Dissolution of compacted montmorillonite at hyperalkaline pH and 70°C: in situ VSI and ex situ AFM measurements
Abstract:In situ measurements were carried out to quantify montmorillonite dissolution rates at a compaction pressure ranging from 0.04 to 10.00 MPa and temperature of 70°C in 0.3M NaOH solution (pH 12.1 at 70°C) using vertical scanning interferometry (VSI) and an auto-compaction cell. Ex situ measurements of the reacted samples using atomic force microscopy (AFM) were performed to quantify the ratio of edge surface area (ESA) to total surface area (TSA) (X ESA = ESA/TSA). Accordingly, the actual ESA for the montmorillonite examined by in situ VSI could be estimated. The X ESA value increases as a function of run duration or compaction pressure. At atmospheric pressure, X ESA is approximately 0.0054 and converges to ∼0.0107 at 10 MPa, An expression that relates reactive surface area and montmorillonite compaction (X ESA/X ESA initial = kX ESA, k: variable factor) is kX ESA = 1.0 + 0.64628 P 0.1527 where P is in MPa. Using the calculated X ESA, dissolution rates from the in situ VSI measurements are obtained. The early dissolution (<1500 min) at less compaction pressure tends to show faster rates (>1.0 6 1011 mol/m2/s) than that at higher compaction pressure. The rates after >1500 min are slower, with values of less than 3 6 1012 mol/m2/s, but there is no significant dependency on the density in the range from 1.0 to 1.7 Mg/m3. These observed rates for compacted montmorillonite are two-orders of magnitude slower (2.63 6 1013 mol/m2/s) than dissolution rates in the suspended state.
Document Type: Research Article
Publication date: 2013-05-01