Langmuir Isotherms for Functionalized Superparamagnetic Nanoparticles with Cobalt(II) Ions Based on Zeta Potentials

Cobalt is a crucial element in modern battery design, and in this work its extraction from aqueous solutions by ethylenediamine-functionalized superparamagnetic nanoparticles (MagNP@en) is reported. For the MagNP@en/Co(II) system a good description of the equilibrium involved has been obtained by means of the Langmuir approach, yielding consistent analytical information about the equilibrium constants and the amounts of sequestered metal ions. However, the elaboration of the Langmuir isotherms from the analytical data has proved rather cumbersome, requiring the monitoring and processing of a large number of samples. For this reason, an alternative approach has been proposed based on the measurement of the zeta potentials of the MagNP@en species along the titration with cobalt(II) ions. Because the zeta potentials are correlated to the charges located at the nanoparticles slipping plane (where the complexation equilibrium takes place), the Langmuir isotherms can be more readily constructed. The alternative approach revealed a contrasting response for nanoparticles in the presence NO3– and SO42– ions, indicating the participation of inner/outer-sphere complexes in the equilibrium for the last ones, thus allowing to discriminate the influence of the counterions that cannot be accessed by the conventional analytical treatment.