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Electron Spin Resonance Investigation of Copper(II) Complexation by Fulvic Acid Extracted from Sewage Sludge¹

ABSTRACT

Electron spin resonance (ESR) spectra are reported for copper (II) – fulvic acid complexes prepared with fulvic acid (FA) samples extracted from two different anaerobically-digested sewage sludges. The nominal Cu/FA molar ratios employed were in the range 0.08 to 80. In the original Cu-FA solutions, quadridentate, nearly square planar complexes with FA donor ligands could be inferred from the ESR spectra only at the smallest Cu/FA molar ratio, whereas at the intermediate ratios mixed Cu-FA-H₂O complexes became more evident and at the highest ratio free Cu²⁺ aquo-ions prevailed. Treatment of the Cu(II)-FA solutions at any Cu/FA molar ratio with the strongly acidic cation exchange resin Bio-Rad AG-MP-50 eliminated excess Cu(II) aquo-ions and, therefore, permitted a better differentiation of classes of FA functional groups participating, to different degrees, in the coordination of Cu(II). The ESR spectra suggested the formation of inner-sphere, polydentate complexes, with the Cu(II) coordinated equatorially by oxygenated and nitrogenated FA ligands. ESR parameters of the resulting Cu(II)-FA complexes showed the existence of binding sites involving both 4 O and 2 O, 2 N ligand atoms, as well as a minor, more nitrogenated site (1 O, 3 N). No saturation of the Cu(II) complexation capacity was observed, but increasing the Cu content involved less and less reactive functional groups of FA in the coordination, shifting the complexation equilibria towards the formation of weaker complexes. Further information about binding mechanisms was developed through a comparative analysis of ESR data obtained for Cu(II)-FA complexes both in aqueous solution and in air-dried state. A significant rearrangement of the Cu-FA systems was suggested to take place upon air-drying of the resin-treated Cu/FA solutions.

¹ Contribution from the Istituto di Chimica Agraria, Univ. di Bari, Bari, Italy; Dep. of Chemistry, Univ. of California, Riverside, and Department of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Associate Professor of Agricultural Chemistry, Assistant Professor of Chemistry, and Professor of Soil Science, respectively.

Received for publication October 12, 1983. Accepted for publication September 10, 1984.