Efficient oil-water separation with amphipathic magnetic nanoparticles of Fe3O4@TiO2

Abstract Recently, oil-water separation materials with special wettability surfaces have attracted intense attention. In this work, amphiphilic magnetic nanoparticles of Fe3O4@TiO2 are prepared by the homogeneous precipitation method. The prepared nanoparticles are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Dynamic light scattering, Zeta potential and contact angle. The prepared magnetic nanoparticles can effectively separate the simulated oily wastewater prepared with 0# diesel. The oil removal performance with magnetic nanoparticles Fe3O4@TiO2 is evaluated by a handheld oil meter after rotating at 250 rpm for a certain time. Results show that the prepared material has the best oil-water separation performance and maintains high magnetism when the weight ratio of TiO2 is 80%. When the dosage is 0.125% (w/v), the oil removal rate is up to 97% after 30 minutes of addition. These prepared materials are suitable for oil-water separation under neutral and acidic conditions and have good recycling performance. This indicates that the prepared amphiphilic magnetic nanoparticles have practical applications in the treatment of oily wastewater. Graphical Abstract


Introduction
As the industrialization process expands, a large amount of oily wastewater is generated during the process of mining, transportation, oil refining and so on. [1][2][3][4] How to deal with oily wastewater efficiently and quickly has drawn immense attention due to it is an extremely common contaminant all over the world. In which, oil-water separation is the key to treat oily wastewater. Many oil-water separation methods have been reported, including gravity separation, [5,6] skimming, [7] meshes and membrane separation, [8] electro-coalescence, [9] and so forth. [10] Meanwhile, many novel materials have been developed to separate oil-water, such as sponges, [11,12] foams, [13] and textiles. [14][15][16] However, most of them have some drawbacks like high cost, non-recyclable or non-reusable, low separation efficiency, time-consuming, secondary pollution, and so on. Therefore, it is still an urgent problem to develop advanced materials with high separation efficiency for practical separation of oil-water.
Nano-titanium dioxide (TiO 2 ) has excellent features of stable chemical properties, low cost, nontoxic and easy to obtain. [17] The application of nano-TiO 2 in the treatment of water and atmospheric pollution has been deeply studied. [18] However, the application of nano-TiO 2 is partially limited due to the difficulty of recycling. Considering that magnetic ferric oxide (Fe 3 O 4 ) core with super para magnetism and large surface area is easily to modify or coat, the nano-TiO 2 encapsulated on the surface of Fe 3 O 4 particles will be a good solution to solve the non-reusable of nano-TiO 2 . For example, Gu FX et al have synthesized hollow rattle-type nanoparticles consisting of magnetic Fe 3 O 4 core and TiO 2 shell. [19] The prepared particles were used as a catalyst for the degradation of methylene blue. Shi WQ et al have investigated the excellent performance of TiO 2 /Fe 3 O 4 and its graphene composites for the photocatalytic removal of uranium from radioactive wastewater. [20] Although the complex material of TiO 2 and Fe 3 O 4 has been developed, the applications of this novel material have not been explored in depth.
Inspired by above works, we exerted significant effort to find a new and practical application for the novel material of Fe 3 O 4 @TiO 2 . In this work, we sought to combine the important feature of magnetic separability for easy particle removal and recycling with the benefits of TiO 2 to create an efficient, reusable nanostructured flocculant. These nanoparticles have the ability to rapidly adsorb contaminants and can be easily recovered from solution. Magnetic nanoparticles Fe 3 O 4 @TiO 2 with different weight ratios of TiO 2 are prepared through the homogeneous coprecipitation method and applied for oil-water separation. The results show that the prepared nanoparticles are hundreds of nanometers, it has the best oil/water separation performance and maintain high magnetism when the weight ratio of TiO 2 is 80%, this novel material is suitable for oil-water separation under neutral and acidic conditions with good recycling performance. Ammonia is added and stirred for another 0.5 h to adjust pH % 11. Then the Fe 3 O 4 water matrix is obtained. After cooling it, the Fe 3 O 4 water matrix is washed with pure water for 3 times. Then sulfuric acid is used to adjust pH % 7. After adjusting the pH value, the Fe 3 O 4 water matrix is obtained through magnetic separation. It is dried in a 60 C oven and then keep sealed.

Materials
Preparation of Fe 3 O 4 @TiO 2 nanoparticles 0.2 g polyethylene glycol are put into a beaker and adding 20 mL of deionized water. Then put 2.4 g Ti(SO 4 ) 2 into the beaker and stir thoroughly. 1.5 g prepared Fe 3 O 4 water matrix is poured into it, and the deionized water is added in order to dilute until it reaches 300 mL. It is stirred at 80 C for 3 h, adding urea in order to adjust pH % 3, and the rotating speed is 350 rpm. Then the precursor of Fe 3 O 4 @TiO 2 is obtained. After cooling it, wash with pure water for 3 times and put into the oven at 60 C for drying. After drying, ground the prepared samples with agate mortar.

Calcination treatment
The dried Fe 3 O 4 @TiO 2 is put into the crucible and roasted at 300 C for 2 h. In the end, the powder of Fe 3 O 4 @TiO 2 is obtained.

X-ray diffraction
The crystal form of Fe 3 O 4 @TiO 2 magnetic nanoparticles is measured by X PERT PRO MPD X-ray diffractive apparatus. X-ray is Cu target, 2h ¼ 20 $ 80 .
FT-IR. The qualitative analysis of Fe 3 O 4 @TiO 2 is carried out by WQF520 Fourier infrared spectrometer, and the sample is made by KBr compression method. The scanning range is 400 cm À1 $ 4000 cm À1 .

Oil content
Oil tech 121 A handheld oil meter is used to measure the concentration of oil in the clarification solution to evaluate the oil-water separation effect.

Zeta potential
The Zeta potential of simulated oily wastewater, Fe 3 O 4 and TiO 2 with a mass ratio of 80% in aqueous solution was measured by Zeta PALS dynamic scale evaluation instrument.
DLS. The effective diameter of magnetic nanoparticles with a mass ratio of 80% of Fe 3 O 4 and TiO 2 in aqueous solution are measured by Zeta Pals dynamic scale evaluation instrument.

Contact angle
The contact angles of Fe 3 O 4 , TiO 2 and Fe 3 O 4 @TiO 2 with water and oil are measured respectively by KRUSS DSA30s interface parameter integrated measuring system.
Oil/water separation experiments 0.010 g, 0.020 g, 0.025 g, 0.030 g and 0.040 g magnetic nanoparticles are weighed and added to a conical flask which contained 20 mL simulated oily wastewater, respectively. Shake on a shaker for a certain time. Under external magnetic field, magnetic nanoparticles are attracted to settle. A syringe is used to extract a certain amount of clarification solution. Then the oil content of the remaining liquid is measured with a hand-held oil meter.

Preparation and characterization of Fe 3 O 4 and Fe 3 O 4 @TiO 2
The crystal structure and chemical composition of the Fe 3 O 4 @80%TiO 2 is recorded by using XRD analysis in the scanning angle of 2h range from 20 to 80 , as shown in Figure 1(a). Evidently five peaks appeared at 30.2, 35.5, 43.1, 57.1 and 62.7 , which are assigned to diffraction from the (220), (311), (400), (511), and (440) crystal planes of the cubic structure of OA-Fe 3 O 4 . [21] The pure anatase phase of TiO 2 (2h ¼ 25.5, 48.5, and 53.6 ) is ascribed to the crystal planes of (101), (200), and (105), [22] which are detected in the XRD pattern, suggesting that the prepared samples consisted of Fe 3 O 4 and TiO 2 nanoparticles (NPs). Moreover, compare the standard JCPDS card of Fe 3 O 4 and TiO 2 , no other impurity peaks are newly generated.
The FT-IR spectrum of the as-prepared Fe 3 O 4 @80%TiO 2 is given in Figure 1(b), which shows typical absorption profiles of Fe 3 O 4 and TiO 2 . The characteristic absorption peaks in the range of 400～ 700 cm À1 are attributed to the Ti-O stretching vibration mode [23] and Fe-O stretching vibration mode. [24] The absorption peak at 1647 cm À1 corresponds to the vibration of the H-O-H bond of water molecules adsorbed on the outer surface of Fe 3 O 4 @TiO 2 . The absorption peak at 2357 cm À1 is attributed to the CO 2.
[ 25] The absorption peak at 3421 cm À1 is the vibration of the O-H (or N À H) bond. These results indicate we have successfully prepared the Fe 3 O 4 @TiO 2 .
The particle size of the Fe 3 O 4 water matrix, the dried Fe 3 O 4 , the uncalcined Fe 3 O 4 and the calcined Fe 3 O 4 @TiO 2 are analyzed by DLS when dispersed in water. From Table  1, the effective diameter of Fe 3 O 4 water matrix is 2786.5 nm, and the dispersity is 26.7%. This is because the nanoparticles are easy to agglomerate, which made the particles larger. After drying, the particle size of Fe 3 O 4 decrease to 1053.1 nm, and the dispersity is 21.3%. The prepared Fe 3 O 4 water matrix is directly used to the next step of preparation and TiSO 4 is added at 350 rpm to make it better coated on Fe 3 O 4 . The final calcined Fe 3 O 4 @TiO 2 magnetic nanoparticles are 922 nm and the dispersity is 28.7%. The calcination is benefit for oil removal performance of magnetic nanoparticles Fe 3 O 4 @TiO 2 , as shown in Figure S1. The morphology of the calcined Fe 3 O 4 @TiO 2 particles is observed by SEM ( Figure S2). Due to ultrasonic dispersion in ethanol, the Fe 3 O 4 @TiO 2 magnetic particles appear to be dispersed into many nanoparticles.
The oil-water separation performance of the prepared Fe 3 O 4 @TiO 2 Optimize the ratio of Fe 3 O 4 and TiO 2 A series of Fe 3 O 4 @TiO 2 composite materials with different mass ratio of TiO 2 (0%, 50%, 58%, 68%, 75%, 80%, and 90%) are prepared. And the oil removal rate for simulated oily wastewater is studied. As Figure 2 shows, when the oil content of wastewater was 650 ppm, the material with 50% TiO 2 has a poor performance for oil-water separation. With the mass ratio of TiO 2 increase, the oil-water separation rate increase. The oil removal rate of pure Fe 3 O 4 , 58% TiO 2 and 68% TiO 2 are almost the same. And at 16 mins, the oil removal rates are 52%, 58% and 62%, respectively. When the mass ratio of TiO 2 is increased to 75% and 80%, the oil removal rate is greatly improved. Within 16 min, the oil removal rates are 71% and 83% respectively. However, when the content of TiO 2 increases to 90%, the magnetic property of the material is weak and it is difficult to achieve magnetic separation. Based on the above experimental results, the optimal mass ratio of TiO 2 is 80% to prepare the Fe 3 O 4 @TiO 2 materials.
Optimize the dosage of Fe 3 O 4 @TiO 2 To optimize the dosage of Fe 3 O 4 @80%TiO 2 , 0.010 g, 0.020 g, 0.025 g, 0.030 g and 0.040 g Fe 3 O 4 @TiO 2 have been added to simulated oily wastewater with 650 ppm oil. Figure 3(a) is the image of the separation of oil and water after the  Figure 3(b). The oil-water separation effect is not ideal when the dosage of Fe 3 O 4 @TiO 2 is 0.010 g (the mass ratio of Fe 3 O 4 @TiO 2 to the volume of oil-water system is 0.05%, that is 0.05% (w/v)). As the dosage of Fe 3 O 4 @TiO 2 are 0.020 g, 0.025 g, 0.030 g and 0.040 g (0.10%, 0.125%, 0.15% and 0.20% (w/v), respectively), the oil removal rate could reach 43%, 52%, 65% and 69% in 5 minutes. The oil removal rate is up to 96% when the dosage of Fe 3 O 4 @TiO 2 is 0.04 g at 15 min, and then keep the balance. When the dosage of Fe 3 O 4 @TiO 2 is 0.025 g, the oil removal rate can also up to 97% at 30 min. Considering the cost, the optimum dosage is 0.025 g.

Optimize pH
The pH of oily wastewater is adjusted to 3, 5, 7, 9 and 11, and the pH of simulated oily wastewater is 7. 20 mL of simulated oily wastewater with concentration of 650 ppm is taken as the treatment object. The experiment is carried out with a magnetic nanomaterial of 0.025 g Fe 3 O 4 @80%TiO 2 . Figure 4 shows that the magnetic nano materials have excellent and rapid oil removal effect under acidic and neutral conditions. In particular, the oil removal effect is more rapid with the increase of acidity. At 10 min, the oil removal rate is 65% at pH ¼ 7, 79% at pH ¼ 5 and 90% at pH ¼ 3. At 15 min, the oil removal rate reaches 96% at pH ¼ 5, 97% at pH ¼ 3, and then reaches equilibrium. When pH ¼ 7, the oil removal rate reaches 97% after 30 min. However, under the alkaline conditions (pH ¼ 9 or 11), the material is difficult to be separated due to it is unabsorbed by the external magnet. Thus, it is unable to judge whether the oil removal effect is still maintained. This indicates that the material is not applicable under alkaline conditions.
Magnetic separation performance Figure 5 shows the magnetic separation process of simulated oily wastewater after adding Fe 3 O 4 @80%TiO 2 . The nanoparticles are put into the simulated oily water and dispersed after shaking. After 30 min, the particles could gather to one side of the container quickly and completely by external magnets. It proves Fe 3 O 4 @80%TiO 2 magnetic nanoparticles have good magnetism, which can realize the rapid recovery of materials and the separation of oil and water.

Recycling performance
From the recycling experiment results (Figure 6), the oil removal rate is above 85% at the third time of recovery. That means the Fe 3 O 4 @80%TiO 2 nanoparticles can be recycled three times, after which the separation efficiency becomes poor. At the fourth time of recovery, the oil removal rate is 84%. At the fifth time of recovery, the oil removal rate is 79.3%. This material has a certain capacity of recycling.   Oil-water separation mechanism of the prepared Fe 3 O 4 @TiO 2 The surface charge of the oily wastewater, Fe 3 O 4, Fe 3 O 4 @58%TiO 2 and Fe 3 O 4 @80%TiO 2 is be evaluated Zeta potential, as shown in Figure 7. The surface charge of dispersed oil droplets in the simulated oily wastewater is À43.1 mV, which could be caused by some anionic surfactant additives in diesel oil. The negative charges may induce the electrostatic repulsive forces between oil droplets and thus stabilize the emulsions. The surface charge of Fe 3 O 4 is positive (16.8 mV). And with the mass ratio of TiO 2 in the sample increase, the surface charge is more positive. The zeta potential is 22.7 mV when the mass ratio of TiO 2 increased to 80%. As the previous reports, [26,27] as the surface charge in the range of À30 $ 30 mV, the electrostatic repulsive forces induced by like charges is not enough to stabilize the emulsions. Therefore, one separation mechanism of the simulated oily wastewater is ascribed to the change in charge of oil droplets after the prepared Fe 3 O 4 @TiO 2 being adsorbed on the surface of oil droplets.

Contact angle experiment
The wettability of Fe 3 O 4 , TiO 2 , Fe 3 O 4 @TiO 2 to diesel oil is tested by contact angle experiments (Figure 8(a)-(c)). The contact angle was 4.3 , 6.5 , 9.6 , respectively. has a strong amphiphilic property, and it has a slightly stronger affinity for oil. Thus, it can absorb emulsified oil droplets dispersed in water and make them aggregate on the surface of the material. Therefore, the other separation mechanism of the simulated oily wastewater may cause by the strong lipophilic properties of the Fe 3 O 4 @80%TiO 2 nanomaterials. The separation ability of Fe 3 O 4 @TiO 2 magnetic nanoparticles for oil is compared with other oil adsorbents which have already been reported in literatures [28][29][30][31][32][33][34] and the results are listed in Table 2. According to the table, silicon dioxide, ferric oxide, and titanium dioxide particles are used as active ingredients for oil-water separation. These materials are lipophilic, and the Fe 3 O 4 @TiO 2 magnetic nanoparticles we prepared are amphiphilic. The separation efficiency is 96% (Ref. [28,29] ) when the oil absorbing material contains silicon, which is not as high as the separation efficiency of Fe 3 O 4 @TiO 2 magnetic nanoparticles. Oil absorption capacity is high (Ref. [30,31] ) when the oil absorbing material contains iron oxide. But it cannot be compared directly with the separation efficiency of Fe 3 O 4 @TiO 2 magnetic nanoparticles. The separation capacity is up to 99% (Ref. [33,34] ) when the oil absorbing material contains Fe 3 O 4 and TiO 2 , which is   2% higher than the separation efficiency of Fe 3 O 4 @TiO 2 magnetic nanoparticles. However, the materials of Fe 3 O 4 / LA-TiO 2 or Fe 3 O 4 @PDA@TiO 2 cannot be directly used for oil-water separation, they are coated on textiles. That means oil-water separation is carried out in the form of membrane separation. There is no third component for Fe 3 O 4 @TiO 2 magnetic nanoparticles. Considering the economic cost and material preparation process, the composite nanoparticles we prepared have the advantages of low price and simple preparation. These are more beneficial for the practical oilwater separation.

Conclusions
A composite nanomaterial of Fe 3 O 4 @TiO 2 was prepared by simple precipitation method and applied to the oil-water separation of emulsified oil. Compared with other oilabsorbing materials containing Fe 3 O 4 and TiO 2 , the composite nanomaterial of Fe 3 O 4 @TiO 2 has the advantages of low price, simple preparation and more suitable for practical applications. The simulated oily wastewater prepared with 0# diesel oil are efficiently separated by prepared magnetic nanoparticles. The prepared material has the best oil/water separation performance and maintain high magnetism when the mass ratio of TiO 2 is 80%. The oil removal rate is up to 97% when the dosage is 0.125% (w/v) after 30 minutes of addition. These prepared materials are suitable for oil-water separation under neutral and acidic conditions and have a certain recycling performance. Applying the Fe 3 O 4 @TiO 2 nanomaterial to oily wastewater separation can give full play to the magnetic separability of Fe 3 O 4 and the absorbance of TiO 2 performance.   The separation efficiency is up to 96% [28] CNTs/SiO 2 92.8 (water contact angle) The separation efficiency is 87.4% [29] Fe 2 O 3 @C 162.9 (water contact angle) Oil absorption capacity up to 3.8 times its weight [30] PLA/c-Fe 2 O 3 148 (water contact angle); 0 (oil contact angle) Oil absorption capacity up to 268.6 g [31] magnetic titanium dioxide foam 152 (water contact angle) It can absorb 64.31 kg oil [32] Fe 3 O 4 /LA-TiO 2 153 (water contact angle); super-oleophilic The separation efficiency is higher than 99% [33] Fe 3 O 4 @PDA@TiO 2 115 (water contact angle) The separation efficiency can reach 99% [34] Fe 3 O 4 @TiO 2 27.8 (water contact angle); 9.6 (oil contact angle) The oil removal rate is up to 97% this work