Authors:
PhD Krzysztof Kowalski | Technical University of Denmark | Denmark
Sanne Skov Nielsen | Orbicon A/S | Denmark
Pernille Erland Jensen | Technical University of Denmark | Denmark
PhD Thomas Larsen | Orbicon | Denmark
Mads Terkelsen | Capital Region of Denmark, Center for Regional Development | Denmark
Lisbeth Ottosen | Technical University of Denmark | Denmark
The Collstrop site, a former wood preservation plant, represents a highly soil contaminated area with contaminants of copper, chromium and arsenic. The contaminants were originally used in the impregnation process. The content of copper, chromium and arsenic (1000-2000 mgCu/kg of soil, 300-600 mgCr/kg of soil and 200-1200 mgAs/kg of soil) are clearly above the Danish clean soil criteria, that are 500 mgCu/kg, 500 mgCr/kg and 20 mgAs/kg. The contaminants are primarily found associated with the fine fraction of the soil (<0.063 mm), where more than 95% m contaminant/m soil can be found (4000-10000 mgCu/kg of fine fraction, 1000-2500 mgCr/kg of fine fraction and 3000-8000 mgAs/kg of fine fraction). The Collstrop site only poses minimal risk to the nearby recipients and the groundwater resource in the area, but it is a standard site for wood preservation. The site is therefore used by The Capital Region of Denmark for testing remediation methods. The objective is that these methods in the future can be applied on similar sites where remediation is required due to risks towards groundwater and recipients.
Removal of ionic contaminants is being performed with help of an electrodialytic remediation (EDI) method. The method is a ex-situ continuous process, where ions are separated from soil slurries with help of applied electric field to electrodes isolated from soil slurries by ion-exchange membranes. Anions, like arsenite and arsenate, are removed to the anode compartment through a anion-exchange membrane and cations, of copper and chromium, are moved into a compartment with cathode through a cation-exchange membrane. Application of the electrodialytic remediation developed at the Danish Technical University has been well described in last decades, but was mostly applied as laboratory and bench scale experiments with very limited mass of treated material (Ottosen & Hansen, 1992). Previous laboratory experiments proved that it is possible to remove Cu from 1360 to 40 mg/kg soil in the part of the soil closest to the anode in 70 days without any enhancement of the system (Ottosen et al. 1997). However As is not removed significantly in this system. This is mainly because the soil is acidified during the remediation, and in acidified soil As will mainly be present in uncharged species (H3AsO3 in case of As(III) and H3AsO4 in case of As(V)). Such uncharged species are not transported in the applied electric field. However increasing pH, to pH=3-4, has significantly increased arsenic species removal, but reaching a removal efficiency of only 60% (Sun et al. 2012). The last and other previous studies have also shown an energy consumption corresponding to between 25 kWh and several thousand kWh per ton of soil at 20% fine fraction. Furthermore the treatment of the finest soil fraction will reduce treated mass and increase initial contaminants content, which is expected to increase the removal efficiency and reduce the process energy consumption to below 200 kWh/ton of soil. Based on previous successful experiences with EDI remediation an upscaling of the process has been proposed to treat 15 tons of contaminated soil according to following on-site procedure:
1. Soil excavation.
2. Separation and soil fractionation in a soil washing facility.
3. Performing on-site EDI remediation on the fine fraction
4. Soil regeneration.
The first results indicates that the two step process, removing first Cr and Cu under strongly acidic conditions and then As under circum-neutral pH, showed that it is possible to remove Cu and Cr within 2 days of the EDI treatment. As expected, more problematic is removal of arsenic that is associated with iron hydroxides. Therefore the main challenge for upscaling is overcoming the issue of arsenic mobilization from the fine fraction with parallel ensuring that arsenic species are in ionized form, as anions, that can be attracted by anode.
The aim of this study is to confirm the feasibility of the EDI remediation and implementation of the proposed procedure for soil treatment. The pilot scale investigations give an opportunity to evaluate the EDI process, find its drawbacks and foremost improve it to developed cost-efficient soil remediation technology.
The project is carried out in collaboration between DTU-Byg at the Technical University of Denmark and Orbicon A/S and a project owner is Center for Regional Development, Capital Region of Denmark. We expect to perform all pilot scale investigations and reporting within spring 2015.
We would like to acknowledge Center for Regional Development, Capital Region of Denmark for funding the project and their support.
References:
Hansen H.K., Ottosen L.M., Kliem B.K., Villumsen A. (1997) Electrodialytic remediation of soils polluted with Cu, Cr, Hg, Pb and Zn. J Chem Technol Biotechnol 70:67–73.
Ottosen, L.M.; Hansen, H.K. (1992) Electrokinetic cleaning of heavy metal polluted soil. Internal report. Fysisk-Kemisk Institut and Institut for Geologi og Geoteknik, Technical university of Denmark.
Sun, T.R., Ottosen, L.M. Jensen, P.E.; Kirkelund, G.M. (2012) Electrodialytic remediation of suspended soil – Comparison of two different soil fractions Journal of Hazardous Materials, vol: 203-204, p. 229-235