DRYING INDUCED ACIDITY AT THE MINERAL-WATER INTERFACE: ATR-FTIR SPECTROSCOPY

Drying induced pH changes were quantified on the surface of Na⁺, Ca²⁺, Mg²⁺ and Al³⁺ saturated smectite and kaolinite clays. This was achieved using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy to measure real time changes to a pH indicator, sorbed to the clay surface, during wetting and drying events. Using this technique it was possible to measure how low the pH of the surface drops during dehydration, the critical water content at which acidification of the surface begins and lastly how reversible the pH decrease is. The results show that only Al³⁺-smectite shows acidification below pH 4.8 with drying. The pH starts to decrease on the Al³⁺-smectite surface even when significantly hydrated (gravimetric water content ~ 125 mg/m²), and falls to between 1.2 and 1.4 when completely air dry. The drying induced pH decrease is completely reversible on rewetting, suggesting large pH oscillations may occur on smectite surfaces with appreciable exchangeable Al³⁺. Aluminium saturated kaolinite did not show significant acidification in response to drying (pH > 3.5), however, a 0.1M AlCl₃ solution evaporated to a final pH of 2.8. The enhanced acidification observed on an Al-smectite clay compared to a solution containing free Al³⁺ ions highlights the role of highly charged surfaces in the hydrolysis reaction that occurs within the hydration shell of exchangeable Al³⁺ ions.