The coupled effect of electrokinetic and ultrasonic remediation of contaminated soil

Transcription

1 The coupled effect of electrokinetic and ultrasonic remediation of contaminated soil H, I. Chungl, I. K. Ohl &M. Kamon2 ldepartment of Civil Engineering, Korea Institute of Construction Technology Korea 2DPM, Kyoto University, Japan Abstract The laboratory tests were conducted using specially designed and fabricated devices to determine the effect of sonication and electrokinetics on seepage as well as on contaminant removal in contaminated soil. The electrokinetic technique was applied to remove mainly the heavy metal and the ultrasonic technique was applied to remove mainly organic substance in contaminated soil. Test soil was Jumunjin(East beach of Korea) fine sand; the test specimen was prepared at loose density. Diesel fiel and Cd were used as a surrogate contaminant for soil flushing test. A series of laboratory experiments involving the simple, ultrasonic, electrokinetic, electrokinetic+uhrasonic flushing test were carried out. An increase in permeability and contaminant removal rate was observed in electrokinetic and ultrasonic flushing tests. Some practical implications of these results are discussed in terms of technical feasibility of in situ implementation of electrokinetic ultrasonic remediation technique. 1 Introduction The contaminated ground can be removed and replaced with non-polluted soils. This method requires installations of sheet-pile walls to protect the surrounding ground. The execution of this method is costly and may also interrupt the ongoing business. Therefore, it is always more desirable to cleanup the pollutant in situ than to remove the contaminated soil. In response to the demand for developing effective and economical cleanup techniques, numerous studies have been conducted over the years. Several clean-up techniques have been

2 496 Browttfield Sites: Assessment, Rehabilitation and De~elopntent developed; examples include the pump-and-treat method, the soil vapor extraction method, and the bioremediation method, to name a few. The pumpand-treat method is ineffective due to the requirement of a large equipment with high energy and slow removal of contaminants. The oil vapor extraction method is not applicable to remove contaminants from saturated soil deposits and ground water, The effectiveness of bioremediation depends greatly on suitable microorganism and nutrients in the subsurface. Therefore, much still remain to be done in order that a generally accepted methodology can be developed for a broad range of applications. Natural concentrations of heavy metals on soil deposits are not high; however, studies have indicated that many areas near urban complexes, metalliferous mines or major roads display abnormally high concentrations of these elements. Cadmium is a silver-white metal, In nature, it is usually found combined with other elements such as oxygen (cadmium oxide), chlorine (cadmium chloride), or sulfur (cadmium sulfate, cadmium sulfide). Cadmium does not corrode easily and has many uses, In industry and consumer products, it is used for batteries, pigments, metal coatings, and plastics. Cadmium or its compounds do not have a deftitive taste or odor. Cadmium gets into the environment from the weathering of rocks and minerals that contain cadmium. Exposure to cadmium can occur in industries that commonly use or produce cadmium such as mining or electroplating. Cadmium exposure can also occur from exposure to cigarette smoke, A variety of options may exist to select a cleanup remedy at a site, however the efficiency and costs of these options may vary widely. Most of the existing remediation technologies are limited to soils with high hydraulic conductivities and are not effective in removing heavy metals adsorbed on soil particles, particularly fine-grained deposits. There exists a need to introduce cost-effective, innovative, and preferably in-situ remediation technologies. Electrokinetic(ek) soil processing is a new, innovative, and cost-effective remediation technology that employs conduction phenomena under electric currents for transport, extraction, and separation. A low level direct electric current (or electric potential difference) is applied across contaminated soil deposits through inert electrode placed in holes or trenches in the soil filled with processing fluid. The driving mechanisms for species transport are ion migration by electrical gradients, pore fluid advection by prevailing electroosmotic flow, pore fluid flow due to any externally applied or internally generated hydraulic potential difference, and diffision due to generated chemical gradients. As a result, cations are accumulated at the cathode and anions at the anode, while there is a continuous transfer of hydrogen and hydroxyl ions across the medium. Various lab-scale studies on the feasibility of the process have shown that heavy metals and other cationic species can be removed from the soil specifically when process enhancement techniques are, such as electrolyte conditioning, are employed. The feasibility and cost effectiveness of electrokinetics for the extraction of heavy metals such as copper, zinc, and cadmium from soils have been demonstrated through bench-scale laboratory studies (Runnels and Larson 1986; Hamed 1990; Pamukcu et al. 1990; Acar et al, 1992 and 1993). Acar et al.

3 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent 497 (1994) demonstrated 90% to 95% removal of Cdz+ from bench-scale kaolinite specimens with initial concentration of lg. Acar and Alshawabkeh (1993) and Acar et al. (1994 ) showed higher removal rates of charged species can be achieved by electric migration rather than electroosmotic flow. Other uses of electrokinetics in environmental geotecbnics include injection of grouts, cleanup chemicals or nutrients for growth of microorganisms essential to biodegradation of specific wastes, contaminant detection, monitoring the physiochemical soil profiling (Acar and Gale 1986; Mitchell 19861; Acar et al. 1989; Acar and Hamed 1991; H. Lee 2000; S. Han 2000), and the use of eletrophoresis for sealing impoundment leaks (Yeung et al. 1994). Numerous researchers have proposed possible mechanisms for the effect of stress waves on fluid flow through porous media. Simikin and Verbitskaya (1989) proposed that cavitation and capillary forces are principally responsible for the movement of fluid in porous media. Capillary forces play an important role in liquid percolation through fine pore channels. They suggested that the liquid films that are adsorbed onto pore walls during the percolation process could be destroyed by mechanical vibrations. This study results show that seismoacoustic fields lower the capillary pressure, and that seismoacoustic wave affects the stratum resulting in increasing oil saturation in the upper part and the water saturation in the lower part of the stratum. According to Suslick (1988) and Frederic (1965), the mechanisms responsible for the observed increase in transport rates and unit-operation processes due to ultrasonic energy can be divided into two categories: (1) fust-order effects on fluid particles involving displacement, velocity, and acceleration, and (2) second-order phenomena including radiation pressure, cavitation, acoustic streaming, and interracial instabilities. Usually, one or more if the second-order effects are responsible for the enhancement in the transport process. Murdoch et al. (1998) conducted falling-head permeability tests on sandy clay samples in ultrasonic cleaning chambers, and found that hydraulic conductivity of the samples increased abruptly about 3700/o after 30 seconds of ultrasonic excitation. They later conducted tests using a sonication probe, and observed an increase in hydraulic conductivity. Iovenitti et al. (1995) studied the effect of acoustic excitation on the porous media, They summarized the potential effects of acoustic wave on the porous grain framework of the soil and pore fluid on Figure 1 and 2. They showed the enhancement of the contaminant recovery by acoustic waves through a series of soil flushing tests.

4 498 Sites: Assessment, Rehabilitation and De~elopnzeril Undisturbed Soil After Application of Acoustic Energy [~,~,:, $:.,.%...-..,,.:,-,,,~-: Vibrational alignment or.+. *&&-_! ~j,++>~,i --:#_.._; : reordering of material to :y>_ i..>$,-, -.:?, :._ <._, f-. J change impedance in flow, Y 7X7.. & direction,, _ :....,,,r. -!.,,..-,,. ~,,...,..,....,:,,,,., :,:,,,..,,,,,.;. Temporary increase in porosity,,,,,. ~.,,..,,... ; :,}..<,.,,.:,-,,,,,..,.:..:.,..:,,, due to particle g&@.md aggregatematerial blocking pore L@?&&%%$ g~a Disintegration of organic or %m~gl. WJ.1.+*P:,*W.- #$ &&.....s,. :4 %I!@?LGb.$k Cavitation (opening, bubbles) produced in clay/silt to increase porosity and permeability Figure 1: Potential effects of acoustic (After Young Uk Kim, 2000) excitation on porous grain tiamework Figure 2: Potential effects of acoustic excitation on pore fluids (After Young Uk Kin 2000)

5 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent Experimental methodology The soil flushing tests were conducted by using the setup shown in Figure 3. The test chamber is made of a plexiglas cylinder having an insider diameter of 10cm with a height of 30cm. The cylinder was filled with contaminated sand. In the side part of the cylinder are installed inlet and outlet tubes. The inlet tube is connected to a reservoir of de-aired water, which is connect to the water tap; and the outlet tube is used to maintain constant heads by allowing overflow of excess water. Outlet tube is connected to a burette for measuring the outflow quantity. The elecrokinetic flushing processor consists of three parts: anode electrode, cathode electrode, and electric power supplier. The graphite electrode is used and situated on the top (cathode part) of soil specimen and the bottom (anode part) of the cylinder. A constant voltage gradient of 1.0 V/cm was applied to the anode and cathode electrodes. Two sheets of filter paper were placed on the upper and lower graphite electrode. The ultrasonic flushing processor consists of three parts: a generator, a converter, and an acoustic horn. The transmitting acoustic horn, which is mounted on top of the soil sample, is used for generation ultrasound. The ultrasonic processor has a maximum power output of 200W with a fi-equenc y of excitation equal to 281cHz. The test setup used in this study was made to combine the electrokinetic flushing processor and the ultrasonic flushing processor, Photography for test setup is shown in Figure 3, and more detailed schematic view of the test setup is shown in Figure 4, The test soil was sand, a fine aggregate, and a natural soil obtained Jumunjin, East beach of Korea. Diesel fuel and Cd were used as a surrogate contaminant to demonstrate the soil contaminated by organic substance and heavy metal. Some physical properties of test soils are shown in Table 1. The flushing tests were conducted for four conditions: simple flushing, electrokinetic(ek) flushing, ultrasonic flushing, electrokinetic(ek)+ ultrasonic flushing. Test conditions are shown in Table 2, Hydraulic gradient for the flushing process was constant to 1.0. In the tests, the soil specimens were thoroughly mixed with diesel fhel of 770ml and Cd of 500ppm(385mg in test soil). The test specimen was then subjected to ultrasonic waves at 28kHz frequency from ultrasonic test setup and to electric power at 1.OV/cm fkom electrokinetic test setup. Tests were continued to maximum 100 minute.

6 500 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent Figure 3: Photography for test setup Generator Converter Acoustic horn D1 Mariotte bottle Cathode(-) Anod~ ~ : J Outlet... Contaroinated D. C. power soil Figure 4: Test setup for soil flushing experiment Sand I Diesel fiel Specific gravity Maximum dry density 1.60 Minimum Specific Volubility dry density gravity in water 1.40 I 0,80 I Centistoke ( Cst, mm2/sec ) I 0.564

7 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent 501 Table 2. Test conditions for soil flushing experiment Test Contaminants Hydraulic gradient Simple flushing Electrokinetic(ek) diesel fuel flushing + i= 1.0 Ultrasonic flushing heavy metal (cadmium) ek+ultrasonic flushing 3 Results and Analysis Under the actions of hydraulic gradient, ultrasonic waves, electroosmosis and electromigration, the pore water was allowed to flow upward through the specimen and contaminant was allowed to migrate upward from the specimen. The effluent was collected in a 500ml polypropylene cylinder. The effluent in the cylinder was allowed to stand overnight for gravitational segregation of oil from water. The volumes of the separated water and oil were then measured. Also, Cd concentration of effluent was measured by laboratory chemical analysis facilities. Firstly, the test results for the accumulated flow volume with time are presented in figure 5. The figure shows the accumulative water flow is varied and increased with time. The accumulated flow volume with time is higher in the case of ultrasonic flushing and ek+ultrasonic flushing tests than in the case of simple and electrokinetic flushing tests. And the accumulated flow volume with time is almost same in the case of simple and electrokinetic flushing tests. Also, the accumulated flow volume with time is almost same in the case of ultrasonic flushing and ek+ultrasonic flushing tests. It means that the ultrasonic process has a role to increase the liquid outflow due to sonication effects, but electrokinetic process has not due to short test duration and sandy soil. Normally, electroosmosis by electric power is not or few developed in a short time and in sandy soil. The test data shown in figure 5 were used to compute the discharge velocity. The computed discharge velocity was used to compute the coefficient of permeability of the test soils. The figure 6 shows the variation of permeability with time. According to figure 6, the permeability of specimen is considerably increased by applying the ultrasonic process. This can be attributed to the combined effects of hydraulic gradient and sonication. The permeability is high in the tests used the ultrasonic process compare with in the tests not used the ultrasonic process. The mean permeability throughout the total test time is calculated from the results in Figure 6. The calculated mean permeability is shown in figure 7. In this figure, we can see that the mean permeabilities of test soils are 2.64 E-04crn/sec for simple flushing, 2.68 E-04cmJsec for electrokinetic flushing, 4.70E-04cmlsec for ultrasonic flushing, 5.14 E-04crn/sec for electrokinetic+ultrasonic flushing, respectively.

8 502 Brow} field Sites: Assessment, Rehabilitation and De~elopntent Q 3500 * 3000 $ 2500 *simple flushing A ~ ek flushing *ultrasonic flushing i o Time(min) Figure 5: Accumulated flow volume with time 6.OE-04 ~ 5.OE-04 ; ~ 4,0E-04 ~ z 3.OE-04 e, I ~ 2. OE-04 *simple flushing 4 ek flushing 2 1.OE-04 *ultrasonic flushing ~ ek+ultrasonic flushing,, O.OE+OO o Time(min) Figure 6: Permeability with time for test sample

9 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent OE-04 T.g 5.OE-04 g x 4.OE-04.s = ~ 3.OE-04 E ~ 2.OE-04 ~ Q-I 1.OE-04 2 O.OE+OO simple ek flushing ultrasonic ek+ultrasonic flushing flushing flushing Figure 7: Mean permeability of test soils The quantity of oil removal is measured from the liquid outflow, and oil removal rate is calculated. The results of calculated oil removal rate is shown in figure 8. The Cd concentration is measured from the liquid outflow and shown in figure 9. Reddi et al (1993) also investigated the effect of ultrasonic energy on enhancement of the permeability of clayey soils. They observed an increase in the permeability of all tests, They attributed the increased permeability to the removal of particles smaller than clay and colloidal size particle in the test specimens due to sonication. Therefore, for the test soils, the increased permeability due to sonication can be attributed primarily to particle agitation and dislodging. In the figure 8, it is suggested that the oil contaminant is removed with time elapsed, and the percent of oil removal is increased with time and reached a constant after about 24 minute. According to this figure, it is shown that the maximum percentages of oil removal rate for all tests are approximately 65Y0. The removal velocity of oil is high in the case of ultrasonic process for a very short time. In the figure 9, it is demonstrated that Cd concentration of outflow is increased with time. The Cd contaminant in soil specimen is steadily removed with time elapsed. The Cd concentration of outflow is high in the cases of electrokinetic and electrokinetic+ ultrasonic flushing process by comparing with in the cases of simple and ultrasonic flushing process. The heavy metal contaminant such as Cd is migrated and removed by electromigration phenomena induced from electrokinetic process. Electrokinetic process is most effective technique to remove heavy metal in contaminated soil.

10 504 Brow} field Sites: Assessment, Rehabilitation and De~elopntent ~ 0.8 ~ Q % > g 0.4 w % * ek+ultrasonic flushing \ 1 1! o Time(rnin) I Figure 8: Oil removal rate with time 25 A * simple flushing ek+ultrasonic flushing o Time(min) Figure 9: Removal of Cd with time 4 Conclusions In this study, the coupled effect of electrokinetic and ultrasonic technique is demonstrated by laboratory tests. A series of tests were conducted for simple flushing, electrokinetic flushing, ultrasonic flushing, electrokinetic+ ultrasonic flushing. The test results showed that the ultrasonic technique can enhance the removal of diesel fuel oil from contaminated soils, the electrokinetic technique can enhance the removal of heavy metal from contaminated soils. And the

11 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent 505 combined new technique of these two techniques can be effective in removal of organic substance and heavy metal from contaminated soils at the same time. References [1] Young Uk Kim, Effect of sonication on removal of petroleum hydrocarbon from contaminated soils by soil flushing method, The Pennsylvania State University, The graduate School, Department of Civil and Environmental Engineering, Thesis, Ph. D, 2000 [2] Hyun-Ho Lee, Electrokinetic remediation of soil contaminated with heavy metal and hydrocarbons, Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Thesis, Ph. D, 2000 [3] Lakshimi N. Reddi, S. Berliner, and K. Y. Lee, Feasibility of ultrasonic engineering of flow in clayey sands, Journal of Enviromnental Engineering, Vol No. 4, 1993 [4] Sang-Jae Han, Characteristics of electroosmosis and heavy metal migration in contaminated soil by electrokinetic technique, Department of Civil Engineering, Chung-Ang University, Seoul, Korea, Thesis, Ph. D, 2000