The influence of bulk composition on the diffusivity of carbon dioxide in Na aluminosilicate melts

Abstract

The bulk diffusivity of dissolved carbon dioxide (CO 2 and CO 3 2- ) in NaAlSi 3 O 8 + nNa 2 O (n = 0- 6.87 wt%) and in NaAlSi 3 O 8 + nH 2 O (n = 0-2 wt%) melts was investigated at 1523 K and 0.5 GPa using the diffusion couple technique. CO 2 contents of the starting glass pairs varied between 0 and 0.2 wt%. Symmetrical concentration-distance profiles of bulk CO 2 were determined by infrared spectroscopy. An error function was fitted to the profiles to obtain apparent chemical diffusion coefficients of bulk CO 2 . In the investigated compositional range, the diffusivity of bulk CO 2 increases exponentially with Na 2 O and H 2 O content and thus exponentially with the ratio of nonbridging oxygen atoms per tetrahedral cations (NBO/ T). The bulk CO 2 diffusivity increases from logD CO₂ = -11.38 (D CO₂ in m2/s) in NaAlSi 3 O 8 melt to logD CO₂ = -10.92 in NaAlSi 3 O 8 melts containing 6.87 wt% Na 2 O excess, and to logD CO₂ = -10.91 in NaAlSi 3 O 8 melts containing 2 wt% H 2 O. These data imply that either: (1) the diffusivities of the CO 2 species (molecular CO 2 and CO 3 2- ) are very similar, or (2) the speciation of CO 2 in the quenched glasses is very different from the speciation in the melt.