Effect of Bentonite Colloid on Cr(VI) Transport in Heterogeneous Porous Media

Co-transport of Cr(VI) with bentonite colloids (Bc) at different flow rate, pH, ion strength (IS), and colloid concentration in heterogeneous sand media was investigated. The results showed that obvious penetration of dissolved Cr(VI) in co-transport occurred at about 0.5PV, which was mainly due to the heterogeneity of the sand column. The maximum dimensionless transport fraction ((Ct/C0)_max) of dissolved Cr(VI) was 0.73 in the co-transport (200 mg·L^-1 Bc and 2.5 ml min^-1), much lower than that in the single transport of Cr(VI) (0.98). The increase of pH promoted the transport of Cr(VI) in co-transport with Bc. Elevated flow rate, ion strength, and Bc concentration reduced Cr(VI) transport. Cr(VI) in the effluent in co-transport with Bc was dominated by dissolved Cr(VI), and a small amount of colloidal Cr(VI) (～4.97%) was observed. The recovery rates of total transported Cr(VI) (dissolved +colloidal) in co-transport with Bc were obviously lower than those in single transport. (Ct/C0)_max values of dissolved Cr(VI) in the co-transport with Bc were also lower than those with other colloids, e.g. kaolin colloid, montmorillonite colloid, and inorganic soil colloid. Application of bentonite may alleviate the transport risk of Cr(VI) to groundwater.