Microcrystalline gibbsite

Although the glycol derivative of boehmite is also prepared from aluminum alkoxides (see Section 111.B.9), this product cannot transform into a-alumina because of the absence of a-alumina nuclei and water. However, the reaction of a mixture of gibbsite (microcrystalline) and aluminum alkoxide in the presence of a small amount of water gives a-alumina with larger crystallite sizes than those obtained from gibbsite alone. 

McBride, M. B., Fraser, A. R., and McHardy, W. J. (1984). Cu2+ interaction with microcrystalline gibbsite—Evidence for oriented chemisorbed copper ions. Clays Clay Miner 32(1), 12-18. 

Figure 12.25 (a) Plot of log 1AP versus pH for 64 samples from a drainage basin affected by acid mine waters from the Leviathan mine, Cali-fornia-Nevada. Points shown as open squares have pH <4.6, plus symbols are those with pH >4.9. The solid lines are theoretical solubilities of amorphous AI(0H)3 and microcrystalline gibbsite. (b) Plot similar to (a) for acid mine drainage from Appalachia (solid circles) and Adirondack lake waters affected by acid precipitation (open circles). From Science 232 54-56, D. K. Nordstrom and J. W. Ball, The geochemical behavior of aluminum in acidified surface waters. Copyright 1986 by Science-AAAS. Used by permission. 

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

The electron micrograph of Figure 10 is of particular interest since it apparently shows edges (rectangular shapes) as well as faces and indicates the relative thickness of the particles. Also, it can be seen that the particles appeared layered much in the manner of microscopic mica particles. The electron micrograph of Figure 11, which is of material from solution C, indicates that microcrystalline gibbsite is ultimately formed even if rM value is as low as 0.94 and pH is near 4. 

Figure 10. Electron micrograph of microcrystalline gibbsite from solution F with negative gold particles adsorbed...

Thus, the electron micrographs show conclusively that colloidal material is formed in the aging study solutions. Further, this material appears to be microcrystalline gibbsite. 

McBride, M.B. 1985a. Influence of glycine on Cu2+ adsorption by microcrystalline gibbsite and boehmite. Clays Clay Miner. 33 397—402. 

McBride, M.B., A.K, Fraser, and W.J. McHardy. 1984. Cu2+ Interaction with microcrystalline gibbsite. Evidence lor oriented chemisorbed copper ions. Clays Clay Miner. 32 12-18. 

Figure 1. Equilibrium solubility of microcrystalline gibbsite at four different temperatures and 1 atm. pressure. Ionic strength 0.010.

In the open system maintained during these titrations the hydrolysis reactions are driven by the continuous addition of A1 ions. The rate of the hydrolysis reactions can be inferred from the rate of OH addition, with appropriate adjustments, and the reaction kinetics can be evaluated. Early in the titration period the formation of monomeric aluminum hydroxy-ions (Ala) is the only reaction thermodynamically feasible as the solution is below saturation with respect to microcrystalline gibbsite. The solid point in Figure 2 represents the equilibrium concentration of Ala with respect to that solid at 25 C, I = 0.01 and pH 4.90, which was reached in about 50 minutes. 

Reaction affinities, precipitation of microcrystalline gibbsite, kcal... 

In titrations at pH 5.20 and 5.00 the size of Alb units, as indicated by the OHb/Alb ratio seems to reach a maximum at 2.4, which could be interpreted as a boundary in the transition of Alb to Ale, microcrystalline gibbsite. Some additional support for this hypothesis can be obtained from experiments of Parthasarathy and Ruffle (8), who used ultrafiltration to separate polymeric Alb from Ale. They observed that the polymeric material passed through a filter having a nominal porosity of 20 X. Preliminary experiments we have made with this technique indicated that our Alb fraction also can pass through this membrane, but Ale docs not. 

Figure 8. Electron micrograph of polymerized aluminum hydroxide particles (microcrystalline gibbsite)...

The points on the left-hand regression line, Figure 10, represent the calculated activity of uncomplexed aluminum present at various pH s based on observed reaction rates of bound OH in the experimental solutions that contained microcrystalline gibbsite. Aging times for these solutions ranged up to about four months. Circles are for solutions initially 10" Af in aluminum and Xs for solutions initially 10 M in aluminum. 

The conclusion that polynuclear hydroxide complex ions do not constitute a significant part of the solute species of aluminum at equilibrium in the solutions studied here is in accord with the conclusions of Frink and Sawhney (4). The microcrystalline gibbsite itself is metastable in the sense that as crystals grow in size with longer aging time their solubility decreases. Although at equilibrium metastable species can be ignored, it can be so diflBcult to attain an equilibrium condition that the concept has little practical usefulness. 

Fig. 3.46 The solubility curves 1 CAH, 2 C AHj, 3 AHj gel, 4 microcrystalline AHj, 5 gibbsite. There is also shown the inteiseclion point (iw) of the curve corresponding to the minimum instability with the one (A), corresponding to the CA solubility. The supersaturation curves (7) CjAHj, (77) Al(OH)j gel are also plotted. (According to [124])...

The transition aluminas contain varying amount of hydroxyl ions and are not well defined stoichiometrically therefore, they cannot be considered as true polymorphs of alumina. The structures of the forms occurring at a given temperature range are determined by the structure of the starting materials, and they are different for gibbsite, bayerite, boehmite, or diaspore. The microcrystalline state of these substances and the poor character of their X-ray diffraction pattern preclude the application of refined methods of structure determination. 

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