Synthesis of Fe₃O₄@mTiO₂ nanocomposites for the photocatalytic degradation of Monocrotophos under UV illumination

Abstract

Novel magnetic mesoporous core/shell nanocomposites Fe₃O₄@mTiO₂ were synthetized and characterized by FE-TEM, EDS, PXRD as well as BET, and subsequently tested as photocatalysts for the degradation of Monocrotophos under UV irradiation. The well-designed nanocomposites exhibit a pure and highly crystalline anatase TiO₂ layer, large specific surface area, and high magnetic response. Photocatalytic degradation of Monocrotophos and the formation of intermediates were identified using HPLC, TOC-V_cpn, and GC-MS. Monocrotophos were found to completely disappear after 45 min of UV illumination. At the end of the treatment, the total organic carbon (TOC) of Monocrotophos was reduced by 82%. Finally, we proposed the photocatalytic degradation pathway for Monocrotophos.

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1. Introduction

In many developing countries, the use of pesticides is necessary for agricultural productivity. The organophosphorous pesticides, in particular, are used widely due to their relatively low price and effective ability to control pests, weeds, and diseases.¹ Most organophosphorous compounds display high toxicity and have been shown to result in high levels of acute neurotoxicity² and carcinogenicity.³ Thus, organophosphorous pesticides are abundant environmental and food chain pollutants and have attracted increasing global attention with respect to human, animal, and insect health.² One solution to this problem is the photocatalytic degradation of organophosphorous pesticides, which already has a 30 year history.⁴ Photocatalysis is a clean and attractive, low-temperature, non-energy-intensive approach for the treatment of pollutants in water and wastewater.⁵ Until now, many studies of the photocatalytic degradation of organophosphorous pesticides have been carried out^6,7 and the mechanism of purification by photocatalysis has been analyzed and documented. The photocatalyst is an essential part of the photocatalytic degradation process. Therefore, finding an efficient photocatalyst is an effective way to overcome this extreme problem, and hence many researchers have paid much attention to developing efficient photocatalysts.

Titanium dioxide (TiO₂) is considered an excellent photocatalyst for the degradation of organophosphorous pesticides due to its biological and chemical inertness, non-toxicity, strong oxidizing power, cost-effectiveness, and long-term stability against photo- and chemical corrosion, as well as its excellent degradation capacity for organic pollutants.^8–11 It is generally recognized that when TiO₂ is irradiated with photons of energy greater than its band-gap energy (3.2 eV), the photons are absorbed and high energy electron–hole pairs (e⁻/h⁺) are created. A portion of electron–hole pairs recombine producing thermal energy, and the remaining portion migrates towards the surface of TiO₂. The partial hole migration to the TiO₂ surface leads to the production of surface-trapped valence band (VB) holes or surface-bound OH radicals. Alternatively, conduction band (CB) electrons can migrate to the surface and react with dioxygen to produce superoxide (or hydroperoxide) radicals, particularly H₂O₂. Such a photochemical production of reactive oxygen species with strong oxidizing power is the basis of the remedial action of TiO₂ photocatalysis. However, recycling is a probable drawback in the case of using pristine titanium dioxide in organophosphorous pesticide wastewater treatment. Sedimentation is the most common route for separation of titanium dioxide particles after pH adjustment and coagulation–flocculation processes. However, after applying this method, a fraction of TiO₂ particles usually remains in the treated wastewater and a further microfiltration step is commonly required for final purification.^12–14 Thus, separation and recovery of TiO₂ from the organophosphorous pesticide wastewater becomes laborious, and adds additional cost and practical implications. In order to produce a recoverable and separable photocatalyst, the TiO₂ photocatalyst is typically deposited onto the surface of an iron oxide (magnetite, maghemite or ferrite) core.^15,16 Functionalization of TiO₂ by incorporating magnetic nanoparticle colloids allows a convenient recovery of magnetic photocatalyst under an external magnetic field. Recent studies demonstrated that magnetite (Fe₃O₄) is the most effective heterogeneous Fenton catalyst, when compared to other iron oxides,^17–19 possibly because it is the only one containing Fe²⁺ in its structure to enhance the production rate of ˙OH.¹⁹ As the two semiconductors connect to each other, the iron oxide spreads and foreign Fe ions diffuse into TiO₂ structures resulting in a decline in the surface area. A loss in surface may also occur, particularly during the heat treatment. As a result, the physicochemical properties induced by the electronic interaction between iron oxide and TiO₂ can be changed.²⁰ However, experimental results indicate that shortening the duration of the heat treatment can, not only significantly lower the extent of interaction between the iron oxide core and the TiO₂ coating, but also limit the extent of oxidation of the iron oxide core.²¹ The TiO₂/Fe₃O₄ photocatalyst that is prepared by acid–sol has higher catalytic activity than that prepared by a homogenous precipitation method due to the size difference of the particles.²² There are various synthetic routes to prepare magnetic TiO₂ photocatalyst, such as the sol–gel process,²³ co-precipitation,²⁴ hydrothermal treatment,²⁵ spray pyrolysis,²⁶ sonochemical synthesis,²⁷ and the wet impregnation method.²⁸ For example, Watson et al.,²⁹ and Akhavan et al.³⁰ have employed a sol–gel process to fabricate the coating of nanosized crystalline TiO₂ directly onto magnetic core particles. On the other hand, it is well known that iron oxide nanoparticles can produce ferrous or ferric ion leaching. Ferrous or ferric ion leaching from Fe₃O₄ nanoparticles, which are coated by a mesoporous TiO₂ layer, can be interpreted by energy level theory: the energy level of Fe³⁺/Fe²⁺ is close to the energy level of TiO₂'s conduction band, and the energy level of Fe³⁺/Fe⁴⁺ is close to the energy level of TiO₂'s valence band. Ferrous or ferric ion leaching from Fe₃O₄ nanoparticles, which are coated by a mesoporous TiO₂ layer, can come into not only an electron capture position but also a hole capture position, which causes the electron–hole pair recombination of TiO₂ to decrease.³¹ Therefore the photocatalytic activity of Fe₃O₄@mTiO₂ core/shell nanocomposites increases. Based on the above discussion, the coexistence of Fe²⁺ and H₂O₂ is possible, that is to say, there should be the possibility of photo-Fenton process in the photodegradation of Monocrotophos. Abbas et al.³² also reported a similar theory, that Fe₃O₄/TiO₂ core/shell nanocubes catalyzed Fenton-like reaction under ultraviolet irradiation.

In this study, we were successful in synthesizing the photocatalyst Fe₃O₄@mTiO₂ core/shell mesoporous nanocomposites. The protocol for the synthesis procedure is illustrated in Scheme 1A. Firstly, the Fe₃O₄ nanoparticles were prepared through a solvothermal reaction. Then, a compact TiO₂ layer was deposited directly on the surface of the Fe₃O₄ nanoparticles using a sol–gel method.^33,34 They were then subjected to hydrothermal treatment, which led to the formation of a tailor-made mesoporous TiO₂ shell. The experimental procedure sketch for the degradation of Monocrotophos using Fe₃O₄@mTiO₂ nanocomposites and magnetic separation is illustrated in Scheme 1B. All the synthesized nanomaterials have been unambiguously characterized by FE-TEM, EDS, PXRD as well as BET. The photocatalyst Fe₃O₄@mTiO₂ was used to decompose Monocrotophos under UV illumination. Comparative experiments were also conducted with commercial TiO₂–P25 in suspension. Monocrotophos and the formation of intermediates during photocatalysis were identified by HPLC, TOC-V_cpn, and GC-MS. Based on the main intermediate species, we proposed photocatalysis mechanisms for Monocrotophos.

Scheme 1 Schematic illustration of (A) synthetic procedure for the fabrication of the Fe₃O₄@mTiO₂ core/shell mesoporous nanocomposites, (B) experimental procedure for the degradation of Monocrotophos using Fe₃O₄@mTiO₂ nanocomposites and magnetic separation.

2. Materials and methods

2.1 Materials

Iron(III) chloride hexahydrate (FeCl₃·6H₂O), tetrabutyl titanate (TBOT, 97%), trisodium citrate (C₆H₅Na₃O₇·2H₂O), ammonium acetate (CH₃COONH₄), ethylene glycol (EG), ammonium hydroxide (NH₃·H₂O, 28%), anhydrous ethanol, and acetonitrile were purchased from Shanghai Chemical Reagents Company and used as received. Monocrotophos of analytical grade (99.9% purity) was purchased from Riedel-de-Haen (Germany). All chemical reagents were of analytical grade and used directly without further purification.

2.2 Synthesis of Fe₃O₄ nanoparticles

The Fe₃O₄ nanoparticles were synthesized according to a modified solvothermal reaction. Typically, 2.750 g FeCl₃·6H₂O, 9.250 g NH₄Ac and 0.971 g sodium citrate were dissolved in 150 mL of ethylene glycol and mixed for 30 min under ultrasound to form a pale brown solution. Then, the solution was stirred vigorously for 1.5 h at 140 °C to form a homogeneous black solution and transferred into a Teflon-lined stainless steel autoclave (150 mL capacity). The autoclave was heated to 200 °C and maintained for 20 h. After reaction, the autoclave was naturally cooled to room temperature; the black magnetic Fe₃O₄ nanoparticles product was collected with the aid of a magnet and washed with deionized water and alcohol. The Fe₃O₄ nanoparticles were dried in vacuum at 60 °C for 24 h or dispersed in ethanol for further use.

2.3 Synthesis of Fe₃O₄@TiO₂ core/shell nanocomposites

The Fe₃O₄@TiO₂ core/shell nanocomposites were synthesized by directly coating a TiO₂ layer on the surface of Fe₃O₄ in a mixed solvent of ethanol and acetonitrile at room temperature by hydrolyzing TBOT in presence of ammonia. Briefly, about 0.085 g of Fe₃O₄ nanoparticles were dispersed in a mixed solvent containing 100 mL of ethanol and 45 mL of acetonitrile with the aid of ultrasound. The suspension was stirred vigorously for 0.5 h and then 0.65 mL NH₃·H₂O was added dropwise at room temperature. This formed a weak alkaline homogeneous suspension. Then, 0.85 mL tetrabutyl titanate was added using a dropper into the suspension, while stirring gently. After reacting for 4.5 h, the black Fe₃O₄@TiO₂ core/shell nanocomposites product was isolated with the aid of a magnet and washed with acetonitrile and alcohol. Finally, the Fe₃O₄@TiO₂ core/shell nanocomposites were dried in vacuum at 60 °C for 24 h or dispersed in ethanol for further use.

2.4 Synthesis of Fe₃O₄@mTiO₂ core/shell mesoporous nanocomposites

Through treating the Fe₃O₄@TiO₂ nanocomposites by a hydrothermal method, mesoporous TiO₂ shells were obtained. Briefly, about 0.100 gas-prepared Fe₃O₄@TiO₂ core/shell nanocomposites were dispersed in 120 mL mixed solvent of 90 mL ethanol and 30 mL deionized water, with the help of ultrasound. Then, 3 mL NH₃·H₂O was added using a dropper to the suspension to form a weak alkaline system. Finally, the suspension was transferred to a Teflon-lined stainless steel autoclave with a capacity of 200 mL. The autoclave was heated to 150 °C and maintained for 24 h. After the reaction, the autoclave was naturally cooled to room temperature; a black paste of Fe₃O₄@mTiO₂ nanocomposites was collected with a magnet and fully rinsed with deionized water and alcohol. Finally, the Fe₃O₄@mTiO₂ core/shell mesoporous nanocomposites were dried in vacuum at 60 °C for 24 h or dispersed in ethanol for further use.

2.5 Characterization

Field-emission transmission electron microscope (FE-TEM) images were obtained using a JEM-2100 2015 FE-TEM equipped with a post-column Gatan imaging filter (GIF-Tridium) at an acceleration voltage of 200 kV. Samples dispersed at an appropriate concentration were cast onto a carbon-coated copper grid. The chemical composition was analyzed by energy dispersive X-ray spectrometry (EDX) by embedding its detector on the TEM. The specific surface area and pore volumes of samples were obtained by nitrogen adsorption–desorption at 77 K using the BET method with a Micromeritics 2012 instrument (ASAP2020 MP, Micromeritics, USA). Powder X-ray diffraction (PXRD) patterns of all samples were obtained on an X'Pert Pro (Panalytical, The Netherlands) diffractometer with Cu-K_α radiation at λ = 0.154 nm operating at 40 kV and 40 mA over the 2θ range 0–80°.

2.6 Photocatalytic activity

2.6.1 Irradiation procedure in aqueous solution. The circulation type photocatalytic reactor used in this study can be seen in our previous paper.³⁵ The solution of Monocrotophos with the appropriate amount of photocatalyst was mechanically stirred before and during the illumination. The suspension was stirred in the dark for 30 min to obtain a good dispersion and reach an adsorption–desorption equilibrium between the Monocrotophos and the photocatalyst surface. After that, the solution was irradiated under UV light with constant mechanical stirring for a predetermined amount of time. All experiments were conducted in replicates. The reaction vessel was used repeatedly after clean-up by a series of washings using deionized water. Monocrotophos is a widely used organophosphorous pesticide and was our target pollutant for photocatalysis. 100 mg L⁻¹ aqueous solution of Monocrotophos was prepared as a stock solution. The stock solution was diluted 5 times to obtain 20 mg L⁻¹ for the Monocrotophos used in this study.

2.6.2 Analysis. At specific time intervals the reaction liquid was withdrawn from the test solution. The solution samples were isolated with the aid of a magnet (to remove the Fe₃O₄@mTiO₂ photocatalyst) or centrifugation (to remove the P-25 photocatalyst). Concentration of the Monocrotophos was determined by a Shimadzu 10AD liquid chromatograph equipped with a variable wavelength UV detector using a reversed phase, 250 mm 4.6 mm, C 18 nucleosil 100-S column. The mobile phase was a mixture of methanol and water (80/20, v/v) with a flow rate of 1.0 mL min⁻¹. The detection was observed at 8.82 min. The elimination of Monocrotophos from the aqueous phase was observed based on the decrease of total organic carbon (TOC) as measured by TOC-V_cpn (Shimadzu, Japan). The organic intermediates of Monocrotophos destruction in the aqueous solutions were detected by the GC-MS system (HP 6890 GC-5972 MSD, with a HP-5 column 0.25 mm × 0.1 m × 30 m) with electron impact. Standard solutions of pure compounds were used for quantitative analysis.

3. Results and discussion

3.1 Characterization of Fe₃O₄, Fe₃O₄@TiO₂, and Fe₃O₄@mTiO₂ nanocomposites

The size and morphology of the synthesized nanomaterials were characterized by FE-TEM. As shown in Fig. 1A, uniform and consistent spherical-shaped nanocomposites with a mean diameter of approximately 250 nm (Fe₃O₄), 460 nm (Fe₃O₄@TiO₂), and 470 nm (Fe₃O₄@mTiO₂) were obtained. A well defined core/shell structure was produced; the TiO₂ shell appeared to be continuous and compact in terms of its structure and the possibility of tuning the thickness was afforded by varying the amount of TBOT that was fed into the reaction. The shell thickness in Fe₃O₄@mTiO₂ was approximately 110 nm, with many seemingly porous shells being produced (Fig. 1A(c and f)). The selective area electron diffraction (SAED) pattern of Fe₃O₄ exhibited regular and clear diffraction rings which may be indexed to the 220, 311, 400, 511, and 440 planes of Fe₃O₄ (JCPDS file no. 19-0629), revealing its polycrystalline nature. In the pattern of Fe₃O₄@TiO₂, diffraction from the TiO₂ phase was not observed, indicating that the TiO₂ shell is amorphous. There are two sets of diffraction spots in the Fe₃O₄@mTiO₂ pattern, which can be assigned as Fe₃O₄ and TiO₂, indicated by blue and red characters, respectively, and indicating that the mesoporous TiO₂ shell is a crystalline phase. This is in agreement with the X-ray diffraction data (Fig. 2B). The compositions of the Fe₃O₄@mTiO₂ nanocomposites were identified by EDX spectrum (Fig. 1B). Ti, Fe, and O were the three main elements found with the weight percentage of 15.30%, 52.20%, and 32.60%, respectively, indicating that the obtained nanocomposites were composed of the target materials. This further showed that it was possible for well-structured Fe₃O₄@mTiO₂ materials to be obtained.

Fig. 1 (A) Representative FE-TEM images of Fe₃O₄ (a and d), Fe₃O₄@TiO₂ (b and e), and Fe₃O₄@mTiO₂ (c and f) with different scale bars (inset: the corresponding SAED pattern). (B) The EDX pattern of Fe₃O₄@mTiO₂ nanocomposites.

Fig. 2 (A) Nitrogen adsorption–desorption isotherms (● = adsorption, ○ = desorption) and BJH pore size distribution curve (inset) for Fe₃O₄@mTiO₂. (B) XRD patterns for Fe₃O₄, Fe₃O₄@TiO₂, and Fe₃O₄@mTiO₂.

As shown in Fig. 2A, the Fe₃O₄@mTiO₂ exhibited typical type IV gas sorption isotherms indicative of the mesoporous character. According to calculations made using the BET model,³⁶ the Fe₃O₄@mTiO₂ nanocomposites resulted in a specific surface area of 151.7 m² g⁻¹. Their corresponding pore size distributions were evaluated using the Barrett–Joyner–Halenda (BJH) model³⁷ and the populations were found to be centered at 12.2 nm (Fig. 2A, inset). Before mesoporous treatment, the specific surface area of Fe₃O₄@TiO₂ was only 22.8 m² g⁻¹ (Fig. S1B†). As a control, the specific surface area of P-25 was also calculated to be 142.6 m² g⁻¹ (Fig. S1A†), proving the large specific surface area of commercial TiO₂ P-25. The PXRD patterns of the synthesized nanocomposites are shown in Fig. 2B. It can be seen that pure Fe₃O₄ nanoparticles have been prepared because of the typical cubic structure of Fe₃O₄ (JCPDS 19-629) and the diffraction peaks are broad, which implies that the original nanoparticles' sizes are small. Prior to the crystallization treatment of the TiO₂ shells, the Fe₃O₄@TiO₂ nanocomposites showed a simple PXRD pattern, the same as Fe₃O₄, which could be ascribed to the typical cubic structure of Fe₃O₄ (JCPDS 19-629). No characteristic TiO₂ crystal peaks were detected, which was indicative of an amorphous TiO₂ shell formed around the Fe₃O₄ core. This also proves that dissolving Fe³⁺ from the Fe₃O₄ core in the preparation process does not cause a change in the crystal structure. After hydrothermal treatment for 24 h, the PXRD pattern for the synthesized Fe₃O₄@mTiO₂ nanocomposites was noticeably different from the former pattern. In addition to the diffraction peaks that were preserved due to the Fe₃O₄ component, the other peaks (marked with “A” in the spectrum) corresponding to 2θ of 25.7, 38.1, 48.7, 54.5 and 63.30 indicate that the anatase phase was predominant. These results are in close agreement with the XRD pattern reported by Chen and Gengyu³⁸ for anatase TiO₂ crystal structure. Yet again, this finding confirmed the occurrence of a structural transition in the TiO₂ shell from an amorphous to a crystalline phase. In addition, we also carried out the XPS, magnetic properties, and zeta potential characterizations of the synthetized nanoparticles; the detailed analysis can be seen in ESI 1, 2 and 4.†

3.2 Photocatalytic activity on Monocrotophos in aqueous solution

The photocatalytic activity of the Fe₃O₄@mTiO₂ nanocomposites was evaluated by the degradation of Monocrotophos under the irradiation of a 100 W high pressure mercury lamp (the schematic of the photocatalytic reactor is the same as in our previous research. Fig. 5, Journal of Hazardous Materials 315 (2016) 11–22). Under dark conditions without UV illumination, however, the concentration of Monocrotophos does not significantly change for every measurement in the presence of Fe₃O₄@mTiO₂ or P-25 nanoparticles. Illumination in the absence of Fe₃O₄@mTiO₂ or P-25 also does not result in the distinct photocatalytic degradation of Monocrotophos. Therefore, the presence of both UV illumination and TiO₂-based materials is necessary for efficient degradation. This result agrees with previous research reports.³⁹ The photocatalytic degradation of Monocrotophos in the presence or in the absence of the two photocatalysts is presented in Fig. 3A. Evidently, illumination of the solution in the absence of photocatalysts demonstrated that the photolytic decomposition of Monocrotophos occurs at a much slower rate. Notably, the concentration of Monocrotophos decreases substantially within only 30 min of mixing Monocrotophos with Fe₃O₄@mTiO₂ under UV illumination. According to Fig. 3A, we can calculate and deduce that about 15.6% of Monocrotophos molecules can be adsorbed on the surface of the Fe₃O₄@mTiO₂ photocatalyst because of the large surface area. Particularly, Fe₃O₄@mTiO₂ appears to give much higher photodegradation efficiency than P-25 at concentration of 100 mg L⁻¹ under UV illumination.

Fig. 3 (A) Changes in the concentration of Monocrotophos under photolysis, P-25 and Fe₃O₄@mTiO₂ conditions. (B) The relationship curves of the ln(C_t/C₀) versus reaction time for Fe₃O₄@mTiO₂ and P-25 photocatalysts. C_t is the Monocrotophos concentration at time t, and C₀ is the concentration in the initial solution.

According to many researchers, the influence of the initial concentration of the solute on the photocatalytic degradation rate of most organic compounds is described by pseudo first order kinetics, which is rationalized in terms of the Langmuir–Hinshelwood model, modified to accommodate reactions occurring at a solid–liquid interface.⁴⁰ Fig. 4B shows the relationship curves of ln(C_t/C₀) versus reaction time for the Fe₃O₄@mTiO₂ and P-25 photocatalysts, where C_t is the concentration of Monocrotophos at the irradiation time t and C₀ is the initial concentration. An exponential degradation of Monocrotophos concentration with the irradiation time is evident, indicating that the photodegradation processes follow a pseudo first order reaction and confirming the pesticide's decomposition due to photocatalysis.⁴¹ The apparent rate constant of Monocrotophos for the reaction with the Fe₃O₄@mTiO₂ as the photocatalyst, directly calculated from the slope of the straight line, is 0.0752 min⁻¹ much higher than 0.0219 min⁻¹ calculated for P-25. The results further implied the good catalytic activity of the synthesized Fe₃O₄@mTiO₂ photocatalyst.

Fig. 4 Photodegradation of Monocrotophos with different Fe₃O₄@mTiO₂ concentrations as function of irradiation time.

3.3 Effect of photocatalyst concentration

The influence of the photocatalyst concentration on the degradation rate of Monocrotophos was investigated by testing different concentrations of Fe₃O₄@mTiO₂ varying from 50 to 300 mg L⁻¹ (Fig. 4). It is obvious that the rate increases with an increase in the amount of catalyst up to a level that corresponds to the optimum light absorption; above this value the rate decreased which is in agreement with previous study.³⁹ Fe₃O₄@mTiO₂ appears to give a higher photodegradation rate at low concentrations and the photodegradation rate at 50 mg L⁻¹ was slightly lower at 100 mg L⁻¹. The photodegradation rate dramatically decreased when the photocatalyst concentration increased to 200 mg L⁻¹ and 300 mg L⁻¹. In small concentrations Fe₃O₄@mTiO₂ appears to be more effective, probably due to its larger surface area. As concentration increases, the light scatters moreso in the Fe₃O₄@mTiO₂ suspensions due to the small particle size, which reduces the photocatalytic efficiency. Thus, any further increase in the amount of photocatalyst will have a negative effect on the photodegradation efficiency.

3.4 Effect of initial concentration

The effect of initial Monocrotophos concentration on photocatalytic degradation was studied in the concentration range from 20 to 80 mg L⁻¹ with constant photocatalyst addition of 100 mg L⁻¹ and the results are depicted in Fig. S5.† It is clear that the degradation rate increases with an increase in initial concentration of Monocrotophos from 20 to 60 ppm, and then decreases. The rate of degradation is related to the formation of the ˙OH radical, which is the critical species in the degradation process. The equilibrium adsorption of reactants on the catalyst surface, and the reaction rate of ˙OH radicals with other chemicals are also significant in the rate of degradation. At higher pesticide concentrations, there would be more adsorption of pesticide on TiO₂ resulting in a less availability of catalyst surface for hydroxyl radical generation. A higher pesticide concentration may also almost adsorb more photons from the UV light thus reducing the photonic energy available for hydroxyl radical generation. Higher initial concentration also produces more intermediates which may competitively inhibit the pesticide degradation. It also depends on the type of catalyst, reactor geometry and irradiation source. (The apparent rate constants k_app and half-life t_1/2 have been calculated and presented in Fig. S5†).

3.5 Effect of pH

The amphoteric behavior of titania influences the surface charge of Fe₃O₄@mTiO₂. The role of pH on the photocatalytic degradation rate was studied in the pH range 3–9 at constant Monocrotophos concentration of 20 mg L⁻¹ and Fe₃O₄@mTiO₂ concentration of 100 mg L⁻¹. The results are shown in Fig. S6.† It is observed that the rate of degradation increases with an increase in pH, exhibiting a maximum at pH 5, and then decreases. This means Monocrotophos degradation appears to be best in a slightly acidic medium.

Titanol (Ti–OH) is present on the surface of TiO₂. It is amphoteric and occurs in acid–base equilibrium as indicated by the following equations. TiOH + H⁺ → TiOH₂⁺, pH < 6.25 TiOH → TiO⁻ + H⁺, pH > 6.25. In acidic environment, H⁺ ions are adsorbed onto the surface of TiO₂, which has large proton exchange capacity. The photogenerated electrons can be captured by the adsorbed H⁺ to form H_ads˙. At higher pH, the surface of the catalyst has a net negative charge due to a significant fraction of total surface sites being present as TiO⁻ and hence the degradation rate is found to be less. Moreover, some acidic intermediates are also formed during the decomposition of Monocrotophos, which may result in the drop in pH. (The apparent rate constants k_app and half-life t_1/2 have been calculated and presented in the Fig. S6†).

3.6 Mechanism analysis for photodegradation of Monocrotophos

A complete degradation of the Monocrotophos molecule leads to the conversion of all its carbon atoms to gaseous CO₂ and the heteroatoms into PO₄³⁻ and NO₃⁻ remain in the solution. In order to study the extent of mineralization of Monocrotophos during photocatalysis, we carried out the TOC measurements. The TOC changes in an aqueous phase of Monocrotophos with P-25 and Fe₃O₄@mTiO₂ are shown in Fig. 5. With Fe₃O₄@mTiO₂ as the photocatalyst, complete disappearance of Monocrotophos was observed after 45 min while TOC was approximately 71% reduced indicating the existence of intermediates. Table 1 lists the main products detected with GC-MS during photocatalysis of Monocrotophos over Fe₃O₄@mTiO₂. Based on the aforementioned results the proposed scheme for the photodegradation of Monocrotophos is presented in Fig. 6. The detailed analysis of the formation mechanism of active species on the TiO₂ surface during the photodegradation of organophosphorous pesticides has already been reported.^42–44 Briefly, it is considered that the photocatalysis mechanism of organophosphorous pesticides by active species (active oxygen or hydroxyl radical) is similar to that of other organic compounds. However, it remains unclear which atomic bond in organophosphorous pesticide molecules is the target of the photocatalytic cleavage. In this work, the C=C double bond in the Monocrotophos molecule is supposed to be easily detached by active species⁴⁵ and, on the basis of the GC-MS detection results that (CH₃O)₂P(O)OH was observed with no observation of nitrogen-containing species, we propose that the cleavage of the C=C double bond, C(O)=N bond, and (CH₃O)₂P(O)O–C bond occur in the early stage of photocatalysis. The cleavage of (CH₃O)₂P(O)O–C bond results in the formation of (CH₃O)₂P(O)OH and (CH₃O)₃P(O). It is possible that (CH₃O)₂P(O)OH was unstable and could transform to (CH₃O)₃P(O) and then subsequently it could be oxidized to CH₃O(OH)₂P(O). The C atom in CH₃O(OH)₂P(O) could be eliminated by an H-abstraction or addition–elimination with ˙OH, and the P atom would undergo oxidation and reduction, resulting in the formation of H₃PO₄ and H₃PO₃. Moreover, the cleavage of C=C double bond and C(O)=N bond caused the formation of OHCH₂C(O)OH, a small molecule organic acid, and the N atom undergoes oxidation being converted to NO₃⁻. Finally, we proposed the photocatalysis scheme for Monocrotophos presented in Fig. 6.

Fig. 5 The changes in the total organic carbon (TOC) concentration of Monocrotophos under P-25 and Fe₃O₄@mTiO₂ conditions.

Table 1 Intermediates of Monocrotophos detected by GC-MS

	Structure	Formula	Molecular weight
Monocrotophos		(CH3O)3P(O)	140
		(CH3O)2P(O)OH	126
		HOCH2C(O)OH	76

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Fig. 6 Expected photodegradation scheme of Monocrotophos with Fe₃O₄@mTiO₂ as photocatalyst.

3.7 Regenerability of the Fe₃O₄@mTiO₂ photocatalyst

In order to examine the stability and regenerability of the Fe₃O₄@mTiO₂ photocatalyst, four successive experiments were conducted under the same experimental conditions using the same Fe₃O₄@mTiO₂. The photocatalyst was isolated from the reaction solution thorough an external magnet and rinsed with absolute alcohol thoroughly after every cycle of the photocatalysis experiment, then reused in the next cycle. As shown in Fig. 7, the photocatalytic efficiency is virtually unaffected by the reaction with Monocrotophos under irradiation of UV light. Even after four reaction cycles, the photocatalytic degradation efficiency remained stable, decreasing by only 2.37%. This observation clearly demonstrates the stability and regenerability of Fe₃O₄@mTiO₂ photocatalyst. In addition, reaction products formed on the surface of the Fe₃O₄@mTiO₂ nanocomposites were removed instantaneously by water molecules which collide with the surface, thus avoiding surface poisoning. Therefore, the photocatalyst of Fe₃O₄@mTiO₂ is determined to be excellent in terms of stability and regenerability.

Fig. 7 Regenerability of Fe₃O₄@mTiO₂ photocatalyst used for photocatalytic degradation of Monocrotophos in aqueous solution.

4. Conclusion

In summary, we have successfully synthesized an Fe₃O₄@mTiO₂ core/shell mesoporous nanocomposites photocatalyst with a magnetite core and a homogeneous mesoporous crystalline TiO₂ shell. The photocatalytic degradation of Monocrotophos under UV illumination was conducted and it was confirmed that Fe₃O₄@mTiO₂ nanocomposites had excellent photocatalytic activity. Monocrotophos and the formation of intermediates during photocatalysis were identified by HPLC, TOC-V_cpn, and GC-MS. Based on the main intermediate species we proposed the photodegradation mechanism for Monocrotophos.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by Food Service Industry Special (201513006), Commonwealth Project of the Ministry of Agriculture (No. 201203069–1), Project of National Functional Food Engineering (JUSRP51501), Program of Qing Lan Project and the Program for New Century Excellent Talents in Jiangnan University, Collaborative Innovation Center of Food Safety and Quality Control in Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13423h

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