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Supplementary Material
Optimization of dye adsorption capacity and mechanical
strength of chitosan aerogels through crosslinking strategy
and graphene oxide addition
M. Salzano de Lunaa,b,*, C. Ascioneb, C. Santilloc, L. Verdolottic, M. Lavorgnac,*,
G.G. Buonocorec, R. Castaldob, G. Filipponea, H. Xiad, L. Ambrosioc
a Department of Chemical, Materials and Production Engineering (INSTM Consortium – UdR
Naples), University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy
b Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, Via
Campi Flegrei 34, 80078 Pozzuoli, Italy
c Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, P.le
E. Fermi 1, 80055 Portici (Naples), Italy
d State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of
Sichuan University, Chengdu 610065, China
Corresponding authors:
*e-mail: [email protected]; phone: +39 0817682407 (Martina Salzano de Luna)
*e-mail: [email protected]; phone: +39 0817758838 (Marino Lavorgna)
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S1. Details on the No-GEL and GEL classification of the aerogels
The crosslinking begins soon after the addition of the GA to the CS solution. Therefore, even the
no-GEL samples are actually gelled to some extent before the freeze-drying step. Nonetheless,
this incipient crosslinking is so negligible not to compromise the meaningfulness of the
distinction between the GEL and no-GEL samples. To confirm this aspect, a frozen GEL sample
and a frozen no-GEL one, both at 15 wt.% of GA (i.e. at the highest investigated crosslinker
content, that represents the most critical situation), were soaked in 0.35 M acetic acid solution
under stirring at room temperature for about one week. The evolution of the sample appearance
during this test is shown in Figure S1.
Figure S1. a) No-GEL (left) and GEL (right) samples soon after freezing. b) Vials containing the
no-GEL (left) and GEL (right) samples soon after immersion in the acetic acid solution. c) Same
vials as in b) after one week.
After one week in the acetic acid solution, the no-GEL sample is completely dissolved, while the
GEL sample keeps its integrity. This result confirms the significant difference between the two
sets of investigated samples, in which ice crystals actually form before ("no-GEL") and after
("GEL") the polymer crosslinking step.
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S2. FTIR analyses on CS aerogels after the thermal treatment
At the end of the freeze-drying process, the chitosan (CS) aerogels were thermally treated at
90°C for 60 min and 120 min. FTIR analyses were carried out to evaluate the effect of the
duration of the thermal treatment on the crosslinking reaction between CS and glutaraldehyde
(GA). The FTIR spectra of representative aerogels prepared with low (5 wt.%) and high (15 wt.
%) GA content were compared to not crosslinked CS in Figure S2. For a qualitative comparison,
the spectra were normalized considering the absorption band at 1153 cm-1, attributed to
asymmetric stretching of the C-O-C bridges, as invariant peak.
Figure S2. FTIR spectra of a, c) no-GEL and b, d) GEL CS aerogels crosslinked with ΦGA = a, b)
5 and c, d) 15 wt.% after thermal treatment of different duration: 0, 60, and 120 min. The
spectrum of not crosslinked CS aerogels is also reported as reference.
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The formation of the new peak attributed to imine N=C bonds (1655 cm-1) and the shoulder of
ethylenic C=C bonds (1562 cm-1) can be appreciated in the spectra of the as-prepared (i.e. before
thermal treatment) no-GEL and GEL aerogels at ΦGA = 5 wt.%. Such chemical modifications,
that are the fingerprint of the crosslinking reaction between CS and GA, become more evident in
the spectra of the samples thermally treated at 90°C for 60 min. After longer treatments (up to
120 min), no further chemical modifications take place, confirming that a thermal treatment of
60 min is enough to complete the crosslinking reaction between CS and GA in both no-GEL and
GEL samples. The same considerations also apply for no-GEL and GEL aerogels at ΦGA = 15 wt.
%.
S3. Skeletal density of CS aerogels
The skeletal density of the aerogels, ρS, was determined by means of helium picnometry
(AccuPyc 1340, Micromeritics, USA). Irrespective of the amount of GA and the aerogel
preparation process (i.e. no-GEL or GEL samples), the skeletal density was found to be
essentially the same for all the tested samples, as also reported by Uragami et al. (Uragami,
Matsuda, Okuno, & Miyata, 1994) for GA-crosslinked chitosan membranes. In particular, ρS of
the aerogels varied between 1.38 ± 0.05 and 1.47 ± 0.08 g cm -3, which is in good agreement with
other skeletal density data of chitosan available in the literature (Uragami, Matsuda, Okuno, &
Miyata, 1994; Riegger et al., 2018; López-Iglesias et al., 2019). Finally, no evident trend as a
function of GA content or crosslinking strategy can be appreciated for the values of ρS.
S4. Linear viscoelastic analyses on CS/GA solutions
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The addition of relatively low GA contents (ΦGA ≤2.5 wt.%) did not result in the formation of a
gel, at least in the investigated conditions (Figure S3). For such a sample, indeed, the viscoelastic
moduli exhibit a clear frequency dependence in all the investigated window, with G′ being
always smaller than G′′.
Figure S3. Frequency-dependent storage and loss moduli for CS solution ΦGA = 2.5 wt.% after
24 hours that GA was added. The error bars represent the standard deviation over three
independent measurements.
S5. Evaluation of gel fraction in CS aerogels crosslinked with different amounts of GA
The percentage extent of crosslinked CS in the aerogels was evaluated gravimetrically.
Specifically, dry samples (~15 mg) were immersed in 0.35 M acetic acid solution (15 mL) to
extract soluble (i.e. not chemically crosslinked) fractions. At fixed time intervals, the aerogels
were carefully collected, washed with bi-distilled water and dried at 40°C under vacuum. The
procedure was repeated until reaching a constant value of sample weight. The gel fraction, ,
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was thus determined by the following relationship (Nieto-Suárez, López-Quintela, & Lazzari,
2016):
(S1)
where WI and WF represent the initial weight of the dry sample and the weight at the end of the
washing procedure, respectively. Five aerogels were tested for each set of samples.
The result of the measurements is shown in Figure S4. The gel fraction is over 90% for both the
investigated sets of samples, irrespective of the amount of crosslinker used. The reaction
between CS and GA leads to complete crosslinking of the chitosan chains (as testified by the
non-dissolution of aerogels in acid water) even at the minimum content of GA considered and
independently to the procedure adopted for aerogel preparation.
Figure S4. Gel fraction for no-GEL (red circles) and GEL (blue squares) CS aerogels
crosslinked with different ΦGA. The error bars represent the standard deviation over five
independent measurements
S6. Evaluation of swelling degree of CS aerogels crosslinked with different amounts of GA
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The swelling properties were evaluated by immersion of the dried aerogels in excess bi-distilled
water. The swollen samples were weighted at various time intervals after removing the excess
surface water was removed using filter paper, and the procedure was repeated until reaching a
constant value. The percentage swelling degree was calculated as:
(S2)
where mS and mD represent the mass of the swollen and dried aerogel, respectively.
The result of the measurements is shown in Figure S5.
Figure S5. Swelling degree in water for no-GEL (red bars) and GEL (blue bars) CS aerogels
crosslinked with different ΦGA. The error bars represent the standard deviation over three
independent measurements.
S7. Fitting of the adsorption equilibrium data to the isotherm models for pristine CS
aerogels
Freundlich (Freundlich, 1906) and Langmuir (Langmuir, 1916) isotherm models were exploited
to fit the experimental adsorption data and understand how the pollutant molecules interacted
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with the CS-based adsorbents. The Freundlich model predicts that the equilibrium adsorption
capacity, qe, depends on the equilibrium dye concentration in liquid phase, Ce, as:
(S3)
where kF represents the adsorption capacity and nF is a parameter that accounts for surface
heterogeneity.
Instead, according to the Langmuir isotherm model:
(S4)
where qL is the saturated monolayer adsorption and bL represents the binding energy or affinity
parameter of the sorption system.
The parameters obtained by the best fitting of the experimental adsorption equilibrium curves to
Equations S3 and S4 are reported in Table S1 and Table S2 for pristine no-GEL and GEL CS
aerogels crosslinked with different ΦGA, respectively.
Table S1. Isotherm fitting parameters for no-GEL CS aerogels at different ΦGA.
Freundlich modelΦGA
[wt.%]kF
[(mg g-1) (mg L-1)-1/nF]nF R2
5 168.5 ± 50.7 4.95 ± 1.38 0.8574
7.5 147.1 ± 45.3 4.85 ± 1.34 0.8602
10 125.7 ± 44.8 4.56 ± 1.35 0.8425
12.5 108.2 ± 5.0 4.32 ± 1.28 0.8459
15 100.0 ± 40.3 4.41 ± 1.39 0.8280
Langmuir modelΦGA
[wt.%]qL
[mg g-1]bL
[L mg-1]R2
5 534.4 ± 30.5 0.157 ± 0.016 0.9529
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7.5 484.2 ± 25.5 0.139 ± 0.017 0.9930
10 458.5 ± 16.0 0.099 ± 0.020 0.9805
12.5 431.8 ± 22.1 0.071 ± 0.010 0.9905
15 410.8 ± 18.2 0.069 ± 0.013 0.9849
Table S2. Isotherm fitting parameters for GEL CS aerogels at different ΦGA.
Freundlich modelΦGA
[wt.%]kF
[(mg g-1) (mg L-1)-1/nF]nF R2
5 160.1 ± 34.9 6.80 ± 1.86 0.8690
7.5 128.4 ± 26.0 5.89 ± 1.27 0.9159
10 123.3 ± 30.1 5.91 ± 1.53 0.8773
12.5 115.5 ± 30.7 5.80 ± 1.59 0.8640
15 107.9 ± 29.4 5.98 ± 1.71 0.8505
Langmuir modelΦGA
[wt.%]qL
[mg g-1]bL
[L mg-1]R2
5 365.1 ± 36.6 0.691 ± 0.098 0.9933
7.5 340.4 ± 21.8 0.269 ± 0.065 0.9806
10 330.2 ± 39.3 0.269 ± 0.036 0.9929
12.5 319.0 ± 30.6 0.222 ± 0.038 0.9882
15 292.7 ± 38.3 0.214 ± 0.029 0.9925
Since the Langmuir isotherm equation was the most suitable model to fit the experimental data,
the maximum adsorption capacity of the pristine no-GEL and GEL CS aerogels can be
considered equal to the saturated monolayer coverage of the adsorbent active sites (i.e. the value
of qL obtained through the best fitting procedure).
S8. Insights into dye removal mechanism
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Although all the investigated aerogels significantly swell in aqueous solutions (see Figure S5),
the dye removal process is essentially governed by adsorption phenomena (rather than to
absorption processes. Targeted experiments were performed to prove this aspect. In particular,
the aerogels collected at the end of the adsorption process were immediately soaked in fresh bi-
distilled water. No evident release of dye molecules due to diffusion-related processes can be
appreciated (see the photo in Figure S6). Evidently, if the removal process was merely based on
absorption, the fresh water would be immediately polluted by the dye molecules released from
the aerogel. On the contrary, UV-vis spectroscopy data of the aqueous medium also confirms
that no dye molecules can hardly detected (Figure S6). Note that UV-vis spectroscopy can detect
a concentration of IC as low as 1 mg L-1. The dye molecules are thus firmly anchored to the
surface of the aerogels even when immersed in excess of fresh bi-distilled water, indicating that
the removal process is related to adsorption phenomena.
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Figure S6. Photo of a representative aerogel (no-GEL CS aerogel at ΦGA = 10 wt.%) soaked in
fresh bi-distilled water at the end of the adsorption process of IC (the dye solution obtained at the
end of the adsorption process is shown on the left side as reference). The UV-vis spectra of the
two solutions that are shown in the photo are also reported.
S9. Gelation kinetics of CS/GO solution with GA
The gelation kinetics in the presence of GO nanosheets in the CS solution is slightly faster
(Figure S7). Nonetheless, the gelation time, tGEL, which is identified as the crossover point of G′
and G′′ over time, is still sufficiently long to allow the distinction between no-GEL and GEL
samples.
Figure S7. Time-dependent storage (full symbols) and loss (empty symbols) moduli for CS/GO
solution at ΦGA = 10 wt.% (red squares). The curves for the pristine CS solution at ΦGA = 10 wt.%
(black circles) are reported for comparison.
S10. Fitting of the adsorption equilibrium data to the isotherm models for nanocomposite
CS/GO aerogels
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Freundlich (Freundlich, 1906) and Langmuir (Langmuir, 1916) isotherm models were used to fit
also the dye adsorption data of no-GEL and GEL CS/GO aerogels. The corresponding
mathematical relationships are presented in Section S7. The parameters obtained by the best
fitting of the experimental adsorption equilibrium curves to Equation S3 and S4 are reported in
Table S3 and Table S4 for no-GEL and GEL nanocomposite CS/GO aerogels, respectively.
Table S3. Isotherm fitting parameters for no-GEL CS and CS/GO aerogels for different dyes.Freundlich model
Sample - dyekF
[(mg g-1) (mg L-1)-1/nF]nF R2
CS - IC a) 125.7 ± 44.8 4.56 ± 1.35 0.8425
CS - MB b) - - -
CS/GO - IC 86.7 ± 26.7 4.10 ± 0.92 0.9098
CS/GO - MB 7.5 ± 3.2 2.30± 0.36 0.9516
Langmuir model
Sample - dyeqL
[mg g-1]bL
[L mg-1]R2
CS - IC a) 458.5 ± 16.0 0.099 ± 0.020 0.9805
CS - MB b) - - -
CS/GO - IC 376.8 ± 32.3 0.062 ± 0.010 0.9887
CS/GO - MB 168.6 ± 9.6 0.005 ± 0.001 0.9727a) Same data reported in Table S1 for pristine no-GEL CS aerogel at ΦGA = 10 wt.%. b) The experimental data of qe were too low to be fitted with isotherm models.
Table S4. Isotherm fitting parameters for GEL CS and CS/GO aerogels for different dyes.Freundlich model
Sample - dyekF
[(mg g-1) (mg L-1)-1/nF]nF R2
CS - IC a) 123.3 ± 30.1 5.91 ± 1.54 0.8773
CS - MB b) - - -
CS/GO - IC 96.9 ± 38.2 5.46 ± 2.03 0.7687
CS/GO - MB 2.4 ± 0.6 2.04 ± 0.17 0.9839
Langmuir model
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Sample - dyeqL
[mg g-1]bL
[L mg-1]R2
CS - IC a) 330.2 ± 39.3 0.270 ± 0.036 0.9929
CS - MB b) - - -
CS/GO - IC 300.9 ± 26.9 0.099 ± 0.02 0.9317
CS/GO - MB 87.2 ± 5.2 0.003 ± 0.001 0.9909a) Same data reported in Table S2 for pristine GEL CS aerogel at ΦGA = 10 wt.%.b) The experimental data of qe were too low to be fitted with isotherm models.
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