Model and experiments on soil remediation by electric fields

The dominant transport process for removing charged species from soils by electric fields is electromigration. In the case of heavy metals, the polarity and magnitude of the charge depends on the pH. Positive ions are generally stable at low pH and negatively charged complexes dominate at high pH. The transport is further complicated by the strong dependence of solubility on pH, with many heavy metals being virtually insoluble in moderately alkaline conditions. It was found experimentally that under certain conditions, strong pH gradients can develop in the soil trapping the metals by a process of isoelectric focusing. A numerical model of the transport and electrochemical processes was extended for the first time to incorporate complexation and precipitation reactions and was found to closely reproduce the experimental findings. The model demonstrates the role played by background ions and electroneutrality in governing the distribution of species, and how the concomitant variations in the electric field result in the virtual cessation of the transport process. The model confirmed that the focusing effect can be eliminated and high metal removal efficiencies achieved simply by washing the cathode.