Functionalized chitosan adsorbents allow recovery of palladium and platinum from acidic aqueous solutions

Article abstract of DOI:10.1039/c9gc00166b

Platinum (Pt) and palladium (Pd) are precious metals considered critical in our society and are needed in a variety of sustainable technologies. Their scarcity urges the increase of recycling from secondary waste streams through new and efficient recovery techniques. Adsorption is an established recovery method for liquid streams, where chitosan shows promising results as a low-cost adsorbent, derived from biomass. This biopolymer is able to capture metals, but suffers from a low stability under acidic conditions and poor adsorbing properties. In this study, three new chitosan derivatives were synthesized and employed for Pd(ii) and Pt(iv) recovery from acidic solutions. Specific and simple modifications were selected based on their known affinities for these metal ions and taking into account the principles of green chemistry. The prepared derivatives consist of 1,10-phenanthroline-2,9-dicarbaldehyde cross-linked chitosan (Ch-PDC), [2,2′-bipyridine]-5,5′-dicarbaldehyde cross-linked chitosan (Ch-BPDC) and glutaraldehyde cross-linked chitosan grafted with 8-hydroxyquinoline-2-carbaldehyde (Ch-GA-HQC). For all derivatives, the adsorption occurred fast and equilibrium reached within 30 min. The Langmuir isotherms revealed a maximum adsorption capacity for Pd(ii) and Pt(iv) of respectively 262.6 mg g^-1 and 119.5 mg g^-1 for Ch-PDC, 154.7 mg g^-1 and 98.3 mg g^-1 for Ch-BPDC and 340.3 mg g^-1 and 203.9 mg g^-1 for Ch-GA-HQC. Such adsorption capacities are considerably higher compared to the biosorbents reported in the literature. Excellent physical properties in homo- and heterogeneous systems and high regeneration performances demonstrate that chitosan-based adsorbents are very promising for Pd(ii) and Pt(iv) recovery from acidic solutions.

Full text of DOI:10.1039/c9gc00166b

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ARTICLE
Functionalized chitosan adsorbents allow recovery of palladium
and platinum from acidic aqueous solutions
Stein Mincke,*a Tsegaye Grima Asere,*b,c Ivar Verheye,a Karel Folens,a Flore Vanden Bussche,a,d
Linsey Lapeire,e Kim Verbeken,e Pascal Van Der Voort,d Dejene A. Tessema,b Fekadu Fufa,c Gijs Du
Lainga and Christian V. Stevensa
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Platinum (Pt) and palladium (Pd) are precious metals considered critical in our society and needed in a variety of sustainable
technologies. Their scarcity urges to increase the recycling from secondary waste streams through new and efficient
recovery techniques. Adsorption is an established recovery method for liquid streams, where chitosan shows promising
results as a low-cost adsorbent, derived from biomass. This biopolymer is able to capture metals, but suffers from a low
stability in acidic conditions and poor adsorbing properties. In this study, three new chitosan derivatives were synthesized
and employed for Pd(II) and Pt(IV) recovery from acidic solutions. Specific and simple modifications were selected based on
known affinities for these metal ions and taken into account the principles of green chemistry. The prepared derivatives
consist of 1,10-phenanthroline-2,9-dicarbaldehyde cross-linked chitosan (Ch-PDC), [2,2-bipyridine]-5,5-dicarbaldehyde
cross-linked chitosan (Ch-BPDC) and glutaraldehyde cross-linked chitosan grafted with 8-hydroxyquinoline-2-carbaldehyde
(Ch-GA-HQC). For all derivatives, the adsorption was very fast and equilibrium reached within 30 min. The Langmuir
isotherms revealed a maximum adsorption capacity for Pd(II) and Pt(IV) of respectively 262.6 mg/g and 119.5 mg/g for Ch-
PDC, 154.7 mg/g and 98.3 mg/g for Ch-BPDC and 340.3 mg/g and 203.9 mg/g for Ch-GA-HQC. Such adsorption capacities
are considerably higher compared to biosorbents reported in literature. Excellent physical properties in homo- and
heterogeneous systems and high regeneration performances demonstrate that chitosan-based adsorbents are very
promising for Pd(II) and Pt(IV) recovery from acidic solutions.
www.rsc.org/
second most abundant natural biopolymer and can be found as
a structural component in the exoskeleton of arthropods such
as crustaceans or insects, or in the cell wall of yeast and fungi.2,4
For that matter, the entorefinery concept is an emerging
biorefinery approach where insects are used to convert organic
and food waste into biodiesel, useful proteins and chitin.5–7
Introduction
The introduction of the Sustainable Development Goals in 2015
gave a renewed attention to the long-term care for planet Earth
and its inhabitants. Concepts of circular economy and
sustainable technologies substantiate these goals in the
processing industries, where the reduction and valorization of
waste are central. In this context, chitosan has been widely
studied, since it is sourced from waste streams and applied as
building block for added value materials. Chitosan is a polymer
of biological origin, consisting of randomly distributed D-
glucosamine and N-acetyl-D-glucosamine units, linked through
a β(1→4) bond. It is mainly obtained from chitin after N-
deacetylation under basic conditions (Fig. 1).1–3 Chitin is the
Fig. 1 Structures of chitin and chitosan1
The unique chemical and biological properties of chitosan, e.g.
biocompatibility, bioactivity and biodegradability, are valued in
many fields.1,8–10 For example, chitosan is used for the
adsorption of metals in wastewater treatment. Biosorption can
be considered as an effective and sustainable process in
comparison to conventional metal recovery techniques, since
cheap chitinous materials, algal biomass or microbial biomass
can be used.11–13 The use of biosorbents in the recovery of
platinum group metals (PGM) from waste streams is of
particular interest for the processing industries. The PGMs are
a group of six noble elements (Ru, Rh, Pd, Os, Ir and Pt) that are
regarded as precious metals. They have similar physical and
a. SynBioC research group, Department of Green Chemistry and Technology, Ghent
University, Campus Coupure, Coupure Links 653, 9000 Ghent, Belgium
b. Department of Chemistry, Welkite University, Gubrei Wabe Bridge 5, Southern
Nations, Nationalities and Peoples' Region, Welkite, Ethiopia
c. Department of Water Resources and Environmental Engineering, Jimma
University, P.O. Box 378, Jimma, Ethiopia
d. Department of Chemistry, Ghent University, Krijgslaan 281 S3, 9000 Ghent,
Belgium
e. Department of Materials, Textiles and Chemical Engineering, Ghent University,
Technologiepark Zwijnaarde 46, 9052 Zwijnaarde, Belgium
*These authors made equivalent contributions
†Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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Fig. 2 Schematic overview of the synthesis of Ch-PDC, Ch-BPDC and Ch-GA-HQC (A) and the chelating and cross-linking compounds (B)
chemical properties which are desirable in a variety of modern and characterized using Secondary Electron Microscopy (SEM),
applications.14,15 Palladium (Pd) and platinum (Pt) are used in Fourier
Transform Infrared (FTIR) spectroscopy,
sustainable technologies, such as fuel cells, catalysts in vehicles, Thermogravimetric analysis (TGA), X-ray powder diffraction
petrochemical conversion processes and electronics. Despite (XRPD) analysis, elemental analysis and Brunauer-Emmett-
the wide scope of applications, the natural availability of Pt and Teller (BET) surface area analysis. The adsorption and kinetic
Pd is limited and, hence, they belong to the most expensive performances were studied together with the effect of the pH
resources. To move towards a sustainable society, these critical and selectivity towards the metal ions. Furthermore, the
metals should be recovered from waste streams and used again regeneration of the derivatives was investigated, indicating
as a secondary source of raw materials.15–20
their reusability.
The use of chitosan as biosorbent for Pd and Pt adsorption first
requires chemical modification since native chitosan lacks
stability at low pH and has only moderate adsorption capacities
towards metal ions.21–23 To increase the stability at low pH,
cross-linking of the polymer with di/polyfunctional reagents is
commonly used. In this respect, glutaraldehyde is by far the
most studied cross-linking agent, amongst many other
possibilities.24–29 Attention should be paid to the type and
degree of cross-linking, together with the cross-linking
conditions, since these parameters affect the adsorption
capacity.24,30,31
Results and discussion
Synthesis and characterization of chitosan derivatives
Chitosan was modified to reduce the acid solubility and to
increase the number of metal sorption sites (Fig. 2A). Therefore,
chitosan (Ch) was dissolved in a 1% acetic acid solution and
derivatized with 1,10-phenanthroline-2,9-dicarbaldehyde
(PDC), [2,2-bipyridine]-5,5-dicarbaldehyde (BPDC), or 8-
hydroxyquinoline-2-carbaldehyde (HQC) (Fig. 2B). These
particular compounds were selected for their known affinity
towards Pd and Pt species.32–35 Since HQC is not able to cross-
link chitosan, glutaraldehyde (GA) was added prior to the
addition of HQC. The amount of cross-linker added was based
on previous optimization experiments available in literature.36
The formed imine bonds were reduced to secondary amines. An
increase of the viscosity was clearly visible after the addition of
the cross-linking agents, indicating efficient cross-linking. The
resulting chitosan derivatives (Ch-PDC, Ch-BPDC and Ch-GA-
This study focuses on the synthesis of acid-tolerant and high-
performance chitosan-based sorbents, able to recover Pd(II)
and Pt(IV) from aqueous media. Native chitosan was derivatized
under homogeneous conditions via a simple one- or two-step
modification involving cross-linking and the addition of a metal
chelating moiety. Three different biosorbents are synthesized
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Fig. 3 Secondary electron (SE) images from native chitosan (Ch) and chitosan derivatives (Ch-PDC, Ch-BPDC and Ch-GA-HQC) at different
magnifications
HQC) were investigated by SEM, FTIR-spectroscopy, XRPD and whereas the single broad peak indicates N-H stretching of
TGA. Further characterization results (elemental analysis and secondary amines as a result of cross-linking and/or grafting.
BET surface area) can be found in supporting information.
Additionally, the peak(s) at 856 cm-1 for Ch-PDC, 835 cm-1, 752
cm-1 and 717 cm-1 for Ch-BPDC and 819 cm-1 and 742 cm-1 for
From the secondary electron (SE) images, it is clear that the Ch-GA-HQC indicate the presence of an aromatic system,
native chitosan flakes (Ch) are the smallest compared to the corresponding to the phenanthroline, bipyridine and quinoline
modified samples (Fig. 3). The difference is explained by the moiety respectively. The sharp peaks at 2981 cm-1 and 1381 cm-
1
addition of a cross-linker, that bridges the chains resulting in
in the spectrum of Ch-GA-HQC correspond to C-H vibration,
larger entities. The Ch-PDC particles appear to be smaller coming from the CH2 cross-linking unit of glutaraldehyde. A
compared to those of Ch-BPDC and Ch-GA-HQC. At higher detailed peak assignment is given in the supporting
magnifications, all chitosan derivatives have a higher surface information. XRPD analysis was performed to investigate the
roughness in comparison with the native chitosan, which has a changes in structural organization by the modification of
smoother surface.
chitosan. XRPD results of native chitosan (A) and the modified
The FTIR-spectra of all derivatives show a single broad peak sorbents (B, C and D) are depicted in Fig. 5 and clearly show the
around 3300 cm-1, compared to two peaks at 3354 cm-1 and amorphous nature of these materials. The characteristic
3288 cm-1 for chitosan (Fig. 4).37 The two peaks for chitosan diffraction peak of chitosan was observed at 20° in all spectra.37
indicate N-H stretching of primary amines (and O-H stretching), However, the use of a cross-linking unit lowers the intensity of
Fig. 5 XRPD spectra of chitosan (A), Ch-PDC (B), Ch-BPDC (C) and Ch-
Fig. 4 FTIR spectra of chitosan (A) and chitosan derivatives Ch-PDC
GA-HQC (D)
(B), Ch-BPDC (C) and Ch-GA-HQC (D)
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catalytic converters. These waste streams contVaiienw AortniclleyOlnolinwe
42
concentrations of precious metals (10 D4O0I: m10.g1/0L3)9/C,9tGhCe0re01fo66rBe
an initial element concentration of 25 mg/L was used.
To study the effect of pH on the adsorption capacity, the metal
speciation and the point of zero charge (pHPZC) of the chitosan
derivatives were investigated. The point of zero charge (pHPZC
)
is considered an important parameter when studying the effect
of pH on the adsorption capacity of materials. Chitosan is
reported to have a pHPZC of 6.3, in which the protonation of the
available amine groups plays an important role.43 For the
modifications performed in this study, the amine groups were
used in order to functionalize and cross-link chitosan. The pHPZC
of the biosorbents was determined and can be found in the
supporting information. All pHPZC values (3.7  4.7) are well in
the pH 1  5 range.
Fig. 6 TG curves of the thermal degradation of chitosan and chitosan
derivatives
For Pd, all derivatives show a slightly increased adsorption
efficiency when the pH increases from 1 to 5 (Fig. 7). The
adsorption capacity is not much affected when the pH rises
above the pHPZC of the materials. This can be ascribed to the
occurrence of different adsorption mechanisms. Adsorption can
occur through electrostatic attraction between negatively
charged metal species and positively charged amine groups (pH
this peak, which indicates a loss in the structural organization.
This is a result of the cleavage of hydrogen bonds that organize
the supramolecular structure of the polysaccharide chains.38,39
In the case of Ch-GA-HQC (D), two compounds (GA and HQC)
cause structural changes, resulting in a lower peak intensity
compared to (B) and (C).
Thermogravimetric analysis was performed to investigate the
effect of cross-linking on the temperature profile of the
materials (Fig. 6). Native chitosan shows a well-defined
decomposition at 300 °C.40,41 The decomposition of all cross-
linked derivatives, on the other hand, starts at a lower
< pHPZC)
44–46 or through coordination of the amine and hydroxyl
groups with the metal species (pH > pHPZC).39,47 By increasing the
pH from 1 to 5, a slight increase of the adsorption capacity for
Pd can be attributed to a slightly better coordination interaction
compared to electrostatic attractions. Besides the pHPZC, the
governing speciation of the metal species also affects the
adsorption. The type of speciation is highly determined by the
pH of the solution and concentration of chloride ions present
during adsorption. Speciation modelling is often used in
literature to investigate the type of speciation of the metal
temperature (200 °C) and proceeds within
a broader
temperature profile. At high temperatures, more residual tar
remains for these cross-linked derivatives. Therefore, it is clear
that cross-linking has a significant effect on the thermal stability
of chitosan. All results mentioned above indicate a successful
reaction of chitosan with the chelating and cross-linking
compounds PDC, BPDC and GA-HQC.
2-
species.48,49 Accordingly, PdCl4 is considered to be the
predominant species at all pH levels used in the experiments.50
The increase in adsorption efficiency, as the pH increases from
1 to 5, can be related to a reduced competition between
Effect of initial pH
2-
The effect of pH on the adsorption of Pt(IV) and Pd(II) was
investigated in separate solutions using the three chitosan
derivatives. The pH was varied between 1 and 5 since most
wastewater streams are acidic, e.g. from acid-leachate of
chloride ions and PdCl4 at higher pH levels. At low pH levels,
the attraction of counter anions, such as chloride, towards
positively charged adsorbents is stronger and involves
2- 36
competition with PdCl4
.
Fig. 7 pH effect on removal of Pd(A) and Pt(B) using Ch-PDC, Ch-BPDC or Ch-GA-HQC adsorbents
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Fig. 8 Langmuir adsorption isotherms of Pd (A) and Pt (B) on Ch-PDC, Ch-BPDC, Ch-GA-HQC and Ch-GA
In contrast to what is observed for Pd, the removal efficiency of Adsorption isotherms
Pt rises when the pH is increased from 1 to 4, but decreases
Figure 8 shows the adsorption isotherms of Pd and Pt, on the
synthesized chitosan derivatives (Ch-PDC, Ch-BPDC, Ch-GA-HQC
and Ch-GA). The parameters resulting from fitting the
experimental data to the Langmuir model are presented in
Table 1. The value of qm (obtained from Langmuir plots) is
consistent with the experimental results, indicating that the
adsorption of Pd(II) and Pt(IV) onto Ch-PDC, Ch-BPDC and Ch-
GA-HQC occurs through a monolayer deposition.44,54 The
adsorption capacity for Pd was found to increase in the order
Ch-BPDC < Ch-PDC < Ch-GA-HQC. Additional functionalization of
again if the pH is further increased to pH 5. Fujiwara et al.51 also
reported that higher adsorption capacity of lysine modified
chitosan for precious metal ions was obtained at pH 1 – 4. The
drop in sorption capacity occurs when the pH rises above the
pHPZC. In this respect, electrostatic attraction is considered the
dominant adsorption mechanism. Furthermore, it is important
to notice that Pt can be present as chloro-complex, because
hydrochloric acid was used in the preparation of the adsorbate
solutions. The distribution of these complexes depends on pH,
chloride concentration and metal concentration. At pH 1  5.5,
Pt is mainly present as PtCl62- and from pH 5.5 to 8, PtCl5(OH)2-
is expected to be the dominant species.50,52 At pH 1, the
interaction between the excess chloride ions and the positively
Ch-GA
by
8-hydroxyquinoline-2-carbaldehyde
(HQC)
ameliorated the Pd adsorption capacity from 177.7 mg/g to
340.3 mg/g. Similar adsorption capacities for Ch-GA were
reported in literature.36,55 Also for Pt adsorption, an increase
was observed through the addition of HQC to Ch-GA, namely
from 180.8 mg/g to 203.9 mg/g. Although the adsorption
capacity of Pt is in general lower than that of Pd, the same order
of materials performance counts. The preference of Pd(II)
adsorption over Pt(IV) was reported earlier for several chitosan-
based adsorbents.56,57 Wang et al.58 also reported the greater
adsorption potential of Pd(II) over Pt(IV) on alginate and algal-
based beads at pH 2.5 in a mono-component system. The
difference in the observed adsorption capacities can be
associated with the difference in the affinity of the adsorbents
towards these metals.48 The highest Pd(II) and Pt(IV) adsorption
capacity is obtained by Ch-GA-HQC, compared to Ch-PDC and
charged adsorbents increased, resulting in
a greater
competition between chloride ions and hexachloroplatinate. At
pH 5, i.e. closer to the pHPZC of the adsorbents, electrostatic
interactions become less pronounced and the recovery
efficiency decreases. Hence, pH 3 was selected for further
adsorption experiments. The suggested removal mechanism at
this pH is mainly based on ion-exchange onto protonated
nitrogen groups of the chitosan-based sorbents.51 Moreover,
performing the adsorption at a low pH value is an important
strategy to obtain selectivity in adsorption away from base
metal ions.53
Table 1 Parameters of adsorption isotherms for Pt(IV) and Pd(II) adsorption onto the synthesized chitosan derivatives for the Langmuir
model with standard error (SE)
Element
Pd
Parameter
qm (mg/g)
KL (L/mg)
RL
Ch-PDC (SE)
262.6 (21.4)
0.006 (0.002)
0.169 - 0.873
0.9538
Ch-BPDC (SE)
154.7 (9.0)
3.53 (1.01)
0.0003 - 0.012
0.8999
Ch-GA-HQC (SE)
340.3 (29.9)
0.005 (0.001)
0.204 - 0.896
0.9695
Ch-GA (SE)
177.7 (7.8)
0.29 (0.08)
0.004 - 0.713
0.8695
R2
Pt
qm (mg/g)
KL (L/mg)
RL
119.5 (2.8)
0.62 (0.12)
0.002 - 0.061
0.9974
98.3 (2.3)
203.9 (10.5)
0.33 (0.10)
0.004 - 0.108
0.8877
180.8 (5.6)
0.20 (0.11)
0.01 - 0.727
0.9385
0.61 (0.12)
0.002 - 0.063
0.9775
R2
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Table 2 Comparison of adsorption capacities of cross-linked chitosan compounds
Adsorption capacity (mg/g)
Adsorbent
Chitosan flakes
pH
2
Pd(II)
62.5
Pt(IV)
66.6
Reference
[60]
L-lysine modified cross-linked chitosan resin
Chitosan cross-linked hydrogels
Glycine modified cross-linked chitosan resin
Thiourea modified chitosan microspheres
Ethylenediamine-modified magnetic chitosan nanoparticles
Azacrown ether cross-linked chitosan
Rubeanic acid grafted cross-linked chitosan
2-aminobenzaldehyde modified chitosan
β-cyclodextrin grafted chitosan
Azobenzene modified chitosan
Glutaraldehyde cross-linked chitosan
Glutaraldehyde cross-linked chitosan
Glutaraldehyde cross-linked chitosan
Sulpur-modified chitosan
Cross-linked sulfoethylated chitosan
Glutaraldehyde cross-linked chitosan
Polyethyleneimine-loaded core-shell chitosan beads
Ch-PDC
Ch-BPDC
Ch-GA-HQC
1, 2
2
2
2
2
4
2
5
6
4.1
2
8
3
2.5
2
2
1
3
3
109.5
192.9
120.4
112.4
138
173.1
352.0
275
202.0
101.6
204.7
166.7
177.7
38.5
42.6
-
-
282.6
154.7
340.3
129.3
231.1
122.5
129.9
171
-
-
-
[51]
[56]
[61]
[45]
[44]
[62]
[63]
[39]
[64]
[65]
[36]
[55]
-
-
-
-
180.8
-
-
300
815.2
119.5
98.3
203.9
This study
[66]
[67]
[68]
[69]
This study
This study
This study
3
Ch-BPDC. This could be ascribed to the difference in the available. The Pd adsorption capacity of the synthesised
synthesis process. In the case of Ch-PDC and Ch-BPDC, the chitosan derivatives were higher or at least comparable to
addition of one compound (PDC and BPDC, respectively) causes earlier reports. Only rubeanic acid chitosan derivatives (352
both cross-linking and an increase in adsorption sites. In the mg/g) were slightly better than Ch-GA-HQC (340.3 mg/g). Note
case of Ch-GA-HQC, two separate compounds are used. that the pH was not the same in both studies and that the
Namely, cross-linking is obtained by glutaraldehyde, whereas adsorption capacity of rubeanic acid modified chitosan
HQC increases the sorption capacity. For all derivatives, the derivatives was strongly influenced by the pH. The Pt adsorption
feasibility of the adsorption system could be evaluated by capacity of the derivatives from this study were comparable to
calculating the separation factor RL. All RL values for Pd and Pt other chitosan-based adsorbents. Polyethyleneimine-loaded
adsorption are in between zero and one, indicating that the chitosan beads showed the highest adsorption capacity for Pt.
adsorption of both precious metals is favourable and The high adsorption can be mainly attributed to the presence of
irreversible.59 60–69
the polyethyleneimine at the surface of the beads, since
Table 2 shows the comparison of the adsorption capacity of Ch- chitosan was only present in the core.69 Generally, cross-linking
PDC, Ch-BPDC and Ch-GA-HQC for Pt(IV) and Pd(II) as a chitosan and modifying it with nitrogen- and sulphur-containing
benchmark towards other chitosan-based adsorbents. Few groups significantly enhanced the Pd(II) and Pt(IV) adsorption
studies examined both the adsorption of Pd and Pt. In this capacity of chitosan in acidic solutions.
respect, Ch-GA-HQC is the best chitosan-based adsorbent
Fig. 9 Plots of pseudo-second-order fit for Pd (A) and Pt (B) adsorption on Ch-PDC, Ch-BPDC and Ch-GA-HQC
6 | Green Chemistry, 2012, 00, 1-3
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was employed to determine if pore diffusion or internal
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diffusion is the rate-determining step in tDhOeIa: d10s.o10r3p9t/iCo9nGpCr0o0c1e6s6sB.
If the sole rate-limiting step is pore or intra-particle diffusion,
the plot of qt vs t0.5 should be a straight line, passing through the
origin.61 However, it is observed from Fig. 11 that the data are
multi-linear and lines are not passing through the origin. The
multi-linear trend indicates two steps in the sorption process:
an initial linear phase due to external mass transfer, versus a
later linear phase due to intra-particle diffusion.71 Immediately
after mixing the adsorbent, bulk diffusion is the rate limiting
factor, which may be related to a limited BET surface area (≤ 2
m2/g) of the materials. (Analysis results in supporting
information). Similar observations were found by the
adsorption of Au(III), Pt(IV) and Pd(II) onto glycine modified
cross-linked chitosan resins,61 and Pd(II) adsorption with soluble
tannins cross-linked Lagerstroemia speciosa leaves powder.71
Fig. 10 Concomitant removal (%) of 25 mg L-1 Pd and 25 mg L-1 Pt by
Ch-PDC, Ch-BPDC and Ch-GA-HQC
Table 3 Parameters of the pseudo-second-order kinetic model for Pd and Pt
Adsorption kinetics
adsorption on Ch-PDC, Ch-BPDC and Ch-GA-HQC with standard error (SE)
In order to investigate the adsorption rate, the effect of contact
time was investigated at pH 3 using Ch-PDC, Ch-BPDC and Ch-
GA-HQC adsorbents. The results of kinetic studies are shown in
Fig. 9. The kinetic curve for Pd and Pt showed a rapid adsorption
within the first 30 min. The equilibrium was reached in 120 min.
The high adsorption rate in the beginning is due to the abundant
active sites on the external surface of the adsorbent, available
for interaction with Pd or Pt. The adsorption process slows
down between 30 and 120 min. The experimental data were
Element Parameter
Ch-PDC
(SE)
24.2 (0.1)
24.3 (0.1)
0.020
(0.001)
0.9991
25.0 (0.4)
25.2 (0.6)
0.009
Ch-BPDC
(SE)
25.2 (0.1)
25.6 (0.1)
0.018
(0.001)
0.9984
24.8 (0.3)
24.5 (0.5)
0.017
Ch-GA-
HQC (SE)
22.8 (0.1)
22.4 (0.3)
0.016
(0.002)
0.9881
24.8 (0.3)
25.4 (0.4)
0.008
Pd
Pt
qe,exp (mg/g)
qe,cal (mg/g)
k2
(g/(mg.min))
R2
qe,exp (mg/g)
qe,cal (mg/g)
k2
fitted with
a non-linear pseudo-first-order (supporting
(g/(mg.min))
(0.002)
0.9574
(0.002)
0.9768
(0.001)
0.9885
information) and pseudo-second-order kinetic model. Since the
pseudo-second-order fitting gave higher correlation
coefficients, only these results are described here (Table 3). The
excellent fit of the pseudo-second-order model suggests that
the main adsorption mechanism is chemisorption. A similar
phenomenon was observed in the adsorption of Pd(II) on
cysteine functionalized 2,3-dialdehyde cellulose,70 Pd(II) on
glutaraldehyde cross-linked chitosan,55 Pt(IV) on chitosan and
Au(III), Pt(IV) and Pd(II) onto glycine modified cross-linked
chitosan resin.23,61 The calculated adsorption rates k2 were high
for Pd (0.016 – 0.020 g/mg.min) and Pt (0.008 – 0.017
g/mg.min). Differences in k2 values can be attributed to a
different molecular geometry of both elements. In a mixed
system, slightly higher adsorption rates for Pd are reflected in a
relatively larger adsorption compared to Pt (Fig. 10), illustrating
that adsorption in the presence of multiple compounds is
kinetically controlled. For Pd, the value of k2 increases in the
order of Ch-GA-HQC < Ch-BPDC < Ch-PDC, whereas Ch-GA-HQC
< Ch-PDC < Ch-BPDC for Pt. The intra-particle diffusion model
R2
This journal is © The Royal Society of Chemistry 20xx
Green Chemistry, 2013, 00, 1-3 | 7
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Fig. 11 Intra-particle diffusion plots of Pd (A) and Pt (B) adsorption on Ch-PDC, Ch-BPDC and Ch-GA-HQC

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DOI: 10.1039/C9GC00166B
Fig. 12 Pd (A) and Pt (B) removal (%) in subsequent adsorption cycles (I, II and III) following desorption
determined using a Wagner & Munz Kofler Hot Bench (type
WME). IR spectra with a S/N-ratio of 30,000:1 were obtained
from samples in neat form with a Quest ATR (Attenuated Total
Reflectance) accessory with diamond crystal puck using a
Shimadzu IRAFFINITY-1S Fourier transform infrared
Regeneration of biosorbents
After Pd or Pt adsorption onto the synthesized chitosan
derivatives, the adsorbents were regenerated by using
thiourea, a common and green reagent,72 under mild conditions
and reused in subsequent adsorption cycles. The adsorption
efficiencies of Pd(II) in the third cycle were 94.1% (Ch-PDC),
99.8% (Ch-BPDC) and 99.6% (Ch-GA-HQC). Those for Pt(IV) were
97.7% (Ch-PDC), 97.6% (Ch-BPDC) and 97.9% (Ch-GA-HQC) (Fig.
12). These values indicate that all three chitosan derivatives can
be regenerated and reused for Pd(II) and Pt(IV) immobilization,
adding to the successful application as adsorbents for resource
recovery. The favourable regeneration allows the adsorbents to
be packed in a column for continuous recovery of Pd and Pt
from a liquid waste stream with intermittent regeneration.
1
spectrophotometer. All H NMR spectra were recorded at 400
MHz, on a Bruker Avance III, equipped with 1H/BB z-gradient
probe (BBO, 5 mm). CDCl3 and DMSO-d6 were used as solvent
and TMS was used as an internal chemical shift standard. NMR
spectra were acquired through the standard sequences
available in the Bruker pulse program library and collected data
processed using TOPSPIN 3.2.
Chitosan modification and characterization
Derivatization of chitosan with PDC or BPDC occurs via a one-
pot, two-step reductive amination. Since HQC is not able to
cross-link chitosan on its own, glutaraldehyde (GA) was added.
A schematic representation of the derivatization of chitosan is
depicted in Fig. 2. All cross-linked compounds were analyzed
using the following techniques: SEM, FTIR spectroscopy, XRPD
analysis, TGA, elemental analysis, BET surface area analysis and
point of zero charge determination. Secondary electron images
were taken on a JEOL JSM-7600F FEG SEM apparatus. Gold
sputtering was used prior to analysis. X-ray powder diffraction
patterns were collected on a Thermo Scientific ARL X’Tra
diffractometer, operated at 40 kV, 30 mA using Cu Kα radiation
(λ = 1.5406 Å). TA Netzsch Model STA 449 F3 Jupiter system was
used to perform thermogravimetrical analysis under N2 flow
(100 mL/min). For CHNS elemental analysis, a Thermo Flash
2000 thermal analyser (Thermo Scientific) is used with V2O5 as
catalyst and methionine as standard. The specific Brunauer-
Emmett-Teller (BET) surface area was measured on a TriStar
3000 analyser (Micromeritics) and a Belsorp Mini apparatus at
77 K. Prior to analysis, the samples were dried at 100°C under
vacuum for 24 h. The pH at the point of zero charge (pHPZC) of
the products was determined in the pH range between 2 and
12.43
Experimental
Materials
Precious metals used in this study are PdCl2 (Merck, Darmstadt,
Germany) and K2PtCl6 (Alfa Aesar, Thermo Fischer (kandel),
GmbH, Germany). 5,5′-dimethyl-2,2′-bipyridine was purchased
from TCI Europe (Zwijndrecht, Belgium) and solid thiourea
(>99%) from Chem-Lab (Zedelgem, Belgium). Chitosan powder
(low molecular weight, 75-85% deacetylated), together with all
other chemicals, were supplied from Merck (Darmstadt,
Germany). All these chemicals were of analytic grade or higher
and used without further purification.
Synthesis and characterization of chelating and cross-linking
compounds
Three metal chelating compounds were synthesized in a one- or
two-step synthesis: 1,10-phenanthroline-2,9-dicarbaldehyde
(PDC), [2,2’-bipyridine]-5,5’-dicarbaldehyde (BPDC) and 8-
hydroxyquinoline-2-carbaldehyde (HQC) (Fig. 2B). The exact
synthesis and characterization of these molecules can be found
in the supporting information. The melting point was
8 | Green Chemistry, 2012, 00, 1-3
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Page 9 of 11
Green Chemistry
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ARTICLE
Synthesis of 1,10-phenanthroline-2,9-dicarbaldehyde cross-linked
chitosan (Ch-PDC) and [2,2-bipyridine]-5,5-dicarbaldehyde cross- at any time t (min) were calculatedDOuIs:i1n0g.103th9/eC9fGoCll0o0w16in6Bg
of Pt or Pd adsorbed per unit mass of the adsorbeVnietw, qArtti(cmle Ogn/lgin)e,
equations (Eq. 1 and Eq. 2), in which C0 and Ct (mg/L) are the
initial Pt(IV) or Pd(II) concentration and the concentration at
time t (min), respectively. V (L) is the solution volume and m (g)
linked chitosan (Ch-BPDC) Chitosan (1 eq., 1 g) was dissolved in
50 mL 1% (v/v) acetic acid (HOAc) at 50 °C for 1 h. Next, 0.4 eq.
of chelating compound PDC or BPDC was added, together with
a 1% (v/v) acetic acid solution to reduce the viscosity and to
insure complete mixing (Entry 1 and 2, Table 4). The viscous
mixture was stirred with an overhead stirrer at 50 °C for 4 h.
Afterwards, 1.2 eq. of NaCNBH3 (10 wt.% in H2O) was added and
stirred overnight at room temperature. The mixture was
purified by dialysis in demineralized water for 5 days. The
dialyzed solution was added to an excess of ethanol, which was
consequently evaporated in vacuo. The resulting concentrated
mixture was precipitated in acetone, centrifuged, filtered and
washed with acetone. Finally, the product was dried in an oven
at 60 °C overnight and resulted in PDC or BPDC cross-linked
is the adsorbent mass.
푪ퟎ ― 푪풕
푨% =
× ퟏퟎퟎ
(퐄퐪. ퟏ)
(퐄퐪. ퟐ)
(
)
푪ퟎ
푽
(
)
풒풕 = 푪ퟎ ― 푪풕
(
)
풎
Effect of initial pH value The influence of pH was investigated at
five different initial pH levels (1.0 ± 0.1 – 5.0 ± 0.1). Therefore,
separate adsorbate solutions with a concentration of 25 mg/L
were prepared as described above. After contacting for 24 h,
the solution was analyzed for their Pt or Pd concentration.
Adsorption isotherms To determine the maximum sorption
capacity for Pt and Pd adsorption, metal solutions with an initial
concentration of 25 mg/L to 1000 mg/L were prepared at pH 3.0
± 0.1 and analyzed after 24 h as described above. The
experimental equilibrium data were evaluated using the
Langmuir isotherm model (Eq. 3). In this model, qe and Ce are
the amount of Pd(II) or Pt(IV) adsorbed (mg/g) and the
adsorbate concentration in the aqueous phase (mg/L), both at
equilibrium. qm (mg/g) and KL (L/mg) are the Langmuir
constants, which are related to the adsorption capacity and
energy of adsorption, respectively. Both qm and KL were
chitosan.
Table 4 Synthesis of chitosan biosorbents using different volumes of acetic
acid solution (1% v/v HOAc)
Entry
Compound (concentration)
PDC (0.83% in DMSO)
BPDC (0.83% in DMSO)
HQC (1.25% in methanol)
Volume HOAc (mL)
1
2
3
60
125
25
Synthesis of glutaraldehyde cross-linked chitosan (Ch-GA) followed
by grafting with 8-hydroxyquinoline-2-carbaldehyde (Ch-GA-HQC)
After the dissolution of chitosan in acetic acid, 0.2 eq of
glutaraldehyde (GA 50% in H2O) was added together with an
extra amount of 1% HOAc solution (Entry 3, Table 4). After 1h,
0.4 eq. of HQC were added and the viscous mixture was stirred
at 50 °C for 4 h. Afterwards, 1.2 eq. of NaCNBH3 (10 wt.% in H2O)
was added and stirred overnight at room temperature and then
purified as described above. Ch-GA was made without the
addition of HQC, and using 0.4 eq. of NaCNBH3 for the
reduction. A lower amount of cross-linker was used compared
to the synthesis of Ch-PDC and Ch-BPDC in order to have
sufficient amine groups available for the modification with HQC.
Batch adsorption studies
calculated by plotting Ce vs qe, using the following equation.
풒풎푲푳푪풆
풒풆 =
(퐄퐪. ퟑ)
(ퟏ + 푲푳푪풆)
The degree of suitability of the resin towards the metal ions was
estimated from the values of the separation factor (RL), which
gives an indication for the possibility of the adsorption to
proceed. RL was calculated via Eq. 4, where KL is the Langmuir
equilibrium constant and C0 the initial concentration of Pt or Pd
.
ퟏ
푹푳 =
(퐄퐪. ퟒ)
ퟏ + 푲푳푪ퟎ
Stock solutions of 1000 mg/L Pt and Pd were prepared by
dissolving K2PtCl6 and PdCl2 powder in 1.1 M HCl.56 All other
concentrations in the adsorption experiments were obtained by
diluting the stock solution with deionized water to obtain a
Pt(IV) or Pd(II) concentration in the range of 25 mg/L to 1000
mg/L. Batch adsorption experiments were performed in
duplicate using 15 mL polyethylene tubes. A quantity of 10 mg
of chitosan derivative (Ch-PDC, Ch-BPDC or Ch-GA-HQC) was
added to 10 mL of a Pt and/or Pd solution. The initial pH was
adjusted using 5 M NaOH or 0.2 M H2SO4. Then, the tubes were
shaken at 200 rpm in a horizontal shaker at 20 ± 1 °C for a
predetermined period of time (from 5 min to 24 h). After phase
separation through sedimentation, the supernatant was filtered
(MACHERY-NAGEL; CHROMAFIL® RC-45/25 syringe filter; pore
size 0.45 µm) and analyzed using inductively coupled plasma
optical emission spectroscopy (ICP-OES; Varian Vista MPX, Palo
Alto, CA, USA). The adsorption efficiency (A%) and the amount
Adsorption kinetics Separate solutions of 25 mg/L Pt and Pd were
prepared and the pH was adjusted to pH 3.0 ± 0.1. The samples
were analyzed after different time intervals ranging from 5 min
to 24 h, as described above. The experimental data were
evaluated using non-linear form of pseudo-first-order and
pseudo-second-order kinetics. This is because of non-linear
regression method is more appropriate compared to linear
method to determine the rate kinetic parameters.73 The non-
linear expressions of the pseudo-first-order and pseudo-
second-order models are given in equations (5) and (6),
respectively, in which k1 (min-1) is the pseudo-first-order rate
constant, k2 (g mg-1 min-1) is the pseudo-second-order rate
constant, qt and qe are the adsorbed amount (mg/g) at any time
t (min) and the adsorbed amount at equilibrium, respectively.
― 풌ퟏ풕
(
)
풒풕 = 풒풆 ퟏ ― 풆풙풑
(퐄퐪. ퟓ)
This journal is © The Royal Society of Chemistry 20xx
Green Chemistry, 2013, 00, 1-3 | 9
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ARTICLE
Green Chemistry
풌ퟐ풒ퟐ풆퐭
Acknowledgements
View Article Online
풒풕 =
(퐄퐪.ퟔ)
T. G. A. (BOF fellowship n°: 01W05414) DisOsI:u1p0p.1o0r3t9e/Cd9bGyC0G0h16e6nBt
University. Dr. Jonathan De Roo, prof. Isabel Van Driessche and
Funda Aliç are warmly thanked for their guidance and help with
the structural characterization measurements (TGA & CHNS).
Further, the authors want to thank Pieter Naert for
proofreading the manuscript and Yoshi Mertens and Jens
Mincke for the graphical contribution.
ퟏ + 풌ퟐ풒풆퐭ퟐ
Pore or internal diffusion was analyzed using the intra-particle
diffusion model, given in Eq. 7.74 In this model, qt is the amount
of Pt(IV) or Pd(II) adsorbed on the adsorbent (mg/g) at time t, kp
(mg/(g.min0.5) is the intra-particle diffusion rate constant.
풕
풒풕 = 풌풑
(퐄퐪. ퟕ)
Regeneration of biosorbents Regeneration performances of the
adsorbents was evaluated in three adsorption-desorption
cycles. For the adsorption experiment, 10 mg of chitosan
derivative was brought into contact with a 25 mg/L Pt(IV) or
Pd(II) solution for 24 h. Then, the Pt or Pd loaded adsorbents
were subjected to desorption by adding 10 mL of 0.25 M
thiourea solution56 in 15 mL polyethylene tubes and shaken at
200 rpm in a horizontal shaker for 24 h at 20 ± 1 °C. Then, the
suspensions were filtered and the Pt or Pd content of the filtrate
was measured. After rinsing with deionized water and drying at
70 °C for 24 h, the Pt and Pd adsorption efficiency of the
regenerated adsorbents was determined in the next sorption
cycle. All experiments were performed in duplicate.
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