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Journal of Hazardous Materials 190 (2011) 810–815
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Binding characteristics of copper and cadmium by cyanobacterium Spirulina
platensis
Linchuan Fang a , Chen Zhou b , Peng Cai b , Wenli Chen a , Xingmin Rong b , Ke Dai b , Wei Liang b ,
Ji-Dong Gu c , Qiaoyun Huang a,b,∗
a
State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, China
Key Laboratory of Subtropical Agricultural Resources and Environment, Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan
430070, China
c
Department of Ecology & Biodiversity, The University of Hong Kong, Pokfulam Road, Hong Kong, China
b
a r t i c l e
i n f o
Article history:
Received 8 February 2011
Received in revised form 24 March 2011
Accepted 31 March 2011
Available online 7 April 2011
Keywords:
Spirulina platensis
Cu(II)
Cd(II)
Sequestration mechanisms
Chemical modifications
XAFS
a b s t r a c t
Cyanobacteria are promising biosorbent for heavy metals in bioremediation. Although sequestration
of metals by cyanobacteria is known, the actual mechanisms and ligands involved are not very well
understood. The binding characteristics of Cu(II) and Cd(II) by the cyanobacterium Spirulina platensis
were investigated using a combination of chemical modifications, batch adsorption experiments, Fourier
transform infrared (FTIR) spectroscopy and X-ray absorption fine structure (XAFS) spectroscopy. A significant increase in Cu(II) and Cd(II) binding was observed in the range of pH 3.5–5.0. Dramatical decrease in
adsorption of Cu(II) and Cd(II) was observed after methanol esterification of the nonliving cells demonstrating that carboxyl functional groups play an important role in the binding of metals by S. platensis. The
desorption rate of Cu(II) and Cd(II) from S. platensis surface was 72.7–80.7% and 53.7–58.0% by EDTA and
NH4 NO3 , respectively, indicating that ion exchange and complexation are the dominating mechanisms
for Cu(II) and Cd(II) adsorption. XAFS analysis provided further evidence on the inner-sphere complexation of Cu by carboxyl ligands and showed that Cu is complexed by two 5-membered chelate rings on S.
platensis surface.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Land utilization of biosolids and applications of fertilizers and
pesticides have contributed to a continuous accumulation of heavy
metals in many aquatic and near-surface systems [1]. The fate of
toxic metallic cations in environment depends largely on their
interactions with microorganisms. The biomass of bacteria, fungi,
yeasts and algae have been reported for effective and economical
removal of a wide variety of toxic heavy metals from wastewater and engineering systems. Metal ions can be immobilized by
functional groups such as carboxyls, phosphomonoesters, phosphodiesters, amines and hydroxyls that are native to the proteins,
lipids, and carbohydrates on the cell walls of organisms [2]. A better
understanding of how metal sorption takes place on the surfaces
of microorganisms on molecular-scale is critical to elucidate the
mechanisms involved in terms of mobility, speciation and bioavailability of metals in geological systems.
∗ Corresponding author at: State Key Laboratory of Agricultural Microbiology,
Huazhong Agricultural University, Wuhan 430070, China. Tel.: +86 27 87671033;
fax: +86 27 87280670.
E-mail address: [email protected] (Q. Huang).
0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2011.03.122
Metal adsorption onto bacterial surfaces has been studied
extensively over the past 25 years. However, much of previous
investigations have been focused either qualitative information
or have quantified adsorption using a bulk-partitioning approach,
making it impossible to provide detailed information about metalbinding mechanisms [3]. Until 10 years ago, applications of XAFS
spectroscopy to environmental science have grown significantly,
resulting in a better delineating of metal adsorption processes and
mechanisms onto bacteria. For example, Panak et al. [4,5] used
time-resolved laser-induced fluorescence spectroscopy (TRLFS), in
conjunction with XAFS, to demonstrate that U(VI) forms inner
sphere complexes only with phosphate groups on cell walls of a
number of Bacillus species at pH 4.5–5.0. Boyanov et al. [6] reported
that Cd binds to the Bacillus subtilis predominantly due to phosphoryl binding below pH 4.4, whereas with increasing pH (4.4–6.5),
adsorption to carboxyl groups becomes increasingly important.
Toner et al. [7] investigated Zn sorption by a bacterial biofilm of
Pseudomonas putida at pH 6.9, and attributed zinc sorption to the
biofilm predominantly to Zn-phosphoryl complexes, with a relatively small contribution from carboxyl-type complexes. Guiné
et al. [2] reported sulfhydryl ligands were responsible for Zn adsorption to three Gram negative bacterial strains at low loadings of
Zn. The above molecular-scale investigations demonstrated that
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L. Fang et al. / Journal of Hazardous Materials 190 (2011) 810–815
sorption of heavy metals onto bacteria is controlled by carboxyl,
phosphoryl and perhaps sulfhydryl functional groups on the cell
wall of the microorganisms.
Among a large amount of information on bacteria, few studies
have been conducted on cyanobacteria. Until recently, cyanobacteria have been found to be of interest in metal adsorption processes.
Cyanobacteria are photosynthetic prokaryotes commonly found
in natural environmental and are suggested to have some added
advantages over other microorganisms for removing heavy metals
because of their large surface area, greater mucilage volume with
high binding affinity and simple nutrient requirements [8]. Laboratory and field studies have shown that cyanobacteria are highly
effective biological sorbents and represent an important sink for
metals in aquatic settings [9–11]. To accurately predict the fate
of metals in cyanobacteria-inhabited environments, a quantitative
and mechanistic understanding of metal-cyanobacteria sorption
reactions is needed. However, our current knowledge of metal
uptake by cyanobacteria is largely empirical and limited by a lack
of molecular-scale information [12]. In this study, a combination of
chemical modifications, metal-binding experiments, infrared spectroscopy and XAFS were performed to gain insights on the chemical
functional groups that may be involved in the binding of Cu(II)
and Cd(II) by cyanobacterium Spirulina platensis. In addition, some
structural information was obtained to describe the complexation
of Cu(II) with the functional groups involved. These investigations
aimed to provide a more comprehensive understanding of the
metal-cyanobacteria sorption reactions.
2. Materials and methods
2.1. Preparation of the cyanobacterium biomass
S. platensis was cultured at pH 7.5 in Medium BG-11
(Supplementary Table S1) under illumination of 400 ␮E m−2 s−1 at
28 ◦ C and 120 rpm. Five-day-cultured cells (late exponential phase)
were harvested and washed three times with deionized distilled
water (DD H2 O), then separated by centrifugation at 12,000 rpm
for 10 min, collected as native S. platensis cells.
2.2. Chemical modification of the nonliving cells
The collected native cells were lyophilized overnight in a Labconco freeze dryer and then autoclaved at 121 ◦ C for 30 min to
obtain the nonliving cells [13,14]. The esterification of the nonliving
cells was carried out according to the method of Gardea-Torresdey
et al. [15]. Specifically, 5.0 g nonliving cells of S. platensis were suspended in 250 mL of 99.9% acidic methanol solution and 3 mL of
concentrated hydrochloric acid (HCl). The suspension was continuously stirred at 60 ◦ C for 48 h and allowed to cool to room
temperature. The pelleted S. platensis cells were then washed three
times with DD water in order to quench the esterification reaction,
after that lyophilized again for further metal binding experiments.
The chemical esterification reaction is shown below, where R represents all of the components in the nonliving cells [15–17].
H+
RCOOH + CH3 OH−→RCOOCH3 + H2 O
Base hydrolysis of the nonliving cells was carried out in order to
increase the availability of carboxyl groups that may be involved
in the binding of heavy metals [18]. Three gram of nonliving cells
was reacted with 100 mL of 0.1 mol L−1 NaOH for 2 h to perform the
following reaction:
H+
RCOOCH3 + NaOH−→RCOO− + CH3 OH + Na+
811
After the reaction, the suspension was centrifuged and the supernatant was discarded. The cells were washed three times with DD
water, centrifuged, and lyophilized for metal binding experiments.
2.3. Fourier transformed infrared spectroscopy
The chemical characteristics of cyanobacterial cells (native and
esterified nonliving cells of S. platensis) were analyzed using a
Fourier transformed infrared spectrometer (Nicolet AVAR 330). All
infrared spectra were recorded over the range of 4000–400 cm−1
and the averaged spectra were obtained at a resolution of 4 cm−1 .
Sample disks were made from 5 mg of cyanobacterial cells encapsulated in 150 mg of KBr.
2.4. Metal solution
All chemicals used in this study were of analytical grade
and solutions were prepared using DD water. Stock solutions
of Cu(II) and Cd(II) (1000 mg L−1 ) were prepared by dissolving
Cu(NO3 )2 ·3H2 O and Cd(NO3 )2 ·4H2 O in DD water. A few drops of
0.1 mol L−1 HNO3 were added to the solutions to prevent the precipitation of Cu(II) and Cd(II) by hydrolysis. The initial pH of the
working solutions was adjusted to 5.0 for Cu(II) and 6.0 for Cd(II)
binding experiment by the addition of 0.1 mol L−1 HNO3 and NaOH
solution.
2.5. Metal adsorption experiments
Biosorption experiments were conducted at 25 ◦ C in batch with
0.01 g of the native and chemical modified S. platensis cells in a
50 mL plastic tube containing 20 mL of working solution volume.
The initial pH of the working solutions was adjusted to 5.0 for Cu(II)
and 6.0 for Cd(II) binding experiment by addition of 0.1 mol L−1
HNO3 and NaOH solution. The final metal concentrations for Cu(II)
and Cd(II) were 100 mg L−1 and 60 mg L−1 , respectively. Potassium
nitrate (0.01 mol L−1 ) was used as a supporting electrolyte for all
experiments. The mixture was shaken for 2 h and then centrifuged.
After centrifugation at 12,000 rpm for 10 min, the concentration of
metal in the supernatant was analyzed by flame atomic absorption
spectrometry (Varian AAS240FS). The difference between the initial
metal ion concentration and the remaining metal ion concentration
was assumed to be adsorbed by S. platensis cells. Adsorption was
also conducted in the range of pH from 2.0 to 5.0 for Cu(II) and 2.0
to 6.0 for Cd(II). No precipitation of metals occurs by hydrolysis in
the pH range explored according to the calculated pH values for
the precipitation of the hydroxyl complexes of copper (5.7) and
cadmium (8.5). All experiments were conducted in triplicate.
2.6. Desorption of Cu(II) and Cd(II)
Desorption of Cu(II) and Cd(II) from previously loaded native
S. platensis was studied by using DD water, 0.1 mol L−1 of EDTA
and 1.0 mol L−1 of NH4 NO3 as eluent. For this purpose, the previously prepared S. platensis cells loaded with Cu(II) or Cd(II) was
added to 20 mL of eluent in a 50 mL plastic tube in the presence
of 0.01 mol L−1 of KNO3 . After 2 h of shaking at 25 ◦ C, supernatants
after centrifugation were analyzed for the Cu(II) and Cd(II) concentrations. The same procedure was repeated three times. Desorption
ratio was calculated from the amount of metal ions adsorbed on the
cyanobacteria and the final metal ion concentration in desorption
medium. Control experiments without cyanobacteria were carried
out in order to determine the extent of adsorption of Cu(II) and
Cd(II) from solution by the plastic tube.
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L. Fang et al. / Journal of Hazardous Materials 190 (2011) 810–815
2.7. Zeta potential measurements
-1
Metal adsorbed (mg g )
The suspension for zeta potential measurements was prepared
by mixing the native S. platensis cells (1 g L−1 ) with 0.01 mol L−1
KNO3 solution (25 mL). The solution pH was adjusted to 2.0–6.0
using 0.1 mol L−1 of HNO3 or NaOH and then analyzed by Zeta
Potential Analyzer (Brookhaven Instruments Corporation, USA).
Three independent zeta potential measurements were collected for
each sample to ensure accurate and reproducible data.
2.8. X-ray absorption spectra measurement
30
Cu
Cd
Zeta potential
60
20
10
50
0
40
-10
30
-20
-30
20
-40
10
-50
0
-60
2
3
4
5
6
pH
Fig. 1. pH profile for Cu(II) and Cd(II) adsorption on the native S. platensis and the
zeta potential of S. platensis in the presence of 0.01 M KNO3 at different pH values
and 25 ◦ C.
Nonliving
70
Native
Hydrolysis
60
Esterification
-1
Metal ions adsorbed (mg g )
Copper K-edge X-ray absorption spectra at 8979 eV were
recorded on beamline U7c at the National Synchrotron Radiation
Laboratory (NSRL, China). The electron beam energy was 0.8 GeV
and the mean stored current was 100 mA. The energy of X-ray was
detuned by using a fixed-exit double-crystal Si (1 1 1) monochromator. Ionization chambers with N2 atmosphere were used to
collect the Fe K-edge spectra at room temperature. To calibrate Xray energies, a Cu foil internal reference was used with the first
inflection point set at 8979.0 eV. To determine the most probable configuration of Cu in our sorption sample, reference spectra
were measured from standard compounds CuCl, CuS, Cu3 PO4 and
0.1 mol L−1 aqueous Cu(OAc)2 ·H2 O (Sigma Chemicals). The XAS
data of the samples and aqueous complex reference were collected using fluorescence mode with a Lytle detector, while the
absorptions of solid standard compounds were measured by transmittance mode. The XAFS data analysis was performed with the
NSRL-XAFS software [19]. The XAFS oscillations were isolated from
the raw, averaged data by removal of the pre-edge background,
approximated by a first-order polynomial. The extracted XAFS
spectra, obtained via spline fitting techniques and normalized using
a Victoreen function, were Fourier transformed (FT) using the k
range 2.6–12.0 Å−1 . The theoretical scattering phases and amplitudes used in data analysis were calculated with the scattering code
FEFF 7 [20] using the five-membered Cu(II) ring structure [21].
Zeta potential (mv)
812
50
40
30
20
Cu(II)
Cd(II)
Treatments
3. Results and discussion
Fig. 2. The adsorptions of Cu(II) and Cd(II) on native, nonliving and chemically
modified nonliving S. platensis in the presence of 0.01 M KNO3 and at 25 ◦ C.
3.1. FTIR spectra analysis
3.2. Effect of pH on metal adsorption
The chemical characteristics of native and esterified S. platensis cells are shown in Supplementary Fig. S1. The spectral features
for bacteria are well established and the bands assignments are
based on the Refs. [22–25]. The spectrum of native cells showed two
high frequency bands (1656 cm−1 and 1540 cm−1 ) corresponding
to amide I and amide II, respectively. A small band corresponding to symmetric stretching of COO− groups was observed at
1402 cm−1 , which may derive from proteins and carboxylated
polysaccharides. Additional information on phospholipids along
with phosphodiesters, free phosphate, and monoester phosphate
functional groups were observed near 1238 and 1082 cm−1 , corresponding to >P O double bond asymmetric stretching frequencies.
These data showed the presence of carboxyl, amino and phosphate groups on the native cells of S. platensis. In contrast to
the native cells, the band near 1402 cm−1 (–COO− ) was apparently disappeared and one new band at 1731 cm−1 (ester carbonyl)
was clearly observed for the esterified nonliving cells. These differences demonstrated that the carboxyl groups on S. platensis
surfaces were transformed through esterification reaction. Furthermore, the bands between 1656–1540 cm−1 and 1238–1082 cm−1
were nearly unaffected after esterification reaction, indicating that
esterification had no effects on other functional groups and the
cyanobacterial structures remained intact.
Proton environment (pH) is an important factor affecting the
biosorption of heavy metals due to the influence of pH on the deprotonation of metal-binding functional groups as well as the surface
charge on the cyanobacteria. As shown in Fig. 1, the amount of
Cu(II) and Cd(II) adsorption on untreated cyanobacteria increased
with pH from 2.0 to 5.0 or 6.0, which is ascribed to the exposition of more negative charges on the S. platensis cells as showed
in the zeta potential curves. Furthermore, significant increases in
the binding of Cu(II) and Cd(II) were also observed in the range
of pH 3.5–5.0, indicating that the functional groups at pKa 3.5–5
play an important role in the adsorption of Cu(II) and Cd(II) on S.
platensis cells. Based on typical deprotonation constants for shortchained carboxylic (4 < pKa < 6), phosphoric (pKa ≈ 7) and hydroxy
(or phenolic) acids (9 < pKa < 11) [26], the increased metal binding at higher pH was due to the deprotonation of carboxylic
groups.
3.3. Effect of chemical modification on metal adsorption
The adsorptions of Cu(II) and Cd(II) on native, nonliving and
chemically modified nonliving cells are presented in Fig. 2. The
binding capacity by the native cells for Cu(II) and Cd(II) were
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L. Fang et al. / Journal of Hazardous Materials 190 (2011) 810–815
813
H2 O
0.8
NH4 NO3
CuCl
EDTA
Normalized Absorbance
Desorption (%)
CuS
0.6
0.4
0.2
Cu3(PO4)2
Cu(OAc)2
S. platensis
0.0
Cu(II)
Cd(II)
Metal ions
Fig. 3. The percentage of Cu(II) and Cd(II) released from S. platensis cells after treatment with DD water, 0.1 mol L−1 of EDTA and 1.0 mol L−1 of NH4 NO3 .
8960
9000
9040
9080
Energy (eV)
Fig. 4. Cu K-edge XANES spectra of sample and reference compounds.
57.8 mg kg−1 and 45.1 mg kg−1 , respectively. Esterified nonliving
cells resulted in the reduction in the binding of Cu(II) and Cd(II) by
55.5 and 45.6%, respectively. After hydrolysis, the binding capacities for Cu(II) and Cd(II) by S. platensis increased slightly due to
the formation of new carboxyl ligands. These results indicated
the carboxyl groups play the most important role in the binding of Cu(II) and Cd(II) from solution which was supported by
the increased adsorption capacity at pH from 3.5 to 5.0. Furthermore, a similar binding capacity between the native and
nonliving cells also suggested that active uptake was insignificant
in the overall mechanism of biosorption. Although cyanobacteria
are Gram-negative in cellular structure, their cell walls contain
a thick structural layer of peptidoglycan and an extended glycoproteins and polysaccharides [27], which are the main source
of reactive carboxyl groups on the surfaces of cyanobacteria
[28]. Therefore, our data demonstrated that carboxyl groups on
the S. platensis cell wall are the dominant sink for Cu(II) and
Cd(II).
3.4. Desorption of Cu(II) and Cd(II)
Fig. 3 shows the percentage of Cu(II) and Cd(II) released from
S. platensis cells after treatment with different desorbents. Only
2.5–3.6% of Cu(II) and Cd(II) on S. platensis surface were removed
by DD water. It suggested that the bound metal ions were not easily released and the contribution of physical adsorption was minor.
The biosorption of Cr3+ , Cd2+ and Cu2+ by blue-green algae Spirulina
species also showed that the maximum contribution of physical
adsorption was only 3.7% [29]. The desorption rate of Cu(II) by
NH4 NO3 and EDTA was 53.7% and 72.7%, respectively. Higher desorption percentage by EDTA and NH4 NO3 was observed for Cd(II)
(58.0% and 80.7%, respectively). Metal ions released by NH4 NO3
can be regarded as the fraction adsorbed by ion exchange, while
that released by EDTA are considered as that adsorbed by complexation [30]. This implied that Cu(II) and Cd(II) was adsorbed
predominantly by ion exchange and complexation on the surface of
S. platensis. In addition, esterified nonliving cells resulted in higher
percentages of reduction in the binding of Cu(II) (55.5%) than Cd(II)
(45.6%), suggesting that carboxyl groups play a more important role
for Cu(II) adsorption. Therefore, the lower desorption percentage
of Cu(II) as compared with that of Cd(II) leads us to speculate that
more carboxyl groups are involved in the complexation with Cu(II)
than Cd(II).
3.5. X-ray absorption fine structure (XAFS) data analysis
The X-ray absorption near edge structure (XANES) has been
used successfully to determine the oxidation state and the chemical coordination environment of ions in different systems [31].
The Cu K-edge XANES spectra from the samples and standards
used in this investigation are shown in Fig. 4. The adsorption edge
of sample was very similar to that of Cu(II) model compounds
Cu(OAc)2 ·H2 O, Cu3 (PO4 )2 and CuS, indicating that the oxidation
state of copper was not changed upon binding to S. platensis surface.
As shown in Fig. 5, the extended X-ray absorption fine structure
(EXAFS) spectra of samples (both in k and R space) were similar
to those of Cu(OAc)2 ·H2 O, but different from those of Cu3 (PO4 )2 ,
CuCl or CuS. The similar spectra between the sample and standard
compounds containing Cu-carboxyl suggest the predominant formation of C–O–Cu bond between copper and the cyanobacterial
cells.
To gain more insight into the molecular structure of the adsorption of Cu on S. platensis surfaces, the EXAFS spectrum was fitted
and presented in Figs. S2 and S3. The coordination number (CN),
atomic separation distance (R), and EXAFS Debye–Waller factor
( 2 ) obtained are listed in Table 1. The results indicated that Cu
was coordinated by 4 O atoms at an average distance of 1.94 Å in
the first shell. The EXAFS studies of well-defined compounds have
shown that Cu(II) is usually 6 coordinated in the first coordination shell, with 4 O or N atoms at distances of 1.90–1.97 Å, and
2 axial O atoms at 2.15–2.78 Å [32,33]. In this research, the axialligand backscattering pair was not included in the fitting routine
because of their high degree of disorder and thus minimal EXAFS
contributions [33,34]. Therefore, these four atoms at shorter distance surrounding Cu are most likely positioned in the equatorial
plane of a Jahn–Teller distorted elongated octahedron. As for the
second shell, Cu was found to be coordinated to 4.1 C atoms at a
distance of 2.76 Å. This provided direct evidence for inner-sphere
complexation of Cu by carboxyl ligands on S. platensis surface. Based
on the known structure of Cu organic monodentate and bidentate
organic Cu complexes [35], the bonding environment of a bis-fivemembered carboxyl chelate with Cu was very similar to our data.
Therefore, our final fitting results strongly suggested that Cu(II) is
complexed by carboxyl groups on S. platensis surface, forming a
structure involving two five-membered chelate rings (Fig. 6). As
compared with the previous XAFS studies reported in Table 1, our
determined the coordination number and atomic separation distance are in agreement with that of the complexation of Cu(II) in
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814
L. Fang et al. / Journal of Hazardous Materials 190 (2011) 810–815
Table 1
EXAFS fit results of Cu adsorption on S. platensis cells. Compared to values reported for Cu in glycine (PMG), aquatic humic acid (HA) and soil organic matter (SOM).
Samples
S. platensis
Shell 1
Shell 2
PMGf
Shell 1
Shell 2
Aquatic HAg
Shell 1
Shell 2
SOMh
Shell 1
Shell 2
Atomic backscatter
R (Å)a
2 (Å2 )b
CNc
E0 (eV)d
Rf e
Cu–O
Cu–C
1.95
2.76
0.0049
0.0081
4.0
4.1
1.50
-8.87
0.041
0.060
Cu–O
Cu–C
1.96
2.81
–
4
3
–
–
Cu–O
Cu–C
1.94
2.88
–
4.4
2.9
–
–
Cu–O
Cu–C
1.92–1.95
2.76–2.84
–
4
2–4
–
–
a
Atomic separation distance.
Debye–Waller factor coefficient.
Coordination number.
d
Energy shift.
e
Residual factor = k(k3 xexp − k3 xcalc )/k(k3 xcalc ), which measures the quality of the model Fourier-filtered contribution (xcalc ) with respect to the experimental contribution
(xexp ). The amplitude reduction factor (S02 ) was set to 0.75. For the first shell, coordination numbers (CN) was fixed 4.0.
f
Ref. Sheals et al. [33].
g
Ref. Lee et al. [34].
h
Ref. Karlsson et al. [21].
b
c
a
CuCl
Cu3(PO4)2
3
k χ(k)
CuS
Cu(OAc)2
S. platensis
Fig. 6. Proposed model for Cu complexed by two 5-membered chelate rings in sample. The more distant axial oxygens (Oax) are not included in the final fits to the
EXAFS data.
2
4
6
8
10
12
-1
k(Å )
b
Fourier Transform Magnitude
CuCl
aquatic humic acid and soil organic matter. The modeling of EXAFS
data for the complexation of solid and dissolved natural organic
matter suggested that Cu forms complexes consisting of one or two
five-membered chelate rings [21]. To our knowledge, data obtained
in this study are the first to directly determine the binding mechanism of Cu(II) on cyanobacteria using XAFS technique, providing
key information about the metal–microorganisms interactions at
molecular level.
4. Conclusions
CuS
Cu3(PO4)2
Cu(OAc)2
S. platensis
0
1
2
3
4
5
R (Å)
Fig. 5. k3 -Weighted EXAFS spectra for Cu in sample and reference compounds. (a)
k space; (b) R space.
Our results demonstrated that the carboxyl groups play a vital
role in the adsorption of Cu(II) and Cd(II) to the cyanobacterium
S. platensis. Ion exchange and complexation are the dominating mechanisms for Cu(II) and Cd(II) adsorption. XAFS analysis
revealed that Cu(II) forms inner-sphere complexes consisting of
two five-membered chelate rings on the cyanobacterial surface.
The elucidation of the binding mechanisms for Cu(II) and Cd(II) on S.
platensis cells at the molecular scale may help to accurately predict
the fate of metals in cyanobacteria-inhabited environments.
Acknowledgments
This work was financially supported by the National Natural Science of Foundation of China (40825002) and Huazhong Agricultural
University Scientific & Technological Self-innovation Foundation
(2009YB005). We gratefully acknowledge Dr. Bo He and Dr. Zhi Xie
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L. Fang et al. / Journal of Hazardous Materials 190 (2011) 810–815
(NSRL, USTC, China) for their helpful technical assistance of XAFS
experiments. We also thank Dr. Torbjörn Karlsson from Swedish
University of Agricultural Sciences for XAFS data processing.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jhazmat.2011.03.122.
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