Transport and re-entrainment of soil colloids in saturated packed column: effects of pH and ionic strength

Purpose Colloid migration in subsurface environments has attracted considerable attention in recent years because of its suspected role in facilitating transport of strongly adsorbed contaminants to groundwater. The influence of bulk solution pH or ionic strength on model colloid (i.e., latex microsphere, amorphous silica colloids) transport is well established, while little attention has been paid to water-dispersible soil colloids. In this study, saturated packed columns were conducted to explore the mechanism of transport and fate of water-dispersible soil colloids and facilitating transport of Cu during transients in solution chemistry. Materials and methods Water-dispersible soil colloids were fractionated from a Cu-contaminated soil sample. Transport of soil colloidal suspensions was conducted with varying pH and ionic strengths, and then, re-entrainment of those retained colloids after completion the transport experiments was conducted by changing pore water solution transient ionic strength and pH conditions. Meanwhile, transport and fate of the Cu strongly adsorbed on the soil colloids were determined under different ionic strength conditions. Results and discussion The transport behavior of soil colloids in porous media was found to depend on the pH and ionic strengths of bulk solution. An increase in solution ionic strength and decrease in solution pH resulted in greater deposition which was revealed by the collision efficiency ( α ). It increased from 0.15 to 1.0 when solution composition changed from 0 to 50 mM NaNO 3 and decreased dramatically from 1.0 to 0.035 as the solution pH converted from 2.97 to 8.94. The results were in agreement with Derjaguin–Landau–Verwey–Overbeek theory. Upon stepwise reduction in ionic strength of eluting fluid or enhancement in its pH, a sharp release of colloids retained in the column occurred in each step. Meanwhile, the value of FRE NaOH that reveals the effect of NaOH solution at pH 11 on the mobilization of retained colloids deposited in the primary minimum increased from 38.6% to 64.6% when the ionic strength of bulk solution changed from 0 to 50 mM NaNO 3 and decreased from 86.7% to 35.8% as the solution pH from 2.97 to 8.94. In addition, the transport and fate of the Cu strongly adsorbed on soil colloids were highly consistent with the results of soil colloids. Conclusions The colloid collision efficiency ( α ) decreased as the pH of bulk solution increased and increased as the ionic strength of bulk sol