title: Transport of colloidal silica in unsaturated sand: Effect of charging properties of sand and silica particles.
abstract: We have studied the transport of colloidal silica in various degrees of a water-saturated Toyoura sand column, because silica particles are widely used as catalyst carriers and abrasive agents, and their toxicity is reported recently. Since water-silica, water-sand, and air-water interfaces have pH-dependent negative charges, the magnitude of surface charge was controlled by changing the solution pH. The results show that, at high pH conditions (pH 7.4), the deposition of colloidal silica to the sand surface is interrupted and the silica concentration at the column outlet immediately reaches the input concentration in saturated conditions. In addition, the relative concentration of silica at the column outlet only slightly decreases to 0.9 with decreasing degrees of water saturation to 38%, because silica particles are trapped in straining regions in the soil pore and air-water interface. On the other hand, at pH 5 conditions (low pH), where sand and colloid have less charge, reduced repulsive forces result in colloidal silica attaching onto the sand in saturated conditions. The deposition amount of silica particles remarkably increases with decreasing degrees of water saturation to 37%, which is explained by more particles being retained in the sand column associated with the air-water interface. In conclusion, at higher pH, the mobility of silica particles is high, and the air-water interface is inactive for the deposition of silica. On the other hand, at low pH, the deposition amount increases with decreasing water saturation, and the particle transport is inhibited.
authors.affiliation: Graduate School of Life & Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan.
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Fujita
authors.affiliation: Graduate School of Life & Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan. Electronic address: kobayashi.moto.fp@u.tsukuba.ac.jp.
authors.forename: Motoyoshi
authors.initials: M
authors.lastname: Kobayashi
mesh_terms: D003102:Colloids / Q000737:chemistry*; D025802:Electric Capacitance; D006863:Hydrogen-Ion Concentration; D016062:Porosity; D012822:Silicon Dioxide / Q000737:chemistry*; D012987:Soil / Q000737:chemistry*; D014867:Water / Q000032:analysis; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D003102:Colloids; D012987:Soil; D014874:Water Pollutants, Chemical; D014867:Water; D012822:Silicon Dioxide
keywords: Colloid transport; Solution pH; Water contents