Removal of Pb2+ ions from synthetic wastewater using functionalized multi-walled carbon nanotubes decorated with green synthesized iron oxide–gold nanocomposite

Abstract

Purification of wastewater before it is discharged into the aquatic environment is important in order to prevent pollution of clean water. This study investigated the applicability of functionalized multi-walled carbon nanotubes (MWCNTs) decorated with gold-iron oxide nanoparticles for the adsorptive removal of Pb²⁺ from synthetic wastewater. CNTs were commercially obtained and functionalized with a mixture of H₂SO₄/HNO₃ acids. The CNTs were coated with gold-iron oxide nanoparticles, to enhance the adsorption of heavy metals. The gold-iron oxide nanoparticles were synthesized by reacting green tea leaf extract with iron chloride (FeCl₂) and gold (III) chloride (HAuCl₄) precursors. The composite was cross-linked using N, N-dimethylformadide (DMF). The adsorbents were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to assess their surface morphology, Fourier transform infrared (FTIR) spectroscopy to identify the functional groups present, X-ray diffraction (XRD) to ascertain the crystallographic structure of the green adsorbent and Raman spectroscopy to  determine the sample purity. SEM results showed highly agglomerated and polydispersed nanoparticles, owing to the presence of phytochemicals in the tea extract and magnetic interaction between the individual particles indicating the successful synthesis of Au/Fe₃O₄ adsorbent. Furthermore, an increase in the amount of Pb²⁺ removed per unit mass (q_e) of adsorbent from 1.233 to 7.266 mg‧g^-1 at 298 K was observed. A high sorption capacity was noticed for MWCNT-Au/Fe₃O₄ as compared to the MWCNT-COOH. The Pb²⁺ removal percentage increased from 50% to 78% with an increase in MWCNT-Au/Fe₃O₄ dosage from 0.02 g to 0.1 g. Adsorption isotherm data fitted well to the Freundlich and Langmuir isotherm models for MWCNT-COOH and MWCNT-Au/Fe₃O₄ adsorbents and the rate of Pb(II) adsorption by MWCNT-Au/Fe₃O₄ encountered an increase with increasing solution temperature and followed the pseudo-second-order model. The synthesized MWCNT-Au/Fe₃O₄ has good potential in removing heavy metals from wastewater.