Perturbation of Imogolite Formation by Humic Substances
- K. Inoue and
- P. M. Huang
Imogolite formation is influenced by ionic factors. This study was conducted to investigate the perturbation of imogolite formation by humic substances. Interactions of hydroxy-Al ions with orthosilicic acid as influenced by humic substances were studied at the Si concentration of 1.6 mM, a Si/Al atomic ratio of 0.5, an OH/Al molar ratio of 2.0, and concentrations from 1 to 1000 mg humic acid (HA) L−1 and 1 to 300 mg fulvic acid (FA) L−1. Chemical analyses of filtrates (<0.01 µm) and x-ray diffraction patterns, infrared (IR) spectra, and electron micrographs of the precipitation products (>0.01 µm) clearly showed that the degree of perturbation of the formation of imogolite increased with the rising concentration of humic substances, leading to the formation of ill-defined aluminosilicate complexes. These complexes are similar in IR spectra to allophane-like constituents or freeze-dried protoimogolite and are associated with humic substances. In addition, pseudoboehmite and hydroxy-Al-humus complexes were present in the reaction products. The formation of imogolite was completely inhibited at concentrations >30 to 50 mg FA L−1 and 300 HA L−1. These results reveal that the pedogenic significance of humic substances in impeding the formation of imogolite and in promoting the formation of ill-defined aluminosilicate or hydroxy-Al-humus complexes in the Andisol A and Spodosol Bh horizons merits attention.
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