Humic Acid Effect on Aluminum Interlayering in Montmorillonite
- A. Singer and
- P. M. Huang
Humic acids (HAs) are a major component of the organic matter of soils, many of which also contain smectites and other expansible clay minerals. However, the effects HAs might exert on Al interlayer formation in expansible clay minerals remain obscure. The objective of this study was to investigate the effect of a HA extracted from a Udic Haploboroll on Al interlayering in montmorillonite. Hydroxy-Al-HA-montmorillonite complexes were formed in which the Al/clay ratio was 600 cmol kg−1, the OH/Al molar ratio 2.25 and HA/clay ratio in the range of 50 to 200 g kg−1. At the higher HA addition levels, the presence of the hydroxy-Al polymers increased the adsorption of humic acid on the clay. X-ray diffraction data indicated distinct interlayering, possibly Al13 polymers, in the complexes formed at pH 4.8 in the absence of HA. With increasing addition levels of HA, hydroxy-Al interlayering was perturbed as indicated by x-ray diffraction, infrared absorption, and cation-exchange capacity data. Perturbation of Al interlayering by HA increased with increasing pH during complex formation. The data obtained from the treatment of the complexes with pyrophosphate indicated an intimate association of the HA with hydroxy-Al-clay complexes. Only partial removal of organic C from the complexes by sodium NaOCl treatment suggested that some of the adsorbed HA was very strongly bound with the hydroxy-Al-clay complexes. Diffuse x-ray diffractograms of the hydroxy-Al-HA-montmorillonite complexes, reduced adsorption of HA during complex formation with collapsed montmorillonite, decreased d-spacing after NaOCl treatment, and the shift to a higher temperature endotherm (575 °C) in thermogravimetric analysis suggested that some of the HA in the complex was apparently located in interlayers. In the hydroxy-Al-HA-montmorillonite complexes prepared with the clay added first and HA last, perturbation of Al interlayering by HA was minimal.
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