A simple starch/ZnO/MWCNT nanocomposite with selective adsorption towards methylene blue (MB) and methyl orange (MO): Theoretical and experimental analysis

The rapid expansion of industrial activities has led to increased emissions of organic dyes, posing significant environmental challenges. Consequently, the development of efficient and reusable adsorbents for dye removal has become a critical focus in water treatment research. In this study, a nanocomposite consisting of multiwalled carbon nanotubes (MWCNTs) and ZnO was synthesized to enhance the adsorption capacity of starch for the removal of methylene blue (MB) and methyl orange (MO) from aqueous solutions. The physicochemical characteristics of the starch/ZnO@MWCNTs (SZM) nanocomposite were thoroughly analyzed using FTIR, XRD, SEM, BET, and TGA techniques. The resulting material exhibited a specific surface area of 291.8 m² g⁻¹ and a pore diameter of 1.79 nm. Batch adsorption experiments were conducted to evaluate the influence of pH (4–10), initial dye concentrations (10–50 mg L⁻¹), and contact time (10–120 min). The maximum adsorption capacities were 465.11 mg g⁻¹ for MB and 228.31 mg g⁻¹ for MO. Data fitting indicated that the adsorption process conformed well to the Langmuir isotherm (R² = 0.9975) and pseudo-second-order kinetic model, suggesting that chemisorption followed by physisorption mechanisms dominate the process. Density Functional Theory (DFT) calculations were employed to elucidate the interactions between the SZM nanocomposite and dye molecules by evaluating quantum parameters, electrostatic potential maps, and binding energies. The results indicate that complexation is more favorable for MB (-224 kJ·mol −1 ) compared to MO (-161 kJ·mol −1 ), confirming a stronger affinity between the SZM NC and the MB dye. Reusability tests demonstrated that the nanocomposite maintains high adsorption efficiency over multiple cycles. Overall, the findings highlight the potential of the SZM NC as a highly effective and reusable adsorbent for the simultaneous removal of cationic and anionic dyes from contaminated water.