ABSTRACT

This study assessed the capacity of four adsorbents, namely, Annonamuricata petal composite (AMC), Acacia xanthophloea stem bark composite (AXC), Funtumiaelastica husk composite (FEC) and nanohybrid (Ag/f-MWCNTs), to remove Cd(II) and Pb(II) ions from aqueous solution.  The nanocomposite materials were fabricated via the agro-waste (Annona muricata petal (AMB), Funtumia elastica husk (FEB), or Acacia xanthophloea stem bark (AXB)) modification of Ag/f-MWCNTs.  The kinetics study showed that the uptake of Cd(II) or Pb(II) ions by AMC, FEC, AXC, AMB, FEB, AXB and Ag/f-MWCNTs was best described by the pseudo-second order kinetics model.  It emerged that the equilibrium data obtained for the uptake of Cd(II) ions by the AMB, AMC, FEC, FEB, AXC and AXB adsorbents fitted the Langmuir and Sips isotherm models while Ag/f-MWCNTs were best described by the Freundlich isotherm model.  However, the adsorption equilibrium data obtained for the removal of Pb(II) ions by the adsorbents AXC, AXB, FEC and FEB were found to fit the Freundlich isotherm model, while the uptake of Pb(II) ions by AMC, AMB and Ag/f-MWCNTs was best described by the Langmuir isotherm.  Among the three-parameter isotherm models used, the Sips isotherm model best described the uptake of Pb(II) ions.  Variables controlling the adsorption process, including contacts time, adsorbents dose and initial concentrations of Cd(II) or Pb(II) ions, were found to have a common trend, as these variables increase, the uptake capacity of the adsorbents increased.  The nanocomposite materials demonstrated enhanced uptake potentials, as their uptake capacities were observed to be greater than the adsorption capacity of Ag/f-MWCNTs.  The estimated Gibbs energy of adsorption for the removal Cd(II) and Pb(II) ions indicates that the adsorbate-adsorbent interactions within the range of temperatures studied was feasible and spontaneous.  A desorption efficiency greater than 70% was reported for all nanocomposites, hence effective adsorption-desorption activities were observed, suggesting that these nanocomposite materials will be effective in treating real wastewaters contaminated with toxic Cd(II) or Pb(II) ions.