Investigation of co-migration of heavy metal with colloid under preferential flow

Most of the heavy metal pollutants in soil come from external industrial and agricultural activities. Surface soil layer is the only access through which the heavy metal can migrate downward. Because of drying shrinkage and animal or plant activities, there are macropores in surface soil, which form preferential flow channels. In some cases, these channels are the only way for the heavy metal to pass through the vadose zone. Colloid has great adsorption capacity of heavy metal and can be transported under preferential flow, so the colloid may accelerate the migration of the heavy metal. The adsorption characteristics of heavy metal cadmium and lead in brown-yellow silty clay in Shanghai and soil colloid are determined by the isothermal adsorption tests. Leaching tests are carried out using the soil columns with repeatable preferential flow characteristics to investigate the colloid-heavy metal co-migration under preferential flow condition. The results show that the soil colloid has higher adsorption capacity of heavy metal than the silty clay. When there is colloid, the outflow of cadmium is 1.49 times higher than that without colloid, and the outflow of lead is 33.88 times higher than that without colloid. By the effect of colloid, the concentrations of heavy metal adsorbed in silty clay and kept in pore solution are both lower than those without colloid, so more heavy metal migrates downward. The migration of heavy metal is obviously accelerated by the colloid under preferential flow condition. Although the source concentration and leaching time of lead are both higher than cadmium, the outflow of lead is only about 1/13 of cadmium. The migration of lead in soil is weak due to high adsorption.