Experimental and DFT studies on the selective adsorption of Pb 2+ and Zn 2+ from aqueous solution by nitrogen-functionalized multiwalled carbon nanotubes

Abstract

Abstract The ligand, 4′-(4-hydroxyphenyl)-2,2′:6′,2″-terpyridine, was bonded to multiwalled carbon nanotubes to afford nitrogen-functionalized carbon nanotubes (MWCNT-ttpy). This material was compared with acid-functionalized multiwalled carbon nanotubes (MWCNT-COOH) for the removal of Pb2+ and Zn2+ from aqueous solution. The comparison was conducted both experimentally and theoretically by means of batch adsorption experiments and density functional theory (DFT) analysis, respectively. The adsorption capacities (qe) of the adsorbents increased with increasing pH, contact time and adsorbent dose. The adsorption of Pb2+ and Zn2+ onto both sorbents was conducted at an optimum pH of 4.5 and 5.5, respectively. For both metal ions, better removal was observed with MWCNT-ttpy than with MWCNT-COOH. In the case of Pb2+, the Langmuir maximum adsorption capacity (qm) increased from 20.60 to 36.23 mg g−1, and for Zn2+, qm increased from 18.51 to 32.60 mg g−1, in going from MWCNT-COOH to MWCNT-ttpy, respectively. DFT analysis supports the experimental results showing a better coordination of Pb2+ to the nitrogen-donor atoms on MWCNT-ttpy than for Zn2+. The pseudo-second order kinetics model fitted both systems well, indicating a bimolecular interaction between the adsorbate cations and sorbents. The adsorption processes were endothermic in nature and spontaneous. DFT studies reveal that the metal-adsorbent interactions proceeded via a coordination/electrostatic mechanism between the nitrogen/oxygen donor atoms contained in the adsorbents and the metal ions. Desorption studies showed high desorption efficiencies, thereby demonstrating the suitability of the adsorbents for reuse. Thus, nitrogen-functionalized carbon nanotubes show promise as a good reusable adsorbent for the removal of Pb2+ and Zn2+ from wastewaters.