On the evaluation of the intrinsic stability of indium-nanoparticulate organic matter complexes

Abstract

HypothesisProper evaluation of the intrinsic stability of metal complexes with humic nanoparticles calls for a robust representation of the particulate electrostatic features. Addressing here the case of trivalent metal association with humics for which a significant electrostatic contribution is expected, we report a robust interpretative approach as an alternative to the conventional but approximative Donnan model applied in popular speciation codes. ExperimentsThe intrinsic chemical binding affinity of Indium to humic nanoparticles is addressed from equilibrium electroanalytical measurements (Absence of Gradient and Nernstian Equilibrium Stripping) in NaClO4 electrolyte (10–100 mM, pH4) at different metal-to-humics concentration ratios. The electrostatic contribution was evaluated using a Poisson-Boltzmann based-approach where the key electrostatic descriptors of humics are involved. FindingsThe electrochemical results interpreted in the light of so identified non-specific indium binding contribution evidences a dramatic impact of electrostatics on indium complexation by humics, with e.g. in 10 mM electrolyte an intraparticulate Boltzmann metal accumulation factor that is ca. 5 times larger than that reported for cadmium and highly-charged fulvics complexants. A successful comparison between theory and experiments is consistently achieved over the tested electrolyte concentrations with the only adjustment of the radius of the metal accumulation spherical volume. The analysis reveals the necessity to consider full equilibration of charged humics with its intraparticulate counterion atmosphere.