Effects of kaolinite colloids on Cd2+ transport through saturated sand under varying ionic strength conditions: Column experiments and modeling approaches

Column experiments were performed under various ionic strengths (0.0-0.9 mM) using 10 mg L-1 of Cd2+ without kaolinite colloids and 10 mg L-1 Cd2+ mixed with 100 mg L-1 kaolinite colloids. The nonequilibrium two-site model (TSM) described the behavior of both Cd2+ transport and Cd2+ co-transported with kaolinite colloids better than the equilibrium model (CDeq) (R-2 = 0.978-0.996). The results showed that an increase in ionic strength negatively impacted the retardation factors (R) of both Cd2+ and Cd2+ mixed with kaolinite colloids. The presence of kaolinite colloids increased the retardation factors of Cd2+ from 7.23 to 7.89, 6.76 to 6.61 and 3.79 to 6.99 for ionic strengths of 0225, 0.45 and 0.9 mM, respectively. On the other hand, the presence of kaolinite colloids decreased the retardation factor of Cd2+ from 8.13 to 7.83 for ionic strength of 0.0 mM. The fraction of instantaneous sorption sites (f) parameters, kinetic constant for sorption sites (alpha) and Freundlich constant (K-f) were estimated from HYDRUS-1D of TSM for Cd2+ transport. The fraction of instantaneous sorption sites was found to increase for an increase in ionic strength. K-f values of Cd2+ transport without kaolinite colloids for 0.0, 0225 and 0.45 mM were found to be higher than those of Cd2+ transport with kaolinite colloids, except for ionic strength of 0.9 mM. Hence, the presence of kaolinite colloids probably retarded the mobility of Cd2+ in porous media for higher ionic strengths. Furthermore, retardation factors and K1 values of both Cd2+ transport and Cd2+ co-transport were shown to decrease when ionic strength increased. Interestingly, according to TSM, the fraction of instantaneous sorption sites tends to increase for an increase in ionic strength, which imply that the mechanism of Cd2+ sorption onto quartz sand can be better described using equilibrium sorption rather than nonequilibrium sorption for an increase in ionic strength. (C) 2015 Elsevier B.V. All rights reserved.