Synthesis of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites via sol-gel chemistry for water treatment by photo-Fenton and photocatalytic reactions

Abstract

MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were synthesized in different ratios of MgAC-Fe₃O₄ and TiO₂ precursor. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray fluorescence spectrometry (XRF), electron spin resonance spectrometry (ESR), Brunauer-Emmett-Teller (BET), photoluminescence (PL), and UV photoelectron spectroscopy (UPS) were used to characterize the nanocomposites. The increase of MgAC-Fe₃O₄, in the hybrid nanocomposites’ core-shell structure, led to the decrease of anatase TiO₂ peaks, thus reducing the photo-Fenton and photocatalytic activities. According to the obtained data, MgAC-Fe₃O₄ [0.05 g]/TiO₂ showed the best photo-Fenton and photocatalytic activities, having removed ~93% of MB (photo-Fenton reaction) and ~80% of phenol (photocatalytic reaction) after 20 and 80 mins, respectively. On the pilot scale (30 L), MgAC-Fe₃O₄ [0.05 g]/TiO₂ was completely removed after 27 and 30 hours by the photo-Fenton and photocatalytic activities, respectively. The synergistic effect gained from the combined photo-Fenton and photocatalytic activities of Fe₃O₄ and TiO₂, respectively, was credited for the performances of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites.

Introduction

Advanced oxidation processes (AOPs) including O₃/H₂O₂, UV/O₃, UV/H₂O₂, H₂O₂/Fe²⁺ and UV/TiO₂ have been utilized for removal of toxic organic compounds in water/waste water, air, and soil. AOPs produce hydroxyl radicals that contain powerful oxidants capable of oxidizing various organic compounds with one or many double bonds¹. Recently, the potential of photocatalytic materials has been extended to other applications such as photocorrosion inhibition, solar water splitting by combined with different materials^2–4. Among AOPs materials, photocatalysis with TiO₂ nanoparticles (NPs) under UV/Visible light has attracted keen interest from the time of its first discovery by Frank and Bard⁵, specifically due to its high oxidative power and chemical stability^6–8. However, the limitation of TiO₂ NPs is their low quantum efficiency of photo-generated electron-hole pairs caused by a high and rapid recombination rate⁹. For example, Degussa P25, a well-known commercial product, can reduce only 14% of phenol at 365 nm in water¹⁰. Another major disadvantage of TiO₂ NPs is that they cannot be recycled post-reaction. Leakages of photocatalytic materials to aqueous solution, moreover, can lead to secondary pollution. These disadvantages can be overcome by coating TiO₂ NPs on the surfaces of magnetic components such as Fe₃O₄ NPs, which can be easily collected from solutions under magnetic fields¹¹.

Fe₃O₄ NPs have attracted interest due to their considerable magnetic behavior and strong pin polarization¹². Many methods of Fe₃O₄-TiO₂ composite synthesis, such as sol-gel, co-precipitation, hydrothermal, sonochemical, and templates routes, have been reported in the literature^12,13. Incorporation of Fe₃O₄ NPs into a TiO₂ matrix can block NP aggregation and improve the durability of catalysts^14,15. However, due to the small band gap of Fe₃O₄ NPs (0.1 eV), Fe₃O₄-TiO₂ composites will increase the rate of electron-hole pairs recombination, with the result that photocatalysis is usually unchanged or even diminished relative to pure TiO₂ NPs¹². To overcome this problem, Zheng et al. demonstrated that special structures such as core-shell microspheres in Fe₃O₄-TiO₂ composites can delay the recombination of photo-induced electrons¹²; other researchers have used noble metal (Au or Ag) or rare elements (e.g. Eu) as electron traps to enhance electron-hole separation and facilitate electron excitation by creating a local hole in the electrical field^13,16,17. In contrast, He et al., after preparing Fe₃O₄-TiO₂ core-shell NPs, indicated that Fe³⁺ released from Fe₃O₄ can be doped into TiO₂ NPs to decrease electron-hole pair recombination and thus increase the photocatalytic performance of Fe₃O₄-TiO₂ core-shell NPs under visible light¹⁸. One remarkable report in this research field is that of Sun et al., who found that a small number Fe₃O₄ NPs loaded onto TiO₂ NPs (Fe/TiO₂ ratio: 1/200) could enhance the degradation of organic dye (Reactive Brilliant Red X3B). They attributed the improved photocatalytic performance of Fe₃O₄-TiO₂ to the synergistic contribution of the photocatalytic and Fenton reactions in the composite¹⁹.

2-D materials have been attracted and extended their applications due to their unique properties²⁰. Photocatalytic materials have also been developed based on 2-D materials such as graphene²¹.

From its first introduction by Mann et al., magnesium aminoclay (MgAC), which is also another types of 2-D materials, has attracted interest in its propylamine functionalities, structures, and high dispersity in water^22–24. Use of MgAC’s high adsorption utility for heavy metal and organic dye removal has been reported²⁵. Besides being utilized as a single agent, MgAC has been conjugated with other materials for environmental-treatment purposes. For example, MgAC has been coated with nZVI for removal of perfluorinated compounds²⁶ and chromium²⁷.

In previous work, we conjugated MgAC with TiO₂ NPs and Fe₃O₄ NPs by different methods for environmental-treatment^28,29 and microalgae-harvesting purposes^30,31. The presence of MgAC in composites was demonstrated to improve the photocatalytic behavior of pure TiO₂ NPs²⁸ as well as the photo-Fenton behavior of Fe₃O₄ NPs²⁹. Based on these successful preliminary results, in the present study, we synthesized MgAC-Fe₃O₄/TiO₂ hybrid composites in order to exploit the advantages of both Fe₃O₄ and TiO₂ in environmental-treatment applications.

Results and Discussion

Photo-Fenton performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on batch scale

Among different MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on the batch scale, the MgAC-Fe₃O₄ [0.05 g]/TiO₂ sample showed the best photo-Fenton performance, more than ~93% of MB having been removed after 20 min at a constant rate of ~0.1175 min⁻¹; this was ~10 and ~100 times higher than the performances of MgAC-Fe₃O₄ [0.1 g]/TiO₂ and MgAC-Fe₃O₄ [0.2 g]/TiO₂, which removed ~92 and ~17% of MB from aqueous solution after 180 min of reaction, respectively (Fig. 1a and Table 1).

Figure 1.

Photo-Fenton (MB, a) and photocatalytic performances (phenol, b) of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on batch scale.

Table 1.

Degradation rates of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites by photo-Fenton and photocatalytic reaction on batch scale.

Sample	Photo-Fenton constant rate against MB 10 ppm (min−1)	Photocatalytic constant rate against phenol 5 ppm (min−1)
MgAC-Fe3O4 [0.05 g]/TiO2	0.1175	0.0039
MgAC-Fe3O4 [0.1 g]/TiO2	0.0130	0.0012
MgAC-Fe3O4 [0.2 g]/TiO2	0.0011	0.0006

We chose the MgAC-Fe₃O₄ [0.05 g]/TiO₂ sample to investigate the photo-Fenton performance when inhibition agents such as humic acid (HA), SO₄²⁻, and HCO₃⁻ existed in the aqueous solution. Among these three inhibition agents, SO₄²⁻ and HCO₃⁻ had stronger inhibitory effects on the photo-Fenton reactions of the hybrid samples than HA, especially when these anionic and organic substances were presence at high concentrations in water (Fig. S2). The presence of HA, SO₄²⁻, and HCO₃⁻ can limit the photo-Fenton reaction of hybrid samples by reacting with generated reactive oxygen species (ROS)³².

Photocatalytic performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on batch scale

Instead of MB, we chose phenol to investigate the photocatalytic performances of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on the batch scale. Phenol is widely utilized in wastewater treatment within various industrial fields such as resins, petrochemicals, paints, textiles, oil refineries, foods, photographic chemicals, antioxidants and flavoring agents³³. Phenol is a highly toxic pollutant that can cause environmental degradation and serious health problems in humans.

Among the samples, MgAC-Fe₃O₄ [0.05 g]/TiO₂ showed the best photocatalytic activity, ~80% of phenol having been removed after 180 min at a constant rate of 0.0039 min⁻¹; MgAC-Fe₃O₄ [0.1 g]/TiO₂ and MgAC-Fe₃O₄ [0.2 g]/TiO₂, by comparison, had removed only 40 and 27% of phenol, respectively (Fig. 1b and Table 1). The photocatalytic activities of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were attributed to the synergistic contributions of the photocatalytic and Fenton reactions in the hybrid nanocomposites¹⁹. In previous studies, Sun et al. indicated that increased Fe₃O₄ NP number could lead to decreased photocatalytic activity. Excess loading of Fe₃O₄ NPs promotes cluster aggregation, reduces O₂ adsorption, and thereby decreases the efficiency of interfacial charge transfer for pollutant degradation¹⁹.

After 5 times recycle, the MgAC-Fe₃O₄ [0.05 g]/TiO₂ sample still remained ~80% of its initial removal efficiencies against phenol (Fig. S3). The deactivation of MgAC-Fe₃O₄ [0.05 g]/TiO₂ could be explained by the accumulation of phenol intermediate products on actives sites of hybrid nanocomposites³⁴. The presence of these intermediate products in the reactor after stopping photocatalytic reaction could be supported by results of TOC below³⁵. The hybrid nanocomposites could not be regenerated by using simple washing methods³⁴. In the future works, the more suitable regeneration methods should be considered to improve the recycling performances.

Photo-fenton and photocatalytic mechanisms

The photocatalytic mechanism of MgAC-Fe₃O₄/TiO₂ was attributed to the presence of anatase TiO₂ in the samples, as follows:

$${\mathrm{TiO}}_2+hv\to {\mathrm{TiO}}_2(e^{-}+h^{+})$$

$$h^{+}+{\mathrm{H}}_2\mathrm{O}\to H^{+}+O{H}^{•}$$

$$h^{+}+{\mathrm{OH}}^{-}\to {\mathrm{OH}}^{•}$$

$$e^{-}+O_2\to {}^{•}{\mathrm{O}}_2^{-}$$

$${\mathrm{Fe}}^{3+}+{\mathrm{TiO}}_2({\mathrm{e}}^{-}+h^{+})\to {\mathrm{Fe}}^{2+}+{\mathrm{TiO}}_2(h^{+})$$

OH^• and ^•O₂⁻, which are produced through the above chain reactions, will degrade pollutant molecules via oxidation and a reduction reaction process, respectively. To investigate the contribution of these ROS to the photocatalytic activity of MgAC-Fe₃O₄/TiO₂, different scavengers have been used: isopropanol for •OH, methanol for both h⁺ and •OH, and p-benzoquinone for ^•O₂^− 36,37. In our system, highly reactive •OH was the main actor in the degradation of the pollutant materials, rather than the valence band h⁺ and the conductive band e⁻ (Fig. S4)³⁶.

On the other hand, the photo-Fenton activities of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were extremely high, due to the presence of H₂O₂ in the reaction:

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {}^{•}\mathrm{OH}+{\mathrm{Fe}}^{3+}+{\mathrm{OH}}^{-}$$

$${\mathrm{H}}_2{\mathrm{O}}_2+hv\to {}^{•}\mathrm{OH}$$

As noted just above, pollutants are degraded mainly by ^•OH, which is generated in a photo-Fenton-like process: direct photolysis of H₂O₂, and photocatalytic oxidation of adsorbed H₂O through holes in the valence band of the TiO₂ surface. Additionally, photo-induced electrons generated from TiO₂ can cause reduction of Fe³⁺ to Fe^2+ 38. The photocatalytic and photo-Fenton activities of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites can be additionally supported by the presence of MgAC, which, with its high adsorption utility, can bring reactants to the surfaces of photo-Fenton agents^24,25.

Photoluminescence spectra could be used to explain the photocatalytic mechanism³⁹. From photoluminescence spectra (Fig. S5), it is clearly seen that, due to the presence of TiO₂ in the hybrid composites, the separated electron and holes were kept longer in excited state than original MgAC-Fe₃O₄ samples^40,41. These results also were used to support the photocatalytic mechanisms of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites. In addition, MgAC-Fe₃O₄ [0.05 g]/TiO₂ have the lowest photoluminescence intensity.

Preliminary evaluation of photocatalytic and photo-Fenton performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on pilot scale

The photocatalytic activities of TiO₂ NP materials have been widely investigated on the batch scale. However, for the purposes of industrial application, these materials need to be up-scaled to pilot-scale reactors. There are a variety of pilot reactors that have been introduced in the literature^26,27. In this study, we used a design of pilot reactor that has been introduced in previous reports^28,29. The design and photos of the pilot reactor are presented in Fig. S6.

Based on the batch-scale results discussed above, MgAC-Fe₃O₄ [0.05 g]/TiO₂ was mass produced for testing of photocatalytic and photo-Fenton performances on the pilot scale. For photocatalytic performance testing, 30 g of MgAC-Fe₃O₄ [0.05 g]/TiO₂ was added to the 30 L pilot reactor to obtain a dosage of 1 g/L (similarly to the batch-scale study). After 30 hours, phenol had been nearly completely removed from the aqueous solution at a constant rate of 0.032 (h⁻¹) (Fig. 2a and Table 2). The extended reaction time might be attributable to the heaviness of the materials, which is quickly self-precipitated. However, this phenomenon makes MgAC-Fe₃O₄ [0.05 g]/TiO₂ easily recoverable after the reaction. In this study, ~80% of the materials was recovered from the reactor by simply stopping the reactor and allowing self-precipitation to occur for 24 hours without any external force (Fig. S7). To shorten the reaction time, we added 50 mL of H₂O₂ to obtain ~15 ppm peroxide in the reactor while reducing the dosage of MgAC-Fe₃O₄ [0.05 g]/TiO₂ from 1 g/L to 0.5 g/L. Under this photo-Fenton condition, phenol was completely removed after 27 hours at a constant rate of increase to 0.036 (h⁻¹) (Fig. 2b and Table 2). It should be noted that in this paper, we present just the preliminary results; the optimal concentrations of photocatalytic materials and H₂O₂ should be discussed in future work investigating the effect of tap water on degradation rate. There is also a requirement for continual reactor upgrading to prevent quick sedimentation in the photocatalytic reaction and, thereby, improve the performance of MgAC-Fe₃O₄ [0.05 g]/TiO₂ on the pilot scale.

Figure 2.

(a) Photocatalytic and (b) photo-Fenton performances of MgAC-Fe₃O₄ [0.05 g]/TiO₂ hybrid nanocomposites on pilot scale.

Table 2.

Phenol-degradation rates of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites by photocatalytic and photo-Fenton reaction on pilot scale.

MgAC-Fe3O4 [0.05 g]/TiO2	Constant rate (h−1)
Photocatalytic	0.0032
Photo-Fenton	0.0036

For phenol of 500 ppm, after 48 h exposure, the LC₅₀ was 6.39 mg/L (5.36–7.53, 95% CI). At the concentrations of 0, 2.5, 5, 7.5, 10, and 20 ppm, the mortality rates were 0, 15, 20, 50, 80, and 100%, respectively (Fig. S1b). Meanwhile, the treated samples showed a mortality rate of ~10% (Fig. S1c). We suspected that the remnant toxicity had come from the leakage of Fe³⁺ ions from the hybrid nanocomposites or the phenol intermediate products after the reactions. So, we conducted both ICP and TOC experiments to investigate the reason behind the toxicity of the treated samples. The ICP results showed that the leakage of iron ions after stoppage of the reaction was negligible (~60 ppb after treatment, lower than the standard 300 ppb for drinking water according to the WHO)⁴². However, the TOC experiments showed that the organic carbon concentration was still very high after the reaction (~50–55%); thus, the toxicity could be attributed to the residual toxic phenol intermediate products (Fig. S8). Therefore, it is necessary to extend the reaction until intermediate products are completely removed, not to the phenol concentration of zero³⁶.

Characterization of hybrid nanocomposites

The magnetic properties of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were preserved after the synthesis process (Fig. S9). The XRD pattern showed that, for the MgAC-Fe₃O₄ sample, the peaks at 29.98°, 35.5°, 42.9°, 56.7°, and 62° belonged respectively to (200), (311), (400), (511), and (440) of Fe₃O₄ magnetite (JCDS 00-021-1272; JCDS: Joint Committee on Power Diffraction Standard)²⁸; meanwhile, for MgAC-Fe₃O₄ [0.05 g]/TiO₂ and MgAC-Fe₃O₄ [0.1 g]/TiO₂, besides the reduced MgAC-Fe₃O₄ peaks, there were additional peaks at 25.5°, 37.8°, 48.3°, 54.2°, and 62.8° belonging respectively to the (101), (004), (200), (105), and (204) planes of the anatase phase (JCDS 00-064-0863, Fig. 3)²⁸. It was apparent that, for MgAC-Fe₃O₄ [0.1 g]/TiO₂, the high-intensity peaks of TiO₂ were reduced, whereas those of MgAC-Fe₃O₄ [0.2 g]/TiO₂ had totally disappeared. It could be concluded that the excess MgAC-Fe₃O₄ loading inhibited the growth of anatase TiO₂ in the hybrid nanocomposites. XRD was double checked by conducting two measurement to confirm the phenomena (Fig. S10). These results were similar with Sun et al.¹⁹. Also, the XPS analysis showed that, for MgAC-Fe₃O₄ [0.05 g]/TiO₂, besides the peaks at ~532, ~285, ~102, and ~52 eV belonging respectively to O 1 s, C1s, Si 2p, and Mg 2p (Fig. S11a)^27,43, there were additional peaks at ~463 eV and ~457 eV that were attributed to the Ti 2p_1/2 and Ti 2p_3/2 of Ti⁴⁺ states of stoichiometric TiO₂, respectively (Fig. S11b)⁴⁴. The Fe 2p_1/2 and Fe 2p_3/2 peaks of Fe₃O₄ existed at ~723.9 and ~710.2 eV, respectively (Fig. S11c)⁴⁵.

Figure 3.

XRD patterns of MgAC-Fe₃O₄ and MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites.

According to the SEM results, MgAC-Fe₃O₄ [0.05 g]/TiO₂ had an aggregated form, with a diameter ranging between 32.82 and 201.80 nm (Fig. 4a). The morphology of MgAC-Fe₃O₄ [0.05 g]/TiO₂ was further investigated by TEM and energy-dispersive X-ray mapping analysis (EDX). In the XRD, TEM, and EDX results, where MgAC-Fe₃O₄ was uniformly distributed in the TiO₂ matrix, MgAC-Fe₃O₄/TiO₂ showed a core-shell-like structure, MgAC-Fe₃O₄ playing the role as the core material and TiO₂ that of the out-layer material (Fig. 4b,c). Lattice fringe spacing of 0.253 nm, belonging to the (311) plane of the Fe₃O₄ NPs, and 0.355 nm of TiO₂ NPs in HR-TEM image confirmed the presence of these particles in the hybrid nanocomposites (Fig. 4d)^46,47. From the XRF analysis results, the ratios between Fe₃O₄ and TiO₂ were ~1:12, 1:5.5, and 1:3.5 for MgAC-Fe₃O₄ [0.05 g]/TiO₂, MgAC-Fe₃O₄ [0.1 g]/TiO₂, and MgAC-Fe₃O₄ [0.2 g]/TiO₂, respectively (Table S1). XRF and XRD confirmed the effects of MgAC-Fe₃O₄ loading on the photocatalytic activities of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites. Sun (2018) indicated that a ratio between Fe and TiO₂ of 1:200 could enhance the degradation of organic dye, increase the loading amount of Fe₃O₄ and, thus, decrease the photocalytic activity¹⁹. The amounts of MgO and SiO in the XRF analysis and of Mg, Si in the EDX mapping analysis were attributed to the presence of amorphous MgAC in the hybrid nanocomposites (Fig. 5c and Table S1).

Figure 4.

Morphology of MgAC-Fe₃O₄ [0.05 g]/TiO₂ hybrid nanocomposite. (a) SEM, (b) TEM, (c) EDX mapping analysis, and (d) HRTEM.

Figure 5.

Scheme of process of MgAC-Fe₃O₄/TiO₂ synthesis.

As for the ESR spectrum, the two main signals at ~2.0 and ~4.3 could be assigned to different Fe(III) sites⁴⁸. The broad signal at ~2.0 could be assigned to the interaction between iron (octahedral sites) and MgAC. This band could belong to the chain -O-Si-O-Fe-O-Si-O- from the reaction between the MgAC and Fe₃O₄ NPs⁴⁹. The second signal at g ~4.3 corresponded to the strongly orthorhombic sites located on the surface or to the isolated Fe(III) ions dispersed in the silica matrix⁵⁰. Additionally, the symmetric broad band at g ~2.0 confirmed the ferromagnetic properties of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites (Fig. S12)⁴⁹. Based on the characteristic data, the suggested structure of MgAC-Fe₃O₄/TiO₂ is provided in Fig. 5. In the synthesis of MgAC-Fe₃O₄, the presence of amino groups in the layered MgAC structure makes for high dispersibility in water²⁵. The formation of the NH₃⁺ and –OH groups on the surfaces of Fe₃O₄ NPs helps them to connect through strong electrostatic interaction. When the calcine process was carried out, besides the decomposition of amine groups on the surface material, there was evident formation of new bond bridges such as (-Fe-O-C₃-Si-O-Mg-O-Si-O-C₃-) within the architecture. (-C₃-O-Si-Mg-O-Si-O-C₃-) bridges within MgAC, for example, are very stable under harsh conditions. The large-interface surface area between Fe₃O₄ and MgAC facilitates formation of new bridges between Fe₃O₄ and MgAC at high temperature. This formation of new bridges suggests possible coordination of the O elements with the carbon elements and replacement of the O atoms in the NH₃ groups⁵¹. Fe₃O₄ NPs are not easy to coat with TiO₂ NPs layers, and so the presence of SiO₂ could overcome this problem^52,53. Moreover, the SiO₂ layer formed in hybrid NPs can make space for the adsorption of polluted materials^53,54. For synthesis of MgAC-Fe₃O₄/TiO₂, adding TiO₂ to the surfaces of synthetic material not only leads to increased photocatalytic activity but also forms a stable inter-connection. We offered some great bridges at the interface of the TiO₂ NPs and Fe₃O₄, such as (-O-Ti-O-Fe-O-Si-O-Mg-O-Si-O-Fe-O-Ti-O-) or (-O-Ti-O-Si-O-Mg-O-Si-O-Ti-O-). Based on the synthetic condition, it was clearly seen that many O atoms existing on the material surface could have resulted in the development of these new bridges. Besides, Si atoms are more electronegative and less polarizable than Fe and Ti atoms; the effective positive charge on Fe and Ti is increased, and the effective negative charge on O is decreased. In other words, the electron density around Fe and Ti atoms is decreased and the shielding effect is weakened, which results in increased binding energy.

From the obtained UPS spectra (Fig. S13), the work functions were calculated to 2.98, 1.87, 2.1, 1.95, and 1.56 eV for MgAC-Fe₃O₄, MgAC-Fe₃O₄ [0.05 g]/TiO₂, MgAC-Fe₃O₄ [0.1 g]/TiO₂, MgAC-Fe₃O₄ [0.2 g]/TiO₂, and MgAC-TiO₂, respectively (Table S2). It was noted that MgAC-TiO₂ was synthesized by using MgAC and titanium butoxide (TB) as precursors in ethanol media, as previously reported²⁸. The decrease of work function could facilitate electron emission and narrow the energy band gap⁵⁵. It was clearly seen that the work function of MgAC-Fe₃O₄ was lower than that of Fe₃O₄, which is around 3.7 eV in the literature⁵⁶; this reduction of work function could be explained by the higher photo-Fenton activity of MgAC-Fe₃O₄ than that of commercial Fe₃O₄, as was demonstrated in a previous report²⁹. The presence of TiO₂ on the surface of MgAC-Fe₃O₄ continued to decrease the work function, the lowest number belonging to MgAC-Fe₃O₄ [0.05 g]/TiO₂ (1.87 eV, optimal sample), which was yet larger than that of MgAC-TiO₂ (1.56 eV), which showed photocatalytic activation under visible light activation²⁸. It could be concluded that, in general, the photocatalytic activity of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposite is higher than that of MgAC-Fe₃O₄ (which has only photo-Fenton activity) but lower than that of MgAC-TiO₂ (which also has photocatalytic activity under visible light irradiation). The advantages of MgAC-Fe₃O₄/TiO₂ relative to MgAC-TiO₂ come from its recycling utility. The difference of photocatalytic activity between the two might come from their respective synthesis processes and material structures. In the case of MgAC-TiO₂, nitrogen from the amino-functional groups of MgAC can be doped into the TiO₂ structure via a calcination process, thus inducing photocatalytic activity under visible light⁵⁷. However, in the case of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites, the nitrogen element is removed by the process of the synthesis of MgAC-Fe₃O₄, as suggested above.

Additionally, the surface areas, pore sizes and pore volumes of the materials were investigated to characterize their surfaces. As can be seen in Table 3, the surface area, pore size and pore volume of the MgAC-Fe₃O₄ sample were the lowest (surface area: 34.790 m²/g; pore size: 3.639 nm; pore volume: 0.0356 cm³/g). In the literature, the adsorption capacity of Fe₃O₄ is lower than that of TiO₂¹⁹. In the present study, the MgAC-TiO₂ sample had the largest surface area, pore size and pore volume (surface area: 87.822 m²/g; pore size: 7.595 nm; pore volume: 0.1510 cm³/g). Surprisingly, for the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites, when TiO₂ existed on the surfaces of the MgAC-Fe₃O₄ particles, the surface area was not significantly changed (exception: MgAC-Fe₃O₄ [0.1 g]/TiO₂), whereas the pore size and pore volume were increased. For instance, for MgAC-Fe₃O₄ [0.05 g]/TiO₂ (the optimal sample), the pore size and pore volume were 9.4163 and 0.0834 cm³/g, 3 times larger than those of MgAC-Fe₃O₄. In general, even though the surface areas of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites are not significantly changed, their absorption capacities, via the increased pore sizes and volumes, are higher than that of MgAC-Fe₃O₄.

Table 3.

BET surface areas, pore sizes and pore volumes of samples in this study.

Sample	BET surface area (m2/g)	Pore size (nm)	Pore volume (cm3/g)
MgAC-Fe3O4	34.790	3.639	0.0356
MgAC-Fe3O4 [0.05 g]/TiO2	35.159	9.4163	0.0834
MgAC-Fe3O4 [0.1 g]/TiO2	70.244	6.652	0.1280
MgAC-Fe3O4 [0.2 g]/TiO2	31.44	10.629	0.0899
MgAC-TiO2	87.822	7.595	0.1510

Discussion

MgAC-Fe₃O₄/TiO₂ samples were synthesized via sol-gel methods. The core-shell structure with Fe₃O₄ as the core component, SiO₂ as the middle layer and TiO₂ as outer layer was suggested. Based on the laboratory scale, the cost of MgAC-Fe₃O₄/TiO₂ materials has been calculated to around 1,479.80 USD/kg, similar to that of MgAC-TiO₂ (1,476.24 USD/kg; Table S3), due to its higher production efficiency (1 g MgAC can produce only 1.08 g of MgAC-TiO₂, whereas 1 g MgAC-Fe₃O₄ can produce 4.4 g of MgAC-Fe₃O₄ [0.05 g]/TiO₂). Another advantage of MgAC-Fe₃O₄/TiO₂ is its quick sedimentation, which enables ~80% of materials to be recovered after 24 hours of self-precipitation. Moreover, with their ferromagnetism was remained after the synthesis process, these hybrid nanocomposites show their recycling potential. However, the remnant toxicity of the treated sample against Daphnia magna indicated the presence of residual phenol intermediate products. The comparison of our study with some remarkable researches in the literature was briefly summarized in the Table 4.

Table 4.

The comparison of our study with some remarkable researches in the literature.

	Preparation Method	Obtained material	Remarkable results
He et al.18	Homogenous precipitation	Core-shell Fe3O4-TiO2	• Obtained Fe3O4-TiO2 is non-toxic • Fe3+ could be doped into TiO2 and activate the photocatalytic activation under visible light activation
Sun et al.19	One-step calcination	Magnetic Fe3O4-TiO2	• The degradation of organic dye by Fe3O4-TiO2 (Fe/TiO2 ratio: 1/200) was enhanced compared to single Fe3O4 and TiO2 • The synergistic of Fe3O4 and TiO2 could be attributed to the high photocatalytic activity
Zheng et al.12	Liquid phase deposition	Waxberry-like microsphere Fe3O4-TiO2	• Diameter: ~500 nm • Shell thickness: ~10–20 nm • Remove 40% of MB (10 ppm) after 60 mins under Xenon lamp (300 W) • Could be recycled after photocatalytic reaction
Stefan et al.13	Ultrasound assisted sol-gel	Fe3O4-TiO2: Eu nanocomposite	• Increase of Eu doping decrease the formation of FeTiO3 • Large surface area and mesoporous strcuture • Remove 85% of RhB (1.0 × 10−5 mol/L) dye after 3 h unter visible light irradiation (400 W halogen lamp)
Alzahani53	Sol-gel	Core shell Fe3O4/SiO2/TiO2	• Under UV light, the photocatalytic performance was higher than commercial TiO2
This study	Sol-gel	MgAC-Fe3O4/TiO2	• The synergistic of Fe3O4 and TiO2 could be attributed to the performances of MgAC-Fe3O4/TiO2 • Core-shell structure with Fe3O4 as core and TiO2 at outer layer is suggested. • However, the photocatalytic under visible light should be activated in the near future

For industrial-scale application purposes, the toxicity potential of MgAC-Fe₃O₄/TiO₂ as well as its photo-Fenton and photocatalytic mechanisms should be more thoroughly investigated. Additionally, it is necessary to find a way to induce the photocatalytic activity of such hybrid nanocomposites under visible light.

Methods

Materials

(3-aminopropyl)trielthoxysilane (APTES; ≥98%, 221.37 g/mol), iron (III) chloride hexahydrate (III) (FeCl₃ · 6H₂O; 97%) and titanium butoxide (TB; 97%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Magnesium chloride hexahydrate (MgCl₂ · 6H₂O; 98%) was purchased from Junsei Chemical (Tokyo, Japan). Ethanol (18 L, 95%) was acquired from Samchun Pure Chemicals (Pyungtack, Korea). NaOH (pellet, 97%) was purchased from Daejung Chemical & Metals (Siheung, Korea). Distilled-deionized water (DI; resistance: >18 mΩ) was employed in all of the experiments.

Synthesis of magnesium aminoclay (MgAC)

For preparation of MgAC, 1.68 g of MgCl₂ · 6H₂O was dissolved in 40 mL of ethanol. Then, 2 mL of 3-aminopropyltrielthoxylane (APTES) was added and stirred for 8 hours to form a white suspension. The resultant suspension was then centrifuged and washed with ethanol (3 × 50 mL) before being dried at 40 °C and ground into powder⁵⁸.

Synthesis of magnesium aminoclay-iron oxide (MgAC-Fe₃O₄) nanocomposites

A total of 0.7 g of MgAC was mixed with 3 g of FeCl₃ · 6H₂O in 40 mL of DI water, to which mixture 10 mL of NaOH 10 M was added. The solution was stirred for 12 hours and then centrifuged and washed with DI water (3 × 50 mL) and dried at 60 °C to form a brown solid. The brown products were ground into powder and calcinated at 500 °C for 3 hours in a furnace (FU-100TG, Samheung Energy, Korea) under 4% H₂/Ar (flow rate: 0.15 L/min) to produce MgAC-Fe₃O₄ nanocomposites²⁹.

Synthesis of magnesium aminoclay-iron oxide/TiO₂ (MgAC-Fe₃O₄/TiO₂) hybrid nanocomposites

In order to prepare MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites, respectively 0.05 g, 0.1 g and 0.2 g of MgAC-Fe₃O₄ were mixed with 1 mL of TB in 40 mL EtOH, to each of which mixtures 0.25 µL of DI water was slowly added; the resultant solutions were denoted MgAC-Fe₃O₄ [0.05 g]/TiO₂, MgAC-Fe₃O₄ [0.1 g]/TiO₂, and MgAC-Fe₃O₄ [0.2 g]/TiO₂, respectively. Each solution was then stirred for at least 12 hours, washed with ethanol (3 × 50 mL) and dried to form a grey solid. The resultant products were then ground and consequently calcinated at 350 °C for 3 hours under air in a muffle furnace to produce MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites²⁸.

Photo-fenton performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on batch scale

A total of 0.1 g of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites was loaded into 100 mL of methylene blue (MB; Sigma-Aldrich, St. Louis, MO, USA) at a concentration of 10 ppm. After obtainment of equilibrium adsorption, 1 mL of H₂O₂ (35%) was added, and a 365 nm (24 W) UV light source was turned on. One (1) mL of treated water was withdrawn after an interval of 20 min, and the MB concentration was observed under UV-Vis spectroscopy (Cary 50-UV Vis Spectrophotometers, Varian Inc., USA) at a wavelength of 664 nm²⁸.

The degradation rate of the organic compounds was determined by the equation

$$-\frac{[C]}{dt}=k[C_o]$$

where C is the concentration of MB at time (t), C_o is the initial MB concentration, and k is the pseudo-first-order rate constant (min⁻¹)¹⁹.

Photocatalytic performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on batch scale

For evaluation of the photocatalytic performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites, 0.1 g of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites was loaded into a petri dish of 100 mL of phenol at 5 ppm (≥99%; Sigma-Aldrich, St. Louis, MO, USA). After obtainment of equilibrium adsorption, a 365 nm wavelength UV light source (~610 µW/cm²) was turned on. Interval samples were withdrawn after each 20 min, and the remaining organic compounds were detected by high-performance liquid chromatography (for phenol; HPLC, Waters Alliance 2695 Separations Module equipped with Waters 2487 Dual λ Absorbance Detector; Waters, Milford, MA, USA) under the mobile phase of water and acetonitrile in a ratio of 40:60 (v/v) with a flow rate of 1 mL/min⁵⁹. For detection of phenol, UV absorption was performed at 270 nm.

The recycle usage experiments were conducted to check the stability of materials. After photocatalyst materials were separated from degraded solution, they were washed with DI water and ethanol, then dried in the oven at 60 °C for 12 hours. Then the materials is ready for another photocatalytic experiments. This method was repeated for 5 times⁵³.

Preliminary evaluation of photocatalytic and photo-Fenton performances of MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites on pilot scale

The photocatalytic performances of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were tested using the systems introduced in a previous report²⁸. Briefly, MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were loaded into a reactor (90 cm (width) × 30 cm (depth) × 60 cm (height)) containing 30 L of tap water contaminated with phenol at 3 ppm and stirred with 3 stirrers (GGM speed control motor, Korea) at 145 rpm. After obtainment of equilibrium adsorption, 18 × 365 nm wavelength UV lamps (light intensity: ~610 µW/cm², 65 cm × 3 cm) were turn on. After an interval of 3 hours, 40 mL of treated water was withdrawn, and the remaining concentration of phenol was determined by HPLC. For evaluation of the photo-Fenton performances of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites, H₂O₂ was supplied after obtainment of equilibrium adsorption.

For an ecotoxicity test, Daphnia magna was incubated in a 16 h light/8 h darkness cycle at 21 ± 1 °C with M4 medium prepared according to OECD Test Guideline 202 (OECD, 2014)⁶⁰. The culture medium was replaced daily, and Chlorella was fed once a day. The Daphnia test was carried out according to OECD Test Guideline 202, and young daphanids aged less than 24 hours were collected and exposed to the test materials for 48 hours after Chlorella feeding for 2 hours. In the case of 500 ppm phenol feedstock, 5 daphanids were exposed to 25 mL of 0, 2.5, 5, 7.5, 10, and 20 ppm phenol to determine the LC₅₀ (Fig. S1a). Ecotoxicity tests of photo-Fenton- and photocatalytic-treated samples against Daphnia magna also were performed. The toxicity of the photo-Fenton and photocatalytic samples also were investigated, first, via inductive coupled plasma atomic emission spectroscopy (ICP-AES; Optima 7300 DV, Pelkin Elmer, USA) for leakage of iron ions and, second, via total organic carbon (TOC, Vario TOC Cube, Elementar, Germany) for the presence of intermediate products, after the reaction.

Characterization

The crystallography of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites was examined using a Rigaku D/max-2500 (18 kW, Japan) incorporating a θ/θ goniometer equipped with a 40 kV, 30 mA CuKα radiation generator. The morphology of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites was investigated by scanning electron microscopy (SEM; SEM-4700, Japan) and transmission electron microscopy (TEM; JEM-2100F, JEOL Ltd., USA). X-ray fluorescence spectrometry (XRF; MiniPal 2, PANanalytical, Almelo, Netherlands) was employed to identify the elemental compositions of as-prepared samples. The surface areas, pore sizes and pore volumes of the materials were investigated by Brunauer-Emmett-Teller (BET; Micromeritics ASAP 2010, USA)²⁵. The photoluminescence (PL) spectra were obtained to investigate the electron-hole fate of the semiconductor (DUT-260, Core Bio System, Korea)⁴⁰. The magnetic properties of the MgAC-Fe₃O₄/TiO₂ hybrid nanocomposites were examined by electron spin resonance spectrometry (ESR; EMXplus/ELEXYS E580, Bruker, USA).

Next, for investigation of the optical properties of the materials, UV photoelectron spectroscopy (UPS; Axis Ultra DLD, Japan) with He I line (21.2 eV) UV source was applied. The work functions of the materials were calculated by the equation

$$WF=hv-|E_{cutoff}-E_F|$$

where hv is the incident energy (21.2 eV), E_cutoff is the secondary electron cutoff energy, and E_f is the Fermi energy⁵⁵.

Author Contributions

Vu Khac Hoang Bui and Young-Chul Lee designed the study, prepared materials, conducted the experiment and wrote the manuscript mainly. Vu Khac Hoang Bui and Tuyet Nhung Pham analyzed the structure of materials. Yejin An and Ki-Tae Kim conducted the ecotoxicity experiments. Hyun Uk Lee helped to explain ESR results. Young-Chul Lee and Ki-Tae Kim are corresponding authors of this work. Duckshin Park, Jin Seok Choi, Oh-Hyeok Kwon and Ju-Young Moon contributed comments on the manuscript before submission.

Data Availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Competing Interests

The authors declare no competing interests.

Supplementary information

Supplementary information accompanies this paper at 10.1038/s41598-019-48398-5.