Effect of the Composition of Anodic Purging Solutionson Electroosmosis

Songhu Yuan1; Meng Tian2; Taiping Li3; and Xiaohua Lu4

Abstract: The objective of this study is to investigate the effect of the composition of anodic purging solutions on electroosmosis in anelectrokinetic �EK� system. The effect of buffering capacity of anodic purging solutions on electroosmosis was first studied. With theincrease of buffering capacity, soil pH and electric current increased, but the maximal cumulative electroosmostic flow �EOF� wasachieved with 0.010 mol/L Na2CO3/NaHCO3 buffer. Na2CO3/NaHCO3 containing NaCl and Na2SO4, respectively, were used as anodicpurging solutions to investigate the effect of cation concentration and anion type on electroosmosis. The increase of cation concentrationled to the increase of soil pH and electric current but the decrease of EOF. At the same cation concentration, Na2SO4 resulted in higherelectric current and greater EOF than NaCl, but similar distribution of soil pH. The present study provides useful information for theselection of purging solution in the EK remediation of contaminated soils.

DOI: 10.1061/�ASCE�0733-9372�2007�133:6�627�

CE Database subject headings: Soil treatment; Abatement and removal; Environmental engineering; Restoration.

Introduction

Electrokinetic �EK� technology is one of the most promisingmethods that apply to the remediation of pollutants from fine-grain soils. It has been verified to be effective in removing bothheavy metals �Hicks and Tondorf 1994; Puppala et al. 1997;Reddy et al. 2004; Zhou et al. 2004; Yeung and Hsu 2005; Gian-nis and Gidarakos 2005� and organic contaminants �Shapiro andProbstein 1993; Ho et al. 1995, 1997; Luo et al. 2005; Ko et al.2000; Li et al. 2000; Saichek and Reddy 2003; Reddy andSaichek 2003; Yuan and Weng 2004�. A comprehensive review ofEK soil remediation has been provided in several previous studies�Acar and Alshawabkeh 1993; Shapiro and Probstein 1993; Pageand Page 2002; Virkutyte et al. 2002; Lageman et al. 2005�.Briefly, with the application of electric field to the polluted site,pollutants will migrate toward anode or cathode by electromigra-tion, electroosmosis, and electrophoresis �Acar and Alshawabkeh1993; Shapiro and Probstein 1993; Virkutyte et al. 2002�. Elec-tromigration describes the transport of ions in the pore fluid with

1Ph.D. Candidate, Environmental Science Research Institute,Huazhong Univ. of Science and Technology, Wuhan 430074, P. R. China.E-mail: yuansonghu622@hotmail.com

2Graduate Student, Environmental Science Research Institute,Huazhong Univ. of Science and Technology, Wuhan 430074, P. R. China.E-mail: haonanrentianmeng@126.com

3Ph.D. Candidate, Environmental Science Research Institute,Huazhong Univ. of Science and Technology, Wuhan 430074, P. R. China.E-mail: litai20000946@sina.com

4Professor, Environmental Science Research Institute, HuazhongUniv. of Science and Technology, Wuhan 430074, P. R. China�corresponding author�. E-mail: lxh@hust-esri.com

Note. Discussion open until November 1, 2007. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on May 1, 2006; approved on January 30, 2007. Thispaper is part of the Journal of Environmental Engineering, Vol. 133,

No. 6, June 1, 2007. ©ASCE, ISSN 0733-9372/2007/6-627–632/$25.00.

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cations moving to cathode and anions to anode. Electroosmosis isthe movement of the pore fluid contained in the soils. Pollutantsdissolved in the fluid can be driven out of the site by electroos-mosis. Electrophoresis is the migration of charged colloids, whichhas negligible effect in a compact soil system.

In an EK system, electroosmosis has great influence on theremoval of pollutants, particularly organic pollutants. In general,for the EK removal of the organic pollutants that cannot dissoci-ate, the pollutants should first be desorbed from soils and thendriven out by the electric field, mainly electroosmosis �Ko et al.2000; Li et al. 2000; Saichek and Reddy 2003�. However, elect-roomosis is affected by many factors such as the intrinsic charac-teristics of the soils and pore solution. Electroosmosis is producedbecause the locally existing excess ions migrate in a plane parallelto the particle surface toward the oppositely charged electrodeand, as they migrate, they transfer momentum to the surroundingfluid molecules via viscous forces �Saichek and Reddy 2003�. TheHelmholz–Smoluchowski �H-S� Eq. �1� has been used to estimateelectroosmostic flow �EOF� �Shapiro and Probstein 1993�. ueo isthe average electroosmostic flow, which is proportional to thepermittivity of the fluid ���, the uniform zeta potential of thesurface ���, and the uniform electric field strength �E�, and it isinversely proportional to the viscosity of the liquid ���. In a givenEK system, the change of �, E, and � is normally minute, so ueo

is mainly determined by �. Generally, more negative � leads tolarger EOF. However, � is related to many factors including pH,ion strength, ion type, and the coordination adsorption of cationsand anions �Hunter 1981; Li 2001�

ueo = ��E/� �1�

In the initial stage of the EK process, � is determined by theintrinsic characteristics of soils, normally negative for kaolin�Acar and Alshawabkeh 1993�. In the EK remediation process,purging solution enters into the soil matrix and replaces the in-trinsic pore solution; anode reaction as Eq. �2� leads to the for-mation of O2 and the decrease of pH; and cathode reaction asEq. �4� leads to the formation of H2 and the increase of pH. The

+ −

electromigration rate of H is 1.76 times that of OH �Shapiro

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and Probstein 1993�. Consequently, low pH dominates in mostregions, suggesting a high value of � �Giannis and Gidarakos2005�. In order to achieve a large and continuous EOF, it isnecessary to condition pH in the anodic compartment. Variouspurging solutions have been used to condition the pH in an anodiccompartment and to enhance the EK removal of pollutants�Puppala et al. 1997; Reddy et al. 2004; Zhou et al. 2004; Yeungand Hsu 2005; Giannis and Gidarakos 2005; Ko et al. 2000; Liet al. 2000; Saichek and Reddy 2003; Reddy and Saichek 2003;Yuan and Weng 2004�.Anode

2H2O − 4e− = O2 + 4H+ �2�

Cl− − 2e− = Cl2 �3�

Cathode

2H2O + 2e− = H2 + 2OH− �4�

For the EK remediation of soils contaminated with heavy met-als and organic compounds, particularly organic compounds, thecomposition of anodic purging solutions pose a great impact onelectroosmosis. But up to now, the impact has not been investi-gated comprehensively. Hamed and Bhadra �1997� studied theinfluence of current density and influent pH on electrokinetics.The current density was controlled constant. The influent pH wasprepared with either HCl ��7�, or NaOH ��7�, and the soil wasmixed with the corresponding influent. The pH of the soil matrixwould be changed frequently due to the low buffering capacity.Researchers found that the pH control in anodic compartmentbenefit electroosmosis �Shapiro and Probstein 1993; Ko et al.2000; Saichek and Reddy 2003; Reddy and Saichek 2003�, butthe effect was not investigated in detail. In addition, a supportingelectrolyte such as NaCl was often included in anodic purgingsolutions �Shapiro and Probstein 1993; Ko et al. 2000�. Cl− maybe oxidized to Cl2 �Eq. �3��, which reduces the generation of O2

and H+ �Eq. �4��. As a consequence, NaCl may increase the alka-linity of the purging solution and Cl2 generated will result insecond pollution. The increase of alkalinity may neutralize moreH+ produced on the anode, suggesting a higher pH and electroos-motic flow.

This study was conducted to provide useful information for theselection of anodic purging solution in the EK remediation pro-cess. The objectives are: �1� to investigate the effect of the buff-ering capacity of anodic purging solutions on electroosmosis inthe EK system; �2� to evaluate the effect of ion strength on elec-troosmosis, practically the concentration of Na+, because of itspredominant contribution to electromigration in the present EKsystem; and �3� to compare and analyze the effect of anion typeon electroomosis.

Materials and Methods

Chemicals and Materials

Kaolin was often used as the model clay in EK laboratory experi-ments �Hunter 1981; Hicks and Tondorf 1994; Hamed and Bhadra1997; Ko et al. 2000; Saichek and Reddy 2003� because of its lowbuffer capacity, low organic content, low cation exchange capac-ity, and inertia �Saichek and Reddy 2003�. In this study, kaolin�Chemical Reagent Company of Guoyao Group, China, chemicalpurity� was used as the simulated clay and its properties are

shown in Table 1. The kaolin used in this study has a high content

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of clay and negligible content of organic materials. Comparedwith the kaolin used in the literature �Hamed and Bhadra 1997;Ko et al. 2000; Saichek and Reddy 2003�, the kaolin herein had ahigher pH value. All the other reagents used were above analyti-cal grade. Deionized water �18.0 m� cm� obtained from a Milli-pore Milli-Q system was used for the preparation of solutions.

EK Experiments

A schematic diagram of the EK setup in this study is shown inFig. 1. A glass cylinder ��5.0 cm�9.0 cm� was used as theEK cell. Perforated graphite ��5.0 cm�0.7 cm� was used asanode and cathode, respectively. An anodic compartment�100 mL capacity� and cathode compartment �50 mL capacity�were assembled in each end. The anodic purging solution wassiphoned from a stocking bottle by a soft rubber tube, whichattached a piston to control the inlet flow. Gas vents were set inboth electrode cells. The electroosmotic flow in the cathode wascollected and measured with a 250 mL cylinder, which was sealedwith gummed tape to reduce the evaporation of water. Constantpotential of 9.0 V was supplied by DC power �GPC-H, 30V/5A,Taiwan Guwei Electronic Ltd., Inc., Taiwan�. The electric currentwas measured by a multimeter connected in the electric circuit.

Table 1. Main Characteristics of Kaolin

Main characteristics Kaolin

0.05–1 mm 0

�0.05 mm 100

Particle size �%�a 0.01–0.05 mm 26.4

0.005–0.01 mm 15.8

0.001–0.005 mm 27.6

�0.001 mm 30.2

OC �g/kg�b 0

CEC �mmol/100 g�c 1.31

pHd 9.60aParticle size was measured by gravimeter �TM-85 soil densimeter,Shanghai Tianle Electromagnetic Institute, China�.bOrganic carbon content, which was measured by potassium dichromatedigestion.cCation exchange capacity, which was measured by ammonium acetatemethod.dpH was measured by the mixture of soil and water �1:1, w/w� with a pHmeter �pHS-25, Shanghai Leici Instrument Factory, China�.

Fig. 1. Schematic diagram of EK reactor

7

In all the EK experiments, approximately 300 g kaolin weremixed in a pan with 100 mL deionized water in a beaker to simu-late the water content of 25%. The anodic compartment, perfo-rated graphite anode, and the EK cell were assembled first. A filterpaper was placed between the electrode and EK cell to preventthe escape of soil. A faction of the moist soil was added followedby compacting with a glass rod to remove air bubbles. Anotherfraction was then added, and the procedure was repeated until theappropriate soil column length was obtained. When the soil wasfully packed into the cell, the perforated graphite cathode andcathode compartment were assembled likewise. The anodic cellwas filled with the preset purging solution. The cathode cell wasfilled with deionized water in any case. After the system wassettled for 12 h, the wires and tubes were connected. A low con-stant voltage of 9 V was supplied by the DC power. All the EKexperiments were carried out at the surrounding temperature of10±2°C. The duration was 150 h for all the runs.

The parameters associated with the EK experiments are pre-sented in Table 2. Experiments of T1–T5 were conducted to ex-amine the effect of buffering capacity on electroosmosis.Na2CO3/NaHCO3 buffering solution was used because of its bet-ter effect on controlling pH higher than NaOH �Yuan et al. 2006�.Since ionic strength may influence the electromigration of ionsand change the double layer thickness of soil surface �Shapiro andProbstein 1993�, additional experiments of T6 and T7 were tested.Practically, the effect of cation concentration and anion type onelectroosmosis was investigated. Ions were introduced to the an-odic compartment, cations were moved to the cathode by elec-tromigration and diffusion, while anions were moved solely bydiffusion. In this study, Na+ contributed to electromigration muchmore significantly than the anions such as CO3

2−, HCO3−, Cl−, and

SO42−. In order to simulate the practical effect of electrolyte, the

purging solutions with the same concentration of Na+ were usedin T6 and T7.

At the end of the EK experiments, soils in the EK cell werepushed out and sliced into six pieces from anode to cathode. Eachpiece was analyzed for pH with a pH meter by mixing a 10 g soil

Table 2. Parameters Associated with EK Experiments

Experimentnumber Anodic purging solution

CNa

�mol/L�

Ionicstrength�mol/L�

T1 De-ionized water �pH 7.00� 0 0

T2 0.005 mol/L Na2CO3/NaHCO3 buffer�pH 10.25�

0.015 0.020

T3 0.010 mol/L Na2CO3/NaHCO3 buffer�pH 10.25�

0.030 0.040

T4 0.025 mol/L Na2CO3/NaHCO3 buffer�pH 10.25�

0.075 0.100

T5 0.050 mol/L Na2CO3/NaHCO3 buffer�pH 10.25�

0.150 0.200

T6 0.010 mol/L Na2CO3/NaHCO3 buffercontaining 0.134 mol/L NaCl

�pH 10.25�

0.164 0.174

T7 0.010 mol/L Na2CO3/NaHCO3 buffercontaining 0.067 mol/L Na2SO4

�pH 10.25�

0.164 0.240

Note: 0.005 mol/L Na2CO3/NaHCO3 buffer consists of 0.005 mol/LNa2CO3+0.005 mol/L NaHCO3, it is the same with the otherbuffer. CNaconcentration of Na+ contained in the anodic purgingsolutions. Ionic strength is calculated from 1/2�cizi

2, where ci andziconcentration and valence of the corresponded ion, respectively.

sample with 10 mL deionized water.

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Quality Control

All the glassware such as vials and bottles were cleaned in ultra-sonic cleaner �20 kHz� assisted by washing liquor for 30 min.Then they were further washed by tap water and deionized waterthree times, respectively. The rubber tube was new for each ex-periment. The graphite electrode was steeped by 2% H2SO4 for24 h, followed by 2% NaOH for 24 h, and then it was rinsed bytap water and deionized water to neutralize. All the testing equip-ment such as multimeter and pH meters were in good conditionand calibrated. Selected EK experiments were performed in du-plicate and good repeatability was obtained.

Results and Discussion

Distribution of pH in Soils

In an EK system, H+ and OH− will be produced on the anode andcathode, respectively. They will electromigrate to the soil matrixand change soil pH temporally and spatially �Yeung and Hsu2005; Giannis and Gidarakos 2005�. The distribution of pH at theend of the EK process is shown in Fig. 2. The initial pH of thesoil was 9.60.

Fig. 2 shows that the soil pH increased from anode to cathode.An acidic front was formed near the anode and led to the dropof pH, while a basic front was formed near the cathode and re-sulted in the increase of pH. This result is in agreement withthe literature �Ko et al. 2000; Li et al. 2000; Saichek and Reddy2003; Reddy and Saichek 2003�. The soil pH increased slightlywith the increase of the buffering capacity of anodic purgingsolutions from 0 to 0.025 mol/L Na2CO3/NaHCO3 �T1–T4�.It is reasonable since greater buffering capacity can neutralizemore H+. A sharp increase of pH was attained when 0.050 mol/LNa2CO3/NaHCO3 buffer �T5� was used. 0.050 mol/L Na2CO3/NaHCO3 had a higher capacity to neutralize H+ and could pro-vide more alkalinity to increase the soil pH. When H+ was greatlyneutralized, the movement of OH− from cathode to anode wasfavored. All these were responsible for the sharp increase ofsoil pH.

When the buffers containing NaCl �T6� and Na2SO4 �T7� wereused as anodic purging solutions, the result is interesting. Com-pared with the background buffer �T3�, much higher pH wasobserved in the overall region. It can be inferred that greaterconcentration of cation in anodic purging solution leads to highersoil pH. When NaCl was contained in the anodic purging solu-

−

Fig. 2. Distribution of pH in soils

tion, Cl may be oxidized to Cl2 on the anode �Eq. �3��. This

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phenomenon was confirmed by the appearance of irritative odorand jade-green color in the anodic compartment. The formation ofCl2 due to the oxidation of Cl− on anode �Eq. �3�� would competewith the formation of H+ due to the oxidation of H2O �Eq. �2��.Cl− may enter into the soil matrix by electroosmosis and diffu-sion. But the direction of electroosmosis and diffusion was con-trary to the direction of electromigration, and the rate of elec-tromigration was rather higher than that of electroosmosis anddiffusion �Acar and Alshawabkeh 1993�, so the concentration ofCl− in the anodic compartment was kept at a high level. As aresult, the generation of Cl2 by the anodic oxidation of Cl− re-duced the generation of H+ by the anodic oxidation of H2O on theanode. In addition, the concentration of Na+ in T6 was muchgreater than in T3. An increase of Na+ concentration resulted in adecrease of the fraction of H+ in the total ions. Thus, the electriccurrent used to transport H+ was reduced and H+ entering intosoils was restricted �Acar and Alshawabkeh 1993; Shapiro andProbstein 1993�. However, these factors had a minute effect onthe formation of OH− on the cathode. So, the region where theacidic front met the basic front shifted toward the anode. WhenNaCl was replaced by Na2SO4, a similar distribution of soil pHwas observed. Because Na2SO4 is the inertia on electrode in nor-mal conditions, the higher pH was probably resulted from thepredominant electromigration of Na+. Compared with NaCl,Na2SO4 achieved a similar distribution of soil pH except for thesection close to the anode. The quantity of H+ produced on theanode was lower in T6 than that in T7 because of the formation ofCl2 by the anodic reaction. So, a lower pH in T7 was observed inthe section close to the anode.

It could be concluded that high buffering capacity and ionstrength benefit controlling soil pH at a high level; Cl− could beoxidized to Cl2 on the anode, which reduced the generation of H+

and produced the second pollution.

Electric Current

In an EK system, electric current is influenced by the conductivityof the soil pore solution, the composition of the solutions in an-odic and cathode compartments, and the soil moisture �Acar andAlshawabkeh 1993; Yuan et al. 2006�. However, all the factorswill change temporally and spatially with increasing EK opera-tion time. The transportation of H+ and OH− may enhance theconductivity of a pore solution, but the precipitation of OH− withmetals such as Ca2+, Mg2+, and Al3+ contained in the soil maydecrease the conductivity. The composition of the solutions inboth anodic and cathode compartments are changed by the elec-

Fig. 3. Electric current changes

trode reactions �Eqs. �2� and �4��. Fig. 3 shows the changes of

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electric current in the experiments. The difference of the initialelectric current was resulted from the different composition of theanodic purging solutions.

It can be seen from Fig. 3 that in T1–T3 the electric currentgradually declines, while in T4–T7 the electric current reaches apeak and then declines. In the EK process, H+ and OH− are pro-duced on the anode and cathode, respectively. H+ is neutralizedby the anodic purging solution and OH− enters into the soil matrixby electromigration. Na+, CO3

2−, and HCO3− were contained in the

anodic compartment. The direction of the electromigration of Na+

was the same as the direction of electroosmosis and diffusion, soNa+ could move to the soil smoothly. However, the direction wascontrary for CO3

2− and HCO3−, causing them to be retained in the

anodic compartment. The electromigration of Na+ and the residueH+ contribute to electric current, whereas when anions such asOH− and CO3

2− move to soil, they will precipitate with cationssuch as Ca2+, Mg2+, and Al3+ contained in kaolin �Saichek andReddy 2003�. This will compete with their electromigration andresult in the decrease of electric current. The other factors such asthe composition of the solutions in anodic and cathode compart-ment and the soil moisture would also influence the electric cur-rent. Therefore, a gradual decrease of electric current was ob-served in T1, T2, and T3. In T4–T7, the quantity of Na+ increasessharply, while the quantity of the cations that can be precipitatedremains constant. Consequently, an increase of electric currentappears in the first stage.

By comparing the electric current in the tests, it was found thatelectric current increases with the increase of buffering capacity�T1–T5�, and cation concentration significantly affected electriccurrent �T6, T7�. As known, the ions contained in the soil porefluid contribute predominantly to electric current. Due to elec-tromigration, Na+ and H+ are driven to the cathode while CO3

2−

and HCO3− are retained in the anodic compartment theoretically.

However, CO32− and HCO3

− will partially move to the soil matrixby electroosmosis. Therefore, the quantity of ions that can trans-port charge increases in T1–T5, which resulted in the increase ofelectric current. When NaCl or Na2SO4 were introduced to thebuffer �T6, T7�, the quantity of Na+ that was contained in theanodic compartment and moved to the soil matrix by electromi-gration obviously increased, so a relatively higher electric currentwas observed in T6 and T7.

By comparing the electric current in T6 and T7, it was foundthat the electric current with Na2SO4 was much higher than withNaCl. The ionic strength of 0.067 mol/L Na2SO4 was higher thanthat of 0.134 mol/L �Table 2�. Cl− could be oxidized to Cl2 on theanode, which reduced the quantity of Cl−. Both Cl− and SO4

2− canenter into the soil matrix by electroosmosis, but the electromigra-tion rate of SO4

2− is higher than that of Cl− �Fu 1990�. Therefore,the movement of SO4

2− in the region close to anode was morefrequent than that of Cl−. All these were responsible for the dif-ferent electric current in T6 and T7. Additionally, it is noteworthythat the electric current in T6 was less than that in T5, although ahigher cation concentration was used in T6. This result also indi-cated that the effect of formation of Cl2 from Cl− on electriccurrent was significant.

Electroosmostic Flow

Kaolin is normally negatively charged. The movement of thepositively charged counterions in the diffusion double layer to-ward the cathode creates an EOF during EK processes. Thedeparting positive ions in the diffusion layer are immediately re-

placed by other positive ions supplied from the bulk aqueous

7

solutions, thereby providing a constant restoration of charge equi-librium in the double layer and continuous EOF �Ko et al. 2000�.

The cumulative EOF is shown in Fig. 4. By comparing thecumulative EOF in T1–T5, it can be seen that the maximalcumulative EOF was obtained when 0.010 mol/L Na2CO3/NaHCO3 buffer �T3� was used as the purging solution, thoughboth soil pH and electric current increased with increasing buff-ering capacity. Both pH and ionic strength have great effects onthe zeta potential of kaolin �Hunter 1981; Acar and Alshawabkeh1993; Li et al. 2000�. With the increase of pH, the quantity ofOH− that adsorbed on the surface of kaolin increases, so zetapotential becomes more negative. On the other hand, the thick-ness of the double layer becomes thinner when the ionic strengthincreases. This will reduce the absolute value of zeta potential,making it near zero �Hunter 1981�. As a result, the comprehensiveeffect of pH and ionic strength contributed to the change of cu-mulative EOF. When the buffering capacity increased from0 to 0.010 mol/L Na2CO3/NaHCO3 buffer, the higher soil pHled to a more negative zeta potential, while the increase of ionicstrength resulted in the decline of the absolute value of the zetapotential. The zeta potential of kaolin was normally negative, sothe decline of the absolute value indicated a more positive valueof zeta potential. It can be suggested that the soil pH predomi-nantly contributed to the zeta potential in low buffering capacity�T1–T3�. When the buffering capacity increased further �T3–T5�,the effect of ionic strength became more significant than that ofsoil pH. Therefore, a decrease of cumulative EOF was observedin T3–T5. The effect of ionic strength was particularly obviouswhen NaCl �T6� or Na2SO4 �T7� was added to the buffer.

It can also be seen from Fig. 4 that Na2SO4 �T7� led to moreEOF than NaCl �T6�. This result is correlated with the change ofelectric current. The same correlation was also reported by severalresearchers �Li et al. 2000; Saichek and Reddy 2003�. Electriccurrent represents the transport of ions when the conductive me-dium has the same resistance. Higher electric current leads tofaster ion transport, which results in faster transport of water byelectroosmosis �Ko et al. 2000�.

In order to achieve a straightforward comparison of EOF withthe literature, the coefficient of electroosmotic conductivity�ke, m2/V s� was used. ke can be calculated from the followingequation �Yeung 2006�

Q = keieA �5�

where Q=electroosmotic flow rate �m3/s�; ie=strength of the ap-plied electric field applied �V/m�; and A=gross cross-sectional

2

Fig. 4. Cumulative EOF

area perpendicular to the direction of flow �m �. The equation

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assumes the positive direction of electroosmotic flow to be fromthe anode toward the cathode. Since ke is independent of soiltype and applied electric field �Yeung 2006�, it is possible tocompare ke obtained from different studies. The variation of thecoefficient of electroosmotic conductivity with time is demon-strated in Fig. 5. The variation of ke stemmed from the variationof chemistry in the soil �Yeung 2006�. The decreasing trend ofke was obtained in all the tests. Fig. 5 also shows that ke wasin the range of 1.1�10−9 to 3.9�10−9 m2/V s. The range of−2.0�10−9 to 4.0�10−9 m2/V s has been reported by Yeung�2006�.

A further comparison was performed using the coefficient ofelectroosmotic water transport �ki�, which gives the volume offluid moved by a unit electric charge and provides information onthe efficiency and economics of the use of electroosmosis totransport water in soils �Yeung 2006�. The following equationcould be used to calculated ki

Q = kiI �6�

where I=electric current. In the tests, the variation of ki with timeis presented in Fig. 6. It could be seen that the higher ki wasobtained when deionized water or a lower buffer solution than0.010 mol/L Na2CO3/NaHCO3 was filled in the anodic com-partment �T1–T4�. In T4, ki was kept at a relatively high valueduring the operation. The increase of ionic strength resulted in thedecrease of ki. The relationship of ki and ke was reported to be:ki=ke / �Yeung 2006�, where electrical conductivity of thesystem. The increase of ionic strength led to the increase of elec-trical conductivity, which subsequently reduced ki. The values of

Fig. 5. Variation of coefficient of electroosmotic conductivity ke

with time

Fig. 6. Variation of coefficient of electroosmotic water transport ki

with time

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ki in this study were a little higher than those in the literature�Yeung 2006�.

Conclusions

This study investigated the effect of the composition of purgingsolutions such as buffering capacity, ionic strength, and anionictype on electroosmosis. The electrokinetic properties of soilpH, electric current, and cumulative electroosmotic flow weremeasured and discussed. The main conclusions were drawn asfollows:1. The soil pH and electric current increased with the increase

of buffering capacity, but the maximal cumulative EOF wasachieved when 0.010 mol/L Na2CO3/NaHCO3 buffer �T3�was used as the purging solution. The comprehensive effectof pH and ionic strength on zeta potential was responsible forthe result.

2. The increase of ionic strength of the anodic purging solutionsled to the increase of soil pH and electric current but thedecrease of cumulative EOF. Compared with Cl−, SO4

2− re-sulted in a similar distribution of soil pH, but higher electriccurrent and more cumulative EOF. Cl− could be oxidized toCl2 at the anode and affect electroosmosis, so the selection ofsupporting electrolyte should be careful.

3. This study provides useful information on the selection ofpurging solutions in the remediation of soils contaminatedwith heavy metals and organic compounds, particularly or-ganic compounds. The buffering capacity should be suffi-cient to neutralize as much H+ as possible, but the ionicstrength should be as small as possible to reduce the absolutevalue of zeta potential and increase the coefficient of elec-troosmotic water transport. Furthermore, anions such as Cl−

should not be included in the anodic purging solutions.

Acknowledgments

This work was supported by the key project of Education Minis-try of China �Grant No. 104250� and the key project of NaturalScience Foundation of Hubei Province �Grant No. 2006ABD005�.

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