Enhanced electrokinetic remediation of multi-contaminated dredged sediment and induced effect on their toxicity
Résumé
The maintenance of harbor waterways generates large amounts of dredged sediments which contain organic and inorganic contaminants. Electrokinetic (EK) remediation is often developed for metal decontamination but shows limitations for polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyles (PCBs) which are nonionic and involve low aqueous solubility. Laboratory studies were devoted to the investigation of EK efficiency on the mobility and the removal of metals (Cd, Cr, Cu, Pb and Zn), PAHs and PCBs from contaminated sediments. Fresh dredged sediments were collected from the old lock of the Tancarville (France) canal managed by Le Havre port authority, and were subjected to enhanced electrokinetic treatments, using mixtures of chelating agents (citric acid CA or ethylenediaminedisuccinic acid EDDS) and surfactants (synthetic Tween 20 TW20 orbiosurfactants) as additives in the processing fluids. Results showed that increasing CA concentration was favorable for both metal and PAH removal. Applying a periodic voltage gradient associated to a low concentration of CA and TW20 provided the best results for Zn, Cd and Pb removal and also for the removal of the 16 priority PAHs. Promising results were obtained with solutions containing rhamnolipids (0.028%) and a viscosin-like biosurfactant produced by Pseudomonas fluorescens Pfa7B (0.025%), associated to a periodic voltage gradient. Although the rhamnolipid and the viscosin-like additives involved a higher electrical current than TW20, metals were less removed from the sediment. The electroosmotic flow was lower when we used biosurfactants, hence a less effective effect on PAH removal was obtained. The harbor sediment was highly resistant to metal and organics mobilization and transport because of an aged contamination, a high buffering capacity, a very low hydraulic permeability and a high organic matter content. The variability of removal results was also due to the surface aeration of the deposited sediments. EK remediation was more efficient for sediments involving low organic matter and carbonate contents, but also oxidation of the material. A negative redox potential of the sediment was not favorable for the mobilization of metals. The efficiency of the EK process was also assessed by measuring the acute toxicity of the EK-treated sediment on Eurytemora affinis copepods exposed to sediment elutriates. Fortunately, the use of CA and biosurfactants did not significantly impact on sediment toxicity. It was also the case with the use of TW20 as enhancing agent. More particularly, E. affinis copepods were significantly sensitive to low pH values and oxidative conditions obtained after the EK-treatment, but also to Cu, and to a lesser extent to Pb amounts.