Optimization of Graphene oxide with methyl-β-Cyclodextrin and carbon Nanotubes with Polypyrrole for removal of lead (ii) and copper (ii) ions from water

Abstract

ABSTRACT Heavy metals pose risks to the humans and the environment. Lead (II) and copper (II) are of particular concern because they can cause irreversible damage to vital body organs. Their sources include leaded paints, old water piping systems, pesticides, fertilizers and metal recycling plants. Various methods such as adsorption have been used to remove these metals from wastewater. The total surface area and surface functionalities of an adsorbent affect the adsorption of the adsorbate. Conventional adsorbents such as clay which are considered to be inexpensive have been found to have moderate surface areas hence do not adsorb heavy metals to appreciable levels. This challenge could be met by exploring advanced adsorbents whose surfaces can be manipulated to introduce functional groups that have an adsorption affinity for heavy metal cations onto their surfaces. Currently, research is focused on designing carbon-based nanomaterials such as graphene oxide (GO) and carbon nanotubes (CNT) for the removal of heavy metals from water. Optimization and derivatization of these adsorbents by other groups such as β-cyclodextrin, enhance their adsorption efficiencies. The objectives of this study were to synthesize, characterize and optimize the adsorption characteristics of GO conjugated with methyl-β-cyclodextrin (GO-mβCD) and oxygenated CNT coated with polypyrrole (oMWCNT/Ppy). GO-mβCD and oMWCNT/Ppy were synthesized, characterized and applied as adsorbents to determine the adsorption behavior of Pb(II) and Cu(II) from aqueous solutions. Pristine and oxidized carbon nanotubes (pMWCNT and oMWCNT) were used as reference adsorbents for oMWCNT/Ppy and graphene oxide (GO) a reference for GO-mβCD. The GO-mβCD was synthesized through in situ aggregation of GO and mβCD while oMWCNT/Ppy was synthesized using in situ oxidative polymerization method. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and thermogravimetric analysis (TGA) and X-ray Diffraction (XRD) showed that oMWCNT/Ppy and GO-mβCD were used to characterize the synthesized adsorbents. The experimental data of means of triplicates were determined at a confidence level of 95%. The adsorption processes fitted better to the Langmuir isotherm than Freundlich isotherm indicating that the adsorption was monolayer. The adsorption data fitted better onto the pseudo-second order kinetic model than pseudo-first order kinetic model implying that the adsorption process involved both the adsorbate and the adsorbent. The adsorption capacities indicated that derivatization of oMWCNT with Ppy significantly (p≤0.05) enhanced its adsorption capacity for both Pb(II) and Cu(II) by 50.1% and 43.9%, respectively. Equally, the derivatization of GO with mβCD makes it a better adsorbent for Pb(II) than pure GO by 43.8% although this was not applicable for Cu(II). The uptake of the metal ions in a single metal system was significantly higher (p≤0.05) than that of binary metal ion system when oMWCNT/Ppy was applied. In addition, the desorption studies indicate that the composite adsorbents, GO-mβCD and oMWCNT/Ppy can be used repeatedly up to five times with minimal loss of their initial adsorption capacity. More heavy metal species and these derivatives need to be investigated to come up with a wider conclusions on the adsorption capacity of these derivatives.