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Environmental Pollution 158 (2010) 2710e2715

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Environmental Pollution

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Electrochemical EDTA recycling with sacrificial Al anode for remediation of Pbcontaminated soil

Maja Pociecha, Domen Lestan*

Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia

Aluminium anode at alkaline pH in conventional electrolytic cell entechnology.

ables efficient recycling of EDTA as a part of soil washing remediation

a r t i c l e i n f o

Article history:Received 5 February 2010Received in revised form19 April 2010Accepted 23 April 2010

Keywords:LeadContaminated soilRemediationEDTAElectrochemical treatment

* Corresponding author.E-mail address: domen.lestan@bf.uni-lj.si (D. Lesta

0269-7491/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.envpol.2010.04.014

a b s t r a c t

Recycling chelant is a precondition for cost-effective EDTA-based soil remediation. Extraction with EDTAremoved 67.5% of Pb from the contaminated soil and yielded washing solution with 1535 mg L�1 Pb and33.4 mM EDTA. Electrochemical treatment of the washing solution using Al anode, current density96 mA cm�2 and pH 10 removed 90% of Pb from the solution (by electrodeposition on the stainless steelcathode) while the concentration of EDTA in the treated solution remained the same. The obtained dataindicate that the Pb in the EDTA complex was replaced by electro-corroded Al after electro-reduction ofthe EDTA and subsequently removed from the solution. Additional soil extraction with the treatedwashing solution resulted in total removal of 87% of Pb from the contaminated soil. The recycled EDTAretained the Pb extraction potential through several steps of soil extraction and washing solutiontreatment, although part of the EDTA was lost by soil absorption.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Recent decades have witnessed the rapid development ofvarious technologies for the remediation of heavy metals contam-inated soils. One of the most promising methods is soil washingwith aqueous solutions of chelants. The advantages of chelantsinclude a high efficiency of metal extraction, good solubility of themetal complexes formed and minor impact on the soil’s physical,chemical and biological properties compared to acid soil extraction(Lim et al., 2004). For Pb contaminated soils, ethylenediamine tet-raacetic acid (EDTA) has often been shown to be the most effectivechelant (Lestan et al., 2008). In practice, however, the use of EDTA infull-scale is prohibited by a large volumes of waste washing solu-tion generated, which must be treated before disposal. Effectivetreatment methods for waste washing solution, particularly recy-cling and reuse of EDTA, are needed.

Although several EDTA recycling procedures have been demon-strated on a laboratory scale, there is currently no practical andcommercially available method. Ager and Marshall (2001) usedzero-valent bimetallic mixtures (Mg0ePd0, Mg0eAg0) to precipitatePb from the solution while liberating EDTA in alkaline pH. Metalsliberated from the EDTA complex were cemented to the surfaces of

n).

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the excess magnesium or removed from the solution as insolublehydroxides. The method is efficient but could be economicallyprohibitive. Hong et al. (1999) separated Pb from EDTA with Na2S,resulting in almost complete recovery of metals through precipita-tion in the formof insolublemetal sulphides. Thismethodhas foundlimited application due to the hazardous nature of the reagents andthe sludge produced. Kim and Ong (1999) recycled chelant fromPb-EDTA solution by substituting Pb with Fe3þ in acidic conditions,followed by precipitation of the released Pb with phosphate nearneutral pH. Fe3þ ions were then precipitated as hydroxides at highpH using NaOH, thus liberating EDTA. The process does not useexpensive or hazardous reagents but it is complicated, with severaloperations involved and a slow kinetics of some reactions.

Electrochemical technologies are simple and efficient methodsfor the treatment of many wastewaters, characterised bya compact size of the equipment, simplicity of operation, and lowcapital and operating costs (Chen, 2004). Johnson et al. (1972)reported that using a Pt anode in a conventional electrolytic celloxidized EDTA into CO2, formaldehyde and ethylendiamine, andcould thus potentially be used for treating waste soil washingsolutions. However, simultaneous recovery of metals and chelantwas not possible with this system. A two chamber cell separatedby a cation-selective membrane was therefore proposed to allowliberation of metals from the complex and to prevent oxidation ofEDTA at the anode. Metals, including Pb, were reduced anddeposited onto the cathode and the EDTA was simultaneously

M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e2715 2711

recycled (Allen and Chen, 1993; Juang and Wang, 2000). Thismethod, however, is prone to operational problems such asmembrane fouling and degradation. We recently proposed elec-trocoagulation for the removal of metals and EDTA froma washing solution obtained after extraction of primarily Pbcontaminated soil (Pociecha and Lestan, 2009). In electro-coagulation, Al (or Fe) ions are generated from the sacrificialanode. The reactions for the electrochemical system at the Alanode are as follows (Eqs. 1 and 2):

Al/Al3þ þ 3e� (1)

Al3þ þ 3H2O/ AlðOHÞ3 (2)

The nascent Al3þ ions are very effective coagulants and formlarge networks of Al-O-Al-OH flocks, with a large surface area andconsiderable absorption capacity (Shen et al., 2003). In our study Pbwas almost entirely removed from the soil washing solution while,to our surprise, some EDTA remained in the washing solution. Thisearly result indicated separation of Pb from EDTA.

In the current work, the feasibility of electrochemical separationof EDTA and Pb in waste soil washing solution using an Al anodeand a single-chamber electrolytic cell was studied.

2. Materials and methods

2.1. Soil properties

Soil contaminated with 3980 � 60 mg kg�1 of Pb was collected from the0e30 cm surface layer of a vegetable garden in the Me�zica Valley, Slovenia. TheMe�zica Valley has been exposed to more than three hundred years of active leadmining and smelting. For standard pedological analysis, the pH in soils wasmeasured in a 1/2.5 (w/v) ratio of soil and 0.01 M CaCl2 water solution suspension.Soil samples were analyzed for organic matter (as C content) by modified Wal-kleyeBlack titrations (ISO 14235, 1998), cation exchange capacity (CEC) by theammonium acetate method (Rhoades, 1982) and soil texture by the pipette method(Fiedler et al., 1964). The following values were obtained: pH 6.57, C content 8.2%,CEC 20.7 mval 100 g�1 of soil, sand 51.0%, silt 42.5%, clay 6.5%. The soil texture wassandy loam.

2.2. Soil washing

The extraction of soil with EDTA solutions was performed in two scales. Toobtain the washing solution for the electrochemical treatment, we placed 0.5 kg ofair-dried soil and 875 mL of aqueous solution (1:1.75 soil: washing solution ratio) of75 mmol of EDTA (disodium salt) per kg of soil (43 mM EDTA), pH 4.3, in 1.5 L flasks.Soil was extracted on a rotating shaker (3040 GFL, Germany) for 72 h at 16 RPM.Approximately 400 mL of the washing solution was decanted from each flask afterthe soil was allowed to settle for 24 h. The decanted washing solution was filtered(filter paper was wide-pored, grade 388, density was 80 g m�2).

The same procedure was used to extract the soil with the recycled EDTA solu-tion, except that centrifugation at 2880 g for 5 min and not decantation was used toseparate the soil from the washing solution, to minimise solution loss. Fine particleswere removed from the solution by filtration as described above.

2.3. Electrochemical treatment of the soil washing solution

The electrolytic cell consisted of an Al anode placed between two stainless steelcathodes (distance ¼ 10 mm), the overall anode surface 63 cm2 and the surfacearea ratio between the cathodes and anode 1:1. The electrodes were placed in500 mL of magnetically stirred soil washing solution in a 1.0 L flask. Current densitywas kept at 96 mA cm�2, and the cell voltage measured with a DC power supply(Elektronik Invent, Ljubljana, Slovenia). The electrode cell was cooled usinga cooling mantle and tap water (flow rate 250 mL min�1) to keep the temperatureof the treated washing solution below 35 �C. The contact time of the electro-chemical treatment was calculated as the ratio of the electrode cell volume to thevolume of the washing solution and multiplied by the operation time (initially30 min of operation time equalled 3.78 min of contact time). During the electro-chemical treatment, the pH of the washing solution was regulated to pH 6, 8 and 10by drop-vice addition of 5 M NaOH and HCl. Samples (20 mL) of washing solutionwere collected periodically and the pH and EC measured immediately. Sampleswere afterwards centrifuged at 2880 g for 10 min and the supernatant stored in thecold for further analysis of Pb and EDTA. The pellet (mainly Al hydroxide precipi-tate) was suspended in 200 mL of deionised water acidified with 37% HCl to pH 1.5.

The resulting solution with some finely suspended precipitate of EDTA, which isinsoluble in acidic media, was stored in the cold for Pb determination. At the end ofthe electrochemical treatment, the cathodes were etched with 30 mL of 65% HNO3

to dissolve and later measure the concentration of electro-deposited Pb. The Alanode was weighed before and after treatment of the washing solution to deter-mine the amount of electro-corroded Al.

During the electrolysis, the surface of the Al anode was passivised by an oxide/hydroxide layer, which increased the potential between the electrodes (Mouedhenet al., 2008). In order to break down this passive layer and reduce the powerconsumption, we applied small amounts of Cl� (as NaCl) when the voltage increasedabove 8 V (Chen, 2004).

To prepare the recycled EDTA solution for soil extractions, we electrochemicallytreated the washing solution at pH 10 for 24 min (contact time) and separated therecycled EDTA solution from the Al hydroxide precipitate by centrifugation at 2880 gfor 30 min.

2.4. Treatment of the soil washing solution with dosing Al-salt

Aweight of 4110 mg of AlCl3 was dosed into 100 mL of the soil washing solutionwith pH 10 and gently stirred for 22.68, 45.36 and 113.4min, which corresponds to 1,2 and 3-times the total contact time of the electrochemical treatment, respectively.The amount of chemically dosed Al was the same as the molar amount of Al electro-corroded from the anode during electrochemical treatment. During the coagulationtreatment with Al dosing, the pH of the washing solution was kept at pH 10, using5 M NaOH. The precipitate was removed from the treated solution by centrifugationat 2880 g for 30 min, and the concentrations of Pb and EDTA in the supernatantmeasured. Afterwards, the pH of the chemically treated washing solution wasadjusted to 4.3 and the solution reused for soil Pb extraction.

2.5. EDTA determination

The concentration of EDTAwas determined spectrophotometrically according tothe procedure of Hamano et al. (1993). The method involves the reaction of EDTA inwashing solution with Fe3þ under acidic conditions to produce the Fe-EDTA chelate(trans-complexation), followed by the removal of excess of Fe3þ by chelate extrac-tion in the aqueous phase using chloroform and N-benzoyl-N-phenylhydroxylamineand the formation of a chromophore with 4,7-diphenyl-1,10-phenanthroline-disulfonic acid. Using a spectrophotometer, absorbance was measured at 535 nmagainst a blank solution with the 4,7-diphenyl-1,10-phenanthroline-disulfonic acidreplaced with an equal volume of distilled water. The limit of EDTA quantificationwas 20 mg L�1.

2.6. Pb determination

Air-dried soil samples (1 g) were ground in an agate mill, sieved througha 160 mmmesh and digested in a glass beaker on a hotplate with 28 mL of aqua regiasolution (HCl and HNO3 in a 3:1 ratio (v/v)) for 2 h at 110 �C. Condensation ofevaporating fumes was achieved via circulation of cool tap water through the glasstubes placed on top of the glass beakers. After cooling, digested samples werefiltered through Whatman no. 4 filter paper and diluted with deionised water up to100 mL. The pseudo-total concentration of Pb was determined by flame (acetylene/air) AAS with a deuterium background correction (Varian, AA240FS). The Pb in thesolutions was determined by AAS directly. A standard reference material used ininter-laboratory comparisons (Wepal 2004.3/4, Wageningen University, Wagenin-gen, Netherlands) was used in the digestion and analysis as part of the QA/QCprotocol. The limit of quantification for Pb was 0.01 mg L�1. Reagent blank andanalytical duplicates were also used where appropriate in order to ensure accuracyand precision in the analysis.

2.7. Statistics

The Duncan multiple range test (Statgraphics 4.0 for Windows) was used todetermine the statistical significance (P < 0.05) between different treatments.

3. Results and discussion

3.1. Soil washing

Soil extraction with 75 mM EDTA per kg of dry soil removed67.5% of Pb. The molar ratio between the Pb initially present in thesoil and the EDTA in the washing solution was 1:3.9. It is knownthat even strong chelants such as EDTA cannot remove heavymetals from the soil entirely, even at high molar ratios of EDTA vs.heavy metal concentration applied (Nowack et al., 2006). However,the Pb residual in soil after stringent soil washing with EDTA isencapsulated in soil minerals or strongly bound to the non-labile

10001200140016001800

g L

-1)

pH 6pH 8pH 10

A

M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e27152712

soil fractions and therefore essentially non-leachable and non-bioavailable (Udovic et al., 2009).

The concentrations of Pb and EDTA in the soil washing solutionbefore treatment in the electrolytic cell were 1535 and12 444 mg L�1 (33.4 mM), respectively. The pH of the washingsolution before treatment was 7.91.

0200400600800

Pb

(m

050100150200250300350400450

0 5 10 15 20 25 30 35

Contact time (min)

Pb

(m

g L

-1)

B

Fig. 1. Removal of Pb from the washing solution (A) and accumulation of Pb in theprecipitate (B). Washing solutions were electrochemically treated at pH 6, 8 and 10.Error bars represent standard deviation from the mean value (n ¼ 3).

Table 1Balance of Pb after electrochemical treatment of the soil washing solution atdifferent pH. Standard deviation from the mean value (n ¼ 3) was calculated.

Treated washing solution Pb balance (%)

In solution Precipitated Electrodeposited S

pH 6 30 � 6 20 � 7 33 � 5 83 � 10pH 8 13 � 2 7 � 1 68 � 5 88 � 7pH 10 11 � 3 15 � 2 62 � 5 88 � 7

3.2. Electrochemical treatment of soil washing solution

Soil washing solutions were treated at various pH (6, 8 and 10).The pH of the solution tended to increase with treatment time,since the electrochemical system generated enough OH� at theelectrode to counteract the Hþ released by the formation of Alhydroxides as a net final product (Canizares et al., 2006). Solutiontreated at pH 10 consequently required very little pH adjustment.The voltage between the electrodes also tended to increase withtreatment time, regardless of the pH of the washing solution. Themain reason for the voltage increase was the passivisation of the Alanode surface by formation of an insulating film of Al oxide(Mouedhen et al., 2008). In order to break down the passive filmand thus to reduce the cell voltage surge and increase of powerconsumption, small amounts of Cl� (as NaCl) were applied (Chen,2004), to keep the voltage close to initial 8 V. The amount of Alconsumed from the Al anode was 14.6 � 2.5, 12.0 � 0.4 and9.4 � 0.4 g L�1 of solutions treated at pH 6, 8 and 10, respectively.The amount of electro-corroded Al decreases with increasing pH.A higher aluminium current efficiency at higher alkaline conditionsthan at neutral is generally found in electrochemical systems (Chen,2004). The electro-conductivity of the washing solution increasedfrom an initial 6.1 to up to 10.0 mS cm�1 (solution with pH 6). Thisincrease followed the increasing concentration of charged Alhydroxide (electrolyte) (data not shown) during the electro-chemical process.

pH is an important operating factor influencing the performanceof electrochemical processes (Chen, 2004). The effect of differentpH of electrochemical treatment on the dynamics of Pb removaland precipitation from the washing solution and on the massbalance of Pb is shown in Fig. 1 and Table 1. During electrochemicaltreatment, metals complexed to EDTA could be removed from thesoil washing solution by absorption on Al hydroxide flocks (elec-trocoagulation). Metals (M) could also be released from the EDTAcomplex after reduction reactions at the cathode (Juang and Wang,2000), Eqs. 3 and 4.

M� EDTA2��!Cell voltage

M2þ þ EDTA4� (3)

M2þ þ 2e� /MðsÞ (4)

Metals liberated from the EDTA complex could then beremoved from the solution by direct electrodeposition on thecathode, precipitation as insoluble hydroxides, or absorption andco-precipitation on Al hydroxide flocks according to the followingreaction (Eq. 5):

AlðOHÞ3 þ M2þ / AlðOHÞO2M þ 2Hþ (5)

Table 1 indicates that, in all electrochemical treatments, themajority of Pb was removed from solution by electrodeposition onthe cathode. However, at pH 8 and 10, a significantly higher amountof Pb was removed this way than at pH 6. This could again beexplained by a higher current efficiency of Al anode electrochemicalsystems at higher pH (Chen, 2004). The phenomenon presumablyalso explains the faster rate of Pb removal from the washing solu-tion treated at pH 10 (Fig. 1).

After treatment at pH 10, the EDTA remained almost entirelypreserved quantitatively in the washing solution, while at pH 6 and8, approximately one half of the initial EDTA was removed (Fig. 2).EDTA was presumably removed from the washing solution byelectrocoagulation adsorption on Al hydroxide flocks, althoughsome EDTA degradation by anodic oxidation (Johnson et al., 1972)might also occur. At pH 6 and 8, negatively charged EDTAcomplexes (M-EDTA2-) are probably partly absorbed on variousmonomeric and polymeric Al hydroxides species formed during theelectrocoagulation process (Nowack and Sigg, 1996). These species,such as Al(OH)2þ, Al(OH)2þ, Al2(OH)24þ, Al6(OH)153þ, Al7(OH)174þ,Al8(OH)204þ, Al13O4(OH)247þ or Al13(OH)345þ, have a long lasting positivecharge before they finally transform into amorphous Al(OH)3according to complex precipitation kinetics (Mouedhen et al.,2008; Rebhun and Lurie, 1993).

Amorphous Al(OH)3 is a typical amphoteric metal hydroxide,which leads in alkaline conditions to the reaction shown in Eq. (6)(Chen, 2004).

AlðOHÞ3 þ OH� / AlðOHÞ�4 (6)

The formation of negatively charged Al hydroxides explains

0

2000

4000

6000

8000

10000

12000

14000

0 5 10 15 20 25 30 35

Contact time (min)

ED

TA

(m

g L

-1)

pH 6pH 8pH 10

Fig. 2. Concentration of EDTA in the washing solutions during electrochemical treat-ment at pH 6, 8 and 10. Error bars represent standard deviation from the mean value(n ¼ 3).

0

10

20

30

40

50

60

70

Original soil Extracted soil

Re

mo

ved

Pb

(%

)

Fresh solutionTreated solutionNon-treated solution

ab

cd d

e

Fig. 3. Removal of Pb from the original and previously extracted soil using fresh EDTAsolution, EDTA solution after soil extraction but not treated, and electrochemicallytreated EDTA solution. Soil washing solutions were treated at pH 10 and, for soilextraction, the solutions’ pH was adjusted to 4.3. EDTA concentration was equal in allsolutions (30 mM). Error bars represent standard deviation from the mean value(n ¼ 3); letters (a, b, c) denote statistically different Pb removal potential within thecategories of original and extracted soil, according to the Duncan test (p < 0.05).

M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e2715 2713

why, at pH 10, negatively charged EDTA was also not be removedfrom the washing solution by electrocoagulation (Fig. 2).

Electrochemical treatment at pH 10 efficiently removed themajority of the Pb from the washing solution (Fig. 1), while theEDTA remained almost completely preserved in the washingsolution (Fig. 2). As explained below, these data indicate thereplacement of Pb from the complex with EDTA, removal of liber-ated Pb from the solution, and formation of Al-EDTA complex(trans-complexation). EDTA is a hexaprotic system. The degree ofEDTA protonation and complexation with metals depends on thepH of the washing solution and the nature of the metal ionspresent. Al has a lower complex formation stability constant (Ks)than Pb (log Ks are 16.3 and 18.0 (at 20 �C and ionic strengthm ¼ 0.1) for Pb and Al, respectively, Martell and Smith, 2003).However, Al ions formed in abundant concentrations during elec-tro-corrosion of the Al anode and Treacy et al. (2000) showed thatthe stability of Al-EDTA complex was higher in a solutionwith pH 9than in solutions with pH 7 and 4. On the other hand, the stability ofPb-EDTA complex decreases in solutions with pH > 8 (Chang et al.,2007). The trans-complexation hypothesis, however, still needs tobe proven.

Our data indicate that using an Al anode enables electro-chemical treatment in a conventional, simple, one-compartmentelectrolytic cell, without significant EDTA degradation. PresumablyAl is oxidized at the anode (Eq. (1)) preferentially to EDTA oxida-tion, due to the high Al reactivity (electro-positivity). The standardelectrode (oxidation) potential of the Al/Al3þ couple is 1.66 V(Evangelou, 1998).

This is the first report on using this type of electrochemicalsystem for EDTA recycling from Pb soil washing solution. Thesignificance of pH for effective treatment was demonstrated.

3.3. Recycling and reuse of the treated EDTA soil washing solution

The efficiency of EDTA recycled from a washing solution elec-trochemically treated at pH 10 to extract Pb from the soil is shownin Fig. 3. After adjustment to pH 4.3 (pH of the fresh EDTA washingsolution), the treated washing solution retained almost 90% of thePb extraction potential (from original soil) compared to freshlyprepared EDTA solution of the same molarity and pH. In thisexperiment (soil was extracted in two separate batches), freshEDTA solution removed 63% of the Pb from the original soil andfurther extraction using the treated solution on the once extractedsoil removed an additional 24% of Pb (Fig. 3). In total, using EDTArecycling, almost 90% of Pb was removed from the soil.

Interestingly, the potential of the treated washing solution toextract Pb from previously (once) extracted soil was even higherthan that of fresh EDTA solution, although the difference was notstatistically significant (P < 0.05), Fig. 3.

Sincewe used a highmolar ratio of EDTA against soil Pb (as usualin soil washing; Nowack et al., 2006) only part of the EDTA in thewashing solution was complexed to Pb (approximately 22% calcu-lated from data on Pb and EDTA concentration in the washingsolution, section 3.1.), someEDTAwaspresumably left in the originalform or in various stages of protonation. Spent non-treatedwashingsolution was therefore expected to retain some Pb extractionpotential, as indeed shown in Fig. 3.

Table 2 shows the potential of recycled EDTA for Pb extractionfrom the original soil through several steps of soil extraction andwashing solution treatment. First, fresh EDTA solution was used(in the 1st ext./treat. step) following by the use of electrochemi-cally treated washing solutions (2nd and 3rd ext./treat. step) forsoil extraction. Washing solution treated once retained 86% andsolution treated twice 69% of the Pb extraction efficiency of freshEDTA solution (calculated from data on percentages of Pb removedfrom the soil presented in Table 2). The decrease of Pb extractionpotential can be explained by the loss of EDTA from the solution(Table 2), mainly due to EDTA absorption into the soil solid phase(Nowack and Sigg, 1996). Some EDTA (<10%) was also lost duringthe solution treatment phase, in which EDTA was either precipi-tated or anodically oxidized. Alternatively, EDTA could also bedegraded by chlorine (Cl2) and hypochlorite (HOCl), which arestrong oxidants and could be generated anodically followingadditions of NaCl into the electrolytic chamber to break down theanodic passive film (Chen, 2004).

3.4. Chemical dosing of Al

Electrochemical treatment of wastewaters with a sacrificial Alanode is an alternative to more commonly used chemical coagu-lation of pollutants by dosing Al-salts. To compare these twomethods, the same amount of Al (as AlCl3) was dosed in thewashing solution (pH 10) as the amount of Al electro-corrodedduring the corresponding electrochemical treatment. Chemicaldosing (Table 3) removed very little Pb from the washing solution,much less than electrochemical treatment (Fig. 1, Table 1), but didremove some EDTA. The reason is that chemical coagulation withAlCl3 removed Pb with EDTA complexes, while electrochemical

Table 2Potential of fresh and electrochemically treated EDTA solutions for Pb extraction from original (previously not-extracted) soil during three consecutive steps of soil extractionand washing solution treatment. Washing solutions were electrochemically treated at pH 10 and, for soil extraction, the pH was adjusted to 4.3. The percentage of lost EDTAafter soil extraction (Lost EDTA e extraction) and after electrochemical treatment of the solution (Lost EDTA e treatment) was calculated.

Soil extraction/Solutiontreatment

Pb removed(%)

Initial EDTAconcentration (mM)

EDTA conc. afterextraction (mM)

EDTA conc. aftertreatment (mM)

Lost EDTA e extraction(mM) (%)

Lost EDTA e

treatment (%)

1. ext./treat. 71 43 32 29 26 9.92. ext./treat. 61 29 23 22 19 6.43. ext./treat. 49 22 15 / 32

M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e27152714

treatment liberated Pb and separated the EDTA. Chemical dosingdid not lead to EDTA recycling; the Pb extraction potential of thechemically treated solution (with pH adjusted to 4.3) was evenlower than the extraction efficiency of non-treated washing solu-tion with the same pH (27 � 3% removed Pb, Fig. 3).

3.5. Cost and safety considerations

An accurate evaluation of the costs associated with soil reme-diation would require a pilot-scale experiment. However, the costof Al and electricity consumption, Al-hydroxide sludge disposal andtreatment of the final spent washing solution (which represent themajor part of the material costs) can be extrapolated from theobtained data. If we assume two re-cycles of EDTAwashing solutiontreatment and reuse, then (including compensation for lost EDTA,Table 2) extraction of 1 ton of soil (with 75 mmol kg�1 of fresh andrecycled EDTA) would require 13.9 kg of EDTA. At a price of 1.3 V

per kg�1 EDTA (information obtained from a major European EDTAproducer) this translates into 18.1 V. Treatment of the washingsolution and EDTA recycling at a constant current density of96 mA cm�2 and an average voltage of 8 V would require approx-imately 115 kWh and, at an approximate cost of 0.1V per kWh, thistranslates into 11.5 V. During the treatment/EDTA recycling,approximately 5 kg of Al would be expected to electro-corrodefrom the anode. The current prices of Al in the open market arebelow 1.6V kg�1, which equals 8V for spent Al. During the process,approximately 20 kg of liquid Al hydroxide sludge was formed,calculated per ton of treated soil. The sludge can be deposited aftertreatment, i.e., after solidification (and stabilisation of metals) withcement. The disposal cost of solid hazardous waste transportationand disposal was assessed to be approximately 200 V per ton(Meunier et al., 2006), which adds an additional 4 V to the totalcost. Since Al hydroxide is the major component of aluminium orebauxite (together with AlO(OH) and Al2O3), it could perhaps bereused in the Hall-Héroult process to recycle aluminium and avoiddisposal costs. The spent washing solution contains EDTA and Pb,which need to be completely removed before safe discharge. Ina previous paper, we proposed an electrochemical advancedoxidation process using a boron-doped diamond anode for EDTAdegradation and removal of Pb from treated solution by (electro)precipitation (Finzgar and Lestan, 2008). The electricity cost for thisoperation was estimated to be 10.3 V ton�1 of soil. The total

Table 3Pb and EDTA removal from the washing solution after chemical dosing of AlCl3 at pH10, and Pb extraction efficiency of chemically treated soil washing solution (after pHwas adjusted to 4.3). The treatment time of chemical dosing was selected as a 1e3multiple of the electrochemical treatment time (tel). Standard deviation from themean value (n ¼ 3) was calculated.

Treatment time(min)

Pb removed(%)

EDTA removed(%)

Pb extraction potential(%)

23 (1 � tel) 3 � 5 34 � 9 26 � 245 (2 � tel) 4 � 5 27 � 4 23 � 6113 (3 � tel) 9 � 3 26 � 3 21 � 0

estimated cost of major material expenses for the treatment of 1ton of contaminated soil thus amounts to approximately 52 V. Thiscost does not include capital investment in the equipment, which,in terms of electrochemical technologies, is characterised as rela-tively low (Chen, 2004). The cost seems favourable compared to thecurrent cost of soil washing, which can go up to 350 V per ton(Summergill and Scott, 2005).

During the proposed remediation technique, some deposition ofAl from the EDTA complex into the soil is expected. Aluminium isknown to reduce plant growth on acid soils, in which Al3þ cationsdisturb root growth and function. However, Al constitutes alumi-nosilicate minerals and sesquioxides and is naturally present in thesoil. It is harmless to plants, immobile and non-toxic in pH-neutralsoils (Andersson, 1988).

4. Conclusions

The following conclusions can be drawn from our study:

� Electrochemical treatment of soil washing solution obtainedafter EDTA extraction of Pb contaminated soil, using an Alanode in a conventional electrolytic cell at pH 10, efficientlyseparated EDTA and Pb.

� Pb was relatively efficiently removed from the treated washingsolution (>85%, Fig. 1), mostly by electrodeposition on thecathode.

� Electrochemical treatment separated EDTA in an active form.We demonstrated that, after treatment, the EDTA solutionretains almost all its Pb extraction potential.

� Less than 10% of EDTA was lost during electrochemical treat-ment. More EDTA was lost from the solution due to absorptiononto the soil solid phases during soil extraction.

� Chemical dosing of Al was not effective in separating Pb andEDTA in the washing solution. We conclude, therefore, thatelectro-reduction of EDTA on a cathode (Eq. (3)) is essential forthe exchange of Pb from the EDTA complex.

� Electrochemical treatment of the washing solution with an Alanode at alkaline pH has potential for cost-effective recyclingand reuse of EDTA as a part of soil washing technologies.

Acknowledgement

This work was supported by the Slovenian Research Agency,Grant J4-9277.

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