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Nov 9, 2016 - Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of. Sciences, 4888 ...
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Enhanced Selective Adsorption of Pb(II) from Aqueous Solutions by One-Pot Synthesis of Xanthate-Modified Chitosan Sponge: Behaviors and Mechanisms Nana Wang,†,‡ Xingjian Xu,† Haiyan Li,† Lizhu Yuan,†,‡ and Hongwen Yu*,† †

Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 4888 Shengbei Road, Changchun 130102, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Sponge-like xanthate-modified chitosan with a three-dimensional network macroporous structure was prepared using a facile one-pot approach. The as-prepared adsorbent possessed remarkable adsorption capacity and excellent mechanical property as well as rapid and intact separation performance. Adsorption properties of Pb(II), Cd(II), Ni(II), and Zn(II) on xanthate-modified chitosan sponge (XCTS) were systematically investigated in single and multiple systems. The experimental data for each heavy metal adsorption well fitted to the pseudosecond-order kinetic model and Langmuir isotherm model. The maximum adsorption capacities of Pb(II), Cd(II), Ni(II), and Zn(II) were 216.45, 92.85, 45.46, and 41.88 mg/g, respectively. The mutual interference effects of heavy metals in multiple systems were investigated using the inhibitory effect and equilibrium adsorption capacity ratios. The results indicated that the coexisting metal ions had a synergistic promoting effect on Pb(II) adsorption. The competitive adsorption behaviors of Pb(II) in multiple systems were successfully described by the Langmuir and Langmuir competitive models. The adsorption capacity of Pb(II) in multiple systems was higher than that in single system while those of Cd(II), Ni(II), and Zn(II) had a significant decrease in multiple systems, especially for Ni(II) and Zn(II). It turned out that Pb(II) could be effectively removed from an aqueous solution in the presence of Cd(II), Ni(II), and Zn(II), whereas the removal of Cd(II), Ni(II), and Zn(II) would be restrained by the presence of Pb(II). The high selective factor and physicochemical properties of these studied heavy metals revealed the selective adsorption sequence: Pb(II) > Cd(II) > Ni(II) > Zn(II). The characteristic analyses showed sulfur and nitrogen atoms participated in the heavy metal adsorption. The interaction mechanism between Pb(II) and coexisting metal ions could be attributed mainly to the direct displacement effect.

1. INTRODUCTION Heavy metals pose serious threats to human health and ecological systems due to their high toxicity, bioaccumulation, and nonbiodegradability.1−3 Many physical and chemical remediation technologies4−10 are developed to remove heavy metals from wastewater, among them, adsorption has been proven to be one of the most efficient and convenient methods for this purpose because it is cost-effective and versatile to operate for removing trace-levels heavy metal ions from aqueous systems.11,12 For the adsorption technology, the wastewater treatment is usually conducted in a suspension of adsorbent, which requires the adsorbent to have good separation performance and be nontoxic. As a consequence, an adsorbent with low cost, high adsorption capacity, easy separation, and environment protection benefit is extraordinarily promising in removing heavy metals from practical wastewater.13,14 Chitosan is a kind of renewable cationic polysaccharide obtained by partial deacetylation of chitin and possesses © 2016 American Chemical Society

abundant amino groups, which grant it excellent biodegradability, nontoxicity, and adsorption properties.15,16 Nevertheless, the poor stability in aqueous acidic media restricts its application as an adsorbent. It is thereby essential to chemically modify chitosan. According to the hard and soft acids and bases classification system,17,18 sulfur served as a soft ligand group has a strong affinity for heavy metals and can form stable complexes. Among sulfur-bearing compounds, xanthate is a wise choice for the modification of chitosan because the modification process is easy to operate, and the corresponding modified product is highly insoluble and has excellent stability constant values of metal complexes formed.19 In our previous work, we used chitosan as a substrate and successfully synthesized a novel porous xanthate-modified Received: Revised: Accepted: Published: 12222

September 2, 2016 October 26, 2016 November 9, 2016 November 9, 2016 DOI: 10.1021/acs.iecr.6b03376 Ind. Eng. Chem. Res. 2016, 55, 12222−12231

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Industrial & Engineering Chemistry Research thiourea chitosan sponge as an adsorbent.20 It exhibited high adsorption capacity, could effectively remove Pb(II) from aqueous solutions and be easily separated. However, the twostep reaction process was time-consuming, and it would expend massive raw materials and energy. To save time and economic costs, we explored a facile and eco-friendly one-pot preparation method and successfully synthesized a xanthate-modified chitosan sponge (XCTS). Compared with our previous work, this new as-prepared adsorbent gained more outstanding properties, including a uniform macroporous structure and excellent mechanical property with higher adsorption capacity as well as rapid and intact separation performance. We then attempted to investigate its specific adsorption behavior. It is common knowledge that single heavy metal species scarcely exist in natural streams and industry effluents in which those toxic and carcinogenic metals such as Pb(II), Cd(II), Ni(II), and Zn(II) are widely detected. These competitive metal ions present in contaminated water can generally compete for the active sites of the adsorbents.21 Moreover, the adsorption behavior and interactive effect among them are not exactly consistent with each other, owing to the combined impacts of the different heavy metal ions and adsorbents.22−25 Therefore, it is essential to improve the selectivity toward certain heavy metal ions and investigate the simultaneous adsorption behaviors and interactions involving two or more metal species.26,27 Thus, the ultimate application of adsorbents in separating a specific metal ion from industrial wastewater could likely be on the way. Apart from determining the characteristics of XCTS, the current work is also to investigate the competitive and selective adsorption properties and elucidate the adsorption mechanisms in the single and multiple systems of aqueous divalent heavy metal ions involving Pb(II), Cd(II), Ni(II) and Zn(II), finally try to provide some valuable information for its further practical application in heavy metal remediation.

scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX) (Philips XL30 ESEM-FEG, FEI Company, Netherlands). The changes of the functional groups on XCTS before and after adsorption were identified by a Fourier transform infrared spectra (FTIR) with a PerkinElmer spectrometer (L1600400 spectrum Two DTGS, PerkinElmer Inc., USA) using KBr pellets in the range of 500−4000 cm−1. The surface chemistry properties of XCTS during the adsorption process were determined by using X-ray photoelectron spectroscope (ESCALAB250, Thermo-VG Scientific Inc., UK) with monochromatized Al Kα excitation. The compression test of XCTS was performed on a universal testing machine (INSTRON-5869, Instron Corporation, USA) with a compression/decompression rate of 1 mm/min. 2.4. Single and Competitive Adsorption Studies. The batch experiments were performed in conical tubes (50 mL) with 30 mL of solutions and 0.01 g of XCTS using a thermostatic water bath rocking shaker (SPH-110x12, Shanghai, China) at 25 °C and 180 rpm for 12 h. The pH was fixed at 5 using dilute HNO3 or NaOH. After reaching equilibrium, the supernatants were filtered through a 0.22 μm filter membrane for analysis of heavy metal ion concentration by inductively coupled plasma emission spectroscopy (ICP-OES, ICP-5000, Focused Photonics, Inc., Hangzhou, China). The detection wavelengths of heavy metal ions Pb(II), Cd(II), Ni(II), and Zn(II) were 220.353, 228.802, 221.647, and 213.856 nm, respectively. All batch experiments were repeated in triplicate. The corresponding standard deviations of adsorption capacity for each heavy metal ion in all the batch experiments were calculated and reflected on the relevant figures in the form of Y error bars. The RSD values were less than 4% for Pb(II), 6% for Cd(II), and 10% for Ni(II) and Zn(II). The adsorption capacity qe (mg/g) is given in eq 1: qe =

2. EXPERIMENTAL SECTION 2.1. Chemical Agents. Chitosan (CTS) 90% deacetylated was purchased from commercial sources and used as received (Zhejiang Golden-Shell Pharmaceutical Co. Ltd., Zhejiang, China). Pb(NO3)2, Cd(NO3)2·4H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, and CS2 were also used without any further purification (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). All analytical grade reagents were used in the present work. Ultrapure water (MingChe-D24UV water purification system, Merck Millipore, Shanghai, China) was employed to prepare all the solutions. 2.2. Preparation of XCTS. Sponge-like xanthate-modified chitosan was prepared using a facile one-pot approach. Typically, 1.0 g of CTS was directly mixed with 50 mL of 14% NaOH solution and 2 mL of CS2, and then stirred at room temperature for 24 h; finally the precipitate was collected by centrifugation (Thermo Scientific Sorvall RC6 Plus Superspeed centrifuge, Thermo Fisher Scientific Inc., Germany) at 10 000 rpm for 15 min every time and washed thoroughly with ultrapure water until the pH was close to neutral. The shaping of the material was executed by dropping the viscous suspension into the glass beaker (100 mL) and freezing at different temperatures: −18 °C and −80 °C. After the freezing step, the material was finally freeze-dried. The product frozen at −80 °C was served as the adsorbent in further batch experiments. 2.3. Physicochemical Characteristics of XCTS. The surface morphology of XCTS was performed by using a

(C0 − Ce)V m

(1)

where C0 and Ce (mg/L) are the initial and equilibrium concentrations of heavy metals, respectively; V (L) is the volume of solution, and m (g) is the dry weight of XCTS. The adsorption kinetics characteristics of the four heavy metals were carried out with the initial concentration of 100 mg/L for 16 h. The kinetic data were analyzed by two kinetic models. The pseudo-first-order (PFO) kinetic model assumes that the adsorption rate is proportional to the difference between the adsorbed amount and the equilibrium adsorption capacity.28 The pseudo-second-order (PSO) kinetic model considers that the main adsorption power is a covalence force formed through the exchange or sharing of electrons between adsorbate and adsorbent.28 The equations are expressed as follows: log(qe − qt ) = log qe − t 1 t = + 2 qt qe K 2qe

K1t 2.303

(2)

(3)

where K1 and K2 are the adsorption rate constants. To investigate the adsorption isotherms, experiments on single adsorption for Pb(II), Cd(II), Ni(II), and Zn(II) were performed with initial concentrations ranging from 10 to 500 mg/L, respectively. Experiments on competitive adsorption were conducted for the following two series: (1) competitive 12223

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Figure 1. Morphology photographs and SEM images of XCTS prepared at −18 °C (a, d) and −80 °C (b, c, and e).

adsorption isotherms in binary systems with initial mass concentration ratios CPb(II)/CM(II) (M(II) represents Cd(II), Ni(II), and Zn(II)) of 1:2, 1:1, and 2:1; and (2) competitive adsorption isotherm in a quaternary system with an initial mass concentration ratio of 1:1:1:1. In a binary system, the adsorption preference of XCTS for Pb(II) over other heavy metal ions can be evaluated by the 26 selectivity coefficient αPb M according to eq 4: αMPb =

DPb DM

qe =

C0 − Ce V × Ce m

(4)

RL =

(5)

log qe =

1 log Ce + log KF n

1 1 + KLC0

(9)

3. RESULTS AND DISCUSSION 3.1. Morphology Analysis. Figure 1 shows the morphological properties of XCTS prepared at −18 °C and −80 °C. The porous sponge-like adsorbents frozen at two different temperatures were successfully prepared. This material could be shaped easily and regularly, and its dimension and shape were tunable. Compared with XCTS prepared at −18 °C (Figure 1a), the XCTS prepared at −80 °C (Figure 1b) had a more superior porous structure and appeared to be more homogeneous and dense. Moreover, the adsorbent prepared at −80 °C exhibited a better flexibility and could remain intact when cut it into pieces (Figure 1c). The mechanical property of XCTS was evaluated through a uniaxial compression test. As shown in Figure S1, the materials prepared at −18 °C and −80 °C could recover the shape and size to some extent after unloading the stress. However, XCTS prepared at −80 °C had a better performance than the one prepared at −18 °C on the tenacity and mechanical strength. The good mechanical property of XCTS prepared at −80 °C endowed it an admirable ability to be intactly separated from practical wastewater or the free production of chemical sludge during

To further explore the adsorption behaviors of XCTS, classical Langmuir, Freundlich, and Temkin models29−31 were employed to analyze the experimental data. The Langmuir isotherm assumes that the adsorbent possesses limited active sites on a monolayer surface which are homogeneous and equivalent, and there is no transmigration of the adsorbate on the adsorbent surface. The Freundlich isotherm describes multilayer adsorption on a heterogeneous surface, assuming that the adsorption heat decreases with the increase of surface coverage of adsorbent. The Temkin model illustrates the indirect adsorbate−adsorbent interactions, and considers that the adsorption heat decreases linearly due to the effects of these interactions and the adsorption is characterized by a uniform distribution of the binding energies. They are represented as Ce C 1 = e + qe qm KLqm

(8)

where qm is the maximal adsorption capacity (mg/g), KL is the Langmuir binding constant, n and KF are the adsorption intensity and Freundlich constant, respectively; bT is the Temkin constant (kJ/mol) and aT is the equilibrium binding constant (L/mg). In addition, the separation factor RL is used to describe the essential characteristics of the Langmuir isotherm, indicating the feasibility of the adsorption process: RL = 0, irreversible; 0 < RL < 1, favorable; RL = 1, linear; or RL > 1, unfavorable. It is defined as below:

where DPb and DM are the distribution ratios of the Pb(II) and other coexisting heavy metal ions, respectively. The distribution ratio is expressed as D=

RT RT ln a T + ln Ce bT bT

(6)

(7) 12224

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Industrial & Engineering Chemistry Research Table 1. Adsorption Isotherm Parameters of Pb(II), Cd(II), Ni(II), and Zn(II) on XCTS in Single Systems heavy metals parameters

Pb(II)

qe (mg/g) qm (mg/g) KL (L/mg) RL R2

216.92 216.45 0.141 0.014−0.415 0.995

KF (mg/g) n R2

116.92 9.497 0.816

aT (L/mg) bT (J/mol) R2

5.0 × 103 170.389 0.885

Cd(II) Langmuir Model 93.41 92.85 0.307 0.007−0.246 1 Freundlich Model 80.35 32.051 0.662 Temkin Model 1.63 × 1013 934.164 0.647

Ni(II)

Zn(II)

45.97 45.46 0.563 0.004−0.151 1

42.22 41.88 0.594 0.003−0.144 1

36.80 27.855 0.795

51.63 27.473 0.779

1.39 × 1010 1593.314 0.787

4.63 × 1013 1523.160 0.775

Figure 2. Relationships between adsorption capacity and initial concentration of four heavy metals in binary and quaternary systems with different initial mass concentration ratios.

the adsorption process, whereas the one prepared at −18 °C would be prone to have a slight collapse under turbulent hydraulic conditions. This might be attributed to the fact that the pores of XCTS prepared at −80 °C were more uniform and smaller. Moreover, the result of SEM analysis also proved that the more homogeneous and organized structure was the characteristic of the sponge prepared at −80 °C (Figure 1e) rather than the one prepared at −18 °C (Figure 1d). This may be due to the temperature change affecting the freezing ramp, the rearrangement of water in the mixture, and finally the macroporous properties of the materials.32,33 On the basis of the above results, we could draw the conclusion that XCTS prepared at −80 °C had better separation performance and mechanical property.

3.2. Adsorption in Single Systems. It is essential to determine the adsorption rate for practical application. The effect of contact time on the adsorption of Pb(II) on XCTS was shown in Figure S2a. The adsorption rate increased rapidly at the initial adsorption stage due to the availability of abundant active sites on the adsorbent surface. As the active sites were unceasingly occupied, adsorption slowed down and finally reached equilibrium. The linear simulation plots of the kinetic models were presented in Figure S2b,c. The corresponding parameters were calculated and listed in Table S1. The results implied that the PSO kinetic model described the experimental data more exactly than the PFO kinetic model, with the higher correlation coefficient R2. This indicated that the adsorption rate of Pb(II) on XCTS depended on the availability of adsorption sites, and chemical adsorption might be the rate 12225

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and Zn(II) could promote Pb(II) adsorption rather than inhibit it. This might be attributed to ion exchange and the physicochemical properties of heavy metals such as ionic radius, hydrated radius, electronegativity, and hydration energy.36 Exceptionally, the adsorption capacity of Pb(II) was not obviously increased in the quaternary system, which might be due to the direct competitive effect on active adsorption sites among these four heavy metal ions. To clearly illustrate the mutual interference effects of heavy metal ions on adsorption of Pb(II) by XCTS, the qe′/qe ratios were employed in this study. The qe′ and qe denote the adsorption capacities at equilibrium in multiple and single systems, respectively. Generally, there are three possible types of interference effect:37 (1) synergism (q′e/qe > 1), the effect of multiple heavy metal ions is greater than that of the individual adsorbate in the mixture; (2) no net interaction (qe′/qe = 1), the presence of other coexisting metal ions has no effect on the adsorption of the individual adsorbate in the mixture; (3) antagonism (q′e/qe < 1), the effect of multiple heavy metal ions is less than that of the individual adsorbate in the mixture. The q′e/qe ratios for the adsorption of Pb(II) in the presence of other coexisting metal ions were shown in Table S2. The qe′/qe values of Pb(II) in the Pb/Ni and Pb/Zn systems with different initial mass concentration ratios were all greater than 1, indicating that the presence of Ni(II) and Zn(II) had a synergistic effect on the adsorption of Pb(II). In the Pb/Cd system, when C0,Pb/C0,Cd was 1:2, the q′e/qe value of Pb(II) (0.97) was close to 1, suggesting that the presence of Cd(II) almost had no significant effect on the adsorption of Pb(II) when the concentration of Cd(II) was much higher than that of Pb(II). However, when Pb(II) had the same or a higher concentration than Cd(II) in the binary system, the q′e/qe values of Pb(II) were 1.06 and 1.04, hence the presence of Cd(II) led to a slight positive effect on the adsorption of Pb(II) on this occasion. Furthermore, the qe′/qe value of Pb(II) in the quaternary system was also close to 1, indicating that the coexistence of Cd(II), Ni(II), and Zn(II) resulted in an insignificant antagonistic effect on the adsorption of Pb(II). On the above analyses, the adsorption of Pb(II) on XCTS is not strongly depressed but may even be slightly promoted in the case of other coexisting metal ions involving Cd(II), Ni(II), and Zn(II). 3.4. Competitive Adsorption in Binary and Quaternary Systems. To understand the nature of competition among Pb(II), Cd(II), Ni(II) and Zn(II), the Langmuir and Langmuir competitive models were both applied to describe the binary and quaternary adsorption equilibrium data. For the Langmuir competitive model,37 the adsorption capacity qe,i of the ith solute from an n-solute mixture is given by

limiting step. The adsorption behaviors of Cd(II), Ni(II), and Zn(II) were similar to that of Pb(II), and 12 h was enough to attain equilibrium (data not shown). The effect of initial concentration on the adsorption capacity and the proverbially linearized simulation plots of the isotherm models of Pb(II), Cd(II), Ni(II), and Zn(II) adsorption on XCTS were presented in Figure S3 (the linearized simulation plots of the Freundlich and Temkin models were not presented owing to the relatively low R2), and the corresponding isotherm parameters were listed in Table 1. Obviously, the adsorption capacity of each heavy metal increased sharply with the increase of the initial concentration due to the deficient adsorbate and abundant binding sites, and then tended toward saturation because the active sites were occupied by the increasing adsorbate. As presented in Table 1, the value of correlation coefficient (R2 > 0.99) obtained from the Langmuir isotherm was higher than those of the Freundlich and Temkin isotherms, indicating that the Langmuir model gave better description of the experimental data of four heavy metals, which suggested that the adsorption of these heavy metals on XCTS was monolayer adsorption. This could be attributed to the uniform distribution of −O−C(S)−S− and −NH2 groups on the adsorbent surface and the complex reactions between these active functional groups and M(II). Moreover, the calculated RL values of four heavy metals were all within the range of 0 < RL < 1, indicating that the adsorption of four heavy metals on XCTS were extraordinarily favorable. Obviously, the maximum adsorption capacities calculated by the Langmuir model followed the order of 216.45 mg/g (Pb(II)) > 92.85 mg/g (Cd(II)) > 45.46 mg/g (Ni(II)) > 41.88 mg/g (Zn(II)), which were in good accordance with the experimental data. This result indicated a more adsorption preference for Pb(II) than other heavy metals by XCTS. 3.3. Mutual Interference Effects of Heavy Metal Ions on Adsorption of Pb(II). The relationships between adsorption capacity and initial concentration of heavy metals in binary and quaternary systems with different initial mass concentration ratios were shown in Figure 2. The adsorption capacity of Pb(II) was proportional to the initial concentration before adsorption saturation and far more than those of other coexisting metal ions in multiple systems. It was noteworthy that the adsorption capacity of Pb(II) slightly increased with increasing C0,Pb/C0,Cd in the Pb/Cd system, whereas the adsorption capacity of Cd(II) exhibited an opposite trend. However, in Pb/Ni and Pb/Zn systems, the adsorption capacity of Pb(II) decreased when C0,Pb/C0,Ni and C0,Pb/C0,Zn increased, especially for the Pb/Zn system, and the adsorption capacities of Ni(II) and Zn(II) were extremely low. According to the q −q inhibitory effect (IE(%) = s q b , qs and qb were the removal b

quantity (mg/g) of heavy metal in single and binary system, respectively),34,35 in the Pb/Cd system, the IE values of Cd(II) caused by Pb(II) were 12.22%, 55.04%, and 80.73% with the C0,Pb/C0,Cd of 1:2, 1:1, and 2:1, respectively. And the IE values of Ni(II) and Zn(II) caused by Pb(II) increased from 76.07% to 86.95% and 83.42% to 90.53% with increasing C0,Pb/C0,Ni in Pb/Ni systems and C0,Pb/C0,Zn in Pb/Zn systems, respectively. Obviously, Pb(II) exerted a significantly inhibitory effect on Zn(II) and Ni(II), while the inhibitory effect on Cd(II) was strongly concentration dependent. Nevertheless, the IE values of Pb(II) caused by coexisting metal ions in all binary systems (except for the Pb/Cd system with C0,Pb/C0,Cd of 1:2, the IE value was 3.2%) were negative, suggesting that Cd(II), Ni(II),

n

qe, i = qm, iKL, iCe, i(1 +

∑ KL,jCe,j)−1 j=1

(10)

where qe,i and qm,i are the equilibrium and maximum adsorption capacities, respectively; KL,i(j) is physical parameter, and Ce,i(j) is equilibrium concentration. When the concentrations of the solutes in the system are sufficiently high, and then the surface coverage is substantially complete, the unit term in eq 10 may be neglected, finally the expression can be linearized through some algebraic manipulation and shown as eqs 11 and 12 for the binary and quaternary systems, respectively: 12226

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Figure 3. (a) Langmuir and (b) Langmuir competitive models plots for the adsorption of Pb(II) on XCTS in multiple systems with the different initial mass concentration ratios.

Ce,1 Ce,2qe,1

=

Ce,1 qm,1Ce,2

+

KL,2 KL,1qe,1

The graphs plotted using eqs 6, 11, and 12 were shown in Figure 3, and the corresponding adsorption model parameters of Pb(II) in multimetal systems were presented in Table S2. All adsorption results were better illustrated by the Langmuir model with much higher R2, suggesting that the Pb(II) adsorption on XCTS in multiple systems was a monolayer adsorption similar to that in a single system. However, it was noteworthy that the R2 values of the Langmuir competitive model of the Pb/Ni and Pb/Zn combinations were greater than 0.93, indicating that the equilibrium data for the interactions of Pb(II) in the binary systems where Ni(II) and Zn(II) were

(11)

Ce,1 qe,1(KL,2Ce,2 + KL,3Ce,3 + KL,4Ce,4) =

Ce,1 qm,1(KL,2Ce,2 + KL,3Ce,3 + KL,4Ce,4)

+

1 qm,1KL,1 (12) 12227

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Industrial & Engineering Chemistry Research present could also be well described by the Langmuir competitive model. This result manifested that there was competitive effect between Pb(II) and either Ni(II) or Zn(II). Fitting the equilibrium adsorption data of Pb/Cd system to the Langmuir competitive model obtained the coefficients of determination: 0.89, 0.64, and 0.73 for the initial concentration ratios of 1:2, 1:1, and 2:1, respectively. These values implied that there was competitive interaction in the adsorption of Pb(II) and Cd(II), but the competition intensity was not as fierce as that in Pb/Ni and Pb/Zn systems. For the quaternary system, the adsorption behavior of Pb(II) on XCTS well fitted the Langmuir competitive model with R2 > 0.93, suggesting that fierce competition also existed among these four heavy metals. 3.5. Selective Behaviors in Binary and Quaternary Systems. Distribution ratios (D), a potential mobility index of a heavy metal ion, could verify the interaction between two heavy metal ions in solution on account of the competition effect. D values for all the binary systems were calculated to evaluate the behaviors of XCTS for heavy metals (Table S3). Obviously, DPb was invariably larger than DM in all the binary systems, indicating that XCTS exhibited intense preference for Pb(II). Furthermore, D decreased with increasing the initial concentrations of heavy metal ions on account that the adsorption sites of XCTS gradually decreased as the heavy metal ion concentration increased, and this effect was important for preferred metals. For Pb/Ni and Pb/Zn systems, DPb of a lower initial mass concentration ratio (1:2) was higher than those of two higher ratios in most cases, owing to the lower Pb(II) equilibrium concentration. Nevertheless, the Pb/Cd system had opposite phenomena. The system with a higher initial mass concentration ratio (2:1) presented a higher DPb, which might be attributed to the stronger binding ability to Cd(II) than Ni(II) and Zn(II) for XCTS, thus it was difficult for Pb(II) to displace Cd(II) adsorbed on the active binding sites. In addition, selectivity coefficient (αPb M ) for all binary systems were calculated and illustrated in Figure 4. Generally, αPb M > 1 implied that Pb was preferred to XCTS in a binary Pb Pb system. As shown in Figure 4, the values of αPb Cd, αNi , and αZn in all binary systems were greater than 1, further demonstrating the obvious preference for Pb with XCTS. The value of αPb Cd was Pb extremely small, while the values of αPb Ni and αZn were fairly high, which indicated the difference of the preference degree for Pb(II) and Cd(II) with XCTS was not as obvious as that of the preference degree for Pb(II) and Ni(II) or Pb(II) and Zn(II). Thus, it also could be concluded that XCTS relatively preferred Cd(II) to Ni(II) and Zn(II). Similarly, Ni(II) had a slight advantage over Zn(II) on the adsorption preference due to the lower αPb Ni in nearly all situations. Taken together, the order of selective adsorption on XCTS was Pb(II) > Cd(II) > Ni(II) > Zn(II). 3.6. Adsorption Mechanisms. To elucidate the adsorption mechanism, FTIR spectra, and XPS studies had been used to investigate the surface interactions involved in the adsorption process. Figure 5 exhibited the FTIR spectra of XCTS before and after the adsorption of heavy metals. For the spectrum of XCTS, the adsorption bands at 3298 and 1420 cm−1 were characteristic of −NH2 stretching vibration and −NH vibration and deformation in −NH2 groups. The peak appearing at 1207 cm−1 was the characteristic absorption peak of −O−C(S)− S−,20,38 indicating that XCTS was successfully synthesized. After the adsorption of Pb(II), Cd(II), Ni(II), and Zn(II), the changes of 3294 and 1420 cm−1 and the reduction of the

Figure 4. Selectivity coefficient (αPb M ) of Pb(II) on XCTS in binary systems with the different initial mass concentration ratios: (a) 1:2, (b) 1:1, and (c) 2:1.

intensity at 1207 cm−1 indicated that the nitrogen atom of −NH2 and the sulfur atom of −O−C(S)−S− were the adsorption sites.39 XPS studies of XCTS before and after adsorption were performed. Compared with XPS survey spectra of XCTS before and after adsorption in single and multiple systems (Figure 6), it could be discerned that all the corresponding peaks of Pb 4f, Cd 3d, Ni 2p, and Zn 2p appeared but Na 1s spectrum disappeared, indicating that ion exchange might occur between them. The photoemission bands Pb 4f7, Pb 4f7/2, Pb 4f5, and Pb 4f5/2 (Figures S4a and S5a) showed that a significant amount of Pb(II) had been adsorbed. High resolution XPS spectra of S 2p and N 1s of XCTS before and after heavy metal adsorption in different systems were compared in Figure S4b,c and Figure S5b,c. For the S 2p spectrum, the BE peak at 162.40 eV corresponding to the thiol group had an obvious change after adsorption and a new peak at around 164.82 eV appeared, which could be attributed to the formation of complexes between S and M(II). Similarly, the typical N 1s peak at 399.70 eV assigned to the amine group shifted after adsorption, and a 12228

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Figure 5. FTIR spectra of XCTS before and after heavy metal adsorption in single, quaternary (a) and binary (b) systems.

Figure 6. XPS spectra of XCTS before and after heavy metal adsorption in single and multiple systems.

Table 2. Physicochemical Properties of the Studied Heavy Metals heavy metal

ionic radius r (Å)

hydrated radius R (nm)

electronegativity Xm

hydration energy E (kJ/mol)

acid classification

Pb(II) Cd(II) Ni(II) Zn(II)

1.19 0.97 0.69 0.74

0.401 0.426 0.404 0.430

2.33 1.69 1.91 1.65

−1481 −1807 −2106 −2064

soft acid soft acid borderline borderline

new peak at around 401.46 eV was observed, suggesting the formation of N−M(II) complexes, in which a lone pair of electrons was donated to the shared bond between N and M(II). As mentioned above, FTIR and XPS studies revealed that the adsorption of heavy metals on XCTS mainly depended on ion exchange and complex reactions with nitrogen and sulfur atoms. However, the selective adsorption behaviors of Pb(II) in multiple systems failed to be illustrated clearly. The more obvious selectivity of Pb(II) to XCTS than Cd(II), Ni(II), and Zn(II) might be attributed to the physicochemical properties of these heavy metals (Table 2). Generally, the facts are that a smaller hydrated radius leads to greater electrostatic attraction, bigger electronegativity results in larger electronic attraction to counterions, and lower hydration energy facilitates the cation to shed its hydration shell more easily.40 Thus, it could be drawn from Table 2 that, for the heavy metal ions having the same valence, Pb(II) could be much more easily selected than Cd(II), Ni(II), and Zn(II) during the adsorption process. Moreover, Pb(II) with the largest atomic weight and two numbers of coordination (2, 4), is paramagnetic and has the highest standard reduction potential in comparison to Cd(II),

Ni(II), and Zn(II), which also makes Pb(II) favorable for adsorption.37 According to Nieboer and Richardson,18 soft acid is prone to forming stable complexes with soft ligand, and vice versa. The covalent index X2mr is usually used to evaluate whether covalent interactions of the heavy metal ions are more important than their ionic interactions. The large X2mr value imply that the metal ion can preferentially interact with the functional group in the following order: S > N > O containing group. The calculated X2mr values decreased as follows: Pb(II) > Cd(II) > Ni(II) > Zn(II), indicating that Pb(II) had a stronger attraction than Cd(II), Ni(II), and Zn(II) to the lone pair electrons of sulfur and nitrogen atoms. In combination with the acid classification of heavy metals, the selective adsorption of XCTS strongly followed the sequence: Pb(II) > Cd(II) > Ni(II) > Zn(II). Therefore, in the selective adsorption experiments, the possible elucidative mechanism was that Pb(II), Cd(II), Ni(II), and Zn(II) were adsorbed at the initiative stage when most of functional groups were available, but the adsorbed Cd(II), Ni(II), and Zn(II) subsequently released from the active sites because of the higher binding ability of XCTS toward Pb(II) over them, and Pb(II) gradually 12229

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separating and concentrating Pb(II) from multiple solutions containing Pb(II), Zn(II), Ni(II), or Cd(II).

displaced these coexisting metal ions due to the more stable complexes formed between Pb(II) and the functional groups.26 3.7. Adsorption Property Comparison between XCTS and the Commonly Used Adsorbents. For industrial application, a desired adsorbent needs to possess a series of excellent characteristics from synthesis to application. The newly prepared adsorbent XCTS is more easily synthesized by a simple reaction process with little reagents under mild operating conditions in comparison to the organic polymer adsorbents such as vinylpyridine-grafted PAAc hydrogel and polysiloxane−graphene oxide gel.14,41 For the natural mineral and carbon-based adsorbents,35,42 they can cause the difficulties in the liquid−solid postseparation and high pressure drop in a fixed-bed column mode while XCTS is free of these shortcomings. Many adsorbents have a high affinity for Pb(II) and also can adsorb other heavy metals,36,43 but the adsorption capacity of Pb(II) significantly decreases with the presence of the coexisting heavy metals.21 As previously discussed, the high adsorption capacity of Pb(II) on XCTS was not suppressed by coexisting heavy metals and even had a slight increase. Moreover, the cost of adsorbents is another important factor. The price of chitosan from industry raw materials is approximately US$ 6.35 × 104/t. In comparison with activated carbon (US$ 7.5 × 104/t), multiwalled carbon nanotubes (US$ 2.5 × 106/t), carboxyl multiwalled carbon nanotubes (US$ 7 × 107/t) and C18 silica (US$ 4.0 × 106/t), the cost of XCTS is much lower.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03376. Stress−strain curves of XCTS, adsorption kinetic parameters of Pb(II) on XCTS, effects of adsorption time and initial concentration on the adsorption of heavy metals on XCTS in single systems and the corresponding kinetics and isotherm models, and XPS spectra of XCTS before and after heavy metal adsorption in binary systems (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 431 85542290. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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, and the Science and Technology Service Network Initiative (STS) Project of Chinese Academy of Sciences.

4. CONCLUSIONS In this study, a facile one-pot preparation method of XCTS was explored and the as-prepared adsorbent was used for selective removal of Pb(II) from aqueous solution. XCTS presented a three-dimensional network structure with relatively regular macropores and efficient separation performance. Results indicated that the shaping of XCTS at lower temperature significantly contributed to the gain of excellent mechanical property. Adsorption of Pb(II), Cd(II), Ni(II), and Zn(II) were conducted in single and multiple systems. The results of the equilibrium adsorption study using several isotherm models showed that adsorption of Pb(II), Cd(II), Ni(II), and Zn(II) on XCTS was a chemical adsorption and homogeneous. Compared with the adsorption capacity in the single system, the adsorption capacity of Pb(II) increased while that of Cd(II) significantly decreased, especially for Ni(II) and Zn(II), there were no obvious adsorption in multiple systems. According to the inhibitory effect and equilibrium adsorption capacity ratios, it was found that Cd(II), Ni(II), and Zn(II) promoted the adsorption of Pb(II), while Pb(II) exerted a significantly inhibitory effect on Cd(II), Zn(II), and Ni(II), namely, the multimetal effect on the adsorption of Pb(II) by XCTS was synergistic whereas the effect of Pb(II) on other heavy metals was antagonism. The isotherm studies disclosed that the adsorption behaviors of Pb(II) followed the Langmuir and Langmuir competitive models. The FTIR and XPS studies verified that the adsorption for heavy metal ions was mainly through forming complexes with nitrogen and sulfur atoms in XCTS. The separation factor indicated the order of selective adsorption on XCTS was Pb(II) > Cd(II) > Ni(II) > Zn(II). And the strongly selective adsorption of Pb(II) was ascribed to the differences in the physicochemical properties of heavy metals and the direct displacement effect. XCTS has unique advantages over commonly used adsorbents, suggesting that it possesses a good potential to be applied in selectively

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