Eco-friendly Ca/α-Fe2O3/ZnO/CuO nanocomposite: Green synthesis, characterization, and high-efficiency heavy metal removal from contaminated sludge

Abstract

This study explores the green synthesis of Ca/α-Fe2O3/ZnO/CuO nanocomposite using Mentha pulegium L. leaf extract as a bioreducing and stabilizing agent for eco-friendly remediation. XRD confirmed a multiphase crystalline structure—cubic Ca, rhombohedral α-Fe2O3, hexagonal ZnO, and monoclinic CuO—with an average crystallite size of 24.42 ± 3.07 nm. SEM revealed ~50 nm agglomerated nanoparticles with rough surfaces, enhancing catalytic reactivity. FTIR identified metal–oxygen bonds (FeO, ZnO, CuO, CaO) and plant-derived functional groups, confirming stabilization. UV–Vis showed strong absorption at 275 nm with direct (3.07 eV) and indirect (1.70 eV) bandgaps, suitable for UV/visible-light applications, and the low Urbach energy (0.211 eV) further reflected the material's high crystallinity. TGA-DSC demonstrated 49% residual mass at 1200°C, reflecting strong thermal stability. A zeta potential of −29.06 mV ensured colloidal stability. The nanocomposite efficiently removed heavy metals from petroleum sludge, achieving 84.1% (Pb), 74.6% (Zn), 72.3% (Se), and 73.7% (Cr), while arsenic removal was relatively lower at 24.9%, highlighting current adsorption challenges. Adsorption involved diffusion and chemical fixation, underscoring potential in remediation, photocatalysis, and optoelectronics