Electrodialytic remediation of heavy metal polluted soil

Abstract

This article presents the historical background, the principle and case studies of electrodialytic remediation of heavy metal polluted soil. Remediation of fine grained soils polluted by heavy metals is problematic for most traditional methods such as pump and treat technologies and soil washing methods. On the other hand, electrochemical soil remediation methods are particularly suited for the fine grained soils. These methods are based on an applied electric current as cleaning agent. The current tends to pass the soil through the finest pores, i.e. next for the smallest soil particles, where the heavy metals are mainly adsorbed. The heavy metals are mobile in the electric field as ions in solution, only. This means that the heavy metals must be desorbed during the process. For some soils it is necessary to add an enhancement solution to aid this desorption, whereas for other soils addition of water is sufficient to the remediation to occur. Electrodialytic soil remediation was applied at laboratory scale for different soil and pollution combination. Soils with a low carbonate content and polluted with Cu, Pb and Zn are shown remediated by the electrodialytic method without use of enhancement solution to improve the mobility of the metal ions. It was shown possible to remove 97% Cu in one experiment and 87% Pb and 69% Zn in another experiment. During the electrodialytic process the soil becomes acidified, thus desorbing these metal ions. Further a carbonate soil polluted by Cu can be remediated using ammonia as enhancement solution. Indeed, Cu forms mobile complexes with ammonia. Thus the soil can be remediated at high pH, at which the carbonates are not dissolved. It was shown that addition of ammonia improved the remediation from nothing removed from the soil totally without ammonia to 73% Cu removed after addition of ammonia. In the case of wood preservation sites where the soil is polluted by Cu and As, ammonia can also be used as enhancement solution, since As is mobile in the alkaline environment. In an experiment 70% As and 53% Cu was removed. Nonetheless, for wood preservation sites polluted with Cu, Cr and As ammonia is not sufficient. Here ammonium citrate showed potential, since the citrate part is mobilizing Cr. Experimentally 33% Cr, 65% Cu and 66% As was removed from the soil at the same time after addition of ammonia citrate. Thus, by carefully choosing the enhancement solution for each case, electrodialytic remediation is possible, even of soils with a fine fraction ranging from 33% to 63% per weight as the experimental soils here. © 2005 Springer-Verlag Berlin Heidelberg.