Magnetically Separable Nanospherical g-C3N4@Fe3O4 as a

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Magnetically Separable Nanospherical g‑C3N4@Fe3O4 as a Recyclable Material for Chromium Adsorption and Visible-LightDriven Catalytic Reduction of Aromatic Nitro Compounds A. Santhana Krishna Kumar,† Jyun-Guo You,† Wei-Bin Tseng,† G. D. Dwivedi,‡ N. Rajesh,*,⊥ Shiuh-Jen Jiang,*,†,∥ and Wei-Lung Tseng*,†,§

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†

Department of Chemistry and ‡Department of Physics, National Sun Yat-sen University, No. 70, Lien-hai Road, Gushan District, Kaohsiung 80424, Taiwan § School of Pharmacy and ∥Department of Medical Laboratory Science and Biotechnology, Kaohsiung Medical University, No. 100, Shiquan First Road, Sanmin District, Kaohsiung 80708, Taiwan ⊥ Department of Chemistry, Birla Institute of Technology and Science, Pilani Hyderabad Campus, Jawahar Nagar, Shameerpet, Telangana, Hyderabad-500078, India S Supporting Information *

ABSTRACT: Graphite-like graphitic carbon nitride (g-C3N4) has gained considerable interest in the past few years. However, merely a few studies have been undertaken regarding the application of g-C3N4 for metal adsorption and visible-lightdriven reduction of aromatic nitro compounds. Here, we describe a versatile method for the preparation of g-C3N4 nanocomposite decorated with magnetite nanoparticles (g-C3N4@Fe3O4NPs) that subsequently showed their efficiency in sequestration of Cr(VI)/Cr(III) and NaBH4-mediated conversion of 2-nitroaniline (2-NA) and 4-nitroaniline (4-NA) under visible-light exposure. The as-synthesized g-C3N4@Fe3O4NPs adsorbent revealed excellent water dispersibility, superior magnetic property, and porous structure. Numerous surface hydroxyls (−OH) and amino groups (−N, −NH, −NH2) enabled gC3N4@Fe3O4NPs to rapidly isolate Cr(VI) from aqueous solution through applying an outer magnetic field. The adsorbed Cr(VI) on the g-C3N4@Fe3O4NPs surface offered a maximum equilibrium adsorption capacity of 555 mg g−1, and their absorption behavior followed the Langmuir isotherm and pseudo-second-order kinetics model. The morphology, surface properties, crystalline structure, and chemical compositions of g-C3N4@Fe3O4NPs were thoroughly investigated. In real-world applications, g-C3N4@Fe3O4NPs was implemented for the determination of total chromium in industrial soil sludge samples. Additionally, NaBH4-induced reduction of 2-NA to 2-aminoaniline and 4-NA to 4-aminoaniline catalyzed by g-C3N4@ Fe3O4NPs (catalyst loading as low as 20 mg) was achieved within 8 min. KEYWORDS: Graphitic carbon nitride, Magnetite nanoparticles, Photocatalytic degradation, Chromium removal, Kinetic study, Nitroaniline reduction, Industrial soil sludge

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the elimination of chromium species in industrial wastewater.1 Traditional techniques for treating Cr(VI)-contaminated wastewater are chemical precipitation, electrodeposition, ion exchange, reverse osmosis, membrane filtration, chemisorption, and physisorption. Among these previously reported techniques, the adsorption method is broadly used due to its high removal efficiency, cost-effectiveness, easy operation, and low secondary pollution impact. In the aim to find suitable adsorbent materials, a few examples of porous Fe2O3/g-C3N4/

INTRODUCTION

Rapid industrialization and technological developments lead to many benefits for humans’ day-to-day life. However, they simultaneously cause several environmental impacts, such as water pollution, water scarcity, climate change, and unhealthy air. Various toxic heavy metal ions are discharged through several industrial activities. In particular, chromium has been considered as a high-priority environmental pollutant due to rapid urbanization and scientific improvements throughout the world. It is recognized that hexavalent chromium [Cr(VI)] is approximately 100-fold more poisonous than trivalent chromium [Cr(III)]. For the sake of avoiding risk to human health, it is urgent to develop a fast and efficient approach for © XXXX American Chemical Society

Received: November 5, 2018 Revised: February 24, 2019 Published: March 7, 2019 A

DOI: 10.1021/acssuschemeng.8b05727 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering graphene ternary nanocomposites,2 mesoporous carbon nitride,3 Fe3O4-modified mesoporous carbon nanospheres,4 Fe 3 O 4 @poly(m-phenylenediamine) particles, 5 MoS 2 @ Fe3O4NPs,6 and mesoporous carbon microspheres7 have been reported. Although these adsorbents have shown great potential to remove Cr(VI) from environmental water and industrial wastewater, their adsorption capacity needs considerable enhancement. It is worthwhile to emphasize that the maximum adsorption capacity of the reported materials (Table S1, Supporting Information) for Cr(VI) ranges from 10 to 357 mg g−1.2−14 Graphitic carbon nitride (g-C3N4), consisting of an sp2-hybridized structure, is an emerging material for many applications due to its high thermal stability (600 °C in air), tunable electronic band gap (∼2.7 eV), metal-free composition, and nontoxicity. Such g-C3N4-related materials have been explored to develop environmental remediation,15 energy conversion,16 and organic synthesis.17 For example, Chen et al. demonstrated that the loading of single Ti atoms on the nitrogen vacancies of g-C3N4 allowed the efficient reduction of N2 to ammonia.18 By contrast, few studies focused on the use of g-C3N4 for the elimination of heavy-metal ions from industrial sewage water, even though g-C3N4 contains the order of tri-s-triazine units with abundant lone-pair electrons.19−24 For example, Shen et al. synthesized g-C3N4NPs through thermal treatment of melamine in a solid solvent and applied it to the efficient adsorption of Ni(II), Cu(II), Pb(II), and Cd(II). 19 Kumar et al. prepared oxidized g-C 3 N 4 nanosheets by acid etching and oxidative decomposition, followed by a decoration of polyaniline nanofiber on their surface.25 The maximum adsorption capacity of the as-made nanosheets was reported to be 178.57 mg g−1 for Cr(VI). However, these g-C3N4 nanomaterials provide low efficiency for the adsorption of heavy-metal ions because of their low ratio of surface to volume. Hence, it is crucial to develop hybrid nanomaterials of g-C3N4 and magnetic NPs with a high magnetism that could rapidly remove Cr(III) and Cr(VI) from aqueous media through applying an external magnetic field. Nitroaromatic compounds are considered to be some of the most hazardous and toxic organic chemicals. They cause severe health hazards, imposing a threat to human, animal, and aquatic lives. Additionally, nitroaromatic compounds have been intensively used in wars and explosive industrial applications, resulting in considerable environmental pollution and ecological degradation.26,27 For safety concerns, a rapid, convenient, and effective strategy is urgently needed for the treatment of such hazardous materials. One of the most effective methods for this requirement is to reduce nitroaromatic compounds to their corresponding aromatic amines. Therefore, it is imperative to explore ecofriendly alternatives to the conversion of nitro compounds to their amino analogues. Noble-metal nanoparticles as an active catalyst have been recently introduced for sodium borohydride (NaBH4)mediated reduction of aromatic nitro compounds.27−29 Since visible-light-induced catalysis has achieved much success in the field of pollutant degradation,30 researchers have been engaged in developing a series of photocatalytic materials, like g-C3N4, TiO2, ZnO, and graphene oxide, instead of metal nanoparticles. In comparison to other photocatalysts, g-C3N4 offers unique characteristics, including low cost, facile synthesis procedure, nontoxic nature, and stable photocatalytic performance.29,30 Nevertheless, unmodified g-C3N4 has poor photocatalytic activity, which could be mainly attributable to (i) fast recombination of electron−hole pairs in a photoexcited g-

C3N4, (ii) restricted response in visible-light exposure, and (iii) relatively small specific surface area.23,24,30 To tackle those problems, numerous methods have been developed to enhance the catalytic activity of g-C3N4 for photoreduction of Cr(VI),31,32 pollutant degradation,33 and hydrogen production.34−37 Examples of approaches include element doping,32,35 copolymerization,33,37 and heterojunction construction.31,36,38 On the other hand, Fe3O4NPs have around 43% absorption in the red portion of the visible color spectrum; this feature could play an essential role in catalysis studies widely employed in environmental remediation. However, bare Fe3O4NPs usually suffer from poor photocatalytic activity as a result of their insufficient holediffusion length, low carrier ability, small absorption coefficient, and short excited-state lifetime. These weaknesses lead to fast recombination of photoinduced charge carriers. As a further improvement, Fe3O4NPs coupled with g-C3N4 could generate a novel material that is expected to show unique optical and photocatalytic properties.30,39−41 Herein, this study reports the preparation of g-C3N4 nanocomposites decorated with magnetite nanoparticles (g-C3N4@Fe3O4NPs) through the integration of thermal polycondensation of melamine and coprecipitation approaches. The formed g-C3N4@Fe3O4NPs have a mesoporous nanostructure with high pore volume and large specific surface area. These features further enable remarkable photocatalytic activity for NaBH4-mediated conversion of 4- nitroaniline (4-NA) to 4-aminoaniline (4-AA) and 2-nitroaniline (2-NA) to 2-aminoaniline (2-AA) and efficient adsorption of Cr(VI). As we all know, there is little-tono information regarding the applications of g-C3N4@ Fe3O4NPs for efficient capture of chromium species from real-world samples and photocatalytic reaction of 4-NA/2-NA and NaBH4 under visible-light exposure. We emphasize that (i) the maximum adsorption capacity of Cr(VI) by g-C3N4@ Fe3O4NPs was significantly superior to that of the previously reported materials (Table S1, Supporting Information) and (ii) g-C3N4@Fe3O4NPs possessed a more efficient photocatalytic activity toward 2-NA and 4-NA than previously reported materials (Table S2, Supporting Information). We suggest that this study offers a facile and viable method for synthesizing enhanced multifunctional nanomaterials, which could pave the way for an alternative strategy in the design and development of high-efficiency nanomaterials for environmental remediation.

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EXPERIMENTAL SECTION

Chemicals. Melamine was purchased from Alfa-Aesar (Ward Hill, MD). FeCl2·4H2O and FeCl3·6H2O were obtained from Showa Chemicals (Tokyo, Japan). 4-NA, 2-NA, NaBH4, and other chemicals were bought from Acros Organics (Geel, Belgium). Milli-Q water (Millipore, Hamburg, Germany) was employed throughout the experiments. Stock solutions of Cr(III) and Cr(VI) were ordered from Merck (1000 mg L−1; Darmstadt, Germany). Synthesis of g-C3N4 Nanospheres. Melamine (3.0 g) in a crucible was directly heated to 450/550 °C for 4 h under N2 atmosphere. The obtained product was cooled to room temperature and then ground into a powder. Subsequently, 1.0 g of the as-made gC3N4 fine powder was dispersed in HNO3 (100 mL, 5 M), and the resulting solution was refluxed for 8 h. The white product was purified by centrifugation (30 min, 6000 rpm) and washing multiple times with Milli-Q water. When the supernatant was near to neutral pH, the purified product was collected via centrifugation. After decanting the supernatant, the precipitate, denoted as exfoliated g-C3N4 nanospheres, was dried at 80 °C.30,40,41 B


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ACS Sustainable Chemistry & Engineering Synthesis of Fe3O4NPs and g-C3N4@Fe3O4NPs. Exfoliated gC3N4 nanospheres (1.0 g) were initially dissolved in Milli-Q water (100 mL), followed by ultrasonication at ambient temperature for 4 h. FeCl2·4H2O (0.703 g) and FeCl3·6H2O (1.838 g) were prepared separately in aqueous solutions of g-C3N4 nanospheres (20 mL). The mixture was gently stirred at 80 °C for 0.5 h, followed by the injection of an ammonia solution (10 mL). After 1 h of stirring, the obtained solution was cooled to ambient temperature and then rinsed multiple times with Milli-Q water. The final product, corresponding to gC3N4@Fe3O4NPs, was stored at 4 °C for further analytical studies.2,6 For further characterization studies, the as-synthesized g-C3N4@ Fe3O4NPs was oven-dried at 80 °C. For the comparison experiment, the synthetic route mentioned above was used to prepare bare Fe3O4NPs. The analysis of g-C3N4@Fe3O4NPs by energy-dispersive X-ray spectroscopy (EDS) shows that the mass fractions of Fe3O4NPs and g-C3N4 in g-C3N4@Fe3O4NPs were 71 and 29 wt %, respectively. Batch Adsorption Study. The experiments associated with the adsorption of chromium species by the present materials were conducted at ambient temperature to determine the optimal conditions. The tested parameters included chromium concentration (50−550 mg L−1), solution pH (1.4−12.1), and contact time between liquid and solid phase (2−12 min). Under the optimal conditions (20 mg L−1 chromium, pH 5.5−9.9, and a 12 min contact time for equilibrium study), subsequent adsorption experiments were conducted for the kinetics study between the present materials and Cr(VI). The individual batch experiments were conducted in a mixture of Cr(VI) (10 mL, 120 mg L−1) for g-C3N4@Fe3O4NPs (0.01 g) and in another mixture of Cr(VI) (15 mL, 50 mg L−1) for Fe3O4NPs (0.03 g) at constant pH values. After equilibration, the asprepared materials (Fe3O4NPs and g-C3N4@Fe3O4NPs) was isolated from aqueous solution through applying an external magnetic field.2,6 The Cr(VI) concentration in the supernatant was then detected by inductively coupled plasma mass spectrometry (ICP-MS; PerkinElmer, Sciex-Elan-6100 DRC PLUS) (Table S3, Supporting Information). The loading of Cr(VI) on the present materials (mg g−1) was determined as qe =

(Cinitial − Cfinal)V W

Figure 1. BET adsorption isotherm of (A) unmodified g-C3N4, (B) bare Fe3O4NPs, and (C) g-C3N4@Fe3O4NPs. Magnetic hysteresis curves of (D) Fe3O4NPs and (E) g-C3N4@Fe3O4NPs. Inset: photo images of g-C3N4@Fe3O4NPs (F) without and (G) with an external magnetic field.

Table 1. Surface Area and Porosity of Three Materials

(Cinitial − Cfinal) × 100 Cinitial

BJH pore volume (cm3 g−1)

pore diameter (nm)

porous g-C3N4 Fe3O4NPs g-C3N4@Fe3O4NPs

4.5 65.1 84.9

0.02 0.18 0.33

17.2 11.1 15.8

We mention that Fe3O4NPs, unmodified g-C3N4, and gC3N4@Fe3O4NPs are related to type H3 hysteresis loops and type IV isotherms, indicating their mesoporous structures.23,30,39 The BET surface areas were determined to be 65.1, 4.5, and 84.9 m2 g−1 for bare Fe3O4NPs, unmodified gC3N4, and g-C3N4@Fe3O4NPs, respectively. Evidently, the BET surface area of g-C3N4@Fe3O4NPs is much superior to that of unmodified g-C3N4,2,23,30 which is attributable to the loading of Fe3O4NPs into the composites. This result implies that Fe3O4NPs are uniformly decorated on the g-C3N4 surface, which is highly desirable for efficient adsorption of chromium species.2 Moreover, the incorporation of Fe3O4NPs into unmodified g-C3N4 led to an enlargement in the pore volume from 0.02 to 0.33 cm3 g−1. From these findings, we point out that the higher pore volume of g-C3N4@Fe3O4NPs provides a higher number of pores on the surface, which is beneficial to provide a superior environment for adsorption and more active sites for photocatalytic applications.42 Additionally, the larger specific surface area of g-C3N4@Fe3O4NPs allows not only a higher affinity toward the adsorbates but also better absorption of visible light during photocatalytic reactions. The pore diameter of g-C3N4@Fe3O4NPs is large enough for the diffusion of chromium species and aromatic nitro compounds, facilitating the adsorption−desorption processes. More importantly, the mesoporous structure of g-C3N4@Fe3O4NPs can behave as light-transfer paths for the introduction of photoenergy onto the inner surface,42 thereby enhancing the photocatalytic activity. These features facilitate g-C3N4@ Fe3O4NPs to capture chromium species from an aqueous solution and to photocatalyze NaBH4-induced reduction of aromatic nitro compounds. A superconducting quantum interference device (SQUID) magnetometer was utilized to detect the magnetic properties of Fe3O4NPs and g-C3N4@ Fe3O4NPs at 300 K. Parts D and E of Figure 1 display that the magnetization saturation values of Fe3O4NPs and g-C3N4@ Fe3O4NPs were 120 and 67.7 emu g−1, respectively.2,30 Compared to bare Fe3O4NPs, a 48% reduction in the magnetization saturation values of g-C3N4@Fe3O4NPs results from the appearance of nonmagnetic g-C3N4 in g-C3N4@

(1)

where Cinitial and Cfinal correspond to the initial and final concentration (mg L−1) of chromium in the liquid phase. W and V are the weight (g) of the present materials and the volume (L) of the solution, respectively.6 The adsorption efficiency (AE) of chromium was estimated as %AE =

material

BET surface area (m2 g−1)

(2)

Catalytic Reduction. An aqueous solution of 2-NA (500 μL, 1 mM) or 4-NA (500 μL, 1 mM) was diluted with Milli-Q water (5 mL) and, after that, were mixed with freshly prepared NaBH4 (500 μL, 1.0 M) with vigorous stirring. Subsequently, Fe3O4NPs (1000 μL, 25 mg) and g-C3N4@Fe3O4NPs (1000 μL, 12 mg) were separately injected into the resultant solution. The obtained mixture was exposed to blue-emitting LED (KESSIL-PR160, Richmond, CA; λmax of 456 nm, optical power of 30 W). The kinetic reaction was investigated by UV−vis spectroscopy as a function of time.26−28 After completion of nitro compound reduction, the catalyst (Fe3O4NPs and g-C3N4@Fe3O4NPs) was collected from the aqueous solution by a magnetic separation, and simultaneously, the color of the solution changes distinctly from light-yellow to colorless.

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RESULTS AND DISCUSSION Comparison of Three Materials. Figure 1A−C shows the N2 adsorption−desorption isotherms of three materials, and Table 1 summarizes their Brunauer−Emmett−Teller (BET) surface area, pore diameters, and pore volume. The pore diameters of unmodified g-C3N4, bare Fe3O4NPs, and gC3N4@Fe3O4NPs were 17.2, 11.1, and 15.8 nm, respectively. C


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ACS Sustainable Chemistry & Engineering

Figure 2. (A−C, E, G, I, K) HR-TEM images, (D, H, L) SAED patterns, and (F, J) size distributions of (A−D) g-C3N4, (E−H) Fe3O4NPs, and (I−L) g-C3N4@Fe3O4NPs.

temperature. When the processing temperature is higher than 450 °C, two separate diffraction peaks at 13.1° and 27.3° were recognized as the hexagonal phase of g-C3N4 (JCPDS No. 871526; Figure S1D, Supporting Information).40 Furthermore, it can be confirmed that all the diffraction peaks well agree with literature data.23,30,40 The XRD pattern of Fe3O4NPs reveals the diffraction peaks at 30.3°, 35.7°, 43.2°, 53.3°, and 56.9° corresponding to the (220), (311), (400), (422), and (511) planes of the crystal lattice of cubic Fe3O4NPs (JCPDS No. 019-0629),6,23,30,40 in sequence (Figure S1E, Supporting Information). The XRD spectrum of g-C3N4@Fe3O4NPs reveals an intensive peak at 2θ = 12.4°, corresponding to tris-triazine units (Figure S1F, Supporting Information). This peak position showed a slight downshift as compared to gC3N4, implying a strong binding of Fe3O4NPs and gC3N4.2,30,43,44 In other words, the porous g-C3N4 is highly anchored onto the Fe3O4NPs surface. Notably, the diffraction pattern of Fe3O4NPs with a cubic phase diffraction pattern was slightly shifted to lower angles after the decoration process with g-C3N4. Moreover, no other impurity phases reflect that g-C3N4@Fe3O4NPs must be a two-phase composite. It is obvious that the as-prepared g-C3N4@Fe3O4NPs consists of Fe3O4NPs and g-C3N4 phases. As shown in the high-resolution transmission electron microscopy (HR-TEM) images, unmodified g-C3N4 possessed a spherical structure (Figure 2A− C).43 The selected area of electron diffraction (SAED) pattern of g-C3N4 (Figure 2D) displays a full, but obscure, diffraction

Fe3O4NPs. Moreover, the curves of magnetization against applied magnetic field at 300 K for Fe3O4NPs and g-C3N4@ Fe3O4NPs had no remanence and zero coercivity with S-like magnetic hysteresis loops. The findings mentioned above clearly illustrate that Fe3O4NPs and g-C3N4@Fe3O4NPs possess normally superparamagnetic behavior. The black and homogeneous g-C3N4@Fe3O4NPs, as shown in Figure 1F, can be attracted easily by an external magnet,2,23,30 resulting in a clear and transparent solution (Figure 1G). This feature suggests that g-C3N4@Fe3O4NPs is well-suited for adsorption of heavy-metal ions and photocatalytic reduction of nitroaromatic compounds. The structural analysis of melamine by X-ray powder diffraction (XRD) has numerous sharp peaks corresponding to the crystalline phase (JCPDS No. 39-1950; Figure S1A, Supporting Information). This diffraction pattern indicates that no other impurity was detected.23,30,40 When the heating temperature of melamine varied from 350, 450, and 550 °C, the main peaks became broader, along with the disappearance of several peaks (Figure S1B−D, Supporting Information). This result indicates that materials could be transformed from crystalline to amorphous. The pattern of g-C3N4 in the XRD spectrum clearly shows a broad peak at 27.3° in line with the phase of g-C3N4, which is identified as the (002) plane with an interplanar distance of 0.32 nm (Figure S1C,D, Supporting Information). These peaks originate from the long-range interlayer stacking of aromatic units with increasing heating D


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surface possesses multiple amine residues (−N, −NH, and −NH2). The isoelectric point of bare Fe3O4NPs was found to be 6.8 (Figure 3B). Thus, as the pH was below the isoelectric point of bare Fe3O4NPs, the protonation of hydroxyl groups caused the formation of −OH2+. By contrast, hydroxyl groups of bare Fe3O4NPs are deprotonated at a pH above the isoelectric point of g-C3N4@Fe3O4NPs, which was measured to be about pH 9.9 (Figure 3C). Intriguingly, the ζ-potential of g-C3N4@Fe3O4NPs fell between that of unmodified g-C3N4 and bare Fe3O4NPs in the pH range from 3.0 to 10.0, reflecting the attachment of functional amino and hydroxyl groups on the g-C3N4@Fe3O4NPs surface. Accordingly, we reason that hydroxyl residues of bare Fe3O4NPs and amino residues of unmodified g-C3N4 in g-C3N4@Fe3O4NPs could be protonated at acidic and neutral pH, leading to electrostatic binding of negatively charged chromium species (HCrO4− and CrO42−) to the positively charged particle surface.3,11 Figure 3D reveals the impact of pH on the Cr(VI) removal by gC3N4@Fe3O4NPs. The removal efficiency of g-C3N4@ Fe3O4NPs for Cr(VI) gradually enhanced with increasing solution pH and reached a saturation level (>99%) above pH 10.0. Given that electrostatic repulsion exists between gC3N4@Fe3O4NPs and CrO42− above pH 10.0, we suggest that electron donor−acceptor interaction between amino residues of g-C3N4@Fe3O4NPs (electron donor) and Cr(VI) (electron acceptor) could also dominate the capture of Cr(VI) on the gC3N4@Fe3O4NPs surface. We emphasize that amino residues of g-C3N4@Fe3O4NPs originate from the contribution of gC3N4. Besides, g-C3N4@Fe3O4NPs exhibited a similar trend for capturing Cr(III) at any tested pH. At pH lower than 4.0, protonated hydroxyl residues of Fe3O4NPs and amino residues of g-C3N4 in g-C3N4@Fe3O4NPs induced strong electrostatic repulsion with Cr3+ and Cr(OH)2+, resulting in poor removal efficiency (Figure 3E). The efficiency of g-C3N4@Fe3O4NPs for the removal of Cr(III) was improved to >99% above pH 4.0. When the solution pH was increased, protonated hydroxyl residues of Fe3O4NPs and amino residues of g-C3N4 in gC3N4@Fe3O4NPs could behave as an electron donor to interact with electron-deficient chromium species [Cr3+, Cr(OH)2+, and Cr(OH)2+].5,6 Additionally, the adsorption of Cr(OH)3 on the g-C3N4@Fe3O4NPs surface occurred at alkaline pH. Under an identical pH condition, deprotonated hydroxyl groups of Fe3O4NPs in g-C3N4@Fe3O4NPs also have an electrostatic attraction with Cr(OH)2+. Taken together, hydroxyl residues of Fe3O4NPs and amino residues of g-C3N4 enable g-C3N4@Fe3O4NPs to contribute an electrostatic attraction and an electron donor−acceptor interaction with chromium species, resulting in a highly efficient removal of chromium species. To further confirm the Cr(VI) adsorption on the g-C3N4@Fe3O4NPs surface, X-ray photoelectron spectroscopy (XPS) was used to examine the chemical composition and elemental oxidation state of a mixture of gC3N4@Fe3O4NPs and Cr(VI). The survey XPS spectrum shows the coexistence of C, O, N, Cr, and Fe elements in gC3N4@Fe3O4NPs (Figure 3F). The high-resolution XPS Cr 2p spectrum reveals that the peaks are located at 574.4 eV (Cr 2p3/2) and 584.1 eV (Cr 2p1/2) (Figure 3G).2,6 The broad Cr 2p3/2 peak can be divided into three peaks at 576.8 eV (K2Cr2O7), 574.9 eV (CrO42−), and 573.8 eV (CrO3), implying the appearance of Cr(VI). Besides, deconvolution of the Cr 2p1/2 peak resulted in two components, including 585.7 eV (Cr2O3) and 583.8 eV [Cr(OH)3]. In addition, small turbid precipitates were also formed during the adsorption

ring corresponding to the representative (002) plane of gC3N4. The representative HR-TEM image reflects that Fe3O4NPs were highly aggregated in an aqueous solution (Figure 2E) and they have an average diameter of 17.5 nm (Figure 2F) with a d-spacing of 0.25 nm (Figure 2G), indicating the presence of the (111) plane of Fe3O4 crystals. Moreover, the SAED pattern indicates that Fe3O4NPs are highly crystalline (Figure 2H). As indicated in the HR-TEM image of g-C3N4@Fe3O4NPs (Figure 2I), the hybrid of g-C3N4 and Fe3O4NPs suppressed the aggregation of g-C3N4@ Fe3O4NPs at these inflates of large particle size distribution (Figure 2J).30,40 The HR-TEM image taken from g-C3N4@ Fe3O4NPs shows clear lattice plane spacing of 0.29 and 0.32 nm that are indexed to the (220) and (002) planes of Fe3O445,46 (Figure 2K). The SAED pattern confirms that gC3N4@Fe3O4NPs has better crystallinity than g-C3N4, mainly owing to the adsorption of Fe3O4NPs onto the g-C3N4 surface (Figure 2L). Field-emission scanning electron microscopy (FESEM) integrated with EDS mapping was further utilized to support the formation of composite g-C3N4@Fe3O4NPs. The FESEM-EDS elemental mapping images show that the carbon element was homogeneously distributed in the composite gC3N4@Fe3O4NPs (Figure S2, Supporting Information). Moreover, the iron element was present in the same composite. Since g-C3N4 only consists of carbon and nitrogen elements, the iron element was suggested to be due to the contribution of Fe3O4NPs. These results strongly recommend that Fe3O4NPs were successfully assembled onto the g-C3N4 surface. Capture of Cr(III) and Cr(VI) by g-C3N4@Fe3O4NPs. Considering that electrostatic attraction could be the key factor that governs the binding of g-C3N4@Fe3O4NPs to Cr(III) and Cr(VI), we estimated the ζ-potentials of g-C3N4@Fe3O4NPs at different pH values and compared them with unmodified gC3N4 and also bare Fe3O4NPs under the identical conditions. The ζ-potential of unmodified g-C3N4 remained positive at pH values higher than 10.0 (Figure 3A), reflecting that the g-C3N4

Figure 3. ζ-Potentials of (A) unmodified g-C3N4, (B) bare Fe3O4NPs, and (C) g-C3N4@Fe3O4NPs at different pH values. Effect of solution pH on the adsorption efficiency of g-C3N4@Fe3O4NPs for (D) Cr(VI) and (E) Cr(III). (F) Total survey of XPS spectrum of gC3N4@Fe3O4NPs. (G) High-resolution XPS Cr 2p spectrum of gC3N4@Fe3O4NPs. The original spectrum is in dark blue. E


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where KF corresponds to the Freundlich constant (L/g) and n presents the Freundlich parameter associated with the degree of heterogeneity in the system.3,6,9 The adsorption data obtained from the three adsorbent materials all fitted well into Langmuir isotherm model (Table S4, Supporting Information). In the case of Cr(VI), bare Fe3O4NPs, unmodified g-C3N4, and g-C3N4@Fe3O4NPs exhibit the maximum adsorption capacity of 50.7, 243.9, and 555.5 mg g−1, respectively (Table S4, Supporting Information). Table S1 (Supporting Information) shows that g-C3N4@Fe3O4NPs provides remarkably higher maximum absorption capacity for Cr(VI) than the recently reported materials. A forward curve was obtained by plotting the adsorbed amount of Cr(VI) (qt) on the g-C3N4@Fe3O4NPs surface against the contact time (Figures S6, Supporting Information). These data reflect that the efficiency of g-C3N4@Fe3O4NPs for the Cr(VI) adsorption is higher than 85%. When qt almost achieved its equilibrium value, we observed a nearly flat distribution of experimental data. Within the first 8−10 min, large amounts of Cr(VI) were adsorbed on the surface of g-C3N4@Fe3O4NPs. Thus, there is a gradually increasing trend for the efficiency of Cr(VI) removal. Furthermore, the adsorption of Cr(VI) by g-C3N4@ Fe3O4NPs reached a saturation level within 12 min that is significantly faster than the previously reported materials. We suggest that accessible sites on the g-C3N4@Fe3O4NPs surface are equal for the adsorption of Cr(VI). The kinetics of Cr(VI) adsorption on the g-C3N4@Fe3O4NPs surface can be fitted using the pseudo-first-order Lagergren equation and the pseudo-second-order model.3−6 The pseudo-first-order rate constant (k1) was calculated using the following Lagergren equation

studies. Taken together, we suggest that g-C3N4@Fe3O4NPs is capable of capturing Cr(VI) and Cr(III) from aqueous solution. Apparently, the hybrid of Fe3O4NPs and porous gC3N4 not only provides efficient capture of Cr(VI) but also induces the photocatalytic reduction of Cr(VI) to Cr(III). As we will prove later, g-C3N4@Fe3O4NPs efficiently catalyzes NaBH4-induced conversion of 2-NA and 4-NA under visiblelight exposure. Previous studies demonstrated that g-C3N4based materials efficiently triggered the photocatalytic conversion of Cr(VI) to Cr(III).47−51 Furthermore, in the Fourier-transform infrared (FT-IR) spectrum of g-C3N4@ Fe3O4NPs, there are considerable changes in the N−H and O−H region after capture of Cr(VI) on the g-C3N4@ Fe3O4NPs surface (Figure S3, Supporting Information). The negatively charged hydrogen chromate ion (HCrO4−) and protonated (−NH3+,−OH2+) groups are involved in an electrostatic attraction onto the adsorbent surface, as observable from the features of FT-IR spectra. The new peak appearing at 890 cm−1 is characteristic of the CrO stretching frequency in the hydrogen chromate ion, while stretching peaks of Cr−O at 991, 906, and 866 cm−1 were assigned to monodentate chromate.2−6 Additionally, the analysis of a mixture of Cr(VI) and g-C3N4@Fe3O4NPs by EDS supports the adsorption occurrences between Cr(VI) and g-C3N4@ Fe3O4NPs (Figure S4, Supporting Information).2−6 Adsorption Equilibrium. The equilibrium relations between Cr(VI) and adsorbents in aqueous solution were examined by varying the concentration of Cr(VI) at a fixed concentration of adsorbents. The adsorbents included bare Fe3O4NPs, unmodified g-C3N4, and g-C3N4@Fe3O4NPs. Figure S5 (Supporting Information) shows that the adsorption of Cr(VI) on the three materials obeys an L-shaped isotherm, leading to a considerable enhancement in the adsorbed Cr(VI) initially. The rising part of the curve at relatively low Ce values and the flat part of the curve at relatively high Ce values provide clear evidence for effective electrostatic interaction between chromium oxyanions and g-C3N4-based adsorbents (g-C3N4 and g-C3N4@Fe3O4NPs). The saturation curve reflects that the three materials have a limited number of adsorption sites for Cr(VI). The most extensively used adsorption isotherm equations are the Langmuir and Freundlich ones that were implemented to specify the observed equilibrium data.2−6 The Langmuir adsorption isotherm represents that the adsorbent surface has a fixed number of homogeneous active sites with equal energies and each adsorbate has the same opportunity to desorb from the adsorbent surface. The amounts of Cr(VI) adsorbed on the surface of the three materials are expressed as qe =

qt = qe(1 − e−k1t )

where qt, qe, and k1 have been mentioned above and t is the contact time. The pseudo-second-order rate constant (k2) was estimated using the following expression: qt =

(3)

where qe (mg g−1) and qmaxL (mg g−1) correspond to the equilibrium adsorption capacity and the maximum adsorption capacity, respectively. Ce (mg L−1) is the equilibrium concentrations between solid and liquid phase, and b is the adsorption energy (Table S4, Supporting Information). Additionally, the Freundlich isotherm describes the adsorption process that takes places on the heterogeneous surface, and this can be described as qe = KFCe1/ n

k 2qe 2 1 + k 2qet

(6)

The adsorption of Cr(VI) by g-C3N4@Fe3O4NPs follows pseudo-second-order reaction kinetics, which is in agreement with the higher values of the regression coefficient for gC3N4@Fe3O4NPs (Table S5, Supporting Information). Moreover, the values of the reduced χ2 and the adjusted R2 demonstrate the applicability of the second-order rate expression (Table S5, Supporting Information). The adsorption rate of g-C3N4@Fe3O4NPs for Cr(VI) could be influenced by two driving forces, including (i) pore or intraparticle diffusion, where chromium ions penetrate the interior pore sites, and (ii) external mass transfer, in which chromium ions are moved from the bulk liquid phase to the surface of gC3N4@Fe3O4NPs. In comparison with two driving forces mentioned above, surface adsorption is dominant, and thus, pore or surface diffusion plays an important role to study adsorption kinetics. Since the adsorption process is relatively rapid, the values of the calculated and experimental qe were matched to the second-order kinetics. Adsorbent Reusability and Its Application. From the greener perspective, it is essential to investigate the regeneration ability of the adsorbent for multiple adsorption/ desorption cycles. The selected reagents should be nontoxic and create less damage to the adsorbent surface. On the basis

qmaxLbCe 1 + bCe

(5)

(4) F


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light irradiation. As a result, g-C3N4@Fe3O4NPs would provide a relatively high photocatalytic activity in comparison to gC3N4 and Fe3O4NPs. The catalytic efficiency of g-C3N4@ Fe3O4NPs was further assessed by catalytic reduction of 2-NA and 4-NA with an excess amount of NaBH4. Figure S9 (Supporting Information) illustrates the mechanism of how gC3N4@Fe3O4NPs catalyzes NaBH4-induced reduction of nitroaromatic compounds under visible-light irradiation. After immediately adding freshly prepared NaBH4 solution, aromatic nitro compounds and BH4− are both attached onto the adsorbent surface through electrostatic attraction.26−28 BH4− behaves as the hydrogen source, while g-C3N4@Fe3O4NPs acts as electron donor. As a result, aromatic nitro compounds are converted to aromatic amino compounds via nitroso and hydroxylamine intermediates. Without the addition of gC3N4@Fe3O4NPs as a catalyst, NaBH4-mediated conversion of 2-NA and 4-NA rarely occurred even after 7 days. Aqueous solutions of 2-NA and 4-NA reveal absorbance peaks at 413 and 380 nm with a bright yellow color, respectively (Figure 4A,D). By contrast, the same concen-

of the above considerations, reducing agents (NaNO2, Na2SO3, and L-ascorbic acid) and strong bases (NaOH and NH4OH) were tested at varying concentrations. These strong bases could elute Cr(VI) from the g-C3N4@Fe3O4NPs surface, thereby generating chromate salts (Na2CrO4, NH4CrO4). Besides, the selected reducing agents could effectively reduce Cr(VI) to Cr(III) (Figure S7A, Supporting Information).6 Among these reagents, aqueous NaOH solution has better performance for desorption studies. The surface functional groups (−N, −NH, −NH2, and Fe−O−) of g-C3N4@ Fe3O4NPs can be efficiently deprotonated with NaOH, leading to Cr(VI) desorption.2−6 Afterward, g-C3N4@Fe3O4NPs was rinsed with Milli-Q water under a mildly acidic environment and provided to the next experiments on Cr(VI) adsorption. The cycles of adsorption−desorption studies were repeated for five successive adsorption−desorption cycles (Figure S7B, Supporting Information). The soil sludge samples were collected from local industrial waste discharge river basins (Erren River, Tainan, Taiwan). For sample pretreatment, a known amount of each soil sludge sample (1.0 g) was separately weighted into a 50 mL polytetrafluoroethylene digestion vessel. After adding 4 mL of concentrated HF and 3 mL of concentrated HNO3 to each sample, the heating program for the microwave oven initiated at 90 °C, gradually rose up to 210 °C over 20 min and 800 W, remained at 150 °C for 15 min, and then cooled to ambient temperature. The volume of the resultant solutions were made up to 20 mL with Milli-Q water, followed by the analysis of total chromium using ICP-MS. The procedure mentioned above was also used for the analysis of blank controls. All experiments were performed in three replicates, and the obtained data are expressed as the average of triplicates. Briefly, an appropriately diluted soil sludge sample (10 mL) that contains chromium was contacted with g-C3N4@Fe3O4NPs (1.0 g) for 12 min at an adjusted pH of 5.5. g-C3N4@Fe3O4NPs was removed from the aqueous solution by a magnetic field, followed by the analysis of the resultant solution using ICP-MS. As indicated in Table S6 (Supporting Information), the removal efficiency of g-C3N4@ Fe3O4NPs for Cr(VI) is more than 90.0% from soil sludge samples. The exhausted adsorbent could also be regenerated using 0.5 M NaOH solution. Application to Catalytic Reduction of 2-NA and 4-NA. We next investigated the optical features of g-C3N4, Fe3O4NPs, and g-C3N4@Fe3O4NPs using UV−vis diffuse reflectance spectroscopy. By plotting (αhν)1/2 against photon energy, the intercept of the tangent line gave rise to the band gap energies of the three materials. Figure S8 (Supporting Information) indicates that the band gap energies of g-C3N4, Fe3O4NPs, and g-C3N4@Fe3O4NPs, estimated by the KubelkaMunk function, were 2.78, 2.19, and 2.07 eV, respectively. The value of the band gap energy of g-C3N4 is in line with previously reported studies.30,40 Note that g-C3N4@Fe3O4NPs exhibits more intense absorption than g-C3N4 and starts to shift significantly to longer wavelengths. An incorporation of Fe3O4NPs into g-C3N4 renders the band gaps to be relatively narrow, signifying that the absorption peak moves into the lower energy region.30,40 Meanwhile, the powder color dramatically changes from bright yellow to pale crimson red with increasing content of Fe3O4NPs loading. Thus, we suggest that the visible-light response of g-C3N4@Fe3O4NPs is wellsuited for photocatalytic performance.30 Furthermore, the loading of Fe3O4NPs onto the unmodified g-C3N4 surface could cause a possible charge-transfer transition under visible-

Figure 4. UV−vis absorption spectra for the NaBH4-mediated reduction of (A) 2-NA to (B) 2-aminoaniline and (D) 4-NA to (E) 4-aminoaniline with g-C3N4@Fe3O4NPs under visible-light irradiation. Time-dependent UV−vis absorption spectra for g-C3N4@ Fe3O4NPs-catalyzed reduction of (C) 2-NA to 2-aminoaniline and (F) 4-NA to 4-aminoaniline in the presence of NaBH4 and visiblelight irradiation. A plot of ln(A/A0) versus reaction time for (G) gC3N4@Fe3O4NPs, (H) g-C3N4@Fe3O4NPs, (I) unmodified g-C3N4, and (J) Fe3O4NPs in the absence (H) and presence (G, I, J) of visible-light irradiation. G


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ACS Sustainable Chemistry & Engineering trations of 2- and 4-aminoaniline gave low absorbance across the visible region (Figure 4B,E). When g-C3N4@Fe3O4NPs was incubated with either 2-NA or 4-NA, the NaBH4-mediated reductions of 2-NA to 2-aminoaniline and 4-NA to 4aminoaniline were both complete within 8 min under visiblelight exposure (Figures 4C,F). It is noteworthy to mention that, after the addition of g-C3N4@Fe3O4NPs, the reduction reaction started immediately without any induction period.27,28 In the absence of visible-light irradiation, it took around 460 and 450 min for g-C3N4@Fe3O4NPs to convert to some extent 2-NA and 4-NA, respectively (Table S2, Supporting Information). When g-C3N4@Fe3O4NPs was replaced by either g-C3N4 and or Fe3O4NPs under the identical conditions (NaBH4 and visible light irradiation), the time for complete reduction of 2-NA and 4-NA catalyzed by g-C3N4 and Fe3O4NPs was found to be relatively long (Table S2, Supporting Information). Figure 4G,I,J shows that plots of the natural logarithm of A/A0 versus catalytic time yielded linear regression lines in the case of g-C3N4@Fe3O4NPs-, gC3N4-, and Fe3O4NPs-catalyzed reaction of 2-NA/4-NA with NaBH4 under visible-light exposure. A0 and A are the absorbance of 2-NA at 413 nm or 4-NA at 380 nm at zero time and at any other time, respectively.52−54 A control experiment without visible-light irradiation was performed for g-C3N4@Fe3O4NPs, in which the catalytic time is linearly correlated with the natural logarithm of A/A0 (Figure 4H). The rate constants of g-C3N4@Fe3O4NPs for catalyzing NaBH4-induced reduction of 2-NA and 4-NA are superior to those of g-C3N4, Fe3O4NPs, and irradiation-free g-C3N4@ Fe3O4NPs. Table S2 (Supporting Information) shows a comparison of kinetic parameters for catalyzing NaBH4mediated reduction of 2-NA and 4-NA among g-C3N4@ Fe3O4 NPs, irradiation-free g-C3N 4@Fe3O 4NPs, g-C3N 4, Fe3O4NPs, and previously reported materials. It is evident that g-C3N4@Fe3O4NPs have a relatively high rate constant, signifying that g-C3N4@Fe3O4NPs is capable of rapidly reducing 2-NA and 4-NA under visible-light exposure. In confirmation of the results mentioned above, time-dependent Raman spectroscopy was implemented to monitor the gC3N4@Fe3O4NPs-catalzyed reaction of 4-NA and NaBH4 under visible-light irradiation. The characteristics bands of 4NA appeared at 866, 928, 1324, 1384, and 1598 cm−1 (Figure 5A). The three bands at 866, 1324, and 1598 cm−1 are separately associated with the NO2 bending, C−N stretching, and −NO2 symmetric stretching.55−57 As the catalytic reaction proceeded, the intensities of the bands at 1598 and 866 cm−1

(indicated as dashed black lines) of 4-NA progressively diminished with time (Figure 5A−D). This result provides clear evidence for the NaBH4-mediated reduction of nitro groups of 4-NA catalyzed by g-C3N4@Fe3O4NPs. For practical applications in heterogeneous systems, we evaluated the reusability of g-C3N4@Fe3O4NPs for catalyzing the reaction of NaBH4 and 2-NA/4-NA under visible-light exposure. Figure 5E shows that g-C3N4@Fe3O4NPs catalyzed four successive cycles for >96% conversion of 2-NA and 4-NA. There is only a slight difference in the XRD pattern between the freshly prepared and used catalysts (Figure S10, Supporting Information). However, the catalytic efficiency for the conversion of 2-NA and 4-NA was reduced to 85 and 75% after the fifth and sixth catalytic cycles, respectively. This result could be attributable to the leaching of a small amount of iron from g-C3N4@Fe3O4NPs to the aqueous solution during the recycling process. For example, the iron loss of g-C3N4@ Fe3O4NPs, determined by ICP-MS, can reach up to 12 wt % after five repetitive cycles.

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CONCLUSION We have demonstrated a facile method for preparing g-C3N4@ Fe3O4NPs based on thermal polycondensation of melamine and coprecipitation of ferric and ferrous salts. This adsorbed material not only exhibits high removal efficiency toward Cr(VI) and Cr(III) but also provides improved photocatalytic activity toward the reaction of aromatic nitro compounds and NaBH4. Given that g-C3N4@Fe3O4NPs has a porous structure, abundant surface functional groups, strong magnetism, electrostatic attraction, and electron donor−acceptor interaction, these features enable them to offer the efficient adsorption of chromium species in a wide pH range during five repetitive cycles of the adsorption−desorption process. The adsorption capacity of g-C3N4@Fe3O4NPs toward Cr(VI) is superior to that of the previously reported adsorbents.2−14,25 We have shown that g-C3N4@Fe3O4NPs is practically applied for the removal of chromium species from real-world samples. Besides, g-C3N4@Fe3O4NPs can efficiently catalyze the NaBH4mediated reduction of 2-NA and 4-NA under the irradiation of visible light. The catalytic activity of g-C3N4@Fe3O4NPs is superior to that of Fe3O4NPs and previously reported materials. Additionally, the proposed material can be conveniently reused in four successive cycles of 2-NA and 4NA reduction through magnetic decantation. We firmly believe that this adsorbent material could have considerable potential to serve as a photocatalytic material for environmental pollutants. This study would pave the new way to design multifunctional materials with high adsorption properties and catalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05727. Additional experimental details, XRD spectra, FT-IR spectra, EDS spectra, FESEM-EDS images, UV−vis DRS spectra, adsorption isotherms, reusability data, catalytic mechanism, removal of chromium from real-world samples, comparison of adsorption capacity and catalytic activity, and analysis of adsorption kinetics. (PDF)

Figure 5. Time-dependent Raman spectra of the g-C3N4@Fe3O4NPscatalyzed reaction of NaBH4 and 4-NA under visible-light exposure: (A) 0 min, (B) 3 min, (C) 5 min, and (D) 8 min. (E) Reusability of gC3N4@Fe3O4NPs as the catalyst for NaBH4-mediated conversion of 2-NA and 4-NA under visible-light exposure. H


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AUTHOR INFORMATION

Corresponding Authors

*N.R.: e-mail, [email protected]. *S.-J.J.: e-mail, [email protected]. *W.-L.T.: e-mail, [email protected]; phone, 011-8867-5254644; fax, 011-886-7-3684046. ORCID

A. Santhana Krishna Kumar: 0000-0002-1681-4426 N. Rajesh: 0000-0003-1546-9904 Shiuh-Jen Jiang: 0000-0002-3205-8093 Wei-Lung Tseng: 0000-0001-9808-5863 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology (MOST107-2811-M-110-009 and MOST1042113-M-110-002) for the financial support of this study.

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