Synthesis of N-doped multi-walled carbon nanotubes derived from plastic waste for adsorption of Pb2+ ions in aqueous solutions

In this research multi-walled carbon nanotubes (MWCNTs) and nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) adsorbents were synthesized using polypropylene plastic as the carbon precursor, with melamine serving as the nitrogen source. A two-stage chemical vapour deposition (CCVD) method was employed for the pyrolysis of polypropylene, utilizing a nickel catalyst supported by CaCO₃. Results showed that the nitrogen is successfully doped on N-MWCNTs as the surface is composed of nitrogen-containing groups, leading to the decrease in the carbon nanotube's outer diameter from 22.9 nm to 21.2 nm, an increase in crystallinity index from 81.40 to 83.19, an increase in specific surface area from 46.6 m² g⁻¹ to 76 m² g⁻¹, and an increase in pore volume from 0.09 cm³ g⁻¹ to 0.18 cm³ g⁻¹, enhancing their potential for adsorption applications. MWCNTs and N-MWCNTs were applied in the removal of Pb²⁺ ions from wastewater and the highest removal efficiency was 88.1 % in N-MWCNTs compared to 80 % in MWCNTS. The optimum conditions of pH 5, an initial concentration of 1 mg L⁻¹, and an adsorbent dose of 0.005 g were used. Langmuir isotherm provided a better fit for the adsorption data with maximum monolayer coverage (q_o) of 7.52 mg g⁻¹ for MWCNTs and 8.91 mg g⁻¹ for N-MWCNTs. The adsorption process followed pseudo-second-order kinetics, indicating that chemical adsorption, involving electron exchange or sharing, is the dominant mechanism for MWCNTs and N-MWCNTs.