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Development of electrokinetic-flushing technology for the remediation of contaminated soil around nuclear facilities
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Journal of Industrial and Engineering Chemistry 14 (2008) 732–738 www.elsevier.com/locate/jiec
Development of electrokinetic-flushing technology for the remediation of contaminated soil around nuclear facilities Gye-Nam Kim *, Yun-Ho Jung, Jung-Joon Lee, Jei-Kwon Moon, Chong-Hun Jung Korea Atomic Energy Research Institute, Daedeokdaero, Yuseong-gu, Daejon, Republic of Korea Received 24 October 2007; accepted 14 May 2008
Abstract The effects of a flushing by a pump on an electrokinetic-flushing remediation of contaminated soil were estimated. The soils were sampled from the sites around nuclear facilities which were built on a high hydro-conductivity of sandstone. An electrokinetic-flushing equipment with a pump was manufactured to estimate the effect of a flushing on an electrokinetic-flushing remediation. In order to select an optimal reagent suitable to the characteristics of a soil near nuclear facilities, 4 experiments were executed with 4 candidate reagents selected from 12 reagents and the results of the experiments are as follows. The removal efficiencies of cobalt and cesium from the contaminated soil with the acetic acid were the highest, which were 92.1% and 83.1%, respectively. The effluent solution volume generated from an electrokinetic remediation was very smaller and it was 5% below that from a soil washing. Next, the results from a comparison of an electrokinetic-flushing remediation and an electrokinetic remediation revealed that the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 5 days were increased by 25% and 35% when compared to those by the electrokinetic remediation, but the effect of a flushing by the electrokinetic-flushing equipment started to decrease after 5 days. The removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. Namely, the higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation. # 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Keywords: Electrokinetic-flushing; Remediation; Soil; Cobalt; Cesium; Reagent; Removal efficiency
1. Introduction South Korea has many nuclear power plants and a research reactor. As such, the soil around these nuclear facilities can become contaminated with radionuclides from a long-term operation of these facilities. The electrokinetic process holds great promise for a remediation of polluted soils, as it has a high removal efficiency and is time effective for a low permeability. An electrokinetic remediation can be used to treat soils contaminated with inorganic species and radionuclides [1,2]. The main mechanisms of a contaminant’s movement in an electrical field involved in electrokinetic technology are an electromigration of ionic species and electroosmosis. Electromigration probably contributes significantly to the removal of contaminants, especially at high concentrations of ionic contaminants and/or a high hydraulic permeability of a soil [3]. The cathode reaction should be depolarized to avoid the generation of
* Corresponding author. Tel.: +82 42 868 8674.
hydroxides and their transport in a soil. The selected liquids, also known as purging solutions, should induce favorable pH conditions in a soil, and/or interact with the heavy metals, so that these heavy metals are removed from a soil [4]. Most radioactive facility sites have been contaminated by the leakage of radioactive waste-solution due to the corrosion of concrete and pipes by a long-term operation of waste-solution tanks and connection pipes, set up in the underground around nuclear power plants. Electroosmosis moves a pore solution in response to an electric field, typically towards a cathode because of the negative surface charge of a soil. The magnitude of the transport velocity due to an electromigration and electroosmosis is directly related to the electric potential gradient. Therefore remediation efficiency can be improved through an increase of the electric potential gradient by a pumping. Recently, researchers have been investigating whether this method can be used to remove subsurface contaminants and they have compiled the published research on the use of electrokinetic techniques to decontaminate fine-grained soils, and discussed some of the problems that occur
1226-086X/$ – see front matter # 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.05.001
G.-N. Kim et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 732–738
during this process [5–7]. An electrolytic reactor was used for a greywater reuse [8]. An electrokinetic remediation has been applied to remove heavy metals from mine trailing soils [9]. Recently researchers have tried to develop soil washing techniques in which soil-bound contaminants are transferred to a liquid phase by a desorption and solubilization. Several washing solutions have been investigated, such as water, acids, bases, chelating agents, alcohols and other additives [10,11]. In practice, acid washing and a chelator soil washing are the two most prevalent removal methods [12–14]. The research reactor in Seoul, South Korea is under decommissioning work. Because the research reactor was constructed on a hard sandstone rock like the other Korea nuclear power plants, the contaminated soil around the research reactor contains a lot of sand and it has a higher hydro-conductivity. It has been supposed that an electrokinetic-flushing remediation is a suitable technology in consideration of the soil characteristics near the nuclear facility, which has merits of both an electrokinetic remediation and a soil flushing method. The object of this study was to estimate the effect of a flushing by a pump on an electrokinetic-flushing remediation. The electrokinetic-flushing equipment with a pump was manufactured. In order to select an optimal reagent suitable to the soil characteristics, four experiments were executed with four candidate reagents using the electrokinetic equipment. In order to estimate the effect of a flushing by a pump on an electrokineticflushing remediation, different hydro-conductivities of soils were decontaminated with different injection flushing rates using the electrokinetic-flushing equipment.
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Table 1 Mineralogical composition and properties of a soil near nuclear facility. Soil components and properties
Value
Soil components SiO2 Al2O3 K2 O Fe2O3 P2O5 CaO Na2O MgO
64.4% 16.7% 9.5% 3.0% 1.8% 1.5% 1.2% 0.4%
Moisture content (pH) Surface 10 cm depth 30 cm depth 40 cm depth 50 cm depth Moisture content (pH) Porosity Bulk density
0.43 1.56 g/cm3
Hydro-conductivity 75–2000 mm (Soil A) 150–2000 mm (Soil B)
1.6 105 m/s 6.7 105 m/s
13.5% (7.5) 11.3% (7.4) 10.2% (6.8) 9.8% (6.6) 9.6% (6.7)
into a horizontal soil cell of 4.5 cm 5.9 cm 14.5 cm. A paper filter was inserted between the electrode compartment and the contaminated soil to protect against an influx of a soil. A pump was used to increase the pressure transport of the pore solution in a soil pore by the following equation. Electrolyte
2. Soil characteristics and experiment method 2.1. Description of a soil South Korea currently has about 20 nuclear power plants and a research reactor. Because most nuclear facilities in Korea have been constructed on a hard sandstone rock, the contaminated soil around a nuclear facility contains a lot of sand, and it has a higher hydro-conductivity. Experimental soil was extracted from the site around the research reactor, since it might be partially contaminated with 60Co and 137Cs. But a soil which was not contaminated was excavated and was artificially contaminated with Co2+ and Cs+ for the experiments. Co2+ can be exchanged with zeolite similar to NaX [15]. The measurement results of the composition and properties of the soil near the nuclear facility used in the experiments are shown in Table 1. Also, Fig. 1 shows a particle size distribution curve of the soil near the nuclear facility.
Fig. 1. Particle size distribution curve of a soil near nuclear facility.
2.2. Experimental set-up Electrokinetic equipment and electrokinetic-flushing equipment were manufactured for the experiments. The electrokinetic-flushing equipment consisted of an acryl soil cell, two electrodes compartments, an electrolyte solution reservoir, an effluent reservoir, a power supply, and a pump and the electrolic equipment is the same as the electrokinetic-flushing equipment except for no pump (Fig. 2). The contaminated soil was placed
Fig. 2. Design diagram of electrokinetic equipment and eletrokinetic-flushing equipment.
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solutions reservoir supplies a chemical solution to an anode electrode compartment. j ¼ ½ðko þ km Þrf þ kh r pC
D rC t2
where j is the molar flux of the species per unit pore area, ko is the electroosmotic permeability, km is the electromigration coefficient, kh is the hydraulic permeability, p is the pressure, C is the molar concentration, D is the diffusion coefficient, and t is a nondimensional tortuosity. The electrode compartments were separated from the soil by a paper filter. A titanium electrode was placed at the end of the cathode electrode compartment and a dimensionally stable anode (DSA) electrode was placed at the end of the anode electrode compartment which can protect against a melting of an anode electrode under a high voltage. The top hole of the electrode compartment was used for adding a strong acid and releasing the produced gas. The soil near the nuclear facility which was not contaminated was excavated and was artificially contaminated with 0.01 M of Co2+ and Cs+. In order to maintain the equality in soil size for several experiments, the soil near the nuclear facility was sieved by 75–2000 mm (Soil A) and 150– 2000 mm (Soil B). 2.3. Remediation experiments Complexes consist of one or more central atoms or central ions, usually metals, with a number of ions or molecules, called ligands, surrounding them and attached to them. Some ligands can form several bonds to a single metal ion. The resulting complexes are called chelates, which are usually more stable than those complexes with a single bond metal–ligand [16]. Ethylenediaminetetraacetic acid can attach to a metal ion for up to six sites, since each of the acetate groups and two nitrogen atoms have the free electron pairs necessary for a coordinate bond formation. The feasibility of this compound as a solubilizing or complexing agent has been reported in several works, especially due to its strong chelating ability for a variety of heavy metals [13]. EDTA is also relatively expensive and given the tones of soils that need remediation, this often leads to an excessively costly remediation [17]. Also, because citric acid is relatively inexpensive, rather easy to handle, and has a comparatively low affinity for alkaline earth metals (Ca, K and Mg), it is a suitable candidate for a soil washing [18]. HNO3 and HCl show a significant potential to extract metal ions from a soil. However, their use is associated with a number of disturbing physical, chemical and biological properties [19]. A soil with a permeability of less than 104 cm/s is considered unsuitable for an in situ washing in which cases an excavation of a contaminated soil followed by an on-site clean-up by a washing can provide a viable alternative [20]. Oxalate was tested as a soil metal extractant because it is biodegradable, naturally occurring and relatively inexpensive, and forms moderately stable metal complexes [21]. Moreover, acetic acid is one of the strongest organic acids and therefore, it is able to attack and dissolve hydrous oxides [22]. Citric acid was used to improve the removal efficiency of the metals from a soil
Fig. 3. Remediation efficiency for 1 day by soil washing versus each electrolyte reagent.
because of it being readily available, relatively inexpensive, and environmentally benign [23]. Shiau et al. [24] showed that citric acid removed up to 80% of the copper from a polluted wood waste [25]. In order to select 4 candidate reagents, soil washing experiments were executed with 12 reagents of 0.01 M for 1 day, respectively. The results are shown in Fig. 3. EDTA, oxalic acid, citric acid, and acetic acid were selected as the candidate reagents for an electrokinetic-flushing remediation on the basis of their removal efficiencies. Then, four different experiments were executed to select an optimal reagent for the removal of Co2+ and Cs+. These experiments (Test IV) were executed with a manufactured electrokinetic equipment (Fig. 2). Namely, the contaminated soil was saturated with water and four different reagents were used as purging solutions in the electrode compartments. Then an optimal electrolyte reagent could be determined by a comparison of the removal efficiencies of Co2+ and Cs+ from these experimental results. In Tests V and VI, the contaminated soil was saturated with water and the selected optimal electrolyte reagent was used as a purging solution in the electrode compartments. Also, electrokinetic-flushing equipment was used to investigate a variation of the removal efficiency along with an increase of the electrolyte reagent rate. In Tests VII and VIII, the contaminated Soil B was saturated with water and the selected an optimal electrolyte reagent was
Fig. 4. A variation of electrolyte flow rate versus remediation time at the cathode compartment during Tests I–IV.
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Table 2 Co2+ and Cs+ removal efficiency, potential gradient, effluent volume under different experiment conditions. Experimental condition
Remediation (days)
Co2+ and Cs+ removal efficiency in the soil cell (%)
Potential gradient (V/cm)
Effluent solution volume (ml/g)
Soil hydro-conductivity (m/s)
Test
Electrokinetic (EDTA)
15
Co (%) Cs (%)
83.5 57.9
2
871 ml (1.4)
1.6 105 m/s
I
Electrokinetic (oxalic acid)
15
Co (%) Cs (%)
90.5 81.9
2
966 ml (1.6)
1.6 105 m/s
II
Electrokinetic (citric acid)
15
Co (%) Cs (%)
86.9 67.1
2
1,027 ml (1.7)
1.6 105 m/s
III
Electrokinetic (acetic acid)
15
Co (%) Cs (%)
92.1 83.1
2
978 ml (1.6)
1.6 105 m/s
IV
Electrokinetic-flushing
15
Co (%) Cs (%)
97.9 86.9
2
1,471 ml (2.4)
1.6 105 m/s
V
Electrokinetic-flushing
15
Co (%) Cs (%)
98.3 87.2
2
2,944 ml (4.8)
1.6 105 m/s
VI
Electrokinetic Soil B
15
Co (%) Cs (%)
92.8 85.1
2
1,465 ml (2.4)
6.7 105 m/s
VII
Electrokinetic-flushing Soil B
15
Co (%) Cs (%)
99.6 92.8
2
1,471 ml (2.4)
6.7 105 m/s
VIII
Soil washing (2nd scrubbing)
15
Co (%) Cs (%)
83.9 40.5
used as a purging solution in the electrode compartments. Likewise, the electrokinetic-flushing equipment was used to investigate a variation of the removal efficiency in a higher hydro-conductivity of a soil. Table 2 shows the remediation method, potential gradient, and different experiment conditions. For Tests I–IV, a constant DC voltage of 29 was applied and the electric current was increased with the test time’s passage. The concentration of EDTA, oxalic acid, citric acid, and acetic acid electrolyte reagents in the electrode compartments was 0.01 M, so the pH for both electrode compartments was around 3. Due to the electrolysis reaction at the electrodes, H+ at the anode and OH at the cathode compartment were generated, and the pH of the solution in the cathode compartment was increased up to 12. The height of the electrolyte reagent in the electrode compartments was kept at the same level to avoid the formation of a hydraulic gradient across a specimen (Tests I–V), and the pore liquid was transported along the cell by an electroosmosis. About 385 cm3 of the soil saturated with water was placed in the soil cell and the total soil weight was 603 g. The electric current across a soil cell, as well as the flow rate and concentration of a pore solution, and the pH in both anode and cathode compartments were periodically measured throughout the experimental period. If the pH in the soil increases to more than 6, the zeta potential of the soil increases. Namely, the surface charge of the soil can change to a positive one and the direction of the electroosmotic flow can be reversed [26]. Therefore in order to keep the pH of the soil near the cathode lower than 6, HCl was periodically put into the cathode electrode compartment. Also, after the completion of an experiment, the soil in a soil cell was divided into 6 sections and dried for more than 3 days to analyze the removal efficiencies of cobalt and cesium from a soil for 15 days. Each
28,400 ml (47.1)
5 g of dry soil and about 10 ml of undiluted nitric acid solution were mixed, and heated at 150 8C on a hot plate for 3 days, then filtered at a 0.2 mm size, diluted to 50 ml, and then the concentrations of the cobalt and cesium in the diluted solution were measured by AA. Also, to measure the pH distribution in a soil cell, 10 g sample from each segment of a soil was mixed with 25 ml of distilled water, and the resulting slurry was thoroughly stirred and allowed to stand for a few minutes. The pH of the supernatant was then determined using a calibrated pH meter. In Test V, the artificially contaminated soil was saturated with water. The acetic acid selected from Tests I–IV was used as an electrolyte reagent. The electrokinetic-flushing equipment was used for this experiment. The pH in the cathode compartment was controlled by continuously adding a strong acid and it was constantly lower than 6. Electric current, pH, and flow rate of a pore solution were periodically measured at the electrode compartments. After an experiment of 15 days, the residual concentrations of cobalt and cesium in a soil were measured by the same method as Tests I–IV. Finally, the difference between the removal efficiencies of an electrokinetic-flushing remediation (Test V) and those of an electrokinetic remediation (Test IV) were compared through an analysis of the residual concentrations in the soil cells. And a difference in their effluent volume was analyzed. In Test VI, the artificially contaminated soil was saturated with water. The selected acetic acid was used as a flushing reagent. The electrokinetic-flushing equipment was used for this experiment and the flushing reagent rate was injected at more than that in Test V. A variation of the removal efficiency of the contaminants by an increase of the flushing reagent rate in a soil cell was estimated. In Tests VII and VIII, the effect of a
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flushing on an electrokinetic-flushing remediation was estimated for a higher hydro-conductivity of a soil (Soil B). Soil B which has a higher hydro-conductivity was injected in a soil cell. The removal efficiency of the contaminants by the electrokinetic-flushing equipment was analyzed after the completion of an experiment for 15 days. 3. Results and discussion 3.1. The selection of an electrolyte reagent suitable for an electrokinetic remediation Table 2 shows the removal efficiencies of Co2+ and Cs+ and the effluent volume as experiment results under different experiment conditions. Fig. 4 shows a variation of the electrolyte flow rate versus the remediation time at the cathode compartment during Tests I–IV. The electrolyte flow rate from the soil cell was reduced with the remediation time. Movement of the pore solution was mainly due to an electroosmosis. When EDTA was used as an electrolyte reagent, the average electrolyte flow rate was 58.1 ml/day. When oxalic acid was used as an electrolyte reagent, the average electrolyte flow rate was 64.4 ml/day. When citric acid was used as an electrolyte reagent, the average electrolyte flow rate was 68.5 ml/day. And when Acetic acid was used as an electrolyte reagent, the average electrolyte flow rate was 65.6 ml/day. Therefore, when citric acid was used as a flushing reagent, the average electrolyte flow rate was the fastest. Also, it was found that the effluent solution volume generated from an electrokinetic remediation it was smaller and was 5% below that from a soil washing. Fig. 5 shows a total Co2+ removal efficiency versus remediation time for Tests I–IV. The removal efficiency rates until 7 days were fast, while those after 7 days were slow. The removal efficiency of cobalt from a soil cell with EDTA was 83.5%, that of cobalt with oxalic acid was 90.5%, that of cobalt with citric acid was 86.9%, and that of cobalt with acetic acid was 92.1%. The removal efficiency of cobalt from the contaminated soil with acetic acid for an electrokinetic remediation was the highest.
Fig. 5. Total Co2+ removal efficiency with remediation time in Tests I–IV.
Fig. 6. Total Cs+ removal efficiency with remediation time in Tests I–IV.
Fig. 6 shows a total Cs+ removal efficiency versus the remediation time for Tests I–IV. The removal efficiency rates were almost constant for 15 days. Therefore it is predicted that the Cs+ desorption time was longer than the Co2+ desorption time and the total Cs+ removal efficiency increases lineally with an increase of the remediation days. Namely, In order to obtain a remediation efficiency of more than 90%, more remediation days were required. The removal efficiency of cesium from a soil cell with EDTA was 57.9%, that of cesium with oxalic acid was 81.9%, that of cesium with citric acid was 67.1%, and that of cesium with acetic acid was 83.1%. Therefore, it was found that the removal efficiency of cesium from a soil cell with the acetic acid was the highest. Results of the above experiments show that the removal efficiency of cobalt and cesium with acetic acid was the highest. It may be a reason that the pH in soil cell appeared to be the lowest value, when acetic acid was used as an electrolyte reagent. Therefore, acetic acid was selected as the optimum reagent and it was used for the following tests. 3.2. A comparison of an electrokinetic-flushing remediation and an electrokinetic remediation Fig. 7 shows the pH distribution in a soil cell and the electrode compartments after 15 days of Tests IV and V. HCl was periodically put into the cathode electrode compartment so that the pHs of the soil cells could be kept below 6. After a completion of Test IV, an average pH in a soil cell was 2.5, while in the case of Test V, an average pH in a soil cell was 3.5. H+ was produced at the anode in a soil cell when using the electrokinetic experiment. Because a soil cell was injected with a lot of acetic acid solution by a pump for Test V, it was predicted that the pHs in a soil cell after Test IV were lower than those after Test V. Fig. 8 shows the total Co2+ and Cs+ removal efficiencies versus the remediation time during Tests IV and V. The effect of an electrokinetic-flushing was increased until 5 days, while it was reduced after 5 days. Namely, the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 5 days were increased by about 25% and 35% when compared to those by the electrokinetic remediation, but the effect of a
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737
Fig. 7. The pH variation in a soil cell after a completion of Tests IV–V.
flushing by the electrokinetic-flushing equipment was decreased after 5 days. Fig. 9 shows the Co2+ and Cs+ distributions versus a normalized distance in a soil cell after Tests IV and V for 15 days. The average removal efficiency of cobalt from a soil cell for Test IV was 92.1% and that of cesium was 83.1%. While, the average removal efficiency of cobalt from a soil cell for Test V was 97.9% and that of cesium was 86.9%. Namely, an electrokinetic-flushing remediation enhanced the removal efficiencies of Co2+ and Cs+ from the contaminated soil by 5.8% and 3.8%, respectively, at 15 days when compared to the electrokinetic remediation. Fig. 10 shows the total Co2+ and Cs+ removal efficiencies versus the remediation time during Tests Vand VI. The effect of an injection flushing rate on an electrokinetic-flushing was estimated. The removal efficiency of cobalt from a soil cell for Test V (injection flushing rate: 2.4 ml/g) was 97.9% and that of cesium was 86.9%. While, the removal efficiency of cobalt from a soil cell for Test VI (injection flushing rate: 4.8 ml/g) was 98.3% and that of cesium was 87.2%. Namely, when the injection flushing rate was doubled, the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 15
days were increased by just 0.4% and 0.3%. Therefore it was suitable for the injection flushing rate for an electrokineticflushing remediation to be 1.5 times (2.4 ml/g) that from electrokinetic remediation. Fig. 11 shows the effect of an electrokinetic-flushing remediation for a higher hydro-conductivity of a soil. Soil B which has a higher hydro-conductivity was located in a soil cell. The removal efficiency of cobalt from a soil cell for Test VII (electrokinetic remediation) was 92.8% and that of cesium was 85.1%. While, the removal efficiency of cobalt from a soil cell for Test VIII (electrokinetic-flushing remediation) was 99.6% and that of cesium was 92.8%. Namely, the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. Therefore, the higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation.
Fig. 8. Total Co2+ and Cs+ removal efficiency with remediation time along reagent flushing rate (Test IV: 1.6 ml/g, Test V: 2.4 ml/g).
Fig. 10. Total Co2+ and Cs+ removal efficiency with remediation time along reagent flushing rate (Test V: 2.4 ml/g, Test VI: 4.8 ml/g).
Fig. 9. Co2+ and Cs+ distributions versus a normalized distance in a soil cell after a completion of Tests V–VI.
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by the electrokinetic-flushing equipment started to decrease after 5 days. (e) The removal efficiencies of Co2+ and Cs + by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. (f) The higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation. References
Fig. 11. Total Co2+ and Cs+ removal efficiency with remediation time in Soil B with a higher hydro-conductivity (Tests VII–VIII).
4. Conclusion Electrokinetic-flushing equipment with a pump was manufactured to investigate the effect of a flushing on an electrokinetic-flushing remediation. In order to select an optimal flushing reagent suitable for the radioactive soil characteristics, four experiments were executed with four candidate-flushing reagents and the results of the experiments are as follows: (a) The removal efficiencies of cobalt and cesium from the contaminated soil with the acetic acid were highest, which were 92.1% and 83.1%, respectively. Therefore, the acetic acid was selected as an optimum reagent. (b) The effluent solution volume generated from an electrokinetic remediation was much smaller and it was 5% below that from a soil washing. (c) The Cs+ desorption time was longer than the Co2+ desorption time and the total Cs+ removal efficiency increased lineally with increased remediation days. Next, the results from a comparison of an electrokineticflushing remediation and an electrokinetic remediation were as follows: (d) The removal efficiencies of Co2+ and Cs + by an electrokinetic-flushing remediation for 5 days was increased by 25% and 35% when compared to those by an electrokinetic remediation, but the effect of a flushing
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Journal of Industrial and Engineering Chemistry 14 (2008) 732–738 www.elsevier.com/locate/jiec
Development of electrokinetic-flushing technology for the remediation of contaminated soil around nuclear facilities Gye-Nam Kim *, Yun-Ho Jung, Jung-Joon Lee, Jei-Kwon Moon, Chong-Hun Jung Korea Atomic Energy Research Institute, Daedeokdaero, Yuseong-gu, Daejon, Republic of Korea Received 24 October 2007; accepted 14 May 2008
Abstract The effects of a flushing by a pump on an electrokinetic-flushing remediation of contaminated soil were estimated. The soils were sampled from the sites around nuclear facilities which were built on a high hydro-conductivity of sandstone. An electrokinetic-flushing equipment with a pump was manufactured to estimate the effect of a flushing on an electrokinetic-flushing remediation. In order to select an optimal reagent suitable to the characteristics of a soil near nuclear facilities, 4 experiments were executed with 4 candidate reagents selected from 12 reagents and the results of the experiments are as follows. The removal efficiencies of cobalt and cesium from the contaminated soil with the acetic acid were the highest, which were 92.1% and 83.1%, respectively. The effluent solution volume generated from an electrokinetic remediation was very smaller and it was 5% below that from a soil washing. Next, the results from a comparison of an electrokinetic-flushing remediation and an electrokinetic remediation revealed that the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 5 days were increased by 25% and 35% when compared to those by the electrokinetic remediation, but the effect of a flushing by the electrokinetic-flushing equipment started to decrease after 5 days. The removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. Namely, the higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation. # 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Keywords: Electrokinetic-flushing; Remediation; Soil; Cobalt; Cesium; Reagent; Removal efficiency
1. Introduction South Korea has many nuclear power plants and a research reactor. As such, the soil around these nuclear facilities can become contaminated with radionuclides from a long-term operation of these facilities. The electrokinetic process holds great promise for a remediation of polluted soils, as it has a high removal efficiency and is time effective for a low permeability. An electrokinetic remediation can be used to treat soils contaminated with inorganic species and radionuclides [1,2]. The main mechanisms of a contaminant’s movement in an electrical field involved in electrokinetic technology are an electromigration of ionic species and electroosmosis. Electromigration probably contributes significantly to the removal of contaminants, especially at high concentrations of ionic contaminants and/or a high hydraulic permeability of a soil [3]. The cathode reaction should be depolarized to avoid the generation of
* Corresponding author. Tel.: +82 42 868 8674.
hydroxides and their transport in a soil. The selected liquids, also known as purging solutions, should induce favorable pH conditions in a soil, and/or interact with the heavy metals, so that these heavy metals are removed from a soil [4]. Most radioactive facility sites have been contaminated by the leakage of radioactive waste-solution due to the corrosion of concrete and pipes by a long-term operation of waste-solution tanks and connection pipes, set up in the underground around nuclear power plants. Electroosmosis moves a pore solution in response to an electric field, typically towards a cathode because of the negative surface charge of a soil. The magnitude of the transport velocity due to an electromigration and electroosmosis is directly related to the electric potential gradient. Therefore remediation efficiency can be improved through an increase of the electric potential gradient by a pumping. Recently, researchers have been investigating whether this method can be used to remove subsurface contaminants and they have compiled the published research on the use of electrokinetic techniques to decontaminate fine-grained soils, and discussed some of the problems that occur
1226-086X/$ – see front matter # 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.05.001
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during this process [5–7]. An electrolytic reactor was used for a greywater reuse [8]. An electrokinetic remediation has been applied to remove heavy metals from mine trailing soils [9]. Recently researchers have tried to develop soil washing techniques in which soil-bound contaminants are transferred to a liquid phase by a desorption and solubilization. Several washing solutions have been investigated, such as water, acids, bases, chelating agents, alcohols and other additives [10,11]. In practice, acid washing and a chelator soil washing are the two most prevalent removal methods [12–14]. The research reactor in Seoul, South Korea is under decommissioning work. Because the research reactor was constructed on a hard sandstone rock like the other Korea nuclear power plants, the contaminated soil around the research reactor contains a lot of sand and it has a higher hydro-conductivity. It has been supposed that an electrokinetic-flushing remediation is a suitable technology in consideration of the soil characteristics near the nuclear facility, which has merits of both an electrokinetic remediation and a soil flushing method. The object of this study was to estimate the effect of a flushing by a pump on an electrokinetic-flushing remediation. The electrokinetic-flushing equipment with a pump was manufactured. In order to select an optimal reagent suitable to the soil characteristics, four experiments were executed with four candidate reagents using the electrokinetic equipment. In order to estimate the effect of a flushing by a pump on an electrokineticflushing remediation, different hydro-conductivities of soils were decontaminated with different injection flushing rates using the electrokinetic-flushing equipment.
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Table 1 Mineralogical composition and properties of a soil near nuclear facility. Soil components and properties
Value
Soil components SiO2 Al2O3 K2 O Fe2O3 P2O5 CaO Na2O MgO
64.4% 16.7% 9.5% 3.0% 1.8% 1.5% 1.2% 0.4%
Moisture content (pH) Surface 10 cm depth 30 cm depth 40 cm depth 50 cm depth Moisture content (pH) Porosity Bulk density
0.43 1.56 g/cm3
Hydro-conductivity 75–2000 mm (Soil A) 150–2000 mm (Soil B)
1.6 105 m/s 6.7 105 m/s
13.5% (7.5) 11.3% (7.4) 10.2% (6.8) 9.8% (6.6) 9.6% (6.7)
into a horizontal soil cell of 4.5 cm 5.9 cm 14.5 cm. A paper filter was inserted between the electrode compartment and the contaminated soil to protect against an influx of a soil. A pump was used to increase the pressure transport of the pore solution in a soil pore by the following equation. Electrolyte
2. Soil characteristics and experiment method 2.1. Description of a soil South Korea currently has about 20 nuclear power plants and a research reactor. Because most nuclear facilities in Korea have been constructed on a hard sandstone rock, the contaminated soil around a nuclear facility contains a lot of sand, and it has a higher hydro-conductivity. Experimental soil was extracted from the site around the research reactor, since it might be partially contaminated with 60Co and 137Cs. But a soil which was not contaminated was excavated and was artificially contaminated with Co2+ and Cs+ for the experiments. Co2+ can be exchanged with zeolite similar to NaX [15]. The measurement results of the composition and properties of the soil near the nuclear facility used in the experiments are shown in Table 1. Also, Fig. 1 shows a particle size distribution curve of the soil near the nuclear facility.
Fig. 1. Particle size distribution curve of a soil near nuclear facility.
2.2. Experimental set-up Electrokinetic equipment and electrokinetic-flushing equipment were manufactured for the experiments. The electrokinetic-flushing equipment consisted of an acryl soil cell, two electrodes compartments, an electrolyte solution reservoir, an effluent reservoir, a power supply, and a pump and the electrolic equipment is the same as the electrokinetic-flushing equipment except for no pump (Fig. 2). The contaminated soil was placed
Fig. 2. Design diagram of electrokinetic equipment and eletrokinetic-flushing equipment.
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solutions reservoir supplies a chemical solution to an anode electrode compartment. j ¼ ½ðko þ km Þrf þ kh r pC
D rC t2
where j is the molar flux of the species per unit pore area, ko is the electroosmotic permeability, km is the electromigration coefficient, kh is the hydraulic permeability, p is the pressure, C is the molar concentration, D is the diffusion coefficient, and t is a nondimensional tortuosity. The electrode compartments were separated from the soil by a paper filter. A titanium electrode was placed at the end of the cathode electrode compartment and a dimensionally stable anode (DSA) electrode was placed at the end of the anode electrode compartment which can protect against a melting of an anode electrode under a high voltage. The top hole of the electrode compartment was used for adding a strong acid and releasing the produced gas. The soil near the nuclear facility which was not contaminated was excavated and was artificially contaminated with 0.01 M of Co2+ and Cs+. In order to maintain the equality in soil size for several experiments, the soil near the nuclear facility was sieved by 75–2000 mm (Soil A) and 150– 2000 mm (Soil B). 2.3. Remediation experiments Complexes consist of one or more central atoms or central ions, usually metals, with a number of ions or molecules, called ligands, surrounding them and attached to them. Some ligands can form several bonds to a single metal ion. The resulting complexes are called chelates, which are usually more stable than those complexes with a single bond metal–ligand [16]. Ethylenediaminetetraacetic acid can attach to a metal ion for up to six sites, since each of the acetate groups and two nitrogen atoms have the free electron pairs necessary for a coordinate bond formation. The feasibility of this compound as a solubilizing or complexing agent has been reported in several works, especially due to its strong chelating ability for a variety of heavy metals [13]. EDTA is also relatively expensive and given the tones of soils that need remediation, this often leads to an excessively costly remediation [17]. Also, because citric acid is relatively inexpensive, rather easy to handle, and has a comparatively low affinity for alkaline earth metals (Ca, K and Mg), it is a suitable candidate for a soil washing [18]. HNO3 and HCl show a significant potential to extract metal ions from a soil. However, their use is associated with a number of disturbing physical, chemical and biological properties [19]. A soil with a permeability of less than 104 cm/s is considered unsuitable for an in situ washing in which cases an excavation of a contaminated soil followed by an on-site clean-up by a washing can provide a viable alternative [20]. Oxalate was tested as a soil metal extractant because it is biodegradable, naturally occurring and relatively inexpensive, and forms moderately stable metal complexes [21]. Moreover, acetic acid is one of the strongest organic acids and therefore, it is able to attack and dissolve hydrous oxides [22]. Citric acid was used to improve the removal efficiency of the metals from a soil
Fig. 3. Remediation efficiency for 1 day by soil washing versus each electrolyte reagent.
because of it being readily available, relatively inexpensive, and environmentally benign [23]. Shiau et al. [24] showed that citric acid removed up to 80% of the copper from a polluted wood waste [25]. In order to select 4 candidate reagents, soil washing experiments were executed with 12 reagents of 0.01 M for 1 day, respectively. The results are shown in Fig. 3. EDTA, oxalic acid, citric acid, and acetic acid were selected as the candidate reagents for an electrokinetic-flushing remediation on the basis of their removal efficiencies. Then, four different experiments were executed to select an optimal reagent for the removal of Co2+ and Cs+. These experiments (Test IV) were executed with a manufactured electrokinetic equipment (Fig. 2). Namely, the contaminated soil was saturated with water and four different reagents were used as purging solutions in the electrode compartments. Then an optimal electrolyte reagent could be determined by a comparison of the removal efficiencies of Co2+ and Cs+ from these experimental results. In Tests V and VI, the contaminated soil was saturated with water and the selected optimal electrolyte reagent was used as a purging solution in the electrode compartments. Also, electrokinetic-flushing equipment was used to investigate a variation of the removal efficiency along with an increase of the electrolyte reagent rate. In Tests VII and VIII, the contaminated Soil B was saturated with water and the selected an optimal electrolyte reagent was
Fig. 4. A variation of electrolyte flow rate versus remediation time at the cathode compartment during Tests I–IV.
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Table 2 Co2+ and Cs+ removal efficiency, potential gradient, effluent volume under different experiment conditions. Experimental condition
Remediation (days)
Co2+ and Cs+ removal efficiency in the soil cell (%)
Potential gradient (V/cm)
Effluent solution volume (ml/g)
Soil hydro-conductivity (m/s)
Test
Electrokinetic (EDTA)
15
Co (%) Cs (%)
83.5 57.9
2
871 ml (1.4)
1.6 105 m/s
I
Electrokinetic (oxalic acid)
15
Co (%) Cs (%)
90.5 81.9
2
966 ml (1.6)
1.6 105 m/s
II
Electrokinetic (citric acid)
15
Co (%) Cs (%)
86.9 67.1
2
1,027 ml (1.7)
1.6 105 m/s
III
Electrokinetic (acetic acid)
15
Co (%) Cs (%)
92.1 83.1
2
978 ml (1.6)
1.6 105 m/s
IV
Electrokinetic-flushing
15
Co (%) Cs (%)
97.9 86.9
2
1,471 ml (2.4)
1.6 105 m/s
V
Electrokinetic-flushing
15
Co (%) Cs (%)
98.3 87.2
2
2,944 ml (4.8)
1.6 105 m/s
VI
Electrokinetic Soil B
15
Co (%) Cs (%)
92.8 85.1
2
1,465 ml (2.4)
6.7 105 m/s
VII
Electrokinetic-flushing Soil B
15
Co (%) Cs (%)
99.6 92.8
2
1,471 ml (2.4)
6.7 105 m/s
VIII
Soil washing (2nd scrubbing)
15
Co (%) Cs (%)
83.9 40.5
used as a purging solution in the electrode compartments. Likewise, the electrokinetic-flushing equipment was used to investigate a variation of the removal efficiency in a higher hydro-conductivity of a soil. Table 2 shows the remediation method, potential gradient, and different experiment conditions. For Tests I–IV, a constant DC voltage of 29 was applied and the electric current was increased with the test time’s passage. The concentration of EDTA, oxalic acid, citric acid, and acetic acid electrolyte reagents in the electrode compartments was 0.01 M, so the pH for both electrode compartments was around 3. Due to the electrolysis reaction at the electrodes, H+ at the anode and OH at the cathode compartment were generated, and the pH of the solution in the cathode compartment was increased up to 12. The height of the electrolyte reagent in the electrode compartments was kept at the same level to avoid the formation of a hydraulic gradient across a specimen (Tests I–V), and the pore liquid was transported along the cell by an electroosmosis. About 385 cm3 of the soil saturated with water was placed in the soil cell and the total soil weight was 603 g. The electric current across a soil cell, as well as the flow rate and concentration of a pore solution, and the pH in both anode and cathode compartments were periodically measured throughout the experimental period. If the pH in the soil increases to more than 6, the zeta potential of the soil increases. Namely, the surface charge of the soil can change to a positive one and the direction of the electroosmotic flow can be reversed [26]. Therefore in order to keep the pH of the soil near the cathode lower than 6, HCl was periodically put into the cathode electrode compartment. Also, after the completion of an experiment, the soil in a soil cell was divided into 6 sections and dried for more than 3 days to analyze the removal efficiencies of cobalt and cesium from a soil for 15 days. Each
28,400 ml (47.1)
5 g of dry soil and about 10 ml of undiluted nitric acid solution were mixed, and heated at 150 8C on a hot plate for 3 days, then filtered at a 0.2 mm size, diluted to 50 ml, and then the concentrations of the cobalt and cesium in the diluted solution were measured by AA. Also, to measure the pH distribution in a soil cell, 10 g sample from each segment of a soil was mixed with 25 ml of distilled water, and the resulting slurry was thoroughly stirred and allowed to stand for a few minutes. The pH of the supernatant was then determined using a calibrated pH meter. In Test V, the artificially contaminated soil was saturated with water. The acetic acid selected from Tests I–IV was used as an electrolyte reagent. The electrokinetic-flushing equipment was used for this experiment. The pH in the cathode compartment was controlled by continuously adding a strong acid and it was constantly lower than 6. Electric current, pH, and flow rate of a pore solution were periodically measured at the electrode compartments. After an experiment of 15 days, the residual concentrations of cobalt and cesium in a soil were measured by the same method as Tests I–IV. Finally, the difference between the removal efficiencies of an electrokinetic-flushing remediation (Test V) and those of an electrokinetic remediation (Test IV) were compared through an analysis of the residual concentrations in the soil cells. And a difference in their effluent volume was analyzed. In Test VI, the artificially contaminated soil was saturated with water. The selected acetic acid was used as a flushing reagent. The electrokinetic-flushing equipment was used for this experiment and the flushing reagent rate was injected at more than that in Test V. A variation of the removal efficiency of the contaminants by an increase of the flushing reagent rate in a soil cell was estimated. In Tests VII and VIII, the effect of a
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flushing on an electrokinetic-flushing remediation was estimated for a higher hydro-conductivity of a soil (Soil B). Soil B which has a higher hydro-conductivity was injected in a soil cell. The removal efficiency of the contaminants by the electrokinetic-flushing equipment was analyzed after the completion of an experiment for 15 days. 3. Results and discussion 3.1. The selection of an electrolyte reagent suitable for an electrokinetic remediation Table 2 shows the removal efficiencies of Co2+ and Cs+ and the effluent volume as experiment results under different experiment conditions. Fig. 4 shows a variation of the electrolyte flow rate versus the remediation time at the cathode compartment during Tests I–IV. The electrolyte flow rate from the soil cell was reduced with the remediation time. Movement of the pore solution was mainly due to an electroosmosis. When EDTA was used as an electrolyte reagent, the average electrolyte flow rate was 58.1 ml/day. When oxalic acid was used as an electrolyte reagent, the average electrolyte flow rate was 64.4 ml/day. When citric acid was used as an electrolyte reagent, the average electrolyte flow rate was 68.5 ml/day. And when Acetic acid was used as an electrolyte reagent, the average electrolyte flow rate was 65.6 ml/day. Therefore, when citric acid was used as a flushing reagent, the average electrolyte flow rate was the fastest. Also, it was found that the effluent solution volume generated from an electrokinetic remediation it was smaller and was 5% below that from a soil washing. Fig. 5 shows a total Co2+ removal efficiency versus remediation time for Tests I–IV. The removal efficiency rates until 7 days were fast, while those after 7 days were slow. The removal efficiency of cobalt from a soil cell with EDTA was 83.5%, that of cobalt with oxalic acid was 90.5%, that of cobalt with citric acid was 86.9%, and that of cobalt with acetic acid was 92.1%. The removal efficiency of cobalt from the contaminated soil with acetic acid for an electrokinetic remediation was the highest.
Fig. 5. Total Co2+ removal efficiency with remediation time in Tests I–IV.
Fig. 6. Total Cs+ removal efficiency with remediation time in Tests I–IV.
Fig. 6 shows a total Cs+ removal efficiency versus the remediation time for Tests I–IV. The removal efficiency rates were almost constant for 15 days. Therefore it is predicted that the Cs+ desorption time was longer than the Co2+ desorption time and the total Cs+ removal efficiency increases lineally with an increase of the remediation days. Namely, In order to obtain a remediation efficiency of more than 90%, more remediation days were required. The removal efficiency of cesium from a soil cell with EDTA was 57.9%, that of cesium with oxalic acid was 81.9%, that of cesium with citric acid was 67.1%, and that of cesium with acetic acid was 83.1%. Therefore, it was found that the removal efficiency of cesium from a soil cell with the acetic acid was the highest. Results of the above experiments show that the removal efficiency of cobalt and cesium with acetic acid was the highest. It may be a reason that the pH in soil cell appeared to be the lowest value, when acetic acid was used as an electrolyte reagent. Therefore, acetic acid was selected as the optimum reagent and it was used for the following tests. 3.2. A comparison of an electrokinetic-flushing remediation and an electrokinetic remediation Fig. 7 shows the pH distribution in a soil cell and the electrode compartments after 15 days of Tests IV and V. HCl was periodically put into the cathode electrode compartment so that the pHs of the soil cells could be kept below 6. After a completion of Test IV, an average pH in a soil cell was 2.5, while in the case of Test V, an average pH in a soil cell was 3.5. H+ was produced at the anode in a soil cell when using the electrokinetic experiment. Because a soil cell was injected with a lot of acetic acid solution by a pump for Test V, it was predicted that the pHs in a soil cell after Test IV were lower than those after Test V. Fig. 8 shows the total Co2+ and Cs+ removal efficiencies versus the remediation time during Tests IV and V. The effect of an electrokinetic-flushing was increased until 5 days, while it was reduced after 5 days. Namely, the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 5 days were increased by about 25% and 35% when compared to those by the electrokinetic remediation, but the effect of a
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Fig. 7. The pH variation in a soil cell after a completion of Tests IV–V.
flushing by the electrokinetic-flushing equipment was decreased after 5 days. Fig. 9 shows the Co2+ and Cs+ distributions versus a normalized distance in a soil cell after Tests IV and V for 15 days. The average removal efficiency of cobalt from a soil cell for Test IV was 92.1% and that of cesium was 83.1%. While, the average removal efficiency of cobalt from a soil cell for Test V was 97.9% and that of cesium was 86.9%. Namely, an electrokinetic-flushing remediation enhanced the removal efficiencies of Co2+ and Cs+ from the contaminated soil by 5.8% and 3.8%, respectively, at 15 days when compared to the electrokinetic remediation. Fig. 10 shows the total Co2+ and Cs+ removal efficiencies versus the remediation time during Tests Vand VI. The effect of an injection flushing rate on an electrokinetic-flushing was estimated. The removal efficiency of cobalt from a soil cell for Test V (injection flushing rate: 2.4 ml/g) was 97.9% and that of cesium was 86.9%. While, the removal efficiency of cobalt from a soil cell for Test VI (injection flushing rate: 4.8 ml/g) was 98.3% and that of cesium was 87.2%. Namely, when the injection flushing rate was doubled, the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 15
days were increased by just 0.4% and 0.3%. Therefore it was suitable for the injection flushing rate for an electrokineticflushing remediation to be 1.5 times (2.4 ml/g) that from electrokinetic remediation. Fig. 11 shows the effect of an electrokinetic-flushing remediation for a higher hydro-conductivity of a soil. Soil B which has a higher hydro-conductivity was located in a soil cell. The removal efficiency of cobalt from a soil cell for Test VII (electrokinetic remediation) was 92.8% and that of cesium was 85.1%. While, the removal efficiency of cobalt from a soil cell for Test VIII (electrokinetic-flushing remediation) was 99.6% and that of cesium was 92.8%. Namely, the removal efficiencies of Co2+ and Cs+ by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. Therefore, the higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation.
Fig. 8. Total Co2+ and Cs+ removal efficiency with remediation time along reagent flushing rate (Test IV: 1.6 ml/g, Test V: 2.4 ml/g).
Fig. 10. Total Co2+ and Cs+ removal efficiency with remediation time along reagent flushing rate (Test V: 2.4 ml/g, Test VI: 4.8 ml/g).
Fig. 9. Co2+ and Cs+ distributions versus a normalized distance in a soil cell after a completion of Tests V–VI.
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by the electrokinetic-flushing equipment started to decrease after 5 days. (e) The removal efficiencies of Co2+ and Cs + by an electrokinetic-flushing remediation for 15 days were increased by 6.8% and 7.7% when compared to that by an electrokinetic remediation. (f) The higher the hydro-conductivity of a soil was, the larger the effect of a flushing was on an electrokinetic-flushing remediation. References
Fig. 11. Total Co2+ and Cs+ removal efficiency with remediation time in Soil B with a higher hydro-conductivity (Tests VII–VIII).
4. Conclusion Electrokinetic-flushing equipment with a pump was manufactured to investigate the effect of a flushing on an electrokinetic-flushing remediation. In order to select an optimal flushing reagent suitable for the radioactive soil characteristics, four experiments were executed with four candidate-flushing reagents and the results of the experiments are as follows: (a) The removal efficiencies of cobalt and cesium from the contaminated soil with the acetic acid were highest, which were 92.1% and 83.1%, respectively. Therefore, the acetic acid was selected as an optimum reagent. (b) The effluent solution volume generated from an electrokinetic remediation was much smaller and it was 5% below that from a soil washing. (c) The Cs+ desorption time was longer than the Co2+ desorption time and the total Cs+ removal efficiency increased lineally with increased remediation days. Next, the results from a comparison of an electrokineticflushing remediation and an electrokinetic remediation were as follows: (d) The removal efficiencies of Co2+ and Cs + by an electrokinetic-flushing remediation for 5 days was increased by 25% and 35% when compared to those by an electrokinetic remediation, but the effect of a flushing
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