Glycine-b-Cyclodextrin–Enhanced Electrokinetic Removalof Atrazine from Contaminated Soils

Guanghui Wang,* Weidong Xu, Xuegang Wang, and Lei Huang

College of Water Resources and Environmental Engineering, East China Institute of Technology, Fuzhou, China.

Received: August 24, 2010 Accepted in revised form: May 28, 2011

Abstract

Glycine-b-cyclodextrin (G-b-CD) was synthesized by the reaction of b-CD with glycine in the presence of KOHand epichlorohydrin. G-b-CD–enhanced solubilization and desorption behavior of atrazine were investigated,and the feasibility of using G-b-CD in electrokinetic (EK) removal of atrazine from contaminated soil wasevaluated for the first time. Bench-scale EK tests were conducted under a voltage gradient of 2.0 V cm - 1 for 10 d,and deionized water, 0.05 M Na2CO3/NaHCO3 buffer solution, 0.05 M Na2CO3/NaHCO3 buffer solution con-taining 2 g/L G-b-CD and 0.05 M Na2CO3/NaHCO3 buffer solution containing 10 g/L G-b-CD were used asanodic flushing solutions, respectively. Results from solubilization experiments showed that the solubility ofatrazine in 30 g/L of G-b-CD was enhanced about 4.2-fold. Desorption efficiency of atrazine in soil increasedwith increasing G-b-CD concentration. Experimental results from EK remediation tests showed that migrationand removal of atrazine in soil were significantly affected by G-b-CD concentrations and cumulative electro-osmotic flow. pH control is beneficial for migration and removal of atrazine. In a test with deionized water (E1),only about 4.4% of atrazine was removed from the soil near the anode, whereas 12% of atrazine was removedusing the deionized water test with pH control (E2). When G-b-CD was added to anodic flushing solution, the2 g/L and 10 g/L G-b-CD flushing solution showed approximately 33% and 40% removal, respectively. The EKprocess combined with G-b-CD flushing and pH buffering may be a good remediation alternative for hydro-phobic organic contaminant removal from contaminated soil.

Key words: atrazine; solubilization; glycine-b-cyclodextrin; electrokinetic remediation

Introduction

Pesticides have become an integral part of today’s in-tensive agriculture. Widespread and large-scale use of

pesticides has led to the global problem of pollution of soiland water resources (Spliid and Kopper, 1998). Atrazine is aselective triazine herbicide used to control broadleaf andgrassy weeds in agricultural lands and on noncropped in-dustrial lands (Ying et al., 2005). Atrazine is highly persistentin soil, the half-life of atrazine in loamy soils ranges from 60 to150 days, and it can persist for longer than 1 year under dry orcold conditions (Ribaudo and Bouzaher, 1994). Several stud-ies in laboratory rodents have observed reproductive, devel-opmental, and immunological toxicological effects caused byatrazine exposure (Ross and Filipov, 2006). The atrazine-contaminated soil became a pollution source for groundwaterand ecosystem. Thus, the remediation of atrazine-contami-nated soil is an important environmental issue. Researchers

have reported the remediation of atrazine-contaminated soilsby in situ chemical oxidation (Mecozzi et al., 2006), dechlori-nation with zero-valent iron (Satapanajaru et al., 2008), phy-toremediation (Singh et al., 2004), bioremediation (Kadianet al., 2008), and enhanced desorption (Rodriguez-Cruz et al.,2006). However, phytoremediation and bioremediation re-quire highly selectivity for plants and bacteria, and both ofthem are time consuming. In addition, desorption and phys-icochemical processes are only applicable to permeable soils.

Electrokinetic (EK) remediation is an emerging technologythat can effectively remove soluble organic pollutants fromlow permeability soils (Acar and Alshawabkeh, 1993). How-ever, the EK removal of hydrophobic organic chemicals(HOCs) from contaminated soils is rather difficult due to thepoor dissolution and minimal desorption efficiency. Thus,various extracting agents have been used to enhance the de-sorption efficiency of HOCs (Boving et al., 2000).

Recently, cyclodextrins (CDs) have received increasing at-tention because of their forming inclusion complex with variousguest molecules with suitable polarity and dimension. So, theyhave been proposed as an alternative agent to enhance watersolubility of hydrophobic compounds (Ko et al., 1999). In ad-dition, CDs present several advantages over solvents and

*Corresponding author: College of Civil and EnvironmentalEngineering, East China Institute of Technology, Fuzhou 344000,Jiangxi, P.R. China. Phone: (86) 794 8258287; Fax: (86) 794 8258828;E-mail: wgh68611@163.com

ENVIRONMENTAL ENGINEERING SCIENCEVolume 29, Number 6, 2012ª Mary Ann Liebert, Inc.DOI: 10.1089/ees.2010.0272

406

nonionic surfactants such as their lower toxicity and their higherbiodegradability (Martin, 2004; Fenyvesi et al., 2005). Cyclo-dextrins (CDs) are cyclic oligosaccharides made up of six toeight a-d-glucopyranose monomers connected at 1 and 4 car-bon atoms. The CDs with 6–8 a-d-glucopyranose units are de-noteda-,b-, and c-CDs, respectively. However,b-CD is the mostaccessible, the lowest priced, and, generally, the most useful, butits application was limited because of low solubility (18.5 g/L).Some modified b-CDs with high solubility have been used toenhance the solubilty of organic pollutants (Hanna et al., 2003).

In this work, a novel, modified b-CD, glycine-b-CD (G-b-CD) with high solubility was synthesized by the reaction of b-CD with glycine in the presence of KOH and epichlorohydrin.The G-b-CD–enhanced solubilization and desorption behav-ior of atrazine in soil were studied. On the base of it, theperformance of G-b-CD on the enhancement of EK removal ofatrazine from contaminated soils was evaluated. Various EKparameters, as well as the distribution of pH and residualatrazine in the soils, were measured. This research providedvaluable information on the feasibility of using solubility-enhanced EK remediation as an effective in situ remediationmethod for removing HOCs from contaminated soils.

Materials and Methods

Materials

The b-CD was obtained from Seebio Biotechnology, Inc.and used without further purification. Atrazine was obtainedfrom Changxing No.1 Chemical Co., Ltd. with a purity of99%. Glycine was analytically pure, purchased from JingchunChemical Reagent Co.. Epichlorohydrin was obtained fromFuchen Chemical Reagent Co.. All other reagents and solventsused were of analytical reagent grade unless otherwise stated.The water used throughout the work was deionized by aMilli-Q Water Purification system.

Soil characteristics and preparationof contaminated soil

An uncontaminated natural soil was collected from Fu-zhou City, China; the soil was air-dried and sieved to obtain

particles less than 2 mm in all experiments. The soil has a pHof 5.80 and organic carbon content of 1.82%. The contami-nated soil was prepared by dissolving an appropriatequantity of atrazine in hexane, and a known weight of soilwas slowly added, with continuous mixing. This slurry wasthoroughly mixed, and the solvent was allowed to evaporateslowly. The contaminated soils had a final concentration of1.10 mg/kg of atrazine, and then the dry contaminated soilwas transferred into a bottle and tumbled for about a weekbefore the experiments.

Preparation of G-b-CD

Epichlorohydrin (10 g) was added dropwise to a solution ofglycine (7.5 g) and potassium hydroxide (6.7 g) in 70 mLdeionized water at 50�C, and b-CD (8.0 g) was subsequentlyadded to the solution just mentioned. The mixture was re-acted at 60�C for 1 h, and pH was adjusted to about 5.5 usingsulfuric acid after the solution had been cooled to room tem-perature. Then, ethanol (280 mL, 95%) was added to themixture. The mixture then became composed of two phases:the top layer, a solution of salt and other small moleculecompounds in 80% ethanol; the bottom layer, a solution of G-b-CD in 80% ethanol. Methanol (350 mL) was added to thebottom layer, and a white cubic crystal was obtained, thewhite crystal was filtered and washed by ethanol, then put ina vacuum drying chamber at 60�C. The reaction scheme forthe synthesis was shown in Fig. 1.

Solubilization experiments

For the solubility measurements, 10 mL of solution con-taining different CD concentrations were poured in 50 mLconical flasks with caps, and the solid atrazine was added inquantities in excess of the solubility limit. The conical flaskswere equilibrated on a reciprocating shaker for 24 h at 25�C,and then the mixture solution was centrifuged at 3488 g for30 min. The solubility of atrazine was determined by mea-suring the solution absorbance at 225 nm after being dilutedwith 50:50 methanol/water. The role of methanol is to de-compose the CD inclusion complexes, thereby keeping the

OH

OH

KOH

OK

OH OK

CH

O

CH2ClCH2

KOHCHClCH2

OH

NH2CH2COOH

KOHOK

OK

OCH2

OCH2

CH

OH

CH2 NHCH2COOH

CH CH2 NHCH2COOH

CH2 NHCH2COOH

O

CH2 CH CH2 NHCH2COOHOH

OH

OH

OH

H2SO4

+

(pH=5.5)

FIG. 1. Reaction scheme for thesynthesis of glycine-b-cyclodextrin(G-b-CD).

ELECTROKINETIC REMEDIATION 407

ultraviolet (UV) spectrum of atrazine unchanged (Wang andBrusseau, 1993).

Batch desorption experiments

The desorption experiments were performed in 50 mL con-ical flasks containing 0.1 g of atrzine contaminated soil and10 mL of the G-b-CD solution with a concentration of 1 g/L.The conical flasks were equilibrated on a reciprocating shakerfor 24 h at 25�C. After equilibration, the mixture solution wascentrifuged at 3488 g for 30 min. Atrazine in aqueous solutionwas determined by HPLC (Shimadzu). The mobile phase wasmethanol/water mixture (70/30, v/v) at a flow rate of 1.0 mLmin - 1. The UV detector was set at 225 nm. Measurements weremade in triplicate in each experiment with errors less than 5%.

EK remediation

The EK remediation test setup used in this study wasshown in Fig. 2. The setup consisted of a cell, two electrodecompartments (60 mL capacity), reservoirs, and a powersupply. A plexiglas cylinder (internal diameter 5.0 cm · 15 cm)was used as the EK cell. Perforated graphite (internal diameter5.0 cm · 0.7 cm) was used as anode and cathode. Each elec-trode compartment contained a filter paper, a porous stone,and a perforated graphite electrode. Approximately 300 g ofatrazine-contaminated soil was mixed with 200 mL of deio-nized water. The moist soil was then added into the EK cell inlayers. A voltage gradient of 2.0 V/cm was applied in all tests.Parameters associated with each experiment were listed inTable 1.

At the end of each test, the anode reservoir and the elec-trode assemblies were disconnected, and the soil specimen

was extruded from the cell. The soil specimen was sectionedinto five equal parts. Each part was weighed and preserved ina glass bottle and was used to analyze atrazine concentra-tions. In addition, pH of each soil section was also determined.

Results and Discussion

Solubilization of atrazine in CD solution

The solubilization effects of CD on atrazine are plotted inFig. 3. The results show that the apparent aqueous solubilitiesof atrazine are linearly increased with increasing CD con-centration. This phenomenon is attributed to the formationof 1:1 inclusion complexes (Ko et al., 1999). The linear rela-tionship can be expressed as follows:

St=S0¼ 1þKfC0 ð1Þ

where St is aqueous-phase concentration of atrazine in thepresence of CD, S0 is concentration of atrazine in the ab-sence of CD, C0 represents the initial concentration of CD,and Kf is the stability constant of inclusion complexes foratrazine with CD, and it was used to evaluate solubilizationcapacity of CD for atrazine. As shown in Fig. 3, the stabilityconstant of inclusion complex for atrazine with G-b-CD andb-CD is 0.11 and 0.060, respectively. Higher binding con-stant was found for atrazine with G-b-CD than with b-CD,which indicates that solubilization capacity of G-b-CD foratrazine is higher than that of b-CD for atrazine, and thesolubility of atrazine in 30 g/L of G-b-CD was enhancedabout 4.2-fold.

Desorption of atrazine in soil

Before the EK tests, batch equilibrium experiments wereconducted to evaluate the effects of G-b-CD on the desorptionof atrazine from contaminated soil. As shown in Fig. 4, theresults showed that desorption efficiency of atrazine in soilincreased with increasing G-b-CD concentration. When 2.0 g/LG-b-CD solution was used, the maximal atrazine desorptionwas obtained as about 67%. The low-polarity cavity of G-b-CD provided a capacity to increase the apparent solubility ofatrazine in soil. Therefore, the desorption efficiency of G-b-CDfor atrazine in soil can be improved by the increase of G-b-CDconcentration.

Variation of electric current and cumulativeelectroosmotic flow during EK process

During the EK process, the electric current was affected bythe conductivity of soil pore solution, the composition of thesolutions in anode and cathode compartments, and soil

Gas Vent Gas Vent

Anode Reservoir

Cathode Reservoir

Soil

Anode Compartment

Cathode Compartment(+) Power Supply (-)

Graphite Electrode

FIG. 2. Schematic diagram of electrokinetic remediationtest setup.

Table 1. Parameters Associated with Electrokinetic Setup

Experiment number Anodic flushing solution Soil moisture (%)Voltage gradient

(VDC/cm)

E1 Deionized water 40% 2E2 0.05 mol/L Na2CO3/NaHCO3 buffer 40% 2E3 10 g/L G-b-CD in 0.05 mol/L Na2CO3/NaHCO3 buffer 40% 2E4 2 g/L G-b-CD in 0.05 mol/L Na2CO3/NaHCO3 buffer 40% 2

G-b-CD, glycine-b-cyclodextrin; VDC, voltage direct current.

408 WANG ET AL.

moisture (Yuan et al., 2006). The variation of electric currentversus elapsed time was shown in Fig. 5a. The electric currentvalue generally reached a peak at the start of testing, and thequantity of ions in the pore solution was greatest due to thedissolution of salts that were associated with the dry soilparticles (Saichek and Reddy, 2003). As the ions electro-migrated toward the electrodes, the current gradually de-clined. After approximately 216 h, a more stable or residual

current value was reached. The initial current of all tests wasdifferent because of the different initial conditions.

Comparing the results of the test with and without pHcontrol, it can be observed that the tests (E2, E3, and E4) em-ploying the 0.05 M Na2CO3/NaHCO3 solution commonlyhad higher current values, and this is most likely a resultof the additional ions introduced by the Na2CO3/NaHCO3

electrolyte. When Na + and CO32 - /HCO3

- ions were intro-duced, the CO3

2 - /HCO3- neutralized some of the H + ions

generated at the anode by the electrolysis reaction; whereasNa + ions electromigrated toward the cathode and increasedthe current. In addition, the current values of E3 and E4 werehigher than those of E2; this was because more ions could bereleased into pore solution with the help of coordination in-teraction of G-b-CD containing amino and carboxyl groups(Meers et al., 2009).

The cumulative electroosmotic flow (EOF) was depictedin Fig. 5b. The flow behavior was dependent on the flushingsolutions and elapsed time. For all tests, the cumulative EOFincreased with increasing elapsed time. Within 240 h EKprocess, for the deionized water test (E1) without pH control,the maximum cumulative EOF was 250 mL; however, for testswith pH control, the maximum cumulative EOF of E2, E3, andE4 was 370, 480, and 565 mL, respectively. When comparedwith deionized water (E1), Na2CO3/NaHCO3 buffer solu-tion (E2) significantly increased the cumulative EOF. It isbecause of this that the Na2CO3/NaHCO3 buffer solutionleads to higher pH than deionized water. Higher pH leadsto more negative f potential, resulting in the increase of the

a

0 2 4 6 8 10 12 14 161.0

1.2

1.4

1.6

1.8

2.0

S t / S 0

Concentration of b-CD (g/L)

St / S0=1.00+0.060 C (R=0.9958)

b

0 5 10 15 20 25 300.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

S t / S 0

Concentration of G-b-CD (g/L)

St/S0=0.85+0.11 C (R=0.9955)

FIG. 3. Solubilization curves of atrazine in (a) b-CD and(b) G-b-CD. (pH = 6.0 and T = 25�C).

0.4 0.8 1.2 1.6 2.048

52

56

60

64

68

Des

orpt

ion

effi

cien

cy (

%)

G-b-CD (g/L)

FIG. 4. Desorption efficiency of atrazine as a function ofG-b-CD concentration. (pH = 6.0 and T = 25�C).

50 100 150 200 2500

5

10

15

20

25

30

Ele

ctri

c C

urre

nt (

mA

)

Elapsed time (h)

E1 E2 E3 E4

a

50 100 150 200 2500

100

200

300

400

500

600b

Cum

ulat

ive

EO

F (m

L)

Elapsed time (h)

E1

E2

E3

E4

FIG. 5. Variation of (a) electric current and (b) cumulativeEOF with elapsed time. EOF, electroosmotic flow.

ELECTROKINETIC REMEDIATION 409

cumulative EOF (Acar and Alshawabkeh, 1993). Comparedwith the test with 10 g/L G-b-CD (E3), the higher cumulativeEOF in the test with 2 g/L G-b-CD (E4) may be due to rela-tively higher dielectric constant and lower viscosity of 2 g/LG-b-CD solution. Similar results were also reported (Maturiand Reddy, 2006).

In addition, the electric current represents the transport ofions when the conductive medium has the same resistance.Higher electric current leads to faster ion transport, whichresults in faster transport of water by electroosmosis (Saichekand Reddy, 2003). Thus, the higher the electric current is, thehigher the cumulative EOF is, and the order of the cumulativeEOF is as follows: E4 > E3 > E2 > E1.

Variation of soil pH during EK process

When voltage potential was applied to the EK cell, theelectrolysis of water produces H + at the anode and OH - atthe cathode. Migration of two ion species toward the oppositeelectrode results in a high pH at the cathode and a low pH atthe anode. For the test without pH control (E1), the H + ionsmigrated through the soil all the way to the cathode region,which is evident because acidic pH values occurred along thelength of the soil profiles. As shown in Fig. 6, the pH value inthe soil was 3.7 near the anode and then gradually increasedto 4.7 toward the cathode. However, the test conducted withpH control (E2) had somewhat higher pH values that rangedfrom 7.4 near the anode to 9.4 near the cathode, which indi-cates that Na2CO3/NaHCO3 had a larger potential to neu-tralize H + produced on the anode. The pH of the tests withG-b-CD ranged from 7.5–7.7 near the anode region and from9.5–9.8 near the cathode region. The pH in both tests (E3 andE4) was almost the same in the corresponding soil sections.

EK removal of atrazine in soil

At the end of EK tests, the residual atrazine concentrationprofiles from anode to cathode were shown in Fig. 7. Thedeionized water test without pH control (E1) exhibited min-imal atrazine migration. It was because of this that its move-ment by electroosmosis was rather difficult due to lowsolubility of atrazine in water. However, the deionized watertest with pH control (E2) caused some contaminant mobili-zation. In my opinion, the Na2CO3/NaHCO3 buffer solution

has no obvious solubilization for atrazine, but the cumulativeEOF in test E2 was higher than that in test E1. Therefore, thestrong flushing action through the small pore spaces wasobtained in test E2, which resulted in the adsorbed atrazineparticles being flushed and transferred toward the cathodebecause of the high cumulative EOF (Saichek and Reddy,2003). When G-b-CD (E3, E4) was used, it was found thatatrazine concentration increased gradually from anode tocathode. The values of C/C0 were below 1 in the region closeto the anode, which implied that atrazine was partly removedfrom the soil; however, the values of C/C0 were above 1 in theregion close to the cathode, which implied that atrazine wasaccumulated there. The maximum atrazine migration wasobserved in the test with 2 g/L G-b-CD solution (E4). Thenormalized concentration near the anode was 0.67, and itincreased gradually to 1.30 in the soil near the cathode. Thismobility may be due to partial solubizlization of atrazine andthe increased soil-solution-contaminant interaction resultingfrom increased EOF. In the test with 10 g/L G-b-CD (E3), thenormalized concentration of atrazine at the section near theanode was 0.60, and it increased gradually to 1.18 in the soilnear the cathode. Compared with 2 g/L G-b-CD (E4), the10 g/L GCD concentration was sufficient for the solubiliza-tion to occur, but mobilization remained minimal because oflow EOF.

At the end of each test, atrazine removal efficiency at a fixedlocation in the EK test was calculated from the total atrazineinitial mass present in soils and the final mass in soils. For thedeionized water test without pH control (E1), only about 4.4%of atrazine was removed from the soil near the anode; how-ever, for the deionized water test with pH control (E2), theremoval efficiency of atrazine was 12%. When G-b-CD wasadded to the flushing solution, for comparison, the 2 g/L and10 g/L G-b-CD flushing solution showed approximately 33%and 40% removal for E4 and E1, respectively.

Conclusions

The G-b-CD has obvious solubilization for atrazine, and itcould enhance desorption of atrazine in soil. Atrazine mi-gration and removal from contaminated soils were signifi-cantly affected by G-b-CD concentrations and cumulativeEOF. In a test with deionized water, little atrazine movement

0.0 0.2 0.4 0.6 0.8 1.0

4

5

6

7

8

9

10

11

12

pH

Normalized distance from anode (cm/cm)

E1

E2

E3

E4

Initial pH=5.8

FIG. 6. Distribution of pH in the soils.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

C/C

o

Normalized distance from anode (cm/cm)

E1 E2 E3 E4

FIG. 7. Distribution of atrazine in the soils.

410 WANG ET AL.

was observed; and only about 4.4% of atrazine was removedfrom contaminated soils. With G-b-CD and Na2CO3/NaH-CO3 buffer solution, obvious movement of atrazine from an-ode to cathode across soils was observed during the EKprocess, particularly in the test with 10 g/L G-b-CD, nearly40% of atrazine was removed. It was encouraging that sus-tained high EOF with G-b-CD solution was able to achievegreat removal of atrazine from contaminated soils. The EKprocess combined with G-b-CD flushing and pH bufferingmay be a good remediation alternative for removing atrazinefrom contaminated soil.

Acknowledgments

This work was financed by the Natural Science Foundationof China (40861017, 50968001). The authors thank the anon-ymous reviewers for their comments.

Author Disclosure Statement

No competing financial interests exist.

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