Article pubs.acs.org/IECR
Preparation and Application of a Xanthate-Modified Thiourea Chitosan Sponge for the Removal of Pb(II) from Aqueous Solutions Nana Wang,†,‡ Xingjian Xu,† Haiyan Li,† Jiali Zhai,† Lizhu Yuan,†,‡ Kexin Zhang,† and Hongwen Yu*,† †
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 4888 Shengbei Rd, Changchun 130102, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: As chitosan has low uptake for Pb(II) and high solubility in monoprotic acids, a simple procedure using nontoxic chitosan as a precursor at room temperature was developed to prepare a xanthate-modified thiourea chitosan sponge (XTCS) by lyophilization for enhancing Pb(II) removal in water. The sponge structure of XTCS improved both stability and separation performance. Adsorption results showed that pH markedly affected the adsorption performance of XTCS. The adsorption process attained equilibrium in 45 min with a maximum adsorption capacity of 188.04 mg g−1 at room temperature. The pseudosecond-order kinetic equation and Langmuir isotherm model were suitable to describe the adsorption behavior of XTCS. The physicochemical characteristics of XTCS were investigated by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results indicated that Pb(II) removal mechanism of XTCS was mainly dependent on complexations of sulfur and nitrogen atoms and ion exchange.
1. INTRODUCTION Lead (Pb) is widely found in nature, but is recognized as being one of the most hazardous and carcinogenic elements. Because of its nonbiodegradability and bioaccumulation, increasing attention has been paid to the pollution of water with Pb(II) as a result of continuous industrial activity. The potential sources of Pb(II) in wastewater include lead-acid batteries, electronic manufacturing, pigments, fertilizers, lead smelting, and mine tailings. 1 Long-term exposure to Pb(II) can lead to dysfunctions of the reproductive system, blood system, nervous system, urinary system, and immune system, especially in children.2 Hence, the U.S. Environmental Protection Agency (EPA) has classified Pb(II) as a priority issue.3 Numerous physicochemical technologies and processes have been developed for Pb(II) removal in order to meet water pollution discharge standards or to achieve an acceptable concentration level. These methods include electrocoagulation, chemical precipitation, membrane filtration, ion exchange, and adsorption.4−7 However, among these strategies, adsorption techniques have been extensively investigated for removing Pb(II) from effluents due to their high efficiency, convenient © 2016 American Chemical Society
operation, low cost, and simple postprocessing. From the perspective of long-term development, it is necessary to develop or improve an adsorbent material that is efficient, eco-friendly, and sustainable. Chitosan, a natural polymer, is biocompatible, biodegradable and has excellent metal-binding capabilities. It possesses a high ratio of hydroxyl group and amine group, making it conductive to the introduction of new functional groups.8,9 Nevertheless, the poor stability and insufficient adsorption capacity of chitosan in acidic solutions limit its further application in industrial effluent treatment. To enhance its stability and improve its adsorption capacity, chemical modification of chitosan is necessary. According to HSAB (hard and soft acids and bases) theory used for the prediction of complexation reactions,10 sulfur serving as a soft ligand group has a very strong affinity for a large number of heavy metals which are Received: Revised: Accepted: Published: 4960
February April 11, April 14, April 14,
20, 2016 2016 2016 2016 DOI: 10.1021/acs.iecr.6b00694 Ind. Eng. Chem. Res. 2016, 55, 4960−4968
Article
Industrial & Engineering Chemistry Research Scheme 1. Synthesis of XTCS
2.2. Methods. 2.2.1. Preparation of XTCS. CTS was first turned into thiourea chitosan and then chemically modified as xanthate chitosan.14,17 The synthesis of XTCS is expressed in Scheme 1. In this process, about 16.1 g of CTS and 15.2 g of NH4SCN were soaked in 150 mL of ethanol in a 250 mL round-bottomed flask. It was then refluxed at 95 °C in an oil bath for 12 h with continuous magnetic stirring. After cooling down to room temperature, the obtained solid product was filtered, washed several times with ethanol, and dried in an oven at 50 °C. This product was thiourea chitosan (TCTS). In the second step, 1.0 g of TCTS was mixed with 50 mL of 14% NaOH solution and 2 mL of CS2. The reactants were stirred at room temperature for 24 h. The precipitate was collected by centrifugation and washed thoroughly with ultrapure water until its pH was close to neutral. After suspension in ultrapure water, the modified material was freeze-dried for 48 h to obtain a spongy product, namely XTCS. This product served as the adsorbent in further experiments. 2.2.2. Physicochemical Characteristics of XTCS. The physicochemical properties of XTCS were characterized by a series of performance tests. A Fourier transform infrared spectra (FTIR) analysis was recorded with a PerkinElmer spectrometer (L1600400 spectrum Two DTGS, America) using KBr pellets in the range of 500−4000 cm−1 to identify the functional groups on CTS, TCTS, and XTCS. An examination of the surface morphology was performed using an XL30 ESEM FEG scanning electron microscope (The Netherlands) equipped with an energy dispersive X-ray analyzer (EDX). X-ray photoelectron spectra (XPS) of XTCS before and after adsorption were acquired using a Thermo VG Scientific Escalab 250 Spectrometer with monochromatized Al Ka excitation (UK). The crystalline phases were examined by X-ray diffraction (XRD) using a Rigaku D/Max-2550 diffractometer (Japan) under Cu Kα radiation (λ = 1.540 56 Å) at 50 kV and 200 mA in the 2θ range 10−80° with a scanning rate of 5° min−1. 2.2.3. Adsorption Equilibrium Experiments. The adsorption behavior of Pb(II) on XTCS was investigated though batch adsorption studies. The batch experiments were performed using a thermostate water bath rocking shaker (SPH-110×12, China) at 25 °C and 180 rpm. For adsorption kinetics, though the preliminary experiment, 4 h was used as the upper adsorption time limit, as this was sufficient for reaching adsorption equilibrium. Herein, 10 mg of XTCS was added into 30 mL of 100 mg L−1 Pb(II) solution. To investigate the adsorption isotherms, 10 mg of XTCS was mixed with 30 mL of different initial concentrations of Pb(II) solution, in the range of 50−500 mg L−1, for 45 min. As the precipitation of Pb(II) occurred at pH ≥ 6, the pH levels were varied from 1 to 5.5. The pH of each solution was adjusted by using 0.1 M HNO3 and 0.1 M NaOH. The effect of temperature on Pb(II) adsorption was studied with 10 mg of XTCS and 30 mL of 100
considered to be soft acids and can form a stable metal−sulfur complex. Adsorbents with introduced sulfur atoms in their functional groups are endowed a dual structure, namely both chelating and ion exchange capability.11 Among various sulfurbearing compounds, xanthates are relatively more favorable because of their easy operation, high insolubility, and strong stability with metal ions. In a xanthation reaction, the hydroxyl group of the polymer undergoes an esterification reaction with CS2, under caustic condition, to form the corresponding xanthate sodium salt.12 Thus, chitosan serves as a potential substrate for the synthesis of xanthate in this study. Some prior researches have focused on the application of chitosan xanthates for removing metal from an aqueous media.11,13−16 In those studies, chitosan was universally cross-linked at first, and then it was chemically modified as xanthate chitosan. These studies have reported that the adsorbent materials are made into different shapes such as powder and beads but the porous spongy-like structure is seldom reported. Although chemical cross-linking can enhance the stability of adsorbent material, parts of amino group are consumed and occupied, which reduces the adsorption capacity. In addition, the separability and mechanical properties require further improvement. To overcome the above shortcomings, this work used a simple procedure to synthesize successfully a novel xanthatemodified thiourea chitosan sponge with high sulfur and nitrogen element content. The as-prepared adsorbent obtains a new chelation group and maintains a free amino group. The macroporosity of xanthate-modified thiourea chitosan sponge is unique compared with chitosan and xanthate-modified chitosan reported. This as-prepared spongy adsorbent has abundant macropores and the dimension and shape of the adsorbent are tunable, which grant it two advantages. One is convenient for separation implying great potential in practical application. Moreover, its porous appearance benefits the mass transfer process and improves the adsorption efficiency. This study conducted a physicochemical characterization of the synthesized material and evaluated its applicability in Pb(II) removal in terms of adsorption conditions, models, and mechanism.
2. EXPERIMENTAL SECTION 2.1. Materials. Chitosan (CTS) with a deacetylation degree of 90% was obtained from Zhejiang Golden-shell Pharmaceutical Co. Ltd. (Zhejiang, China) and used without any further purification. Pb(NO3)2 used as a source of Pb(II) and CS2 were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). NH4SCN was purchased from Tianjin Guangfu Technology Development Co. Ltd. (Tianjin, China). Other chemicals used (unless specifically noted) were of analytical reagent grade (AR grade) from Beijing Chemical Works or Sinopharm Chemical Reagent Co. Ltd. Ultrapure water was employed to prepare all the solutions. 4961
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Figure 1. (a and b) Morphology photographs of XTCS, SEM images of CTS (c) and XTCS (f) before and (i) after Pb(II) adsorption, and EDX analyses of XTCS (e) before and (h) after Pb adsorption. The insets d, g, and j correspond to high-magnification images.
mg L−1 Pb(II) at different temperatures, in the range of 0−55 °C, for 45 min. After equilibrium was reached, the adsorbent was separated from the liquid by filtration with a 0.22 μm filter membrane. The Pb(II) concentration in the filtrate was examined by inductively coupled plasma emission spectroscopy (ICP-OES, ICP-5000, Focused Photonics, Inc., China). All batch experiments were repeated in triplicate. The adsorption capacity of Pb(II) on XTCS at equilibrium, qe (mg g−1), was calculated from the mass balance equation given in eq 1: qe =
(c0 − ce)V M
around 3443 cm−1 can be indicative of −OH group stretching vibration overlapping with an amine stretching band; (ii) small shoulder bands at 2917 and 2875 cm−1 are assigned to asymmetric and symmetric −CH2 groups; (iii) the adsorption bands at 1650 and 1598 cm−1 are characteristic of the secondary amide CO of the remaining acetamide groups due to the incomplete deacetylation of chitosan and the −NH bending vibration of −NH2 groups; (iv) the strong peaks at 1422 and 1382 cm−1 are connected with aliphatic CH bending vibration and the C−H deformation; and (v) the adsorption peaks of the pyranosidic ring appear in the 1200− 800 cm−1 region, reflecting asymmetric stretching of the C OC linkage and C−N stretching as well as skeletal vibrations of CO stretching related to primary and secondary alcohols.18,19 As depicted in the FTIR spectra of TCTS, a new broad peak at 1618 cm−1 due to the overlap of δ (−NH2) substitutes for the adsorption peaks at 1650 and 1598 cm−1. Moreover, a new vibration band appears at 1528 cm−1, representing the characteristic absorbance peak of thiourea group.17 This suggests that thiourea group is successfully grafted onto the main chain of CTS. Compared to TCTS, XTCS has a peak at 1632 cm−1 that is obvious and sharp. Also,
(1) −1
where c0 and ce (mg L ) are the initial and equilibrium concentrations of the Pb(II) solution, V (L) is the volume of solution, and M (g) is the dry weight of XTCS.
3. RESULTS AND DISCUSSION 3.1. Characterization. The FTIR spectra of CTS, TCTS, and XTCS before and after Pb(II) adsorption are analyzed and shown in Figure S1. A series of characteristic bands for the pristine chitosan can be seen: (i) a broad absorption peak 4962
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Figure 2. Effects of (a) pH and (b) temperature on the adsorption of XTCS for Pb(II).
a new peak at 1205 cm−1 appears that is connected with the characteristic absorbance peak of −OC(S)S−,20 indicating that TCTS has been modified by xanthate. After Pb(II) adsorption on TCTS, the peak of the thiourea group is shifted to 1542 cm−1 and the bands at 1419 and 1378 cm−1 are replaced by the band at 1400 cm−1 which is similar to CTS loaded with Pb(II). Compared with TCTS-Pb, the same peak at 1539 cm−1 appears, suggesting that the thiourea group may be involved in Pb(II) adsorption. Moreover, the band at 1205 cm−1 is shifted to 1207 cm−1 and becomes weaker. This indicates that −OC(S)S− group may participate in the adsorption process. Figure 1 illustrates the results of SEM-EDX. As depicted in Figure 1a,b, the adsorbent XTCS prepared in this study has a porous sponge structure with large volume and low weight (about 5.57 mg/cm3). This material can be shaped easily and made into different dimensions and shapes, which is convenient for separation. The morphological features keep original structure after cutting into pieces. A comparison of Figure 1c,f shows that CTS and XTCS are closely similar in their lamellar structure, with a size in the range of 50−100 μm. The inset of Figure 1d shows that the surface of CTS is relatively rough, whereas the surface of the XTCS is smoother with fewer cracks (Figure 1g). This result suggests that the surface of CTS is successfully modified. From the results of EDX (Figure 1e), the addition of sodium and sulfur elements indicates that the xanthate sodium salt of CTS is likely obtained. After adsorption, a change in the surface topography of XTCS (Figure 1i) and the appearance of Pb (Figure 1h) verify that Pb(II) is successfully adsorbed on XTCS. In addition, there are massive cubic lattices with the same shape and dimension, uniformly distributing in the range of 50−80 nm on the surface of XTCS (Figure 1j). This may provide an indication of an S··· Pb complex. An XRD analysis, shown in Figure S2, is used to verify further the nature of cubic lattices on XTCS after the adsorption of Pb(II). It is known that chitosan exhibits a strong characteristic peak at 20°,21,22 which corresponds to its crystal form. However, XTCS shows a weaker and broader peak at 20°. This is attributed to the introduction of new groups into the polysaccharide structure, which impairs the crystallinity of the chitosan derivative. Usually, the crystallinity parameter of CTS can be used to judge its accessibility to internal sites for both water and metal ions. A weaker crystallinity would give rise to even greater possibilities for metal ions adsorption
properties.11 An XRD analysis of XTCS loaded with Pb(II) confirms that the uniform cubic lattices are lead sulfide (PbS, PDF# 65-0346), suggesting that Pb(II) is successfully complexed by sulfur. 3.2. Adsorption Study. 3.2.1. Effect of pH and Temperature. The pH value is one of the most significant environmental parameters used to investigate adsorption, as it affects not only the chemical characteristics of heavy metals such as hydrolysis, precipitation, and complexation reactions but also their speciation and adsorption availabilities.23 The pH value also affects the protonation of the binding sites of adsorbent in the acid solution. Therefore, pH should be the first parameter to be optimized. Here, no buffer solutions were used and no efforts were made to maintain the pH. No significant changes in the initial and equilibrium pH were observed during the adsorption experiments. Figure 2a illustrates the adsorption capacity of XTCS for Pb(II) under various pH conditions. It is obvious that the adsorption capacity decreases at pH ranging 1 from 3 and increases with increasing pH in the range of 3−5.5. The result shows a “V” shape of curve with the adsorption capacity being the lowest at pH 3. It is reported that the xanthate easily decomposes in an acid solution. The instability of xanthate in the acid range is due to the monomolecular decomposition of xanthic acid (HX). The xanthate (X−), xanthic acid (HX), and protonated xanthic acid (H2X+) could keep in equilibrium in the acid solution, as follows: H+ + X− ↔ HX + H+ ↔ H 2X +
Therefore, the existence of an ion-pair activated complex and protonated xanthic acid along with xanthate could decrease the decomposition rate in highly acid solutions.24 In the pH range of 1−3, most of X− groups of XTCS were protonated, and H2X+ group was the predominant species. Therefore, Pb(II) could be bound to XTCS through the exchange of protons on H2X+ groups. And the adsorption capacity of Pb(II) decreased with the reducing number of H2X+. When pH was over 3, the decomposition rate of xanthate gradually decreased and it predominated in the solution, which would contribute to the complexation interaction between xanthate and Pb(II). Moreover, a higher pH would reduce the competition between hydrogen and Pb(II). Both of them gave rise to an increase of the adsorption capacity with increasing the solution pH. Thus, the maximum adsorption capacity occurred at pH over 5. Remarkably, it had an obvious adsorption capacity at pH 1, 4963
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Figure 3. (a) Effect of time on the adsorption of XTCS. Adsorption kinetics model fitted for (b) pseudo-first-, (c) pseudo-second-order equations, and (d) intraparticle diffusion model.
3.2.2. Adsorption Kinetics. Figure 3a shows the relationship between contact time and the adsorption capacity in the process of Pb(II) adsorption on XTCS. A rapidly increasing trend for the Pb(II) adsorption capacity is observed within the initial 45 min, and then it drastically slows and attains equilibrium. Upon analysis of Figure 3a, the kinetics adsorption is seen to be composed of three stages: initial rapid uptake, gradual uptake, and equilibrium uptake. At first, the abundant vacant surface sites on the adsorbent are effective for Pb(II) adsorption and the mass transfer resistance on the surface is relatively weak; thus, the adsorption is fast and instantaneous. This stage is termed as external surface adsorption. With an increase in time, fewer available binding sites are present on the external surface of the adsorbent. As a result, Pb(II) must migrate further to reach the available internal binding sites, and this strengthens the mass transfer resistance and slows the adsorption rate. At this moment, intraparticle diffusion plays a dominant role in the adsorption process instead of external surface adsorption. After being in a state of equilibrium uptake, a final approximate plateau region occurs without any apparent changes in adsorption capacity with prolonged adsorption time. No significant difference in adsorption capacity was observed after 45 min, suggesting that equilibrium was achieved. Hence, 45 min was used as the adsorption time in subsequent studies. To investigate the kinetic mechanism of Pb(II) adsorption by XTCS, three kinetic models were applied to interpret the experimental data. The pseudo-first-order (PFO) kinetic model is based on the assumption that the adsorption rate is proportional to the difference between the adsorbed amount and the equilibrium adsorption capacity; the pseudo-second-
indicating that this adsorbent had an excellent chemical stability, which was attributed to the high insolubility of xanthate.25 This was also supported by the FTIR spectra of XTCS before and after soaking in the aqueous solution at pH 1. As depicted in Figure S3, there was no obvious difference in the structure of functional groups. According to Brownian movement, the temperature has a direct influence on the adsorption process. The effect of temperature on the adsorption capacity of XTCS for Pb(II) is presented in Figure 2b. Obviously, the adsorption capacity initially increases as the temperature increases, and then it decreases over a short temperature range until flattening out. Room temperature is a favorable condition for Pb(II) adsorption on XTCS. This phenomenon can be explained through the characteristics of the integrated adsorption process, including fast diffusion and slow complexation.2 With assistance from the increasing temperature, Pb(II) gains more energy to overcome the mass transfer resistance, and the diffusion rate is enhanced from the bulk solution to the surface of adsorbent. Apart from that, the increase in temperature might alter the size of the transmission channels of the adsorbent, causing intraparticle diffusion and strengthening the chemical affinity between Pb(II) and the adsorbent.26 Therefore, the amount of Pb(II) adsorbed on the adsorbent is initially increased. With temperature increases continuously, the adsorption capacity declines. This might be attributed to the fact that, the faster the reaction between Pb(II) and the active functional groups, the more labile the complex of Pb(II) and XTCS. This is due to the stronger driving force under higher temperatures. 4964
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Industrial & Engineering Chemistry Research Table 1. Adsorption Kinetic Parameters of Pb(II) on XTCS pseudo-first-order model qe (mg g−1)
K1 (min−1)
33.49
1.96 × 10−2
Kid,1 (mg g−1 min−0.5)
Cid,1
45.83
0
R2
pseudo-second-order model qe (mg g−1)
R2
0.8928 176.99 intraparticle diffusion model
K2 (g mg−1 min−1)
R2
1.69 × 10−3
0.9999
Kid,2 (mg g−1 min−0.5)
Cid,2
R2
Kid,3 (mg g−1 min−0.5)
Cid,3
R2
9.76
104.05
0.9722
0.98
160.03
0.9316
Figure 4. (a) Effect of initial concentration on the adsorption of XTCS. Adsorption isotherm models fitted for (b) Langmuir, (c) Freundlich, and (d) Temkin isotherms.
The results of the corresponding kinetic parameters for Pb(II) adsorption are listed in Table 1. The linear simulation plots are also shown in Figure 3b,c. According to the correlative coefficients, the adsorption kinetics of XTCS for Pb(II) can be fitted well with the PSO kinetic model, with the correlation coefficient R2 being nearly unity (R2 = 0.9999). This is much better than the fit with the PFO kinetic model. This result indicates that the rate of Pb(II) adsorption on XTCS depends on the availability of adsorption sites and is controlled by chemical adsorption. Similar results have been reported in several studies with respect to the adsorption of Pb(II) and other metal ions on chitosan or modified chitosan derivatives.15,27,28 Furthermore, the calculated K2 value was lower, indicating that the affinity for adsorbent active sites would be higher and the uptake process of Pb(II) would be faster and more favorable.29 In the present study, the plot of qt versus t0.5 had three linear portions (Figure 3d), indicating that the adsorption of Pb(II) on XTCS contained three steps in the following order: external surface adsorption, mesopore diffusion, and micropore diffusion.30 As shown in Table 1, the order of Kid value was as follow: Kid,1 > Kid,2 ≫ Kid,3, suggesting that the key step in
order (PSO) kinetic model considers chemisorption to be the rate limiting step, that is, a covalence force formed through the exchange or sharing of electrons between the adsorbate and adsorbent is the main adsorption power; the Weber and Morris intraparticle diffusion model is usually used to investigate the diffusion mechanism of the adsorption process. The most used linear forms of these models are represented by the following eqs 2, 3, and 4,22 respectively: log(qe − qt ) = log qe −
K1t 2.303
(2)
t 1 t = + 2 qt q K 2qe e
(3)
qt = K idt 0.5 + C id
(4)
where K1 and K2 are the adsorption rate constants, qt (mg g−1) is the adsorption capacity at time t (min), Kid (mg g−1 min−0.5) is the intraparticle diffusion coefficient and Cid (mg g−1) presents the effect of boundary layer thickness. 4965
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Industrial & Engineering Chemistry Research Table 2. Adsorption Isotherm Parameters of Pb(II) Ion on XTCS Langmuir isotherm
Freundlich isotherm
Temkin isotherm
qm (mg g−1)
KL (L mg−1)
R2
KF (mg g−1)
1/n
R2
aT (L mg−1)
bT (J mol−1)
R2
189.04
0.2060
0.9991
147.05
0.0407
0.9097
1.125 × 109
2.834
0.9258
adsorption intensity and the distribution of the heterogeneous active sites, bT is the Temkin constant related to the heat of adsorption (kJ mol−1) and aT is the equilibrium binding constant corresponding to the maximum binding energy (L mg−1). The proverbially linearized simulation plots of these isotherm models are shown in Figure 4b−d, and the corresponding isotherm constants are calculated and given in Table 2. The value of R2 (0.9991) obtained from the Langmuir isotherm was higher than those of Freundlich isotherm (0.9097) and Temkin isotherm (0.9258), indicating that the Langmuir isotherm model was the most adequate model for describing the relationship between the adsorption capacity of Pb(II) and equilibrium concentration in the solution. The Temkin adsorption potential aT was quite high, indicating the high potential for XTCS and Pb(II). The low Temkin constant bT in the study implied a weak interaction between adsorbate and adsorbent. The value of 1/n was less than 0.5, which indicated that the adsorption of Pb(II) onto XTCS was favorable under the study conditions. The predicted qm of Pb(II) by XTCS was 189.04 mg g−1, which was in good accordance with the saturated adsorption capacity (188.04 mg g−1) obtained from the experimental data. The adsorption capacity of Pb(II) obtained in our study was higher than many other modified chitosan derivatives, as shown in Table 3. To compare with
the adsorption process was related to the external surface adsorption.31 Compared to the first two rate constants, Kid,3 was extremely lower, suggesting that the adsorption was tending to equilibrium state. In other words, pore diffusion was not dominant mechanism in this system, as supported by the relatively low values of Kid,2 and Kid,3. Cid denoted the effect of the boundary layer thickness, where the larger Cid value corresponded to the greater effect of the surface diffusion. Meanwhile, a high R2 value suggested that intraparticle diffusion also exhibited an important effect on the adsorption process of Pb(II) on XTCS. On the basis of the kinetic modeling, it was indicated that both surface binding and intraparticle diffusion affected the adsorption of Pb(II) on XTCS. The external resistance to mass transfer was more significant at the early stage of adsorption process rather than pore diffusion. 3.2.3. Adsorption Isotherm. The relationship between the initial Pb(II) concentration and adsorption capacity in the process of Pb(II) adsorption on XTCS is presented in Figure 4a. The curve sharply increases at lower Pb(II) concentrations, then it mildly increases with a further increase in Pb(II) concentration, finally leveling out at concentrations over 200 mg L−1. This indicates that the adsorption attains saturation and the adsorption capacity remains constant. On one hand, this phenomenon can be attributed to the continuously decreasing mobility of Pb(II) resulting from the gradual increase in Pb(II) concentration; on the other hand, it can be ascribed to the limited adsorbent dosage with inadequate binding sites. It is generally known that equilibrium adsorption isotherms could reveal the surface properties and affinity of the adsorbent. The Langmuir isotherm model is one of the most widely used theoretical models assuming that the adsorbent surface possesses limited active sites with identical energy and the adsorbate is situated on one site with no transmigration; the Freundlich isotherm is another empirical model assuming that the adsorption of adsorbate on the adsorbent surface is nonmonolayer adsorption; the Temkin isotherm model assumes that the adsorption heat of molecules decreases linearly rather than logarithmic adsorption occurs due to the effects of some adsorbate−adsorbent interactions, and that the adsorption is characterized by a uniform distribution of the bonding energies.18 The isotherms can be given by the following forms, respectively: ce c 1 = + e qe KLqm qm (5) 1 log ce n
(6)
RT RT ln a T + ln ce bT bT
(7)
log qe = log KF + qe =
Table 3. Maximum Equilibrium Adsorption Capacity of Pb(II) on Various Chitosan Derivatives adsorbent chitosan functionalized with xanthate flakes15 xanthate-modified thiourea chitosan sponge carboxylated cellulose nanofibrils-filled magnetic chitosan hydrogel beads32 chitosan cross-linked with epichlorohydrin triphosphate9 trioctylmethyl ammonium carboxymethylated chitosan33 GLA-cross-linked metal-complexed chitosans34 xanthate-modified magnetic chitosan13 chitosan-tripolyphosphate beads19 Ca(II) imprinted chitosan microspheres35 polyaniline grafted chitosan22
T (°C)
t (min)
25
240
4
322.6
25
45
5
188.04
25
200
4.5
171
25
720
5
166.94
20
360
4.4
143.3
25
240
5
105.26
25 27 25 25
100 30 60
5 4.5 5 6
pH
qm (mg g−1)
76.9 57.33 49.90 13.23
XTCS, CTS and TCTS prepared in our work were investigated for the adsorption of Pb(II) under the same conditions. The adsorption amounts were less than 30 mg g−1 and even lower (data not shown). This result indicated that XTCS might be a promising adsorbent material to remove Pb(II) from aqueous solutions. To demonstrate the feasibility of the adsorption process, the dimensionless constant RL, referred to as a separation factor, is used to describe the essential characteristics of the Langmuir isotherm. It is expressed by the following equation:
where KL (L mg−1) is the Langmuir constant indicating the affinity of adsorbent for metal ions, qm (mg g−1) is the maximum adsorption capacity on the adsorbent, KF (mg g−1) is the Freundlich isotherm constant indicating maximum adsorption capacity, n is the dimensionless constant indicating 4966
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Industrial & Engineering Chemistry Research RL =
1 1 + KLc0
(8)
The RL value obtained in our study under different initial Pb(II) concentrations was between 0.01 and 0.09, indicating that the adsorption of Pb(II) on XTCS was a favorable process and the interaction between Pb(II) and XTCS was likely to be high affinity. 3.3. Adsorption Mechanism. Adsorption reactions, considered as surface complex reactions, normally occur at solute and solid phases. These complex interactions are composed of (1) chemical binding reaction, described as a specific adsorption with more selectivity and less reversibility, occurs between metal ions and the surface functional groups; (2) electrostatic binding reaction, described as a nonspecific adsorption with less selectivity and weak reversibility, occurs between metal ions and the oppositely charged surface functional groups.36 To elucidate further the adsorption mechanism of Pb(II) on XTCS, XPS studies of XTCS before and after Pb(II) adsorption were performed. The results are shown in Figure S4. For the computer deconvolution XPS spectra of N 1s, before adsorption, there were two peaks at 398.65 and 399.5, assigned to −NH− and −NH2 groups,13 respectively. After adsorption, a new peak at 401.72 eV was observed, which was attributed to the fact that a lone pair of electrons on nitrogen was donated to form the bond between N and Pb, namely, −NH2···Pb complex. Before adsorption, the S 2p spectra presented three peaks at the binding energies of 161.45, 162.64, and 167.97 eV. It can be obviously seen from the spectrum that the binding energy positions at 161.45 and 167.97 eV after adsorption did not show any noticeable change; however, a significant S 2p binding energy at 162.64 eV corresponding to the thiol group disappeared. Furthermore, two new peaks at 160.45 and 164.05 eV were observed after Pb(II) adsorption.13 These results indicated that the sulfur of thiol group on XTCS participated in complexation with Pb. In the comparison of the XPS spectra of XTCS (Figure 5a), an Na 1s spectrum but not a Pb 4f spectrum was distinctly seen before adsorption; however, the result was just the opposite after Pb(II) adsorption. From the computer deconvolution XPS spectra of Pb 4f (Figure 5b), the binding energy positions at 137.25, 138.2, and 142.6 eV corresponded to Pb 4f7, Pb 4f7/2, and Pb 4f5/2, respectively. The appearance of binding energies was in accordance with the research results that bidentate complex could be formed between metal ions and sulfur on sulfhydryl containing adsorbents and similar to the small lead sulfide clusters,36 suggesting that Pb(II) could react with two thiol groups to form bidentate complex. Furthermore, the change in the element contents before and after Pb(II) adsorption in EDX analysis, especially for Na, indicated that ion exchange might play a role in removing Pb(II) from aqueous solutions.
Figure 5. (a) XPS of XTCS and (b) computer deconvolution spectra of Pb 4f after adsorption.
most appropriate to describe Pb(II) adsorption behavior of XTCS and the adsorption process was much favorable. Results of SEM-EDX and XPS revealed that the removal mechanism of Pb(II) was mainly due to complexation and ion exchange. Compared with CTS or XCTS, the as-prepared XTCS exhibited remarkable stability and Pb(II) removal efficiency. XTCS is very inexpensive and eco-friendly, and its synthesis procedure is simple and also inexpensive. It is expected that the as-prepared XCTS could be a promising adsorbent in the removal of Pb(II) from wastewater.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00694. FTIR spectra of samples before and after modification and Pb(II) adsorption, XRD analysis, computer deconvolution spectra of N 1s and S 2p before and after adsorption (PDF).
4. CONCLUSIONS XTCS obtained from chemically modified chitosan was successfully synthesized in our study. This adsorbent had a porous appearance, prominent acid resistance, excellent separability, and possessed a good ability for Pb(II) removal. Pb(II) adsorption kinetic was best described by a pseudosecond-order kinetic model. The primary rate-limiting step was chemical adsorption, but intraparticle diffusion also influenced the adsorption process. The Langmuir isotherm model was the
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AUTHOR INFORMATION
Corresponding Author
*H. Yu. Tel./fax: +86 431 85542290. E-mail:
[email protected]. cn. Notes
The authors declare no competing financial interest. 4967
DOI: 10.1021/acs.iecr.6b00694 Ind. Eng. Chem. Res. 2016, 55, 4960−4968
Article
Industrial & Engineering Chemistry Research
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21277142), “Cross-disciplinary Collaborative Teams Program for Science, Technology and Innovation” of Chinese Academy of Sciences, the “Hundred Talents Project” of the Chinese Academy of Science, the Important Deployment Project of Chinese Academy of Sciences (KZZD-EW-TZ-16), CAS Interdisciplinary Innovation Team.
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DOI: 10.1021/acs.iecr.6b00694 Ind. Eng. Chem. Res. 2016, 55, 4960−4968
