Influence of Selected Organic Ligands on the Formation of Allophane and Imogolite1
- K. Inoue and
- 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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