Us Enhanced Ek For Removal Ph And Phenanthrene In Soil

Engineering Geology 77 (2005) 233 – 242 www.elsevier.com/locate/enggeo

Ultrasonically enhanced electrokinetic remediation for removal of Pb and phenanthrene in contaminated soils Ha Ik Chunga,*, Masashi Kamonb,1 a

Geotechnical Engineering Research Department, Korea Institute of Construction Technology, 2311 Daewhadong, Ilsangu, Goyangsi, Gyeonggido, Republic of Korea b Graduate School of Global Environmental Studies, Kyoto University, Yosida Honmarchi, Sakyoku, Kyoto 606-8501, Japan Received 1 June 2003; accepted 1 July 2004 Available online 13 September 2004

Abstract Electrokinetic and ultrasonic remediation technologies were studied for the removal of heavy metal and polycyclic aromatic hydrocarbon (PAH) in contaminated soils. The study emphasized the coupled effects of electrokinetic and ultrasonic techniques on migration as well as clean-up of contaminants in soils. The laboratory soil flushing tests combined electrokinetic and ultrasonic technique were conducted using specially designed and fabricated devices to determine the effect of both techniques. 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. A series of laboratory experiments involving electrokinetic and electrokinetic and ultrasonic flushing tests were carried out. Natural clay was used as a test specimen and Pb and phenanthrene were used as contaminants. An increase in out flow, permeability and contaminant removal rate was observed in electrokinetic and ultrasonic tests. Some practical implications of these results are discussed in terms of technical feasibility of in situ implementation of electrokinetic ultrasonic remediation technique. D 2004 Elsevier B.V. All rights reserved. Keywords: Polycyclic aromatic hydrocarbon; Electrokinetic and ultrasonic remediation technologies; Phenanthrene

1. Introduction Several clean-up techniques have been developed for contaminated soils; examples include pump-and* Corresponding author. Tel.: +82 31 910 0216; fax: +82 31 910 0211. E-mail addresses: [email protected] (H.I. Chung)8 [email protected] (M. Kamon). 1 Tel.: +81 75 753 5114; fax: +81 75 753 5116. 0013-7952/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2004.07.014

treat, soil vapor extraction, soil washing, water flushing, steam extraction, bioremediation, etc. The pump-and-treat method is ineffective due to the requirement of large equipment with high energy and slow removal of contaminants. The soil 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

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order that a generally accepted methodology can be developed for a broad range of applications. Electrokinetic soil processing is a new, innovative and cost-effective remediation technology that employs conduction phenomena under electric currents for transport, extraction and separation in cohesive soils. A low level direct electric current 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 diffusion due to generated chemical gradients as shown in Fig. 1. 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 laboratory and field studies on the feasibility of the electrokinetic process have shown that heavy metals and other cationic species can be removed from the contaminated soil. The feasibility and cost effectiveness of electrokinetics for the extraction of heavy metals such as lead, copper, zinc and cadmium from

soils have been demonstrated by many researchers (Mitchell, 1986; Runnels and Larson, 1986; Hamed, 1990; Pamukcu et al., 1990; Pamukcu, 1994; Acar et al., 1992, 1994, 1989; Acar and Alshawabkeh, 1993; Acar and Hamed, 1991; Yeung, 1993; Yeung et al., 1994; Lee, 2000; Han, 2000; Reddy and Saichek, 2002). Numerous researchers (Iovenitti et al., 1995; Reddi et al., 1993; Simikin and Verbitskaya, 1989) have proposed possible mechanisms for the effect of acoustic waves on fluid flow through porous media. The 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. The liquid films adsorbed onto pore walls during the percolation process can be destroyed by mechanical vibration. The seismoacoustic fields lower the capillary pressure and seismoacoustic wave affects on increasing of water saturation and flow in soil stratum. 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) effects on fluid particles involving displacement, velocity and acceleration, and (2)

Fig. 1. The effects of electrokinetic phenomena on porous soil media (Chung and Kang, 1999).

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Fig. 2. The effects of ultrasonic phenomena on porous soil media (Kim, 2000).

phenomena including radiation pressure, cavitation, acoustic streaming and interfacial instabilities. An increase in hydraulic conductivity and contaminant removal was observed from the effect of acoustic excitation on cohesionless soils. Various researchers have shown the enhancement of the contaminant migration and recovery by acoustic waves through laboratory and field tests (Simikin and Verbitskaya, 1989; Suslick, 1988; Frederick, 1965; Murdoch et al., 1998; Iovenitti et al., 1995; Kim, 2000). They summarized the potential effects of acoustic wave

on the porous grain framework of the soil and pore fluid on Figs. 2 and 3. Natural concentrations of heavy metals and polycyclic aromatic hydrocarbons (PAHs) on soil deposits are not high; however, studies have indicated that many areas near urban complexes, gas station, metalliferous mines or major roads display abnormally high concentrations of these elements. So it is important to prevent the transport of these contaminants into the environments by remediating the source zones where high concentrations of heavy metals and PAHs exist. The

Fig. 3. The effects of ultrasonic phenomena on pore fluids (Kim, 2000).

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objective of this paper is to present a laboratory investigation that was performed to evaluate the coupled effects of electrokinetic and ultrasonic remediation technologies. Bench-scale electrokinetic and electrokinetic and ultrasonic experiments were conducted using natural clay that was spiked with Pb as a representative heavy metal and phenanthrene as a representative PAH. Pb is a positive charged ionic contaminant, on the other hand, phenanthrene is a neutrally charged nonionic contaminant. The measured parameters during the tests included the hydrogen ion concentration (pH) values, electrical potential, electroosmotic flow and at the conclusion of the experiments, the soil was extruded and pH values and residual Pb and phenanthrene concentrations were measured along the length of each soil specimen. The results were analyzed to assess the electrokinetic and ultrasonic remedial efficiency.

2. Experimental methodology Two types of experiments including electrokinetic remediation experiment (EK) and electrokinetic and ultrasonic remediation experiment (EK+US) were carried out. The electrokinetic remediation processor consisted of five parts: anode and cathode electrode, test chamber, inlet and outlet, supply water reservoir and electric power supplier. The graphite electrode is

used and situated on the one side (cathode part) and the other side (anode part) of the test chamber. A constant current of 50 mA was applied to the anode and cathode electrodes. The test chamber is made of a plexiglass and it has a 10 cm wide, a 10 cm height and a 20 cm length. The chamber was filled with contaminated soil. The inlet and outlet tubes were installed at both parts of the chamber. The inlet tube is connected to a reservoir and outlet tube is connected to a burette for measuring the outflow quantity. A reservoir was filled de-aired and de-ionized water. The solutions in test chamber were circulated constantly by pump to check the anolyte and catholyte pH. The electrokinetic and ultrasonic remediation processor consisted of eight parts: anode and cathode electrode, test chamber, inlet and outlet, supply water reservoir, electric power supplier, generator, converter and 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 200 W with a frequency of excitation equal to 30 kHz. The detailed schematic view of the experimental setup is shown in Fig. 4. Natural clay sampled at Inchon City area in Korea was used as a soil specimen. Table 1 contains the physical and chemical properties of this clay used in this experiment. The index properties of this soil were specific gravity 2.58, liquid limit 32.8%, plastic limit 22.5%, percent finer than #200 sieve 90%, specific

Fig. 4. Test setup for electrokinetic and ultrasonic experiment.

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242 Table 1 Physical and chemical properties of test clay Items

Values

Specific gravity Percent finer than no. 200 sieve Specific surface area (cm2 g 1) Coefficient of uniformity Hydraulic conductivity (cm s 1) Electrical conductivity (As cm 1) pH

2.58 90.0 5249 4.5 210 7 1500 6.5–7.13

Chemical constituents (%) SiO2 Al2O3 Fe2O3 The others

69.20 13.97 4.30 12.53

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measurement was made in the anode and cathode reservoir. The time-dependent water movement through the soil due to electroosmosis was measured on the outflow by graduated cylinder. After the electrokinetic and electrokinetic and ultrasonic tests, the cell was disassembled from the electrokinetic system and the soil specimen was extruded and sliced in 10 sections. The value of pH and the concentration of Pb and phenanthrene were measured in each slice to assess the change of chemical property in the soil specimen and the efficiency of the process in lead and phenanthrene removal.

3. Results and discussion surface area 5,249 cm2/g, coefficient of uniformity 4.5, activity 0.114 and coefficient of permeability 2d 10 7 cm/s. The chemical properties were organic matter content 6.74%, initial pH 6.5–7.13 and initial electrical conductivity 1500 s/cm. Natural clay is classified into CL in USCS soil classification system. Natural clay was dried, crushed, pulverized and mixed with Pb and phenanthrene solution. Pb and phenanthrene were used as a surrogate contaminant to demonstrate the soil contaminated by heavy metal and PAH. For the preparation of contaminated soil, the admixed soil specimens were thoroughly mixed with lead of 500 mg/kg concentration and phenanthrene of 500 mg/kg concentration. The mixtures were stirred with stainless steel spoons and all mixing operations were performed within glass beakers. The motivation for this mixing technique was to ensure that the Pb and phenanthrene would be distributed evenly throughout the soil. For each clay sample that was prepared, the water content was optimum moisture content, the dry density was 95% of maximum dry density, and the degree of saturation was 100%. The mixture was cured in a bowl more than 24 h. The mixtures were compacted in a test cell. The test specimen was then subjected to ultrasonic waves at 30 kHz frequency from ultrasonic test setup and to electric current at 50 mA from electrokinetic test setup. Tests were continued to maximum 15 days (360 h). The hydraulic head was not applied to the cells, the water level of inlet and outlet is same, so hydraulic gradient was zero for all tests. A constant current of 50 mA was applied to the cells and the voltage was measured on a daily basis during experiment. The pH

3.1. Cumulative outflow Under the actions of electroosmotic flow and electromigration by electric power and acoustic flow by ultrasonic waves in fine soil, the pore water and contaminant in fine soil is allowed to flow and migrate from the inlet (anode side) to outlet (cathode side). The effluent was collected at cathode side in a 500-ml polypropylene cylinder. The accumulated flow volume with time is presented in Fig. 5. The basic pattern of flow was relatively consistent for each experiment. The figure shows the accumulative water flow is varied and increased with time. The outflow began a little later in the beginning stage of experiment on the application of electrical and ultrasonic energy due to time elapse on the development of their phenomena in soil porous media. The outflow for electrokinetic remediation test was steadily increased in the beginning stage with an

Fig. 5. Accumulated flow volume with time.

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increase in the operating duration up to 250 h during experiment, and continuously constant to the end of experiment. On the other hand, the outflow for electrokinetic and ultrasonic remediation test was steadily increased in the beginning stage with an increase in the operating duration to the end of experiment of 360 h. In the case of electrokinetic remediation process, a lead hydroxide is formed in soil pore and reduced ionic strength and soil pore space with time, so the outflow is reduced to the end of experiment. But in the case of electrokinetic and ultrasonic remediation process, the movement of molecule and the porosity and permeability of soil is occurred, so the outflow is not reduced and steadily increased to the end of experiment as shown in Fig. 5. The accumulated flow volume with time is higher for electrokinetic and ultrasonic remediation test than for electrokinetic remediation test. It means that the ultrasonic process has a role to increase the liquid outflow due to sonication effects. The accumulated quantity of outflow was 120 ml/h for electrokinetic remediation test and 143 ml/h for electrokinetic and ultrasonic remediation test after 360 h. The final accumulated quantity of outflow increased with 19% due to the coupled effects for electrokinetic and ultrasonic test by comparing with for electrokinetic test. 3.2. pH of anolyte and catholyte The pH of anolyte and catholyte was measured by pH meter from the influent and effluent. Fig. 6 shows a plot of the anolyte and catholyte pH with respect to time. Anolyte pH was measured at the inflow reservoir

Fig. 6. Anolyte and catholyte pH with time.

Fig. 7. Final pH distribution across the soil specimen.

and catholyte pH was measured at the outflow reservoir during an electrokinetic remediation test. The anolyte pH dropped to values of 2–3 and the catholyte pH rose to value of 11–13 upon the start of the electrokinetic test regardless of different two types of tests. Subsequently, the pH remained relatively constant with further processing. This results from the electrolysis of water. The oxygen gas and hydrogen ion is generated and decreased the pH at the anode, on the other hand hydrogen gas and hydroxide ion is generated and increased pH at the cathode. 3.3. pH of soil The profiles of pH across the soil specimen by different remediation tests are shown in Fig. 7. This demonstrates that the acid front generated at the anode advances steadily towards the cathode, while base front generated at the cathode remains in the cathode. The acid front progressed to the cathode by advection, migration and diffusion and neutralized the base at the cathode. The fronts have met at a normalized distance of approximately 0.8 from the anode in the case of electrokinetic test. On the other hand, the whole area of soil specimen was acidified in the case of electrokinetic and ultrasonic test. The soil pH remained between 4 and 8 for the entire length of the soil specimens for electrokinetic test, but the soil pH remained between 3 and 6 for electrokinetic and ultrasonic test. The final pH value across the soil specimen slightly decreased in the case of enhancement test by introducing ultrasonic wave to electric field due to increase of outflow by applied ultrasonic energy.

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3.4. Electrical potential gradient The variation of electrical potential across the soil specimen for electrokinetic test and electrokinetic and ultrasonic test during experiments is presented typically in Fig. 8. The electrical potential was steadily increased during all experiment. The increase of electrical potential in the tests is probably associated with formation of a lead hydroxide, which will reduce ionic strength and soil pore space. Thus, the resistance in the soil specimen is increased and electrical potential is increased with elapsed time. The final electrical potentials by electrical currents from this figure were about 11 V/cm in the electrokinetic test and about 10 V/cm in the electrokinetic and ultrasonic test. This result shows that the electrical potentials slightly decreased about 10% in the case of electrokinetic and ultrasonic experiment due to removal of lead hydroxide and decrease of electrical resistance in soil due to the movement of molecule and the increasing of porosity and permeability by vibration, cavitation and sonication effects. 3.5. Pb concentration in soil specimen The soil specimen was extruded and cut into 10 small sliced specimens with same length after conducting the experiment to measure pH and concentration throughout total soil specimen length. Pb concentrations of each sliced soil specimen were measured by laboratory chemical analysis facilities. The Pb contaminant was allowed to migrate from the inlet (anode side) to outlet (cathode side) under the

Fig. 8. Electrical potential with time.

Fig. 9. Normalized concentration of Pb in soil specimen.

actions of electroosmosis and electromigration by electric fields and migration by ultrasonic waves. From the results of these phenomena, finally the contaminant is accumulated to the cathode zone or wholly passed the cathode zone and gone outside from the soil specimen. Fig. 9 demonstrates the normalized Pb concentration (final concentration C/initial concentration Co) profile across the soil specimen with different experimental conditions. The results show that the Pb ion is migrated and transported toward the cathode zone, and removed from the soil specimen. Thus the soil specimen is cleaned due to extraction of contaminant compared with initial condition. The Pb concentration is relatively higher at the anode zone than at the cathode zone. Overall, considering the initial concentration of Pb in the soil, two types of remediation techniques removed significant amounts of Pb. The normalized Pb concentration is varied with experimental conditions. The normalized Pb concentration in soil specimen is low in the case of electrokinetic and ultrasonic remediation process by comparing with in the case of electrokinetic remediation process. The heavy metal contaminant such as Pb is easily migrated and removed by electroosmosis and electromigration phenomena induced from electrokinetic process. The removal efficiency of heavy metal by electrokinetic and ultrasonic technique is higher than that by electrokinetic technique. The residual concentration of Pb in soil specimen is average 0.12 C/Co for electrokinetic test and average 0.09 C/Co for electrokinetic and ultrasonic test.

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3.6. Phenanthrene concentration in soil specimen Phenanthrene concentration in soil specimen was measured from 10 small sliced specimens in each experiment. The phenanthrene contaminant was allowed to migrate from the inlet to outlet under the actions of electroosmosis and ultrasonic wave. Since phenanthrene is a neutrally charged contaminant, electromigration is not act for migration of phenanthrene toward outlet compartment. Fig. 10 demonstrates the normalized phenanthrene concentration profile across the soil specimens determined at the conclusion of experiments. The results show that the phenanthrene is migrated and transported toward the cathode zone, and removed from the soil specimen. The phenanthrene concentration is relatively higher at the anode zone than at the cathode zone. Considering the initial concentration of phenanthrene in the soil, significant amounts of phenanthrene removed for all techniques. The PAHs such as phenanthrene is migrated and removed by electric and sonic phenomena induced by electrokinetic and ultrasonic process. The normalized phenanthrene concentration in soil specimen is low in the case of electrokinetic and ultrasonic process by comparing with in the case of electrokinetic process. The removal efficiency of phenanthrene by electrokinetic and ultrasonic technique is higher than that by electrokinetic technique. Reddi et al. (1993) also investigated the effect of ultrasonic energy on enhancement of the transportation of water in porous soils. They observed an increase in the permeability of soil specimen. The increased permeability due to sonication can be

The contaminant removal efficiency is calculated from the inverse of residual concentration of contaminant in soil specimen. The removal efficiency of Pb is average 88% for electrokinetic test and average 91% for electrokinetic and ultrasonic test as shown in Fig. 11. This results show that the removal efficiency of Pb in electrokinetic and ultrasonic process is increased about 3.4% comparing with electrokinetic process due to ultrasonic effect. Fig. 12 shows the removal efficiency of phenanthrene is average 85% for electrokinetic test and average 90% for electrokinetic and ultrasonic test. This results show that the removal

Fig. 10. Normalized concentration of phenanthrene in soil specimen.

Fig. 12. Removal efficiency of phenanthrene in soil specimen.

Fig. 11. Removal efficiency of Pb in soil specimen.

attributed to water and contaminant migration. The residual concentration of phenanthrene in soil specimen is average 0.15 C/Co for electrokinetic remediation test and average 0.1 C/Co for electrokinetic and ultrasonic remediation test. 3.7. Contaminants removal rate

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efficiency of phenanthrene in electrokinetic and ultrasonic process is increased about 5.9% comparing with electrokinetic process. The mass balance was calculated from the amount of contaminants in soil, influent, and effluent. The mass balances in electrokinetic test are 92% (final amount of Pb in soil and effluent: 12% and 80%, respectively, error 8%) for Pb contaminant and 90% (final amount of phenanthrene in soil and effluent: 9% and 81%, respectively, error 10%) for phenanthrene contaminant. And the mass balances in electrokinetic and ultrasonic test are 95% (final amount of Pb in soil and effluent: 15% and 80%, respectively, error 5%) for Pb contaminant and 96% (final amount of phenanthrene in soil and effluent: 10% and 86%, respectively, error 4%) for phenanthrene contaminant. The high mass balances over 90% were obtained in all experiments. The 5–10% error is acceptable for the accuracy needed in this study. When ultrasonic energy is applied in contaminated soil, the viscosity of fluid phase decreased and flow rate increased, the molecular movement increased and sorbed contaminants mobilized, and the cavitation developed and porosity and permeability increased. Thus, the removal efficiency of contaminant is higher for electrokinetic and ultrasonic test than for simple electrokinetic test. From these results, it can be suggested that the electroosmosis and ultrasonic migration phenomena are efficient for the removal of nonionic matter, and the electromigration phenomenon is efficient for the removal of ionic matter. Thus, the contaminant removal rate is highest in electrokinetic and ultrasonic soil flushing system due to the coupled effect of electrokinetic and ultrasonic technique. This could suggest that introduction of enhancement techniques, addition of ultrasonic process onto electrokinetic process, could be effective for increasing of contaminant removal rate from the contaminated soil.

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and electrokinetic and ultrasonic remediation. The main conclusions drawn from the results of this investigation were as follows: 1.

The water and contaminant in porous soil media is allowed to flow and migrate under the actions of electroosmotic flow and electromigration by electric power and acoustic flow by ultrasonic waves for electrokinetic and ultrasonic remediation process. 2. The accumulated outflow and contaminant removal rate are higher by addition of vibration, cavitation and sonication effects in the case of electrokinetic and ultrasonic remediation process than in the case of electrokinetic remediation process. 3. The accumulated quantity of outflow was 120 ml/ h for electrokinetic remediation test and 143 ml/h for electrokinetic and ultrasonic remediation test after 360 h, so the quantity of outflow increased with 19% due to the coupled effects of electrokinetic and ultrasonic phenomena. 4. The final pH value across the soil specimen slightly decreased due to increase of outflow and further advance of acid front in the case of enhancement test by introducing ultrasonic wave to electric field. 5. The removal rates of Pb and phenanthrene are average 88% and 85% for electrokinetic test and average 91% and 90% for electrokinetic and ultrasonic test, thus the removal efficiencies in electrokinetic and ultrasonic process are increased about 3.4% for Pb and 5.9% for phenanthrene by the coupled effects of electrokinetic and ultrasonic phenomena comparing with simple electrokinetic process. 6. The new remedial technique combined with electrokinetic and ultrasonic process can be effectively applied for the removal of ionic and nonionic contaminants from the ground contaminated with various contaminants.

4. Conclusions The objective of this laboratory investigation was to evaluate the coupled effect of electrokinetic and ultrasonic technique for extraction of ionic and nonionic matter from contaminated soils. A series of tests were conducted for electrokinetic remediation

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