Enhanced photocatalytic activity of magnetic BaFe12O19 nano

Subscriber access provided by Kaohsiung Medical University

Kinetics, Catalysis, and Reaction Engineering

Enhanced photocatalytic activity of magnetic BaFe12O19 nano-platelets than TiO2 with emphasis on reaction kinetics, mechanism and reusability Sandesh S Raut, Santhosh Kumar Adpa, Amruta Jambhale, Ashutosh C. Abhyankar, and Prashant Shripad Kulkarni Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02859 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Enhanced photocatalytic activity of magnetic BaFe12O19 nano-platelets than TiO2 with emphasis on reaction kinetics, mechanism and reusability Sandesh S. Raut,† Santhosh Kumar Adpa,‡ Amruta Jambhale,‡ Ashutosh C. Abhyankar,‡ Prashant S. Kulkarni*,† †Energy

and Environment Laboratory, Department of Applied Chemistry, Defence Institute of Advanced Technology (DU), Ministry of Defence, Pune - 411 025, India.

‡Magnetic

Materials Laboratory, Department of Materials Engineering, Defence Institute of Advanced Technology (DU), Ministry of Defence, Pune - 411 025, India.

ABSTRACT: Magnetically separable, barium hexaferrite (BaFe12O19) nano-platelets were synthesized at various temperatures by cost-effective, molten salt technique and its photocatalytic activity was compared with commercially available TiO2. BaFe12O19 nanostructures were characterized by using UV-Vis-DRS, FE-SEM, XRD, BET and Raman and further, subjected to photocatalytic degradation of an organic pollutant, hexahydro-1,3,5trinitro-1,3,5 triazine (RDX) under UV and visible light. The reaction parameters, degradation kinetics and mechanism were thoroughly studied and optimum reaction conditions were evaluated. The degradation products were analyzed by HPLC, LCMS and TOC. The BaFe12O19-800 °C nano-platelets (0.6 g.L-1) with UV-Vis lamp irradiation were efficient and economical to degrade 40 mg.L-1 of RDX below the discharge limits (˂ 0.035 mg.L-1) in 300 min whereas TiO2 showed 4 mg.L-1 of unreacted RDX at similar conditions.

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

Kinetic rate of BaFe12O19 and TiO2 were observed to be 0.017 and 0.003 min-1, respectively. Magnetic separation of BaFe12O19 for recovery and recycling is also reported.

KEYWORDS: Photocatalysis, organic pollutant, BaFe12O19 nano-platelets, magnetically separable, recycling. 1.

INTRODUCTION

Photocatalysis is proven to be the most promising technology of the 21st century for sustainable chemical processes. It can contribute to solve the chemical, energy and environmental issues and sustainable production of raw materials, in the near future. It has a good chemistry with sunlight, which is an intrinsic infinite source of environmental energy that can be used to promote chemical transformations which are generally performed with expensive chemical treatments at elevated temperatures. Semiconducting metal oxide nanostructures such as titanium dioxide (TiO2), zinc oxide (ZnO) and magnetite (Fe3O4) based nanomaterials are well-known photocatalysts from many decades and are used for total oxidation of hydrocarbons to clean the environment.1-3 Photocatalytic degradation4,5 of organic contaminants6-9 is most commonly done using TiO2 as it is relatively inexpensive and active in high energy UV light, due to its wide band gap (Eg = 3.2 eV).10,11 Although TiO2 is cheap, utilization of high energy UV light can be expensive and is not easily separable from aqueous media.5,12-15 To address these issues, several researchers have focused on the fabrication of magnetic semiconducting oxide nanophotocatalysts.16 In particular, magnetic iron oxide-based nanostructures such as magnetite (Fe3O4), ferrites MFe2O4 (M = Co, Ni, Zn) and hexaferrite MFe12O19 (M = Sr, Ba) with welldefined structures have been studied for removal of solutes from wastewater.14,17 Xie et al. synthesized magnetic composite photocatalyst Bi2O3-SrFe12O19 with magnetic saturation value (MS) of 25.42 emu.g-1 and Eg value of 2.72 eV and tested for the photo-degradation 2 ACS Paragon Plus Environment

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


methylene blue dye.18 Haw et al. investigated the photocatalytic activity of magnetically active CoFe2O4 core/shell particles (MS = 0.2111 emu.g-1, Eg = 4 eV) for the decomposition of an organic dye.19 Kritapas et al. developed magnetically active TiO2/SiO2/Mn-Zn ferrite (MS = 20 emu.g-1) composites for photo-decomposition of methylene blue dye.20 Among the family of ferrites, BaFe12O19 is a hexagonal (M-type magneto-plumbite, space group P63/mmc) ferromagnetic ceramic with easy magnetization along c-axis. It is used in many interesting magnetic device applications21-28 due to its low production cost, high chemical stability, corrosion resistance, high resistivity, superior anti-erosion properties and mechanical hardness. However, there are no studies on the photocatalytic degradation of toxic organic pollutants with hexagonal barium ferrite nano-platelets. A photocatalyst like BaFe12O19 is easily separable and reusable due to its superior magnetic and optical properties. RDX (hexahydro-1,3,5-trinitro-1,3,5 triazine) is commonly used for the fabrication of space and defence related devices such as projectiles and munitions.4,29,30 During the largescale production of RDX, wastewater generated at the munitions facilities mainly contains the nitroaromatics and nitramines.4,31,32 These compounds are very toxic to the aquatic environment and also, produce mutagenic effects on a living organism through the food chain.4,33,34 As a consequence, the USEPA and WHO have put stringent standards for the presence of RDX in aqueous stream (0.035 mg.L-1) and drinking water (0.002 mg.L-1). The reported studies on degradation of RDX have used homogeneous and heterogeneous methods, but seem to be less practical. It is because homogeneous methods require additional costly chemical reagents which again get released into wastewater. Heterogeneous catalysis is a best alternative and researchers have studied catalysts like TiO2, ZnO and WO435 however, a magnetic catalyst having various advantages was never explored for degradation of RDX. The present study, first time reveals the photocatalytic degradation of an organic pollutant, RDX with magnetically separable BaFe12O19. Herein, we synthesized BaFe12O19 nano-



Page 4 of 31

platelets (MS = 58 emu.g-1, Eg~ 2.1eV) at various temperatures by cost-effective, molten salt method26 and its photocatalytic activity was compared with commercially available TiO2-P25. The study reveals the impact of various parameters such as concentration of the BaFe12O19, the effect of run time, pH and recycling of catalyst on the degradation products of RDX. The results presented in this study highlights that BaFe12O19 nano-platelets can be used as an effective catalyst for degradation of a selected test compound and is reusable due to easy magnetic separation.

2.

EXPERIMENTAL SECTION

2.1 Chemicals Iron nitrate (Fe(NO3)3.9H2O, 99.99%), barium nitrate (Ba (NO3)2, 99.9%) and citric acid (C6H8O7, 99.9%) were purchased from Alfa-Aesar, India whereas potassium chloride (KCl), sodium chloride (NaCl) salts were purchased from Merck. Pure RDX was obtained from the ordnance factory, Khadki, Pune, India. The TiO2 powder used as the photocatalyst was commercially available Degussa P-25 (Degussa Chemical Co., Germany). 2.2 Synthesis of hexagonal BaFe12O19nano-platelets The BaFe12O19 was synthesized by a molten salt method.24,27 In a typical synthesis, a gel precursor was prepared using Ba(NO3)2, Fe(NO3)3.9H2O and C6H8O7 by dissolving in deionized (DI) water in a molar ratio of 1:12:13. The aqueous solution was heated at 100 °C with continuous stirring until a viscous brown gel was formed. The obtained viscous brown gel was sintered at 400 °C for the purpose of decomposition of an organic precursor. The resulted powder obtained from the previous step was mixed with potassium chloride (KCl) and sodium chloride (NaCl) in a weight ratio of 1:1:1 and sintered at different temperatures ranging from 800 to 1000 °C for 8 h to get different sizes of single crystalline phase of 4 ACS Paragon Plus Environment

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BaFe12O19 nano-discs. The obtained powder was washed with DI water and ethanol for 2 to 3 times to remove the traces of salt and impurities. 2.3 Solid phase characterization The crystallographic studies were carried out by x-ray diffraction, using Bruker AXS, Germany (Model D8 advanced) diffractometer in the scanning range 2θ from 15° to 80° with Cu-Kα radiations (λ=1.54 Å). Raman spectra of all the samples were recorded by a LabRAM HR800 laser Raman spectroscopy using 514.5 nm argon ion laser with laser power less than 10 mW on the surface of the sample. The morphology and microstructure of the materials were studied with the help of Field Emission Scanning Electron Microscopy (FE-SEM; SIGMA, Carl Zeiss). The chemical composition and elemental mapping of samples were analyzed using energy dispersive spectroscopy (EDAX) (EDS, INCA OXFORD). Optical properties were measured by using UV-Vis-DRS spectrophotometer (Carry-5000-1.12) in the range of 200 to 800 nm. Specific surface area (SBET) of catalyst was calculated from the Brunauer-Emmett-Teller (BET) analyser using Microtrac, SAA BET Single-Point, Nikkiso, USA, instrument. 2.4 Batch experiments of photocatalysis Photodegradation experiments were performed in the photocatalytic reactor (Trident Labortek, Thane, India) system. It consisted of a cylindrical Pyrex glass container with 1L capacity, 10 cm diameter, 30 cm height and UV-Vis lamp as a source of light. Photocatalytic reactions were performed by UV (Model/part KTL-UV-250, 250 W, wavelength 254 nm, quartz mercury vapour lamp, 61 cm PTFE covered lead wires, 121 mm arc length, 15 cm cord connected to power supply, life 1000 h) and visible light (Model/part KTL-Vis-300, 300 W, wavelength 350-1100 nm, halogen- tungsten quartz lamp, life 2000 h) lamps, separately; manufactured and supplied by Lelesil, Trident Labortek. The RDX was dried for 18 h in a 5 ACS Paragon Plus Environment


Page 6 of 31

desiccator before being dissolved in the deionized water. All the photocatalytic experiments were performed by dissolving 40 mg.L-1 of RDX in deionized water. 500 ml aqueous solution of RDX was subjected to the photocatalytic reactor with catalysts having different sintered temperatures and concentrations of BaFe12O19. Cooled water was circulated in an outer jacket by the chilling unit to maintain the constant temperature of 25 °C of the reaction mixture. The reaction was run with continuous stirring at 450 rpm. Another batch with TiO2-P25 was also run at similar experimental conditions to compare the activity of BaFe12O19. pH of the reaction mixture was 7.4 at zero time interval. Photolysis reaction of RDX was performed without addition of catalyst. Samples were periodically withdrawn for HPLC-MS and total organic carbon (TOC) analysis (Shimadzu, Japan). 2.5 Analytical methods of photocatalysis The initial and degraded aqueous solution of RDX at different time interval was withdrawn, catalyst separated, filtered and used for injections in HPLC (Hitachi, Germany). The alltimaC18 column (Reserve Phase, particle size 5 μm), pump L-2130 and diode Array Detector L2455 (UV, wavelength 215 nm) were used. Samples were injected with a manual injector having 20 µl loop. A well known HPLC analysis (USEPA 8330) method was followed for the analysis of RDX samples.33 Acetonitrile and water in the proportion of 80:20 was used as a mobile phase at a flow rate of 0.5 ml min-1. The stabilized pressure of the column was 24.3 MPa. Further, details of the HPLC method are given in Table S1. Mass fragments were analyzed by the mass analyzer (Advion USA, model NY 14850). The TOC of degradation of RDX was analyzed by using TOC analyzer. 3.

RESULTS AND DISCUSSION

3.1 Structural analysis


Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The structure of BaFe12O19 was examined by X-ray diffraction and Raman spectroscopy at room temperature. The XRD patterns of the samples formed under different temperatures are shown in Figure 1. The peaks of the prepared BaFe12O19 at 700 °C correspond to highly crystalline BaFe12O19, but some peaks of the hematite (Fe2O3) phase can be seen in Figure 1. The main peak of hematite appears at 33.6 and the relative intensity of hematite peaks decreases with increasing reaction temperature. All the diffraction peaks observed at 800, 900 and 1000 oC confirmed the formation of single phase BaFe12O19 (JPCDS 039-1433, a = 3.253 Å, c = 21.209 Å). The obtained BaFe12O19 samples were of hexagonal phase with space group P63/mmc. No impurity peaks were detected, which indicated that single phase BaFe12O19 had formed at 800 °C. With the increase of annealing temperature, the primary peaks became stronger and sharper indicating an enhancement of crystalline nature of BaFe12O19 nanostructures by the supply of sufficient thermal energy. Further, the full-width at half-maximum (FWHM) of the planes is found to decrease with the increase in annealing temperature. The grain size of the BaFe12O19 nanostructures is increased with increase in temperature and reaches highest value for the BaFe12O19 annealed at 1000 °C (Table S2). The observed 2θ positions corresponding to the (110) (107) and (114) planes found to be decreased with increase in annealing temperature. At 700 oC, Raman peaks attributing to α-Fe2O3 were observed whereas, above 800 °C no peaks of α-Fe2O3 were seen which confirms the single phase and purity of BaFe12O19 (Figure 2). The peaks appeared at and above 800 °C synthesized samples show the characteristics of M-type hexaferrite. According to the group theory, there are 42 Raman-active modes namely (11 A1g + 14 E1g + 17 E2g), for the D6h symmetry possessed by M-type hexaferrite. The peaks observed at 475 and 621 cm-1 are assigned to A1g vibrations of Fe-O bonds at the octahedral 2a, 12k, and 4f2 sites, respectively. The peak appeared at 684 and 713 cm-1 are due to A1g 7 ACS Paragon Plus Environment


Page 8 of 31

vibrations of bipyramidal 2b and tetrahedral 4f1 sites, respectively. While peak appeared at 420 cm-1 is a result of octahedral 12k dominated. The peaks at 289 and 529 cm-1 are as a result of E1g vibration whereas peak at 335 cm-1 is due to E2g vibration at the octahedral 12k site.36,37 The observed Raman spectra has been indexed and the comparative statement of observed vibrational modes is listed in Table S3. 3.2 Morphological analysis Surface morphology of the BaFe12O19 is investigated using FE-SEM and the micrographs are shown in Figure S1(a-c). The homogeneous flake-like morphology of the sample annealed at 800 °C, with hexagonal-shaped crystals, is revealed in Figure S1(a). At 800 °C, the size of BaFe12O19 is about 200 to 400 nm while the thickness of those flakes has a narrow distribution, in the range of 50-80 nm. The influence of the annealing at 900 °C is not obvious as shown in Figure S1(b). For the sample annealed at 1000 °C, the morphology changes remarkably with non-uniform bulk granular like structures, as shown in Figure S1(c). This shows that the higher annealing temperature will deteriorate the hexagonal growth of BaFe12O19. In Figure S1(d), the EDAX spectrum reveals the presence of Ba, Fe, and O elements in the BaFe12O19 materials at the correct proportion without any impurities. The extracted EDAX quantitative data are given in the inset. The surface area of BaFe12O19-800 °C as well as TiO2 particles was calculated from BET surface area analyzer which showed surface areas of 13.85 and 10.23 m2g-1, respectively. 3.3 Optical analysis The optical and band gap properties of BaFe12O19 synthesized at different temperatures are evaluated from the diffused reflectance spectroscopy and depicted in Figure 3(a-b) and S2. The diffused reflectance spectra were analysed using Kubelka-Munk equation


Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝑭(𝑹) =

(𝟏 ― 𝐑)𝟐

(1)

𝟐𝑹

where, R is the absolute reflectance of the sample and F(R) is Kubelka-Munk function. The optical band gap (𝑬𝒈) of the BaFe12O19 nanostructures was calculated by using Tauc’s model.38 The optical band gap (𝐸𝑔) of the BaFe12O19 nanostructures is determined by extrapolating the linear portion of (𝐹(𝑅)ℎ𝜈)2 to the photon energy ℎ𝜈 axis. From Figure 3(b), the optical band gap (𝐸𝑔) of samples was found to be 2.1, 2.09 and 2.03 eV for 800, 900 and 1000 °C synthesized BaFe12O19 nanostructures, respectively which indicate slight decrement in E g value with temperature. The absorbance spectra of BaFe12O19-800 °C were recorded that showed λmax value at 571 nm (Figure S2) whereas TiO2 showed 370 nm as per the data provided by supplier. The narrowing of band gap energy is due to the enhanced crystallization and formation of new defect centres. Thus, these materials, indeed, can absorb low energy light (visible light). As the band gap energies of the BaFe12O19 samples are greater than the required theoretical energy for water splitting (1.23 eV), they are also suitable candidates for the role of a UV-Vis light photocatalysis. Vibrating Sample Magnetometer (VSM) was used to determine magnetic saturation (Ms) of BaFe12O19-800 °C, and the observed value was found to be 59 emu.g-1 (Figure S3). It is important to recover the photocatalyst after the degradation studies for reuse. 3.4 Photocatalytic degradation of RDX using BaFe12O19 Photocatalytic degradation of 500 ml (40 mg.L-1) aqueous solution of RDX was carried out with BaFe12O19. The catalyst loading was 1 g.L-1 for each sintered BaFe12O19 under continuous UV light irradiation. It was found that as sintering temperature of BaFe12O19 increases above 800 °C, photocatalytic activity goes on decreasing as BaFe12O19 (800 °C) > BaFe12O19 (900 °C) > BaFe12O19 (1000 °C). BaFe12O19-800 °C degraded 98.1% RDX within 9 ACS Paragon Plus Environment


Page 10 of 31

240 min, while BaFe12O19-900 °C and 1000 °C shown degradation of 90 and 63% respectively within same time. After 240 min, un-degraded RDX was measured by HPLC and it was found to be 0.76 (BaFe12O19-800), 4 (BaFe12O19-900) and 14.8 mg.L-1 (BaFe12O191000) as shown in Figure 4. As synthesized BaFe12O19 at 800 °C showed better photocatalytic properties than other prepared nanostructures. Enhanced catalytic activity is due to high surface area and low band gap of 2.1 eV of BaFe12O19 (800 °C) nano-platelets. BaFe12O19 crystallizes in a hexagonal magneto plumbite structure with 64 ions per unit cell on 11 different symmetry sites (Figure S4). The 24 Fe3+ atoms are distributed over five distinct sites: three octahedral sites (12k, 2a, and 4f2), one tetrahedral site (4f1), and one bipyramidal site (2b). The hexagonal symmetry of the crystal structure generates large magneto crystalline anisotropy along C axis which may also be reflected in increased catalytic activity.28 Further, TOC was used to find out remained C-C or C-N intermediate linkages. The initial RDX concentration shown by TOC was 10.52 mg.L-1 and it goes on decreasing with reaction time. Highest active photocatalyst, BaFe12O19-800°C shows 83.27% carbon loss after 240 min. It means that though there was nearly complete disappearance of RDX within 240 min (as shown by HPLC), but all the organic content was not fully degraded (Table. 1). It’s due to attack of OH radicals on the bonds of the molecule to fragment it into another species and this fragmentation continuously goes on to yield different, smaller derivatives. The initial pH (7.4) of the aqueous reaction mixture after 30 min of irradiation became 4.9 and goes on decreasing with time. Throughout the reaction, pH remained acidic from 7.4 to 3.1 which indicate that acidic substances (HCOOH) were produced during the photocatalytic degradation of RDX with BaFe12O19. It is well known that the photocatalytic degradation of RDX follows pseudo-first order rate equation.39 10 ACS Paragon Plus Environment

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝒅𝑪 𝒅𝒕

= ―𝒌𝑪 or 𝒍𝒏

( ) = ―𝒌𝒕 𝑪

(2)

𝑪𝒐

where, Co and C are the concentration of RDX before and after the reaction, k is the observed pseudo-first order rate reaction. The effect of type of catalyst on the photocatalytic degradation of RDX is fitted with equation 2 and is shown in Figure S5. When each catalyst concentration was taken 1 g.L-1, the kinetic rate constants were found to be 0.017, 0.009 and 0.006 min-1 for BaFe12O19-800 °C, 900 °C and 1000 °C, respectively. Hence, the higher kinetic rate was observed for BaFe12O19-800 °C. The efficiency of the catalyst is determined by the total number of free carriers i.e. electron/hole, on the surface of the photocatalyst. BaFe12O19 at temperature 800 °C and 900 °C shows hexagonal nano-platelets morphology, whereas at 1000 °C BaFe12O19 get deformed from hexagonal shape. It means that the crystal structure also affects. Due to this condition, a number of free charges on the surface of catalyst largely reduces and deteriorates the photocatalytic activity. In the case of discs obtained at a temperature of 800 °C, the transportation length of electron/hole from crystal interface to the surface is short, which accelerates the migration rate of electron/hole to the surface of the disc in order to participate in the process and yield a better result. Thus, smaller the particle size with an accessible surface area of the photocatalyst as that of BaFe12O19-800 °C discs, lead to increasing rate of photo-degradation of organic pollutant although the band gap is 2.1 eV. 3.5 Effect of pH on photocatalytic degradation As photocatalytic degradation of RDX is performed in an aqueous medium, the study of pH is necessary because photocatalytic reactions are generally pH dependent. Due to the fragmentation of organic frames by ∙OH radicals, lot of ions and intermediate species get generated which alters the pH of reaction mixture. All the experimental conditions were kept constant with BaFe12O19-800 °C. Three reactions were run separately and the pH was 11 ACS Paragon Plus Environment


Page 12 of 31

monitored. In this study, neutral (7) and basic pH (10) was maintained by adding 1N NaOH and acidic pH (3) by 1N HCl. Samples were withdrawn after 240 min for HPLC analysis and it was observed that neutral pH gives total degradation of 100% of RDX while pH 3 and 10 results in degradation of 72 and 84% respectively (Figure S6). At initial stage, the rate of formation of ∙OH radicals is higher. It means that higher number of ∙OH radicals leads to attack on C-C or C-N bond and acidic intermediates are formed. The chloride ions may act as scavenger of ∙OH radicals and thus, the number of ∙OH radicals decreases. When the pH of an aqueous medium is increased from acidic to basic, it was observed that photocatalytic degradation also increases due to the production of large amount of ∙OH radicals. The photogenerated holes get scavenged by oxidizing species like H2O and OH¯ while electrons reduces the O2 species. The surface of photocatalyst gets positively charged at acidic pH (pH < 6.25) and negatively charged at alkaline pH (pH > 6.25). The higher degradation rate of RDX at neutral pH is because of the point of zero charge (pzc) at pH 6.25.6,39 The attraction and repulsion charge between BaFe12O19, RDX and ∙OH radicals also affects the degradation rate. It may be applied to BaFe12O19 which shows amphoteric character at pzc. 3.6 Effect of catalyst loading The initial rate of photocatalytic activity of many contaminants depends on the dosage of the photocatalyst. In order to determine the effect of concentration of BaFe12O19 on the degradation of RDX, varying quantities of BaFe12O19-800 °C were used for a 40 mg.L-1 solution of RDX with an irradiation time of 240 min at similar experimental conditions. Figure 5 shows the photocatalytic degradation of RDX with the dosage of 0.2 to 1.4 g.L-1. As the loading of the catalyst increased from 0.2 to 0.6 g.L-1 the final degradation concentration shifted from 4.4 to 0.6 mg.L-1. The degradation rate increased initially due to the increase in active sites contributing to the enhancement of the rate of reaction. However, as the amount of catalyst is further increased from 0.6 to 1.4 g.L-1, the final concentration of RDX is shifted 12 ACS Paragon Plus Environment

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from 0.6 to 2 mg.L-1. This is due to the interaction within radicals and increasing opaqueness of solution because of the larger amount of suspended particles and therefore, increasing the light scattering.40,41 The standard deviation or error for the degradation plot of catalyst concentration was 0.38464 and is given Table S4. In case of catalyst loading, the rate constants were found to be 0.009, 0.018, 0.017 and 0.012 min-1 for 0.2, 0.6, 1 and 1.4 g.L-1 of BaFe12O19 -800 °C respectively (Figure S7). These rate constants suggest that 0.6 g.L-1 of BaFe12O19-800 °C is the optimum catalyst concentration which has a higher rate of degradation. Thus, at higher concentration, the increasing number of particles can block UV light from reaching the surface of the catalyst. It results into decreased penetration depth of photons and thus, fewer sites of the catalyst could be activated. The second possibility is that when the concentration of ∙OH radicals increases from a certain number, radicals get react with each other rather than attacking the target molecule. Finally, a number of active ∙OH radicals becomes less and leads to slow degradation.39 3.7 Comparison of photocatalytic activity under UV and visible light We performed two typical photocatalytic reactions under UV and visible light separately in which BaFe12O19-800 oC and TiO2-P25 were chosen as catalysts and increased the irradiation time upto 420 min. It is because the catalyst shows absorption of light in visible region. The reasons for this selected reaction were i) to evaluate the effect of run time and light source (irradiation) for maximum possible photocatalytic degradation, ii) to achieve the drinking water limit of RDX as given by WHO and USEPA and, iii) to compare the activity of BaFe12O19-800 °C with commercially available TiO2 for maximum irradiation. For this purpose, 40 mg.L-1 aqueous solution of RDX was subjected to photocatalysis with 1 g.L-1 of each catalyst. It was observed that under UV light irradiation, BaFe12O19-800 oC could 13 ACS Paragon Plus Environment


Page 14 of 31

degrade all the RDX within 300 min (Figure 6) while TiO2 shows 3.1 mg.L-1 of unreacted RDX after 420 min. In case of visible light irradiation, BaFe12O19-800 oC gave total degradation of RDX in 420 min whereas TiO2 showed 4.3 mg.L-1 of unreacted RDX. It is important to mention that at zero-time interval the TOC was 10.52 mg.L-1 and it decreased to 0 and 1.8 mg.L-1 for BaFe12O19-800 oC and TiO2-P25 respectively after 420 min under UV light while in visible light TOC were 0 and 2.3 mg.L-1 for BaFe12O19-800 oC and TiO2 respectively. The higher photocatalytic activity of BaFe12O19 than TiO2 is due to higher absorption of light thereby producing maximum number of electrons and photogenerated holes. In both the case, BaFe12O19 and TiO2, recombination of excited electrons and photo generated holes can’t be avoided because there is no foreign element in the crystal lattice to shift the Fermi energy level. The Fermi level presents near the conduction band and therefore, it resends the excited electrons to the photogenerated holes. Photocatalytic activity almost depends on band gap and light harvesting efficiency.42 The optical property states that BaFe12O19 has lower band gap (2.1 eV) than TiO2 (3.2 eV). Therefore, it absorbs the energy from wide spectra i.e. varying from UV to the visible range. It means that more absorption of light (low band gap) leads to produce more excited electrons from valence to conduction band. Here, BaFe12O19 excites more electrons than TiO2 as justified by UV-Vis-DRS (optical properties). In both the photocatalysts, excited electrons recombine with photogenerated holes, but again irradiation of light pushes them to higher energy level. Electron excitation cycle of BaFe12O19 keeps more electrons and photogenerated holes to react with O2 and H2O, for generation of hydroxyl radicals. Therefore, only the numbers of excited electrons are considered in the degradation rate of both the catalysts. Choi et al. reported total photocatalytic degradation of RDX with TiO2 within 80 min however, four UV lamps were used.39 TiO2 loaded cloth under natural sunlight is also tested for degradation of RDX (40 14 ACS Paragon Plus Environment

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mg.L-1) but showed incomplete degradation after 120 min.7 Thus, in present case, due to the low band gap, BaFe12O19-800 °C shows better efficiency than TiO2-P25 and achieves the degradation limit of RDX posed by WHO and USEPA whereas the commercially available catalyst couldn’t succeed. 3.8 Mechanism of Degradation BaFe12O19 is a low band gap semiconductor and the photodegradation of an organic pollutant is initiated by the photoexcitation of the semiconductor. The electron-hole pair is obtained from photoexcitation of BaFe12O19. The positive holes formed at valence band (h+) exhibit a strong oxidising potential of 3.0 V and can be adsorbed by the OH- ions from the H2O molecules on the surface of the BaFe12O19 discs, yielding hydroxyl radical (∙OH) with Eo= 3.06 V (Eq. 3 to 5). At the same time, conduction band contains free electrons which are ejected from the valence band and these electrons react with O2 to form superoxide radicals. Although, photocatalysis is an oxidative process, but actually, it leads through both oxidation and reduction mechanisms. The electron deficient valence band always play a role in oxidation of H2O to generate strong oxidizing agent (∙OH) whereas the conduction band of catalyst easily reduces O2 or any other functional group of selected organic compound. + ― 𝑩𝒂𝑭𝒆𝟏𝟐𝑶𝟏𝟗 +𝒉𝝂→𝒉𝝂𝒃 + 𝒆𝒄𝒃

(3)

+ + 𝑯𝟐𝑶 →𝑶𝑯 ― + 𝑯 + 𝒉𝝂𝒃

(4)

+ + 𝑶𝑯 ― →𝑯𝑶. 𝒉𝝂𝒃

(5)

∙OH

is a strong oxidising agent and reacts with organic contaminants to oxidise it non-

selectively into completely mineralized species. Due to the oxidative hydroxyl radical and reductive conduction band, degradation path can’t follow a specific selective route. Therefore, the researchers have predicted two different routes for the degradation mechanism.39,43,44 By following the reported works and mass analysis of our reaction 15 ACS Paragon Plus Environment


Page 16 of 31

intermediates and products, we have depicted the degradation mechanism as shown in Figure 7. Two routes have been proposed in which one follows N attack while the other at C. Both the routes progressed simultaneously and given mixture of all the intermediates.44 The conduction band (e-& O2-) reacts with NO2 of RDX and leads to the formation of reductive intermediates whereas oxidative attack of hydroxyl radicals (∙OH) leads to breaking of C-N bonds and finally liberates formate ions. Figure 7 shows that when there is N attack, it forms nitroso derivatives of RDX like hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) (m/z 174), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX) (m/z 190) and hexahydro-1-nitroso-3,5dinitro-1,3,5-triazine (MNX) (m/z 206). It again loses NO2- and NO ions to form methylene dinitramine (m/z 136) and its nitroso derivatives (m/z 120 & 104). These nitramines further react with ∙OH and carbon get converted into formate species (m/z 30 & 45) while N into hydrazine (m/z 32) and subsequently forms N2 and NH4+. The NH4+ ions can be further oxidized to NO3- by ∙OH radicals. NO2 radicals were formed by both N attack and C attack which further reacts with ∙OH to release NO3- ions. In case of C attack, it immediately gives C-N bond brakeage and oxidized carbon of RDX gets converted into formaldehyde (HCHO) and formic acid (HCOOH). It is followed by the simultaneous release of NO2- and evolution of CO2 which decreases the carbon content. When photocatalytic reaction starts, RDX (m/z 222) gives its parent abundant peaks at m/z 221 and 223. The mineralization of RDX and its intermediates (nitroso, nitramine and hydrazine) into formaldehyde, formic acid, NH4+, NO3- and finally CO2 is confirmed by mass spectrometry. The mineralization of nitrogen content of RDX exists as NO3- and N2. The evolution of N2 is important from the water reusability point of view. In Figure 7, m/z ratio of mass fragments is written by theoretically calculating a particular intermediate where actual values from mass analysis at different time interval are given for the confirmation of all the 16 ACS Paragon Plus Environment

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intermediates (Figure S8-S12). The RDX presents in the environment for a longer time and its degraded intermediates such as MNX, DNX and TNX are also toxic. The toxicity of these intermediates was evaluated on earthworms and lowest observed effect concentration in soil was 50- 100 mg.kg-1.45,46 Zhang et. al. observed lethal concentrations, LC20 and LC50 for TNX were lower than for MNX. It shows TNX is more toxic than MNX. Hence, all the toxic intermediates were mineralized. 3.9 Reproducibility and reusability of the photocatalyst Due to the magnetic property of the BaFe12O19, it is very easy to recover the photocatalyst after the degradation studies and reuse it. The reproducibility of magnetically separable BaFe12O19-800 °C as a photocatalyst was examined for RDX degradation during the experimentation of three cycles. Each experiment was carried out under the identical conditions of 40 mg.L-1 solution of RDX under the UV illumination for 360 min. Initially, 1 g.L-1 of BaFe12O19-800 °C concentration was taken and it was recycled for further two subsequent reactions. After each experiment of degradation, the concentration of energetic material in feed stream was adjusted to its initial value. A very slow decrease in the photocatalytic activity with reusability of the catalyst was observed (Figure S13). After the first run, RDX was 100% degraded within 360 min and the catalyst was further separated magnetically, washed with water and dried in oven at 80 °C. The second run with the same catalyst shows 99.5% photo-degradation of RDX while the third run could give 98%. The loss of catalyst during recycling was negligible due to magnetic separation. Initial, 1 g.L-1 of BaFe12O19 magnetic catalyst was found to be 0.985 g.L-1 after the third cycle. The normal decreased photocatalytic activity is due to deposition of various organic and inorganic ions on the surface of the catalyst. The active sites of hexagonal BaFe12O19 get blocked and hence, interaction with light to eject the electron gets lowered. It ultimately increases the gap between valence to conduction band and affects the degradation. 17 ACS Paragon Plus Environment


4.

Page 18 of 31

CONCLUSIONS

The crystalline BaFe12O19 nano-platelets exhibiting a hexagonal system were synthesized by a molten salt technique at different temperatures and used for photocatalytic degradation of the energetic material, RDX under UV and visible light irradiation. All the synthesized catalysts were well characterized by XRD, Raman, UV-Vis-DRS, FE-SEM and EDX spectra. The optical band gap (Eg) found to be decreased with increase in synthesis temperature due to crystalline properties. The efficiency of the photocatalytic activity of BaFe12O19-800 oC was found to be more than the commercial TiO2-P25. The degradation kinetics of RDX was analyzed by HPLC, LCMS, and TOC. It was observed that 0.6 g.L-1 of BaFe12O19-800 °C with a single UV lamp irradiation was efficient and economical to degrade 40 mg.L-1 solution of toxic energetic compound. Moreover, the compound was degraded below the discharge limits of WHO and USEPA. Other effective parameters like concentration of catalyst, the effect of run time, pH and recycling is productively studied and optimum reaction conditions are evaluated. Photocatalytic degradation rate is higher in UV irradiation than visible. As BaFe12O19 exhibit magnetic properties, it is very easily separable and hence, decreases the additional release of contaminants of catalyst into wastewater. With the consideration of this work, presented BaFe12O19 shows efficient photocatalytic activity than TiO2 and other reported magnetic iron oxide-based nanostructures. This difference is due to crystal structure (atomic arrangement), low band gap (῀ 2.1eV), high magnetic saturation (59 emu.g-1) and hexagonal nano-platelet morphology of BaFe12O19. It is well known that the properties were always decided by atomic arrangement, phase formation or crystal structure and geometry. Therefore, BaFe12O19 nano-platelets come with new approaches in photocatalysis and hence, it may be called as an eco-friendly photocatalyst as, it saves the living beings from the modern polluted environment. Finally, the application of catalytically and magnetically active


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BaFe12O19 hexagonal nano-platelets can be extended to various fields particularly from the sustainable chemistry point of view. ASSOCIATED CONTENT Supporting Information HPLC conditions (Table S1) and chromatograms (S14-S19), XRD parameters and spectra, Raman lattice vibrations, magnetic properties of BaFe12O19 with curve and UV-DRS parameters are given. Kinetic rate constant of degradation of RDX at various conditions is given with plotting. Mass analysis of degraded product from time 0 to 360 min is showed with mass fragment spectra. All these materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected], Tel. +91 20 24304161, Fax: +91 20 24389509.

ACKNOWLEDGEMENTS The financial support from DRDO (ERIP/ER/1003883/M/01/908/2012/D, R&D/1416, dated, 28-3-2012) New Delhi, India, is gratefully acknowledged. Dr. Sanjay P. Kamble, Senior Scientist, NCL, Pune for his help in instrumental analysis.



Page 20 of 31

REFERENCES 1. Connelly, K. A.; Idriss, H. The photoreaction of TiO2 and Au/TiO2 single crystal and powder surfaces with organic adsorbates: Emphasis on hydrogen production from renewable. Green Chem. 2012, 14, 260-280. 2. Cargnello, M., Fornasiero, P., Handbook of Green Chemistry; Wiley Online Library: Weinheim, 2012. 3. Jo, W. K.; Santosh Kumar.; Isaacs, M. A.; Lee, A. F.; Karthikeyan, S. Cobalt promoted TiO2/GO for the photocatalytic degradation of oxytetracycline and Congo red. Appl. Catal. B: Environ. 2017, 201, 159-168. 4. Lee, S. J.; Son, H. S.; Lee, H. K.; Zoh, K. D. Photocatalytic degradation of explosives contaminated water. Water Sci. & Tech. 2002, 46, 139-145. 5. Tian, F.; Hitchman, M. L.; Shamlian, S. H. Photocatalytic and photoelectrocatalytic degradation of the explosive RDX by TiO2 Thin Films Prepared by CVD and anodic oxidation of Ti. Chem. Vap. Dep. 2012, 18, 112-120. 6. Son, H. S.; Lee, S. J.; Cho, I. H.; Zoh, K. D. Kinetics and mechanism of TNT degradation in TiO2 photocatalysis. Chemosphere 2004, 57, 309-317. 7. Liu, Z.; He, Y.; Li, F.; Liu, Y. Photocatalytic treatment of RDX wastewater with nano-sized titanium dioxide. Environ. Sci. Pollut. Res. 2006, 13, 328-332. 8. Chen, W. S.; Huang, Y. L. Removal of dinitrotoluenes and trinitrotoluene from industrial wastewater by ultrasound enhanced with titanium dioxide. Ultrasonics Sonochemistry 2011, 18, 1232-1240. 9. Perchet, G.; Merlina, G.; Revel, J. C.; Haﬁdi, M.; Richard, C.; Pinelli, E. Evaluation of a TiO2 photocatalysis treatment on nitrophenols and nitramines contaminated plant wastewaters by solid-phase extraction coupled with ESI HPLC-MS. J. Hazar. Mater. 2009, 166, 284-290. 10. Shetty, R.; Chavan, V. B.; Kulkarni, P. S.; Kulkarni, B. D.; Kamble, S. P. Photocatalytic degradation of pharmaceuticals pollutants using N-doped TiO2 photocatalyst: Identification of CFX degradation intermediates. Indian Chemical Engineer 2016, 1-13. 20 ACS Paragon Plus Environment

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11. Zeng, X.; Wanga, Z.; Menga, N.; McCarthy, D. T.; Deletic, A.; Pan, J. H; Zhang, X. Highly dispersed TiO2 nanocrystals and carbon dots on reduced graphene oxide: Ternary nanocomposites for accelerated photocatalytic water disinfection. Appl. Catal. B: Environ. 2017, 202, 33-41. 12. Han, C.; Pelaez, M.; Likodimos, V.; Kontos, A. G.; Falaras, P.; O’Shea, K.; Dionysiou, D. D. Innovative visible light-activated sulfur doped TiO2 films for water treatment. Appl. Catal. B: Environ. 2011, 107, 77-87. 13. Shin, G. B.; Kim, Y. K.; Photocatalytic degradation of 2,4,6-trinitrotoluene in wastewater using a thin-film TiO2 reactor. Environ. Eng. Res. 2008, 13, 28-32. 14. Zhu, Q.; Zhang, Y.; Zhou, F.; Lv, F.; Ye, Z.; Fan, F.; Chu, P. K. Preparation and characterization of Cu2O–ZnO immobilized on diatomite for photocatalytic treatment of red water produced from manufacturing of TNT. Chem. Eng. J. 2011, 171, 61-68. 15. Han, C. H.; Li, Z. Y.; Shen, J. Y. Photocatalytic degradation of dodecylbenzenesulfonate over Cu2O-TiO2 under visible irradiation. J. Hazard. Mater. 2009, 168, 215-219. 16. Balu, A. M.; Baruwati, B.; Serrano, E.; Cot, J.; Martinez, J. G.; Varma, R. S.; Luque, R. Magnetically separable nanocomposites with photocatalytic activity under visible light for the selective transformation of biomass-derived platform molecules. Green Chem. 2011, 13, 2750-2758. 17.

Sadeghpour, F.; Nabiyouni, G.; Ghanbari, D. Photo degradation of acid blue, black and brown: photo catalyst and magnetic investigation of CoFe2O4-SnO2 nanoparticles and nano composites. J. Mater. Sci: Mater. Electron. 2016, 27, 12160-12173.

18. Xie, T.; Liu, C.; Xu, L.; Yang, J.; Zhou, W.; Novel heterojunction Bi2O3/SrFe12O19 magnetic photocatalyst with highly enhanced photocatalytic activity. J. Phys. Chem. C, 2013, 117, 24601-24610. 19. Haw, C.; Chiu, W.; Rahman, S. A.; Khiew, P.; Radiman, S.; Shukor, R. A.; Hamid, M. A. A.; Ghazali, N. The design of new magnetic photocatalyst nanocomposites (CoFe2O4-



Page 22 of 31

TiO2) as smart nanomaterials for recyclable Photocatalysis applications. New J. Chem. 2016, 40, 1124-1136. 20. Laohhasurayotin, K.; Pookboonmee, S.; Viboonratanasri, D.; Kangwansupamonkon, W. Preparation of magnetic photocatalyst nanoparticles-TiO2/SiO2/Mn-Zn ferrite and its photocatalytic activity influenced by silica interlayer. Materials Research Bulletin 2012, 47, 1500-1507. 21. Pang, Y. L.; Lim, S.; Ong, H. C.; Chong, W. T. Research progress on iron oxide-based magnetic materials: Synthesis techniques and photocatalytic applications. Ceram. Int. 2015, 42, 9-34. 22. Martirosyan, K. S.; Galstyan, E.; Hossain, S. M.; Wang, Y. J.; Litvinov, D. Barium hexaferrite nanoparticles: Synthesis and magnetic properties. Mater. Sci. & Eng. 2011, B176, 8-13. 23. Lisjak, D.; Drofenik, M. The low-temperature formation of barium hexaferrites. Journal of the European Ceramic Society 2006, 26, 3681-3686. 24. Dursun, S.; Topkaya, R.; Akdogan, N.; Alkoy, S. Comparison of the structural and magnetic properties of submicron barium hexaferrite powders prepared by molten salt and solid state calcination routes. Ceram. Int. 2012, 38, 3801-3806. 25. An, G. H.; Hwang, T. Y.; Kim, J.; Kang, N.; Kim, S.; Choi, Y. M.; Cho, Y. H. Barium hexaferrite nanoparticles with high magnetic properties by salt-assisted ultrasonic spray pyrolysis. Journal of Alloys and Compounds 2014, 583, 145-150. 26. Li, W.; Qiao, X.; Li, M.; Liu, T.; Peng, H. X. La and Co substituted M-type barium ferrites processed by sol-gel combustion synthesis. Materials Research Bulletin 2013, 48, 4449-4453. 27. Kumar, S.; Datt, G.; Santhosh Kumar, A.; Abhyankar, A. C. Enhanced absorption of microwave radiations through flexible polyvinyl alcohol-carbon black/barium hexaferrite composite films. J. Appl. Phys. 2016, 120, 164901. 28. Moitra, A. ; Kim, S.; Kim, S. G.; Erwin, S. C.; Hong, Y. K.; Park, J. Defect formation energy and magnetic properties of aluminum-substituted M-type barium hexaferrite. Computational Condensed Matter. 2014, 1, 45-50. 22 ACS Paragon Plus Environment

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29. Akhavan, J. Chemistry of Explosives; 2nd ed.; Royal Society of Chemistry, 2004. 30. Arpe, H. J. Ullmann's Encyclopedia of Industrial Chemistry; 5th ed.; VCH; Weinheim, 1991. 31. Best, E. P.; Sprecher, S. L.; Larson, S. L.; Fredrickson, H. L.; Bader, D. F. Environmental behaviour of explosives in groundwater from the Milan army ammunition plant in aquatic and wet land plant treatments. Removal, mass balances and fate in groundwater of TNT and RDX. Chemosphere 1999, 38, 3383-3396. 32. Alnaizy, R.; Akgerman, A. Oxidative treatment of high explosives contaminated wastewater. Water Res. 1999, 33, 2021-2030. 33. Bhosale, V. K.; Patil, N. V.; Kulkarni, P. S. Treatment of energetic materials contaminated wastewater using ionic liquids. RSC Adv. 2015, 5, 20503-20510. 34. Toxicological

profile

of

2,4,6-

trinitrotoluene

(TNT).

ATSDR;

1995,

http://www.atsdr.cdc.gov/toxprofiles/tp.asp?id=677&tid=125. 35. Mahbub, P.; Nesterenko, P. N. Application of photo degradation for remediation of cyclic nitramine and nitroaromatic explosives. RSC Adv. 2016, 6, 77603-77621. 36. Kreisel, J.; Lucazeau, G.; Vincent, H. Raman spectra and vibrational analysis of BaFe12O19 hexagonal ferrite. Journal of solid state chemistry 1998, 137, 127-137. 37. Rane, V. A.; Meena, S. S.; Gokhale, S. P.; Yusuf, S. M.; Phatak, G. J.; Date, S. K. Synthesis of low coercive BaFe12O19 hexaferrite for microwave applications in low-temperature co-fired ceramic. Journal of Electronic Materials 2013, 42, 4. 38. Asanuma, T.; Matsutani, T.; Liu, C.; Mihara, T.; Kiuchi, M. Structural and optical properties of titanium dioxide films deposited by reactive magnetron sputtering in pure oxygen plasma. J. Appl. Phys. 2004, 95, 6011. 39. Choi, J. K.; Son, H. S.; Kim, T. S.; Stenstrom, M. K.; Zoh, K. D. Degradation kinetics and mechanism of RDX and HMX in TiO2 photocatalysis. Environmental Technology 2006, 27:2, 219-232. 40. Munusamy, S.; Aparna, R.; Prasad, R. Photocatalytic effect of TiO2 and the effect of dopants on degradation of brilliant green. Sustainable Chemical Processes 2013, 1:4.



Page 24 of 31

41. Akpan, U. G.; Hameed, B. H. Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: A review. J. Hazar. Mater. 2009, 170, 520-529. 42. Ozawa, K.; Emori, M.; Yamamoto, S.; Yukawa, R.; Yamamoto, S.; Hobara, R.; Fujikawa, K.; Sakama, H.; Matsuda, I. Electron hole recombination time at TiO2 single-crystal surfaces: Influence of surface band bending. J. Phys. Chem.Lett. 2014, 5, 1953-1957. 43. Piccinini, P.; Minero, C.; Vincenti, M.; Pelizzetti, E. Photocatalytic interconversion of nitrogen-containing benzene derivatives. J. Chem. Soc. Faraday Trans. 1997, 93, 1993-2000. 44. Naja, G.; Halasz, A.; Thiboutot, S.; Ampleman, G.; Hawari, J. Degradation of hexahydro1,3,5-trinitro-1,3,5-triazine (RDX) using zerovalent iron nanoparticles. Environ. Sci. Technol. 2008, 42, 4364-4370. 45. Zhang, B.; Kendall, R. J.; Anderson, T. A. Toxicity of the explosive metabolites hexahydro1,3,5-trinitroso-1,3,5-triazine

(TNX)

and

hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine

(MNX) to the earthworm Eiseniafetida. Chemosphere 2006, 64, 86-95. 46. Halasz, A.; Spain, J.; Paquet, L.; Beaulieu, C.; Hawari, J. Insights into the formation and degradation Mechanisms of methylenedinitramine during the incubation of RDX with anaerobic sludge. Environ. Sci. Technol. 2002, 36, 633-638.


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURES

Figure 1. XRD patterns of BaFe12O19 synthesized by molten salt method at various temperatures.



Page 26 of 31

Figure 2. Raman Spectrum of BaFe12O19 synthesized by molten salt method at different temperature (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C.


Page 27 of 31

800 900 1000

2 (F(R)h) (a.u.)

(a)

40

(b)

Reflectance %

35 30 25 20 15 10 5 0 200

300

400

500

600

700

Wavelength ( ) nm

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

h (eV) Figure 3 (a).Variation of (hν F(R))2 with photon energy (hν) for BaFe12O19 (b) diffuse reflectance versus wavelength of nano-structured BaFe12O19 1.0 0

BaFe12O19 800 C

0.8

0

BaFe12O19 900 C 0

BaFe12O19 1000 C

Conc. C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photolysis

0.6

0.4

0.2

0.0 0

60

120

180

240

Time (min) Figure 4. Photocatalytic degradation of RDX (40 mg.L-1) by BaFe12O19 (1 g.L-1) sintered at different temperatures and photolysis (without catalyst) at 25 °C



1.0 -1

0

-1

0

-1

0

-1

0

0.2 g L BaFe12O19 800 C 0.6 g L BaFe12O19 800 C

0.8

Conc. C/C0

1.0 g L BaFe12O19 800 C 1.4 g L BaFe12O19 800 C

0.6

0.4

0.2

0.0 0

60

120

Time (min)

180

240

Figure 5. Effect of concentration of BaFe12O19-800 °C on degradation of RDX (40 mg.L-1) at 25 °C 40

BaFe12O19 UV TiO2 P25 UV

30

BaFe12O19 Vis

-1

TiO2 P25 Vis .

RDX mg L

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

20

10

0 0

60

120

180

240

300

360

420

Time (min) Figure 6. Comparative photocatalytic activity in UV and visible light (BaFe12O19-800 oC & TiO2-P25 (1 g.L-1), RDX (40 mg.L-1) at 25 °C) 28 ACS Paragon Plus Environment

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NO2

e-cb /

N

O2N

O2

N attack

N

N

O2N

NO2

NO2

NO

NO

N

N

N

N

N

NO2-

NO2

NH4 + N2 C attack

N

O2N

N

ON

NO

N

NO-

-

NH4 + N2

H

H

N

N

N

N

O2N

NO2

NO

TNX m/ z 174

H

m/ z 176

BaFe12O19

N

DNX m/ z 190

MNX m/ z 206

RDX m/ z 222

N

O2N

NO

N

N

ON

NO

N

N

NO

m/z 144

m/z 160

hv .OH H N

O2N

H N

O2N

NO2

H N

NO

ON

H N

NO

H

N C. N

H N

m/ z 104

m/ z 120

m/ z 136

NO2

H N

CH2O

N

O2N

NH2NH2 + CH2O

+ NH4 + N2

NO2

m/ z 32

m/ z 221

.NO2

NO2-

NO3-

NO2 N

O2N

N

H .C .N

OH NO2

.CHOH

HCHO

m/ z 30

HCOO-

CO2

m/ z 45

m/ z 238

Figure 7. Mechanism of photocatalytic degradation of RDX by BaFe12O19



Page 30 of 31

TABLE Table.1. TOC analysis of RDX in mg.L-1 Time (min) 0

BaFe12O19800 °C 10.52

BaFe12O19900 °C 10.52

BaFe12O191000 °C 10.52

60

7.26

8.46

9.47

120

4.78

5.79

8.38

180

2.03

4.02

7.22

240

1.76

2.88

6.81


Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic 256x170mm (96 x 96 DPI)

ACS Paragon Plus Environment

Enhanced photocatalytic activity of magnetic BaFe12O19 nano

Recommend Documents