Synthesis, characterization, and application of a novel Cd–In layered double hydroxide for ultrasound-assisted Congo Red adsorption

Congo Red (CR) is a persistent anionic dye that is resistant to biological degradation, thereby requiring effective adsorption strategies for its removal. In this study, Cd–In layered double hydroxide (LDH) was synthesized via a co-precipitation method and characterized using XRD, FTIR, SEM-EDX, HR-TEM, BET-BJH, XPS, and TGA analyses. The material exhibited a layered structure with mesoporous morphology and a specific surface area of 29.618 m2/g. Adsorption performance was evaluated under conventional shaking and ultrasonic-assisted conditions. Ultrasonication significantly accelerated adsorption kinetics and enhanced the maximum adsorption capacity up to 170.36 mg/g, owing to cavitation-induced particle dispersion and improved mass transfer. The optimum adsorption occurred at pH 8, close to the pHpzc value (8.25), highlighting the key role of electrostatic interactions between the positively charged LDH surface and CR anions. Kinetic data were best described by the pseudo-second-order model, while equilibrium data followed the Freundlich isotherm, indicating a heterogeneous multilayer adsorption. Thermodynamic analysis confirmed that the process was spontaneous and exothermic. Desorption and regeneration studies revealed the coexistence of electrostatic and non-ionic interactions, with partial reusability of the adsorbent and minimal Cd release under the optimum adsorption conditions, demonstrating its potential for environmentally safe wastewater treatment applications.