Evolution of Waste Iron Rust into Magnetically Separable g-C3N4

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Evolution of Waste Iron Rust into Magnetically Separable g-C3N4-Fe2O3 Photocatalyst: An Efficient and Economical Waste Management Approach Santosh Babasaheb Babar, Nana L Gavade, Harish Shinde, Prasad G. Mahajan, Ki Hwan Lee, Narayan Mane, Ashish Deshmukh, Kalyanrao M. Garadkar, and Vijaykumar M. Bhuse ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00936 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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ACS Applied Nano Materials

Evolution of Waste Iron Rust into Magnetically Separable g-C3N4-Fe2O3 Photocatalyst: An Efficient and Economical Waste Management Approach Santosh Babar†§, Nana Gavade§, Harish Shinde§, Prasad Mahajan■, Ki Hwan Lee■, Narayan Mane ◊, Ashish. Deshmukh ◊, Kalyanrao Garadkar*§ and Vijaykumar Bhuse*† †

Thin Film Research Laboratory, Department of Chemistry, Government Rajaram College,

Kolhapur, Maharashtra, India-416004. §

Nanomaterials Research Laboratory, Department of Chemistry, Shivaji University,

Kolhapur, Maharashtra, India-416004. ■

Department of Chemistry, Kongju National University, Gongju, Chungnam 32588,

Republic of Korea. ◊

Cellular Stress Response Laboratory, Cell Biology Division, Department of Zoology,

Shivaji University, Kolhapur, Kolhapur, Maharashtra, India-416004. * Corresponding Author. Tel.: +91 0231 2609167; Fax: +91 0231 2692333. Email: [email protected] (K. M. Garadkar), [email protected] (V. M. Bhuse)

ABSTRACT The corrosion of iron structures gives rise to a serious safety, environmental pollution, and economic issues. However, current technologies are neither efficient to impede the corrosion completely nor effective in the recycling of waste iron rust. To best recycle of waste iron rust, we report a simple, cost-free and sustainable strategy to exploit iron rust as a Fe-precursor for the synthesis of magnetic Fe2O3 nanoparticles (NPs) via simple grinding and calcination process. The process efficiently transforms bulky iron rust into ferromagnetic Fe2O3 NPs, which exist in both α, and γ phases. Synthesized materials were characterized by XRD, SEM-EDS, TEM, XPS, UV-Vis. DRS and VSM. We also explored the catalytic ability of rust-derived Fe2O3 as a lowcost material for the fabrication of magnetically separable g-C3N4-Fe2O3 composite as a photocatalyst. It is interesting to find that the g-C3N4-Fe2O3 composite exhibited superior photocatalytic activity than that of individual g-C3N4 and Fe2O3 under sunlight towards Methyl Orange and Textile Effluent. Moreover, the g-C3N4-Fe2O3 composite exhibited excellent

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reusability without loss of photocatalytic activity after successive five runs and more importantly, photocatalyst could be recovered magnetically. The histological studies on the gills of freshwater fish Tilapia revealed that toxic dye solution induces damage to secondary gill lamellae whereas the photodegraded products were found to less toxic and did not cause any structural alteration in the gill architecture. This innovative process of waste recycling offers a cost-free and large-scale approach to transform waste iron rust into magnetically separable Febased photocatalyst for environmental remediation. KEYWORDS: Corrosion,

Iron Rust, Magnetic Fe2O3, g-C3N4-Fe2O3, Photocatalyst,

Heterojunction, Photodegradation. 1. IntroductionOver the past centuries, iron has been one of the crucial industrial materials and plays an essential role in many sectors, which includes transport, manufacturing industries, construction and other utilities. The extensive use of iron components pushes forward the recurrent development of a country and improvement in human civilization. The inevitable corrosion of iron occurs when the iron comes in contact with moisture from the air. Iron corrosion is a complex and slow process that depends on environmental conditions such as humidity, pH, temperature, and pollution level.1 Every year an enormous amount of rust is formed due to corrosion on iron-containing parts such as pipes, machines, wires, building goods etc. The corrosion of iron parts not only cause economic loss but also leads to dreadful issues like diminishing the tensile strength that makes it more breakable.

2,3

On the basis of the report by

NACE International, “The global cost of corrosion is estimated to be US$ 2.5 trillion, which is equivalent to 3.4% of the global GDP (2013)”.4 Consequently, different anti-corrosion methods have been developed to prevent the corrosion of iron parts in different corrosive surroundings.5-8 However, these methods fail to control the corrosion completely and leading to the formation of rust at a slower rate. Despite the fact that corrosion of iron is a slow and natural process, its substantial formation and existence may lead to the formation of million tons of rusty waste. The literature review indicated the presence of few reports conducted on recycling and refurbishment of waste iron rust into potentially useful Fe2O3 NPs. For instance, Mhamane et al. examined Listorage properties of interconnected α-Fe2O3 derived from iron-based wires collected from building scrap waste.9 Zhu et al synthesized α-Fe2O3 nanospheres through hydrothermal


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treatment of rust in HNO3 solution.10-13 These approaches not only help to improve waste management but also open the new sustainable and potential way for the synthesis of Fe2O3 NPs from waste. However, the current handling approaches on the synthesis of Fe2O3 NPs or recycling of iron rust invite and necessitate harsh chemical treatments, high energy, high temperature, and are most suitable only for small-scale synthesis. Hence, novel cost-free, reliable and safer approaches that enable the large-scale synthesis of Fe2O3 NPs are one of the leading issues. These significant studies suggest an opportunity for synthesis of Fe2O3 NPs using waste resources for the development of magnetically separable photocatalyst. This also clearly suggest that the novel, more rational, affordable, and safer approaches for the synthesis of Fe2O3 NPs from waste iron rust has not been intensely used for the photocatalysis. More recently, significant studies have been focused on a new kind of fascinating polymeric graphitic nitride C3N4 (g-C3N4) due to its low-cost, metal-free and nontoxic photocatalytic behavior under visible light. The g-C3N4 is easily fabricable which has a narrow band gap of 2.7eV, appealing electronic structure, high chemical, and thermal stability due to the presence of π-conjugated frameworks.14-17 However, it exhibits poor photocatalytic performance and has limited applications in practical fields due to following issues (i) Fast recombination of electron-hole pairs, (ii) Difficulty in recycling, (iii) Loss of catalyst due to high dispersible nature (iv) Complicated separation of the catalyst from the reaction medium using conventional separation techniques.18-20 To address the above issues, the coupling of magnetic Fe2O3 (n-type) with the g-C3N4 (n-type) to form a g-C3N4-Fe2O3 n-n type heterojunction which can not only enhances the photocatalytic performance but also provides convenient magnetic separation of the catalyst under an external magnetic field.

21-24

Fe2O3 is promising material due to its significant

properties such as good absorption in the visible region, suitable bandgap (2.1 eV), low cost, low toxicity, abundance, high electrical conductivity, and magnetic nature.25-26 These properties and well matched overlapping band structures of Fe2O3 with g-C3N4, which promote the photoinduced charge separation at g-C3N4-Fe2O3 interface owing to the type II heterojunction which enhances photocatalytic performance under the visible light compared to the pure g-C3N4.27 In the present work, we have adopted the unique approach towards the one-step largescale synthesis of Fe2O3 NPs using waste film of rusted iron (Collected from the corroded portion of iron beams). A simple method of cleaning, washing, grinding followed by calcination was



used to prepare Fe2O3 NPs. This synthesis method has many advantages like simple, requires no chemicals, large scalability, cost-effective and can be the first step towards the commercialization of the Fe2O3 NPs synthesis. We term these iron oxide materials as the rustderived Fe2O3 NPs. According to literature survey, till date no reports are available on the use of waste iron rust for the synthesis of Fe2O3 NPs and its coupling with g-C3N4 for photocatalysis application which resolves two issues of waste management and water pollution simultaneously. Further, we evaluated the photocatalytic performance of the rust-derived C3N4-Fe2O3 composite for the degradation of a model pollutant, Methyl Orange (MO), and Textile Effluent (TE) under sunlight irradiation. Our finding suggests that the magnetically separable g-C3N4-Fe2O3 photocatalyst exhibited superior photocatalytic performance compared to g-C3N4 and Fe2O3 NPs. The ease of C3N4-Fe2O3 separation using magnet make it reusable and hold great sustainability for the wastewater treatment as it keeps catalytic behavior up to the fifth cycle. The histological studies of cytotoxicity of dye on the gills of freshwater fish Tilapia before and after photodegradation was carried out to confirm the toxicity of photodegraded products. 2 Experimental 2.1. Materials and chemicals Thiourea, Iso-propanol, and Ethylene Diamine Tetra Acetic Acid (EDTA) were purchased from SD Fine-Chemical Ltd. Methyl Orange and 1, 4-Benzoquinone were purchased from Sigma-Aldrich. Formalin, Xylene, Ethanol, DPX (Distyrene, Plasticizer, and Xylene), Eosin and Haematoxylin were purchased from Fisher Scientific, India. The textile effluent was collected from textile industry located in the industrial area of Solapur, Maharashtra, India. Films of waste iron rusts were collected from the corroded beams. 2.2 Preparation of Fe2O3 from waste rust iron The waste iron rusts collected from the corroded beams were washed several times with deionized water to remove the dust particles and any kind of water-soluble impurities. Later, this rusted iron is subjected to a drying process at 80° C and pulverized into fine powder in a mortar for 60 min. Finally, iron rust powder was calcined at 450° C in an air atmosphere for 2 hr. to obtain crystalline Fe2O3 NPs. The obtained rust sample with and without calcination designated as Rust and Fe2O3 respectively.


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2.3. Synthesis of g-C3N4 and g-C3N4-Fe2O3 composites Graphitic carbon nitride (g-C3N4) was synthesized according to the reported method.28 Typically, thiourea was placed in an alumina crucible with a cover and heated at 450 °C for 2 h in a muffle furnace. The obtained yellow color product was grounded to a fine powder and denoted as g-C3N4. Composite of g-C3N4-Fe2O3 was synthesized by grinding 4.0 g thiourea and 0.2 g Fe2O3 in a mortar for 15 min. Finally, the mixture was transferred in an alumina crucible with a cover and calcined at 450°C for 2 h in a muffle furnace. After completion of the calcination, the obtained product was washed with deionized water and dried in an oven at 80° C. This sample is designated as g-C3N4-Fe2O3. The schematic representation of the synthesis of gC3N4-Fe2O3 composite from waste iron rust is shown in Scheme 1.

Scheme 1: Schematic representation of the synthesis of magnetic g-C3N4-Fe2O3 from waste iron rust 2.4. Characterization techniques XRD of as-synthesized g-C3N4, Fe2O3 NPs, g-C3N4-Fe2O3 were recorded on a Panalytical Diffractometer with CuKα radiation (λ = 1.5406 Å) in the range of 2θ, 20 to 80°. The chemical composition, morphology and the crystalline structure of the samples were investigated using Field Emission Scanning Electron Microscope (FE-SEM) with Energy Dispersive X-Ray Spectroscopy (EDS) (MIRA3 LMH, TESCAN, USA) and High-Resolution Transmission Electron Microscopy (Tecnai G2, F30). X-ray Photoelectron Spectroscopy (XPS) was carried out using the XPS spectrometers (MultiLab. ESCA 2000). The Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Bruker spectrometer in the frequency range of 4000– 400 cm−1. To analyze the light absorption of the photocatalysts, UV−Vis. Diffuse Reflectance Spectra (DRS) were recorded using a scan UV−Vis. spectrophotometer (LABINDIA Analytical



UV-3092).

Photoluminescence

(PL) spectra

of the

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samples

were

recorded

on

a

spectrofluorometer (JASCO, Model FP.750, Japan). The magnetic study was carried out at room temperature using Vibrating Sample Magnetometer (Lakeshore VSM 7410) by varying magnetic field from −15000 to +15000 Oe 2.5. Evaluation of Photocatalytic Performance The photocatalytic performances of as-prepared samples were evaluated with MO and TE as a model target pollutants beneath direct natural sunlight irradiation. The photocatalytic experiments were executed on the bright sunny days from 11 a.m. to 2 p.m. (Kolhapur city, Maharashtra, India). Typically, for the photocatalytic experiments, a 100 mg photocatalyst was dispersed in 100 mL aqueous suspension of the dye solution (MO = 20 ppm). Prior to irradiation, the suspensions were magnetically stirred for 30 min in the dark, thereby allowing the system to obtain the equilibrium adsorption of dye molecules onto the surface of the photocatalyst. At certain time intervals of irradiation, 3 mL of irradiated dye solution was periodically withdrawn, and the photocatalyst was magnetically separated from the suspension using a permanent magnet. The equilibrium concentration of the dye in the reaction solution for each sample was monitored using UV-visible Spectrophotometer (Shimadzu, Model-UV-3600) by measuring the absorbance intensity of the respective dyes at 464 and 664 nm for MO and TE, respectively during the photocatalytic degradation process. To study the recyclability and sustainability of the photocatalyst, after each run, the photocatalyst was collected, washed with DI water, and redispersed into fresh dye solution for the next cycle. 2.6 Cytotoxicological study of dye on fish (Tilapia mossambica) gills. The experimental fishes Tilapia mossambica were harvested from the fish reservoir of the Department of Zoology, Shivaji University, Kolhapur. The fishes were acclimatized in the optimally aerated glass aquaria and fed ad libitum with a standard fish meal. After a span of 30 days, the fishes were transferred to experimental aquaria. The average length and weight of fishes were 7.1 ± 0.25 cm and 5 ± 0.3 g respectively. The fishes were divided into three groups. Group I was of control. In this group, the fishes were maintained in an aquarium containing potable water from the well. Group II was exposed to non-degraded dye solution; where fishes were maintained in an aquarium containing MO dye solution (20 ppm) and Group III was


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exposed to photodegraded dye solution; where fishes were maintained in an aquarium containing a photodegraded solution of MO (20 ppm). After completion of 96 hours of exposure, the fishes were sacrificed. Gills of each fish were fixed in neutral buffered formalin for 24 hours and washed under running tap water for the next 24 hours. This was followed by dehydration of gills through ascending grades of ethanol, clearing in Xylene and embedding in the paraffin wax. Sagittal sections of 6µ thick were cut on a rotary microtome and mounted on clean grease free microslides. The sections were dewaxed in Xylene hydrated in descending grades of ethanol and stained with Haematoxylin-Eosin and mounted in DPX. The sections were observed under Microscope (Lawrence and Mayo, UK) at 400X magnification. 3. Results and Discussion 3.1. XRD analysis. The chemical composition, crystallinity and phase purity of the red colored waste iron rust, Fe2O3 NPs, pure g-C3N4, and g-C3N4-Fe2O3 composite were identified using XRD analysis. The XRD pattern of rust and Fe2O3 are shown in Figure 1a. The XRD patterns clearly suggest that the waste iron rust contains a mixture of iron oxide hydroxides [FeOOH] and iron oxide [γFe2O3] structure corresponding to JCPDS file no. 75-1594 and 39-1346 respectively.29 On heating, iron rust loses water molecule and transformed into crystalline Fe2O3 NPs.30 The XRD patterns of Fe2O3 NPs display the main characteristic diffraction peaks of γ-Fe2O3 located at 30.18°, 57.28°, and 62.94°, which correspond to the (220), (511), and (440) planes, respectively. Along with additional peaks located at 24.26°, 33.17°, 40.82° and 49.57° match well with the (012), (104), (113) and (024) planes of α-Fe2O3 (JCPDS file no. 33-0664). Thus, we can conclude that the synthesized Fe2O3 NPs consist of both α and γ phases possess excellent magnetization values than the α-Fe2O3.31 This is attributed to the calcination of iron rust at 450° C is efficient for the phase transformation of thermodynamically unstable γ-Fe2O3 into thermodynamically stable α-Fe2O3 and iron oxide hydroxide [FeOOH] to iron oxide [Fe2O3] phase by losing water molecule.

32-33

The average crystallite size of the as-synthesized Fe2O3

NPs was about 17 nm, calculated from the Debye-Scherrer equation based on the measurements of the FWHM of the strongest (311) peaks. Figure 1b portrays the representative XRD patterns of the g-C3N4, Fe2O3 and g-C3N4-Fe2O3. Two distinct peaks are observed at 13.17° (100) and 27.4° (002) for the g-C3N4 sample, which corresponds to the in-plane structural packing of tri-s-



triazine units and the interlayer stacking of conjugated aromatic systems respectively.

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34

When

Fe2O3 NPs is coupled with g-C3N4, the crystal phase of Fe2O3 did not change, and the diffraction peak at 27.4° (002) of g-C3N4 was also present in the XRD pattern of in g-C3N4-Fe2O3 composite. In addition, it should be noted that the relative intensity of g-C3N4-Fe2O3 composite diffraction peak of 27.4° (002) become weaker due to the introduction of Fe2O3 nanoparticles. However, the diffraction peak at 13.17° (100) for g-C3N4 disappeared due to the attached Fe2O3 NPs may prevent van der Waals and π–π stacking interactions between graphitic sheets of C3N4 during the reaction.

35

A similar result was also found during the preparation of Fe2O3/g-C3N4

photocatalysts, which revealed that the diffraction intensity of the peak at 27.4° became weaker with Fe2O3 content, indicating that the introduction of Fe2O3 hampers the growth of the crystal.36 We believe that the presence of Fe2O3 NPs may also lead to restriction of crystallization of gC3N4. Furthermore, no other peaks are observed in the XRD pattern, indicating the g-C3N4-Fe2O3 composite to be a two-phase hybrid.

Figure 1: (a) XRD patterns of Rust and Fe2O3 (b) XRD patterns of g-C3N4, Fe2O3, and g-C3N4Fe2O3. 3.2. SEM-EDS analysis. The composition and structure of the as-obtained iron rust were further confirmed by Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-Ray Spectroscopy (EDS) (Supporting Information Figure S1). From EDS (Figure S1a) it is demonstrated that the separated rust composed of C, N, O, Al, Si, S, Cr, and Fe. The major elemental contributions in the rust are Fe and O. However, we believe that the other additional elements are presents with


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smaller percentages, which are attributed to the doping or coating of anticorrosive films (Al, Si, and Cr) over the surface of iron during the manufacturing of iron components.37-38 While the Pt peaks arise from the coating of the sample with conducting Pt during SEM analysis and less intense C peak are originated from the carbon-coated copper grid. Figure S1b shows the SEM images of the as-obtained iron rust. It is evident that as-obtained iron rust particles are found in a cluster and made up of a large number of interconnected irregularly shaped particles. We also performed XPS analysis of the as-prepared g-C3N4-Fe2O3 composite. However, no other additional peaks of Al, Si, S, and Cr are present in the survey spectrum of the g-C3N4-Fe2O3 composite, which can be ascribed to that XPS analyses only give the surface layer composition and XPS probe only 1-10 nm depth close to the surface. EDS analysis is more appropriate for the average composition of the sample because of its deeper (microns) investigation. This spectrum is well complemented with the XRD, which shows no additional diffraction peaks assigned to oxides of Al, Si, or Cr in XRD pattern. The SEM image (Figure S2a) of the g-C3N4-Fe2O3 composite reveals that the g-C3N4 has a typical two-dimensional aggregated morphology. In addition, the elemental mapping (Figure S2b-e) indicated the uniform distribution of C, N and O atoms across the entire structure. Interestingly, it also demonstrates the Fe atoms uniformly distributed all over the g-C3N4, which indicates that the g-C3N4-Fe2O3 composite was fabricated successfully. 3.3. X-ray Photoelectron Spectroscopy. XPS spectrum analysis was carried out to determine the valence states and chemical environment of the constituent elements on the surface of the g-C3N4-Fe2O3 composite. Figure 2a represents the full survey spectrum of C, N, O and Fe elements were detected in the g-C3N4Fe2O3 composites. Peaks allocated to S species cannot be observed, further indicating that the sulfur from thiourea was totally liberated during heat treatment.39 The C1s spectra in Figure 2b clearly represent two peaks at the binding energies of 284.7 and 288.3 eV, respectively. The former peak could be associated with graphitic C=C bonds, and the latter peak is assigned to the tertiary carbon C−(N)3 in the g-C3N4 lattice. Figure 3c represents N1s peak at 398.7, 400.4, and 401.1 eV can be clearly resolved, which are ascribed to sp2 hybridized nitrogen bonded to carbon (C−N=C) in triazine-ring, tertiary nitrogen bonded to



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carbon atoms in the form of (N–(C)3) groups and amino (C–N–H) groups originating from the imperfect polymerization respectively.

40

Figure 2d represents the asymmetric peak of O1s can

be divided into two peaks located at 530.1 eV and 533.2 eV, which can be attributed to the oxygen in the crystal lattice of Fe2O3 (Fe−O bond), and chemisorbed water on the catalyst respectively. Figure 2e shows a Fe 2p 3/2 peak at 710.6 eV, a 2p 1/2 peak at 724.6 eV, and a 2p 3/2 satellite peak at 719 eV, corresponding to binding energies of Fe3+.41 In addition, the satellite peak of Fe2+ located at 716 eV was not found, thus indicating the absence of Fe2+ species.

9

Considering together the XPS and XRD results, one may conclude that the successful formation of a composite consisting principally of the Fe2O3 phase structure from iron rust.

Figure 2: XPS survey spectrum of g-C3N4-Fe2O3 and the high-resolution binding energy spectra of, b) C 1s, c) N 1s d) O 1s and e) Fe 2p.


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3.4. FT-IR analysis. FT-IR spectra were performed to explore the structural information of the Rust, Fe2O3, pure g-C3N4, and g-C3N4-Fe2O3 (Figure S3). For rust, the absorption peak at 3151 cm−1 can be assigned to the stretching vibration of –OH. The peak at 1637 cm−1 corresponds to the bending vibration of the adsorbed water molecules. The peaks at 891 and 797 cm−1 are attributed to the Fe–O–H bending vibrations in FeOOH. Two bands at 609 and 476 cm−1 are assigned to the Fe– O stretching and bending vibrations in FeOOH.42 For Fe2O3, the two peaks at 470 and 545 cm−1 are assigned to the stretching and bending vibration of the Fe–O bond.43 However, broad peak located at 2900-3600 cm−1 corresponds to the–OH group associated with adsorbed water molecules. For g-C3N4, the peak at 808 cm−1 was related to the out-of-plane bending vibration of the s-triazine ring. The strong bands in the region of 1200–1600 cm−1are assigned to the typical stretching vibration of the C–N heterocycles. The peaks at 1241, 1318, and 1425 cm−1 are attributed to the aromatic C−N stretching. The broad peaks located at 2900–3600 cm−1 are assigned to the uncondensed amino groups N–H and O–H adsorbed H2O molecules.44-45 Notably, the characteristic peaks of both g-C3N4 and Fe2O3 appeared and absorption peak intensity of g-C3N4 especially at 808 cm−1 is decreased slightly in g-C3N4-Fe2O3.23 The FT-IR results clearly demonstrate the complete formation of g-C3N4-Fe2O3 composite. This would enhance the photocatalytic activity. 3.5. TEM analysis. The detailed morphologies of and microstructures of as-obtained iron rust, Fe2O3 NPs, and g-C3N4-Fe2O3 composite have been further investigated by TEM and HRTEM. Figure 3a-b shows the low magnification TEM images of as-obtained iron rust, it can be seen that the cluster of iron rust is made up of a large number of interconnected irregularly shaped NPs, agree well with the SEM analysis. However, strong mechanical grinding and calcination of iron rust would break up the clusters to yield Fe2O3 NPs with sizes ranging from 20 to 100 nm (Figure 3c-d). Furthermore, the interplanar distances of 0.252 and 0.370 nm measured out in the HRTEM image (Figure 3e) can be indexed to the lattice fringes of the γ-Fe2O3 (311) and α-Fe2O3 (110) planes, respectively, which is prominently consistent with the XRD results. Therefore, it is clearly observed that the heat treatment of iron rusts at high temperature can be transformed thermodynamically unstable γ- Fe2O3 into stable α-Fe2O3 and the resulting Fe2O3 NPs consists of



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both α and γ phase. Figure 3f shows the SAED pattern of the Fe2O3 NPs with bright spots in the diffraction circle confirms the crystalline nature of the Fe2O3 particles.

Figure 3: (a–b) TEM images of as-obtained iron rust, (c-d) TEM, (e) HR-TEM image and (f) SAED pattern of Fe2O3. To understand the feasibility of breaking of bulky iron rust into nanosized Fe2O3, we looked into the crucial role of mechanical grinding and calcination. It has been considered that the corrosion of iron is basically an electrochemical and slow process undergoes the following steps. 2 Fe + 2 H2O + O2

2 Fe(OH)2

(1)

4 Fe(OH)2 + O2 + H2O

4 Fe(OH)3

(2)

2 Fe(OH)3

Fe2O3.n H2O + (3-n) H2O

(3)

Initially, the corrosion of iron takes place in the presence of water and oxygen molecules gives rust in the form of FeOOH and Fe2O3 particles.46 Because of the slow process of corrosion, the initial size of rust particles is very small maybe in the range of nanometer. However, the


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continuous process of corrosion eventually leads to the aggregation of rust NPs into the cluster were observed in TEM (Figure 3a-b). Subsequently, these clusters convert into a passive oxide layer and slow down the further process of corrosion. Considering the slow process of corrosion and TEM results, we can conclude that the corrosion may not only form rust NPs but also clusters. These clusters consist of an aggregation of rust NPs which are loosely held together via the weak bonding.47-48 Fortunately, this weak nature of bonding clearly suggests that clusters can break easily by applying enough mechanical energy. For this reason, the simple mechanical grinding provides an efficient energy to break the aggregation. In the present study, we have adopted a simple and environmental friendly top-down approach involving mechanical grinding which can be easily transform clusters of rust into rust NPs without using any sophisticated techniques such as high-speed ball-milling. Further calcination of iron rust NPs at 450°C is efficient for the conversion of FeOOH to Fe2O3 NPs which is well concurrence with the TEM images of Rust and Fe2O3 NPs. The TEM images of g-C3N4-Fe2O3 composite (Figure 4a-c) reveals that g-C3N4 has a two dimensional aggregated morphology and irregular structure with a larger particle size while Fe2O3 NPs have well dispersed over the surface of g-C3N4, which confirms the formation of heterojunction in the g-C3N4-Fe2O3 composite. Although having strong mechanical sonication before the TEM analysis, g-C3N4-Fe2O3 composite displayed strong interfacial connections between g-C3N4 and Fe2O3 NPs rather than a physical mixture of two distinct phases of g-C3N4 and Fe2O3. In addition, we observed similar interplanar distances in the HRTEM images (Figure 4d-e) can be indexed to the lattice fringes of the γ-Fe2O3 (311) and α-Fe2O3 (110) planes, respectively. The SAED pattern (Figure 4f) demonstrates that the g-C3N4-Fe2O3 possess good crystallinity. Moreover, this strong interfacial connection and the presence of heterojunction favor electron migration paths and promote the separation of photogenerated electron-hole pairs, leading to an enhanced photocatalytic performance.49



Figure 4: (a-c) TEM images, (d-e) HR-TEM images and (f) SAED pattern of g-C3N4-Fe2O3 3.6 UV–Visible Diffuse Reflectance Spectra The optical absorption range plays an important role in the photodegradation of dye pollutants, as visible light constitutes a large fraction of the solar energy. The optical properties of as-prepared pure g-C3N4, Fe2O3 NPs, and g-C3N4-Fe2O3 composite were investigated by UVVis. diffuse reflectance spectroscopy and are shown in Figure 5a. Pure g-C3N4 sample shows the characteristic spectrum with its fundamental absorption edge at 460 nm that corresponding to band gap energy of 2.7 eV, which is consistent with the previous result.14 While the Fe2O3 NPs exhibit significant absorption in the visible region. It is noteworthy that the g-C3N4-Fe2O3 composite sample show more intense absorption in the visible region than the pure g-C3N4 and the absorption edge shifts towards redshift. This obvious difference is due to the introduction of Fe2O3 NPs onto g-C3N4 with the color change of the sample from yellow to light grey. This analogous phenomenon can be attributed to the visible light response of the photocatalysts, interfacial interaction and feasible charge-transfer transition between the g-C3N4 and Fe2O3


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conduction or valence band that help to enhance photocatalytic activity under sunlight irradiation.

Figure 5: (a) UV–Vis. diffuse reflectance spectra and (b) Band gap energy of g-C3N4, Fe2O3 and g-C3N4-Fe2O3. The optical band gap energy of all samples were determined by the Tauc plot, (αhυ)2 versus hυ from the UV-Vis. spectrum that can describe using the following equations.50 αhυ A hυ 4 where α is the absorption coefficient, h is Planck's constant, υ is light frequency, A is proportionality constant, Eg is band gap energy and n=0.5 for the allowed direct transition. The estimated band gap values of the samples are about 2.7, 1.81, and 2.31 eV, corresponding to gC3N4, Fe2O3 NPs, and g-C3N4-Fe2O3 composite respectively (Figure 5b). 3.7. Photoluminescence Spectra Photoluminescence (PL) spectral analysis is used to measures the extent of migration, transfer and recombination rate of photoinduced electron-hole pairs in the composite material. The PL spectra of photocatalysts with an excitation wavelength of 260 nm for Fe2O3, 365 nm for pure g-C3N4 and g-C3N4-Fe2O3 composite at room temperature are shown in Figure S4. It can be seen that the appearance of strong emission band at 498 and 460 nm for the Fe2O3 and pure gC3N4 material which indicates fast recombination rate of the photoinduced electron-hole pairs due to narrow band gap. When the Fe2O3 was combined with g-C3N4, g-C3N4-Fe2O3 composite exhibits a lower PL intensity due to the formation of efficient heterojunction where interfacial



charge transfer from the CB of g-C3N4 to the CB of Fe2O3 occurs.51 Consequently, it significantly suppresses the photoinduced electron-hole pair recombination rate and increases the charge separation efficiency in the g-C3N4-Fe2O3 composite. Therefore, the g-C3N4-Fe2O3 composite is the best composition for utilization of sunlight and displays the outstanding photocatalytic activity towards degradation of dye molecules. 3.8. Magnetic behavior of Fe2O3 and g-C3N4-Fe2O3. Magnetic properties of Fe2O3 NPs along with g-C3N4-Fe2O3 composite were studied at room temperature using a Vibrating Sample Magnetometer (VSM) with an applied field of 15000 Oe and the results are shown in Figure 6. The saturation magnetization (Ms) value of Fe2O3 NPs was 0.0446 emug−1 with 8.889× 10−3 emu remnant magnetization (Mr) and 198.13 G coercive force (Hc) revealing the ferromagnetic character. In addition, the saturation magnetization value observed for the g-C3N4-Fe2O3 composite was 0.038 emug−1, which is lower than that of the magnetic Fe2O3 NPs. This can be attributed to the presence of non-magnetic nature of g-C3N4.52 Although the saturation magnetization value of the g-C3N4-Fe2O3 composite is low, it is quite enough to separate the composite from the solution by simple bar magnet (inset of Figure 6) and recorded in Video S1. This result clearly states that this kind of rust-derived Fe2O3 NPs may be an excellent cost-free source for designing Fe-based magnetic composite.

Figure 6: Magnetic hysteresis loop of Fe2O3 and g-C3N4-Fe2O3. Inset of the figure is a separation of the photocatalyst from the treated solution using an external magnetic field.


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3.9.1 Photocatalytic Performance The reaction kinetics and photocatalytic activity of pure g-C3N4, Fe2O3, and g-C3N4Fe2O3 were separately explored for the degradation of a model pollutant, MO, and TE under natural sunlight irradiation. Total concentrations of dye were simply determined from the maximum absorbance (λmax. = 464 nm for MO, and λmax. = 664 nm for TE) measurements using UV-Visible spectra. The initial concentration (C0), the final concentration (C), and the degradation (D %) has a mathematical relation as follows %

0 – × 100 5 0

The photocatalytic degradation process was fitted to a pseudo-first-order, and the value of the rate constant (k) is equal to the corresponding slope of the fitting line;

0 6

Where k represents the rate constant (min−1). The adsorption ability and photocatalytic activity of the g-C3N4, Fe2O3 NPs and g-C3N4Fe2O3 composite photocatalysts for degradation of MO within 120 min under sunlight irradiation are shown in Figure 7a. The Figure 7c shows, in absence of a catalyst, the photodegradation of MO was negligible under sunlight irradiation, signifying the high stability of MO under sunlight irradiation. It can be seen that the pure g-C3N4 and g-C3N4-Fe2O3 composite possess superior adsorption ability in comparison to bare Fe2O3. The enhanced dye adsorption ability for pure gC3N4 and the g-C3N4-Fe2O3 composite is due to the fact that g-C3N4 possesses large surface area that exhibits more adsorption sites for electrostatic attraction between the aromatic dye molecules with the aromatic region of the g-C3N4 sheets.35 The rate constant (k) was found to be 2.34× 10−4 min−1, 3.39 × 10−3 min−1 and 3.35 × 10−2 min−1 for Fe2O3, g-C3N4, and g-C3N4-Fe2O3 respectively.



Figure 7: (a and b) UV-visible spectral changes and (c and d) Kinetic linear simulation curves for photodegradation of MO and TE solution under sunlight irradiation with g-C3N4-Fe2O3 composite During the photocatalytic experiments, no significant degradation of dye was observed with Fe2O3 though Fe2O3 has high absorption in the visible spectrum. The poor photocatalytic performance of the bare Fe2O3 ascribed to the fast recombination of electron-hole pairs.53 However, g-C3N4 exhibited 44% photocatalytic activity, compared to only 7% with Fe2O3 during the same time. This is due to the unique electronic structure, moderate band gap, and involvement of photogenerated electron-hole pairs in the degradation process. Remarkably, a maximum photocatalytic activity of a g-C3N4-Fe2O3 composite was found to be 99 % for photodegradation of MO within only 120 min under same experimental conditions. These results illustrates that the incorporation of Fe2O3 could play the key role in improving the photocatalytic


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activity of g-C3N4 photocatalyst, which could be attributed to the vital synergistic effect between the interface of g-C3N4 and Fe2O3 and the quality of the effective heterojunction formed in the gC3N4-Fe2O3.51 During the photodegradation reaction, efficient transfer of photogenerated electrons from g-C3N4 to the Fe2O3 and reversal holes transfer from Fe2O3 to g-C3N4, which impede the recombination of electron-hole pairs, and as a result, a better photocatalytic performance is achieved. For a better understanding of the versatility of the g-C3N4-Fe2O3 photocatalyst, the photodegradation of the TE under sunlight was also performed under identical conditions (Figure 7b). The rate constants (k) were found to be 3.8× 10−3, 7.77× 10−4 and 3.9× 10−2 min−1 for pure gC3N4, Fe2O3 and g-C3N4-Fe2O3 respectively within 90 min under sunlight irradiation as shown in Figure 7d. The Figure 7d demonstrates that g-C3N4-Fe2O3 achieved 98% photocatalytic activity, which is much higher and more prominent than with Fe2O3 (13%) or pure g-C3N4 (43%). These results suggest the versatility and effectiveness of the g-C3N4-Fe2O3 photocatalyst in the sunlightinduced photodegradation of organic dye pollutants. For the practical application, the recyclability and stability of the catalyst are crucial for photocatalytic reactions. Consequently, the photocatalytic stability of the C3N4-Fe2O3 was evaluated by recycling experiments were carried out for the photodegradation of MO and TE under same photocatalytic condition. After each cycle, the catalyst was separated from the reaction medium with a magnet, washed thoroughly with DI water and fresh dye solution was added for a further cycle. After five times recycling experiments, no obvious decrease of photocatalytic activity of the g-C3N4-Fe2O3 photocatalyst can be seen in Figure S5. It can be seen that the MO and TE degradation activity decline by only 8 and 5% after five consecutive cycles. All these results clearly suggest that as-prepared g-C3N4-Fe2O3 photocatalyst possess excellent stability during photochemical reactions. 3.9.2 Plausible mechanism for the photocatalytic degradation of MO and TE dyes over gC3N4-Fe2O3. According to the above results, a possible photocatalytic mechanism of the as-prepared pure g-C3N4, Fe2O3 NPs and g-C3N4-Fe2O3 composite under sunlight irradiation was proposed. Scheme 2 illustrates the mechanism of electron-hole separation and transportation at the g-C3N4-



Fe2O3 photocatalyst interface for Methyl Orange (MO) and Textile Effluents (TE) degradation under sunlight irradiation. According to the photocatalytic dye degradation process, the photocatalytic performance of the g-C3N4-Fe2O3 composite may be related to the appropriate heterojunction (“staggered” type II junction) formed between the g-C3N4 and Fe2O3 with different energy levels. As well known, Fe2O3 is a typical n-type semiconductor, while g-C3N4 is also n-type material. Based on the UV-Vis. analysis, the conduction band (ECB) and the valence band (EVB) of the g-C3N4 and Fe2O3 at the point of zero charge can be determined by the following equations. 0.5 7 " + E% 8 where X is the semiconductor’s absolute electronegativity, and the values of X for g-C3N4 and Fe2O3 are 4.73 and 5.88 eV, respectively; Ee is the energy of free electrons on the hydrogen scale ( 4.5 eV); EVB is the valence band (VB) potential; ECB is the conduction band (CB) potential and Eg is the band gap of the semiconductor.54-55 The estimated band gap values of g-C3N4 and Fe2O3 are about 2.7 and 1.81 eV, respectively (Figure 5b). Therefore, the calculated CB and VB positions are −1.12 and +1.58 eV whereas they are +0.47 and +2.28 eV for Fe2O3, respectively. Under sunlight irradiation, both g-C3N4 and Fe2O3 are excited to produce electrons (e−) in the conduction band and holes (h+) in the valence band due to the narrow bandgap. Then, owing to the heterojunction established between g-C3N4 and Fe2O3, and the fact that the ECB of Fe2O3 (+0.47 eV) is lower than that of g-C3N4 (−1.12 eV), the photogenerated electrons in CB of gC3N4 had a tendency to diffuse to the CB of Fe2O3 via the interface. Whereas the photogenerated holes in the VB of Fe2O3 could migrate to g-C3N4.23-24 This process could effectively improve the separation efficiency of the photogenerated electron-hole pair and the lifetime of the excited electron-hole pair. Subsequently, photogenerated and separated electrons in the CB of g-C3N4 and Fe2O3 further react with surface chemisorbed oxygen molecules to form strong oxidative species •O2−, which plays an important role in mineralizing and degrading the adsorbed molecules of dyes. However, the photogenerated holes in the VB of g-C3N4 and the holes transferred from Fe2O3 to the VB of g-C3N4 could not oxidize H2O to generate oxidative species •OH due to the redox potential of •OH/H2O (+2.68 eV) higher than the EVB value of g-C3N4


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(+1.58 eV).56 Hence, the photogenerated holes would directly oxidize the adsorbed dye molecules of the target pollutant directly.

Scheme 2: A plausible mechanism for the photodegradation of MO and TE over the g-C3N4Fe2O3 composite under sunlight irradiation. To disclose the roles of the active species, responsible for photodegradation of MO and TE over the g-C3N4-Fe2O3 composite, the effect of scavenger on the photodegradation rate was studied and the results are shown in Figure S6. Here iso-propanol (IPA), 1, 4-Benzoquinone (BQ) and Ethylene Diamine Tetra Acetic Acid (EDTA) were used as the scavengers of the hydroxyl radical (•OH), superoxide radical (•O2−) and hole (h+), respectively. 57-58 It is clear that moderate decrease of the photodegradation (MO 85 % and TE 82%) in presence of IPA. However, a substantial decrease in the photodegradation (MO 28 % and TE 30%) were observed upon the addition of BQ. Meanwhile, a similar change of the photodegradation rate (MO 45% and TE 50%) was observed in the presence of EDTA. Hence photogenerated hole (h+) and superoxide radical (•O2−) radicals are the main active species are responsible for photodegrdadation.



3.9.3 Cytotoxicological Study of Dye on Fish Gills of Tilapia mossambica Gills are the respiratory organ in fish and are covered by a thin layer of epithelial cells through which exchange of gases, regulation of ionic and acid-base balance and release of nitrogenous waste take place.59 The gills of fish have a large surface that makes direct contact with the aquatic environment and particularly sensitive to chemical and physical changes. Therefore, histological alterations in the fish gills are used as a tool in the study of environmental toxins.60 The histological observations of control Tilapia mossambica gills (Figure 8A) showed the normal appearance of gill primary lamella (PL), secondary lamella (SL), surface epithelial cells (SE), mucus-secreting goblet cells (G), undifferentiated basal cells (UB) with wellmaintained cell morphology. In the present study, there was damage to the surface epithelium of the gills of fishes exposed to MO for 96 hours shown in Figure 8B. Oedema to primary lamellae indicates inflammation.61 The dye also caused distortion of the undifferentiated cell mass (DUB) present adjacent to secondary lamellae. The secondary lamellae showed atrophy (DS) and necrotic cells (N) after exposure to MO. Rao et al. have demonstrated similar degeneration of secondary lamellae in fishes exposed to monocrotophos.62 The increase in cell size of goblet cells (HG) could be a compensatory and protective strategy adapted after exposure to MO. The mucus keeps the surface sticky and covered so that the surface epithelium gets protection from the direct effect of dye molecules.63 The gills of fishes (Figure 8C) maintained at photodegraded dye solution exhibited histological architecture like that of a control group without any oedema to primary gill lamellae, no shrinkage of secondary lamellae, no hypertrophy of mucus-secreting goblet cells and without any necrotic cells. There were no pathological alterations in the gills of the fishes exposed to photodegraded dye solution, indicating that the solution does not carry any toxic effect.


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Figure 8: Histological images of Gill of Tilapia mossambica fish: (A) Control, (B) Exposed to MO 20ppm for 96 hours, (C) Exposed to Photodegraded dye 20 ppm for 96 hours. Primary Lamella (PL), Secondary Lamella (SL), Undifferentiated Basal Cells (UB), Surface Epithelial Cells (SE), Mucus Secreting Goblet Cells (G).Oedema to Primary Gill Lamella (EPL), Distorted Secondary Lamellae(DS), Hypertrophy of Goblet Cells (HG), Necrotic Cells (N) Distorted Undifferentiated Basal Cells (DUB). 4. Conclusions We have successfully demonstrated an innovative strategy to transform waste iron rust into Fe2O3 NPs for the synthesis of magnetically separable g-C3N4-Fe2O3 composite photocatalyst by the simple grinding and calcination method. The method reported here is a facile, sustainable and inexpensive (i.e., without using any expensive chemical treatments or process) to any other Fe- based waste source. This study demonstrated for the first time the potential use of rust-derived Fe2O3 NPs in a g-C3N4-Fe2O3 composite for the photodegradation of MO and TE as a model pollutant under natural sunlight. The as-prepared g-C3N4-Fe2O3 composite exhibits remarkable photodegradation efficiency (MO= 99% and TE= 98%) than that of pure g-C3N4 (MO= 44 % and TE= 43%) and Fe2O3 (MO= 7% and TE= 13%). In addition, gC3N4-Fe2O3 photocatalyst was magnetic enough to be easily separated and reused without loss of photodegradation efficiency up to five successive runs. Such an enhanced photodegradation efficiency could be ascribed to the high visible-light-absorption efficiency, synergistic effect and well-established heterojunction between the g-C3N4 and Fe2O3, which was favorable for efficient



separation of photoinduced electron-hole pairs. We believe that our work not only provides a new avenue for waste management but also develop an affordable and sustainable platform to produce magnetically separable photocatalyst from any Fe-based waste source for environmental remediation. ■ ASSOCIATED CONTENT Supporting Information The SEM and corresponding elemental mappings images of C3N4-Fe2O3, FT-IR and PL spectra of Rust, Fe2O3, g-C3N4, and g-C3N4-Fe2O3, Reusability of the g-C3N4-Fe2O3 for MO and TE degradation under sunlight irradiation, and Effects of different scavengers on the photodegradation of MO and TE in the presence of the g-C3N4-Fe2O3 under sunlight irradiation. The highly efficient separation of the composite from the solution by simple bar magnet (AVI). The video is attached as a Supporting Information in AVI format

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K. M. Garadkar), *E-mail: [email protected] (V. M. Bhuse) ORCID K. M. Garadkar: 0000-0002-5733-9895 Notes The authors declare no competing financial interest.

5. ACKNOWLEDGMENT


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We are thankful to SAIF-IIT Bombay and SAIF-IIT Madras for providing the TEM and VSM facilities. We are also thankful for providing SEM-EDS and XPS facilities through a grant of Mid-Career Researcher Program (NRF-20161A2B4016552) and National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP).

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Evolution of Waste Iron Rust into Magnetically Separable g-C3N4

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