Engineered hierarchical WO₃/activated carbon nanocomposite for sequestration of some heavy metals in mining wastewater

This paper presents the green synthesis of tungsten trioxide (WO₃) nanoparticles and activated carbon from Piliostigma thonningii pods, followed by the synthesis of a WO₃/AC nanocomposite for efficient removal of Cd(II), Fe(III), Cr(VI), and Pb(II) ions from mining wastewater. UV-Vis spectroscopy confirmed the presence of a characteristic peak for WO₃ at 435.56 nm. Dynamic light scattering spectroscopy confirmed the hydrodynamic diameter of the WO₃ nanoparticles as 87.16 nm and the WO₃/AC composite as 77.47 nm. XRD confirmed the monoclinic structure of WO₃ and the presence of turbostratic carbon. SEM images confirmed the effective dispersion of WO₃ within the activated carbon matrix. BET confirmed the highest surface area for the WO₃/AC composite as 764.75 m²/g, along with a pore diameter of 2.98 nm for the composite. The textural properties of the composite were higher than those of the individual materials. Adsorption studies confirmed the efficient removal of metal ions within 60 min, with a removal efficiency of 96.86% for Cd(II), 95.78% for Fe(III), 97.98% for Cr(VI), and 91.39% for Pb(II).The equilibrium data fit the Langmuir isotherm model adequately (R² = 0.9965–0.9993), while the maximum adsorption capacities for Cr, Fe, Cd, and Pb were found to be 238.64 mg/g, 156.78 mg/g, 82.45 mg/g, and 64.12 mg/g, respectively. The kinetics of the adsorption process fit the pseudo-second-order model (R² = 0.995–0.9996), indicating that the adsorption process is chemisorption-controlled. The thermodynamics of the adsorption process also confirmed that the process is spontaneous and endothermic, as evidenced by the negative values of ΔG°, ranging from −8.86 kJ/mol to −43.35 kJ/mol, while the positive values of ΔH° also confirmed the endothermic nature of the adsorption process. Additionally, the composite has more than 55% removal efficiency after six regeneration cycles, thus proving that the composite is reusable. Overall, the findings indicate that the WO₃/AC nanocomposite is a sustainable, high-performance, and reusable adsorbent with strong potential for practical application in heavy-metal-contaminated wastewater treatment.