Simultaneous Extraction and Separation of Phosphorus and Heavy Metals from Freshwater Sediments using Electrodialysis – Influence by Sediment Characteristics

This paper explores the use of 3-compartment and 2-compartment electrodialytic (ED) cells for the simultaneous extraction and separation of P and heavy metals from freshwater sediments, to recover a P-product without heavy metals and sediments fit for recycling. Eutrophic sediments from two different freshwater sources, one less contaminated and one highly contaminated with heavy metals, were used. The extraction of heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) was higher from the highly contaminated sediment than from the less contaminated one. The ED treatment (3-compartment and 2-compartment) of both sediments obtained the highest extraction for Zn (about 85 %) and the lowest for Cr (2 - 11%), whereas the extraction of P was between 35 and 50 %. The separation and extraction of P and heavy metals was better with the 3-compartment ED cell than the 2-compartment ED cell, potentially allowing the utilization of the P-extracted solution. Sequential extraction was performed to assess the operationally defined pools of P and heavy metals that could be extracted during the ED treatment. HCl-soluble P was fully extracted during ED treatment. Heavy metals associated with the exchangeable and reducible fractions were extracted significantly, and those with the oxidizable fraction were only partially extracted. After the ED treatment, the less contaminated sediment was suitable for recycling, for eg. as a soil improver, but not the highly contaminated sediment. Optimization of ED treatment is suggested to enhance the extraction of P and heavy metals, especially from the dominant and difficult-to-mobilize oxidizable fraction.