Dependence of Zeta Potential and Soil Hydraulic Conductivity on Adsorbed Cation and Aqueous Phase Properties

Abstract

This paper deals with the effects of pH, electrolyte concentration, and exchangeable Na percentage (ESP) on electrophoretic mobility (EM)/zeta potential (ζp) of clay particles and hydraulic conductivity (HC) of the clay/sand mixtures. The soils taken from Japan and Kazakhstan were used for obtaining clay fractions. For EM determinations, clay suspensions were prepared at a concentration of 4 g of clay per 100 mL of distilled water (DW) or electrolyte solution. The electrophoretic mobilities were measured using Burton apparatus with water-cooling system and converted into ζp. For HC measurements, clay/sand mixtures were designated 16:84. Columns of these mixtures were prepared by packing 120 g of mixtures into 50-mm diameter plastic cylinders to a bulk density of about 1.4 g cm−3 Results showed clearly that the mobility was very sensitive to the ion valence adsorbed on the clay. The negative electrophoretic mobilities of homoionic Na– Ariake soil (AS) clay and Na–Kzyl-Orda soil (KS) clay were 2.13 × 10−8 and 2.14 × 10−8 m2 s−1 V−1, respectively, whereas Ca clays flocculated. The ζp values of AS clay and KS clay, as a function of the ESP, varied between −12.83 and −26.84 mV, and −5.68 and −27.00 mV, respectively, at ESP > 30. Although the smectitic AS clay was less sensitive than the micaceous KS clay to pH changes during electrophoresis experiment, its HC also was affected by pH changes. Decreased pH from 7 to 5 could easily result in two to three times high HC values for both clay/sand mixtures. The EM of both soil clays was similar at pH 10 to 12, and exchangeable Na percentages 90 to 100. Sharp increases in EM and decreases in HC of AS clay were observed at exchangeable Na percentages 50 and 60, respectively. Similar trends related to EM were also obtained for the KS clay. However, salt concentration of the suspension solution did not have consistent effect on the EM values. This behavior of the clays was consistent with HC observation. The results indicated that HC of the clay/sand mixtures could be correlated to ζp. The saturated HC of the mixtures was found to change as an exponential function of the ζp of clay particles.