Separation and Purification Technology 98 (2012) 216–220

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Separation and Purification Technology

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Electrokinetic remediation of chromium- and cadmium-contaminated soilfrom abandoned industrial site

Ping Lu, Qiyan Feng ⇑, Qingjun Meng, Tao YuanSchool of Environmental Science and Spatial Informatics, China University of Mining and Technology, Xuzhou 221116, Jiangsu, ChinaJiangsu Key Laboratory for Resource and Environmental and Information Engineering, Xuzhou 221116, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 September 2011Received in revised form 30 June 2012Accepted 10 July 2012Available online 20 July 2012

Keywords:Electrokinetic remediationChromium (Cr)Cadmium (Cd)Abandoned industrial site

1383-5866/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.seppur.2012.07.010

⇑ Corresponding author at: School of Environmentmatics, China University of Mining and Technology, Xu

E-mail address: lupinguncc@hotmail.com (Q. Feng

This paper presents a systematic bench-scale study on removals of Chromium (Cr) and Cadmium (Cd)from abandoned industrial site in China with concentration of 510 mg/kg for Cr and 200 mg/kg for Cd.The polarity exchange and conventional electrokinetic remediation were conducted with treatment timeof 192 h to assess the removal efficiency and transient behavior of Cr and Cd. All the experiments weresubjected to a voltage gradient of 1 V/cm. Results showed that, polarity exchange electrokinetic remedi-ation led to 88% of Cr and 94% of Cd removals when exchange polarity interval was 48 h. Seventy percentof Cr and 82% of Cd were removed when exchange polarity interval was 96 h. The removal efficiency wascomparable with conventional electrokinetic remediation with removals of 57% of Cr and 49% of Cd. SoilpH was controlled between 5 and 7 by polarity exchange electrokinetic remediation. Additional chemi-cals and complex equipment were not required.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Chromium (Cr) and Cadmium (Cd) are the common contami-nants in abandoned industrial site in China. Accumulated Cr andCd in soil are harmful to human health by contaminating ground-water and agriculture activity.

Electrokinetic remediation is a technique treating heavy metalspolluted soil by applying a low direct current (in order of mA/cm2 ofthe cross-sectional area) or a low potential gradient (in order of V/cm of the distance between the electrodes) to electrodes that are in-serted into the ground. Contaminants are transported by electroos-mosis and electromigration to either the cathode or anode, wherethey are extracted by one or more of the following methods:electroplating, adsorption onto the electrode, precipitation or co-precipitation at the electrode, pumping water near the electrode,or complexing with ion-exchange resins [1–3]. The application ofa direct electric current to soil may result in soil chemical character-istics changes, such as soil pH, zeta potential and electrolyteconcentration, etc. which might impact the performance of theelectrochemical remediation. In conventional electrokinetic treat-ment process, an acidic environment is generated at the anode side(pH is 2), the capability of the electrokinetic remediation to cleanthe polluted site is thus declined [4–6]. Additionally, an alkalineenvironment is generated at the cathode side (pH is 11) which re-

ll rights reserved.

al Science and Spatial Infor-zhou 221116, Jiangsu, China.

).

sults in the precipitation of heavy metals. The phenomena thatOH- ions generated at the cathode by the reduction of water andleading to the precipitation of the metals are called ‘‘focusing ef-fect’’, and it is the main barrier for the electrokinetic treatment ofheavy metal contaminated soil [7–9].

Many studies have been proposed to control the soil pH and en-hance the metals removals [10,11]. Adding reagents such as com-plexing agents can help to form stable complexes in a wide rangeof pH to improve metal solubility [5,12]. Adding acid in the cathodechamber controlled soil pH and avoided the formation of an alka-line environment [13–15]. Using of ionic exchange membraneswas evaluated, by which the flux of ions moving into/out of the soilwas controlled [16–20]. An electrokinetic remediation by reversingpolarity of the power was reported previously to remove manga-nese (Mn) from spiked kaolin with the voltage gradient of 3 V/cm[21]. The polarity was reversed according to a pH indicator colortransition (e.g. exchange polarity once soil turned to alkaline, andchange polarity back to original position once pH was decreased).These studies improved the metal removals compared withconventional electrokinetic remediation, but adding chemicals oradditional materials may lead to secondary contamination, andprobably make higher consumption than the conventional electro-kinetic treatment. A new ‘‘polarity exchange’’ technique was pro-posed, which was easy to operate relative to the previous study[22]. The new polarity exchanging was not only based on the pHvariations, but also dependent on the metal distribution in soil.Additional chemical was not required. New ‘‘polarity exchange’’was applied for sole metal removal from soil, but no research on

P. Lu et al. / Separation and Purification Technology 98 (2012) 216–220 217

the multiple metals removals from soil using polarity exchangeelectrokinetic remediation was reported so far [22].

Removals of multiple metals (Cr and Cd) from abandonedindustrial site under new ‘‘polarity exchange’’ electrokinetic reme-diation condition were first evaluated in this study. Different polar-ity exchange intervals with 96 h and 48 h were conducted based onthe distribution of metals in soil after conventional electrokineticremediation and soil pH variation. Conventional electrokineticremediation was conducted as a preliminary experiment to deter-mine the metal distribution and pH variation after treatment. Theobjective of this work was to evaluate the performance of new‘‘polarity exchange’’ electrokinetic remediation on Cr and Cd rem-ovals from soil.

2. Method and material

2.1. Experimental set-up

Fig. 1a shows set-up of the electrokinetic experiments. Electrodechambers, electrode reservoirs, cylindrical electrokinetic cell, andDC power were the main components. Collected soil was filled inthe cylindrical electrokinetic cell. Five sample collection ports werelocated on top of the soil cell and named as S1–S5. An ammeter wasused to measure the voltage. Two electrode chambers were placedat each end of the soil cell to simulate one-dimensional transport ofcontaminants under an introduced electric potential. Fig. 1b showsthe details of the electrode chamber and reservoir. Electrode cham-bers were connected with the soil cell with screws. Filter paper ando-ring were installed between electrode chambers and the soil cellto avoid leakage. Electrode chambers were connected with the elec-trode reservoirs with pipes. Fresh solution stored in electrode reser-voirs was refilled to the electrode chambers by recirculation pumpsonce the waste solution in electrode chambers was dumped. Graph-ite electrode was inserted into each electrode chamber and con-nected with DC power. Graphite electrode with surface area of130 cm2 (2 � 13 � 10 cm) covered the whole soil cross-section toprovide electric current evenly. Gas vents on top of the chamberwere used for air release. The cell was made of polyethylene with

Fig. 1. Schematic diagram of experimental set-up ((a) experimental set-up and (b)detailed components of chambers and soil cell).

1.5 cm thick. The electrode distance was 15 cm and the cross diam-eter was 9.3 cm.

2.2. Experimental condition

Soil was collected from an abandoned industrial site. Weeds,leaves and rocks were discarded before samples collection. Top soil(5–20 cm) was collected using plastic spade. The sample was holdin the black plastic bag and transported to the lab. Measured con-centration was 510 mg/kg for Cr and 200 mg/kg for Cd. Soil mois-ture content was 13.5%. Initial soil pH was 7.4. Cation exchangecapacity was 8.2 cmol(+)/kg. Organic content was 10.2 g/kg. Foreach electrokinetic test, approximately 1500 g of dry soil samplewas tamped into the electrokinetic cell. Tap water was used aselectrolysis solution. The waste solution for each polarity exchangeperiod was collected in a vessel.

A constant DC voltage gradient of 15 V (1 V/cm) was applied inall experiments for a series of treatment times of 24 h, 48 h, 96 h,and 192 h. A 1 V/cm voltage gradient was applied for removal effi-ciency and energy consumption concern according to previousstudies [6,20,22,23].

Table 1 shows the experimental design. Nine electrokineticexperiments were conducted. EK1–EK4 were conventional electro-kinetic treatment experiments with treatment time of 24 h, 48 h,96 h, and 192 h without polarity exchange in order to evaluatethe Cr6+, Cr3+, and Cd2+ distributions and pH variations after eachtreatment time. The polarity exchange interval was determinedby the distribution and concentration of Cr6+, Cr3+, and Cd2+ afterconventional electrokinetic remediation. EK5–EK9 were the new‘‘polarity exchange’’ electrokinetic treatment experiments, whichwere also conducted in 192 h. Polarity direction was exchangedperiodically in order to enhance removal efficiency during thenew ‘‘polarity exchange’’ electrokinetic remediation. Anode reac-tion of water electrolysis can take place in the original cathodechamber. H+ neutralized the alkaline soil, and a weak acid condi-tion was produced at the soil section where metals precipitated.The re-dissolved metal ion then transported towards the oppositeelectrode chamber and were extracted in a new chamber. contam-inated electrode compartment solution went to waste and the newsolution was refilled.

Soil pH was measured at five different sections by a pH meter(soil/water = 1/2.5). Samples were air-dried, passed through a100-mesh screen (U 0.149 mm), and digested with HF–HNO3–HClO4 for determination of heavy metal concentration. An atomicabsorption spectrophotometer (AAS) was used to determine theconcentrations of total Cr and total Cd according to USEPA methods7190 and 7130. Cr6+ was extracted according to EPA Method 3060 A.Concentration of Cr3+ was determined by the difference between to-tal Cr and Cr6+. Two standard soil samples (kaolin with given exter-nal addition of heavy metals) were analyzed for quality control. Inorder to ensure the accuracy and repeatability of the test results,(1) new electrodes and tubes were used for each experiment, (2)

Table 1Protocol of electrokinetic treatments.

Experiments Exchange polarity interval (h) Treatment time (h)

EK1 Conventional technique 24EK2 Conventional technique 48EK3 Conventional technique 96EK4 Conventional technique 192EK5 96 192EK6 48 48EK7 48 96EK8 48 144EK9 48 192

0

2

4

6

8

10

12

S1 S2 S3 S4 S5

pH

Soil Section

Initial 48 hours 96 hours

Fig. 2. Soil pH profile after 48 hours’ and 96 hours’ conventional electrokineticremediation treatment.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 24 48 72 96 120 144 168 192

Cur

rent

den

sity

(m

A/c

m2 )

Time (h)

Fig. 4. Electric current intensity profile for conventional electrokinetic remediation.

12

120

140

Cd Cr (VI) Cr (III) pH

218 P. Lu et al. / Separation and Purification Technology 98 (2012) 216–220

the electrokinetic test setup components were soaked in a diluteacid solution for 24 h and then rinsed with tap water followed byultrapure water, and (3) chemical analyses were performed induplicate.

0

2

4

6

8

10

0

20

40

60

80

100

S1 S2 S3 S4 S5

pH

C/C

0(%

)

Soil Section

Fig. 5. Time dependent electric current intensity profile with polarity reverseinterval of 48 h.

3. Results and discussion

3.1. Conventional electrokinetic remediation

Application of a direct current in the electrokinetic cell led tocharge transport of the dissolved ionic species and soil pH varia-tion. Fig. 2 shows the pH profile after 48 h and 96 h treatment. Re-sults showed S1–S3 regions were acidic with pH value less than 5,and S4–S5 regions were alkaline with pH values above 10. Soil pHdecreased to 3 at the soil section close to anode (S1), and increasedto 11 at the soil section close to cathode (S5), since electrolysis ofwater produced H+ at anode and OH� at cathode [2]. Similar pHvariation was reported in previous study [21,22].

Fig. 3 shows the standard concentration of Cd2+, Cr6+, and Cr3+ at24 h, 48 h, 96 h, and 192 h. Fifty-seven percent of total Cr and 49% oftotal Cd were removed in 192 h. Concentration of Cr6+ was

Cd-24h

Cr(III)-48h

Cr(VI)-96h

Cr(III)-24h

Cr(VI)-48h

Cd-192h

Cr(VI)-24h

Cd-96h

Cr(III)-192h

Fig. 3. Cr6+, Cr3+, and Cd2+ remnant after co

significant higher at S1 relative to other soil sections (S2–S5). Thehigh Cr6+concentration at S1 might relate to the adsorption of Cr6+

in acid condition, and the oxidation of Cr3+ to Cr6+. Potential oxida-tion reaction for Cr3+ is shown as in Eq. (1). Previous study proved

Cd-48h

Cr(III)-96h

Cr(VI)-192h

Sample collection

SS5

0

2

4

6

8

S1S2

S3S45

points

20

40

60

80

100

120

C/C

0(%

)C

/C0 (%

)

nventional electrokinetic remediation.

00.10.20.30.40.50.60.70.80.9

1

0 30 60 90 120 150 180

Cur

rent

den

sity

(m

A/c

m2 )

Time (h)

Fig. 6. Time dependent electric current intensity profile with polarity reverseinterval of 48 h.

P. Lu et al. / Separation and Purification Technology 98 (2012) 216–220 219

soil pH was significantly lowered near the anode resulting higheradsorption and slower migration of Cr6+ [24]. The concentrationvariation indicated that Cr6+ accumulated at anode. Similar resultthat Cr6+ accumulated at anode was reported previously [24]. Mostof Cr3+ and Cd2+ precipitated at S4 and S5 because of the alkaline soilcondition. Concentration variations indicated that Cr3+ and Cd2+

moved to cathode and finally accumulated. The same results wereobtained in previous study that Cr3+ and Ni2+ (Cd2+ and Ni2+ (nickel)had similar behavior in studied soil) migrated towards the cathodeand accumulated either as precipitates or adsorbates at the sectionsclose to the cathode where the high pH conditions exist [24]. No sig-nificant differences were observed on Cr3+, Cr6+, and Cd2+ distribu-tions at 96 hours’ treatment and 192 hours’ treatment, whichindicated the migrations of ions were retarded. The decreasedmigrations of ions were also proved by current density variation.Fig. 4 shows current density variation over treatment time. Currentdensity started at 0.1 mA/cm2, and increased up to 0.8 mA/cm2,then decreased after that until less than 0.1 mA/cm2 after 96 hours’treatment.

0

2

4

6

8

10

12

0

20

40

60

80

100

120

140

S1 S2 S3 S4 S5

pH

C/C

0(%

)

Soil Section(a. 48 hours)

Cd Cr (VI) Cr (III) pH

0

2

4

6

8

10

12

0

20

40

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100

120

140

S1 S2 S3 S4 S5

pH

C/C

0(%

)

Soil Section(b. 96 hours)

Cd Cr (VI) Cr (III) pH

Fig. 7. Cr6+, Cr3+, and Cd2+ distribution (bars) and pH profile (line) in soil after pola

2Cr3þ þ 3O2 þ 6H2Oþ 6e� ! 2Cr6þ þ 12OH� ð1Þ

3.2. Polarity exchange electrokinetic remediation

The simple way to reduce pH in S4 and S5 can be achieved byexchanging the polarity. Oxidation of water would take place inthe alkaline zone and produce hydrogen ions. The polarity was ex-changed every 96 h and every 48 h based on distributions of metalsafter conventional electrokinetic remediation and soil pH. Thesolution in both chambers was changed at the same time withpolarity exchange to keep the extracted metal away from soil.

EK5 was conducted with polarity exchange interval of 96 h. Itwas important to refill new solution at the time of exchangingpolarity. Fig. 5 shows the remnant metal concentration in soiland pH profile after 192 hours’ treatment. The exchange of polaritywas conducted at the 96-h. After reversing polarity, previous cath-ode turned to anode where the oxidation of water took place. H+

was produced, soil pH was lowered, precipitates (Cr3+ or Cd2+ pre-cipitate) re-dissolved, and Cr3+ and Cd2+ migrated towards the newcathode and finally were removed. Soil pH (S1–S5) was controlledin the range of 6 and 7. A similar pH profile in the soil by exchangepolarity was reported previously [22].

Seventy percent of total Cr and 82% of the Cd were removed.Removals were enhanced compared with the conventional tech-nique with the same voltage gradient and the same treatmenttime. Improved removals were attributed to the absorption onCr6+ in acid soil and precipitates of Cr3+ and Cd2+ in alkaline soilwere alleviated by exchanging the polarity. The remnants Cr6+,Cr3+, and Cd2+ were mostly in S3. Metals concentration variationsindicated that operation for a longer time at reversing polarityled to the easier migration of Cr6+, Cr3+, and Cd2+ near the two polessoil sections (S1, S2, S4, and S5) than other sections. High Cr6+ com-position in S3 relative to other sections and relative to Cr3+ andCd2+ was obtained. This high Cr6+ concentration could due to a

0

2

4

6

8

10

12

0

20

40

60

80

100

120

140

S1 S2 S3 S4 S5

pH

C/C

0(%

)

Soil Section(c. 144 hours)

Cd Cr (VI) Cr (III) pH

0

2

4

6

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10

12

0

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60

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140

S1 S2 S3 S4 S5

pH

C/C

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)

Soil Section(d. 192 hours)

Cd Cr (VI) Cr (III) pH

rity exchange electrokinetic remediation with polarity reverse interval of 48 h.

220 P. Lu et al. / Separation and Purification Technology 98 (2012) 216–220

partial oxidation of Cr3+ to Cr6+ at S3 under bench-scale experimen-tal condition (as shown in Eq. (1)). The oxidation of Cr3+ to Cr6+ wasattributed to the oxygen that was produced at the electrodes whichcan alter the redox conditions of the pore water [24]. Oxidation/reduction might be significant in bench-scale experiment due toan increased impact of the boundaries on the overall process.

Polarity exchange interval was decreased to 48 h. Experimentswere conducted in 192 h. Fig. 6 shows the time-dependent currentintensity variation in ‘‘polarity exchange’’ electrokinetic remedia-tion with exchange interval of 48 h. The current intensity increasedto a peak of 1 mA/cm2. Electric current peak was due to the polarityexchange and the re-dissolved precipitates. The electric currentintensity dropped to less than 0.1 mA/cm2 at the 192 h, since thedissolved ion concentration decreased over time.

Fig. 7a–d shows Cr6+, Cr3+, and Cd2+ distribution and soil pHafter polarity exchange with interval of 48 h. Soil pH at S1 wasabove 5, which was higher than the conventional technique. SoilpH at S5 was decreased compared with the conventional tech-nique. The pH in S3 was high relative to other sections after polar-ity exchange treatment. Soil pH were in the range of 5–7.

Approximate 88% of total Cr was removed, and 94% of total Cdwas removed by polarity exchange with interval of 48 h. Removalswere enhanced compared with the polarity exchange with intervalof 96 h under the same voltage gradient and the same treatmenttime. Removal of Cr was about 10% higher than Cd in conventionalelectrokinetic remediation while the removal efficiency of Cr wasabout 10% lower than Cd in exchange polarity experiments. Remo-vals indicated Cd migration was more significant than Cr underpolarity exchange condition, while Cr migration was more signifi-cant than Cd under conventional electrokinetic remediation condi-tion. Accumulation of Cr6+ in S3 was the hindering mechanism forthe removal of total Cr under polarity exchange condition.

4. Conclusions

Application of conventional electrokinetic remediation obtaineda removal of 57% of the initial Cr and 49% of the initial Cd. The newpolarity exchange technique increased removals of the heavy met-als and led to soil pH in the range of 5–7. Seventy percent of Cr and82% of Cd were removed by polarity exchange technique withpolarity exchange interval of 96 h. Eighty-eight percent of totalCr and 94% of total Cd were removed by polarity exchange tech-nique with polarity exchange interval of 48 h. The improved rem-ovals were attributed to exchanging polarity which preventedabsorption on Cr6+ in acid soil and precipitates of Cr3+ and Cd2+

in alkaline soil. Additional chemicals and complex equipmentswere not required for the new polarity exchange electrokineticremediation process.

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