The suitability of electrokinetic remediation for removing heavy metals from dredged marine sediments with high acid buffering capacity was investigated. Laboratory-scale electrokinetic remediation experiments were carried out by applying two different voltage gradients to the sediment (0.5 and 0.8 V/cm) while circulating water or two different chelating agents at the electrode compartments. Tap water, 0.1 M citric acid and 0.1 M ethylenediaminetetraacetic acid (EDTA) solutions were used respectively. The investigated metals were Zn, Pb, V, Ni and Cu. In the unenhanced experiment, the acid front could not propagate due to the high acid buffering capacity of the sediments; the production of OH(-) ions at the cathode resulted in a high-pH environment causing the precipitation of CaCO3 and metal hydroxides. The use of citric acid prevented the formation of precipitates, but solubilisation and mobilisation of metal species were not sufficiently achieved. Metal removal was relevant when EDTA was used as the conditioning agent, and the electric potential was raised up to 0.8 V/cm. EDTA led to the formation of negatively charged complexes with metals which migrated towards the anode compartment by electromigration. This result shows that metal removal from sediments with high acid buffering capacity may be achieved by enhancing the electrokinetic process by EDTA addition when the acidification of the medium is not economically and/or environmentally sustainable.
SUMMAR YRecent MT soundings carried out in the Penninic Alps of Valais have shown the presence of a very good, outcropping conductor. Extremely high conductivity was attributed to the presence of graphite.To verify this assumption, the electrical properties of borehole black shales were measured under simulated physical conditions (electrical frequency, hydrostatic confining pressure, internal fluid pressure, temperature). These measurements showed that under all physical conditions (electrical frequency, in the 0.005-200 Hz interval; hydrostatic confining pressure up to 39 MPa; internal fluid pressure up to 23 MPa; temperature up to 180 uC) one of the samples studied was very conductive (resistivity less than 2 Vm under all physical conditions). Interestingly, despite similar macroscopic aspect, other samples from a nearby borehole were found to be only slightly conductive. Chemical, mineralogical and petrographic investigations revealed that the enhanced electrical conductivity is mostly due to textural characteristics (such as grain size and carbon film distribution at the grain boundaries) rather than to chemical differences.
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