The morphologically controlled synthesis of aluminum-based surfactant mesophases by the homogeneous precipitation method using urea is demonstrated, together with a model for the templating mechanism pathway. Dodecyl sulfate surfactant initially forms a layered mesophase with an interlayer spacing of 3.5 nm in which the surfactant molecules are arranged as a bilayer between the linked sheets of aluminum oxyhydroxide groups. The aluminate sheets consist mostly of octahedral Al but contain some fraction of tetrahedral Al dependent on urea concentration. On further hydrolysis of urea, the layered mesophase is transformed into a hexagonal form through the interlayer condensation and cross-linking of the Al-OH groups in any adjacent aluminate sheets. The structural transition accompanies the rearrangement of the bilayered surfactant molecules into a rodlike assembly, followed by the additional growth into enlarged particles. The hexagonal mesophase resulting from the lamellae containing comparatively large amounts of tetrahedral Al forms a ringed or curved wormlike morphology, while the phase grown from octahedral Al-enriched forms appears in versatile patterns including spherical, funneled, and tubular particles. Such morphologically versatile characteristics of the hexagonal mesophases reflect those of their precursors produced by the folding of aluminum-based flexible sheets of aluminum oxyhydroxide octahedral groups mixed with their tetrahedral ones at a fraction dependent on urea concentration.

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IntroductionRecently, attention has been focused on the recovery of gold from aqueous solutions from the standpoints of gold recovery from industrial resources, such as electronics parts and plating materials. [1][2][3][4][5][6][7] Previously, one of the authors reported that persimmon tannin (PT) gel, a newly developed biosorbent, adsorbed large amounts of gold from a solution containing hydrogen tetrachloroaurate(III). 8 The gold adsorption by the gel was rapid, endothermic, and obeyed the Langmuir isotherm. On the other hand, the gel also adsorbed large amounts of uranium, 9 iron, 10 and vanadium. 11 The gel contained a number of adjacent hydroxyphenyl groups, which are suitable ligands for hard acids, such as uranyl, iron(III), and vanadyl ions, based on the HSAB idea. 12,13 Because Au(III) ion is softer than these cations, hydroxyphenyl groups in the gel are not suitable.Preliminary experiments revealed that colloidal Au(0) was formed in a HAuCl4 solution containing PT gel. In this note, therefore, further experiments were conducted to analyze the mechanism of gold adsorption. ExperimentalThe persimmon tannin (PT) solution used throughout this study was obtained from Tomiyama Shoten, Kyoto. PT gel was prepared as previously described. 8,11 Hydrogen tetrachloroaurate(III) tetrahydrate and other chemicals used for this study were obtained from Nacalai Tesque, Inc. (Kyoto, Japan). Gold adsorption experiments in a column system were conducted as follows: 1000 ml of a solution (pH 3) containing 0.1 mM HAuCl4 was passed through a column (diameter 8 mm, bed volume 8.9 ml) of PT gel at 150 and 300 ml/h, collecting 100 ml for each fraction. The pH of the gold solution was adjusted to 3 by 0.1 M HCl and 0.1 M NaOH. To avoid an optical reduction of Au(III) ion, adsorption experiments were conducted under dark conditions. The gold content in each fraction was determined by a inductively coupled plasma quantometer (Shimadzu ICPQ 1000II). A colloidal Au(0) solution was prepared by adding 20 µM of formaldehyde to a solution containing 0.05 mM HAuCl4. After adjusting the pH to the desired value by 0.1 M HCl and 0.1 M NaOH, the mixture solution was allowed to stand. The time when the gold solution became colored purplish red was recorded as the time for colloid formation. Fifty milliliters of the resulting colloidal Au(0) solution were passed through a column (diameter 8 mm, bed volume 5 ml) of PT gel at 100 ml/h. The gold adsorption ratio (%) was estimated from the gold content in each elute. The optical absorption spectra were measured using an absorption spectrophotometer (Japan Spectroscopic Co. Ltd., JASCO V-550). Powder X-ray diffraction (XRD) patterns of PT adsorbed gold were recorded with a powder X-ray diffractometer (Shimadzu XD-D1) using Cu Kα1 radiation (λ = 1.54056 Å) over a 2θ range from 1˚ to 80˚. Results and DiscussionThe effect of the flow rate on gold adsorption by PT gel was examined in a column system, the results of which are listed in Table 1. When the HAuCl4 solution (pH 3) was passed through the PT g...

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