Perturbation of Imogolite Formation by Humic Substances

Abstract

AbstractImogolite 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.