Adsorption of Cr(lll), Pb(ll) and Cd(ll) ions from aqueous solution by Monkey bread (Piliostigma thonningii) seed pod

Abstract The potential of piliostigma thonningii seed pod (a plant waste) as an adsorbent in the removal of Cd(II), Cr(III) and Pb(II) ions from aqueous solution was investigated in this study. The influences of initial metal ion concentration, pH and temperature on the adsorption process were investigated in a batch study. Maximum percentages of each of the metals adsorbed were 89% for Cd(II), 92 % for Cr(III) and 98 % for Pb(II) ions at a fixed temperature of 303 K. The trend of the heavy metal adsorption follows: Pb(II) > Cr(III) > Cd(II), indicating that lead(ll) was best adsorbed by the adsorbent. From the two isotherm models applied to the experimental adsorption data, Freundlich model was found to give best description to the experimental data. The feasibility of the process was assessed using different thermodynamic parameters viz; ΔG_ads, ΔH_ads and ΔS_ads. Results obtained showed that the ΔH values (J/mol) were 30.49, 35.94 and 47.40 while ΔS values (J mol^-1 K^-1) were 99.02, 117.07 and 154.73 for Cd(II), Cr(III) and Pb(II) ions respectively, indicating the endothermic nature of the process and further revealed increased randomness at solid/solution interface. This increased randomness at the solid-solution interface thus indicates that physisorption is the likely predominant mode of the adsorption process. The negative ΔG values at the different studied temperatures suggest that the adsorption process was spontaneous.

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