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NANOMETER-SCALE CHEMICAL MODIFICATION OF NANO-BALL ALLOPHANE

Abidin, Zaenal and Matsue, Naoto and Henmi, Teruo (2007) NANOMETER-SCALE CHEMICAL MODIFICATION OF NANO-BALL ALLOPHANE. Clays and Clay Minerals, 55 (4). pp. 443-449.

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Abstract

Nano-ball allophane is a hydrous Al silicate with a hollow-sphere morphology that contains some defects or pores along the spherule walls. Enlargement of the pore openings by dilute alkali treatment was confirmed by cation exchange capacity determinations using various alkylammonium cations as replacement cations. An allophane sample with a low Si/Al ratio (0.67) was equilibrated with 10 mM CaCl2 (pH = 6.0) and the Ca2+ retained was extracted using aqueous 1 M NH4Cl or alkylammonium chloride salts. The Ca2+ extracted by NH4 + was 15.1 cmolc kg1, butCH 3NH3 + (mean diameter = 0.38 nm) only extracted 7.9 cmolc kg1 of Ca2+. After 10 mM NaOH treatment (0.25 g:100 mL) of the allophane, the Ca2+ extracted by NH4 + was 29.7 cmolc kg1, 29.6 cmolc kg1 by CH3NH3 +, and 29.4 cmolc kg1 by (CH3)2NH2 +. The extraction of Ca2+ by the large C2H5NH3 + cation (mean diameter = 0.46 nm) only decreased to 26.1 cmolc kg1, indicating that pore diameters were enlarged from ~0.35 to 0.45 nm. The significanti ncrease in Ca2+ retention after NaOH treatment was attributed to the dissociation of increased numbers of newly exposed silanol groups in the enlarged pores. The low Si/Al ratio of the NaOH-dissolved material (0.35) and the decreased intensity of the 348 cm1 IR band also suggested selective dissolution of the pore region. For allophane with a high Si/Al ratio (0.99) and much accessory polymeric Si, dissolution of polymeric Si and of the pore region occurred simultaneously. Alkali treatment produced a smaller increase in pore size and Ca2+ retention for allophanes with large Si/Al ratios than for allophanes with small Si/Al ratios. It was concluded that by altering the dilute alkali treatment conditions and varying the Si/Al ratio of allophane, the extent of structural modification or pore enlargement of the hollow spheres mightbe controlled.

Item Type:Article
Subjects:Material Science > Nanostructured materials
ID Code:4598
Deposited By:CSMNT
Deposited On:08 Apr 2009 07:49
Last Modified:08 Apr 2009 07:49

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