Nitrogen functionalized hierarchical microporous/mesoporous carbon with a high surface area and controllable nitrogen content for enhanced lead(ii) adsorption

Abstract

Nitrogen-doped hierarchical microporous/mesoporous carbons (NHMCs) were conveniently fabricated for enhanced lead adsorption through a sol–gel method and NaOH-assisted activation with a soluble melamine–phenol–formaldehyde resin as a precursor. Characterization techniques, including Brunauer–Emmett–Teller, TEM, FTIR spectroscopy, and XPS analysis, were used to study the surface physicochemical properties of carbons. The prepared carbons feature high surface areas of up to 1463 m² g⁻¹, controllable nitrogen content of 0–4.27%, and an interconnected 3D microporous/mesoporous network with peak micropore width around 0.7–0.9 nm, and 3–8 nm pore channel. The nitrogen atoms were mostly bound to the carbon in forms of pyrrolic, pyridine, and amine-like nitrogen groups, providing adsorption sites for Pb(II) easily binding to the surface through strong chemisorptive bonds or ion-exchange. Compared with CMK3, NHMCs exerted excellent adsorptive removal performance toward Pb(II) in adsorption capacity (up to 212 mg g⁻¹), equilibrium time (30 min), and adsorption conditions (pH 4.5–6). The adsorption data at various initial concentrations fit well with the Langmuir and pseudo-second order model. Even after five consecutive adsorption–desorption cycles, no obvious decrease in Pb(II) adsorption was detected, thereby implying that the nitrogen-doped 3D hierarchical NHMCs was suitable for efficient Pb(II) removal.