Electrochemically enhanced oxidation reactions in sandy soil polluted with mercury

For remediation of soils contaminated with heavy metals, the electrodialytic remediation (EDR) method is a highly relevant method, see e.g. Hansen et al. (Hansen HK, Ottosen LM, Kliem BK, Villumsen A. Electrodialytic remediation of soils polluted with Cu, Cr, Hg, Pb, and Zn. J Chem Tech Biotechnol 1997;70:67-73). During the process the heavy metals are transferred to the pore water in dissolved form or attached to colloids and move within the applied electric field. The method is found to be useful in many soil types, but has its strength in fine-grained soils. It is exactly in such soils that other remediation methods fail. Four cell experiments were made in order to investigate how relevant the method is for a more sandy soil and if it is suitable for non-ionic heavy metals such as elemental mercury. The duration was 27 days for two of the experiments and two experiments lasted 54 days, and the mercury within the soil was initially 1200-1900 mg kg(-1), of which 84% was elemental Hg. To monitor the process the pseudo-total mercury concentration was distinguished between elemental mercury and non-metallic mercury species by thermodesorption. During the electrodialytic treatment an increase of the content of non-metallic mercury occurred and a corresponding decrease of the content of elemental mercury which indicates a transformation of the latter species into any other non-metallic species. Generally, oxidation of Hg by dissolved oxygen in a solution is kinetically inhibited and thus quite slow. The redistribution of Hg was closely connected to a decrease of soil pH during the experiments. This corresponds very well to the thermodynamic calculations from which it was found that a decrease in the pH of the soil will result in an increase in the oxidation rate of elemental Hg. Results from this investigation show that the electrodialytic remediation method alone is not efficient in situations with sandy soils containing elemental mercury. As a solution for this problem it is suggested to add chloride to the soil system. Chloride would act as a complexing agent avoiding precipitation and enhancing the dissolution of precipitates as well as elemental mercury.