g-C3N4 nanocomposite: An efficient magnetic nanoadsorbent for adsorptive removal of organic pollutants

Journal Pre-proof Fe3O4 nanoparticles functionalized GO/g-C3N4 nanocomposite: An efficient magnetic nanoadsorbent for adsorptive removal of organic pollutants Shraban Ku Sahoo, Sandip Padhiari, S.K. Biswal, B.B. Panda, G. Hota PII:

S0254-0584(20)30090-0

DOI:

https://doi.org/10.1016/j.matchemphys.2020.122710

Reference:

MAC 122710

To appear in:

Materials Chemistry and Physics

Received Date: 16 November 2019 Revised Date:

16 January 2020

Accepted Date: 21 January 2020

Please cite this article as: S.K. Sahoo, S. Padhiari, S.K. Biswal, B.B. Panda, G. Hota, Fe3O4 nanoparticles functionalized GO/g-C3N4 nanocomposite: An efficient magnetic nanoadsorbent for adsorptive removal of organic pollutants, Materials Chemistry and Physics (2020), doi: https:// doi.org/10.1016/j.matchemphys.2020.122710. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Graphical abstract Fe3O4 nanoparticles functionalized GO/g-C3N4 nanocomposite: An efficient magnetic nanoadsorbent for adsorptive removal of organic pollutants Shraban Ku. Sahoo1,2,Sandip Padhiari1, S. K. Biswal2, B. B. Panda3and G. Hota1* 1 2

Department of Chemistry, NIT Rourkela, Odisha, India, 769008

School of Applied Science, Centurion University of Technology and Management, Odisha 3

Department of Chemistry, I.G.I.T. Sarang, Odisha

* Corresponding author Dr.GarudadhwajHota Dept. of Chemistry, NIT Rourkela, Orissa, India, 769008 Email: [email protected],[email protected] Ph: 91-661-2462655, Fax: 91-661-246265

Fe3O4 nanoparticles functionalized GO/g-C3N4 2D/2D nanocomposite: An efficient magnetic nanoadsorbent for adsorptive removal of organic pollutants Shraban Ku. Sahoo1,2, Sandip Padhiari1, S. K. Biswal2, B. B. Panda3and G. Hota1* 1 2

Department of Chemistry, NIT Rourkela, Odisha, India, 769008

School of Applied Sciences, Centurion University of Technology and Management, Odisha, India 3

Department of Chemistry, I.G.I.T. Sarang, Odisha, India

Abstract: Contamination of toxic organic pollutants is a worldwide problem and needs to develop an ecofriendly and highly effective adsorbent material for its removal. Recently, 2D graphene oxide (GO) and graphitic carbon nitride (g-C3N4) is mostly used as an adsorbent for the efficient removal of organic pollutants. Here we have prepared a novel 2D/2D GO/g-C3N4 sheets decorated with Fe3O4 nanoparticles (GO/g-C3N4-Fe3O4) using the hydrothermal method. The structural properties, formation, morphology, and bonding were analyzed by different analytical techniques. Then the obtained GO/g-C3N4-Fe3O4 nanocomposite was used as an adsorbent to eliminate both toxic tetracycline (TC) antibiotic and methylene blue (MB) dye. The adsorption of TC and MB were pH-dependent and maximum adsorption capacity (120 mg/g) was achieved at pH = 3 for TC and (220 mg/g) at pH = 9 for MB. The high adsorption efficiency of GO/g-C3N4Fe3O4 for TC and MB was mainly due to π-π and hydrogen bonding interaction. The introduction of Fe3O4 nanoparticles onto 2D/2D GO/g-C3N4 not only increases the adsorption capacity but also can make it easily separable from treated water. Adsorption data obtained were best fitted to the Langmuir and pseudo-second-order kinetics model. The adsorbed TC and MB on GO/gC3N4-Fe3O4 surface were recovered and can be reused up to 5 cycles. ______________________________________________________________________________ Key Words: graphene oxide, graphitic carbon nitride, tetracycline, methylene blue, adsorption *Corresponding author Email: [email protected]; [email protected] Ph: 91-661-2462655. Fax: 91-661-2462022

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1. Introduction Nowadays, environmental pollution and energy crisis are two major issues across the globe due to the rapid growth of population and industries[1–3]. Among various environmental pollutions, water pollution is more significant. Water contaminated with organic pollutants (including dyes, antibiotics, pesticides, phenol type compounds) and inorganic pollutants (including heavy metals and fluoride) has become serious environmental problems in both rural and urban areas[4–8]. Surface water is being polluted due to the discharge of various industrial wastes mainly paint, paper, food, pharmaceuticals, textiles, and leather [9]. Organic aromatic dyes can easily be transported within the aqueous solution due to their high solubility properties and may cause many serious health problems for living organisms [4,10,11]. Methylene blue (MB) is a widely used organic dye in various industries. Its presence in water causes several health problems including eye burns, gastrointestinal and skin diseases. It also increases chemical oxygen demand that’s harmful to aquatic organisms [12,13]. Except for organic dyes pharmaceutical antibiotics are also extensively used worldwide for human therapy, farming, and fungistat in medical treatment. However, most of the antibiotics are poorly metabolized and adsorbed by the environmental living organism. As a result, this creates a serious environmental problem including acute and chronic toxicity towards human health and dissemination of antibioticresistant among microorganisms [10,14,15]. Tetracycline (TC) is one of the most commonly used antibiotics and also ranked second in manufacture and usage worldwide [16,17]. Therefore, the removal of organic pollutants like MB and TC from aqueous medium is necessary for safety and healthy environment. Among various water remediation techniques, adsorption technique is found to be more convenient, effective and widely used to remove various pollutants from water [10,17]. A variety of adsorbents including clay, activated carbon, zeolite, different types of nanomaterials (carbon nanotube and metal oxide) are used for the removal of MB and TC [17– 21]. However, there is still challenging task for the researchers to develop a better adsorbent considering the high adsorption capacities, more chemical stability and easy separation. Recently, graphene oxide (GO) and its derivatives show much more attraction toward the removal of organic pollutants [4]. GO has a unique 2D structure having high surface area, more chemical stability, high functionality (containing epoxy, hydroxyl and carboxyl groups) and environmental friendly [22]. These properties of GO make it a potential candidate for adsorptive 2

removal of organic pollutants [15,23,24]. Very recently, surface modification strategies have been applied to further improve the adsorption efficiency of GO. Although different metal oxide nanomaterials have exhibited good adsorption behaviors[25], magnetic metal oxide nanocomposite can have additional advantages of its separation from treated water by using an external magnet [26–28]. Magnetic metal oxide decorated with high surface area GO substrate can solve all individual drawbacks of GO as well as a metal oxide in the adsorption field. So recently, GO-based magnetic nanocomposites are found to be promising adsorbent toward the removal of MB and TC. For example, reduced graphene oxide (rGO)-TiO3-Fe3O4 [29], magnetic GO modified zeolite [30],magnetic GO/polypyrrole [31],GO-MnFe2O4 [32] and rGO-Fe2O3 [33] were used for MB and TC adsorption. These composites not only increase the adsorption capacity but also increase the chemical stability during the adsorption process along with easy separation from treated water. Very recently, graphitic carbon nitride (g-C3N4) a 2D carbon material is also being used for the effective removal of organic as well as inorganic pollutants. Like GO, it also contains surface functional groups (-NH2, =N-, and -NH-) and has a π-conjugated planar layer structure. These chemical properties can offer abundant interaction sites for different types of pollutants [34–36]. For example, Cai et al.,[34], Shen et al.,[35], Zhou et al.,[36], Gan et al.,[37] and Zhu et al.,[38] have used g-C3N4 as an adsorbent to remove different pollutants. Considering the fact of multilayered heterostructure adsorbent materials towards high removal efficiency of organic pollutants, here we intend to prepare 2D/2D GO/g-C3N4 layered materials for adsorption applications. In this work, magnetic Fe3O4 nanoparticles were decorated on the 2D/2D GO/g-C3N4 surface (GO/g-C3N4-Fe3O4) using the hydrothermal method. Then the prepared GO/g-C3N4-Fe3O4 nanocomposite was characterized by FTIR, XRD, Raman, XPS, FE-SEM, HR-TEM, N2 adsorption/desorption isotherm, zeta potential analysis, and VSM analytical techniques. The obtained GO/g-C3N4-Fe3O4 nanocomposite was used as the adsorbent for the removal of highly toxic aromatic organic pollutants (MB and TC) from water. The batch adsorption experiments were performed to calculate the maximum adsorption capacity of MB and TC on GO/g-C3N4Fe3O4. The possible mechanism of MB and TC adsorption on the GO/g-C3N4-Fe3O4 surface has been proposed.

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2. Materials and method 2.1. Materials All the chemicals and reagents used for the experiment were of analytical grade and used as such purchased. The distilled water (DW) was used in all adsorption experiments. Urea (NH2CONH2), ferric and ferrous chloride anhydrous (FeCl3 and FeCl2), sodium acetate (NaAc), polyethylene glycol (PEG) were purchased from Himedia Pvt. Limited, India. Methylene blue (MB), tetracycline hydrochloride (TC), sodium hydroxide pellet (NaOH), hydrochloric acid (HCl), diethylene glycol (DEG) were purchased from Merck, India.

2.2. Preparation of 2D/2D GO/g-C3N4 layer decorated with Fe3O4 nanoparticle (GO/g-C3N4-Fe3O4) GO was synthesized using modified Hummer’s method and the detailed procedure has been described in our previously published article [39]. Then g-C3N4 was prepared by thermal polymerization technique using urea as a precursor. In this procedure 10g of urea was put in an aluminium crucible and sintered at 500 °C for 3h in a muffle furnace with a heating rate of 5 °C/min. The yellow g-C3N4 was collected from the furnace after cooling down to room temperature. Again for the synthesis of GO/g-C3N4-Fe3O4, we have used a simple sonication technique followed by the hydrothermal method. Typically,1g of GO and 0.5g of g-C3N4 was put in 30 mL of DEG and sonicated for 1h to form 2D/2D GO/g-C3N4 double layer. Then Fe3O4 nanoparticles were decorated on it using the hydrothermal method[40]. In this method, FeCl2, FeCl3, NaAc, and PEG were dissolved in 40 mL DEG by maintaining the molar ratio of FeCl2: FeCl3: NaAc: PEG as 1:2: 25: 1.5. After that, this prepared mixture was mixed with previously prepared GO/g-C3N4 mixture under sonication for another 30 min. The resulting mixture was put in the autoclave at 200°C for 10h. Then the mixture was centrifuged and washed several times with DW and ethanol. The black residue obtained was dried at 60°C to obtain GO/g-C3N4-Fe3O4. Also, we have synthesized 2D/2D GO/g-C3N4 and Fe3O4 nanomaterials using the above procedure for comparison study.

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2.3. Characterization Method The crystallinity of the prepared nanocomposite was performed by X-ray diffraction (XRD, RigakuUltima-IV) using CuKα radiation. The functionality and bonding with composition were analyzed using Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer-1000) and X-ray photoelectron spectroscopy (XPS, PHI 5000, VersaProbe III) respectively. The morphological feature and particle size along with the EDX spectrum and imaging were determined by field emission scanning electron microscopy (FESEM, Nova-450) and transmission electron microscope (TEM, FEI-Tecnai G2) with energy-dispersive X-ray spectroscopy (EDX). The BET and pore size distribution of nanocomposite was analyzed by N2 adsorption-desorption study using a Quantachrome instrument (Autosorb-IQ). Zeta potential study was also performed for the determination of surface charge using a Melvern Nano ZS. Vibrating sample magnetometer (VSM, Lakeshore-7404) was used to analyze the magnetic properties. The final concentration of MB and TC after the adsorption test was determined by a UV−visible spectrophotometer (PerkinElmer, Lambda 750S).

2.4. Adsorption Experiments To study the effect of pH, contact time, initial concentration and temperature, a set of adsorption experiments were carried out for TC and MB adsorption on GO/g-C3N4-Fe3O4 surface. On the basis of the effect of adsorbent dose experiments by taking a different dose (0.005 to 0.06 g), adsorption equilibrium occurred at 0.03 g for both TC and MB and this amount of adsorbent was used for all adsorption experiments. In the equilibrium adsorption tests, 0.03g of GO/g-C3N4Fe3O4 was added to 50mL of 50 mg/L TC solution and 100 mg/L MB solution in 100 mL blue capped bottles. The pH of the solution was 3 for TC and 9 for MB adsorption. All the adsorption experiments were performed using an orbital shaker (Remi RS 12 plus) with continuous shaking (250rpm) at room temperature (25ºC). The nanocomposite after adsorption of TC and MB was separated out by a simple external magnet due to its magnetic properties. Then the adsorbent free solution was collected for analysis. The final concentration of TC and MB after adsorption was measured by a UV-visible spectrometer with maximum wavelength at 360nm and 664 nm respectively.

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The effect of pH on adsorption capacity for TC and MB removal was examined by changing the pH from 3 to 10. The various pH of the solution was maintained using 0.1M of NaOH and HCl solutions. In order to study the kinetic parameter, the effect of contact time experiment was performed by changing the contact time from 30 to 360 min for TC and 30 to 240 min for MB. All the above adsorption tests were carried out at constant concentration (50 mg/L for TC and 100 mg/L for MB). Adsorption isotherm were also studied using different concentration of solution i.e. 20, 50, 80, 120 and 150, 180 mg/L for TC and 50, 100, 150, 200, 250, 300 mg/L for MB with constant dose, pH and time. All the above adsorption tests were carried out at room temperature. Further, the thermodynamic study was also examined at different temperature i.e. 298K, 308K, and 318K. The equilibrium adsorption capacity (qe in mg/g) was calculated using equations 1 and % removal (%R) was calculated using equation 2 [39].

qe =

(C 0 − C e )V

%R =

(1)

m

(C0 − Ct ) C0

× 100

(2)

C0 and Ct (mg/L) are the initial concentration and final concentration at time t respectively. Ce (mg/L) is the concentration at equilibrium. V is the volume (L) of solution and m is the mass (g) of the adsorbent. All the adsorption experiments were repeated three times and the average value of the data were considered for final results.

3. Results and discussion 3.1. Surface characteristics of prepared adsorbents The FT-IR analysis of GO, g-C3N4, GO/g-C3N4 and GO/g-C3N4-Fe3O4 nanocomposites were carried out in the spectral range of 4000-400 cm-1 and the spectra are presented in Fig. 1a. Several specific peaks of GO observed at about 1042cm-1, 1420cm-1,1617cm-1 and 1740cm-1 are attributed to C-O, carboxyl O=C-O, C=C stretching vibration of epoxide, and O=C stretching vibration, respectively [39]. The peak at 3414 cm-1 and 1227 cm-1 indicate the OH stretching and bending vibrations respectively. The broad peak at 3140cm-1 was ascribed to the N–H stretching 6

for g-C3N4. The peaks at 1640, 1565, 1400, 1310, and 1230 cm-1 of g-C3N4 correspond to C−N heterocycles. Further, the peak at 815 cm-1 refers to the stretching vibration of triazine units[41– 43]. For the IR spectra of GO/g-C3N4, due to the formation of g-C3N4 layers on to GO surface, the respective peaks of g-C3N4 only observed. The peak at 580cm-1 corresponds to the Fe-O stretching vibration of Fe3O4 which is present only GO/g-C3N4-Fe3O4 nanocomposite [44]. The formation and phase analysis of the prepared nanocomposites were analyzed by XRD. From the XRD patterns of GO, g-C3N4 and GO/g-C3N4 (Insert) and (i) Fe3O4, (ii) GO-Fe3O4 and (iii) GO/g-C3N4-Fe3O4 (Fig. 1b), the peak at 10.27° of GO was ascribed to the inter-planar arrangement value of (001). The g-C3N4 has two intensive peaks at 13.14° corresponds to (100) plane for tristriazine units and 27.47° represents (002) plane for aromatic parts which is accordance with the JCPDS no 87-1526[41,45].In the XRD spectra of GO-Fe3O4, GO/g-C3N4Fe3O4, the GO peak at 10.27º vanished which suggests that there is a defect on GO lattice after introducing Fe3O4 and g-C3N4. Also the peaks at around 18.31, 30.43, 35.59, 43.32, 53.77, 57.12, and 62.79° are ascribed to (111), (220), (311), (400), (422), (511) and (440) crystal planes of cubic Fe3O4, respectively with JCPDS no 01-1111[31,40].The extra peak at 27.52ºin case of GO/g-C3N4-Fe3O4 is ascribed to (002) plane of g-C3N4, which indicates that the composites consist of both cubic Fe3O4 and g-C3N4.

Fig. 1. (a) FT-IR spectra and (b) XRD patterns of GO, g-C3N4 and GO/g-C3N4 (Insert) and (i) Fe3O4, (ii) GO-Fe3O4 and (iii) GO/g-C3N4-Fe3O4 nanocomposites.

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Raman spectroscopy is an influential technique to determine the defects and disordered present on the surface of carbon-based materials. Fig. 2 presents the Raman spectra of GO, g-C3N4, GO/g-C3N4 and GO/g-C3N4-Fe3O4. For GO, two remarkable peaks at around 1352 and 1590 cm-1 are assigned to D and G bands respectively. D band gives information about the defect present in sp3 carbon atoms whereas G band provides defect present in sp2 carbon atoms on the GO surface. When compared with pristine GO, the G band of GO/g-C3N4 remarkably shifted from 1590 to 1612 cm-1 and to 1620 cm-1 for GO/g-C3N4-Fe3O4. Similarly, the D band shifted from 1352 to 1366 cm-1 for GO/g-C3N4 and 1370 cm-1 for GO/g-C3N4-Fe3O4 [39,46]. These shifting of G and D bands were due to the covalent doping of g-C3N4 on GO layer and more shifting in the case of GO/g-C3N4-Fe3O4 is due to more defects on GO sheet after the formation of Fe3O4 nanoparticles. Again a number of peaks in between 1000 to 2000 cm-1 are found in the case of GO/g-C3N4 and GO/g-C3N4-Fe3O4 due to the presence of g-C3N4 [47].

Fig. 2. Raman spectra of GO, GO/g-C3N4 and GO/g-C3N4-Fe3O4. In order to study the surface morphology of prepared composites, we have performed FE-SEM and TEM analysis. The FE-SEM image of prepared GO (Fig. 3a) and g-C3N4 (Fig. 3b) showed the characteristic wrinkled shaped and 2D layered sheets structure. However, g-C3N4 consists of shorter length graphitic planes as compared to GO. GO/g-C3N4 (Fig. 3c) possesses a more compact sheet-like structure. A similar type of morphology was also found for GO/g-C3N4-Fe3O4 (Fig. 3d) along with the distribution of fine spherical Fe3O4 nanoparticles on the surface of GO/g-C3N4. 8

Fig. 3.FE-SEM images of (a) GO, (b) g-C3N4, (c) GO/g-C3N4 and (d) GO/g-C3N4-Fe3O4. TEM was further used to study more detail on the microstructures and to determine the particle size of Fe3O4 nanoparticles onto the 2D/2D GO/g-C3N4 surface. Fig. 4a and 4b show the TEM images of GO/g-C3N4-Fe3O4 at different resolutions along with the average particle size curve. From Fig. 4a, double layer 2D/2D structure formation was observed, in which g-C3N4 was sandwiched between GO sheets. Again Fig. 4b indicates the formation of fine nanoparticles on the 2D/2D GO/g-C3N4 surface. For the determination of average particle size, we have also measured the diameter of 200 particles using Image-J software and the size distribution curve as shown in Fig.4b (insert). The average particle size of nanoparticle was found to be 12 nm. To further confirm of crystallinity phase, the HR-TEM and SAED were performed and shown in Fig. 4c. From the SAED pattern of GO/g-C3N4-Fe3O4 (Insert Fig. 4c), the pattern of the ring comes from the planes of GO/g-C3N4, while the bright dots confirm the crystalline and cubic structure of Fe3O4 [48].

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The HRTEM image (Fig. 4c) showed the formation of spherical nanoparticles with one type of lattice fringe. The d-spacing of 0.242 nm corresponds to the (311) lattice plane of the cubic structure of Fe3O4 and this result also in accordance with the XRD result. From this observation, the formation of Fe3O4 nanoparticles on the GO/g-C3N4 layer is confirmed [49].

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Fig. 4. (a) and (b) TEM images with particle size distribution curve (insert), (c) HR-TEM image with SAED pattern (insert), (d) EDX spectrum, (e) selected area for elemental mapping and (f) elemental mapping of individual element of GO/g-C3N4-Fe3O4 nanocomposite. TEM-EDX spectrum of GO/g-C3N4-Fe3O4 (Fig. 4d) indicates the presence of C, O, N and Fe elements with 90.88, 4.89, 1.45 and 2.78 atomic percentages respectively. This experiment suggests the formation of Fe3O4 functionalized GO/g-C3N4 nanocomposite, which is also consistent with the XPS results. Further, the TEM-EDX elemental mapping of this nanocomposite (Fig. 4e and f) revealed the presence of C, N, Fe and O elements with a different color on the required selected region.

Fig. 5. (a) XPS survey for whole range, High resolution XPS spectra of (b) C 1s region, (c) O 1s region, (d) N 1s region, (e) Fe 2p region and (f) Fe 2p3/2 region. To obtain the information about the different chemical state, bonding and elemental on the surface of GO/g-C3N4-Fe3O4 nanocomposite, XPS analysis was carried out [1] and the results obtained are shown in Fig. 5a-f. As shown in Fig. 5a, the XPS survey spectrum of GO/g-C3N4Fe3O4 shows C, O, N, and Fe elements. Fig. 5b reveals the high-resolution XPS spectrum of C 1s. From this Fig., the peak centered at 284.2, 284.8 and 287.8 eV correspond to C-C, C=O and C-O bond of GO respectively [50] and a peak at 285.9 eV corresponds to C-N bond of g-C3N4 11

[51]. The peaks at 529.9 and 532.2 eV in the high-resolution O 1S spectrum (Fig. 5c) refer to M(Fe)-O due to metal oxide formation and C-O bond respectively. The N1s spectrum (Fig. 5d) can provide further information regarding the C–N covalent interaction between g-C3N4 and GO. The peaks centered at 397.9, 398.9 eV may be assigned to C–N=C and N-(C)3 respectively [45]. The peak at 401.4 eV can be assigned to secondary amine (C–N–H) and which is formed the covalent bond between g-C3N4 and GO [1,52]. This indicates the existence of GO and g-C3N4 in prepared nanocomposite through covalent linking. Again from the high-resolution spectrum of Fe 2p (Fig. 5e), the peak centered at 710.6 and 724.3 eV, represent Fe 2p3/2 and Fe 2p1/2 respectively. From Fe 2p3/2XPS spectrum (Fig. 5f) the two peak at 710.1 eV refers to Fe2+ and 711.6 eV refers to Fe3+ [53,54]. This result confirms the iron oxide decorated on GO/g-C3N4 surface is Fe3O4.

Fig. 6. (a) N2 adsorption/desorption isotherms and (b) pore size distribution of GO, g-C3N4, GO/g-C3N4 and GO/g-C3N4-Fe3O4; (c) VSM curve and (d) Zeta potential curve of GO/g-C3N4Fe3O4. 12

The N2 adsorption/desorption isotherms along with pore size distribution of GO, g-C3N4, GO/gC3N4 and GO/g-C3N4-Fe3O4 are presented in Fig. 6a and 6b respectively. From Fig.6a, the amount of N2 adsorbed onto and desorbed from GO/g-C3N4-Fe3O4 is considerably greater than that of GO, g-C3N4 and GO/g-C3N4. The increasing order of the amount of N2 adsorbed and desorbed is g-C3N4< GO < GO/g-C3N4< GO/g-C3N4-Fe3O4 and it is also the order of surface area. The surface area was found to be 120 m2/g for GO/g-C3N4-Fe3O4 which is higher as compared to other prepared materials, i.e. 92.22 m2/g for GO/g-C3N4, 72.12 m2/g for GO and 42.18 m2/g for g-C3N4. The average pore size for GO, g-C3N4, GO/g-C3N4 and GO/g-C3N4Fe3O4 was found to be 4.42, 3.96, 3.88, and 3.72 nm respectively. The magnetic properties of GO/g-C3N4-Fe3O4 nanocomposite were examined using VSM at room temperature and represented in Fig. 6c. The VSM exhibits an S-like curve with the saturation magnetization value of 28.4 emu/g and this value is enough to separate adsorbent by an external magnet from the treated water. The zeta potential studies of the GO/g-C3N4-Fe3O4nanocomposite were carried out by changing pH from 3 to 8 and have calculated pHIEP value. The result obtained is displayed in Fig. 6d. The pHIEP value was found to be 5.24.

3.2. Adsorption studies of TC and MB 3.2.1. Effect of dose and pH Fig. 7a and 7b show the effect of dose on TC and MB adsorption by GO/g-C3N4-Fe3O4 nanocomposite. The % removal increases with an increase in the dose up to 0.03 g for both TC and MB. The increase in % removal maybe because of the increase in the sum of active sites with adsorbent dosage. However, further increase in the adsorbent dose, the % removal all most constant because at the higher adsorbent dose the concentration of TC and MB molecules present in the solution is not enough to cover all adsorption sites on the adsorbent surface. Thus equilibrium adsorbent dosage for both TC and MB was 0.03 g and this amount was used for all adsorption experiments. From Fig.7a and 7b, it was also found that the equilibrium adsorption capacity (qe in mg/g) decreases with the increasing adsorbent dose. As pH can change the surface charge and properties of adsorbent, hence the pH of the solution is an important parameter that can greatly affect the TC and MB adsorption on adsorbents surface. The effect of pH ranging from 2.0 to 10 on the adsorption capacity of TC and MB on GO/g-

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C3N4-Fe3O4 is shown in Fig. 7c. From this Fig., it is clearly observed that the qe value is maximum at higher pH value (pH=9) for MB and lower pH value (pH=3) for TC. For better understanding, the above phenomenon, the pHIEP value and structural formula of TC and MB at different pH were performed. As seen in Fig. 6d, the pHIEP value was 5.24. The pH value is greater than 5.24, the surface of the adsorbent becomes negative. As we know MB is a positive charge dye and it attracts more at pH greater than 5.24 by electrostatic interaction between MB and negatively charges adsorbent surface that achieved higher adsorption at higher pH and lower adsorption value at lower pH[10,24]. As seen in Fig. 7c, for TC, the adsorption capacity is maximum at pH= 3.0. TC has three pKa values i.e.3.3, 7.8 and 9.6 and shown in Fig. 7d. According to this Fig., the cationic TC (TCNH(CH3)2+) would be repelled with the positive adsorbent surface at pH < 3.3. At pH > 7.8, the electrostatic repulsion may exist between anionic TC (TNH- or TO-) with the cationic adsorbent surface. However, the maximum adsorption occurs at pH=3[15,55]. Hence, it is concluded that electrostatic interaction was not the main driving force in the TC adsorption. Other mechanisms were the key factors for TC adsorption which was described in the mechanism section.

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Fig. 7. (a) Effect of adsorption dose of TC adsorption (condition: concentration of solution (50 mg/L), pH=3 and contact time (300 min)) and (b) effect of adsorption dose of MB adsorption (condition: concentration of solution (100 mg/L), pH=9 and contact time (180 min)), (c) effect of pH of TC and MB adsorption using 0.03 g adsorbent dose on GO/g-C3N4-Fe3O4 and (d) chemical formula of TC with different pKa values.

Fig. 8 (a) Effect of time with nonlinear curve of pseudo second and first order kinetic models for TC adsorption (condition: dose (0.03g), concentration of solution (50 mg/L), pH=3) and (b) MB adsorption (condition: dose (0.03g), concentration of solution (100 mg/L), pH=9) by GO/gC3N4-Fe3O4.

3.2.2. Effect of contact time and adsorption Kinetics The effect of contact time with the nonlinear curve of pseudo-second and first-order kinetic models for TC and MB adsorption onto GO/g-C3N4-Fe3O4 nanocomposite was illustrated in Fig. 8a and 8b respectively. The adsorption capacity of TC and MB by GO/g-C3N4-Fe3O4 is found to be increased with the increase of contact time from 30 to 270 min due to the availability of more surface binding sites on the adsorbent surface. After that, the adsorption capacity remains constant and reached equilibrium at 300 min due to insufficient surface binding sites on the adsorbent. Similarly, the adsorption capacity of MB increases up to 150 min and reached an equilibrium value at 180 min. To further investigate adsorption behaviors, pseudo-first-order and

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second-order models were applied to fit adsorption kinetics data. The pseudo-first-order and pseudo-second-order models were described as following equations, respectively [56,57]. q t = q e (1 − e k1t ) qt =

qe k 2t qe k 2t + 1

(3) (4)

Where qe and qt refer to the adsorption capacity (mg/g) at equilibrium and time t respectively, k1 is the pseudo-first (min-1) and k2 is the pseudo-second-order rate constant (g mg-1min-1). Fitting parameters including the values qe, qt, k1, k2 were calculated by using nonlinear regression coefficients (R2) and represented in Table S1. Based on the R2 value along with a comparison of qe experimental and calculated value, the adsorption data were best fitted with the pseudosecond-order kinetic models for both TC and MB adsorption by GO/g-C3N4-Fe3O4.

3.2.3. Effect of initial concentration and adsorption isotherms In order to study the interaction between TC/MB and different adsorbents including GO, g-C3N4, GO/g-C3N4 and GO/g-C3N4-Fe3O4, adsorption isotherms were carried out with different concentration (20, 50, 80, 100, 150 and 200 mg/L for TC and 50, 100, 150, 200, 250 and 300 mg/L for MB) at room temperature. The results were presented in Fig. 9a for TC and Fig. 9b for MB. From there, it has been seen that the adsorption capacity of all adsorbents for TC and MB increases sharply with the increase in the initial concentration and attains the adsorption saturation gradually. As can also be seen, the adsorption capacity of TC and MB of GO/g-C3N4Fe3O4 was much more as compared to GO/g-C3N4, GO and g-C3N4 and this may be due to higher surface area and more active sites. Furthermore, for a better understanding, of the adsorption process, two widely used isotherm models (Langmuir and Freundlich) were used. The adsorption capacity was also calculated for TC and MB adsorption using these models. In theory, the Langmuir model is used to present the homogeneous or monolayer adsorption behaviors onto the adsorbent surface. Fig. 9a and 9b show the nonlinear Langmuir isotherm for TC and MB adsorption by GO, g-C3N4, GO/g-C3N4 and GO/g-C3N4-Fe3O4. The Freundlich adsorption model is applied to describe the heterogeneous or multilayer adsorption behavior onto the adsorbent surface. The nonlinear Freundlich adsorption model for both TC and MB adsorption is shown in Fig. S1. The mathematical equation of nonlinear Langmuir and nonlinear Freundlich isotherms models are express in equations 5 and 6 [56,57]. 16

qe =

q 0 bCe 1 + bCe

(5)

Where, Ce (mg/L) is the concentration at equilibrium, qe (mg/g) is the adsorption capacity at equilibrium and q0 (mg/g) is the maximum adsorption capacity. Again b (L/mg) is a constant called Langmuir constant. qe = K F Ce

1/ n

(6)

Where KF (mg/g) is the Freundlich constant and dimensionless n is the Freundlich exponent. The n value indicates the degree of nonlinearity between solution concentration and adsorption as follows: if n = 1, then adsorption is linear; if n < 1, then adsorption is a chemical process; if the values of n within the range of 1–10 represent favorable adsorption process. Table 1 represents the adsorption isotherm parameters for TC and MB which are determined from the nonlinear regression curve of fitted adsorption data with the adsorption isotherm models. Based on the R2 values of the two models, it is shown that the Langmuir model is best fitted with our experimental data for both TC and MB adsorption. This confirms that the adsorption process refers to the monolayer adsorption onto a homogeneous adsorbent surface. To further examine the adsorption process is favorable or not the dimensionless constant RL, can be calculated by using equation no (7) from the Langmuir isotherms model [39].

RL =

1 1 + bC0

(7)

C0 is the initial concentration (mg/L). We have calculated RL values for all three prepared nanomaterials and found between 0 to 1; which reveals the feasibility of TC and MB adsorption process. The maximum adsorption capacity for TC and MB was found to be 107.28 mg/g and 187.36 mg/g for GO/g-C3N4-Fe3O4 which are higher as compared to GO/g-C3N4 (100.04 mg/g for TC and 174.23 mg/g for MB) and that of pristine GO and g-C3N4. The obtained maximum adsorption capacity value of GO/g-C3N4-Fe3O4 towards TC and MB adsorption was compared with the other reported GO and g-C3N4 based adsorbents and represented in Table 2. It is observed that the prepared GO/g-C3N4-Fe3O4 is a good and efficient adsorbent for both TC and MB adsorption.

17

Fig. 9. Nonlinear Langmuir isotherm plot for (a) TC adsorption (condition: dose (0.03 g), pH=3 and contact time (300 min)); and (b) MB adsorption (condition: dose (0.03 g), pH=9 and contact time (180 min)) by GO/g-C3N4-Fe3O4, GO/g-C3N4, GO and g-C3N4. Table 1. The parameters for the Langmuir and Freundlich adsorption isotherms of TC and MB on GO/g-C3N4 and GO/g-C3N4-Fe3O4 (condition: dose (0.03 g), pH=3 and contact time (300 min) for TC and dose (0.03 g), pH=9, contact time (180 min) for MB). Adsorbent

Adsorbed

Langmuir

Freundlich

Molecules

q0 (mg g-1)

b (Lg-1)

R2

KF (mg g-1)

n

R2

TC

100.04

0.30

0.95

44.92

5.83

0.68

MB

174.23

0.37

0.95

93.73

8.08

0.67

GO/g-C3N4-

TC

107.28

0.60

0.98

53.94

6.40

0.75

Fe3O4

MB

187.36

1.29

0.99

114.37

9.45

0.75

GO/g-C3N4

Table 2. Comparison of TC and MB adsorption on various GO and g-C3N4 based adsorbent. Adsorbent

Molecules

Maximum adsorption capacity (mg/g)

Magnetic GO modified zeolite[30]

MB

82.14

RGO/α-Fe2O3[33]

TC

18.48

g-C3N4[34]

MB

42.10

18

GO-TiO2[15]

TC

117.98

GO/calcium alginate composite fibers[17]

TC

131.60

GO/poly(vinylidine fluoride)

TC

18.03

GO/g-C3N4-Fe3O4 (in this study)

TC

107.28

GO/g-C3N4-Fe3O4 ( in this study)

MB

187.36

nanofibers[58]

3.2.4. Thermodynamic Studies Thermodynamic study plays a key role in estimating adsorptive mechanisms [59]. Hence, the thermodynamic factors like standard Gibbs free energy change (∆G°), entropy change (∆S°) and enthalpy change (∆H°) and the equilibrium constant (kc) were calculate using thermodynamic equations. The equations were given below [56,57,59].

kc =

Cs Ce

(8)

∆G° = −RT ln kc

(9)

∆G° = ∆H ° − T∆S °

(10)

ln k c =

∆S ° ∆H ° − R RT

(11)

Where Cs is the amount of TC and MB adsorbed onto the adsorbent surface at equilibrium (mg L-1), Ce is the equilibrium concentration (mgL-1), R (8.314 J mol-1) is gas constant and T (Kelvin) is the absolute temperature. The values of Kc are obtained from the intercept of the plot ln(Cs/Ce) vs Cs. The curve of lnkc vs 1/T called van't Hoff plot was showed in Fig.S2. From this plot, the ∆H° and ∆S° were calculated from the slope and intercept respectively and listed in Table S2. The negative value of ∆G° for both TC and MB adsorption recommended that the adsorption process was spontaneous. The positive values of ∆H° (kJmol-1) revels endothermic nature. Furthermore, the positive values of ∆S° reveal the degree of irregularity increases at the solidliquid interface. From Table S2, it is also observed that the ∆G° values increases with

19

temperature (from 293 to 323K) indicating high temperature is favored for the better TC and MB adsorption [15,39].

3.3. Regeneration of adsorbent To evaluate the reusability of TC and MB adsorption onto GO/g-C3N4-Fe3O4, desorption behavior is examined using a 1M NaOH solution for TC and 1M HCl solution for MB. The Fig. 10a and 10b have represented the desorption-adsorption cycles of TC and MB onto GO/g-C3N4Fe3O4 respectively. From the above Fig., it is clearly observed that, after five consecutive cycles, the adsorption capacity of adsorbent for both TC and MB sorption slightly reduced with increasing regeneration cycles. The removal efficiencies remained steady at above 78% for TC and 82 % for MB after five successive cycles. These results indicate that GO/g-C3N4-Fe3O4 has good chemical stability and reusability and is a promising candidate for the cyclic removal of TC and MB.

Fig. 10. Recycling of GO/g-C3N4-Fe3O4 in the removal of (a) TC and (b) MB (condition: dose (0.03g) concentration (50 mg/L) and time (300 min) for TC and dose (0.03g) concentration (100 mg/L) and time (180 min) for MB in adsorption-desorption experiments).

3.4. Possible adsorption mechanism In the adsorption process, adsorbate species attach to adsorbents surface by a number of interactions depending on the adsorbent and adsorbate/adsorbed species including electrostatic, π-π, hydrogen bonding, Vander Waals, hydrophobic interaction, and so on[10]. Here, adsorbed 20

species were MB and TC and adsorbent was GO/g-C3N4-Fe3O4. All are aromatic molecules having π electrons. So π-π interaction mostly favors the adsorption of TC and MB onto GO/gC3N4-Fe3O4. Both GO and g-C3N4 have π electron and both can form π-π interaction with TC and MB. When both are present in one material, the π-π interaction sites increases, as a result, the adsorption capacity also increases. In this work, we have also found that the adsorption capacity of GO/g-C3N4-Fe3O4was found to be higher as compared to pristine GO and g-C3N4 and GO/g-C3N4. Other additional interactions are also possible between TC/MB with the prepared adsorbent. MB is a cationic dye and it exists as positively charged in aqueous solution at the pH range of 1.5 to 11. From the adsorption experiment of MB, it was found that the maximum adsorption occurs at pH=9. At pH=9 the adsorbent surface became highly negative charge (pHIEP=5.24). Therefore, MB gets adsorbed on the adsorbent surface by the electrostatic interaction between the negative charge surface of the adsorbent and the positive charge MB. Again due to the presence of the hydroxyl group in adsorbent, the hydrogen bonding also possible between H-atom and N-atom in MB[10,34].On the other hand, TC contains various functional groups (phenol, alcohol, ketone, and tertiary amine and amide) so it has different pKa values shown in Fig. 7d. At acidic pH < 3.3, TC becomes positively charged by the tertiary ammonium functional groups. The carboxyl groups of GO contain negatively charged at pH range 2 to 10. Therefore, at acidic pH, the TC adsorption is favored by the ionic interaction between negatively charged carboxyl groups of GO and positively charge tertiary ammonium group of the TC and achieves the maximum adsorption capacity. The TC exists in the zwitterionic form (neutral) in the pH range 3.3 to 7.68. Thus, electrostatic interaction cannot possible at this pH range. At much higher pH, the TC molecule becomes negatively charged. Therefore, the electrostatic repulsive forces between negatively charge carboxyl group and the negatively charged TC molecule lower the adsorption capacity. Again like MB, TC also can form, the hydrogen bonding between H-atom of the hydroxyl group of GO-based adsorbent and O-atom in TC [15,55,60]. From the above explanation, the TC and MB adsorption happen onto GO/g-C3N4-Fe3O4 may be due to three types of possible interactions such as π-π interaction, electrostatic interaction and hydrogen bonding and all are shown in Fig. 11.

21

Fig. 11. Possible interaction of TC and MB with GO/g-C3N4-Fe3O4 surface.

4. Conclusion In this study, Fe3O4 nanoparticles were decorated on higher surface area 2D/2D GO/g-C3N4 sheets (GO/g-C3N4-Fe3O4) using the simple hydrothermal method and used as an ecofriendly and highly efficient adsorbent to remove both toxic antibiotic (TC) as well as an organic dye (MB). The structure, morphology and surface properties were analyzed by FTIR, XRD, Raman, XPS, FESEM, HRTEM, BET and zeta potential studies. The magnetic properties were also analyzed by VSM measurement at room temperature. The adsorption of both TC and MB by GO/g-C3N4-Fe3O4 was highly pH-dependent and the maximum adsorption capacity was found to be pH=3 for TC and pH=9 for MB. The GO/g-C3N4-Fe3O4 2D/2D nanocomposites showed the highest adsorption capacity with 107 mg/g for TC and 187 mg/g for MB and these value are higher than that of GO/g-C3N4, pristine GO and g-C3N4 materials. The increase in adsorption capacity for TC and MB onto GO/g-C3N4-Fe3O4 due to an increase in π-π interaction compare to individual GO and g-C3N4. Again the TC and MB adsorption capacity was found to be more after introducing Fe3O4 on GO/g-C3N4. Functionalization of Fe3O4 onto GO/g-C3N4 can also be useful for easy separation of 22

adsorbent from treated water using an external magnet with excellent recyclability and reusability. The thermodynamic study reveals that the adsorption process for both TC and MB was spontaneous and endothermic in nature. The adsorption mechanism for TC and MB removal was because of π-π interaction and hydrogen bonding between TC/MB molecules and GO/g-C3N4-Fe3O4.

Acknowledgments The authors are thankful to NIT Rourkela for providing all facilities for this work.

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Highlights Magnetically separable and reusable GO/g-C3N4 decorated Fe3O4 nanocomposite was synthesized. GO/g-C3N4-Fe3O4 can effectively remove MB and TC organic pollutants from water. The regenerated adsorbent can be effectively reused up to 5 successive cycles without major loss in its efficiency. The mechanism of TC and MB adsorption is mainly due to π – π and electrostatic interaction.

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All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

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This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

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The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript

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Author’s name Shraban K. Sahoo Sandip Padhiari Dr. S. K. Biswal Dr. B. B. Panda Dr. G. Hota

Affiliation NIT Rourkela, Odisha, India NIT Rourkela, Odisha, India Dept. of Chemistry, CUTM, BBSR, India Dept. of Chemistry, IGIT Sarang, Odisha,India NIT Rourkela, Odisha, India