Journal of Hazardous Materials 262 (2013) 464471
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Journal of Hazardous Materials
jou rn al hom epage: www.elsevier.com/locate/jhazmat
Chelant extraction of heavy metals from contaminated soils using
new selective EDTA derivatives
Tao Zhang a , Jun-Min Liu b , Xiong-Fei Huang a , Bing Xia a ,
Cheng-Yong Su b , Guo-Fan Luo a , Yao-Wei Xu b , Ying-Xin Wu a ,
Zong-Wan Mao c , Rong-Liang Qiu a,d,
a School of Environmental Science and Engineering, Sun Yat-sen
University, Guangzhou 510275, PR Chinab MOE Key Laboratory of
Bioinorganic and Synthetic Chemistry, State Key Laboratory of
Optoelectronic Materials and Technologies, School of Chemistry
andChemical Engineering, Sun Yat-Sen University, Guangzhou 510275,
PR Chinac MOE Key Laboratory of Bioinorganic and Synthetic
Chemistry, School of Chemistry and Chemical Engineering, Sun
Yat-Sen University, Guangzhou 510275, PR Chinad Guangdong
Provincial Key Laboratory of Environmental Pollution Control and
Remediation Technology, Guangzhou 510275, PR China
h i g h l i g h t s
Two EDTA derivatives were synthesized to enhance the selectivity
of chelators. PDTA had the highest stability constants for Cu2+ and
Ni2+ . PDTA had the highest overall selectivity for trace metals
over major cations.
a r t i c l e i n f o
a b s t r a c t
Article history:Received 13 April 2013Received in revised form
30 July 2013Accepted 26 August 2013Available online 1 September
2013
Keywords:EDTA derivatives Potentiometric titration
Cu(II)SelectivityExtraction
Soil washing is one of the few permanent treatment alternatives
for removing metal contaminants. Ethylenediaminetetraacetic acid
(EDTA) and its salts can substantially increase heavy metal removal
from contaminated soils and have been extensively studied for soil
washing. However, EDTA has a poor utilization ratio due to its low
selectivity resulting from the competition between soil major
cations and trace metal ions for chelation. The present study
evaluated the potential for soil washing using EDTA and three of
its derivatives: CDTA (trans-1,2-cyclohexanediaminetetraacetic
acid), BDTA (benzyldiaminete- traacetic acid), and PDTA
(phenyldiaminetetraacetic acid), which contain a cylcohexane ring,
a benzyl group, and a phenyl group, respectively. Titration results
showed that PDTA had the highest stability constants for Cu2+ and
Ni2+ and the highest overall selectivity for trace metals over
major cations. Equi- librium batch experiments were conducted to
evaluate the efcacy of the EDTA derivatives at extracting Cu2+ ,
Zn2+ , Ni2+ , Pb2+ , Ca2+ , and Fe3+ from a contaminated soil. At
pH 7.0, PDTA extracted 1.5 times more Cu2+ than did EDTA, but only
75% as much Ca2+ . Although CDTA was a strong chelator of heavy
metal ions, its overall selectivity was lower and comparable to
that of EDTA. BDTA was the least effective extractant because its
stability constants with heavy metals were low. PDTA is potentially
a practical washing agent for soils contaminated with trace metals
2013 Elsevier B.V. All rights reserved.
1. Introduction
Heavy metal contamination of soils, resulting from rapid indus-
trialization, increased urbanization, modern agricultural
practices, and inappropriate waste disposal methods, has become a
serious problem worldwide [13]. The available remediation
technologies for heavy metal-contaminated soils are mainly divided
into two
Corresponding author at: School of Environmental Science and
Engineering, SunYat-sen University, Guangzhou 510275, PR China.
Tel.: +86 20 84113454;fax: +86 20 84113616.E-mail address:
[email protected] (R.-L. Qiu).
groups: namely immobilization, such as in situ chemical xation,
and separation, such as soil washing. Chelant-enhanced soil wash-
ing is a technology that is potentially useful for the economically
feasible remediation of contaminated soils [48].The chelating agent
ethylenediaminetetraacetic acid (EDTA) and its salts have been
extensively studied for their potential use in soil washing [912].
They have been reported to appreciably increase the dissolution and
mobilization of cationic heavy metals [13,14]. EDTA has low
biodegradability in soil and a high efciency of metal extraction
through the formation of thermodynamically sta- ble and soluble
metalEDTA complexes [15,16]. In addition, recent advances in
recovery and recycling techniques of used EDTA have enhanced its
appeal [17].
0304-3894/$ see front matter 2013 Elsevier B.V. All rights
reserved.http://dx.doi.org/10.1016/j.jhazmat.2013.08.069
Soil properties.
SoilpHOM (%)CEC (cmol kg1 )Sand (%)Silt (%)Clay (%)Cu (mg kg1
)Ni (mg kg1 )Pb (mg kg1 )Zn (mg kg1 )
GD7.35.19.5613110138826507236
466T. Zhang et al. / Journal of Hazardous Materials 262 (2013)
464471
A majority of the literature focused on demonstrating the reme-
diation capabilities of EDTA has found that extraction of heavy
metals was faster and more complete with increased quantities of
added EDTA. Competition between the major cations of the soil
(e.g., Ca2+ , Mg2+ , and Fe3+ ) and minor cations for chelation by
EDTA may be one of the factors affecting the efciency of trace
metal extraction [1823]. As illustrated in previous studies
[24,25], when Ca solubility in calcareous soils is raised, the
effectiveness of EDTA extraction is diminished signicantly,
increasing the cost of reme- diation. For non-calcareous soils, Fe
and Al dissolution may be more crucial, in view of their high
tendency for complexation (i.e., large stability constants).
Excessive addition of chelating agent can cause extensive
dissolution of soil minerals and organic matter, leading to
alteration of soil physical and chemical properties and even dis-
integration of soil structure, which could render the soil unt for
future use for vegetation or construction. Therefore, there has
been a growing need to develop highly selective chelating agents
for the extraction of heavy metal ions from polluted soils.In
previous studies, EDTA has been modied to improve its selectivity
in chelating target metal ions [2628]. Highly selective EDTA
derivatives have a wide range of application in the elds of
analytical chemistry, biology and medicine, as well as in many
industrial processes. In this study, two new EDTA derivatives were
designed and synthesized with the goal of enhancing its selectivity
as a chelating agent. These EDTA derivatives contain a phenyl or
benzyl group directly bonded to the nitrogens of the ethylenedi-
amino group, and thus potentially are more sterically constrained
than the parent compound. The objective of this study was to char-
acterize these EDTA derivatives in aqueous solution and assess
their potential as selective washing agents. Batch experiments were
con- ducted to determine their efcacy in the simultaneous
extraction of trace metal ions and major cations from contaminated
soils and to investigate the extraction mechanisms.
2. Materials and methods
2.1. Soil Characteristics
Soil samples were collected from 0.7 to 1.7 m below the ground
surface at a demolished industrial site in South China, air-dried
atroom temperature (2030 C), and passed through a 2 mm sieve.The
soil properties in Table 1 were the average of three
replicates.Various soil physical and geochemical characterization
tests were carried out. The physical and chemical characteristics
of the soil are shown in Table 1. The metal concentrations in soil
were determined by acid digestion with HFHClO4 HNO3 and induc-
tively coupled plasma optical emission spectrometry (ICP-OES)
measurement (5300DV, Perkin Elmer). Soil pH was determined using a
1:5 soil-to-water ratio and pH meter. The CEC of the sample
was determined by the ammonium acetate/sodium acetate
method[29]. Organic matter content was determined by heating the
dried samples at 350 C for 5 h [30]. The particle size distribution
wasestablished by mechanical sieving, followed by the hydrometer
method [31].
2.2. Synthesis of EDTA derivatives
Phenyldiaminetetraacetic acid (PDTA) was synthesized as illus-
trated in Fig. 1 according to a previously published procedure
[32]. A solution of 1,2-diaminobenzene (1.1 g, 10 mmol), ethyl bro-
moacetate (6.7 mL, 60 mmol), sodium iodide (1.3 g, 8.5 mmol), and
diisopropylethylamine (8.3 mL 50 mmol) in 10 mL acetonitrile was
reuxed under nitrogen for 7 h, then cooled and poured into 50 mL
water. The resulting mixture was extracted with dichloromethane(3
40 mL). The extract was dried and was concentrated to givebrown
oil, which was puried by ash chromatography usingpetroleum
ether/ethylacetate (10:1, v/v) as eluant. The product obtained was
2.1 g (45%) of a white solid (A) (Fig. 1); a mixtureof ethanol,
complex A, and NaOH (14.0 M) were reuxed at 78 Cfor 24 h. The
solution was cooled to room temperature, ethanol was evaporated,
and the residue was washed with 6.0 M HCl solution and dried under
high vacuum to give a white solid (B) (Fig.
1).Benzyldiaminetetraacetic acid (BDTA) was synthesized as illus-
trated in Fig. 2. To a mixture of 1,2-bis(bromomethyl)benzene (1.0
mmol) in 5.0 mL dry CH3 CN was added diethyliminodiacetate (2.0
mmol) and anhydrous K2 CO3 (10 mmol). The resulting reac- tion
mixture was stirred at room temperature for 5 h, then the solids
were ltered off and solvent was removed under vacuum. The residue
was again dissolved in ethyl acetate and washed with water and
saturated NaOH solution, which was puried by ash chromatography
using petroleum ether/ethylacetate (10:1, v/v) as eluant. Light
yellowish oil (A) (Fig. 2) was obtained by evaporation of the
organic phase. Then the residue was dissolved in the solu- tion of
KOH. The mixture was stirred at room temperature for 24 h, and then
resulting mixture was poured into ice water, after whichit was
acidied with concentrated HCl (pH 1) to obtain a whitesolid (B)
(Fig. 2).
2.3. Potentiometric titration
An automatic titrator (Metrohm 702GPD Titrino) coupled to a
Metrohm electrode was used and calibrated according to the Gran
method [33,34]. The electrode system was calibrated with buffers
and checked by titration of HClO4 with NaOH solution(0.01 M). The
thermostated cell contained 25 mL of 1.0 mM species (free lig-
and/metal ligand complex) in aqueous solution with ionic strength
maintained at 0.10 M by potassium nitrate. All titrations were
car-ried out on the aqueous solutions under nitrogen at 298 0.1 K,
and
Fig. 1. Synthesis of PDTA.
Fig. 2. Synthesis of BDTA.
initiated by adding xed volumes of 0.10 M standard NaOH in small
increments to the titrated solution. Triplicate measurements were
performed, for which the experimental error was below 1%. The
titration data were tted from the raw data with the Hyperquad2000
program to calculate the log and the pKa values of
species[3538].
2.4. Batch experiments
In the batch experiments, contaminated soil was mixed with a
measured volume of chelating agent solution in 50-mL polyethyl- ene
tubes. To probe the inuence of EDTA derivative concentration on
metal mobility in the washed soil, every 1.000 g of soil was mixed
with 20 mL chelating agent solution of different concentrations
(0.005 M, 0.01 M, 0.02 M, 0.05 M and 0.1 M) in tubes. The tubes
were agitated using a thermostatic shaker at 180 rpm at room
tempera-ture (25 2 C) for a given time. The suspensions were
centrifugedat 5000 rpm for 10 min and the supernatants were then
lteredthrough a 0.45 m membrane. All solutions were stored in amber
vials at 4 C prior to analysis.Another two sets of batch
equilibrium experiments were con- ducted to study the inuence of
solution pH and liquid-to-soil ratio on metal extraction by the
EDTA derivatives. An initial chelator concentration of 0.02 M was
used in both sets. In the rst set, theliquid-to-soil ratio was 20:1
(i.e., 20 mL g1 soil) while the solutionpH were adjusted using NaOH
or HNO3 to pH values of 3.0, 4.0, 5.0,6.0, 7.0, 8.0, 9.0, 10.0,
11.0, and 12.0. In the second set, the solu- tion pH was kept at pH
6.0 while ve liquid-to-soil ratios of 2.5:1(2.5 mL g1 ), 5:1 (5 mL
g1 ), 10:1 (10 mL g1 ), 20:1 (20 mL g1 ), and40:1 (40 mL g1 ) were
studied. These ratios were achieved using aconstant solution soil
mass of 1.0 g.
2.5. Analytical methods
The 1 H NMR spectra of EDTA derivatives was obtained by
Mercury-Plus 300 1 H Nuclear Magnetic Resonance (VARIAN, USA). We
determined the molecular weight of PDTA and BDTA as 339 and364,
respectively, with electro spray ionization mass spectroscopy
(negative ion mode). The heavy metals of concern (Cu, Ni, Zn and
Pb) and soil component elements (Ca and Fe) were measured by induc-
tively coupled plasma optical emission spectrometry (ICP-OES).
3. Results and discussion
3.1. Acidbase properties of EDTA derivatives
The protonation constants of PDTA and BDTA in water were
determined with a view to assessing their acidbase properties,
since these properties control what species exist in solution at
various pH values. Both PDTA and BDTA have six potential sites that
can bind with a proton, including the two nitrogens of the
ethylenediamino groups and the four carboxyl groups. However, only
three deprotonation events were observed by potentiometry (Table
2). For comparison, Table 2 also includes the literature val- ues
and determined values for EDTA. The agreement of the obtained
pKa values with the protonation constants of EDTA (2.67, 6.16,
and10.26) is acceptable.The pKa1 value reects the acidity of the
carboxyl groups, and the pKa2 and pKa3 values characterize the
basicity of the amino nitrogens. The pKa2 and pKa3 values of CDTA
and BDTA were similar to those of EDTA. The pKa3 value of CDTA was
higher as a result of the electron-donating effect of the
cyclohexane ring attached to the amino nitrogens.The obtained pKa1
value of PDTA (2.79 0.01), correspondingto the protonation of
carboxyl groups, was lower than that ofEDTA (3.05 0.02). The lower
pKa1 value of PDTA indicates that theacidity of the carboxyl groups
was increased due to the electron-withdrawing effect of the
aromatic group.Similarly, PDTA had lower pKa2 and pKa3 values in
comparison with EDTA, indicating that the basicity of the nitrogen
atoms is signicantly lower for PDTA relative to EDTA [39]. This may
be attributed to the lower electronic density of the nitrogens in
the former due to the presence of a phenyl group, since the
electron- withdrawing effect is increased when an aromatic group is
directly bonded to nitrogens of an ethylenediamino group [32].In
total, three deprotonation events were observed for the free
ligands PDTA and BDTA (Fig. 3 and Table 2). At pH < 4.0, the
Fig. 3. Distribution plots of species of the free ligands PDTA
(1.0 mM) and BDTA (1.0 mM).
Table 2Deprotonation constants (pKa ) of EDTA, CDTA, PDTA, and
BDTA.
EquilibriumEDTAaCDTAaEDTAbPDTAbBDTAb
H3 Y /H2 Y2 (pKa1 )2.673.533.05 0.022.79 0.013.56 0.03
H2 Y2 /HY3 (pKa2 )6.166.156.42 0.023.70 0.028.31 0.05
HY3 /Y4 (pKa3 ) 10.26 12.40 10.50 0.03 5.95 0.03 10.00 0.06 a
Literature values.b Determined values.
468T. Zhang et al. / Journal of Hazardous Materials 262 (2013)
464471
percentage of species H3 Y/H2 Y2 was higher for BDTA than
forPDTA, which is consistent with the pKa2 values of BDTA and PDTA.
Similarly the percentage of BDTA existing as the species HY3
(i.e.,with one nitrogen deprotonated) at pH < 5.0 was
approximately zero, in accordance with the high pKa3 values of
BDTA.
3.2. Selectivity of EDTA derivatives
The metalligand (ML) complexation equilibrium constant, KML ,
expresses the ligands afnity toward the target metal (reaction
(I)).
M2+ 2 ML + 2K+ H2 Y 2H + MY (I)
The equilibrium constants of PDTA and BDTA complexes with a
range of metals were determined by pH potentiometric titration,
with the calculated results for the [MPDTA] complex summa- rized in
Table 3. The [MBDTA] complex, however, did not exist in
distribution plots of species, except for CdBDTA(OH), which
demonstrated that MBDTA was not thermodynamically stable (Table 4).
We deduced that the structure of BDTA allowed it to form
Table 3Equilibrium constants (log KML ) of EDTA and derivatives
in aqueous solution.
MetalsEDTAaCDTAaPDTAbPb(II)17.8820.2420.30
0.05Cu(II)18.7021.9224.84 0.07Cd(II)16.3619.8418.18
0.03Zn(II)16.4419.3517.99 0.09Ni(II)18.5220.2023.28
0.05Hg(II)21.5024.7919.73 0.09Ca(II)10.6113.1511.25 0.05 Mg(II)
8.83 11.07 10.41 0.08 a Literature values.b Determined values.
a seven-membered ring complex with most metal ions, but not a
stable chelate compound with a ve-membered or six-membered
ring.Table 3 also shows that PDTA has higher selective capability
for Cu2+ and Ni2+ , relative to Cd2+ , Zn2+ , Ca2+ , and Mg2+ ,
than do the conventional chelators EDTA and CDTA. The differences
in the log KML values for Cu2+ and Ni2+ complexes with PDTA, CDTA,
and
Fig. 4. Extraction efciency of different concentrations of
chelating agents (liquid/solid = 20; contact time, 24 h;
temperature, 25 C).
Table 4Equilibrium constants of MBDTA in aqueous solution at 298
0.1 K.
Species Equilibrium constants
log log KMLCdL 7.41 0.08
Table 5pKtarget and SR values of EDTA and EDTA
derivatives.Chelating agents pKtarget SR EDTA 18.23 1.88CDTA 21.06
1.74PDTA 20.72 2.11CdL(OH) 4.67 0.07 3.57 CaL(OH) 0.35 0.02 MgL(OH)
0.30 0.05 ZnL(OH) 1.28 0.04 NiL(OH) 2.27 0.04 PbL(OH) 1.89 0.02
HgL(OH) 0.90 0.05
The selectivity of chelating agents toward heavy metals can be
quantitatively computed on the basis of the selectivity ratio (SR),
which is dened as: pKtarget CuL(OH)
SR =
pKambient
(3)
EDTA are related to the steric constraints created by the
benzene ring in PDTA and the cyclohexane ring in CDTA [40] that are
not
According to formula (3), a strong chelator will have large
pKtarget (>12) and pKambient values, whereas a strong and selec-
tive chelator toward heavy metals will have a large pKtarget and a
relatively small pKambient , and thus a large SR value (>2)
[42].present in EDTA. Because the radii of Ni2+ and Cu2+ are
similar,
The pKtarget
and SR were calculated and summarized in Table 5,PDTA does not
exhibit good selectivity between the two [41].In evaluating EDTA
derivatives, we determined the equilibrium constants (log KML ) of
PDTA for complexation with six divalent tar- get metals, including
Pb, Cu, Cd, Zn, Ni, and Hg [42], and with two ambient divalent
cations, Ca and Mg.pKPb + pKCu + pKCd + pKZn + pKNi + pKHg
which may serve to guide in the selection of chelators that are
both effective and selective toward the six target metals. The
results demonstrated that CDTA and PDTA were strong chelators, each
having as large a pKtarget (>12) as EDTA. More interestingly,
PDTA, compared with EDTA and CDTA, had a larger SR value (>2),
showing that it has a higher selectivity toward heavy
metals.pKtarget =
pKambient =
(1)6
pKCa + pKMg (2)2
3.3. Comparison of heavy metal removal with different
chelators
3.3.1. Inuence of concentrationEDTA and its derivatives in a
range of concentrations were used to extract the heavy metals Cu,
Ni, Pb, and Zn from a contaminated
Fig. 5. Extraction efciency using chelator solutions with
different liquid/soil ratios (contact time, 24 h; temperature, 25
C).
Fig. 6. Inuence of pH on extraction of metals using chelators
(initial concentration, 0.02 M; liquid/solid = 20; contact time, 24
h; temperature, 25 C).
soil. The extent of removal of the four metals increased with
increasing concentration of the chelators from 0.005 to 0.10 M, but
the rate of increase for most metalchelator combinations was less
steep above a chelator concentration of 0.02 M (Fig. 4). At a
chela- tor concentration of 0.02 M, the extraction of Cu was 62% by
PDTA,59% by CDTA, 48% by EDTA, and 8.6% by BDTA, while at the high-
est chelator concentration (0.10 M), these values increased to
only73%, 65%, 60%, and 10%, respectively. Over the entire
concentra- tion range, addition of PDTA caused the extraction
increase slightly compared with EDTA due to the low concentration
of Ni in the soil, from 4.4% to 15% by PDTA and from 2.2% to 11% by
EDTA, respec- tively. Pb extraction by PDTA increased from 2.0% to
10%, while the corresponding Pb values for EDTA were much higher,
increasing from 8.1% to 21%. Zn showed a trend similar to that of
Pb, with extraction values reaching 13% for PDTA and 28% for EDTA
at the highest chelator concentration. In light of these results, a
chelator concentration of 0.02 M was selected for further
studies.As the chelator concentration was increased, only a small
frac- tion of the added chelator was incorporated into metalchelant
complexes, while the excess remained in the free form or was com-
plexed with other cations (Ca, Mg, Fe, Al, etc.) [43]. Similarly,
when EDTA is used for soil washing, not all EDTA added to soil
binds the target metals, because other ions in the soil such as Ca
and Fe also interact with EDTA due to their high concentrations and
the relatively high stability of their complexes [44]. Above 0.02
M, PDTA was the most efcient of the tested chelators at extracting
Cu from soil. This result can be attributed not only to stronger
chela- tion of Cu by PDTA compared to the other chelators, but also
to the weaker chelation between PDTA and other soil cations (Ca,
Fe, etc.) and heavy metals, as evidenced by the pKML constants of
theircomplexes (Table 3). In fact, PDTA had the second lowest Pb
and
Zn extraction efciencies, only slightly higher than those of
BDTA, which is unable to form a stable chelate compound with a ve-
or six-membered ring.
3.3.2. Inuence of liquid-to-soil ratioAt a constant chelator
concentration of 0.4 mmol g1 soil, theextraction efciency of the
target metals was measured as the liquid-to-soil ratio increased
from 2.5:1 to 40:1. For EDTA and CDTA, increasing the
liquid-to-soil ratio over this range generally increased the
efciency of extraction of Cu (from 21% to 54% by EDTA and from 24%
to 61% by CDTA), Ni (from 0 to 12% by EDTA and from 0% to 10% by
CDTA), Pb (from 8% to 18% by EDTA and from
Fig. 7. Distribution plots of species of 1.0 mM Cu:PDTA as a
function of pH at 0.10 M KNO3 and (298 0.1) K.
Fig. 8. Solubilization of Ca and Fe by EDTA and EDTA derivatives
(a: initial concentration, 0.02 M; liquid/solid = 20; contact time,
24 h; temperature, 25 C. b: initial pH of 6.0;liquid/solid = 20;
contact time, 24 h; temperature, 25 C).
6% to 17% by CDTA), and Zn (from 8% to 17% by EDTA and from 6%
to24% by CDTA), respectively (Fig. 5). However, for PDTA and BDTA,
extraction of the four metals was found to be independent of the
liquid-to-soil ratio.The extraction efciency, as determined by the
mole ratio of extracted potentially toxic trace metals to chelant
inputs [45], increased with increasing liquid-to-soil ratio because
a larger amount of organic matter could be dissolved from a greater
mass of soil [8,43]. In addition, Cu has a high afnity toward
organic mat- ter, and the hydrophobic aromatic groups present in
PDTA may have facilitated soil organic matter dissolution, making
Cu quickly extractable and independent of the liquid:soil
ratio.
3.3.3. Inuence of solution pHAs shown in Fig. 6, extraction
efciency was highest at pH 3.0, and generally decreased as solution
pH was increased from pH 3.0 to pH 12.0. At pH 12.0, 24% of Cu was
removed by PDTA but almost none was removed by EDTA and BDTA.The
important mobilization processes of heavy metals in soil include:
(1) acidication, (2) competitive adsorption of other metal ions or
anions, (3) reductive and non-reductive dissolution of the solid
phase, and (4) complexation of metal ions by ligands [46]. Metals
bound to soil hydrous oxides can often be retrieved simply by
lowering pH because protons can promote oxide dissolution. Hydrogen
ions are also weak competitive cations which can replace the
adsorbed heavy metals via a cation exchange mechanism, andas the H+
ion concentration increases, the particle surface generallybecomes
increasingly protonated and acquires a positive charge, thus
promoting desorption of metals [13]. Therefore this is why heavy
metal extraction is higher at lower values of solution pH. In
addition, the pH-potentiometric titration results showed that the
equilibrium species of Cu at a 1:1 metal:PDTA ratio are pri- marily
ML, ML(OH), and ML(OH)2 (Fig. 7). The concentration of free Cu2+
was very low in aqueous solution, and Cu is almost completely
complexed above pH 3.5. As pH is increased, one water molecule is
deprotonated but remains in the inner coordination sphere of the
complexed Cu2+ , forming a stable chelate compound, which explains
why even at a solution pH of 12.0, 24% of Cu was removed by PDTA.
In practice, however, the pH is normally controlled within the
range of 5.09.0 in order to eliminate adverse changes in the soils
chemical and physical structure brought about by too acidic or
alkaline a solution [14]. As pH increased from 5.0 to 9.0, the
extrac- tion efciency of Cu by PDTA decreased from 72% to 56%, but
the decrease was from 52% to 35% for EDTA. Thus, at constant pH,
PDTA is more efcient at extracting Cu from soil than is EDTA.
3.3.4. Solubilization of major soil mineral elementsThe
solubilization of soil mineral elements (i.e., Ca, and Fe) by EDTA
and its derivatives was also measured (Fig. 8). EDTA solubi- lized
a large amount of Ca from the soil, with values that decreasedfrom
13 400 mg kg1 to 4510 mg kg1 as pH rose from pH 4.5 to pH9.0, while
the amount of Ca solubilized by PDTA was considerably lower (10 500
mg kg1 to 2900 mg kg1 over the same pH range).Similarly, at pH 6.0
and a chelator concentration of 0.05 M, EDTAsolubilized more Fe
than did PDTA, with values of 227 mg kg1 and102 mg kg1 ,
respectively.Some researchers have found that co-dissolution of
soil Ca is an important factor that can result in a low degree of
heavy metal complexation by EDTA [20,21]. Another study showed that
Ca2+ is the main competitive cation, because CaCO3 is strongly
dissolved in the EDTA leaching solution under acidic conditions
[20,25]; thus, concentrations of Ca2+ in the leaching solution are
very high com- pared to those of the target heavy metals. However,
at pH 7.0, PDTA solubilized 25% less Ca and extracted 54% more Cu
from the con- taminated soil than did EDTA (Figs. 6 and 8). These
results can be partly attributed to the low metal selectivity of
EDTA, caus- ing potential chelation of all of the exchangeable
cations present in the soil [18], and partly to the selective
complexing ability of PDTA for Cu. Thus, the minerals present in
soil may not interfere with PDTA extraction of Cu, conrming the
potential applicabil- ity of PDTA for remediation of heavy
metal-contaminated soil containing high concentrations of mineral
elements such as Ca and Fe.
4. Conclusions
Two new EDTA derivatives, BTDA and PDTA, were synthesized and
their metalligand complexation equilibrium constants and selective
capabilities in aqueous media were investigated, along with those
of EDTA and CDTA. Titration results showed that PDTA had the
highest stability constants for Cu and Ni and the highest overall
selectivity for trace metals over major cations. Equilibrium batch
experiments were conducted to evaluate the efcacy of the EDTA
derivatives at extracting Cu, Zn, Ni, Pb, Ca, and Fe from a con-
taminated soil. At pH 7.0, PDTA extracted 1.5 times more Cu than
did EDTA, but only 75% as much Ca. Although CDTA was a strong
chelator of heavy metals, its overall selectivity was lower and
com- parable to that of EDTA. BDTA was the least effective
extractant because its stability constants with heavy metals were
low. PDTA is potentially a practical washing agent for soils
contaminated with trace metals, especially Cu.
Acknowledgements
The project was supported by National Natural Science Foun-
dation (No. 41171374), 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 Provincial
Key Laboratory of Environmental Pollution Control and Remediation
Technology (No. 2011K0007).
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