Application of a heterogeneous physical model for the adsorption of Cd2+, Ni2+, Zn2+ and Cu2+ ions on flamboyant pods functionalized with citric acid

Abstract A heterogeneous physical model with two bindings sites (HPMTBS) was successfully used to explain the adsorption of Cd2+, Ni2+, Zn2+, and Cu2+ ions on flamboyant pods functionalized with citric acid. Experimental results and statistical physics modeling showed that carboxylic and phenolic functionalities of this adsorbent played a relevant role for the adsorption of Cd2+, Ni2+, Zn2+, and Cu2+ ions. Calculations performed with HPMTBS suggested that the removal of these cations was a multi-ionic adsorption process at tested operating conditions. This adsorbent was more effective to remove Cd2+, Ni2+, Zn2+ and Cu2+ ions at high solution temperatures thus indicating an endothermic process with adsorption energies ranging from 20.3 to 29.5 kJ/mol, which were associated to physisorption. The total adsorption capacities varied from 0.10 to 0.32, 0.28 to 0.39, 0.19 to 0.25 and 0.20 to 0.31 mmol/g for Cd2+, Ni2+, Zn2+ and Cu2+, respectively. This difference in adsorption capacities could be attributed to the ionic radius of tested adsorbates. This functionalized biomass can be considered as an alternative adsorbent for facing water pollution by heavy metal ions.