Electrokinetic ion transport through unsaturated soil: 1. Theory, model development, and testing

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• Understanding ion transport in unsaturated soils is crucial for environmental management and contaminant remediation.
• Electromigration is a significant process influencing solute movement in soils, particularly under electrical gradients.
• Existing models often simplify soil conditions, necessitating more comprehensive approaches for heterogeneous unsaturated environments.

Purpose of the Study:

• To develop and validate an electromigration transport model for non-reactive ions in unsaturated soils.
• To investigate the influence of moisture content on ionic mobility and transport velocity.
• To provide a predictive tool for ion movement in complex soil systems.

Main Methods:

• Developed a time-independent electromigration transport model incorporating ionic mobility, electrolyte concentration, and tortuosity.
• Estimated ionic mobility using chemical activity coefficients and calculated tortuosity from electrical conductivity data.
• Validated the model against laboratory experiments using red dye No. 40 migration in unsaturated soil under a constant current.

Main Results:

• The electromigration transport model demonstrated agreement with laboratory-measured ion migration velocities.
• Model predictions and experimental results showed a peak transport velocity at moisture contents below saturation.
• Observed competing effects of current density and tortuosity influencing transport velocity with decreasing moisture content.

Conclusions:

• The developed electromigration model effectively predicts non-reactive ion transport in heterogeneous unsaturated soils.
• Moisture content significantly impacts ion transport dynamics, with optimal conditions for electromigration occurring below full saturation.
• The model provides valuable insights for managing solute transport in environmental and geotechnical applications.