Optimizing Dye Removal from Water by Coupling Mg–Al Layered Double Hydroxide Adsorption with UV/H2O2 Oxidation

Abstract Textile dye removal remains challenging because conventional wastewater treatment plants are not specifically designed to eliminate persistent colorants. In this study, calcined Mg–Al layered double hydroxide (LDH) adsorption, UV irradiation, and H₂O₂ oxidation were evaluated individually and in combination for the removal of two model textile dyes with different ionic characteristics: methyl orange (MO, anionic) and crystal violet (CV, cationic). The coupled treatment was optimized using a Box–Behnken experimental design and a hierarchical reduced polynomial model including selected mixed third-order terms. Among the individual treatments, LDH adsorption showed the highest removal efficiency. For MO, 99% removal was achieved within 20 min using 2.5 mg mL⁻1 LDH. In contrast, CV adsorption was slower and required 12 h and 5 mg mL⁻1 LDH to reach 99% removal. UV irradiation and H₂O₂ oxidation applied separately showed limited effectiveness, with removal efficiencies below 30% after 1 h for both dyes. The hierarchical reduced polynomial model obtained for MO and CV showed coefficients of determination of 0.9630 and 0.9987, respectively, and were used to identify the optimal treatment conditions. For MO, removal above 99% was achieved using 3.2 mg mL⁻1 LDH and 2 h of UV irradiation without H₂O₂. A comparable removal efficiency could also be achieved using 4.2 mg mL⁻1 LDH alone, confirming that adsorption was the dominant removal mechanism for this anionic dye. In contrast, near-complete CV removal required the combined application of 8.7 mg mL⁻1 LDH, 32 min of UV irradiation, and 5.7% H₂O₂ stock solution. A generalized condition consisting of 4.5 mg mL⁻1 LDH, 1 h of UV irradiation, and 3.5% H₂O₂ stock solution achieved removal efficiencies above 98% for both dyes. The LDH also maintained MO removal efficiencies above 90% over ten consecutive reuse cycles and achieved 94% removal after thermal regeneration. Overall, these findings highlight the potential of the LDH/UV/H₂O₂ system as a versatile treatment strategy for dye-contaminated water, with its performance strongly influenced by the ionic characteristics of the target dye.