Fe3O4: Bifuctional Composites

Nov 8, 2016 - Bifunctional mesoporous g-C3N4/Fe3O4 composites were prepared and used for the simultaneous visible-light catalysis and adsorption of ...
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Magnetically Separated meso-g‑C3N4/Fe3O4: Bifuctional Composites for Removal of Arsenite by Simultaneous Visible-Light Catalysis and Adsorption Shenghua Chi, Chunnuan Ji,* Suwen Sun, Hua Jiang, Rongjun Qu, and Changmei Sun School of Chemistry & Materials Science, Ludong University, Yantai 264025, China ABSTRACT: Bifunctional mesoporous g-C3N4/Fe3O4 composites were prepared and used for the simultaneous visible-light catalysis and adsorption of arsenic from aqueous solution. The as-prepared composites were characterized by wide-angle X-ray powder diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet−visible diffuse reflectance spectroscopy, a vibrating sample magnetometer, and Brunauer−Emmett−Teller surface analysis. Experimental results showed that meso-g-C3N4/Fe3O4 composites could oxidize As(III) to As(V), which was in turn effectively removed by the composites. The photocatalytic activity was significantly enhanced by the synergistic effect between gC3N4 and Fe3O4. In addition, a reasonable visible-light catalytic oxidation mechanism of As(III) was investigated.

composite-based photocatalysts, such as CdS,23 TiO2,24 Ag2O,25 ZnFe2O4,26 and Fe3O4,27,28 have been developed. It is well-known that Fe3O4 not only is an excellent adsorbent for arsenic removal owing to its high adsorption capacity but also exhibits matched energy band structure (ECB = 1 V vs NHE) with g-C3N4.29,30 Therefore, it is proposed that the construction of heterojunctions between Fe3O4 and g-C3N4 may be a good strategy for improving photocatalytic performance. The inclusion of Fe3O4 endows meso-g-C3N4/Fe3O4 composites with the effect of simultaneous photocatalytic oxidation and adsorption properties, thus significantly improving the efficiency of arsenite removal. To our best knowledge, the combination of photocatalysis and adsorption based on meso-g-C3N4/Fe3O4 composites has not been attempted for arsenic removal. In this study, meso-g-C3N4/Fe3O4 composites were obtained and characterized. Their photocatalytic activity and adsorption performance for arsenic removal were investigated in detail under xenon lamp irradiation. The plausible visible-light catalytic oxidation mechanism of meso-g-C3N4/Fe3O4 composites for As(III) was also discussed.

1. INTRODUCTION Arsenic contamination in natural water has raised great concerns because long-term exposure to arsenic may pose great risks to human health.1 Much work has been done to explore efficient techniques for removing arsenite from groundwater and surface water. The main forms of As in natural water are arsenite (As(III)) and arsenate (As(V)). Between them, arsenite exhibits toxicity that is much higher than that of arsenate.2 Furthermore, the removal of As(III) is not efficient in adsorption processes because arsenite usually exists as uncharged forms and has poor affinity for adsorbents.3 Thus, a common technique for the removal of arsenic usually involves a preoxidation step prior to complete removal.4,5 To date, various oxidants of As(III), such as manganese dioxide,6 hydrogen peroxide,7 ozone,8 Fenton reagent,9 TiO2/α-Fe2O3,10 and Fe-deposited titanate,11 have been used for arsenic oxidation. Among them, a photocatalyst based on TiO2 is considered to be a promising approach for As(III) oxidation owing to its high efficiency.12−14 However, the band gaps of most TiO2-based photocatalysts are generally wide, which results in the scant utilization of visible light. Furthermore, the low adsorption capacity of TiO2 for arsenic may cause the insufficient removal of arsenic.15 Recently, g-C3N4 has attracted considerable attention owing to its narrow band gap (2.7 eV, ECB = 1.42 V vs NHE). Also, it can be excited under visible light.16,17 However, the photocatalytic activity of pure graphitic carbon nitride is limited by lower efficiency caused by the recombination of photoinduced electron−hole pairs.18−20 The construction of heterojunctions between two semiconductors is considered as an effective method to promote the separation of carriers, which can improve the photocatalytic activity.21,22 Thus, various g-C3N4 © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Chemicals. Melamine, FeCl3·7H2O, FeCl2·7H2O, NH3 solution (25%), tert-butanol (t-BuOH), 2-propanol (i-PrOH), and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. These reagents were used as Received: June 6, 2016 Revised: October 26, 2016 Accepted: October 28, 2016

A

DOI: 10.1021/acs.iecr.6b02178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research received. NaAsO2 and Na2HAsO4·7H2O (Sigama-Aldrich Co.) were used to prepare standard solutions (1000 mg/L). Deionized water was used in the entire experimental process. 2.2. Synthesis of meso-g-C3N4/Fe3O4 Composites. gC3N4 was obtained according to the method reported in the literature.14 Typically, mealmine was placed in an alumina crucible with a cover and heated at 550 °C for 4 h in a muffle furnace. The synthesis of meso-g-C3N4/Fe3O4 was conducted as follows:31 1.0 g of g-C3N4 powder was added to 250 mL of ethanol/water (1:2, V/V) solution. The mixture was sonicated for 5 h at normal temperature. The ferric and ferrous aqueous solutions were prepared by dissolving FeCl3·6H2O (0.2 g, 0.037 mol) and FeCl2·4H2O (0.73 g, 0.018 mol) in 20 mL of deionized water. The above-prepared aqueous solutions were added to the suspension of g-C3N4. Aqueous ammonia (2 mL) was added to the mixture under stirring at 80 °C. After being stirred for 30 min, the mixture was allowed to cool and was filtered. Then the sample was rinsed with deionized water and dried at 80 °C. To elucidate the role of g-C3N4 and Fe3O4 on the photocatalytic activity, samples with different amounts of Fe3O4 in the composites were prepared. The samples are denoted as FeOCN-x, where x is the ratio of Fe3O4 to g-C3N4 (W/W). In addition, pure g-C3N4 and Fe3O4 were also prepared for comparison. 2.3. Charaterizations. The phase structure was determined by X-ray diffraction (XRD) with Cu Kα radiation (D/max IIIB). Nitrogen sorption−desorption experiments were conducted by an automatic physisorption analyzer (ASAP 2020), and the obtained data was analyzed with Brunauer−Emmett− Teller (BET) and Barrett−Joyner−Halenda (BJH) methods. Diffuse reflection spectra were analyzed by an ultraviolet− visible (UV−vis) spectrophotometer (Shimadzu UV-2550). The morphologies of the composites were observed on a JEM3010 electron microscope (JEOL). The magnetic property of the samples was determined by a magnetometer (VSM, LDJ9600). 2.4. Phototcatalytic Test. A 500 W Xe lamp (Beijing Perfect Light Company, China) was used as the visible-light source. Photocatalytic experiments were carried out as follows: 50 mg of as-prepared FeOCN-x was added to 100 mL of As(III) solution with various concentrations. After being sealed and immersed in a water bath, the mixture was shaken continuously during irradiation at 25 °C. A double-channel atomic fluorescence spectrometer (AFS920) was used to determined the arsenic concentrations including arsenite and arsenate, which is denoted as As(T). To measure the concentration of arsenite, the citrate buffer solution (pH 5) was applied according to a previous report.32 Then, the concentration of arsenate was calculated as follow: CAs(V) = CAs(T) − CAs(III). Each set of experiments was performed in triplicate. The effects of coexisting anions such as Cl−, SO42−, NO3−, CO32−, and SiO32− on the removal of arsenite and arsenic were investigated. The arsenite concentration was kept at 100 μg/L. The concentrations of the coexisting ions in the binary mixture system were set at three levels (0.1, 1.0, and 10 mM). In the recycling experiments, NaOH (1 M) solution was chosen to regenerate the absorbed composites. After being shaken with NaOH (1 M) solution and washed with deionized water, the composites were reused for another adsorption− desorption cycle.

For the experiments of oxidation mechanism of As(III), tertbutanol (t-BuOH), iso-propanol (i-PrOH), and carbon tetrachloride were used as the active species scavengers.11

3. RESULTS AND DISCUSSION 3.1. Characterization of meso-g-C3N4/Fe3O4 Composites. The structures of pure Fe3O4, pure g-C3N4, and meso-gC3N4/Fe3O4 composites were confirmed by XRD and are shown in Figure 1. It can been seen that six diffraction peaks in

Figure 1. XRD patterns of g-C3N4, Fe3O4, and FeOCN-x.

the range 2θ = 30.6°−63.2° are observed, which can be indexed as (220), (311), (400), (422), (511), and (400) reflections of Fe3O4.33 For graphitic carbon nitride, a characteristic peak at 27.5° is observed, which can be indexed as the graphitic interlayer (002) peak.16 The characteristic peak of g-C3N4 is also presented in FeOCN-x, and the relative diffraction intensity of g-C3N4 decreases gradually with increasing Fe3O4 content. These results indicate that the FeOCN-x is a twophase composite. The transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images of pure Fe3O4, pure g-C3N4, and FeOCN-16 are shown in Figure 2. The pure g-C3N4 shows sheetlike structure with significant aggregation (Figure 2a), whereas the pure Fe3O4 nanoparticles have sizes ranging from 5.4 to 17.8 nm (Figure 2b). Figure 2c shows that the Fe3O4 nanoparticles are scattered on g-C3N4. Figure 2d shows the HRTEM image recorded from FeOCN-16. It can be noted that there are two different lattice fringes in the resulting composite system, which indicates the existence of the heterojunction structure between g-C3N4 and Fe3O4.31 The UV−vis diffuse reflectance spectroscopy spectra of pure g-C3N4, Fe3O4, and FeOCN-x are given in Figure 3a. The pure g-C3N4 shows the adsorption onset of 458 nm with a band gap of 2.71 eV.16 However, the as-prepared FeOCN-x has obvious red shifts in the absorption edge. The absorbance of the composites is higher than that of pure g-C3N4 because of the presence of heterojunctions between Fe3O4 and g-C3N4. These results indicate that the as-prepared composites can be excited under visible-light irradiation, with enhanced photocatalytic activity and efficiency. B

DOI: 10.1021/acs.iecr.6b02178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. TEM and HRTEM images of (a) g-C3N4, (b) Fe3O4, and (c, d) FeOCN-16.

Figure 3. (a) UV−vis spectra of pure g-C3N4 and FeOCN-x and (b) nitrogen adsorption isotherms of pure g-C3N4, Fe3O4, and FeOCN-16.

Moreover, the pore structures of pure g-C3N4, Fe3O4, and FeOCN-16 were also investigated by BET and BJH methods through N2 adsorption at 77 K. The results are presented in Figure 3b and Table 1. The adsorption−desorption isotherms in Figure 3b show that FeOCN-16 is type IV according to the IUPAC classification, suggesting the presence of mesopores.34 As shown in Table 1, the specific surface areas of pure Fe3O4 and g-C3N4 are 107.17 and 15.70 m2/g, respectively. The surface area and pore diameter of FeOCN-x slightly increase with the increase of the loaded amount of Fe3O4 in the composites, which implies that Fe3O4 is mainly scattered on the surface of graphitic carbon nitride. This is desirable for efficient adsorption of As(V). The hysteresis loops and maximal saturation magnetization (MSM) of FeOCN-x along with the pure Fe3O4 were determined, and the results are presented in Figure 4. As depicted in Figure 4, no hysteresis is observed. In addition, the remanence and coercivity of the composites are close to zero.

Table 1. Physicochemical and First-Order Kinetic Model for Photocatalysis for As(III) by g-C3N4, Fe3O4, and FeOCN-x visible-light photocatalysis

material g-C3N4 Fe3O4 FeOCN-8 FeOCN12 FeOCN16 FeOCN20

C

BET surface area (m2 g−1)

BJH pore volume (cm3 g−1)

pore diameter (nm)

k1 (min−1)

R2

15.70 107.17 23.06 31.70

0.08 0.48 0.13 0.18

19.26 14.96 21.56 22.32

0.0002 0.0020 0.0048 0.0059

0.966 0.891 0.940 0.957

36.47

0.21

23.65

0.0066

0.963

36.67

0.21

23.66

0.0038

0.897

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Figure 4. Magnetic hysteresis loops of pure Fe3O4 and FeOCN-x.

heterojunction structure. On the one hand, because the conduction band (CB) level of Fe3O4 is lower than that of gC3N4,31 the photoinduced electrons can transfer from the CB of g-C3N4 to that of Fe3O4. The charge recombination can be effectively inhibited, which leads to prolonging the lifetime and enhancing the photocatalytic performance. On the other hand, the high affinity of Fe3O4 makes it easy to adsorb arsenic onto the surface of the meso-g-C3N4/Fe3O4, which is beneficial to the arsenic oxidation and the simultaneous adsorption on the surface of the hybrid composite system of g-C3N4 and Fe3O4. From Figure 5, we also note that the loaded amounts of Fe3O4 in the g-C3N4/Fe3O4 composites have obvious influence on the arsenic removal. The removal efficiency of As(III) and As(T) is 90.0% and 92%, respectively, when the loaded amount of Fe3O4 is 8%. As the loaded amounts of Fe3O4 increases to 12% and 16%, the removals of As(III) and As(T) are increased from 97% to 100% and from 97% to 98%, respectively. While the amount of Fe3O4 is about 20%, the removals of As(III) and As(T) slightly decline to 95%. The decrease of the arsenic removal in this condition may be ascribed to too much Fe3O4 deposited on the surface of g-C3N4. This is not beneficial to absorb visible light; thus, the photocatalytic oxidation declines. To further demonstrate the photocatalytic oxidation rate of the meso-g-C3N4/Fe3O4 composites toward As(III), the Langmuir−Hinshelwood model (as shown in eq 1) was used to analyze the experimental data in Figure 5.14

These experimental results show that FeOCN-x has a typical superparamagnetic property. Also, the MSM values of Fe3O4, FeOCN-8, FeOCN-12, FeOCN-16, and FeOCN-20 are 43.74, 7.34, 8.27, 15.13, and 19.37 emu/g, respectively. Clearly, the saturation magnetization decreases with increased Fe3O4 loading. Furthermore, the meso-g-C3N4/Fe3O4 composites can be separated with a permanent magnet (the inset in Figure 4). This is of great importance for regeneration and reutilization of the composites. 3.2. Photocatalytic Oxidation and Adsorption. The photocatalytic oxidation−adsorption of arsenic was conducted at 25 °C around pH 7.0 with or without irradiation. The plot of the photocatalytic oxidation−adsorption kinetics of g-C3N4, Fe3O4, and FeOCN-x is depicted in Figure 5. As presented in Figure 5a, only 17% of As(III) is removed by pure g-C3N4 without visible-light irradiation, which illustrates that pure g-C3N4 has little adsorption affinity for As(III). As expected, a weak photocatalytic performance for As(III) is observed under irradiation; the removal of As(III) and As(T) reached about 41% and 17%, respectively. The lower photocatalytic performance may be ascribed to the fast recombination of the photogenerated carrier.18,19 In comparison with pure g-C3N4, higher removals of As(III) and As(T) of pure Fe3O4 are observed (Figure 5b). The As(III) removal of pure Fe3O4 with or without irradiation is about 85% and 80%, respectively. The lesser difference in the As(III) removal means that the photocatalic oxidation activity of Fe3O4 for As(III) is weak. In addition, this result indicates that Fe3O4 exhibits good adsorption performance for As(III), which is in accordance with previous reports.35 Therefore, the higher arsenic removal can be ascribed to the high affinity of Fe3O4 for arsenic. From Figure 5c,d, it can be noted that obvious increase of As(III) removal under irradiation is obtained. In the case of FeOCN16, the removal values of As(III) and As(T) without irradiation are 89% and 86%, respectively. Meanwhile, when FeOCN-16 was used for the arsenic removal under the irradiation, the As(III) and As(T) removal values increase remarkably to above 100% and 98%, respectively. The arsenic removal enhancement of the composites can be ascribed to the synergistic effect of the

ln(C0/C) = kt

(1)

where C0 (mg/L) is the initial concentration of As(III), C (mg/ L) the residual concentration of As(III) at time t (min), and k the reaction rate constant. The determined reaction rate constants and correlation coefficients (R2) are listed in Table 1. The sequence of the photocatalysis rate constants is as follows: FeOCN-16 > FeOCN-12 > FeOCN-8 > FeOCN-20 > Fe3O4 > g-C3N4. Moreover, FeOCN-16 shows the highest value of the photocatalysis rate constant (0.0066 min−1), which is about 16.5 and 3.0 times than that of pure Fe3O4 and g-C3N4, respectively. Because FeOCN-16 has the highest photocatalysis rate constant and arsenic removal, it is chosen as the photocatalyst−adsorbent in the following experiments. D

DOI: 10.1021/acs.iecr.6b02178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Removal kinetics of pure g-C3N4, Fe3O4, and FeOCN-x with and without irradiation at pH 7.0 (a, pure g-C3N4; b, pure Fe3O4; c, FeOCN8; d, FeOCN-12; e, FeOCN-16; f, FeOCN-20) (As(III) 5 mg/L; amount of pure g-C3N4, Fe3O4, and FeOCN-x, 50 mg; pH 7.0).

3.3. Effect of Coexisting Anions on Arsenic Removal. Generally, arsenic-contaminated water contains abundant amounts of various anions, which may affect the photocatalysis activity and adsorption of FeOCN-16 for arsenic. In this study, several anions commonly present in natural water, such as Cl−, SO42−, NO3−, HCO3−, and SiO32−, were chosen as foreign ions. Considering the practical treatment conditions of the ground and drinking water, the effects of individual anions including Cl−, SO42−, NO3−, HCO3−, and SiO32− on the removal of As(III) and As(T) were evaluated at three levels (0.1, 1.0, and 10 mM) with pH 7.0.36 As presented in Figure 6, the coexisting ions display little effect on the removal of arsenite. When the concentrations of Cl−, SO42−, NO3−, HCO3−, and SiO32− are 10

mM, the removal of arsenite is 95%, 94%, 96%, 91%, and 89%, respectively. As for As(T) removal, hindering by HCO3− and SiO32− is observed as compared to Cl−, SO42−, and NO3−. When the concentrations of HCO3− and SiO32− are 10 mM, the removal of As(T) is 91 and 87%, respectively. The decrease in arsenic removal is due to the chemical similarity between the two foreign ions and arsenic, which leads to significant competition.36 Similar adverse effects of HCO3− and SiO32− on metal oxides adsorbents were also reported elsewhere.37 3.4. Photocatalytic Oxidation Mechanism of As(III). To investigate the possible oxidation mechanisms of As(III) in the photocatalytic process, three kinds of oxidative species scavengers, including t-BuOH (5 mM), i-PrOH (5 mM), and E

DOI: 10.1021/acs.iecr.6b02178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Effect of coexisting anions on the removal of As(III) and As(T) by FeOCN-16 (As(III), 100 μg/L; FeOCN-16, 50 mg; pH 7.0).

CCl4 under N2 purging, were chosen to study the oxidation mechanism of As(III) by FeOCN-16.31,38,39 The results are shown in Figure 7.

Figure 8. XPS spectra of FeOCN-16-As(III)-D, FeOCN-16-As(V)-D, and FeOCN-16-As(III)-L.

confirms that Fe(II) existing in FeOCN-16 can generate radical species in oxic systems and that partial As(III) is oxidized to As(V).42 Thus, FeOCN-16 is capable of facilitating As(III) oxidation in the absence of light to some extent. On the other hand, as indicated in Figure 8c, when the experiment was conducted under irradiation, the fraction of As(III) and As(V) is changed to 54.8 and 45.2%, respectively. This implies that the formation of heterojunction structure between Fe3O4 and gC3N4 is beneficial to enhance the oxidation efficiency of As(III). On the basis of the above-mentioned analysis, a reasonable oxidation mechanism of As(III) is presented in Figure 9. When FeOCN-16 is added into the arsenic-contaminated water, arsenic in the solution is adsorbed onto the surface of FeOCN-16. Under visible-light irradiation, the excited-state electrons in the CB of Fe3O4 transferred from CB of g-C3N4 can react with the O2 adsorbed onto the surface of FeOCN-16 to generate O2·−, which can oxidize As(III) to As(V). Because of the excellent adsorption property of Fe3O4 for As(V), As(V) can be removed efficiently by meso-g-C3N4/Fe3O4 composites. 3.5. Desorption and Reusability. To investigate the reusability of the as-prepared composites, five adsorption− desorption cycles were conducted by using 1 M NaOH solution as the eluant to regenerate the adsorbent.10 The removal of As(III) and As(T) is presented in Figure 10. It can be noted that the removals of As(III) and As(T) are 94% and 80% respectively, even after five cycles. This indicates that meso-g-

Figure 7. Removal of As(III) by the addition of the active species capture (As(III), 5 mg/L; FeOCN-16, 50 mg; pH 7.0).

In can be seen from Figure 7 that when t-BuOH and i-PrOH are conducted as radical scavengers, little effect on As(III) photooxidation is observed. These results imply that hydroxyl radicals and trapped holes are not the main oxidant species in the photocatalytic process.40 However, when CCl4 under N2 purging is chosen as O2·− radical quencher, an obvious change in the removal of As(III) is observed. This indicates that O2·− is the main species for oxidation of As(III) in this system. To further confirm the ionic species of arsenic on the surface of FeOCN-16, the As 3d spectra of the selected samples were conducted by X-ray photoelectron spectroscopy (XPS). The results are shown in Figure 8. Among these samples, FeOCN16-As(III)-D and FeOCN-16-As(V)-D were obtained by placing the As(III) and As(V) solution to FeOCN-16 away from light, respectively. Meanwhile, FeOCN-16-As(III)-L was obtained by mixing FeOCN-16 with As(III) solution under visible-light irradiation. From spectra a in Figure 8, it can be seen that the As 3d spectra represent two ionic species with binding energies at 44.03 and 45.23 eV, respectively.41 Furthermore, the fraction of As(III) and As(V) on the surface of FeOCN-16-As(III)-D is about 79.2 and 20.8%, respectively. The presence of As(V) F

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REFERENCES

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Figure 9. Reasonable oxidation mechanism of As(III) by FeOCN-16.

Figure 10. Recycle of FeOCN-16 for removal of As(III) and As(T) (As(III), 5 mg/L; FeOCN-16, 50 mg; pH 7.0).

C3N4/Fe3O4 composites are very stable and can be regenerated with 1 M alkali solution.

4. CONCLUSION In summary, meso-g-C3N4/Fe3O4 composites were successfully prepared and used for the photocatalytic oxidation and removal of As(III). The synergistic effect between g-C3N4 and Fe3O4 can effectively enhance the removal of As(III) under visiblelight irradiation because of the existence of a heterojunction structure. Meanwhile, the combination of the excellent adsorption properties of Fe3O4 with the photocatalytic activity of g-C3N4 is beneficial to the efficient removal of As(III). Moreover, meso-g-C3N4/Fe3O4 can be easily regenerated with 1 M alkali solution and possesses excellent adsorption properties even after five recycles.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of Shandong Province (ZR2014EMM016) G

DOI: 10.1021/acs.iecr.6b02178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b02178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX