Application of the electrokinetic technique, for the reclamation of an industrial area contaminated by elemental Hg: an in-situ innovative approach

The possibility of application of electrochemistry in the removal of organic and inorganic contaminants from soils and sludges has been widely investigated during the last decade, after the pioneering work of Lageman and Pool (e.g. [1] and literature therein). The wealth of the published results has clearly demonstrated that movement of ionic species by electro-migration can result in an effective removal of contaminants of different nature from different kinds of dispersing media. However, the large majority of experiments has been carried out under laboratory conditions, with the scope of identifying correlations between electric variables, nature of the contaminant and of the dispersing medium on the one side and rate of removal on the other. The extension of experiments to real cases involves, in turn, a number of additional components, like non-homogeneity, chemical and morphological, in real soils, and the difficulty to control, on a larger scale, water balance across the treated soil and the physicochemical properties of the aquatic medium at electrodes, as well as in the soil porosity. The present work represents an attempt of pollutant removal from an area of roughly 50 square meters situated within a site where a chloro-alkali plant was originally established, and then recently removed, in one of the most important petrochemical sites in Italy. The target pollutant was Mercury, mainly present as a metal. The area to be reclaimed was covered by concrete paving slabs, below which an unsaturated soil thickness of less than one meter was affected by Hg pollution. The request of carrying out the reclamation process, maintaining the site structure made an electrokinetic approach particularly suitable. On the other hand, the presence of Hg as a metal, made any non-electrochemical decontamination attempt practically inapplicable in the described site morphology. The electrode array consisted of four rows of cathodes intercalated with three rows of anodes. The distance between cathode and anode rows was 1.25 m, as well as the intercathodic and interanodic distances. The electrodes were located in wells, 1 m deep and with a 0.2 m diameter, suitably constructed to control soil humidity and electrokinetic flow. In order to mobilize elemental Hg, it had to be transformed into an ionic complex. This was accomplished by adding 1 g/l of NaI to the catholyte. Under the influence of the electrical DC field, the Iodide ions were transported into the soil and formed a negative charged HgI complex which was then electromigrated to the anode(s). The solutions from anodic and cathodic wells were then fed into a so-called EnViro-Cell where the Mercury was recovered. Under these conditions, the remediation system could be run under closed loop conditions, with minimal water and energy consumption. Along the decontaminating flow path, six monitoring well couples were located close to the cathodes and their corresponding anodes, in order to trace Hg complex displacement due to electromigration. During roughly the first one-month and a half treatment period, quite significant Hg concentration changes were observed in the monitoring well solution, strongly dependent on the well position. In some wells the pollutant concentration reached quickly the concentration of few thousands of micrograms per liter of solution. In agreement with the observed inhomogeneity of distribution of Hg across the treated area, increase in its concentration as a function of time was strongly dependent on well location. Electromigration of other cationic species was documented as well. The results so far obtained prove the efficiency of the electrokinetic reclamation in Hg and other heavy-metal species from soils, in quite complex in-situ situations. [1] Electro-reclamation: theory and practice, R. Lageman; W. Pool, G. Seffinga Chemistry & Industry, (1989), 18, 585-90.