Kasmen Dogo, Isaac Alhamdu Baba, Habila Samson Paul, Manase Auta, Ambali Saka Abdulkareem, Uduak George Akpan, M.A. Olutoye, C. E. Egwim, Sunday Albert Lawal, Elizabeth Jumoke Eterigho, Kehinde Shola Obayomi
Rapid industrialization has increased the discharge of heavy-metal-laden petroleum wastewater into water bodies, posing severe health and ecological risks due to the persistence, bioaccumulation, and toxicity of these contaminants. However, the simultaneous removal of these persistent inorganic pollutants from water bodies remains a major challenge. To mitigate this issue, we designed multi-walled carbon nanotubes (MWCNTs) impregnated with magnetite at different mixing ratios (magnetite/MWCNTs@1:1, magnetite/MWCNTs@2:1, and magnetite/MWCNTs@1:2 ) via a two-step process involving chemical vapor deposition (CVD) followed by wet impregnation to remove Cr(VI), Cd(II), Pb(II), and Ni(II) from petroleum-contaminated wastewater. The uniform dispersion of magnetite on the MWCNTs matrix, along with the resulting synergistic effects, was confirmed through physicochemical characterization. The impregnation of magnetite improved the surface potential of the MWCNTs, offering magnetic separability, synergistic adsorption effects, and metal-binding sites with increased BET surface area. In a batch adsorption system, the influence of contact time, adsorbent dosage, and temperature revealed that the adsorption performance followed the order 1:2 > 1:1 > 2:1 , with the magnetite/MWCNTs@1:2 nanocomposite exhibiting optimum removal efficiencies of 100% for Ni(II) , 100% for Cd(II) , 89.97% for Pb(II) , and 83.86% for Cr(VI) . The adsorption kinetics and isotherm fitting confirmed that the adsorption process was more consistent with pseudo-second-order kinetic and Langmuir isotherm models, suggesting the dominance of chemical adsorption on homogenous surfaces. The magnetite/MWCNTs@1:2 nanocomposite exhibited maximum capacities of 585.3 mg/g for Cr(VI), 658.1 mg/g for Cd(II), 551.1 mg/g for Pb(II), and 468.6 mg/g for Ni(II). Thermodynamic examination revealed that the adsorption reactions were spontaneous, feasible, and proceeded endothermically. The cyclic experiment demonstrated that magnetite/MWCNTs with different mixing ratios still maintained > 75% removal efficiencies after seven cycles, confirming their potential in practical applications. Remarkably, the iron leaching from magnetite/MWCNTs@1:2 is only 0.06 mg/L, which is significantly below the WHO/EPA guideline limit for drinking water of 0.3 mg/L. Overall, this study demonstrates for the first time how the magnetite/MWCNTs mixing ratio governs the surface and structural properties of the nanocomposites, enabling effective heavy metal ions adsorption and recovery from petroleum-contaminated wastewater • Magnetite/MWCNTs with different mixing ratios via wet impregnation were designed. • Engineered magnetite/MWCNTs enabled enhanced heavy metals adsorption. • Magnetite/MWCNTs with mixing ratio 1:2 exhibited superior adsorption capacities. • The magnetite/MWCNTs composites still maintained >75% efficiencies after 7 cycles. • Thermodynamics studies revealed the endothermic nature of the adsorption process.