Effective Affinity Distribution for the Binding of Metal Ions to a Generic Fulvic Acid in Natural Waters

The effective distribution of affinities (Conditional Affinity
Spectrum, CAS) seen by a metal ion binding to a humic substance
under natural water conditions is derived and discussed within the
NICA−Donnan model. Analytical expressions for the average affinity
of these distributions in general multi-ion mixtures are reported
here for the first time. These expressions enable a simple evaluation
of the effect of all interfering cations on the affinity distribution
of a given one. We illustrate this methodology by plotting the affinity
spectra of a generic fulvic acid for 14 different cations in the presence
of major inorganic ions and trace metals at pH and concentration values
representative of a river water. The distribution of occupied sites
and their average affinity at the typical freshwater conditions are
also reported for each ion. The CAS allows us to distinguish three
groups of cations: (a) Al, H, Pb, Hg, and Cr, which are preferentially
bound to the phenolic sites of the fulvic ligand; (b) Ca, Mg, Cd,
Fe(II), and Mn, which display a greater effective affinity for carboxylic
sites, in contrast to what would be expected from their individual
complexation parameters; and (c) Fe(III), Cu, Zn, and Ni, for which
phenolic and carboxylic distributions are overlapped.