Adsorption of Tetracycline with Reduced Graphene Oxide Decorated with MnFe2O4 Nanoparticles
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Nanomaterials were widely used as efficient adsorbents for environmental remediation of tetracycline pollution. However, the separation of the adsorbents posed the challenge to their practical applications. In this study, we grew magnetic MnFe2O4 nanoparticles on the reduced graphene oxide (rGO) to form MnFe2O4/rGO nanocomposite with a one-step method. When used as the absorbent of Tetracycline, it exhibited an adsorption capacity of 41 mg/g. The adsorption kinetics and isotherm were fitted well with the pseudo-second order model and Freundlich model, respectively. The MnFe2O4/rGO nanocomposite could be easily extracted from the solution with the external magnetic field and regenerated with acid washing.
KeywordsGraphene oxide Tetracycline MnFe2O4 Adsorption
Reduced graphene oxide
Transmission electron microscopy
Owing to its low toxicity with a broad spectrum of activity, tetracycline (TC) is one of the most widely used antibiotics in the world . However, increasing concern has been raised in the recent years because TC is poorly degraded through metabolism. As a result, residual TC is directly discharged into the environment through feces and spread into nearby waterbodies and soil with water, causing the non-point pollution of those areas [1, 2, 3]. After the residue TC is accumulated in the human body, it exhibits chronic toxicity. Meanwhile, it can influence the aquatic photosynthetic organisms and indigenous microbial populations [4, 5]. To treat TC-polluted water, adsorption has been emerging as a promising method because it is efficient and cost-effective. The adsorbents used in adsorption include smectite clay , montmorillonite , diatomite , activated carbon , alumina , and carbon nanotube . More recently, graphene-based nanomaterials has been used as the most effective adsorbents due to the existence of π-π interaction, H-bond, and cation-π bond between TC and graphene-based materials [12, 13]. Thus, these nanomaterials exhibit high adsorption capacities of TC. For example, theoretical maximum of adsorption capacity (qm) of graphene oxide and reduced graphene oxide can reach 313 and 558 mg/g, respectively [14, 15]. Graphene-based composite even exhibit higher adsorption capacities. TiO2/GO composite exhibits a qm value of 1805 mg/g . However, the separation of absorbents based on nanomaterials from polluted water poses a challenge to their practical applications. To facilitate the separation of the absorbent, magnetic absorbents were used. Our group demonstrated that thiol-functionalized magnetite/graphene oxide hybrid could be used as a reusable adsorbent for Hg2+ removal . Chandra et al. utilized water-dispersible magnetite-reduced graphene oxide composites for arsenic removal . In this study, we utilized Mn in the formation of GO to synthesize magnetic MnFe2O4/rGO composite with a one-pot method. MnFe2O4/rGO as the adsorbent exhibited relatively high adsorption capacity of 41 mg/g with an initial TC concentration of 10 mg/L. The magnetic adsorbent could be extracted from the water solutions easily with the help of the external magnetic field and reused after it was regenerated by soaking it in HCl aqueous solution.
Materials and Methods
Synthesis of GO
GO was prepared with a modified Hummer’s method. Briefly, H2SO4 (75.0 ml, 98 wt%) was slowly add in a flask with 1.0 g flake graphite and 0.75 g NaNO3 with mechanical stirring in an ice-water bath. After 10 min, 4.5 g KMnO4 was added gradually in the flask. With continuous and vigorous stirring, the mixture became pasty brownish, and then it was diluted with deionized water. H2O2 aqueous solution (20 ml, 30 wt%) was then slowly added into the mixture to form the GO mixture with Mn2+ ions.
Synthesis of MnFe2O4/rGO Composite
We synthesized the MnFe2O4/rGO composite as reported previously . Briefly, the above mixture was further diluted to 3000 ml with deionized water. FeCl3 (9.237 g) was dissolved in 400 ml deionized water, and then added into the mixture. Ammonia aqueous solution (30 wt%) was added to adjust its pH to 10 in 2 h. After the mixture was heated to 90 °C, hydrazine hydrate (98 wt%, 30 ml) was added slowly and stirred for 4 h, resulting in a black suspension. The suspension was cooled and was separated with magnets, washed with deionized water and ethanol several times, and finally dried in vacuum at 60 °C.
Characterization of MnFe2O4/rGO Composite
X-ray diffraction (XRD) analysis was conducted with a diffractometer (Bruker D8 Discover) with Cu Kα radiation (40 kV, 40 mA). The morphology of samples was observed by a transmission electron microscope (TEM, JEOL 2100F). In this study, the vibrating sample magnetometer (VSM 7410, the Lake Shore) was used for the analysis of magnetic property of the nanocomposite.
Determination of the Concentration of TC
where C0 (mg/L) and Ct (mg/L) are the concentration of TC residues in the solution in the beginning and at time t, respectively. V (mL) stands for the volume of the solution, and it is 30 mL in this study, and m (g) is the weight of the MnFe2O4/rGO sample used.
Results and Discussion
Synthesis and Characterization of MnFe2O4/rGO
Adsorption of TC on MnFe2O4/rGO
Where K2 in this equation stands for rate constant of the pseudo-second-order kinetics.
Kinetic models and related parameters used to fit the curves of adsorption
TC concentration (mg L−1)
First-order kinetics model
Second-order kinetics model
K1 × 10−3
K2 × 10−3
(g mg−1 min−1)
Adsorption isothermal parameters fitted with Freundlich and Langmuir models
Overall, we believed that rGO mainly contributed the adsorption of TC. Firstly, the size of MnFe2O4 reached several tens of nanometers; it could not contribute a lot to the overall surface area. Secondly, the overall adsorption capacity was ~ 40 mg/g in TC with an initial concentration of ~ 10 mg/mL. This value was almost the same with the reported adsorption capacities of GO . The appearance of magnetic MnFe2O4 made the extraction and recycling of the adsorbent, rGO, easily.
MnFe2O4/rGO nanocomposite was successfully synthesized with one-pot method. The nanocomposite could be used as efficient adsorbents of TC with the adsorption capacity of 41 mg/g when the initial TC concentration was 10 mg/L. The kinetics and isotherm of the adsorption process was described as the pseudo-second-order model and Freundlich model, respectively. The magnetic adsorbents can be separated and regenerated, indicating the MnFe2O4/rGO nanocomposite can be a promising reusable adsorbents for environmental remediation for TC pollution.
We acknowledge the financial support received from National Key Research and Development Program of China (2017YFA0204600), the National Natural Science Foundation of China (41473071), the High Level Talent Project of “Six Talents Summit” in Jiangsu Province (JNHB-008), Jiangsu Province Key Laboratory of Environmental Engineering (KF2015006, ZX2017014), Natural science Foundation of Jiangsu Province, China (no. BK20171199), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, 164320H116) for this study.
HT, FW, and ZB conceived the project and supervised the study. JB, SY, and HZ performed the experiments. YZ and YC analyzed the data. JB, HT, and ZB wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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