Adsorption of copper to different biogenic oyster shell structures

Applied Surface Science 311 (2014) 264–272
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Applied Surface Science
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Adsorption of copper to different biogenic oyster shell structures
Qiong Wu a , Jie Chen a , Malcolm Clark b , Yan Yu a,∗
a
b
College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China
Marine Ecology Research Centre, School of Environment, Science and Engineering, Southern Cross University, P.O. Box 157, Lismore, NSW 2480, Australia
a r t i c l e
i n f o
Article history:
Received 12 October 2013
Received in revised form 11 May 2014
Accepted 11 May 2014
Available online 19 May 2014
Keywords:
Adsorption
Copper ion
Oyster shell
Prismatic layer
Nacreous layer
a b s t r a c t
The removal of copper from solution by oyster shell powder was investigated for potential wastewater
treatment uses. In particular, adsorption behavior differences between the prismatic (PP) and nacreous
(NP) shell layers, and how this affects copper removal, were investigated. Experimental results indicated
that copper adsorption was highly pH-dependent with optimal copper removal at pH 5.5, where the
powdered whole raw shell (RP) removed up to 99.9% of the copper within 24 h at a 10 mg/L initial copper concentration. Langmuir and Freundlich models were used to analyze the isotherm PP, NP and RP
data. These results showed a strong homogeneous Langmuir model for low initial copper concentrations
(5–30 mg/L) to both RP and PP layer, while strong agreement with a heterogeneous Freundlich model for
high initial copper concentrations (30–200 mg/L); nevertheless, a homogeneous Langmuir model provided the best fit for the more dense NP layer across the initial concentration range (5–200 mg/L). The
distribution coefficient (Kd ) value of PP layer for each initial concentration investigated was substantially
higher than the NP layer and it was also found that the PP layer dominated the adsorption process with
an adsorption capacity of 8.9 mg/g, while the adsorption capacity of the NP layer was 2.6 mg/g. These
differences are believed to be because of the more porous structure of the PP layer, which was confirmed by scanning electron microscopy, infrared spectroscopy, energy-dispersive X-ray spectroscopy,
and thermogravimetry–differential thermal analyses.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Copper contamination of water is of considerable concern
worldwide, although some industries (particularly in developing
countries) still dispose of copper-rich effluents. These effluents can
have serious implications not only for the environment but also
for human populations via the food chain, because excess Cu2+
may cause several diseases and disorders that can be fatal [1–3].
However, copper carbonate precipitation is an effective way of
reducing copper concentrations, and copper is also known to adsorb
carbonate minerals [4]. Moreover, with the development of oyster cultivation in China, large quantities of oyster shell residues
are dumped, where for each dozen oysters consumed (∼1 kg)
370–700 g of the shell residue is produced [5]. This shell residue,
often with attached meat, must be deposed of because of undesirable odors, and fly and mosquito attraction. Most Chinese market
oyster meat traders pay for waste oyster shell removal, which
reduces profitability. Hence, an oyster shell reuse option in industrial and/or environmental applications provides an opportunity to
∗ Corresponding author. Tel.: +86 591 22866540; fax: +86 591 22866534.
E-mail address: yuyan 1972@126.com (Y. Yu).
http://dx.doi.org/10.1016/j.apsusc.2014.05.054
0169-4332/© 2014 Elsevier B.V. All rights reserved.
add value to the waste, thereby increasing profitability for Chinese
oyster meat traders.
Biogenic CaCO3 is believed to be a good substitute for geological CaCO3 as an adsorbent of trace-metal ions. Biogenic carbonates
(e.g., oyster shell) have several advantages over geologic CaCO3 .
Firstly, there is no need to establish a mine and exploit fixed earth
resources (the oyster shell is the mine) [6]. Secondly, biogenic carbonates typically have a twisted open aragonite structure, and
are high in the Mg–CaCO3 phase rather than the more compact
calcite structure [7] and thirdly, oyster shell has three welldefined layers: a cuticle, a prismatic (PP) and nacreous (NP) layers
[8–10].
The cuticle is mainly composed of cutin and organic matter
that covers the surface of the whole oyster shell. After the cuticle
is removed, a layered composite structure (>80%) of calcium carbonate remains, which is composed of PP and NP layers [5,11–13].
Hence, oyster shells have many mineralogical and geochemical
properties, such as high absorbability, exchange capacity, and
reactive surface area that can be used in environmental and
industrial applications [14–16]. Several studies have applied waste
oyster shell to remove phosphate [16,17], hydrogen sulfide [18],
and dissolved contaminant cations (e.g., Pb, Zn, Cd) [7] from
wastewater. However, the use of oyster shell as an adsorbent for
Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
aqueous copper and the role the different shell layers play in cation
adsorption has not been previously investigated.
This work investigates the use of the porous and layered oyster shell structure to develop an efficient copper removal material
that not only reduces the environmental effects, but also converts
and adds value to the waste material. Copper adsorption to the PP
and NP layers of the oyster shell is compared to better understand
the adsorption preferences, and to determine whether different
adsorption mechanisms are active. In addition, different geochemical conditions, such as pH and initial copper concentration, are
investigated to optimize adsorption.
265
using several techniques. The crystalline phase(s) of each sample was determined with a Philips X’pert-MPDX-ray diffractometer
(XRD) using Cu K␣1 radiation generated at 40 kV and 40 mA
with a scan rate of 5◦ min−1 . The cured material microstructures
were examined using a Philips XL30 scanning electron microscope (SEM), where the chemical compositions were determined by
energy-dispersive X-ray spectroscopy (EDAX). The functional surface groups on the samples were determined by infrared absorption
spectroscopy (IR) using a TJ270-30A infrared spectrophotometer
(Tianjin, China). Thermogravimetry–differential thermal analysis
(TG–DSC, TGA-Q600) was performed to determine weight changes
during heating and decalescence (heat release).
2. Materials and methods
3. Results and discussion
2.1. Sample preparation
Oyster shells intended for waste disposal at Xiyangxincun market, Fuzhou City were collected, scrubbed clean to remove residual
meat and sediments, and air-dried. Powdered whole raw shell (RP)
and the separated PP and NP layers were investigated to compare
the layers and their contribution to whole shell adsorption. PP and
NP layer separation was performed by taking the raw shell and
rinsing it with a 5% NaClO solution for 1 day to remove the surface
organic matter (cuticle) and attached sediments, and then scraping
using a knife to physically separate the PP and NP. The cleaned RP
and separated PP and NP shell materials were air-dried for 24 h,
and then ground using an agate mortar to generate a fine powder,
which was then sieved to <80 (177 ␮m) mesh particle size.
2.2. Solution preparation
The analytical grade copper nitrate dihydrate (Cu(NO3 )2 ·2H2 O)
was supplied by the Chinese medicine chemical reagent company
(Shanghai, China). The copper solution was prepared by dissolving
Cu(NO3 )2 ·2H2 O in deionized water at a concentration of 1000 mg/L,
which was then further diluted to a range of concentrations with
deionized water.
2.3. Adsorption experiments
All working volumes of metal solutions were fixed at 50 mL and
ambient temperature (25 ◦ C). The initial copper solution concentrations were prepared at 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 and
100 mg/L for 0.5 g RP at pH 5.5 for 24 h. To investigate the adsorption
isotherm, two equilibrium models were analyzed: the Langmuir
and Freundlich isotherm equations. The effect of pH on adsorption
was analyzed by placing 0.3 g of PP or NP into a copper solution of
10 mg/L for 24 h with the pH changed from 4 to 8 in 0.5 increments.
The optimum removal pH of 5.5 was then selected for further studies. To compare the adsorption differences between the PP and NP
layers, 0.3 g of PP or NP was placed in copper solutions of 50, 100,
150, 200, 300, or 400 mg/L for 24 h at the optimum pH (5.5). To
allow for any secondary adsorption to the container surface, several control experiments without adsorbent were performed, and
showed that no secondary adsorption occurred.
The equilibrium adsorption capacity (qe , mg/g) and removal efficiency (R, %) for copper were determined using
qe =
R=
(C0 − Ce ) · V
m
C0 − Ce
× 100%
C0
(1)
(2)
where C0 is the initial copper concentration (mg/L), Ce is the concentrations of copper at the equilibrium time (mg/L), m is the mass
of adsorbent (g), and V is the volume of copper solution (L).
3.1. Adsorption of copper to RP
Some reports have reported the excellent adsorption capacity of
oyster shell for phosphate, hydrogen sulfide, and boron [14,18,19].
However, few studies have investigated the adsorption behavior
of copper to oyster shell. In this study, several important factors
for the adsorption of copper to oyster shell, such as initial copper
concentration and metal affinity are discussed. The data obtained
were used to develop the adsorption isotherm.
3.1.1. Effect of initial copper concentration on adsorption
Fig. 1 shows the effect of initial copper concentration on adsorption to RP. Clearly, RP exhibits a high adsorption capacity for
copper, with a maximum removal of 99.9% with an initial copper concentration of 10 mg/L, and maintains >90% removal with
an initial copper concentration of 25 mg/L, although this percentage rapidly decreases as the copper concentration further increases
(Fig. 1). With initial copper concentrations of 50 and 100 mg/L,
the removal efficiencies were 45% and 29% (not shown in Fig. 1),
respectively. However, as the percentage adsorbed decreased with
2.4. Characterization
All solution pHs were adjusted using a small amount of 0.1 M
HCl or NaOH and monitored with a pH electrode (Model 225,
pHISE Meter, Denver Instruments, Denver, CO, USA) calibrated
with standard buffer solutions (pH 4, 6.86, and 9). The equilibrium copper concentrations were measured by atomic adsorption
spectroscopy (TAS-986, PGeneral, Beijing, China). Characterization of the adsorbent pre- and post-adsorption was performed
Fig. 1. Effect of initial copper concentrations on copper adsorption parameters qe
and removal percentage to RP.
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Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
increasing initial concentration, the equilibrium adsorption capacity increased, indicating increasing occupation of available binding
sites, which is in agreement with Amarasinghe and Rahman et al.
[20,21]. They reported that adsorption of heavy metals only takes
place at specific sites of the sorbents [21,22]. When the initial concentration increases, there are insufficient binding sites for the
heavy metal, leading to a reduction in metal adsorption.
In addition, the initial copper concentration directly affects
the diffusion of copper from the solution to RP surfaces because
diffusion is in equilibrium, and the concentration is controlled.
Moreover, the zeta () potential at pH 5.5 (Figure S1, Supplementary Material) indicates that the RP is negatively charged
with a -potential of −18.04 mV, thereby providing an electrostatic attraction between the RP surfaces and copper ions. As
the initial concentration increases, the copper diffusion rate to
the RP surfaces increases with a larger potential difference. With
the surface accumulation of metal ions, the surface charge will
change with the isoelectric point (IEP, pH where the overall surface
charge is 0 mV) increasing as active binding sites reach saturation
[23], and the rate of copper diffusion also decreases. In addition,
once the surfaces of porous materials become loaded, the adsorbates diffuse into the internal pore structure at a relatively slow
rate [24,25]. In this work, the increasing initial concentration and
the amount of copper sorbed to RP increased but did not reach
saturation.
3.1.2. Metal affinity of adsorption
To evaluate the metal affinity of adsorption, the distribution
coefficient (Kd , L/kg) was calculated using
Kd =
qe
.
ce
(3)
Kd is defined as the ratio of the mass loading in the solid phase to
the concentration in the equilibrium solution after a specific reaction time, and the results are shown in Table 1 [26,27]. A high Kd is
usually associated with high metal retention in sorbents through
adsorption and chemical reactions, whereas a low Kd indicates a
high amount of metal remaining in solution [26,27]. It was observed
that the Kd value reached 9183.3 L/kg at 5 mg/L copper concentration, while it decreased to 118.2 L/kg at 100 mg/L. This reveals that
more copper ions were adsorbed to the oyster shell at low initial
concentrations, and more remained in solution at high concentrations, which is in agreement with the high removal efficiency
at low initial concentrations observed above. In addition, a high
Kd value indicates a high affinity between the copper ion and the
sorbent [26], which means that at low copper ion concentrations
the copper ions are more easily adsorbed to specific adsorption
sites.
Fig. 2. Langmuir models for RP, where three fits are provided, the fit for low copper
concentrations (5–30 mg/L), for high concentrations (30–200 mg/L), and a Langmuir
fit to all data (5–200 mg/L).
adsorption. The dimensionless constant separation factor (RL ) is
defined as [30,31]
RL =
1
1 + KL · C0
(5)
where C0 is the initial concentration of adsorbate (mg/L). RL is considered to be a more reliable indicator of adsorption. There are four
possibilities for the RL value: (i) for favorable adsorption, 0 < RL < 1;
(ii) for unfavorable adsorption, RL > 1; (iii) for linear adsorption,
RL = 1; and (iv) for irreversible adsorption, RL = 0 [31].
The Freundlich isotherm equation in its linear form can be
expressed as
lg qe = lg KF +
1
lg Ce
n
(6)
where KF is the intercept and n is the derivative of the slope, which
are the Freundlich constants representing the adsorption capacity
and the adsorption intensity, respectively. In general, the greater
the value of KF the greater the heterogeneity, and the larger the
value of n (n > 1), the more spontaneous the adsorption process is.
The two models were calculated for three different initial concentration ranges (5–200, 5–30, and 30–200 mg/L), as shown in
Figs. 2 and 3. The Langmuir R2 for the complete initial concentration range 5–200 mg/L was 0.9084. However, the R2 for the range
5–30 mg/L was 0.9932, and larger than the other two concentration
3.1.3. Adsorption isotherm
Two of the most commonly used adsorption isotherm models
to investigate and describe solution removal processes and mechanisms are the Langmuir and Freundlich models. The Langmuir
model assumes a completely homogeneous surface [28], where
adsorption to the surface has the same activation energy, whereas
the Freundlich model is suitable for highly heterogeneous surfaces
[29].
The Langmuir isotherm equation in its linear form can be
expressed as
1
1
1
1
=
·
+
qm · KL Ce
qm
qe
(4)
where Ce is the equilibrium concentration (mg/L), qe is the amount
adsorbed to the solid (mg/g), qm is the maximum saturation
capacity at the isotherm temperature (mg/g), and KL (L/mg) is
the adsorption equilibrium constant related to the energy of
Fig. 3. Freundlich models for RP, where three fits are provided, the fit for low
copper concentrations (5–30 mg/L), for high concentrations (30–200 mg/L), and a
Freundlich fit to all data (5–200 mg/L).
Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
267
Table 1
Distribution coefficient (Kd ) for RP for different initial copper concentrations (C0 ).
C0 (mg/L)
Kd (L/kg)
5
9188.3
10
6452.9
15
1116.2
20
573.1
25
572.3
30
500
35
473.8
40
549.4
45
189.9
50
181.7
100
118.2
Table 2
Parameters and regression coefficients (R2 ) for the two equilibrium models for RP.
C0 (mg/L)
Langmuir
5–200 mg/L
5–30 mg/L
30–200 mg/L
Freundlich
qm (mg/g)
KL (L/mg)
R2
n
KF (L/mg)
R2
3.05
2.01
8.22
3.61
6.36
0.09
0.9084
0.9932
0.9697
2.64
3.15
2.23
1.43
1.37
1.19
0.9801
0.9905
0.9793
ranges. This indicates that the removal process for 5–30 mg/L was
best fitted with the Langmuir model; that is, the adsorption behavior is single-layer adsorption. In addition, the maximum saturation
capacity for 5–30 mg/L was 6.36 mg/g (Table 2) and the values
of RL for 5–30 mg/L ranged from 0.005 to 0.03 (Table 3), revealing that adsorption under low initial concentrations is a favorable
process.
The Freundlich R2 values for all three concentration ranges were
high, with values of 0.9801, 0.9905, and 0.9793, suggesting that
the Freundlich model was in good agreement with the experimental data. In addition, the values of n were all >1, indicating
that copper removal from the solution to the solid was preferential and spontaneous. However, considering that the Langmuir R2
for 5–30 mg/L was higher, the adsorption process for low initial
concentrations (5–30 mg/L) can be described by a Langmuir model
while a Freundlich model describes adsorption for high initial concentrations (30–200 mg/L), suggesting a more precipitation driven
removal.
3.2. Adsorption of copper to PP and NP
Although Fig. 1 shows the adsorption capacity of the whole oyster shell for copper in solution, it can be inferred that the adsorption
capacity has a close relationship with the physical structure of the
oyster shell [14–16]. Hence, separation of the two major layers in
the oyster shell (the PP and NP layers) was performed to determine
their separate roles in the adsorption process, which may provide
a greater understanding of the adsorption mechanisms requiring
further investigation.
3.2.1. Effect of pH on adsorption
Solution pH is a primary factor governing metal ion adsorption
[32–34], because pH has a significant effect on metal speciation, and
particularly on MOH+ , which is the most readily adsorbed species
through the pKa of hydration [35–38]. In addition, pH affects the
metal adsorption mechanisms (surface precipitation vs. adsorption), surface charge polarity across the isoelectric point, and the
adsorption capacity [39].
The effect of pH on copper adsorption to NP and PP is shown
in Fig. 4, and indicates that the PP has a greater adsorption capacity than NP for all pH values from 4 to 8. For both PP and NP, the
copper removal reached a maximum at pH 6, which is close to the
copper pKa of hydration [40], with removal estimates of 99.6% and
95.1%, respectively. Despite the difference in the adsorption capacity, the overall trend was similar for both PP and NP: copper removal
Table 3
Separation factor (RL ) for RP for different initial copper concentrations
(C0 = 5–30 mg/L).
C0 (mg/L)
RL
5
0.03
10
0.015
15
0.01
20
0.008
25
0.006
30
0.005
Fig. 4. Effect of pH value on copper removal efficiency to NP and PP layers for a
10 mg/L initial copper concentration.
rapidly increased as the pH was increased from 4 to 6 and then
reached a plateau for pH > 6. This result agrees well with the copper speciation (Table 4) [41], while as the pH increases from 4 to
6 there is less H+ available to compete with Cu2+ and/or Cu(OH)+
for the same adsorption sites on the surface of the adsorbent. Furthermore, as the pH increases, Cu2+ will hydrolyze to Cu(OH)+ ,
which is the species most readily adsorbed [35–38] (Figure S2),
where at pH 5.5 copper remains below the solubility of the kinetically favored precipitates. However, when the pH increases to 7
or 8, precipitation is more likely to dominate copper removal (Figure S2) than adsorption, although adsorption and co-precipitation
are considered to be analogous [37,38]. In addition, copper exists
both as Cu(OH)2 and Cu(OH)+ at pH 6–7 (Table 4) and it becomes
difficult to determine the effects of precipitation and adsorption
without sophisticated techniques such as synchrotron EXAFS and
X-ray adsorption near-edge structure spectroscopy. Therefore, the
optimum pH was selected as pH 5.5, which is well below the pH of
possible precipitation (Figure S2).
In the adsorption experiments, it was also found that the postcopper adsorption pH decreases with increasing initial copper
concentration, which can be explained by
2M OH+ + Cu2+ → 2(M OCu)+ + 2H+
(7)
M OH+ + Cu(OH)+ → (M O)Cu(OH)+ + H+
(8)
Table 4
Copper speciation in water for different pH values.
pH range
Cu2+ speciation dominance
pH < 4.0
4.0 ≤ pH < 5.0
5.0 ≤ pH < 6.0
6.0 ≤ pH
Cu2+
Cu2+ , CuOH+
CuOH+ , Cu(OH)2
Cu(OH)2
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Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
3.2.2. Effect of initial copper concentration on adsorption
The equilibrium adsorption of copper to PP and NP for different
initial copper concentrations (Fig. 5) shows that both PP and NP
saturation occurs at about 150 mg/L, where the measured equilibrium adsorption capacity values are 8.9 mg/g and 2.6 mg/g (0.89%
and 0.26%), respectively. The adsorption capacity of PP is about 3.5
times higher than that of NP, but it is similar to the results obtained
for RP (data not shown). This is most likely because of the different microstructures in the PP and NP layers. Several studies have
reported that the NP layer is denser than the PP layer because the
PP layer is a porous structure [8–10], and this porous structure provides a larger surface area and contribution to the capacity than the
NP layer.
Fig. 5. Effect of initial copper concentration on surface copper loadings (qe ) of the
NP and PP shell layers.
2M OH + Cu2+ → (2M O)Cu + 2H+
M OH + Cu(OH)+ → (M O)Cu(OH) + H+
(9)
(10)
The hydrolysis of CaCO3 in PP contributes to the negatively charged
PP surface. Hence, high Cu2+ and Cu(OH)+ concentrations increase
diffusion to the PP surface through electrostatic attraction that then
react with OH− and OH (Eqs. (7)–(10)).
Fig. 6. Langmuir and Freundlich models for the NP layer (5–200 mg/L initial concentrations): (a) Langmuir; (b) Freundlich.
3.2.3. Adsorption isotherm and metal affinity
Adsorption isotherm and copper affinity were investigated to
compare fully between PP and NP shell layers. Langmuir, Freundlich and distribution coefficient models were applied to fit the
adsorption data of PP and NP layers. Fig. 6 shows the linear curve
fitting of Langmuir and Freundlich isotherm models for the NP
layer across the complete initial concentration range (5–200 mg/L).
Clearly, the adsorption of copper to the NP layer correlates better
with a Langmuir isotherm (R2 = 0.9995) than a Freundlich isotherm
(R2 = 0.6274). These results suggest that adsorption behavior of NP
layer is a monolayer adsorption and that Cu2+ was adsorbed to the
surface of the NP layer only.
However, for the PP layer, two isotherm models were calculated for two different initial concentration ranges (5–30 and
30–200 mg/L), as shown in Fig. 7a and b. For the concentration range 5–30 mg/L, there was a better fit for the Langmuir
Fig. 7. Langmuir and Freundlich models for the PP layer (5–200 mg/L initial concentrations): (a) Langmuir; (b) Freundlich.
Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
269
Fig. 8. Distribution coefficients (Kd ) for the PP and NP layers for different initial
copper concentrations (C0 ).
isotherm (R2 = 0.9675) than the Freundlich isotherm (R2 = 0.9258).
Whereas, the reverse is true for the higher concentration range
30–200 mg/L, with R2 values of 0.8357 (Langmuir) and 0.9804
(Freundlich). Hence, the adsorption process of PP layer for low
initial concentrations (5–30 mg/L) can be described by a homogeneous Langmuir model, but a heterogeneous Freundlich model best
describes adsorption for high initial concentrations (30–200 mg/L).
This behavior is most likely ascribed to the porous structure of the
PP layer [8]. At low initial concentrations, there are abundant binding sites favorable for Cu2+ removal to the surface of PP layer [22],
in which case the adsorption is generally single-layer. However, as
initial Cu2+ concentration increases, active binding sites reach saturation [21], and Cu2+ either diffuses into the internal pores of PP
layer because of potential difference [25], or begins to precipitate as
Fig. 10. IR spectra. (a) Pre-adsorption (PP) and post-adsorption (PP(x)) for PP. (b)
Pre-adsorption (NP) and post-adsorption (NP(x)) for NP.
Fig. 9. XRD patterns and TG–DSC curves of PP and NP layers: (a) pre-adsorption XRD patterns for PP and NP; (b) post-adsorption XRD patterns for PP and NP; (c) pre-adsorption
TG–DSC curve for PP; and (d) pre-adsorption TG–DSC curve for NP.
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Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
poly-nuclear species. This result for the PP layer is consistent with
that of RP, which agrees with that PP layer forms most structure of
the oyster shell and that the PP layer controls the Cu2+ binding to
the whole shell (RP).
In addition, the distribution coefficient (Kd ) also differs greatly
between PP and NP layers. The Kd values for the initial concentration range 50–400 mg/L are presented in Fig. 8, where the PP
layer exhibits a significant decrease in Kd values with increasing initial concentration, while the NP layer shows only a slight
decrease. This response (Fig. 8) shows that copper adsorption to
PP layer is much more concentration dependent than the NP layer.
In addition, the higher PP than NP layer Kd values, suggest that
copper is more easily adsorbed and/or precipitated to the PP layer,
which agrees with the higher adsorption capacity of the PP layer
(Fig. 5).
3.3. XRD and TG–DSC analysis
The pre- and post-adsorption XRD analyses of the PP and NP
layers (Fig. 9a and b) confirm that their crystal structure is calcite.
Fig. 11. SEM images and EDAX patterns pre- and post-adsorption. (a) Pre-adsorption and (b) post-adsorption for PP. (c) Pre-adsorption and (d) post-adsorption for NP.
Q. Wu et al. / Applied Surface Science 311 (2014) 264–272
Furthermore, the major X-ray fluorescence peaks of the PP and NP
layers pre- and post-adsorption differ only in peak intensity (apart
from the additional copper peak), indicating the difference in Cu
adsorption capacity. Both the XRD and XRF data suggest a highly
crystalline calcite structure (a = b = 4.989 Å, c = 17.062 Å) is retained
post-copper adsorption for both the PP and NP layers. Moreover,
although the saturation loadings are high, particularly for the
PP materials, there is insufficient copper to form a new phase
detectable by XRD, where concentrations of 2–5% are required for
adequate peak detection.
In addition, the proportion of organic matter and CaCO3 in the PP
and NP layers before copper adsorption was evaluated by TG–DSC
(Fig. 9c and d). Two distinct mass loss steps are observed in the
PP and NP layers: 0.68% and 1.44% (the loss of organic matter and
water) for temperatures up to 530 ◦ C, and then 41.52% and 33.13%
(the loss of CaCO3 ) from 530 to 800 ◦ C, respectively. The NP layer
had greater mass loss from 300 to 530 ◦ C than the PP layer, indicating a higher proportion of organic matter, which is also seen
in the XRD patterns (Fig. 9a and b) where the unloaded materials show a broad organic hump from 15 to 25 2. However, the
PP layer has a greater mass loss from 530 to 800 ◦ C, indicating a
greater proportion of CaCO3 in the PP layer than the NP layer. A
clear peak is observed at 770 ◦ C for both the PP and NP layer, where
CaCO3 endothermically decomposes to CaO with CO2 evolution
[42].
3.4. IR analysis
To determine the functional groups responsible for metal
uptake, IR analyses of the PP and NP layers were performed preand post-adsorption (Fig. 10). The IR spectra show a number of
adsorption peaks. The carbonate group internal C–O vibrations are
observed at 706, 874, and 1424 cm−1 [8,42], and there is also a
strong band at 1797 cm−1 that can be attributed to the C O groups
of the carbonate ions [8]. The IR adsorption peak at 1159 cm−1
(Fig. 10) is assigned to organic C C bonds, while the IR bands at
1009 and 1691 cm−1 are assigned to organic molecules or organic
matter and calcite structure bonds. O H and/or N H stretching
modes are also found in the region 3000–3500 cm−1 , which again
indicates the presence of organic matter. The peaks between 2500
and 3000 cm−1 are characteristic of carbonate compounds. Both the
PP and NP layer materials contained organic matter and N H and/or
O H bonds (Fig. 10). The IR peaks in the region 3000–3500 cm−1
(Fig. 10a) have a greater proportional change in the PP rather than
the NP layer materials between pre- and post-adsorption, which is
most likely because of the greater copper adsorption by PP than NP
(Fig. 5).
The pre- and post-adsorption PP IR spectra show some significant differences, indicating that physical and/or chemical changes
occur because of copper adsorption. First, a new peak appears
at 2875 cm−1 , whereas the peak at 3475 cm−1 disappears. Second, there is increased peak strength at 2515 cm−1 post-adsorption
(Fig. 10). These IR spectra changes are most likely because of the
ion-exchange between Cu2+ and OH− (Eqs. (7) and (8)). Furthermore, the peak at 1641 cm−1 (Fig. 10a) undergoes a blue-shift to
1691 cm−1 and the peak strength increases for the peaks at 1009,
1077, and 1159 cm−1 , suggesting that the conjugate action from
the organic matter may be affected by the ion-exchange or electrostatic attraction on PP surfaces during the adsorption process.
Similar results were observed for the NP layer, where the peak at
3457 cm−1 disappears (Fig. 10b), indicating that chemical changes
also occurred during adsorption of Cu2+ to the NP layer (possible surface coating, most likely CuCO3 precipitation). The peak at
2973 cm−1 undergoes a blue-shift to 2924 cm−1 , which may be
caused by physical adsorption and surface coating development
(again most likely CuCO3 precipitation).
271
3.5. SEM and EDAX analysis
SEM and EDAX of the surface structures of PP and NP pre- and
post-copper adsorption are shown in Fig. 11, where the PP layer
consists of an open-weave structure with a significant amount of
open 2–10-␮m pores (Fig. 11a and b). The pore walls consist of calcite lamellae, giving the PP layer a large specific surface area for
copper adsorption. However, the NP layer shows a dense parallellayered lamellar structure where the pore spaces between lamellae
are in the order of 100 nm (Fig. 11c). Fig. 11c shows surface etching
because of organic matter removal. The tight packing of the lamellae in the NP layer provides a greatly reduced surface area compared
with the PP layer materials (Fig. 11a and b), thereby limiting copper adsorption. Furthermore, the EDAX results show that although
both EDAX spectra show Cu signals, the Cu signal is much more
intense for PP materials than for the NP layer materials (Fig. 11b and
d). Hence, the EDAX and SEM data indicate that the shell structure
(Fig. 11) of PP provides greater porosity and surface area availability
than NP, which is the main reason for the higher copper adsorption
to PP than to NP (Fig. 5). This would suggest that the increased surface area for copper adsorption is also mainly responsible for the
differences in the XRD patterns (Fig. 9) and IR spectra (Fig. 10).
4. Conclusion
The results indicate that oyster shell is an effective and
potentially low-cost adsorbent for copper removal from aqueous
solution. The solution pH, solution concentration, metal affinity,
and zeta potential of the oyster shell greatly affected the adsorption
process, with an optimum adsorption pH of 5.5 and an overall negative surface charge facilitating the adsorption process. Electrostatic
attraction is the PP layer driving force for copper adsorption to the
whole oyster shell through displacement of attached hydroxyl moieties on the calcite surfaces. The adsorption process of both RP and
the more porous PP layer was best described by Langmuir and Freundlich models under low and high initial concentrations, respectively. In addition, copper ions are more readily removed to the PP
layer than the NP layer. Adsorption isotherm, equilibrium and metal
affinity data all demonstrated that it is the PP layer that plays the
dominant role in the adsorption of copper rather than the NP layer.
This dominance is most likely from the open network-like structure
of the PP layer, which provides a substantially larger surface area for
copper removal. This open network-like structure and large surface
area facilitating adsorption was also reflected in the XRD patterns,
TG–DSC generated, and IR responses. These data suggest that applications using waste oyster shell may benefit and be enhanced by
separation of the PP and NP layers, where the PP layer is used for
adsorption applications and the NP layer is used elsewhere.
Acknowledgments
This work was supported by the National Natural Science Fund
Project of China (No. 51102047) and the Fujian Provincial Natural
Science Fund for Distinguished Young Scholars (2012J06011).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.
2014.05.054.
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