Accepted Manuscript
Title: Simultaneous adsorption of heavy metal ions and anions
from aqueous solutions on chitosan − Investigated by
spectrophotometry and SEM-EDX analysis
Author: Mandy Mende Dana Schwarz Christine Steinbach
Regine Boldt Simona Schwarz
PII:
DOI:
Reference:
S0927-7757(16)30671-9
http://dx.doi.org/doi:10.1016/j.colsurfa.2016.08.033
COLSUA 20919
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date:
Revised date:
Accepted date:
1-3-2016
9-8-2016
20-8-2016
Please cite this article as: Mandy Mende, Dana Schwarz, Christine Steinbach, Regine
Boldt, Simona Schwarz, Simultaneous adsorption of heavy metal ions and anions
from aqueous solutions on chitosan − Investigated by spectrophotometry and SEMEDX analysis, Colloids and Surfaces A: Physicochemical and Engineering Aspects
http://dx.doi.org/10.1016/j.colsurfa.2016.08.033
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Simultaneous Adsorption of Heavy Metal Ions and Anions from Aqueous
Solutions on Chitosan – investigated by Spectrophotometry and SEM-EDX
Analysis
Mandy Mende*, Dana Schwarz¤, Christine Steinbach*, Regine Boldt*, Simona
Schwarz*
*Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden,
Germany
¤ Charles University in Prague, Faculty of Science, Department of Organic
Chemistry, Hlavova 2030/8 128 43 Prague 2, Czech Republic
1
GA
2
HIGHLIGHTS




Removal of heavy metal ions from aqueous solution with chitosan flakes
Detailed characterization of surface structure by SEM
Formation of crystal-like structures on chitosan surface
Validation of simultaneous adsorption of salt cations and anions by SEM-EDX
Abstract
Chitosan (flakes with a degree of deacetylation of 90 %) was used as adsorbent for
heavy metal ions in solution (copper, iron, nickel). The adsorption capacities were
determined in dependence on the adsorption time and the initial metal salt
concentration. With increasing adsorption time as well as the initial metal salt
concentration the adsorption capacities increased. Highest adsorption capacity was
achieved for copper(II)ions with 110 mg/L. Iron(II)- and nickel(II)ions adsorbed with
an adsorption capacity of 80 mg/L.
The surface of chitosan flakes were investigated before and after the adsorption
process by SEM and SEM-EDX, respectively. The formation of crystal-like structures
was observed by SEM analysis for the investigation of copper(II)sulfate and
iron(II)sulfate. It has been noticed that iron(II)ions oxidized before the adsorption on
chitosan occurs. In comparison, the adsorption of nickel salt resulted in a smooth
layer on the chitosan surface. SEM-EDX analysis revealed that sulfate adsorbs on
the chitosan surface besides the metal cations used.
Keywords: Heavy Metal Ion Adsorption, Chitosan, Natural Polymers, Waste
Water Treatment
1. Introduction
Pollution by heavy metals is a serious threat for the environment, aquatic ecosystem,
and human health. Hence, heavy metals are no biodegradable and potentially toxic
at very low concentrations [1]. The removal of inorganic components/particles by
various methods including precipitation, ion exchange, reverse osmosis, and
membrane processes have been intensively investigated during the last years [2]. It
is much more difficult to remove soluble components like dye [3,4], surfactants, or
metal ions due to economic constraints, or strict regulatory requirements. Besides the
well known high cost adsorbents such as activated carbon and some ion-exchange
resins, natural materials which are available in large quantities as industrial waste
products might be a beneficial displacement towards more effective and cheaper
adsorbents [5,6].
3
Chitosan is a biodegradable polysaccharide and originates from the natural polymer
chitin [7] . Latter is the second most abundant polymer in nature next to cellulose and
is mainly obtained from the shells of crustacean which accumulate as waste in large
quantities countries located on the Pacific and Atlantic coasts with fishery factories
(e.g. Russia, China, Nicaragua, or Vietnam). At an industrial level, the acetyl groups
in the natural chitin polymer structure are converted into secondary amino groups by
the treatment with sodium hydroxide. Furthermore, it can also be converted by
enzymatic deacetylation [8]. Hence, chitosan is most often a mixture of both
functional groups (see Scheme 1). The field of applications for chitosan is huge and
increases constantly (e.g. fungicide, Biomedical and Pharmaceutical, dieting, or
paper industry) [9]. Particularly the implementation as an adsorber material in
wastewater and drinking water treatment for various kinds of impurities (e.g. heavy
metal ions, anions, pharmaceutics, or dyes) has attracted a lot of attention within the
last years [10,11,5]. The adsorption capacity depends on several parameters, such
as the degree of deacetylation, the molecular weight, crystallinity, and particle size
[12]. In addition, the adsorption process is influenced by solution properties like pH,
metal ion concentration, and the composition of the solution (i.e. ionic strength and
concentration of the different types of ions) [13] .
Various types of adsorption mechanisms are known (e.g. ion-exchange,
complexation,
coordination/chelation,
electrostatic
interactions,
acid-base
interactions, hydrogen bonding, hydrophobic interactions, physical adsorption, or
precipitation) and most of the time several of them proceed at the same time [1] . The
long known good adsorption of heavy metal ions on chitosan is mainly attributed to
the presence of amine groups (-NH2) exhibiting coordination sites for metals such as
Cu(II), Ni(II), Zn(II), Cd(II), Hg(II), Cr(III), V(IV), and U(VI) [14-19]. Additionally, the
hydroxyl group in C3 position of the chitosan unit is available as binding site too [20].
For a low pH-value in solution, chitosan acts as a weak basic anion exchange resin
due to the protonated amino group [21]. Metal anions like arsenate [22], and
chromate can interact with the positive charge of the protonated amino groups by
electrostatic interaction. With increasing pH-value the overall positive charge on the
surface of the polymer decreases. Hence, the formation of chelate complexes
between metal cations and the lone pair of electrons on the nitrogen atom of the
uncharged amino groups is preferred. Most studies investigating the formation of
chelate complexes were carried out with copper ions. Essentially two models have
been proposed, the pendant model and bridge model. Latter was proposed by
Schlick [23] suggests a coordination number of 4 of the chelate complex forming a
square planar structure (see Figure 1a). That means that the metal cation is
coordinated by four nitrogen atoms and hence intramolecular as well as
intermolecular interactions occur.
The pendant model was proposed by Ogawa and Oka [24] and assumes an
interaction between the metal cation and one amino group only (see Figure 1b)
based on an x-ray study of chitosan complexes with different metal cations (i.e.
4
copper, zinc, and cadmium). Domard [25] supported this model by potentiometric
and dichroic data. He suggested a [Cu NH2 (OH)2]0 copper complex with one amino
group as ligand and two hydroxyl groups, which is the only uncharged structure. The
fourth binding site could be occupied by a water molecule or the hydroxyl group in C3
position of the chitosan unit.
However, there are several studies that show diversities from these two models.
Rhazi et al. [26] found a dependence of the coordination number of the chelate
complex on the pH of the solution. In a pH range of 5.3 to 5.8 the coordination
number of the complex ([Cu(-NH2)]2+, 2OH-, H2O) is 1 resulting in a similar complex
formation as shown in Figure 1b. At a higher pH the coordination number is 2 (see
Figure 1c).
Piron and Domard [27] gave evidence that the adsorption of strontium(II) and
barium(II)ions on chitosan is possible in the presence of carbonate due to the
formation of ternary complexes. They assumed that the interaction between chitosan,
carbonate, and strontium ions do not base on electrostatic interaction because the
ion pair strontium and carbonate is formed first, followed by the complexation to the
amino group on chitosan.
In this work the simultaneous adsorption of heavy metal ions (copper, iron, nickel)
and the salt anion (sulfate) is studied. The adsorption capacity of the chitosan flakes
for the investigated metal ions was determined at different concentration and in
dependence on time. SEM-EDX analysis was a very useful tool to show the
simultaneous adsorption of the metal cations and the corresponding anion (sulfate),
which leads to the formation of crystal-like structures at higher concentrations.
2. Experimental
2.1 Chemicals
Chitosans
All chitosans were purchased from the BioLog GmbH, Germany, and used as
received. Chitosan flakes with a degree of deacetylation of 90 % were used in all
experiments. The wide particle size distribution of the flakes was in mm range.
Salts
Used salts (i.e. copper(II)sulfate (anhydrous) and nickel(II)sulfate hexahydrate were
purchased from Sigma-Aldrich. Iron(II)sulfate heptahydrate was purchased from Carl
Roth GmbH + Co. KG, Germany. The salts were used as received.
2.2 Adsorption experiments
The adsorption capacities were determined in dependence on the initial
concentration of heavy metal ions in solution, as well as on the time of adsorption
(contact time between adsorbent and adsorbate). The adsorption investigations were
carried out as batch experiments. Adsorption capacities were calculated from
5
absorbance measurements. The concentration of heavy metal ions in solution was
measured before and after the adsorption procedure at a defined time.
To study the effect of contact time on heavy metal adsorption 20 mL of the heavy
metal solution with an initial cation concentration of 180 mg/L were mixed with 0.1 g
chitosan flaks in a 100 mL baker. The suspensions were stirred for the desired time
at 25 °C. For all adsorption experiments the pH was not adjusted. The initial pHvalues of copper, iron, and nickel sulfate solutions were 5.5, 4.5, and 6.0,
respectively.
Experiments to elucidate the effect of initial metal ion concentration on equilibrium
were realized by adding 20 mL of metal ion solutions with different initial cation
concentration to 0.1 g chitosan. The samples were stirred with a magnetic stirrer for
24 h at 25 °C.
2.3 Analytical Methods
Spectrophotometry
We used the DR2800 of the HACH Lange GmbH, Germany, to determine the metal
cation concentration in solution. It is a visible light spectrophotometer with preprogrammed test methods, so-called cell tests. During the testing process each
sample is rotated and 10 measurements were done to give a concentration value (or
transmittance, or absorbance respectively).
In our studies the cell tests for copper, iron and nickel were relevant. Copper(II)ions
in solution are reduced by ascorbic acid. Then it is converted with Bathocuproine
disodium salt to an orange complex. Iron(II)ions and 1,10-phenanthroline react to an
orange-red complex. Iron(III)ions may be present in solution are reduced by ascorbic
acid to iron(II)ions before. The nickel ions presented in solution form an orangebrown precipitate with dimethylglyoxime in alkaline solution.
SEM
SEM images were detected with the scanning electron microscope Ultra plus from
Carl Zeiss NTS, Germany. All samples were prepared on a graphite carrier and
coated with a 3 nm layer of platinum.
SEM-EDX analysis
With the combination of scanning electron microscopy and energy dispersive x-ray
spectroscopy chitosan surfaces before and after adsorption processes were
analyzed. It was done with the Ultra Plus from Carl Zeiss NTS, Germany, equipped
with an EDX-Detector XFlash Quad 5060F from Bruker Nano GmbH, Germany.
3. Results and Discussion
Adsorption experiments
The adsorption of copper, nickel, and iron on Chitosan flakes with a deacetylation
degree of 90 % were studied. The heavy metal ions copper, nickel, and iron are
common impurities in water and hence they were utilized for the adsorption
6
investigations. Sulfate was the anion for all three salts investigated. Copper
concentrations above 1.3 mg/L may cause serious concerns to human health [28-30].
The critical concentration of nickel ions is about 10 times lower. Copper, nickel, and
iron are one of the most widely used heavy metals.
Figure 2 shows the adsorption efficiency of copper, iron, and nickel ions on chitosan
flakes in dependence of the time of adsorption. Copper exhibits the highest
adsorption efficiency followed by iron(II), and nickel(II)ions. The adsorption efficiency
was 95 % for copper ions. Iron and nickel have an adsorption efficiency of about 75
% only. Within the first hour of adsorption, the rate of adsorption is very fast and
decreases towards reaching the adsorption equilibrium. The slope until equilibrium
reached is steep for copper and less for nickel. For a real heavy metal waste water
the selective separation of copper and nickel could be successful because of the time
effect during the adsorption process. Until now only pure salt ions were investigated.
For further investigations in the future the adsorption of mixtures and real systems
are of interest. The adsorption equilibrium has been reached after 24 hours for all
samples.
Furthermore, the adsorption capacities of the heavy metal ions were studied for an
adsorption time of 24 hours to detect the effect of initial cation concentration on the
adsorption process (s. Figure 3). The highest adsorption capacity was achieved for
copper ions with 110 mg/g whereas the adsorption capacities of iron and nickel with
80 mg/g were lower in comparison to copper. Iron and nickel exhibit similar
adsorption behavior. This sequence matches very well to the adsorption experiment
shown in Figure 2. The adsorption of copper on chitosan has been studied in a few
publications before [31] . Recently we investigated different morphologies of chitosan
like powder, flakes and chitosan beads (alginate coated with chitosan) with regard to
the influence of particle size on adsorption capacities of chitosan [31]. It was found,
that the adsorption capacities differ significantly in dependence on the surface area.
The adsorption capacity follows the order: powder > flakes > chitosan beads. The
achieved adsorption capacity of 150 mg/g for copper ions adsorbed on chitosan
powder is relatively high in comparison to other adsorbents. However, how reliable is
the comparison with other studies from other groups and with other materials as so
many parameters have an influence on the adsorption capacity. In the literature it is
difficult to find consistent information about the adsorption capacities of heavy metal
ions on chitosan. A good compilation of references with absorption capacities of
chitosan and its derivatives for copper are found in Kyzas et al. [32]. However, as
mentioned before chitosan is a natural polymer resulting in a variety of chitosan
materials with different properties.
A comparison with other adsorber materials for copper ions is shown in table1.
Typically adsorber materials are bentonite (30 mg/g and 7.72 mg/g) [33, 34], clay,
silica gel (0.52 mg/g) [35], modified activated carbon (140 mg/g and 16 mg/g) [36, 37]
and others. In the most cases the adsorption capacity is much lower than for
7
chitosan, especially for natural adsorber material like tea waste (47.9 mg/g) [38], peel
(1.3 mg/g) [39], or birch wood (1.46 mg/g) [40] . Only for modified activated
carbon the adsorption capacity for copper ions with 140 mg/g [36] is in the
range of the copper adsorption on chitosan powder with 150 mg/g [31]. We will
publish the results soon. Recently we coated inorganic materials like clay or
silica with chitosan to obtain an efficiently use with a defined layer thickness,
surface consistency, and particles size. We will publish the results soon.
SEM-EDX analysis
All solid salts used are colored. Copper sulfate pentahydrate is blue, nickel sulfate
hexahydrate and iron(II)sulfate heptahydrate are green to turquoise. The colors of the
salt solutions depend on salt concentration. After the adsorption experiments, the off
white chitosan material has always undertaken the color of the heavy metal salt in
case of the copper and nickel salt. In case of iron(II)sulfate chitosan flakes appear in
an orange to brown color. It is necessary to get a better understanding of the
adsorption model and thereby the reason for color changes.
For this purpose, the chitosan samples were characterized by SEM and SEM-EDX
analysis before and after adsorption of heavy metal ions. Figure 4 shows the SEM
image of the pure and untreated chitosan surface before heavy metal adsorption. We
observe a rough uneven surface.
SEM images of the chitosan surface after copper adsorption in dependence of the
initial concentration (0.1g/L; 1.0g/L; 3.0g/L; 5.0g/L) of the solution are displayed in
Figure 5 a-d.
As expected from the color change, the surface structure changes significantly in
dependence of the initial copper sulfate concentration. At high copper salt
concentrations crystal-like structures occur on the chitosan surface and the more
comprehensive and developed are the crystal-like structure formed. Furthermore,
with increasing the initial copper sulfate concentration we observed a change of the
structure shape from plates to needles too (Fig. 5 a – d). This could be an
explanation for the change in the intensity of the blue color of the chitosan flakes with
higher initial salt concentration as the refraction of light at the surface changes. The
color of crystal-like structure (blue) suggests to us that beside the adsorption of the
heavy metal ions, sulfate was adsorbed too. EDX analysis confirmed the adsorption
of both ions resulting in the crystal-like structure (Fig. 5e). The elemental distribution
images from SEM-EDX analysis revealed that the whole chitosan surface is covered
by copper as well as by sulfur. Furthermore, the deposition of copper and sulfur
increases with increasing the initial salt concentration, which is displayed by the peak
8
areas of the EDX spectra. In the case of pure chitosan no copper and sulfur was
found on the surface. From this result we deduced that during the adsorption process
copper(II)ions as well as sulfate anions were adsorbed on the chitosan surface.
Figure 6 presents SEM images (Fig. 6a-d) and element distribution images (Fig. 6e)
of chitosan surface after the treatment with iron(II)sulfate for initial salt concentrations
of 0.05 g/L; 1.0 g/L; 3.0 g/L, and 5.0 g/L. We made similar observations compared to
the copper adsorption on chitosan. With increasing initial salt concentration a crystallike thicker covering on the chitosan surface is formed and the color of chitosan
flakes changes from orange to dark brown. The micro structure of the covering is like
needles and exhibits even at low initial salt concentrations (0.05 g/L iron(II)sulfate;
Fig. 6a).
From SEM-EDX analysis (Fig. 6e) sulfur was found to deposit uniformly on the whole
chitosan surface in contrast to iron. However, iron was detected only at certain sites
on the chitosan surface. Probably iron(II)ions oxidize before adsorption on chitosan
surface occurs. Iron would be adsorbed as iron oxide.
In Figure 7 a and b the SEM images of the chitosan surface after the adsorption of
nickel(II)ions with an initial nickel(II)sulfate concentration of 1.0 g/L (7a) and 3.0 g/L
(7b) are displayed. Despite the relatively high initial salt concentrations we do not
observe a formation of crystal-like surface structure. In the case of the lower initial
salt concentration (Fig. 7a; 1.0 g/L) the surface structure resembles to the pure
chitosan (see Fig. 4). However, the color change of the chitosan flakes during the
adsorption process display macroscopically that nickel and possibly sulfate ions were
adsorbed due to the same color of the pure solid salt used.
SEM-EDX analysis confirmed this presumption. From EDX spectra we found that
nickel as well as sulfur adsorbed on chitosan surface. Additionally, the elemental
distribution image shows the uniformly distribution of nickel as well as sulfur over the
whole chitosan surface. As in the case of adsorption of copper on chitosan we
conclude that the cations and anions of the salt adsorbed simultaneously. When
iron(II)sulfate was mixed with chitosan flakes the salt anion (sulfate) adsorbs too, but
the oxidation of iron(II)ions seems to be very fast and results in adsorption of iron
oxide. Feasible reasoning could be the investigated pH range (5-6). Due to the
coexistence of protonated and unprotonated amine groups in this pH range salt
cations are able to form chelate complexes with unprotonated amine groups and salt
anions are able to adsorb on chitosan surface by electrostatic interaction
simultaneously. So we propose a slightly different absorption mechanism that is
schematically illustrated in Figure 8 compared to those which are shown in Figure 1.
9
4. Conclusion
Chitosan with a degree of deacetylation of 90 % is a good adsorbent for heavy metal
ions in solution. First the dependence of the adsorption capacity on the adsorption
time and the initial metal salt concentration was investigated. With increasing
adsorption time as well as the initial metal salt concentration the adsorption capacity
increases. Highest adsorption capacity was achieved for copper(II)ions with 110
mg/L. Iron(II)- and nickel(II)ions adsorbed with an adsorption capacity of 80 mg/L.
By SEM analysis the formation of crystal-like structures was observed at higher
adsorption time and higher initial salt concentration, when copper(II)sulfate and
iron(II)sulfate were used. The adsorption of the nickel salt resulted in a smoother
layer on the chitosan surface.
SEM-EDX analysis revealed that sulfate adsorbs on chitosan surface besides the
metal cations used. In the case of copper and nickel sulfate all elements (i.e. copper,
nickel, and sulfur) are uniformly distributed over the whole chitosan surface. This fact
gives evidence that both cation as well as anion of the used salts adsorbed on
chitosan surface. The distribution of the element iron on chitosan surface is not
uniformly over the whole chitosan surface due to the rapid oxidation of the iron(II)ions
on air resulting in partially adsorbed iron oxide. Sulfur was uniformly distributed on
chitosan surface too. However, iron was only detected at certain areas on chitosan.
The simultaneous adsorption of salt cations and anions used can be interpreted due
to the investigated pH range between 5 and 6 in which the adsorption experiments
were carried out. The coexistence of protonated and unprotonated amine groups
allows that salt cations form chelate complexes with unprotonated amine groups and
salt anions adsorb on chitosan surface by electrostatic interaction.
5. Acknowledgement
This work was supported by the Central Innovation Programme (ZIM) of the Federal
Ministry of Economy and Energy (BMWi) (KF 2022812RH1). The authors thank
Heppe Biolog GmbH from Germany for the support of the materials and discussion
and cooperativeness.
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aqueous solutions using hazelnut shell activated carbon, Chem. Eng. J. 148 (2009) 480-487.
[38] B.M.W.P.K. Amarasinghe, R.A. Williams, Tea waste as a low cost adsorbent for the removal of Cu
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12
a
b
c
Figure 1: Coordination models of Cu(II)-Chitosan complex: a – Bridge model; b
– Pendant model; and c – pH > 5.8 [23,26].
13
adsorption efficiency / %
100
75
50
25
Cu2+
Fe2+
Ni2+
0
0
1
2
20
30
40
50
60
70
80
90
100
tads / h
Figure 2: Adsorption efficiency of the heavy metal ions copper, iron, and nickel
in dependence of adsorption time with an initial cation concentration of 180
mg/L and initial pH values of 5.5, 4.5, and 6.0, respectively.
14
125
100
q / mg/g
75
50
Cu2+
Fe2+
Ni2+
25
0
0
50
100
250
500
CMe
2+
-eq
750
1000
1250
1500
/ mg/L
Figure 3: Adsorption isotherm of the heavy metal ions copper, iron, and nickel
on chitosan flakes, adsorption time 24 h, 25 °C and initial pH values of 5.5, 4.5,
and 6.0, respectively.
15
Figure 4: SEM image of the pure surface of a chitosan flakes.
Figure 5: a-d) SEM images (black and white, 5k x magnification) of chitosan
surface after copper adsorption in dependence on initial salt concentration of
copper(II)sulfate (a: 0.1 g/L; b: 1.0 g/L; c: 3.0 g/L; d: 5.0 g/L) after 24 hours time
of adsorption; e) SEM-EDX element distribution images for Cu and S and EDX
spectra of pure chitosan surface (orange line), chitosan surface after treatment
with an initial CuSO4 concentration of 0.1 g/L for 24 h (red line); and chitosan
surface after treatment with an initial CuSO4 concentration of 4.0 g/L for 24 h
(green line).
16
e
Figure 6: a) – d) SEM images (black and white, 5k x magnification) of chitosan
surface after iron adsorption in dependence on initial salt concentration of
iron(II)sulfate, (a: 0.05 g/L; b: 1.0 g/L; c: 3.0 g/L; d: 5.0 g/L), adsorption time 24
h; e) SEM-EDX element distribution images for Fe and S and spectra of pure
chitosan (orange line) and chitosan after treatment with initial iron(II)sulfate of
0.1 g/L and an adsorption time of 24 h (red line).
17
Figure 7: a)-b) SEM images with 2k x magnification (black and white) of
chitosan surface after nickel adsorption in dependence on initial salt
concentration of nickel(II)sulfate, (a: 1.0 g/L; b: 3.0 g/L), adsorption time 24 h,;
c) SEM-EDX element distribution images for Ni and S and spectra of chitosan
after adsorption process with 1g/L NiSO4 and an adsorption time of 24 h.
18
An2‐
Me
2+
NH2
An2‐
An2‐
Me2+
2‐
An
NH3
Me
2+
NH2
Me2+
2‐
Me2+
An
NH3
NH2
Figure 8: Schematically adsorption mechanism of heavy metal cations and salt
anions on chitosan by formation of chelate complexes and electrostatic
interactions, respectively.
19
Scheme 1: Structure of chitosan with a small portion of acetyl groups originating from
the natural chitin structure, and the secondary amino groups as the main functional
groups of the chitosan structure besides the hydroxyl groups (n=0,1; m=0,9).
20