Intraparticulate Metal Speciation Analysis of Soft Complexing Nanoparticles. The Intrinsic Chemical Heterogeneity of Metal-Humic Acid Complexes.

The counterion condensation-Donnan (CCD) model for the electrostatic features of soft, charged nanoparticles (NPs) is applied to the determination of the intrinsic stability constants, K̅, for inner-sphere Cd(II) and Cu(II) complexes with humic acid NPs. The novel CCD model accounts for the strong ion condensation potential for higher valency counterions within the intraparticulate double layer zone of the soft NP. The approach offers new insights into the intrinsic heterogeneity of the HA complexes, as revealed by the intraparticulate speciation as a function of the true degree of inner-sphere complexation, θ. The ensuing intrinsic heterogeneity parameters, Γ, for CdHA and CuHA complexes are in very good agreement with those obtained from dynamic electrochemical stripping chronopotentiometric measurements. The overall intraparticulate metal ion speciation is found to depend on θ: at low θ the strong inner-sphere complexes predominate whereas at higher θ values, electrically condensed M may be an equally significant or even larger fraction of the particle-associated M.