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Decontamination of gravels contaminated with uranium
Annals of Nuclear Energy 72 (2014) 367–372
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Decontamination of gravels contaminated with uranium Gye-Nam Kim a,⇑, Uk-Ryang Park a, Seung-soo Kim a, Wan-Suk Kim a, Jei-Kwon Moon a, Jae-hyuk Hyun b a b
Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yusong-gu, Daejeon 305-353, Republic of Korea Environmental Engineering, Chungnam National University, 99 Daehak-ro, Yusong-gu, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 January 2014 Received in revised form 28 May 2014 Accepted 30 May 2014
Keywords: Decontamination Uranium Gravel Washing Electrokinetic Electrodialytic
a b s t r a c t Gravel washing equipment and electrokinetic–electrodialytic decontamination equipment were manufactured to decontaminate gravel contaminated with uranium. The removal efficiency according to the gravel size and weight and the removal efficiency according to the lapsed time using the manufactured equipment were investigated through several experiments. The volume of gravel in the high uranium concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The larger the gravel size, the higher the contaminated concentration of gravel. The average uranium (238U) concentration of gravel after the first washing was about 1.45 Bq/g, and the average removal efficiency of gravel after the third washing was about 37%. In addition, the removal efficiency of the contaminated gravel was not related to its size. The contaminated concentration of the gravel decreased with an increasing gravel weight. In addition, the removal efficiency of contaminated gravel was not related to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, the 238U in the gravel was removed by about 40%, 65%, 72%, and 81%. The more the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. Finally, the gravel with a size of less than 10 cm was treated by soil washing and electrokinetic decontamination methods with soil, and gravel with size of more than 10 cm but less than 20 cm was treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Gravel with a size of more than 20 cm is treated by a gravel washing method, and gravel contaminated with a high concentration of uranium was treated by crushing and ball mill washing methods. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The soil around nuclear facilities is contaminated with radionuclides during the operation and decommissioning of those facilities, especially during a nuclear accident such as the one at Chernobyl in Ukraine and the one at Fukushima in Japan. Korea has a lot of soil contaminated with uranium generated during its operation of nuclear facilities. Soil with a size of less than 10 cm is usually decontaminated using soil washing and electrokinetic technologies. However, it is difficult to use soil washing technology for decontamination of gravel with a size of more than 10 cm. It is impossible to scrub gravel in a washing tank, because the gravel sinks to the bottom (Fedje et al., 2013; Bisone et al., 2012; Gryschko et al., 2005; Tandy et al., 2004; Voglar and Lestan, 2013). In addition, when electrokinetic decontamination technology
⇑ Corresponding author. Tel.: +82 42 868 8674; fax: +82 42 868 2499. E-mail address: [email protected] (G.-N. Kim). http://dx.doi.org/10.1016/j.anucene.2014.05.031 0306-4549/Ó 2014 Elsevier Ltd. All rights reserved.
(Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005) is applied to gravel with a size of more than 10 cm, the removal efficiency of the radionuclides from the gravel is reduced, because the electro-osmotic flux at the surface of the gravel in an electrokinetic cell is reduced owing to a reduction of the particle surface area, which is attributable to the large size of the gravel (Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005; Kim et al., 2011; Kim et al., 2010, 2008). Meanwhile, there have been few studies on the decontamination of gravel with a size of more than 10 cm. The volume ratio of gravel whose size is more than 10 cm in the total volume of soil at KAERI was about 20%. Therefore, it is necessary to study the decontamination of gravel contaminated with radionuclides. Vitoria et al. used three mechanical gravel cleaning methods: (1) tractor rotovating, using a Dowdeswell Powervator 35 rotovator, with a width of 90 cm, behind a Ford 1220 four-wheel drive tractor, (2) high pressure jet washing, using a KEW 5203 KD pressure washer, in which water was pumped at 150 bar through a hand-held lance with jets of 5 mm and 1 mm diameter, and (3)
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pump washing, using a Pacer pump with a 3 Hp Briggs and Stratton engine. The most effective method for cleaning river gravels appeared to be pump washing (Victoria et al., 1999). Pereira et al. used vegetable oil biodiesel for cleaning oiled shores. Pure oil biodiesels (rapeseed and soybean) were significantly more effective in the cleanup of oiled sands (up to 96%) than recycled waste cooking oil biodiesel (70%) (Pereira and Mudge, 2004). Clement et al. used a physic-chemical method for the treatment of dredged sediment. The positive effects of physic-chemical treatment are the reduction of sediment mass, materials easier to handle, reduction of visual and odor nuisances, better settability, the removal of ammonia emissions and associated ecotoxicological risks, removal of zinc and nickel emissions, and a reduction of sediment toxicity to amphipods and chrironomids (Clement et al., 2010). Min et al. used thermal and mechanical treatment to clean aggregates from concrete. Most pollutants are easily separated from the contaminated concrete waste, which concentrates mainly in the porous fine cement powder. Removal on pollutants in concrete was influenced by the heating temperature and crushed aggregate size. Heating temperature played an important role in moving the contaminants from the concrete waste (Min et al., 2010). Cho et al. used dry washing technology for the treatment of polluted railroad ballast gravel. A dry washing method removes the pollutants on the surface of ballast gravels by blasting media on the gravel. The technology efficiently removed heavy metals from contaminated gravel for a short time (Cho et al., 2012). In this study, the gravel contaminated with uranium was sampled at an area near a nuclear facility in Korea. The contamination characterization of gravels separated from soil was analyzed. The gravel washing equipment and electrokinetic–elctrodialytic decontamination equipment were manufactured to decontaminate the contaminated gravel. The removal efficiency according to the gravel size and weight, and the removal efficiency according to the lapsed time by electrokinetic–electrodialytic equipment, was investigated through several experiments. The optimum experiment conditions for uranium decontamination by the gravel washing and electrokinetic–elctrodialytic decontamination equipment were found. Finally, a process to decontaminate the contaminated gravel was developed for the self-disposal of radioactive gravel waste on the basis of the experimental decontamination results. 2. Materials and methods 2.1. Characteristics of contaminated gravels About 30% of the volume of soil contaminated with uranium, which was excavated at an area near a nuclear facility, was gravel. The gravel contaminated with uranium is shown in Fig. 1. High and low concentration groups of contaminated gravels are shown in Table 1. The volume of gravel in the high uranium concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The reason is considered to be that uranium can infiltrate into the deep side of those rocks, because quartz that consists of a crystal cluster, lamprophyre that consists of biotite and amphibole, and schist that consists of biotite and muscovite are easy to be split. Washing after using a crushing method should be selected to decontaminate gravel in the high concentration group for an improvement of the decontamination efficiency.
Fig. 1. Gravel contaminated with uranium.
Table 1 High and low concentration groups of contaminated gravel. Concentration (Bq/g)
High-concentration
Low-concentration
1 2 3 4 5
5.32 7.68 4.56 6.72 7.25
0.82 1.47 2.18 1.7 1.85
Average
6.31
1.60
waste solution collection box, nitric acid solution box, and drum hoist, as shown in Fig. 2. The trammel circulates gravels in its inside at a fixed rpm, and the nozzles in the trammel spray a nitric acid solution to wash the contaminated gravel. The gravel injection box injects contaminated gravel into the trammel, and the gravel collection box collects the washed gravel. The waste solution collection box collects waste solution released during gravel washing, the nitric acid solution box supplies the nozzle in the trammel with nitric acid solution, and the drum hoist transports contaminated gravel to the gravel injection box. Images taken before and after gravel washing are shown in Fig. 3. The optimum experimental conditions of the gravel washing equipment were obtained through several experiments: the optimum rpm of the gravel washing equipment, and the optimum concentration of nitric acid as a washing solution.
2.2. Manufacturing of gravel washing equipment The gravel washing equipment was manufactured to wash the contaminated gravel. The gravel washing equipment consisted of a trammel, nozzle, gravel injection box, gravel collection box,
Fig. 2. Manufactured gravel washing equipment.
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electrokinetic–electrodialytic decontamination equipment was manufactured as shown in Fig. 5 for a shortening of the decontamination period. 2.5. Radioactivity measurement for gravel
Fig. 3. Before and after gravel washing.
2.3. Experiments to decontaminate gravels contaminated with uranium Gravels near the uranium conversion facility in Korea Atomic Energy Research Institute (KAERI) were contaminated with uranium. The size of the gravel was mainly 5–30 cm, and the weight was mainly 150–4000 g. The uranium concentration of the contaminated gravel was 0.5–10.0 Bq/g. The required uranium (238U) concentration for self-disposal was below 0.43 Bq/g. Thus, it was necessary to reduce the uranium concentration of the gravel using washing and electrokinetic decontamination. 2.4. Manufacturing electrokinetic–electrodialytic decontamination equipment Using electrokinetic decontamination technology, the removal efficiency of radionuclides from gravel decreased, because the electro-osmotic flux at the surface of the gravel in the electrokinetic cell decreased due to the large size of the gravel. In addition, the decontamination period was lengthened because the electrolyte in the anode room was contaminated with uranium owing to a higher hydraulic conductivity of gravels in the gravel cell of the electrokinetic equipment. Meanwhile, the electrodialytic method has generally been used for treating waste solution and soil remediation, which attaches an ion exchange membrane at the anode room or cathode room (Sun et al., 2012; Nystrom et al., 2005; Jensen et al., 2012; Jakobsen et al., 2004). Fig. 4 shows a schematic diagram of electrokinetic–electrodialytic decontamination. An anion exchange membrane was attached at the anode room partition to prevent an infiltration of uranium ions. In this study, the
Fig. 4. Schematic diagram of electrokinetic–electrodialytic decontamination.
The original uranium concentration for gravel was measured using a Multi-Channel Analyzer (MCA) with a standard tube of 1000 cc, QCY48 (Amersham), manufactured by the Korea Reach Institute Standards and Sciences. The MCA operates in a pulse height analyzer mode. The scintillation counter measures the pulse height distribution from a gamma ray source. The amplitude of an incoming analog pulse is digitized by an analog digital converter (ADC), and the digital value is used as the address of the incremented memory location. Thus, the screen display of the number of counts vs. the channel number is really a histogram of the number of counts vs. the pulse height, i.e., a pulse height spectrum. The time required to measure the radioactivity concentration of a gravel sample using the MCA was estimated to be 4–8 h. The original radioactivity concentration of the washed gravels before electrokinetic–electrodialytic decontamination was calculated. The electrokinetic–electrodialytic experiment was stopped at pre-determined interim times, the gravel samples were extracted from the gravel cell, and the radioactivity concentration of the gravel samples were then measured using the MCA. The gravel samples were returned to their original locations in the gravel cell, and the experiment was continuously restarted. The removal efficiency of the nuclides was calculated as a ratio of the original gravel concentration, and the gravel concentrations were measured at pre-determined interim times. After completion of the electrokinetic–electrodialytic decontamination experiments for radioactive gravels with initial concentrations of 0.5–6.5 Bq/g, the residual concentration of gravel was calculated. Finally, the decontamination period required for decontaminating the initial gravel concentration to a clearance concentration level (238U: 0.43 Bq/g) was estimated with the experimental electrokinetic– electrodialytic results. 3. Results and discussion From the results of the washing experiments with gravel washing equipment, it was concluded that the optimum rpm of the gravel washing equipment was 10 ppm, the optimum concentration of nitric acid as a washing solution was 1.0 M, and the optimum spray rate by nozzle was 50 L/min. Table 2 shows the results of the removal efficiency according to the gravel size by washing using the manufactured gravel washing equipment. The larger the gravel size, the more the contaminated concentration of gravel decreased. The reason is considered to be that the uncontaminated volume in a larger size of gravel is larger, because it is difficult for uranium to infiltrate into the deep side of gravel. The average concentration of gravel after the first washing was about
Fig. 5. Manufactured electrokinetic–electrodialytic decontamination equipment.
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Table 2 Removal efficiency according to the gravel size by gravel washing. Gravel size
Ci (Bq/g)
1st
2nd
3rd
Removal efficiency (%)
5 cm (154 g) 5 cm (266 g) 10 cm (766 g) 10 cm (784 g) 15 cm (944 g) 15 cm (1074 g)
2.25 2.93 2.54 2.12 1.9 1.2
1.04 2.14 1.83 1.67 1.32 0.68
1.01 2.10 1.76 1.61 1.27 0.67
1.0 2.09 1.76 1.60 1.22 0.65
56 29 31 25 36 46
Average
2.16
1.45
1.40
1.39
37.2
Table 3 Removal efficiency according to the gravel weight by gravel washing (drum 1). Gravel weight (g)
Ci (Bq/g)
1st
2nd
Removal efficiency (%)
1328 1526 1656 1876 2200 2252 3242
0.97 3.71 1.6 0.61 0.4 0.72 0.76
0.91 2.1 1.2 0.4 0.2 0.32 0.72
0.9 1.6 1.18 0.37 0.18 0.3 0.54
7 43 26 39 50 58 29
Average
1.25
0.84
0.72
42
manufactured gravel washing equipment. Table 4 shows the results of the removal efficiency according to the gravel weight by washing the gravel in drum 2 using the manufactured gravel washing equipment. The higher the gravel weight, the more the contaminated concentration of the gravel decreased. The reason is considered to be that the uncontaminated volume in a higher weight of gravel is larger, because it is difficult for uranium to infiltrate into a deep side of gravel. The contamination concentration and removal efficiency of the gravel depend on the rock type. It is impossible to decontaminate deeply contaminated gravel by washing or electrokinetic–electrodialytic decontamination. The optimum number of washings for contaminated gravel is considered to be two. In addition, the removal efficiency of contaminated gravel was not related to its weight. Fig. 6 shows images of the gravel used in Table 4. 3.1. Ball mill washing after crushing
Table 4 Removal efficiency according to the gravel weight by gravel washing (drum 2). Gravel weight (g)
Ci (Bq/g)
1st
2nd
Removal efficiency (%)
1214 1244 1382 1432 1458 1760 2728 3250 4440 4906
3.68 1.5 6.49 0.89 1.2 1.45 0.95 0.73 0.71 0.79
1.86 1.47 4.26 0.68 0.67 0.99 0.85 0.63 0.28 0.54
2.03 1.23 3.68 0.67 0.59 0.98 0.84 0.54 0.27 0.47
45 18 43 25 51 32 12 26 62 41
Average
1.84
1.22
1.13
39
Gravel in the high concentration group should use a washing after a crushing work to improve the decontamination efficiency. Gravel contaminated with a high concentration of uranium was crushed to below 1 mm size, as shown in Fig. 7. The crushed gravel was then put in a ball mill, and washed and crushed for 3–4 h. The removal efficiencies of gravel by ball mill washing are shown in Table 5. The average removal efficiency after the second washing by a ball mill was more than 90%. 3.2. Electrokinetic–electrodialytic decontamination Uranium (UO2+ 2 ) in the contaminated gravel in the electrokinetic– electrodialytic decontamination equipment was removed by electroosmosis, electro-migration, and a hydraulic pressure flow, as in the following equation:
j ¼ ½ðko þ km ÞRrI þ kh rpC
1.45 Bq/g. The average removal efficiency of the gravel after the third washing was about 37% and the removal efficiency of the third gravel washing was very little in comparison with those of the first and second washings. In addition, the removal efficiency of the contaminated gravel was not related to its size. Table 3 shows the results of the removal efficiency according to the gravel weight by washing the gravel in drum 1 using the
D
s2
rC;
ð1Þ
where j is the molar flux of the species per unit pore area, ko the electro-osmotic permeability, km the electro-migration coefficient, R the electric resistance, I the electric current, kh the hydraulic permeability, P the pressure, C the molar concentration, D the diffusion coefficient, and s is a non-dimensional tortuosity. To increase the removal velocity of radionuclides from gravel, electrokinetic–electrodialytic decontamination equipment was manufactured. This equipment mixed the electrokinetic concept and electrodialytic concept. That is, an anion exchange membrane was attached at the net plate of the anode room of the electrokinetic equipment to prevent the transfer of positive ions into the
Fig. 6. Images of gravel used in Table 4.
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Fig. 7. Crushing and ball mill washing for gravel contaminated with high concentration.
Table 5 Removal efficiencies of gravel by ball mill washing. Gravel weight (g)
Ci (Bq/g)
1st
2nd
Removal efficiency (%)
550 650 850
4.8 3.6 3.45
0.95 0.75 0.64
0.36 0.32 0.27
92.5 91.1 92.2
anode room, thus preventing the solution in the anode room from being contaminated with UO22+. The pure solution in the anode room can shorten the time required to remove uranium from gravel surface. The experimental electrokinetic–electrodialytic conditions were as follows. The gravel volume in the gravel cell was 400 L, the electric current was 150–200 A, the electric voltage was 15–20 V, the electrolyte inflow rate was 130–160 mL/min, the temperature in electrokinetic–electrodialytic experiment was below 65 °C, and L (electrolyte volume, mL)/S (gravel weight, g) in the gravel cell was about 0.33. Table 6 shows the removal efficiency of uranium from gravel according to the lapsed time by
electrokinetic–electrodialytic equipment. When the decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, 238U in the gravel was removed by about 40%, 65%, 72%, and 81%. The more the decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. In addition, the more the initial concentration of 238U increased, the more the removal efficiency of 238U increased. A process to decontaminate contaminated gravel was developed for the self-disposal of radioactive gravel waste on the basis of preceding experimental decontamination results. The decontamination process for gravel contaminated with uranium is shown in Fig. 8. Gravel contaminated with uranium can be treated by four methods. First, gravel with a size of less than 10 cm is treated by soil washing and electrokinetic decontamination methods with soil. Second, gravel with a size of more than 10 cm but less than 20 cm is treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Third, gravel with a size of more than 20 cm is treated by a gravel washing method. Fourth, gravel contaminated with a high concentration of uranium is treated by crushing and ball mill washing methods.
Table 6 Removal efficiency according to the lapsed time by electrokinetic–electrodialytic equipment. Lapsed time
Origin (Bq/g)
5 Days (%)
10 Days (%)
15 Days (%)
20 Days
Removal efficiency
2.3 1.7 1.3
42 39 38
67 64 63
74 71 70
83% (0.39 Bq/g) 80.6% (0.31 Bq/g) 80% (0.26 Bq/g)
Average
1.7
40
65
72
81% (0.32 Bq/g)
Fig. 8. The decontamination process for gravel contaminated with uranium.
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4. Conclusions The volume of gravel in the high concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The larger the gravel size, the more the contamination concentration of the gravel decreased. The average concentration of gravel after the first washing was about 1.45 Bq/g. The average removal efficiency of gravel after the third washing was about 37%, and the removal efficiency of the third gravel washing was very little in comparison with those of the first and second washings. In addition, the removal efficiency of the contaminated gravel was not related to its size. The higher the gravel weight, the more the contamination concentration of the gravel decreased. The contamination concentration and removal efficiency of the gravel depend on the rock type. The optimum number of washings for contaminated gravel is considered to be two. In addition, the removal efficiency of contaminated gravel is not related to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, 238U in gravel was removed by about 40%, 65%, 72%, and 81%. The more the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. Conclusively, gravel with a size of less than 10 cm should be treated by soil washing and electrokinetic decontamination methods with soil, and gravel with a size of more than 10 cm but less than 20 cm should be treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Gravel with a size of more than 20 cm should be treated by a gravel washing method, and gravel contaminated with a high concentration of uranium should be treated by crushing and ball mill washing methods. Acknowledgement This project was carried out under the Nuclear R&D Program by MOST in KOREA. References Bisone, S., Blais, J.F., Drogui, P., Mercier, G., 2012. Toxic metal removal from polluted soil by acid extraction. Water Air Soil Pollut. 223 (7), 3739–3755. Cho, Y.M., Lee, J.Y., Kwon, T.S., Jung, W.S., Park, D.S., Lee, I.H., Kim, B.K., Jung, W.H., Lee, C., Yoon, Y.K., Yang, Y.M., Kim, H.M., Lim, J.I., 2012. Field application of dry
washing technology for polluted rail road ballast gravel. Fall Meeting Korea Soc Railway. A139. Clement, B., Vaille, G., Moretto, R., Vernus, E., Abdelghafour, M., 2010. Effects of physic-chemical treatment of a dredged sediment on its ecotoxicity after discharge in laboratory gravel pit microcosms. J. Hazard. Mater. 175, 205–215. Dong, W., Ball, W.P., Liu, C., Wang, Z., Stone, A.T., Bai, J., Zachara, J.M., 2005. Influence of calcite and dissolved calcium on uranium(VI) sorption to a Hanford subsurface sediment. Environ. Sci. Technol. 39 (20), 7949–7955. Fedje, K.K., Yillin, L., Stromvall, A.M., 2013. Remediation of metal polluted hotspot areas through enhanced soil washing – evaluation of leaching methods. J. Environ. Manage. 128, 489–496. Gryschko, R., Kuhnle, R., Terytze, K., Breuer, J., Stahr, K., 2005. Soil extraction of readily soluble heavy metals and as with 1 M NH4NO3-solution. J. Soils Sediments 5 (2), 101–106. Jakobsen, M.R., Fritt-Ramussen, J., Nielsen, S., Ottosen, L.M., 2004. Electrodialytic removal of cadmium from wastewater sludge. J. Hazard. Mater. 106, 127–132. Jensen, P.E., Ottosen, L.M., Allard, B., 2012. Electro dialytic versus acid extraction of heavy metals from soil washing residue. Electrochim. Acta 86, 115–123. Kaneta, T., Tanaka, S., Taga, M., Yoshida, H., 1992. Effect of addition of glucose on micellar electrokinetic capillary chromatography with sodium dodecyl sulphate. J. Chromatogr. 609, 369–374. Kim, G.N., Jung, Y.H., Lee, J.J., Jung, C.H., 2008. Development of electrokineticflushing technology for the remediation of contaminated soil around nuclear facilities. J. Ind. Eng. Chem. 14 (6), 732–738. Kim, G.N., Lee, S.S., Shon, D.B., Lee, K.W., Chung, U.S., 2010. Development of pilotscale electrokinetic remediation technology to remove 60Co and 137Cs from soil. J. Ind. Eng. Chem. 16 (6), 986–991. Kim, G.N., Shon, D.B., Park, H.M., Lee, K.W., Chung, U.S., 2011. Development of pilot scale electrokinetic remediation technology for uranium removal. Sep. Purif. Technol. 80, 67–72. Min, B.Y., Choi, W.K., Lee, K.W., 2010. Separation of clean aggregates from contaminated concrete waste by thermal and mechanical treatment. Ann. Nucl. Energy 37, 16–21. Nystrom, G.M., Ottossen, L.M., Villumsen, A., 2005. Test of experimental set-ups for electrodialytic removal of Cu, Zn, Pb and Cd from different contaminated harbor sediments. Eng. Geol. 77, 349–357. Pereira, M.G., Mudge, S.M., 2004. Cleaning oiled shores: laboratory experiments testing the potential use of vegetable oil biodiesels. Chemosphere 54 (3), 297– 304. Sun, T.R., Ottosen, L.M., Jensen, P.E., 2012. Pulse current enhanced electrodialytic soil remediation: comparison of different pulse frequencies. J. Hazard. Mater. 237–238, 299–306. Tandy, S., Bossart, K., Mueller, R., Ritschel, J., Hauser, L., Schulin, R., Nowack, B., 2004. Extraction of heavy metals from soils using biodegradable chelating agents. Environ. Sci. Technol. 38 (3), 937–944. Victoria, J.S., Simom, H., Vaughan, T.L., 1999. The influence of three methods of gravel cleaning on brown trout Salmo trutta egg survival. Hydrol. Process. 13 (3), 477–486. Voglar, D., Lestan, D., 2013. Pilot-scale washing of Pb, Zn and Cd contaminated soil using EDTA and process water recycling. Chemosphere 91 (1), 76–82. Yang, G.C.C., Chang, Y.I., 2011. Integration of emulsified nanoiron injection with the electrokinetic process for remediation of trichloroethylene in saturated soil. Sep. Purif. Technol. 79 (2), 278–284.
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Decontamination of gravels contaminated with uranium Gye-Nam Kim a,⇑, Uk-Ryang Park a, Seung-soo Kim a, Wan-Suk Kim a, Jei-Kwon Moon a, Jae-hyuk Hyun b a b
Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yusong-gu, Daejeon 305-353, Republic of Korea Environmental Engineering, Chungnam National University, 99 Daehak-ro, Yusong-gu, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 January 2014 Received in revised form 28 May 2014 Accepted 30 May 2014
Keywords: Decontamination Uranium Gravel Washing Electrokinetic Electrodialytic
a b s t r a c t Gravel washing equipment and electrokinetic–electrodialytic decontamination equipment were manufactured to decontaminate gravel contaminated with uranium. The removal efficiency according to the gravel size and weight and the removal efficiency according to the lapsed time using the manufactured equipment were investigated through several experiments. The volume of gravel in the high uranium concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The larger the gravel size, the higher the contaminated concentration of gravel. The average uranium (238U) concentration of gravel after the first washing was about 1.45 Bq/g, and the average removal efficiency of gravel after the third washing was about 37%. In addition, the removal efficiency of the contaminated gravel was not related to its size. The contaminated concentration of the gravel decreased with an increasing gravel weight. In addition, the removal efficiency of contaminated gravel was not related to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, the 238U in the gravel was removed by about 40%, 65%, 72%, and 81%. The more the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. Finally, the gravel with a size of less than 10 cm was treated by soil washing and electrokinetic decontamination methods with soil, and gravel with size of more than 10 cm but less than 20 cm was treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Gravel with a size of more than 20 cm is treated by a gravel washing method, and gravel contaminated with a high concentration of uranium was treated by crushing and ball mill washing methods. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The soil around nuclear facilities is contaminated with radionuclides during the operation and decommissioning of those facilities, especially during a nuclear accident such as the one at Chernobyl in Ukraine and the one at Fukushima in Japan. Korea has a lot of soil contaminated with uranium generated during its operation of nuclear facilities. Soil with a size of less than 10 cm is usually decontaminated using soil washing and electrokinetic technologies. However, it is difficult to use soil washing technology for decontamination of gravel with a size of more than 10 cm. It is impossible to scrub gravel in a washing tank, because the gravel sinks to the bottom (Fedje et al., 2013; Bisone et al., 2012; Gryschko et al., 2005; Tandy et al., 2004; Voglar and Lestan, 2013). In addition, when electrokinetic decontamination technology
⇑ Corresponding author. Tel.: +82 42 868 8674; fax: +82 42 868 2499. E-mail address: [email protected] (G.-N. Kim). http://dx.doi.org/10.1016/j.anucene.2014.05.031 0306-4549/Ó 2014 Elsevier Ltd. All rights reserved.
(Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005) is applied to gravel with a size of more than 10 cm, the removal efficiency of the radionuclides from the gravel is reduced, because the electro-osmotic flux at the surface of the gravel in an electrokinetic cell is reduced owing to a reduction of the particle surface area, which is attributable to the large size of the gravel (Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005; Kim et al., 2011; Kim et al., 2010, 2008). Meanwhile, there have been few studies on the decontamination of gravel with a size of more than 10 cm. The volume ratio of gravel whose size is more than 10 cm in the total volume of soil at KAERI was about 20%. Therefore, it is necessary to study the decontamination of gravel contaminated with radionuclides. Vitoria et al. used three mechanical gravel cleaning methods: (1) tractor rotovating, using a Dowdeswell Powervator 35 rotovator, with a width of 90 cm, behind a Ford 1220 four-wheel drive tractor, (2) high pressure jet washing, using a KEW 5203 KD pressure washer, in which water was pumped at 150 bar through a hand-held lance with jets of 5 mm and 1 mm diameter, and (3)
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pump washing, using a Pacer pump with a 3 Hp Briggs and Stratton engine. The most effective method for cleaning river gravels appeared to be pump washing (Victoria et al., 1999). Pereira et al. used vegetable oil biodiesel for cleaning oiled shores. Pure oil biodiesels (rapeseed and soybean) were significantly more effective in the cleanup of oiled sands (up to 96%) than recycled waste cooking oil biodiesel (70%) (Pereira and Mudge, 2004). Clement et al. used a physic-chemical method for the treatment of dredged sediment. The positive effects of physic-chemical treatment are the reduction of sediment mass, materials easier to handle, reduction of visual and odor nuisances, better settability, the removal of ammonia emissions and associated ecotoxicological risks, removal of zinc and nickel emissions, and a reduction of sediment toxicity to amphipods and chrironomids (Clement et al., 2010). Min et al. used thermal and mechanical treatment to clean aggregates from concrete. Most pollutants are easily separated from the contaminated concrete waste, which concentrates mainly in the porous fine cement powder. Removal on pollutants in concrete was influenced by the heating temperature and crushed aggregate size. Heating temperature played an important role in moving the contaminants from the concrete waste (Min et al., 2010). Cho et al. used dry washing technology for the treatment of polluted railroad ballast gravel. A dry washing method removes the pollutants on the surface of ballast gravels by blasting media on the gravel. The technology efficiently removed heavy metals from contaminated gravel for a short time (Cho et al., 2012). In this study, the gravel contaminated with uranium was sampled at an area near a nuclear facility in Korea. The contamination characterization of gravels separated from soil was analyzed. The gravel washing equipment and electrokinetic–elctrodialytic decontamination equipment were manufactured to decontaminate the contaminated gravel. The removal efficiency according to the gravel size and weight, and the removal efficiency according to the lapsed time by electrokinetic–electrodialytic equipment, was investigated through several experiments. The optimum experiment conditions for uranium decontamination by the gravel washing and electrokinetic–elctrodialytic decontamination equipment were found. Finally, a process to decontaminate the contaminated gravel was developed for the self-disposal of radioactive gravel waste on the basis of the experimental decontamination results. 2. Materials and methods 2.1. Characteristics of contaminated gravels About 30% of the volume of soil contaminated with uranium, which was excavated at an area near a nuclear facility, was gravel. The gravel contaminated with uranium is shown in Fig. 1. High and low concentration groups of contaminated gravels are shown in Table 1. The volume of gravel in the high uranium concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The reason is considered to be that uranium can infiltrate into the deep side of those rocks, because quartz that consists of a crystal cluster, lamprophyre that consists of biotite and amphibole, and schist that consists of biotite and muscovite are easy to be split. Washing after using a crushing method should be selected to decontaminate gravel in the high concentration group for an improvement of the decontamination efficiency.
Fig. 1. Gravel contaminated with uranium.
Table 1 High and low concentration groups of contaminated gravel. Concentration (Bq/g)
High-concentration
Low-concentration
1 2 3 4 5
5.32 7.68 4.56 6.72 7.25
0.82 1.47 2.18 1.7 1.85
Average
6.31
1.60
waste solution collection box, nitric acid solution box, and drum hoist, as shown in Fig. 2. The trammel circulates gravels in its inside at a fixed rpm, and the nozzles in the trammel spray a nitric acid solution to wash the contaminated gravel. The gravel injection box injects contaminated gravel into the trammel, and the gravel collection box collects the washed gravel. The waste solution collection box collects waste solution released during gravel washing, the nitric acid solution box supplies the nozzle in the trammel with nitric acid solution, and the drum hoist transports contaminated gravel to the gravel injection box. Images taken before and after gravel washing are shown in Fig. 3. The optimum experimental conditions of the gravel washing equipment were obtained through several experiments: the optimum rpm of the gravel washing equipment, and the optimum concentration of nitric acid as a washing solution.
2.2. Manufacturing of gravel washing equipment The gravel washing equipment was manufactured to wash the contaminated gravel. The gravel washing equipment consisted of a trammel, nozzle, gravel injection box, gravel collection box,
Fig. 2. Manufactured gravel washing equipment.
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electrokinetic–electrodialytic decontamination equipment was manufactured as shown in Fig. 5 for a shortening of the decontamination period. 2.5. Radioactivity measurement for gravel
Fig. 3. Before and after gravel washing.
2.3. Experiments to decontaminate gravels contaminated with uranium Gravels near the uranium conversion facility in Korea Atomic Energy Research Institute (KAERI) were contaminated with uranium. The size of the gravel was mainly 5–30 cm, and the weight was mainly 150–4000 g. The uranium concentration of the contaminated gravel was 0.5–10.0 Bq/g. The required uranium (238U) concentration for self-disposal was below 0.43 Bq/g. Thus, it was necessary to reduce the uranium concentration of the gravel using washing and electrokinetic decontamination. 2.4. Manufacturing electrokinetic–electrodialytic decontamination equipment Using electrokinetic decontamination technology, the removal efficiency of radionuclides from gravel decreased, because the electro-osmotic flux at the surface of the gravel in the electrokinetic cell decreased due to the large size of the gravel. In addition, the decontamination period was lengthened because the electrolyte in the anode room was contaminated with uranium owing to a higher hydraulic conductivity of gravels in the gravel cell of the electrokinetic equipment. Meanwhile, the electrodialytic method has generally been used for treating waste solution and soil remediation, which attaches an ion exchange membrane at the anode room or cathode room (Sun et al., 2012; Nystrom et al., 2005; Jensen et al., 2012; Jakobsen et al., 2004). Fig. 4 shows a schematic diagram of electrokinetic–electrodialytic decontamination. An anion exchange membrane was attached at the anode room partition to prevent an infiltration of uranium ions. In this study, the
Fig. 4. Schematic diagram of electrokinetic–electrodialytic decontamination.
The original uranium concentration for gravel was measured using a Multi-Channel Analyzer (MCA) with a standard tube of 1000 cc, QCY48 (Amersham), manufactured by the Korea Reach Institute Standards and Sciences. The MCA operates in a pulse height analyzer mode. The scintillation counter measures the pulse height distribution from a gamma ray source. The amplitude of an incoming analog pulse is digitized by an analog digital converter (ADC), and the digital value is used as the address of the incremented memory location. Thus, the screen display of the number of counts vs. the channel number is really a histogram of the number of counts vs. the pulse height, i.e., a pulse height spectrum. The time required to measure the radioactivity concentration of a gravel sample using the MCA was estimated to be 4–8 h. The original radioactivity concentration of the washed gravels before electrokinetic–electrodialytic decontamination was calculated. The electrokinetic–electrodialytic experiment was stopped at pre-determined interim times, the gravel samples were extracted from the gravel cell, and the radioactivity concentration of the gravel samples were then measured using the MCA. The gravel samples were returned to their original locations in the gravel cell, and the experiment was continuously restarted. The removal efficiency of the nuclides was calculated as a ratio of the original gravel concentration, and the gravel concentrations were measured at pre-determined interim times. After completion of the electrokinetic–electrodialytic decontamination experiments for radioactive gravels with initial concentrations of 0.5–6.5 Bq/g, the residual concentration of gravel was calculated. Finally, the decontamination period required for decontaminating the initial gravel concentration to a clearance concentration level (238U: 0.43 Bq/g) was estimated with the experimental electrokinetic– electrodialytic results. 3. Results and discussion From the results of the washing experiments with gravel washing equipment, it was concluded that the optimum rpm of the gravel washing equipment was 10 ppm, the optimum concentration of nitric acid as a washing solution was 1.0 M, and the optimum spray rate by nozzle was 50 L/min. Table 2 shows the results of the removal efficiency according to the gravel size by washing using the manufactured gravel washing equipment. The larger the gravel size, the more the contaminated concentration of gravel decreased. The reason is considered to be that the uncontaminated volume in a larger size of gravel is larger, because it is difficult for uranium to infiltrate into the deep side of gravel. The average concentration of gravel after the first washing was about
Fig. 5. Manufactured electrokinetic–electrodialytic decontamination equipment.
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Table 2 Removal efficiency according to the gravel size by gravel washing. Gravel size
Ci (Bq/g)
1st
2nd
3rd
Removal efficiency (%)
5 cm (154 g) 5 cm (266 g) 10 cm (766 g) 10 cm (784 g) 15 cm (944 g) 15 cm (1074 g)
2.25 2.93 2.54 2.12 1.9 1.2
1.04 2.14 1.83 1.67 1.32 0.68
1.01 2.10 1.76 1.61 1.27 0.67
1.0 2.09 1.76 1.60 1.22 0.65
56 29 31 25 36 46
Average
2.16
1.45
1.40
1.39
37.2
Table 3 Removal efficiency according to the gravel weight by gravel washing (drum 1). Gravel weight (g)
Ci (Bq/g)
1st
2nd
Removal efficiency (%)
1328 1526 1656 1876 2200 2252 3242
0.97 3.71 1.6 0.61 0.4 0.72 0.76
0.91 2.1 1.2 0.4 0.2 0.32 0.72
0.9 1.6 1.18 0.37 0.18 0.3 0.54
7 43 26 39 50 58 29
Average
1.25
0.84
0.72
42
manufactured gravel washing equipment. Table 4 shows the results of the removal efficiency according to the gravel weight by washing the gravel in drum 2 using the manufactured gravel washing equipment. The higher the gravel weight, the more the contaminated concentration of the gravel decreased. The reason is considered to be that the uncontaminated volume in a higher weight of gravel is larger, because it is difficult for uranium to infiltrate into a deep side of gravel. The contamination concentration and removal efficiency of the gravel depend on the rock type. It is impossible to decontaminate deeply contaminated gravel by washing or electrokinetic–electrodialytic decontamination. The optimum number of washings for contaminated gravel is considered to be two. In addition, the removal efficiency of contaminated gravel was not related to its weight. Fig. 6 shows images of the gravel used in Table 4. 3.1. Ball mill washing after crushing
Table 4 Removal efficiency according to the gravel weight by gravel washing (drum 2). Gravel weight (g)
Ci (Bq/g)
1st
2nd
Removal efficiency (%)
1214 1244 1382 1432 1458 1760 2728 3250 4440 4906
3.68 1.5 6.49 0.89 1.2 1.45 0.95 0.73 0.71 0.79
1.86 1.47 4.26 0.68 0.67 0.99 0.85 0.63 0.28 0.54
2.03 1.23 3.68 0.67 0.59 0.98 0.84 0.54 0.27 0.47
45 18 43 25 51 32 12 26 62 41
Average
1.84
1.22
1.13
39
Gravel in the high concentration group should use a washing after a crushing work to improve the decontamination efficiency. Gravel contaminated with a high concentration of uranium was crushed to below 1 mm size, as shown in Fig. 7. The crushed gravel was then put in a ball mill, and washed and crushed for 3–4 h. The removal efficiencies of gravel by ball mill washing are shown in Table 5. The average removal efficiency after the second washing by a ball mill was more than 90%. 3.2. Electrokinetic–electrodialytic decontamination Uranium (UO2+ 2 ) in the contaminated gravel in the electrokinetic– electrodialytic decontamination equipment was removed by electroosmosis, electro-migration, and a hydraulic pressure flow, as in the following equation:
j ¼ ½ðko þ km ÞRrI þ kh rpC
1.45 Bq/g. The average removal efficiency of the gravel after the third washing was about 37% and the removal efficiency of the third gravel washing was very little in comparison with those of the first and second washings. In addition, the removal efficiency of the contaminated gravel was not related to its size. Table 3 shows the results of the removal efficiency according to the gravel weight by washing the gravel in drum 1 using the
D
s2
rC;
ð1Þ
where j is the molar flux of the species per unit pore area, ko the electro-osmotic permeability, km the electro-migration coefficient, R the electric resistance, I the electric current, kh the hydraulic permeability, P the pressure, C the molar concentration, D the diffusion coefficient, and s is a non-dimensional tortuosity. To increase the removal velocity of radionuclides from gravel, electrokinetic–electrodialytic decontamination equipment was manufactured. This equipment mixed the electrokinetic concept and electrodialytic concept. That is, an anion exchange membrane was attached at the net plate of the anode room of the electrokinetic equipment to prevent the transfer of positive ions into the
Fig. 6. Images of gravel used in Table 4.
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Fig. 7. Crushing and ball mill washing for gravel contaminated with high concentration.
Table 5 Removal efficiencies of gravel by ball mill washing. Gravel weight (g)
Ci (Bq/g)
1st
2nd
Removal efficiency (%)
550 650 850
4.8 3.6 3.45
0.95 0.75 0.64
0.36 0.32 0.27
92.5 91.1 92.2
anode room, thus preventing the solution in the anode room from being contaminated with UO22+. The pure solution in the anode room can shorten the time required to remove uranium from gravel surface. The experimental electrokinetic–electrodialytic conditions were as follows. The gravel volume in the gravel cell was 400 L, the electric current was 150–200 A, the electric voltage was 15–20 V, the electrolyte inflow rate was 130–160 mL/min, the temperature in electrokinetic–electrodialytic experiment was below 65 °C, and L (electrolyte volume, mL)/S (gravel weight, g) in the gravel cell was about 0.33. Table 6 shows the removal efficiency of uranium from gravel according to the lapsed time by
electrokinetic–electrodialytic equipment. When the decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, 238U in the gravel was removed by about 40%, 65%, 72%, and 81%. The more the decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. In addition, the more the initial concentration of 238U increased, the more the removal efficiency of 238U increased. A process to decontaminate contaminated gravel was developed for the self-disposal of radioactive gravel waste on the basis of preceding experimental decontamination results. The decontamination process for gravel contaminated with uranium is shown in Fig. 8. Gravel contaminated with uranium can be treated by four methods. First, gravel with a size of less than 10 cm is treated by soil washing and electrokinetic decontamination methods with soil. Second, gravel with a size of more than 10 cm but less than 20 cm is treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Third, gravel with a size of more than 20 cm is treated by a gravel washing method. Fourth, gravel contaminated with a high concentration of uranium is treated by crushing and ball mill washing methods.
Table 6 Removal efficiency according to the lapsed time by electrokinetic–electrodialytic equipment. Lapsed time
Origin (Bq/g)
5 Days (%)
10 Days (%)
15 Days (%)
20 Days
Removal efficiency
2.3 1.7 1.3
42 39 38
67 64 63
74 71 70
83% (0.39 Bq/g) 80.6% (0.31 Bq/g) 80% (0.26 Bq/g)
Average
1.7
40
65
72
81% (0.32 Bq/g)
Fig. 8. The decontamination process for gravel contaminated with uranium.
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4. Conclusions The volume of gravel in the high concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The larger the gravel size, the more the contamination concentration of the gravel decreased. The average concentration of gravel after the first washing was about 1.45 Bq/g. The average removal efficiency of gravel after the third washing was about 37%, and the removal efficiency of the third gravel washing was very little in comparison with those of the first and second washings. In addition, the removal efficiency of the contaminated gravel was not related to its size. The higher the gravel weight, the more the contamination concentration of the gravel decreased. The contamination concentration and removal efficiency of the gravel depend on the rock type. The optimum number of washings for contaminated gravel is considered to be two. In addition, the removal efficiency of contaminated gravel is not related to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, 238U in gravel was removed by about 40%, 65%, 72%, and 81%. The more the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. Conclusively, gravel with a size of less than 10 cm should be treated by soil washing and electrokinetic decontamination methods with soil, and gravel with a size of more than 10 cm but less than 20 cm should be treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Gravel with a size of more than 20 cm should be treated by a gravel washing method, and gravel contaminated with a high concentration of uranium should be treated by crushing and ball mill washing methods. Acknowledgement This project was carried out under the Nuclear R&D Program by MOST in KOREA. References Bisone, S., Blais, J.F., Drogui, P., Mercier, G., 2012. Toxic metal removal from polluted soil by acid extraction. Water Air Soil Pollut. 223 (7), 3739–3755. Cho, Y.M., Lee, J.Y., Kwon, T.S., Jung, W.S., Park, D.S., Lee, I.H., Kim, B.K., Jung, W.H., Lee, C., Yoon, Y.K., Yang, Y.M., Kim, H.M., Lim, J.I., 2012. Field application of dry
washing technology for polluted rail road ballast gravel. Fall Meeting Korea Soc Railway. A139. Clement, B., Vaille, G., Moretto, R., Vernus, E., Abdelghafour, M., 2010. Effects of physic-chemical treatment of a dredged sediment on its ecotoxicity after discharge in laboratory gravel pit microcosms. J. Hazard. Mater. 175, 205–215. Dong, W., Ball, W.P., Liu, C., Wang, Z., Stone, A.T., Bai, J., Zachara, J.M., 2005. Influence of calcite and dissolved calcium on uranium(VI) sorption to a Hanford subsurface sediment. Environ. Sci. Technol. 39 (20), 7949–7955. Fedje, K.K., Yillin, L., Stromvall, A.M., 2013. Remediation of metal polluted hotspot areas through enhanced soil washing – evaluation of leaching methods. J. Environ. Manage. 128, 489–496. Gryschko, R., Kuhnle, R., Terytze, K., Breuer, J., Stahr, K., 2005. Soil extraction of readily soluble heavy metals and as with 1 M NH4NO3-solution. J. Soils Sediments 5 (2), 101–106. Jakobsen, M.R., Fritt-Ramussen, J., Nielsen, S., Ottosen, L.M., 2004. Electrodialytic removal of cadmium from wastewater sludge. J. Hazard. Mater. 106, 127–132. Jensen, P.E., Ottosen, L.M., Allard, B., 2012. Electro dialytic versus acid extraction of heavy metals from soil washing residue. Electrochim. Acta 86, 115–123. Kaneta, T., Tanaka, S., Taga, M., Yoshida, H., 1992. Effect of addition of glucose on micellar electrokinetic capillary chromatography with sodium dodecyl sulphate. J. Chromatogr. 609, 369–374. Kim, G.N., Jung, Y.H., Lee, J.J., Jung, C.H., 2008. Development of electrokineticflushing technology for the remediation of contaminated soil around nuclear facilities. J. Ind. Eng. Chem. 14 (6), 732–738. Kim, G.N., Lee, S.S., Shon, D.B., Lee, K.W., Chung, U.S., 2010. Development of pilotscale electrokinetic remediation technology to remove 60Co and 137Cs from soil. J. Ind. Eng. Chem. 16 (6), 986–991. Kim, G.N., Shon, D.B., Park, H.M., Lee, K.W., Chung, U.S., 2011. Development of pilot scale electrokinetic remediation technology for uranium removal. Sep. Purif. Technol. 80, 67–72. Min, B.Y., Choi, W.K., Lee, K.W., 2010. Separation of clean aggregates from contaminated concrete waste by thermal and mechanical treatment. Ann. Nucl. Energy 37, 16–21. Nystrom, G.M., Ottossen, L.M., Villumsen, A., 2005. Test of experimental set-ups for electrodialytic removal of Cu, Zn, Pb and Cd from different contaminated harbor sediments. Eng. Geol. 77, 349–357. Pereira, M.G., Mudge, S.M., 2004. Cleaning oiled shores: laboratory experiments testing the potential use of vegetable oil biodiesels. Chemosphere 54 (3), 297– 304. Sun, T.R., Ottosen, L.M., Jensen, P.E., 2012. Pulse current enhanced electrodialytic soil remediation: comparison of different pulse frequencies. J. Hazard. Mater. 237–238, 299–306. Tandy, S., Bossart, K., Mueller, R., Ritschel, J., Hauser, L., Schulin, R., Nowack, B., 2004. Extraction of heavy metals from soils using biodegradable chelating agents. Environ. Sci. Technol. 38 (3), 937–944. Victoria, J.S., Simom, H., Vaughan, T.L., 1999. The influence of three methods of gravel cleaning on brown trout Salmo trutta egg survival. Hydrol. Process. 13 (3), 477–486. Voglar, D., Lestan, D., 2013. Pilot-scale washing of Pb, Zn and Cd contaminated soil using EDTA and process water recycling. Chemosphere 91 (1), 76–82. Yang, G.C.C., Chang, Y.I., 2011. Integration of emulsified nanoiron injection with the electrokinetic process for remediation of trichloroethylene in saturated soil. Sep. Purif. Technol. 79 (2), 278–284.