Publication

Groundwater suspensions consist of colloidal particles of nonuniform size, yet most published studies of colloid straining in porous media have been conducted with monodisperse colloidal suspensions. In this technical note, we report the results of column experiments on the straining of monodisperse suspensions (3.1 or 5.1 mm microspheres) and a bidisperse suspension (containing 3.1 and 5.1 mm microspheres) within water-saturated quartz sand. The experiments are similar to those described by Xu et al. (2006) in which chemical conditions were prescribed to minimize physicochemical deposition and straining represented the dominant mechanism of colloid immobilization. We find that the straining rates of the monodisperse suspensions can be quantified with published, empirically based expressions that use the ratio of colloid diameter to sand grain diameter as the predictor variable. The straining kinetics of the bidisperse suspension cannot be quantified with these empirical expressions when the mass-averaged microsphere size is used for the colloid diameter. Analysis of the data from the experiments with the bidisperse suspension suggests that retention of the 5.1 mm microspheres constricted pore spaces with dimensions between 3.1 and 5.1 mm, which enhanced the straining of the 3.1 mm microspheres. A model that accounts for this effect and that uses one fitting parameter in coordination with parameters estimated from column experiments with the monodisperse suspensions describes the observations made in the bidisperse experiment with good success.