Electrodialytic Remediation of Toxic Elements and P Recovery from Sediments of Eutrophic Fresh-Waters in 3-Compartment Batch and Stack Setup

This workpresents a first screening of electrodialytic remediation (EDR) of eutrophic freshwater sediments contaminated by heavy metals. Sediments were collected from two heavily contaminated waterbodies north of Copenhagen, Denmark (Raadvad millpond and Bagsvaerd lake), and a less contaminated lake in an agriculturally impacted area of Jutland, Denmark (Bygholm lake). EDR at laboratory scale in batch cells (treating 100 g sediment) was experimented on all three sediments, and the two most contaminated sediments were further subject to experimental treatment in a stack setup (500/1000 g sediment) feasible for upscaling to continuous process. The results showed that all investigated elements can be extracted from freshwater sediments by EDR. The removal order was in general Zn>Cd>Pb>Cu>Ni>As>Cr. The highest removal percentages were obtained in the batch experiments for the most contaminated sediment: Zn (95%)>Cd (92%)>Pb (76%)>Cu, Ni (73%)>As (56%)>Cr (19%). Because the removal efficiency was lower for elements at low concentrations, it is uncertain whether Cd and As can be remediated to below regulatory limits, which are very low due to their high toxicity. The carbonate content was the main governing parameter, with longer treatment needed for sediments with higher carbonate content. Removal was observed from the least carbonaceous sediment (Raadvad) primarily at pH < 2, and no removal was observed at pH > 4.8. The carbonate-rich sediments (Bygholm and Bagsvaerd), on the other hand, never reached pH < 6, but nevertheless 40% Cd was removed from the Bygholm sediment, and 16% Pb was removed from the Bagsvaerd sediment. This indicates that elements in carbonate-rich sediments are partially bound in the carbonates themselves, and thus mobilized as the carbonates dissolve. However, a longer treatment time or pretreatment to dissolve the carbonate would anyhow be necessary to obtain sufficient removal from such sediments. The stack setup showed superior to the batch treatment in regards to energy consumption and remediation time, which were reduced significantly. The value of the treated sediment for amendment of agricultural soils was reduced due to depletion of P during the electrodialytic treatment. Means to recover P from the electrolyte should be developed.