evaluation of the potential redistribution of chromium fractionation in contaminated soil by citric...

$: Evaluation of the potential redistribution of chromium fractionation in contaminated soil by citric acid/sodium citrate washing$
Arabian Journal of Chemistry (2012) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Evaluation of the potential redistribution of chromium

fractionation in contaminated soil by citric acid/sodium

citrate washing

Gang Lia, Xuelian Yang

b, Lili Liang

a, Shuhai Guo

a,*

a Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, PR Chinab Shenyang University, Shenyang 110044, PR China

Received 5 April 2012; accepted 4 October 2012

*

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KEYWORDS

Chromium fractionation;

Soil remediation;

Citric acid/sodium citrate

washing;

BCR;

Redistribution

Corresponding author. Tel.

-mail address: Shuhaiguo@

er review under responsibilit

Production an

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Abstract A new four-step approach, which removed fractions one by one based on the sequential

extraction procedure proposed by the Community Bureau of Reference (BCR), was designed to

study the variation of chromium (Cr) chemical fractionation after citric acid/sodium citrate (CA/

SC) washing. Particular attention was paid to the potential redistribution of the acid soluble Cr

fraction. The results indicated that CA/SC washing decreased the content of the reducible (R2)

and oxidizable (R3) fractions during the four steps. During Step 1, the ratio of the acid extractable

(R1) fraction with total Cr increased significantly. Through designing Steps 2, 3 and 4, it was proved

that R1 was released from the R2, R3 and R4 (residual) fractions during washing. This indicated

that CA/SC washing may be favorable for converting Cr from being difficult to extract into being

easier extraction chemical fractionation. The results of this work revealed the high potential risk for

the application of CA/SC washing in soil polluted with Cr because of the redistribution of its frac-

tionation.ª 2012 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Chromium (Cr) is widely used in various important industrialapplications, such as leather tanning, dyeing and chromium

plating (Reddy et al., 1997; Rai et al., 2005), and as a result

8397 0449.

(S. Guo).

Saud University.

g by Elsevier

. Production and hosting by Elsev

0.016

G. et al., Evaluation of the poteshing. Arabian Journal of Chem

has been released into the agricultural environment. In soil,chromium can occur as Cr(III) and/or Cr(VI), mainly depend-ing on pH and redox conditions. The two forms behave quite

differently, with Cr(III) being much less soluble and thereforeless mobile than Cr(VI) (Fibbi et al., 2012). The hexavalentform of Cr usually exists as soluble anionic species over a widepH range, such as chromate (CrO2�

4 ), hydrochromate

(HCrO�4 ), and dichromate (Cr2O2�7 ), and possesses significantly

higher levels of toxicity than other valence states (Sharma andForester, 1995; Bartlett, 1991). Compared to other heavy

metals, the removal of chromium is more complex due tochanges of valence states and fractionations. Generally, owingto the high toxicity of Cr(VI), in soil, particular attention has

been paid to changes in the soluble and exchangeable Cr

ier B.V. All rights reserved.

ntial redistribution of chromium fractionation in contaminatedistry (2012), http://dx.doi.org/10.1016/j.arabjc.2012.10.016

mailto:[email protected]

http://dx.doi.org/10.1016/j.arabjc.2012.10.016


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2 G. Li et al.

fraction due to its bioavailability (Bhattacharyya et al., 2005;Barrera-Diaz et al., 2012).

Washing has been considered as a successful method for the

removal of heavy metals from contaminated soil, a processthat usually employs different chelating agents (Schramelet al., 2000; Reddy and Chinthamreddy, 2000). Among the

agents, citric acid (CA) has been a major focus for soil remedi-ation research because of its ability and efficiency in mobilizingmetal cations, coupled with only a minor impact on the phys-

ical and chemical properties of the soil (Romkens et al., 2002).Compared with the effect of inorganic acid on slags compoundwaste (Moutsatsou et al., 2006.), the effect of organic acid onpolluted soil appears to be better. The results of Wen et al.

(2009) indicated that CA can be rapidly degraded, with 20%degradation occurring between 1 and 4 d, depending on the le-vel of soil contamination, and with 70% degradation occurring

within 20 d. In contrast, the authors found ethylenediaminetet-raacetic acid (EDTA) to be more persistent in the soils; only14% of the EDTA was degraded after 20 d. Several studies

have been published in which CA is reported as advantageousin the use of chelate-assisted phytoextraction, because it is bio-degradable and rapidly degrades to carbon dioxide and water

(Evangelou et al., 2007; Yan et al., 1996; Huang et al., 1998b).Jean et al. (2007) carried out batch experiments to investigatethe mobilization of Cr in soil, and indicated that CA was themost effective for Cr mobilization. In addition, Peters (1999)

also showed that CA was effective, while other chelatingagents like gluconate, oxalate, and ammonium acetate wereineffective at removing heavy metals from Aberdeen Proving

Ground soils.The most serious issue associated with the use of CA as a

washing agent is that the process may change the distribution

of metals’ chemical fractionation. This can lead to the releaseof a soluble and exchangeable fraction, which poses a greatenvironmental risk because of its known characteristic of bio-

availability (Kotas and Stasicka, 2000). In general, sequentialextraction schemes have been used to determine the fraction-ation of Cr in soils. The BCR sequential extraction procedure,which has been developed under the auspices of the European

Community Bureau of Reference (Rauret et al., 1999; Davidsonet al., 1998), involves the separation of elements into four mainfractions: ‘‘acid extractable’’, ‘‘reducible’’, ‘‘oxidizable’’ and

‘‘residual’’. In fact, the approach has been certified as a usefultool for monitoring the relative changes in Cr element parti-tioning (Reddy et al., 2001).

A study of the mobilization of Cr from a contaminated soilwas conducted by Jean et al. (2007) to examine the effect ofCA on the distribution of metals, and concluded that onlyslight modification could be found. However, the authors also

reported that Cr was extracted from the fractions R3 and R4

Table 1 Main characteristics of BCR sequential extraction steps.

Extraction

step

Reagents Opera

R1 CH3COOH (0.11 mol L�1, 16 h) Excha

adsor

R2 NH2OHÆHCl (0.5 mol L�1), pH 1.5, 16 h Reduc

R3 H2O2 (8.8 mol L�1, 3 h), CH3COONH4

(1 mol L�1), pH 2, 16 h

Oxidi

R4 Aqua regia Resid

Please cite this article in press as: Li, G. et al., Evaluation of the potesoil by citric acid/sodium citrate washing. Arabian Journal of Chem

and was redistributed in both fractions R1 and R2. This wasattributed to the Cr bound to iron-oxides possibly not beingextracted effectively from the R2 fraction, and thus was ex-

tracted with the last fraction (R4), but also fraction R3. Thispart of Cr was therefore redistributed in fractions R1 andR2 during the mobilization with chelants. However, no mechanis-

tic details were uncovered to help understand the observed effects.In addition, few studies have assessed the effects of CA washingon the fractionation of Cr. In this context, our study used a four-

step washing experiment to investigate the effects of CA onchanges in Cr chemical fractionation in actual contaminated soil,as well as to evaluate the potential risk of acid extractable Cr.

2. Materials and methods

2.1. Soil sampling and characterization

Cr-contaminated soil samples were collected from the top soillayer (5–20 cm) near a deposition site of chromite ore process-

ing residue (COPR) in Shenyang, Liaoning Province, north-eastern China (42�04’04’’ N, 123�30’15’’ E), where a fertilizerplant was previously located. In order to reduce the influence

of remnant Cr slag, before sampling, a superficial layer of soil(0–5 cm) was discarded, and then the 5–20 cm layer was sam-pled manually. This thickness was selected due to the site soil

contamination rate. After air drying, the soil sample wasground and passed through a 20-mesh sieve to remove stonesand large particles, before being mixed to ensure uniformity

in preparation for determining Cr concentrations. The soilsample was stored in plastic bags at room temperature for sub-sequent experiments. The content of sand, silt, and clay was18.8%, 46.7%, and 34.5%, respectively. Soil properties were:

pH, 8.5; organic content, 1.32%; cation exchange capacity,60.94 cmol/kg (determined by standard methods; Lu, 2000).The total Cr content in the soil was 768.53 ± 7.37 mg kg�1,

and the Cr(VI) content was 74.61 ± 0.35 mg kg�1 (See Table 1).

2.2. Experimental procedure

Citric acid/sodium citrate (CA/SC) washing of Cr in the soilwas carried out in batch experiments. In order to investigatethe redistribution of Cr fractionation, a four-step experimentwas designed (Fig. 1). A 100.00 g homogeneous soil sample

was used in the experimental procedure.Step 1:First, a 10.00 g soil sample was used to analyze the content

of four fractions (R1, R2, R3 and R4). Then, another 10.00 gsoil sample was washed using CA/SC solution. The main rea-son for this was to reduce test error during the preliminary

tionally defined fractions

ngeable and weak acid-soluble species: soil solution, non-specifically

bed species, carbonates

ible: retained in iron/manganese oxyhydroxides

zable: retained in organic matter and sulfides

ual: remaining non-silicate bound metals



NH2OH.HCl

Extract R1 CH3COOH

H2O2

CH3COONH4Extract R3

CA/SC washingStep 1:

Step 2:

Step 3:

Step 4:

R1S1 R2S1 R3S1 R4S1R1 R2 R3 R4

R1S2 R2S2 R3S2 R4S2

R1S3 R2S3 R3S3 R4S4

R1S4 R2S4 R3S4 R4S3

R2 R3 R4

R3 R4

Extract R2

CA/SC washing

CA/SC washing

CA/SC washingR4

Figure 1 Schematic representation of the washing and extraction

of fractions one-by-one. Black panes indicate the extracted

fractions.

Evaluation of the potential redistribution of chromium fractionation in contaminated soil 3

experiment. The residual 80.00 g sample was prepared forStep 2.

The washing tests were conducted in 100 mL polyethylenetubes. The tubes containing the 10.00 g sample and a measuredvolume of 0.2 mol L�1 CA/SC (20 mL) were agitated using a

Vortex shaker (QILINBEIER, QL-861, Jiangsu). The tubeswere then placed in a shaking incubator (ZDP-250, Shanghai)at a speed of 150 rpm at room temperature (25 ± 2 �C). Eachtube was shaken first for 5 h, then 10 h, then 15 and 20 h. Thesuspensions were centrifuged at 4500 rpm for 20 min and thesupernatants were then filtered through a 0.45 lm membranefor Cr analysis. The residual solid was extracted and the fracti-

onations of Cr were analyzed. The four fractions were namedas R1s1, R2s1, R3s1 and R4s1.

Step 2:

Following Step 1, R1 was extracted from the residual80.00 g soil sample. Then, 10.00 g of the treated soil samplewas used to analyze the content of the four fractions (R1,

R2, R3 and R4). Another 10.00 g soil sample was washedusing CA/SC solution, and the residual 60.00 g was preparedfor Step 3. The process of CA/SC washing in Step 2 was the

same as in Step 1. Then, the residual solid after washing wasanalyzed and the fractions were named as R1s2, R2s2, R3s2and R4s2.

Step 3:

Following Step 2, R2 was extracted from the residual60.00 g soil sample. A 10.00 g soil sample from this 60.00 gwas then used to analyze the content of the four fractions

(R1, R2, R3 and R4). Another 10.00 g soil sample was thenwashed using CA/SC solution. The residual 40.00 g was pre-pared for Step 4. The process of CA/SC washing in Step 3

was the same as in Step 1. Then, the residual solid after wash-ing was analyzed and the fractions were named as R1s3, R2s3,R3s3 and R4s3.

Step 4:

Following Step 3, R3 was extracted from the residual40.00 g soil sample. A 10.00 g soil sample from this 40.00 gwas then used to analyze the content of the four fractions

(R1, R2, R3 and R4). And another 10.00 g soil sample wasused to be washed by CA/SC solution. The process of CA/SC washing in Step 4 was the same as in Step 1. Then, the

residual solid after washing was analyzed and the fractionswere named as R1s4, R2s4, R3s4 and R4s4.


2.3. Analysis

The chemical fractionation of Cr in the soil was quantified bythe BCR sequential extraction procedure (Pazos-Capeanset al., 2005). Furthermore, 1.00 g soil samples were used. Cr

was analyzed after sample centrifugation and filtration(0.45 lm cellulose nitrate filter-Sartorius). Depending on theconcentration, a flame atomic absorption spectrometer (Var-ian SpectrAA 220, USA) or graphite furnace atomic absorp-

tion spectrometer (Varian SpectrAA 800, USA) was used.Soil pH was monitored using a pH meter (PHS-3B, China).All reagents used in the experiments were of analytical grade

and were purchased from Shenyang Chemistry Reagent Cor-poration, China. All plastic and glassware were soaked in a5% HNO3 solution overnight and rinsed with distilled water

before use. Three soil samples were used for quality controlof all the analytical results.

3. Results and discussion

For tests conducted during Step 1, the total amounts of Crrecovered during the BCR procedure for the initial soil were

98.43, 221.00, 153.20, and 231.19 mg kg�1, respectively. Thedistribution of Cr chemical fractionation after CA/SC(0.2 mol/L) washing is also shown in Fig. 2. As indicated,the whole washing procedure was considered as two phases

by taking 5 h as boundaries, and it is clear that there was a dra-matic difference between them. The removal efficiency of R2,R3 and R4 was extremely quick during the beginning of the

washing process, but after 5 h washing, they showed slightchanges. However, it is important to note that, for the washedsoil, the percentage of R1 increased from 12.83% to 28.54%.

After 10 h, the ratio of R1 (R1s1) with Cr(total) was the biggestpercentage at 30.88%. Moreover, it was 2.41 times that for thecontrolled sample. In addition, the contents of R2s1, R3s1 andR4s1 were lower than those before washing. Therefore, the ef-

fect of washing for R2 and R3 was to achieve satisfactory re-sults in a short period of time. At initial concentrations of221.00, 153.20 mg kg�1, the removal ratios of R2 and R3 were

53.62% and 33.62% respectively by washing for 5 h, and65.43% and 58.77% respectively by washing for 20 h. ForR4, the removal ratio was 53.57% by washing for 20 h.

During Step 2, although R1 had been removed by aceticacid, the amounts of Cr recovered in R1, R2, R3 and R4 were16.07, 257.68, 312.05, and 125.36 mg kg�1, respectively. The

levels of R2 and R3 were 36.25% and 43.88% of the total Cr.The effect of CA/SC washing on the distribution of Cr fraction-ation is illustrated in Fig. 3. After 5 h of washing, the content ofR1s2 was found to increase distinctly (176.79 mg kg�1). This va-

lue was close to 11 times that of the amount of R1 in the controlsample in Step 2. Moreover, a small range of variance occurredin R4, whereas a large range of variance occurred in R2 and R3.

Specifically, they were decreased from 257.68 to 79.02 (R2s2)and 312.05 to 174.11 mg kg�1 (R3s2), respectively. It is thus rea-sonable to assume that much acid soluble Cr in R2 and R3 had

been released and was able to be extracted by the first BCR pro-cedure. After washing for 10 h, the concentrations of R1s2, R2s2and R3s2 decreased slightly compared to those after 5 h ofwashing.

Fig. 4 shows the distribution of Cr fractionation after Step3. The amount of R2 extracted was 98.63 mg kg�1. Owing to



0

30

60

90

120

150

180

210

240

270

0 5 10 15 20

Time (h)

Con

cent

ratio

n (m

g/kg

)

0

5

10

15

20

25

30

35

40

45

50

(%)

R1 R2 R3 R4 Percent of R1 in total Cr

Figure 2 Variation of the four fractions of the BCR sequential extraction procedure for Cr in the contaminated soils by CA/SC washing

(R1, Acid soluble; R2, Reducible; R3, Oxidizable; R4, Residual).

0

50

100

150

200

250

300

350

0 5 10Time (h)

Con

tent

of f

ract

ions

(mg

kg-1

)

R1 R2 R3 R4


after Step 2.

4 G. Li et al.

the experimental design, theoretically there was not any R1 orR2. This was perhaps as a result of the effect of acetic acid,which was used to determine the content of R1. In response

to the release, after reacting for 16 h, acetic acid led to theredistribution of Cr fractionations. After CA/SC washing for5 h, the contents of R2 and R3 were decreased from 98.63

and 272.24 (R2s3) to 16.24 and 80.00 mg kg�1 (R3s3), respec-tively. However, R1 increased from 2.39 to 124.18 mg kg�1

(R1s3). Again, the content changes of R2 and R3 were similar

in the Step 1 and Step 2.During Step 4, theoretically only R4 remained in the soil.

Fig. 5 shows the distribution of Cr fractionation, indicating

the contents of R1, R2, R3 and R4 were 0, 11.56, 19.97 and68.47 mg kg�1, respectively. After washing for 5 and 10 h,the contents of R2 and R3 both decreased. R2s4 decreasedfrom 11.56 to 6.18 (5 h) and 6.57 mg kg�1 (10 h). R3s4


decreased from 19.97 to 9.71 (5 h) and 11.14 mg kg�1 (10 h).The content of R1s4 increased obviously, from 0 to 14.37(5 h) and 11.92 mg kg�1 (10 h). The content of R4s4 decreased

slightly after 5 h of washing.The proportions of Cr chemical fractionations after wash-

ing in Step 2, Step 3 and Step 4 are shown in Fig. 6. It can

be seen that the ratio of R4 remained almost constant by wash-ing for either 5 or 10 h for the three steps. The proportions ofR2 and R3 decreased significantly. After 5 h of washing, the

proportion of R2 decreased from 36.23% to 14.46%,19.27% to 5.03% and 11.56% to 6.18%. Furthermore, theproportion of R3 decreased from 43.88% to 31.86%,

53.20% to 24.76% and 19.97% to 9.76%. The proportion ofR1 increased significantly after washing. Comparing the con-trol sample and the sample washed for 5 h, the proportion ofR1 increased from 2.26% to 32.35%, 0.47% to 38.43%, and



0

50

100

150

200

250

300

0 5 10

Time (h)

Con

tent

of

frac

tions

(m

g kg

-1)

R1 R2 R3 R4


after Step 3.

0

20

40

60

80

100

120

0 5 10

Time (h)

Con

tent

of

frac

tions

(m

g kg

-1)

R1 R2 R3 R4


after Step 4.


0 to 14.37%. In addition, there is a little change in the propor-

tion of variation of Cr fractions from 5 to 10 h washing.Previous studies have shown that the efficiency of Cr frac-

tionation by CA/SC washing is better than other forms of

washing, such as DTPA (Diethylene triamine pentacetate acid)and EDTA (Pichtel and Pichtel, 1997; Sun et al., 2001; Honget al., 2002; Muhlbachova, 2011). Here, during the washingprocess, the ratio of R1 with total Cr increased before 5 h

and decreased after 5 h. This means that there was a redistribu-tion of Cr fractionation, i.e. R1 increased from 98.43 to141.20 mg kg�1 and the amount recovered in R2 decreased

from 221.00 to 76.40 mg kg�1. A similar result was reportedin the earlier work by our group (Liang et al., 2011) as wellas by other researchers. Jean et al. (2007), for example, re-

ported that the content of R1 was increased during the reme-diation of Cr-Ni-contaminated soil by CA. However, theratio of R1 with total Cr in their samples was too low


(0.069%) to verify the hypothesis. The authors mentioned that,

during the sequential extractions, the Cr bound to the iron-oxides might not have been extracted effectively from the R2fraction, and thus it could have been extracted with the last

fraction (R4), but also in fraction R3. This part of Cr mighttherefore have been redistributed in the R1 and R2 fractionsduring the mobilization with chelants. In fact, other heavymetals have also been shown to decrease exchangeable frac-

tions, Lei et al. (2008) found that after EDTA extraction, theconcentrations of exchangeable Pb, Cd, Cu, and Zn increased.The concentrations of carbonate, iron and manganese oxides,

organic matter, and the residue of heavy metals decreased. Inaddition, Manouchehri et al. (2006) reported that the stoichi-ometric ratio of reagent/total metal needs to be investigated

with respect to all the extractable cations present in the soil.Therefore, the quantification analysis of Cr fractionationseems to be a better method for complex soil system.



0%

20%

40%

60%

80%

100%

R4

R3

R2

R1

5h 10h 5h 10h 5h 10hStep 2 Step 3 Step 4

Figure 6 Ratio of the four fractions of the BCR sequential extraction procedure for Cr in the contaminated soils by CA/SC washing

after Steps 2, 3 and 4.

6 G. Li et al.

In this study, the result of the BCR procedure could directly

reflect the redistribution of Cr chemical fractionation. We ob-served that R4, R3 and R2 were decreased by washing. How-ever, contrary to the literature (Wuana et al., 2010), CA

removed most of the metals hitherto associated with theexchangeable and reducible fractions, such as Ni, Zn andCd, extracted Cr in R4 using CA washing needs to be investi-

gated in the soil. In this study, because there is a d-value be-tween measured R4 and real R4, Step 4 should beconsidered as the key step for calculating the redistributionrate. According to Fig. 5, the content of R4 was almost con-

stant. We can hypothesize that 86.00 mg kg�1 (R4s3) was thereal value of R4 in the contaminated soil.

DR4 ¼ R4 ðin original soilÞ �R4 ðR4s3Þ

Whereas, we have known that residual fraction was difficult to

be washed. Therefore, DR4 can be considered as the undetectedcontent of R2 and R3. For Step3 and Step 2, the removal con-tents of total Cr were 188.6 and 164.7 mg kg�1, respectively.

However, when comparing the calculated actual decreased con-tents of R2 and R3, it was found that R1 increased 121.8 and150.7 mg kg�1. Moreover, after washing in Step 1, if the value

of R1 decreased by the washing process, then R2 and R3 hadbeen decreased and transformed into R1. Therefore, Step 4,Step 3 and Step 2, all proved true in converting R2 and R3.The slow leaching of Cr therefore corresponds to dissolution

of the mineral matrix and most likely to oxide dissolution.A previous report (Jean et al., 2008) indicated that the effec-

tiveness of the chelants in solubilizing Cr is not directly related

to its complexation constants. It can be explained by: (i) itsability to solubilize the mineral matrix containing the metals.According to Di Palma (2009), the mechanism of metal extrac-

tion involves a two-step dissolution-chelation process where,after metal salts dissolute due to the strong acidity of the leach-ing solution, chelation occurs. When the soil was acidified by

citric acid, the dominant species of Cr(III) for pH < 4.5 areCr3+(aq) and Cr(OH)2+ (Andjelkovic et al., 2012); and (ii)the competition of CA with Cr(VI) on the surface sites. Asthe fact that the oxidation and reduction of soluble chromium


added to soils depend on the soil structure (Kozuh et al., 2000),

the desorption of Cr(VI) also depends on the soil structure.However, as a complex process, CA/SC washing brought onthe redistribution of Cr in the four fractions of the BCR pro-

cedure. Moreover, the fact that increase of R1 content may beattributed to the properties of minerals. In the initial stages ofwashing, water soluble and acid-leachable concentrations of

Cr were removed. Subsequently, Cr adsorbed on mineralswas released gradually by dissolution with washing agents.This process enhanced the ratio of R1 in total Cr.

4. Conclusions

CA/SC washing was successful for the removal of soil Cr, espe-

cially oxidizable and residual fractions of soil Cr, but the sol-uble and exchangeable Cr fraction increased. After washing for5 h, the content of total Cr remained almost constant. Resultsof the four steps indicated that the agent can extract the R2

and R1 contained in R4 and R3.Changes in Cr fractionation in contaminated soil by CA/SC

washing mean that the process not only improves soil Cr re-

moval, but also releases the soluble and exchangeable fractionof Cr in contaminated soil simultaneously. The main reasonmay be that the agent dissolved the minerals of the soil, and

the washing procedure may affect the inaccessible Cr, whichwas bound-up in a soil crystal lattice.

This is an advisable choice for the remediation of Cr-pollutedsoil in the future, especially for decreasing the secondary

pollution of the soluble and exchangeable fraction, whichwas contained in R4 and R3.

Acknowledgments

This study was supported by the National High TechnologyResearch and Development Program of China (No.

2009AA063101) and the Knowledge Innovation Project Key-Direction Project Sub-project of Chinese Academy of Sciences




(No. KZCX2-EW-407). We are grateful to the editors and

reviewers for their helpful suggestions about our study.

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evaluation of the potential redistribution of chromium fractionation in contaminated soil by citric...

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