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Copper(II) and Iron(III) Complexation by the Carboxylate Group of Humic Acid¹

ABSTRACT

Infrared (IR) spectroscopy, between 2,000 and 1,000 cm^–1, was used to identify the mode by which carboxylates of a soil humic acid (HA) complexed Cu²⁺ and Fe³⁺. Carboxylic acid groups of HA (R-COOH) were converted to carboxylates (R-COO^-) by forming coordinate bonds with Cu²⁺ and Fe³⁺. The separation of the antisymmetrical and symmetrical stretching vibrations of R-COO^- was obtained from the IR spectra of the metal-HA complexes. The values for the metal-HA complexes were significantly larger than the corresponding reference value of for uncomplexed -COO^-, demonstrating that R-COO^- formed a unidentate complex (i.e., a single M-O bond) with Cu²⁺ and Fe³⁺. The formation of bidentate or bridging complexes between R-COO^- of HA and Cu²⁺ or Fe³⁺ was not observed.

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, respectively, Dep. of Agronomy, Purdue Univ., W. Lafayette, IN 47907. The Senior Author is now Assistant Professor, Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI 48824.

Received for publication February 20, 1981. Accepted for publication June 18, 1981.

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