The variable charge of dioctahedral smectites.

Area of Science:

• Clay mineralogy
• Geochemistry
• Materials science

Background:

• Smectites possess cation exchange capacity (CEC) from permanent structural charge and pH-dependent variable charge.
• Variable charge, primarily from edge aluminol groups, is crucial but rarely quantified accurately.
• Understanding variable charge is key for applications involving smectite clays.

Purpose of the Study:

• To compare methods for measuring variable charge in dioctahedral smectites (e.g., montmorillonites).
• To assess the variability of smectite charge across different geological deposits.
• To identify factors contributing to differing variable charge values.

Main Methods:

• Calculation of variable charge for a model montmorillonite particle.
• Measurement using two copper-triethylenetetramine (Cu(trien)) based cation exchange capacity (CEC) methods.
• Potentiometric titration to determine variable charge.

Main Results:

• Calculated variable charge was ~10% of CEC for a model particle.
• CEC-based methods yielded 2-14% (pH 4-6) and 10-30% (pH 4-9) variable charge.
• Potentiometric titration indicated higher variable charge (15-35%).
• All methods showed comparable trends in distinguishing materials with different variable charges.

Conclusions:

• Variable charge significantly contributes to the overall CEC of smectites.
• Smectite chemical composition (e.g., Mg content) and surface characteristics influence variable charge.
• The tested methods are suitable for differentiating smectites based on their variable charge.