The effect of pH on the bonding of Cu2+ and chitosan-montmorillonite composite

Accepted Manuscript Title: The effect of pH on the bonding of Cu2+ and chitosan-montmorillonite composite Authors: Chao Hu, Guoyuan Li, Yiyu Wang, Fengyi Li, Guangguang Guo, Hongqing Hu PII: DOI: Reference:

S0141-8130(17)30581-0 http://dx.doi.org/doi:10.1016/j.ijbiomac.2017.05.065 BIOMAC 7554

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

15-2-2017 2-5-2017 15-5-2017

Please cite this article as: Chao Hu, Guoyuan Li, Yiyu Wang, Fengyi Li, Guangguang Guo, Hongqing Hu, The effect of pH on the bonding of Cu2+ and chitosan-montmorillonite composite, International Journal of Biological Macromoleculeshttp://dx.doi.org/10.1016/j.ijbiomac.2017.05.065 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.

The effect of pH on the bonding of Cu2+ and chitosan-montmorillonite composite Chao Hua,b, Guoyuan Lib, Yiyu Wangb, Fengyi Lia, Guangguang Guoa, Hongqing Hua* a

College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China

b

Department of Life Sciences and Technologies, Hubei Engineering University, Xiaogan 432000,

China *Corresponding author. Tel: +86 027 87280271. Fax: +86 027 87288578. E-mail address: [email protected]

Highlights: 

Bonding with amino in chitosan contribute main adsorption capacity



Amino bond with copper ion near pH 5.0



Hydroxyl was more active at pH 3 than at pH 5.

Abstract: Based on the contradict statements on the mechanisms of the bonding between metal ions and chitosan based adsorbent, this paper present studies on the Cu2+ adsorption by the chitosan-montmorillonite composite. The in situ attenuated total reflectance Fourier transform infrared spectroscopy and two dimensional correlation analysis were used to record the spectra during the adsorption and the pH perturbation, and analyze the interaction sequence of the groups related to the adsorption, respectively. The results showed that the bonding with -NH3+ might attribute the adsorption capacity greater than that with hydroxyl. The hydroxyl of chitosan (or composite) take part in the Cu2+ adsorption at pH 3.0 to 5.0 and were more active at lower pH. The amino replaced hydroxyl as the bonding sites with the change of -NH3+ to -NH2 by proton exchange when pH was near 5.0. Keywords: ATR-FTIR, 2D-COS, metal complexation, chitosan, Cu2+

1 Introduction: Both chitosan and montmorillonite are low cost, natural and ecologically safe materials. In the composite of chitosan-montmorillonite, there are abundant hydrophilic groups 1

such as amino and hydroxyl from chitosan (Crini & Badot, 2008; Ngah et al., 2011). The functional groups in chitosan contribute to the bonding of metal ions and make the composite a promising adsorbent for removal of metal in aqueous media, meanwhile, the montmorillonite improves the acid stability and mechanical properties of the chitosan. Therefore, the composite has attracted widely attention in recent decades (Fu & Wang, 2011; Futalan et al., 2011; Ngah et al., 2011; Pereira et al., 2013; Wang & Chen, 2014; Yang et al., 2016). The surface area of chitosan is about 3.7 m2/g, suggesting chemisorption rather than physisorption on the chitosan (Dalida et al., 2011; Ng et al., 2002). According to the previous reports, the mechanisms for chemisorption included the following issues: 1) the coordination-chelating by the amino or combination of hydroxyl clusters; 2) complexation in acidic medium; 3) ion exchange, such as the proton exchange on the protonated amino (Varma et al., 2004; Yan & Lin, 2015). Because the chitosan-montmorillonite composite is saturated with chitosan, the metal ions adsorption on the composite might be similar to that on pure chitosan in aqueous media. Varma et al. (2004) expressed the primary hydroxyl was not involved in chelating, but the Cu2+ was coordinated by secondary hydroxyl and amine coordinated. Many researches agreed that the amine and hydroxyl groups were the main active sites for adsorbing Cu2+ or Pb2+ (Kamari et al., 2011; Ngah & Fatinathan, 2010; Pereira et al., 2013; Swayampakula et al., 2009). However, some researches proved the amino groups played the major or unique role in the divalent metal ions adsorption (Chethan & Vishalakshi, 2013; Guibal, 2004; Kannamba et al., 2010; Yan & Bai, 2005), but others indicated that the hydroxyl acted as major binding site for Cu2+ (Huang et al., 2015; Ngah et al., 2013).The contradiction of the statements might be due to the different nature of metals and experimental conditions. Herein, it is important to differentiate the active site for the metal ions and investigate the mechanism for the modification and application of the composite. Through the in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and two-dimensional correlation analysis (2D-COS), this study presents a new perspective for the researches from which to understand the adsorption 2

mechanisms of Cu2+ on the chitosan-montmorillonite composite at different pH. The adsorption isotherm of the copper ion on the chitosan-montmorillonite composite and the pH dependency were measured. The ATR-FTIR and 2D-COS were used to study the interaction correlation of the groups and the Cu2+ at various pH, the mechanisms of the adsorption could be distinguished with the analysis.

2 Materials and methods 2.1 Materials The chitosan (80% deacetylation degree, medium molar mass, and viscosity of 200-800 cP) was purchased from Sigma-Aldrich. The natural montmorillonite was supplied by Southern Clay Products, Inc. (Gonzales, TX, USA) and saturated with sodium chloride. The mineral was washed with deionized water until no Cl- remained, and freeze-dried at -60 ºC (FD5-3T, SIM, USA). The 500 ml chitosan solution with a concentration of 1 g/L (dissolved in 10 mmol/L HCl) was added into the 500 ml 1% (w/w) sodium montmorillonite dispersion, adjusted to pH 5.0, and stirred for 12 h. The mixture dispersion was centrifuged at 9000 r/min for 30 min and the sediment was mixed with 500 ml 1 g/L pH 5.0 chitosan solution twice. The montmorillonite saturated with chitosan was centrifuged again and the sediment was freeze-dried (-60 ºC), grinded to pass through 60 mesh and stored in a desiccator for use (An & Dultz, 2007; Darder et al., 2003; Hu et al., 2016). All reagents of analytical grade were purchased from Sinopharm Chemical Co. (Shanghai, China) and the deionized (DI) water (resistance of 18.2 MΩ cm) was obtained with Aquapro water system (AWL-2001-U). All experiments were conducted at room temperature.

2.2 Methods 2.2.1 The effect of pH on the composite-Cu adsorption Two grams chitosan-montmorillonite composite was dispersed in 1L DI water and stirred constantly for use. The pH of Cu2+ solution (20 mg/L) was adjusted to 3-5.5 to avoid the precipitation of chitosan and ensure the protonation of the amino group (An 3

& Dultz, 2007), the pH of 20 mg/L Cu2+ solution (Cu(NO3)2) was adjusted to 3-5.5, respectively. 10 ml Cu(NO3)2 solution at pH 3-5.5 was added into 50 ml centrifuge tube with 1 ml dispersion of the composite, and the DI water was added to make the volume 20 ml. The tubes were shaken at 200 r/min for 24 h and centrifuged at 9000 r/min for 30 min. The samples were conducted in triple, and the concentration of Cu2+ in the supernatants were measured on atomic absorption spectrophotometer (SpectrAA 220FS, Varian, USA). 2.2.2 The isotherm adsorption of Cu2+ on the composite One milliliter dispersion of composite (pH 5.0, 2 mg/L) was added into 5 ml Cu(NO3)2 solution with Cu2+ concentration range from 2- 100 mg/L (pH 5.0) in the 50 ml tube. The following process was the same as section 2.2.1. The adsorbed mass of Cu2+ was calculated by the difference between the metal concentrations in initial and equilibrium solution. The adsorption capacities were plotted with the equilibrium concentrations, and the plots were fitted with isotherm equations as followed: 1) Langmuir equation with the linear form as: 1 𝑞𝑒

1

=𝑞

𝑚𝑙

+ 𝑏𝑞

1

(1)

𝑚𝑙 𝐶𝑒

where qml is the theoretical maximal adsorption capacity, mg/g; qe is the adsorbed amount at equilibrium, mg/g; Ce is the equilibrium concentration of metal in the supernatant, mg/L; b is a constant related to the affinity of binding sites, ml/mg. It could be used to calculate the dimensionless constant RL, the equation is expressed by: 1

𝑅𝐿 = 1+𝑏𝐶

(2)

0

where C0 is the initial concentration of metal and b is the constant in Langmuir fitting, ml/mg. RL was the separation factor. When RL = 0, 0-1, 1 or > 1, it indicates whether the isotherm is irreversible, favorable, linear and unfavorable, respectively (Gerente et al., 2007). 2) Freundlich equation with the linear form as: 1

log 𝑞𝑒 = log 𝐾𝐹 + 𝑛 log 𝐶𝑒

(3) 4

where KF is the maximum adsorbed amount of metal; 1/n presents the intensity of metal on the absorbent surface (Gerente et al., 2007). 2.2.3 The in situ ATR-FTIR analysis of Cu2+ adsorption on the composite The powder of the composite was grinded for 1 h and passed through 200 mesh sieve. 1 g of the powder was dispersed in 100 ml DI water and 0.5 ml dispersion was deposited on an ATR cell (0.3×7.2×1.0 cm3), which was composed of a ZnSe crystal surface. The dispersion in cell was subsequently dried at room temperature. The pH of Cu(NO3)2 solution (100 ml, 200 mg/L Cu) was adjusted to 5.0 and 3.0, respectively. Sodium nitrate (NaNO3) was added into the Cu(NO3)2 solution to reach a final concentration of 3.15 mmol/L Na+ in the solutions. The composite film was initially immersed with 3.15 mmol/L NaNO3 solution and the FTIR spectrum was recorded as a background. The ATR-FTIR spectroscopy was conducted in a Bruker Vertex 70 FTIR spectrometer with a deuterated triglycine sulfate (DTGS) detector. The background spectra were recorded from 4000 to 800 cm-1 with 128 scans at a resolution of 2 cm-1, and then an accumulated 1 ml Cu(NO3)2 (200 mg/L, pH 5.0) was added into the cell. For each spectrum of the samples, 256 scans were recorded and averaged. The spectra of the composite-Cu were continuously recorded 30 times, about 210 min. The software OPUS 5.0 (Bruker, Germany) was used for analyses. 2.2.4 The in situ ATR-FTIR analysis of the adsorption from pH 5.0 to 3.0 A 0.4 ml chitosan solution (1 g/L, pH 5.0) was added into the cell and the settings of the IR were the same as section 2.2.3. The background spectra were recorded and averaged with 128 scans after the adding of NaNO3 solution (3.15 mmol/L, pH 5.0) and then an accumulated 0.4 ml Cu(NO3)2 solution (200 mg/L, pH 5.0) was added. The spectra were recorded 30 times and each spectrum was averaged from 256 scans. About 200 minutes after the Cu(NO3)2 solution (pH 5.0) was added, an accumulated 1 ml Cu(NO3)2 solution (200 mg/L Cu, pH 3.0) were added into the cell. The spectra were recorded 30 times the same as section 2.2.3 during the change of the pH. 2.2.5 The in situ ATR-FTIR analysis of the adsorption from pH 3.0 to 5.0 5

A 0.4 ml chitosan solution (1 g/L, pH 3.0) was added into the cell and the settings were set up same as section 2.2.3. The background spectra was recorded and averaged with 128 scans after the addtion of NaNO3 solution (3.15 mmol/L, pH 3.0) and then an accumulated 0.4 ml Cu(NO3)2 solution (200 mg/L, pH 3.0) was added. The spectra were recorded 30 times with 256 scans for each spectrum. An accumulated 0.8 ml Cu(NO3)2 solution (200 mg/L, pH 5.0) and 0.1 ml sodium hydroxide (5 mmol/L) were added sequentially into the cell. The spectra were recorded 30 times same as the section 2.2.4 during the change of the pH. 2.2.5 Two-dimensional correlation analysis (2D-COS) The computation of the 2D FTIR correlation analysis was performed with a 2D Shige software (version 1.3, Shigeaki Morita, Kwansei-Gakuin University, 2004-2005). To discuss the sequential order of intensity change of bands V1 and V2, the 2D correlation computation gives two types of plots: synchronous, A (V1, V2) and asynchronous, B (V1, V2). An auto peaks (V1=V2) in synchronous plot is responsible for the change of band intensity attribute to perturbation, while the cross peaks (V1≠V2) gives the correlation among the auto peaks. For the cross peaks, the same signs at V1 and V2 indicate the simultaneous change of the band intensity, or opposite signs show the intensity changes accompanied another. The sequential order could be obtained from the sign (red or blue) of synchronous correlation peaks (A (V1, V2)) and asynchronous correlation peaks (B (V1, V2)). There will be three possibilities: 1) V1 prior to V2, if A (V1, V2) and B (V1, V2) show same signs (all red or all blue); 2) V2 prior to V1, if A (V1, V2) and B (V1, V2) show opposite signs (red and blue); 3) V1 and V2 occurs simultaneously, if B (V1, V2) is 0 in the asynchronous plots. The cross peak arise when V1 and V2 change independently of each other (Noda, 2014; Noda & Ozaki, 2004; Yan et al., 2013; Zhang et al., 2010).

3 Results and discussion 3.1 The adsorption of Cu2+ on the composite When the initial concentration of Cu2+ was 10 mg/L, the adsorbed amount increased from 3.14 to 13.99 mg/g with pH range from 3.5 to 5.5 (Fig. 1a). An abrupt increasing 6

was observed from 5.78 (pH 4.5) to 13.51 mg/g (pH 5.0). We speculated that the bonding mechanism changed during the pH from 4 to 5. As shown in Fig. 1b, the adsorbed amount increased with increasing equilibrium concentration. When the equilibrium concentration was 77.27 mg/L, the adsorption capacity reached the maximum at 28.45 mg/g. Both Langmuir and Freundlich were fitted well with the adsorption data (Tab. 1). The correlation constant R2 with Freundlich (0.9986) was higher than that with Langmuir (0.9828), indicated that the adsorption sites could be heterogeneous and the mechanism might be multiple. The RL was between 0.2011 and 0.9159 (Tab. 2), indicated the adsorption was favorable with the experimental feeding concentration of Cu2+. 3.2. The in situ ATR-FTIR spectra of the Cu2+ adsorption 3.2.1. Spectra analysis of the Cu2+ adsorption on composite From 1300 to 800 cm-1(Fig. 2a), the bands intensity at 1033 and 1011 cm-1 decreased constantly with time increasing. From 0 to 200 min, the intensity of bands at 1649, 1582, 1532, 1410, 1381 and 1326 cm-1 changed obviously (Fig. 2b). The band at 1649 cm-1 increased while the others decreased. The peaks at 1033 and 1011 cm-1 were attributed to the stretching of secondary hydroxyl group at C3 position and C-OH at C6 position in chitosan (from the spectrum following, stretching of Si-O could be excluded here), respectively. The bands at 1582 and 1532 cm-1 were attributed to the asymmetric and symmetric bending vibration of primary amines (-NH3+), respectively. Other bands such as 1410, 1381 and 1326 cm-1 were attributed to the bending of C-N, C-H and O-H, respectively. The intensity at 1649 cm1

increased with the time increasing, which was attributed to the in-plane bending of N-

H. There were no obvious changes for the groups on montmorillonite, indicating that the montmorillonite was covered and filled with chitosan. The adsorption mechanism at pH 5.0 might be speculated as: 1) the hydroxyl at C3 and C6 and amino at C2 position in chitosan take part in the interaction with Cu2+; 2) the decreased intensity of protonated amino (-NH3+) indicated it bonded with Cu2+ through proton exchange. 3.2.2. Spectra analysis of the Cu2+ adsorption on chitosan at pH 3.0 and 5.0 7

At fixed pH 3.0 and 5.0, the in situ ATR-FTIR spectra of the Cu2+ adsorption on chitosan displayed the similar change tendency of the bands intensity (Fig. 2c and d). Bands at 1447, 1408, 1252, 1229, 1068, and 895 cm-1 in Fig.2c and 1448, 1408, 1249, 1228, 1071, and 895 cm-1 in Fig. 2d showed the intensity decreasing during the time increasing (adsorption process). The bands were attributed to the vibration of δO-H (primary or secondary hydroxyl), δC-H (primary or secondary hydroxyl), νC-N (one of double peak of primary amine), νC-N (one of double peak of primary amine), νC-OH (primary or secondary hydroxyl), δC-H (deformation vibration of β pyranose at C1 position), respectively. The similar tendency of the intensity changes at pH 3.0 and 5.0 indicated that the O-H, C-H, C-N, C-O, and N-H on chitosan were related to the adsorption of Cu2+ at both pH 3.0 and 5.0. The O-H and C-O vibration might be assigned to the O-H and C-OH on C3 or C6, and C-N, N-H might be attributed to C-NH2 or -NH-C=O on C2 position in chitosan. The decreased intensity of C-O, C-N and C-H might be the changes of bonding energy affected by the near bonding between N, O and Cu2+. With the time increasing, the most obvious changes were the intensity decreasing of OH. It could be considered as the evidence for the interaction of Cu2+ and -OH on chitosan. There were no band change for the amine (-NH2) and no obvious difference between the adsorption at pH 3.0 and 5.0. However, the adsorbed amount jumped from 5.78 to 13.51 mg/g when the pH increased from 4.5 to 5.0 (Fig. 1a). That could be speculated that the sorption mechanisms might change with the pH increasing. 3.2.3. Spectra analysis of the Cu2+ adsorption on chitosan with pH perturbation When pH changed from 5.0 to 3.0 during the adsorption of Cu2+ (Fig. 2e), the bands intensity of 1072 and 1052 cm-1 (νC-OH, primary and secondary hydroxyl) decreased, while that of 1346 cm-1 (δC-N, C-NH2) increased obviously. On the other hand, when pH changed from 3.0 to 5.0 during the adsorption of Cu2+ (Fig. 2f), the intensity of bands at 1445 cm-1 (stretching of O-H), 1404 and 1384 cm-1 (bending of C-H), 1251 and 1228 cm-1 (stretching of C-N), 1073 and 1048 cm-1(stretching of C-OH) increased, while that of 1560, 1541, and 1516 cm-1(3 bands attributed to bending of N-H on primary amine) decreased. These changes during the pH perturbation indicated that amine and hydroxyl 8

might act differently at different pH. The Cu2+ was more inclined to interact with amino at pH 5.0 while the pH decreased to 3.0, the Cu2+ was more inclined to interact with hydroxyl.

3.3. The 2D-COS on the spectra All data of the spectra related to Fig. 2a (1700-1500 cm-1), Fig. 2e (1400-1000 cm-1) and Fig. 2f (1600-900 cm-1) were imported in program Shige, respectively. The synchronous and asynchronous plots were obtained in Fig. 3 a, c, e and b, d, f, respectively. From the Fig. 3a, the correlation of the bands at 1532, 1582 and 1646 cm -1 could be observed and the signs were summarized in Tab. 3. The sequential order could be obtained from the signs of synchronous (A (V1, V2)) and asynchronous (B (V1, V2)) correlation peaks as "V1 prior to V2", "V2 prior to V1", and "V1 and V2 occur simultaneously" when A (V1, V2) and B (V1, V2) have same signs, A (V1, V2) and B (V1, V2) have opposite signs, and B (V1, V2) is 0 in the asynchronous plot, respectively(Noda, 2014; Yan et al., 2013). The sequence of bands changes (Tab. 3) proved that the vibration of -NH3+ was prior to –NH. This indicated the proton exchange interaction between Cu2+ and -NH3+ and the forming of -NH2-Cu on the surface of the composite at pH 5.0. Tab. 4 was the summary of signs from Fig. 3c and d. The bands 1346 and 1052-1072 cm-1 showed correlation in Fig. 3c and the zero sign in asynchronous plot (Fig. 3d), which expressed the increasing intensity of 1346 cm-1 (δC-N, C-NH2 on primary amine) was simultaneous to the decreasing intensity of 1052-1072 cm-1 (νC-OH, primary or secondary hydroxyl). That indicated the C-N was freed with the Cu2+ bonding on COH when the pH was changed from 5.0 to 3.0. Similarly, table 5 indicated the sequence priority as 1541→1048 cm-1, 1541→1251 cm1

, 1541→1404 cm-1, 1251→1404 cm-1, 1048→1404 cm-1 and 1048 ~ 1251 cm-1("→"

and "~" mean "prior to" and "simultaneous to", respectively). That demonstrated the interaction with -NH3+ was prior to –OH, and C-OH was simultaneous to C-N when the pH was changed from 3.0 to 5.0. The intensity of C-N and C-OH were simultaneous 9

when one increased and another decreased when pH was changed from 3.0 to 5.0 or from 5.0 to 3.0, indicated the bonding possibly shifted from N to O, or O to N when the pH was changed from 5.0 to 3.0 or 3.0 to 5.0, respectively. From the results above, the adsorption mechanism of Cu2+ on chitosan or chitosanmontmorillonite composite could be concluded as the interaction of -NH2 and -OH with Cu2+. Hydroxyl was a functional group in the adsorption of Cu2+ from pH 3.0 to 5.0 and more active at pH 3.0 than that at pH 5.0. At pH 5.0, -NH2 joined to the adsorption and was the main adsorption sites for the bonding of Cu2+. The -NH2 was protonated by H+ and interacted with Cu2+ through proton exchange at pH 5.0, while the proton exchange was not available when the pH was 3.0 due to competition of H+. From pH 3.0 to 5.0, the -NH2 was more and more active to bond the Cu2+ than -OH, while the activity was reversed from pH 5.0 to 3.0. Even though the -OH took part in the bonding of Cu2+ at pH 3.0 to 5.0, the adsorbed amount at pH 3.0 (Fig. 1a) was obvious lower than that at pH 5.0. It could be deduced that the proton exchange with -NH3+ was the main adsorption mechanism.

4 Conclusion From the in situ ATR-FTIR spectra and 2D-COS, the adsorption capacity of Cu2+ on chitosan or chitosan-montmorillonite composite could be concluded mainly through the proton exchange interaction with the protonated -NH3+. The -NH2 group could be protonated as -NH3+, and the -NH3+ reversed to -NH2 after the proton exchange for the bonding with Cu2+ when the pH was near 5.0. The -OH was more active at pH 3.0 than at pH 5.0. The contribution made by -OH to the adsorption capacity was less than that by -NH2.

Acknowledgements The research was financially supported by National Key Technology Support Plan (2015BAD05B02).

10

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13

b 30

12

25

Adsorbed Amount(mg/g)

Adsorbed amount (mg/g)

a 14

10 8 6

20 15 10

2+

Cu adsorption on composite Langmuir fitting Freundlich fitting

5

4

2+

Cu adsorption on composite 2

3.5

4.0

4.5

5.0

0

5.5

0

20

40

60

80

Equilibrium concentration(mg/L)

pH

Fig. 1. The adsorption studies of Cu2+ on the composite (a. the effect of pH on the adsorption with the initial concentration of 20 mg/L Cu2+; b. the isotherm adsorption of Cucomposite at pH 5.0)

14

2+

2+

a. Cu adsorbed on the composite at pH 5.0 -1 (1300-800 cm )

0 200 min

1000

900

1700

800

2+

895

Relative Absorbance

1071

0

1068

200 min 1000

1300

-1

2+

0

1200

1326

1410 1381

1400

d. Cu adsorbed on chitosan at pH 3.0

865

1252 1229

Relative Absorbance

1447 1408 1400

1500

Wavenumber cm

c. Cu adsorbed on chitosan at pH 5.0

1600

1600

-1

800

1600

-1

Wavenumber cm

1400

1200

200 min 1000

Wavenumber cm

800

-1

2+

2+

f. Cu adsorbed on chitosan at pH from 3.0 to 5.0

1073

e. Cu adsorbed on chitosan at pH from 5.3 to 3.0

1048

1100

Wavenumber cm

1249 1228

1200

1448 1408

1300

1582 1532

Relative Absorbance

200 min

Relative Absorbance

1033

1011

1646

b. Cu adsorbed on the composite at pH 5.0 -1 (1700-1300 cm ) 0

1600

1400

1200

Wavenumber cm

1052

1072

1600

1000 -1

1400

200 min

1251 1228

1560 1541 1516 1445 1404 1384

200 min

Relative Absorbance

0

1346

Relative Absorbance

0

1200

Wavenumber cm

1000 -1

Fig. 2. In situ ATR-FTIR spectra of Cu2+ adsorbed on composite at pH 5.0 (a.1300-800 cm-1; b. 1700-1300 cm-1) and on chitosan at various pH (c. 5.0, d. 3.0, e. from 5.0 to 3.0, f. from 3.0 to 5.0) with increasing sorption time region from 1600-800 cm-1, respectively 15

Fig. 3. Synchronous (a, c, e) and asynchronous (b, d, f) 2D FTIR correlation spectra of Cu2+ adsorption on composite at pH 5.0 (a, b), on chitosan from pH 5.0 to 3.0(c, d) and from pH 3.0 to 5.0 (e, f)

16

Tab. 1 The fitting results with Langmuir and Freundlich models Langmuir R qmL b 2+ Cu -composite 0.9828 34.9063 0.0447 Constants

Freundlich R KF 1/n 0.9986 3.1590 0.5085

2

2

Tab. 2 The factor RL in the fitting with Langmuir Initial concentration/mg·L-1 2.00 4.31 6.29 8.03 9.64 20.81 45.50 67.25 86.75 2+ Cu -composite 0.9159 0.8351 0.7764 0.7313 0.6938 0.5121 0.3243 0.2451 0.2011 RL

Tab.3 Results of 2D-COS on the signs of each cross peak in synchronous and asynchronous spectra (Fig. 3a and b) ("+" and "-" present same and opposite sign in Fig. 3a and b, respectively;" →" and "~" mean "prior to" and "simultaneous to", respectively) Wavenumber Synchronous (A) Asynchronous (B) (cm-1) 1532 1582 1646 1532 1582 1646 1532 + + + 1582 + + + 1646 + +

Sequences of bands Sequence of groups 1532→1582 1582→1646 1532→1646

-NH3+→-N-H

Tab.4 Results of 2D-COS on the signs of each cross peak in synchronous and asynchronous spectra (Fig. 3c and d) Wavenumber Synchronous (A) Asynchronous (B) Sequences of bands Sequence of groups (cm-1) 1346 1052-1072 1346 1052-1072 1346 1052-1072

+ -

+

0

1346~1052-1072

0

-NH3+ ~ -OH

Tab.5 Results of 2D-COS on the signs of each cross peak in synchronous and asynchronous spectra (Fig. 3e and f) Wavenumber (cm-1)

Synchronous (A)

Asynchronous (B)

1541 1404 1251 1048 1541 1404 1251 1048

1541

+

-

-

-

+

1404

-

+

+

+

-

1251

-

+

+

+

-

-

1048

-

+

+

+

-

-

+

+

+

+ 0

0

17

Sequences of bands

Sequence of groups

1541→1048 1541→1251 1541→1404 1251→1404 1048→1404 1048 ~ 1251

-NH3+→C-OH C-OH ~ C-N