Uptake of Pb(II) and Cd(II) on Chitosan Microsphere Surface

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Uptake of Pb(II) and Cd(II) on Chitosan Microsphere Surface Successively Grafted by Methyl Acrylate and Diethylenetriamine Haifeng Zhang, Qifeng Dang, Chengsheng Liu, Dongsu Cha, Zhenzhen Yu, Wenjing Zhu, and Bing Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00480 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Applied Materials & Interfaces

Uptake of Pb(II) and Cd(II) on Chitosan Microsphere Surface Successively Grafted by Methyl Acrylate and Diethylenetriamine

Haifeng Zhang,† Qifeng Dang,† Chengsheng Liu,*,† Dongsu Cha,‡ Zhenzhen Yu,† Wenjing Zhu,† and Bing Fan§

†

College of Marine Life Sciences, Ocean University of China, 5 Yushan Road,

Qingdao 266003, P.R. China ‡

The Graduate School of Biotechnology, Korea University, Seoul 136-701, South

Korea §

Qingdao Aorun Biotechnology Co., Ltd., Room 602, Century Mansion, 39

Donghaixi Road, Qingdao 266071, P.R. China

KEYWORDS: chitosan-based microsphere, methyl acrylate, diethylenetriamine, high adsorption capacity, Pb(II), Cd(II)

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ACS Paragon Plus Environment


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ABSTRACT: A novel adsorbent, CS-MA-DETA microspheres, for uptake of heavy metal ions from aqueous solutions was first fabricated via two-step grafting methyl acrylate (MA) and diethylenetriamine (DETA) onto chitosan (CS) microsphere surface in the absence of cross-linkers. CS-MA-DETA microspheres of 3.04 µm in mean diameter were of uniformly wrinkle-like topography sketched out by SEM, whose surface after decoration by MA and DETA was stable and beneficial to metal ion capture. Its chemical composition, microstructure, and thermal property were characterized by elemental analysis, FTIR, XRD, BET, and TGA techniques, and the achieved quantitative results mainly included C/N ratio (4.76), crystallinity (31.20%, 19.75% of CS), specific surface area (27.806 m2 g−1), pore diameter (3.452 nm), and mass loss at the first stage (3%, around 10% of CS), which indicated a successful synthesis, well-defined structure, and good thermostability. Adsorption tests of CS-MA-DETA microspheres were performed in Pb(II) and/or Cd(II) solution(s) at various pH values, contact time, and initial concentrations, exhibiting an excellent adsorption capability. Its maximum adsorption capacity calculated by Langmuir model was 239.2 mg Pb(II)/g, or 201.6 mg Cd(II)/g, which was higher than those of most available CS-based adsorbents. Furthermore, several adsorption kinetic and isotherm models were employed to investigate its uptake behavior, implying that it was mainly a monolayer adsorption and chemisorption process. Five-cycle reusability tests demonstrated CS-MA-DETA microspheres could be repeatedly used without significant capacity loss (< 10%). Additionally, several potential bonding modes and adsorption sites for both metal ions were also proposed. Overall, CS-MA-DETA microspheres with outstanding adsorption performance towards Pb(II) and/or Cd(II) might serve as a new absorbent for wastewater purification.

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1. INTRODUCTION Heavy metal pollution has increasingly drawn much attention from all over the world, due to its strong diffusibility, high toxicity, non-biodegradability, and cumulative property.1−3 Generally, heavy metals from various origins are blended into water body, accumulated in aquatic organism, and further transferred to human body through food chains, causing various physical diseases or disorders.1,4 Exposure of heavy metals even at trace levels is a risk for human beings, for example, Pb(II) is accumulated in kidney and nervous system, leading to cancer or deformity;5 Hg(II) disturbs nervous system, causing Minamata disease;6 Cd(II) damages skeletal system, resulting in skeletal disease.7,8 For these reasons, it has always been an urgent task for researchers engaged on environmental security to seek more reasonable ways of treating water pollution. Conventional methods, such as chemical precipitation,9 ion exchange,10 membrane filtration,11 electrochemical treatment,12 activated carbon adsorption,13 and chelating resin,14 for heavy metal ion removal from contaminated water have been extensively investigated and applied, by which much polluted water body has had an effective control or remediation to some extent. However, such techniques are often costly or ineffective, especially in treating water contaminated by heavy metal ions at trace levels. Compared with conventional methods, bio-adsorption is recently considered as the most advisable method for heavy metal removal. During bio-adsorption, the bio-adsorbent plays a vital role, not only due to its good adsorption property and economic performance, but also because of no secondary pollution.15–22 Among available bio-adsorbents, chitosan (CS) is generally regarded as a suitable candidate, mainly due to its unique properties such as hydrophilicity, biodegradability, biocompatibility, non-toxicity, and adsorption property.23 Besides, 3



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CS can be easily obtained through deacetylation reaction of chitin that is the second abundant natural polymer behind only cellulose, and is usually extracted from shells of crab, prawn, and insect.24,25 Notably, CS possesses plentiful primary amine (–NH2) and hydroxyl (–OH) groups, which can exert good capability to chelate with metal ions. Despite these advantages, CS in the form of powders is still of some defects, such as inadequate absorption capacity, low specific surface area, irregular shape difficult to recycle, poor stability in acidic media, and weak mechanical strength, which limit its widespread applications in heavy metal removal.26 Over the last few decades, some effective attempts have been made to overcome the disadvantages mentioned above. To enhance CS adsorption capacity for metal ions, as well as to improve its stability in acidic media, the introduction of groups with abundant coordination atoms (e.g. nitrogen, oxygen, and sulfur) is generally considered as an optimal choice.27,28 Wu et al. introduce melamine onto CS backbone to improve its capability of chelating with Cu(II) or Cu(I).28 Zhu et al. prepare xanthate-modified magnetic CS whose maximum adsorption capacities for Pb(II), Cu(II), and Zn(II) are 76.9, 34.5, and 20.8 mg g−1, respectively.29 More notably, many researchers have proved the modified CS containing multi amine groups can exhibit a strong tendency to chelate with heavy metal ions in aqueous solutions. For improving mechanical strength of CS powders or particles, various chemical or physical modifications have been performed, especially modifications by cross-linkers. Zheng et al. report carboxymethyl chitosan microspheres for Cu(II) removal using adipic acid dihydrzide as a cross-linker.30 Besides, George et al. fabricate poly(itaconic acid)-grafted CS adsorbents with two cross-linkers for Pb(II) and Cd(II) uptake.31 Undeniably, the addition of cross-linkers can enhance CS mechanical strength, and also its stability in acidic media, but several studies have indicated the participation of 4


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cross-linkers brings about a negative effect on its adsorption capacity, due to the loss of adsorption sites involved in cross-linking reaction.32 In order to increase specific surface areas of CS-based absorbents, reducing their particle sizes to micro- or nano-scale is often employed, based on which, numerous functional groups that act as active sites for heavy metal ion adsorption can be exposed outside.33 For example, nanoporous magnetic cellulose/chitosan composite microspheres for Cu(II) absorption exhibit excellent uptake performance.26 Also, magnetic nano-based particles of diethylenetriamine-functionalized CS show great adsorption capabilities, due to their large specific surface areas.34 If the adsorption capacity of a CS-based adsorbent is related to the number of amino groups and specific surface area partly or completely, then increasing its amino groups and/or specific surface area will result in a higher adsorption capacity. Here, a new strategy was proposed by our laboratory (Scheme 1). First, CS microspheres with the uniformly wrinkle-like topography and the mean diameter of less than 10 µm were obtained via a spray drying method. With above achievements, multiple purposes could be taken into account: (1) to increase specific surface area; (2) to enhance mechanical strength, without the participation of any cross-linker; (3) to expose more grafted sites to a certain extent; (4) to facilitate reusing operation, unlike nanoparticles or powders. Secondly, using Michael addition,35 methyl acrylate (MA) grafted CS (CS-MA) microspheres were prepared by grafting MA onto CS microsphere surface. Finally, diethylenetriamine-grafted CS-MA (CS-MA-DETA) microspheres were fabricated via substitution reaction. In this study, CS-MA-DETA microspheres were characterized by Fourier transform infrared spectroscopy (FTIR), elemental analysis, X-ray diffraction (XRD), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) method, and 5



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scanning electron microscope (SEM). Batches of experiments were performed to evaluate its adsorption capacities for Pb(II) and/or Cd(II) in either single or binary metal ion solutions at various pH values, contact time, and initial concentrations. Furthermore, three kinds of adsorption kinetics and two kinds of adsorption isotherms were employed to further study the uptake behavior of CS-MA-DETA microspheres. Its adsorption–desorption performance was also investigated. Finally, several probable bonding modes and adsorption sites for both metal ions were proposed.

Scheme 1. Schematic Preparation Procedures of CS-MA and CS-MA-DETA Microspheres. (CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan.)

2. MATERIALS AND METHODS 2.1. Preparation of CS Microspheres. Information on materials and chemicals used in this work can be seen in Text S1. 6


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CS solution was prepared by dissolving 10 g of CS in 1000 mL of acetic acid solution (1%, v/v). The impurity was removed from the obtained solution using a vacuum filter. Then, CS microspheres were obtained via a spray dryer (SD-Basic, Labplant Ltd., UK) at inlet temperature of 160 °C and outlet temperature of 90 °C.36 2.2. Fabrication of CS-MA-DETA Microspheres. On the basis of CS microspheres, CS-MA-DETA microspheres were fabricated, as shown in Scheme 1. The fabrication process could be divided into two steps: (1) preparation of CS-MA microspheres; (2) fabrication of CS-MA-DETA microspheres. MA was grafted to amino groups on CS microsphere surface by Michael addition reaction,37 in a heterogeneous system. Specifically, 5 g of CS microspheres and 200 mL of methanol were successively poured into a flask-3-neck. Then, the flask was sealed, and the mixture was stirred with a magnetic stirrer at 50 °C under a constant nitrogen supply. Subsequently, 20 mL of MA was added into the flask through a syringe. After 48 h, the microspheres were filtered via a vacuum filter, and washed with alcohol for five times. Then, the sample was transferred to a oven, and dried at 60 °C to a constant weight. Finally, the obtained CS-MA microspheres were stored in a desiccator for the following use. DETA was grafted to terminal ester groups on CS-MA microsphere surface by substitution reaction. Briefly, 1.5 g of CS-MA microspheres and 150 mL of methanol were in turn transferred into a three-necked flask. The mixture in the sealed three-necked flask was continuously stirred at 50 °C under a constant nitrogen protection. Then, 20 mL of DETA was dropwise added into the flask using a syringe within 2 h. After 72 h, the microspheres were separated by centrifugation (8000 × rpm, 10 min), and washed with alcohol for five times. After that, the sample was dried in an oven at 60 °C to a constant weight. The desired CS-MA-DETA microspheres 7



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were stored in a desiccator for further experiments. Information about the characterization of materials involved in this work can be seen in Text S2. 2.3. Adsorption Experiment. All the adsorption experiments in single or binary system were conducted in a thermostatic shaker at 25 °C and 200 rpm to determine the adsorption capabilities, kinetic and thermodynamic parameters of CS-MA-DETA microspheres. After adsorption, the adsorbent was separated from the solution using 0.45 µm syringe filter. The residual concentration of metal ions was measured by a flame atomic absorption spectrophotometer (AA-6880F/AAC, Shimadzu, Japan). All the adsorption experiments were carried out in triplicate. The adsorption capacity was calculated according to Eq. (1).

‫=ݍ‬

(஼బ ି஼೐ )௏

(1)

௠

where q (mg g−1) is the adsorption capacity, C0 (mg L−1) and Ce (mg L−1) are respectively initial and final concentrations of metal ions, V (L) is the volume of the metal ion solution, and m (g) is the mass of the dried adsorbent.

2.3.1. Effect of pH. Briefly, 30 mg of CS-MA-DETA microspheres was added into a conical flask containing 30 mL of Cd(II) or Pb(II) solution (250 mg L−1), then the mixture was adjusted to a desired pH value with 0.1 mol L−1 NaOH or HNO3 aqueous solution. All the experiments were conducted in Cd(II) or Pb(II) solution with initial pH 1.0–6.0 for 360 min in triplicate.

2.3.2. Effect of Contact Time. Each experiment was performed through adding 30 mg of CS-MA-DETA microspheres into a conical flask containing 30 mL of Cd(II) or Pb(II) solution (250 mg L−1) with initial pH 5.0. Twelve parallel experiments were respectively carried out at various contact time (10, 20, 30, 40, 50, 60, 90, 120, 150, 180, 210, and 240 min), and repeated three times. 8


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2.3.3. Effect of Initial Concentration. Generally, 30 mg of CS-MA-DETA microspheres was added into a conical flask containing 30 mL of Cd(II) or Pb(II) solution at pH 5.0 and various initial concentrations (10, 20, 30, 40, 50, 100, 150, 200, 250, 300, 350, and 400 mg L−1). Twelve parallel experiments were respectively conducted for 360 min, and repeated three times.

2.3.4. Selective Adsorption. The tests of selective adsorption were carried out in a binary ion mixture system of Cd(II) and Pb(II). A total of 50 mg of CS-MA-DETA microspheres was added into 100 mL of the mixed solution with 250 mg L−1 of initial concentration of Cd(II) and Pb(II) at initial pH 5.0, after continuous stirring for 240 min, the residual concentration of Cd(II) or Pb(II) was measured. Each test was repeated three times.

2.3.5. Desorption and Regeneration. The reusability of CS-MA-DETA microspheres was evaluated via sequential cycles of adsorption–desorption. After adsorption for Cd(II) or Pb(II), the microspheres were collected and washed thoroughly with deionized water to remove unabsorbed metal ions on the surface. The metal-loaded CS-MA-DETA microspheres were regenerated in 0.5 mol L−1 of HCl solution, and shaken at 25 °C for 2 h. After desorption and separation, the adsorbent was repeatedly washed with deionized water until no Cd(II) or Pb(II) was detected in the filtrate. The desorbed absorbent was further used in the next adsorption cycle. Each experiment was carried out for five times under the same conditions.

3. RESULTS AND DISCUSSION 3.1. Characterization of CS-MA-DETA Microspheres.

3.1.1. FTIR Analysis. FTIR spectra of CS, CS-MA, and CS-MA-DETA microspheres are displayed in Figure 1a. For CS microspheres, the main characteristic absorption bands appeared at 3404–3459 (stretching vibration O–H), 2925 and 2875 9



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(asymmetric and symmetric stretching vibration of C–H),37 1655 (amide I), 1599 (stretching vibration of –NH2), 1420 (bending vibration of –CH2–), 1372 (bending vibration of C–H), 1316 (amide III), 1155 (–C–O–C– stretching vibration), and 1069 cm−1 (C–O stretching vibration). Compared with the spectrum of CS microspheres, a new band at 1729 cm−1 emerged in the CS-MA microspheres’ spectrum, which was associated with the bending vibration of O–C=O (ester bond). Besides, the bands at 2877 (stretching vibration of C–H) and 1375 cm−1 (bending vibration of C–H) became stronger. These changes above implied that MA was successfully grafted to CS microspheres. In the spectrum of CS-MA-DETA microspheres, the band at 1729 cm−1 (O–C=O) significantly decreased in intensity, while the band at 1655 cm−1 (amide I) considerably increased, compared with those in the CS-MA microspheres’ spectrum. Simultaneously, the absorption at around 1599 cm−1 was somewhat enhanced, which was attributed to the stretching vibration of –NH2. Based on the above findings, it is clear that DETA was grafted to CS-MA microspheres, namely, CS-MA-DETA microspheres were successfully fabricated.

3.1.2. Elemental Analysis. Elemental analysis, which is usually used to detect the change in elemental content and evaluate the reaction outcome, was employed to determine the elemental composition in CS, CS-MA, or CS-MA-DETA microspheres. As shown in Table 1, the nitrogen content in CS-MA microspheres was lower than that in CS microspheres, which was attributed to the introduction of MA that increased the content of carbon and oxygen, resulting in the increase of C/N ratio. Such results indicated that MA was successfully grafted onto CS microsphere surface. Compared with CS-MA microspheres, CS-MA-DETA microspheres had higher nitrogen content and lower C/N ratio that were ascribed to the introduction of DETA onto CS-MA microsphere surface. Based on elemental analysis results, as well as on 10


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FTIR analyses, it was able to conclude that CS-MA-DETA microspheres were successfully prepared through two-step grafting reaction.

Figure 1. (a) FTIR spectra of CS, CS-MA, and CS-MA-DETA microspheres. (b) XRD patterns of CS, CS-MA-DETA, CS-MA-DETA-Pb(II), and CS-MA-DETA-Cd(II) microspheres. (c) TGA curves of CS and CS-MA-DETA microspheres. (d) FTIR spectra of CS-MA-DETA microspheres before adsorption and after five-time adsorption for Cd(II) and Pb(II). (CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan; CS-MA-DETA-Pb(II) and CS-MA-DETA-Cd(II) microspheres stand for Pb(II)- and Cd(II)-loaded CS-MA-DETA microspheres.)

Table 1. Elemental Analysis Results of CS, CS-MA, and CS-MA-DETA Microspheres. Elemental analysis

Sample a C (%)

H (%)

N (%)

C/H

C/N

CS microspheres

39.9600

8.3183

6.5433

4.8039

6.1060

CS-MA microspheres

41.3900

7.7550

6.2770

5.3375

6.5923

CS-MA-DETA microspheres

41.6400

8.3180

8.7567

5.0061

4.7555

a

CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS;

MA, methyl acrylate; CS, chitosan. 11



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3.1.3. X-ray Diffraction. To comparatively investigate the crystalline properties of CS, CS-MA-DETA, CS-MA-DETA-Pb(II), and CS-MA-DETA-Cd(II) microspheres, XRD technique was employed, and their XRD patterns are shown in Figure 1b. For CS microspheres, two peaks at around 9.2 and 21.3° were major characteristic peaks of CS. In the diffractogram of CS-MA-DETA microspheres, the characteristic peak at 9.2° completely disappeared, while the peak at around 21.3° shifted to 20.5°, and its intensity increased but width decreased. Generally, the peak intensity and width in the XRD pattern of a sample have close correlation with its crystallinity. Compared with CS microspheres, the stronger and sharper peak appeared at 20.5° implied that there were more crystalline phases in CS-MA-DETA microsphere matrix. The crystallinity of CS-MA-DETA microspheres was 31.20%, while that of CS microspheres was 19.75%, which was calculated according to the method reported by Osorio Madrazo et al.38 Compared with CS microspheres, more crystalline regions appeared in CS-MA-DETA matrix, due to the introduction of amino groups that not only increased the stereoregularity and symmetry of saccharide chains, but also reinforced inter- and intra-molecular hydrogen bonds. Such results implied the excellent stability of CS-MA-DETA microspheres fabricated without cross-linkers. Additionally, XRD patterns of the materials after Pb(II) and Cd(II) adsorption (CS-MA-DETA-Pb(II) and CS-MA-DETA-Cd(II) microspheres) are presented in the inset in Figure 1b. More noticeably, the diffraction peak intensity of CS-MA-DETA-Pb(II) microspheres, as well as of CS-MA-DETA-Cd(II) microspheres, significantly decreased as compared with that of CS-MA-DETA microspheres. Both patterns exhibited broad diffraction peaks, indicating that either CS-MA-DETA-Pb(II) or CS-MA-DETA-Cd(II) microspheres were of amorphous structure. This fact could be attributed to the dominant adsorption mechanisms (chelation, electrostatic interaction, etc.) between 12


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the adsorbent and metal ions.31

3.1.4. TGA Measurement. TGA curves of CS and CS-MA-DETA microspheres are shown in Figure 1c. Obviously, CS microspheres mainly had two stages of mass loss during heating process. The first stage was at the range of 80–100 °C, with mass loss about 10%, which was due to the loss of water physically adsorbed into CS microspheres. The second stage began at 320.8 °C, with a high value of mass loss about 50%, which was attributed to the decomposition and ablation of CS molecule chains. Similarly, CS-MA-DETA microspheres also had two stages of mass loss in heating process. Compared with CS microspheres, CS-MA-DETA microspheres had mass loss of 3% at the first stage, which was ascribed to the elimination of water adsorbed and bound water at the moderate temperature. The mass loss at the second stage starting at 300 °C was 55%, due to the decomposition of CS-MA-DETA molecule chains. Notably, for CS-MA-DETA microspheres, the mass loss of 3% at the first stage was lower than that of CS microspheres, mainly due to the introduction of the grafted polymer that reinforced the surface of microspheres, resulting in less water adsorbed into microsphere matrix. Such change might endow microspheres with solid shells beneficial to practical applications.

3.1.5. BET Measurement. Nitrogen adsorption–desorption isotherms of CS microspheres (Figure 2a) provided information on its microstructure. According to IUPAC classification, the adsorption isotherms exhibited type IV characteristics, indicating that CS microspheres were typical mesoporous materials.39,40 Hysteresis loop of CS microspheres showed a parallel plate slit pore structure. For CS-MA-DETA microspheres, its adsorption isotherms (Figure 2b) exhibited type II characteristics, indicating that CS-MA-DETA microspheres were of typical reversible multilayer adsorption. Figure 2c and d showed the pore size distribution curves of CS 13



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and CS-MA-DETA microspheres, respectively. The surface parameters of CS and CS-MA-DETA microspheres were calculated from the adsorption branch of the isotherm using Barrett-Joyner-Halenda (BJH) method, as shown in Table S1. Clearly, the specific surface area, pore diameter, and pore volume of CS-MA-DETA microspheres were 27.806 m2 g−1, 3.452 nm, and 0.025 cc g−1, respectively, indicating a large specific surface area beneficial to uptake of metal ions. Notably, CS-MA-DETA microspheres had a smaller specific surface area than CS microspheres, which was perhaps due to two facts that the concave surface of CS microspheres was partly filled by grafting compound and, more crucially, polished by a magnetic stirring bar over a long time during two-step reaction. However, CS-MA-DETA microspheres had a slightly larger mean pore diameter than CS microspheres, mainly because of the fact that the grafting reaction partly caused the collapse of pores that made the cumulative pore volume increased.

Figure 2. Nitrogen adsorption–desorption isotherms of CS (a) and CS-MA-DETA (b) microspheres; BJH–adsorption pore size distribution curves of CS (c) and CS-MA-DETA (d) 14


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microspheres. (CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan.)

3.1.6. SEM Observation. The images of CS and CS-MA-DETA microspheres at different magnifications were recorded via SEM, by which the surface morphology and texture of each sample was sketched out. As shown in Figure 3, SEM images showed that each sample was spherical or very nearly spherical in shape, and had a integral and regular surface, which could facilitate the separation and recycling of samples. For CS microspheres (Figure 3a, b, and c), its surface with uniform shallow sulci exhibited homogeneity and smoothness without any conspicuous protuberances, on which there were large numbers of exposed amino groups in favor of grafting reaction. Furthermore, its mean, maximum, and minimum microsphere diameters were 3.04 ± 0.02, 9.52, and 0.82 µm (Table S2), respectively, which was statistically calculated by an optical microscope (OLYMPUS CX31 RTSF, Tokyo, Japan). The size difference of microspheres was mainly due to various influential factors during the process of spray drying. As shown in Figure 3d, e, and f, compared with CS microspheres, the sulci on the surface of CS-MA-DETA microspheres seemed to be shallower, due to the outcomes of filling by grafting compound and abrasion by the magnetic stirring bar during two-step grafting reaction, which was consistent with BET results. After five-time adsorption for Pb(II) (Figure 3g, h, and i), although the sulci on the surface of CS-MA-DETA microspheres seemed to be more flattened, like being covered by a layer of thin film, the wrinkle-like topography was still able to be identified, which demonstrated that the new-prepared adsorbent, without participation of any cross-linker, was of good mechanical strength and reusability that was also confirmed by the mean microsphere diameter (2.89 ± 0.03 µm) after five-cycle reuse, without significant loss in particle size (Table S2). 15



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Figure 3. SEM images of CS，CS-MA-DETA, and Pb(II)-loaded CS-MA-DETA microspheres. (a, b, and c: pictures of CS microspheres; d, e, and f: pictures of CS-MA-DETA microspheres; g, h, and i: pictures of CS-MA-DETA microspheres after five-time adsorption for Pb(II); pictures of each group above are in turn taken at magnifications of 3k, 15k, and 30k ×. CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan.)

3.2. Adsorption Behavior. 3.2.1. Effect of pH. The initial pH of aqueous solutions is an important parameter that greatly influences the adsorption property of an adsorbent.41 It can affect not only the surface structure of an adsorbent but also the degree of ionization of the metal in solutions, which further impacts interactions between adsorbents and metal ions. As some literature reported, the adsorption of CS-based sorbents is highly related to the pH of metal ionic liquid that affects the lone electron pairs on CS amino groups that are easily protonated at a low pH. In this section, the effect of pH on adsorption of CS-MA-DETA microspheres was investigated, and the results are displayed in Figure 4a. Clearly, for both Cd(II) and Pb(II), the adsorption capacities of CS-MA-DETA 16


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microspheres exhibited a similar pH-dependence. The adsorption capacities for Cd(II) and Pb(II) increased with the increase of pH from 1 to 5, and from 1 to 4, respectively, which could be explained that with pH increasing, the free lone electron pair on nitrogen atom is more suitable for coordination with metallic cations. Subsequently, the adsorption capacities decreased with the further increase of pH, which might be attributed to the deprotonated amino groups. Such results above implied that both uptake behavior was mainly controlled by the coordination reaction. At a lower pH, on the one hand, the higher the protonated degree of amino groups was, the stronger the electrostatic repulsion between adsorption sites and metallic ions was; on the other hand, the adsorbent surface was covered with hydronium ions that strongly compete with metal ions for adsorption sites; as a result of which, the adsorption capacity of CS-MA-DETA microspheres for either Cd(II) or Pb(II) was very low. At pH 5 and 4, the adsorption capacities for Cd(II) and Pb(II) respectively reached maximum values, about 160 and 190 mg g−1. Additionally, for both metal ions, no adsorption platform was observed, due to the formation of precipitation with the further increase of pH. 3.2.2. Effect of Contact Time and Adsorption Kinetics. Figure 4b exhibits the effect of contact time on Cd(II) and Pb(II) adsorption on CS-MA-DETA microspheres. The adsorption capacity for Cd(II) or Pb(II) increased rapidly within the first 100 min, because of a large number of binding sites on CS-MA-DETA microspheres. During this period, about 87.5% of Cd(II) and 90% of Pb(II) were adsorbed, and the adsorption capacities were respectively 140 and 170 mg g−1. Then, the adsorption capacities increased slowly with the increase of contact time till reaching adsorption equilibrium. Because the metal ion diffusion into pores and the adsorption by interior surface were a slow process, after almost all facial adsorption sites of CS-MA-DETA 17



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microspheres were occupied.

Figure 4. Effects of initial pH (a), contact time (b), and initial concentrations (c) on adsorption

capacities towards Cd(II) and Pb(II) on CS-MA-DETA microsphere surface; selective adsorption (d) toward Cd(II) and Pb(II). (a: initial ion concentration 250 mg L−1, contact time 6 h, sample dosage 1 g L−1, temperature 25 °C, stirring speed 200 rpm; b: initial ion concentration 250 mg L−1, initial pH 5.0, sample dosage 1 g L−1, temperature 25 °C, stirring speed 200 rpm; c: initial pH 5.0, contact time 6 h, sample dosage 1 g L−1, temperature 25 °C, stirring speed 200 rpm; d: initial concentration 250 mg L−1, initial pH 5.0, contact time 4 h, sample dosage 0.5 g L−1, temperature 25

°C,

stirring

speed

200

rpm.

CS-MA-DETA,

DETA

grafted

CS-MA;

DETA,

diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan.)

To explore adsorption behavior, both pseudo-first-order and pseudo-second-order kinetic models were used to fit adsorption data, as shown in Figure S1a, b, c, and d. Generally, the linear pseudo-first-order and pseudo-second-order models are respectively expressed by Eq. (2) and (3).42–44 ݈݊(‫ݍ‬௘ െ ‫ݍ‬௧ ) = ݈݊(‫ݍ‬௘ ) െ ݇ଵ ‫ݐ‬ ௧ ௤೟

=

ଵ ௞మ ௤೐మ

൅

(2)

௧

(3)

௤೐

18


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where qe (mg g−1) is the adsorption capacity at equilibrium, qt (mg g−1) is the adsorption amount at time t (min), k1 (min−1) and k2 (g mg−1 min−1) are adsorption rate constants of pseudo-first-order and pseudo-second-order kinetic models, respectively. The pseudo-first-order and pseudo-second-order kinetic parameters of adsorption for Cd(II) and Pb(II) on CS-MA-DETA microspheres are exhibited in Table 2. The values of correlation coefficient (R2) of Cd(II) and Pb(II) indicated that the data were fitted better by pseudo-second-order kinetic model than by pseudo-first-order kinetic model, suggesting that the whole adsorption process was mainly controlled by chemisorption that was involved in valence forces for sharing or exchanging electrons through coordination or chelation between CS-MA-DETA microspheres and heavy metal ions. Table 2. Kinetic Model Parameters of the Adsorption for Cd(II) and Pb(II) on CS-MA-DETA Microspheres.a Kinetic model

Parameter

Cd(II)

Pb(II)

Pseudo-first-order

k1 (min−1)

2.13×10−2

2.12×10−2

qe, cal (mg g−1)

153.2

154.3

ܴଵଶ

0.9847 −1

Pseudo-second-order

−1

k2 (g mg min )

1.40×10

2.07×10−5

qe, cal (mg g−1)

191.2

219.3

ܴଶଶ Intraparticle diffusion

0.9977

0.9936

−1

−1/2

20.67

25.09

−1

−1/2

6.39

5.50

−1

−1/2

0.08

2.62

kp1 (mg g min kp2 (mg g min kp3 (mg g min C1 (mg g−1)

) ) )

–36.2

–37.4

−1

77.6

119.8

−1

161.2

149.8

C2 (mg g ) C3 (mg g ) a

0.9656 −4


MA, methyl acrylate; CS, chitosan.

For further confirmation of adsorption behavior, experimental data were also fitted via Weber and Morris kinetic model, which could provide information whether 19



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intraparticle diffusion was a key step in adsorption process or not. Weber and Morris kinetic model can be expressed as Eq. (4).45 భ

‫ݍ‬௧ = ݇௣ ‫ ݐ‬మ ൅ ‫ܥ‬

(4)

where qt (mg g−1) is the adsorption capacity at time t (min), kp (mg g−1 min−1/2) is the rate constant of intraparticle diffusion, and C (mg g–1) is an intraparticle diffusion constant. As shown in Figure S1e and f, three linear portions emerged in all curves, suggesting that more than one stage influences the adsorption process. Table 2 shows various parameters of Weber and Morris kinetic model. The values of kp obeying the order of kp1 > kp2 > kp3 might be attributed to the adsorption steps of exterior surface, intraparticle diffusion and equilibrium, respectively. The values of C calculated by Eq. (4) can reflect whether intraparticle diffusion (C = 0) is a rate limiting step or not.46 The results in Table 2 indicated that intraparticle diffusion was not a key rate limiting step in these adsorption processes. 3.2.3. Effect of Initial Concentration and Adsorption Isotherm. Figure 4c displays the effect of various initial concentrations on Cd(II) and Pb(II) adsorption on CS-MA-DETA microspheres. For either Cd(II) or Pb(II), the adsorption capacity was increased quickly at the first stage with initial concentration increasing, and then reached at a adsorption platform. The fast increase in metal ion loading amount on CS-MA-DETA microspheres could be explained by the greater driving force due to a higher concentration gradient pressure. The equilibrium isotherm model is the fundamental in describing interactive behavior between metal ions and adsorbents. The adsorption isotherms of CS-MA-DETA microspheres for Cd(II) and Pb(II) were determined by Langmuir and Freundlich isotherms, as shown in Figure S2. The Langmuir and Freundlich isotherms are respectively described by Eq. (5) and (6).47,48 20


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஼೐ ௤೐

=

ଵ ௤೘ ௄ಽ

൅

஼೐

(5)

௤೘

భ ೙

‫ݍ‬௘ = ‫ܭ‬ி ‫ܥ‬௘

(6)

where Ce (mg L−1) is the equilibrium concentration of metal ions in solution, qe (mg g−1) is the adsorption capacity at equilibrium, qm (mg g−1) is the maximum adsorption capacity, KL (L mg−1) and KF (mg1−1/n L1/n g−1) are Langmuir and Freundlich constants, respectively, n is Freundlich exponent related to adsorption intensity. The parameters of Langmuir and Freundlich isotherms are shown in Table 3. The values of correlation coefficient (R2) of Cd(II) and Pb(II) indicated the data were fitted better by Langmuir isotherm than by Freundlich isotherm, suggesting that Langmuir isotherm model could well interpret the adsorption procedure, which was mainly a monolayer adsorption on surface with the finite number of identical sites. For evaluating adsorption process, the favorability of adsorption for Cd(II) and Pb(II) on CS-MA-DETA microspheres was further studied by detecting RL, which was calculated by Langmuir Eq. (7).45,46 ܴ௅ =

ଵ

(7)

ଵା௄ಽ ஼బ

where C0 (mg L−1) is the initial concentration of metal ions, KL (L mg−1) is Langmuir constant, RL demonstrates the adsorption nature, which can be divided as irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), and unfavorable (RL > 1).46 As shown in Table 3, KL > 0, thus, all the values of RL were in the range of 0–1, indicating that the adsorption process for Cd(II) or Pb(II) was favorable. Additionally, the maximum adsorption capacity calculated by Langmuir model was 239.2 mg Pb(II)/g or 201.6 mg Cd(II)/g, which was higher than those of most available CS-based adsorbents. The comparison of the maximum adsorption capacities of

21



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CS-MA-DETA microspheres for Cd(II) and Pb(II)) with various CS-based absorbents reported by journals is presented in Table S3. Table 3. Isotherm Model Parameters of the Adsorption for Cd(II) and Pb(II) on CS-MA-DETA Microspheres.a Isotherm model

Parameter

Cd(II)

Pb(II)

Langmuir

qm (mg g−1)

201.613

239.234

KL (L mg−1)

0.025

0.021

R2

0.994

0.997

n

1.615

1.565

KF (mg1−1/n L1/n g−1)

8.080

8.998

R2

0.958

0.958

Freundlich

a


MA, methyl acrylate; CS, chitosan.

3.2.4. Selective Adsorption. Figure 4d shows the selective adsorption for Cd(II) and Pb(II) of CS-MA-DETA microspheres in the binary metal ion solution at pH 5.0 and different contact time. The results indicated the adsorption for Pb(II) was better than that for Cd(II) in the binary metal ion solution, which could be explained by the ଶ covalent index ܺ௠ ‫( ݎ‬here, ܺ௠ stands for electronegativity, r stands for ionic ଶ radius).29 The covalent index ܺ௠ ‫ ݎ‬is well correlated to the adsorption capacity for ଶ metal ions.49 Generally, the greater the ܺ௠ ‫ ݎ‬value is, the more the covalent bonds

between the adsorbent and metal ions form, and the larger the adsorption capacity on ଶ the adsorbent surface is.50 According to HSAB theory,51 the higher the ܺ௠ ‫ ݎ‬value

was, the more the characteristics of soft acid was presented. Pb(II) ion belonged to soft ion that tended to combine with amino groups on CS-MA-DETA microspheres, while Cd(II) belonged to intermediate ion that was of less binding capacity with amino groups than Pb(II). 3.2.5. Reusability. Reusability is one of the crucial factors to evaluate a new 22


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adsorbent. Here, desorption experiments were conducted by using 0.5 mol L−1 of HCl solution as an eluent. The adsorption–desorption tests were repeated five times, and the results showed that the adsorption capacities of CS-MA-DETA microspheres for Cd(II) and Pb(II) slightly decreased with the increase of cycle times (Figure 5). However, the adsorption capacities were all more than 90% during five-time adsorption–desorption cycles, which illustrated that CS-MA-DETA microspheres almost had no obvious loss of adsorption capacity. Such results were also in accordance with SEM images (Figure 3g, h, and i) and the mean microsphere diameter (2.89 ± 0.03 µm) after the fifth recycle (Table S2), which demonstrated the newly-fabricated adsorbent was of good mechanical strength, chemical stability, and good reusability.

Figure 5. Results of five consecutive adsorption−desorption for the reuse of CS-MA-DETA microspheres for Cd(II) and Pb(II) adsorption. (initial concentration 250 mg L−1, initial pH 5.0, contact time 4 h, sample dosage 1 g L−1, temperature 25 °C, stirring speed 200 rpm.)

3.3. Bonding Mode and Adsorption Site. To further understand the bonding modes of M2+ (M2+ stands for Pb(II) or Cd(II)), and to preliminarily identify the possible adsorption sites of M2+ on CS-MA-DETA microspheres, FTIR spectra before and after M2+ adsorption were recorded, as shown in Figure 1d. More clearly, almost similar profiles could be observed in FTIR spectra of Pb(II)- and Cd(II)-loaded 23



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CS-MA-DETA microspheres, implying that both metal ions very likely had the same adsorption sites and/or binding modes. The characteristic band at 3387 cm−1 (stretching vibration of O–H) of CS-MA-DETA microspheres shifted to 3396 or 3421 cm−1, with a significant decrease in intensity after M2+ adsorption, suggesting that M2+ might coordinate with oxygen atom on CS-MA-DETA microspheres.52 The band at 1599 cm−1 (stretching vibration of –NH2) of CS-MA-DETA microspheres was changed to 1555 or 1540 cm−1 (amide II) after M2+ adsorption, which could be explained that –NH2 was involved in ion adsorption. Besides, the bands at 1419, 1375, and 1321 cm−1 of CS-MA-DETA microspheres were combined to form one broad band at around 1384 cm−1 after M2+ adsorption, which might be explained that the coordination reaction with M2+ had the influence on C–H bonds of saccharides chains. The band at 2872 cm−1, which was assigned to both C–H and O–H stretching vibration, also decreased in intensity after M2+ adsorption, indicating that the oxygen atom was involved in ion adsorption.53 Additionally, the bands at 1161 (–C–O–C– stretching vibration) and 1083 cm−1 (O–H stretching vibration) moved to 1153 and 1068 cm−1, respectively,54 which indicated that hydroxyl groups were also involved in adsorption reaction. Notably, a new band appeared at 826 cm−1 in the spectrum of the materials after M2+ adsorption, which might be attributed to the formation of N–M2+ or O–M2+ bond. As is known to all, both nitrogen and oxygen atoms have lone electron pairs that can bind metal ions through electron pair sharing to form the complex.53 Thus, it was reasonable to assume that nitrogen and oxygen atoms should be main adsorption sites for M2+ adsorption on CS-MA-DETA microspheres. Rangel-Mendez et al. consider metal ions with empty orbital function as a Lewis acid are capable of accepting electron pairs, while –NH2 and –OH groups as Lewis bases donating their electron pairs.55 At slightly acid pH, –NH2 and –OH groups on 24


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CS-MA-DETA microspheres donated their electron pairs to M2+ to form complex through coordinate covalent bond. However, at lower pH, there were acid-base type reactions between amino groups and free orbitals in protons that could replace M2+ via an ion exchange process, where (1) an electrostatic repulsion existed between protonated amino groups and M2+; (2) the protons strongly competed with M2+ for adsorption sites, leading to a low adsorption capacity of M2+.

Figure 6. Schematic preparation procedure of CS-MA-DETA microspheres and probable bonding modes and adsorption sites for Cd(II) and Pb(II) on CS-MA-DETA microsphere surface. (CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan.) 25



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Based on FTIR results and analysis above, as well as on the report from Debbaudt et al.,56 a series of binding modes and adsorption sites were proposed, which were that (1) M2+ was bonded to two –NH2 (or –NH), (2) to one –NH2 and one C6–OH, (3) to two C3–OH, (4) to two O (C6–OH and C3–OH), and (5) to four –NH2. The schematic adsorption sites and binding modes are illustrated in Figure 6.

4. CONCLUSION In this work, a novel absorbent, CS-MA-DETA microspheres, was successfully fabricated via a two-step way to graft MA to amino groups on CS microsphere surface first, and to graft DETA to terminal ester groups of MA on CS-MA microsphere surface afterwards, without any cross-linker, which was confirmed by FTIR and elemental analysis. XRD and TGA results showed its well-defined crystallinity and good thermostability. BET and SEM results illustrated the new absorbent possessed a large specific surface area and uniformly wrinkle-like morphology. Furthermore, adsorption tests in single or binary metal ion solution were conducted to evaluate its adsorption capacities for both metal ions, and the maximum adsorption capacities for Cd(II) and Pb(II) were respectively 201.6 and 239.2 mg g−1. Its uptake behavior could be well described by Langmuir isotherm and pseudo-second-order kinetic models, indicating that it was mainly a monolayer adsorption and chemisorption process. The analyses of bonding modes and adsorption sites for both ions showed a multi-mode and multi-site adsorption. Additionally, Cd(II)- and Pb(II)-loaded CS-MA-DETA microspheres could be easily regenerated and recycled at least five times. Overall, CS-MA-DETA microspheres with outstanding adsorption performance towards Pb(II) and Cd(II) might serve as a promising adsorbent.

■ ASSOCIATED CONTENT 26


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Supporting Information Summary of materials and chemicals used; characterization of materials; surface parameters; statistics of microsphere diameters; comparison of maximum adsorption capacities; linear fitting plots of kinetic and isotherm models

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86 0532 82032586. Fax: +86 0532 82032586. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by the Science and Technology Development Funds of Qingdao Shinan (2015-5-015-ZH), the Applied Basic Research Program for Youngster of Qingdao (15-9-1-42-jch), the Shandong Province Natural Science Foundation (ZR2014CQ052), and the National Natural Science Foundation of China (NSFC, 31400812).

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ABSTRACT GRAPHIC

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Uptake of Pb(II) and Cd(II) on Chitosan Microsphere Surface

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