Facile Preparation of Magnetic Chitosan Coprecipitated by Ethanol

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Article Cite This: ACS Omega 2018, 3, 5725−5734

Facile Preparation of Magnetic Chitosan Coprecipitated by Ethanol/ NH3·H2O for Highly Efficient Removal toward Cr(VI) Weiquan Cai,*,†,‡,§ Wenhui Xue,‡,§ and Yihong Jiang‡ †

School of Chemistry and Chemical Engineering, Guangzhou University, 230 Guangzhou University City Outer Ring Road, Guangzhou 510006, P. R. China ‡ School of Chemistry, Chemical Engineering & Life Sciences, State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Luoshi Road 205#, Wuhan 430070, P. R. China S Supporting Information *

ABSTRACT: The adsorption ability of chitosan (CS) usually decreases with the introduction of magnetic particles, and thus magnetic CS has to be chemically modified further to improve its adsorption performance. Herein, a novel magnetic chitosan composite (Fe3O4−CS3) with porous structure and evenly distributed Fe3O4 was successfully prepared via the reduction of the solubility of CS by ethanol (physical reaction), and followed the coprecipitation of the mixture of FeCl3/FeCl2/CS by ethanol/NH3·H2O. Without any modification, its maximum adsorption capacity toward Cr(VI) ions can achieve 242.1 mg/ g (≈468.6 mg/g of CS). This significant progress could be ascribed to the very fast precipitate rate of CS due to the decrease in solubility induced by ethanol. Ethanol causes rigorous solidification of CS, so that there is not enough time to densify, resulting in a looser CS matrix with a larger pore size. After adsorption, this Fe3O4−CS3 could not only be easily separated but also be effectively regenerated by NaOH solution in a rather wider concentration range, showing great potential in the field of heavy metal wastewater treatment. improve its performance.15,16 On the other hand, magnetic CS prepared by coprecipitation seems to show a better adsorption capacity toward heavy metal ions. For method II, the Fe3O4 nanoparticles are relatively uniformly dispersed in the continuous CS phase. For example, the maximum adsorption capacity of Fe3O4−CS prepared by a facile one-step method can reach 63.5 mg/g toward Cu(II).17 Although in the peer literatures based on method I, the maximum adsorption capacities of CS-bound Fe3O4 nanoparticles18 and xanthatemodified magnetic CS19 are 21.5 and 34.5 mg/g toward Cu(II), respectively. Unfortunately, there is no specialized research on the effects of the introduction ways on the adsorption performance of CS toward heavy metal ions in one single study. Both for methods I and II, CS should be dissolved in an acid solution for the following blend with magnetic particles.20,21 In this step, the amino (−NH2) of CS is protonated into −NH3+. To finally obtain solid-state products, a CS-precipitated process, which could be divided into three types based on the precipitation mechanism, is involved: cross-linking reaction induced by cross-linker,22,23 deprotonation of −NH3+ induced by alkali,24,25 and solubility decrease induced by alcohol.26,27 Among them, the former two are induced by chemical

1. INTRODUCTION Magnetic chitosan (CS) composite has been extensively investigated for adsorbing heavy metal ions because of its characteristic of easy magnetic separation,1−4 and the controllable modification of CS via the free amino and hydroxyl groups for highly efficient removal.5,6 Its most used preparation procedure is first to obtain the magnetic micro/nanoparticles, followed by CS blending (labeled as method I).7,8 Meanwhile, a few explorations appear in preparing magnetic CS by a simple one-step method, which includes coprecipitating FeCl3/FeCl2/ CS solution by alkali (labeled as method II).9,10 For example, after adding a small amount of CS powder to FeCl3/FeCl2 mixture at 50 °C, alkali precipitation was carried out once the aqueous ammonia was dripped, resulting in CS-coated Fe3O4 nanoparticles.11 It seems that different ways of introducing magnetic particles to CS influence the dispersion situation of the composites, as well as their adsorption performance. Especially, if CS is in excess in method I, the magnetic particles are inclined to aggregate or remain on the surface of CS.12,13 Yu et al.14 added Fe3O4 nanoparticles into the CS/FeCl3 solution to synthesize magnetic CS−Fe(III) complex; its scanning electron microscopy (SEM) image shows that the Fe3O4 spheres are anchored on the surface of the CS−Fe(III) resins with high density. This is the main reason why magnetic CS prepared by method I usually exhibits poor adsorption capacity, and thus various chemical modifications were conducted to © 2018 American Chemical Society

Received: March 4, 2018 Accepted: May 11, 2018 Published: May 28, 2018 5725

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Figure 1. SEM images with different magnifications of (a) Fe3O4−CS1, (b) Fe3O4−CS2, and (c) Fe3O4−CS3, as well as the corresponding energydispersive X-ray spectroscopy (EDS) elemental mapping of Fe and N.

around 2 μm, respectively. Furthermore, Figure 1a2−c2 shows that Fe3O4−CS1 and Fe3O4−CS2 look like dense microparticles, whereas Fe3O4−CS3 consists of loose, porous blocks with many fragments. This phenomenon may result from the difference in the speed of precipitating CS among glutaraldehyde (GLA), NH3·H2O, and ethanol. From the point view of precipitation mechanism, cross-linking reaction induced by glutaraldehyde and deprotonation of −NH3+ induced by NH3· H2O are chemical changes, which require much energy to maintain. However, after adding ethanol, the CS solubility decreases and simultaneously precipitates because CS is insoluble in ethanol, and this physical precipitation is much easier and faster than that of the other two precipitation processes. This theoretical analysis agrees well with the experimental observations: flocculent precipitate slowly developed in the case of glutaraldehyde or NH3·H2O, whereas transparent gel quickly filled the whole beaker once the ethanol was added. Fast precipitate rate causes too hurried solidification of CS so that there is not enough time to densify, resulting in loose, porous blocks with large size, just like Fe3O4−CS3. Similarly, CS−Fe(III) complexes precipitated by ethanol usually have many cracks.14 Furthermore, elemental mapping of microstructures by SEM with energy-dispersive X-ray spectroscopy (EDS) was shown in Figure 1a3−c3 for element Fe (characteristic element of Fe3O4) and Figure 1a4−c4 for element N (characteristic element of CS). The presence of Fe and N further confirms the successful preparation of Fe3O4−CS. However, the uniform distribution

reactions, whereas the latter is attributed to the physical reaction, which is common in the fabrication of CS−metal ion complexes (such as CS−Fe(III)).28,29 Therefore, it is of great significance to investigate the effects of the precipitation types on the adsorption performance of CS. To the best of knowledge, there is no report on preparing magnetic CS with alcohol. Herein, we report a facile in situ method to prepare a magnetic CS composite (Fe3O4−CS3) with enhanced adsorption performance toward Cr(VI) ions via coprecipitating FeCl3/FeCl2/CS mixture by ethanol/NH3·H2O. The Fe3O4−CS3 shows better performance than that obtained by mixing pre-prepared Fe3O4 nanoparticles, and than that precipitated by NH3·H2O alone, respectively. This Fe3O4−CS3 can not only be easily separated after adsorption, but also be effectively regenerated by NaOH solution in a rather wider concentration range, showing a great potential in the field of heavy metal wastewater treatment.

2. RESULTS AND DISCUSSION 2.1. Structure and Morphology. The X-ray diffraction (XRD) patterns and Fourier transform infrared (FT-IR) spectra of Fe3O4−CS1, Fe3O4−CS2, Fe3O4−CS3, and pure CS in Figure S1 show that CS was successfully bounded with well-crystallized Fe3O4 in each case. The SEM images of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 in Figure 1 show that all of them present irregular microparticles. Comparatively, the particles size of Fe3O4−CS3 around 10 μm is much greater than that of Fe3O4−CS1 around 1 μm and that of Fe3O4−CS2 5726

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Figure 2. TEM images with different magnifications of (a) Fe3O4−CS1, (b) Fe3O4−CS2, and (c) Fe3O4−CS3.

Figure 3. Formation illustrations of (a) Fe3O4−CS1, (b) Fe3O4−CS2, and (c) Fe3O4−CS3.

of the elements in micronscale could not precisely characterize the disperse state on the nanoscale. Their high-magnification

SEM images are shown in Figure 1a5−c5. The surface of Fe3O4−CS1 in Figure 1a5 is rough and some tiny particles with 5727

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Figure 4. (a) ζ-Potential curves of Fe3O4−CS at different pH, (b) effect of the initial pH on the adsorption of Cr(VI) by Fe3O4−CS (adsorption conditions: C0 = 100 mg/L, V = 25 mL, m = 25 mg, t = 4 h, T = 25 °C), (c) FT-IR curves of Fe3O4−CS before and after Cr(VI) adsorption, (d) vibrating sample magnetometer (VSM) magnetization curves of Fe3O4−CS (inset shows the photos of the adsorption system before and after adsorption attracted by an external magnet).

Scherrer equation (see the Supporting Information). The similar TEM images of Fe3O4−CS2 and Fe3O4−CS3 show that addition of ethanol does not change the distribution of Fe3O4 in CS. 2.2. Formation Mechanism. As shown in Figure 3, Fe3O4 nanoparticles were prepared by coprecipitating FeCl3/FeCl2 in the presence of ammonia as follows30

diameters of about 15 nm (in circle 2#) could be discernible on it, just like the particles scattered out of the CS phase (in circle 1#). The literature also indicates that if CS is in excess, the added magnetic particles are inclined to aggregate or remain on the surface of CS.13,14 However, the surface of Fe3O4−CS2 in Figure 1b5 is relatively smooth without extensive attachments. Figure 1c5 clearly confirms that Fe3O4−CS3 consists of loose, porous blocks with many tiny fragments. Transmission electron microscope (TEM) images in Figure 2 further clarify the distribution of Fe3O4 in CS. Figure 2a shows that the well-defined Fe3O4 nanoparticles (∼15 nm) in Fe3O4−CS1 slightly aggregate, resulting in a nonuniform dispersion. The high contrast ratio between Fe3O4 nanoparticles and CS matrix implicitly proves that Fe3O4 nanoparticles anchor on the surface of CS, indicating that the particles in red circles (Figure 1a5) are Fe3O4. By comparison, Fe3O4 nanoparticles in Fe3O4−CS2 and Fe3O4−CS3 uniformly disperse without an obvious color difference. Furthermore, for samples obtained from the two coprecipitation processes in Figure 2b,c, the Fe3O4 nanoparticles show irregularly shaped nanoparticles, just like tiny fragments embedded in CS. The sizes of Fe3O4 nanoparticles in the two cases are both around 10 nm, which is consistent with the calculation based on the

NH3·H 2O ⇄ NH4 + + OH−

(1)

Fe3 + + 3OH− → Fe(OH)3

(2)

Fe2 + + 2OH− → Fe(OH)2

(3)

2Fe(OH)3 + Fe(OH)2 → Fe3O4 + 4H 2O

(4)

Then, after blending Fe3O4 with the CS solution, the mixture was cross-linked by Schiff’s reaction between aldehyde group of glutaraldehyde (denoted as r-CHO) with amine group of CS (denoted as R-NH2), resulting in insoluble polymers31 r‐CHO + R‐NH 2 → r‐CHN‐R + H 2O

(5)

Thus, Fe3O4−CS1 yielded. Different from injecting the Fe3O4 nanoparticles to the acid CS solution directly, Fe3O4 nano5728

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Figure 5. Adsorption kinetics (a), the corresponding pseudo-first-order model (b), pseudo-second-order model (c), and intraparticle diffusion model (d) of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 (reaction conditions: C0 = 100 mg/L, V = 100 mL, m = 100 mg, T = 25 °C).

particles in Fe3O4−CS2 and Fe3O4−CS3 arose in situ. Before the formation of Fe3O4 in FeCl3/FeCl2/CS solution, Fe3+ ions would first form a complex with the hydroxyl group (−OH) of protonated CS via chelate bond, as shown in Figure 3b,c. In other words, Fe3+ ions are uniformly fixed on the CS, and this limits their mobility. Therefore, once Fe2+ and NH3·H2O were successively added, Fe3O4 nanoparticles in smaller size uniformly emerged, just following 1−4. At the same time, the soluble protonated CS (denoted as R-NH3+) was deprotonated into insoluble CS (R-NH2) by NH3·H2O R‐NH3+ + OH− → R‐NH 2 + H 2O

2.3. Adsorption Performance. 2.3.1. Effects of pH. The pH dependence of adsorbing Cr(VI) is largely connected with the surface charge of the adsorbent and the speciation of Cr(VI) ions. Effects of pH value on the ζ-potentials of the samples are shown in Figure 4a. It is clear that, as the pH increases, their surface potentials turn from positive charge to negative charge at the isoelectric points (pHZPC). When the pH < pHZPC, the amino group of Fe3O4−CS was protonated into −NH3+, making its surface electropositive. With increasing pH, −NH3+ of Fe3O4−CS would be gradually deprotonated, even negatively charged at pH > pHZPC. The pHZPC of Fe3O4−CS2 and Fe3O4−CS3 are 9.74 and 9.08, respectively, which are much higher than 5.01 of Fe3O4−CS1. This could be attributed to the fact that the surface of Fe3O4−CS1 is seriously covered by Fe3O4 nanoparticles, which carry numerous electronegative hydroxyl groups, resulting in low pHZPC. The high pHZPC of Fe3O4−CS2 and Fe3O4−CS3 can yield a stronger electrostatic attraction between the adsorbents and the Cr(VI) anions, and thus their Cr(VI) uptake efficiencies increase. Cr(VI) exists in various forms such as H2CrO4, HCrO4−, and CrO42−, depending on the pH.32 When the pH < 2, H2CrO4 and HCrO4− coexist; when the pH is between 2 and 4, primarily HCrO4− exists; at a higher pH, CrO42− appears and increases gradually; and when the pH > 9, HCrO4− disappears,

(6)

Based on the two simultaneous processes, Fe3O4 nanoparticles uniformly embed into CS composite (Figure 3b), resulting in Fe3O4−CS2. While for the precipitants of ethanol/NH3·H2O containing ethanol which is much easier and faster than that of NH3·H2O to precipitate the aqueous CS, ethanol dominates the precipitation process of CS by decreasing its solubility seriously. Fast precipitate rate causes rigorous solidification of CS, so that there is not enough time for Fe3O4−CS3 to densify, resulting in loose, porous blocks with large size (Figure 3c). Meanwhile, Fe3O4 nanoparticles arose by coprecipitation, and protonated CS was deprotonated by NH3·H2O. 5729

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closer values between qe and qe,exp, the adsorption data fit the pseudo-second-order model better than those of the pseudofirst-order model, indicating that the adsorption of Cr(VI) onto each Fe3O4−CS may be controlled by chemical adsorption. Furthermore, in comparison with Fe3O4−CS1 and Fe3O4− CS3, the value of k2 for Fe3O4−CS2 shows faster adsorption kinetics, which is consistent with the result of adsorption equilibrium time. To investigate whether surface or pore diffusion is the controlling step in the adsorption process or not, intraparticle diffusion model (see the Supporting Information) was used.22 Three linear portions in the plots of qt vs t1/2 (Figure 5d) indicates that the adsorption of Cr(VI) on Fe3O4−CS occurs via three steps. As shown in Table S4, the intraparticle diffusion rate constant in each case follows the same order: kid,1 > kid,2 > kid,3. The rate-limiting steps could be described as follows: (1) the first sharp portion corresponded to the surface diffusion stage, in which a large amount of Cr(VI) ions from the bulk solution are quickly adsorbed onto the surface protonated amino groups of Fe3O4−CS; (2) when the surface active sites are completely occupied, the Cr(VI) gradually enters the interior surface of the pores in the second stage, in which the intraparticle diffusion (pore diffusion) is the rate-controlling step; and (3) the intraparticle diffusion rate of the third portion is almost zero, suggesting that the final equilibrium stage is reached. As shown in Table S4, kid,1 and kid,2 of Fe3O4−CS3 are much higher than those of Fe3O4−CS1 and Fe3O4−CS2, indicating that Cr(VI) adsorption on Fe3O4−CS3 is the fastest. Especially, kid,2 of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 are 0.625, 0.743, and 1.596 mg/(g min)1/2, respectively, which are closely related to intraparticle diffusion. However, it is Fe3O4−CS3 that spends the most time to reach its adsorption equilibrium. On the one hand, the more Cr(VI) ions the adsorbent adsorbs, the more time it takes. On the other hand, due to the biggest average pore size of 10.03 nm and the largest pore volume of 0.04 cm3/g of Fe3O4−CS3, its adsorption toward Cr(VI) ions greatly involves in pore diffusion (with high kid,2) and takes more time for Cr(VI) ions to reach its pore surface. 2.3.5. Adsorption Isotherms. Their adsorption isotherms at 25 °C are depicted in Figure 6a. With the increasing concentration of Cr(VI), their adsorption capacities rise sharply and then tend to be saturated. Obviously, Fe3O4−CS2 shows a much higher adsorption capacity (152.5 mg/g) than Fe3O4− CS1 (82.1 mg/g). More interestingly, the adsorption performance of Fe3O4−CS2 could be significantly enhanced via precipitating FeCl3/FeCl2/CS mixture by ethanol/NH3·H2O (i.e., the adsorption capacity of Fe3O4−CS3 can reach 235.4 mg/g). Langmuir and Freundlich models (see the Supporting Information) were used to simulate the data,37 as shown in Figure 6b,c, respectively. Langmuir model assumes that a monomolecular layer is formed when adsorption takes place without any interaction between the adsorbed molecules.38 However, Freundlich isotherm is an empirical equation that assumes that the adsorption process takes place on heterogeneous surfaces or surfaces containing sites of varied affinities.39 Their parameters calculated from the models are listed in Table S5. It shows that the Langmuir isotherm fits better to the data with all R2 > 0.98, indicating that the Cr(VI) adsorption of each sample is a monolayer adsorption. The maximum adsorption capacities of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 are 97.9, 156.2, and 242.1 mg/g, respectively. Compared with CS, which contains

leaving only CrO42−. Thus, the negatively charged Cr(VI) ions have a strong interaction with the positively charged adsorbents. The effect of the initial pH on the Cr(VI) adsorption of Fe3O4−CS is shown in Figure 4b. It is obvious that with increasing pH, the adsorption capacities of the three samples all decrease notably. Therefore, pH = 2 was chosen as the optimum pH for the following experiments. 2.3.2. Adsorption Mechanism. The highly pH-dependent adsorption process is mainly based on the electrostatic attraction between HCrO4− and −NH3+ on the surface of Fe3O4−CS. To further elucidate the effect of functional groups on the adsorption mechanism,33 the surface interaction involved in the adsorption process was characterized by FTIR, as shown in Figure 4c and Table S2. Taking Fe3O4−CS2 as an example, its FT-IR after adsorption exhibits a significant shift from 3447.7 to 3432.3 and 1633.5 to 1628.4 cm−1, whereas other peaks shift slightly. This reveals that the amino groups may participate in the Cr(VI) removal process. As mentioned earlier, the −NH2 on the surface of Fe3O4−CS are easily protonated to a positively charged −NH3+ under acidic condition and favorable for anion adsorption through electrostatic interaction. Another obvious feature after adsorption is that a new peak appears at ∼940 cm−1, which is the characteristic peak of Cr(VI)−O,34 indicating that the adsorption process on Fe3O4−CS (R-NH2) is as follows35 R‐NH 2 + H+ → R‐NH3+

(7)

2R‐NH3+ + HCrO4 − → R‐NH3+ ··· HCrO4 − ··· H3+N‐R (8)

2.3.3. Magnetic Separation. The magnetization curves of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 are shown in Figure 4d. Their corresponding saturation magnetizations are 51.48, 28.5, and 11.8 emu/g, respectively. Because of the nonmagnetic CS, its content in Fe3O4−CS as well as the combination between CS and Fe3O4 impacts the magnetic moment. For Fe3O4−CS1, its CS content of 37.06% (see Figure S2) is lower than 42.91% for Fe3O4−CS2 and 51.66% for Fe3O4−CS3, and the Fe3O4 nanoparticles are anchored on the surface of CS, resulting in the highest saturation magnetization. However, compared with Fe3O4−CS2, the agglomerate size of Fe3O4− CS3 is much larger, and the porous structure further quenches the magnetic moment, resulting in the lowest saturation magnetization. Even so, the magnetic moment of Fe3O4−CS3 is large enough for magnetic separation (see the inset of Figure 4d). The particles of Cr(VI)-loaded Fe3O4−CS3 can be quickly separated within 30 s by the permanent magnet, and the solution became limpid accordingly. 2.3.4. Adsorption Kinetics. Adsorption of Cr(VI) on Fe3O4−CS as a function of the contact time is presented in Figure 5a. For the three adsorbents, the very fast Cr(VI) adsorption at the initial stage (0−30 min) implies an external surface diffusion; during the second period (0.5−1 h), the Cr(VI) adsorption is milder and more gradual, whereas the last stage (1−8 h) is the equilibrium state. Furthermore, Fe3O4− CS2 requires a rather shorter time to reach the adsorption equilibrium (within 1 h). To investigate the controlling mechanism of the adsorption process, the pseudo-first-order and pseudo-second-order models (see the Supporting Information) were used to assess the data.36 The plots of the kinetic models are shown in Figure 5b,c, and the corresponding parameters are listed in Table S3. Because of the higher correlation coefficients (R2) and the 5730

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tion capacity of CS singly, which turns to be 264.16, 364.01, and 468.64 mg/g, respectively. In other words, the CS in Fe3O4−CS3 shows the most efficient adsorption toward Cr(VI). It is generally recognized that the greater the surface area, the larger the saturated adsorption amount. Herein, their Brunauer−Emmett−Teller (BET) surface areas (see Table S1) from high to low are 47.7 m2/g (Fe3O4−CS1) > 32.4 m2/g (Fe3O4−CS2) > 17.3 m2/g (Fe3O4−CS3). However, their adsorption capacities follow the reverse order of the BET surface area. This anomaly may result from their construction features. During the process of injecting the Fe3O4 nanoparticles to the CS solution, the tiny particles were inclined to anchor on the surface of Fe3O4−CS1, resulting in a high surface area but leaving the valuable CS surface seriously covered. However, for Fe3O4−CS2 prepared from coprecipitating the FeCl3/FeCl2/CS mixture by NH3·H2O, the Fe3O4 nanoparticles were uniformly dispersed in the continuous CS phase, making more CS participate in the electrostatic adsorption reaction. This is the reason why Fe3O4−CS2 shows a much better adsorption performance than Fe3O4−CS1. On the other hand, compared with Fe3O4−CS2, the adsorption capacity of Fe3O4−CS3 was significantly enhanced via precipitating FeCl3/FeCl2/CS mixture by ethanol/NH3·H2O. Fast precipitate rate induced by ethanol causes too hurried solidification of CS, so that there is not enough time to densify, resulting in loose blocks with large pore size, just like Fe3O4− CS3. Adsorption is a physicochemical process that involves mass transfer of ions from liquid phase to the adsorbent’s surface, as well as the inner side. So, too narrow pores are not favorable for ion diffusion, especially for big ions such as HCrO4−. The BJH results reveal that the average pore size (see Figure S3 and Table S1) of Fe3O4−CS3 (10.0 nm) is much bigger than that of Fe3O4−CS2 (5.2 nm), and thus Fe3O4−CS3 shows a much better adsorption performance toward Cr(VI) than Fe3O4−CS2. A comparison has been made between the prepared Fe3O4− CS and the previously reported CS-based adsorbents for Cr(VI) adsorption (see Table S6). The results show that without any modification, Fe3O4−CS3 gains advantage over many other similar adsorbents (even the modified ones), indicating that the method we explored can greatly improve the adsorption efficiency of CS, and thus provides a better platform for the further chemical modification of magnetic CS. 2.3.6. Desorption and Reusability. Usually, NaOH is used to regenerate heavy metal ions-loaded CS-based adsorbents because OH− ionized from NaOH can neutralize the protonated amino group of CS, resulting in the desorption of the adsorbed ions. Taking Cr(VI)-loaded Fe3O4−CS as an example, the desorption process could be proposed as follows. R‐NH3+ ··· HCrO4 − ··· H3+N‐R + 3OH− → 2R‐NH 2 + CrO4 2 − + 3H 2O

Figure 6. Adsorption isotherms (a), the corresponding Langmuir model (b), and Freundlich model (c) of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 (adsorption conditions: C0 = 50−1000 mg/L, V = 25 mL, m = 25 mg, t = 4 h, T = 25 °C).

(9)

Effect of concentration of NaOH solution on the desorption of the Cr(VI)-loaded Fe3O4−CS is shown in Figure 7a. It is obvious that the optimal desorption rates of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 are 85.96, 96.97, and 94.75%, respectively, at C(NaOH) = 0.05 mol/L. Therefore, 0.05 mol/L was further used to evaluate their reusability toward Cr(VI) (Figure 7b), and the corresponding adsorption−desorption cyclic process is proposed in Figure 8. After three adsorption− desorption cycles, the adsorption capacities of Fe3O4−CS1, Fe3O4−CS2, and Fe3O4−CS3 are reduced by 27.8, 16.4, and

numerous free −NH2 and −OH groups, the contribution of Fe3O4 in the adsorbtion of Cr(VI) is negligible. Considering that the CS contents (see Figure S2) in Fe3O4−CS1, Fe3O4− CS2, and Fe 3 O 4 −CS3 are 37.06, 42.91, and 51.66%, respectively, it is of great significance to describe their adsorption performance by calculating the equivalent adsorp5731

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Figure 7. (a) Desorption of Cr(VI)-loaded Fe3O4−CS in various NaOH solutions (0.001−0.5 mol/L, 100 mL). (b) Adsorption capacity of Fe3O4− CS for Cr(VI) during the cyclic experiments (initial conditions: C0 = 100 mg/L, V = 100 mL, m = 100 mg, t = 4 h, T = 25 °C).

Figure 8. Schematic illustration of the Cr(VI) adsorption−desorption cycle of Fe3O4−CS.

13.6%, respectively. Therefore, Fe3O4−CS3 with slight losses after recycling for three times possesses the largest potential for Cr(VI) removal.

g (≈468.6 mg/g of CS). Fast precipitate rate induced by ethanol causes too hurried solidification of CS, so that there is not enough time for Fe3O4−CS3 to densify, resulting in loose blocks with large pore size.

3. CONCLUSIONS Three magnetic CS composites prepared from different blending/precipitating processes show significant difference in terms of Cr(VI) removal. For Fe 3 O 4 −CS1, the Fe 3 O 4 nanoparticles incline to anchor on the surface of the CS, resulting in high surface area but leaving the valuable CS surface seriously covered. While for Fe3O4−CS2 prepared from precipitating the mixture of FeCl3/FeCl2/CS by NH3·H2O, the Fe3O4 nanoparticles uniformly disperse in the continuous CS phase, making more CS participate in the electrostatic adsorption reaction. Accordingly, it shows a much better adsorption performance than Fe3O4−CS1. Most noticeably, Fe3O4−CS3, via precipitating FeCl3/FeCl2/CS mixture by ethanol/NH3·H2O, shows a significantly enhanced adsorption performance with the highest adsorption capacity of 242.1 mg/

4. EXPERIMENTAL SECTION 4.1. Preparation of Fe3O4−CS. 4.1.1. Fe3O4−CS1. Fe3O4 nanoparticles were prepared by a coprecipitation method. Typically, 10 mmol FeCl3·6H2O and 5 mmol FeCl2·4H2O were added into 50 mL deionized water under stirring for 30 min at 40 °C. Then, 10 mL ammonia solution was added into the above solution dropwise, and the precipitate was separated by a magnet and washed by deionized water and ethanol, respectively. One gram of CS was dissolved in 50 mL acetic solution (2% v/v) and then the as-prepared Fe3O4 nanoparticles were added into it. After the mixture was sonicated for 30 min, 15 mL GLA (cross-linker) was added into it for cross-link reaction under mechanical stirring for 2 h. The precipitate was washed with 5732

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ethanol and then distilled water and dried in a vacuum oven at 60 °C. 4.1.2. Fe3O4−CS2. One gram of CS was dissolved in 50 mL FeCl3 aqueous solution containing 10 mmol FeCl3·6H2O, and the mixture was stirred for 2 h. Then, 5 mmol FeCl2·4H2O was added into the CS−Fe(III) solution under stirring for 2 h at 40 °C. Then, 10 mL ammonia solution was added into the above solution dropwise, resulting in Fe3O4−CS precipitate, which was separated by a magnetic field, washed by ethanol, and dried in a vacuum oven at 60 °C. After complete grinding, it was redispersed in 50 mL of GLA/ethanol solution (10 ml GLA + 40 mL ethanol) for a cross-link reaction under mechanical stirring for 2 h. The precipitate was washed with ethanol and deionized water and dried in a vacuum oven at 60 °C. 4.1.3. Fe3O4−CS3. Its preparation process was similar to that of Fe3O4−CS2, except that the mixture containing 10 mL ammonia and 100 mL ethanol was added into the FeCl3/ FeCl2/CS solution instead of “10 mL ammonia solution”. 4.2. Characterization. Detailed information on the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscope (TEM), thermogravimetric analysis, Brunauer−Emmett−Teller (BET), ζ-potential, and vibrating sample magnetometer (VSM) are available in the Supporting Information. 4.3. Batch Adsorption Experiments. The different concentrations of Cr(VI) ions were prepared from K2Cr2O7, and the pH value was adjusted by HCl or NaOH. All the experiments were performed on a rotary shaker (180 r/min) at 25 °C. The residual Cr(VI) concentration was measured by a UV−vis spectrophotometer (UV-1240, Shimadzu, Japan). The adsorption capacities (mg/g) of the samples were calculated as follows qt =

(C 0 − C t ) V m

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-20-39366905. ORCID

Weiquan Cai: 0000-0001-8744-3979 Author Contributions §

W.C. and W.X. contributed equally to this work.

Author Contributions

All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21476179), 100 talents project of Guangzhou University, 2016 Wuhan Yellow Crane Talents (Science) Program and the Fundamental Research Funds for the Central Universities (2018-zy-186).

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(10)

where C0 and Ct (mg/L) are the Cr(VI) concentrations at the initial time and time t, respectively; m (g) and V (L) represent the weight of the adsorbents and volume of the solution, respectively; and qe (mg/g) and Ce (mg/L) are the equilibrium adsorption capacity of the adsorbent and the equilibrium concentration of Cr(VI), respectively. The other adsorption experiments were conducted in terms of the effects of pH, adsorption kinetics, adsorption isotherm, desorption, and reusability, as described in the Supporting Information.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00393. Part experimental details; characterizations of XRD, FTIR, thermogravimetry, N2 adsorption−desorption technique and their discussions; pseudo-first-order model, pseudo-second-order model, intraparticle diffusion model, Langmuir model, Langmuir model and the corresponding parameters; comparison of the maximum Cr(VI) adsorption capacities of various chitosan-based adsorbents, schematic illustration of the Cr(VI) adsorption−desorption cycle of Fe3O4−CS (PDF) 5733

DOI: 10.1021/acsomega.8b00393 ACS Omega 2018, 3, 5725−5734

Article

ACS Omega

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DOI: 10.1021/acsomega.8b00393 ACS Omega 2018, 3, 5725−5734