Hongmei Zanga,
Chunyan Miaob,
Jianying Shangc,
Yingxin Liub and
Juan Liu*a
aThe Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China. E-mail: juan.liu@pku.edu.cn
bSchool of Gemmology, China University of Geosciences, Beijing, 100083, China
cDepartment of Soil and Water Sciences, China Agricultural University, Beijing 100193, China
First published on 1st May 2018
The catalytic reactivity of synthetic bare magnetite nanoparticles, activated carbon supported magnetite (AC-Mt), and graphene oxide supported magnetite (GO-Mt) for heterogeneous Fenton-like oxidation of methylene blue (MB) were compared, in order to investigate how the structural features of the support impact catalytic activity of the nanocomposites. The different effects of AC and GO on MB removal rate, hydroxyl radical (˙OH) production, iron leaching, and surface deactivation have been systematically studied. The rate constant of MB removal by AC-Mt was 0.1161 min−1, one order of magnitude larger than the value of bare magnetite nanoparticles (0.0566 min−1). The higher catalytic activity of AC-Mt might be attributed to the larger reactive surface area of well-dispersed magnetite for ˙OH production and the recharge of the magnetite surface by the AC support via Fe–O–C bonds. However, the removal rate of MB by GO-Mt was one order of magnitude slower than that of bare magnetite nanoparticles under the same experimental conditions, presumably due to the wrapping of GO around magnetite nanoparticles or extensive aggregation of GO-Mt composites. These findings revealed the significant influence of support structure on the catalytic activity of carbon-supported magnetite nanocomposites, which is important for the development of efficient magnetite-based catalysts for wastewater treatments.
Magnetite, a widespread iron oxide mineral in natural sediments,5 is a promising heterogeneous Fenton-type catalyst for wastewater treatment because of its good biocompatibility and unique ferromagnetism. The mixed-valence states for iron ions in magnetite structure may facilitate the initiation and propagation stages of Fenton processes, making magnetite a good catalyst for heterogeneous Fenton-like reactions. Nevertheless, surface oxidation of magnetite during Fenton reactions may lead to a gradual decrease in catalytic reactivity.6,7 Moreover, magnetite particles tend to aggregate in aquatic environments, which may evidently reduce available surface active sites for catalysis.8 Thus, immobilization of magnetite particles onto high-surface-area supports, such as mesoporous silica, clay minerals, activated carbon, zeolites, etc., has been proposed as an environmentally-friendly solution to these problems.4
Among all substrates used for magnetite-based catalysts in heterogeneous Fenton reactions, carbonaceous materials have aroused wide interest because of their high specific surface area, stability in acidic/basic media, tunable surface chemistry and structure, as well as environmental compatibility.9–11 Carbonaceous materials with different morphologies, such as one-dimensional carbon nanotubes, two-dimensional graphene sheets, and three-dimensional activated carbon with high porosity, have been proposed as the support for magnetite-based catalysts.12 The reactivity of these nanocomposites as heterogeneous Fenton catalysts has been widely studied in the treatments of various contaminants,9 but most of the studies focused on the interrelationship between chemical properties of carbon materials (such as metal impurities, surface functional groups, etc.) and catalytic efficiency of the carbon-magnetite nanocomposites. Very limited studies have studied how structural features of carbonaceous materials affect the catalytic activity of the whole nanocomposites. A recent study found that the ratio of magnetite to graphene oxide in graphene oxide-Fe3O4 nanocomposites led to different nanocomposite structures and catalytic activities for the heterogeneous Fenton-like reaction.12 Magnetite nanoparticles (Mt NPs) were dispersed on GO nanosheets at low GO loading (5 wt%), while Mt NPs were covered by GO stacking in the nanocomposite with higher GO loading (15 wt%).12 Moreover, Zhou et al. reported that, in the synthetic graphene-Mt composites with a 13.3 wt% graphene content, the graphene nanosheets enwrap Mt NPs.13 Further studies are needed to investigate the relationships between nanocomposite structure and their catalytic activity for development of highly efficient Fenton-like catalyst.
In order to investigate how substrate structure influences the catalytic activity of the carbon-supported nanocomposites, in this study, we selected graphene oxide as the representative two-dimensional lamellar support and activated carbon as the typical three-dimensional porous support to prepare carbon-supported magnetite nanocomposites. We synthesized activated carbon supported magnetite (AC-Mt) and graphene oxide supported magnetite (GO-Mt) nanocomposites with the same magnetite-to-substrate ratio of 1:
1, and compared the catalytic reactivity of synthetic bare magnetite nanoparticles, AC-Mt, and GO-Mt in heterogeneous Fenton-like oxidation of methylene blue (MB), a model recalcitrant dye pollutant. Moreover, aggregation state, adsorption capacity for MB, Fe leaching, ˙OH production, changes in surface chemistry of these catalysts have been systematically studied. The findings in the present study revealed that the geometrical structure of carbonaceous support can significantly impact the dispersion, available surface sites, Fe leaching, and surface oxidation of the supported magnetite nanoparticles, leading to obviously different catalytic activities in heterogeneous Fenton reactions.
The change of surface chemical properties of Mt, AC-Mt, and GO-Mt before and after Fenton reactions were analyzed by X-ray photoelectron spectroscopy (XPS). All XPS spectra were recorded with a Kratos AXIS-Ultra spectrometer (Kratos Analytical, Manchester, UK) employing a monochromatic Al Kα radiation (150 W) and a low-energy electron flooding for charge compensation. The carbon 1s line at 284.6 eV (for hydrocarbon or hydrocarbon groups) was used to calibrate the binding-energy scale for XPS measurements.
In order to compare the ability of different catalysts to generate hydroxyl radical (˙OH) in the Fenton-like reactions, ˙OH was quantified according to the method reported in the previous study.17 More details were shown in Section S3.† Furthermore, to study the role of ˙OH in MB removal, parallel experiments of Fenton-like reactions were conducted with the addition of 100 mM methanol as a hydroxyl radical scavenger.18 The MB concentrations in the system were monitored over time to evaluate the effect of ˙OH scavenging on MB removal.
The aggregation state of Mt NPs, GO and GO-Mt, respectively, at 25 °C under the similar solution conditions for the Fenton experiments were investigated by dynamic light scattering (DLS) on a Zetasizer (Nano ZS90, Malvern, UK) operating with a He–Ne laser at a wavelength of 633 nm and a scattering angle of 90°. A certain volume of the stock suspensions was added to 5 mL degassed HCl solution (pH = 3) in 10 mL centrifuge tubes. The mixture were sonicated (40 kHz, 300 W) for 20 min just before measurements. All measurements were conducted in three runs with at least fifty measurements for each run.
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Fig. 1 XRD patterns of synthetic Mt (A), AC-Mt (B), and GO-Mt NPs (C) with the reference pattern of magnetite (PDF no. 19-0629) (D). |
The synthetic Mt NPs were nearly spherical with a diameter of ∼11 nm (Fig. S2†). The representative SEM images of the pristine AC, GO, AC-Mt, and GO-Mt are shown in Fig. 2. Compared to pristine AC (Fig. 2A), AC-Mt composite showed a much rougher surface with brighter aggregates all over the sample (Fig. 2B), indicating the well-dispersion of magnetite NP aggregates on the surface or in the holes of AC. The EDS spectrum of AC-Mt (Fig. S3†) indicated that the chemical composition of the bright aggregates on AC surface included Fe, O, and C, indicating the formation of Mt NPs.
The synthetic GO exhibited a sheet-like morphology with large lateral surface and wrinkled edge (Fig. 2C), while GO-Mt (Fig. 2D) showed the wrapping of magnetite NPs by GO nanosheets. A similar phenomenon that GO nanosheets fold and wrapped goethite (FeO(OH)) NPs was observed after mixing GO and goethite suspensions.23 The possible explanation is that the GO nanosheets are not thick enough to stably support NPs and apt to fold around NPs or form crimps at the edge of NPs. In addition, to synthesize the GO-Mt nanocomposites with the same magnetite-to-substrate ratio (1:
1) as AC-Mt, a relatively high GO loading was used during the synthesis, which inevitably led to the wrapping of GO around Mt NPs.12 The SEM images of AC-Mt and GO-Mt (Fig. 2) directly show that using AC or GO as the support for carbon-supported magnetite catalysts could lead to the different structures of nanocomposites. The wet chemical analysis (Section S2†) confirmed that the mass ratio of magnetite to support in GO-Mt and AC-Mt was about 1
:
1.
It is worth mentioning that the adsorption capacity of 13 mg L−1 AC-Mt (AC:
Mt = 1
:
1) was similar to that of 6.5 mg L−1 pristine AC (1/2 AC in Fig. 3A), indicating that MB was mainly adsorbed by the AC support in the case of AC-Mt. In addition, the specific surface area (Table S1†) of AC (962.0 m2 g−1) was almost twice of AC-Mt (512.1 m2 g−1), which agreed with the similar adsorption capacity of 6.5 mg L−1 pristine AC and 13 mg L−1 AC-Mt, as shown in Fig. 3A.
However, the adsorption capacity of 13 mg L−1 GO-Mt (GO:
Mt = 1
:
1) was much lower than 6.5 mg L−1 (1/2 GO in Fig. 3B) or 13 mg L−1 pristine GO. The lower adsorption capacity of GO-Mt than pristine GO might be related to the extensive aggregation of GO-Mt that resulted in the decrease of reactive sites for MB adsorption. The hydrodynamic diameters (Dh) of 13 mg L−1 GO-Mt in the suspension at pH 3 was 1677 ± 95 nm, evidently larger than the Dh of 13 mg L−1 pristine GO (833 ± 212 nm) or bare Mt NPs (116 ± 3 nm). Mt and GO are opposite charged at pH 3. Extensive aggregation of GO-Mt can be induced via electrostatic attraction between Mt and exposed GO surface,25 resulting in the decrease of surface sites available for MB adsorption.
MB removal by pristine AC or GO in the presence of H2O2 (data not shown) were similar to their corresponding adsorption curves of MB (Fig. 3), so the support in AC-Mt or GO-Mt could not catalyze Fenton-like oxidation of MB by themselves under the conditions of this study.
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Fig. 4 The percentage of Fe released from 13 mg L−1 catalysts as a function of time in the solution for heterogeneous Fenton reactions. |
In all heterogeneous Fenton-like experiments, before MB and H2O2 were added, catalysts were pre-equilibrated with the solution at pH 3 for 20 minutes, because the rates of Fe leaching from these catalysts obviously decreased after 20 minutes. Fig. 4 shows that 0.51, 0.44, and 0.09 mg L−1 Fe ions were, respectively, released from bare Mt NPs, AC-Mt, and GO-Mt at 20 minutes. Previous studies have proven that the dissolved iron ions mostly are Fe3+ in heterogeneous Fenton-like system with magnetite at pH = 3,28,29 so Fenton oxidation of 10 mg L−1 MB with 0.44, 0.51, and 0.09 mg L−1 Fe3+ ions, respectively, were studied in order to evaluate the contribution of Fe-leaching induced homogeneous Fenton reaction to MB removal by the different catalysts. As shown in Table 1, the higher Fe3+ concentration resulted in the higher oxidation rates of MB. Thus, the rate constants of MB removal by iron ions released from bare Mt, AC-Mt, and GO-Mt were 0.0114, 0.0078, and 0.0023 min−1, respectively.
Catalysts | Heterogeneous Fenton | ˙OH production | Iron leaching | Homogeneous Fenton | ||
---|---|---|---|---|---|---|
kapp (min−1) 1st stage | kapp (min−1) 2nd stage | Within 5 min (μM min−1) | Within 25 min (μM min−1) | mg L−1 | kapp (min−1) | |
AC-Mt | 0.1161 | 3.58 | 0.83 | 0.44 | 0.0078 | |
Mt | 0.0566 | 2.27 | 0.45 | 0.51 | 0.0114 | |
GO-Mt | 0.0075 | 0.0282 | 1.1 | 0.46 | 0.09 | 0.0023 |
−ln(C0/Ct) = kappt | (1) |
Because the mass ratio of magnetite to support in the synthetic GO-Mt or AC-Mt composites was 1:
1, control experiments with 6.5 mg L−1 bare Mt was conducted to investigate the effect of initial Fe3O4 concentration on the kinetics of MB removal. The initial rate constant of MB removal with 6.5 mg L−1 bare Mt was 0.0511 min−1 (R2 = 0.95) that was very close to the value (0.0566 min−1) of 13 mg L−1 bare Mt (Table 1). Thus, the different removal rates of MB in the experiments with bare Mt and the supported Mt nanocomposites were not mainly due to the different amount of magnetite in the catalysts.
The initial rates of heterogeneous Fenton-like reactions with any of the synthetic catalysts were evidently larger than the corresponding rates of the homogeneous Fenton reactions induced by released iron ions (Table 1). For example, the rate constant of MB removal by 13 mg L−1 AC-Mt was 0.1161 min−1 that was two orders of magnitude faster than the rate constant (0.0078 min−1) of the homogeneous Fenton oxidation by 0.44 mg L−1 iron ions released from 13 mg L−1 AC-Mt. Moreover, although bare Mt released more iron ions than AC-Mt, the initial rate of MB removal by AC-Mt was obviously higher than bare Mt. Thus, MB removal by these catalysts was mainly attributed to heterogeneous Fenton-like process, not Fe-leaching induced homogeneous Fenton process. The smaller initial rate of MB removal by GO-Mt was probably related to the wrapping of magnetite NPs by GO nanosheets or extensive aggregation of GO-Mt nanocomposites, which inhibited the interaction between magnetite and H2O2 or the diffusion of reactive oxygen species and MB. As shown in Fig. 5 and Table 1, the rate constant of MB removal by GO-Mt increased to 0.0282 min−1 after ∼20 minutes, showing a two-stage reaction. It might be related to the continuous increase of iron ions released from GO-Mt or ˙OH concentration after 20 minutes (Fig. 4 and 6A).
The initial TOC content in MB solution was 5.9 mg L−1. After 4 hours heterogeneous Fenton oxidation, the TOC concent in the systems with AC-Mt, GO-Mt and Mt was 1.9, 2.1 and 2.4 mg L−1, respectively. As shown in Fig. 5A, MB was completely removed after 4 hours reactions in all cases. However, the TOC removal efficiency by AC-Mt, GO-Mt and Mt was 68%, 64% and 59%, respectively, which were consistent with the previously reported results about MB removal by magnetite-based catalysts via heterogeneous Fenton reaction.31–33 The residual TOC may be attributed to some small molecular intermediates generated from the catalytic reactions.31
The reusability of bare Mt, AC-Mt, and GO-Mt was studied, respectively, in seven consecutive cycles of use. The percentage of MB removal after seven successive cycles by all catalysts was still more than 85% of MB, indicating a good recyclability of all catalysts for the heterogeneous Fenton-like oxidation of MB. The removal efficiency of MB during the first four hours in each cycle was compared in Fig. S4.† The gradually decrease of initial oxidation rates, or catalytic efficiency, with the increase of cycles was observed in all cases. It might be related to Fe leaching from magnetite, the oxidation of magnetite surface, or poisoning of the active catalytic sites by the adsorbed organic species.29,34,35
The concentration of hydroxyl radicals in the system with AC-Mt was 17.9 μM at 5 minutes and increased to 20.8 μM at 25 minutes (Fig. 6A) that were obviously higher than the values of bare Mt and GO-Mt. In AC-Mt, Mt NPs were well-dispersed in AC that has a large specific surface area and porous structure. Relative to aggregated Mt NPs in solution, AC-Mt has more active surface sites for the reaction with H2O2, resulting in more ˙OH production. Besides, methanol was used as a scavenger for ˙OH during heterogeneous Fenton-like experiments (Fig. 6B). The degradation profiles of MB by the catalysts became similar to the corresponding adsorption curves (Fig. 3). Thus, ˙OH was the predominant reactive species to oxidize MB in the heterogeneous Fenton-like reaction with bare Mt or Mt-based nanocomposites.
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Fig. 7 XPS Fe 2p peaks of AC-Mt (A), GO-Mt (B), and Mt (C) before (left) and after (right) seven consecutive cycles of use. |
The C 1s spectra of AC-Mt and GO-Mt (Fig. 8) can be fitted into five components: CC (284.6 eV), C–C (285.6 ± 0.1 eV), C–O (286.5 eV), C
O (288.0 ± 0.2 eV), and O–C
O (289.1 ± 0.1 eV).12,39 The percentage of C
C in the C 1s spectra of AC-Mt decreased from 69% to 25% after Fenton reactions (Table 2), and also that of GO-Mt decreased from 48% to 29%. Correspondingly, the percentages of C–C and C
O in the C 1s spectra of AC-Mt and GO-Mt obviously increased after Fenton reactions. These changes in the C 1s spectra probably suggest that AC or GO support in the nanocomposites was oxidized to some extent during Fenton reactions. As mentioned above, ˙OH was the dominant reactive oxygen species for MB removal. It was proposed that Fe(II) on magnetite surface linked to the carbonaceous support might be the active sites for ˙OH production.40 However, the oxidation of surface Fe(II) on magnetite was commonly accompanied with ˙OH production. The O 1s spectra of AC-Mt and GO-Mt (Fig. 9A and B) could be fitted into four components: Fe–O (530.0 ± 0.2 eV), Fe–O–C (531.2 ± 0.2 eV), COO/C
O (532.3 eV), C–O–C (533.4 eV),4,12 and that of bare Mt (Fig. 9C) could be fitted into three components: Fe–O (530.2 eV), C–O (531.6 eV), O–H (533.1 eV).41 In AC or GO supported magnetite, electrons could be transferred from the support to Fe(III) on magnetite surface through Fe–O–C bonds and regenerate Fe(II),40 which can alleviate surface deactivation and facilitate ˙OH production. Although AC and GO are both carbonaceous materials with the ability to recharge magnetite surface via Fe–O–C bonds, their different structures could significantly influence the number of Fe–O–C bonds formed between the support and Mt NPs. In AC-Mt, magnetite NPs were well dispersed on the AC support with a porous structure, but magnetite NPs in GO-Mt were wrapped by GO nanosheets and tended to aggregate. Thus, there were more Fe–O–C bonds formed in AC-Mt than in GO-Mt, as shown in the O 1s spectra of AC-Mt and GO-Mt (Fig. 9 and Table 3). The results indicate that immobilizing magnetite nanoparticles onto carbonaceous materials with different structures can obviously influence catalytic activity, surface deactivation, and stability of the nanocomposite catalysts in heterogeneous Fenton-like reactions.
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Fig. 8 C 1s peak of AC-Mt (left) and GO-Mt (right) before (A and B) and after (C and D) seven consecutive cycles, respectively. |
C![]() |
C–C | C–O | C![]() |
O–C![]() |
|
---|---|---|---|---|---|
Binding energy/eV | 284.6 | 285.6 ± 0.1 | 286.5 | 288.0 ± 0.2 | 289.1 ± 0.1 |
AC-Mt | 69% | 6% | 15% | 4% | 6% |
AC-Mt after 7 runs | 25% | 38% | 20% | 10% | 7% |
GO-Mt | 48% | 21% | 15% | 9% | 7% |
GO-Mt after 7 runs | 29% | 38% | 20% | 8% | 5% |
Fe–O | Fe–O–C | COO; C![]() |
C–O–C | |
---|---|---|---|---|
Binding energy/eV | 530.0 ± 0.2 | 531.2 ± 0.2 | 532.3 | 533.4 |
AC-Mt | 56% | 27% | 11% | 6% |
GO-Mt | 56% | 20% | 15% | 9% |
Fe–O | C–O | O–H | — | |
Binding energy/eV | 530.2 | 531.6 | 533.1 | — |
Mt | 61% | 33% | 6% | — |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra02286k |
This journal is © The Royal Society of Chemistry 2018 |