Electrokinetic remediation: The use of mercury speciation for feasibility studies applied to a contaminated soil from Almadén

We have chosen the BCR (Bureau Communautaire de Référence) sequential extraction procedure as a speciation analysis to determine the mobility of mercury in the contaminated soil of the Almadén (Spain) mining district. This soil has a high mercury concentration (1000 mg kg⁻¹) in some areas. In previous works, the relationship between the weak-acid soluble fraction and the amount of contaminant recoverable by acid-enhanced electrokinetic remediation (EKR) was shown. In this study, after testing that this relationship is maintained, we test if similar relationships with chelating agents could be established. Recently, several chelating agents were tried and iodide was shown to be quite efficient for the removal of mercury. Thus we have carried out iodide-enhanced EKR experiments at the lab (16 g of soil) and semi-pilot (2 kg) scales. From these experiments it can be concluded that the amount of mercury recoverable by this technique is similar to the one recovered in the batch extraction experiments. Thus, this is another evidence that this kind of experiments can be used for the feasibility studies of the remediation technique. It is also shown that, although the removal efficiency is similar to the one obtained previously for in situ flushing, the time required to achieve this efficiency would be several orders of magnitude shorter for this soil.Nevertheless, the residual Hg present in the weak-acid soluble fraction of the speciation analysis of the soil after the EKR treatment increases with respect to the original contaminated soil, indicating a possible increase of the risks associated to the contaminated site. Therefore, a second EKR treatment is applied to the same soil, this time using acid-enhanced EKR. It is shown that, although almost no Hg was recovered with this technique for the original soil, an important amount can be recovered after the first treatment (iodide-enhanced EKR). Also it is shown that the design and operation of this second technique should be studied carefully due to the rather complex chemistry of the species involved.