Abstract

Mango stone (DMS) and cocoa pod (CPC), two agricultural waste materials evaluated for the removal of toxic cadmium ion Cd²⁺ and Pb²⁺ ions from aqueous media in batch adsorption mode. Adsorption variables such as the nature and composition of electrolyte, pH, adsorbent dose and equilibration time were optimized for the maximum accumulation of the metals to the biosorbents. The optimum contact time of 15 min for Cd²⁺ and 40 min for Pb²⁺ were found to be independent of the adsorbent in use but on the metal been adsorbed. The presence of alkali electrolyte ions Na⁺ and K⁺ reduced the uptake of Cd²⁺ and Pb²⁺ as these electrolytes ions competed and reduced the available adsorption sites for heavy metal removal, the effect been more pronounced in DMS than with CPC. Kinetic data obtained in this study fitted satisfactorily to the pseudo-second-order rate equation (R² >0.995). Other tested kinetic models however proved useful for describing the adsorption mechanisms at different points in the biosorption process. The Langmuir, Freundlich, Dubinin- Radushkevich (D-R) and Temkin models were applied to describe the isotherms in the study. Based on the assumptions of these equilibrium models and the results obtained, an attempt was made at describing the surface adsorption characteristics of the biosorbents relative to the metals in question. Biosorption capacity of both biomasses for both metals compared well with those of earlier workers.

Key words: Adsorption, kinetics, equilibrium, isotherms, cocoa pods, mango stone.