RESEARCH ARTICLE

Enhanced electrokinetic remediation of chromium-contaminated soil using approaching anodes

Shucai LI1,2, Tingting LI1, Gang LI1, Fengmei LI1,2, Shuhai GUO (✉)1

1 Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2012

Abstract As a new technology used for the cleaning ofchromium-contaminated soil, worldwide interest in ele-trokinetic (EK) remediation has grown considerably inrecent times. However, owing to the fact that chromiumexists as both cationic and anionic species in the soil, it isnot an efficient method. This paper reports upon a study inwhich a process using approaching anodes (AAs) was usedto enhance the removal efficiency of chromium byeletrokinetics. Two bench-scale experiments to removechromium from contaminated soil were performed, oneusing a fixed anode (FA) and the other using AAs. In theAAs experiment, the anode moved toward the cathode by7 cm every three days. After remediation, soil pH, totalchromium, and fractionation of chromium in the soil weredetermined. The average removal efficiency of totalchromium was 11.32% and 18.96% in the FA and AAsexperiments, respectively. After remediation, acidic soilconditions throughout the soil were generated through theuse of AAs, while 80% of the soil remained neutral oralkalic when using the FA approach. The acidic soilenvironment and high field intensity in the AAs experi-ment might have favored chromium desorption, dissolu-tion and dissociation from the soil, plus the mobility ofchromium in the soil was also enhanced. The resultsdemonstrate that AAs used in the process of EKremediation can enhance the efficiency of chromiumremoval from soil.

Keywords approaching anodes, chromium-contaminatedsoil, electrokinetics, chromium fractionation

1 Introduction

Electrokinetic (EK) remediation is an effective method for

the removal of heavy metals such as Pb, Cu, and Cd fromcontaminated soils [1]. Under a direct current (DC)electrical field, anionic ions migrate toward the anodeand cationic ions migrate toward the cathode [2,3].Although most heavy metals in soil exist as cationicspecies only, chromium exists mainly as both cationic andanionic species, depending on its different oxidation states.Thus, it is far more complex to clean chromium-contaminated soil using the EK method than soilscontaminated with other heavy metals.Hexavalent chromium Cr(VI) exists mainly as soluble

anionic species in a wide pH range in soils, includingchromate (CrO2 –

4 ), hydrochromate (HCrO –4 ) and dichro-

mate (Cr2O2 –7 ). On the contrary, trivalent chromium Cr(III)

exists mainly as Cr H2Oð Þ3þ6 in acidic soils (pH< 4),whereas at pH< 5.5, it exists mainly as Cr OHð Þ H2Oð Þ2þ5and Cr OHð Þ2 H2Oð Þþ4 . At pH 5.5, Cr(III) begins toprecipitate as Cr(OH)3 [4–6].Reddy and Chinthamreddy [7] performed a laboratory

investigation to evaluate the effects of the initial form ofchromium on the EK remedial efficiency for contaminatedclays with a fixed anode (FA). After remediation,accumulation of Cr(III) occurred in the soil near thecathode when using Cr(III) alone, but the accumulation ofCr(III) occurred in the middle part of the soil cell whenusing a combination of Cr(III) and Cr(VI). These resultswere mainly caused by a sharp jump in pH. During the EKprocess, because of the electrolysis of water, H+ ionsgenerated at the anode migrated toward the cathode, andOH– ions generated at the cathode migrated toward theanode. Therefore, an acid front near the anode and analkalic front near the cathode were formed. Where the twofronts met, a neutral reaction occurred and a sharp jumpformed between the anode and cathode. Thus, precipitationof Cr(III) took place and accumulated in the pH jump areawhere pH was above 5.5, resulting in the mobility ofCr(III) in the soil becoming lower. This was an obstacle toenhancing the removal efficiency of chromium from soil

Received August 8, 2010; accepted February 27, 2011

E-mail: shuhaiguo@iae.ac.cn

Front. Environ. Sci. Eng.DOI 10.1007/s11783-012-0437-4

by electrokinetics.To improve the removal efficiency of chromium

(especially Cr(III)) from contaminated soil by EK, it isnecessary to retain soil acidity, which helps to avoid theabove mentioned pH jump. To achieve this, some studieshave used acids to neutralize the OH– ions generated at thecathode [8–13], while in other studies, to avoid the pHjump in the soil, a conductive solution [14] or cationexchange membrane [12] has been used between thecathode and the soil. This retards the migration of OH– ionsfrom the catholyte to the soil, causing a low soil pH to beretained. The results from studies such as these indicatethat the mobility of Cr(III) in soil is influenced mainly bysoil pH, and that low pH can enhance obviously themobility of Cr(III).Based on the results of Reddy and Chinthamreddy [7],

we hypothesized that if the anode is mobile during the EKremediation process, approaching the cathode step by step,the pH jump area should move toward the cathode,resulting in enhanced mobility and removal efficiency ofchromium. To test this, two EK remediation experimentswere designed for the remediation of chromium-contami-nated soil collected from a chromite ore processing residue(COPR) deposition site. One experiment used FAmethodology, while the other used approaching anodes(AAs) for the remediation process. The objectives of thestudy were: 1) to investigate any differences in thedistribution of chromium species after remediation usingthe two approaches; and 2) to verify whether using AAs inthe EK remediation process can enhance the removalefficiency of chromium.

2 Materials and methods

2.1 Soil sample

Chromium-contaminated soil was collected from the topsoil layer (0–20 cm) near a COPR deposition site inShenyang, Liaoning Province, north-eastern China. Afterair drying, the soil sample was ground and passed througha 20-mesh sieve. The characteristics of tested soil prior toEK remediation are shown in Table 1.

2.2 Electrokinetic apparatus

Two bench-scale EK experiments were carried out in arectangular-shaped reactor (Fig. 1). The reactor was madeof Plexiglass with an inner length of 43 cm, a width of 10

cm, and a height of 10 cm. The setup included twoelectrode reservoirs (inner length of 4 cm) and a soil cell(inner length 35 cm). A pair of graphite electrodes wasused, and a DC power supply (Jeledar, Taiyuan, China)was applied across the cathode and anode with constantelectric potential (48V). The soil cell was divided into fiveequal sections, numbered as I (near the anode reservoir),and then II, III, IV and V (toward the cathode reservoir).The soil cell and cathode/anode rooms were separated by amulti-orifice plate.For the experiment using AAs, the soil sample (passed

through a 20-mesh sieve) was placed in the soil cell andsaturated with distilled water. Sodium acetate and aceticacid buffer solution (0.2 mol$L–1, pH 4) was used as thecatholyte, and NaCl solution (0.1 mol$L–1) was used as theanolyte. During the EK process, the electrolytes wererefreshed every 12 h. The approaching anode migratedfrom section I to section Vevery three days by a distance of7 cm. After 15 days, the soil samples in each section werecollected and stored in a refrigerator at 4°C.Conditions were the same for the FA experiment as they

were for the AAs experiment, apart from the obviousdifference that the anode was fixed.

2.3 Chemical analysis

Soil pH was measured at a 1∶2.5 ratio of soil to water by apH meter (PHS-3C, Rex, Shanghai, China). Cr (VI) in thesoil was digested according to the USEPA 3060A method[15], and Cr(VI) in the soil sample digests was determinedwith 1,5-Diphenylcarbazide at a wavelength of 540 nmusing a UV-VIS spectrophotometer (UV2100, UNIC,Shanghai, China). Total chromium in the soil wasmeasured by a Varian AA240 atomic absorbance spectro-meter (AAS) after digesting with aqua regia. AAS was alsoused to determine chromium in electrolytes collected fromthe reservoirs. Chromium species in the soil were extractedusing an improved Bureau Community of Reference three-step sequential extraction procedure (BCR-SEP) [16]. Allreagents used in the experiments were AR or GR grade.Three reference soil samples were used for quality controlof all the analytical results.

3 Results and discussion

3.1 Variations of soil pH during electrokinetic remediation

Figure 2 shows the variation in pH through the soil cell

Table 1 Physicochemical characteristics of tested soil prior to EK remediation

pH OM a) /% CEC b)/(cmol$kg–1)soil texture/(µm, %) chromium/(mg$kg–1)

< 2 2–50 50–830 total Cr Cr(VI)

8.31 1.32 60.94 38.8 50.7 10.5 692.123�11.852 71.187�0.812

Note: a) OM: organic content; b) CEC: cation exchange capacity

2 Front. Environ. Sci. Eng.

during the EK process using both methodologies (FA andAAs). After FA remediation, soil pH values showed agradual increase from section I to section V, with valuescompared to initial conditions having decreased to 3.11 insection I, but having increased to 12.38 in section V. Thisresult was due to the H+ and OH– ions generated by theelectrolysis of water during the EK remediation process[3].In the EK experiment with AAs, after three days the

variation in soil pH through the cell was similar to that inthe FA experiment after 15 days. Compared to initialconditions, soil pH decreased to 4.67 near the anode andincreased to 10.69 near the cathode. However, the variationin pH with AAs (3d) was not as obvious as that with the FA(15d) because of the shorter treatment time. This result is inaccordance with the findings of Shen et al. [17].With the approach of the anode from section I to section

V, the distribution of pH was different from that using theFA. After six days, the soil pH in section II decreased to3.46, with a constant acidic state finally forming through-out the soil after 15 days. Figure 3 shows the variations ofelectric current during the EK remediation process, and itindicates that the electric current in the experiment withAAs was higher than that in the experiment with FA. Thefield intensity was enhanced because of the distancebetween the anode and cathode becoming shorter duringthe EK remediation process using AAs. Thus, more H+

ions generated at the anode resulted in lower pH, andtherefore pH throughout the soil reached below 6 by theend of the AAs experiment.

3.2 The residual total chromium

The average content of total chromium was calculated

Fig. 1 Schematic diagram of the EK apparatus

Fig. 2 Variation of soil pH during EK remediation process Fig. 3 Variations of electric current during the EK remediation process

Shucai LI et al. Enhanced eletrokinetic remediation of chromium-contaminated soil 3

using the test results of the five soil sections. A massbalance analysis of the chromium was performed, and therecovery rates of total chromium for the FA and AAstests were around 101.02% and 99.29%, respectively.Figure 4 displays the total chromium content in the soilcell after EK remediation using both the FA and AAsexperiments. After EK remediation in the FA experiment,the peak of total chromium was found in section II(723.123�12.745 mg$kg–1) with the value exceeding theoriginal chromium content of the soil sample.Such a result can occur where the acid and base fronts

meet during the EK remediation process, causing afocusing effect of the heavy metal in the soil [3]. As lowsoil pH (of around 4) in section I favored Cr(III)dissociation and desorption from the soil, Cr(III) in thesoil in section I migrated into section II under the electricfield, and Cr(III) that might have been held in the soil asprecipitation due to the higher soil pH (of around 7) insection II for Cr(III) would have begun to precipitate asCr(OH)3 when the pH was higher than 5.5 [4–6]. Similarresults were found by Reddy and Chinthamreddy [7].In the experiment using AAs, the total chromium

content in sections II, III and V was significantly lower,but was higher in sections I and IV. Relatively higher totalchromium in section I might have resulted from therelatively short remediation period of only 3 days. Thepeak of total chromium was in section IV (627.723�3.164mg$kg–1). Owing to acidic soil conditions having beenformed near the approaching anode during the EKremediation process, chromium (especially Cr(III)) des-orbed from the soil and migrated toward the cathode underthe electric field. As a result, total chromium content insection IV was higher than in all other sections. Comparedto FA EK remediation, the peak of total chromium in thesoil cell not only moved toward the cathode, but alsodecreased significantly. As a consequence, the removalefficiency of total chromium was enhanced.

3.3 Residual hexavalent chromium

The content of Cr(VI) in the different sections of thesoil cell after EK remediation for 15 days is shown inFig. 5. After remediation, the average residual Cr(VI)content of the soil was 13.363�1.226 mg$kg–1 and6.539�0.534 mg$kg–1 in the FA and AAs experiments,respectively. The average removal efficiency of Cr(VI)with AAs was approximately 90.81%, which was higherthan in the FA experiment (81.23%). The variation in Cr(VI) seen in the different sections for AAs treatment wasrelated to pH. Acidic soil conditions formed during the EKremediation with AAs, which favored the reduction of Cr(VI) by reducing materials (such as Fe2+ or organicmaterials) in the soil. As the reduction of Cr(VI) isaccompanied by the consumption of H+ ions [5,6,18], theefficiency of soils in reducing Cr(VI) increased withdecreasing pH [18].Reddy and Chinthamreddy [5] reported removing

Cr(VI) from soil via EK remediation. They found thatgreater Cr(VI) reduction occurred in the soil near theanode, and that this enhanced rate of reduction was due tothe availability of excess H+ ions to participate in the redoxreactions. The results of Yang et al. [19] indicated thatCr(VI) reduction by tartaric acid is more remarkable at alower pH range (pH 3–4) than a higher pH range (pH 4.5–5.5).

HCrO –4 þ 7Hþ þ 3e –⇌Cr3þ þ 4H2O (1)

HCrO –4 þ 3Fe2þ þ 7Hþ⇌Cr3þ þ 3Fe3þ þ 4H2O (2)

3.4 Chromium fractionation

Chromium species in the soil after EK remediation in bothexperiments is presented in Figs. 6 and 7. As can be seen

Fig. 4 Distribution of total chromium in soil after remediation Fig. 5 Content of Cr(VI) in soil after remediation

4 Front. Environ. Sci. Eng.

from Fig. 6, the content of chromium in the easily-reducible fraction in section II was higher than that in theoriginal soil sample. Chromium coming from section I mayhave precipitated due to the high soil pH in section II andexisted as an easily-reducible fraction [20]. In Fig. 7, it canbe seen that the content of chromium in the oxidisablefraction in sections III and IV decreased obviously afterEK remediation. It is possible that some organic matter inthe soil might have been oxidized near the anode duringthe EK process [21,22] when using AAs, and thus thechromium bound to the organic matter may have convertedto the more mobile fraction. The mass balance of totalamount of chromium in four forms and the remaining totalchromium in each section were calculated. The relativemass balance errors were less than 16%.After remediation in the AAs experiment, the average

content of chromium in the soluble, exchangeable andcarbonate fraction was higher than in the FA experiment;whereas, the average content of chromium in the residuefraction was not only lower in the AAs experiment than theFA experiment, but was also lower than the content prior toremediation. This demonstrates that most of the chromiumin the residue fraction had converted to other fractionsduring the AAs remediation process. This process alsooccurred after EK remediation in the FA experiment, butonly a small amount of chromium in the residue fractionconverted to other fractions. As we know, species ofchromium in soil and sludge can convert during the EKremediation process [17,20,23–25], and metal complexesor precipitates can be partially solubilized in acidic soilconditions near the anode, allowing for migration to occurunder a DC electric field [17,20]. The low soil pH and highfield intensity conditions during the EK remediationprocess in the AAs experiment favored desorption anddissolution of chromium from the soil, therefore theinversion of chromium in the residue fraction to otherfractions was more significant. If a proper treatment

technology is followed in the EK process using AAs,chromium could be removed from soil much moreeffectively, and the content of chromium in this particularcase–in the soil at the COPR deposition site used for thisstudy–could be sufficiently improved to reach thenecessary standards of environmental quality.

4 Conclusions

Based on the results from this study, the followingconclusions can be drawn:1) Acidic soil condition was formed after EK remedia-

tion using AAs. Low pH generated throughout the soil anda high field intensity between electrodes during the EKremediation process with AAs favored chromium deso-rption, dissolution and dissociation from the soil.2) The average removal efficiency of total chromium

and Cr(VI) was 11.32% and 81.23% for EK experimentwith FA, respectively. In the EK experiment with AAs, itwas 18.96% for total chromium and 90.81% for Cr(VI).3) After EK remediation using AAs, chromium in the

soluble, exchangeable and carbonate fraction increased,and the mobility of chromium in the soil was alsoenhanced. Therefore, the removal of chromium wasimproved.

Acknowledgements This work was supported by the National HighTechnology Research and Development Program of China (No.2009AA063101) and the Knowledge Innovation Project Key-DirectionProject Sub-project of Chinese Academy of Sciences (No. 09ZY441YZ).

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