Electron Spin Resonance Investigation of Copper(II) Complexation by Soil Fulvic Acid

Electron Spin Resonance (ESR) spectra are reported and interpreted for Cu(II)-fulvic acid complexes prepared by mixing Cu(ClO₄)₂ with a representative soil fulvic acid (FA) at Cu/FA molar ratios in the range 0.02 to 20. The data indicated the formation of anisotropic Cu(II) complexes, the metal cation being immobilized in inner-sphere coordination with ligands arranged in a distorted octahedral environment. The soil FA showed a high potential binding capacity for Cu(II) added at various ratios in aqueous solution, the ligation being derived from both oxygenated and nitrogenated functional groups. The ESR spectra of the Cu-FA complexes were investigated under varying conditions, among which were the Cu/FA ratio, treatment with a strongly acidic cation exchange resin, elimination of the solvent by air-drying, and recording temperature. For the lowest Cu/FA ratio (0.02), calculated ESR parameters suggested the existence of three different coordinating environments for copper ions: (i) 4 O, (ii) 3 O, 1 N, and (iii) 2 O, 2 N. When the Cu/FA molar ratio was increased to 0.2, a small excess of Cu appeared to result in the formation of additional mixed Cu-H₂O-FA complexes as predominant species. When the Cu/FA ratio was increased beyond 0.2, only a dipolar-broadened ESR absorption produced by a large excess of Cu²⁺ aquo-ions was observable. However, treatment of the Cu-FA solutions with a cation exchange resin was not only able to reduce excess Cu²⁺ ions and leave the Cu-FA complexes nearly unaltered, but also to yield better-resolved ESR spectra. The ESR spectra of all the Cu/FA solutions after passage through the resin were similar and were characterized by two components consistent with 4 O and 2 O, 2 N coordinating sites for the copper ions. A highly structured pattern in the g region exhibited by the ESR spectrum at the highest Cu/FA ratio was attributed to superhyperfine coupling of the Cu²⁺ unpaired electron to N-ligand nuclei. The ESR spectra of dried Cu-FA solutions not treated with resin were similar to those of the corresponding aqueous solutions, whereas drying the solutions after resin treatment resulted in ESR spectra and spin Hamiltonian parameters consistent with a binding site involving 3 O, 1 N ligand atoms. The implied rearrangements of ligands in the complexes after the removal of water emphasize the importance of soil relative humidity to the Cu(II) complexation mechanisms of fulvic acid.