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Conditions for Al₁₃ Polymer Formation in Partially Neutralized Aluminum Solutions¹

ABSTRACT

The influence of synthesis methodology on the formation of the Al₁₃([AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺) polymer was investigated to better establish boundary conditions for its formation. The amount of Al₁₃ polymer formed in solutions ranging in Al concentration from 3.34 x 10^–4 to 3.34 x 10^–2 mol L^–1 was found to be highly dependent on OH/Al ratio (ñ), neutralization rate, mixing conditions, and initial Al concentration. For the 3.34 x 10^–3 mol Al L^–1 solution synthesized at 1.2 cm³ min^–1, the Al₁₃ polymer formed only when ñ exceeded 1.5, whereas this polymer predominated in a solution 3.34 x 10^–2 mol L^–1 with respect to Al at ñ 0.4. There was no evidence for Al₁₃ polymer formation in solutions 3.34 x 10^–4 mol Al L^–1 with ñ = 2.5 synthesized at the 1.2 cm³ min^–1 base injection rate; however, the formation of this species could be induced in the 3.34 x 10^–4 mol Al L^–1 solution by increasing the base injection rate to 100 cm³ min^–1. Utilizing an Erlenmeyer flask instead of a beaker for the neutralization reaction resulted in different amounts of Al₁₃ formation, and this was believed to result from differential mixing conditions. The data suggest that the formation of the Al₁₃ polymer in partially neutralized Al solutions is an artifact of synthesis conditions requiring inhomogeneous pH conditions at the point of base injection resulting in the formation of the Al(OH)^-₄ ion. It would appear that this polymer is of little general significance to secondary Al mineral formation in acidic weathering environments.

¹ Contribution from the Savannah River Ecology Lab. of the Univ. of Georgia.

² Assistant Professor, Univ. of Georgia, Savannah River Ecology Lab., P.O. Drawer E. Aiken, SC 29802.

Received for publication July 2, 1986.