Mobilization and Deposition of Variably Charged Soil Colloids in Saturated Porous Media.

Understanding colloid mobilization and retention in subsurface is important for predicting colloid-facilitated transport of contaminants and developing remedial strategies. Colloid attachment and straining are considered as the key retention mechanisms of colloids in porous media.  However, the behavior and transport of colloids in varying physical and chemical conditions is yet to be fully understood. This study investigated transport behavior of water dispersible colloids (WDC) with different surface charges, extracted from volcanic ash soil (VAS) from Nishi-Tokyo, Japan and Red-yellow soil (RYS) from Okinawa, Japan. WDC solutions containing colloids with diameter <1µm was applied at water-saturated flow conditions through 10cm-column packed with 0.1-0.5mm Toyoura sand or 0.42-0.85mm (Narita sand) size fraction at different colloid concentrations, flow rates and pH conditions. The colloidal solutions were characterized from the measurement of turbidity, zeta potential, and particle size distribution. 0.001M NaBr was used as a conservative tracer and the pH was adjusted using 0.1M HCl. Mechanisms of colloid transport and retention were studied by measuring colloid effluent concentration, deposition profile, and particle size distribution.

The colloid characterization results for VAS-WDC showed significant change in colloidal stability and zeta potential with change in pH indicating pH dependent surface charge, while a less significant change was observed in case of RYS-WDC. The preliminary results of this study showed: a) Toyoura sand irreversibly retained more WDC (40%) compared to Narita sand (10%). b) Low flow rate (10 times higher residence time) caused more reversible entrapment of WDC compared to high flow rate condition. c) Additional effects of low solution pH in stronger colloid retainment, especially for VAS-WDC was observed. d) The solution concentration of WDC had minimum effects on transport and deposition. The experimental results also indicated that it is highly important to use a low energy method for the correct determination of deposited colloid profiles.