Fulvic acids: structure and metal binding

SUMMARY Equilibrium calculations have been carried out at 25°C and an ionic strength of 0.04 to determine the predominant metal binding sites on a well‐characterized fulvic acid for magnesium(ll), calcium(ll), manganese(ll), iron(lll), copper(ll) and zinc(II). Statistical estimates of the concentration of 14 sites are obtained from a random molecular model of fulvic acid. Protonation and metal formation constants are estimated from those of simple model ligands and corrected to the above conditions. Results indicate the importance of phthalate sites for all metals except iron(III), salicylate sites for iron(lll) and acetylacetonate sites for copper(ll). Iron (III) ions are found to form ML 2 type complexes extensively. The affinity of the metal ions for fulvic acid as a whole is found to decrease in the order iron(III) > copper(II) > zinc(II) > manganese(Il) > calcium(II) > magnesium (II).