Hierarchical mesoporous TiO₂ microspheres for the enhanced photocatalytic oxidation of sulfonamides and their mechanism

Abstract

Sulfonamides, a group of antibacterial agents, are not readily biodegradable and as a result have often been detected in wastewater effluents, rivers and lakes. This work investigated the photocatalytic degradation of sulfadimethoxine (SDM) and related sulfonamides in H₂O₂ containing aqueous suspensions of mesoporous TiO₂ microspheres exposed to simulated solar light irradiation. The three-dimensional mesoporous TiO₂ microsphere catalyst was fabricated by a facile method via a one-step solvothermal process without templates. The performance of mesoporous TiO₂ microspheres exceeded the commercially available P25 TiO₂ catalyst in the photocatalytic degradation of sulfonamides. Sulfadimethoxine degradation increased and reached the optimal H₂O₂ concentration of 5.9 mM. Particularly, the disappearance of SDM as well as the formation and decomposition of some degradation intermediates were determined by high-performance liquid chromatography-mass spectroscopy (HPLC-MS) and high-performance liquid chromatography-selective ion recording (HPLC-SIR) data modules, and possible photocatalytic degradation mechanisms were proposed. The hydroxylation and cleavage of the S–N or C–N bonds through ^•OH attacks on the aromatic rings and aminopyrimidine rings under simulated solar light irradiation with the assistance of H₂O₂ played important roles during SDM photocatalytic degradation.

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1. Introduction

Sulfa drugs or sulfonamides are synthetic antimicrobial agents commonly used in human and veterinary medicine. Sulfonamides are among the most frequently detected antibiotics in surface waters.¹ They are not readily biodegradable in the natural environment,² leading to the common detection of such compounds in groundwater, secondary wastewater effluent, landfill leachate, and soils irrigated with reclaimed water.^3–5 Research indicates that sulfonamides in water may be removed by a chemical oxidation processes.^6–8 Photocatalytic oxidation is a promising technology because it is efficient in carbon mineralization and can utilize sunlight as an energy source.⁹ Titanium dioxide (TiO₂) induced photocatalysis is a technology widely employed in water and wastewater treatment recently to eliminate organic compounds due to its particular optical properties, innocuity, low cost, and durability in terms of photo and chemical corrosion.^10,11 Most efforts have been dedicated to the UV-light-driven photocatalytic oxidation process that typically employed TiO₂. As the photocatalyst, TiO₂ is a high band gap (∼3.2 eV) semiconductor material and can be used for photocatalysis by the illumination of ultraviolet (UV) light. However, UV light only accounts for a small portion (∼5%) of the solar spectrum in comparison to the visible region (∼45%). Designing new photocatalytic materials with visible light activation is therefore of great interest. To achieve this purpose, efforts have been devoted to shift the optical response of TiO₂ from the UV to the visible spectral range to effectively utilize solar energy. The addition of oxidizing agents such as H₂O₂ has proved to be an effective approach.^12–14 It is known that the chemisorption of H₂O₂ on the surface of TiO₂ results in the formation of the yellow complex “Titanium peroxide”.¹⁵ Although the interaction between H₂O₂ and TiO₂ is well-documented, the oxidation mechanism of organic compounds in the TiO₂/H₂O₂ system under solar light irradiation is not well-understood.¹⁴

Significant advances have been made in the utilization of TiO₂ nanoparticles in the photocatalytic degradation of organic waste on the laboratory scale; however, the obvious drawback of TiO₂ nanoparticles is their small size, making separation difficult and restricting their applications on large scales. Mesoporous materials, with relatively large particles, have attracted a lot of attention because they have large surface areas (up to 2000 m² g⁻¹) and uniform mesopores between 2–20 nm that give them with high chemical sorption kinetics.¹⁶ Mesoporous nanocrystalline titania has proved to afford more active sites for adsorption and photocatalytic reactions, providing excellent activity for the photo-oxidation of formaldehyde and acetone mixtures in the gas phase at room temperature.¹⁷ Researchers reported the high efficiency of photo-degrading dyes in water by using mesoporous anatase TiO₂ catalyst.^18–20 Mesoporous TiO₂ microspheres were fabricated and showed much better photoactivity than that of commercial photocatalyst P25 TiO₂ (Degussa, Germany) due to its large surface area, highly crystallized mesoporous wall, and its greater number of active sites for concentrating the substrate.^21–23

While several previous studies have reported photocatalytic oxidation of sulfonamides in UV, UV/TiO₂, Vis/TiO₂ and UV/TiO₂/FeCl₃ systems,^7,24–27 in this paper, mesoporous TiO₂ microspheres were employed to investigate their photoactivity on sulphonamide degradation. This is the first to report the photocatalysis of sulfadimethoxine (SDM) in the suspension of mesoporous TiO₂ microspheres under simulated solar light irradiation. The influence of H₂O₂ was evaluated and a degradation mechanism was proposed. Under the optimal photoreaction conditions, the photodegradation intermediates of SDM were examined by high performance liquid chromatography-mass spectroscopy (HPLC-MS) and high-performance liquid chromatography-selective ion recording (HPLC-SIR) data module. Possible mechanisms of product formation during photocatalytic degradation are also proposed under the present experimental conditions.

2. Experimental

2.1. Materials and reagents

Sulfonamides standards with purity > 99% were purchased from Sigma-Aldrich and used as received (Table 1). Degussa P25 (anatase : rutile = 80 : 20, particle size 21 nm, BET area 50 m² g⁻¹) was supplied from Degussa AG. Acetonitrile and methanol (HPLC grade) were obtained from Dikma Scientific (Dikma, Beijing, China). Other chemicals used were all in analytical grade and purchased from Tianjin Chemical Reagents Company (Tianjin, China). Ultrapure water was prepared with a Milli-Q water purification system (Millipore, Bedford, MA, USA).

Table 1 Structure and pK_a of selected sulphonamides

Sulphonamides	Acronym	CAS	Structure	pKa1	pKa2	Ref.
Sulfacetamide	SAAM	144-80-9		1.8	5.4	28
Sulfapyridine	SPD	144-83-2		2.74	8.29	28
Sulfamethazine	SMZ	57-68-1		2.3	7.4	29
Sulfisoxazole	SIA	127-69-5		1.5 ± 0.2	5.0 ± 0.7	29
Sulfadimethoxine	SDM	122-11-2		2.9 ± 0.5	6.1 ± 0.2	29

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2.2 Preparation of mesoporous TiO₂ microspheres

The mesoporous TiO₂ microspheres were synthesized via a one-step solvothermal process without templates. An appropriate amount of NaOH was completely dissolved in 40 ml ethanol and 2 ml TiCl₃ was added dropwise to the solution with vigorous stirring. The resultant transparent solution was put into a Teflon-lined autoclave vessel and maintained at 150 °C for 24 h. After autoclaving, the content was naturally cooled to room temperature and centrifuged to collect the solids that were rinsed thoroughly with distilled water and ethanol for several times. The powder was dried at 60 °C for 6 h, and then calcined at 400 °C for 2 h. This product was denoted as TiO₂ 3D_M.

2.3 Photocatalysis experiments

Photocatalytic reaction experiments were performed in a photochemical reactor system as described previously (ESI Fig. S1†). The reaction vessel was made of quartz and was suitable to accommodate an immersion well. Simulated sunlight irradiation was provided by an 800 W xenon lamp and UV irradiation was provided by a 300 W medium-pressure mercury lamp (Institute of Electric Light Source, Beijing), which was positioned in a cylindrical quartz cold trap. The reaction solution was stirred by a magnetic stirrer at the bottom of the reactor. Two hundred milliliters of aqueous suspension containing TiO₂ and sulfonamides were equilibrated in darkness for at least 30 min prior to irradiation. After collecting an initial aliquot from the equilibrated suspension in the dark, a batch reaction was initiated by placing the suspension in the collimated light path. One-milliliter aliquots of suspension were periodically collected from the irradiated batch reactor, and TiO₂ was separated from the aqueous phase by centrifugation (3000 rpm, 10 min).

2.4 Analytical quantification procedures

The HPLC separation was performed using a Waters 2695 HPLC separation module (Waters, USA) equipped with a Kromasil C18 column (250 × 4.6 mm, particle size 5 μm). The column was maintained at 25 °C during the sample analysis. The mobile phase consisted of eluent A (acetonitrile) and eluent B (0.1% formic acid in ultrapure water). The flow rate was kept at 0.4 ml min⁻¹, and the injection volume was 10 μL. The separation of antibiotics was achieved with a gradient program as follows: 0–10 min: 15–50% A; 10–16 min: 50–80% A; 16–21 min: 80% A; 21–26 min: 80–15% A; 26–30 min: 15% A. The system was re-equilibrated for 10 min between runs.

The tandem MS analyses were carried out on a Micromass Quattro triple-quadrupole mass spectrometer equipped with an electrospray ionization (ESI) source and operated in the positive ion mode. The optimal conditions for the MS system were determined as follows: source temperature 90 °C, desolvation temperature 300 °C, desolvation gas flow 500 L h⁻¹, cone gas flow 70 L h⁻¹ and capillary voltage 4.0 kV. Multiple responses monitoring (MRM) analysis was used to verify the five sulfonamides in the reaction solution. The optimal collision energy, cone voltage and transitions chosen for MRM were determined (ESI, Table S1†). For each analyte, quantification was based on the response of a single product ion (Table S1†).

Organic reaction intermediates were identified using liquid chromatography with tandem mass spectrometry (LC/MS/MS). MS analyses were conducted in positive mode with ESI over a mass range of 50–400 m/z, and an ion trap MS was used to separate ions with different m/z values. Selective ion recording (SIR) mode with a dwell time of 200 ms was used to acquire MS spectra and MS/MS spectra of analyte and its intermediates.

3. Results and discussion

3.1 Characterization of prepared catalysts

The mesoporous TiO₂ microspheres, with or without calcination, exhibit similar characteristics, as shown on wide angle XRD patterns (Fig. 1) which can be indexed to 25.3 (101), 37.2 (004), 48.9 (200), 54.0 (105), 55.3 (211), 62.4(204) and 68.7 (112). The data are consistent with the reported values of JCPDS No. 21-1272, suggesting that anatase is the only crystalline phase in the samples. The mean crystal sizes of the mesoporous TiO₂ microspheres were calculated from the breadth of the (101) XRD peaks of the anatase phase according to Scherrer formula as 9.1 and 5.0 nm, respectively for mesoporous TiO₂ samples with and without calcination. The size and shape of the products were confirmed by Field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) images (Fig. 2a, 2b). It shows that mesoporous TiO₂ has rough surfaces, and the microspheres are formed by the aggregation of the nanoparticles. The TiO₂ 3D_M has a nanoporous structure, and the pores are attributed to the interparticle spaces, and the nanoparticles are monocrystals (Fig. 2c, 2d).

Fig. 1 XRD patterns of the catalysts: (a) TiO₂ 3D_M calcined at 400 °C, (b) TiO₂ 3D_M uncalcined, and (c) P25 TiO₂.

Fig. 2 Field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy images (HRTEM) of the 400 °C calcined TiO₂ 3D_M surface: (a) (b) low magnification, (c) (d) high magnification.

According to the IUPAC classification, the N₂ adsorption/desorption isotherms of the mesoporous TiO₂ microspheres (ESI, Fig. S2†) show a typical IV pattern with hysteresis loop, which is related to the capillary condensation associated with large mesopores.³⁰ The Barrett–Joyner–Halenda (BJH) pore size distribution profiles (Fig. S2 insert†) reveals a small hysteresis loop, implying the presence of mesopores at a high relative pressure (P/P₀) range between 0.4 and 0.8.³¹ The formation of the mesoporous structure can be ascribed to the aggregation of TiO₂ particles. Moreover, the hysteresis loops for the sample prepared with calcination at 400 °C shifts to a higher P/P₀ range and the areas of the hysteresis loops become bigger than that of the uncalcined, indicating the increased mean pore size and pore volume with increasing calcination temperatures.³¹ The properties of TiO₂ 3D_M are as follows: mean particle size of ca. 11.6 nm, crystallite size of ca. 9.1 nm, and specific surface area of 140.9 m² g⁻¹. The TiO₂ 3D_M shows a stronger absorption in the UV-vis light region compared to that of P25 TiO₂. See diffuse reflectance spectra of TiO₂ 3D_M and P25 TiO₂ for supplementary information on the absorption edges of the two catalysts (Fig. S3†). An intensive absorption band with a steep edge was observed, indicating that the apparently higher adsorption of mesoporous TiO₂ microspheres might be from their reflective behavior. The enhancement of absorbance in the UV-vis region increases the number of photogenerated electrons and holes to participate in the photocatalytic reaction which will enhance the photocatalytic activity of the catalyst.³²

3.2 Photodegradation of SDM and related antimicrobials in aqueous solution

When sulfonamides in reaction solutions were photo-oxidized in the presence of TiO₂ 3D_M under simulated solar light irradiation, the substrates dramatically decreased with time and eventually disappeared after 60-min exposure. Chromatograms in Fig. S4, ESI† showed the evolution of substrates during the photo-catalytic oxidation process. The dark control showed minimal SDM adsorption to TiO₂ surfaces and almost no chemical reaction occurs without sunlight (Fig. 3a). The reaction solution irradiated with simulated solar light and UV light only degraded 16.4% and 33.1% of SDM, respectively, in the 60 min exposure. In the presence of TiO₂ 3D_M and irradiated with simulated solar light, 81.6% of SDM in the reaction solution was degraded within 60 min, a better performance than that of the commercial P25 TiO₂ (Fig. 3a). The UV-irradiated TiO₂ 3D_M catalytic oxidation completely removed the SDM and four sulfonamides analogues in the reaction solutions in 20 min (Fig. 3b). The TiO₂ 3D_M exhibited much higher photocatalytic activity than the P25 TiO₂. For the TiO₂ suspension under UV irradiation, the oxidative activity of TiO₂ particles is mostly attributed to the species formed through reactions initiated by the photo-generated electron (e⁻)-hole (h⁺) pairs, such as hydroxyl radicals, hydroperoxyl and other peroxyl radicals, valence-band holes in TiO₂, and hydrogen peroxide.³³ A photon absorbed by an organic molecule causes an electronic excitation of the molecule following the basic photochemical law of Gilbert and Baggott.³⁴ The UV irradiation accelerated the degradation of all five sulfonamides (Fig. 3b). The photocatalytic process is initiated when a semiconductor such as TiO₂ absorb photons with an energy equal or superior to their band gap, promoting the valence band electrons to the conduction band energy level. The electrons in the conduction band (e_CB⁻) can recombine with the photo-generated vacancies (holes, h_VB⁺) decreasing the efficiency of the photocatalytic process. TiO₂ 3D_M showed a photoresponse in both the UV and solar light regions, accordingly, TiO₂ 3D_M has greater speed and effectiveness for degrading sulfonamide analogues under UV irradiation, which gives higher populations of the photoexcited e⁻ and h⁺ to participate in the photocatalytic reaction in this system.³⁵

Fig. 3 (a) Photocatalytic degradation of SDM (b) UV-TiO₂ 3D_M catalyzed photo degradation rates for five sulfonamide analogues. Reaction conditions: C_o = 0.1 mM, 0.5 g L⁻¹ of TiO₂ 3D_M and P25 TiO₂, and 25 °C

The photocatalytic degradation of sulfonamides as a function of the irradiation time by TiO₂ 3D_M and P25 TiO₂ was observed to follow first-order kinetic reaction:⁹

where k_app represents the apparent degradation rate constant, which was determined by plotting ln(C/C₀) versus reaction time t. The values of k_app for SDM in TiO₂ 3D_M and P25 TiO₂ suspensions under simulated solar light are 0.029 and 0.018 min⁻¹, respectively, indicating the preferable photocatalytic performance of TiO₂ 3D_M which possessed a suitable conformation of pores allowing light waves to penetrate deep inside the photocatalyst and leading to a high mobility of charge.^36,37 The smaller the pore sizes, the stronger the penetration of light waves. The large specific surface area of TiO₂ 3D_M also facilitates the absorption and utilization of solar light, which is also essential for the photocatalytic degradation.^20,38 The experimental outcomes confirmed that the photocatalytic effects of TiO₂ 3D_M were stronger than that of P25 TiO₂.

The formation of the hydroxyl radicals on the surface of the mesoporous TiO₂ microspheres catalysts under simulated solar light irradiation was monitored by photoluminescence (PL) with terephthalic acid as a probe molecule which can readily react with ^•OH to produce highly fluorescent product, 2-hydroxyterephthalic acid.^39,40 The experimental procedure was similar to the photocatalytic process, except that the sulfonamides solution was replaced by the 5 × 10⁻⁴ M terephthalic acid solution in 2 × 10⁻³ M NaOH. The fluorescence spectra of the formed 2-hydroxyterephthalic acid were measured by a Hitachi F-4500 spectrophotometer excited at 315 nm. A higher recombination rate was observed from the higher PL intensity during the reaction process (Fig. S5, ESI†), indicating the formation of hydroxyl radicals during the photocatalytic degradation the present system.

3.3 The effect of H₂O₂ on the photodecomposition of sulfonamides and its degradation mechanism

Adding powerful oxidizing species such as hydrogen peroxide (H₂O₂) and potassium peroxydisulfate (K₂S₂O₈) to TiO₂ suspensions may increase the photo-oxidation rates.⁴¹ H₂O₂ plays dual roles in enhancing the semiconductor-sensitized photocatalytic degradation of organic compounds by acting as an electron scavenger to prevent the recombination of e⁻ and h⁺ and as a direct source of hydroxyl radicals.^26,42

The standard conditions for the photocatalytic degradation experiments involved 0.1 mM SDM in the presence of 0.5 g L⁻¹ TiO₂ 3D_M exposed to 30 min of simulated solar light irradiation. When H₂O₂ added, from 1.47 to 11.76 mM, the substrate in the reaction solution initially disappeared with amount of H₂O₂ added up to C = 5.88 mM, beyond which less substrate was degraded with more H₂O₂ added (Fig. 4a). As the SDM was almost completely eliminated at initial H₂O₂ concentrations of 5.88 mM, it was selected as the optimal H₂O₂ treatment for sequent experiments. The resulting first-order rate constants k are 0.060, 0.066, 0.089, 0.092 and 0.095 min⁻¹ for SAAM, SPD, SMZ, SIA and SDM, respectively (Fig. S6).

Fig. 4 Photo-catalytic oxidation of 0.1 mM SDM with presence of 0.5 g L⁻¹ TiO₂ 3D_M under simulated solar light for 30 min (a) H₂O₂ concentrations and (b) different reaction conditions with initial H₂O₂ concentrations of 5.88 mM.

Comparing outcomes of the treatment combinations, the TiO₂-3D_M + H₂O₂ without irradiation, H₂O₂ with irradiation only, and P25 TiO₂ + H₂O₂ with irradiation removed 5%, 21%, and 45% of SDM in the reaction solution in 30 min of contact time, respectively (Fig. 4b). The TiO₂-3D_M + H₂O₂ with irradiation treatment however resulted in a pronounced enhancement of the reaction and SDM was almost completely removed after 30 min of contact. The H₂O₂ by itself was ineffective in degrading SDM. The percentages of SDM degradation employing 30 min of simulated solar light irradiation in the presence of H₂O₂ only, P25 TiO₂ + H₂O₂ and TiO₂-3D_M + H₂O₂ were 21%, 45%, 99.5%, respectively (Fig. 4b). The fabricated mesoporous TiO₂ microspheres were efficient at SDM removal, maybe due to their large surface area, highly crystallized mesoporous wall, and greater number of active sites for concentrating the substrate. According to the fluorescence spectral results (Fig. S5, ESI†), hydroxyl radicals were formed on the surface of the mesoporous TiO₂ microsphere catalysts under simulated solar light irradiation. Additional hydroxyl radicals might be produced by the breakdown of H₂O₂ on the surface of TiO₂ under the irradiation of solar light (eqn (2)), while a possible reaction of H₂O₂ with the photogenerated intermediates cannot be excluded. The enhancement of substrate degradation at H₂O₂ dosages from 1.5 to 5.9 mM is the result of H₂O₂ photolysis by solar light that generates free radicals,⁴³ likely the dominant rate-improving mechanism in this process. H₂O₂ is a better electron acceptor than oxygen and may contribute to the enhanced substrate degradation in a minor way.^9,44 The electrons on the conduction band of TiO₂ can initiate the decomposition of H₂O₂ generating hydroxyl radicals, as shown in eqn (3), and forming ^•OH radicals via superoxide according to eqn (4).^14,42 The excessive H₂O₂ may compete with the organic compound for the adsorption sites on the catalyst surface, resulting in a “chromatographic peaking effect” of the pollutant concentration in the solution during the initial stages of the photocatalytic process.²² Meanwhile, the excess H₂O₂ however also may act as a hole that scavenges the ^•OH and reacts with the electron-hole to form O₂ or HO₂^• (eqn(5)). The HO₂^• not only serves as an exogenous active oxygen source reacting with organic compounds, but also scavenges the ^•OH (eqn(6)). It is imperative that the H₂O₂ concentration be optimal to achieve the maximum photocatalytic degradation of the substrate. The proposed photocatalysis process was illustrated in Scheme 1.

H₂O₂ + TiO₂ → 2^•OH (2)

H₂O₂ + e⁻ → OH⁻ + ^•OH (3)

H₂O₂ + O⁻₂ → ^•OH + OH⁻ + O₂ (4)

H₂O₂ + HO^• → H₂O + HO^•₂ (5)

HO₂^• + HO^• → H₂O + O₂ (6)

Scheme 1 Proposed photocatalysis process of the sulfonamides in aqueous suspension.

3.4 Degradation intermediates of SDM

The HPLC-ESI-SIR mass spectrometry chromatogram (Fig. 5) shows the potential organic intermediates 10 min after SDM in a reaction solution subjected to TiO₂-3D_M + H₂O₂ photocatalytic oxidation. Fig. S7, ESI† provided independent verification of the peaks in Fig. 5. Seven products were identified by the molecular ions and mass fragment peaks and by comparison with HPLC-MS library data (Fig. 6 and Fig. S8†). The molecular structure of SDM is displayed in Table 1; it has a [M+H]⁺ 311, which was eluted as peak 7 in Fig. 5 (R_t = 22.65 min, Fig. S8†). Calza et al.²⁴ reported that in the photocatalytic degradation of SDM using P25 TiO₂ powder only three intermediates were confirmed which corresponded to peak 4, peak 5, and peak 1 or 6 in Fig. 5. Two peaks holding [M+H]⁺ 327 (Fig. 5 peak 4, R_t = 20.43 min and peak 5, R_t = 21.91 min) present retention times shorter than the parent molecule, suggesting a proof for the occurrence of ^•OH radicals in the present system and the formation of a hydroxylated structure by an ^•OH radical attack,⁴⁵ which was also confirmed by the experimental results (Fig. S5, ESI†). The intermediates holding [M+H]⁺ 156 (Fig. 5 peak 1, R_t = 18.61 min and peak 6, R_t = 22.56 min) were found in our experiment, following the fragmentation pathway identified by the MS/MS spectra. The intermediate occurs through the cleavage of the S–N bond, and is attributed to the species 2,5-cyclohexadiene-1-thione-4-imino-dioxide and 2,6-dimethoxy-4-aminopyrimidine (Fig. 6).

Fig. 5 HPLC-ESI-SIR mass spectrometric chromatogram showing degradation intermediates of SDM photolysis in the presence of TiO₂ 3D_M and H₂O₂ and after 10 min of exposure to simulated solar light irradiation. Peaks are cross-referenced with outcomes of liquid chromatography mass spectrometry (HPLC-MS) shown in Fig. S7.†

Fig. 6 Proposed products of photocatalytic degradation of SDM in the presence of mesoporous TiO₂ microspheres and H₂O₂ exposed to simulated solar light irradiation.

In this study, the photocatalytic degradation of SDM produces additional products compared with the referenced literatures.^24,27 One important direct pathway is through excitation of SDM to its triplet state followed by self-quenching by ground state SDM to produce the SO₂ extrusion product 4-(6-imino-2,4-dimethoxypyrimidin-1(6H)-yl) aniline, which has been identified by other researchers^29,46 as [M+H]⁺ 247 mass unit corresponding to peak 2, R_t = 19.35 min in Fig. 5. One more intermediate product with [M+H]⁺ 172 with the m/z lost by 140, corresponding to peak 3, R_t = 19.72 min in Fig. 5 and was identified as 4-amino-benzene sulphonamide. Its formation may be attributed to the cleavage of the C–N bond, the mechanism used to explain the formation of sulfamethoxazole degradation intermediates by TiO₂ and TiO₂/FeCl₃ in aquatic environments.²⁵ The last compound with the [M+H]⁺ 341 mass unit is somewhat suspect as it would be formed from O-addition only to the phenyl ring or addition to both rings (peak 8, R_t = 24.51 min in Fig. 5).⁴⁶ The possible bisoxidation intermediate (SDM+32) in the experimental system was identified by an electrospray ionization (ESI) source and in the positive ion mode.

4. Conclusions

In this work, pure ordered mesoporous TiO₂ microspheres were successfully synthesized via a one-step solvothermal process without templates. The obtained TiO₂ 3D_M are highly active for the photodegradation of sulfadimethoxine and related sulfonamides antimicrobial agents. Due to their specific properties, these products have advantages over commercial P25 TiO₂ for photocatalytic activities. Our research has shown that SDM can undergo significant photocatalytic degradation by mesoporous TiO₂ microspheres with the assistance of H₂O₂ under simulated solar light irradiation, suggesting that these products will provide the possibility of being employed in the cleaning up of industrial pollutants. The seven intermediates of SDM identified by LC/ESI-MS and LC/ESI-SIR in this study have not been simultaneously proposed in previous research. However, although photocatalysis is efficient in degrading SDM, some of its photoproducts may exhibit residual antimicrobial activity which is the focus of the future research.

Acknowledgements

This work was supported by National Natural Sciences Foundation of China (20977051).

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Footnote

† Electronic supplementary information (ESI) available: Figures: (S1) The experimental apparatus, (S2) N₂ adsorption/desorption isotherms of the mesoporous TiO₂ microspheres, (S3) UV-vis reflection spectra of the catalysts, (S4) Mass chromatograms of five sulfonamides solutions after different simulated solar light irradiation time with TiO₂ 3D_M, (S5) Temporal fluorescence spectral, (S6) Kinetic linear simulation curve of five sulfonamides photocatalytic degradation by mesoporous TiO₂ microspheres under simulated solar light irradiation with the assistance of H₂O₂, (S7) Liquid chromatography mass spectrometry (LC-MS) chromatogram for SDM photocatalysis, (S8) Mass spectra corresponding proposed structures of six main peaks and SDM in TIC. Table: (S1) Analyte acronyms, molecular weights, ions monitored for LC–MS/MS, and conditions of cone voltages and collision energies. See DOI: 10.1039/c2ra01164f

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