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

Abstract

• Functionalization of flamboyant pods with citric acid was used to remove Cd 2+ , Ni 2+ , Zn 2+ and Cu 2+ . • Carboxylic and phenol functionalities played an important rolein heavy metal adsorption. • Endothermic adsorption of tested heavy metals was associated to physisorption. • Adsorption mechanism was analyzed via a physical model with two adsorption sites. A heterogeneous physical model with two bindings sites (HPMTBS) was successfully used to explain the adsorption of Cd 2+ , Ni 2+ , Zn 2+ , and Cu 2+ 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 Cd 2+ , Ni 2+ , Zn 2+ , and Cu 2+ 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 Cd 2+ , Ni 2+ , Zn 2+ and Cu 2+ 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 Cd 2+ , Ni 2+ , Zn 2+ and Cu 2+ , 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.