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Association of Microcrystalline Goethite and Humic Acid in Some Oxisols from Brazil

Reforestadora Simpson, Ltda., DIAGONAL 6, 17-97 ZONA 10, Guatemala

S. B. Weed^*

Dep. of Soil Science

L. H. Bowen

Dep. of Chemistry, North Carolina State Univ., Raleigh, NC 27695-8204

* Corresponding author.

ABSTRACT

Oxisols from the cocoa-growing region of the state of Bahia, Brazil, may contain large amounts of Fe oxides and significant amounts of organic C to depths exceeding 2 m. The nature of the association of organic C and crystalline oxides was the objective of this study. Alkali extracts (0.1 M NaOH) of soil material taken from different depths in three Typic Hapludox and one Humic Hapludox were characterized by chemical analysis, x-ray diffraction, Mössbauer spectroscopy, electron microscopy, surface area determination (N₂ adsorption), Fourier-transform infrared spectroscopy, and phosphate adsorption. Microcrystalline goethite was the only inorganic phase accompanying the humic acid in the alkaline solution and humic acid was essential for goethite to be extracted. Mössbauer spectra confirmed the absence of Fe oxides other than goethite in the humic-acid extracts although hematite and ilmenite were identified in the total soil material. The goethite particles were 10 to 15 nm in diameter and nearly equidimensional. Infrared spectra of the humic acid-goethite complex indicated that the COOH group was in the salt form, suggesting bonding through this group to the goethite surface. Phosphate adsorption was markedly higher on the humic-acid-free goethite than on the complex, indicating competition for adsorption on the goethite by the humic acid. Apparently, the interaction between the humic acid and the goethite involves iigand exchange in addition to coulombic attraction, and may significantly modify the properties of the Fe oxide.

This work was supported in part by the National Science Foundation, Grant EAR-850656.

Received for publication May 23, 1991.

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