Transport and interactions of kaolinite and mercury in saturated sand media.

To evaluate the potential of Hg release and co-transport by colloids, it is important to understand how Hg, colloids and Hg-loaded colloids migrate in soils. Hg sorption by kaolinite and sand were nonlinear and fit the Langmuir model, with the maximum Hg sorption capacity being 1.2mg/g kaolinite and 0.11 mg/g sand. Co-transport of Hg and kaolinite was evaluated using: (1) 1 or 100 mg/L Hg or 100mg/L kaolinite, (2) 1 or 100 mg/L Hg mixed with 100mg/L kaolinite, (3) 1 or 100 mg/L Hg presorbed onto kaolinite, and (4) 25 0mg/L kaolinite in Hg-loaded sand columns. The presence of kaolinite (100 mg/L) reduced Hg's mobility through sand column by increasing deposition rate of Hg-loaded kaolinite. At 100 mg/L Hg, soluble Hg dominated Hg transport; but at 1 mg/L Hg, colloidal Hg (Hg sorbed on kaolinite) affected Hg transport. Preloading 100 mg/L Hg onto kaolinite (0.43 mg/g) reduced kaolinite's mobility with low recovery rate (78%), with Hg retardation (R=1) in Hg-loaded kaolinite being lower than Hg retardation at 100 mg/L Hg (R=1.287). The Hg recovery rate (93%) from Hg-loaded kaolinite at 1mg/L Hg was higher compared to 22% from 1 mg/L Hg. Kaolinite can serve as a carrier to enhance Hg transport in porous media, with 250 mg/L kaolinite mobilizing ~2.4% Hg presorbed onto sand media. Correlation analysis revealed that desorbed Hg was significantly correlated with kaolinite (r=0.81, P<0.0001). Hence kaolinite enhanced Hg transport in the sand media serving both as a carrier (Hg was loaded before transport) and as mobile colloids stripping Hg off the sand media (Hg was loaded during transport).