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Electrodialytic Remediation of Soil Polluted with Copper from Wood Preservation Industry† L I S B E T H M . O T T O S E N , * ,‡ HENRIK K. HANSEN,‡ SØREN LAURSEN,‡ AND ARNE VILLUMSEN§ Department of Chemistry and Department of Geology and Geotechnical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark
Electrodialytic soil remediation is a newly developed method for removal of heavy metal from polluted soil. The method is based on a combination of the electrokinetic movement of ions in soil with the principle of electrodialysis. The principle was tested in six experiments using laboratory cells on a copper-polluted Danish loamy sand. The duration of the experiments was varied, and the development of concentration profiles in the soil after the remediation was investigated for two different dc currents (0.1 and 0.2 mA/ cm2 of soil). The rate of Cu removal was about doubled when doubling the current. It was found that Cu content was reduced to a level below 100 mg of Cu/kg of dry soil in the section closest to the anode and that the Cu removed was accumulated in the next section in the direction of the cathode. The accumulation zone was moving in the direction of the cathode during the application of an electric current. When the remediation ended, all Cu could be found in the cathode compartment, and results showed that it was possible to decontaminate the soil from 1360 to below 40 mg of Cu/kg of dry soil.
This paper deals with Cu as a soil pollutant. In a polluted soil, Cu can be found in various forms: soluble ions, ions at exchangeable sites, inorganic and organic complexes in soil solution, as stable organic complexes in soil humus, adsorbed by hydrous oxides, or in the crystal lattice of soil minerals (2). Any prediction of the possibility of removing Cu from a soil, with its complex composition and the variety of Cu forms, by applying an electric current seems an overwhelming task. Thus, it is necessary to work for a better understanding, and there seems to be two ways to start the work. The first is to use synthetic, homogeneous model systems and laboratoryprepared soil samples in order to get a better understanding of a part of the problem, and the second is to apply the current to the complex system that a soil is, trying to identify the main parameters of importance. The work presented here is based on the second way, mainly because it is uncertain if the results obtained in pure systems can be adapted to “true” soil remediation. The doubt of this is based on the findings in refs 3 and 4, where it is showed that Cu is easier to extract from laboratory-prepared soils than from soils sampled from polluted sites. In the literature, different experiments with electrokinetic soil remediation (EK) or electrodialytic soil remediation (ED) to Cu-polluted soils are described:
EK (5): EK (6): ED (1): ED (7): ED (8): ED (9):
silty quarts sand; laboratory prepared Argillaceous sand and river mud Danish sandy clay Danish loamy sand Czech soil polluted from accumulator industry Danish loamy sand and Danish sand
In each of these soils, Cu was mobilized by the applied electric current. The purpose of this work is to demonstrate the use of electrodialytic soil remediation to remove copper from a sandy loam and to identify parameters of importance to the remediation process.
Introduction
Electrodialytic Remediation
The environmental problem of heavy metal pollution in soils is of increasing concern. This is due to a greater understanding of the toxicological importance of heavy metals in the ecosystem combined with the lack of methods to remediate such soils. Electrodialytic soil remediation is a method under development that can be used to remove heavy metals from soils. The method is closely connected to electrokinetic remediation, and the cleaning agent of both methods is an electric current that is applied to the soil. The two methods differ in the separation between the soil and the electrode compartments. In electrokinetic remediation, passive barriers are used whereas in electrodialytic remediation ion-exchange membranes are used. The main purpose for using ion-exchange membranes is that no ions can enter the soil from the electrode compartments, which means that the current is carried by ions that were originally in the soil. Hereby no current is wasted for carrying ions from one electrode compartment to the other. A research team from the Technical University of Denmark has been investigating electrodialytic soil remediation since 1992 (1) and has applied for a patent on the method.
A schematic presentation of a laboratory cell for electrodialytic remediation is shown in Figure 1. The cell consists of three compartments; two electrode compartments and a soil compartment placed between these. The catholyte is separated from the soil by a cation-exchange membrane, and the anolyte is separated from the soil by an anion-exchange membrane. When the current is passed through this cell, no current carrying ions can pass from the electrode compartments into the soil due to the ion-exchange membranes, while ions can be transported from the soil into the electrode compartments. In this system, current is thus prevented from being wasted by carrying highly mobile ions from one electrode compartment through the soil and into the other electrode compartment. Furthermore, competition between highly mobile ions from the electrode compartments and less mobile ions in the soil is avoided, so that the latter are affected by an increased force. Soil pH is a parameter of importance for the mobility of Cu in an electric field, because an increased amount of Cu is present as ions, when soil pH is decreased. In electrokinetic soil remediation, where passive separators are placed between soil and electrolyte solution, the soil is acidified due to the electrode process at the anode, where H+ ions are produced and carried into the soil. Meanwhile a basic front is developed from the cathode side due to the electrode process at the cathode, see for example refs 10 or 11. Cu ions moving in
†
Dedicated to Jørgen Birger Jensen (1937-1995). * Corresponding author e-mail:
[email protected]; fax: +45 45 93 48 08. ‡ Department of Chemistry. § Department of Geology and Geotechnical Engineering.
S0013-936X(96)00588-3 CCC: $14.00
1997 American Chemical Society
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FIGURE 1. Schematic diagram of the laboratory cell for electrodialytic soil remediation (KAT ) cation exchange membrane, AN ) anion exchange membrane). the direction of the cathode meet the alkaline front and will be precipitated as insoluble copper hydroxide and oxide. This can be avoided by continuous acidification of the catholyte, but this means introduction of new ions to the soil. Using the principle of electrodialytic soil remediation, development of both fronts from the electrode compartments are avoided due to the ion-exchange membranes. Still an acidification of the soil has been found (9). This production of H+ ions occurs in the interface between soil and anionexchange membrane due to water dissociation (12). What happens is apparently that the flux of anions arriving from the soil to the anion-exchange membrane is inadequate as compared with the applied current. Only anions can contribute to an electrical current through the membrane, and thus OH- ions from the dissociation of water can compensate for the deficiency in the anion flux, while H+ ions moving in the opposite direction act similarly as a compensation of too low flux of anions from the soil side. If the current density is too high, water will dissociate in the interface between the soil and the cation-exchange membrane too. This will only happen at a higher current because more cations are mobile in the soil than anions (the cation-exchange capacity of the soil is higher than the anionexchange capacity). The movement of water in the electric field, electroosmosis, has been recognized by other teams as being important for the elektrokinetic soil remediation process (13). The flushing effect of the electroosmosis on the soil is restricted by the ion-exchange membranes so that the electroosmotic flushing is of smaller importance to the electrodialytic soil remediation.
Experimental Conditions Six electrodialytic remediation experiments were made with samples of soil taken from the same spot at a polluted site. The experiments were carried out with variations in applied current and duration, but otherwise under identical conditions. The dc current through the soil was constant. Two groups of experiments were done, one with an applied current of 5 mA and one with an applied current of 10 mA, corresponding to 0.1 and 0.2 mA/cm2, respectively, referring to the area of the ion-exchange membranes. The following experiments were done:
experiment
duration (d)
current (mA)
A B C D E F
26 40 76 12 22 70
5 5 5 10 10 10
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The duration of the experiments was varied in order to show the changes in concentration, pH, and water content within the soil at different times. During the experiments, the voltage drop over the cell was measured several times a day, and pH was measured in the electrolytes in the twoelectrode compartments every morning. When pH in the anolyte was higher than 3, it was adjusted to below 3 with a small addition of concentrated HNO3. Analytical. The Cu concentrations in the soil were measured in the following way: 2.5 g of dry soil, 50 mL of concentrated HNO3, and 150 mL of distilled water were boiled on a heating plate until the volume was reduced to below 50 mL. The liquid was separated from the solid particles through a nuclepore filter and diluted to 250 mL. All soil samples were duplicated. The analyses were made by atomic absorbance spectrophotometry. Soil pH was measured by mixing 10 g of dry soil and 25 mL of distilled water. After 1 h of contact time, the pH was measured using a Radiometer pH electrode (reference electrode REF401 and glass electrode pHG201). Description of the Soil. The experimental soil was a Danish soil, sampled from the top layer on a site, that was highly polluted from a former wood preservation plant. Wood preservation took place at the site for 40 years. This soil was selected as experimental soil because both pollution type and soil type are representative for a large number of Danish sites. The soil, which is loamy sand deposited during the last glaciation, is polluted with Cu, Cr, and As. This investigation only considers Cu. The initial soil concentration was 1360 mg of Cu/kg of dry soil. The soil contained 17.4% of water on the day the sample was taken, and in the laboratory, the soil was stored in a bucket with a close-fitting lid. The initial pH of the soil was 5.8. The experimental soil is characterized more carefully in ref 9. Laboratory Equipment. The electrodialytic cell used was constructed for laboratory experiments. It was made of glass. The soil compartment was 15 cm long and had an inner diameter of 8 cm. The ion-exchange membranes were obtained from Ionics: cation CR67 HMR 412 and anion 204 SXZL 386. Platinized electrodes, obtained from Bergsøe Anti Corrosion, were placed in the electrode compartments. In each electrode compartment, 1 L of electrolyte solution (0.01 M NaNO3 with pH adjusted initially to 3 using HNO3) was circulated. For the circulating system, peristaltic MasterFlex pumps with norprene tubes were used. The pumps were operating at flow rate of about 45 mL/min.
Results Visual changes could be seen during the experiments. In every experiment a layer developed on the cathode, indicating that electrode processes other than electrolysis of water occurred. Initially the layers were red, probably a precipitation of Cu, because a similar red layer was formed when the electrodes were simply placed in an electrolyte solution containing only Cu(NO3)2 (12). After the red layer, other differently colored and shaped layers developed in the electrodialytic experiments, but the thickness of the layers increased. In experiment G, the layer was hard, metal-like, and weighed 7.5 g when the experiment was finished. This layer consisted of about 14% Cu. Unfortunately, the total composition of elements of this layer was not measured. In experiment C, the soil collapsed in the cathode end because the soil was loosely packed, and only two-thirds of the cationexchange membrane was in contact with the soil. The voltage of the working electrodes was measured several times each day. The variation is shown in Figure 2. At the end of each experiment, the soil was segmented into 10 slices of equal thickness. In each slice pH, water content, and Cu concentration were measured. The resulting profiles in the soil are given in Figures 3-5.
FIGURE 2. Readings of voltage between working electrodes during the experiments. (A) Experiments A-C where 5 mA (dc) was applied to the electrodes and (B) experiments D-F where 10 mA (dc) was applied to the electrodes.
Discussion Voltage Drop between Working Electrodes. The readings of voltage as a function of time is shown in Figure 2. It is seen that the voltage was varying considerably during the experiments, i.e., the electric resistance within the cell was not constant. The change in electric resistance was caused by changes in the system due to the applied current. The passage of current gave rise to various effects influencing the resistance in the cell: (I) The resistance in the interface between the cathode and the electrolyte solution might have been influenced by the layer that developed on the cathode. (II) The water content in the soil was not constant (see Figure 3), and the soil was dewatered, which can cause an increased resistance. (III) Due to the drying of the soil, the area of contact between wet soil and ion-exchange membranes was reduced. This effect was more pronounced at the anionexchange membranes and caused an increase in electric resistance. (IV) The water dissociation at the membranesoil interfaces may cause an increased voltage. It can be seen that the voltage increases in the beginning of the soil remediation process, and after a certain number of days the voltage is decreasing again. This decrease might be an indication of H+ ions starting to arrive in the cationexchange membrane from the soil. Movement of Water within the Soil. Initially, the water content of the soil was 17.4% by weight. Figure 3 shows the profile of water content in the soil at the end of each experiment, and it can be seen that the water content decreased in each experiment except in experiment A. The
FIGURE 3. Final water content in the soil. The straight line indicates the initial water content. (A) Experiments A-C where 5 mA (dc) was applied to the electrodes and (B) experiments D-F where 10 mA (dc) was applied to the electrodes. movement of water is due to electroosmosis. Apart from experiment A, the water content through the soil is almost constant. This means that a steady state has been reached with equal amounts of water flowing through the anionexchange membrane and being transported through the soil by electroosmosis. Experiment A was the one with the shortest duration time, and thus the steady-state situation was not reached here yet. In experiment C, a sharp decrease in water content in slice 1 can be seen. This is probably because of the collapsing of the soil next to the cation-exchange membrane, resulting in an increased current density here. The steady-state water content seems to be about 14%-15% with a slight tendency of a lower water content in the case to the highest current density. Development of pH Profile in the Soil. From Figure 4, it is obvious that soil pH has been changed by the applied electric field. The acidification starts at the anion-exchange membrane due to water dissociation, and the acidic front is moving in the soil toward the cathode. The development of the acidic front in the soil is marked in the experiments, where 5 mA was applied to the soil, and it seems as if pH is decreased in the whole soil volume. From the figures illustrating the pH profiles after application of 10 mA, it can be seen that in experiment D a development of both an acidic front from the anion-exchange membrane and a basic front from the cation-exchange membrane occurred. In this experiment, pH was increased in slice 1 after 12 days of current. Also after 22 days of current, experiment E, the increase was found, but the pH here was closer to the initial value. This might be because the limiting current density of the system at both ion-exchange mem-
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FIGURE 4. Profiles of soil pH developed after application of current for different times. The straight line indicates the initial value. (A) Experiments A-C where 5 mA (dc) was applied to the electrodes and (B) experiments D-F where 10 mA (dc) was applied to the electrodes. branes was exceeded. After a longer time it can be seen that the basic front is equalized and that pH has been decreased in all soil slices (experiment F). Even though soil pH is between 3 and 4 when the remediation is ended, it should be possible to re-establish the soil so it should be possible for plants to grow. In this case, it is necessary to add lime to the soil in order to increase soil pH and also nutrients should be added. Mobility of Cu. Cu was mobilized in all six experiments, see Figure 5. From the concentration profiles, it can be seen that Cu is removed as cation, as expected, because it was the slice closest to the anode that was remediated first. The final concentration of Cu in some soil slices was below 40 mg of Cu/kg of dry soil, which is in the range of the Dutch A level of 36 mg of Cu/kg of dry soil (15). It was found that the copper released from the slices closest to the anion-exchange membrane was accumulated in the slices that followed. This zone of accumulation can be correlated to soil pH, because the accumulation starts mainly where a “pH jump” from a low value to a higher value was found. This underlines that pH is of crucial importance for the successful removal of copper from the soil. Comparing the two series of experiments, it is seen that by increasing the current by a factor 2 the rate of copper removal is also increased by a factor of approximately 2. In experiment F, where the highest current (10 mA) was passed through the soil for the longest time (70 days), the treatment almost achieved the ultimately desired results since the concentration was below the Danish critical level of 200 mg of Cu/kg of dry soil (16). Slices 1 and 2 have higher concentrations as compared with this value, but if the current
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FIGURE 5. Final copper profiles in the soil. The straight line indicates the initial copper concentration in the soil. (A) Experiments A-C where 5 mA (dc) was applied to the electrodes and (B) experiments D-F where 10 mA (dc) was applied to the electrodes. had been passed through the soil for a few days more, the 15 cm of soil would have been sufficiently depleted for Cu.
Acknowledgments The Danish company AS Bioteknisk Jordrens has demonstrated confidence in the electrodialytic soil remediation method by yielding financial support to the project. Thanks are due to C. F. Jensen for his help in the analytical work.
Literature Cited (1) Ottosen, L. M.; Hansen, H. K. Electrokinetic cleaning of heavy metal polluted soil; Internal report, Department of Physical Chemistry & Department of Geology and Geotechnical Engineering, The Technical University of Denmark: Denmark, 1992 (in English). (2) Baker, D. E.; Senft, J. P. In Heavy Metals in Soils, 2nd ed.; Alloway, B. J., Ed.; Blackie Academic & Professional: London, 1995; pp 179-205. (3) Tuin, B. J.; Tels, M. Environ. Technol. 1990, 11, 935-948. (4) Yarlagadda, P. S.; Matsumoto, M. R.; VanBenschoten, J. E.; Kathuria, A. J. Environ. Eng. 1995, 121, 276-286. (5) Runnels, D. D.; Larson, J. L. Ground Water Monit. Rev. 1986, 6 (3), 85-91. (6) Lageman, R. Environ. Sci. Technol. 1993, 27 (13), 2648-2650. (7) Ribeiro, A.; Villumsen, A.; Jensen, J. B.; Re`fega, A.; Vieira e Silva, J. M. Proceedings of the 15th World Congress of Soil Science; Acapulco, Mexico, July 10-16, 1994; International Soil Science Society: Madison, WI, 1994; pp 210-211. (8) Jensen, J. B.; Kubes, V.; Kubal, M. Environ. Technol. 1994, 15, 1077-1082. (9) Ottosen, L. M. Ph.D. Dissertation, Department of Geology and Geotechnical Engineering, The Technical University of Denmark, Denmark, 1995 (in English).
(10) Acar, Y. B.; Gale, R. J.; Putnam, G. A.; Hamed, J.; Wong, R. L. J. Environ. Sci. Health 1990, A25 (6), 687-714. (11) Acar, Y. B.; Gale, R. J.; Hamed, J.; Putnam, G. A. Transport. Res. Rec. 1990, 1288, 23-33. (12) Hansen, H. K. Ph.D. Dissertation, Department of Physical Chemistry, The Technical University of Denmark, Denmark, 1995 (in English). (13) Pamukcu, S.; Kahn, L. I.; Fang, H. Y. Transport. Res. Rec. 1990, 1288, 41-45. (14) McCarter, W. J. Ge`otechnique 1984, 34 (2), 263-267. (15) Alloway, B. J., Ed. Heavy Metals in Soils, 2nd ed.; Blackie Academic & Professional: London, 1995; p 357.
(16) Publication from The Danish Ministry of Environment and Energy. Miljøministeriet Miljøstyrelsen, Branchevejledning no. 4, 1992 (in Danish).
Received for review July 8, 1996. Revised manuscript received January 17, 1997. Accepted January 31, 1997.X ES9605883
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Abstract published in Advance ACS Abstracts, April 1, 1997.
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