Alteration kinetics of basaltic glass in acidic conditions: The effect of temperature and pH on glass dissolution and alteration layer formation

Le résumé fourni par la source

Quantifying basaltic glass alteration kinetics is essential for modelling carbon sequestration and global biogeochemical cycles. However, current literature rates diverge by up to two orders of magnitude due to surface area normalization uncertainties (BET vs. geometric) and conflicting reports on the pH-dependence of activation energies ( E a ). In this study, we resolve these inconsistencies by combining Vertical Scanning Interferometry (VSI) and in situ synchrotron X-ray Reflectivity (XRR) to decouple glass network dissolution from Amorphous Silica-rich Surface Layer (ASSL) growth between 5 and 60 °C and pH 2–5. Our results demonstrate that both dissolution and ASSL growth proceed linearly over time, following a surface-reaction-controlled regime. We show that the surface retreat method (VSI) provides a robust, normalization-independent proxy for network breakdown, yielding rates consistent with Si release. Furthermore, XRR data reveal that steady-state ASSL densities decrease with increasing growth rates, suggesting the formation of more porous structures at low pH. Contrary to common assumptions in long-term field rates, we find no evidence of surface passivation: ASSLs remain permeable and do not act as diffusion barriers over weeks-long experiments, accounting for up to 80% of the total altered volume. The temperature dependence of the dissolution rate ( r ) is best described by a pH-dependent Arrhenius law: r = A pH . exp − E a , pH RT a H + n where A pH is the pH-dependent preexponential factor, R is the gas constant, a H+ is the activity of protons, n the reaction order with respect to protons, which is close to 1 on the investigated temperature range, and where the apparent E a decreases from 62.9 ± 2.0 kJ/mol at pH 2 to values near zero (< 20 kJ/mol) as the pH approaches the point of zero charge of the glass. By providing quantitatively decoupled kinetics for substrate dissolution and layer formation, this work offers a high-resolution framework to refine predictive models of basalt weathering in both natural and engineered acidic environments.

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