Oxford University
Civil Engineering
Department of Engineering Science
The Use of Electrokinetics to Enhance the Degradation of Organic Contaminants in Soils
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M.J. Harbottle†, G.C. Sills†, I.P. Thompson‡ and S.A. Jackman†‡
† Department of Engineering Science
University of Oxford
Parks Rd., Oxford, OX1 3PJ, UK
Tel. (0)1865 273165, Fax (0)1865 273907
http://www.eng.ox.ac.uk
‡ NERC Centre for Ecology and Hydrology
Mansfield Road, Oxford, OX1 3SR, UK
http://www.ceh.ac.uk
Background
The aim of the study is to investigate the potential for remediating soil pollution by moving organic contaminants and bacteria by electrokinetics. Electrokinetic phenomena have been used to solve a wide range of geotechnical and geo-environmental problems (an explanation of the major phenomena involved). They include consolidation through dewatering of soft ground for construction purposes (Bjerrum, Moum and Eide [1967]), ground improvement through chemical alteration of clay properties (Gray and Schlocker, [1969]) and, more recently, the mobilization of contaminants (both metals and organics) to enable their removal from soil (Acar, Li and Gale [1992] and Jackman, Maini, Sharman, Sunderland and Knowles [in press]). This latter problem has largely been investigated under laboratory conditions, although work has also been performed in the field over the last couple of decades (Lageman, Pool and Seffinga [1989]). The effects of electrokinetics are expected to enhance the processes of bioremediation (where bacteria [naturally occurring or artificially inoculated] use pollutants as a source of energy and carbon, thereby degrading it). The movement of both pollutants and microbes due to these effects will increase contact, and improve bioavailability of the contaminants by helping to move those chemicals protected by the soil particles to areas where degradation can occur. The effect of diffusing high (toxic) or low (below the necessary threshold) concentrations of contaminants in the soil will also improve the efficiency of the bioremediation technique.
Early Work
Initial studies concentrated on preparation and contamination techniques, in order to produce known, repeatable soil states. The next stage was to investigate the movement of a model organic pollutant, pentachlorophenol (PCP), in a clayey soil sample. Pentachlorophenol is a highly chlorinated organic molecule that has been extensively used in wood preservation, and also in certain biocides. It is highly toxic to many living organisms, including humans, and is designated a priority pollutant by the United States Environmental Protection Agency. An electric field was applied to contaminated soil, and the resulting movement of the PCP was tracked using high performance liquid chromatography (HPLC).
The initial apparatus was set up as shown.
In this set of experiments, pentachlorophenol was used in the form of its monosodium salt. Assuming that the ionic form persisted in the soil, it would be subject to the effects of both electromigration and electroosmosis. However, electromigration has been shown to give much faster flows than electroosmosis (up to 300 times, Acar and Alshawabkeh, 1993), and so the negatively charged contaminant would be expected to flow preferentially towards the anode. However, the electric field electrolyses the water at the anode to produce hydrogen ions and an increase in the acidity of the soil, with a tendency for association of the hydrogen and the negative PCP ions. This produces a protonated molecule, which is then subject to electroosmosis only, flowing usually towards the cathode (although Eykholt and Daniel [1994] indicated that in highly acidic conditions, electroosmotic flow may slow and possibly turn towards the anode). Thus, a balance may be formed between protonated PCP molecules and negative PCP ions flowing in different directions, depending on the relative forces imparted by electromigration and electroosmosis. This hypothesis is illustrated in the example results displayed below, using plots of normalised (with respect to the initial average concentration of PCP) PCP concentration with the sample divided into five segments.
Experiment PCP3 (length of test = 140hours)
Experiment PCP4 (length of test = 90 hours)
As shown in the results, the shorter experiment, PCP4, had a low concentration of PCP at the cathode end of the soil, indicating that the PCP was still largely ionic and this negative ion was attracted towards the anode via electromigration. In PCP3, which were very similar experiments, the opposite occurred, with a low concentration at the anode end. This would be the case if the initial negative PCP ions had by this time associated with the hydrogen ions produced at the anode, creating a covalent molecule, which would be subject to electroosmosis only and so would travel towards the cathode. Further information and results are available in Harbottle etal [2001], which was presented at the 3rd Symposium on Electrokinetic Remediation in April 2001.
Current Work
Experiments are being performed which combine the potential of the electric field to move the model contaminant with the potential of degradative bacteria to break it down. The test apparatus has been redeveloped and expanded, to allow the use of six separate soil specimens during each experiment, three of which are used as controls. A photograph of the current testing arrangement.
Results of a recent experiment are presented on this page.
Future Work
The current experiments agree with the hypothesis that the electrokinetic effect can enhance organic contaminant degradation by increasing its bioavailability. However, they are not conclusive, and there are several points that need to be addressed, such as fate of the total PCP content of the soil, which will be investigated using a radiolabelling method. The effect of soil organic matter and the sorption (and desorption) of PCP is also an important factor.
References
BJERRUM, L.; MOUM, J.; EIDE, O. Géotechnique 1967,
17:214-235
GRAY, D. H.; SCHLOCKER, J. Clays and Clay Minerals 1969,
17:309-322
ACAR, Y. B.; LI, H.; GALE, R. J. Journal of Geotechnical Engineering 1992,
118:1837-1852
JACKMAN, S. A.; MAINI, G.; SHARMAN, A. K.; SUNDERLAND, G.; KNOWLES, C.
J. Biotechnology and Bioengineering 2001, 74:1:40-48
LAGEMAN, R; POOL, W.; SEFFINGA, G. Chemistry & Industry,
18 September 1989 p585
ACAR, Y. B.; ALSHAWABKEH, A. N. Environmental Science and Technology 1993,
27:2638-2647
EYKHOLT, G. R.; DANIEL, D. E. Journal of Geotechnical Engineering 1994,
120:797-815
HARBOTTLE, M. J.; SILLS, G. C.; THOMPSON, I. P.; JACKMAN, S. A. Proceedings
of the 3rd Symposium and Status Report on Electrokinetic Remediation 2001,
Karlsruhe, Germany.
