Facilitated transport of cadmium with montmorillonite KSF colloids under different pH conditions in water-saturated sand columns: Experiment and transport modeling.

This study investigated the impact of pH on the migration of cadmium(II) ions (Cd2+) in relation to montmorillonite KSF colloids through a water-saturated sand column (WSSC). The sorption isotherms of Cd2+ on colloids and sand at pH values of 3, 6, and 8 were characterized by batch experiments. Cd2+ sorption by colloids and sand fit well with the Freundlich model. In the column experiments, increasing the pH increased the retardation factors and KF values of Cd2+ both with and without the presence of the colloids. The amount of Cd2+ sorbed onto the montmorillonite KSF colloids in the column effluent increased from 0.29 to 0.97 mg as the pH increased. The colloid increased Cd2+ mobility and acted as a carrier at a high solution pH. The increasing level of Cd2+ sorbed on colloids as the pH increased resulted in a long tailing of the breakthrough curve (BTC) of the total Cd, indicating that the total Cd was controlled by rate-limited reactions. These findings indicate that when the solution pH was greater than the point of zero charge (PZC) of the colloids (pH > 6), the system tended to follow a nonequilibrium two-site (TSM) model rather than an equilibrium (CDeq) model. This implies that the PZC of the colloids in the groundwater system is the main factor in predicting facilitated Cd2+ transport.