Influence of Citric Acid on the Formation of Short-Range Ordered Aluminosilicates

K. Inoue1 and P. M. Huang
Department of Soil Science, Saskatchewan Institute of Pedology, University of Saskatchewan Saskatoon, Saskatchewan S7N 0W0, Canada
1 Permanent address: Laboratory of Soil Science, Department of Agricultural Chemistry, Faculty of Agriculture, Iwate University, Morioka, Japan 020.

Abstract: Reactions between hydroxy-Al ions and orthosilicic acid as influenced by citric acid were studied at an initial Si concentration of 1.6 × 10−3 M, Si/Al molar ratios of 0.5 and 1.0, OH/Al molar ratios of 1.0–3.0, and citric acid/Al molar ratios of 0–0.3. In the absence of citric acid and at OH/Al ratios of 1.0–2.8, imogolite (>0.01 µm) was dominant in the precipitates. At citric acid/Al ratios of 0.01–0.1, imogolite and/or pseudoboehmite were dominant in the precipitates at OH/Al ratios of 1.0 and 2.0, and imogolite and/or ill-defined aluminosilicate complexes at OH/Al ratio of 2.8. Instead of allophane or “proto-imogolite” allophane being the predominant species in the precipitates, the formation of ill-defined aluminosilicate complexes at OH/Al ratio of 3.0 was steadily promoted by increasing the solution citric acid/Al ratios from 0 to 0.3. The freeze-dried soluble products (<0.01 µm) ranged from silica gel to “proto-imogolite,” depending upon the basicity of Al and the level of citric acid of the parent solution. The amount of “proto-imogolite,” increased with increasing citric acid/Al ratios from 0 to 0.1 in solution. Complexing low molecular weight organic acids, such as citric acid, impeded the formation of the short-range ordered aluminosilicates, allophanes and imogolite.

Key Words: Allophane • Aluminosilicate • Citric acid • Hydroxy-Al • Imogolite • Noncrystalline • Pseudoboehmite

Clays and Clay Minerals; August 1985 v. 33; no. 4; p. 312-322; DOI: 10.1346/CCMN.1985.0330406
© 1985, The Clay Minerals Society
Clay Minerals Society (www.clays.org)