Conditions for Al13 Polymer Formation in Partially Neutralized Aluminum Solutions

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 × 10⁻⁴ to 3.34 × 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 × 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 × 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 × 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 × 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.