Comparison of Co2+ adsorption by chitosan and its triethylene-tetramine derivative: Performance and mechanism

Accepted Manuscript Title: Comparison of Co2+ adsorption by chitosan and its triethylene-tetramine derivative: performance and mechanism Author: Bing Liao Wei-yi Sun Na Guo Sang-lan Ding Shi-jun Su PII: DOI: Reference:

S0144-8617(16)30575-6 http://dx.doi.org/doi:10.1016/j.carbpol.2016.05.053 CARP 11122

To appear in: Received date: Revised date: Accepted date:

3-2-2016 22-4-2016 14-5-2016

Please cite this article as: Liao, Bing., Sun, Wei-yi., Guo, Na., Ding, Sanglan., & Su, Shi-jun., Comparison of Co2+ adsorption by chitosan and its triethylene-tetramine derivative: performance and mechanism.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.05.053 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.

Comparison of Co2+ adsorption by chitosan and its triethylene-tetramine derivative: performance and mechanism 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).

Abstract A cross-linked chitosan derivative (CCTS) was synthesized via cross-linking of epichlorohydrin and grafting of triethylene-tetramine. The adsorption performance and capacity of the raw chitosan (CTS) and its derivative were also investigated for removal of Co2+ from aqueous solution. A maximum adsorbed amount of 30.45 and 59.51 mg/g was obtained for CTS and CCTS, respectively under the optimized conditions. In addition, the adsorption kinetics for the adsorption of Co2+ by CTS and CCTS were better described by the pseudo second-order equation. The adsorption isotherm of CCTS was well fitted by the Langmuir equation, but the data of the adsorption of Co2+ onto CTS followed Freundlich and Sips isotherms better. Furthermore, the adsorbent still exhibited good adsorption performance after five regeneration cycles. Finally, Co2+ removal mechanisms, including physical, chemical, and electrostatic adsorption, were discussed based on microstructure analysis and adsorption kinetics and isotherms. Chemical adsorption was the main adsorption method among these mechanisms.

Keywords: Chitosan; derivative; cobalt; adsorption

1. Introduction Nowadays, wastewater containing heavy metals is discharged into the environment directly or indirectly, causing a potential threaten to human, plants and animals (Fu & Wang, 2011). Typical toxic heavy metals of public concern from wastewater include Cu, Zn, Co, Ni, Pb, Ag, Hg, Cr, and Cd (Babel & Kurniawan, 2003; Nagajyoti, Lee, & Sreekanth, 2010). Among them, cobalt is a toxic metal derived from waste streams of the nuclear and coal fired power plants and many other industries such as mining, metallurgical, electroplating, paints, petroleum and electronics (Tofan, Teodosiu, Paduraru, & Wenkert, 2013). This cobalt-containing wastewater has been representing a long-term negative influence on human beings and aquatic ecosystems due to its chronic toxicity to lung, heart and liver, resulting in asthma, heart failure, cancer and genetic variation (Gupta, Kushwaha, & Chattopadhyaya, 2011; Monier, Ayad, Wei, & Sarhan, 2010; Repo,

Warchol, Kurniawan, & Sillanpää, 2010). The permissible limits of cobalt in the irrigation water and livestock watering are 0.05 and 1.0 mg/dm3, respectively (Environmental Bureau of Investigation, Canadian Water Quality Guidelines) (Rengaraj & Moon, 2002). Also, the concentration of cobalt in

the discharge wastewater should not be higher than 1 mg/dm3 (discharge standards for copper, cobalt, nickel industrial wastewater (GB25467 -2010)) (Environmental protection department of the

People's Republic of China, 2010). With a better awareness of the problems associated with cobalt, research studies related to the methods of removing cobalt from wastewater have drawn attention increasingly.

Various techniques have been developed to remove cobalt from wastewater. For example, chemical precipitation is widely used for removing cobalt at relatively medium or high levels, but it is ineffective for low level solution. However, ion exchange, solvent extraction, membrane, and electrochemical are generally effective for the low level solution, but the high investment and operation cost, and secondary-pollution limited their industrial application (Evans et al., 2012; Fu & Wang, 2011; Manohar, Noeline, & Anirudhan, 2006). In recent years, adsorption is proved to be a promising process for the treatment of cobalt-containing wastewater, especially in trace concentrations, because of its low cost, extensive resources and non-secondary-pollution (Fu & Wang, 2011; Gupta, Carrott, Ribeiro Carrott, & Suhas, 2009). Chitosan (CTS), derived from deacetylation of chitin, is the second most abundant natural polymer after cellulose. Great efforts have been made on the application of chitosan in medicine,

food, chemical, cosmetic, water treatment, metal extraction and recovery, chemical, biological and biomedical engineering because of its special characteristics of biological functions and compatibility, security, biodegradability, non-toxicity, adsorption properties, etc. (Kumar, 2000). In particular, as an adsorbent to remove heavy metals from water, chitosan has attracted considerable interest due to the presence of amino and hydroxyl groups, which are the main active sites. However, chitosan is unstable and easily soluble in acidic solutions because of the protonation of amino groups; on the other hand, the adsorption capacity of chitosan for heavy metals is still insufficient (Guibal, 2004; Varma, Deshpande, & Kennedy, 2004). In order to enhance its stability and mechanical properties as well as the adsorption capacity, physical and chemical modifications of chitosan including dissolution of the biopolymer as gel beads or microcrystalline chitosan, grafting

and

crosslinking

(conventional

crosslinking

agents

contains

glutaraldehyde,

epichlorohydrin and ethyleneglycon diglycidyl ether) were conducted to increase the number of coordination atoms in chitosan molecule. Moreover, various materials including chloroacetic acid, ethylenediamine, ethylene diamine tetra acetic acid, diethylenetriamine, triethylene tetramine, etc., have been used to modify the cross-linked chitosan and improve the selective adsorption of metal ions (Zohuriaan-Mehr, 2005). At the same time, research on the chitosan and its derivatives used in the adsorption of cobalt from wastewater have drawn extensive interest. For example, Monier et al. (2010) synthesized cross-linked magnetic chitosan-isatin Schiff’s base resin and studied the adsorption performance of Cu(II), Co(II) and Ni(II), for which the adsorption capacities could achieve 103.16, 53.51 and 40.15 mg/g, respectively (Monier et al., 2010). Chang et al. (2006) investigated a magnetic chitosan nanoparticles prepared by the carboxymethylation of chitosan and followed by binding on the surface of Fe3O4 nanoparticles via carbodiimide activation. The adsorption capacity for Co(II) was obtained at 27.5mg/g (Chang, Chang, & Chen, 2006). Chen et al. (2012) reported that a novel xanthate-modified magnetic chitosan and its adsorption capacity for Co(II) was 18.5mg/g, much higher than that of the magnetic chitosan, 2.98mg/g (Chen & Wang, 2012). Repo et al. (2010) immobilized the ligands of ethylene diamine tetra acetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA) onto polymer matrices of chitosan to prepare adsorbents for adsorption of Co(II) and Ni(II). Metal uptake by EDTA-chitosan was 63.0mg/g for Co(II) and 71.0mg/g for Ni(II) and by DTPA-chitosan 49.1mg/g for Co(II) and 53.1mg/g for Ni(II)

(Repo et al., 2010). In view of adsorption performance of the different chitosan-based material for treatment of heavy metals from aqueous solution, there are still some shortcomings presented in the preparation and application of various chitosan adsorbents. On the one hand, crosslinking of the raw chitosan can improve its stability under acidic conditions, however, its adsorption capacity will be reduced as the active sites of the chitosan were occupied and consumed, moreover, some cross-linked chitosan even loses their adsorption ability. Therefore, modification of cross-linked chitosan for grafting new functional groups containing coordination atom N, S or O to enhance the adsorption capacity is necessary (Yu, He, & Gu, 2000). On the other hand, separation of adsorbent from solution after adsorption is also a vital point from the economic point and practical application of view. The traditional methods such as filtration, sedimentation and centrifugation have the disadvantage of diseconomy, consuming time and complex operation (Chen et al., 2012; Monier, 2012). Although magnetic separation technology is a kind of efficient, rapid way to easily remove the adsorbent from the reaction medium using an external magnetic field in recent years, it still is not an optimal method for development of practical application as it is also uneconomic and complex (Hritcu, Popa, Popa, Badescu, & Balan, 2009). At the same time, these studies were primarily focused on evaluating the improvement of adsorption capacities compared with the raw chitosan, however, the adsorption mechanism of the modified chitosan has not been revealed clearly. Besides, very few researches have been found on the adsorption of cobalt by chitosan modified with triethylene-tetramine. Therefore, a newly triethylene-tetramine derivative of chitosan was synthesized via cross-linking with epichlorohydrin in this work, which had good stability under acidic solution with high nitrogen element content and great separation performance after adsorption as spherical particles were used as adsorbent during the adsorption process. The differences of equilibriums, kinetics, and thermodynamics of Co2+ adsorption onto CTS and CCTS were studied. The effects of adsorption time, the initial pH value and temperature on adsorption of cobalt by CTS and CCTS were also investigated. In addition, regeneration performance of the CTS and CCTS was explored for five cycles, and fourier-transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to elucidate the mechanism of Co2+ adsorption on the CTS and CCTS.

2. Experimental

2.1 Materials Chitosan, with a deacetylation level of about 90%, was purchased from Kelong Chemical Co., Ltd (Chengdu, China). The reagents epichlorohydrin, triethylene tetramine, isopropanol alcohol, N, N-dimethylformamide, acetic acid, sodium hydroxide, and sulfuric acid were purchased from Kelong Chemical Co., Ltd (Chengdu, China). Absolute ethyl alcohol was supplied by Chengdu Changlian Chemical Reagent Co., Ltd. All chemicals used in this study were of analytical reagent grade. A cobalt stock solution (1000 mg/L) was prepared by adding CoSO4 to deionized water. The test solutions were prepared via subsequent dilution of the stock solution. 2.2 Methods 2.2.1 Synthesis of triethylene-tetramine-modified cross-linked chitosan 2.2.1.1 Preparation of chitosan beads (CTS) Chitosan (10 g) was dissolved in a 3 % (V/V) acetic acid (300 mL), and the mixture was stirred at room temperature until completely dissolved to obtain a transparent gel. Then the prepared mixture was dropped into 1 mol/L NaOH solution (500 mL) to form the chitosan beads after stirring for 4h and then washed with ethanol and distilled water to neutral. 2.2.1.2 Preparation of epoxy chitosan (E-CTS) The chitosan beads were dipped into 150 mL isopropanol alcohol at 40 ℃ with stirring for 0.5 h, and then 70 mL epichlorohydrin was added slowly. After that, the mixture was stirred for 10 h, and washed with ethanol and distilled water to remove the unreacted epichlorohydrin. 2.2.1.3 Preparation of triethylene-tetramine cross-linked chitosan (CCTS) The epoxy chitosan was dipped into 150 mL N,N-dimethylformamide at 50 ℃ with stirring for 30 min, and then 50 mL triethylene tetramine was slowly dropped into the solution. The mixture was stirred for 4 h and washed with ethanol and distilled water to remove the unreacted triethylene tetramine. The synthesis route of CCTS was shown in Fig. S1. 2.2.2 Adsorption To investigate the ability of the CTS and its derivative adsorbent to remove cobalt from aqueous solutions, batch experiments were conducted by reacting cobalt 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. In the tests involving the pH effect, the initial pH value of the solution was adjusted to 1.0-6.0 with either diluted NaOH or H2SO4 and the tests were performed at the initial cobalt concentration of 200 mg/L at 30 ℃. The adsorption kinetic experiments were conducted by varying the initial cobalt concentration (20, 50 and 200 mg/L) at 30 ℃ and the samples were taken at different time intervals (10, 30, 60, 120, 360, 720, 1440 min). Adsorption isotherm studies were performed at the initial cobalt concentrations of 5, 10, 20, 50, 100, 150 and 200 mg/L at 30, 45 and 60 ℃. 2.2.3 Desorption and regeneration studies For the desorption and reuse study, the spent CCTS adsorbent was added into the 30 mL 1 mol/L H2SO4 and shaken at 30℃ for 24 h. Then the CCTS was filtered and dipped into 30 mL 1 mol/L NaOH for 6 h to activate the functional groups. Finally, the CCTS was washed with distilled water to neutral. Adsorption test was carried out with regenerative adsorbent. The above process was repeated for five times. 2.3 Analysis The cobalt concentration was determined by an ICP-MS (NEXION 300X, PE) equipped with an auto sampler (SC2 DX, ESI). The surface area of chitosan and its derivative was measured by N2 adsorption isotherms at 77K by an SSA 4300 pore size and specific surface area analyzer (Beijing Builder Electronics Co., Ltd, China). The samples were heated at 373K for 2h in vacuum for degassing before measuring the isotherm. It was found that the surface area of the chitosan and its derivate were 1.8 and 1.2 m2/g, respectively. The pore volumes were 0.0017 and 0.0012 cm3/g, respectively. The main elements of chitosan and its derivative were analyzed by EA 3000 elemental analyzer (LEEMAN LABS INC., USA). The surface morphology of the chitosan and triethylene-tetramine grafted chitosan was observed by scanning electron microscopy (SEM) with a JSM-7500F scanning electron microscope (JEOL, Japan). The Fourier-Transform Infrared Spectroscopy (FTIR) was recorded on a spectrometer (FTIR 6700, Nicolet, USA) over the wavenumber range 400–4000 cm-1 at a solution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) was applied to determine the surface chemical composition, using a XSAM-800 spectrometer (KRATOS, UK) with Al (1486.6 eV) under ultra-high vacuum (UHV) at 12 kV and 15 mA. The removal efficiency of Co2+ and adsorption capacity of the adsorbent were separately

calculated by the following equations: η=

𝐶0 −𝐶 𝐶0

Q=

× 100%

(𝐶0 −𝐶)𝑉 𝑚

(1) (2)

whereη is the removal efficiency of Co2+, %; Q is the cobalt adsorption capacity per unit weight of adsorbent, mg/g; C0 and C are the initial and equilibrium cobalt concentrations, respectively, mg/L; V is the volume of aqueous solution, L; and m is the mass of adsorbent used, g.

3. Results and discussion 3.1 Influence of operating parameters on adsorption 3.1.1 Effect of contact time The effect of contact time on the adsorption at different initial concentrations of Co2+ was studied and the results were shown in Fig. S2. It was seen that the amounts of adsorption increased with increase of contact time and adsorption equilibrium was nearly obtained when the contact time was at 2 h. It was also found that the adsorption capacity of CTS and CCTS increased along with the increase of initial concentration of Co2+ and the uptake capacity of CCTS was always much higher than these of CTS at the same condition, which indicated that the introduction of triethylene-triamine into CTS may enhance its adsorption capacity for Co2+ removal. 3.1.2 Effect of initial pH To the best of our knowledge, the pH of aqueous solution is considered as an important parameter as it strongly affects the adsorption property for heavy metals through reacting with the active functional groups of adsorbent (Juang & Shao, 2002). Fig. 1 showed the effect of pH on the adsorption process of Co2+ by CTS and CCTS. It was observed that the adsorption capacity increased with the increase of pH from 1 to 4.5, and the maximum adsorption capacities were 30.45 and 59.51 mg/g for CTS and CCTS at pH 4.5, respectively. The adsorption capacity of the CTS and CCTS for Co2+ in this study were compared with other reported chitosan-based materials as summarized in Tab. 1. From the Tab. 1, it turned out that the prepared adsorbent in this work showed good adsorption performance for Co2+ removal as its adsorption capacity was higher than most of the other adsorbent mentioned. Based on the previous research, the protonation in acid solution and chelation with heavy

metals by the amino on the adsorbent existed in the adsorption process and the reaction was expressed as (Juang & Shao, 2002; Kuang, Wang, Liu, & Wu, 2013): M 2+ + 𝑛𝑅𝑁𝐻2 → 𝑀(𝑅𝑁𝐻2 )𝑛 2+ H+ + 𝑅𝑁𝐻2 → 𝑅𝑁𝐻3 +

(3) (4)

When the pH of the solution was less than 4.5, the uptake capacity of both the CTS and CCTS decreased rapidly, because the protonation of active sites was the major reaction so that the ability of interaction with Co was inhibited. With the increase of pH value, the removal efficiency increased. However, when the pH increased to about 6.0, the removal efficiency dropped a little bit, because the increasing of pH may make small amount of chitosan derivative chains become -O- so as to reduce the uptake capacity of the adsorbents (Juang & Shao, 2002).

3.1.3 Effect of temperature In order to investigate the effects of reaction temperature on the adsorption of Co2+ onto CTS and CCTS as well as adsorption thermodynamics for the adsorption, experiments were carried out at different temperature. Fig. 2 showed the effects of temperature on the adsorption process of CTS and CCTS and the results indicated that the adsorption capacity of CTS for Co removal increased with the increase of temperature, however, the adsorption capacity of CCTS decreased with the increase of temperature. It was also found that all the adsorption capacities of CCTS at different temperature were higher than these of CTS at the same condition. Determination of the Gibbs energy change is an important method to explore whether the adsorption is spontaneous or not, and a higher negative value of the energy change represents a more energetically favorable adsorption. Based on thermodynamic law, the Gibbs energy change (ΔG ) can be calculated by the following equation (Liu, 2009): ∆G = −RT ln 𝐾𝑎

(5)

in which Ka is the thermodynamic equilibrium constant; T is the absolute temperature in kelvins; R is the gas constant with a value of 8.314 J·mol-1·K-1. We also know that the relationship of Gibbs energy change between enthalpy (ΔH) and entropy change (ΔS) of adsorption can be expressed as: ∆G = ∆H − T∆S

(6)

Combining Eq. (5) and (6) reveals the following equation: ∆𝐻

ln 𝐾𝑎 = − 𝑇𝑅 +

∆𝑆 𝑅

(7)

In the study of adsorption process, the determination of thermodynamic equilibrium constant still remain uncertain, as it is related to the characteristics of the adsorbate and the adsorption mechanism. In this work, according to Van’t Hoff equation (Kuang et al., 2013), 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 Co onto CTS and CCTS. The results were displayed in Fig. S3 and Table S1. 𝑄

∆𝐻

ln( 𝐶𝑒) = − 𝑇𝑅 + 𝑒

∆𝑆 𝑅

(8)

As shown in Table S1, in the case of CCTS, the negative value of ΔH and ΔS indicated that the adsorption process was exothermic accompanied by a lowering in the entropy, which was a common observation in the most of the metal ions adsorption (Tang, Zhang, Guo, & Zhou, 2007; Yi, Wang, & Ye, 2006). For CTS, the positive value of theΔH revealed that the sorption was endothermic, and it was consistent with the increasing sorption as the temperature increases. This may be due to that Co(II) was dissolved well in water and the hydration sheath of Co(II) had to be destroyed before its sorption on the CTS. The energy for this dehydration process was higher than that of the exothermicity of cations to attach to the solid CTS surface (Liu, Chen, Hu, Wu, & Wang, 2011; Sheng et al., 2010). So higher temperature would be beneficial for the adsorption of Co onto the CTS. 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 with increasing temperature stated that the adsorption reaction became more favorable at higher temperature, on the other hand, higher temperature would hinder the adsorption of Co2+ by CCTS. 3.2 Adsorption kinetics In adsorption process, the kinetics of the adsorption is defined as the solute removal rate, which controls the residence time of the sorbate on the surface of the solid liquid interface. In particular, the residence time of the adsorption process is considered as an important parameter to determine whether the adsorption process reach equilibrium and to evaluate the adsorption quantity of adsorption process (Febrianto et al., 2009). In order to investigate the adsorption mechanism and potential rate-controlling steps including mass transport and chemical reaction

process, kinetic models such as pseudo-first-order, pseudo-second-order kinetic models and intra-particle diffusion model are widely used to fit the experimental data. Pseudo first-order and second-order kinetic models based on adsorption capacity of the solid-liquid phase are generally expressed as follows (Febrianto et al., 2009): ln(Q𝑒 − Q𝑡 ) = ln Q 𝑒 − K1 t 1

1

Q𝑒 −Q𝑡

= 𝑄 + K2 t 𝑒

(9) (10)

Also, the linear form of Eq. (9) and (10) can be stated by: 1 𝑄𝑡 𝑡 𝑄𝑡

𝐾

1

= 𝑄 1𝑡 + 𝑄 𝑒

=

𝑡 𝑄𝑒

𝑒

+

1 𝐾2 𝑄𝑒 2

(11) (12)

where Qt and Qe are the uptake capacities of adsorbent at time t and equilibrium, respectively, mg/g; K1 and K2 are the equilibrium rate constant of the pseudo first-order and second-order kinetics, respectively, (1/h) and (g/mg h); t is the contact time of adsorption process, h. Furthermore, the model of intra-particle diffusion is also used to evaluate the rate-controlling step, which can be expressed as: 𝑄𝑡 = 𝐾3 √𝑡 + 𝐶

(13)

where K3 is the rate constant of the intra-particle diffusion, mg/g. h0.5. As shown in Fig. S2, adsorption process was conducted at different initial concentrations for removal of Co with CTS and CCTS as a function of contact time. The values of the corresponding kinetic parameters of different models obtained by calculating the slope and intercept of Eq. (11) to (13) by plotting of 1/Qt versus 1/t, t/Qt versus t and Qt versus t0.5, respectively and the results were shown in Table S2. The results demonstrated that pseudo second-order model was more suitable for fitting the experimental data than the pseudo first-order model for both CTS and CCTS based on the correlation coefficients, though their calculated equilibrium adsorption capacities (Qe) were close to the experimental values. This suggested that chemical reaction was the rate-limiting step in the whole adsorption process (Kuang et al., 2013; Tang et al., 2007). In addition, correlation coefficients of intra-particle diffusion for CTS and CCTS were relatively low, indicating that intra-particle diffusion was not the rate-controlling step. The results for the adsorption kinetics study of CTS and CCTS for Co2+ adsorption demonstrated that the uptake of Co2+ by the

adsorbent was chemical adsorption. The adsorption mechanism will be discussed in the later section. 3.3 Adsorption isotherms With the purpose of exploring the interactive behavior of the solution and adsorbent, adsorption isotherms like Langmuir and Freundlich are widely applied in the heavy metals adsorption to correlate adsorption equilibrium. In this work, Langmuir, Freundlich and Sips isotherm models were chosen to determine the adsorption equilibrium and further to explain the adsorption mechanism of Co2+ onto CTS and CCTS. Their non-linear form equations are expressed as follows (Febrianto et al., 2009; Gerente, Lee, Cloirec, & McKay, 2007): 𝑄𝑒 = 𝑄𝑚𝑎𝑥

𝐾𝐿 𝐶𝑒 1+𝐾𝐿 𝐶𝑒

𝑄𝑒 = 𝐾𝐹 𝐶𝑒 1/𝑛

(14) (15)

(𝐾 𝐶 )𝛾

𝑠 𝑒 𝑄𝑒 = 𝑄𝑚𝑎𝑥 1+(𝐾 𝐶

𝛾 𝑠 𝑒)

(16)

where Qe and Qmax are adsorption capacity at equilibrium and the maximal adsorption capacity for monolayer adsorption, respectively, mg/g; KL, KF and KS are Langmuir, Freundlich and Sips constant, respectively, L/mg, mg/g and L/mg; Ce is equilibrium concentration of Co2+, mg/L; γis heterogeneous characteristic parameter for Sips. In order to test the experimental data better, linear forms of Langmuir and Freundlich are usually used for fitting the experimental data, which are represented by the following equations: 𝐶𝑒 𝑄𝑒

=𝑄

𝐶𝑒

𝑚𝑎𝑥

+𝐾

1

𝐿 𝑄𝑚𝑎𝑥

1

log 𝑄𝑒 = log 𝐾𝐹 + 𝑛 log 𝐶𝑒

(17) (18)

The results of fitting plot of Langmuir, Freundlich and Sips were shown in Table S3. For CTS, the Freundlich and Sips isotherm models appeared to be the better fitting models for the experimental adsorption isotherm data, which was also reflected by higher correlation coefficient than the Langmuir isotherm model, indicating that the surface structure of the CTS was heterogeneous and multilayer adsorption occurred (Febrianto et al., 2009). In terms of CCTS, Langmuir isotherm model was more suitable for describing the experimental date with higher correlation coefficient than that of Freundlich and Sips isotherm models, demonstrating that monolayer adsorption took place on homogeneous surface (Tang et al., 2007). In Langmuir

isotherm model, the decrease of KL value with temperature rise signifies the exothermicity of the adsorption process, indicating the adsorption process is physical adsorption, however, chemical adsorption has the opposite trend (Febrianto et al., 2009; Padmavathy, 2008). Therefore, the decrease of KL value in Tab. S3 revealed that adsorption of Co2+ by CCTS may also be along with physical adsorption. That was to say, electrostatic attraction and hydrogen bonding may also occur during the adsorption process. 3.4 Desorption and re-adsorption performance Desorption and re-adsorption performance is an important index for adsorbent as it directly affects the adsorption efficiency, adsorption capacity and investment cost used in the treatment of wastewater. The results of adsorption performance after 5 cycles of adsorption-desorption with CCTS for removal of Co2+ in Fig. 3 showed that the adsorbent still remained high adsorption capacity through 5 cycles of regeneration, indicating that the adsorbent was a promising material with high stability and reusability applied in the removal of cobalt-containing effluent. 3.5 Adsorption mechanism 3.5.1 Elemental analysis The elemental analysis results of the chitosan and its derivative were shown in Tab. S4. It could be seen obviously that the content of nitrogen of CCTS was much higher than that of CTS, which indicated that the introduction of the amino group of triethylene-triamine was successfully cross-linked with epichlorohydrin on chitosan. The decrease of mass ratio of carbon and nitrogen from 5.47 to 3.29 also showed that triethylene-tetramine had been grafted on chitosan successfully. 3.5.2 SEM analysis The SEM images of CTS and its derivatives were shown in Fig. 4. From the Fig. 4A, the surface of the CTS was porous structure, which would provide more adsorption sites on the surface and inner of the material and may enhance the adsorption capacity. Cross-linking of epichlorohydrin improved the stability of the CTS, but plugged some pore of the CTS, which was bad for the adsorbate to pass the channel of the surface of the CTS. The chitosan grafted with triethylene-triamine in Fig. 4B displayed an extensive three-dimensional network with less porous structure and smaller pore diameter, which was mainly due to the grafting of triethylene-triamine

on the surface of CTS (Kuang et al., 2013). The results agreed with the BET analysis of the chitosan and its derivative provided in the section 2.3. Compared with elemental analysis and the SEM images of CTS and CCTS, it was found that introduction of triethylene-triamine increased the nitrogen content of chitosan derivative, which may provide more functional groups so as to improve adsorption capacity, however, covered with triethylene-triamine 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.

3.5.3 Infrared spectra analysis Fig. 5 showed the infrared spectra analysis results of chitosan and its derivatives. It was shown that all the IR spectra of the CTS, E-CTS and CCTS were similar and the characteristic peak of 894 cm-1 which corresponded to paranoid ring remained unchanged, indicating that the preparation process of CCTS did not break up the paranoid ring of the CTS (Shigemasa, Matsuura, Sashiwa, & Saimoto, 1996). From the spectra of CTS, the peak at 3430 cm-1 corresponded to stretching vibration of N-H and O-H bonds, and the peak at 2919 and 2872 were assigned to the symmetric and asymmetric stretching vibration of the C-H bond. The peaks at 1655 and 1604 cm-1 were attributed to stretching vibration of C=O and bending vibration of N-H, respectively (Kuang et al., 2013; Tang et al., 2007). For the IR of E-CTS, the peak at 1604 cm-1 disappeared and a new peak appeared at 739 cm-1 which was assigned to stretching vibration of C-Cl. This demonstrated successful epoxy open-loop crosslinking reaction on the amine functional group of CTS. From comparison of IR spectra of E-CTS 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 in correspondence to N-H bond, revealing that the triethylene-triamine was successfully grafted on the chitosan (Yi et al. 2006). The FTIR spectra of the CTS before and after Co adsorption indicated that the wavenumbers at 3430, 1655 and 1604 cm-1 assigned to the N-H stretching vibration, in-plane bending vibration and out-plane wagging vibration, respectively, moved to 3394, 1652 and 1599 cm-1. This suggested the formation of N-Co bond in the adsorption process. Moreover, the wavenumbers at 1156 cm-1 corresponding to O-H stretching vibration (C3-OH) changed to 1154 cm-1, showing that the oxygen atoms in the hydroxyls may also be involved in the adsorption reaction. Another O-H

stretching vibration (C6-OH) at 1031 cm-1 wavenumber did not change. The results of the FTIR spectra of the CTS for Co2+ adsorption indicated that both the amino and hydroxyl groups were involved in the adsorption of Co2+, which was consistent with the results by Guan and Cheng (Guan & Cheng, 2004). Comparison of the FTIR spectra of CCTS before and after Co adsorption from Fig. 5, it was seen that significant changes in the FTIR spectra were found at the wavenumbers of 3427, 1655, 1580, and 660 cm-1 after adsorption, which were assigned to the N-H stretching vibration, in-plane bending vibration and out-plane wagging vibration, respectively. The N-H stretching vibration wavenumber at 3427 cm-1 before adsorption was shifted to 3395 cm-1 after adsorption, indicating that the Co2+ may coordinate with the nitrogen atom on the CCTS. This was due to the formation of N-Co bond, which reduced the vibration intensity of N-H bond. Moreover, the decrease of the wavenumbers at 1655, 1580, and 660 cm-1 after adsorption also suggested that the attachment of Co2+ onto nitrogen atom took place during the adsorption process (Jin & Bai, 2002; Li & Bai, 2005). At the same time, the wavenumber at 1153 cm-1 assigned to the O-H stretching vibration (C3-OH) changed to 1114 cm-1, indicating that the oxygen atoms in the hydroxyls may also be involved in the adsorption reaction. Another wavenumber at 1031 cm-1 assigned to the O-H stretching vibration (C6-OH) did not change before and after adsorption, suggesting that this oxygen atom group (C6-OH) did not adsorb Co2+ in the adsorption process (Jin & Bai, 2002). 3.5.4 XPS study To further verify the findings from the FTIR spectra, XPS analysis was employed. Fig. S4-S5 and Tab. 2 showed the results of the fit assumptions of O 1s, N 1s and Co 2p spectra of the CTS and CCTS before and after Co2+ adsorption. According to the results from Guan and Cheng (2004), the binding energy of the Co 2p at 797.2 and 780.7 eV for CTS shifted to lower energy region. These downward shifts may be explained as the increase in the electronic density around cobalt(II) resulting from the electron receiving from the O and N atoms in the chitosan units (Guan & Cheng, 2004). The binding energy of O 1s and N 1s at 532.1 and 398.9 eV changed to 532.4 and 399.6 eV, respectively. This indicated that the Co(II)–O and Co(II)–N coordinate bonds were formed. The XPS spectra of O 1s for CCTS before adsorption of Co2+ at peaks of 531.4 and 532.7 eV were assigned to C-OH and bond water, respectively (LoáJacono, 1992). After Co2+ adsorption, the peak at 531.4 changed to 531.9 eV and a new peak at 530.6 eV was observed, which was

assigned to Co-O (Guan & Cheng, 2004; LoáJacono, 1992). Furthermore, comparison of the N 1s XPS data of a and b showed that there were peaks at 397.9 and 399.0 eV before Co2+ adsorption, which were assigned to C-N or -NH2. After CCTS adsorbed Co2+, however, the peaks were moved to 398.6 and 399.5 eV, respectively. Similar behavior was reported by Li and Bai (2005) and Chen and Wang (2012). This indicated that some N atoms existed in a more oxidized state due to Co adsorption, in which a lone pair of electrons in the nitrogen atom was donated to the shared bond between the N and Co2+. Finally, it was attributed to the formation of complexes between Co and –NH2. So the XPS spectra provided evidence of Co2+ binding to nitrogen atoms and oxygen atoms, which is in good agreement with the FTIR results. XPS spectra of Co 2p after Co2+ adsorption by CCTS was shown in Fig. S5. The appearance of peaks for Co 2p3/2 at 780.3, 783.4 eV and peak for Co 2p1/2 at 795.4, 801.6 eV can be observed. The two peaks for Co 2p3/2 observed after Co2+ adsorption had quite a large shift towards to the lower energy region, compared to the XPS data of CoSO4 by Seleţchi (Seleţchi, Negrilǎ, Duliu, & Ţurcaş, 2007), whose binding energies was 784 eV. This was due to the increase in the electronic density around Co2+ resulting from the electron receiving from the other atoms in the modified CCTS units, which provided evidence to the chelation between CCTS and Co2+ ions during the adsorption process as well.

3.5.5 Adsorption mechanism In the adsorption process of Co2+ uptake by CTS, adsorption kinetic study showed that the adsorption rate was controlled by chemical reaction. That was to say, the adsorption process was dependent on the number of available adsorption sites on the adsorbent surface, and was eventually controlled by the binding of Co2+ to the surface or internal of the adsorbent. Moreover, adsorption isotherm of the CTS fitted well by Freundlich model revealed that the surface structure of the CTS was heterogeneous and multilayer adsorption took place on the surface and internal of the adsorbent. Finally, the main functional groups such as amino and hydroxyl on the CTS involved in the coordination of Co2+ were identified. Therefore, the Co2+ adsorption by CTS was a chemical adsorption with both amino and hydroxyl groups participated in the multilayer adsorption on the surface and internal of the adsorbent. The possible adsorption mechanism was proposed in Fig. 6.

However, on the basis of the results obtained for Co2+ adsorption by CCTS, the adsorption mechanism was different. Monolayer adsorption occurred on the surface of the CCTS according to the Langmuir model fitting of the experiment data and the decrease of the KL value indicated physical adsorption may also take place on the surface of the adsorbent. The kinetic study and XPS and FTIR analysis demonstrated that the adsorption process was controlled by chemical reaction involved in the chelation of amino and hydroxyl functional groups. Therefore, the possible adsorption mechanism of Co2+ uptake by CCTS was postulated in Fig. 6 (Cojocaru, Zakrzewska-Trznadel, & Jaworska, 2009; Guibal, 2004; Hu et al., 2010; Nair, Panigrahy, & Vinu, 2014; Negm, Sheikh, El-Farargy, Hefni, & Bekhit, 2014; Rangel-Mendez, Monroy-Zepeda, Leyva-Ramos, Diaz-Flores, & Shirai, 2009; Varma et al., 2004). Co2+ adsorption on the CTS shown in Fig. 6a indicated that both the amino and hydroxyl chelated with Co2+. However, in Fig. 6b for CCTS adsorbing Co2+, physical, chemical, and electrostatic adsorption occurred based on microstructure analysis and adsorption kinetics and isotherms. Chemical adsorption was the main adsorption method among these mechanisms. (Guibal, 2004; Hu et al., 2010).

4. Conclusions In

this

work,

a

chitosan

derivative

was

produced

with

epichlorohydrin

and

triethylene-tetramine. The results of effect of different condition parameters on the adsorption process showed that pH greatly affected the adsorption capacity and adsorption equilibrium was obtained at about 2 h. Langmuir and Freudlich isotherm models were more suitable for describing the adsorption process of CCTS and CTS, respectively, which showed the difference of adsorption mechanism for CTS and CCTS for removal of Co2+ from aqueous solution. Adsorption kinetics showed that the adsorption process of both the CTS and CCTS were better fitted by pseudo second-order kinetics model, indicating that the chemical adsorption was the rate-limiting step rather than mass transport. The adsorbent still exhibited good adsorption performance after regeneration for five times. Elemental analysis and FTIR spectra analysis were used to determine the structure and characteristics of chitosan and its derivative, which were shown in agreement with expected results. The FTIR and X-ray photoelectron spectroscopy (XPS) analysis revealed that the amino and hydroxyl groups in CTS were the main functional groups in the chemical

adsorption of Co2+, however, the adsorption of Co2+ by CCTS was a chemical adsorption process along with physical adsorption. The results suggest that CCTS is a good candidate as an adsorbent for the removal of Co2+ from aqueous solutions.

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.

References

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80 60 50

60

40

50

30

40

20

30 Adsorption capacity of CTS Adsorption capacity of CCTS Removal percentage of CTS Removal percentage of CCTS

10

20

Removal percentage (%)

Adsorption capacity (mg/g)

70

10

0 0 1

2

3

4

5

6

7

pH Fig. 1 Effect of initial pH on the removal percentage and adsorption capacity of CTS and CCTS (initial concentration 200 mg/L, adsorbent dose 2 g/L, contact time 24 h, temperature 303 K). 22

Adsorption capacity (mg/g)

20

18

CTS CCTS

16

14

12

10 30

40

50

60

70

Temperature (℃) Fig. 2 Effect of reaction temperature on the adsorption capacity of CTS and CCTS (pH 5.85, initial concentration 50 mg/L, adsorbent dose 2 g/L, contact time 24 h).

80

Adsorption capacity (mg/g)

70 60 50 40 30 20 10 0 0

1

2

3

4

5

Reusable times Fig. 3 Effect of reusable times on the adsorption capacity of adsorption of Co2+ (pH 5.85, initial concentration 200 mg/L, adsorbent dose 2 g/L, contact time 24 h, temperature 303 K).

Fig. 4 The SEM images of CTS (A) and CCTS (B). Co-CCTS 3395

2919

2853

1635

1459 11141062 1031

617

Co-CTS 2921

2876

1652 1599 1379

897

1154 1075 1031

3394

CCTS 3427

2921

2855

1655 1580

E-CTS

813 660 1153 1031 14671379 1066 739

CTS 1655 1604 3430

4000

3500

2919 2872

3000

2500

2000

Wavenumber/cm

1500

1156

894 1031

1000

500

-1

Fig. 5 Infrared spectra analysis of CTS, E-CTS, CCTS and Co-CCTS.

Fig. 6 Possible mechanisms for Co2+ coordination onto CTS (a) and CCTS (b).

Table 1. Adsorption performance for the adsorption of Co2+ onto different chitosan-based adsorbents. Adsorbent

pH

Isotherm

Kinetics

Adsorption

Ref.

capacity (mg/g) EDTA/ DTPA-chitosan

2.1

Pseudo-second-order

Langmuir

63.0/49.1

Repo et al., 2013

chitosan-modified

6.0

Pseudo-second-order

Langmuir

220

Shaker, 2015

6.0

Pseudo-second-order

Both

10.6

Gupta, Kushwaha,

poly(methacrylate) nanoparticles hydroxyapatite/chitosan composite

Langmuir and

&

Freundlich

Chattopadhyaya, 2012

xanthate-modified

5.0

Pseudo-second-order

Langmuir

18.5

Chen & Wang,

magnetic chitosan

2012

chitosan–montmorillonite

-

Pseudo-second-order

Temkin

150

wang et al., 2014

Chitosan–chloroacetic

7.0

Pseudo-second-order

Langmuir

59.1

Negm et al., 2015

5.5

Pseudo-second-order

Langmuir

27.5

Chang, Chang, &

acid magnetic chitosan nanoparticles

Chen, 2006

cross-linked magnetic

6.0

Pseudo-second-order

Langmuir

60

Monier, Ayad,

chitosan-isatin Schiff’s

Wei, & Sarhan,

base resin

2010

triethylene-tetramine

4.5

Pseudo-second-order

Langmuir

59.51

This work

4.5

Pseudo-second-order

Freundlich

30.45

This work

modified chitosan chitosan -: not mentioned.

Table 2. Binding energies (BE) of O 1s, N 1s, Co 2p1/2 and Co 2p3/2 obtained from the XPS spectra of CTS and CCTS before (a) and after (b) Co2+ adsorption. Binding energies (BE)

Samples O 1s a

532.1

b

532.4

a

531.4

b

531.9

N 1s

Co 2p1/2

Co 2p3/2

398.9

CTS 399.6

398.8

797.2

532.7

397.9

399.0

-

530.6

398.6

399.5

795.4

801.4

780.7

785.6

-

-

780.3

783.6

CCTS 801.6