Impacts of ionic strength on facilitated transport of Cd by kaolinite colloid in saturated sand column

Abstract

This study investigated the impacts of ionic strength (IS) on co-transport of Cd and kaolinite colloid through water-saturated sand column. Batch equilibrium sorption experiments were conducted to obtain the sorption isotherms of kaolinite clay and sand under different IS of CaCl2 (0.0 – 3.6 mM). The results displayed that Cd sorption by kaolinite clay and sand was nonlinear and fitted with Freundlich model. Column experiments were performed under different IS of CaCl2 (0.0 – 0.9mM) using 10 ppm of Cd without kaolinite colloid and 10 ppm Cd mixed with 100 ppm kaolinite colloid. According to column test, the results showed that the increase in IS reduced retardation factors of both Cd and Cd mixed with kaolinite colloid. In addition, the relative concentration (Ci/C0) of Cd-bearing kaolinite colloid in the column effluent was not higher than 0.25 under IS of 0.0 mM, and this relative concentration (Ci/C0) of Cd-bearing kaolinite colloid decreased as increasing in IS. Futhurmore, the amount of Cd-facilitated kaolinite colloid in the column effluent (Cd sorbed onto kaolinite colloid) decreased from 0.15 mg to 0.04 mg with increasing IS from 0.0 to 0.9 mM. The presence of kaolinite colloid increased the retardation factors of Cd from 3.46 to 3.71, 3.16 to 3.38 and 2.23 to 3.33 under IS of 0.225, 0.45 and 0.9 mM., respectively; on the other hand, kaolinite colloid decreased the retardation factor of Cd from 4.04 to 3.93 under 0.0 mM. Hence, kaolinite colloid can increase Cd mobility under extremely low IS condition; on the contrary, kaolinite colloid tends to decrease Cd mobility at high IS.