Effective affinity distribution for the binding of metal Ions to a generic fulvic acid in natural waters
Huidobro Redondo, César
Mas i Pujadas, Francesc
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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.