Different cadmium adsorption behavior of juniper wood and bark sorbents

Abstract

In acidic aqueous solutions, cadmium (Cd) adsorption on sorbents made from juniper bark was greater than on sorbents made from juniper wood. Increase in Cd adsorption with increase in solution pH in the range 2-5, for both types of sorbents, suggested that surface carboxylic groups (RCOO^-) might be involved in the adsorption of Cd cations. The difference in adsorption behavior of the two types of sorbents appears to be related to the concentration of carboxylic groups available for interaction with Cd cations on the sorbent surface. Diffuse reflectance infrared Fourier transform (DRIFT) spectra showed that the surface concentration carboxyl groups were higher for bark compared to wood. However, metal displacement experiments showed that the calcium (Ca) content of the sorbents might also be important in the Cd adsorption process. The amount of Ca displaced by Cd adsorption was greater for bark compared to wood.

Full text

Eun Woo Shin, K.G. Karthikeyan, Jin Suk Chung et al. KORUS’2005 434
Different Cadmium Adsorption Behavior
of Juniper Wood and Bark Sorbents
Eun Woo Shin1), K. G. Karthikeyan2), Jin Suk Chung1), Ik-Keun Yoo1)
1) School of Chemical Engineering and Bioengineering, University of Ulsan, San 29,
Mugeo-dong, Nam-gu, Ulsan 680-749 South Korea,
Phone: +82-52-259-2253, Fax: +82-52-259-1689, E-mail: ewshin@mail.ulsan.ac.kr;
2) Department of Biological Systems Engineering, University of Wisconsin–Madison, 460 Henry Mall,
Madison, Wisconsin 53706 U. S. A.
Abstract - In acidic aqueous solutions, cadmium (Cd)
adsorption on sorbents made from juniper bark was greater
than on sorbents made from juniper wood. Increase in Cd
adsorption with increase in solution pH in the range 2-5, for
both types of sorbents, suggested that surface carboxylic
groups (RCOO-) might be involved in the adsorption of Cd
cations. The difference in adsorption behavior of the two
types of sorbents appears to be related to the concentration
of carboxylic groups available for interaction with Cd
cations on the sorbent surface. Diffuse reflectance infrared
Fourier transform (DRIFT) spectra showed that the surface
concentration carboxyl groups was higher for bark
compared to wood. However, metal displacement
experiments showed that the calcium (Ca) content of the
sorbents might also be important in the Cd adsorption
process. The amount of Ca displaced by Cd adsorption was
greater for bark compared to wood.
I. INTRODUCTION
Cadmium is one of the heavy metals that are
considered to be toxic to humans and aquatic life. Over
the last two decades there has been a sharp rise in the
global use of cadmium for batteries, and a steady decline
in its use for other applications, such as pigments, PVC
stabilizers, and plating. This trend in the use of cadmium
products and compounds has inspired the establishment
of a number of international agreements to manage and
control releases of cadmium to the environment and limit
human and environmental exposure to cadmium. Chronic
exposure to cadmium can cause kidney damage in
mammals and humans [1, 2]. One of the major sources of
surface water contamination by heavy metals such as
cadmium is urban and agricultural stormwater runoff.
There is therefore a great need for new and cost-effective
processes for preventing excess concentrations of these
toxins from accumulating in our streams, ponds and
lakes.
Various processes, including chemical precipitation
and reverse osmosis, have been developed for removal of
cadmium from wastewater. However, when applied to
non-point sources of cadmium contamination such as
stormwater runoff, these processes can be expensive to
implement. Consequently, there is growing interest in the
use of sorbents made from low-cost renewable materials
such as solid wood waste or bark.
Several natural sorbents including algal biomass[2-6],
peat moss[7, 8], bark [9-13] and sugar beet pulp[14] have
been investigated for their ability to sequester Cd from
water. Adsorption of Cd from aqueous solutions can take
place via two mechanisms, ion-exchange and
complexation. In the ion-exchange mechanism, Cd binds
to anionic sites by displacing protons from acidic groups
or existing light metals from anionic sites at high pH [2,
7, 8]. This mechanism explains well the observation
made during heavy metal uptake experiments such as the
release of light metal ions. Complexation mechanism
views Cd sequestration as the coordination of Cd to
surface functional groups. Cd adsorption is considered
complex formation where Cd is designated as the central
atom and surrounding atoms as the ligand(s) [3].
However, in both cases, the extent of Cd adsorption from
aqueous solutions is strongly influenced by the surface
chemistry of the sorbent on one hand, and the solution
chemistry on the other hand. For example it has been
suggested that uptake of copper by wood takes place by
several mechanisms: reaction between CuII species and
surface carboxylic groups (RCOO-); hydrogen bonding
of hydrated, Cu(H2O)62+ ions with cellulose; and
formation of complexes with surface hydroxyl groups of
lignin [12]
Recently, Min et al [15] reported on the use of juniper
fiber for removal of Cd from aqueous solution. The
juniper fiber consisted of a mixture of wood and bark. To
improve the adsorption capacity of the juniper fiber, they
treated it with base. They observed that base-treatment of
the juniper fiber not only increased Cd adsorption
capacity, but also that adsorption of Cd was greater for
bark compared to wood. However, there was no
explanation as to why bark performed better than wood.
They focused on improvement of sorption capacity and
change in a carboxylate functional group by base-
treatment. In this study, juniper wood and juniper bark
sorbents were prepared separately, and their adsorption
behavior towards Cd in aqueous solutions of different pH
values was investigated. An attempt was made to
correlate their adsorption behavior with the surface
chemistry of each type of sorbent. For characterizing
adsorption behavior, Cd adsorption kinetics, isotherms,
and edge were performed and Ca and Na displacement
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KORUS’2005 Eun Woo Shin, K.G. Karthikeyan, Jin Suk Chung et al. 43
5
were monitored simultaneously during adsorption
experiments. The surface chemical composition of each
type of sorbent was characterized by inductively coupled
plasma atomic emission spectrometry (ICP-AES), and by
diffuse reflectance infrared Fourier transform (DRIFT)
spectrometry.
II. EXPERIMENTAL
A. Materials and characterizations
A small diameter tree, juniper (Juniperus
monosperma) was randomly collected from New Mexico,
and shredded into small chips at the Forest Products
Laboratory. Bark was separated from wood chips and
then, each was ground to pass through a 3-mm screen
using a Wiley mill. Juniper wood and bark were denoted
JW and JB, respectively.
Elemental analysis of Cd, Ca and Na were conducted
with an inductively coupled plasma atomic emission
spectrometer (ICP-AES, ULTIMA, Jobin Yyon Inc.,
Edison, New Jersey) at the Forest Products Laboratory.
Diffuse reflectance infrared Fourier transform (DRIFT)
spectra were monitored using a Mattson Galaxy 5020
(Mattson Instruments, Madison, WI) equipped with a
Harrick Scientific diffuse reflectance accessory (Harrick
Scientific Co., Ossining, NY). Each spectrum was
acquired with 4000 scan between 400 and 4000 cm-1
(resolution = 4 cm-1). Prior to analysis, samples were
finely ground using a Wiley mill and sieved with a 0.18
mm screen. For comparison, each spectrum was baseline
corrected at 400, 840, 2000, and 4000 cm-1 and
normalized against the 1320cm-1 band associated with the
C-H bending mode [16]. Zeta potential of the biosorbents
was measured with a ZETASIZER 3000HS (ATA
Scientific Ltd., Lucas Heights, Australia). 25 mg of the
sample was suspended in 40 ml of deionized water and
pH of solution was adjusted between 2 and 10 using
either 0.1 M KOH or 0.1 M HNO3. After pH adjustment,
suspensions were equilibrated in a shaker for 4 hrs and
then, the zeta potential of solutions was measured.
B. Adsorption tests
Cd adsorption isotherms were acquired through batch
experiments. Samples weighing between 0.02 and 1.0 g
were placed in 40 mL plastic tubes with 35 mL of 20
mg/L Cd ion solution (C0). The initial Cd solution was
prepared by serial dilution of standard 1,000 mg/L
reference solution (Fisher Scientific, Pittsburgh, PA). The
initial pH of the solution was adjusted to 5.6 ± 0.1. The
sealed bottles were shaken in a shaker for 4 hrs at 298 K.
The supernatants of the solutions were filtered by 0.45-
µm (pore size) membrane filters and then measured for
dissolved cadmium concentration by ICP-AES. The final
concentration (Ce), measured in milligrams per liter,
differed according to varying sample amount in the
solutions. The uptake capacity (qe), the amount of
cadmium ions adsorbed at equilibrium (mg/g), was
calculated by mass balance between C0 and Ce.
Adsorption kinetic experiments were performed in
1000 mL solutions with 1.0 g of samples at pH 5.0 ± 0.1.
The initial Cd concentration of the solution was 20 mg/L,
and the pH of the solution was constantly maintained
during the experiment using 0.1M HNO3 and 0.1M KOH.
The suspension was stirred by a magnetic bar and
supernatants were taken at various times during the 2 hrs
experiment. The Cd concentrations of the filtered
solutions were measured with ICP-AES.
Cd uptake (qe) and rate constant (k) of adsorption were
determined from the pseudo second-order rate equation.
This model assumes that adsorption follows the
Langmuir equation [17].
The kinetic rate equations can be written as follows:
(
2
tet
dq kq q
dt =−
)
(1)
where qt, and qe is the amount of cadmium adsorbed at
time t and equilibrium (mg/g), respectively and k is the
equilibrium rate constant of the second order sorption
(g/mg min).
Adsorption edge experiments were conducted in 40 mL-
sample tubes containing 30 mg of biosorbents and 30 mL
of 20 mg/L-Cd solution whose pH is different. The
sample tubes were placed in a shaker for 4 hrs for
equilibrium and then the pH of solutions and final Cd
concentration were measured.
III. RESULTS AND DISCUSSION
A. Elemental analysis
Ca and Na contents in the samples are shown in
Table 1. Those light metals are well known as the counter
ions that involve in ion-exchange mechanism for Cd
removal [18]. In both the biosorbents, Na content is much
smaller than Ca content, which implies that the role of Na
in metal displacement with Cd is negligible. In addition,
Ca amount in JB is 945 µmol, which is 15 times higher
than that in JW.
TABLE 1
CONTENS OF CALCIUM AND SODIUM IN EACH SAMPLE
Ca Na
Samples mg/g µmol/g mg/g µmol/g
JW 2.47 61.7 0.40 17.4
JB 37.9 945 0.75 32.6
B. Zeta potential
All the points in the graphs show negative charge. In
other words, the surface charge of the adsorbents are
negative even around pH 2.5. This indicates that those
adsorbents can act as a cation remover at low pH.
However, the difference in zeta potential between JB and
JW is negligible in pH range (2.5-5.5) of adsorption
condition.
C. Cadmium adsorption behavior
Kinetics: Fig. 1 and Table 2 show adsorption kinetic
results and parameters fitted to the pseudo second order
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Eun Woo Shin, K.G. Karthikeyan, Jin Suk Chung et al. KORUS’2005 43
6
equation. Generally known, the adsorption of Cd onto the
lignocellulosic adsorbents was so fast that the equilibrium
was reached within 30 min. The adsorption amount at
equilibrium (qe) for JB and JW are 91.6 and 24.8 µmol/g,
respectively, indicating that adsorption capacity of JB for
Cd removal is about four times higher than that of JW.
Those values are consistent with those obtained from
other experiments in this study and reported in previous
study [14].
0 20406080100
0
20
40
60
80
100
Cd uptake (µmol/g)
Time (min)
JB
JW
pseudo second order fittings
Fig. 1. Cadmium adsorption kinetics for JB and JW. (Initial Cd
concentration = 20 ppm, pH = 5.5)
TABLE 2
KINETIC PARAMETERS FROM THE FIT OF KINETIC DATA TO THE
PSEUDO-SECOND-ORDER EQUATION
Kinetic parameters
Samples k (µmol/min) qe (µmol/g) R2
JB 0.934 91.6 0.997
JW 0.649 24.8 0.962
Isotherms: adsorption isotherms are shown in Fig. 2. In
General, isotherms are fitted to the Langmuir equation. In
this study, however, the Langmuir fitting was not
employed because maximum Cd adsorption amount
would be overestimated as results of poor fittings.
Instead, the Cd uptake around 12 mg/L of final
concentration was compared as adsorption capacities. In
the isotherms, the Cd uptake around 12 mg/L for JB and
-4 0 4 8 1216202428
0
20
40
60
80
100
120
Cd uptake (µmol/g)
Final Concentration (mg/L)
JB
JW
Fig. 2. Cadmium adsorption isotherm for JB and JW
JW are about 98 µmol/g and 16 mol/g respectively,
showing a good match with the results from adsorption
edges shown in next section. The values are slightly
different from the capacity obtained from kinetics
because of pH effect on adsorption. In the adsorption
isotherm, the initial solution pH was 5.6 before being
added to the adsorbents. However, the final solution pH
for JW and JB were 4.8 ± 0.1 and 5.3 ± 0.1, respectively
since the pH of solution during the test was not adjusted.
When compared with qe from kinetic experiment where
pH condition was 5.0, the Cd uptake of JW in the
isotherm was low and that of JB relatively high. The
difference in final pH condition affected adsorption of Cd
onto the adsorbents.
Adsorption edges (pH effect on the adsorption): Fig. 3
shows adsorption capacities of both the adsorbents for Cd
at different pH conditions. On the whole, the adsorption
capacities of both the adsorbents increased with pH of
solution because protons suppressed the adsorption of Cd
in a low pH condition where protons compete
predominantly against Cd.
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
0
20
40
60
80
100
120
Cd uptake (µmol/g)
pH of solution
JB
JW
Fig. 3. Dependence of cadmium uptake on the pH of solution
In Figure 4, the adsorption capacities of JB and JW
around pH 2 are almost zero whereas the difference in the
adsorption capacities above pH 5 is more than a factor of
three. This implies that the adsorption sites on both the
adsorbents might be originated from the same surface
functional group. According to literatures [3, 19],
carboxyl group and sulfonic acid groups are identified as
main adsorption sites on the adsorbents. The acidity
constant (pKa) of carboxyl group is between 3.5-5.5,
indicating that it is weak acidic site [19, 20]. In contrast,
the pKa of sulfonic acid is around 1.5, which is strong
acidic site[18]. In other research [15], the difference in
adsorption capacities between sulfonated juniper wood
and untreated juniper wood was larger at a low pH
condition than at a high pH condition because the
sulfonated juniper wood contained the sulfonic group that
is strong adsorption site even at a low pH condition.
Therefore, the disappearance of adsorption capacities of
both the biosorbents around pH 2 can be interpreted that
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KORUS’2005 Eun Woo Shin, K.G. Karthikeyan, Jin Suk Chung et al. 43
7
the surface functional group on both the adsorbents might
be the carboxyl group that is only the functional group
responsible for Cd adsorption.
1450 1500 1550 1600 1650 1700 1750 1800 1850
0
1
2
3
4
5
6
7
8
9
10
11
12 17421664
1605
1590
JB
JW
Intensity
Wavenumber (cm-1 )
B
500 1000 1500 2000 2500 3000 3500
-2
0
2
4
6
8
10
12
14
16
A
Fig. 4. Diffuse reflectance infrared Fourier transformation
spectra of JB and JW.
D. Diffuse reflectance infrared Fourier transformation
(DRIFT)
To understand the role of functional groups responsible
for Cd adsorption, DRIFT spectra of juniper wood and
juniper bark were obtained. Figure 7A shows the whole
spectra of both the adsorbents. The bands assignments are
presented in Table 3. The IR bands consist of four
regions: the broad hydrogen band (3200 cm-1-3600 cm-1),
C-H stretching region (2800-3000 cm-1), carbonyl group
stretching (1550 – 1750 cm-1), and the finger print bands
(below 1550cm-1). Most remarkable difference between
two spectra appears in intensity of carbonyl functional
group around 1600-1700 cm-1. The bands intensity of
TABLE 3
IR BAND ASSIGNMENTS
Positions Assignments References
3000-3600 O-H stretching [2, 21]
2930 C-H stretching [2, 21]
1743 C=O stretching from ester [15, 22]
1660 C=O stretching from ketone and
aldehyde [15, 22]
1630 Water [15, 22]
1605 COO- stretching (asymmetric) [2, 15, 21, 22]
1590 Aromatic ring vibration [15, 22]
carbonyl groups for JB is much larger than that for JW.
The detailed IR bands for carbonyl functional groups are
shown in Figure 7B. In this region, there are four major
bands: aromatic skeleton vibration at 1590 cm-1,
carboxylate (COO-) asymmetric stretching at 1605 cm-1,
carbonyl group from ketone and aldehyde at 1664 cm-1,
and carbonyl group from ester at 1742 cm-1. Among those
functional groups, COO- stretching IR band has been
mentioned to be responsible for heavy adsorption onto
biosorbents [2, 14]. In Figure 7B, the band height at
1605 cm-1 for JB is 8.6 while the band height for JW 3.4,
indicating that JB contained much more carboxylate
functional group than JW. Therefore, higher Cd
adsorption capacity of JB is caused by more carboxylate
content in JB.
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