PNNL

Abstract

Motivated by the major industrial relevance of Al solution chemistry under high pH conditions, this work provides new insight into the speciation of aluminate Al(OH)4- and its equilibria with the dimeric species Al(OH)82- and Al2O(OH)62- using experimental and simulation solution phase vibrational data. Ab-initio molecular dynamics provides the first simulated IR and Raman spectra that account for solvent ensemble fluctuations in solution, anharmonicity and peak broadening, and definitive assignments of modes through decomposition of vibrational motions projected onto internal coordinates. Fitting of the experimental solution–phase data to linear combinations of the predicted spectra was performed for: 1) Al(OH)4- and the lowest energy dimer Al2O(OH)62-, vs. 2) Al(OH)4- with both dimeric species. Importantly, these fits reveal overlapping bands that prevent definitive assessment of either the specific dimer that is present in solution, nor that there is even only a single dimer species in equilibrium with the monomer. The two dimers of aluminate represent spectroscopic chameleons that challenge the general assumptions made regarding aluminate speciation in alkaline solutions and have impact upon the future study of crystallization processes based upon aluminate oligomer pre-nucleation species.