Enhanced sulfamethoxazole ozonation by noble metal-free catalysis based on magnetic Fe₃O₄ nanoparticles: catalytic performance and degradation mechanism

Abstract

In this research, Fe₃O₄ nanoparticles were prepared by a low-cost route free of other agents, and applied in the catalysis of sulfamethoxazole (SMX) ozonation. It was proven that Fe₃O₄ nanoparticles significantly enhance SMX ozonation. Using a kinetics analysis, when Fe₃O₄ particles were added to the ozonation process, the reaction rate constant increased by 51% when the pH was 5. Moreover, we also identified that Fe₃O₄ enhanced the SMX ozonation removal rate by changing the degradation pathway. It was found that addition of Fe₃O₄ improved the production of Lewis acid active sites in SMX. These kinds of site in SMX are much easier to attack, which leads to a higher SMX removal rate and lower operational costs for the Fe₃O₄-based catalytic ozonation process compared to an O₃ oxidation process. Finally, the SMX degradation pathways were classified for the first time, based on ozone oxidation types to give a guide for the quick and direct oxidation of SMX and other pollutants.

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1 Introduction

Pharmaceutical contamination of environmental samples is considered to present a potential risk for aquatic and terrestrial organisms and is regarded as a rising environmental issue of global concern. Among the various available pharmaceutical compounds, antibiotics have been found in the effluent of a number of sewage treatment plants as well as in surface water and groundwater in the USA, UK, Canada, Germany and China.^1–3 Sulfamethoxazole (SMX), a commonly used sulfonamide antibiotic, can be used to treat some of the most frequent illness outbreaks like coccidiosis, diarrhoea and gastroenteritis. Thus, there is large-scale global consumption of SMX in the animal food industry.⁴ In 2007, SMX was the 6th most universally prescribed antibiotic in Canada.⁵ In addition, SMX is one of the most commonly used antibiotics in China.⁶ Due to insufficient pollution control, large amounts of residual SMX has entered into the environment and poses a great threat to human beings. Although in recent years, the pollution control of discharged antibiotics and their by-products have attracted increasing attention from governments and researchers, SMX removal rates are presently low at between 20% and 30% in wastewater treatment plants.⁷ As a result, untreated SMX residues have caused severe pollution in surface water and underground aquifers in recent years. Furthermore, these residual antibiotics have led to animals and people evolving resistant genes, which could cause antibiotics to final lose function for the treatment of diseases. Therefore, it is necessary to control the discharge concentration of SMX by increasing its removal rate using modified techniques for wastewater treatment processes.

Recently, ozone oxidation has proven to be efficient for the removal of refractory pharmaceuticals like sulfonamides and other organic pollutants and for the enhancement of their biodegradability.^4,8–14 However, due to their selectivity being limited to the oxidation of organic compounds, the high cost of ozone addition and a poor mass transfer rate in aqueous solution, ozonation processes have not been widely used for practical wastewater treatment. As a consequence, ozone has usually been combined with other technologies, such as ultraviolet/O₃, H₂O₂/O₃, ultrasound/O₃ and catalyst/O₃, in order to improve its utilization rate and save on operation costs.

Modern chemistry and chemical engineering rely heavily on catalytic processes that have dramatically initiated and sustained global industrialization and also played important roles in green chemistry and wastewater treatment. During the past few decades, magnetic Fe₃O₄ nanoparticles have been the focus of much research because of their considerable properties, such as large surface area to volume ratio, biocompatibility, non-toxicity, simple modification, low cost and recyclability, as well as their high catalytic activity.¹⁵ In the field of environmental chemistry, magnetic Fe₃O₄ nanoparticles are used to catalyze a wide variety of reactions, such as intramolecular C–N cross-coupling reactions, the growth of carbon nanotubes, the decarboxylative cross-ketonisation of aryl- and alkylcarboxylic acids, and heterogeneous Fenton reactions. It has been reported that many Fe₃O₄-containing composite catalysts, including Au–Fe₃O₄,¹⁶ Pd–Fe₃O₄,¹⁷ Ag@Pd–Fe₃O₄ (ref. 18) and single crystalline Fe₃O₄ (ref. 19) have also been shown to exhibit high catalytic activity for the reduction of organic pollutants including pharmaceuticals. However, in all these cases, Fe₃O₄ is used only as a support for noble metals due to its magnetic properties, but not as a core part of the catalyst. Whether the Fe₃O₄ nanoparticles themselves have any catalytic activity in the reduction of pharmaceuticals is seldom considered and is still an open question. These catalysts with noble metal coatings have experienced sustainability issues due to their expense, scarcity in nature and deactivation in direct application. In addition, the loss and leaching of toxic metals has caused severe secondary pollution in the environment. Therefore, trialling Fe₃O₄ nanoparticles free of noble metals and other agents to catalyze pharmaceutical ozonation is economical and essential for the development of green chemistry and environmentally friendly catalysis. Furthermore, the effects of Fe₃O₄ on catalytic ozonation and its performance should be sufficiently confirmed as well.

Therefore, in this study, the catalytic ozonation of SMX using Fe₃O₄ nanoparticles as a catalyst was investigated. To clarify the effect of Fe₃O₄ on SMX degradation, three main aspects were concentrated on: (i) the catalytic performance of Fe₃O₄, by examining the role Fe₃O₄ plays in the oxidation system; (ii) the structure and characteristics of Fe₃O₄, studied by the means of a variety of techniques like TEM, SEM, FTIR, etc.; (iii) the degradation mechanism of SMX, determined by calculating the kinetic constants and speculating on the possible degradation pathways of SMX.

2 Methods and materials

2.1 Preparation of the nanocatalyst

Fe₃O₄ nanoparticles were prepared by the chemical co-precipitation of Fe³⁺ and Fe²⁺ ions in a molar ratio of 2 : 1.²⁰ Typically, a solution was prepared by adding FeCl₂·4H₂O (3.68 g) and FeCl₃·6H₂O (10 g) to deionized H₂O (150 mL) under an Ar atmosphere. The resultant solution was left to stir for 30 min at 80 °C. 25 mL of 25% aqueous NH₃·H₂O was then added dropwise with vigorous stirring to produce a black solid product. The resultant mixture was heated in water bath for 2 h at 60 °C and the black magnetite solid product was then washed with deionized H₂O, filtered and dried in an oven at 105 °C for 24 h.

2.2 Analytical methods

Sulfamethoxazole (C₁₀H₁₁N₃O₃S, analytical reagent grade, 99.0%) was purchased from Sigma and used as received without further purification. The SMX concentration was determined by UPLC (Waters, C₁₈, λ = 265 nm). The mobile phase was a mixture of acetonitrile and 0.1% formic acid with a volume ratio of 30 : 70. A 2 μL volume was injected using an auto sampler. The intermediates of SMX degradation were measured using a HPLC-MS/MS (Finnigan, LCQ-DECA-XP-MAX) instrument, which was equipped with a Zorbax SB-C18 HPLC column (150 mm × 2.1 mm × 3.5 μm, Agilent). In the MS analysis, the ionization for the MS was operated in APCI mode with the negative ion mode. In this paper, the method applied for the kinetics was based on a reaction comparison between the degradation rate of SMX and that of a reference compound. Fumaric acid (FA) was chosen to be the reference compound for the comparison reaction (details shown in ESI†).

The powder XRD pattern for the Fe₃O₄ catalyst was recorded with a Bruker D8 advance diffractometer. A transmission electron microscope (TEM, Hitachi H-7650) with an accelerating voltage of 100 kV, was used to observe the morphology of the catalyst. Fourier transform infrared (FTIR) spectrometry was carried out on a Spectrum One spectrometer in order to obtain information about the valence of the catalyst.

2.3 Catalytic tests

The degradation experiments were carried out in a conical flask (containing 200 mL of reaction solution) at 25 °C. The dosage of catalyst was 1.0 g L⁻¹ and the concentration of SMX was 50 mg L⁻¹ (≈0.2 mmol L⁻¹). All of the experiments were carried out under constant stirring to make sure the catalyst was well dispersed. H₃PO₄ and NaOH were used to adjust the pH of the solution. Before adding O₃, the suspension containing the catalyst and SMX was stirred for 0.5 h in order to achieve adsorption equilibrium. The degradation reaction was then initiated by introducing O₃ (2 g h⁻¹) into the SMX solution. At a given interval of degradation, the concentration of a sample was determined by UPLC. After adsorption and degradation the magnetite was collected and dried in a vacuum oven at 105 °C for 24 h.

3 Results and discussion

3.1 Catalytic performance

To understand the role of Fe₃O₄ in the SMX ozonation removal process, various sets of experiments were conducted under different operating parameters, including ozonation only, ozonation/Fe₃O₄, and adsorption of SMX onto Fe₃O₄.

As shown in Fig. 1, it was observed that the SMX concentrations decreased with increasing reaction time during the two processes; ozonation only and ozonation/Fe₃O₄. After ozonation for 5 min, compared with the ozonation only process, which had a SMX degradation efficiency of 85%, the SMX degradation efficiency had reached 97% in the presence of the Fe₃O₄ catalyst. The experimental results showed that the SMX removal followed apparent pseudo-first-order kinetics, (d[SMX])/dt = k[SMX], where [SMX] is the SMX concentration at time t and k is the pseudo-first-order rate constant. More importantly, the pseudo-first-order rate constant of the catalytic SMX ozonation with Fe₃O₄ was 0.61 min⁻¹, which was 1.6 times higher than that of the ozonation only (0.38 min⁻¹). In this study, the Fe₃O₄ improved the SMX pseudo-first-order rate constant by 60%, while Hou et al.²¹ found that their Fe₃O₄ catalyst enhanced the SMX pseudo-first-order constant by 38% with the observed pseudo-first-order constant changing from 0.18 min⁻¹ to 0.25 min⁻¹ ([SMX] = 50 mg L⁻¹; pH = 7.0; [Fe₃O₄] = 0.3 g L⁻¹) in 20 min. Meanwhile, Gonçalves et al.^22,23 found that multi-walled carbon nanotubes could provide a 25–50% increase in SMX ozonation after 3 h, which indicates that in the current study, Fe₃O₄ has much better catalytic performance. Ozone has a strong affinity towards Lewis acid sites and the Fe₃O₄ nanoparticles can offer more Lewis acid sites on their surfaces, enhancing the redox process involved in ozone adsorption/decomposition.^24,25 Therefore, in this study, the ozone decomposition rate was enhanced in the catalyst suspensions, and the removal rate was consequently improved.

Fig. 1 Fitting and degradation curves for SMX degradation using the O₃ and O₃ + Fe₃O₄ systems.

In order to determine the enhancement mechanism of the SMX degradation, SMX adsorption experiments on the catalyst surface were conducted under the same experimental conditions as used in the catalytic activity tests, but in the absence of ozone. Although a small amount of SMX adsorption onto the Fe₃O₄ catalyst was observed (data not shown), this adsorption was too weak to affect the degradation efficiency in the Fe₃O₄ catalytic ozonation process. However, the degradation rate and the observed rate constant were indeed enhanced in the catalytic process. Consequently, the Fe₃O₄ nanoparticles certainly demonstrated catalytic effects on the SMX ozonation process rather than adsorption. In order to reveal how the Fe₃O₄ nanoparticles catalyze the degradation of SMX, the characterization of the catalyst is analysed in the next section.

3.2 Characterization of the Fe₃O₄ catalyst

The surface chemistry of the Fe₃O₄ nanoparticles was studied using FTIR spectroscopy. In the spectrum show in Fig. S1,† the set of broad peaks centered at 574 cm⁻¹ can be ascribed to stretching vibrations of the Fe–O bonds in Fe₃O₄ (ref. 26) and the presence of vibration bands at 3400 (O–H stretching) demonstrate the successful synthesis of Fe₃O₄ nanoparticles. The XRD pattern of the Fe₃O₄ nanoparticles is shown in Fig. S2.† Diffraction peaks are seen at 2θ = 30.2°, 35.6°, 42.3°, 53.6°, 57.1° and 62.6°, which can be indexed to the cubic-phase of Fe₃O₄, which is in agreement with the literature²⁷ and corresponds well to the standard card for magnetite (PDF no. 65-3107). TEM and SEM images demonstrated that the Fe₃O₄ nanoparticles were quasi-spherical in shape, and had a nearly uniform distribution of particle size (3–5 nm, Fig. S3 and S4†). An EDS (energy dispersive X-ray fluorescence spectrometer) coupled with a scanning electron microscope (SEM) was used for microanalysis on single spots or on small areas using an accelerating voltage of 20 kV. Elemental SEM/EDS analysis shows the presence of iron and oxygen, suggesting that the Fe salt is pure. The results showing elements in the SEM/EDS sample as given by the experimental engineer are shown in Table S1.†

3.3 The degradation mechanism of SMX

3.3.1 The kinetic enhancement effect of the nanocatalyst. In order to evaluate the increase of the reaction rate with the assistance of the nanocatalyst, competitive kinetics experiments were designed and operated. The reaction constants for the O₃-only and the catalytic ozonation process were both determined. Detailed calculation methods and formula are stated in our previous paper²⁸ and the results are shown in Table 1. It can be seen from Table 1 that the nanocatalyst improved the ozone oxidation rate. On the one hand, the nanocatalyst increased the ozone oxidation rate by 51% when the pH was 5. The Fe₃O₄ nanocatalyst can catalyze the SMX degradation by producing more Lewis acid active sites and ozone has been demonstrated to strongly bond with Lewis acid active sites.^25,29 Therefore, the more Lewis acid active sites means more O₃ molecules being attracted to oxidize with SMX, contributing to a higher SMX degradation efficiency. This hypothesis is verified in section 3.3.2 which discusses pathway analysis. On the other hand, under neutral conditions, the nanocatalyst enhancement increased the oxidation rate by 34%. Ozone indirect-oxidation (˙OH oxidation) is predominant under neutral conditions, and the nanocatalyst causes the ozone to produce more ˙OH and improves the oxidation rate. In addition, when pH > 5.7 (pK_a2), the protonated form of SMX is the predominant form, the structure of which has a higher reactivity towards the oxidant.^4,21 Above all, the nanocatalyst directly enhances the SMX ozonation process, but in different ways, because there are various ozone oxidation mechanisms including direct and indirect oxidation. Furthermore, the oxidation rate was inversely proportion to t_1/2, which means that the increase in the kinetic rate led to shorter reaction times. Therefore, our proposed methods reduce ozone input and operation costs.

Table 1 The reaction constants (L mol⁻¹ s⁻¹) under various conditions^a

pH	Ratio kSMX/kFA	kFA	kSMX	Ratio kSMX(Fe3O4+O3)/kSMX(O3)	kSMX(Fe3O4+O3)
a kFA: ozonation rate of FA; kSMX: ozonation rate of SMX; kSMX(Fe3O4+O3): oxidation rate of SMX in the catalytic system.
5	1.35	1 × 10^5	1.35 × 10^5	1.51	2.04 × 10^5
7	2.58	1.5 × 10^5	3.87 × 10^5	1.34	5.19 × 10^5

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The SMX degradation curves indicate that the SMX degradation reaction follows a pseudo-first-order kinetic model under the experimental conditions (both O₃ and catalytic ozonation), as shown in Fig. 1. It can also be seen that the ozone oxidation SMX degradation reaction fits this kinetic model well. However, in the catalytic ozonation system, it shows a clear deviation from the fitting curve. As intermediate accumulation occurs with access to the Fe₃O₄ catalyst, the degradation of the parent SMX molecules is initially affected, making the degradation rate slower than the theoretical value. The rate then becomes higher than the theoretical value with prolonged reaction time due to SMX and its intermediates being gradually degraded, indicating that the Fe₃O₄ nanoparticles not only catalyze SMX degradation and enhance the oxidation of its intermediates, but it also intensifies competition between SMX and its intermediates for O₃ contact to achieve a higher pollutant removal rate. Therefore, it could be reasonably assumed that the Fe₃O₄ nanocatalyst had an influence on the SMX ozonation degradation, its intermediates (either species or quantity) and pathways. For deeper understanding of the role that Fe₃O₄ plays in this process, the possible SMX degradation pathways needed to be clarified.

3.3.2 Influence of Fe₃O₄ on the degradation pathways of SMX. In order to investigate the influence of Fe₃O₄ on the degradation mechanism of SMX, the intermediates were carefully identified by HPLC-ESI-MS-MS. Total ion chromatographs and mass spectra of SMX and its intermediates, which were produced in the O₃ and Fe₃O₄ catalytic ozonation process, are shown in Fig. 2. It can be seen from the total ion chromatographs that there are two different peaks in the Fe₃O₄ catalytic ozonation process when compared to the O₃-only oxidation system, representing at least two different kinds of major intermediate. As shown in the mass spectra, the peak at m/z 252 is the deprotonated ion of SMX. Following this precursor ion, in the O₃-only oxidation system, seven other products at m/z 282, 268, 226, 179, 156, 115, 113, 107 and 97 were detected. Meanwhile, in the Fe₃O₄ catalytic ozonation process, other two products at m/z 348 and 298 were detected. Based on the results obtained in the current work and literature,^23,28,30 these major fragments correspond to the fragmentation of by-products through the loss of H and the addition of O. Therefore, the structures of possible intermediates and their major fragments could be determined. The actual fragments of SMX detected are listed and compared in Table 2. From a comparison of the intermediate species between the O₃-only and Fe₃O₄ catalytic ozonation processes, we found that Fe₃O₄ catalyzed the degradation of SMX to form intermediates with Lewis acid active sites, like those at m/z of 348 and 298. Therefore, it was found that the introduction of the Fe₃O₄ nanocatalyst to the O₃ oxidation system increased the amount of Lewis acid active sites in SMX, which attracted more ozone to quickly and directly attack SMX, improving the efficiency of oxidation, enhancing the SMX removal rate and reducing operational costs.

Fig. 2 Total ion chromatographs and mass spectra for the O₃-only and Fe₃O₄ catalytic ozonation processes.

Table 2 SMX degradation fragment ions from the O₃-only and Fe₃O₄ catalytic ozonation processes^a

m/z	Structure	O3	O3 + Fe3O4
a D: fragmentation detected; ND: fragmentation not detected.
97		D	D
107		D	D
113		D	D
115		D	D
156		D	D
179		D	D
226		D	D
268		D	D
282a		D	D
282b		D	D
298		ND	D
348		ND	D

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3.3.3 The degradation pathways of SMX. The intermediates identified in this study have provided firm evidence for the occurrence of various degradation reactions. Due to the dissimilar biodegradability and toxicity of the different intermediates and for the purpose of accessing the direct transformation of pollutants, we first divided the possible SMX ozonation degradation pathways into two parts: direct oxidation and indirect oxidation,^31,32 based on the two different oxidation types of ozonation. Direct oxidation means that O₃ molecules directly oxidize SMX to break its chemical bonds, such as selectively breaking down double bonds in the compound. Meanwhile, indirect oxidation is when O₃ molecules decompose to form other oxidants like ˙OH, which attack the target so that they make SMX decompose or become more active by addition on an oxygen atom onto the ring. As can be seen in Table 2, a comparison of the ion fragments between the O₃-only process and Fe₃O₄ catalytic ozonation process showed some similarities and difference in the products formed from the SMX degradation pathways with some variation in the ion m/z ratios. The possible SMX degradation pathways are shown in Fig. 3. Similar degradation reactions in these pathways include: (i) S–N bond cleavage (P-97, P-156); (ii) oxidation of the amino group on the benzene ring and bond cleavage of the isoxazole ring (P-226); (iii) loss of the amino benzene ring and addition of a double bond in the isoxazole ring (P-179); (iv) oxidation of the benzene ring to p-benzoquinone (P-107) and then to maleic acid (P-115); (v) oxidation of the amino group on the benzene ring (P-282a); (vi) oxidation of the methyl group on the isoxazole ring (P-282b); and (vii) hydroxylation of the isoxazole ring (P-268). However, there were two other pathways present in the Fe₃O₄ catalytic ozonation process, which were not found in the O₃-only process: (1) oxidation of the methyl group and O addition on the double bond in the isoxazole ring (P-298); and (2) addition of a sulfonic acid group onto the isoxazole ring (P-348). Based on the above analysis of the pathways, direct ozone oxidation mainly occurred at the S–N bond and isoxazole ring. This is because ozone can selectively attack activated isoxazole rings or double bonds, both of which are present in SMX. Furthermore, aromatic compounds, such as those contained in SMX, have a high delocalization of electrons and exhibit greater reactivity towards ozone. Degradation pathways (i) and (iv) correspond to those in Alexandra’s study,²³ which demonstrated one pathway for turning SMX into a small organic acid. However, in the current study, other main pathways have been listed giving a deeper understanding of SMX degradation.

Fig. 3 Proposed degradation pathways for SMX in the Fe₃O₄ catalytic ozonation process.

The enhancement mechanism of the nanocatalyst was also identified. Although the direct and indirect types of ozone oxidation were both observed in the O₃-only and Fe₃O₄ catalytic ozonation processes, it was found that the introduction of Fe₃O₄ to the ozonation improved the production of Lewis acid active sites, which attracted greater attention from O₃ bubbles, so making SMX easier to attack and resulting in a higher SMX removal rate, shorter reaction time, smaller ozone input and lower operational costs for the catalytic ozonation process. Furthermore, degradation pathways of SMX were classified along with the different ozone oxidation types. Once the intermediates with lower toxicity and fewer hazards are determined, we could regulate the degradation of SMX-like pollutants directly and quickly towards these known intermediates by changing the operational conditions to control the ozone oxidation type, which will have benefits for targeted SMX-like pollutant removal.

4 Conclusions

Fe₃O₄ nanoparticles free of any noble metals or other agents have proven to be an effective method to facilitate and enhance SMX ozonation in water, with evidence that they change the degradation pathway of SMX. Calculation of the kinetic constants also demonstrated that the SMX degradation rate was improved with the aide of Fe₃O₄. At the same time, new insights into SMX ozonation degradation pathways were proposed for the first time, according to the ozone oxidation type, which gives guidelines for the direct degradation of SMX and other pollutants to less toxic and less harmful products by controlling the operational conditions. In summary, this reusable, stable and O₃ and Fe₃O₄-based nanocatalyst could be used to remove pharmaceuticals and organic pollutants, which is significant for the development of advanced technologies for environmental remediation from both an economic and environmental point of view.

Acknowledgements

This work was financially supported by the Harbin Institute of Technology Fund for young top-notch talent teachers (AUGA5710052514). The authors also gratefully acknowledge financial support from the National Key Technology Support Program (2014BAD02B03), the Department of Education Fund for Doctoral Tutors (20122302110054), the State Key Laboratory of Urban Water Resource and Environment (2014TS06), and the Postdoctoral Science-Research Foundation of Heilongjiang Province (LBH-Q12106).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25994k

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