Equilibriums and kinetics studies for adsorption of Ni(II) ion on chitosan and its triethylenetetramine derivative

Accepted Manuscript Title: Equilibriums and kinetics studies for adsorption of Ni(II) ion on chitosan and its triethylenetetramine derivative Author: Bing Liao Wei-yi Sun Na Guo Sang-lan Ding Shi-jun Su PII: DOI: Reference:

S0927-7757(16)30275-8 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.04.043 COLSUA 20600

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

23-2-2016 14-4-2016 19-4-2016

Please cite this article as: Bing Liao, Wei-yi Sun, Na Guo, Sang-lan Ding, Shi-jun Su, Equilibriums and kinetics studies for adsorption of Ni(II) ion on chitosan and its triethylenetetramine derivative, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.04.043 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.

Equilibriums and kinetics studies for adsorption of Ni(Ⅱ) ion on chitosan and its triethylenetetramine derivative

Bing Liao, Wei-yi Sun, Na Guo, Sang-lan Ding, Shi-jun Su*

College of Architecture and Environment, Sichuan University, Chengdu, 610065,

China



Corresponding author. Tel./fax: +86 28 8546 0916.

E-mail address: [email protected] (S.J. Su).

Graphical Abstract:

Highlights 

Triethylenetetramine modified crosslinked chitosan was used for adsorption of Ni2+ from aqueous solution.



The surface structure of chitosan was heterogeneous and multilayer adsorption took place on the surface and inner of the adsorbent.



Monolayer adsorption was dominant for the adsorption of Ni2+ by the chitosan derivative adsorbent.



Both the oxygen and nitrogen atoms in the chitosan derivative were involved in the adsorption of Ni2+.

Abstract The adsorption of Ni(II) ions from aqueous solution by chitosan (CTS) and its triethylenetetramine derivative beads (CCTS) was investigated in a batch adsorption system. Chitosan beads were crosslinked with epichlorohydrin and then grafted with triethylenetetramine to obtain sorbents that are insoluble in aqueous acidic solution and improve adsorption capacity, respectively. Elemental analysis, scanning electron microscopy (SEM), and Fourier-transform infrared (FTIR) spectroscopy were used to analyze the structure and characteristics of chitosan and its derivative. It showed that the derivative of chitosan possessed good stability in acidic solution and achieved optimal adsorption capacity at pH higher than 4.5 and the max adsorption capacity of chitosan and chitosan derivate were 58.09 and 91.44 mg/L, respectively. Their adsorption processes could be best described by the pseudo second-order model, which suggested that the rate-limiting step may be the chemical adsorption rather than the mass transport. The experimental data of adsorption of Ni2+ by chitosan derivate was fitted well by the Langmuir isotherm model with a high correlation coefficient (R2＞ 0.99), showing that monolayer adsorption took place on the surface of chitosan derivate absorbents, while the adsorption isotherm was well fitted by Freundlich for chitosan. Furthermore, the chitosan derivate exhibited good adsorption performance after regeneration for 5 cycles. At last, FTIR and XPS analysis showed that both the nitrogen and oxygen functional groups were involved in the adsorption of Ni2+.

Keywords: Chitosan; derivative; nickel; adsorption

1. Introduction An increasing demand for nickel in different industrial applications like mineral processing, electroplating, smelting, battery production, etc., leads to more and more discharge of nickel-containing wastewater, which is harmful to human health and environment. High-level exposure to nickel for human is linked to dermatitis, lung fibrosis, cardiovascular and kidney diseases, reduced lung function and lung cancer [13]. As nickel is a non-biodegradable heavy metal with a high toxicity present in wastewater, it is of great need to develop effective and environmentally friendly methods to remove and recover the nickel from the aqueous solution. Different conventional physic-chemical treatment methods including chemical precipitation, ion exchange, membrane filtration, and electrochemical have been widely used in the last decades for nickel removal. However, these processes have significant disadvantages such as incomplete removal, high cost and second-pollution problem, which hinder their promotion and development in the treatment of nickel-contaminated wastewater [4, 5]. Recently, numerous researches have been carried out to explore the potential of cheaper and more efficient technologies for nickel removal in order to reduce the second-pollution produced and improve the removal efficiency of treatment. Moreover, adsorption is proved to be a promising method as alternative treatment because of its low cost, extensive resources and non-secondary-pollution. Nowadays, many studies on potential of different adsorbents including mineral, zeolite, industrial waste, agricultural waste, biomass, and polymer materials, etc., used for treatment of nickel wastewater have been performed [6-12]. Chitosan, derived from chitin deacetylation, is the second most abundant natural polymer after cellulose and has been widely used in biomedical, chemistry, printing and dyeing, agriculture, and environment. In particular, chitosan and its derivatives present good property in the treatment of wastewater, especially for removal of heavy metals from aqueous solution, because of their functional groups such as amino and hydroxyl groups. Otherwise, chitosan has its special characteristics like extensive sources, low cost, biodegradability, non-toxicity, etc., which make it a viable alternative adsorbent material for removal of heavy metals without second-pollution [13-15]. However,

earlier studies focusing on adsorption characteristics of chitosan for uptake of heavy metals showed that raw chitosan is unstable and easily soluble in acidic solutions and its adsorption capacity is still insufficient, which hinder its promotion and development in the application. In recent years, a number of chitosan derivatives have been synthesized through physical and chemical modifications to improve their stability under acidic conditions and their adsorption capacity. Also, chitosan derivatives modified by different functional compounds for removal of nickel-contaminated wastewater were studied [16-22]. For example, Monier et al. [23] synthesized a modified magnetic chitosan chelating resin and the adsorption capacities of Cu, Co, Ni were 103.16, 53.51, 40.15 mg/g, respectively. Zhou et al. [24] prepared a magnetic chitosan microsphere modified with thiourea and its uptake capacities for Hg, Cu, Ni were 625.2, 66.7, 15.3 mg/g, respectively. Tran et al. [25] investigated that magnetic chitosan composite beads were prepared through co-precipitation followed by hydrothermal treatment and its adsorption capacity for Pb, Ni were 63.33 and 52.55 mg/g, respectively. Among the chitosan derivatives containing nitrogen, sulfur and oxygen and phosphorus functional groups, chitosan modified by amide compounds showed good adsorption performance and selectivity for heavy metals, especially for heavy metals which are difficult to remove such as nickel and cobalt. Although modification of the chitosan by amino compounds can improve the stability and adsorption capacity, there are still some shortcomings in the adsorption of heavy metals. On the one hand, cross-linked chitosan can improve their stability under acidic conditions, it, in turn, may also reduce the adsorption capacity due to its consuming and wrapping part of the functional groups of the chitosan such as amino and hydroxyl. Therefore, grafting on the cross-linked chitosan with extra more functional groups such as coordination atom N is necessary to improve its adsorption capacity. On the other hand, separating adsorbent from solution is also a problem in the traditional methods such as filtration, sedimentation and centrifugation as they are timeconsuming and uneconomic. Therefore, it’s necessary to discover other methods to separate the adsorbent from solution easily. In view of the above problems present in the treatment of heavy metals wastewater

using chitosan and its derivative as adsorbents, a newly triethylenetetramine grafted chitosan was synthesized via cross-linking with epichlorohydrin in this work, which had high nitrogen element content and good separation performance after adsorption as the adsorbent when spherical particles were used as adsorbent during the adsorption process. In addition, a comparative study of the adsorption behavior of nickel between the original chitosan and its derivative was also investigated through studying the influences of different parameters such as pH, adsorption time, and initial nickel concentration on the adsorption capacity, the adsorption kinetics, isotherms, and thermodynamics. Finally, mechanism for adsorption of nickel on chitosan derivative and desorption performance were discussed.

2. Experimental 2.1 Materials Chitosan, with a deacetylation level of about 90%, was supplied by Kelong Chemical Co., Ltd (Chengdu, China). Epichlorohydrin, N, N-dimethylformamide, triethylene tetramine, isopropanol alcohol, sodium hydroxide, acetic acid, and sulfuric acid were obtained from Kelong Chemical Co., Ltd (Chengdu, China). Absolute ethyl alcohol was supplied by Chengdu Changlian Chemical Reagent Co., Ltd. Chemicals used in this study were of analytical reagent grade. The desired concentration of nickel was prepared from the stock solution using Milli-Q water and the nickel stock solution (1000 mg/L) was prepared by NiSO4. 2.2 Methods 2.2.1 Synthesis of triethylenetetramine-modified cross-linked chitosan In order to improve the stability of chitosan in acidic solution and make the adsorbent

easily

separated

from

the

aqueous

solution

after

adsorption,

triethylenetetramine-modified cross-linked chitosan was synthesized. 2.2.1.1 Preparation of chitosan beads (CTS) 10 g of chitosan powder was dissolved in 300 mL of 3% (in volume) acetic acid under magnetic stirring at room temperature until completely dissolved to obtain a transparent gel. Then the mixture was added dropwise dropped into 500 mL of 1 mol/L

NaOH. Chitosan beads were obtained after stirring for 4h and washed with ethanol and distilled water to neutral. 2.2.1.2 Preparation of epoxy chitosan (ECTS) Chitosan beads were dipped into 150 mL of isopropanol alcohol at 40 ℃ with stirring for 30 min, and then 70 mL of epichlorohydrin was added slowly. After that, the mixture was stirred for 10 h, and washed with ethanol and distilled water to remove the unreacted epichlorodydrin. 2.2.1.3 Preparation of triethylenetetramine cross-linked chitosan (CCTS) The epoxy chitosan was dipped into 150 mL of N,N-dimethylformamide at 50 ℃ with stirring for 30 min, and then 50 mL of triethylenetetramine was slowly dropped into the solution. The mixture was stirred for 4 h and washed with ethanol and distilled water to remove the unreacted triethylenetetramine. 2.2.2 Adsorption experiments Batch experiments were conducted by reacting nickel solutions (25 mL) in 100 mL conical flasks with the adsorbent (0.02 or 0.05 g). The conical flasks were placed in a thermostatic water bath vibrator and shaken at 120 rpm and 30 ℃ for 24 h. The initial pH value of the aqueous solution was adjusted with either 1M NaOH or 1M H2SO4. The adsorption capacity of adsorbent and removal efficiency of Ni2+ was calculated by the following equations: Q=

𝐶0 −𝐶𝑒

η=

𝐶0 −𝐶

𝑚 𝐶0

𝑉

× 100%

(1) (2)

where Q is the nickel adsorption capacity per unit weight of adsorbent, mg/g;η is the removal efficiency of Ni2+ in percentage , %; C0 and C are the initial and equilibrium nickel concentrations, respectively, mg/L; V is the volume of aqueous solution, L; and m is the mass of adsorbent used, g. 2.2.3 Regeneration studies For the desorption and regeneration study, the spent CCTS adsorbent was added into the 30 mL 1mol/L H2SO4 and shaken at 30℃ for 24h. Then the CCTS was filtered and dipped into 30 mL 1mol/L NaOH for 6h to activate the functional groups for reuse,

subsequently, washed with distilled water to neutral. Finally, adsorption test was carried out with regenerative adsorbent. The above process was repeated for five times. 2.3 Analysis The nickel concentrations before and after adsorption were determined by an ICPMS (NEXION 300X, PE) equipped with an automatic sampler (SC2 DX, ESI). The running parameters of ICP-MS are listed below: RF power of 1200 W, plasma gas flow rate 15 L/min, nebulizer gas flow rate 0.94 L/min, analog stage voltage -1900 V, pulse stage voltage 950 V, scan mode peak hoping, MCA channels 1, dwell time 50 ms, 141 integration time 1000 ms, and readings were taken as triplicates. Elemental analysis was used to determine the main elements (C, H, N) of chitosan, epoxy chitosan and triethylenetetramine grafted chitosan with a Euro EA 3000 elemental analyzer (LEEMAN LABS INC., USA). In order to determine the synthetic route of the desired compounds, FTIR analysis was used to examine the change of the peak value of chitosan, epoxy chitosan and triethylenetetramine grafted chitosan, which was recorded on a Nicolet 6700 FT-IR spectrometer (Thermo Electron Corporation, USA) after the samples were mixed with KBr in agate mortar and pressed to a thin plate over the wavenumber range 4000–400 cm-1 at a solution of 4 cm−1. The surface morphology of the chitosan and triethylenetetramine grafted chitosan was observed by scanning electron microscopy (SEM) with a JSM-7500F scanning electron microscope (JEOL, Japan). The SEM preparation process was as follows: first, samples suspended in distilled water and the dispersion was dropped on aluminum foil and dried at ambient atmosphere; then the samples mounted on metal grids, using double-sided adhesive tape, and finally coated by gold under vacuum. X-ray photoelectron spectroscopy (XPS) was applied to determine the surface chemical composition of the chitosan and triethylenetetramine grafted chitosan, using a XSAM-800 spectrometer (KRATOS, UK) with Al (1486.6 eV) under ultra-high vacuum (UHV) at 12 kV and 15 mA. 3. Results and discussion 3.1 Characteristics of the chitosan derivative

3.1.1 Elemental analysis To evaluate the change of element content and explore the production process, elemental analysis was used to determine the element contents of raw chitosan and its derivative and the result was shown in Table 1. It showed obviously that the nitrogen content of CCTS was much higher than that of CTS and the carbon and hydrogen content as well as total percentage of CCTS were also higher than that of CTS. This was due to the introduction of epichlorohydrin and triethylenetetramine onto the chitosan, indicating that the triethylenetetramine was successfully grafted onto chitosan. 3.1.2 Infrared spectra analysis Fig. 1 showed the infrared spectra analysis results of chitosan and its derivatives. The FTIR spectrum of CTS in Fig. 1 indicated the presence of predominant peaks at 3430 cm-1 (O-H and N-H stretching vibration), 2919 cm-1 and 2872 cm-1 (symmetric and asymmetric stretching vibration of C-H bond), 1655 cm-1 (stretching vibration of C=O bond), 1604 cm-1 (bending vibration of N-H bond), 894 cm-1 (pyranoid ring) [26]. All the peaks near 894 cm-1 of the three materials remained unchanged, which indicated that the preparation process of CCTS did not break up the pyranoid ring of the CTS. The peak of 739 appeared assigned to stretching vibration of C-Cl, which demonstrated successful epoxy open-loop crosslinking reaction on the amine function group of CTS. By comparison of IR spectra of ECTS and CCTS, the peak at 739 cm-1 disappeared and the peaks at 813, 1379 and 1467 cm-1 were attributed to C-N-C bond, and the peak at 1580 cm-1 was corresponding to N-H bond, indicating that the triethylenetriamine was successfully grafted on the chitosan [27, 28]. 3.1.3 SEM analysis Adsorption material with porous structure and large specific area is good for treatment of heavy metals as it may provide more active adsorption sites [29]. The SEM images of CTS and CCTS resin were shown in Fig. 2. By comparing the images of CTS and CCTS, the CTS surface displayed a porous structure, however, after cross-linking, the CCTS surface exhibited slight agglomerate phenomenon with rough and less porous structure. This was due to the cross-linking reaction, which coated the surface of CTS with triethylenetetramine. Compared with elemental analysis and the SEM images of

CTS and CCTS, it was found that introduction of triethylenetriamine increased the nitrogen content of chitosan derivative, which provided more functional groups so as to improve adsorption capacity, however, covered with triethylenetriamine on the surface of the material also changed the surface characteristics such as roughness, specific surface area, and porous structure and further influenced the adsorption performance. 3.2 Effect of pH To study the effect of pH on the adsorption process with CTS and CCTS, experiments were conducted in the pH range 1.0-6.5 and the result was shown in Fig. 3. It turned out that the adsorption capacity increased quickly with the increase of pH value of solution from 1.0 to 3.5 and stayed almost constant along with further increasing of pH value. This was due to the competition between protonation and chelation in the adsorption process under different pH value of the aqueous solution. At low pH, most of the amino groups of the adsorbents were ionized and presented in the form of NH3+, and electrostatic repulsion may exist between Ni2+ and NH3+, which prevented the adsorption of Ni2+ onto the adsorbents [30]. When the pH increased, more and more amino groups would chelate with Ni2+, which resulted in the increasing of uptake capacity. 3.3 Adsorption kinetics Adsorption experiments were performed at different initial concentrations of Ni2+ as a function of reaction time and the result were stated in Fig. 4. As shown in Fig. 4, the amount of adsorption capacity for both CTS and CCTS increased with the increasing of contact time and adsorption equilibrium was almost reached when the reaction time was 2 h under low initial concentrations of Ni2+. When the initial concentration of Ni2+ was higher, the adsorption rate increased quickly at first, and then went down slowly until adsorption equilibrium. This was due to the fact that higher initial concentration of adsorbate provided more driving power of adsorption of Ni2+ onto adsorbents, which made the adsorbate easier be captured [29]. Moreover, it was also observed through comparing the adsorption capacity of CTS and CCTS at the same experimental conditions that the adsorption capacity of CCTS was always higher

than that of CTS, which indicated that introduction of triethylenetetramine improved the adsorption capacity of CTS derivative. In order to explore the adsorption mechanism and potential rate-controlling step, adsorption kinetics models such as pseudo first-order, pseudo second-order and intraparticle diffusion kinetics are necessary to fit the experiment data. Generally, the linear form of pseudo first-order kinetic model is expressed by [31]: 1 𝑄𝑡

𝐾

1

= 𝑄 1𝑡 + 𝑄 𝑒

(3)

𝑒

where Qt and Qe are the adsorption capacity at time t and equilibrium, mg/g; K1 is the rate constant of the pseudo first-order kinetic model, h-1; t is the contact time, h. The constants of the equation are determined by the slope and intercept of the linear plot of 1/Qt versus 1/t and the results were shown in Fig. 5 and Table 2. The pseudo second-order model can be presented as follow [31]: 𝑡 𝑄𝑡

𝑡

=𝑄 +𝐾 𝑒

1

2 𝑄𝑒

2

(4)

where K2 is the rate constant of the pseudo second-order adsorption model, g/mg. h. The values of K2 and Qe are calculated from the intercept and slope of the linear plot of t/Qt versus t, respectively and the results were presented in Fig. 6 and Table 2. As shown in Table 2, the calculated values of Qe by using pseudo second-order model were closer to the experimental values than that of pseudo first-order model (the experimental values were 8.95, 18.05 and 57.41 mg/g for CTS at the initial concentration of 20, 50 and 200 mg/L, respectively and 10.07, 26.16 and 90.80 mg/g for CCTS at the initial concentration of 20, 50 and 200 mg/L, respectively). Furthermore, the correlation coefficient (R2) of pseudo second-order kinetic were also high than that of first-order model, which indicated that the pseudo second-order kinetic was more suitable for fitting the experimental data and the rate-limiting step of adsorption process was controlled by a chemical reaction. In addition, the model of intraparticle diffusion was also used to fit the experiment data to determine whether the intraparticle diffusion is the rate-controlling step of the adsorption process. It can be expressed as: 𝑄𝑡 = 𝐾3 √𝑡 + 𝐶

(5)

where K3 is the rate constant of the intraparticle diffusion, mg/g. h0.5. The slope and intercept of the linear plot of Qt versus t0.5 can be used to determine the constants of the equation (5) and the results were stated in Fig. 7 and Table 2. From the Table 2, the correlation coefficient of intraparticle diffusion were relatively low, which suggested that the intraparticle diffusion kinetic was unsuitable and the rate-controlling step may be chemical reaction. 3.4 Adsorption isotherms and thermodynamics In order to investigate the adsorption performance of Ni2+ onto CTS and CCTS under different initial concentrations and adsorption isotherms, experiments were conducted as a function of initial concentration for adsorption of Ni2+ at different temperatures and the results were presented in Fig. 8. It was shown that the adsorption capacities of Ni2+ on both CTS and CCTS increased as initial concentration and reaction temperature got higher and the reaction temperature had a little influence on the adsorption process. Furthermore, the adsorption capacity of CCTS was always higher than that of CTS in adsorption of Ni2+ at the same condition. Furthermore, adsorption isotherm was used to explore the interactive behavior between solution and adsorbents. As we all know, Langmuir and freundlich isotherm models are widely used in describing the adsorption equilibrium in heavy metals adsorption. Their linear form of equations can be expressed as follows [32]: 𝐶𝑒 𝑄𝑒

=𝑄

𝐶𝑒

𝑚𝑎𝑥

+𝐾

1

𝐿 𝑄𝑚𝑎𝑥

1

log 𝑄𝑒 = log 𝐾𝐹 + 𝑛 log 𝐶𝑒

(6) (7)

where Qe and Qmax are the equilibrium and maximal adsorption capacity, respectively, mg/g; Ce is the equilibrium concentration of Ni2+, mg/L; KL and KF are constants of Langmuir and Freundlich, respectively, L/mg and mg/g. The experimental data of adsorption of Ni2+ onto CTS and CCTS was fitted by linear plot of Langmuir and Freundlich isotherm models as Ce/Qe versus Ce and log Qe versus log Ce, respectively, and the results were shown in Fig. 9-10 and Table 3. The corresponding parameters obtained in Table 3 indicated that Langmuir isotherm was more suitable to explain the adsorption of Ni2+ by CCTS as its correlation coefficient

(R2＞0.99) was higher than that of Freundlich isotherm model, which showed that the monolayer adsorption may take place on the homogeneous surface of CCTS adsorbent[32]. At the same time, it was also found that the experimental data of adsorption of Ni2+ by CTS was fitted better by Freundlich isotherm model than Langmuir, indicating that the surface structure of the CTS may be heterogeneous. From the results, we also found that the heterogeneous surface structure of CTS may be great benefit for adsorption process and adsorption capacity. When the initial concentration of Ni2+ was low, the adsorption reaction took place on the active sites of the surface, however, raising the concentration of Ni2+ would force Ni2+ into the inner sites of the adsorbent through the pore canal so as to enhance the adsorption capacity. But for CCTS, the adsorption mainly took place on the surface as monolayer adsorption was dominant in the adsorption process [32]. In the study of adsorption process, the determination of thermodynamic equilibrium constant still remains uncertain, as it is related to the characteristics of the adsorbate and the adsorption mechanism. In this work, according to Van’t Hoff equation [33], the relationship of Ka and T can be represented as Eq. 8 and plotting of ln (Qe/Ce) versus (1/T) was applied to investigate the adsorption thermodynamics of Ni2+ onto CTS and CCTS. The results were displayed in Table 4. 𝑄

∆𝐻

ln( 𝐶𝑒) = − 𝑇𝑅 + 𝑒

∆𝑆

(8)

𝑅

where Qe and Ce are the adsorption capacity and the concentration of Ni2+ at equilibrium, respectively, mg/g and mg/L; Δ H and Δ S are enthalpy and entropy changes, respectively, kJ/mol and J/mol K; R is the universal gas constant, 8.314 J/mol K; T is the absolute temperature, K. Gibbs free energy was calculated by the following equation: ∆G = ∆H − T∆S

(9)

As shown in Table 4, the value of ΔH and ΔS for both CTS and CCTS were positive, which showed that the adsorption process was endothermic along with the increase in the entropy. The endothermicity was explained as the Ni2+ in the aqueous solution displace more than one water molecule and destroy the hydration sheath for its

adsorption and formation of coordination complex with CCTS. Meanwhile, the positive ΔS values identified the increasing trend in randomness at solid–solute interface during adsorption caused by this water release [34]. The negative value of ΔG for both CTS and CCTS confirmed that the adsorption process was spontaneous. The observed increase in the negative value of ΔG for CTS and CCTS with increasing temperature stated that the adsorption reaction became more favorable at higher temperature [27]. 3.5 Desorption and re-adsorption performance Fig. 11 showed the desorption property of CCTS for removal of Ni2+ from aqueous solution. It was found that the adsorption capacity decreased gradually with the increase of cycle number. After 4 cycles of adsorption-desorption process, the adsorption capacity still remained high with reduction of adsorption capacity by 5%, however, the adsorption capacity started to reduce quickly after then and in the final cycle the adsorption capacity reduced to about 57% of the initial one. Considering the results obtained above, the adsorbent presented good adsorption performance after regeneration and greatly potential for being used in the removal of Ni2+ from wastewater. 3.6 Adsorption mechanism As all we know, the amino and hydroxyl groups of chitosan derivatives are the main active functional groups in adsorption removal of heavy metals. To understand the adsorption mechanism of nickel onto CCTS and the possible sites of nickel binding to CCTS, FTIR spectra analysis was used to determine the changes of the bond intensity before and after adsorption. It was found that in Fig. 12 the wavenumbers of 3427, 1655 and 660 cm-1 before adsorption was shifted to 3386, 1645, 616 cm-1 after adsorption, respectively, indicating that the nitrogen atom of CCTS chelated with nickel. This was due to the formation of N-Ni bond, which reduced the vibration intensity of N-H bond so as to reduce the wavenumber of bonds related to N-H [35, 36]. Furthermore, the wavenumbers at 1153 cm-1assigned to O-H stretching vibration changed to 1111 cm-1, indicating that the oxygen atoms in the hydroxyls may also be involved in the adsorption reaction [37]. To further verify the results of adsorption mechanism from FTIR analysis, XPS analysis was also carried out to elucidate the surface adsorption mechanism in this work.

Fig. 13 and Fig. 14 showed the O 1s and N 1s, Ni 2p spectra of CCTS (a) before adsorption and (b) after adsorption of Ni2+, respectively. Comparison of the BE of oxygen and nitrogen element of CCTS before and after Ni2+ adsorption, the peaks at 531.4 and 532.7 eV of oxygen atom assigned to C-OH or bond water shifted to 531.6 and 530.3 eV, respectively; the peaks at 397.9 and 399 eV of nitrogen atom assigned to C-N or –NH2 moved to 398.3 and 399.3 eV, respectively. This may be explained that the electronic density around O and N atoms decreased because they were easily to lose electron which were migrated to Ni2+ after chelation between –OH, -NH2 and Ni2+ [38]. This indicated that some O and N atoms existed in a more oxidized state due to Ni adsorption. Finally, it was attributed to the formation of complexes between Ni and – OH, –NH2. So the XPS spectra provided evidence of Ni2+ binding to nitrogen atoms and oxygen atoms, which agreed with the FTIR results. In Fig.14, the appearance of peaks for Ni 2p3/2 at 854.4 and 859.8 eV and peak for Ni 2p1/2 at 871.8 and 878.4 eV can be observed. Compared with the BE of free Ni2+ in the NiSO4 solution at 856.80 eV [38], the BE at 854.4 eV of Ni2+ on the adsorbent surface decreased by 2.4 eV. This indicated that the Ni2+ gained electron and formed complexes between Ni and –OH, –NH2. Therefore, the FTIR and XPS analysis suggested that both the oxygen and nitrogen atoms in the CCTS participated in the adsorption of Ni2+. Combined with the adsorption kinetics and isotherms analyses, it was concluded that the adsorption of Ni2+ by CCTS adsorbent was chemical adsorption, taken place on the homogeneous surface of CCTS adsorbent for monolayer adsorption with functional groups of oxygen and nitrogen atoms involved in the chelating with Ni2+ from aqueous solution. 4. Conclusions In this work, a chitosan derivative grafted with triethylenetetramine was successfully synthesized and the adsorption performance of chitosan and its derivative for removal of Ni2+ from aqueous solution was also studied comparatively. Both CTS and CCTS exhibited good stability and high adsorption capacity at optimal pH higher than 4.5. Adsorption kinetics study showed that the pseudo second-order kinetic model described the experimental data better, indicating that the chemical adsorption may be

the rate-limiting step. Langmuir isotherm was more suitable to fit the adsorption isotherm data for adsorption of Ni2+ by CCTS and Freundlich isotherm was the better one to fit the experimental data of adsorption process with CTS, which presented different adsorption mechanism between the adsorbent and the adsorbate. Compared to the unmodified chitosan, the chitosan derivate exhibited better removal performance and higher adsorption capacity. Furthermore, after five cycles adsorption-desorption performance, the CCTS still showed good adsorption capacity, indicating that the adsorbent was a good substitution as an efficient adsorbent in the removal of Ni2+ from wastewater. The FTIR and XPS analysis suggested that both the oxygen and nitrogen atoms in the CCTS participated in the adsorption of Ni2+.

Acknowledgements This project is supported by the National Natural Science Foundation of China (NSFC-51374150 and NSFC-51304140) and Science and Technology Plan Projects of Sichuan Province, China (Grant No. 2014SZ0146 and 2015HH0067). Also, the authors acknowledge the grammatical support by Co-co Chen, Greensboro Day School North Carolina.

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chitosan-grafted-poly(2-amino-4,5-

pentamethylene-thiophene-3-carboxylic acid N'-acryloyl-hydrazide) chelating resin for removal of Cu(II), Co(II) and Ni(II) metal ions from aqueous solutions, Int. J. Biol. Macromol. 48 (2011) 558-565. [17] N. Gupta, A.K. Kushwaha, M.C. Chattopadhyaya, Adsorptive removal of Pb2+, Co2+ and Ni2+ by hydroxyapatite/chitosan composite from aqueous solution, J. Taiwan Inst. Chem. E. 43 (2011) 125-131. [18] A.G. Hadi, Adsorption of Nickel Ions By Synthesized Chitosan, Adsorption, 6 (2012). [19] R. Jayakumar, N. Nwe, S. Tokura, H. Tamura, Sulfated chitin and chitosan as novel biomaterials, Int. J. Biol. Macromol. 40 (2007) 175-181. [20] T.W. Tan, X.J. He, W.X. Du, Adsorption behaviour of metal ions on imprinted chitosan resin, J Chem Technol Biot. 76 (2001) 191-195. [21] Y.K. Twu, H.I. Huang, S.Y. Chang, S.L. Wang, Preparation and sorption activity of chitosan/cellulose blend beads, Carbohydr. Polym. 54 (2003) 425-430. [22] Y. Yi, Y. Wang, F. Ye, Synthesis and properties of diethylene triamine derivative of chitosan, Colloids and Surfaces A 277 (2006) 69-74. [23] M. Monier, D.M. Ayad, Y. Wei, A.A. Sarhan, Adsorption of Cu(II), Co(II), and Ni(II) ions by modified magnetic chitosan chelating resin, J. Hazard. Mater. 177 (2010) 962-970. [24] L. Zhou, Y. Wang, Z. Liu, Q. Huang, Characteristics of equilibrium, kinetics

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CCTS

813 1580

1379 1467

ECTS

739

CTS 1655 1604

894

2919 2872 3430

4000

3500

3000

2500

2000

Wavenumber cm

1500

1000

500

-1

Fig. 1 Infrared spectra of CTS, ECTS and CCTS.

Fig. 2 The SEM images of CTS (A) and CCTS (B).

100

80

80

60

60

40

40 Removal percentage of CTS Removal percentage of CCTS Adsorption capacity of CTS Adsorption capacity of CCTS

20

20

0

Adsorption capacity (mg/g)

Removal percentage (%)

100

0 1

2

3

4

5

6

7

pH

Fig. 3 Effect of pH on the adsorption capacity and removal percentage of Ni2+ onto CTS and CCTS. (initial concentration 200 mg/L, adsorbent dose 2 g/L, contact time 24 h, temperature 303 K).

Adsorption capacity (mg/g)

100

CTS, 20 mg/L CTS, 50 mg/L CTS, 200 mg/L CCTS, 20 mg/L CCTS, 50 mg/L CCTS, 200 mg/L

80

60

40

20

0 0

5

10

15

20

25

Time (h)

Fig. 4 Effect of contact time on the adsorption capacity of CTS and CCTS. (pH 5.92, initial concentration 20, 50 and 200 mg/L, adsorbent dose 2 g/L, contact time 24 h, temperature 303 K).

0.30

0.30

20mg/L 50mg/L 200mg/L

0.25

0.20

1/Qt

0.20

1/Qt

20mg/L 50mg/L 200mg/L

0.25

0.15

0.15

0.10

0.10

0.05

0.05

0.00

0.00 0

1

2

3

4

5

6

0

1

2

3

4

5

6

1/t (h-1)

-1

1/t (h )

(a)

(b)

Fig. 5 Linear plot of pseudo first-order kinetic model for adsorption of Ni2+ on CTS (a) and CCTS (b).

3.0 2.5

2.5

20mg/L 50mg/L 200mg/L

2.0

20mg/L 50mg/L 200mg/L

2.0

t/Qt

t/Qt

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0

0

5

10

15

20

t (h)

25

0

5

10

15

20

25

t (h)

(a) (b) Fig. 6 Linear plot of pseudo second-order kinetic model for adsorption of Ni2+ on CTS (a) and CCTS (b).

100

20mg/L 50mg/L 200mg/L

60

80

40

Qt (mg/g)

Qt (mg/g)

50

30

60

20mg/L 50mg/L 200mg/L

40

20 20

10

0 0

1

2

3

4

0.5

5

0 0

1

2

3

4

5

t0.5

t

(a)

(b)

Fig. 7 Linear plot of intraparticle diffusion kinetic model for adsorption of Ni2+ on CTS (a) and CCTS (b).

80

Adsorption capacity (mg/g)

70 60 50 40 CTS, 30 ℃ CTS, 45 ℃ CTS, 60 ℃ CCTS, 30 ℃ CCTS, 45 ℃ CCTS, 60 ℃

30 20 10 0 0

50

100

150

200

Initial concentration (mg/L)

Fig. 8 Effect of initial concentration on the uptake of Ni2+ onto CTS and CCTS.

3.5

2.0

30℃ 45℃ 60℃

3.0

30℃ 45℃ 60℃

1.5

2.0

Ce/Qe

Ce/Qe

2.5

1.5 1.0

1.0

0.5

0.5

0.0

0.0 -20

0

20

40

60

80

100

Ce (mg/L)

120

140

160

-20

0

20

40

60

80

100

120

140

Ce (mg/L)

(a)

(b)

Fig. 9 Langmuir fitting plots for adsorption isotherms of Ni2+ onto CTS (a) and CCTS (b).

1.8

2.0 1.8

30℃ 45℃ 60℃

1.6

1.6

logQe

log Qe

1.4

1.2

1.4

30℃ 45℃ 60℃

1.2

1.0 1.0 0.8

0.6 -1.0

0.8

-0.5

0.0

0.5

1.0

1.5

log Ce

2.0

2.5

0.6 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

logCe

(a)

(b)

Fig. 10 Freundlich fitting plots for adsorption isotherms of Ni2+ onto CTS (a) and CCTS (b).

120

Adsorption capacity (mg/g)

100

80

60

40

20

0 1

2

3

4

5

6

Times

Fig. 11 Effect of adsorbent regeneration times on the adsorption capacity of Ni2+ by CCTS.

CCTS-Ni 616 1645

2920

1111 1064

2855

CCTS

3386 660 1655 2921

1580

2855

1153 1066

1031

3427

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber cm

Fig. 12 FTIR spectra of CCTS before and after Ni2+ adsorption.

10000

7000

(a) (b)

8000

O 1s Intensity (cps)

Intensity (cps)

6000 5000 4000 3000 2000

O 1s

6000

4000

2000

1000 0

520

525

530

535

540

545

520

525

Binding Energy (eV)

530

535

540

545

550

Binding Energy (eV) 4000

(a)

3500

2500

Intensity (cps)

Intensity (cps)

3000

N 1s

2000

(b)

3000

N 1s

2500

2000

1500

1500 1000 385

1000 390

395

400

Binding Energy (eV)

405

410

390

395

400

405

410

415

Binding Energy (eV)

Fig. 13 O 1s and N 1s spectra of CCTS before (a) adsorption and after (b) adsorption of Ni2+.

3600

Intensity (cps)

3400

Ni 2p

3200 3000 2800 2600 2400 2200 840

850

860

870

880

890

Binding Energy (eV)

Fig. 14 Ni 2p spectra of CCTS after adsorption of Ni2+.

Table 1. Elemental analysis results of chitosan and its derivative. wt. % Compound C

H

N

C/N

total

CTS

39.95

6.53

7.3

5.47

53.78

CCTS

41.03

7.89

12.48

3.29

61.4

Table 2. Adsorption kinetic parameters of different models for Ni2+ adsorption on CTS and CCTS. Pseudo-first-order

Pseudo-second-order

Intraparticle diffusion

Initial concentration

Q（mg/g） K1(1/h) e

R2

Q（mg/g） K2(g/mg e

50

200

Kp(mg/g

C

R2

h0.5)

h)

（mg/L）

20

R2

CTS

8.28

0.2625

0.9244

9.07

0.2567

0.9997

1.1100

4.55

0.6439

CCTS

10.28

0.3158

0.9972

10.23

0.3568

0.9998

1.1692

5.69

0.5234

CTS

15.91

0.2577

0.9478

18.37

0.0905

0.9993

2.3662

8.30

0.7871

CCTS

23.72

0.2906

0.9429

26.73

0.0741

0.9998

3.5918

12.02

0.7190

CTS

49.68

0.2704

0.9615

58.34

0.0234

0.9976

7.5378

25.22

0.8185

CCTS

73.26

0.2725

0.9992

92.94

0.0116

0.9970

13.1231

34.19

0.8917

Table 3. Adsorption isotherm parameters of Ni2+ onto CTS and CCTS for different models. Temperature （℃）

Langmuir

Freundlich

Qmax（mg/g）

KL(L/mg) R2

n

CTS

45.48

0.0851

0.9637

3.08 8.6892

0.9868

CCTS

70.77

0.1950

0.9941

2.82 14.23

0.9163

CTS

47.26

0.0898

0.9710

2.99 8.8014

0.9898

CCTS

71.48

0.2466

0.9946

2.98 16.10

0.8458

CTS

49.93

0.0860

0.99657 3.11 9.6070

0.9976

CCTS

73.10

0.2709

0.9975

0.8866

KF(mg/g)

R2

30

45

60 2.82 15.60

Table 4. Thermodynamic parameters at different temperature. Δ

T (K)

(kJ/mol)

G

Δ (kJ/mol)

ΔS (J/mol

H K)

CTS

-2.41

14.49

55.77

CCTS

-3.17

16.53

65.01

CTS

-3.24

CCTS

-4.14

CTS

-4.08

CCTS

-5.12

303

318

333

Table 5. Binding energies (BE) of O 1s, N 1s, Ni 2p1/2 and Ni 2p3/2 obtained from the XPS spectra of CCTS before (a) and after (b) Ni2+ adsorption. Samples

Binding energies (BE) O 1s

N 1s

Ni 2p1/2

a

531.4

532.7

397.9

399.0

-

b

531.6

530.3

398.3

399.3

871.8

878.4

Ni 2p3/2 -

-

854.4

859.8