To evaluate the potential of Hg release and co-transport by colloids, it is important to understand how colloids and Hg-loaded colloids migrate in soils. We examined Hg sorption onto quartz sand and kaolinite colloids at 0.5-100 mg L^-1 Hg via batch experiments. The transport behaviors of Hg, kaolinite, and Hg-loaded kaolinite in water-saturated sand columns were evaluated via column experiments. Hg sorption by kaolinite and sand were nonlinear and fit the Langmuir model, with the maximum Hg sorption capacity being 1.2 mg g^-1 kaolinite and 0.11 mg g^-1 sand. Hg transport in sand media had higher retardation (R=1.14-1.15) and lower mass recovery (87-93%) than kaolinite transport (R=1 with 96% recovery). Preloading kaolinite with Hg (0.073 and 0.43 mg g^-1) reduced kaolinite’s mobility with 8.4 and 24% Hg-loaded kaolinite being retained in the column. Kaolinite at 250 mg L^-1 mobilized 14% and 0.95% Hg presorbed on the sand media at 1 and 100 mg L^-1 Hg. Correlation analysis revealed that desorbed Hg was significantly correlated with kaolinite (r= 0.811 and r= 0.807, 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).