Experimental determination of the effect of the ratio of B/Al on glass dissolution along the nepheline (NaAlSiO4)–malinkoite (NaBSiO4) join

The dissolution kinetics of five glasses along the NaAlSiO 4 –NaBSiO 4 join were used to evaluate how the structural variations associated with boron–aluminum substitution affect the rate of dissolution. The composition of each glass varied inversely in mol% of Al 2 O 3 (5–25 mol%) and B 2 O 3 (20–0 mol%) with Na 2 O (25 mol%) and SiO 2 (50 mol%) making up the remaining amount, in every case Na/(Al + B) = 1.0. Single-pass flow-through experiments (SPFT) were conducted under dilute conditions as a function of solution pH (from 7.0 to 12.0) and temperature (from 23 to 90 °C). Analysis of unreacted glass samples by 27 Al and 29 Si MAS-NMR suggests Al (∼98% [4] Al) and Si-atoms (∼100% [4] Si) occupy a tetrahedral coordination whereas, B-atoms occupy both tetrahedral ( [4] B) and trigonal ( [3] B) coordination. The distribution of [3] B fractionated between [3] B(ring) and [3] B(non-ring) moieties, with the [3] B(ring)/ [3] B(non-ring) ratio increases with an increase in the B/Al ratio. The MAS-NMR results also indicated an increase in the fraction of [4] B with an increase in the B/Al ratio. The 27 Al peak maxima shift to lesser values with an increase in the B/Al ratio which suggests mixing between the [4] Al and [3] B sites, assuming avoidance between tetrahedral trivalent cations ( [4] Al–O– [4] B avoidance). Unlike the 27 Al and 11 B spectra, the 29 Si spectra illustrate a subtle shift to more negative chemical shift (chemical shift range between −88 and −84 ppm) and increases in the spectral widths as the B/Al ratio increases. Raman spectroscopy of unreacted glass samples was also used to cross-check the results collected from MAS-NMR and suggested that NeB4 (the glass sample with the highest B content) may consist of B–Na enriched and Al–Si enriched micro-domains, which affected the measured dissolution rates. Results from SPFT experiments suggest a forward rate of reaction and pH power-law coefficients, η , that are independent of B/Al under these neutral to alkaline test conditions for all homogeneous glasses. The temperature dependence shows an order of magnitude increase in the dissolution rate with a 67 °C increase in temperature and suggests dissolution is controlled by a surface-mediated reaction, as indicated by the activation energy, E a , being between 44 ± 8 and 48 ± 7 kJ/mol. Forward dissolution rates, based on Na and Si release, for homogeneous glasses are independent of the B/Al ratio, whereas dissolution rates based on Al and B release are not. Normalized dissolution rates, based on B release, increase with the molar fraction of [3] B(ring). Finally in accord with previous studies, the data discussed in this manuscript suggest rupture of either the Al–O or Si–O bonds as the rate-limiting step controlling the dissolution of these glasses.