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Formation of complexes between functionalized chitosan membranes and copper: A study by angle resolved XPS
Accepted Manuscript Formation of complexes between functionalized chitosan membranes and copper: A study by angle resolved XPS Belén Jurado-López, Rodrigo Silveira Vieira, Rodrigo Balloni Rabelo, Marisa Masumi Beppu, Juan Casado, Enrique Rodríguez-Castellón PII:
S0254-0584(16)30752-0
DOI:
10.1016/j.matchemphys.2016.10.018
Reference:
MAC 19226
To appear in:
Materials Chemistry and Physics
Received Date: 8 August 2015 Revised Date:
4 October 2016
Accepted Date: 15 October 2016
Please cite this article as: B. Jurado-López, R.S. Vieira, R.B. Rabelo, M.M. Beppu, J. Casado, E. Rodríguez-Castellón, Formation of complexes between functionalized chitosan membranes and copper: A study by angle resolved XPS, Materials Chemistry and Physics (2016), doi: 10.1016/ j.matchemphys.2016.10.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Formation of complexes between functionalized chitosan membranes and copper: a study by angle resolved XPS
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Belén Jurado-Lópeza, Rodrigo Silveira Vieirab, Rodrigo Balloni Rabeloc, Marisa Masumi Beppuc, Juan Casadod, Enrique Rodríguez-Castellóna* a
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Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain b Chemical Engineering Department, Universidade Federal do Ceará, UFC, 60455-760 Fortaleza, CE, Brazil c School of Chemical Engineering, University of Campinas, UNICAMP, P.O. Box 6066, 13081-970 Campinas, SP, Brazil d Departamento de Química-Física, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
Abstract
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Chitosan is a biopolymer with potential applications in various fields. Recently, it has been used for heavy metals removal like copper, due to the presence of amino and hydroxyl groups in its structure. Chitosan membranes were crosslinked with epichlorohydrin and bisoxirano and functionalized with chelating agents, such as, iminodiacetic acid, aspartic acid and tris-(2-amino-ethyl) polyamine. These membranes were used for copper adsorption and the formed complexes were characterized. Thermal and crystalline properties of chitosan membranes were studied by TG-DCS and X-ray diffraction. Raman, XPS and FT-IR data confirmed that copper is linked to the modified chitosan membranes by the amino groups. The oxidation state of copper-chitosan membranes were also studied by angle resolved XPS, and by UV-Vis diffuse reflectance spectroscopy.
Keywords: Biomaterials; polymers; X-ray photo-emission spectroscopy (XPS); Raman spectroscopy and scattering; thermogravimetric analysis (TGA); chemisorption
Corresponding author: Enrique Rodríguez Castellón, Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain. E-mail address: [email protected]
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1. Introduction The mitigation of heavy metal pollution is an important issue in ensuring human and environmental health. Heavy metals are natural components of the earth's crust and they cannot be easily degraded. In very low concentrations, some heavy metals are
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essential to maintain the metabolism of the human body. However, at medium and high concentrations they can lead to poisoning and are harmful because they tend to bioaccumulation.
To minimize the heavy metal amount in wastewaters, a previous treatment is
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necessary, in order to minimize the impact in plants and humans [1]. In recent years, many studies have been focused into using low-cost adsorbent materials to purify water
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contaminated with metals [2,3]. This technology is known as an economic and ecofriendly option.
Copper can be found in many wastewater sources including, electroplating, circuit
manufacturing, wire drawing and
polishing, paint manufacturing, wood
preservatives, and protection of ship hulls. Typical range concentrations are between
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thousands of ppm from plating waste and one ppm from cleaning processes [4-6]. The use of chitosan in heavy metal adsorption is supported by numerous studies [7-9], due to its capacity for metal ion removal because of its chelating ability [10].
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Several researchers have investigated chitosan as an adsorbent for toxic metals such as chromium, nickel, zinc, mercury, lead, cadmium, copper, and uranium [8,9].
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The chelating properties of chitosan are mainly attributed to the availability of
amino
(-NH2) and hydroxyl (-OH) groups that can coordinate transition metal ions
[11]. Chitosan surface can be modified by physical or chemical treatments, which are highly influenced by its deacetylation degree and molecular weight [12]. This material can be modified by functionalization, crosslinking, grafting, in order to enhance its reactivity for defined species or to modify its solubility in acidic medium, which is an important characteristic for adsorbents [13-17]. Chitosan is an excellent adsorbent for metal ions, which adsorption capacity is higher than those observed in many commercial chelating resins [18-20].
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ACCEPTED MANUSCRIPT In this study, chitosan membranes were crosslinked and functionalized with poly-chelating agents aiming a better copper ion complexation (Figure 1). Figure 2 shows the different crosslinked chitosan membranes used in this study: CHI-ECH (crosslinked with epichlorohydrin), CHI-ECH-IDA membrane (crosslinked with epichlorohydrin and functionalized with iminodiacetic acid) CHI-ECH-ASP membrane
TRIS
(crosslinked
with
epichlorohydrin
and
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(crosslinked with epichlorohydrin and functionalized with aspartic acid), CHI-ECHfunctionalized
with
tris((2-
aminoethyl)amine) and CHI-BIS-TRIS (crosslinked with bisoxirane and functionalized with tris((2-aminoethyl)amine). After production, these modified membranes were
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complexed with copper ions.
The main objective of this study was the characterization of five crosslinked
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chitosan membranes that had been functionalized with different chelating agents and investigate the oxidation state of copper after metal ion adsorption into the functionalized membranes. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal stability and characteristics of the thermal decomposition of modified chitosan membranes. X-ray diffraction patterns were measured in order to evaluate the crystalline/amorphous character of the
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chitosan membranes. Angle resolved XPS (ARXPS), Raman and infrared spectroscopy were used to characterize the surface functional groups of crosslinked chitosan membranes after copper adsorption. The possible reduction or oxidation of Cu(II) after
spectroscopy.
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adsorption process was also studied by XPS and UV-Vis diffuse reflectance
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2. - Experimental. 2.1. Materials
Chitosan membranes were prepared using chitosan with molecular weight of
9.90 × 105 g/mol (measured using gel permeation chromatography) was purchased from Sigma (USA), with a minimum deacetylation degree of 85% and the thickness of the membrane was 95 ± 15 µm, measured using a digital pachymeter. Iminodiacetic acid (IDA), L-aspartic acid (Asp), bromoacetic acid and tris(2-aminoethyl)amine (TREN) were supplied by Sigma–Aldrich (USA). Epichlorohydrin and acetic acid were provided
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ACCEPTED MANUSCRIPT by Merck (Germany). All solutions were prepared with deionized water (18MΩ cm) obtained from a Milli Q (Millipore) purification system. 2.1.1 Preparation of chitosan membranes Chitosan membranes were prepared according to the methodology described by
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Beppu et al. [12]. Chitosan was dissolved in acetic acid (2.5% (w/w)) and the resulting solution was casted into plastic Petri dishes and left to dry at 60 ºC during 24 hours, to evaporate all the solvent, obtaining the chitosan membranes. After this time, the membranes were immersed for 24 hours in a NaOH solution (1 mol L-1) to neutralize
membranes were rinsed with water at 4 ºC.
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the amino groups of chitosan that had been protonated into acid solution. The
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2.2. Crosslinking and functionalization of chitosan membranes
2.2.1 -CHI-ECH-IDA. Crosslinked with epichlorohydrin and functionalized with iminodiacetic acid
A NaOH solution (4 g of NaOH were dissolved into 30 mL of ultrapure water)
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was mixed with 2 g of iminodiacetic acid and 1.95 mL of epichlorohydrin. This mixture was stirred for 4 hours at 60 ºC. After this period, this solution was cooled to 0 ºC, and 8 g of NaOH were added. The obtained solution was called as “A solution” [20].
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A small portion of 7.0 g of chitosan membrane, previously obtained, was heterogeneously crosslinked in the “A solution” during 16 hours at 65 ºC. Finally the
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crosslinked chitosan membrane was rinsed with ultrapure water [21]. 2.2.2 - CHI-ECH-ASP: Crosslinked with epichlorohydrin and functionalized with aspartic acid.
The crosslinked chitosan membrane was functionalized with aspartic acid
following the procedure described by Chaga et al. [22]. Firstly, a chitosan membrane was heterogeneously crosslinked with epichlorohydrin: 7.0 g of chitosan membrane were soaked during 15 minutes at room temperature with 46 mL of NaOH solution (2.0 mol L-1), 4.6 mL of epichlorohydrin and 0.17 g of NaBH4. After this time, a new solution (prepared with 46 mL of NaOH and 23 mL of epichlorohydrin) was dropped on 4
ACCEPTED MANUSCRIPT the previous solution and stirred for 16 hours at room temperature. After this period, the CHI-ECH was rinsed with ultrapure water, followed with Na2CO3 2 mol L-1. An aspartic acid solution (8 g of aspartic acid was dissolved into 50 mL of Na2CO3 1 mol L-1) was prepared (pH 11.5). The CHI-ECH membrane was immersed (7.0 g) in this solution for 24 hours, obtaining CHI-ECH-ASP. Finally to remove the unreacted
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aspartic acid residues the film was rinsed with water. 2.2.3. - CHI-ECH-TRIS. Crosslinked with epichlorohydrin and functionalized with tris(2-aminoethyl)amine. method
followed
to
functionalize
the
CHI-ECH,
using
tris(2-
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The
aminoethyl)amine as chelating agent, was described by Boden et al. [23] 5 mL of TRIS
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(96% w/v) solution were dissolved into 25 mL of ultrapure water. 7.0 g of CHI-ECH membrane, as obtained in 2.1.2.2 item, were immersed into this solution during 48 hours under continuous stirring. Finally it was rinsed with ultrapure water in order to remove the unreacted functionalization agent.
2.2.4.-CHI-BIS-TRIS: Crosslinked with bisoxirano (BIS) and functionalized with tris(2-
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aminoethyl)amine.
The crosslinking process used to obtain the CHI-BIS-TRIS sample was the method proposed by Boden et al. [23]. Chitosan membrane (7.0 g) was immersed into 20 mL of NaOH solution (0.6 mol L-1) and 20 mL of 1,4-butenodiol diglycidyl ether
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that contained 40 mg of NaBH4. The mixture was kept at room temperature under continuous stirring during 48 hours. After this time, in order to remove the unreacted
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bisoxirano, the membrane was rinsed with ultrapure water. 5 mL of TRIS solution (96% w/v) were added to 25 mL of ultrapure water. This solution was added to 7.0 g of chitosan membrane that previously had been crosslinked with bisoxirano. To complete the functionalization, the suspension was kept at room temperature, during 48 hours, under continuous stirring. After this time, the suspension was rinsed with ultrapure water to eliminate the residues of the functionalization agent. 2.3. Adsorption of copper on functionalized crosslinked chitosan membranes. After the crosslinked chitosan membranes were functionalized, 0.30 g of each membrane were immersed into 25 mL of Cu(II) solution (500 mg L-1 as sulfate), with 5
ACCEPTED MANUSCRIPT continuous stirring at 20 ºC with a pH=5.0 during 60 hours. The membranes were washed and then dried to follow the characterization experiments. 2.4. Membranes characterization XPS analysis: X-ray photoelectron spectroscopy (XPS) was used to surface chemical
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characterization of functionalized crosslinked chitosan membranes before and after Cu incorporation. A Physical Electronics spectrometer (PHI 5700) was used, with an X-ray Mg Kα radiation (300W, 15 kV, 1253.6 eV) as the excitation source. A concentric
hemispherical analyser operated in the constant pass energy mode at 29.35 eV, using a
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sample of 720 µm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV FWHM at a binding energy (BE) of 84.0 eV. The spectrometer energy scale was calibrated using Cu 2p3/2, Ag 3d5/2, and Au 4f7/2 photoelectron lines at
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932.7, 368.3, and 84.0 eV, respectively.
The residual pressure inside the analysis chamber was maintained at 5×10−6 Pa or lower during the measurement. The XPS signals were decomposed into different contributions using a software package (PC-Access ESCA-V6.0F). Each spectral region was scanned several sweeps up to a good signal to noise ratio.
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A small portion of each membrane was placed in the specimen chamber for testing. The spectra obtained were always fitted using Gauss–Lorentz curves for each peak to determine the binding energy of the elements presents at the samples. The calibration of the binding energy of the spectra was performed with the C 1s peak of the
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aliphatic and adventitious carbons, which is at 284.8 eV. To determinate more accurately the binding energy (BE) of the different element
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core levels, the first spectra of each sample was obtained with a short acquisition time of 10 min to examine C 1s, Cu 2p and Cu LMM XPS regions in order to avoid, as much as possible the photo-reduction of Cu2+ species [24,25]. ARXPS: To obtain additional information, several spectra of each membrane
sample at different take-off angles (α) [26] were recorded to study in-depth the membranes. This enables us to analyse different depths depending the elements probed by varying the analysis angle between 15 and 75º [27]. In these spectra, the chitosan membranes were irradiated for a maximum time of 20 min, and the take-off angles were 15, 30, 45, 60 and 75o. The depth probed depends on the kinetic energy (KE) of the 6
ACCEPTED MANUSCRIPT emerging electrons. This kinetic energy is specific for each element and can be studied at the universal curve of electron mean escape depth (or inelastic mean free path, IMFP). In this study a X-ray source of Mg Kα was used (1253.6 eV) and the KE of C 1s is 968.8 eV, so that, the IMFP for C1s is from about 1.5 nm (when a take angle of 15º was used) to 6 nm (with a take-off angle of 75º). This fact is due to take-off angle of 15º
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is around one quarter of 75º (λ = (KE)3/4). When the N 1s core level signal is analysed, the studied range of depth is from 1.5 to 6 nm, similar to that of C 1s because the KE of N 1s is around 854.0 eV. However, as the KE of Cu 2p is around 320 eV, the depth studied is from 0.9 to 3.5 nm. This means that these different depths must be considered
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in ARXPS analysis.
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Thermal analysis: The thermal degradation behavior of functionalized crosslinked chitosan membranes was studied by means of dynamic thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A thermal analyzer METLER TOLEDO model TGA/DSC 1 was used. This analyzer is able to carry out simultaneously TGA and DSC measurements using approximately 10 µg of sample. The balance was kept in a thermostatic water bath at 22 °C and protected by a continuous
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flow of 20 mL min-1 of N2. The thermobalance has coupled a quadrupole mass spectrometer model THERMOSTAR GSD320 of Pfizer. The experiments were carried out in alumina crucibles of 70 µl in the temperature range 30-1000 ºC, with an air flow
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of 50 mL min-1 and a heating rate of 10 °C min-1. XRD: The XRD technique was used to compare the crystalline structure of
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chitosan membranes and to observe the modifications on the membrane crystallinity induced by the different crosslinking agents. X-ray diffraction (XRD) patterns were collected on a PANanalytical X’Pert Pro automated diffractometer. XRD patterns were recorded in Bragg-Brentano reflection configuration by using a Ge(111) primary monochromator (Cu Kα1) and the X’Celerator detector with a step size of 0.0167º (2θ). The patterns were recorded between 2 and 80º in 2 θ. The total measurement time was about 30 min. Raman analysis: Raman spectra with the excitation laser at 785 nm were collected by using the 1×1 camera of a Bruker Senterra Raman microscope by averaging spectra during 50 minutes with a resolution of 3–5 cm-1. A CCD camera 7
ACCEPTED MANUSCRIPT operating at 50 ºC was used for the Raman detection. Raman scattering radiation was collected in a back–scattering configuration. UV-Vis-NIR Diffuse reflectance spectroscopy: The diffuse reflectance spectra were recorded, at room temperature, in the ultraviolet-visible-near infrared region (250850 nm) with a Shimadzu spectrophotometer model MPC-3100. The spectra were
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obtained using the functionalized crosslinked chitosan membranes (before and after copper adsorption) and using barium sulfate as a white reference.
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3. Results and discussion.
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3.1. XPS
XPS survey spectra of all functionalized crosslinked chitosan membranes after copper adsorption detected the presence of C, O, N, Cu and Si. The presence of Si in some membranes is due to surface contamination. High resolution spectra were recorded at a given take-off angle of 45º. These spectra were recorded using a short irradiation time
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(10 min) to avoid as much as possible the photo-reduction of Cu(II) [24,25]. Table 1 summarizes the binding energies that appear in the XPS spectra and their assignation after an irradiation time of 10 min for all the samples studied. The range at which appear the binding energies for the main functional groups, are similar in all
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membranes, independent of crosslinking or the functionalization agent used.
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3.2 ARXPS Analysis.
ARXPS permits the study of the chemical surface composition and the chemical
state by varying the depth with a minimum damage by using different take-off angles, registering the principal elements of each membrane and its relative atomic composition. Using different take-off angles, the C 1s signals regarding coppercrosslinked chitosan membranes could be decomposed in three contributions as shown in Figure 3. The contribution at low binding energy (C1) was mainly assigned to adventitious carbon or C-C chemical bonds. The contribution C2 was assigned to C-O, C-N or C-O-C, and the contribution at higher binding energy (C3) was assigned to OC=O and O-C-O groups. 8
ACCEPTED MANUSCRIPT Table 2 summarizes the identification of the contributions observed on the resolved spectra of the C 1s signal for the different crosslinked chitosan membranes studied and the binding energy at which are located the contributions. The C 1s contribution for the CHI-ECH-Cu, CHI-ECH-IDA-Cu and CHI-ECHASP-Cu samples appears at 284.8+0.1eV and is assigned to adventitious carbon and C-
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C moieties. This contribution exhibited a decrease of its relative intensity at high takeoff angles, where the depth of the studied layer is higher, and therefore, the contribution due to adventitious contamination is less relevant. The relative atomic concentration of the C2 contribution (which appears at a BE of 286.4+0.1 eV for the CHI-ECH-IDA-Cu)
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increased when the take-off angle was higher. This can be explained because this contribution is due to C-N and C-O bonds, and, as expected, the relative atomic
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concentration of N increases with increasing depth of the layer studied. C3 contribution, which is located at 288.1+0.1 eV (for CHI-ECH-ASP-Cu membrane) and belonging to O-C=O and O-C-O bonds, remains constant at different take-off angles. For CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu, the behavior patterns of contributions C1 and C2, are not the same as in the other crosslinked membranes because of the use of tris((2-aminoethyl)amine) as chelating agent. The relative atomic
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concentration of the contribution C3 can be considered as constant with the depth of the studied layer in these membranes. However, the relative atomic concentration corresponding to the C1 contribution increases with the increase of the take-off angle.
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Conversely, the relative atomic concentration of the contribution of C2 decreases when the studied take-off angle increases and hence the depth of the layer studied. Figure 4 shows the evolution of the atomic concentration C2/C1 ratio as a function of the take-
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off angle and the depth in nm for the CHI-ECH-TRIS sample. There is a different distribution on depth. For samples before copper adsorption, at the external surface and at low take-off angle, the C2/C1 value is high, indicating a higher concentration of polar groups. After Cu incorporation the C2/C1 ratio presented a low constant value at different take-off angles. This fact seems to indicate a very homogeneous distribution on depth of the copper complex in spite of different depth probed for Cu. When the relative atomic concentration of O is considered, the observed values depend on the relative atomic concentration of total C (that is, the addition of C1, C2
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ACCEPTED MANUSCRIPT and C3 contributions), and these values are lower when the relative atomic concentration values of C are higher. Figure 5 shows the N 1s signals for the CHI-ECH-ASP sample before and after the copper adsorption. Before the copper adsorption (Fig 5-a), the N 1s spectrum shows two contributions. One is centered at 399.8 + 0.1eV, and it is attributed to the N atom in
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the –NH2 and/or the –NH- groups on the surfaces of the chitosan [24]. The other contribution is located at 401.7+0.2 eV and belongs to the protonated species (-NH3+). After the adsorption of Cu, the N 1s spectrum showed a symmetric peak at a BE
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of 399.0 eV. This means that Cu is fixed to the crosslinked chitosan membrane through nitrogen atom, with the formation of the complex R-NH-Cu. In this case the protonated
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species has disappeared and the BE for N 1s signal has been shifted [28].
Table 3 lists the evolution of the N 1s and Cu 2p3/2 signals at the highest and lowest take-off angles. In all cases, an increase in the relative atomic concentration of N is observed when the take-off angle changes from 15 to 75 º. This increase is more evident for CHI-ECH-Cu, CHI-ECH-IDA-Cu and CHI-ECH-ASP-Cu samples. This N
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1s signal appears at 399.3-399.9 eV range and it is assigned to the amino groups. The higher relative atomic concentration value for high take-off angle is due to the absence of surface contamination in depth. A similar behavior was observed in the case of Cu.
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The observed increase in the relative atomic concentration of Cu is also more evident for CHI-ECH-Cu, CHI-ECH-IDA-Cu and CHI-ECH-ASP-Cu samples. This increase was not significant for CHI-ECH-TRIS-Cu as also observed in the case of N. In some
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cases the relative observed increases in percentage are not similar due to the different depth probed for N 1s and Cu 2p at 75 º (6.0 and 3.5 nm, respectively). In the case of samples CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu samples the
presence of groups with long alkyl chain, can also photoemit as adventitious carbon, causing a small change in the relative atomic concentration.
Figure 6 shows the Cu 2p3/2 signals for all the samples. It was observed an asymmetric main peak that can be decomposed in two contributions that appear at about
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ACCEPTED MANUSCRIPT 933.0 eV for Cu+ and at 935-936 eV for Cu2+ [9]. The ARXPS study of the Cu 2p signal was done with 20 min of irradiation time. The accumulation of the irradiations provokes that all the Cu 2p spectra showed copper reduced species. There
are
significant
differences
depending
of
the
crosslinking
or
functionalization agent used. However, the irradiation time used produced a
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modification of the Cu 2p spectra and evident conclusions concerning the copper chemical state cannot be established. As expected, the irradiation not only produces a shift of the main Cu 2p3/2 peak at lower binding energies, but also the shake-up satellite
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practically disappears due to the reduction of Cu2+.
In the case of membrane CHI-ECH-TRIS-Cu, the asymmetric Cu 2p3/2 signal can be decomposed in two contributions at 933.3+0.2 eV and 935.8+03 eV that are assigned
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to Cu+ and Cu2+ species, respectively. When the take-off angle increases, the contribution due to the presence of Cu2+ species decreases. This behavior is completely different for the CHI-BIS-TRIS-Cu membrane, where the contribution of the Cu+ species decreases with increasing the take-off angle studied. Probably, ECH forms a more stable complex with Cu2+.
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Table 4 shows a summary of the peaks located in the region Cu 2p3/2, for the CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes obtained at different take-off angles, and the area under each peak.
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The contribution of the shake-up satellite mainly due to the presence of Cu2+ has been considered for the calculation of the relative percentages of Cu2+ and Cu+.
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3.3 Thermal analysis
The thermogravimetric analysis is very helpful to understand the stability of the
copper complexes on the membranes and to obtain information about the moisture content and the percentages of inorganic and organic components. The TG curves showed three weight losses. Table 5 shows the temperatures ranges for each chitosan membrane, describing the dehydration, decomposition and combustion of the studied materials. It was observed that each thermal process depends of chitosan modification. There was first an endothermic effect, which was attributed to water evaporation, that would be associated to amino and hydroxyl groups of chitosan 11
ACCEPTED MANUSCRIPT chain [12]. The next effect was exothermic, attributed to chitosan membrane decomposition and also elimination of volatile products. The last effect was also exothermic and it could be related to the complexed chitosan combustion. The thermal stability followed this order: CHI-ECH-ASP-Cu > CHI-ECH-TRISCu > CHI-ECH-Cu > CHI-ECH-IDA-Cu > CHI-BIS-TRIS-Cu, showing that the
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functionalization with higher organic molecules increased the thermal stability.
The DSC curves (results not showed here) show changes in the intensity of exothermic/endothermic peaks and their positions are influenced by the chemical
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modification. The DSC curves show a broad endothermic peak around 100 ºC related to the water evaporation. The exothermic peak, which appears in the temperature range between about 250 ºC and 380 ºC, is attributed to the decomposition of the crosslinked
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chitosan membrane. And the last exothermic peak is due to the combustion of the sample. Once formed the carbonized remainder, the copper could be reduced to Cu+, or even to Cu0. This agrees with thermogravimetric studies for chlorine acetate and copper
3.4. XRD analysis
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acetate [29,30].
The XRD powder patterns of the each chitosan membrane are shown in Fig. 7. The XRD diagrams reveal poor crystallinity for all the crosslinked chitosan membranes
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studied. One reflection line at 23.9º was observed in all cases, and assigned to the presence of Cu2O [31,32]. The XRD of CHI was also included in Figure 7 as reference
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and it does not show the reflection line due to the presence of Cu2O 3.5. Vibrational Infrared and Raman Data The Raman spectra of modified chitosan complexed with Cu(II) are shown in
Figure 8. In this case the band at 1043 cm-1 is the strongest and the spectral changes are slightly, meaning that metal coordination took place through the amino group, which acted independently the others parts of the polymer. Comparing the samples crosslinked with ECH and BIS the difference is due the length of aliphatic spacer (2 methylenes in CHI-ECH-TRIS-Cu and 4 in CHI-BIS-TRIS-Cu). There is also a difference for the chitosan crosslinked with ECH, depending of functionalization agent. 12
ACCEPTED MANUSCRIPT Figure 9 displays the Raman spectra for CHI-ECH-ASP before and after copper adsorption. The spectrum of CHI-ECH-ASP is characterized by a background likely due to fluorescence from the sample which fully disappears after treatment with copper salt highlighting the drastic change on the optical properties of the membranes (fully transparent in the sample free of copper and blue color with divalent copper).
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The spectrum of CHI-ECH-ASP is dominated by one main Raman dispersion at 975 cm-1 accompanied by rather undefined weak bands in the high wavenumber region. In the sample with copper this band significantly shifts by 68 cm-1 up to 1043 cm-1 and the band shape slightly modifies its aspect becoming a sharper band. In addition, the
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bands in the region 1500-1100 cm-1 also change but these modifications are much less pronounced. We tentatively assign the Raman band at 975 cm-1 in CHI-ECH-ASP as
bond from the amine groups [33].
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emerging from a vibrational mode mainly involving the stretching of the C-N single
Complexation with copper provokes the main changes of this band indicating that coordination of the divalent cation should proceed through the lone electron pair of the nitrogen. This is in agreement with: i) the large change in the wavenumber upshift might reveal that coordination is reinforcing somehow this C-N bond; and ii) this
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Raman band gets shaper upon coordination in accordance with the specific effect of the metallic divalent cation [34].
Interestingly, in the spectrum with copper, a clear weak signal is detected
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around 1650-1630 cm-1 due to the stretch of the carbonyl group in the sample. This band correlates with that at 1630 cm-1 in the infrared spectrum of the same
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copper sample which is unaltered in the infrared spectrum of the untreated CHI-ECHASP highlighting again that the chemical effect of copper does not involve the carboxylic groups. Other secondary changes can be mentioned between the spectra of the natural and copper treated chitosan but with much less structural implications.
3.6. DR/UV-Vis-NIR Analysis Electronic spectra of the different samples studied were recorded in solid state (diffuse reflectance) in the UV-Vis-NIR region to obtain information concerning the 13
ACCEPTED MANUSCRIPT coordination of the copper complexes and the oxidation state of copper for the studied membranes. Figure 10 shows the DR/UV-Vis-NIR spectra for all chitosan membranes. The observed color is due to the presence of Cu2+complexes, mainly due to various electronic transitions between the ground state and excited states (due to unfolding of the fundamental term of the free ion 2D), the Cu2+ species has a d9
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configuration, which provides a flexible stereochemistry, which involves highly distorted geometries, with significant differences in bond lengths and angles with
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respect to the regular geometry [35].
For each sample, it can observe a band which is located in the region 14970-
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14045 cm-1. This band may be assigned to1A1g -->1A2g (dx2-dy2 ->dxy) transitions. According to the literature [36], the geometry of copper complex inside the crosslinked chitosan membrane, is due to the octahedral geometry of the metal ion of copper, which has an oxidation state of Cu2+. The octahedral geometry is distorted by the Jahn Teller effect.
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4. Conclusions
The study of crosslinked chitosan membranes using ARXPS technique confirms the previously proposed structure in the case of membranes CHI-ECH-Cu, CHI-ECH-
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IDA-Cu and CHI-ECH-ASP-Cu complexed, thus confirming the chemisorption of this metal. In the case of CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes, the tris(2-
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aminoetil)amine chelating agent amine is the most voluminous and the study of the surface composition of these membranes using this technique does not give rise to clear conclusions. The chemical state of copper was followed by using XPS, where copper is mainly present as Cu2+ as confirmed by UV-Vis diffuse reflectance. The study of the interaction of the metal with the biopolymer showed that complexation increased the crystallinity degree of the materials. The infrared and Raman spectra obtained for samples with and without copper suggested a mechanism of adsorption through covalent interactions more strongly to the amino groups. Acknowledgements
14
ACCEPTED MANUSCRIPT The authors acknowledge financial support from Projects CTQ2015-68951-C3-3R and CTQ2012-30703 of Ministerio de Economía y Competitividad (Spain) and FEDER
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funds and Project of Excellence P12-RNM-1565 (Junta de Andalucia, Spain).
References
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[2] E. Igberase, P. Osifo, A. Ofomaja. J. Environ. Chem. Eng. 2 (2014) 362-369.
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[3] Y. Baowei, X. Jing, L. Hia-Hui , Y. Sheng-Tao , L. Jianbin, Z. Qinghan, W. Jing, L. Rong, W. Haifang, L. Yuanfang. J. 11(2013) 1044-1050.
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[7] K.D. Trimukhe, A.J. &Varma. Carbohyd. Polym. 71(1) (2008) 66-73.
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[8] Y. Sag. Separ. Purif. Methods. 30(1) (2001) 1-48. [9] R.S. Vieira, M. L. M. Oliveira, E. Guibal, E. Rodríguez-Castellon, M. M. Beppu. Colloid Surface A. 374 (2011) 108-114. [10] K.H. Chu. J. Hazard. Mater. B90: (2002) 77-95. [11] M. H. Mostafa, Z. Khaled Al- Wakeek, S. Sayed Abd El Rehim, H. Abd El. J. Environ. Chem. Eng. 1, (2013) 566-573. [12] M. M. Beppu, C.C. Santana. Mater. Res. 5 (2002) 47-50. [13] N. Li, R. Bai. Sep. and Purif. Technol. 42 (2005) 237-247. 15
ACCEPTED MANUSCRIPT [14] E. Guibal. Sep. Purif. Technol. 38, (2004) 43-74. [15] W. S. W. Ngah, S. Fathinathan. Chem. Eng. J. 143, (2008) 62- 72.
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[19] A. J. Varma, S. V. Deshpande, J. F. Kennedy. Carbohyd. Polym. 55 (2004) 77-93.
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[20] R. S. Vieira, E. Meneghetti, P. Baroni, E. Guibal, V.M. González de la Cruz, A. Caballero, E. Rodríguez-Castellón, M.M. Beppu. Mater. Chem. Phys. 146 (2014) 412417. [21] T. Sano, I. Murase. Sumitoma Chemical Company. US Patent 4.200.735. 06 mar. 1978, 26 fev. 1979.
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[22] G. Chaga, J. Hopp, P. Nelson. Biotechnol. Appl. Bioc. 29 (1999) 19-24. [23] V. Boden, J.J. Winzerling, M. Vijayalakshmi, J. Porath. J. Immunol. Methods. 181 (1995) 225-232.
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[25] E. Moretti, E, M. Lenarda. L. Storaro, A. Talon, T. Montanari, G. Busca, E. Rodríguez-Castellon, A. Jiménez-López, M. Turco, G. Bagnasco, R. Frattini. Appl. Catal. A: Gen. 335 (2008) 46-55. [26] A.P.Pijpers, R.J Meier. Chem. Soc. Rev. 28, 1999. [27] G.P. López, D.G. Castner, B.D.Ratner, Surf. Interface Anal. 17 (1991) 267. [28] W. Jiang, S. Chen, B. Pan, Q. Zhang, L. Teng, Y. Chen, L. Liu. J. Hazard. Mater. 276 (2014) 295-301.
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ACCEPTED MANUSCRIPT [29] F.Walts, J. Walstenholme. An introduction to surface analysis by XPS and AES Ed. Wiley. England. (2003) 93-111. [30] H. Peng-Zhi, L. Si-Dong, O. Chun-Yan, L. Cheng-Peng, Y. Lei, Z. Chao-Hua. J. Appl. Polym. Science. 105 (2007) 547-551.
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[31] R. Guan, H. Hashimoto, K.H. Kuo. Acta Crystallogr. B, 40 (1984) 560-566. [32] M. Salavati-Niasari, F. Davar, N. Mir. Polyhedron. 27 (2008) 3514–3518.
[33] A.C. Oyrton, Jr. Monteiro. Claudio Airoldi. Int. J. of Biol. Macromol. 26 (1999)
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119-128.
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[34] C. Jin-Yi, Z. Pei-Jiang, L. Jia-Lin, L. Su-Qin. Carbohyd. Polym. 67 (2007) 623629.
[35] S. Balboa, R. Carballo, A. Castiñeiras, J.M. González-Pérez, J. Niclós-Gutiérrez. Polyhedron. Volume 27, Issue 13, (2008) 2921-2930.
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New York, 1984.
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[36] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed. Elservier Science,
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CAPTIONS
Table captions
Table 1: Assignments of main spectral bands based on their binding energies (BE) in eV for the crosslinked chitosan membranes complexed with Cu(II) for a take-off angle of 45º and after an irradiation time of 10 min. Table 2: Assignments of main spectral bands of C 1 speaks based on their binding energies (BE) for the crosslinked chitosan membranes complexed with Cu(II), using as take-off angles 15º and 75º. 17
ACCEPTED MANUSCRIPT Table 3: N 1s and Cu 2p3/2 binding energies (in eV) and relative atomic concentrations at take-off angles of 15 and 75º, after the adsorption. Table 4: Evolution of the Cu2p3/2 signal as a function of the take-off angle in ARXPS experiments for samples CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu.
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Table 5: TG-DSC data for the studied membranes.
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Figure captions
Figure 1. Schematic representation of the adsorption mechanism.
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Figure 2. Proposed structures for crosslinked and functionalized chitosan membranes. Figure 3. ARXPS spectra for C 1s for samples CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHIECH-ASP-Cu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu.
Figure 4. Evolution of the atomic concentration C2/C1 ratio as a function of the take-
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off angle for samples CHI-ECH-TRIS and CHI-ECH-TRIS-Cu. Figure 5. ARXPS spectra for the CHI-ECH-ASP crosslinked chitosan membrane before
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and after Cu adsorption.
Figure 6. ARXPS Cu 2p spectra for CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-
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ASP-Cu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes. Figure 7. X-ray diffraction pattern for modified chitosan membranes (CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-ASP-Cu, CHI-ECH-TREN-Cu and CHI-ECH-TRENBIS-Cu) and pure chitosan (CHI). Figure 8. 785 nm Raman spectra of the five membranes treated with copper nitrate treated in the 1700-700 cm-1 spectral region. Figure 9. 785 nm Raman spectra of CHI-ECH-ASP and copper treated CHI-ECH-ASP in the 1700-700 cm-1 spectral region.
18
ACCEPTED MANUSCRIPT Figure 10: UV-Vis-NIR spectra of crosslinked chitosan membranes.
Samples
C 1s
C2
C3
CHI-BIS-
ECH-
ECH-
ECH-
ECH-
TRIS-Cu
Cu
IDA-Cu
ASP-Cu
BIS-Cu
284.8
284.8
284.8
284.8
284.8
(53%)
(54%)
(37%)
(33%)
(67%)
286.3
286.3
286.3
286.1
286.2
(35%)
(33%)
(48%)
(54%)
(23%)
288.0
287.9
288.3
288.0
288.1
(12%)
(13%)
(15%)
(13%)
(10%)
933.2
933.3
933.3
933.2
933.1
Si 2p
Functional Group
C-C; adventitious carbon
C-N; C-O; C-O-C
C=O; O-C-O
Cu+
(29%)
(39%)
(55%)
(10%)
(34%)
935.8
935.4
935.4
935.8
935.5
(71%)
(61%)
(45%)
(90%)
(66%)
532.6
532.6
532.5
532.8
532.5
C-O; O-H
399.8
399.9
399.7
399.4
399.6
-NH2
102.0
102.1
----
----
SiO2
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N 1s
CHI-
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O 1s
Cu2p3/2
CHI-
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Cu 2p
CHI-
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C1
CHI-
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Signal
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Table 1. Assignments of main spectral bands based on their binding energies (BE) in eV for the modified chitosan membranes complexed with Cu(II) using a take-off 45º and after an irradiation time of 10 min.
102.2
Cu2+
19
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Table 2. Assignments of main spectral bands of C 1s peaks based on their binding energies (BE) for the modified chitosan membranes complexed with Cu(II), data expressed corresponding to the limits angles 15 ° and 75 °.
C2
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CHI-BISTRIS-Cu
15º
284.8 (71%)
284.8 (61%)
284.8 (53%)
284.4 (8%)
284.7 (26%)
75º
284.7 (55%)
284.8 (53%)
284.7 (44%)
284.8 (18%)
284.9 (27%)
15º
286.4 (21%)
286.5 (28%)
286.3 (38%)
286.2 (82%)
286.0 (60%)
75º
286.4 (33%)
286.4 (35%)
286.3 (46%)
286.2 (69%)
286.7 (58%)
15º
288.2 (8%)
288.2 (11%)
288.1 (9%)
288.7 (10%)
288.0 (14%)
75º
288.1 (12%)
288.2 (12%)
288.2 (10%)
288.4 (13%)
288.2 (15%)
Asignation
C-C; adventitious C
C-N; C-O; C-OC
O-C=O ; O-C-O
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C3
CHI-ECHTRIS-Cu
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C1
Take-off CHI-ECH-Cu CHI-ECH-IDA- CHI-ECHCu ASP-Cu angle
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Band
20
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Table 3: N 1s and Cu 2p3/2 binding energies (in eV) and relative atomic concentrations at take-off angles of 15 and 75º, after the adsorption. N 1s
Cu 2p
15º take off angle
75º take off angle
CHI-ECH-Cu
399.8+0.2
4.4
6.5
CHI-ECH-IDACu
399.9+0.3
4.9
7.6
CHI-ECH-ASPCu
399.7+0.4
3.1
5.1
CHI-ECHTRIS-Cu
399.3+0.2
7.6
CHI-BIS-TRISCu
399.6+0.1
6.3
BE (eV) Cu 2p3/2
% Relative atomic concentration 15º take off angle
75º take off angle
1.14
1.84
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% Relative atomic concentration
BE (eV) N 1s
934.3+1.1
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Sample
0.81
1.31
933.1+0.3
0.82
1.24
8.2
933.3+0.2 935.8+0.3
5.93
5.96
7.9
933.1+0.1 935.4+0.04
3.16
4.05
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933.2+0.1
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Table 4. Evolution of the Cu 2p3/2 signal as a function of the take-off angle in ARXPS experiments for samples CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu. CHI-ECH-TRIS-Cu
45º
60º
Relative area (%)
Chemical species
933.5
3
933.2
41
Cu+
936.1
97
935.4
59
Cu 2+
933.3
10
933.1
66
Cu+
936.0
90
935.3
34
Cu 2+
933.2
16
933.1
935.7
84
935.4
933.2
17
935.6
83
933.2
23
935.5
77
67
Cu+
33
Cu 2+
933.1
70
Cu+
935.5
30
Cu 2+
933.1
67
Cu+
935.4
33
Cu 2+
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75º
Cu 2p3/2
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30º
Relative area (%)
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15º
Cu 2p3/2
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Take-off angle
CHI-BIS-TRIS-Cu
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Table 5. TG-DSC data for the studied membranes. Loss of water ∆t (ºC)
First decomposition
%weightloss
∆t (ºC)
% weightloss
∆t (ºC)
% weightloss
(375-590)
39.35
CHI-ECH-Cu
(50-175)
9.77
(175-375)
45.52
CHI-ECH-
(43-165)
8.44
(165-370)
44.58
(40-190)
8.78
(190-405)
43.97
(50-185)
6.16
(185-495)
(43-163)
5.34
IDA-Cu
CHI-ECH-
CHI-BIS-
(163-510)
33.70
(405-630)
40.72
47.43
(495-630)
43.2
(510-662)
39.39
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TRIS-Cu
44.49
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TRIS-Cu
(370-580)
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ASP-Cu
CHI-ECH-
Second decomposition
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Sample
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Chitosan membrane
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Chelating agent
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Crosslinking agent
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Figure 1. Schematic representation of the adsorption mechanism.
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Figure 2. Proposed structures for crosslinked and functionalized chitosan membranes.
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Figure 3. ARXPS spectra for C 1s for samples CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHIECH-ASP-Cu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu.
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CHI-ECH-TRIS-Cu
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CHI-ECH-ASP-Cu
CHI-BIS-TRIS-Cu
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depth (nm) 1,50
2,25
3,00
3,75
4,50
5,25
6,00
10 9
CHI-ECH-TRIS CHI-ECH-TRIS-Cu
8 7
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C2/C1
6 5 4 3 2
0 10
20
30
40
50
70
80
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Take-off Angle (º)
60
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1
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Figure 4. Evolution of the atomic concentration C2/C1 ratio as a function of the takeoff angle for samples CHI-ECH-TRIS and CHI-ECH-TRIS-Cu.
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N 1s CHI-ECH-ASP
404
403
402
401
400
399
398
397
415
B.E. (eV)
410
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405
I(u.a)
I (a.u)
º º º º º 0 5 5 6 0 7 5 3 4 1 406
15º 30º 45º 60º 75º
N 1s CHI-ECH-ASP-Cu
405
400
395
390
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BE (eV)
5-a
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5-b
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Figure 5. ARXPS spectra for the CHI-ECH-ASP crosslinked chitosan membrane before (a) and after (b) Cu adsorption.
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Cu 2p CHI-ECH-Cu
Cu 2p CHI-ECH-IDA-Cu
60º
I (u.a)
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60º
I (u.a.)
I (u.a)
75º
45º
45º
30º
30º
15º
971 961 951 941 931 961
951
941
971
961
951
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971
Binding Energy (eV)
931
941
931
Binding Energy (eV)
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Binding Energy (eV)
Cu 2p CHI-ECH-TRIS-Cu
Cu 2p CHI-BIS-TRIS-Cu
75º
75º
I (u.a.)
60º
60º
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45º
45º
30º
30º
970
960
950
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15º
940
930
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Binding Energy (eV.)
Figure 6. ARXPS Cu
970
960
950
15º 940
930
Binding Energy (eV.)
2p spectra for CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-ASPCu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes.
ACCEPTED MANUSCRIPT Figure 7. X-ray diffraction pattern for modified chitosan membranes (CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-ASP-Cu, CHI-ECH-TREN-Cu and CHI-ECH-TREN-BIS-Cu) and pure chitosan (CHI)..
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CHI-ECH-Cu CHI-ECH- IDA-Cu CHI-ECH-ASP-Cu CHI-ECH- TREN-Cu CHI-ECH-TREN-BIS-Cu
1800
EP
1600
CHI
1200 1000
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Intensity (c/s)
1400
800 600 400 200
0 0
20
40
2Theta (degrees)
60
80
3000
2500
796 716
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946 895
1110
1463 1374 1325 1266
1650
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SC
raman intensity / a. u.
2940 2882
2754
2000
CHI-ECH-Cu CHI-ECH-IDA-Cu CHI-ECH-ASP-Cu CHI-ECH-TRIS-Cu CHI-BIS-TRIS-Cu
1043
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2000
1500
1000
-1
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raman shift / cm
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Figure 8. 785 nm Raman spectra of the five membranes treated with copper nitrate treated in the 1700-700 cm-1 spectral region.
ram an shift / cm
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946 895
1110
975 897
1046
1102
1257 1266
1043
1000
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1500
1325
2000
1463
3000
1650
CHI-ECH-ASP -Cu
2000
CHI-ECH-ASP
2500
1379 1374
1457
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-1
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Figure 9. 785 nm Raman spectra of CHI-ECH-ASP and CHI-ECH-ASP-Cu in the 1700-700 cm-1 spectral region.
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30000
28000
26000
24000
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SC
Abs(a.u)
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CHI-ECH-Cu CHI-ECH-IDA-Cu CHI-ECH-ASP-Cu CHI-ECH-TRIS-Cu CHI-BIS-TRIS-Cu
22000
20000
18000
16000
14000
Wavelength (cm-1)
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Figure 10. UV-Vis-NIR spectra of crosslinked chitosan membranes.
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ACCEPTED MANUSCRIPT Highlights
1.- Chitosan membranes were crosslinked with epichlorohydrin and bisoxirano and functionalized with chelating agents.
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2.- The chelating agent were iminodiacetic acid, aspartic acid and tris-(2-amino-ethyl) polyamine. 3.- These functionalized membranes were used for copper adsorption and studied by angle resolved XPS, Raman, TG-DCS, FT-IR and XRD.
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4.- Spectroscopic data confirmed that copper is linked to the modified chitosan membranes by the amino groups.
S0254-0584(16)30752-0
DOI:
10.1016/j.matchemphys.2016.10.018
Reference:
MAC 19226
To appear in:
Materials Chemistry and Physics
Received Date: 8 August 2015 Revised Date:
4 October 2016
Accepted Date: 15 October 2016
Please cite this article as: B. Jurado-López, R.S. Vieira, R.B. Rabelo, M.M. Beppu, J. Casado, E. Rodríguez-Castellón, Formation of complexes between functionalized chitosan membranes and copper: A study by angle resolved XPS, Materials Chemistry and Physics (2016), doi: 10.1016/ j.matchemphys.2016.10.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Formation of complexes between functionalized chitosan membranes and copper: a study by angle resolved XPS
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Belén Jurado-Lópeza, Rodrigo Silveira Vieirab, Rodrigo Balloni Rabeloc, Marisa Masumi Beppuc, Juan Casadod, Enrique Rodríguez-Castellóna* a
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Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain b Chemical Engineering Department, Universidade Federal do Ceará, UFC, 60455-760 Fortaleza, CE, Brazil c School of Chemical Engineering, University of Campinas, UNICAMP, P.O. Box 6066, 13081-970 Campinas, SP, Brazil d Departamento de Química-Física, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
Abstract
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Chitosan is a biopolymer with potential applications in various fields. Recently, it has been used for heavy metals removal like copper, due to the presence of amino and hydroxyl groups in its structure. Chitosan membranes were crosslinked with epichlorohydrin and bisoxirano and functionalized with chelating agents, such as, iminodiacetic acid, aspartic acid and tris-(2-amino-ethyl) polyamine. These membranes were used for copper adsorption and the formed complexes were characterized. Thermal and crystalline properties of chitosan membranes were studied by TG-DCS and X-ray diffraction. Raman, XPS and FT-IR data confirmed that copper is linked to the modified chitosan membranes by the amino groups. The oxidation state of copper-chitosan membranes were also studied by angle resolved XPS, and by UV-Vis diffuse reflectance spectroscopy.
Keywords: Biomaterials; polymers; X-ray photo-emission spectroscopy (XPS); Raman spectroscopy and scattering; thermogravimetric analysis (TGA); chemisorption
Corresponding author: Enrique Rodríguez Castellón, Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain. E-mail address: [email protected]
1
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1. Introduction The mitigation of heavy metal pollution is an important issue in ensuring human and environmental health. Heavy metals are natural components of the earth's crust and they cannot be easily degraded. In very low concentrations, some heavy metals are
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essential to maintain the metabolism of the human body. However, at medium and high concentrations they can lead to poisoning and are harmful because they tend to bioaccumulation.
To minimize the heavy metal amount in wastewaters, a previous treatment is
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necessary, in order to minimize the impact in plants and humans [1]. In recent years, many studies have been focused into using low-cost adsorbent materials to purify water
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contaminated with metals [2,3]. This technology is known as an economic and ecofriendly option.
Copper can be found in many wastewater sources including, electroplating, circuit
manufacturing, wire drawing and
polishing, paint manufacturing, wood
preservatives, and protection of ship hulls. Typical range concentrations are between
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thousands of ppm from plating waste and one ppm from cleaning processes [4-6]. The use of chitosan in heavy metal adsorption is supported by numerous studies [7-9], due to its capacity for metal ion removal because of its chelating ability [10].
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Several researchers have investigated chitosan as an adsorbent for toxic metals such as chromium, nickel, zinc, mercury, lead, cadmium, copper, and uranium [8,9].
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The chelating properties of chitosan are mainly attributed to the availability of
amino
(-NH2) and hydroxyl (-OH) groups that can coordinate transition metal ions
[11]. Chitosan surface can be modified by physical or chemical treatments, which are highly influenced by its deacetylation degree and molecular weight [12]. This material can be modified by functionalization, crosslinking, grafting, in order to enhance its reactivity for defined species or to modify its solubility in acidic medium, which is an important characteristic for adsorbents [13-17]. Chitosan is an excellent adsorbent for metal ions, which adsorption capacity is higher than those observed in many commercial chelating resins [18-20].
2
ACCEPTED MANUSCRIPT In this study, chitosan membranes were crosslinked and functionalized with poly-chelating agents aiming a better copper ion complexation (Figure 1). Figure 2 shows the different crosslinked chitosan membranes used in this study: CHI-ECH (crosslinked with epichlorohydrin), CHI-ECH-IDA membrane (crosslinked with epichlorohydrin and functionalized with iminodiacetic acid) CHI-ECH-ASP membrane
TRIS
(crosslinked
with
epichlorohydrin
and
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(crosslinked with epichlorohydrin and functionalized with aspartic acid), CHI-ECHfunctionalized
with
tris((2-
aminoethyl)amine) and CHI-BIS-TRIS (crosslinked with bisoxirane and functionalized with tris((2-aminoethyl)amine). After production, these modified membranes were
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complexed with copper ions.
The main objective of this study was the characterization of five crosslinked
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chitosan membranes that had been functionalized with different chelating agents and investigate the oxidation state of copper after metal ion adsorption into the functionalized membranes. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal stability and characteristics of the thermal decomposition of modified chitosan membranes. X-ray diffraction patterns were measured in order to evaluate the crystalline/amorphous character of the
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chitosan membranes. Angle resolved XPS (ARXPS), Raman and infrared spectroscopy were used to characterize the surface functional groups of crosslinked chitosan membranes after copper adsorption. The possible reduction or oxidation of Cu(II) after
spectroscopy.
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adsorption process was also studied by XPS and UV-Vis diffuse reflectance
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2. - Experimental. 2.1. Materials
Chitosan membranes were prepared using chitosan with molecular weight of
9.90 × 105 g/mol (measured using gel permeation chromatography) was purchased from Sigma (USA), with a minimum deacetylation degree of 85% and the thickness of the membrane was 95 ± 15 µm, measured using a digital pachymeter. Iminodiacetic acid (IDA), L-aspartic acid (Asp), bromoacetic acid and tris(2-aminoethyl)amine (TREN) were supplied by Sigma–Aldrich (USA). Epichlorohydrin and acetic acid were provided
3
ACCEPTED MANUSCRIPT by Merck (Germany). All solutions were prepared with deionized water (18MΩ cm) obtained from a Milli Q (Millipore) purification system. 2.1.1 Preparation of chitosan membranes Chitosan membranes were prepared according to the methodology described by
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Beppu et al. [12]. Chitosan was dissolved in acetic acid (2.5% (w/w)) and the resulting solution was casted into plastic Petri dishes and left to dry at 60 ºC during 24 hours, to evaporate all the solvent, obtaining the chitosan membranes. After this time, the membranes were immersed for 24 hours in a NaOH solution (1 mol L-1) to neutralize
membranes were rinsed with water at 4 ºC.
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the amino groups of chitosan that had been protonated into acid solution. The
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2.2. Crosslinking and functionalization of chitosan membranes
2.2.1 -CHI-ECH-IDA. Crosslinked with epichlorohydrin and functionalized with iminodiacetic acid
A NaOH solution (4 g of NaOH were dissolved into 30 mL of ultrapure water)
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was mixed with 2 g of iminodiacetic acid and 1.95 mL of epichlorohydrin. This mixture was stirred for 4 hours at 60 ºC. After this period, this solution was cooled to 0 ºC, and 8 g of NaOH were added. The obtained solution was called as “A solution” [20].
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A small portion of 7.0 g of chitosan membrane, previously obtained, was heterogeneously crosslinked in the “A solution” during 16 hours at 65 ºC. Finally the
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crosslinked chitosan membrane was rinsed with ultrapure water [21]. 2.2.2 - CHI-ECH-ASP: Crosslinked with epichlorohydrin and functionalized with aspartic acid.
The crosslinked chitosan membrane was functionalized with aspartic acid
following the procedure described by Chaga et al. [22]. Firstly, a chitosan membrane was heterogeneously crosslinked with epichlorohydrin: 7.0 g of chitosan membrane were soaked during 15 minutes at room temperature with 46 mL of NaOH solution (2.0 mol L-1), 4.6 mL of epichlorohydrin and 0.17 g of NaBH4. After this time, a new solution (prepared with 46 mL of NaOH and 23 mL of epichlorohydrin) was dropped on 4
ACCEPTED MANUSCRIPT the previous solution and stirred for 16 hours at room temperature. After this period, the CHI-ECH was rinsed with ultrapure water, followed with Na2CO3 2 mol L-1. An aspartic acid solution (8 g of aspartic acid was dissolved into 50 mL of Na2CO3 1 mol L-1) was prepared (pH 11.5). The CHI-ECH membrane was immersed (7.0 g) in this solution for 24 hours, obtaining CHI-ECH-ASP. Finally to remove the unreacted
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aspartic acid residues the film was rinsed with water. 2.2.3. - CHI-ECH-TRIS. Crosslinked with epichlorohydrin and functionalized with tris(2-aminoethyl)amine. method
followed
to
functionalize
the
CHI-ECH,
using
tris(2-
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The
aminoethyl)amine as chelating agent, was described by Boden et al. [23] 5 mL of TRIS
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(96% w/v) solution were dissolved into 25 mL of ultrapure water. 7.0 g of CHI-ECH membrane, as obtained in 2.1.2.2 item, were immersed into this solution during 48 hours under continuous stirring. Finally it was rinsed with ultrapure water in order to remove the unreacted functionalization agent.
2.2.4.-CHI-BIS-TRIS: Crosslinked with bisoxirano (BIS) and functionalized with tris(2-
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aminoethyl)amine.
The crosslinking process used to obtain the CHI-BIS-TRIS sample was the method proposed by Boden et al. [23]. Chitosan membrane (7.0 g) was immersed into 20 mL of NaOH solution (0.6 mol L-1) and 20 mL of 1,4-butenodiol diglycidyl ether
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that contained 40 mg of NaBH4. The mixture was kept at room temperature under continuous stirring during 48 hours. After this time, in order to remove the unreacted
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bisoxirano, the membrane was rinsed with ultrapure water. 5 mL of TRIS solution (96% w/v) were added to 25 mL of ultrapure water. This solution was added to 7.0 g of chitosan membrane that previously had been crosslinked with bisoxirano. To complete the functionalization, the suspension was kept at room temperature, during 48 hours, under continuous stirring. After this time, the suspension was rinsed with ultrapure water to eliminate the residues of the functionalization agent. 2.3. Adsorption of copper on functionalized crosslinked chitosan membranes. After the crosslinked chitosan membranes were functionalized, 0.30 g of each membrane were immersed into 25 mL of Cu(II) solution (500 mg L-1 as sulfate), with 5
ACCEPTED MANUSCRIPT continuous stirring at 20 ºC with a pH=5.0 during 60 hours. The membranes were washed and then dried to follow the characterization experiments. 2.4. Membranes characterization XPS analysis: X-ray photoelectron spectroscopy (XPS) was used to surface chemical
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characterization of functionalized crosslinked chitosan membranes before and after Cu incorporation. A Physical Electronics spectrometer (PHI 5700) was used, with an X-ray Mg Kα radiation (300W, 15 kV, 1253.6 eV) as the excitation source. A concentric
hemispherical analyser operated in the constant pass energy mode at 29.35 eV, using a
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sample of 720 µm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV FWHM at a binding energy (BE) of 84.0 eV. The spectrometer energy scale was calibrated using Cu 2p3/2, Ag 3d5/2, and Au 4f7/2 photoelectron lines at
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932.7, 368.3, and 84.0 eV, respectively.
The residual pressure inside the analysis chamber was maintained at 5×10−6 Pa or lower during the measurement. The XPS signals were decomposed into different contributions using a software package (PC-Access ESCA-V6.0F). Each spectral region was scanned several sweeps up to a good signal to noise ratio.
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A small portion of each membrane was placed in the specimen chamber for testing. The spectra obtained were always fitted using Gauss–Lorentz curves for each peak to determine the binding energy of the elements presents at the samples. The calibration of the binding energy of the spectra was performed with the C 1s peak of the
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aliphatic and adventitious carbons, which is at 284.8 eV. To determinate more accurately the binding energy (BE) of the different element
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core levels, the first spectra of each sample was obtained with a short acquisition time of 10 min to examine C 1s, Cu 2p and Cu LMM XPS regions in order to avoid, as much as possible the photo-reduction of Cu2+ species [24,25]. ARXPS: To obtain additional information, several spectra of each membrane
sample at different take-off angles (α) [26] were recorded to study in-depth the membranes. This enables us to analyse different depths depending the elements probed by varying the analysis angle between 15 and 75º [27]. In these spectra, the chitosan membranes were irradiated for a maximum time of 20 min, and the take-off angles were 15, 30, 45, 60 and 75o. The depth probed depends on the kinetic energy (KE) of the 6
ACCEPTED MANUSCRIPT emerging electrons. This kinetic energy is specific for each element and can be studied at the universal curve of electron mean escape depth (or inelastic mean free path, IMFP). In this study a X-ray source of Mg Kα was used (1253.6 eV) and the KE of C 1s is 968.8 eV, so that, the IMFP for C1s is from about 1.5 nm (when a take angle of 15º was used) to 6 nm (with a take-off angle of 75º). This fact is due to take-off angle of 15º
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is around one quarter of 75º (λ = (KE)3/4). When the N 1s core level signal is analysed, the studied range of depth is from 1.5 to 6 nm, similar to that of C 1s because the KE of N 1s is around 854.0 eV. However, as the KE of Cu 2p is around 320 eV, the depth studied is from 0.9 to 3.5 nm. This means that these different depths must be considered
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in ARXPS analysis.
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Thermal analysis: The thermal degradation behavior of functionalized crosslinked chitosan membranes was studied by means of dynamic thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A thermal analyzer METLER TOLEDO model TGA/DSC 1 was used. This analyzer is able to carry out simultaneously TGA and DSC measurements using approximately 10 µg of sample. The balance was kept in a thermostatic water bath at 22 °C and protected by a continuous
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flow of 20 mL min-1 of N2. The thermobalance has coupled a quadrupole mass spectrometer model THERMOSTAR GSD320 of Pfizer. The experiments were carried out in alumina crucibles of 70 µl in the temperature range 30-1000 ºC, with an air flow
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of 50 mL min-1 and a heating rate of 10 °C min-1. XRD: The XRD technique was used to compare the crystalline structure of
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chitosan membranes and to observe the modifications on the membrane crystallinity induced by the different crosslinking agents. X-ray diffraction (XRD) patterns were collected on a PANanalytical X’Pert Pro automated diffractometer. XRD patterns were recorded in Bragg-Brentano reflection configuration by using a Ge(111) primary monochromator (Cu Kα1) and the X’Celerator detector with a step size of 0.0167º (2θ). The patterns were recorded between 2 and 80º in 2 θ. The total measurement time was about 30 min. Raman analysis: Raman spectra with the excitation laser at 785 nm were collected by using the 1×1 camera of a Bruker Senterra Raman microscope by averaging spectra during 50 minutes with a resolution of 3–5 cm-1. A CCD camera 7
ACCEPTED MANUSCRIPT operating at 50 ºC was used for the Raman detection. Raman scattering radiation was collected in a back–scattering configuration. UV-Vis-NIR Diffuse reflectance spectroscopy: The diffuse reflectance spectra were recorded, at room temperature, in the ultraviolet-visible-near infrared region (250850 nm) with a Shimadzu spectrophotometer model MPC-3100. The spectra were
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obtained using the functionalized crosslinked chitosan membranes (before and after copper adsorption) and using barium sulfate as a white reference.
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3. Results and discussion.
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3.1. XPS
XPS survey spectra of all functionalized crosslinked chitosan membranes after copper adsorption detected the presence of C, O, N, Cu and Si. The presence of Si in some membranes is due to surface contamination. High resolution spectra were recorded at a given take-off angle of 45º. These spectra were recorded using a short irradiation time
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(10 min) to avoid as much as possible the photo-reduction of Cu(II) [24,25]. Table 1 summarizes the binding energies that appear in the XPS spectra and their assignation after an irradiation time of 10 min for all the samples studied. The range at which appear the binding energies for the main functional groups, are similar in all
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membranes, independent of crosslinking or the functionalization agent used.
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3.2 ARXPS Analysis.
ARXPS permits the study of the chemical surface composition and the chemical
state by varying the depth with a minimum damage by using different take-off angles, registering the principal elements of each membrane and its relative atomic composition. Using different take-off angles, the C 1s signals regarding coppercrosslinked chitosan membranes could be decomposed in three contributions as shown in Figure 3. The contribution at low binding energy (C1) was mainly assigned to adventitious carbon or C-C chemical bonds. The contribution C2 was assigned to C-O, C-N or C-O-C, and the contribution at higher binding energy (C3) was assigned to OC=O and O-C-O groups. 8
ACCEPTED MANUSCRIPT Table 2 summarizes the identification of the contributions observed on the resolved spectra of the C 1s signal for the different crosslinked chitosan membranes studied and the binding energy at which are located the contributions. The C 1s contribution for the CHI-ECH-Cu, CHI-ECH-IDA-Cu and CHI-ECHASP-Cu samples appears at 284.8+0.1eV and is assigned to adventitious carbon and C-
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C moieties. This contribution exhibited a decrease of its relative intensity at high takeoff angles, where the depth of the studied layer is higher, and therefore, the contribution due to adventitious contamination is less relevant. The relative atomic concentration of the C2 contribution (which appears at a BE of 286.4+0.1 eV for the CHI-ECH-IDA-Cu)
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increased when the take-off angle was higher. This can be explained because this contribution is due to C-N and C-O bonds, and, as expected, the relative atomic
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concentration of N increases with increasing depth of the layer studied. C3 contribution, which is located at 288.1+0.1 eV (for CHI-ECH-ASP-Cu membrane) and belonging to O-C=O and O-C-O bonds, remains constant at different take-off angles. For CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu, the behavior patterns of contributions C1 and C2, are not the same as in the other crosslinked membranes because of the use of tris((2-aminoethyl)amine) as chelating agent. The relative atomic
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concentration of the contribution C3 can be considered as constant with the depth of the studied layer in these membranes. However, the relative atomic concentration corresponding to the C1 contribution increases with the increase of the take-off angle.
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Conversely, the relative atomic concentration of the contribution of C2 decreases when the studied take-off angle increases and hence the depth of the layer studied. Figure 4 shows the evolution of the atomic concentration C2/C1 ratio as a function of the take-
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off angle and the depth in nm for the CHI-ECH-TRIS sample. There is a different distribution on depth. For samples before copper adsorption, at the external surface and at low take-off angle, the C2/C1 value is high, indicating a higher concentration of polar groups. After Cu incorporation the C2/C1 ratio presented a low constant value at different take-off angles. This fact seems to indicate a very homogeneous distribution on depth of the copper complex in spite of different depth probed for Cu. When the relative atomic concentration of O is considered, the observed values depend on the relative atomic concentration of total C (that is, the addition of C1, C2
9
ACCEPTED MANUSCRIPT and C3 contributions), and these values are lower when the relative atomic concentration values of C are higher. Figure 5 shows the N 1s signals for the CHI-ECH-ASP sample before and after the copper adsorption. Before the copper adsorption (Fig 5-a), the N 1s spectrum shows two contributions. One is centered at 399.8 + 0.1eV, and it is attributed to the N atom in
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the –NH2 and/or the –NH- groups on the surfaces of the chitosan [24]. The other contribution is located at 401.7+0.2 eV and belongs to the protonated species (-NH3+). After the adsorption of Cu, the N 1s spectrum showed a symmetric peak at a BE
SC
of 399.0 eV. This means that Cu is fixed to the crosslinked chitosan membrane through nitrogen atom, with the formation of the complex R-NH-Cu. In this case the protonated
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species has disappeared and the BE for N 1s signal has been shifted [28].
Table 3 lists the evolution of the N 1s and Cu 2p3/2 signals at the highest and lowest take-off angles. In all cases, an increase in the relative atomic concentration of N is observed when the take-off angle changes from 15 to 75 º. This increase is more evident for CHI-ECH-Cu, CHI-ECH-IDA-Cu and CHI-ECH-ASP-Cu samples. This N
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1s signal appears at 399.3-399.9 eV range and it is assigned to the amino groups. The higher relative atomic concentration value for high take-off angle is due to the absence of surface contamination in depth. A similar behavior was observed in the case of Cu.
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The observed increase in the relative atomic concentration of Cu is also more evident for CHI-ECH-Cu, CHI-ECH-IDA-Cu and CHI-ECH-ASP-Cu samples. This increase was not significant for CHI-ECH-TRIS-Cu as also observed in the case of N. In some
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cases the relative observed increases in percentage are not similar due to the different depth probed for N 1s and Cu 2p at 75 º (6.0 and 3.5 nm, respectively). In the case of samples CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu samples the
presence of groups with long alkyl chain, can also photoemit as adventitious carbon, causing a small change in the relative atomic concentration.
Figure 6 shows the Cu 2p3/2 signals for all the samples. It was observed an asymmetric main peak that can be decomposed in two contributions that appear at about
10
ACCEPTED MANUSCRIPT 933.0 eV for Cu+ and at 935-936 eV for Cu2+ [9]. The ARXPS study of the Cu 2p signal was done with 20 min of irradiation time. The accumulation of the irradiations provokes that all the Cu 2p spectra showed copper reduced species. There
are
significant
differences
depending
of
the
crosslinking
or
functionalization agent used. However, the irradiation time used produced a
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modification of the Cu 2p spectra and evident conclusions concerning the copper chemical state cannot be established. As expected, the irradiation not only produces a shift of the main Cu 2p3/2 peak at lower binding energies, but also the shake-up satellite
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practically disappears due to the reduction of Cu2+.
In the case of membrane CHI-ECH-TRIS-Cu, the asymmetric Cu 2p3/2 signal can be decomposed in two contributions at 933.3+0.2 eV and 935.8+03 eV that are assigned
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to Cu+ and Cu2+ species, respectively. When the take-off angle increases, the contribution due to the presence of Cu2+ species decreases. This behavior is completely different for the CHI-BIS-TRIS-Cu membrane, where the contribution of the Cu+ species decreases with increasing the take-off angle studied. Probably, ECH forms a more stable complex with Cu2+.
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Table 4 shows a summary of the peaks located in the region Cu 2p3/2, for the CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes obtained at different take-off angles, and the area under each peak.
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The contribution of the shake-up satellite mainly due to the presence of Cu2+ has been considered for the calculation of the relative percentages of Cu2+ and Cu+.
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3.3 Thermal analysis
The thermogravimetric analysis is very helpful to understand the stability of the
copper complexes on the membranes and to obtain information about the moisture content and the percentages of inorganic and organic components. The TG curves showed three weight losses. Table 5 shows the temperatures ranges for each chitosan membrane, describing the dehydration, decomposition and combustion of the studied materials. It was observed that each thermal process depends of chitosan modification. There was first an endothermic effect, which was attributed to water evaporation, that would be associated to amino and hydroxyl groups of chitosan 11
ACCEPTED MANUSCRIPT chain [12]. The next effect was exothermic, attributed to chitosan membrane decomposition and also elimination of volatile products. The last effect was also exothermic and it could be related to the complexed chitosan combustion. The thermal stability followed this order: CHI-ECH-ASP-Cu > CHI-ECH-TRISCu > CHI-ECH-Cu > CHI-ECH-IDA-Cu > CHI-BIS-TRIS-Cu, showing that the
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functionalization with higher organic molecules increased the thermal stability.
The DSC curves (results not showed here) show changes in the intensity of exothermic/endothermic peaks and their positions are influenced by the chemical
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modification. The DSC curves show a broad endothermic peak around 100 ºC related to the water evaporation. The exothermic peak, which appears in the temperature range between about 250 ºC and 380 ºC, is attributed to the decomposition of the crosslinked
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chitosan membrane. And the last exothermic peak is due to the combustion of the sample. Once formed the carbonized remainder, the copper could be reduced to Cu+, or even to Cu0. This agrees with thermogravimetric studies for chlorine acetate and copper
3.4. XRD analysis
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acetate [29,30].
The XRD powder patterns of the each chitosan membrane are shown in Fig. 7. The XRD diagrams reveal poor crystallinity for all the crosslinked chitosan membranes
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studied. One reflection line at 23.9º was observed in all cases, and assigned to the presence of Cu2O [31,32]. The XRD of CHI was also included in Figure 7 as reference
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and it does not show the reflection line due to the presence of Cu2O 3.5. Vibrational Infrared and Raman Data The Raman spectra of modified chitosan complexed with Cu(II) are shown in
Figure 8. In this case the band at 1043 cm-1 is the strongest and the spectral changes are slightly, meaning that metal coordination took place through the amino group, which acted independently the others parts of the polymer. Comparing the samples crosslinked with ECH and BIS the difference is due the length of aliphatic spacer (2 methylenes in CHI-ECH-TRIS-Cu and 4 in CHI-BIS-TRIS-Cu). There is also a difference for the chitosan crosslinked with ECH, depending of functionalization agent. 12
ACCEPTED MANUSCRIPT Figure 9 displays the Raman spectra for CHI-ECH-ASP before and after copper adsorption. The spectrum of CHI-ECH-ASP is characterized by a background likely due to fluorescence from the sample which fully disappears after treatment with copper salt highlighting the drastic change on the optical properties of the membranes (fully transparent in the sample free of copper and blue color with divalent copper).
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The spectrum of CHI-ECH-ASP is dominated by one main Raman dispersion at 975 cm-1 accompanied by rather undefined weak bands in the high wavenumber region. In the sample with copper this band significantly shifts by 68 cm-1 up to 1043 cm-1 and the band shape slightly modifies its aspect becoming a sharper band. In addition, the
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bands in the region 1500-1100 cm-1 also change but these modifications are much less pronounced. We tentatively assign the Raman band at 975 cm-1 in CHI-ECH-ASP as
bond from the amine groups [33].
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emerging from a vibrational mode mainly involving the stretching of the C-N single
Complexation with copper provokes the main changes of this band indicating that coordination of the divalent cation should proceed through the lone electron pair of the nitrogen. This is in agreement with: i) the large change in the wavenumber upshift might reveal that coordination is reinforcing somehow this C-N bond; and ii) this
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Raman band gets shaper upon coordination in accordance with the specific effect of the metallic divalent cation [34].
Interestingly, in the spectrum with copper, a clear weak signal is detected
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around 1650-1630 cm-1 due to the stretch of the carbonyl group in the sample. This band correlates with that at 1630 cm-1 in the infrared spectrum of the same
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copper sample which is unaltered in the infrared spectrum of the untreated CHI-ECHASP highlighting again that the chemical effect of copper does not involve the carboxylic groups. Other secondary changes can be mentioned between the spectra of the natural and copper treated chitosan but with much less structural implications.
3.6. DR/UV-Vis-NIR Analysis Electronic spectra of the different samples studied were recorded in solid state (diffuse reflectance) in the UV-Vis-NIR region to obtain information concerning the 13
ACCEPTED MANUSCRIPT coordination of the copper complexes and the oxidation state of copper for the studied membranes. Figure 10 shows the DR/UV-Vis-NIR spectra for all chitosan membranes. The observed color is due to the presence of Cu2+complexes, mainly due to various electronic transitions between the ground state and excited states (due to unfolding of the fundamental term of the free ion 2D), the Cu2+ species has a d9
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configuration, which provides a flexible stereochemistry, which involves highly distorted geometries, with significant differences in bond lengths and angles with
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respect to the regular geometry [35].
For each sample, it can observe a band which is located in the region 14970-
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14045 cm-1. This band may be assigned to1A1g -->1A2g (dx2-dy2 ->dxy) transitions. According to the literature [36], the geometry of copper complex inside the crosslinked chitosan membrane, is due to the octahedral geometry of the metal ion of copper, which has an oxidation state of Cu2+. The octahedral geometry is distorted by the Jahn Teller effect.
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4. Conclusions
The study of crosslinked chitosan membranes using ARXPS technique confirms the previously proposed structure in the case of membranes CHI-ECH-Cu, CHI-ECH-
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IDA-Cu and CHI-ECH-ASP-Cu complexed, thus confirming the chemisorption of this metal. In the case of CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes, the tris(2-
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aminoetil)amine chelating agent amine is the most voluminous and the study of the surface composition of these membranes using this technique does not give rise to clear conclusions. The chemical state of copper was followed by using XPS, where copper is mainly present as Cu2+ as confirmed by UV-Vis diffuse reflectance. The study of the interaction of the metal with the biopolymer showed that complexation increased the crystallinity degree of the materials. The infrared and Raman spectra obtained for samples with and without copper suggested a mechanism of adsorption through covalent interactions more strongly to the amino groups. Acknowledgements
14
ACCEPTED MANUSCRIPT The authors acknowledge financial support from Projects CTQ2015-68951-C3-3R and CTQ2012-30703 of Ministerio de Economía y Competitividad (Spain) and FEDER
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funds and Project of Excellence P12-RNM-1565 (Junta de Andalucia, Spain).
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[25] E. Moretti, E, M. Lenarda. L. Storaro, A. Talon, T. Montanari, G. Busca, E. Rodríguez-Castellon, A. Jiménez-López, M. Turco, G. Bagnasco, R. Frattini. Appl. Catal. A: Gen. 335 (2008) 46-55. [26] A.P.Pijpers, R.J Meier. Chem. Soc. Rev. 28, 1999. [27] G.P. López, D.G. Castner, B.D.Ratner, Surf. Interface Anal. 17 (1991) 267. [28] W. Jiang, S. Chen, B. Pan, Q. Zhang, L. Teng, Y. Chen, L. Liu. J. Hazard. Mater. 276 (2014) 295-301.
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ACCEPTED MANUSCRIPT [29] F.Walts, J. Walstenholme. An introduction to surface analysis by XPS and AES Ed. Wiley. England. (2003) 93-111. [30] H. Peng-Zhi, L. Si-Dong, O. Chun-Yan, L. Cheng-Peng, Y. Lei, Z. Chao-Hua. J. Appl. Polym. Science. 105 (2007) 547-551.
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[31] R. Guan, H. Hashimoto, K.H. Kuo. Acta Crystallogr. B, 40 (1984) 560-566. [32] M. Salavati-Niasari, F. Davar, N. Mir. Polyhedron. 27 (2008) 3514–3518.
[33] A.C. Oyrton, Jr. Monteiro. Claudio Airoldi. Int. J. of Biol. Macromol. 26 (1999)
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119-128.
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[34] C. Jin-Yi, Z. Pei-Jiang, L. Jia-Lin, L. Su-Qin. Carbohyd. Polym. 67 (2007) 623629.
[35] S. Balboa, R. Carballo, A. Castiñeiras, J.M. González-Pérez, J. Niclós-Gutiérrez. Polyhedron. Volume 27, Issue 13, (2008) 2921-2930.
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New York, 1984.
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[36] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed. Elservier Science,
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CAPTIONS
Table captions
Table 1: Assignments of main spectral bands based on their binding energies (BE) in eV for the crosslinked chitosan membranes complexed with Cu(II) for a take-off angle of 45º and after an irradiation time of 10 min. Table 2: Assignments of main spectral bands of C 1 speaks based on their binding energies (BE) for the crosslinked chitosan membranes complexed with Cu(II), using as take-off angles 15º and 75º. 17
ACCEPTED MANUSCRIPT Table 3: N 1s and Cu 2p3/2 binding energies (in eV) and relative atomic concentrations at take-off angles of 15 and 75º, after the adsorption. Table 4: Evolution of the Cu2p3/2 signal as a function of the take-off angle in ARXPS experiments for samples CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu.
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Table 5: TG-DSC data for the studied membranes.
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Figure captions
Figure 1. Schematic representation of the adsorption mechanism.
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Figure 2. Proposed structures for crosslinked and functionalized chitosan membranes. Figure 3. ARXPS spectra for C 1s for samples CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHIECH-ASP-Cu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu.
Figure 4. Evolution of the atomic concentration C2/C1 ratio as a function of the take-
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off angle for samples CHI-ECH-TRIS and CHI-ECH-TRIS-Cu. Figure 5. ARXPS spectra for the CHI-ECH-ASP crosslinked chitosan membrane before
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and after Cu adsorption.
Figure 6. ARXPS Cu 2p spectra for CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-
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ASP-Cu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes. Figure 7. X-ray diffraction pattern for modified chitosan membranes (CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-ASP-Cu, CHI-ECH-TREN-Cu and CHI-ECH-TRENBIS-Cu) and pure chitosan (CHI). Figure 8. 785 nm Raman spectra of the five membranes treated with copper nitrate treated in the 1700-700 cm-1 spectral region. Figure 9. 785 nm Raman spectra of CHI-ECH-ASP and copper treated CHI-ECH-ASP in the 1700-700 cm-1 spectral region.
18
ACCEPTED MANUSCRIPT Figure 10: UV-Vis-NIR spectra of crosslinked chitosan membranes.
Samples
C 1s
C2
C3
CHI-BIS-
ECH-
ECH-
ECH-
ECH-
TRIS-Cu
Cu
IDA-Cu
ASP-Cu
BIS-Cu
284.8
284.8
284.8
284.8
284.8
(53%)
(54%)
(37%)
(33%)
(67%)
286.3
286.3
286.3
286.1
286.2
(35%)
(33%)
(48%)
(54%)
(23%)
288.0
287.9
288.3
288.0
288.1
(12%)
(13%)
(15%)
(13%)
(10%)
933.2
933.3
933.3
933.2
933.1
Si 2p
Functional Group
C-C; adventitious carbon
C-N; C-O; C-O-C
C=O; O-C-O
Cu+
(29%)
(39%)
(55%)
(10%)
(34%)
935.8
935.4
935.4
935.8
935.5
(71%)
(61%)
(45%)
(90%)
(66%)
532.6
532.6
532.5
532.8
532.5
C-O; O-H
399.8
399.9
399.7
399.4
399.6
-NH2
102.0
102.1
----
----
SiO2
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N 1s
CHI-
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O 1s
Cu2p3/2
CHI-
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Cu 2p
CHI-
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C1
CHI-
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Signal
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Table 1. Assignments of main spectral bands based on their binding energies (BE) in eV for the modified chitosan membranes complexed with Cu(II) using a take-off 45º and after an irradiation time of 10 min.
102.2
Cu2+
19
ACCEPTED MANUSCRIPT
Table 2. Assignments of main spectral bands of C 1s peaks based on their binding energies (BE) for the modified chitosan membranes complexed with Cu(II), data expressed corresponding to the limits angles 15 ° and 75 °.
C2
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CHI-BISTRIS-Cu
15º
284.8 (71%)
284.8 (61%)
284.8 (53%)
284.4 (8%)
284.7 (26%)
75º
284.7 (55%)
284.8 (53%)
284.7 (44%)
284.8 (18%)
284.9 (27%)
15º
286.4 (21%)
286.5 (28%)
286.3 (38%)
286.2 (82%)
286.0 (60%)
75º
286.4 (33%)
286.4 (35%)
286.3 (46%)
286.2 (69%)
286.7 (58%)
15º
288.2 (8%)
288.2 (11%)
288.1 (9%)
288.7 (10%)
288.0 (14%)
75º
288.1 (12%)
288.2 (12%)
288.2 (10%)
288.4 (13%)
288.2 (15%)
Asignation
C-C; adventitious C
C-N; C-O; C-OC
O-C=O ; O-C-O
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C3
CHI-ECHTRIS-Cu
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C1
Take-off CHI-ECH-Cu CHI-ECH-IDA- CHI-ECHCu ASP-Cu angle
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Band
20
ACCEPTED MANUSCRIPT
Table 3: N 1s and Cu 2p3/2 binding energies (in eV) and relative atomic concentrations at take-off angles of 15 and 75º, after the adsorption. N 1s
Cu 2p
15º take off angle
75º take off angle
CHI-ECH-Cu
399.8+0.2
4.4
6.5
CHI-ECH-IDACu
399.9+0.3
4.9
7.6
CHI-ECH-ASPCu
399.7+0.4
3.1
5.1
CHI-ECHTRIS-Cu
399.3+0.2
7.6
CHI-BIS-TRISCu
399.6+0.1
6.3
BE (eV) Cu 2p3/2
% Relative atomic concentration 15º take off angle
75º take off angle
1.14
1.84
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% Relative atomic concentration
BE (eV) N 1s
934.3+1.1
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Sample
0.81
1.31
933.1+0.3
0.82
1.24
8.2
933.3+0.2 935.8+0.3
5.93
5.96
7.9
933.1+0.1 935.4+0.04
3.16
4.05
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933.2+0.1
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Table 4. Evolution of the Cu 2p3/2 signal as a function of the take-off angle in ARXPS experiments for samples CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu. CHI-ECH-TRIS-Cu
45º
60º
Relative area (%)
Chemical species
933.5
3
933.2
41
Cu+
936.1
97
935.4
59
Cu 2+
933.3
10
933.1
66
Cu+
936.0
90
935.3
34
Cu 2+
933.2
16
933.1
935.7
84
935.4
933.2
17
935.6
83
933.2
23
935.5
77
67
Cu+
33
Cu 2+
933.1
70
Cu+
935.5
30
Cu 2+
933.1
67
Cu+
935.4
33
Cu 2+
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75º
Cu 2p3/2
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30º
Relative area (%)
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15º
Cu 2p3/2
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Take-off angle
CHI-BIS-TRIS-Cu
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Table 5. TG-DSC data for the studied membranes. Loss of water ∆t (ºC)
First decomposition
%weightloss
∆t (ºC)
% weightloss
∆t (ºC)
% weightloss
(375-590)
39.35
CHI-ECH-Cu
(50-175)
9.77
(175-375)
45.52
CHI-ECH-
(43-165)
8.44
(165-370)
44.58
(40-190)
8.78
(190-405)
43.97
(50-185)
6.16
(185-495)
(43-163)
5.34
IDA-Cu
CHI-ECH-
CHI-BIS-
(163-510)
33.70
(405-630)
40.72
47.43
(495-630)
43.2
(510-662)
39.39
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TRIS-Cu
44.49
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TRIS-Cu
(370-580)
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ASP-Cu
CHI-ECH-
Second decomposition
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Sample
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ACCEPTED MANUSCRIPT
Chitosan membrane
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Chelating agent
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Crosslinking agent
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Figure 1. Schematic representation of the adsorption mechanism.
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Figure 2. Proposed structures for crosslinked and functionalized chitosan membranes.
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Figure 3. ARXPS spectra for C 1s for samples CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHIECH-ASP-Cu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu.
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CHI-ECH-TRIS-Cu
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CHI-ECH-ASP-Cu
CHI-BIS-TRIS-Cu
ACCEPTED MANUSCRIPT
depth (nm) 1,50
2,25
3,00
3,75
4,50
5,25
6,00
10 9
CHI-ECH-TRIS CHI-ECH-TRIS-Cu
8 7
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C2/C1
6 5 4 3 2
0 10
20
30
40
50
70
80
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Take-off Angle (º)
60
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1
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Figure 4. Evolution of the atomic concentration C2/C1 ratio as a function of the takeoff angle for samples CHI-ECH-TRIS and CHI-ECH-TRIS-Cu.
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N 1s CHI-ECH-ASP
404
403
402
401
400
399
398
397
415
B.E. (eV)
410
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405
I(u.a)
I (a.u)
º º º º º 0 5 5 6 0 7 5 3 4 1 406
15º 30º 45º 60º 75º
N 1s CHI-ECH-ASP-Cu
405
400
395
390
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BE (eV)
5-a
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5-b
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Figure 5. ARXPS spectra for the CHI-ECH-ASP crosslinked chitosan membrane before (a) and after (b) Cu adsorption.
ACCEPTED MANUSCRIPT Cu 2p CHI-ECH-ASP-Cu
Cu 2p CHI-ECH-Cu
Cu 2p CHI-ECH-IDA-Cu
60º
I (u.a)
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60º
I (u.a.)
I (u.a)
75º
45º
45º
30º
30º
15º
971 961 951 941 931 961
951
941
971
961
951
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971
Binding Energy (eV)
931
941
931
Binding Energy (eV)
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Binding Energy (eV)
Cu 2p CHI-ECH-TRIS-Cu
Cu 2p CHI-BIS-TRIS-Cu
75º
75º
I (u.a.)
60º
60º
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45º
45º
30º
30º
970
960
950
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15º
940
930
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Binding Energy (eV.)
Figure 6. ARXPS Cu
970
960
950
15º 940
930
Binding Energy (eV.)
2p spectra for CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-ASPCu, CHI-ECH-TRIS-Cu and CHI-BIS-TRIS-Cu membranes.
ACCEPTED MANUSCRIPT Figure 7. X-ray diffraction pattern for modified chitosan membranes (CHI-ECH-Cu, CHI-ECH-IDA-Cu, CHI-ECH-ASP-Cu, CHI-ECH-TREN-Cu and CHI-ECH-TREN-BIS-Cu) and pure chitosan (CHI)..
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CHI-ECH-Cu CHI-ECH- IDA-Cu CHI-ECH-ASP-Cu CHI-ECH- TREN-Cu CHI-ECH-TREN-BIS-Cu
1800
EP
1600
CHI
1200 1000
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Intensity (c/s)
1400
800 600 400 200
0 0
20
40
2Theta (degrees)
60
80
3000
2500
796 716
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946 895
1110
1463 1374 1325 1266
1650
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SC
raman intensity / a. u.
2940 2882
2754
2000
CHI-ECH-Cu CHI-ECH-IDA-Cu CHI-ECH-ASP-Cu CHI-ECH-TRIS-Cu CHI-BIS-TRIS-Cu
1043
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2000
1500
1000
-1
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raman shift / cm
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Figure 8. 785 nm Raman spectra of the five membranes treated with copper nitrate treated in the 1700-700 cm-1 spectral region.
ram an shift / cm
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946 895
1110
975 897
1046
1102
1257 1266
1043
1000
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1500
1325
2000
1463
3000
1650
CHI-ECH-ASP -Cu
2000
CHI-ECH-ASP
2500
1379 1374
1457
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-1
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Figure 9. 785 nm Raman spectra of CHI-ECH-ASP and CHI-ECH-ASP-Cu in the 1700-700 cm-1 spectral region.
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30000
28000
26000
24000
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SC
Abs(a.u)
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CHI-ECH-Cu CHI-ECH-IDA-Cu CHI-ECH-ASP-Cu CHI-ECH-TRIS-Cu CHI-BIS-TRIS-Cu
22000
20000
18000
16000
14000
Wavelength (cm-1)
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Figure 10. UV-Vis-NIR spectra of crosslinked chitosan membranes.
12000
ACCEPTED MANUSCRIPT Highlights
1.- Chitosan membranes were crosslinked with epichlorohydrin and bisoxirano and functionalized with chelating agents.
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2.- The chelating agent were iminodiacetic acid, aspartic acid and tris-(2-amino-ethyl) polyamine. 3.- These functionalized membranes were used for copper adsorption and studied by angle resolved XPS, Raman, TG-DCS, FT-IR and XRD.
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4.- Spectroscopic data confirmed that copper is linked to the modified chitosan membranes by the amino groups.