Cu2+-Adsorption Characteristics of Aluminum Hydroxide and Oxyhydroxides

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Abstract
The nature of Cu 2+ adsorption by boehmite, gibbsite, and noncrystalline alumina was studied over a range of equilibrium pH (4.5-7.5) and Cu 2§ concentration (10 a-10 -8 M) by electron spin resonance (ESR). Available chemisorption sites at pH 4.5 were the most numerous for noncrystalline alumina (-1 mmole/100 g), less for boehmite, and least for gibbsite as indicated by the relative strength of the rigid-limit ESR signal attributed to Cu z+ adsorbed at discrete sites. The chemisorption process involved immobili- zation of Cu 2+ by displacement of one or more H20 ligands by hydroxyl or surface oxygen ions, with the formation of at least one Cu-O-AI bond. As the pH was raised from 4.5 to 6.0, essentially all of the solution Cu 2+ appeared to be adsorbed by the solids. However, the noncrystalline alumina and boehmite chemi- sorbed much of the total adsorbed Cu 2+ (10 mmole/100 g), whereas precipitation or nucleation of Cu(OH)~ in the gibbsite system was indicated. Precipitated Cu z+ was more readily redissolved by exposure to NH3 vapor than chemisorbed Cu 2+.