Ionic Migration and Spatiotemporal Distribution Characteristics of Salinity in Clay Using Electro-osmosis Based on Conductivity

[Objective] This study investigates how soluble salts in clay redistribute under an applied direct-current electric field during electro-osmotic drainage and how the redistribution affects dewatering and consolidation efficiency. The study quantifies the spatiotemporal evolution of salt content through bulk electrical conductivity, distinguishes the individual effects of salinity and water content on conductivity, and infers ion-migration trends and their implications for the combined dewatering and desalination performance of saline clays. [Methods] Laboratory conductivity calibrations were conducted on remolded clay across practical ranges of water content and soluble-salt concentration. Based on these data, an empirical relationship was established that linked soil bulk conductivity to pore-fluid salinity while explicitly incorporating water content, enabling the conversion of measured conductivities into estimates of salt content. Subsequently, a one-dimensional electro-osmotic consolidation test was conducted. Segmented voltage, local conductivity, cumulative drainage, and current were monitored and recorded. Using this calibration, time-lapse conductivity profiles were processed to reconstruct salt-content distributions and their evolution. This method could provide a framework to monitor and interpret coupled ionic transport and water removal during electro-osmosis. [Results] Calibration showed that conductivity increased with both salinity and water content. However, when water content was considered, salinity accounted for a larger share of the variance in bulk conductivity. Accordingly, conductivity served as a reliable in-situ indicator of salt content during electro-osmosis. The electro-osmotic test revealed a distinct zonation of salt content consistent with electromigration toward the cathode. At the anode, salt content declined rapidly during the first 2 hours and then stabilized at approximately 2.0 g/L until the end of the test. In the mid-section, salt content also decreased over the first 2 hours, showing the smallest reduction among the three regions, followed by an increase and subsequent decline. By 6 hours, it temporarily exceeded the initial salinity. This peak reflected the convergence in the middle zone of cation fluxes migrating from anode to cathode and anion fluxes moving in the opposite direction. After 6 hours, the mid-section salinity decreased progressively and, at the end of the test, fell below that of the anode region. The cathode experienced the most pronounced change, showing a continuous decline throughout energization. By 8 hours, the cathodic zone had nearly approached a salt-free state. During electro-osmosis, the soil potential field was strongly modulated by both water content and salinity, producing spatially differentiated potential distributions that evolved over time. Water content and drainage rate exhibited non-uniform dynamics among regions and ultimately formed a moisture gradient of Anode [Conclusion] This study delineates the migration and distribution patterns of soluble salts in high-salinity clays under electro-osmotic drainage, offering a new perspective for treatment and practical guidance for engineering application. Operationally, a critical point is reached when salinity in the cathodic zone drops to a very low level. Continuing energization beyond this point leads to sharply diminished drainage efficiency and disproportionately increased energy consumption. At the design stage, measuring soil electrical conductivity and conducting pre-tests to characterize the salinity-moisture relationship are recommended, thereby informing the required energization time. In practice, continuous conductivity monitoring provides a comprehensive indicator of overall dewatering progress. Wider adoption of these insights is expected to facilitate broader and more effective application of electro-osmosis in geotechnical engineering.