Experimental determination of the effect of the ratio of B/Al on glass dissolution along the nepheline (NaAlSiO 4)–malinkoite (NaBSiO 4) 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 2O 3 (5–25 mol%) and B 2O 3 (20–0 mol%) with Na 2O (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 27Al and 29Si 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 27Al 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 27Al and 11B spectra, the 29Si 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 dissolut