Electrokinetically induced removal of heavy metals from an aged, contaminated sludge – a laboratory experiment

Abstract

Soil pollution is a universal environmental issue and the clean-up of such contaminated soils can be a costly and time consuming task. Traditional methods such as pump and treat or soil flushing may not provide an effective solution, in particular for fine grained, clayey and highly impermeable soils and contaminants that are relatively immobile. The aim of this study was focused on the removal of heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) from a coagulant treated, aged, contaminated sludge by means of electroremediation. Also a side experiment has been conducted in which the use of coagulant polymers on the removal of heavy metals from a soil spiked with Cd, Cr, Cu, Ni, Pb, Zn was investigated. A sequential extraction was conducted in order to study the metal distribution of both the contaminated sludge and the spiked soil. Maximum removal percentages during the experiment using a constant current of 1 mA/cm2 over ~180 hours with a corresponding 0.5 V/cm potential gradient were > 15 % for Cd (21%), Cu (15%), Zn (23%); Ni and Pb showed removal < 10%; Cr demonstrated no removal. Varying the electrode material was of no influence for the transfer of current from the power supply into the system. In contrast, absence of NaCl in the electrode reservoirs demonstrated a major decrease in the efficiency of the applied current on the removal of heavy metals form the contaminated sludge. A difference between the direction of electroosmosis in presence and absence of coagulant polymers in the spiked soil sample was observed on the direction of electroosmosis. However, the effect of the opposite electroosmotic movement of water on the contaminant transport was not quantified. With this study it was impossible to extrapolate the effect of the electrical current and potential gradient due to multiple parameter changes per experiment. It was recommended that batch-scale on-site application of electroremediation is preferred above in-situ field implementation, since more control, with emphasis on pH, of the system environment, i.e. the contaminated sample including electrolyte fluids and electrodes, is possible during batch-scale on-site application of electroremediation.