Subsurface migration of copper and zinc mediated by soil colloids

Colloid migration in subsurface environments has attracted special attention lately because of its suspected role in facilitating transport of contaminants to groundwater. This study investigated the potential role of water-dispersible soil colloids (WDCs) with variable composition in transporting Cu and Zn through undisturbed soil columns. Copper or Zn solutions without colloids (controls) and combined with suspensions of montmorillonitic, mixed, illitic, and kaolinitic colloids with a range of surface properties were applied at a constant flux into undisturbed soil columns. The soil columns represented upper solum horizons of Maury (fine, mixed, mesic Typic Paleudalf) and Loradale (fine-silty, mixed, mesic Typic Argiudoll) soils with contrasting porosities and organic C (OC) contents. Colloid and metal recoveries in the eluent varied with metal, colloid, and soil properties. The presence of colloids typically enhanced metal transport by 5- to 50-fold over that of the control treatments, with Zn being consistently more mobile than Cu. The greatest metal transport potential was shown by colloids with high negative surface charge and OC content and the lowest by colloids with large particle size, low negative surface charge, and high Fe- and Al-hydroxyoxide contents. Although the dominant transport mechanism was metal sorption by colloids and cotransport, the additional soluble Cu and Zn transported in the presence of colloids suggests involvement of physical exclusion, competitive sorption, or increased metal solubilization processes. Increased amounts of OC content in the soil column appeared to overshadow the effects of macroporosity on the transport of both metals, especially Cu. These findings have important ramifications on the use of contaminant transport prediction models and the application of efficient remediation technologies.