Effective affinity distribution for the binding of metal Ions to a generic fulvic acid in natural waters

Abstract

The effective distribution of affinities (Conditional Affinity Spectrum, CAS) seen by ametal ion bindingto 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 acidfor14 different cationsinthe presence ofmajorinorganic ions and trace metals at pH and concentration values representative of a river water. The distribution of occupied sites andtheir average affinity atthetypicalfreshwater 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 affinityfor carboxylic sites, in contrastto 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.

We gratefully acknowledge support of this research by the Spanish Ministry of Education and Science (Projects CTQ2006-14385 and CTM2006-13583) and from the “Comissionat d’Universitats i Recerca de la Generalitat de Catalunya”. S. Mongin was supported by an FPI-MEC grant from the Spanish Government.