Thermodynamics of Hg²⁺ and Ag⁺ adsorption by 3-mercaptopropionic acid-functionalized superparamagnetic iron oxide nanoparticles

Superparamagnetic iron nanoparticles (SPION) have been functionalized with 3-mercaptopropionic acid (3-MPA), characterized and applied for the removal of Ag⁺, Hg²⁺ and Pb²⁺ metal ions from aqueous solutions by iron oxide (Fe₃O₄). The heavy metal adsorption has been investigated by means of ICP-OES and isothermal titration calorimetry. Experimental data ware better fitted by Langmuir rather than Freundlich isotherms, and the thermodynamic parameters for the adsorption process of the metal ions on the functionalized SPION nanoparticles (SPION@3-MPA) were obtained. Isothermal titration calorimetry (ITC) is applied to monitor heavy metal adsorption on SPION@3-MPA: the process results to be exothermic for Hg²⁺ and Ag⁺, while it is weakly endothermic in the case of Pb²⁺, and the adsorption enthalpies and entropies have been obtained. The values of the thermodynamic parameters suggest that the Ag⁺ and Hg²⁺ ions interact strongly with the thiol groups, while the Pb²⁺ ions seem to be adsorbed by the material mostly via electrostatic interaction. When compared to other thiol-functionalized materials, the obtained SPION@3-MPA NP can be considered a competitive adsorbent for Ag⁺ and Hg²⁺ ions. The comparison between the ICP-OES adsorption rate and the enthalpy trend obtained by ITC supports shows that the latter technique can be a good tool for a fast testing of materials to be applied for heavy metal separation from solutions.