Engineering p-n heterostructure in MgO-Al2O3-CuO ternary metal oxide composites for sonocatalytic removal of pollutants

Currently, the development of environmentally friendly catalysts with high efficiency in degrading organic pollutants in water is of great importance. In this study, a novel p-n heterojunction sonocatalyst based on a ternary MgO-Al₂O₃-CuO composite was designed and synthesized via a facile self-combustion sol-gel method. The crystalline, optical, and morphological properties of the synthesized materials were characterized using XRD, DRS, FE-SEM, EDS, elemental mapping, BET, FT-IR, and pH_zpc techniques. The optimized MgAlCu oxide composite (with a 1:2:2 molar ratio) exhibited a crystallite size of ~ 20.6 nm (determined by XRD) and a reduced band gap (1.88 eV) and enhanced charge separation due to the built-in p-n junction between CuO and Al₂O₃, which significantly suppressed electron-hole recombination. Its sonocatalytic performance was evaluated through the degradation of methylene blue (MB). Under optimal conditions (pH = 6.5, catalyst dose = 0.8 g L^-1, dye concentration = 10 mg L^-1, ultrasound power = 100 W), the degradation efficiency reached 85% within 60 min, representing a 12-fold and 7-fold enhancement compared to sonolysis and adsorption alone, respectively. Radical scavenging experiments confirmed the dominant role of hydroxyl radicals in the degradation mechanism. Kinetic studies revealed that the process follows the Langmuir-Hinshelwood model, with an adsorption equilibrium constant (K_ads) of 0.007625 mg L^{- 1} and a surface reaction rate constant (K_c) of 5.7563 mg L^{- 1} min^{- 1}. The composite also demonstrated excellent stability over seven consecutive cycles and favorable economic feasibility. This work introduces a rationally designed ternary metal oxide p-n heterostructure as a highly efficient, stable, and scalable sonocatalyst for advanced wastewater treatment.