Transport and Deposition of Variably Charged Soil Colloids in Saturated Porous Media

Abstract

A series of column experiments was conducted to investigate the transport and deposition of variably charged colloids in saturated porous media. Soil colloids with diameters <1 μm were extracted from a volcanic‐ash soil from Nishi‐Tokyo (referred to here as VAS colloids) and a red‐yellow soil from Okinawa (RYS colloids) in Japan. The VAS colloids exhibited a negative surface charge with a high pH dependency, whereas the RYS colloids exhibited a negative surface charge with less pH dependency. The soil colloids were applied as colloidal suspensions to 10‐cm‐long saturated sand columns packed with either Narita sand (mean diameter = 0.64 mm) or Toyoura sand (mean diameter = 0.21 mm) at different flow rates, suspension concentrations, and pH conditions. Both sands exhibited a negative surface charge in the measured pH range, but the pH dependency was more pronounced for the Toyoura sand. Breakthrough curves and deposition profiles for soil colloids were strong functions of the hydrodynamics, solution pH, and surface charge of the colloids and sand grains. Greater deposition was typical for lower flow rates and lower pH. The deposition of VAS colloids in both sands under low‐pH conditions exhibited depth‐dependent retention, with high colloid deposition at the column inlet. The RYS colloids were mainly deposited uniformly along columns, and depth‐dependent retention was observed only in Toyoura sand at low pH. Results suggest that decreasing solution pH enhanced surface charge neutralization of both soil colloids and sand grains, thereby increasing colloid deposition. This study emphasizes that the pH‐dependent surface charge of both mobile colloids and receiving porous media needs more consideration in models for colloid and colloid‐facilitated transport in soil.