Kaolinite and Cd2+ Transport and Interaction in Sand Media: Batch and Column Experiments

Batch sorption and column transport experiments were used to investigate kaolinite colloids and Cd(II) ions (Cd2+) co-transport in water-saturated sand media. Batch sorption was used to evaluate Cd2+ sorption onto kaolinite clay as a function of the initial Cd2+ concentrations and solution ionic strengths as well as determining the equilibrium sorption isotherms. For very low ionic strength (~0 mM), the Langmuir isotherm was the more suitable isotherm for the experimental results while at higher ionic strength (> 0.45 mM) the Freundlich isotherm was the more suitable isotherm. The sorption parameters, KF, KL, n, and qm, were found to decrease with increasing ionic strength. For column transport experiments, Cd2+-bearing kaolinite colloids were found to be retained more in the column for an increase in the solution ionic strength. For example, the steady state breakthrough rate of Cd2+-bearing kaolinite colloids was 0.2 Ci/C0 for an ionic strength of 0.0 mM compared to about 0.05 Ci/C0 for ionic strengths of 0.45 and 0.9 mM. This increased Cd2+-bearing kaolinite colloid retention trend with increased solution ionic strength was likely due to the reduced zeta-potential and increased size of the colloid particles at higher ionic strengths. Consequently, the kaolinite-facilitated Cd2+ concentration decreased with increasing solution ionic strength. In summary, solution ionic strength had a strong effect on the transport behavior of kaolinite-facilitated Cd2+ and Cd2+-bearing kaolinite colloids through the sand column