Residual Copper(II) Complexes in Purified Soil and Sewage Sludge Fulvic Acids: Electron Spin Resonance Study

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Abstract
Electron spin resonance (ESR) spectra were obtained for both frozen (77 K) aqueous solutions and solid samples of purified fulvic acid (FA) extracted from sewage sludge and from soil organic matter. Large scan range (5000 G) ESR spectra indicated the presence of organic free radical species of semiquinone nature and of iron(III) in sites of highly rhombic character. Narrower scan range (2000 G) ESR spectra were characterized by absorptions of the axiaI type near g = 2, which indicated the presence of residual copper(II) in a dx2−y2 groundstate complexed with ligands arranged in a tetragonal environment. The coordination of the copper(Cu) was shown to be of the equatorial type, involving both oxygen‐(O) and nitrogen‐(N) ligand atoms of FA. The main resonances in the spectrum obtained for the frozen solution of the soil FA were consistent with a principal binding site of 4 O atoms, whereas the minor ESR component in the same spectrum was assigned to a binding site involving 3 O‐and 1 N‐ligand atoms. The reverse order of importance was found in the ESR spectrum of the solid sample, indicating that more N‐containing functional groups of the FA were involved in the binding of residual copper when the FA was not dissolved in water. The ESR parameters for the sludge FA samples indicated that the main resonances were consistent with a 3 O‐1 N‐binding site and the minor ones with 4 O‐or 2 O‐and 2 N‐coordinating environments. ESR spectra of the sludge FA run over a very narrow scan range (1000 G) suggested that an intensely structured pattern at g⊥ could be assigned to a superhyperfine coupling of the unpaired Cu electron to two equivalent N ligand nuclei.