Dissolution of Common Phyllosilicates In Acid Sulfate Systems.

The development of sulfidic sediments resulting from rising ground water levels and their exposure due to natural and human activities presents a major threat to the soil and water systems in Australia and many other countries. The exposure and subsequent oxidation of sulfidic sediments results in the release of enormous amounts of acidity into the soil and water; the acidity (sulfate form) combined with high salinity that is common in inlands systems produces a unique biogeochemical environment. Chemical weathering of phyllosilicate minerals is the only process that neutralizes the acidity over long periods of time in such environments. The present study was conducted to evaluate the dissolution behaviour of phyllosilicates in saline-acidic systems.

Laboratory dissolution experiments were conducted using the clay fraction (< 2 µm) of common soil minerals: kaolinite, illite and montmorillonite in flow-through reactors at 25 °C. The original and residual mineral samples were characterized by X-ray diffraction, X-ray fluorescence spectroscopy, transmission electron microscopy and specific surface measurement. Two types of input solutions (I =0.25, 0.01) were prepared by using NaCl solution in the pH range 1–4; pH maintained using H2SO4. An output solution sample was collected after every 24 h and the concentrations of Al and Si, and solution pH were determined. The dissolution rate (Rj) of the minerals was calculated from the steady state concentration of Al and Si.

The dissolution experiments showed a rapid release of cations during first 100–200 hours of the experiments with a preferential Al over Si release in most of the experiments. Afterwards the cation concentrations decreased until steady state was attained. Stoichiometry of mineral dissolution was estimated using the elemental ratios. A stoichiometric dissolution of the minerals was observed at the higher ionic strength (I=0.25). Kaolinite dissolution was also stoichiometric at the lower ionic strength (I=0.01), however, illite and montmorillonite showed RSi > RAl at the lower ionic strength at pH 4 and pH 2–4, respectively. The pH dependence of dissolution rates was determined by the linear regression of plot of log RSi against pH; fractional reaction orders of 0.48 and 0.53 for kaolinite, 0.32 and 0.36 for illite and 0.22 and 0.26 for montmorillonite were obtained at the higher and lower ionic strengths, respectively. The results showed that montmorillonite dissolves at a faster rate than kaolinite and illite; and the ionic strength of the solution has a significant effect on the stoichiometry of montmorillonite and illite dissolution reactions.