristic shifts in aromatic, –N=N– (≈1500 cm−1), and –SO32− (1180–1040 cm−1) bands, which, corroborating the data provided by SEM/EDX analysis, completes the adsorption of DR 23 on the X4 matrix. TG/DSC analysis showed modifications in thermal behavior after adsorption without compromising resin stability, supporting strong dye–resin interactions. Overall, the integrated kinetic, isotherm, spectroscopic, and thermal analyses demonstrate that X4 is stable and an adsorbent with desorption capability using chemical agents, highlighting its potential for mitigating the environmental and toxicological risks associated with azo dye contamination in wastewater.