Engineering novel ceramic metal oxides with residual carbon for efficient sequestration of Auramine O from wastewater

Auramine O is a hazardous cationic dye that threatens aquatic ecosystems by suppressing photosynthesis, and it poses health risks to humans through toxicity and bioaccumulation. Its chemical persistence allows it to remain in water bodies and enter food chains, which amplifies environmental and medical concerns. Here, we report a facile Pechini sol-gel route to synthesize CuO/MgCr₂O₄/C and CuO/MgCr₂O₄/MgO/C at 600 and 800 °C nanohybrids labeled MCC600 and MCC800, respectively, for efficient Auramine O removal. XRD confirms monoclinic CuO and cubic MgCr₂O₄ in MCC600 and reveals additional cubic MgO in MCC800 with average crystallite sizes of 60.97 nm and 75.35 nm, respectively, which reflects thermally driven grain growth. EDX verifies C, O, Mg, Cr, and Cu, with carbon decreasing and chromium increasing at higher temperature due to enhanced organic burnout and phase development. FE-SEM shows cauliflower-like agglomerates with fine nanospheres for MCC600 and a more uniform granular texture for MCC800, while HR-TEM reveals ultrafine quasi-spherical clusters for MCC600 and larger faceted polyhedral grains for MCC800. The capacities reach 442.48 mg/g for MCC600 and 298.51 mg/g for MCC800. Kinetic and thermodynamic analyses demonstrate pseudo-first-order behavior with exothermic spontaneous and physical adsorption that follows the Langmuir isotherm. Nearly complete acid-driven regeneration and stable multicycle reuse are achieved, and high performance is retained in real wastewater, which confirms suitability for practical treatment.