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The Mechanism of Copper(II) Binding by Humic Acid: An Electron Spin Resonance Study of a Copper(II)—Humic Acid Complex and Some Adducts with Nitrogen Donors¹

ABSTRACT

Electron spin resonance (ESR) spectroscopy was used to study the mechanism of Cu²⁺ binding by humic acid (HA) extracted from a Chalmers silt loam soil. The Cu-HA complex and some adducts with nitrogen donors were examined. The ESR data presented showed that the addition of pyridine (py) and 1,10-phenanthroline (phen) to the Cu-HA complex resulted in the substitution of coordinated H₂O ligands by the nitrogen donor ligands without displacement of HA oxygen donor ligands also coordinated to the Cu²⁺ center. The addition of a more basic nitrogen donor, ethylenediamine (en), resulted in the displacement from Cu²⁺ of both coordinated H₂O ligands and HA oxygen donors, forming Cu(en)₂²⁺. The g|| and A|| values obtained for the py and phen adducts revealed that there were two HA oxygen donors coordinated equatorially to the Cu²⁺ center in the original Cu-HA complex. The characterization of the adducts with phen and en was consistent with the formation of a Cu²⁺ chelate in the original Cu-HA complex, where the two equatorial Cu-HA oxygen bonds would occupy cis- positions. The crystal field strength of HA oxygen donor ligands coordinated to the Cu²⁺ center was stronger than that of H₂O.

¹ Contribution from the Purdue Agric. Exp. Stn., West Lafayette, IN 47907. Journal Paper no. 8388. This research was supported in part by Grant no. R804547030 from the U.S. Environmental Protection Agency.

² Graduate Research Assistant and Professors, Dep. of Agron., Purdue Univ.; and Professor of Inorganic Chemistry, Dep. of Chem., Illinois State Univ., Normal, IL 61761. The senior author is now Assistant Professor, Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824.

Received for publication October 27, 1980. Accepted for publication January 5, 1981.