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Journal of Hazardous Materials 239– 240 (2012) 128– 134
Contents lists available at SciVerse ScienceDirect
Journal of Hazardous Materials
j our na l ho me p age: www.elsev ier .com/ locate / jhazmat
nhanced remediation of Cr(VI)-contaminated soil by incorporating aalcined-hydrotalcite-based permeable reactive barrier with electrokinetics
ia Zhanga, Yunfeng Xua, Wentao Lia, Jizhi Zhoua, Jun Zhaoa, Guangren Qiana,∗∗, Zhi Ping Xub,∗
School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200072, PR ChinaAustralian Research Council Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane,LD 4072, Australia
i g h l i g h t s
Combing electrokinetic with permeable reactive barrier to remove Cr(VI) from simulated soil.Cr(VI) removal efficiency has been remarkably enhanced in the combined system.Electrokinetic concentrates chromate under the electric field.Permeable reactive barrier media (calcined hydrotalcite) stores chromate.
r t i c l e i n f o
rticle history:eceived 24 April 2012eceived in revised form 16 August 2012ccepted 18 August 2012vailable online 25 August 2012
a b s t r a c t
This paper describes the enhanced Cr(VI)-contaminated soil remediation via a combination of electroki-netics (EK) with a calcined-hydrotalcite-based permeable reactive barrier (PRB). First, this combinationproved to be feasible, and remarkably facilitated Cr(VI) remediation in a column test. Then, lightly-to-severely (0.16–1.65 mg/g) Cr(VI)-contaminated soil was remediated in a simulated test with the calcinedhydrotalcite as the PRB under an voltage of 10–30 V (i.e. an electric field intensity of 0.7–2.0 V/cm). The
eywords:ermeable reactive barrierlectrokineticsydrotalcitehromate removal
observations demonstrated that both PRB and EK are critical to efficient remediation and the high de-contamination efficiency is supposedly attributed to the synergistic effect, for which EK concentratesanionic chromate to the anode region and PRB media (calcined hydrotalcite) absorbs and immobilizes it.Thus we have shown that the combined PRB–EK system is highly adaptive and effective in remediationof a larger area contaminated with chromate and various anionic pollutants.
oil contamination and remediation
. Introduction
Cr(VI) is a highly toxic material, being both a mutagen and auspected carcinogen, and is quite soluble in water almost overhe entire pH range [1]. It is a soil and groundwater pollutant atites of some industrial processes such as chrome plating, textile,eather tanning as well as mine tailings, and moreover it may bedsorbed in acid soil and reduced to the far less mobile Cr(III) inresence of ferrous iron, sulphide, soil organic matter and elec-ric potential. In recent year, rapid development in these industriesives rise to worldwide Cr(VI) soil contamination in various degree1,2]. Therefore, its effective remediation is urgently desirable. As a
esult, many methods have been proposed for Cr(VI) remediation,ncluding geochemical fixation, soil flushing/chromium extraction,ermeable reactive barriers, electrokinetics and phytoremediation∗ Corresponding author. Tel.: +61 7 33463809; fax: +61 7 33463973.∗∗ Corresponding author. Tel.: +86 21 66137758; fax: +86 21 66137758.
E-mail addresses: [email protected] (G. Qian), [email protected] (Z.P. Xu).
304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.08.039
© 2012 Elsevier B.V. All rights reserved.
[3]. Among them, permeable reactive barriers (PRB) and electroki-netics (EK) seem to be more readily applicable and effective.
Permeable reactive barriers (PRB) is a passive remediationtechnology, and has been proven effective in in situ Cr(VI)-contaminated groundwater treatment [1,4,5]. A PRB consists ofpermanent, semi-permanent or replaceable reactive media placedacross the groundwater flow path. When streaming through themedia, contaminants are converted into less harmful compoundsor being immobilized inside the PRB via some reactions [1,5]. Asa result, reactive media selection is always the focus for a studyof PRB. Reactive media is the main PRB body. Its efficiency andduration of efficiency are always the keys to a PRB design and oper-ation. The general reactive media often used is zero-valence iron,and has been used to treat Cr(VI)-contaminated soil remediation viareductive precipitation [2,6,7]. However, the consequent precipita-tion due to iron corrosion usually decreases the PRB permeability
and prohibits the filler activity, which affects the longevity of thebarrier materials [8,9]. Thus, one choice is to use sorbents as thePRB media to adsorb contaminants. Particularly, a family of anionicclay materials, hydrotalcite-like materials (HTs) have shown a high
us Materials 239– 240 (2012) 128– 134 129
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J. Zhang et al. / Journal of Hazardo
dsorption capability for various anions [10–13], such as chro-ate (Cr(VI)), exhibiting the potentiality as PRB reactive media to
emove this soil contaminant. HTs are exemplified by the natu-ally existing hydrotalcite (Mg6Al2(OH)16CO3·4H2O) [14]. Its mostmportant property is the high anion exchange capacity. Further-
ore, the calcined form can be spontaneously reconstructed intoT-like structure in water, which adsorbs anions into the interlayer
rom aqueous phase, promising a high removal of anionic pollutions15]. As our earlier work has proved that Cr(VI) remediation coulde achieved by calcined hydrotalcite [16]. In addition, the adsorbentan be regenerated conveniently by anion exchange with NaHCO3olution and then calcination [12], i.e. HTs could be reused as theRB reactive media after regeneration.
On the other hand, electrokinetics (EK) is an active remediationethod of soils polluted with charged species, and tested feasible in
r(VI) remediation in both bench and field-scale [17,18]. In EK, elec-rodes are installed vertically or horizontally in a contaminated soilrea. Between the electrodes, a low direct current voltage gradients applied. As a result, the pollutant ions move in soil pores towardhe electrodes by electromigration and electroosmosis [19,20]. EKs especially useful in a low osmosis soil where PRB works limit-dly due to low groundwater hydraulic conductivity. However, itsemoval efficiency is usually hindered by acidification of the con-aminated soil. It is reported that if cooperating with a barrier whichas a strong acid neutralizing capacity, this acidification would beuppressed, thus enhancing EK efficiency [20]. HT adsorbent is anntacid material, and it is our hypothesis that merging a calcined-ydrotalcite-based PRB with EK will give rise to a synergistic effecto increase the remediation efficiency.
Therefore, this article aims to verify our hypothesis that Cr(VI)emediation can be synergistically enhanced through combining aRB with EK. To this end, we set up two specific PRB–EK remediationystems to simulate the Cr(VI) removal from Cr(VI)-contaminatedoils, as detailed in Fig. 1. The former set-up was utilized to provehe feasibility of PRB–EK combination, and the latter to simulatehe Cr(VI) remediation in a much broader area. To the best of ournowledge, although the synergy of PRB and EK has been reported21–24] using zero-valent iron as the PRB reactive media, PRB–EKynergistic mechanism from a broader area has not been reportedr emphasized in public literature. We found that the combinedRB–EK system much more efficiently removes Cr(VI) from theontaminated soil.
. Materials and methods
.1. Materials
Hydrotalcite (Mg/Al = 3, HINWOUN CHEMICAL Co. Ltd.) was uti-ized as the PRB reactive media after calcination under 723 K for
h. The calcined hydrotalcite was named as CHT, with a nominalhemical formula of Mg3.0AlO4.5 (MW = 172).
Loam and kaolin were chosen to mix with Cr(VI) as the contam-nated soils because they stand for low and high osmosis, and their
aximum water content is about 40% and 56% (saturation), respec-ively. In general, loam or kaolin was first dried and milled through00 meshes sieve. Then, the contaminated soils were spiked at 0.16r 1.65 mg(Cr(VI))/g (dried solid soil) by adding Cr(VI) solution tory loam or kaolin at a solid/liquid weight ratio of 3:1. Namely the
nitial water weight content in columns was 25%, which ensuredhat all liquid was captured with the solid soil. Finally, the mix-ures were thoroughly mixed by mechanical rabbling and stored
n refrigerator for further use. The pH of these simulated contami-ated soils was about 6.0. About 130 g of loam or 1000 g of kaolinas used in experiments with or without 10 g of CHT. Both contam-nated soils were easily filled in the designed equipment with slight
Fig. 1. (A) PRB–EK reactive column; (B) expanded PRB–EK reactive column(L = length; W = width; H = height; unit = millimeter).
tapping. Filter paper was used to isolate CHT from the contaminatedsoil and the anode.
2.2. PRB–EK system
In the PRB–EK reactive column (Fig. 1A), the middle column(about 70 cm3) was filled with 130 g of contaminated loam, whichhas a higher osmosis, and about 10 g of CHT was placed in theleft side (anode). When the flowing solution (0.01 M KCl) wasintroduced from the right inlet via a constant-flow pump at a flowrate of 0, 0.1, 0.2 or 0.3 ml/min (0.00, 1.85 × 10−4, 3.70 × 10−4, and5.55 × 10−4 cm/s, since general loam owns a hydraulic conductivityof about 1.50 × 10−3 cm/s), a 30 V direct current (DC) was suppliedby a direct-current (DC) source (Model TPR-3C) to create an elec-tric field intensity of about 2 V/cm. Three additional tests werealso run under 0.1, 0.2 and 0.3 ml/min but without voltage sup-ply. After running for about 3, 6, 9, 12, 18 and 24 h, the experimentwas stopped, and all contaminated soil was collected. The Cr(VI)amount left in the collected soil was then analyzed and thus theremoval percentage was calculated.
2.3. Simulation of PRB–EK remediation
As can be referred to Fig. 1B, the middle cubic was radiallyexpanded (i.e. expanded column) to simulate the real situationwhere soil was contaminated in a large area.
Two experiments, e.g. with and without CHT filler, were con-ducted to examine the synergistic effect of PRB–EK combination(Fig. 2). The cubic inside was filled with CHT, and nine samplingpoints (A–I) were selected on the surface displayed by the diagonal
130 J. Zhang et al. / Journal of Hazardous Materials 239– 240 (2012) 128– 134
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20
40
60
80
100
Rem
ov
al
(%)
Time (h)
0 ml/min / 30 V
0.1 ml/min / 0 V
0.1 ml/min / 30 V
0.2 ml/min / 30 V
0.3 ml/min / 30 V
0.2 ml/min / 0 V
0.3 ml/min / 0 V
Fig. 3. Cr(VI) removal profile in the PRB–EK reactive column under different running
ig. 2. PRB–EK system to treat Cr(VI) in an expanded area. A–I stand for samplingoints. (Note that A–I were on the same vertical plane.)
tripes between two graphite electrodes. Both cases were appliedith 30 V and about 10 g of CHT to treat about 1000 g of lightlyr(VI)-polluted soil (0.16 mg/g). In order to avoid destruction of thelling structure in the system, sampling was taken very carefully.
n general, 100 mg of wet sample was taken from each point con-inuously, and then Cr(VI) was extracted from the collected soil forhe amount determination.
To investigate the influence of electrokinetic voltage on reme-iation, a voltage of 10, 20 or 30 V was applied in the remediationf a severely Cr(VI)-polluted soil (1.65 mg/g). Different from therevious vertical plane sampling points, the corresponding samp-
ing points were reselected in the middle horizontal plane in thisxperiment, as shown in Supplementary Data Fig. S1. Similarly,r(VI)-contaminated wet soil was taken and Cr(VI) was extracted
or analysis. All column test parameters were summarized inupplementary Data Table S1.
.4. Cr(VI) concentration analysis
The wet sample taken from the sampling points (100 mg) or thehole column was dried at 60 ◦C overnight, and the dried sampleas weighed. Cr(VI) in the dried sample (∼50 mg) was extracted by
eacting with 10 ml of 5% HNO3 under ultrasonication for 1 h andhe Cr concentration in the extracted solution was determined bynductively coupled plasma - atomic emission spectroscopy (ICP-ES) (Model Prodidy, Leeman).
. Results and discussion
.1. Feasibility of PRB–EK combination
Fig. 3 presents Cr(VI) removal (%) in the combined PRB and EKystem under different conditions. In this test, Cr(VI) removal per-entage was calculated by comparing the Cr(VI) amount left in theoil after treatment for some time with the initial Cr(VI) amountn soil. If no DC voltage was applied, the higher flow rate resultedn a quicker removal of Cr(VI) from the column, as expected. Forxample, when the flow rate was 0.1 ml/min, it took 20 h to remove0% Cr(VI). The same Cr(VI) removal percentage took only 8 h at theow rate of 0.3 ml/min. When only DC voltage was applied withoutny flowing, the Cr(VI) removal seemed a bit fast, with 80% Cr(VI)emoved by 4 h. The removal efficiency with the voltage applied and
HT filled in the left side of the column seems much higher than theeports elsewhere [25,26]. Peng and Tian removed 34% Cr(VI) fromlectroplating sludge under an electric field intensity of 1.5 V/cmor 5 days [25], and Hanay et al. removed 34% Cr(VI) from sewageconditions. ‘0.1 ml/min/0 V’ stands for running at a flow rate of 0.1 ml/min, under avoltage of 0 V and with 10 g of the CHT barrier adjacent to the anode.
sludge under 2.0–3.3 V/cm for 8 days [26]. Their water contentswere both about 70%, bigger than this work (33.3%). In our case,we removed 80% Cr(VI) under 2.0 V/cm within 4 h in the PRB–EKsystem.
Increased rates of Cr(VI) removal were observed in the combinedPRB–EK system. As shown in Fig. 3, it took only 3 h to remove >99%Cr(VI) from the soil under 30 V at the flow rate of 0.1–0.3 ml/min.Note that the effect of the flow rate was constrained within 3 h,with a higher flow rate resulting in a bit higher removal percentageat the same time point. It was obvious that combination of PRBand EK significantly speeded up the Cr(VI) removal. It seems thatCrO4
2− anion adsorbed in the soil is exchanged with aqueous anions(such as HCO3
−, Cl− and H2PO4−/HPO4
2−) in the flowing water andthus slowly removed from the soil. Under the electric field, CrO4
2−
anion also migrates to the anode, which does not only superposethe movement of CrO4
2− anion toward the anode, but also helpthe anion diffuse to facilitate the anion exchange process as well.Therefore, combination of PRB and EK synergistically enhances theeffectiveness of Cr(VI) removal.
It is known that electrokinetics (EK) actively moves CrO42− ions
to the anode through the aqueous channel in the soil matrix. Rela-tively, PRB is a passive process through diffusion and absorption ofCrO4
2− onto the PRB media. So when PRB is placed at the anodeproximity, CrO4
2− ions that are concentrated toward the anoderegion via EK can be quickly absorbed by PRB media placed nearby,which reduces the Cr(VI) concentration gradient and acceleratesthe Cr(VI) removal.
To further test the feasibility of the combined PRB and EK sys-tem in removing Cr(VI) in the practical contaminated soil wherethe aqueous flow can almost be ignored, we expanded the soil col-umn in the pathway while kept the other parts unchanged (referto Fig. 1B), and investigated the Cr(VI) removal profile at a fewparticular points in the contaminated area (Fig. 1B).
3.2. Applicability of PRB–EK combination
After feasibility has been confirmed in the PRB–EK reactive col-umn tests, two experiments (with and without CHT filler) wereconducted under 30 V without any nominal aqueous flow for 70 h
to test applicability of PRB–EK combination in a much more broadcontaminated area (Fig. 1B and Supplementary Data Fig. S1). TheCr(VI) removal profiles of sampling points A–I during the remedi-ation time under these conditions are presented in Fig. 4 where
J. Zhang et al. / Journal of Hazardous Materials 239– 240 (2012) 128– 134 131
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When CHT was placed in the anode region, the Cr(VI) removal
ig. 4. Cr(VI) concentration profile at sampling points A–I in the expanded columnlled without (a) or with (b) CHT after 1, 26, 34, 45 and 70 h.
i/C0 stands for detected residual Cr concentration/initial Cr con-entration (0.16 mg/g). These profiles could approximately exhibithe Cr(VI) flux tendency of the whole process. When there was noHT filled in the anode region, Cr(VI) continuously moved fromoints C, F and I to points A, D, and G. We observed that the Cr(VI)oncentration at points C, F and I was reduced quickly from 1.00i.e. 100% Cr remaining in the soil) to 0.02–0.06 (2% and 6%, respec-ively) by 26 h. The Cr(VI) concentration at points B, E and H waseduced, but not as quickly as that at points C, F and I (Fig. 5B(a) and(a)). To the contrary, the Cr(VI) concentration at points A, D and Generally increased, and reached a much higher value by 45–70 h.or example, the concentration was increased from 100% to 351%t point D by 45 h. The concentration increase became clearer after
0 h (Fig. 5A(a)). All these observations indicate that CrO42− ionas accumulated in the anode region which was also evidenced by
ts characteristic yellow color, especially around point D. Therefore,
Fig. 5. Cr(VI) concentration changing with the operation time at (A) A, D and G; (B)B, E and H; (C) C, F and I filled without (a) or with (b) CHT.
the accumulated CrO42− ion in the anode region probably diffuses
back to points A, D and G, resulting in a higher Cr(VI) concentrationat these points. In particular, as G was located vertically below A,there was more water in the pores of soil around point G due to thegravity, resulting in a higher Cr(VI) concentration at G than A.
profiles were very much different. In the whole test period,the Cr(VI) concentration at all points was continuously reduced(Fig. 5A(b), B(b) and C(b)). More remarkably, the concentration at
132 J. Zhang et al. / Journal of Hazardous Materials 239– 240 (2012) 128– 134
0.0
0.2
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0.6
0.8
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0.4
0.6
0.8
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20V
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II
30V
I
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0 50 10 0 15 0 200 250
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Time (h)
Fe
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Table 1Cr(VI) breakthrough concentrations extracted from anode cell.
Cases Flowing (ml/min) Initial Cr(VI) (mg/g) Cr(VI) (mg/L)
0.1 ml/min 30 Va 0.1 0.16 1.030.2 ml/min 30 Va 0.2 0.16 1.220.3 ml/min 30 Va 0.3 0.16 1.8630 V with CHTb 0.0 0.16 5.7330 V with CHTb 0.0 1.65 15.2320 V with CHTb 0.0 1.65 29.51
b
migration of Cr(VI) anion, which helps the anion diffusion from theexchange site and thus facilitates the next exchange. To the con-trary, the compound adsorption rate (˛) seems to decrease undera high voltage, but not so significantly. This could be understood
Table 2Elovich equation fitting parameters.
1/ ̌ ̨ (1/min) R2
I 10 V 13.4 10.8 0.943I 20 V 20.9 10.1 0.946I 30 V 24.5 9.72 0.934II 10 V 14.7 8.43 0.952II 20 V 23.3 6.41 0.966
ig. 6. Removal percentage at points I, II and III in the expanded column underlectrokinetic voltage of 10, 20 and 30 V.
oints D and G was reduced from 100% to around 20% by 45 h, andhat at point A to 19% by 70 h (Fig. 5A(b)). This demonstrates thatearly all CrO4
2− ions in the soil matrix were moved to the anode,nd the soil was de-contaminated. The efficient de-contaminations presumably assisted by the presence of CHT in the anode region.
hen CrO42− ion is moved to this region, it can be quickly absorbed
y CHT either through surface adsorption or intercalation [16], sohat there is not much free CrO4
2− ion in aqueous phase that isvailable for back diffusion.
Thus the combined PRB and EK system can be used to effectivelye-contaminate the lightly Cr-contaminated soil (0.1–0.2 mg/g)27]. To extend the applicability of this combined system to treateverely Cr-contaminated soil (>0.4 mg/g) [27], we thus used a soilontaining 1.65 mg/g Cr(VI) and investigated the removal profilest a voltage of 10, 20, or 30 V.
.3. Influence of electrokinetic voltage
In these experiments, three sampling points (I, II and III) weree-selected in the same horizon (Supplementary Data Fig. S1). Ashown in Fig. 6, the Cr(VI) removal profiles seem quite similart three sampling points under each voltage. Application of 10 Vould only remove 70–75% Cr(VI), while application of 20 or 30 Vemoved nearly all Cr(VI) from the severely contaminated soil afterreatment for 250 h. We note that under a voltage of 30 V, it took00–150 h to reach 90% Cr(VI) removal at these three points. This
s a bit longer than that in the case of lightly contaminated soilhere it took about 40 h for the Cr(VI) removal at point E to reach
90% (Fig. 5C). When the initial Cr(VI) concentration is increased,oving more CrO4
2− takes a longer time to obtain an removal per-entage similar to those in Figs. 4 and 5 under the same electriceld intensity.
In these tests, 10 g of CHT was used as the reactive media,nd thus could adsorb about 29.1 mmol CrO4
2− ions in theory=10,000/172/2, CHT MW per one positive charge is 172) if CrO4
2−
s the only intercalated anion in the reconstructed CHT. However, in
10 V with CHT 0.0 1.65 0.20
a Loam was used for its high osmosis.b Kaolin was used for its low osmosis.
1000 g of the contaminated soil with 0.16 or 1.65 mg/g Cr(VI), thetotal amount of CrO4
2− is 3.2 or 31.7 mmol. Therefore, CHT usedin the PRB is enough to capture all Cr(VI) in the lightly contami-nated soil, but not enough to capture all Cr(VI) (at most 92%) in theseverely contaminated soil even though competitive adsorption ofCO3
2−/Cl−/OH− is ignored. This could be the reason why under avoltage of 10 V the Cr(VI) removal was only 70–75% (Fig. 6). Onthe other hand, it seems to be contradicted to achieve >90% Cr(VI)removal under 20 or 30 V. We explained that Cr(VI) anion breaksthrough the PRB under 20 or 30 V (but not under 10 V). When thesolution that passed through the PRB media was collected after thetest, the measured Cr(VI) concentration was only 0.20 mg/L underthe voltage of 10 V, while it increased to 15–30 mg/L under 20 or30 V (Table 1 and Supplementary Data Table S1). The Cr(VI) break-through has been also observed in the cases of treating the lightlycontaminated soil (Table 1 and Supplementary Data Table S1).
Elovich equation (1) is widely used in soil adsorption anddesorption kinetics [28]:
q =(
1ˇ
)× ln(˛ˇ) +
(1ˇ
)× ln(t) (1)
where q is the removal percentage, ̨ and ̌ are constant. 1/ˇ(dimensionless quantity) and ̨ (1/min) stand for the compounddesorption and adsorption rate, respectively. In this research, weused this equation to simulate the complicated Cr(VI) removalprocess at these three points, and listed fitting constants inTable 2. Considering the disturbance of many samplings at thesepoints on the column filling structure, the fitting coefficient(R2 = 0.934–0.971) is significantly high enough to demonstrate thatthe fitting model is suitable to simulate the PRB–EK process. Asshown in Table 2, a higher voltage always led to a higher desorp-tion rate. For example at point II, voltage increasing from 10 to 30 Venhanced the desorption rate (1/ˇ) from 14.7 to 25.6. It is mostlikely that desorption of Cr(VI) anion from the soil occurs throughthe anion exchange with aqueous HCO3
−, Cl− and H2PO4−/HPO4
2−.Our data (1/ˇ) indicate that desorption is enhanced under theelectric field. The enhancement could be attributed to the electro-
II 30 V 25.6 5.81 0.960III 10 V 13.3 8.08 0.945III 20 V 22.4 6.86 0.971III 30 V 25.1 6.43 0.952
us Materials 239– 240 (2012) 128– 134 133
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J. Zhang et al. / Journal of Hazardo
ecause Cr(VI) migrates more quickly under a high electric fieldnd thus has less chance to be adsorbed back by the soil throughhe reverse anion exchange.
.4. Synergistic effect of PRB and EK
Under the electrical field, CrO42− moves from the contaminated
oil to the anode at the force of electrokinetic flux due to electro-igration and electroosmosis. This is the reason why the Cr(VI)
oncentration in the cathode region decreases to a very small valuerom 1 to 70 h (points C, F and I in Fig. 4). In the middle part, orig-nal CrO4
2− moves out from region BEH toward the anode (regionDG), while at the same time, some CrO4
2− moves from the cath-de region (CFI) to the middle part (region BEH). Therefore, ther(VI) concentration in the middle region is kept nearly unchanged
n the beginning period (1–26 h), and then decreases afterwards26–70 h) when no CrO4
2− moves from region CFI.However, at points A, D and G, the Cr concentration slightly
ecreased just in the beginning period (20–30 h). When there iso CHT filled, CrO4
2− accumulated in the anode region, whichhen increased the Cr(VI) concentration at these three points, aslearly presented in Fig. 5A(a). After 45–70 h, the concentrations athree points reached 0.56 (D), 0.36 (G) and 0.14 (A) mg/g and wereept unchanged. This is because the CrO4
2− movement toward thenode region is hindered by the diffusion from the anode region tooints D, G and A due to the high Cr concentration gradient whenhere is no CHT filled (Supplementary Data Table S1). When CHT islled in the anode region, CHT acts as a reservoir and stores the con-entrated CrO4
2−. The mechanism for CHT to store CrO42− has been
iscussed elsewhere [16], mainly through following reactions:
Mg3.0AlO4.5 + x/nAn− + 4.5H2O
→ Mg3.0Al(OH)8.0(An−)x/n(OH)1−x + xOH− (2)
Mg3.0Al(OH)8.0(An−)x/n(OH)1−x + y/2CrO42−
→ Mg3.0Al(OH)8.0(CrO42−)y/2(An−)(x−y)/n(OH)1−x + y/nAn−
(3)
here An− is any anions available for intercalation, includingCO3
−, OH− and Cl−. Reaction (2) takes place when CHT is placed inhe anode region and contacts with the environment water, whichs so-called reconstruction of the layered structure. This reconstruc-ion completes within 4–6 h [29]. When CrO4
2− continues to moveoward CHT, it is supposedly immobilized via Reaction (3). Reac-ion (3) is an anion exchange, involving the surface adsorption andhe interlayer intercalation. After CrO4
2− anion exchange, HCO3−,
H− and Cl− would be exchanged out to the bulk solution. As cane seen in Fig. 7, the layered structure has been reconstructed afterhe tests. The interlayer d-spacing is about 0.76–0.77 nm, very closeo that of Mg3Al–Cl–HT (0.78 nm) [30], Mg3Al–CO3–HT (0.76 nm)31] and Mg3Al–OH–HT (0.78 nm) [32]. This reveals that the mainnions in the interlayer are not CrO4
2− as Mg3Al–CrO4–LDH shouldave a d-spacing of 0.95–1.0 nm [16]. Approximately, the contam-
nated soil contains 3.2 mmol CrO42− (6.4 mmol electron) that is
nly about 11% of the anion capacity of 10 g CHT (58.2 mmol). Onhe other hand, when the contaminated soil contains 10 timesr(VI) (1.65 mg/g), the total Cr(VI) is 31.7 mmol, which, togetherith Cl−/CO3
2− competitive intercalation, explains why there isnly 60% Cr(VI) removed under the voltage of 10 V (Fig. 6). Underhe voltage of 20 or 30 V, some CrO4
2− ions are forced to break
hrough the PRB media to the solution around the anode (Table 1).As Table 1 shows, Cr(VI) breakthrough is an ordinaryhenomenon. Especially, when the total Cr(VI) (initialr(VI) = 1.65 mg/g) exceeds theoretical maximum amount of
Fig. 7. XRD patterns of original HT, CHT and CHTs collected after the column tests.
10 g CHT. After CHT adsorption reached this maximum amount,Cr(VI) began to break through. However, when the exact time ofbreakthrough was not determined, as the solution in the anodecell was only collected after the test was finished.
It is worth mentioning that EK would induce reduction of Cr(VI)to Cr(III) that is readily immobilized via precipitation in the neutralmedium, which would hinder the complete Cr removal from thecontaminated soil. Our removal kinetic data at point F (Fig. 5C(b))reveals that Ci/C0 reduced by approx. 94% in the period of 1–10 h(from 1.04 to 0.06), while it only reduced by approx. 28% in theperiod from 58 to 70 h (from 0.040 to 0.029). This means that itis more difficult to remove Cr in the later period. The most likelyreason is that some mobile Cr(VI) is reduced to immobilized Cr(III)species. Similarly, at point E (Fig. 5B(b)) the removal percentage(62% from 15 to 23 h) decreased to 18% in the period of 58–72 h.The Cr(VI) reduction to Cr(III) has also been reported by Cappai et al.[20], Weng et al. [33] and Reddy and Chinthamreddy [34] using EKto remediate the Cr(VI)-contaminated soil. However, the reductionin our system seems to be very much limited. For example, the Ci/Co
value at points D, E, and F was only 0.063, 0.050 and 0.029 (Fig. 5),respectively, after 70 h of PRB–EK treatment. This means that thereduction is at most 6% in the current case. This estimation is closeto the finding (0–30%) by Cappai et al. [20], who used transformedred mud as a PRB reactive media. However, this estimation is muchsmaller than a maximum reduction of 100% by Weng et al. [33] andReddy and Chinthamreddy [34], who used zero-valent iron as thereactive media or EDTA as the electrolyte in a large portion of EKcell where Cr reduction was promoted by zero-valent iron apartfrom EK.
More interestingly, the presence of the CHT filler has a strongacid neutralizing capacity. This property could suppress acidifi-cation of the contaminated soil that was widely observed duringEK treatment and capable of hindering EK removal efficiency.Therefore, the neutralizing capacity would promote the whole EKefficiency [35].
Predictably, the combined PRB–EK remediation method couldalso used to treat soils contaminated with various negativelycharged species, such as AsO4
3− and H2PO4−/HPO4
2−.
4. Conclusions
In conclusion, Cr(III)-contaminated soils in the range of0.16–1.65 mg(Cr)/g(soil) were soundly remediated under a work-ing voltage of 20–30 V. The combination of PRB and EK in the
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34 J. Zhang et al. / Journal of Hazard
imulated set-up significantly enhanced the efficiency of Cr(III)nion removal, demonstrating the strong synergy between PRB andK. At the end of the remediation, PRB reactive media and EK facilityould be recycled. Therefore, the combined PRB–EK system shows aigh promising in effective remediation of soils contaminated withhromate as well as many other ionic pollutants, such as phos-hate and arsenate, particularly when the water flow cannot bestablished through the contaminated soil.
cknowledgements
This project is financially supported by National Nature Scienceoundation of China, No. 20677037, No. 20877053, National Majorcience and Technology Program for Water Pollution Control andreatment 2009ZX07106-01, 2008ZX0742-002 and Shanghai Lead-ng Academic Discipline Project No. S30109. Dr. Xu acknowledgeshe financial support from the Australian Research Council for theRC Center of Excellence for Functional Nanomaterials and ARCiscovery project (DP0870769).
ppendix A. Supplementary data
Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.jhazmat.2012.08.039.
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