Equilibrium study of single and binary adsorption of lead and mercury on bentonite-alginate composite: Experimental investigation and application of two theoretical approaches

The adsorption of Pb(II) and Hg(II) ions on bentonite-alginate composite (BAC) is investigated in both single compound and binary systems, at three different temperatures. For a better interpretation and comparison, all the adsorption equilibrium isotherms are recorded under identical conditions. A simple examination of all adsorption equilibrium isotherms shows that the bentonite-alginate composite is more effective for Pb (II) adsorption than for Hg (II) and the adsorption capacity is reduced in binary system, reflecting a competitive effect between the Pb (II) and Hg (II). To explain the single and binary processes and the corresponding mechanisms, Hill and competitive Hill models are applied. The adsorption geometry can be determined at different temperatures by an estimation of the number of ions captured per BAC adsorbent site. Based on these models, it is shown that the affinity sequence in all adsorption systems is: Pb (II) - Hg (II). This behavior can be explained and corroborated by an energetic analysis deduced from these models. Finally, in order to deepen the interpretation of single-compound and binary adsorption, the conductor-like screening model (COSMO-RS) was tested and applied to calculate the total interaction energies between heavy metals and BAC adsorbent. The evaluation of energies deduced from COSMO-RS model indicated that the electrostatic misfit interaction energy may play the main role in the heavy metals ions adsorption in tested systems.