Constructing Carbon-Coated Fe3O4 Hierarchical Microstructures with a Porous Structure and their Excellent Cr(VI) Ion Removal Properties

xiaoping wu Zhejiang Sci-Tech University https://orcid.org/0000-0001-7163-5765 Lin Cheng Zhejiang Sci-Tech University Chang-Sheng Song Zhejiang Sci-Tech University Yi-Zhe Zhang Zhejiang Sci-Tech University Xiao-Jing Shi Tongji Zhejiang College Xiao-Yun Li Zhejiang Sci-Tech University Ping Lin Zhejiang Sci-Tech University Shun-Li Wang Zhejiang Sci-Tech University Peng Wang Zhejiang Sci-Tech University Ling-Bo Xu Zhejiang Sci-Tech University Li Jin Zhejiang Sci-Tech University Can Cui (  cancui@zstu.edu.cn ) Zhejiang Sci-Tech University https://orcid.org/0000-0002-8429-5875


Introduction
Due to the toxicity of heavy metals and their non-degradability in the human body, water polluted with heavy metals has been a serious global environmental problem [1][2][3][4] . For example, Cr(VI) is a toxic heavy metal ion that can be easily absorbed by the human body and can enter the body through digestion, inhalation, skin and mucous membranes [5][6][7][8] . Cr(VI) has long-lasting environmental hazards, and has been identi ed as one of 17 highly most dangerous toxic substances. The oral lethal dose of Cr(VI) compounds is about 1.5 g. A Cr(VI) concentration above 0.1 mg/L in water will lead to poisoning of the drinking people. Long-term or short-term exposure or inhalation of Cr(VI) compounds may lead to cancer. A 0.1 mg/L Cr(VI) in irrigation water can inhibit the germination of rice seeds. The toxicity of Cr(VI) to the human body is similar to that of arsenic.
Many sewage treatment technologies, such as membrane ltration, biological oxidation, chemical precipitation, ion exchange, electrochemical and ion exchange methods, have been developed to remove pollutants in water [19][20][21][22][23][24][25][26] . However, these methods have some disadvantages such as a complicated operation process, high running cost and secondary pollution. Compared with other treatment technologies, adsorption separation technology has been widely used in waste water treatment because of its rich adsorbent resources, simple process and low cost. Therefore, the adsorption separation method is the most active research and application in recent years to remove toxic pollutants in sewage treatment methods [27][28][29][30][31] . It is well known that the adsorbent material is very important in the adsorption separation technology, but the existing adsorbent material still needs to be further improved, especially how to realize the fast separation and improve the adsorption performance of the material. With the development of magnetic materials and nanotechnology, the rapid separation and adsorption of materials can be realized in theory, so it brings new vitality to waste water treatment technology [32][33][34][35][36] . At present, the development, research and application of magnetic nanomaterials have been paying great attention. Magnetic nanomaterials have many advantages, such as small particle size, large speci c surface area, abundant surface active sites, strong magnetism, and so on. When used as an adsorbent, it can achieve high adsorption e ciency, quickly reach adsorption equilibrium, and under the effect of external magnetic eld, it can be quickly separated from the liquid phase, avoiding common centrifugation or ltration treatment, and thus the di culty and cost of the operation are greatly reduced.
Therefore, magnetic nanomaterials have been widely used in the eld of waste water treatment and attracted much attention [14,15,32−42] .However, nanomaterials are easy to agglomerate because of their small size, which makes the preparation and application of nanomaterials di cult. Therefore, in recent years, magnetic adsorbents have been used to deal with water pollution problems, from the initial use of single magnetite as magnetic adsorbents to the synthesis of various surface modi ed functionalized magnetic nano-adsorbents to e ciently deal with pollutants in wastewater. Surface modi cation can not only improve the agglomeration of particles, but also modify the surface of nanomaterials according to the demand, so that the dispersibility, surface activity and compatibility with other materials can be improved, to make it more appropriate for the application [40][41][42][43][44][45][46][47] . In addition to introducing magnetism and surface modi cation, porous structure is also a widely used and effective method to improve the absorption capacity. The magnetic adsorbent is made into a porous structure, which increases the surface area, reduces the diffusion resistance, promotes the transfer of materials, and is bene cial to increase the adsorption capacity and speed up the adsorption [45][46][47][48][49][50][51][52] . Thus, at lower cost and facile synthetic method for the preparation of porous adsorbents with high speci c surface area architectures for highly adsorption performance is still in great demand.
In this paper, we report a simple, template-free, and environmentally friendly route for the controlled Shanghai Chemical Co.) and Potassium dichromate (K 2 Cr 2 O 7 ) (A.R., Guangzhou Chemical Co.). All of the reagents used in the experiment were directly used without further puri cation.

Growth of Fe 3 O 4 @C hollow spheres
In a typical experiment, rst, 0.30 g (0.11 mmol) of FeCl 3 ·6H 2 O were initially dissolved in EG (30 ml) and stirred vigorously at room temperature to give an orange solution. Next, 0.29 g (2 mmol) of Triethylene tetramine (HETA) was added into the orange solution. The resultant mixture was dispersed by continuously stirring the solution for about 30 min. After addition, the solution was transferred into a Te on-lined stainless-steel autoclave with a volume of 40 ml, and subsequently sealed and heated at 160℃ for 6 h in an oven. After the heat treatment, the autoclave was cooled to room temperature naturally. The products were collected by centrifuging and washed several times with deionized water and absolute ethanol, and then dried at 60°C for 5h. The products were heated to 450°C at 7°C min − 1 for 3h in N 2 in a horizontal furnace, leading to black Fe 3 O 4 @C powders.
Growth of Fe 3 O 4 @C ower-like and egg-shaped micro-structures: When the hexamethylene tetramine (HETA) and triethylene diamine (TEDA) were used to replace HETA, and other experimental conditions are consistent with the above, the morphologies of Fe 3 O 4 @C will form the ower-like and egg-shaped microstructures respectively.

Characterization.
The morphology and the size of the as-prepared Fe 3 O 4 @C products were characterized using a Hitachi S-5500 Field-emission scanning electron microscope (FE-SEM, Tokyo, Japan), JEOL JEM-2010 highresolution transmission electron microscope (HRTEM, Kyoto, Japan). Phase identi cation and structure analysis of the sample were carried out by Raman Spectroscopy (HORIBA, XploRA PLUS) and XRD using a Philips X' Pert Pro MPD X-ray diffractometer with Cu Ka radiation (λ = 0.154056 nm) operated at 40 kV and 40 mA in the 2θ range from 10-90° with a step size of 0.04° and a sampling time of 0.5s. The infrared (IR) spectrum was recorded using an AVATAR360 Fourier transform IR spectrophotometer at room temperature. Thermogravimetric and scalable differential thermal analysis (TG-SDTA) was carried out at a heating rate of 20°C min − 1 in N 2 gas at a owing rate of 50 ml min − 1 using a PYRIS 1 TGA system. The speci c surface areas of materials were performed by using surface area porosity analyzer (BSD-PS1).

Adsorption experiments.
K 2 Cr 2 O 7 was used as the source of Cr (VI). The different concentrations of Cr(VI) ions were prepared and the pH value was adjusted by HCl or NaOH. The fabricated different morphology of Fe 3 O 4 @C microstructures were used as adsorbents for the removal of Cr(VI) ions from aqueous solutions. The adsorption experiments were performed on a digital water bath at 25°C. The adsorbent was then separated from the mixture by a permanent magnet. To determine Cr (VI) ions removal by the adsorbent, the Cr(VI) concentration in the remaining solution was tested with a UV-vis spectrophotometer (UV-2600, Shimadzu). Ultraviolet light of 354 nm was used to irradiate the sample. The morphology and size of the as-prepared Fe 3 O 4 @C hollow spheres are shown in Fig. 1(a-f). Figure 1a displays a representative overview of the Fe 3 O 4 @C hollow spheres, which shows that the prepared samples are composed of large-scale hollow spheres with diameters of 3µm. Figure 1b,  showed that the synthetic product contained about 10 Wt% carbon. Combined with the highmagni cation SEM and TEM images, we believed that there was a layer of carbon on the surface of Fe 3 O 4 sheet, so the hollow sphere should be Fe 3 O 4 @C hollow spheres, which we will further prove by Raman test later.

Results And Discussion
The compositions of Fe 3 O 4 @C hollow spheres were further con rmed by Fourier transform infrared spectroscopy are shown in Fig. 3. For comparison, unannealed sample (Fe 3 O 4 @(HMTA) n ) (Fig. 3a) and In the research process, we found that carbon sources had a great in uence on the morphology of hierarchical Fe 3 O 4 @C micro-structures. To identify the role of carbon source in the formation of Fe 3 O 4 @C micro-structures, the products obtained under the circumstance of different carbon source (HMTA, HETA, TEDA) were characterized by SEM and TEM, as shown in Fig. 5. Figure 5a-c shows the SEM and TEM images of the product obtained in the solvothermal system with the carbon source is HMTA. It can be seen that the products is composed of hierarchical ower-like micro-structures with the diameters of about 4.5µm. When the carbon source is changed to HETA, the self-assembled architecture became apparently hollow spheres with an average diameter of 3µm as shown in Fig. 5d-f. As shown in Fig. 5g-i, as the carbon source converts to TEDA, the products are uniform egg-like Fe 3 O 4 @C composite microspheres composed of akes with sizes of about 3.5µm. In addition, the products with three different morphologies were characterized by XRD and the results showed that the products with three different morphologies were Fe 3 O 4 as shown in Fig. 6.
In order to prove the existence of carbon layer on the surface of Fe 3 O 4 , it is further con rmed by the Raman spectroscopy analysis in the range from 500 to 2000 cm − 1 as shown in Fig. 7. Which showed that a D band at 1324-1361cm − 1 and a G band at 1573-1587cm − 1 herein originate from nite-size crystals of graphite and amorphous C sp2 sites, respectively, and the band around 688cm − 1 was observed, which correspond to the Fe-O bond for spinel Fe 3 O 4 particles. This result is in agreement with the XRD result, con rming that the samples of three different morphologies composite are composed of the magnetite Fe 3 O 4 phase and amorphous carbon. Therefore, the results indicate that the carbon source played an important role in the formation and the assembly of hierarchical Fe 3 O 4 @C micro-structures. Of course, we're not looking at why carbon sources affect the morphology of Fe 3 O 4 . Our next job is to understand how and why the Fe 3 O 4 samples grow into such multiform structures.

Study on Cr(VI) ion adsorption properties of three different morphologies Fe 3 O 4 @C micro-structures.
We have carried out heavy metal ion adsorption test on the Fe 3 O 4 @C hollow spheres, the PH value of the solution is an important parameter in the adsorption of heavy metals, which can not only change the existing form of heavy metal ions, but also change the charge of the functional group on the surface of the adsorbent. Therefore, it is necessary to investigate the effect of the PH value of the solution to the adsorption performance. To get a full understanding about the effect of PH value on the adsorption property of materials, we performed a number of experiments with various PH values (1,2,3,4,7,12,13) are shown in Fig. 8. It can be seen that the reduction e ciency of Cr(VI) increases by increasing the acidity of the solution and the maximal removal was at PH value is equal to 2. However, when the PH value is alkaline, there is almost no adsorption of Cr(VI) ions. We think that there are two reasons for the good adsorption performance under acidic conditions. First, under acidic conditions, Fe 3  In waste water treatment, the adsorption e ciency of adsorption materials is an important parameter. Therefore, we investigated the adsorption performance of Fe 3 O 4 @C composite micro-structures with different morphologies at room temperature when the PH value is equal to 2 (Fig. 9). For instance, when 50 mg of as-prepared Fe 3 O 4 @C composite micro-structures were dispersed into a 100 ml solution of   (Fig. 10,11).
The porous structures of ower-like, hollow spheres and egg-like Fe 3 O 4 @C composite micro-structures are incurred by N 2 adsorption-desorption measurements and the results are shown in Fig. 10. Interestingly, the speci c surface area of ower-like Fe 3 O 4 @C composite micro-structures is the largest, but its adsorption property is the worst, we think the ower-like Fe 3 O 4 @C porous pore between main carbon particles(the pore size is concentrated at 3.6nm), Cr(VI) don't have much chance of coming into contact with the Fe 3 O 4 and result in low adsorption performance. The speci c surface area of the egg-like Fe 3 O 4 @C is only about half of that of the other two structural materials. We believe that the "egg yolk" is solid, leading to a sharp decrease in the speci c surface area, thus resulting in poor adsorption performance of Cr(VI) ions. The pore size of hollow spheres Fe 3 O 4 @C is mainly concentrated at 12 nm.
We believe that the pore size of hollow spheres Fe 3 O 4 @C is mainly between carbon particles and Fe 3 O 4 , that chromium ions can fully contact with Fe 3 O 4 , so that its adsorption performance is relatively good.
In order to exclude the in uence of functional groups on the surface of the material on adsorption performance, infrared tests were conducted on the samples of three different morphologies for comparison, as shown in Fig. 11. The results show that the surfaces of the three structures are consistent after annealing, and there is no special functional group on the surface, so the effects of surface functional groups on adsorption properties are excluded. In conclusion, it is believed that the speci c surface area and pore size of the material are the important factors affecting the adsorption performance of heavy metals in Fe 3 O 4 @C composite micro-structures.

Conclusion
In summary, we demonstrate a novel and facile synthetic route for the synthesis of multi-morphology Fe 3 O 4 @C magnetic composite micro-structures, including ower-like, hollow spheres, and egg-like. This simple method does not need the subsequent complicated workup procedure required for the removal of the template or seed. On the basis of synthesis reactions, carbon source acted as an important parameter for the synthesis, reactions and the shape controller for the crystal growth of ower-like, hollow spheres, and egg-like micro-structures. The results indicate that this method may also be further extended to control the growth of versatile Fe 3 O 4 @C crystals, which may nd application in many elds.
Simultaneously, these Fe 3 O 4 @C magnetic composite hollow micro-spheres were shown to be an excellent Cr(VI) ions adsorbent, which allows them to serve as ideal candidates for environmental remediation materials.

Declarations
Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the nal version of the manuscript.