优先流作用下的胶体–重金属共迁移试验研究

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

土壤中的重金属污染物大部分源自外界的工、农业排放,地表土层是重金属迁移至地下的必经途径。由于干缩和动植物活动,地表土中存在大孔隙,形成污染物运移的优势通道,在一些情况下,这些通道是重金属穿过包气带向下迁移的唯一途径。针对胶体对重金属具有较大吸附能力且在优先流条件下具有移动性,从而可能加快重金属污染物迁移这一问题,通过等温吸附试验确定了上海表层褐黄色粉质黏土与土壤胶体对重金属镉、铅的吸附特征,制备具有可重复性优先流特征的试验土柱,进行优先流条件下胶体–重金属共迁移土柱淋滤试验,分析得到胶体–重金属共迁移规律。试验结果表明,胶体对重金属的吸附能力明显高于粉质黏土。有胶体情况下,镉出流量是无胶体情况下镉出流量的1.49倍,铅的出流量是无胶体情况下的33.88倍,有胶体情况下土上吸附的重金属和孔隙溶液中的重金属浓度都低于无胶体的情况,更多的重金属随胶体向下迁移,优先流条件下胶体明显加快了重金属的迁移。虽然铅的源浓度和淋滤时间都大于镉,但铅的出流量仅约为镉的1/13,由于易发生吸附,铅在土中的迁移能力较弱。

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.