Boosting Methyl Orange Removal via Hydrothermal Engineering of CuZnAl LDHs

Layered double hydroxides (LDHs) are promising adsorbents for dye-contaminated water owing to their tunable layer charge and exchangeable interlayers. In this study, ZnAl, CuAl, and CuZnAl LDHs were synthesized under identical urea-assisted hydrothermal conditions using metal ratios corresponding to those of compositionally related co-precipitated materials reported previously. Their structural, textural, adsorption, and regeneration properties were systematically evaluated using methyl orange (MO) as an anionic model pollutant. In batch tests, CuZnAl exhibited near-quantitative removal across pH 2–11, CuAl achieved near-quantitative removal at pH ≥ 3, whereas ZnAl showed a narrower effective pH range with a marked decline under alkaline conditions. Adsorption kinetics ( C 0 = 50–500 mg L −1 ) were well described by the pseudo-second-order model. Equilibrium analysis indicated high adsorption capacities, with Langmuir monolayer capacities of 497.5 mg g −1 (ZnAl), 609.8 mg g −1 (CuAl), and 666.7 mg g −1 (CuZnAl), while Freundlich fitting suggested favorable adsorption on heterogeneous sites. Regeneration by alkaline Na 2 CO 3 /NaOH washing enabled efficient reuse: CuZnAl and CuAl maintained MO removal efficiencies of 98.2% and 97.2%, respectively, after five cycles, whereas that of ZnAl decreased to 69.3%. Post-adsorption XRD and FTIR results indicated retention of the LDH framework and partial interlayer participation of MO, while SEM revealed composition-dependent changes in aggregate morphology. The adsorption behavior reflected the combined effects of metal composition, basal spacing, textural properties, aggregate architecture, and surface/interlayer chemistry. Among the investigated materials, hydrothermal CuZnAl provided the most favorable balance of adsorption capacity, pH tolerance, and regeneration performance.