ww.sciencedirect.com

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 8

Available online at w

journal homepage: www.elsevier .com/locate/watres

Electrochemical treatment of spent solution after EDTA-basedsoil washing

David Voglar, Domen Lestan*

Center for Soil and Environmental Science, Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101,

1000 Ljubljana, Slovenia

a r t i c l e i n f o

Article history:

Received 11 February 2011

Received in revised form

24 October 2011

Accepted 19 January 2012

Available online 25 January 2012

Keywords:

Soil remediation

Heavy metals

Electrochemical advanced oxidation

process

Electro-Fenton

Anodic oxidation

* Corresponding author. Tel.: þ386 01 320 31E-mail address: domen.lestan@bf.uni-lj.s

0043-1354/$ e see front matter ª 2012 Elsevdoi:10.1016/j.watres.2012.01.018

a b s t r a c t

The use of EDTA in soil washing technologies to remediate soils contaminated with toxic

metals is prohibitive because of the large volumes of waste washing solution generated,

which must be treated before disposal. Degradation of EDTA in the waste solution and the

removal of Pb, Zn and Cd were investigated using electrochemical advanced oxidation

processes (EAOP) with a boron-doped diamond anode (BDDA), graphite and iron anodes

and a stainless-steel cathode. In addition to EAOP, the efficiency of electro-Fenton reac-

tions, induced by the addition of H2O2 and the regulation of electrochemical systems to pH

3, was also investigated. Soil extraction with 15 mmol kg�1 of soil EDTA yielded waste

washing solution with 566 � 1, 152 � 1 and 5.5 � 0.1 mg L�1 of Pb, Zn and Cd, respectively.

Treatments of the waste solution in pH unregulated electrochemical systems with a BDDA

and graphite anode (current density 67 mA cm�2) were the most efficient and removed up

to 98 � 1, 96 � 1, 99 � 1% of Pb, Zn and Cd, respectively, by electrodeposition on the cathode

and oxidatively degraded up to 99 � 1% of chelant. In the electrochemical system with an

Fe anode operated at pH 3, the chelant remained preserved in the treated solution, while

metals were removed by electrodeposition. This separation opens up the possibility of

a new EDTA recycling method from waste soil washing solution.

ª 2012 Elsevier Ltd. All rights reserved.

1. Introduction Chelating agents desorb metals from soil by forming strong

Soil contamination with toxic metals is ubiquitous worldwide

and represents a serious health and environmental problem,

since toxic metals are not degradable and persist in soils for

centuries. The demand for soil treatment techniques is

consequently growing and the development of efficient

remediation technologies has become one of the key research

activities in environmental science and technology.

Soil washing using chelating agents is one of the most

promising and studied metal removal remediation tech-

niques. It involves the separation of toxic metals from soil

solid phases by solubilizing the metals in a washing solution.

62; fax: þ386 01 423 1088.i (D. Lestan).ier Ltd. All rights reserved

and water-soluble metal-chelant coordination compounds

(complexes). These complexes are very stable, prevent the

precipitation and sorption of metals and do not release their

metal ions unless there is a significant drop in soil pH. While

many different chelants (mostly aminopolycarboxylic acids)

have been tested for soil washing, di-sodium salt of ethyl-

enediamine tetraacetate (EDTA) is the most frequently used

because of its efficiency, availability and relatively low cost

(Lestan et al., 2008).

In practice, the use of EDTA in full-scale soil washing is

prevented by the large volumes of waste washing solution

generated, which must be treated before disposal. However,

.

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 82000

toxic waste waters containing complexed EDTA cannot be

treated using conventional methods, such as filtration, floc-

culation and participation (Jiraroj et al., 2006). Other treat-

ments to remove EDTA from the spent washing solution are

required.

Finzgar and Lestan (2006) used a combination of ozone and

UV, an advanced oxidation process (AOP). AOP generated

hydroxyl radicals (�OH) for the oxidative decomposition of

EDTAemetal complexes (�OH are one of the most powerful

oxidants, second only to fluorine). The released metals were

then removed from the washing solution by absorption.

However, colouration and particles in the washing solution

absorb and scatter UV light and disturbed the process. Di

Palma et al. (2003) proposed reverse osmosis for the separa-

tion of EDTA complexes. However, the soil colloidal particles

tend to clog the membranes. In yet another study, Tejowulan

and Hendershot (1998) separated EDTA using an anion

exchange resin. Pociecha and Lestan (2010) proposed electro-

coagulation with an Al sacrificial anode but could not achieve

complete EDTA removal from the solution.

Using electricity to treat waste waters was first proposed in

the UK in 1889. Today, electrochemical technologies are

known as simple and efficient methods for the treatment of

many waste waters, characterized by the compact size of the

equipment, the simplicity of operation, and low capital and

operating costs (Chen, 2004). Electrochemical oxidation of

contaminants through anodically generated chlorine and

hypochlorite is well known. Electrochemical oxidation of

contaminants in waste waters oxidation can also occur

directly on anodes, by generating active oxygen absorbed into

the oxide lattice on the anode. This process is usually called

direct anodic oxidation. Active oxygen can cause the complete

combustion of organic compounds and the formation of

selective oxidation products. Anodic oxidation does not need

the addition of chemicals to waste water, which is an

advantage over other electro-oxidation processes. In an early

study, Johnson et al. (1972) reported that a Pt anode used in

a conventional electrolytic cell oxidized EDTA into CO2,

formaldehyde and ethylenediamine and could thus poten-

tially be used for treating waste soil washing solutions.

The important parameter of an anodic oxidation process is

the anode material. Various anode materials have therefore

been studied: graphite, Pt, various noble metal oxides (PbO2,

IrO2, TiO2, SnO2) on titanium substrate, boron-doped diamond

anode (BDDA) (Troster et al., 2002). BDDA in particular is also

extraordinarily chemically inert and suitable for treating

various waste waters. Yamaguchi et al. (2006) used BDDA to

oxidise EDTA through sequential removal of the acetate groups

until an unidentified small size hydrocarbon product was

formed.The feasibility ofusingBDDAfor the treatmentof spent

soilwashing solutionafterEDTAextractionof Pb, Zn,CdandCu

from contaminated soils was studied by Finzgar and Lestan

(2008) and Pociecha et al. (2009). However, BDDA are expen-

sive and still difficult to manufacture on an industrial scale.

The aim of the current study was to compare the efficiency

of electrochemical systems with three different types of

anodic materials: BDDA, graphite and Fe in two processes:

electrochemical AOP (EAOP), and electro-Fenton (induced by

H2O2 addition and pH regulation), to treat waste soil washing

solution containing EDTA and Pb, Zn and Cd as toxic metals.

1.1. Theoretical background

During the anodic oxidation process, if the oxygen overvoltage

is not sufficiently high molecular oxygen is mainly produced

during water electrolysis. In EAOP, however, the anode

material has sufficient oxygen overvoltage before H2 (cathode)

and O2 (anode) form. This electrochemical window allows the

production of hydroxyl radicals (�OH) at the anode according

to equation below, directly from the electrolyzed water at

a high current efficiency (Kraft et al., 2003; Oliveira et al., 2007).

H2O / �OH þ e� þ Hþ (1)

In general, �OH is more effective for oxidation of contami-

nants than active oxygen in the anode oxide lattice. BDDA has

an extreme oxygen overvoltage of >3 V.

In the last decade, a special type of electrochemical reac-

tions, the electro-Fenton system, has attracted considerable

research interest (Pratap and Lemley, 1998; Tezcan Un et al.,

2006). Traditionally, the Fenton system is a mixture of

ferrous salt and hydrogen peroxide. In electro-Fenton,

a ferrous ion is added to the system or produced from

a sacrificial iron anode via an oxidation reaction:

Fe / Fe2þ þ 2e� (2)

Hydrogen peroxide is either added to the electrolytic cell or

electro-generated from two-electron reduction of sparged

oxygen on the cathode:

O2 þ 2Hþ þ 2e� / H2O2 (3)

Ferrous ion reacts with hydrogen peroxide to produce �OH,

which then participates in pollutant (i.e., EDTA) oxidative

degradation.

H2O2 þ Fe2þ / Fe3þ þ �OH þ OH� (4)

The regeneration of Fe3þ from the reduction at the cathode

is also important in electro-Fenton:

Fe3þ þ e� / Fe2þ (5)

2. Materials and methods

2.1. Soil samples and analysis

Soil was collected from 0 to 25 cm surface layer of fallow land

in the Me�zica Valley, Slovenia. The Me�zica Valley has been

exposed to more than 300 years of active lead mining and

smelting. Soils in the valley, including 6600 ha of agricultural

land, are polluted primarily with Pb but also with Zn and Cd.

For standard pedological analysis, the pH in soils was

measured 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 by modified WalkleyeBlack titrations (ISO 14235.

Switzerland, 1998), cation exchange capacity (CEC) by the

ammonium acetate method (Rhoades, 1982) and soil texture

by the pipette method (Fiedler et al., 1964).

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 8 2001

2.2. Soil washing

To obtain the washing solution for electrochemical treatment,

we placed 0.5 kg of air-dried soil and 875 mL of aqueous

solution of 15 mmol EDTA (di-sodium salt) per kg of soil

(8.6 mM EDTA) in 1.5 L flasks. The soil was extracted on

a rotating shaker (3040 GFL, Germany) for 24 h at 17 RPM and

separated from the washing solution by centrifugation at

2113g for 6min. Fine particles were removed from the solution

by filtration (filter paper density was 80 g m�2). To get a more

concentrated solution, we placed 0.5 kg of air-dried soil and

875 mL of solution obtained after extraction of the first soil

batch, in 1.5 L flasks. Soil was extracted on a rotating shaker,

the washing solution separated and filtrated as described

above.

2.3. Electrochemical treatment of the soil washingsolution

The electrolytic cell consisted of a BDDA, graphite and Fe

anode, each separately placed between two stainless steel

cathodes (distance¼10 mm), the overall anode surface was

45 cm2 and the surface area ratio between the cathodes and

anode 1:1. The electrodes were placed in 500 mL of magneti-

cally stirred soil washing solution in a 750 mL flask (Fig. 1).

Current and current density were kept at 3 A and 67mA cm�2,

respectively, and the cell voltage measured with a DC power

supply (Elektronik Invent, Ljubljana, Slovenia). The electrode

cell was cooled using a cooling mantle and tap water to keep

the temperature of the treated washing solution below 35 �C.The contact time of electrochemical treatment was calculated

as the ratio of the electrode cell volume of the washing solu-

tion multiplied by the operation time (initially 30 min of

operation time equalled 3.78 min of contact time). During the

process, the pH of the washing solution was left unregulated

or regulated to pH 3 by drop-wise addition of H2SO4. In the

second process, 3 g L�1 H2O2 was added to 500 mL of

magnetically stirred soil washing solution and the

Fig. 1 e Scheme of the electrolytic bath.

electrochemical treatment carried out with parameters as

described above. Samples (20 mL) of washing solution were

collected periodically and pH and EC measured immediately.

Samples were afterwards centrifuged at 2113g for 5 min and

supernatant stored in the cold for further analysis of metals

and EDTA. At the end of the electrochemical treatment, the

cathodes were etched with 30mL of 65% HNO3 to dissolve and

later measure the concentration of electro-deposited metals.

The anodes were weighed before and after treatment of the

washing solution to determine the amount of electro-

corroded anodes. To keep the voltage near 8 V and reduce

the power consumption, we applied small amounts of NaCl as

electrolyte when the voltage increased over the set value.

2.4. EDTA determination

The concentration of EDTA was determined spectrophoto-

metrically according to the procedure of Hamano et al. (1993).

Themethod involves the reaction of EDTA inwashing solution

with Fe3þ under acidic conditions to produce Fe-EDTA chelate

(trans-complexation), followed by the removal of excess of

Fe3þ by chelate extraction in the aqueous phase, using chlo-

roform and N-benzoyl-N-phenylhydroxylamine and the

formation of a chromophore with 4,7-diphenyl-1,10-

phenanthroline-disulfonic acid. Using a spectrophotometer,

absorbance was measured at 535 nm against a blank solution

with the 4,7-diphenyl-1,10-phenanthroline-disulfonic acid

replaced with an equal volume of distilled water. The limit of

EDTA quantification was 20 mg L�1.

2.5. Metal determination

Air-dried soil samples (1 g) were ground in an agate mill,

digested in aqua regia (28 mL), diluted with deionized water up

to 100mL, and Pb, Zn and Cd analyzed by flame (acetylene/air)

AAS with a deuterium background correction (Varian,

AA240FS). Metals in solutionwere determined by ASS directly.

A standard reference material used in inter-laboratory

comparisons (Wepal 2004.3/4, Wageningen University,

Wageningen, The Netherlands) was used in the digestion and

analysis as part of the QA/QC protocol. The limits of quanti-

fication (LQ) were 0.1, 0.01 and 0.02 mg L�1 for Pb, Zn and Cd,

respectively. Reagent blank and analytical duplicates were

also used where appropriate to ensure accuracy and precision

in the analysis.

2.6. Statistics

The Duncan multiple range test was used to determine the

statistical significance (P < 0.05) among different treatments.

The computer program Statgraphics 4.0 for Windows was

used.

3. Results and discussion

3.1. EDTA soil extraction

The following pedological properties of the soil used in the

experiment were measured: pH 7.0, organic matter 7.2%, CEC

0 5 10 15 20 25

Contact time (min)

0

100

200

300

400

500

600

Pb

(m

g L

-1)

BDDA

Graphite

Fe

0

40

80

120

160

Zn

(m

g L

-1)

0

1

2

3

4

5

6

Cd

(m

g L

-1)

0

10

20

30

40

50

60

70

Fe

(m

g L

-1)

Fig. 2 e Concentrations of Pb, Zn, Cd and Fe in the spent

soil washing solution during electrochemical treatment

using BDDA, graphite and Fe anodes at pH left unregulated.

Error bars represent standard deviation from the mean

value (n [ 3).

20.3mg100 g�1 of soil, sand 38.5%, silt 55.4%, clay 6.1%.The soil

texture was sandy loam. The soil contained 1431 � 39 mg kg�1

Pb, 587 � 14 mg kg�1, Zn and 12.7 � 0.3 mg kg�1 Cd.

Soil extraction with 15 mmol EDTA per kg of dry soil

(8.6 mM) was used to remove the toxic metals: Pb, Zn and Cd.

The molar ratio between the concentration of toxic metals

initially present in the soil and EDTA applied in the soil

washing solution was 1:2.3. It is known that, after soil

washing, only part of the EDTA is complexed with toxic

metals, the rest remains in differently protonated forms

depending on solution pH (Wong et al., 1997), or complexed

with major soil cations. EDTA forms stable complexes with

soil iron, with a stability constant of complex formation (log

Ks) ¼ 14.3 and 25.0 (at 25 �C and ionic strength, m ¼ 0.1) for Fe2þ

and Fe3þ, respectively, and with Ca (log Ks ¼ 10.7) and Mg (log

Ks ¼ 8.7) (Martell and Smith, 2003).

Tomake better use of the uncomplexed part of the EDTA in

the soil washing solution and to prepare waste washing

solution with more concentrated toxic metals, we applied the

same solution twice. After extraction of the fresh soil,

a second extraction of fresh soil with the washing solution

was therefore made, with 45 and 24% of Pb, 29 and 16% of Zn

and 49 and 27% of Cd being removed from the soil after the

first and second extraction, respectively. The lower

percentage of Zn removed from the soil compared to the

removal of Pb and Cd can be only partly explained by the lower

log Ks of ZneEDTA (16.5), (Martell and Smith, 2003) compared

to the log Ks of PbeEDTA (18.0). Zn was presumably more

difficult to remove since 47% of the Zn in the soil from this

contaminated site was associated with the residual soil frac-

tion of the Tessier’s sequential extraction procedure (Udovic

et al., 2007) and therefore not accessible for extraction even

under strong acidic, reducing or oxidating conditions.

Before treatment in the electrolytic cell, the concentrations

of toxic metals: Pb, Zn, Cd and major soil cations: Fe, Ca and

Mg in the spent soil washing solution were 566 mg L�1

(2.7 mM), 152 mg L�1 (2.3 mM), 5.5 mg L�1 (0.05 mM),

23.1 mg L�1 (0.41 mM), 229 mg L�1 (5.7 mM), and 52.9 mg L�1

(2.2 mM), respectively. The concentration of EDTA was

2809 mg L�1 (7.6 mM), which indicates that the EDTA was

completely complexed with the metals after the second soil

extraction and that some metals were present in the washing

solution in other forms, i.e., complexedwith dissolved organic

matter. Twelve% of the initial EDTA was retained in the soil

after extractions. EDTA and metal complexes that are formed

during soil extraction are absorbed by soil minerals, especially

crystalline iron oxides (Nowack and Sigg, 1996). The pH of the

solution was 8.15.

3.2. Electrochemical treatment using EAOP

We first treated the waste soil washing solution in an elec-

trolytic cell with BDDA, graphite and Fe anodes, without pH

regulation or the addition of chemical. As shown in Fig. 1,

using BDDA and graphite was equally effective in the removal

of Pb. Pb was also the toxic metal that was present in the

highest concentration in the waste solution. Graphite seemed

slightly more efficient for Zn and BDDA for Cd removal. Both

electrochemical systems quickly removed Fe from the solu-

tion. The system with the Fe anode was less efficient,

Table 1 e Initial and final pH and initial and average electro-conductivity (EC) of the solution, amount of electrolyte (NaCl)added to the system to control voltage, percentage of consumed anode material (Dm), specific consumption of the anodematerial relative to themass of removed toxic metal and percentage of Pb, Zn and Cd removed by electrodeposition on thecathode during and after electrochemical treatment of the spent soil washing solution with BDDA, graphite and Fe anodesunder four different conditions: (i) pH left unregulated, (ii) pH left unregulated and the addition of H2O2, (iii) pH regulated tothe value 3 and (iv) pH regulated to the value 3 and H2O2 addition. Standard deviation from the mean value (n [ 3) wascalculated where this was applicable.

Electrode/treatment

pHinitial

[/]pHfinished

[/]ECinitial

[mS]ECaverage

[mS]NaCladded[g L�1]

Dm electrodes[%]

Specificconsumptions

[g g�1]

Electrodeposited metals [%]

Pb Zn Cd

BDDA

pH unregulated 8.2 7.5 1.6 11.9 � 4.1 1.5 � 0.3 0.31 � 0.05a,3 0.0004a,1 99 � 1a,2 96 � 1b,2 99 � 1b,2

H2O2 8.2 8.1 1.6 16.3 � 4.5 1.5 � 0.2 0.12 � 0.05a,1 0.0004a,1 96 � 1ab,1 96 � 1c,2 97 � 1b,1

pH 3 3.0 3.0 2.8 15.9 � 4.5 2.6 � 0.1 0.14 � 0.05a,1 0.0003a,1 97 � 1b,1 91 � 1a,1 97 � 1b,1

pH 3, H2O2 3.0 3.0 2.1 10.6 � 6.9 2.5 � 0.1 0.22 � 0.05a,2 0.0003a,1 96 � 1a,1 92 � 1b,1 97 � 1a,1

Graphite

pH unregulated 8.2 7.9 1.3 4.9 � 3.1 1.4 � 0.2 3.0 � 0.1c,12 0.0009a,1 99 � 1a,2 97 � 1b,2 99 � 1b,2

H2O2 8.2 6.8 1.7 7.6 � 4.1 1.6 � 0.2 3.8 � 0.2b,2 0.0009a,1 96 � 1a,1 91 � 1b,1 98 � 1b,2

pH 3 3.0 3.0 2.1 12.6 � 3.6 1.6 � 0.1 2.7 � 0.1b,1 0.0009b,1 96 � 1b,1 97 � 1b,2 98 � 1c,12

pH 3, H2O2 3.0 3.0 2.0 10.4 � 3.5 1.6 � 0.3 2.9 � 0.2b,2 0.001b,2 96 � 1a,1 90 � 1a,1 99 � 1b,2

Fe

pH unregulated 8.2 9.8 1.7 10 � 3.7 1.9 � 0.3 2.7 � 0.1b,1 0.020b,2 96 � 1a,2 92 � 1a,2 90 � 1a,2

H2O2 8.2 12.0 1.7 8.5 � 4.9 1.7 � 0.2 2.9 � 0.1b,1 0.020b,3 97 � 1b,3 89 � 1a,1 89 � 1a,1

pH 3 3.0 3.0 2.1 10.9 � 4.6 1.4 � 0.1 2.7 � 0.2b,1 0.019c,1 94 � 1a,1 97 � 1b,4 91 � 1a,3

pH 3, H2O2 3.0 3.0 2.1 11.9 � 5.1 1.3 � 0.2 3.1 � 0.2c,1 0.017c,1 99 � 1b,4 93 � 1b,3 97 � 1a,4

a,b,cdenote statistical differences within the group of anode materials, Duncan test ( p < 0.05).1,2,3,4denote statistical differences within the group of treatment conditions, Duncan test ( p < 0.05).

0

2

4

6

8

0 5 10 15 20 25

Contact time (min)

Resid

ual ch

elatin

g activity

(m

M)

BDDA

Graphite

Fe

Fig. 3 e Residual chelating activity after EDTA degradation

in the spent soil washing solution during electrochemical

treatment using BDDA, graphite and Fe anodes at pH left

unregulated. Error bars represent standard deviation from

the mean value (n [ 3).

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 8 2003

particularly in Pb removal (Fig. 2). During the treatment, the

concentration of Fe in the solution first decreased due to

electrodeposition on the cathode, then started to increase due

to Fe electro-corrosion of the anode and then again decrease

due to the formation and precipitation of insoluble Fe-

hydroxides. The pH of the solution at the breaking point of

Fe concentration and precipitation (after 10 min of contact

time), (Fig. 2) was 9.5 and then increased to a final 9.8 (Table 1).

Metals (M) were removed from the treated washing solution

almost completely by electrodeposition on the stainless-steel

cathode (Table 1):

M2þ þ 2e� / M(s) (6)

Evidently the use of stainless steel for cathode material

permitted efficient electrodeposition of metals, despite the

low hydrogen over-potential of the stainless steel, which

favours hydrogen evolution at the cathode over metal reduc-

tion (Huang et al., 2000).

In relation to the EDTA degradation pathway in the elec-

trolytic cell, Johnson et al. (1972) reported anodic oxidation of

EDTA into CO2, formaldehyde (CH2O) and ethylenediamine

(C2H4(NH2)2), which is a precursor for EDTA synthesis and is

itself a well-known chelating agent. We did not assess the

degradation products analytically and the spectrophotometric

method we used for EDTA determination is sensitive not only

to EDTA but also to other chelating agents. We therefore

measured and expressed EDTA degradation as the molar

residual chelating activity.

In terms of EDTA degradation and removal of residual

chelating activity from the waste soil washing solution,

electrochemical systems with BDDA and a graphite anode

were equally effective, while the system with an Fe anode

lagged behind (Fig. 3). The graphite anode otherwise shows

smaller values of over-potential of O2 evolution than BDDA

(Chen, 2004). This means that, with increasing current

density, increased production of O2 and hence a decrease in

anodic oxidation of EDTA and residual chelating activity could

occur. With BDDA, the effect of O2 evolution side reactions are

0

100

200

300

400

500

600

pH unregulated

H2O2

pH 3

pH 3, H2O2

0

40

80

120

160

0

1

2

3

4

5

6

0

5

10

15

20

25

0 5 10 15 20 25Contact time (min)

Fe (

mg

L

-1)

Pb

(m

g L

-1)

Zn

(m

g L

-1)

Cd

(m

g L

-1)

Fig. 4 e Concentrations of Pb, Zn, Cd and Fe in the spent

soil washing solution during electrochemical treatment

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 82004

minimal and efficient anodic oxidation of various organic

compounds has been reported (Chen, 2004; Chen et al., 2003)

to take place on its surface at current densities similar or

higher than the one we used (67mA cm�2). On the other hand,

we measured a significant decline in the removal efficiency of

chelating activity from the soil washing solutions when we

used a graphite anode at lower current densities (data not

shown). The removal of EDTA and residual chelating activity

may in part be attributed to chlorine (Cl2), hypochlorite (HOCl)

and also chlorohydroxyl radicals (�HOCl), which are strong

oxidants (Gotsi et al., 2005) and could be generated anodically

following the additions of NaCl into the electrolytic chamber

(Table 1) in order to control voltage (the operational range was

7.1e7.7 V) and power consumption. The contribution of

chloride-based radical oxidation or direct anodic oxidation by

active oxygen species might thus reduce the difference in the

EAOP performance of BDDA and graphite anodes. For

example: Serikawa et al. (2000) observed a strong catalytic

effect of chloride ion in the electrochemical system in the

mineralization of organic pollutants in waste water to CO2. In

addition, electrochemical processes have also been applied for

disinfection of chloride-containing swimming-pool water. A

much higher disinfection performance against bacteria in

comparison to directly added hypochlorite has been observed

(Troster et al., 2004).

During the process with BDDA and graphite anodes, the pH

of the solution slightly decreased, while with Fe anode, the

electrochemical system generated enough OH� and the pH

increased by almost two pH units during the treatment time

(Table 1) e hence the precipitation of Fe-hydroxides (Fig. 2).

The electro-conductivity of the washing solution increased

but fluctuated frequently during the treatment in all three

electrochemical systems, indicating a complexity of electro-

chemical reactions. As expected, due to the Fe electro-

positivity (the standard electrode oxidation potential of the

Fe/Fe2þ couple is 0.44 V; Evangelou, 1998) substantial electro-

corrosion of Fe was measured in the system with an Fe

anode. The percentage of graphite consumed from the anode

was at the approximately the same rate (Table 1). However,

the specific consumption of graphite relative to the mass of

removed Pb, Zn and Cd was 0.0009 g g�1, much lower than the

specific consumption of Fe 0.02 g g�1. A slight loss of material

was also measured from the BDDA (Table 1), which was

otherwise declared to be chemically inert, even under extreme

conditions (Kraft et al., 2003).

The efficiency of BDDA and graphite anodes for the treat-

ment of the spent soil washing solution was almost identical.

After treatment, the discharge solutions were clear and

almost colourless, with 0.7 and 0.1 mg L�1 of Pb, 1 and

1.3mg L�1 of Zn, 0.3 and 1.2mg L�1 of Cd and 0.57 and 1.05mM

of the residual chelating activity, respectively for the systems

with BDDA and graphite anodes.

with a BDDA anode under four different conditions: (i) pH

left unregulated, (ii) pH left unregulated and the addition of

H2O2, (iii) pH regulated to the value 3 and (iv) pH regulated

to the value 3 and H2O2 addition. Error bars represent

standard deviation from the mean value (n [ 3).

0

100

200

300

400

500

600P

b(m

g L

-1)

pH unregulated

H2O2

pH 3

pH 3, H2O2

0

40

80

120

160

Zn

(m

g L

-1)

0

1

2

3

4

5

6

Cd

(m

g L

-1)

0

5

10

15

20

25

0 5 10 15 20 25

Contact time (min)

Fe (m

g L

-1)

Fig. 5 e Concentrations of Pb, Zn, Cd and Fe in the spent

soil washing solution during electrochemical treatment

with a graphite anode under four different conditions: (i)

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 8 2005

The feasibility of EAOP with BDDA for spent soil washing

solution was also evaluated by Finzgar and Lestan (2008), with

similar reported EDTA (residual chelating activity) and metal

removal dynamics and treatment efficiency: final 0.47 mg L�1

Pb, 1.03mg L�1 Zn, below the limits of quantification of Cd and

0.02 mM of residual chelating activity was achieved. Kraft

et al. (2003) used BDDA to treat industrial waste water elec-

trochemically, with a higher EDTA concentration than in soil

washing solutions. They reported efficient degradation, even

at a current density as low as 7.5mA cm�2. However, the EDTA

in their case was in non-complexed, acidic form.

3.3. Electrochemical treatment using electro-Fenton

In an attempt to improve the performance of tested EAOP

systems, we tried to induce an electro-Fenton-like type of

reaction by H2O2 addition and pH regulation. H2O2 can be

added in electro-Fenton either externally or formed in situ by

the reduction of the dissolved O2 on the surface of cathode. In

the latter case, the cathode is a working electrode, made from

platinum (which is expensive) or graphite. A high usage of O2

for H2O2 production has also been obtained through a gas

diffusion cathode made of porous carbon-

polytetrefluorethylene (Brillas et al., 1996).

Since the treatment of waste soil washing solutions

requires not only oxidative degradation of EDTA complexes

but also removal of released toxic metals, we used external

H2O2.addition and a stainless steel cathode for metal electro-

precipitation. The applied H2O2 concentration (3 g L�1) was

reported as optimal for electro-Fenton by Mert et al. (2010).

The pH is another important factor for electro-Fenton

processes. Jiang and Zhang (2007) reported that an acidic pH

was optimal, with values in the range between pH 2 and 4.

As shown in Figs. 4 and 5, the addition of H2O2 and pH

control to pH 3 did not improve Pb removal from thewaste soil

washing solution using the systems with BDDA and graphite

anodes. A slight improvement was observed only for the

system with an Fe anode (Fig. 6). Furthermore, significantly

less Zn (and in the system with an Fe anode also less Cd) was

removed from all systems when the washing solution was

treated at pH 3 (Figs. 4e6). In all electrochemical systems

tested, Pb, Zn and Cd were removed from the treated solution

by electrodeposition on the cathode (Table 1).

In electrochemical systems with BDDA and graphite

anodes and regulated to pH 3, the rate of Fe removal from the

washing solution was significantly slower than from pH

unregulated systems (Figs. 4 and 5). In pH unregulated

systemswith an Fe anode, the Fe concentration in the solution

also decreased during the treatment (Fig. 6) due to the alkaline

conditions in the cell (Table 1) followed by precipitation of Fe-

hydroxides. The system with an Fe anode and pH 3, however,

produced increasing concentration of Fe ions in the solution,

by electro-corrosion of the (sacrificial) Fe anode. Due to the

high Fe concentration, this was the electrochemical system in

pH left unregulated, (ii) pH left unregulated and the

addition of H2O2, (iii) pH regulated to the value 3 and (iv) pH

regulated to the value 3 and H2O2 addition. Error bars

represent standard deviation from the mean value (n [ 3).

0

100

200

300

400

500

600P

b(m

g L

-1)

pH unregulated

H2O2

pH 3

pH 3, H2O2

0

40

80

120

160

Zn

(m

g L

-1)

0

1

2

3

4

5

6

Cd

(m

g L

-1)

0

1000

2000

3000

4000

5000

6000

0 5 10 15 20 25

Contact time (min)

Fe (

mg

L

-1)

Fig. 6 e Concentrations of Pb, Zn, Cd and Fe in the spent

soil washing solution during electrochemical treatment

with an iron anode under four different conditions: (i) pH

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 82006

which, after the addition of H2O2, we expected to induce most

of the electro-Fenton type reactions and improve EDTA

oxidative degradation. However, removal of residual chelating

activity from thewashing solution in this systemwas not only

less efficient than in the systems with BDDA and graphite

anodes but, after a certain treatment time, the chelating

activity in the treated solution started to increase again,

almost up to the initial EDTA concentration (Fig. 7). A possible

explanation of why electro-Fenton with an Fe anode might be

less efficient than ordinary EAOP with BDDA and graphite

anodes, is undesired side reactions consuming the key

reagent in oxidative degradation, hydroxyl radicals. The

excess Fe2þ can react as follows (Chitra et al., 2003; Pozzo et al.,

2005).

Fe2þ þ �OH / FeOH2þ (7)

Unwanted reactions of H2O2 could also lead to a consider-

able loss of hydroxyl radicals:

H2O2 þ �OH / �O2H þ H2O (8)

In Eq. (8), the �O2H is also an oxidant, but less effective than�OH (Pozzo et al., 2005).

The initial (and temporary) removal of chelating activity

from the washing solution treated in the system with an Fe

anode at pH 3 (and the possible reason for the V-shape curve,

Fig. 7), may be the precipitation of EDTA. EDTA is known to be

poorly soluble in acidic media below pH 3 and to precipitate in

its protonised (H4EDTA) form (Allen and Chen, 1993; Wong

et al., 1997). Additional precipitation of EDTA could be

further promoted by the formation of a low pH condition near

the anode, due to electrode polarisation (Gyliene et al., 2004).

Later in the process, the increasing Fe concentration after

electro-corrosion of the Fe anode (Fig. 6) presumably resulted

in the formation of soluble FeeEDTA complexes (Fe forms

strong complexes with EDTA in acidic conditions; Lim et al.,

2005). In other tested electrochemical systems, EDTA precip-

itation was presumably prevented by efficient EDTA degra-

dation. Johnson et al. (1972) also reported that, with a Pt anode

and similar acidic sulphate solution, EDTA was anodically

oxidized into many compounds, including CO2. Jiraroj et al.

(2006) investigated the effect of pH in EDTAePb degradation

with H2O2/UV AOP and also found a faster degradation rate

when the starting pH was acidic (pH 3).

Preservation of EDTA and simultaneous removal of Pb (Zn

and Cd were removed less efficiently) in the electrochemical

system with an Fe anode regulated to pH 3 indicated the

possibility of a novel EDTA recycling method. Although Juang

and Wang (2000) concluded that simultaneous recovery of

metals and complexing agent from their mixture was not

feasible using a conventional, single compartment electrolytic

cell, our result indicates the contrary; EDTA recovery in such

conditions is possible when an Fe anode is used. Presumably,

left unregulated, (ii) pH left unregulated and the addition of

H2O2, (iii) pH regulated to the value 3 and (iv) pH regulated

to the value 3 and H2O2 addition. Error bars represent

standard deviation from the mean value (n [ 3).

0

2

4

6

8

pHunregulated

H2O2

pH 3pH 3, H2O2

0

2

4

6

8

Re

sid

ua

l c

he

la

tin

g a

ctiv

ity

(m

M)

0

2

4

6

8

0 5 10 15 20 25

Contact time (min)

BDDA

Graphite

Fe

Fig. 7 e Residual chelating activity after EDTA degradation

in the spent soil washing solution during electrochemical

treatment with BDDA, graphite and Fe anodes under four

different conditions: (i) pH left unregulated, (ii) pH left

unregulated and the addition of H2O2, (iii) pH regulated to

the value 3 and (iv) pH regulated to the value 3 and H2O2

addition. Error bars represent standard deviation from the

mean value (n [ 3).

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 8 2007

Fe is oxidized at the anode (Eq. (5)) preferentially to EDTA

oxidation, due to the relatively high Fe reactivity (electro-

positivity).

Could this result lead to the novel EDTA recycling method?

Our further preliminary study indeed indicated that, after

further alkalinisation with NaOH to pH > 10 to replace the Fe

in the EDTA complex with Na and to precipitate insoluble Fe-

hydroxides, the recycled Na2eEDTA had the same soil metal

extraction power as freshly prepared EDTA solution, with

minimal EDTA loss during the recycling process.

4. Conclusions

The efficiency of toxic metal removal and EDTA degradation

in waste soil washing solutions by EAOP depends on many

parameters. In this paper, the type of electrode material,

reactions involved, the addition of H2O2 and pH regulation

were studied. pH unregulated electrochemical systems with

BDDA and graphite anodes removed metals and chelating

agent from the solution most effectively. Since NaCl was

added as an electrolyte to reduce electricity consumption, the

contribution of chlorine and hypochlorite oxidation, as well as

direct EDTA oxidation at the anode, may reduce the expected

difference in EAOP performance of the BDDA and graphite

anodes. In the end, however, due to the relatively low price of

graphite sheets (48.7 $ kg�1 from a USA distributor) and

despite the relatively high graphite consumption rate,

graphite would be our preferred choice for anodic material.

Interestingly, we observed preservation of chelating activity

(presumably of EDTA) in the electrochemical system with an

Fe anode operated at pH 3, while toxic metals were removed

from the treated washing solution. This result indicates the

possibility of EDTA recycling. To recycle the chelating agent

could significantly reduce the cost of EDTA-based soil reme-

diation and there is currently no practical and commercially

available method. The feasibility of the novel EDTA recycling

method, using simple single-chamber electrochemical system

with Fe anode, will be examined in our further studies.

Acknowledgement

This work was supported by the Slovenian Research Agency,

Grant L1-2320.

r e f e r e n c e s

Allen, H.E., Chen, P.H., 1993. Remediation of metal contaminatedsoil by EDTA incorporating electrochemical recovery of metaland EDTA. Environmental Progress 12, 284e293.

Brillas, E., Mur, E., Casado, J., 1996. Iron (II) catalysis of themineralization of aniline using a carbon-PTFE O2-fed cathode.Journal of the Electrochemical Society 143, 49e53.

Chen, G., 2004. Electrochemical technologies in wastewatertreatment. Separation and Purification Technology 38, 11e41.

Chen, X., Chen, G., Gao, F., Yue, P.L., 2003. High performance Ti/BDDA electrodes for pollutant oxidation. Journal ofEnvironmental Science and Technology 37, 21e26.

Chitra, S., Paramasivan, K., Sinha, P.K., Lal, K.B., 2003. Treatmentof liquid waste containing ethylenediamine tetraaceticacid byadvanced oxidation processes. Journal of Advanced OxidationTechnologies 6, 109e114.

Di Palma, L., Ferrantelli, P., Merli, C., Bianifiori, F., 2003. Recoveryof EDTA and metal precipitation from soil washing solutions.Journal of Hazardous Materials 103, 153e168.

wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 9 9 9e2 0 0 82008

Evangelou, V.P., 1998. Environmental Soil and Water Chemistry.John Wiley & Sons Inc., New York.

Fiedler, H.J., Hoffmann, Fr, Schmiedel, H., 1964. Die Untersuchungder Boden, first ed, Band 1. Theodor Steinkopff, Dresden undLeipzig.

Finzgar, N., Lestan, D., 2006. Heap leaching of Pb and Zncontaminated soil using ozone/UV treatment of EDTAextractants. Chemosphere 63, 1736e1743.

Finzgar, N., Lestan, D., 2008. The two-phase leaching of Pb, Zn andCd contaminated soil using EDTA and electrochemicaltreatment of the washing solution. Chemosphere 73,1484e1491.

Gotsi, M., Kalogerakis, N., Psillakis, E., Samaras, P.,Mantzavinos, D., 2005. Electrochemical oxidation of olive oilmill wastewaters. Water Research 39, 4177e4187.

Gyliene, O., Aikaite, J., Nivinskiene, O., 2004. Recovery of EDTAfrom complex solution using Cu(II) as precipitant and Cu(II)subsequent removal by electrolysis. Journal of HazardousMaterials 116, 119e124.

Hamano, T., Mitsuhashi, Y., Kojima, N., Aoki, N., Shibata, M.,Ito, Y., Oji, Y., 1993. Sensitive spectrophotometric method forthe determination of ethylenediaminetetraacetic acid infoods. Analyst 118, 909e912.

Huang, C.P., Hsu, M.-C., Miller, P., 2000. Recovery of EDTA frompower plant boiler chemical cleaning wastewater. Journal ofEnvironmental Engineering e ASCE 126, 919e924.

ISO 14235, 1998. Soil Quality e Determination of Organic Carbonby Sulfochromic Oxidation. International Organization forStandardization, Geneve, Switzerland.

Jiang, C.-C., Zhang, J.-F., 2007. Progress and prospect in electro-Fenton process for wastewater treatment. Journal of ZhejiangUniversityeScience A 8, 1118e1125.

Jiraroj, D., Unob, F., Hagege, A., 2006. Degradation of PbeEDTAcomplex by H2O2/UV process. Water Research 40, 107e112.

Johnson, J.W., Jiang, H.W., Hanna, S.B., James, W.J., 1972. Anodicoxidation of ethylenediaminetetraacetic acid on Pt in acidsulphate solution. Journal of the Electrochemical Society 119,574e580.

Juang, R.-S., Wang, S.-W., 2000. Metal recovery and EDTArecycling from simulated washing effluents of metal-contaminated soils. Water Research 34, 3795e3803.

Kraft, A., Stadelmann, M., Blaschke, M., 2003. Anodic oxidationwith doped diamond electrodes: a new advanced oxidationprocess. Journal of Hazardous Materials 103, 247e261.

Lestan, D., Luo, C.-l., Li, X.-d., 2008. The use of chelating agents inthe remediation of metal-contaminated soils. EnvironmentalPollution 153, 3e13.

Lim, T.T., Chui, P.C., Goh, K.H., 2005. Process evaluation foroptimisation of EDTA use and recovery for heavy metalremoval from a contaminated soil. Chemosphere 58,1031e1040.

Martell, A.E., Smith, R.M., 2003. NIST Critically Selected StabilityConstants of Metal Complexes; Version 7.0. NIST,Gaithersburg.

Mert, B.K., Yonar, T., Kilic, M.Y., Kestioglu, K., 2010. Pre-treatmentstudies on olive oil mill effluent using physicochemical,

Fenton and Fenton-like oxidations processes. Journal ofHazardous Materials 174, 122e128.

Nowack, B., Sigg, L., 1996. Adsorption of EDTA and metaleEDTAcomplexes onto goethite. Journal of Colloid and InterfaceScience 177, 106e121.

Oliveira, R.T.S., Salazar-Banda, G.R., Santos, M.C., Calegaro, M.L.,Miwa, D.W., Machado, S.A.S., Avaca, L.A., 2007.Electrochemical oxidation of benzene on boron-dopeddiamond electrodes. Chemosphere 66, 2152e2158.

Pociecha, M., Sircelj, H., Lestan, D., 2009. Remediation of Cu-contaminated soil using chelant and EAOP. Journal ofEnvironmental Science and Health Part AeEnvironmentalScience and Engineering 44, 1136e1143.

Pociecha, M., Lestan, D., 2010. Electrochemical EDTA recyclingwith sacrificial Al anode for remediation of Pb contaminatedsoil. Environmental Pollution 158, 2710e2715.

Pozzo, A.D., Ferrantelli, P., Merli, C., Petrucci, E., 2005. Oxidationefficiency in the electro-Fenton process. Journal of AppliedElectrochemistry 34, 391e398.

Pratap, K., Lemley, A.T., 1998. Fenton electrochemical treatmentof aqueous atrazine and metolachlor. Journal of Agriculturaland Food Chemistry 46, 3285e3291.

Rhoades, J.D., 1982. Cation exchange capacity. In: Page, A.,Miller, R.H., Keeney, D.R. (Eds.), Methods of Soils Analysis. Part2: Chemical and Microbiological Properties. Series AgronomyNo. 9. ASA e SSSA, Madison, pp. 149e157.

Serikawa, R.M., Isaka, M., Su, Q., Usui, T., Nishumura, T., Sato, H.,Hamada, S., 2000. Wet electrolytic oxidation of organicpollutants in wastewater treatment. Journal of AppliedElectrochemistry 30, 875e883.

Tejowulan, R.S., Hendershot, W.H., 1998. Removal of trace metalsfrom contaminated soils using EDTA incorporating resintrapping techniques. Environmental Pollution 103, 135e142.

Tezcan Un, U., Ugur, S., Koparal, A.S., Bakur Ogutveren, U., 2006.Electrocoagulation of olive mill wastewaters. Separation andPurification Technology 52, 136e141.

Troster, I., Fryda, M., Herrmann, D., Schafer, L., Hanni, W.,Perret, A., Blaschke, M., Kraft, A., Stadelmann, M., 2002.Electrochemical advanced oxidation process for watertreatment using DiaChem� electrodes. Diamond and RelatedMaterials 11, 640e645.

Troster, I., Schafer, I., Fryda, M., Matthee, T., 2004.Electrochemical advanced oxidation process using DiaChemelectrodes. Water Science and Technology 49, 207e212.

Udovic, M., Plavc, Z., Lestan, D., 2007. The effect of earthworms onthe fractionation, mobility and bioaccessibility of Pb, Zn andCd metals before and after soil leaching with EDTA.Chemosphere 70, 126e134.

Wong, J.S.H., Hicks, R.E., Probstein, R.F., 1997. EDTA-enhancedelectroremediation of metal-contaminated soils. Journal ofHazardous Materials 55, 61e79.

Yamaguchi, Y., Yamanaka, Y., Miyamoto, M., Fujishima, A.,Honda, K., 2006. Hybrid electrochemical treatment forpersistent metal complexes at conductive diamond electrodesand clarification of its reaction route. Journal of theElectrochemical Society 153, 1123e1132.