Adsorption of Aluminum by a Smectite: I. Surface Hydrolysis During Ca2+-Al3+ Exchange
1977
Bloom, P. R. | McBride, M. B. | Chadbourne, B.
The exchange of Ca by Al in montmorillonite suspensions adjusted to low pH (∼4) was investigated by measurement of Ca, Al, and pH in the solution phase before and after the addition of clay. Adsorption of Al was accompanied by a release of protons to solution, indicating that the clay surfaces promoted hydrolysis of Al³⁺ when only partial coverage of exchange sites by Al had occurred. Calculations showed a two- to nine fold increase in the degree of hydrolysis at the surface relative to the value for solution. In suspensions with pKₛₚ values near 33.3, Al(OH)₃ was precipitated on interlamellar surfaces. However, at lower pH, the predominant form of adsorbed Al was Al³⁺, with hydroxy -Al monomers and/or polymers also present. The preference of the ions for the clay surface followed the order: hydroxy-Al>Al³⁺>Ca²⁺>H₃O⁺. The exchange systems were complicated by proton uptake by the montmorillonite, resulting from H₃O⁺-Ca²⁺ exchange as well as proton adsorption by edges concurrent with slow mineral dissolution reactions.
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