A Hydrothermal Synthesis of Fe3O4@C Hybrid Nanoparticle and Magnetic Adsorptive Performance to Remove Heavy Metal Ions in Aqueous Solution
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Advanced core-shelled material with a high specific area has been considered as an effective material to remove heavy metal from aqueous solutions. A core-shelled Fe3O4@C hybrid nanoparticle aggregates with environmental-friendly channel in the study. Moreover, the higher exposure of adsorption sites can be achieved for the special configuration that higher Brunauer-Emmet-Teller (BET) surface area reaches up to 238.18 m2 g−1. Thus, a more efficiently heavy metal ion removal is obtained, Pb (II), Cd (II), Cu (II), and Cr (VI) up to 100, 99.2, 96.6, and 94.8%, respectively. In addition, the products are easy to be separated from the aqueous solutions after adsorption, due to the relative large submicrometer size and the enhanced external magnetic fields introduced by the iron-based cores. We provide an ideal mode to remove heavy metal ions using core-shelled Fe3O4@C under the water treatment condition. A new approach is clarified that core-shell nano/micro-functional materials can be synthesized well on large scales which are used in many fields such as environmental remediation, catalyst, and energy.
KeywordsComposites Nanomaterials Heavy metal ions Adsorption
Atomic absorption spectroscopy
Fourier transform infrared spectroscopy
High-resolution transmission electron microscope
Joint Committee on Powder Diffraction Standards
Selected area electron diffraction
Scanning electron microscope
Transmission electron microscope
With the voice of environmental protection is constant and surging, there is increasing attention to its toxic effect that refers to human health and environmental pollution by heavy metal contamination [1, 2, 3]. Removing heavy metal elements from industrial wastewater prior to discharge becomes crucial . To date, ion exchange, coagulation precipitation, and series of traditional technologies were applied to remove heavy metals from wastewater during the past decades [5, 6]. The realization that conventional techniques are significantly created benefits for human, and it also exposed drawbacks characterized both from management and technical sides, which are expensive operation cost, additional toxic sludge generation, and incomplete metal removal [7, 8, 9]. On the other hand, controlling particle size, the morphology of adsorbent materials, has proved to be one of the efficient and innovating solutions to conquer those kinds of problems. Hollow spheres, nanowires, and nanotubes have got a better adsorption performance to contribute to the heavy metal removal in References [10, 11, 12, 13]. Further, researches are focused on the new material that holds core-shell structure with core-void-shell feature. It has been proved for its advantages compared with the same size of solid counterparts, such as the validity of that in the changes of surface areas, refractive index, lower density, and accommodate volume, which lead to a great contribution both from the aspects of properties and functions [14, 15]. Thus, this unique structure with tunable shape, composition, and interior architecture is an exciting direction to pursue environmental remediation.
Many literatures are engaged about core-shell structure materials. Guo  had made cage-bell Ag@TiO2 materials, and the study expressed better photocatalytic and electrochemical properties. Liu  prepared core-shell Fe3O4 polydopamine nanoparticles that exhibited nice potential in the field of medicine support, catalyst carrier, and carbon adsorbent. To our best knowledge, the assistance of removable or sacrificial templates (e.g., polymer silica , spheres , carbon , and ionic liquids ) of the desirable shells are important in general synthesis of core-shell hybrid nanoparticle structures. However, the most available core-shell structure materials are synthesized by multi-template processes that mainly focus on the relatively simple configurations, like one composition in single-shell particles. Moreover, there are still lacking in heavy metal ion removal method with a general approach accompany with further strengthen the prepare feasibility of the advanced materials with core-shell structures, including time and cost in the construction process of complex nanostructures that were limited by synthesis templates and multi-template routes, which have become a desire both from technical and eco-benefit aspect.
The synthesis of the magnetic functional nanocomposite is an effective and handy way to solve the separation between the adsorbent and solution for expanding the magnetic separation . Increasingly covalently immobilized polymer, novel molecule, and inorganic material are put into the surface of magnetic nanoparticle in this process; they are useful technique routes for toxic heavy metal ion wastewater treatment as well . For instance, novel synthesized chitosan-modified magnetic nanocomposites  and monodisperse Fe3O4@silica core-shell structure composite magnetic nanoparticle core-shell microspheres  are reported. Despite these magnetic nanocomposites achieved easily separated from solution through the adsorption process based on the external magnetic, it still needs to be further considered the special conditions such as the applicability of strong acid wastewater.
There are lots of studies about carbon-based nanostructured materials recently. Wildgoose  presented these kinds of materials hold obvious advantages in terms of cost, alkali corrosion resistance, specific surface area, and adsorption capacity. Uchida  pointed out that the carboxylic functional groups can easily generate on the surface of carbon then further enhance the adsorption capacity of heavy metal ions. However, the fatal flaw of the difficulty of removing them from a solution that caused by the small size of carbon particles limited its application in heavy metal wastewater treatment. Considering carbon-coated magnetic nanoparticle is a toward media in wastewater treatment, it showed advanced impacts on adsorption capacity and separation property in the external magnetic field. Much more attention has increased [28, 29, 30, 31, 32, 33, 34, 35]. Zhang  prepared magnetic hollow carbon nanospheres forward used in chromium ion removal. To remove heavy metal, Wang groups  reported a case study of Fe nanoparticle materials. These previous studies indicated that the corresponding future work must refer to highly efficient heavy metal ion adsorbents with easy separation and large adsorption capacity. Meantime, it should be pointed out that there are rare studies on core-shelled Fe3O4@C hybrid nanoparticle aggregates up to now.
In this study, we prepared carbon microspheres with magnetic cores. Also, a concise strategy was proposed to synthesize core-shelled Fe3O4@C hybrid nanoparticle aggregates, which is an advanced material for heavy metal ion removal with the strength purity, surface areas, and adsorption capacity. Compared with traditional production technology of Fe3O4 materials, the benefits are obvious. It not only expressed a larger surface area and steady configuration, but also the removal template which is not affected by product morphology. Our research provides a higher degree of the active sites [38, 39]. The adsorbent could be easily separated when the external magnetic fields are introduced, which are caused by the iron-based nanoparticles [40, 41]. Therefore, the obtained core-shelled Fe3O4@C hybrid nanoparticle aggregates show superior adsorption capacity for heavy metal ions with the route eco-friendly, mass-production, and cost benefits.
Materials and Synthesis
Synthesis of Core-Shelled Fe3O4 Hybrid Nanoparticle Aggregates
The phase composition of prepared material was analyzed by X-ray diffraction (XRD), which was taken at 2θ = 20°–90° by Rigaku D/max-A diffractometer with Co Kα radiation. FTIR (Fourier transform infrared spectroscopy, Thermo Nicolet AVATARFTIR 360) was carried out to record the samples’ FTIR characters within the ranged 400–4000 cm−1 as well. AMRAY 1000B SEM (Scanning electron microscope), HR-TEM (High-resolution transmission electron microscope, JEOL JEM-2100) (200 Kv), and selected area electron diffraction (SAED) were implemented to describe the sample’s morphology, the microstructure feature, and the lattice structure. In addition, Micromeritics Tristar apparatus at 350.15 °C was taken to measure the processes of nitrogen adsorption and desorption; Brunauer-Emmet-Teller (BET) was used to discuss the specific surface areas; atomic absorption spectroscopy (AAS) quantitative analysis will be implemented by Hitachi Z2000 spectrophotometer, which fitted with hollow cathode lamps and acetylene-air flame. The magnetic performance of prepared material was measured by vibrating-sample magnetometer (VSM).
Heavy Metal Ion Removal Experiments
At room temperature, the conduct of a series of experiments was considered to remove heavy metal ions. First of all, Pb (II), Cd (II), Cu (II), and Cr (VI) are added in four closed containers; after that, 0.1 M HCl and 0.1 M NH3•H2O are used to adjust the pH to 3, and then, the adsorption solutions with the final volume of 50 mL and concentration of 10 mg L−1 were obtained. Subsequently, under a continuous stirring condition, 20 mg as-prepared Fe3O4@C sample was added to the above solutions. During the adsorb reaction process, almost 1.0 mL of each aforementioned solution was leached over the various periods (0, 0.5, 1, 1.5, 2, 4, 6, 10, and 24 h, respectively) by means of a pin tube utilization which equipped with membrane filter, attenuated at last to be applied to AAS measure.
Result and Discussion
Physicochemical Characteristics of Core-Shelled Fe3O4@C Nanosphere
Uptake of Heavy Metal Ions by Fe3O4@C
FTIR Spectra of the Heavy Metal Loaded Fe3O4@C
A variety of texts were carried out to discuss the adsorption capacity and kinetics of the Fe3O4@C hybrid nanoparticle aggregates in this section. Pb2+ was prepared for removal experiment from the aqueous solutions at pH = 3. After that, prepared the volume of 50 mL of the 10, 30, and 50 mg L−1 initial solutions, 20 mg adsorbents, which added to the Pb2+ solutions in 100 mL conical flasks in the adsorption of 30 °C respectively. With the different periods (0, 7, 14, 21, 28, 35, 60, 180, 480, and 1440 min), approximately 1 mL was extracted from each solution, they will be used for AAS analysis. Eq. (1) shows the pseudo-second-order kinetic rate model :
The fitted parameters for equilibrium and kinetic adsorption of Pb (II) by Fe3O4@C sample
k2 × 10−3 (g mg−1 min−1)
Removal (%, 24 h)
where the theoretical maximum monolayer sorption capacity is represented as qmax (mg g−1), taking k L to express the Langmuir constant (L mg−1), and C e is the concentration of Pb (II) initially. Whereas, the Langmuir isotherms do not reach the ideal consequence for our study that means it is not well suitable. Corresponding to the linear form as Eq. (3) is another common empirical model as the Freundlich isotherm, which has the hypothesis that, accompany with rising of the site occupation degree, more sturdy binding sites are tied up in advance and its intention decrease correspondingly .
Freundlich isotherm parameters of the Fe3O4@C
K F (mg g−1)(L mg−1)1/n
The dimensions of activation energy were taken to determine the form of adsorption. Usually having a specific scope (0–40 kJ mol−1) for the activation energy in the process of physisorption , a longer range was needed in chemisorption process by contrast. The activation energy was 34.92 kJ mol−1 here. It indicated that the adsorption process of Pb2+ onto the Fe3O4@C is classified into physisorption.
The compound of core-shelled Fe3O4@C hybrid nanoparticle aggregates is achieved through adopting pacific and moderate steps environmentally based on the solvothermal synthesis method and obtained the calcinations ultimately at 450 °C. Via the carbon-based hybrid core-shell nanostructures, a greater degree of exposure efficiency of adsorption sites can be realized efficiently for the adsorbate when compared to a solid one, which will deliver adsorption properties better to eliminate the heavy metal ions. Additionally, the iron-based cores make the adsorbents be separated easily from the aqueous solutions. Under this device (cheaper, less complicity, and higher productivity), a new approach is clarified that core-shell nano/micro-functional materials can be synthesized well on a large scale which are used in many fields such as environmental remediation, catalyst, and energy.
This study was supported by grants from the National Natural Science Foundation of China (Project No.21467030, No. 51464044, No.21761035, and No.21463028), the Key Scientific Research Fund of Department of Education of Yunnan Province (Project No. 2015Z118), and the Scientific Research Fund of Department of Science and Technology of Yunnan Province (Project No. 2016FD059, 2015FB107).
Availability of Data and Materials
The authors declare that the data and material are clear and available.
XY and HG provided technical support, such as experiment design and data check. SJ, CM, and HL performed the experiments. SJ wrote the manuscript. LF contributed to the language and done the VSM experiment. All authors read and approved the final version of this study.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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