Isothermal titration microcalorimetry to determine the thermodynamics of metal ion removal by magnetic nanoparticle sorbents

Isothermal titration microcalorimetry, a novel approach, was applied to determine key thermodynamic parameters directly for the adsorption of metal ions to a magnetic nanoparticle sorbent (Mag-Ligand), measuring the energy exchange during the binding process. We demonstrate that the interactions are enthalpically driven and energetically favorable, with exothermic binding reactions, giving the selectivity sequence: Cr3+ < Cu2+ < Zn2+ < Ga3+ < Ce3+ < Cd2+ < In3+ < Hg2+ < Pb2+. The sorption kinetic and isothermal studies demonstrated effective and fast removal of nine different metal ions including three rare earths, in competitive and non-competitive conditions across a range of pH and water hardness. Sorption is dominated by the complexation reactions between metal ions and Mag-Ligand and adsorption of metal ions onto Mag-Ligand. Maximum sorption capacity can be predicted based on the thermodynamic data, although kinetics plays a role during competitive sorption. Furthermore, the magnetic behavior of Mag-Ligand results in fast and efficient solid-liquid separation after the sorbent has extracted the metal ions from the contaminated waters, and Mag-Ligand can be regenerated for reuse by a simple acid wash, providing a more sustainable, fast, convenient, and efficient approach for heavy metal remediation.