1-s2.0-S0045653514002434-main

Technical Note
Influence of the selective EDTA derivative phenyldiaminetetraacetic acid
on the speciation and extraction of heavy metals from a contaminated
soil
Tao Zhang
a,b
, Hang Wei
b
, Xiu-Hong Yang
b,c,
⇑
, Bing Xia
b
, Jun-Min Liu
a
, Cheng-Yong Su
a,
⇑
,
Rong-Liang Qiu
b,c
a
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering,
Sun Yat-sen University, Guangzhou 510275, PR China
b
School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China
c
Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, PR China
highlights
PDTA significantly enhanced extraction of Cu from contaminated soil.
Complexation-promoted and dissolution of SOM were the mechanisms for Cu extraction.
High selectivity of PDTA for Cu
2+
avoided unwanted dissolution of soil minerals.
article info
Article history:
Received 21 October 2013
Received in revised form 14 February 2014
Accepted 15 February 2014
Keywords:
Chelating agent
Soil washing
Dissolution
Soil organic matter
Heavy metal speciation
abstract
The development of more selective chelators for the washing of heavy metal contaminated soil is
desirable in order to avoid excessive dissolution of soil minerals. Speciation and mobility of Cu, Zn, Pb,
and Ni in a contaminated soil washed with phenyldiaminetetraacetic acid (PDTA), a derivative of EDTA,
were investigated by batch leaching test using a range of soil washing conditions followed by sequential
extraction. With appropriate washing conditions, PDTA significantly enhanced extraction of Cu from the
contaminated soil. The primary mechanisms of Cu extraction by PDTA were complexation-promoted dis-
solution of soil Cu and increased dissolution of soil organic matter (SOM). PDTA showed high selectivity
for Cu(II) over soil component cations (Ca(II), Mg(II), Fe(III), Mn(II), Al(III)), especially at lower liquid-to-
soil ratios under PDTA deficiency, thus avoiding unwanted dissolution of soil minerals during the soil
washing process which can degrade soil structure and interfere with future land use. PDTA-enhanced soil
washing increased the exchangeable fractions of Cu, Zn, and Pb and decreased their residual fractions,
compared to their levels in unwashed soil.
Ó2014 Elsevier Ltd. All rights reserved.
1. Introduction
The available technologies for remediating heavy metal
-contaminated soils are largely divided into two groups: immobili-
zation (e.g., in situ chemical fixation) and separation, which in-
cludes soil washing (Khodadoust et al., 2004). Soil washing
separates contaminants from the bulk soil in either or both of
the following ways: solubilizing contaminants using chelating
agents or acid, or concentrating contaminants into a small volume
of soil through particle size separation (FRTR, 1994; Maturi and
Reddy, 2008). Among the chelating agents that can be used to sol-
ubilize contaminants, EDTA presents the following advantages: a
low degree of biodegradability in groundwater (Mulligan et al.,
2001) and soil (Sims et al., 1984; Peters, 1999; Abumaizar and
Smith, 1999), and a high capacity for complexing with heavy met-
als (Sims et al., 1984; Tejowulan and Hendershot, 1998; Martinez
and Motto, 2000). Thus, EDTA is a promising washing agent for me-
tal-contaminated sites.
However, a significant part of heavy metals usually remains at
aged sites in washed soils, especially if the soil is rich in organic
matter or clay minerals, inasmuch as these soil solid phases often
http://dx.doi.org/10.1016/j.chemosphere.2014.02.039
0045-6535/Ó2014 Elsevier Ltd. All rights reserved.
⇑
Corresponding authors. Address: School of Environmental Science and
Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Tel.: +86 20
8411 3454; fax: [phone] (X.-H. Yang).
E-mail address: [email protected] (X.-H. Yang).
Chemosphere 109 (2014) 1–6
Contents lists available at ScienceDirect
Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere

have a strong affinity for the target heavy metals (Reddy and
Chinthamreddy, 2000; Pichtel et al., 2001; Finzgar and Lestan,
2007; Zhang et al., 2008; Zou et al., 2009). As reported in previous
studies, in aged contaminated sites where heavy metals are
primarily bound to oxides or organic matter, EDTA-promoted dis-
solution can play a substantial role in the metal removal (Yip
et al., 2009). EDTA-promoted dissolution from oxides occurs in
two steps: fast adsorption of free or complexed EDTA onto specific
surface sites via surface complexation, which can destabilize
metal–oxygen bonds in the mineral structure, followed by rate-
limiting metal detachment from the oxide.
If heavy metals are present in chemically stable mineral forms
or bound to non-labile soil fractions, they are less mobile and less
bioavailable, and hence less toxic. The mobility and bioavailability
of these heavy metals in washed soils may change with different
combinations of washing conditions, which have been neglected
in most research on soil washing with EDTA, although EDTA has
been proven to effectively increase the availability of heavy metals
in the soil phase when being used to enhance the phytoextraction
efficiency of potential metal-accumulators. In fact, enhanced
mobility of heavy metals and nutrient deficiency have been sug-
gested as major concern about EDTA application by technical
meetings of European Union Member State Representatives (ECB,
2004).
At high concentrations, EDTA has been found to dissolve indig-
enous oxides, carbonates, and organic matter, and to appreciably
alter both the physical structure and chemical properties of soils
(Tsang et al., 2007), which could render the soil unfit for future
use for vegetation or construction. However, if too dilute, EDTA is
unable to release the majority of the labile fractions of metals.
Therefore, there is a need to develop highly selective chelating
agents for the extraction of heavy metal ions from polluted soils.
In earlier work comparing the soil washing abilities of EDTA
with three of its derivatives, we showed that phenyldiaminetetra-
acetic acid (PDTA), which contains a phenyl group, had the highest
stability constants for Cu and Ni and the highest overall selectivity
for trace metals over major soil cations (Zhang et al., 2013). To
avoid chelator-induced mental movement into groundwater, there
is a need for further research on how metal mobility and speciation
are influenced by chelating agent concentration and liquid-to-soil
ratio. Therefore, the objective of this study is to investigate the
influences of soil-to-solution ratio and PDTA concentration on
the mobility and speciation of heavy metals in a contaminated soil
subjected to PDTA-enhanced soil washing.
2. Materials and methods
2.1. Materials and soil characteristics
PDTA used in this research was synthesized by our group
according to a previously described method (Wang and Qian,
2006). Pure water obtained using a Milli-Q system was used for
the preparation of all solutions.
The studied soil was collected from 0.7 to 1.7 m below the
ground surface at a demolished industrial site in the north of
Guangzhou city, China, air-dried at room temperature (20–30 °C),
and passed through a 2-mm sieve. The soil was comparable to san-
dy loam (61% of sand, 30% of silt, 9% of clay by mass) according to
particle size distribution obtained by sieving and hydrometer
methods, and organic matter content was determined by heating
the dried samples at 350 °C for 5 h (Ball, 1964). The soil pH was
7.3, measured at a 1:5 soil-to-water ratio. The cation exchange
capacity (CEC) of the soil was 9.5 cmol kg
1
, as determined by
NH
4
–Na exchange (Van Reeuwijk, 1992). The metal concentrations
in soil were determined by acid digestion with HF–HClO
4
–HNO
3
and inductively coupled plasma optical emission spectrometry
(ICP-OES) (5300DV, PerkinElmer). The physical and chemical char-
acteristics of the soil are shown in Table 1.
2.2. Batch experiments
In the batch experiments, 1 g of contaminated soil was mixed
with a measured volume of chelating agent solution at pH 6.0 in
50-mL polyethylene tubes and shaken at 180 rpm in a thermostatic
shaker at room temperature (25 ± 2 °C) for 2 h. To probe the influ-
ence of PDTA concentration and liquid-to-soil ratio on metal
mobility, a range of chelating agent concentrations (0.5–20 mM)
and liquid-to-soil ratios (5:1 to 20:1) was used. Detailed operating
parameters are listed in Table 2. The washing solution and soil
were separated by centrifuging at 5000 rpm for 10 min, and heavy
metals of concern (Cu, Ni, Pb, and Zn) and soil component elements
(Ca, Fe, Mg, Al, and Mn) in the supernatant were measured by ICP-
OES. In order to investigate the effect of the PDTA soil washing
treatments on heavy metal speciation, metal species in the washed
soil were fractionated by a traditional sequential extraction
scheme (detailed extraction reagents and operating conditions
are listed in Table SM-1 in Supplementary Material (SM)). All
experiments were performed in at least triplicate.
3. Results and discussion
3.1. Heavy metal removal by PDTA washing
Fig. 1 shows the removal efficiencies of Cu, Ni, Pb, and Zn for the
different PDTA washing treatments. As a whole, the extraction effi-
ciencies of these metals were low. One potential reason was that
for most of the treatments (Exp. 1–6), the mass of added chelating
agent was less than the stoichiometric requirement for 1:1 metal-
chelating agent complexation which was about 2 mM PDTA at a
20:1 solution:soil ratio. Moreover, the strong bonding between
these metals and soil particles, which are rich in clay minerals
and organic matter in the aged site, can result in low metal
removal.
As shown in Fig. 1, the removal efficiencies of the target metals
except Pb generally increased with decreasing liquid-to-soil ratio.
With the same dosage of chelator, decreasing the liquid-to-soil ra-
tio meant the chelating agent concentration was increased. The use
of a high concentration of chelating agent can increase the extrac-
tion efficiency of trace metals, especially for Cu. In general, the me-
tal-chelating agent complex is the dominant mechanism for most
cationic metal removal, and this process often depends on chelat-
ing agent concentrations, especially when the amount of chelating
agent is insufficient for complete complexation (Peters, 1999). Un-
der PDTA-deficiency conditions (Exp. 1–6), decreasing the liquid-
to-soil ratio from 20:1 to 5:1 increased the extraction efficiency
of Cu from 3% to 5% and from 4% to 9% when the mass of chelating
agent was 0.01 and 0.02 mmol g
1
soil, respectively. Similar results
Table 1
Characteristics of the studied soil.
Soil properties Value
pH 7.3
Organic matter content (%) 5.1
CEC (cmol kg
1
) 9.5
Sand (%) 61
Silt (%) 30
Clay (%) 9
Metal content (mg kg
1
)
Cu 1388
Ni 26
Pb 507
Zn 236
2T. Zhang et al. / Chemosphere 109 (2014) 1–6

have been obtained by varying the solution volume or the soil mass
(Yan et al., 2010). As Cu-PDTA is a relatively strong complex (Zhang
et al., 2013) and there was insufficient PDTA to complex with all
the heavy metals adsorbed by soil particles, the readily extracted
fraction of Zn and Pb were displaced from PDTA via metal ex-
change with adsorbed Cu (Tsang et al., 2009).
With a higher PDTA-to-metal molar ratio (under the operating
conditions of Exp. 7–9), the number of moles of PDTA available
for extracting Cu from the soil increased, resulting in much higher
extraction efficiency of Cu than other metals. Under excessive che-
lating dosage conditions, metal resorption was indiscernible, and
enhanced soil mineral elements (such as Ca and Mg) dissolution
might have influence on chelating agent speciation (Yip et al.,
2009) due to the higher selective capability of PDTA for Cu(II)
(logK
Cu-PDTA
= 24.8), relative to Ca(II), and Mg(II) (logK
Ca-PDTA
= 11.3,
logK
Mg-PDTA
= 10.4). Thus, the effects of liquid-to-soil ratio on metal
extraction were statistically minor under sufficient PDTA.
Previous studies have indicated that polyvalent metals ions may
serve as cross-linking agents within the organic phase by binding
to multiple functional groups from different strands of humic mac-
romolecules (Yang et al., 2001; Tipping, 2002). As a consequence,
SOM (soil organic matter)-bound heavy metals can also be re-
moved from the soil along with the released SOM (McBride et al.,
1997). The quantity of SOM released from the soil by PDTA wash-
ing increased with PDTA concentration when the solution:soil ratio
was held constant (Fig. 2). Expressed as UV adsorption intensity,
the dissolved SOM increased from 0.41 in Exp. 1 (0.5 mM PDTA)
to 3.32 in Exp. 7 (5 mM PDTA). In contrast, the released SOM by
distilled water was 0.039. Since the concentration of SOM is com-
monly found to be linearly correlated to UV absorption (APHA,
1998), it is evident that PDTA greatly enhances the release of
SOM from the soil into solution. Compared to other heavy metals
(e.g. Ni, Pb, Zn), Cu shows a stronger affinity for organic matter
(Tandy et al., 2006), which suggested the dissolution of SOM from
soils is another probable mechanism for Cu extraction by PDTA.
3.2. Dissolution of soil component elements during PDTA washing
The concentrations of soil component elements in solution after
PDTA washing are listed in Table 3. The most significant dissolu-
tion of Ca occurred during PDTA-enhanced soil washing, while
comparatively few amounts of Fe, Mn, Al, and Mg were released.
Similar phenomena have been observed for soil washing with
EDTA (Theodoratos et al., 2000; Kim et al., 2003; Palma and
Mecozzi, 2007; Polettini et al., 2007; Qiu et al., 2010). As chelating
agent concentration is increased, the excess amount might form
complexes with soil component elements, leading to the enhanced
release of Al, Mn, and Fe from soil (Tsang et al., 2007). The amount
of Ca decreased from 1031 to 96 mmol kg
1
when the PDTA con-
centration increased from 0.5 to 5 mM, while the amounts of Fe,
Mn, Al, and Mg solubilized by PDTA increased with increasing con-
centration of PDTA. It has been reported that the Ca complex itself
can mediate Fe and Al dissolution and that this reaction has slower
kinetics than that of Ca dissolution (Nowack and Sigg, 1997). Con-
sequently, Ca can be re-adsorbed on soil particles, while Fe and Al
concentrations in the washing solution slightly increase (Palma,
2009).
Under PDTA deficiency and at a constant dosage of PDTA, the
amount of soil component elements solubilized by PDTA generally
decreased with decreasing liquid-to-soil ratio, especially for Ca,
and competition between Ca and heavy metals for chelation by
PDTA may be present. Ca was dissolved initially due to proton-pro-
moted dissolution in an acidic environment, since the initial pH of
the PDTA solution was 6.0, lower than that of the soil (pH = 7.3).
The dissolved Ca can quickly complex with PDTA (Palma and Ferran-
telli, 2005), but an exchange reaction subsequently occurs between
Ca-PDTA and Cu-PDTA, because the highest overall selectivity for
trace metals over major cations (Zhang et al., 2013). Thus, the
extraction efficiency of heavy metals increased, but the solubiliza-
tion of soil component elements decreased with decreasing li-
quid-to-soil ratio under PDTA deficiency, which could avoid the
destruction or alteration of the soil structure.
3.3. Effect of PDTA washing on metal speciation
The metals in the soils washed in Exp. 1, 4, 5, 6, and 7 as well as
in the unwashed soil were fractionated by sequential extraction to
investigate how metal speciation is affected by PDTA-enhanced
Table 2
Washing combinations of PDTA-enhanced soil washing.
Index Liquid-to-soil ratio PDTA concentration (mM)
Exp. 1 20:1 0.5
Exp. 2 10:1 1.0
Exp. 3 5:1 2.0
Exp. 4 20:1 1.0
Exp. 5 10:1 2.0
Exp. 6 5:1 4.0
Exp. 7 20:1 5.0
Exp. 8 10:1 10.0
Exp. 9 5:1 20.0
Fig. 1. Metal removal among different washing combinations (the operating
conditions of Exp. 1–9 are listed in Table 2).
Fig. 2. Dissolution of soil organic matter during PDTA-washing.
T. Zhang et al. / Chemosphere 109 (2014) 1–6 3

soil washing. The metal fractions are defined in Table SM-1. The re-
sults presented in Fig. 3 are expressed as percentages of the indi-
vidual metal contents in the unwashed soil. This soil was
collected from a former industrial site that had been inactive for
10 year, wherefore most weakly bound exchangeable metals had
been leached or transformed by weathering into more strongly
bound forms, resulting in the very low proportions of all heavy
metals of interest in the exchangeable fractions (S1), especially
those of Ni and Pb. Ni, Pb and Zn in the unwashed soil were primar-
ily bound to Fe/Mn oxides of low crystallinity (S3) or in the residual
fraction (S5). The Cu in the unwashed soil was predominantly in
the most stable fraction (S5), followed by the S4 fraction, the S3
fraction and the S2 fraction, while S1 was negligible.
The low level of metals in S1 was an important reason for the
generally low observed metal extraction efficiencies. In the other
four fractions, metal speciation was found to vary between the dif-
ferent metals, suggesting that there were different mechanisms for
metal retention in soils and metal removal by PDTA.
The higher extraction efficiency of Cu compared to Ni, Pb, and
Zn was likely attributable to the high Cu loading and more Cu in
the weakly bound S2 and S3 fractions. Increased PDTA concentra-
tion generally decreased the S2 of Cu, Pb, and Zn in washed soil.
Decreased liquid-to-soil ratio under PDTA deficiency increased
the S2 of Cu but had little effect on the other metals due to the en-
hanced exchange reaction between Ca-PDTA and Cu-PDTA. When
the metal in S3 fraction is extracted by washing, portions of Fe,
Mn, and Al oxides binding the heavy metals are released into solu-
tion (Table 3). Larger portions of Cu and Ni in the S3 fraction were
substantially reduced than that of Zn by PDTA washing at higher
PDTA concentrations, due to the formation of more stable Cu-PDTA
and Ni-PDTA complexes despite the highest proportion of Zn in S3
fraction.
As illustrated in Fig. 3, the proportion of Cu in the S4 fraction of
washed soil decreased markedly as PDTA concentration increased
or liquid-to-soil ratio decreased due to the enhanced release of
SOM from the soil into the solution (see Fig. 2). The metal fractions
Table 3
Release of soil component elements during PDTA-enhanced soil washing.
Index Concentration of soil componential elements in washing solution (mmol kg
1
)
Fe Mn Al Mg Ca
Exp. 1 0.021 ± 0.004 0.045 ± 0.008 0.065 ± 0.045 3.453 ± 0.096 1030.7 ± 0.3
Exp. 2 0.007 ± 0.006 0.050 ± 0.008 0.006 ± 0.003 3.241 ± 0.041 57.7 ± 0.2
Exp. 3 0.005 ± 0.001 0.030 ± 0.006 0.037 ± 0.004 3.364 ± 0.400 57.4 ± 1.3
Exp. 4 0.055 ± 0.028 0.128 ± 0.015 0.368 ± 0.016 3.803 ± 0.132 953.5 ± 1.4
Exp. 5 0.047 ± 0.018 0.080 ± 0.060 0.060 ± 0.013 3.724 ± 0.087 1019.7 ± 3.7
Exp. 6 0.011 ± 0.003 0.109 ± 0.098 0.072 ± 0.003 3.567 ± 0.934 83.3 ± 0.2
Exp. 7 0.483 ± 0.019 0.241 ± 0.029 0.883 ± 0.029 4.269 ± 0.152 95.6 ± 0.1
Fig. 3. Metal fractions affected by PDTA-enhanced soil washing (the metal percentages are calculated based on their individual contents in the unwashed soil).
4T. Zhang et al. / Chemosphere 109 (2014) 1–6

in S2, S3, or S4 were found to increase for some washing combina-
tions for all heavy metals of interest, indicating that portions of
these metals were redistributed during the washing procedure.
Some fractions were destabilized and readsorbed on the carbon-
ates, Fe/Mn/Al oxides, or SOM and sulfides. The residual metal frac-
tion (S5) was expected to be very stable and cannot generally be
removed or changed. However, in Exp. 7 of this study, where a high
PDTA dosage was employed, the S5 fraction of all four metals was
lower than it was in unwashed soil. Similar results were obtained
in a study of soil washing with EDTA (Lei et al., 2008).
A caveat that must be kept in mind when interpreting the re-
sults of the speciation experiments is that, as has been reported
in previous study (Yong and Mulligan, 2004), the determination
of heavy metal fractions by sequential extraction is only operation-
ally defined, and is generally considered to be more qualitative
than quantitative in providing insight into metal distribution and
mobility. The apparent metal redistribution revealed by sequential
extraction reflects only the increase in the chemical availability of
these metals after PDTA washing.
4. Conclusions
Heavy metal speciation and mobility in a contaminated soil
washed with PDTA, a derivative of EDTA, were investigated by
batch leaching tests using a range of soil washing conditions fol-
lowed by sequential extraction. With appropriate washing condi-
tions, PDTA significantly enhanced extraction of Cu from
contaminated soil. The primary mechanisms of Cu extraction by
PDTA were complexation-promoted dissolution of soil Cu and in-
creased dissolution of SOM. PDTA showed high selectivity for Cu(II)
over soil component cations (Ca(II), Mg(II), Fe(III), Mn(II), Al(III)),
especially at lower liquid-to-soil ratios under PDTA deficiency, thus
avoiding unwanted dissolution of soil minerals during the soil
washing process. PDTA-enhanced soil washing increased the
exchangeable fractions of Cu, Zn, and Pb and decreased their resid-
ual fractions, compared to their levels in unwashed soil.
A cost-effective strategy for the use of this agent would be a
continuous washing system where the contaminants were precip-
itated from the washing solution and the PDTA was reused. Soil
adsorption and biodegradation of PDTA should also be investigated
before this method is used in soil remediation.
Acknowledgements
The project was supported by National Natural Science Founda-
tion (No. 41171374, 21272292), National Funds for Distinguished
Young Scientists of China (No. 41225004), Guangdong Province
Higher Vocational Colleges & Schools Pearl River Scholar Funded
Scheme, the Ministry of Environmental Protection of China (No.
201109020) and the Research Fund Program of Guangdong Provin-
cial Key Laboratory of Environmental Pollution Control and Reme-
diation Technology (No. 2011K0007).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.chemosphere.
2014.02.039.
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