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This article in SSSAJ

  1. Vol. 50 No. 6, p. 1623-1633
    Received: Oct 3, 1985

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Influence of Selected Organic Ligands on the Formation of Allophane and Imogolite1

  1. K. Inoue and
  2. P. M. Huang2



The role of organic ligands of different ability to complex with Al3+ in affecting the formation of allophane and imogolite and the nature of the precipitation (>0.01 µm) and soluble (<0.01 µm) products was investigated at initial Si concentration of 1.6 × 10−3 mol.L−1, Si/Al molar ratios of 0.5 and 1.0, OH/Al molar ratios of 2.0, and 3.0, and organic ligand/Al molar ratios of 0 to 0.2. DL-aspartic and p-hydroxybenzoic acids showed a very weak impeding effect on the formation of allophane and imogolite; salicylic acid showed a weak to moderate impeding effect on the formation. Tannic, DL-tartaric, citric, and DL-malic acids with a strong affinity for Al greatly perturbed the interaction of hydroxy-Al ions with orthosilicic acid at Si/Al ratio of 0.5, OH/Al ratio of 2.0, organic ligand/Al ratios of 0.03 to 0.1, and thus hindered the subsequent formation of imogolite, leading to the formation of pseudoboehmites, hydroxy-Al-organic complexes, and/or ill-defined aluminosilicate complexes. These strong complexing organic ligands also hampered the formation of allophane at Si/Al ratio of 1.0, OH/Al ratio of 3.0, and organic ligand/Al ratios of 0.1 to 0.2, resulting in the formation of ill-defined aluminosilicate complexes and/or hydroxy-Al-tannate complexes. Chemical composition, size, number, and nature of the functional groups, and concentration of organic ligands played a vital role in perturbing the formation of allophane and imogolite in the precipitation products. The order of effectiveness of organic ligands was: tannic >> DL-tartaric > citric > DL-malic >> salicylic > DL-aspartic ≥ p-hydroxybenzoic. Organic ligands with a strong affinity for Al significantly modified the nature of the soluble products which range from “proto-imogolite” sol complexed with organic acids to hydroxy-Al-organic complexes.

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