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

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 m(2) 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.