| Humic acid extracted by 0.1M NaOH treatment of soil material from four Oxisols from Brazil contains colloidal, crystalline, highly Al-substituted goethite as determined by X-ray diffraction and Mossbauer spectroscopy. Neither hematite nor ilmenite was detected in any of the extracts, though each is present in the total soil material. When the total soil material is first treated with Na-hypochlorite to destroy organic matter and is then extracted with 0.1M NaOH the goethite does not disperse. The goethite found with the humic acid is of very small particle size, 10 to 15nm in diameter, as estimated by X-ray diffraction and surface area (BET/N{dollar}sb2{dollar}). X-ray diffraction data indicate that the particle size of the humic acid-goethite is 25 to 40% smaller than that of the goethite remaining with the humin. Surface area of the humic acid/goethite complex from the 0-10cm profile depth is less than or equal to 1m{dollar}sp2{dollar}g{dollar}sp{lcub}-1{rcub}{dollar}; after destruction of the humic acid with Na-hypochlorite the measured surface area of the residual geothite is greater than 100m{dollar}sp2{dollar}g{dollar}sp{lcub}-1{rcub}{dollar}. Fourier transform infrared spectra of the humic acid/goethite complex indicate that the carboxyl group is in salt form suggesting bonding to the goethite surface. Electron microscopy shows the goethite particles to be embedded in a matrix of humic acid. Phosphate adsorption by the goethite contained within the humic acid/goethite complex is much less than that by the "free goethite," i.e., goethite treated for destruction of the humic acid, suggesting strong bonding of the humic acid to the goethite surface whereby surface hydroxyl groups are displaced. The conclusion is that this interaction involves ligand exchange mechanisms between the two surfaces in addition to coulombic attraction and significantly modifies the properties of the Fe oxide, possibly favoring particle movement in the profile and explaining the high C content with depth in certain variable charge soils. |
