Synthesis and characterization of magnetite nanoparticles for photocatalysis of nitrobenzene

Journal Pre-proofs Original article Synthesis and characterization of magnetite nanoparticles for photocatalysis of nitrobenzene Victor Alfredo Reyes Villegas, Jesús Isaías De León Ramírez, Esteban Hernandez Guevara, Sergio Perez Sicairos, Lilia Angelica Hurtado Ayala, Bertha Landeros Sanchez PII: DOI: Reference:

S1319-6103(19)30128-0 https://doi.org/10.1016/j.jscs.2019.12.004 JSCS 1096

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Journal of Saudi Chemical Society

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22 August 2019 10 December 2019 12 December 2019

Please cite this article as: V. Alfredo Reyes Villegas, J. Isaías De León Ramírez, E. Hernandez Guevara, S. Perez Sicairos, L. Angelica Hurtado Ayala, B. Landeros Sanchez, Synthesis and characterization of magnetite nanoparticles for photocatalysis of nitrobenzene, Journal of Saudi Chemical Society (2019), doi: https://doi.org/ 10.1016/j.jscs.2019.12.004

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Synthesis and characterization of magnetite nanoparticles for photocatalysis of nitrobenzene Victor Alfredo Reyes Villegasa, Jesús Isaías De León Ramíreza, Esteban Hernandez Guevaraa, Sergio Perez Sicairosb, Lilia Angelica Hurtado Ayalaa, Bertha Landeros Sanchez*a a

Universidad Autónoma de Baja California, Facultad de Ciencias Químicas e Ingeniería, Calzada Tecnológico 14418,

Mesa de Otay, Tijuana, B.C. 22390 [email protected],

[email protected],

[email protected], [email protected], [email protected] b

Centro de Graduados e Investigación en Química. Tecnológico Nacional de México/Instituto Tecnológico de Tijuana.

Apdo. Postal 1166. Tijuana, B. C. 22000, México. [email protected] *Corresponding autor: E mail: [email protected] Tel: +52 664 265 4845 Abstract: The present work shows the photocatalytic degradation of nitrobenzene (NB) using Fe 3O4 magnetic nanoparticles (MNP) as a photocatalyst in the presence of UV light. The MNP were synthesized by an ultrasonicassisted reverse co-precipitation (US-RP) method using FeSO4, FeCl3 and NH4OH as precursors. The prepared nanoparticles were characterized by UV-vis spectroscopy, attenuated total reflectance Fourier transformed infrared spectroscopy (ATR FT-IR), Raman spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Dynamic light scattering (DLS), Zeta potential, Vibrating sample magnetometer (VSM) and Magnetic thermogravimetric analysis (MTGA). The successive decrement in the absorbance at 265 nm shows the effective decrease in NB concentration measured by UV-vis spectroscopy. The reaction intermediates detected by gas chromatography/mass spectrum (GC/MS) were 2-nitrophenol (2-NPh), 3-nitrophenol (3-NPh) and 4-nitrophenol (4NPh). The prepared MNP showed an optimal NB degradation at an initial pH of 2 and 100 ppm of the photocatalyst. Keywords: Magnetite, Nanoparticles, Nitrobenzene, Photocatalyst, Photodegradation

1 Introduction: Nitrobenzene (NB) is a carcinogenic pollutant, [1] after inhalation exposure, nitrobenzene causes tumors in the liver, kidneys and thyroid gland of rats, and in lungs and mammary glands of mice. In humans, the symptoms of NB intoxication are a burning sensation in the mouth and throat, nausea, vomiting, dizziness, coordination disorders, cyanosis, a smell of bitter almonds in the exhaled air, restlessness, tachycardia, a drop in blood pressure, collapse, signs of paralysis, unconsciousness and coma [2]. NB is widely used in explosives, pesticides, paper pulp production and colorant industrial processes [3], perfumes, pharmaceuticals, dyes, synthetic rubber, and plastics [4]. NB is produced annually on the order of 225,000 metric tons, and it has been estimated that as much as 9,000 metric tons of nitrobenzene are discharged annually into natural waters [5]. Conventionally, NB in aqueous media is processed using physical, chemical and biological techniques. The general treatments are composed of adsorption and biodegradation. These techniques can be effective, but it occurs secondary pollution, resulting in high processing cost [6]. However, NB is refractory to conventional chemical oxidation because the electron-withdrawing property of the nitro group is notably strong. In addition, NB is also resistant to conventional biological treatments due to the compound’s toxic and mutagenic effects on biological systems [3]. The electron-deficient character of the nitro-group prevents mineralization of NB by microorganisms [1]. Advanced Oxidation Processes (AOP), generally based on the generation of highly reactive species such as hydroxyl radicals (HO•), are of great interest for degradation of pollutants that are difficult to eliminate with conventional treatments [7]. Among AOPs, heterogeneous photocatalysis seems to be an attractive method as it has been successfully employed for the degradation of various families of organic pollutants. The reason for the increased interest for the photocatalytic method is that the process may use

atmospheric oxygen as the oxidant, it can be carried out under ambient conditions and may lead to total mineralization of organics to CO2, water and mineral acids [8]. Semiconductor photocatalysis has proven to be a promising technology for the removal of various organic pollutants, including nitroaromatic compounds, from groundwater and waste streams [9]. The combination of a contaminant with a semiconductor photocatalyst, followed by irradiation of photons energetic enough to produce an electron/hole pair, is also a well-known treatment for purification of air and water [10]. Fe3O4 nanoparticles have been used as a photocatalyst for: methylene blue, methyl red, Congo red, water splitting, methylene orange, Levofloxacin, Rhodamine B and Cr (VI) [11–20]. Further, the enhancement of photodegradation efficiency of the organic molecules in UV/nano-Fe3O4 system may be attributed to the rapid transfer of the photo-generated electrons resulting in the effective separation of the electrons and holes. Hole itself is a strong oxidant that can oxidize the OH- and H2O that are adsorbed on the Fe3O4 surface, yielding the HO• free radical. The HO• radical adsorbed on Fe3O4 surface is a strong oxidant and it not only oxidizes the organic compound adsorbed, but also diffuses into the bulk solution and oxidizes the organic compounds. After a series of oxidative processes, the organic compound finally may be converted into innocuous end-products viz CO2, H2O and NH3 [12]. In addition, many physical, chemical, biological, and hybrid methods are available to synthesize different types of nanoparticles. The nanoparticles formed using each method show specific properties. Green synthesis of nanoparticles makes use of environmental friendly, non-toxic and safe reagents [21]. Biological methods rely on reduction-oxidation reactions, in which microbial enzymes or plant phytochemicals are responsible for the reduction of salts into MNP. Such biosynthetic routes are generally considered eco-friendly (green chemistry) and the products obtained using such procedures tend to show good biocompatibility. However, the yield of such methods is low, and the size distribution is broad [22].

Modified co-precipitation method is a valuable method due to high control over size, favorable kinetics, cheap, narrow size distribution and environmental friendly [23]. Sulistyaningsih et al. (2017) found that the crystal size of the magnetite synthesized by reverse co-precipitation method and sonochemical treatment was smaller than the ordinary co-precipitation and mechanical treatment [24]. Wang et al. (2010) demonstrated that the introduction of ultrasonic irradiation was a simple method to assist the preparation of efficient Fe3O4 MNPs catalyst with the reverse co-precipitation method. Improving significantly the catalytic activity of Fe3O4 MNPs, which could be tuned by adjusting the initial concentration of NH3·H2O and the molar ratio of Fe2+/Fe3+ during the preparation process. The significantly improved catalytic activity was attributed to smaller particle size, larger BET surface area and higher dispersibility of the Fe3O4 MNPs, which were achieved by the ultrasonic irradiation during the preparation of Fe3O4 MNPs catalyst [25]. The modified sonochemical method could be a good candidate to prevent toxicity problems in typical sonochemical synthesis process, given that, it uses inexpensive and non-toxic metal salts as reactants [26]. In this work, the synthesis and characterization of a nanosized superparamagnetic Fe3O4 for the photodegradation of NB in an aqueous solution at various pH values is reported. Since MNP have been used for water purification due to its function as a photocatalyst under UV light for organic and inorganic molecules, in addition to its facile nanosynthesis under the US-RP method and its time saving magnetic separation from suspension. Intermediate products were detected by GC/MS, which suggested an oxidazing activity supporting an effective electron hole separation and possible innocuos end-products (due to the lack of evident degradation products) as mentioned above.

2 Experimental : 2.1 Synthesis and characterization of Fe3O4 nanoparticles Fe3O4 nanoparticles were synthesized by an ultrasonic-assisted reverse co-precipitation method (US-RP) [25], with the only modification that it was carried out in an Ultrasonic Cleaner UC-450 (TPC Advanced Technology) , the product was dried over night at room temperature for about 12 hours. The UV-Vis spectra were obtained with a UV-Vis DR 6000, HACH spectrophotometer for a dispersion of 5 mg L-1 of nanoparticles in deionized water, ethylene glycol (EG), n-Propanol [27], and a solution of deionized water with 9.3 mg mL-1 of surfactant of tetramethylammonium hydroxide (TMAH) and 1 mM NaCl [28]. The energy band gap for direct and indirect transition was calculated from the UV-Vis spectra using the Davis and Mott expression [29]. The Infrared (IR) spectrum was obtained from the nanoparticle powder with a Nicolet IS10 Thermo Scientific FTIR Spectrometer with ATR. The Raman spectrum was recorded with a confocal Raman microscopy (alpha300R; WITec) using a Nd:YVO4 laser with an excitation wavelength of 532 nm with a power of 0.26 µW . The Raman dispersion was obtained with a 100X objective, both gratings of 672 lines mm-1 (4 cm-1) and 1800 lines mm-1 (1 cm-1), and an acquisition velocity of punctual spectra of ≤ 10 µs with 10 accumulation in 10 seconds. The center of the characteristic Raman peaks was obtained through a Lorentzian fit. The X ray diffraction (XRD) pattern of the nanoparticles were recorder with a Bruker D8 Advance diffractometer (Bruker Co., Billerica, MA, USA), with de Cu Kα1, λ = 1.5406 Å at 30 mA and 30 kV radiation, with 2θ steps of 0.02⁰ per 1.2004 s from the Bragg angle of 5⁰ a 75⁰ (2θ). The Scherrer equation was used to estimate the crystal size with full-width at Half-maximum obtained from the (311) plane.

The morphology of the sample was characterized utilizing a field emission scanning electronic microscope (SEM; JEOLJ7600F, Tokyo, Japan) with an acceleration voltage of 1 keV and using the SE detector. The hydrodynamic diameter (Dh) and the zeta potential measurements were obtained with a Zetasizer Nano ZS from Malvern Instruments. The Dh was obtained by DLS and the size distribution is reported in intensity. The zeta potential was measured in a pH range from 3 to 12 and was adjusted using HCl and NaOH. Both Dh and zeta potential were set in automatic measurement duration with 3 measurements and no delay between measurements at 25 ⁰C. The magnetic properties were determined by the magnetization curve from the obtained graph of magnetization (emu g-1) as function of the applied field H at 300 K and 3T with a vibrating sample magnetometer (VSM) in a Quantum Design MPMS3. The thermogram to stablish the Curie temperature (Tc) was obtained by a magnetic thermogravimetric analysis (MTGA) in a thermobalance TA model Q5000 with a permanent magnet, to a temperature of 950 ⁰C with a ramp of 10 ⁰C min -1 under an Ar atmosphere.

2.2 Photocatalytic degradation of nitrobenzene The photocatalytic experiments and NB degradation determinations were carried out as detailed by Reynoso-Soto et al. (2013) [30] with minor modifications. Briefly, the photocatalytic experiments of NB degradation were performed at pH 7 with a total volume of 250 mL of NB and MNP solution at 15 ppm and 100 ppm respectively, under UV irradiation with UV lamps of 254 nm(4.88 eV). The nanoparticles used were removed from the sample by magnetic separation. Unless specified elsewhere the reaction conditions were carried out as mentioned above. The specific energy consumption (SEC) was calculated from the following expression(Eq. 1): SEC=(L*NP*t⁄m)

(1)

L = number of lamps, NP = nominal power of lamps (KW), t = time for maximum percentage of degradation (h) and m = mass of NB degraded (kg), calculated from initial concentration of NB and concentration of NB at time “t” [31]. The intermediate compounds of the NB photocatalytic degradation were identified by chromatographymass spectrometry (GC-MS) using the extraction method of Cai et al. (2017) [32] . GC-MS data were recorded on a Thermo Scientific TRACE 1310 (GC) and Thermo Scientific single quadrupole ISQ LT (MS), with a column model TG-SQC (30 m × 0.25 mm inner diameter, 0.25 µm film thickness). The detector temperature was 240 °C, the injector temperature was 250 °C, and transfer line temperature was 250 °C; oven temperature started at 70 °C for 3 min, increased at a 40 °C/min rate up to 280 °C, with a hold time of 7 min. Helium was employed as carrier gas, at 1 mL/min flow. The percentage of the degradation products detected was calculated from the chromatogram using the area under of the peaks corresponding to NB and the nitrophenols (NPhs) in proportion to the NB% detected by UV-Vis spectroscopy. The percentage of intermediates of reaction corresponds to the sum of these.

3 Results and discussions: 3.1 Nanoparticle characterization 3.1.1 Synthesis Magnetite (Fe3O4) is one of many forms of iron oxide and exhibits the strongest magnetism of any transition metal oxide. It is also known as black iron oxide, magnetic iron ore, loadstone, ferrous ferrite, or Hercules stone [33]. Figure 1a shows an aqueous solution with the black nanomaterial synthesized by US-RP method, and under a magnetic field (Fig. 1b). Within less than 60 sec the solution is completely clear due to its magnetic properties.

The characterization of the synthesized Fe3O4 magnetic nanoparticles (MNP) by UV-vis spectroscopy is shown in Figure 1c where the reagents (curve a and b), are way more different than of the synthesized nanomaterial (curve c). This suggesting in a qualitative manner a successful synthesis of a Fe3O4 phase since, Gan et al. (2011) presented a UV-vis spectrum where the Fe3O4 nanoparticles did not show any obvious absorbance peaks in ultraviolet-visible spectroscopy [34], as the nanoparticles synthesized (curve c).

3.1.2 UV-vis spectroscopy UV-vis measurements were carried out to determine a suspension where accurate measurements for the band gap energy, Dh and the IEP of the MNP could be made, since the dispersions of MNP tend to aggregate (as explained later on). Khordad, R. (2016) performs a theoretical calculations of the absorbance of spherical iron nanoparticles dispersed in EG and in n-propanol [27], and in both solvents the maximum absorbance increases. Hu et al. (2010) demonstrated that with a solution of 9.3 mg mL-1 TMAH & 1 mM NaCl it could achieve big enough repulsive forces (electrostatic repulsion and steric repulsion) to counter the magnetic attraction, to keep magnetite suspension stable [28]. Figure 2 presents the UV-vis spectra obtained of the synthesized MNP dispersed in EG (), n-Propanol (), TMAH/NaCl () and water (). The spectra show an absorbance peak in the UV region at the wavelength (λ) of 215 nm ( and ) and 221 nm ( and ), similar to the spectrum reported by M. Awwad and M. Salem, (2013) with a maximum peak of 230 nm [35]. With decreasing particle size, the maximum absorbance is found at shorter wavelength, observing a blueshift with decreasing wavelength. A broad size distribution results in a broad absorption maximum, and vice versa [36]. This explains the broad peaks of the spectra in Figure 2, and the blueshift observed for the maximum a bsorbance of the MNP dispersed in EG () and the surfactant TMAH ().

3.1.2.1

Band Gap Energy of Direct and Indirect transition in Fe3O4 Nanoparticles

In pursuit of the optimal wavelength of the lamp for the photodegradation, band gap energies were calculated. The optical energy band gap for indirect and direct transition was determined by plotting (hνα)1/2 and (hνα)2 as a function of photon energy (hν), respectively. The respective values for the powders were obtained by extrapolating to (hνα)1/2 = 0 for the indirect transition and (hνα)2 = 0 for direct transition as shown in Figure 3. In Table 1 are shown the obtained results and a comparison with the results reported by El-Diasty et al. [29] and El Ghandoor et al. [37]. The difference may be attributed to the quantum size effect, with the size of the particles affecting their band gap energy [29]. The energy band gap of nanomaterials is inversely proportional to their sizes, therefor it’s easy to say that the energy band gaps of nanomaterials can be controlled by controlling their sizes [37].

3.1.3 ATR FT-IR spectroscopy Figure 4a shows the FT-IR spectrum of the synthesized MNP, which presents two peaks at 630 cm -1 and 598 cm-1 corresponding to the bonding of Fe-O, and two more bands in the wavenumber of 3363 cm-1 and 1633 cm-1 due to stretching and bending of H2O molecules respectively, these results are consistent with the reported by Ebrahiminezhad et al. (2012) [38]. The rest of the bands are due to iron oxides formed on the surface of the nanoparticles [39]; such as lepidocrocite (751, 795, 892, 1022, 1157), akaganeite (900), goethite (788, 1018), schwertmannite (1040-1070, 1110-1140) [40,41].

3.1.4 Raman spectroscopy Magnetite is a poor Raman scatterer, particularly at the low laser powers which are required to keep the sample from undergoing laser irradiation induced phase transformation [42]. Very low powers of illumination are needed to avoid the transformation of magnetite-like into the hematite-like structure. With the illumination power below 0.2 mW, the typical Raman signatures of the magnetite structure (strong ca. 660–700 cm-1 peak and weak bumps at ca. 344 and 540 cm-1) are obtained. The second order

feature (related to the resonance character of the Raman spectrum under green excitation) peaks at ∼1400 cm-1 as expected for pure magnetite-like materials [43]. Figure 4b shows the Raman spectra and the deconvolution of the peaks corresponding to magnetite which are 344 cm-1, ~ 540 cm-1, between 660 y 700 cm-1 and ~ 1400 cm-1..

3.1.5 X ray diffraction analysis Figure 5a presents the diffraction peaks which could be indexed to an inverse spinel structure of Fe 3O4 according to the literature (JCPDS Card No. 19-0629). The strong and sharp peaks suggested that MNP are highly crystalline. However, the broadening in the reflection peaks was due to the particles size at nano domain [44]. Table 2 shows the resemblance in the intensities and angles between the synthesized nanoparticles and the JCDPS Card 19-0629. The application of the Scherrer’s equation to the reflection 311 with 2θ ~ 35.58° of the MNP indicated that the crystallite size was 16 nm [45].

3.1.6 Size and shape of nanoparticles From the results of XRD (Fig. 5a) and SEM (Fig. 5c) the calculated crystallite and particle size were of 16 nm and 15.5 nm respectively, being in mutual agreement. The Dh obtained for our MNP by DLS was of 350 nm ± 83.10 and a polydispersity index (PDI) of 0.402 (Fig. 5b). Wang, Nan et al. (2010), reported that the magnetic nanoparticles size synthesized by the US-RP method was 16.5 nm, ~15 nm and 345.5 nm obtained from the analysis of XRD, SEM and DLS respectively [25]. The slight difference of the MNP size between the reference and the synthesized nanoparticles could be due to the use of a distinct ultrasonic bath. However, Dh corroborated the trend of aggregation of the MNP in solution which is explained up next.

3.1.7 Zeta potential measurement In aqueous systems, iron oxide particles are hydrated, and Fe–OH groups cover completely their surface. Hydrated iron oxides have amphoteric character [46]. The typical isoelectric point reported for magnetite

is pH 6.8 [47]. The pure oxide surface exhibits a positive charge at a pH lower than the PZC, while it has negative charge above it [48]. Figure 6 shows that the IEP of our MNP is consistent with the literature, using the same surfactant (TMAH) as previously published to be ranging between a pH of 6.5-7.3 [49,50]. Dispersion and the prevention of agglomeration is due to balancing three primary attractive forces: 1) magnetic attractive forces; 2) the gravitational force; and 3) London dispersion or Van der Waals forces [51]. In the absence of stabilizers, magnetite clusters aggregate and readily settle out of suspension. The rates of aggregation and sedimentation depends on particle concentration and aqueous conditions such as pH and ionic strength [52].

3.1.8 Magnetic properties Figure 7a shows the magnetization curve from the VSM obtained at 300 K corresponding to the MNP, presenting a magnetic saturation (MS) of 76.89 emu g-1 at the highest measured magnetic field, a coercivity (HC) and magnetic remanence of (MR) of practically 0 Oe and 0 emu g-1 respectively and no hysteresis cycle. Nanoparticles that have no hysteresis in the magnetization curve with both MR and HC being zero are superparamagnetic [53]. The lack of magnetization saturation at high fields is a well-known effect due to the small particle size and the high surface area, which lead to some spin canting. The Ms value is slightly lower than the reported value for bulk Fe3O4 (92 emu g−1 at room temperature) that can be assigned to surface effects, i.e., spin canting [54]. Figure 7b presents the MTGA of the synthesized MNP, the sharp drop in the mass (or force) detected around 568 °C is attributed to the Curie point of Fe3O4 [55]. Magnetite has a Curie temperature (TC) of 858 K (~584.85 °C) [56]. TC of magnetic nanostructures is complicated issue, by definition, the Curie point is a singularity, and it is therefore not possible to define a well-defined sharp TC in a nanoparticle [57]. When one or more dimensions in the system are extremely small (below 50 nm), the growth of the spin correlation length will be eventually limited by the smallest dimension and the system will display a reduced transition temperature (TC) following the known finitesize scaling effect [57,58].

3.2 Nitrobenzene photodegradation Photocatalytic properties of MNP were evaluated for NB (15 ppm) degradation. MNP concentration ranged from 50-150 ppm. The percentages of degradation of NB (Fig.8a) were 45, 63 and 42, for 50, 100 and 150 ppm of MNP respectively. The increase of photocatalyst concentration did not improve the degradation of NB, however the turbidity, light scattering and agglomeration of solid particles increased. As a result, screening effect due to particle excess becomes important which masks some parts of the photosensitive surface. Hence, the penetration depth of the photons decreases and less catalyst particles can be activated. Consequently, the production of hydroxyl radicals and the degradation efficiency decrease [59]. In addition, this decrease could also be because of the scavenging of reactive radicals by concentrated iron species (Eq. 2) which reduces the amount of •OH and causes the degradation efficiency to drop [60]. 𝐹𝑒 2+ + 𝑂𝐻 • → 𝐹𝑒 3+ + 𝑂𝐻 −

(2)

The solution pH, PZC and pollutant nature can also influence the distribution of the pollutant molecules. NB has a pKa value of 3.98 where the anionic form dominates upon increasing the solution pH and consequently, its molecules can be more easily attracted to the catalyst surface at pH below its PZC [61]. In this sense expecting a greater % of degradation below the PZC of the MNP and with the NB in its anionic form. Figure 8b illustrates NB removal in percentage varying the initial pH of the solution. For the pH of 2 (), 7 () and 10 () the degradation (%) observed was 73, 63 and 69 respectively. Additionally, the final pH of the reactions was measured to be ~3.8, ~7 and ~8 for the acid, neutral and basic respectively. Magnetite nanoparticles are amphoteric molecules that have better dispersion in aqueous mediums above and below the pHpzc (6.5 - 7.3) preventing agglomeration as discussed earlier. MNP tend to aggregate near their pHpzc, which decreases the available active surface sites [59], explaining the less effective degradation at neutral pH and increasing the degradation percentage in the acid and basic

solution. To ensure a better dispersion taking place in the acid with respect to the basic solution the time required to remove the nanoparticles from solution with an external magnetic field was measured, determining no appreciable difference in any of the two solutions (data not shown), suggesting that the role of dispersion in the improvement of the photocatalytic properties of MNP in acidic condition may not be significant. However, the MNP possess superparamagnetic characteristics meaning that the nanoparticles have a single domain, as consequence each individual crystallite have its own magnetic moment [62]. Magnetic field effects (MFE) research on heterogeneous reaction systems, photocatalysis in particular, is rather limited, where there is still debate on the reproducibility of the MFE as well as elucidating the mechanism [63]. Nevertheless, it has been predicted that the behavior of electron/hole and radicals can be affected by the Lorentz force in the magnetic field, which can influence the performance of a photocatalytic process. The investigation of magnetochemistry proved that the magnetic field has an effect on free radical reactions, and influence the intermediates and the product distribution of the reactions [64]. In addition, Xie et al. (2013) proposed for the first time the possible mechanism of auto-enhance photocatalytic activity by a magnetic field effect originated by the photocatalyst itself [65], and Feng et al. (2019) reported that an enhanced photocatalytic activity of BiOI/ MnxZn1-xFe2O4 might be owing to the magnetic field effect of magnetic photocatalyst and the enhancement of light absorption ability [66]. Being possible that the difference in degradation between the acid and basic solution is originating from a MFE and/or the intermediate distribution of the reactions. On the other hand, the activity of the catalyst surface is increased in an acidic pH by increasing the yielding rate of hydroxyl radicals due to that these are more easily formed at low pH [6]. Additionally, at a low pH the surface attraction between the pollutant and the nanoparticles is stronger, making the hydroxyl radicals more efficient, and possibly having a MFE role in this increase. This explains the photocatalytic activity of the Fe3O4 MNP at different pH values evaluated.

Due to the absorption at near the visible light range between 300-517 nm (4.12-2.42eV), we tested the MNP photocatalytic activity in presence of a visible light source of 575 nm (2.16 eV) at the condition where more NB was removed. At these conditions a removal of 26 % was reached which is less than all the reactions in presence of 254 nm UV light. This as consequence of the MNP lower extinction coefficient at 575 nm, since a photocatalysts with higher extinction coefficients require lower catalyst concentrations [67]. In Figure 8c are showed the results of specific energy consumption. The lowest energy consumption to degrade 1 kg of NB was reached at the pH of 2 (), with a value of 93,344.03 KW∙h/kg of NB followed by the pH 7 (),10 () and 2 with 575 nm lamps () with the values of 107,928.68, 99,451.71 and 260,858.49 KW∙h/kg of NB respectively. Due to that as discussed above at a lower pH the degradation is more efficient. Since the same light source was used, we compared the results reported by Li et al. (2011) were a photocatalyst TiO2/SiO2/NiFe2O4 (1000 ppm) was used reaching a 100% degradation of 50 ppm of NB in 5 hours using a 253.7 nm wavelength [68]. Using our MNP a degradation of 73 % of NB at 2 hours was reached with 100 ppm and pH 2, having an effective degradation in less reaction time. In addition the use of MnFe2O4 (200 ppm), MgFe2O4 (200 ppm) [69] and ZnFe2O4 [70] ferrite photocatalyst in the literature for the degradation of NB obtained a degradation of 90%, 85% and 73% respectively in 4 hours, while our synthesized ferrite (FeOFe2O3) degraded 73 % in 2 hours, the use of less of our photocatalyst made the reaction more efficient. These results show a new effective facile synthesis of a photocatalyst that can be magnetically separated from the reaction solution for NB photodegradation having better or similar % removal then most of the photocatalysis reactions performed at alike conditions (Table 3). The GC-MS study (Fig. 9) showed that the degradation products obtained were 2-NPh, 3-NPh and 4-NPh. At the reaction of pH 7 all three by-products were detected, whereas at pH 10 and 2 only two (Table 4)

were determined. The detected by-products in the system were all hydroxylated products. These results are consistent to previous reported photocatalysis degradations of NB [71]. Since their concentrations were quite low this indicated that they were only intermediate products. The NB oxidation pathway consists in that, hydroxyl radicals could be added onto the aromatic ring of NB and form hydroxy cyclohexadienyl radicals, which could undergo many different reactions soon afterward. When the hydroxyl radical was added in the ortho, meta and para position of the NB ring, oxidation or disproportionation of the hydroxy cyclohexadienyl radicals could yield 2-NPh, 3-NPh and 4-NPh, respectively. However, we cannot exclude the possible formation of other unidentified products in the system because these products detected could be further oxidized and finally mineralized by hydroxyl radicals as previous studies reported [3], being the main mechanism proposed an oxidation process which leads to mineralization. Additionally, degradation of NB by an iron catalyst (Fe3O4 or Fe0) without the most common intermediate product aniline (Table 4) was a novel finding as well as its mineralization. This last information provides important data, since aniline is detrimental to public health because of its carcinogenicity and raises ecosystem imbalance due to its high toxicity to aquatic life. Considering the persistent nature of aniline and its wide spreading globally, wastewater containing aniline requires proper treatment prior to discharge [72].

4 Conclusions: In this work, the synthesized magnetite nanoparticles by an ultrasonic-assisted reverse co-precipitation method show a crystallite size of 16 nm, amphoteric nature and presented a superparamagnetic behavior facilitating separation from a solution in less than 1 min by using only a magnet (magnetic separation) and possibly generating MFE in photocatalysis. These nanoparticles also showed a photocatalytic activity under a light source of 254 nm for photodegradation and mineralization of NB, as well as a potential band gap for a photocatalytic activity under visible light below 517 nm. At pH of 2 the photocatalytic activity

was the highest due to higher rate of hydroxyl radicals reaching a removal percentage of 73 % in 2 hours. The photodegradation reactions exhibited an oxidation process and mineralization corroborated by GCMS by detecting only intermediate products such as 2-NPh,3-NPh and 4-NPh. In summary, this is a potential photocatalyst due to the use of common reactants, facile synthesis, superparamagnetic behavior, photocatalytic oxidation capacity and mineralization of organic compounds. As well as possibly MFE and a photocatalytic surface activation potential in both UV and visible light.

Acknowledgements: We would like to acknowledge Adriana Tejeda (IIM-UNAM) for the support in carrying out X-ray measurements; Omar Novelo (IIM-UNAM) for the support in the scanning electron microscopy characterization, Jonathan Zamora and I. Betancourt (IIM-UNAM) for their supports in the magnetic thermogravimetric analysis and vibrating sample magnetometer measurements, María Elena Villafuerte Castrejón (IIM-UNAM) and Gerardo Cesar Diaz Trujillo (UABC) for an investigation residency in the Investigation Institute of Materials (IIM) and Arturo Estolano (UABC) for the support in the gas chromatography mass spectrum analysis. This work was supported by the National Council of Science and Technology (CONACYT) [CB2015-253128]. Declarations of interest: none

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Figure Caption Figure 1: Magnetic separation of the generated black product suspended in water without (a) and with (b) a magnetic field. UV-Vis spectra (c) of the utilized reagents: FeSO4 (curve a), FeCl3 (curve b) and Fe3O4 MNP (curve c). Figure 2: Comparison of UV-Vis spectra of Fe3O4 magnetic nanoparticles (MNP) dispersed in different mediums: Ethylene glycol (), n-Propanol (), TMAH 9.3 mg mL-1 & 1 mM NaCl () and distilled water (). Figure 3: Determination of the optical energy band gap for (a) indirect and (b) direct transition of MNP (Fe3O4 nanoparticles) dispersed in water (), EG (), TMAH (), n-Propanol (). The inserts in both graphs correspond to the band gap calculations observed at energies below 4.5 eV. Figure 4: Spectra of synthesized MNP: (a) ATR FT-IR and (b) Raman spectrum. Figure 5: Phase identification and size of MNP: (a) X Ray diffraction diffractogram, (b) hydrodynamic diameter (Dh) distribution by DLS technique and (c) scanning electron microscope (SEM) image with particle size distribution histogram. Figure 6: Isoelectric point (IEP) localization of MNP (Fe3O4) through zeta potential measurements, at different pH values in a TMAH 9.3 mg mL-1 & 1 mM NaCl solution. Figure 7: Magnetization curve (a) obtained by vibrating sample magnetometer (VSM) at 300 K and determination of the Curie point by magnetic thermogravimetric analysis (b) of MNP (Fe3O4). Figure 8: Photocatalysis of nitrobenzene at 15 ppm and 25 ⁰C and 254 nm UV lamps unless specified at (a) varying the concentration of the MNP (Fe3O4) photocatalyst from 50 ppm (), 100 ppm () to 150 ppm () at pH 7, (b) varying the initial pH from 2 (), 7 () to 10 () at 100 ppm of MNP and at pH 2 with 570 nm light (), (c) the specific energy consumption of the reactions from 0 to 2 hours.

Figure 9: Mass spectroscopy (a) detected molecules from the nitrobenzene (NB) degradation were NB, 2-nitrophenol (2-NPh), 3-nitrophenol (3-NPh) and 4-nitrophenol (4-NPh). Gas chromatography (b) of the nitrobenzene photodegradation extracts from the reactions at pH 7, pH 10 and pH 2.

Table 1: Calculated direct and indirect band gaps (eV) for MNP in different mediums Band Gap (eV) MNP Direct

Indirect

EG

2.47

5.27

2.42

5.16

n-Propanol

2.88

5.32

2.82

5.24

Water

3.8

5.4

3.78

TMAH

4.12

5.57

3.86

5.46

Reference

2.87 [37]

5.5 [29]

1.92 [37]

5 [29]

5.32

Note : EG= Ethylen glycol, TMAH= Tetramethylammonium hidroxide

Table 2: Comparison of JCPDS 00-019-0629 and synthesized F3O4 magnetic nanoparticles (MNP)

Formula

JCPDS

2θ JCPDS

2θ Sample

d (Å) JCPDS

d (Å) Sample

I JCPDS

I Sample

H

K

L

Fe3O4

00-019-0629

30.05

30.14

2.97

2.96

30

32.55

2

2

0

35.42

35.58

2.53

2.52

100

100

3

1

1

43.05

43.28

2.10

2.09

20

28.43

4

0

0

53.40

53.58

1.71

1.71

10

15.93

4

2

2

56.94

57.26

1.62

1.61

30

30.9

5

1

1

62.52

62.8

1.48

1.48

40

40.25

4

4

0

Table 3: Comparison of different photocatalyst for the nitrobenzene (NB) degradation Photocatalyst

Concentration

Radiation

Solution

NB Conc.

Degradation

Reference

AuNPs/HPW/TiO2-NTs

200 mg/L

Visible Light

Water

40 ppm

4 h 0.0078 Kapp

[66]

1% TiO2/Li, Pr: Y2SiO5

1.5 g/L

TFL

Waste Water

5 ppm

4 h 97.08%

[67]

TiO2-SWCNT

0.1 g

365

Water

50 ppm

4 h 100%

[68]

4 plates

320-500

Water

2.51 x 10-4 M

4 h 88.45%

[69]

TiO2

0.1 M

250-400

Water

50 ppm

0.5 h 42%

TiO2-Arginina*

0.1 M,3x10-2 M *

250-400

Water

50 ppm

0.5 h 62 %

50ppm

0.5 h 84 %

Glass Plate (0.5 wt.%)Ti1-xFexO2-δ

TiO2-Arginina*

0.1 M,3x10-2 M *

250-400

Water+ Ph 50 ppm

SrFeO3-δ

1 g/L

MVL

Water

50 ppm

6 h 99%

MnFe2O4

200 ppm

280-450 nm

Water

50 ppm

4 h ~ 90%

MgFe2O4

200 ppm

280-450 nm

Water

50 ppm

4 h ~ 85%

TiO2/SiO2/NiFe2O4

1000 ppm

253.7 nm

Water

50 ppm

5 h 100%

50 ppm

4 h 77 %

50 ppm

4 h 73 %

50 ppm

4 h 70 %

[10]

[4] [64]

TiO2 ZnFe2O4

UV

Water

ZnFe2O4-TiO2 1% graphitic carbon TiO2

200 ppm

MVL

Water

50 ppm

4 h 96 %

(1 wt. % Co)–TiO2

100 mg/L

320-500 nm

Water

2.52 x 10-4 M

4 h 81.03%

(1 wt. % NI)–TiO2

100 mg/L

320-500 nm

Water

2.52 x 10-4 M

4 h 78.49%

(0.5 wt. % Fe)–TiO2

250 mg/L

320-500 nm

Water

0.37 x 10-4 M

4 h 99.73%

Fe3O4 nanoparticles

100 mg/L

253.7 nm

Water pH 2

15 ppm

2 h 73.13%

[63]

[65]

[70]

[71]

This Work

Note: Au NPs= gold nanoparticles, TiO2-NTs= TiO2 nanotubes, HPW= H3PW12O40, TFL=Triphosphor Light, SWCNT= single walled carbon nanotubes, MVL=Mercury Vapor Lamp, Ph=phenol, * corresponds to Arginine. All the reaction where it is not specified the pH of the solution was near neutral.

Table 4: Comparison of nitrobenzene (NB) reduction using iron as a catalyst Catalyst

Concentration

NB Conc.

Conditions

Degradation

Product

Reference

Fe0

5g

1.63 mM

Water pH 3

45 min 1.23 mM

AN 0.71 mM

[1]

CMC stabilized Fe0 nanoparticles

0.05 g/L

49 ppm

Water

30 min 99%

AN 98%

[28]

Fe3O4

1 g/L

40 µM

Water pH 7.2 MOPS Buffer

60 min 99%

AN 99%

[74]

Fe0 /Fe3O4/FeCl2

2.0 g Fe/L

200 ppm

Water pH 3-9

30 min. >95 %

AN >93%

[75]

Water pH 7

120 min. 63.25%

2-NPh, 3-NPh,4NPh (8.06%)

Water pH 10

120 min. 68.64%

3-NPh, 4-NPh (<3.01%)

Water pH 2

120 min. 73.13%

2-NPh, 4-NPh (3.01%)

Fe3O4 nanoparticles

100 mg/L

15 ppm

This Work

Note: CMC:carboximethylcellulose, AN= aniline, 2-NPh= 2-nitrophenol, 3-NPh= 3-nitophenol, 4-NPh= 4-nitophenol