Edited by: Angang Dong, Fudan University, China
Reviewed by: Huaibin Shen, Henan University, China; Yanbin Cui, Institute of Process Engineering (CAS), China
This article was submitted to Nanoscience, a section of the journal Frontiers in Chemistry
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
In this study, two kinds of composites with the structure of graphene oxide (GO) sheets wrapped magnetic nanoparticles were investigated on their regeneration. The composites have a similar core-shell structure, but the interactions between the core and shell are quite different, which are electrostatic and covalent. They were characterized by scanning/transmission electron microscopy, power X-ray diffraction, and vibrating sample magnetometer analysis. Their morphologies and structures of the samples had been revealed using methylene blue and Pb(II) as adsorbates during regeneration. The results showed that the composites with covalent bonding interaction could maintain a stable core-shell structure and present a good regeneration performance for adsorption-desorption of methylene blue and Pb(II). The composites with electrostatic interaction could approximately preserve its core-shell structure and could be recyclable for adsorption-desorption of methylene blue, however, they would disintegrate its core-shell structure during adsorption/desorption of Pb(II), thus greatly decreasing their regeneration performance. The regeneration mechanisms of the composites were analyzed, which could provide a useful theoretical guide to design the GO sheets wrapped magnetic nanoparticles composites.
Due to the unique structure and excellent characteristics, graphene and its derivatives have attracted more and more interests in the scientific community (Geim and Novoselov,
To further improve the performance, some researchers have attempted to wrap Fe3O4 nanoparticles (NPs) with GO sheets and the obtained GO@Fe3O4 displayed excellent adsorption performance toward pollutants (Wei et al.,
We have synthesize magnetic GO composites as environmental materials recently (Hu et al.,
In this paper, our investigations are focused on the regeneration processes and the mechanisms of two kinds of Fe3O4@GO composites, which had been successfully synthesized in our previous study. Although both of the Fe3O4@GO samples have similar a core-shell structure, in which GO sheets are tightly connected with magnetic NPs, the interactions linking the core and shell are quite different, which are electrostatic and covalent, respectively. Methylene blue and Pb(II) were used as typical adsorbates to elucidate the evolution of the morphologies and structures of both samples during regeneration in detail. To the best of our knowledge, it is firstly reported to systematically investigate the regeneration mechanisms of the Fe3O4@GO composites, and the study could provide a theoretical guide for improving the regeneration of GO-based composites, thus accelerating their practical application.
Graphite (100 mesh, XFnano), ferric chloride hexahydrate (FeCl3·6H2O, Sinopharm), ethylene glycol (Sinopharm), polyethylene glycol (PEG 4000, Sinopharm), sodium acetate trihydrate (NaAc·3H2O, Sinopharm), tetraethyl orthosilicate (Sinopharm), poly(diallyldimethylammonium chloride) (PDDA, Sinopharm), 3-aminopropyl triethoxysilane (APTES, Sinopharm), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC, Sinopharm), n-hydroxysuccinimide (NHS, Sinopharm), ammonia water (28 wt.%, Sinopharm), hydrochloric acid (Sinopharm), lead nitrate [Pb(NO3)2, Sinopharm], methylene blue (MB, Sinopharm).
The synthesis procedures of GO wrapped Fe3O4 composites by electrostatic interaction (Fe3O4@GO-e) were described elsewhere in detail (Hu et al.,
The detailed synthesis processes of GO sheets wrapped Fe3O4 composites by covalent bonding (Fe3O4@GO-c) could be found elsewhere (Hu et al.,
Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-6360LV or Hitachi S4800. Transmission electron microscope (TEM, Tecnai G2 F20, FEI, USA) was utilized to investigate the morphology and microstructure of the sample. The powder X-ray diffraction (XRD) patterns of the samples were collected from a Bruker D8 advanced diffractometer using Cu-Kαradiation (λ = 0.1514 nm) in the 2θ range of 10–80°. The magnetic experiments were performed on a Lakeshore 7407 vibrating sample magnetometer at room temperature.
MB, a common dye pollutant, and Pb(II), a typical heavy metal ion, were used as adsorbates for the study. The adsorption experiments were carried out on a shaker with a shaking speed of 200 rpm at 30°C.
For MB adsorption tests, 50 mg of the sample and 50 mL of MB solution (150 mg/L, pH = 8) were mixed in a 100 mL air-tight glass conical flask. The adsorption equilibrium was reached after 2 h of agitation. Subsequently, the adsorbent was separated using a hand-held permanent magnet. The supernatant was collected for concentration measurements by UV-vis spectrophotometry. The adsorption capacity was calculated based on the following formula:
where qe refers to the adsorption equilibrium capacity, C0 and Ce denote the initial and equilibrium concentrations, respectively, V is the solution volume, and M represents the adsorbent's mass.
For Pb(II) adsorption, the experimental procedures were same as the above ones except for modifications of the following parameters. The initial concentration of Pb(II) solution was 300 mg/L, and its pH value was adjusted to 6 before adsorption tests. The adsorption equilibrium time was set as 12 h. The Pb(II) concentration in the supernatant was measured by atomic absorption spectrophotometry and the adsorption capacity toward Pb(II) was obtained according to Equation (1).
The MB-loaded and Pb(II)-loaded samples were utilized to evaluate the regeneration performance. For MB desorption, the regeneration of the sample was carried out by immersing it in ethanol solution under mechanical stirring for 30 min. For Pb(II) desorption, the regeneration of the sample was performed by soaking it in the presence of 0.01 M of HCl under ultrasonication for 30 min.
SEM/TEM could intuitively reveal the morphologies and textures of the samples. The SEM/TEM images of the Fe3O4@GO-e sample are shown in
The regenerability of the sample has much to do with its evolution of morphology and structure during adsorption-desorption recycling.
SEM images of the samples after five cycles
Nevertheless, the Fe3O4@GO-e sample had quite different morphologies and structures after adsorption-desorption recycling toward MB and Pb(II). From
XRD technique is a powerful tool for structure characterization, and the structure variation of the Fe3O4@GO samples mainly lies in whether the GO sheets could still wrap the magnetic NPs or disintegrate from the NPs. The XRD patterns of Fe3O4@GO samples before and after recycling are shown in
XRD patterns of Fe3O4@GO-e and Fe3O4@GO-c
The magnetism of novel carbon materials plays a pivotal role for their application as adsorbents (Ren et al.,
The maximum saturation magnetisms of the Fe3O4@GO samples before and after five cycles of adsorption-desorption toward MB and Pb(II).
Fe3O4@GO-e | 61.0 | 59.5 | 59.1 |
Fe3O4@GO-c | 57.1 | 55.8 | 55.3 |
The adsorption-desorption tests for the samples were repeated five times using MB and Pb(II) as the adsorbates, and the results are displayed in
Adsorption-desorption recycling tests toward
It is well-known that the performance of a composite is highly related to its structure. Therefore, it could be reasonably deduce that the significant variation of the Fe3O4@GO-e sample in adsorption capacity toward Pb(II) is attributed to the disintegration of its core-shell structure. Indeed, during desorption of Fe3O4@GO-e toward Pb(II), there existed black GO sheets in solution, which could not be separated by an external magnet, fully demonstrating that the GO sheets had been separated from the composites and dispersed in solution. The slight decrement in adsorption capacities in other cases could be explained that the pre-adsorbed amounts could not be totally released from adsorption sites (Zhang et al.,
The structure evolution of the adsorbents during regeneration is closely related to their regeneration performance. Therefore, it is very beneficial to improving the structural stability by adoption of pertinent measures.
In this study, both Fe3O4@GO samples have a similar core-shell structure, in which the magnetic NPs are tightly wrapped by GO sheets. However, the interactions between the core and shell are completely different, which are electrostatic and covalent. The Fe3O4@GO-c sample has a stable structure due to the firm covalent bonding that could resist the acid and ethanol environment during regeneration, resulting in good regeneration performance toward MB and Pb(II). For the Fe3O4@GO-e sample, its structure is not very stable owing to the weak electrostatic connection, but it could still maintain its core-shell structure in ethanol solution, thus resulting in a reasonable regeneration performance toward MB.
Unfortunately, when desorbing Pb(II) in acid solution, the Fe3O4@GO-e sample would disassemble due to the following reasons: (1) H+ ions could attract GO sheets with negative charges; (2) A great number of H+ ions in solution could endow GO sheets with positive charges, and it would repel the magnetic NPs with the same charges. The sketch illustrating the structure evolution during regeneration is presented in
Sketch illustrating the structure evolution of Fe3O4@GO-c and Fe3O4@GO-e.
Two kinds of composites with the structure of GO sheets wrapped magnetic nanoparticles composites had been successfully synthesized, and their regeneration had been investigated using MB and Pb(II) as adsorbates. Both samples have a similar core-shell structure, and the linking forces between core and shell are electrostatic and covalent, respectively. During regeneration, the GO@Fe3O4-c sample could resist the erosion from ethanol and acid solution, and could well maintain its core-shell structure. After five cycles, it still holds the adsorption capacity ~83% toward MB and ~89% toward Pb(II), respectively. The GO@Fe3O4-e sample could preserve ~77% adsorption capacity toward MB, and could roughly keep the core-shell structure after five cycles. However, it would completely disassemble its core-shell structure, resulting in only ~29% adsorption capacity toward Pb(II) after five cycles. The related regeneration mechanism and the structure evolution during regeneration had been proposed, which could provide a theoretical guide for designing and improving the GO-based composites.
The datasets generated for this study are available on request to the corresponding author.
ZH and XZ conducted the most experiments. JL and YZ performed the characterization and data analysis. All authors involved the analysis of experimental data and manuscript preparation.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21576075, 21376069).