Nitrogen-functionalised carbon nanotubes as a novel adsorbent for the removal of Cu(ii) from aqueous solution

Abstract

This study investigated the introduction of 4′-(4-hydroxyphenyl)-2,2′:6′,2′′-terpyridine (HO-Phttpy) onto the surface of multiwalled carbon nanotubes (MWCNTs) to obtain nitrogen-functionalized MWCNTs (MWCNT-ttpy). This novel material was characterised and tested for its possible use in the remediation of wastewater contaminated with heavy metal ions. Its efficacy was compared with that of acid-functionalized MWCNTs (MWCNT-COOH) for the removal of the heavy metal ion Cu²⁺ through adsorption. HO-Phttpy was first synthesized, followed by the functionalization of MWCNT-COOH to afford MWCNT-ttpy. MWCNT-ttpy showed significant textural enhancement due to an increase in the extent of functionalization. This was demonstrated by an increase in the surface area and pore volume of MWCNT-ttpy, from 126.8 to 189.2 m² g⁻¹ and 0.692 to 1.252 cm³ g⁻¹, respectively, relative to MWCNT-COOH. Its application for Cu²⁺ removal showed a marked increase in uptake (q_e), i.e. 19.44 to 31.65 mg g⁻¹, compared with MWCNT-COOH. This is attributed to the introduction of more active/chelating sites for adsorption. Adsorption experiments were conducted at pH 5 at which equilibrium was reached after 360 min. The results showed that the adsorption process was best described by the pseudo-second order model. Among the isotherms tested, the Langmuir isotherm provided the best fit for the equilibrium data. Thermodynamic studies revealed that the adsorption process was spontaneous and endothermic. Desorption studies demonstrated a better removal efficiency of Cu²⁺ from MWCNT-ttpy, indicating its possible regeneration and the recovery of the Cu²⁺ adsorbate for reuse. Thus, MWCNT-ttpy shows superior properties for wastewater remediation compared to MWCNT-COOH.