Electrokinetic approach to assess the behaviour of a contaminated marine sediment

Abstract

The present study focuses on the laboratory-scale application of an electrokinetic technique for the remediation of a contaminated marine sediment dredged from a harbour. The challenge of the study was the treatment of the silty sediment containing heavy metals and organic matter, as methods to remove heavy metals or organic matter separately do not usually yield satisfactory results for the mixture of both. The sediment was characterised in terms of elemental and mineralogical composition, heavy metals, total organic carbon, particle size distribution and zeta-potential. Five configurations of remediation tests have been explored. Four of these tests were employing electric field, two of which were carried out with carbon steel electrodes (CSE) and the other two with activated Ti electrodes (ATE). One test was conducted without electrical current, and it was used as a benchmark. In some tests, acetic acid was added to the catholyte to enhance heavy metal mobility. The pH, conductivity and composition of catholyte and anolyte were characterized as function of test duration. Electrophoretic measurements showed that the zeta-potential of raw, untreated sediment samples was negative across all the range of pH. In the tests with CSE, electroosmotic flow (EOF) towards the cathode was observed when acetic acid was added. In other tests, i.e. CSE without acetic acid and ATE with acetic acid, the flow initially directed towards the cathode but then reversed. In the case of ATE without acetic acid, the EOF aimed only at the anode. Without current, no heavy metals were removed. In the tests without acetic acid, the metals concentrated in the anolyte, most probably due to electroosmotic flow or migration of negatively charged aqueous metal complexes. Arsenic and a small amount of Cu and Zn were found in the catholyte. When acetic acid was added to the catholyte, and with ATE electrodes, all the metals were found in the catholyte, whilst Ni was not detected in the anolyte. The formation of an acidic front through the sediment and the control of the EOF appear to be the key factors for the successful remediation via electrokinetic technique. It has been demonstrated that electroremediation can be efficient for removal of all studied metals as all of them could be mobilised using Ti electrodes. The emission of metals such as Ni and Cr from dissolution of carbon steel anode must be taken into account in electrochemical remediation tests.