Acid-Base Properties of Aqueous Illite Surfaces

Area of Science:

• Geochemistry
• Surface Chemistry
• Environmental Science

Background:

• Understanding the acid-base properties of clay minerals like illite is crucial for predicting their behavior in natural water systems.
• The surface complexation model is a key tool for describing mineral-water interactions.
• Previous studies have explored the surface chemistry of various clay minerals, but specific details for illite require further investigation.

Purpose of the Study:

• To investigate the acid-base properties of illite/water suspensions using the constant capacitance surface complexation model.
• To determine appropriate surface protonation models for aqueous illite surfaces.
• To compare the effectiveness of one-site and two-site protonation models in describing experimental data.

Main Methods:

• Potentiometric titrations were performed on illite/water suspensions.
• Solubility experiments were conducted to complement titration data.
• The constant capacitance surface complexation model was applied to analyze the acid-base behavior.

Main Results:

• Proton reactions in illite suspension supernatants can be represented by reactions of Al(H2O)63+ and Si(OH)4.
• Two surface protonation models were proposed: a one-site model (pKa = 4.12-4.23) and a two-site model (pKaI = 4.17-4.44, pKaII = 6.35-7.74).
• Both models successfully described experimental data across varying illite concentrations and ionic strengths.

Conclusions:

• The one-site and two-site protonation models provide good descriptions of illite surface acid-base properties.
• The one-site model can be viewed as a simplification of the two-site model.
• Illite surface behavior, characterized by deprotonation reactions, is similar to that of amorphous silica (SiO2).