Copper (II) and iron (III) complexation by humic acid-like polymers (melanins) from soil fungi

Abstract

The reactivity of humic acid-like polymers (melanins) synthesized in culture media by five soil fungi toward added Cu 2+ and Fe 2+ ions, the total amounts of metals adsorbed, the type of complexes formed and their water- and HC1-stability have been investigated in different experimental conditions by atomic absorption, infrared and electron spin resonance spectroscopy. The humic fungal polymers show a generallyhigh, but quantitatively different adsorption capacity toward Cu 2+ and Fe 3+ ions, principally in function of their total acidity and nitrogen content. Both weak surface sites, water-stable, but easily disrupted by protonexchange, and highly stable inner sites resistant to extensive proton-exchange are involved by the melanins in the complexation of metal ions. The geometry of the inner-sphere binding sites and the type of ligands coordinating the metals have a similar nature in complexes obtained under different conditions. Fe 3+ ions are generally preferentially adsorbed with respect to Cu 2+ ions when the two ions are simultaneously interacted with the humic polymers.