Comparison of metal cations promotion on the flocculation-preloading-electroosmosis (FPE) of bentonite slurry dewatering

The mechanism by which metal cations affect bentonite dewatering via flocculation-preloading-electroosmosis (FPE) remains poorly understood. This study comprehensively examines the effects of cation valence and molar ratio on electroosmotic drainage and consolidation from both macroscale and microscale. Laboratory tests identified an optimal cation molar ratio for achieving maximum dewatering efficiency. The free swelling rate decreased with increasing cation valence and molar ratio. Al3+ yielded superior flocculation and settlement, whereas Na+ had minimal effect. At low molar ratios (e.g., 0.06 mol/kg Na+), the formation of pore channels and the effective stress were reduced, which delayed secondary consolidation under a 2 kPa preload yet allowed primary settlement to continue under 5 kPa. During electroosmosis, a higher molar ratio lowered the initial water content, thereby enhancing drainage efficiency and accelerating the stabilization of outflow. Moreover, Mg2+ and Al3+ reduced the water content through precipitation (e.g., Mg(OH)2), which improved the shear strength unless inhibited at suboptimal concentrations near the cathode. This work establishes a framework for optimizing cation-enhanced bentonite dewatering.