The current efficiency understood as the fraction of the current transferred by a specific element is given in table IV for all investigated elements. The calculation was based on the assumption that the individual elements were transferred with the following valences: Pb2+, Mn4+, Ca2+, Mg2+, Fe3+, Al3+, K+, Na+. In experiment K1
Kinetics almost all the current was accounted for by calcium transport, which again shows how dissolution of carbonates and removal of Ca was the prevailing process during the first stage of EDR. As the remediation proceeded, less and less current was transferred by the investigated soil cations due to the preferential transfer of the produced hydrogen ions. Although concentrations of Ca, Pb, and Mn were reduced significantly, only Ca-transfer constituted a significant fraction of the current transfer due to the relatively small initial concentrations of Pb and Mn. Conversely, Fe and Al ions, which were not particularly reduced in concentration, constitute a substantial fraction of the current transfer due to their initially high concentrations.
Table IV Current efficiency (‰) Element
Time Pb Al Ca Fe K Mg Mn Na Total
188 0.10 25 900 23 9 20 1 1 980
330 0.59 17 567 7 5 19 5 -2 618
503 0.67 22 373 9 4 15 4 -5 422
671 0.45 29 279 30 3 14 3 -10 348
838 0.30 24 224 27 2 12 2 -8 283
930 0.36 22 202 19 2 11 2 -9 248
4 Conclusions
Several potential applications of soil-fines after electrodialytic remediation in suspension exist depending on the characteristics of the remediated product. The process of electrodialytic remediation of Pb-contaminated soil fines can be divided into four phases: In phase 1) the soil buffer capacity is being eliminated by the production of hydrogen-ions at the surface of the anion-exchange membrane where water-splitting takes place. During this phase soil-carbonates are extracted, resulting in complete extraction of Ca and partial extraction of Mg and K. The carbonate extraction results in a corresponding loss of soil mass, and imply a concentration of elements unaffected by EDR during this phase, including Pb. In this phase the major current transfer can be accounted for by Ca. During phase 2) a sharp pH-decrease of the soil-slurry takes place along with increased conductivity. During this phase Pb-removal occurs at a high rate and a significant fraction of the Pb is dissolved in the soil-solution. Along with Pb, also Mn is extracted. Mg is continuously being extracted during this phase, however at a much lower rate than that of Pb and Mn. In phase 3) pH stabilizes at 1-2, while the conductivity continues to increase and the voltage between working electrodes decreases. During this phase Pb is extracted at a lower rate simultaneous with low-rate extraction of Mn and Mg. Furthermore Fe and Al-oxides start to act as buffers, resulting in some extraction of these elements as well. In phase 4) extraction of Pb and most soil-cations has ceased, and the primary transport is that of hydrogen-ions complemented by a continuing slow dissolution of Fe and Al-oxides. It is recommended to terminate remediation as soon as Pb-extraction ceases to limit the dissolution of Fe and Al-minerals. Due to intrusion from the electrolytes, the soil content of Na is continuously increasing during remediation, and a careful choice of electrolytes in order to meet requirements by the succeeding application of soil-fines is necessary. From this soil 97% of the Pb could be extracted, reducing the final Pb-concentration to 25mg/kg. The overall order of removal-rate found was: Ca > Pb >
Mn > Mg > K > (Al and Fe). In order to establish a complete evaluation of any
Kinetics
potential applications of the soil-fines after remediation, this investigation should be complemented by investigations of the fate of phosphate, nitrate, chloride and organic matter as well as the mineralogical condition of the fines after remediation.
Acknowledgements
The authors wishes to thank Ebba C. Schnell for assistance with the analytical work.
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