Colloid Transport in Unsaturated Sand: Effect of Charging Characteristics of Colloidal Particle and Soil Matrix.

To predict and control the transport of contaminants in subsurface condition, it is necessary to understand the fate of colloidal particles like clay minerals or microorganisms adsorbing contaminants. Many column experiments performed so far demonstrated that the solution chemistry significantly affects colloid transport in soil. However, we have little information about colloid transport in unsaturated porous media especially for the system with well-characterized colloidal particles and soil matrix. In this study, we conducted column experiments on the transport of colloidal silica particles in unsaturated Toyoura sand. Both silica and Toyoura sand have pH-dependent negative charge due to the deprotonation of silanol groups, and thus the magnitude of surface charge can be controlled by changing solution pH. Water content of sand was adjusted by controlling suction and applied flux of colloidal suspension gradually.

Our experimental results showed that, transport behavior was affected by solution pH, and thus by surface charge property. At higher pH, where repulsive force due to high charge amount of Toyoura sand and silica particles develops, deposition of colloidal particles to solid matrix is inhibited. As a result, silica particles instantly flow out from the column. In addition, deposition amount is slightly increased with decreasing saturation probably due to the increase of air-water interface. On the other hand, at low pH, where sand and colloid have less charge, reduced repulsive force leads colloidal silica to attach to the sand. Thus, the effluent concentration of silica slowly increases at early stages. And deposition amount of silica is effectively increased by lowering water saturation. Our finding is that dependence of colloid transport behavior on water contents is also significantly affected by charging properties of colloidal silica and Toyoura sand.