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Copper(II) Binding by Humic Acid Extracted from Sewage Sludge: An Electron Spin Resonance Study¹

ABSTRACT

Electron spin resonance (ESR) spectroscopy was used to study the binding of Cu²⁺ by humic acid (HA) extracted from sewage sludge. Electron spin resonance spectral data of the Cu-HA complex and some adducts with nitrogen donors (pyridine, 1,10-phenanthroline, ethylenediamine) showed that Cu²⁺ formed two equatorial bonds with HA oxygen donor ligands. The ESR data presented were consistent with the formation of a Cu-chelate in the original Cu-HA complex. The ligand field strength of the HA oxygen donor ligands coordinated to Cu²⁺ was stronger than that of H₂O. The above results were consistent with an earlier ESR characterization of a soil Cu-HA complex. The Cu²⁺ ion also appeared to form axial bonds with HA donor ligands originating from proteinaceous materials associated with the sludge HA fraction. Characterization of an adduct with glycylglycine showed that the dipeptide formed coordinate honds with Cu²⁺ bound to HA.

¹ Contribution from the Purdue Agric. Exp. Stn., West Lafayette, IN 47907, Journal Paper no. 8584. This research was supported in part by Grant no. R804547030 from the U.S. Environmental Protection Agency.

² Graduate Research Assistant, and Professors, respectively, Dep. of Agronomy, Purdue Univ.; and Professor of Inorganic Chemistry, Dep. of Chemistry, Illinois State Univ., Normal, IL 61761. The Senior Author is now Assistant Professor, Dep. of Crop & Soil Sciences, Michigan State Univ.; East Lansing, MI 48824.