Modeling of heavy metal migration in sand/bentonite and the leachate pH effect

Numerical simulations were carried out using a multicomponent reactive solute transport model to study the migration of four heavy metals (Cd2+, Pb2+, Cu2+ and Zn2+) in a sand/bentonite mixture. The numerical model is capable of simulating simultaneous processes of water flow, advective–dispersive solute transport, and chemical reactions such as acid–base reaction, complexation, ion exchange, precipitation and dissolution. The migration of the heavy metals was simulated in a hypothetical sand/bentonite clay column which was assigned realistic physical and chemical properties and boundary conditions to model one-dimensional contaminant transport through a landfill barrier. The input leachate properties used in the model were modified from those of an actual leachate containing the heavy metals. The numerical simulations were focused on the concentration profiles of these heavy metals in the simulated barrier and the effect of the leachate pH on their mobilities. Numerical results indicate that under a nearly neutral leachate, the mobility of the four heavy metals follows the order Cu2+>Zn2+>Pb2+>Cd2+. Under a more acidic leachate, the mobility order changes to: Cd2+>Zn2+>Cu2+>Pb2+. However, the differences in mobility of the four heavy metals were relatively small. The leachate pH has a significant effect on the migration of Cd2+ and Pb2+, but has only limited effect on the mobility of Cu2+ and Zn2+.