Facile Cross-Link Method To Synthesize Magnetic Fe3O4@SiO2

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Facile Cross-Link Method To Synthesize Magnetic Fe3O4@SiO2− Chitosan with High Adsorption Capacity toward Hexavalent Chromium Yihong Jiang,† Weiquan Cai,*,‡,† Wenjun Tu,† and Mengyuan Zhu† †

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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 ‡ School of Chemistry and Chemical Engineering, Guangzhou University, 230 GuangZhou University City Outer Ring Road, Guangzhou 510006, P. R. China S Supporting Information *

ABSTRACT: The magnetic Fe3O4@SiO2−chitosan (MFSC) with an extremely high adsorption capacity of 336.7 mg g−1 toward toxic Cr(VI) was synthesized successfully by a simple cross-link method. The physicochemical properties of the Fe3O4, Fe3O4@SiO2, and MFSC were characterized comparatively via X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, N2 adsorption−desorption, X-ray photoelectron spectroscopy analysis, and vibrating sample magnetometer. Their adsorption processes are well-fitted by the pseudo-second-order kinetic model; their equilibrium isotherms are better matched with the Langmuir isotherm. Further recycling experiments of MFSC show that its adsorption capacity of 78.6 mg g−1 still remains 81.7% of the original value (96.2 mg g−1), even after five times reuse. The interference adsorption experiments show that MFSC has stronger affinity for Cr(VI) in the solution containing Cr(VI), Cd(II), Cu(II), Zn(II), and Ni(II). The high adsorption capacity, good selectivity, and easy separation of MFSC clearly indicate that it is a potential recyclable adsorbent for Cr(VI) removal from wastewater.

1. INTRODUCTION With the development of society, heavy-metal pollution in wastewater has become a worldwide environmental issue. Chromium is one of the most toxic ions in nature with two stable oxidation states, including trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). The latter is one of the most hazardous heavy metal ions because of its high carcinogenicity and toxicity.1 The quality standard of Cr(VI) for drinking water is no more than 0.05 mg L−1, and thus its effective removal is crucial in wastewater treatment. The routine methods for Cr(VI) removal include chemical precipitation, reverse osmosis, reduction, and adsorption. In particular, adsorption is recognized as one of the most frequently used methods due to its low cost, easy operation, and high efficiency.2 Chitosan (CS), the second most sufficient natural biopolymer, is known as one of the most promising adsorbents owing to its biodegradability and metal-binding capability.3 However, its low acid resistance and poor mechanical strength are obstacles for its application in wastewater treatment. Moreover, the separation and recycling of the conventional powder adsorbents are challenging. The introduction of magnetic Fe3O4 can help pollutant-loaded adsorbent to © XXXX American Chemical Society

separate from wastewater system. For example, Krishna Kumar et al.4 developed a one-step synthesis route of magnetic MoS2@Fe3O4NPs with the maximum adsorption capacity for chromium(VI)/(III) of 218.18 mg g−1; Gong et al.5 prepared a stable magnetic carbon using the activated sludge as a green precursor from a municipal sewage treatment plant, and its adsorption capacity for chromium can reach 203 mg g−1. Recently, our dynamic adsorption experiment for Cr(VI) shows that the maximum adsorption capacity of the magnetic activated carbon obtained via chemical activation and following hydrothermal magnetization is 131.0 mg g−1 using sustainable peanut shell as carbon source.6 Herein a unique magnetic Fe3O4@SiO2−CS (MFSC) with high adsorption capability, selectivity, and easily separated characteristic for Cr(VI) was successfully synthesized. First, magnetic cores were prepared by a solvothermal reduction method. Then, silica layers were coated on the magnetic cores to form Fe3O4@SiO2. The silica layers can not only protect the magnetic cores from corrosion and oxidation but also largely Received: August 19, 2018 Accepted: November 27, 2018

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DOI: 10.1021/acs.jced.8b00738 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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The residual concentration of Cr(VI) was also measured by UV−vis spectra with a UVmini-1240 spectrophotometer (Shanghai, China). 2.4. Static Adsorption Experiments. The original Cr(VI) solution was prepared via dissolving 2.829 g K2Cr2O7 in 1.0 L of deionized water. Static adsorption was studied by mixing 100 mg adsorbent and 100 mL of Cr(VI) solution (100 mg L−1) in a rotary shaker (25 °C, 180 r min−1). 0.1 M NaOH or 0.1 M HCl solution was used to adjust the pH value of the solution. The adsorption capacities qt (mg g−1) of MFSC are determined using the following equation

increase their surface areas. Finally, the coated cores were embedded in chitosan by a facile cross-link method. The physicochemical properties and the adsorption performance of the as-prepared Fe3O4, Fe3O4@SiO2, and MFSC were also studied comparatively.

2. EXPERIMENTAL SECTION 2.1. Materials. FeCl3·6H2O, glycol, anhydrous sodium acetate (NaAc), ethanol, ammonia, hexadecyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), acetic acid, glutaraldehyde, and K2Cr2O7 are of analytical grade from the Sinopharm Chemical Reagent (Shanghai, China). Chitosan with a viscosity of 100−200 mPa·s and deacetylation degree of 95% was purchased from the Aladdin Reagent (Shanghai, China). An original 1000 mg L−1 Cr(VI) solution was prepared by dissolving K2Cr2O7 in deionized water and subsequently diluted. 2.2. Synthesis of MFSC. Step 1. Synthesis of Fe3O4 microspheres. The Fe3O4 microspheres were synthesized by a solvothermal reduction method.7 2.7 g FeCl3·6H2O was first dissolved in 80 mL of glycol with vigorous stirring. After the solution become transparent, 7.2 g NaAc was added to it. Then, the mixed solution was heated to 200 °C and kept for 16 h in a 100 mL Teflon-lined autoclave. The precipitation was separated via a magnet and rinsed with deionized water for several times and ethanol for one time successively. The resultant was vacuum-dried at 60 °C for 12 h. Step 2. Synthesis of Fe3O4@SiO2. The Fe3O4@SiO2 was synthesized according to the previously reported work.8 0.2 g of the as-prepared Fe3O4 was first dispersed in a solution of 60 mL of ethanol and 80 mL of deionized water for 10 min under ultrasonication vibration. Next, 1140 μL of ammonia and 0.28 g of CTAB were added into the mixture and magnetically stirred for 30 min. 400 μL of TEOS was then added dropwise, and the resulting mixture was stirred for 6 h vigorously. The precipitation was magnetically separated and vacuum-dried at 60 °C for 12 h. Step 3. Synthesis of MFSC. The appropriate mass fraction of CS and volume fraction of glutaraldehyde were decided as 1.25 and 10%, respectively, in the exploration experiment (see Figures S1 and S2). 0.1 g Fe3O4@SiO2 was soaked into 5 mL of 10% glutaraldehyde under ultrasonication vibration for 5 min. 50 mL of 1% (vt) acetic acid solution was used to dissolve 0.625 g chitosan. The two mixtures were then mixed and stirred until the gel formed. The gel was washed with ethanol several times and then vacuum-dried at 60 °C for 24 h to obtain MFSC. 2.3. Materials Characterization. A D8 ADVANCE X-ray diffractometer (Bruker, Germany) was used to obtain the X-ray diffraction (XRD) patterns with nickel-filtered Cu Kα radiation (λ = 0.15406 nm) at a voltage of 40 kV and a current of 40 mA. Fourier transform infrared (FT-IR) spectra were obtained via a Nexus (Thermo Nicolet, USA) in the range of 500−4000 cm−1. Scanning electron microscope (SEM) images were carried out on a JSM-5610LV scanning electron microscope. Nitrogen adsorption−desorption isotherms were tested on an ASAP2020 (Micromeritics, USA) analyzer to measure the pore-structure properties. The magnetic properties were determined via a PPMS-9T vibrating sample magnetometer (VSM, Quantum, USA). The concentrations of Cr(VI), Cu(II), Cd(II), Zn(II), and Ni(II) were measured by an inductively coupled plasma optical atomic emission spectrometer (ICP-OES) model Optima 4300DV (PerkinElmer, USA).

qt = (C0 − Ct )V /m

(1)

where C0 and Ct are the Cr(VI) concentrations at the beginning and at a time t in solution (mg L−1), respectively, V represents the volume of the solution, and m represents the mass of the adsorbent (g). The adsorption isotherms were performed by dispersing 50 mg MFSC in 50 mL of Cr(VI) solution at different concentrations of 0.5−800 mg L−1. The multiple cation adsorption of Cr(VI), Cu(II), Cd(II), Zn(II), and Ni(II) was undertaken by mixing 100 mg adsorbent with 100 and 50 mg L−1 Cr(VI) solutions at the same concentrations of the above cations. The residual metal concentrations were analyzed by ICP-OES. The Cr(VI) solution containing different anions was prepared by dissolving a certain amount of sodium salts of Cl−, C2O42−, SO42−, and HPO42− in a Cr(VI) solution of 100 mg L−1, respectively. To evaluate the reusability of MFSC, the used adsorbent was separated with a permanent magnet and then eluted with NaOH solution of 0.005 mol L−1 for 12 h. After washing via deionized water three times, the adsorbent was mixed with Cr(VI) solution (100 mL, 100 mg L−1) at pH 2 for the next adsorption process. The above adsorption−desorption process was repeated five times. 2.5. Dynamic Modeling of the Breakthrough Curves. The dynamic adsorption capacity of MFSC for Cr(VI) was determined by a fixed-bed column system, which was designed using a glass U-tube with a 1.5 cm internal diameter. The MFSC was loaded into the U-tube and filled the lower layer. A 10 mg L−1 Cr(VI) solution at a rate of 1.0 mL min−1 flowed from one side of the U-tube via a peristaltic and digital-pump under ambient condition (see Figure S3). The effluent was collected from the other side of the U-tube at desired time intervals and was then analyzed by the UV−vis spectrophotometer. The experiments were performed in triplicate to obtain the mean values for analysis.

3. RESULTS AND DISCUSSION 3.1. Samples Characterization. Figure 1 presents the XRD patterns of the Fe3O4 microspheres, Fe3O4@SiO2 microspheres, and MFSC. The diffraction peaks of (220), (311), (400), (422), (511), and (440) centering at 30.0, 35.5, 43.2, 53.5, 57.0, and 62.6°, respectively, are typical features of Fe3O4 with a cubic-phase and face-centered-cubic structure (JCPDS card no. 19-0629),6 indicating the existence of Fe3O4 in all samples and its stability during the modification process. The peaks in Figure 1b,c broaden at 20 and 70°, which may be attributed to the amorphous SiO2 coating on the surface of Fe3O4. Corresponding to the peak of chitosan in Figure 1d, the peak in Figure 1b between 10 and 30° broadens a lot, indicating that the chitosan takes up a great proportion in B



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Figure 1. XRD patterns of the Fe3O4 (a), Fe3O4@SiO2 (b), MFSC (c), and chitosan (d).

MFSC. Therefore, MFSC contains three substances, Fe3O4, SiO2, and CS. Their FT-IR spectra are shown in Figure 2. The absorption bands at 3436 and 1640 cm−1 for all of the samples are

Figure 3. Nitrogen adsorption−desorption isotherms (a) and the pore-size distributions (b) of the Fe3O4, Fe3O4@SiO2, and MFSC.

Table 1. Texture Properties of the Fe3O4, Fe3O4@SiO2, and MFSC

Figure 2. FT-IR patterns of the Fe3O4 (a), Fe3O4@SiO2 (b), MFSC (c), and Cr(VI)-loaded MFSC (d).

samples

SBET (m2 g−1)

V (cm3 g−1)

W (nm)

Fe3O4 Fe3O4@SiO2 MFSC

24.0 121.0 0.14

0.07 0.08 0.0003

6.7 2.8 2.8

The magnetization curves of Fe3O4@SiO2 and MFSC are comparatively shown in Figure 4. Their corresponding

attributed to the stretching vibration of O−H from the adsorbed water.9 The band for Fe3O4 at 585 cm−1 results from the stretching vibration of Fe−O bond.10 For Fe3O4@SiO2, apart from the band at 585 cm−1, the absorption band at 1083 cm−1 is attributed to Si−O vibrations,11 indicating that the silica layers were coated successfully onto the surface of Fe3O4 particles. For MFSC and Cr(VI)-loaded MFSC, the absorption bands at 2970, 2880, 1430, 1320, 1275, and 1245 cm−1 are attributed to the vibration of C−H bond, whereas the absorption band at 897 cm−1 is attributed to the stretching vibration of C−O−C. The band at 1640 cm−1 is greatly strengthened due to the bending vibration of the N−H bond in CS.12 This result further suggests the successful combination of CS with Fe3O4@SiO2. Their textural properties were tested by N2 adsorption− desorption. Figure 3 shows that the N2 adsorption−desorption curves of Fe3O4 and Fe3O4@SiO2 show notable hysteresis loops at the relative pressure (P/P0) of 0.4 to 1.0, suggesting that they are mesoporous materials. Table 1 shows that the specific surface areas of Fe3O4 and Fe3O4@SiO2 are 24.0 and 121.0 m2 g−1, respectively. This suggests that the silica coating helps to increase the surface area of the magnetic core. This is beneficial to the coating of chitosan. Interestingly, the specific surface area of MFSC is only 0.14 m2 g−1 after cross-linking of Fe3O4@SiO2 with CS due to the fact that its surface is filled with aminos and hydroxyls.

Figure 4. Magnetization hysteresis loops of the Fe3O4@SiO2 and MFSC.

saturation magnetizations are 52.7 and 3.2 emu g−1, respectively. Normally, more Fe3O4 means poorer adsorption capability, and thus the addition of Fe3O4 should be as little as possible if its magnetization intensity is enough for separation. For MFSC, although its magnetization is fairly low, it can still be separated magnetically from the suspension after adsorption. C



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approaches 7. This implies that MFSC can remain in high capacity in a wide range of pHs. Considering that the average pH of actual electroplating wastewater containing chromium is ∼2, pH 2 was chosen in the following experiments. 3.3. Adsorption Kinetics. The effects of the contact time on their adsorption amounts are comparatively shown in Figure 6a. It was found that their adsorption processes are

The SEM images of MFSC, Fe3O4@SiO2, and Fe3O4 are shown in Figure 5. Figure 5a,b shows that the Fe3O4 particles

Figure 6. Adsorption kinetics (a) and adsorption isotherms (b) of the Fe3O4, Fe3O4@SiO2, and MFSC (conditions of adsorption kinetics: C0 = 100 mg L−1, V = 100 mL, m = 100 mg, T = 25 °C; conditions of adsorption isotherms: C0 = 0.5−800 mg L−1, V = 50 mL, m = 50 mg, t = 24 h, T = 25 °C).

Figure 5. SEM images of the Fe3O4 (a,b), Fe3O4@SiO2 (c,d), and MFSC (e,f).

with pretty rough surfaces present well-dispersed microspheres with diameters of 200−400 nm. However, the surfaces of Fe3O4@SiO2 particles become smooth (Figure 5c,d), indicating the successful coating of silica layers on Fe3O4 microspheres. As presented in Figure 5e,f, some magnetic Fe3O4@ SiO2 particles mount on the surface of the cross-linked CS, whereas others embed themselves in its cross-linked network structure. 3.2. Effects of pH. Cr(VI) exists as various species at different pHs, and the pH can remarkably affect the adsorption performance of MFSC. When the pH is lower than 2, H2CrO4 and HCrO4− coexist in the solution; when the pH reaches 4, HCrO4− is the main species; and when the pH is between 4 and 7, CrO42− appears and coexists with HCrO4−. H2CrO4 can hardly interact with the MFSC, whereas the negatively charged HCrO4− and CrO42− can easily be attracted to the protonated amino groups on MFSC. Hence MFSC has higher adsorption capacity within pH 2−4 than pH 1. However, the number of protonated amino groups decreases with increasing pH, resulting in the lower adsorption capacity at higher pH value. As shown in Figure S4, the adsorption capacity of MFSC increases slightly with increasing pH at first and then reaches a platform within pH 2−4; finally, it declines sharply as the pH

composed of three stages including fast adsorption stage within 0.5 h, slow adsorption stage between 0.5 and 2.0 h, and approaching equilibrium stage between 2.0 and 24.0 h. In particular, their adsorption efficiency after 30 min can reach 85 (8.6 mg g−1), 87 (17.5 mg g−1), and 88% (84.7 mg g−1), respectively, of their adsorption capacities (10.1, 20.1, and 96.2 mg g−1, respectively) within 24 h, indicating that all of them have fast adsorption kinetics. Obviously, MFSC presents the highest adsorption capacity, which is 4.8 times that of the Fe3O4@SiO2 and 9.5 times that of the Fe3O4, respectively, suggesting that cross-linking with CS remarkably improves the adsorption capacity of the magnetic cores due to its surface groups. The formulas for pseudo-first-order and pseudo-secondorder are as follows, respectively13 ln(qe − qt ) = ln qe − k1t

(2)

t /qt = 1/(k 2qe 2) + t /qe

(3)

−1

where qe and qt (mg g ) refer to the adsorption capacity and real-time adsorption amount at time t (min), respectively, and D



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k1 (min−1) and k2 (g mg−1 min−1) are the corresponding rate constants, respectively. The correlated adsorption parameters on the samples according to the above equations are summarized in Table S1 (also see Figure S5). It was shown that on the basis of the correlation coefficient (R2), the pseudo-second-order kinetics fits better with the data than the pseudo-first-order kinetic, suggesting that the rate-controlling step depends on chemical adsorption. Moreover, the corresponding qe,cal of 9.99, 19.95, and 95.88 mg g−1 for Fe3O4, Fe3O4@SiO2, and MFSC agree well with their qe,exp (10.1, 20.1, and 96.2 mg g−1, respectively). 3.4. Adsorption Isotherms and Regeneration Performance. Their adsorption isotherms are shown in Figure 6b (also see Figure S6). Their adsorption capacities increase remarkably at the beginning and then slowly reach a plateau. Compared with the other two samples, MFSC has the maximum adsorption capacity of 336.7 mg g−1, which is much higher than those of Fe3O4 (15.4 mg L−1) and Fe3O4@ SiO2 (31.9 mg L−1), suggesting that MFSC has more active sites for Cr(VI) adsorption. To simulate the data further, the Langmuir and Freundlich isotherm models are presented as follows13 qe = (kLCeqm)/(1 + kLCe)

(4)

qe = k f Ce1/ n

(5)

tion capacity of MFSC becomes 78.6 mg g−1, which is 81.7% of its original adsorption capacity (96.2 mg g−1). 3.5. Adsorption Thermodynamics. To explore the influence of temperature on the adsorption of MFSC toward Cr(VI), its adsorption isotherms at 25, 35, and 45 °C and the homologous thermodynamic parameters are shown in Figure S8 and Table S3. As can be seen from Table S3, the negative value of ΔG° suggests that the adsorption is a spontaneous process. Furthermore, ΔG° becomes smaller with increasing the temperature, suggesting that high temperature can provide stronger adsorption impetus and promote the adsorption spontaneity. 3.6. Interference Tests of Coexisting Cations and Anions. The practical industrial wastewater always contains several kinds of ions, and thus the as-prepared adsorbent should have a high selectivity toward Cr(VI). Herein the tests were performed to see the adsorption capacity of MFSC under the interference of different cations including Cr(VI), Cd(II), Cu(II), Zn(II), and Ni(II) and anions including Cl−, C2O42−, SO42−, and HPO42−. As shown in Figure 7a, MFSC shows

where qe and qm refer to the adsorption capacity and the maximum adsorption capacity of the adsorbent, respectively, kL is a constant correlated to the adsorption energy, Ce is the equilibrium concentration of an adsorbent in solution (mg L−1), kf is a constant, and n is the heterogeneous factor. Table S2 shows the correlated parameters calculated from the two models. The values of qm calculated from the Langmuir model are approximate to the data. In addition, the values of R2 for the Langmuir model are all >0.99, suggesting that the adsorption process is monolayer adsorption. The higher value of kf in eq 5 suggests that MFSC can better adsorb Cr(VI). It is noteworthy that the maximum adsorption capacity of MFSC toward Cr(VI) is comparable to and even higher than most reported magnetic chitosan adsorbents in Table 2 due to the exposure of more active sites resulting from cross-linking of CS. Furthermore, MFSC with easily separated characteristic shows good reusable performance (see Figure S7). After successive adsorption−desorption for five cycles, the adsorpFigure 7. Effects of coexisting cations (a) and anions (b) on Cr(VI) adsorption of the MFSC (V = 100 mL, m = 100 mg, t = 24 h, T = 25 °C).

Table 2. Comparison of the Adsorption Capacities of MFSC and Some Reported Magnetic Chitosan Adsorbents for Cr(VI) adsorbent

qm (mg g−1)

CGM CS/rGO/MT magnetic gelatin Fe3O4@SiO2−mPD/SP MCh−Fe PDFMC GTMAC Fe3O4−SiO2−CTS−PEI ORPF chitosan/MWCNT/Fe3O4 MFSC

82.1 87.0 106.4 119.1 144.9 176 233.1 236.4 266.6 360.1 336.7

conditions 20 15 25 30 30 25 25 25 30 45 25

°C, °C, °C, °C, °C, °C, °C, °C, °C, °C, °C,

pH pH pH pH pH pH pH pH pH pH pH

3 2 2 4 3 3 2.5 2.5 2 2 2

reference

much higher removal efficiency toward Cr(VI) (95.42 and 93.4% at the initial concentration of 50 and 100 mg L−1 cations, respectively.) than the other metal ions. Figure 7b shows that Cl− and C2O42− rarely affect the Cr(VI) removal rate on MFSC. On the contrary, higher concentration of SO42− and HPO42− reduce the removal efficiency by nearly 20% due to the fact that they have similar ionic radii with Cr(VI) in form of HCrO4−. Their analogous molecular sizes cause powerful competition for active sites with Cr(VI) on the surface of MFSC.24,25 3.7. Possible Adsorption Mechanism. The XPS spectra of MFSC and Cr(VI)-loaded MFSC are shown in Figure 8a to

14 15 16 17 18 19 20 21 22 23 this work E



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then −NH3+ interacts with Cr(VI) to form −NH2 metal, which indicates the electrostatic interaction between MFSC and Cr(VI). After cross-linking, MFSC forms the cross-linked network structure with good water solubility, which is not only beneficial for water to enter the gel network but also convenient for the Cr(VI) ions to interact with the active groups of MFSC. A possible adsorption mechanism of MFSC toward Cr(VI) involving electrostatic interaction and reduction is proposed in Figure 9. The corresponding processes are as follows:,28,29

Figure 9. Adsorption mechanism of MFSC toward Cr(VI).

Electrostatic interaction: R−NH 2 + H+ → R−NH3+ +

−

(6)

+

NH3 + HCr2O7 → NH3 ···HCr2O7

−

(7)

Reduction: 3R−CH 2OH + 2HCrO4 − + 8H+ → 3R−CHO + 2Cr 3 + + 8H 2O

(8)

On one hand, the amino groups on the surface of MFSC from CS are protonated under acid conditions and then become attractive to the anions in the suspension. Herein the major species of Cr(VI) in our pH range is HCrO4− ions. They are easily adsorbed by protonated amine groups via electrostatic interaction, which is in accordance with the results of interface tests and XPS spectrum. On the other hand, part of the HCrO4− ions are reduced to Cr(III) by reducing their hydroxyl groups. Furthermore, MFSC shows small specific surface area and pore volume, indicating that its excellent adsorption properties for Cr(VI) result from the active sites rather than the specific surface area. 3.8. Dynamic Modeling of the Breakthrough Curves. Adsorption of Cr(VI) on MFSC was further studied in a continuous fixed-bed column (see Figure S3). On the basis of the Adams−Bohart model and the Yoon−Nelson model28 for MFSC, their corresponding breakthrough curves and fitting parameters are shown in Figure 10 and Table S4, respectively. The correlation coefficient (R2) of 0.997 obtained from the Yoon−Nelson model is higher than the R2 of 0.965 obtained from the Adams−Bohart model, indicating that the former is a satisfactory predictor for the breakthrough curves. Moreover, MFSC performs a good dynamic adsorption capacity (qtotal) of 2.3 mg g−1 at an initial Cr(VI) solution of 10 mg L−1, and the

Figure 8. High-resolution of XPS spectra: full range (a), Cr 2p (b), and C 1s (c) of MFSC before and after adsorption.

further elucidate the adsorption mechanism. After adsorption, typical peaks of Cr appear in the XPS spectrum of MFSC, and the significant peak at 576.52 eV is attributed to Cr(III) (see Figure 8b),26 which is the characteristic of reduction of Cr(VI). Furthermore, the main peak of CO (286.89 eV for C 1s, 531.13 eV for O 1s) greatly increases after adsorption, whereas the peak of C−O (285.48 eV for C 1s, 531.99 eV for O 1s) decreases (see Figure 8c and Figure S9a), suggesting that the reduction of Cr(VI) results from the oxidation of C O to CO. Two fresh peaks appear at 803 and 946 cm−1 are characteristics of Cr(III)−OH and Cr(VI)−O, respectively.27 The XPS spectrum of N 1s (see Figure S9b) shows that the intensity of the −NH2 metal peak (398.65 eV) increases, whereas the intensity of the other two peaks (−NH3+ at 401.19 eV and −NH2 at 398.28 eV) declines. This may be because −NH2 is protonated in acid solution, resulting in −NH3+, and F



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time for half of the adsorbate breakthrough is 320 min, indicating its application potential for wastewater treatment.

4. CONCLUSIONS The MFSC with an extremely high adsorption capacity for Cr(VI) was synthesized successfully by a facile cross-link method. Its adsorption process agrees well with the pseudosecond-order model, which is the characteristic of chemical adsorption. Its maximum adsorption capacity of 336.7 mg g−1 is much better than those of Fe3O4, Fe3O4@SiO2, and many reported magnetic chitosan materials. Besides, MFSC shows excellent adsorption selectivity for Cr(VI) with the existence of coexisting cations including Cr(VI), Cd(II), Cu(II), Zn(II), Ni(II), and coexisting anions including Cl−, SO42− , HPO42−, and C2O42−. The adsorption mechanism involves electrostatic attraction and the oxidation−reduction reaction between MFSC and Cr(VI). This composite adsorbent with easily separated characteristic shows high adsorption capacity and good reusable adsorption performance toward Cr(VI), suggesting its application potential for treatment of heavymetal wastewater. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00738. Effects of chitosan and glutaraldehyde concentrations on the adsorption capacities of MFSC, supplementary static adsorption data, the Adams−Bohart model, and the Yoon−Nelson model (PDF)

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Figure 10. Adsorption breakthrough curves of Cr(VI) on MFSC based on Adams−Bohart model and Yoon−Nelson model.

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Article

AUTHOR INFORMATION

Corresponding Author

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

Weiquan Cai: 0000-0001-8744-3979 Funding

This work was financially supported by the National Natural Science Foundation of China (No. 21476179), One Hundred Talents Project of Guangzhou University (No. 6918ZX10016), and 2016 Wuhan Yellow Crane Talents (Science) Program and the Fundamental Research Funds for the Central Universities (No. 2018-zy-186). Notes

The authors declare no competing financial interest. G



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Facile Cross-Link Method To Synthesize Magnetic Fe3O4@SiO2

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