Highly efficient and recyclable graphene oxide-magnetite composites for isatin mineralization

Article abstract of DOI:10.1016/j.jallcom.2017.07.087

A series of graphene oxide-magnetite (GO-Fe₃O₄) composites with various GO/Fe₃O₄ weight ratios (i.e., GO/Fe₃O₄ = 1/20, 2/20, 3/20 and 4/20) was successfully synthesized via chemical precipitation of Fe₃O₄ nanoparticles on GO nanosheets. The chemical and physical properties of as-synthesized GO-Fe₃O₄ composites were characterized by XRD, TEM and FT-IR. Results from XRD and TEM revealed that cubic-phase Fe₃O₄ nanoparticles were in situ deposited on the surface of GO nanosheets resulting in GO-Fe₃O₄ composites. The C[sbnd]O[sbnd]Fe bridging coordination mode was determined by FT-IR, demonstrating the Fe₃O₄ nanoparticles were well coupled with GO nanosheets by coordination bond. TEM images revealed that two types of geometrical structures of GO-Fe₃O₄ composites were formed by loading different amounts of GO. With low GO loadings (i.e., GO/Fe₃O₄ = 1/20, 2/20 and 3/20), single layer structure GO-Fe₃O₄ composites were obtained. At a high GO loading (i.e., GO/Fe₃O₄ = 4/20), stacking structure of GO-Fe₃O₄ composite was formed. The as-prepared GO-Fe₃O₄ nanocomposites exhibited an excellent catalytic performance in the degradation of isatin in the presence of H₂O₂. With GO/Fe₃O₄ weight ratio of 3/20, GO-Fe₃O₄ composites showed superior degradation efficiency of isatin, mainly due to the effective functional combination between GO nanosheets and Fe₃O₄ nanoparticles.

Full text of DOI:10.1016/j.jallcom.2017.07.087

Accepted Manuscript
Highly efficient and recyclable graphene oxide-magnetite composites for isatin
mineralization
Zhengyuan Zhou, Minhua Su, Kaimin Shih
PII:
S0925-8388(17)32452-0
10.1016/j.jallcom.2017.07.087
JALCOM 42503
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 April 2017
Revised Date: 14 June 2017
Accepted Date: 8 July 2017
Please cite this article as: Z. Zhou, M. Su, K. Shih, Highly efficient and recyclable graphene oxide-
magnetite composites for isatin mineralization, Journal of Alloys and Compounds (2017), doi: 10.1016/
j.jallcom.2017.07.087.
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Highly Efficient and Recyclable Graphene Oxide-Magnetite Composites for
Isatin Mineralization
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Zhengyuan Zhou a, Minhua Su a, b, c *, Kaimin Shih a*
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a Department of Civil Engineering, The University of Hong Kong, Pokfulam Road,
Pokfulam, Hong Kong SAR, P.R. China
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b School of Environmental Science and Engineering, Guangzhou University,
Guangzhou, 510275, P.R. China
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c Guangdong Provincial Key Laboratory of Radioactive Contamination Control and
Resources, Guangzhou University, Guangzhou, 510275, P.R. China
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ABSTRACT: A series of graphene oxide-magnetite (GO-Fe3O4) composites with
various GO/Fe3O4 weight ratios (i.e., GO/Fe3O4 = 1/20, 2/20, 3/20 and 4/20) was
successfully synthesized via chemical precipitation of Fe3O4 nanoparticles on GO
sheets. The chemical and physical properties of as-synthesized GO-Fe3O4 composites
were characterized by XRD, TEM and FT-IR. Results from XRD and TEM revealed
that cubic-phase Fe3O4 was in situ deposited on the surface of GO resulting in
GO-Fe3O4 composites. The C-O-Fe bridging coordination mode was determined by
FT-IR, demonstrating the Fe3O4 nanoparticles were well coupled with GO sheets by
coordination bond. TEM images revealed that two types of geometrical structures of
GO-Fe3O4 composites were formed by loading different amounts of GO. With low
GO loadings (i.e., GO/Fe3O4 = 1/20, 2/20 and 3/20), a single layer structure
GO-Fe3O4 composite was obtained. At a high GO loading (i.e., GO/Fe3O4 = 4/20),
stacking structure of GO-Fe3O4 composite was formed. The as-prepared GO-Fe3O4
nanocomposites exhibited an excellent catalytic performance in the degradation of
isatin in the presence of H2O2. With GO/Fe3O4 weight ratio of 3/20, GO-Fe3O4
composites showed superior degradation efficiency of isatin, mainly due to the
effective functional combination between GO and Fe3O4.
Keywords: Graphene oxide; Magnetite; Catalytic; Isatin; Mineralization
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1. Introduction
Over the past few years, more than 10,000 types of commercial dyes with an annual
production of 7 × 105 tons per year are globally produced [1]. Annually, over 50% of
these dyes are consumed in textile industry all over the world [2,3]. After used, 10-20%
of textile dyes are directly discharged as aqueous effluents, leading to a pressing
environmental issue [4-7]. Dye is known to be a class of reluctant pollutant and
dyeing wastewater is one of the most refractory industrial wastewater. Therefore, it is
difficult to degrade dye via conventional wastewater treatment methods [8]. Effective
removing dye from the wastewater stream has been considered as a huge challenge in
the field of wastewater treatment [9].
A number of wastewater treatment technologies, including physical (adsorption [10]
and membrane [11]), chemical (coagulation [12], oxidation [13,14] and photocatalysis
[15]), biological (aerobic [16] and anaerobic [17,18] degradation) and electrochemical
[19] methods, have been developed and adopted for dyeing wastewater treatment.
Because of its high efficiency, Fenton/Fenton-like process, which used Fenton’s
reagents (i.e., Fe2+ and H2O2) at low pH, has been widely applied in dyeing
wastewater treatment [20]. However, homogeneous Fenton process will inevitably
produce a large amount of iron-rich sludge. In recent years, recyclable heterogeneous
Fenton-like catalysts such as hematite (α-Fe2O3) [21,22], magnetite (Fe3O4) [23] and
goethite (δ-FeOOH) [24,25] have attracted a great attention since iron-rich sludge will
be remarkably reduced in the case of using magnetically recyclable catalysts. Within
these iron-based catalysts, magnetite (Fe3O4) is a promising catalyst due to its
recyclability and superior catalytic performance [25-28]. Only using Fe3O4 in
Fenton-like process, the removal efficiency of organics is often unsatisfactory [29,30].
Coupling with materials could generally enhance the catalytic properties of Fe3O4
based catalysts [29,30]. It has been reported that carbon coating (humic acid [29],
polyhydroquinone [30], sodium carboxymethyl cellulose-graft-poly(acrylic acid) [31]
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and multiwall carbon nanotubes [32-35]) could serve as an electron shuttle in
Fenton-like reaction, which could remarkably increase the catalytic performance of
magnetite nanoparticles [36]. Zubir et al. [37] coupled magnetite nanoparticle with
graphene oxide and applied this composite in the degradation of Acid Orange 7 (AO7)
via a Fenton-like process. Although the catalytic performance of GO-magnetite
composite has been experimentally documented in a number of studies [37-40], the
formation mechanism of GO-magnetite composite has not been fully revealed yet. As
a new Fenton-like catalyst, the knowledge of its chemical and physical properties,
catalytic behavior and practical application is timely needed.
Isatin and its derivatives were commonly used in textile industry [41], resulting in
high COD value, colority and biotoxity in dyeing wastewater [42]. Isatin cannot be
efficiently removed by traditional biodegradation processes (e.g., aerobic and
anaerobic degradation) due to its particular chemical structure [43]. In this study,
isatin (2,3-indolinedione) is therefore used as an organic pollutant for the evaluation
of the catalytic property of as-prepared GO-magnetite composites.
This work synthesized a series of GO- Fe3O4 composites with different GO and Fe3O4
loading ratios. The influences of GO loading ratio on the morphology, crystallization
and catalytic property of GO-Fe3O4 nanocomposites were investigated. FT-IR
technique was employed to reveal the coordination behavior of C-O-Fe in the
as-synthesized nanocomposite. The formation mechanism for GO-Fe3O4
nanocomposites was proposed. The catalytic performance of GO-Fe3O4
nanocomposites in the degradation of isatin is studied.
2. Experimental
2.1 Materials
Ammonia solution (NH3·H2O, 30-33 wt.%) was purchased from E. Merck, LTD.
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Concentrated sulfuric acid (H2SO4, 95 wt.%), di-phosphorus pentaoxide (P2O5), ferric
chloride hydrate (FeCl3·6H2O) and hydrochloric acid (HCl, 35.4 wt.%) were
purchased from BDH Merck Chemical Co.. Ferrous chloride hydrate (FeCl2·6H2O),
potassium permanganate (KMnO4) and potassium persulfate (K2S2O8) were purchased
from Sigma-Aldrich. Graphite (325 mesh) and manganese dioxide (MnO2, 75 wt.%)
was purchased from Beijing HWRK Chem Co., LTD. Hydrogen peroxide (H2O2, 33
wt.%) was purchased from Riedel-de Haen AG. Isatin (C8H5NO2) was purchased
from ACROS Organics. GD120 glass fiber was purchased from Toyo Roshi Kaisha,
Ltd..
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2.2 Preparation of Graphene Oxide (GO) Sheets
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Graphene oxide (GO) was prepared via a modified two-step Hummers method [44].
At the first pre-oxidation step, 20 ml 95 wt.% H2SO4, 3.0 g K2S2O8, 3.0 g P2O5 and
3.0 g graphite were mixed in a 100 mL Erlenmeyer flask. The mixture was
homogenized by magnetic stirring. The reaction system was maintained at 80 °C for 6
h through a water bath. Next, the mixture was diluted by adding 500 ml deionized
water and this as obtained was statically aged at ambient temperature for 12 h. Black
precipitate was collected by vacuum filtration with glass fiber filter paper and dried at
105 °C in an oven for 24 h. For the second oxidation step, 3.6 g preoxidized sample
(black precipitate from the first step), 120 ml 95% H2SO4 and 15 g KMnO4 were
slowly added into a 250 mL Bunsen flask under magnetic stirring in an ice-bath to
10 °C. After the reaction was kept at 35 °C for 6 h, 500 ml deionized water with 20ml
35 wt.% H2O2 was slowly added into the reaction mixture. The resulting solution was
centrifuged by centrifuge (Eppendorf 5810R) at 4,000 rpm for 10 min and washed
with 3wt. % HCl aqueous solution and deionized water for several times. The final
product was diluted in deionized water and exfoliated by ultrasonic treatment for 2 h
to form GO solution (0.08 g/L).
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2.3 Preparation of Graphene Oxide-Magnetite (GO-Fe3O4) Composites
For the synthesis of GO-Fe3O4 composites, 0.041 g FeCl2·6H2O and 0.0932 g
FeCl3·6H2O were added to 100 ml GO solutions (0, 0.02, 0.04, 0.06 and 0.08 g/L)
with magnetic stirring under Ar atmosphere in a three-necked flask. 20 ml NH3·H2O
(30-33 wt.%) was dropwise added into FeCl2-FeCl3-GO solution to keep the pH at
10.0. The reactive system was maintained at 50 °C by water bath. After 2 h reaction,
black product was collected by magnetite and washed with deionized water for
several times. Final products were ultrasonically dispersed in deionized water to form
a homogeneous solution. The concentration of Fe3O4 in all as-obtained homogeneous
solution is estimated to be 3 g/L. Fe3O4 without coupling with GO was denoted as
Catalyst 1 and the as-prepared GO-Fe3O4 composites were denoted as Catalyst 2-5 for
GO/Fe3O4 weight ratios of 1/20, 2/20, 3/20 and 4/20, respectively. The as obtained
Catalysts were characterized by XRD (D8, Bruker AXS, Germany), SEM (Hitachi
S-4800 field emission scanning electron microscope, Japan), TEM (FEI Tecnai G2 20
Scanning Transmission Electron Microscope, USA) and FI-TR (Perkin-Elmer
Spectrum One FT-IR spectrometer, USA). The XRD patterns were recorded over a 2θ
range from 5 to 80° with a step width of 0.02° and a sampling time of 0.3 s per step.
The FT-IR measurement used a potassium bromide (KBr) pellet method.
2.4 Catalytic Study
To examine the catalytic activities of the as-prepared catalysts (i.e., Fe3O4 and
GO-Fe3O4 composites), 2 ml of as-prepared homogeneous solution with catalyst was
added into 8 ml of 50 ppm isatin aqueous solution, then 4 ml of 0.125 M H2O2 was
added into 25 ml Erlenmeyer flasks. Then, the reaction solutions were diluted with
deionized water to 20 ml. The pH in the reaction system was adjusted to 2.5, 3.5 and
5.0 by 0.1 M sulfuric acid. The reaction system was magnetically stirred in water bath
at different reaction temperature (25 °C, 35 °C, 45 °C and 55 °C). At different reaction
time (15, 30, 45, 60, 120 and 180 min), MnO2 powders (0.05 g) were added to
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reaction solutions to terminate the reaction. The final reaction solutions were filtered
by nylon syringe filter (0.45 ꢀm, Filtrex) for UV-vis and TOC measurement.
3. Results and Discussion
3.1 Characterization of GO-Fe3O4
The particle size and combination behavior of newly generated Fe3O4 in the
as-prepared GO-Fe3O4 composites were revealed by TEM and SAED. In the SAED
patterns (Fig. S1e, S1h, S1k, S1n and 1q) of the as-obtained products, various distinct
electron diffraction rings of the GO-Fe3O4 composites were observed. These
diffraction rings are corresponding to the (220), (311), (400), (511) and (440) lattice
plane of cubic-phase Fe3O4 [45], respectively, confirming the black particles (Fig.
1a-1e) were Fe3O4. The particle sizes of the newly generated Fe3O4 were approximate
10 nm (Fig. S1d, Fig. S1g, Fig. S1j, Fig. S1m and Fig. S1p), as revealed by TEM.
With an increase of GO loading, the trend of distribution density of Fe3O4 in the
GO-Fe3O4 composites tends to decrease (Fig. S1f, Fig. S1i, Fig. S1l and Fig. S1o).
For instance, with a great GO/Fe3O4 ratio (i.e., 4/20), GO sheets overlapped
significantly with Fe3O4 (Fig. 1e). In Fig. 2a and Fig. 2b, the GO boundaries were
able to be clearly observed and these boundaries were overlapped. We inferred that
the GO-Fe3O4-GO bedded layer structure might be formed [37].
XRD pattern of the unexfoliated GO is shown in Fig. 3. It can be seen that GO
without ultrasonic exfoliation had a single broad diffraction peak at 2θ of 10°. This
(001) diffraction peak corresponded to an interlayer spacing of 0.88 nm, indicating the
presence of oxygen functionalities in the product [46]. With an ultrasonic treatment,
GO sheets were well exfoliated and become a few-layer structure. Consequently, in
XRD pattern of as-synthesized GO-Fe3O4 products (Fig. S1b), the (011) diffraction
peak of GO disappeared. A series of diffraction peaks at 30.0°, 35.5°, 43.0°, 53.5°,
57.0° and 63.0° was observed in the XRD pattern of GO-Fe3O4 composites (Fig. 3).
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These diffraction peaks were indexed to be (220), (311), (400), (422), (511) and (400)
diffraction planes of cubic Fe3O4 [47], respectively. The intensity of these Fe3O4
peaks progressively became weakened when the GO loading ratio increased, which
indicated GO may render the crystallization of Fe3O4 during the process of forming
GO-Fe3O4 composites. When dried same amount of GO-Fe3O4 samples at 105 °C for
24 h, the dried products exhibited different appearance at various GO loadings (Fig.
S2). For the samples with low GO loadings (e.g., GO/Fe3O4 of 1/20), their appearance
are very similar to the product which only composed of Fe3O4 nanoparticles. With GO
loading increased, the appearance of GO-Fe3O4 is different, for instance, folding fan,
fishbone and paper dust for GO/Fe3O4 of 2/20, 3/20 and 4/20, respectively (Fig.
S2a-2e).
FT-IR is a powerful tool to verify the vibrational stretching frequency of GO, Fe3O4
and GO-Fe3O4 composites. Fig. 4 showed the collated FT-IR spectra of GO, Fe3O4
and GO-Fe3O4 composites. The strong peak at 3401 cm-1 was attributed the O-H
stretching vibration in GO [48]. Due to the O-H stretching vibration of hydrogen
bonded carboxyl groups, peak broadening effect was commonly observed in all FT-IR
spectra. In the FT-IR spectra of GO sample, two peaks at 1720 and 1620 cm-1 were
observed, which are derived from the C=O stretching vibration of carboxylate and
conjugated carbonyls [48,49]. A peak at 1231 cm-1 suggested the C-O stretching
vibration of epoxy ring in the as-prepared GO. Note that the characteristic peak at
1091 cm-1 was often attributed to the C-O stretching of alcohols. The peak at 618 cm-1
was in-plane deformation vibrations of C-CHO for aromatic aldehydes. The peak at
585 cm-1 was the typical Fe-O stretching vibration of Fe3O4. Two significant
vibrations were observed in the FT-IR spectra of GO-Fe3O4 composites, most likely
due to the carboxyl groups, such as an antisymmetric stretching (νa(COO-)) at 1540
cm-1 and a symmetric stretching (νs(COO-)) at 1407 cm-1. The observation of the
signals of these two vibrations further indicated that carboxyl groups of GO sheets
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were deprotonated [50]. The ∆ values of both νa(COO-) and νs(COO-) was 133 cm-1
which were close to the ionic values [51], implying that the carboxyl group
coordinated with Fe2+ or Fe3+ ions in bridging coordination mode (Fig. S3). Notice
that the as above FT-IR results gave a strong evidence for the formation of C-O-Fe
coordination bond between GO sheets and Fe3O4 nanoparticles. The coordination
behavior between GO and Fe3O4 was derived from the chemical bond. Notably, in a
Fenton process, C-O-Fe coordination bond played a pivotal role for electron shuttle,
achieving a higher efficiency for the removal of organics [36]. Higher GO loading
would provide more carboxylate group for the nucleation and growth of Fe3O4
crystallites, thus resulting in smaller Fe3O4 nanoparticles. As a consequent, the
diffraction intensities of Fe3O4 decreased as GO loadings increased. Based on the
observation from our devised synthesis scheme and the results of FT-IR, a possible
formation mechanism for GO-Fe3O4 composites was illustrated in Fig. S3. First, iron
(II, III) ions were absorbed on the surface of GO nano-sheets, forming C-O-Fe
coordination bonds. Then, the nuclei of Fe3O4 crystallization were generated. With
further addition of ammonia solution, Fe3O4 crystallites rapidly grew on the surface of
GO. The pH for the synthesis system was maintained at 11.0 to ensure the complete
precipitation of iron (II, III) ions (eq. 1).
+
Fe2+ + 2Fe3+ + 8NH3·H2O = Fe3O4 + 8NH4 + 4H2O
(1)
As previously indicated, the amount of GO loading tends to alter the quantity of
carboxyl groups for Fe3O4 crystallization. Higher loading of GO will produce smaller
Fe3O4 nanoparticles in GO-Fe3O4 composites. When GO loading is as higher to 4/20
of GO/Fe3O4, the free carboxyl group on GO surface will form C-O-Fe coordination
bond with Fe3O4 nanoparticles leading to a stacking structure. The peaks at around
3100 cm-1 for C-H stretching vibration of aromatic ring, 800 cm-1 for C-H bending
vibration of aromatic ring and 750 cm-1 for C-H bending vibration of aromatic ring
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were observed (Fig. 4). These peaks indicated the distance of two grapheme oxide
nanosheets was close enough to form a strong interaction among the aromatic rings of
two grapheme oxide nanosheets in GO-Fe3O4 composites. This is the main reason to
form two substantially different microstructures of GO-Fe3O4 composites. The overall
results from TEM ， XRD and FT-IR evidently demonstrated that GO-Fe3O4
composites were successfully synthesized in this study. With the overall available
results, a route for the synthesis of GO-Fe3O4 nanocomposites was given in Scheme 1.
3.2. Effect of GO Loading on Catalytic Activities
The catalytic performance of as-synthesized GO-Fe3O4 composites was examined by
removing isatin in the presence of H2O2. Different loadings of GO formed various
GO-Fe3O4 composites, and their catalytic activities may be different. In the isatin
removal system only added H2O2 or as-prepared GO-Fe3O4 composite, the
concentration of isatin was still very high, indicating isatin cannot be effective
removed within 3-h reaction (Fig. S4). In contrast, in the presence of both GO-Fe3O4
composite and H2O2, over 80 % of isatin could be removed (Fig. 5). Generally, with
greater loading of GO in the catalyst, its catalytic activities should be higher. For
instance, near 95 % of isatin was removed by GO-Fe3O4 (3/20) composite. With the
combination of GO and Fe3O4, the catalytic activities of GO-Fe3O4 composites were
significantly enhanced, as less than 40% of isatin was removed in Fe3O4-H2O2 system
after 180 min. In order to further assess the mineralization efficiency of isatin by
GO-Fe3O4 composites, total organic carbon (TOC) analysis was performed and the
results were shown in Fig. 6. As can be seen, all GO-Fe3O4 composites exhibited
higher TOC removal efficiency of isatin rather than that of only using Fe3O4
nanoparticles. Within all the catalysts, Catalyst 4 showed the highest TOC removal
efficiency (i.e., 60%). The removal efficiency of isatin by different catalysts was
concluded as follows: Catalyst 4 > Catalyst 3 and 5 > Catalyst 2 > Catalyst 1. Since
the weight of Fe3O4 was identical in all the GO-Fe3O4 composites, the varied GO
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loading is the key factor for the removal of isatin. By increasing GO/Fe3O4 weight
ratios, the catalytic activities correspondingly increased from Catalyst 1 to Catalyst 4.
The reasons could be attributed as follows: (i) Fe3O4 nanoparticles were well
dispersed on GO surface via coordination bonds thus markedly reducing aggregation
effect and enhancing effective surface area [52]. It has been demonstrated that the
enhancement of mass transfer of reactants toward the active sites could stimulate the
degradation of organics in the Fenton-like process [53,54]. (ii) Strong interactions
between GO and Fe3O4 via C-O-Fe bonds could facilitate the migration of electron
among Fe3O4 nanoparticles and GO nano-sheets. GO has been well recognized as a
π-conjugated system, electrons generated from Fenton reaction could be effectively
transferred to GO surface [55,56], which would significantly enhance the catalytic
activities of GO-Fe3O4 composites. The reductive GO could also facilitate the electron
transports thus accelerate the redox cycle among the active sites [36,57]. Ferrous ions
could be regenerated during the reaction, mostly due to the reducibility of GO. Notice
that Catalyst 5 is in highest concentration of GO but did not show the highest catalytic
efficiency on isatin degradation. It is most likely attributed to the particular GO-Fe3O4
bedded structure (Fig. 2a and Fig. 2b), which could reduce the effective surface of
product thus hinder hydroxyl radical generation. Among the as-obtained catalysts,
GO-Fe3O4 composite with weight ratio of 3/20 (Catalyst 4) showed the highest
catalytic activity for isatin degradation in this study. As indicated previously, Catalyst
4 had a single GO-Fe3O4 layer structure and with less Fe3O4 aggregation. Catalyst 4 is
therefore used for further catalytic study.
It was reported that the intermediate products may be absorbed or bound with the
catalyst via GO-coupling or ionic interactions during the degradation reaction [52].
Fig. S5 showed the FT-IR spectra of Catalyst 4 collected at 120 min and 180 min in
the Fenton process. The FT-IR spectrum of the used Catalyst 4 was nearly identical
with that of fresh Catalyst 4, indicating that the potential intermediate products from
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isatin degradation were not bound or absorbed by the catalyst. The intermediate
products remained in the solution. Therefore, the degradation of isatin was mainly due
to the Fenton oxidation with GO-Fe3O4 composite as catalyst.
3.3. Effect of pH on Isatin Degradation
As to Fenton-like reaction, pH is well known to be one of the most critical parameters.
The effect of pH on the catalytic activities of Catalyst 4 was thoroughly investigated.
Fig. 7 showed the results from the degradation of isatin by Catalyst 4 at pH 2.5, 3.5,
5.0 and 8.0, respectively. At pH 2.5 and 3.5, over 90% of isatin was removed within a
short reaction time (i.e., 30 min). At pH 5.0, the removal efficiency was relatively
high at the initial stage (first 15 min) and degradation rate tended to be much slower
in the rest. In the case of pH 8.0 (without adding sulfuric acid), only 15% of isatin was
degraded over a period of 180 min. The experimental results demonstrated that the
highest removal efficiency of isatin by Catalyst 4 via heterogeneous Fenton-like
process could be achieved at pH 3.5. In Fenton process, interactions among H2O2 and
Fe-based materials will generate hydroxyl radical which could attack and completely
destroy the pollutant due to its high oxidation potential (2.8 eV). Tert-butanol is a kind
of hydroxyl radical scavenger, which could rapidly scavenge the hydroxyl radicals
during the degradation reaction. With the addition of tert-butanol, the degradation of
isatin was significantly impeded, further implied the generation of hydroxyl radical
was the major process for isatin degradation.
3.4. Effect of Temperature and Ionic Strength on Isatin Degradation
Since dyeing water is usually with high salinity, and its temperature may vary
significantly, the effect of ionic strength and temperature was explored to assess the
application of GO-Fe3O4 composites in the industrial dyeing wastewater treatment
(Fig. 8 and Fig. 9). As to the influence of temperature, the degradation of isatin was
undertaken at 25 °C, 35 °C, 45 °C, and 55 °C. Fig. 8 showed the results as a function
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of temperatures. By increasing the temperatures from 25 °C to 55 °C, the degradation
rate of isatin was accelerated remarkably, which is consistent with previous studies
[58,59]. For the ionic strength effect, it can be seen from Fig. 9 that the degradation
efficiency of isatin gradually decreased when NaCl concentration was increased from
0 mg/L to 3000 mg /L, indicating high concentrations of NaCl tend to hinder the
catalytic activity of GO-Fe3O4 composites.
3.5. Durability and Reusability of GO-Fe3O4 Composites
The durability and reusability of GO-Fe3O4 composites were examined by conducting
the degradation of isatin with the same catalyst recycled from various reaction cycles.
Fig. S6 showed the results from 5 successive reactions. After the reaction, Catalyst 4
was magnetically collected from the aqueous solution and washed with deionized
water for several times. In next Fenton process, the recycled Catalyst 4 was used.
Nearly all isatin could be removed by either fresh or recycled Catalyst 4 (Fig. S6),
suggesting the recycled Catalyst 4 could maintain a highly efficient removal of isatin.
XRD, TEM and FT-IR were employed to characterize the recycled Catalyst 4 to
further investigate its stability. The phase structure and morphology of the recycled
Catalyst 4 are almost identical (Fig. S7 and Fig. S8). Though some new peaks
appeared at 1648, 1279, 1070 and 841 cm-1 in FT-IR spectrum of these recycled
sample (Fig. S9), these peaks belonged to C=O stretching of carbonyl group (1648
cm-1) and C-O stretching (1279 cm-1), C-O-C asymmetric stretching (1070 cm-1) and
ring vibration (841 cm-1) of epoxides. It suggested that some hydroxyl radicals were
absorbed by GO sheets. In addition, the TOC of the mixed solution increased with ca.
0.3 mg/L after Catalyst 4 was mixed with H2O2 (without isatin) at pH 3.0 for 15 h.
Comparing with GO loading of 45 mg/L for Catalyst 4, less than 1% of GO was
oxidized after 5 cycles Fenton reaction, demonstrating the excellent reusability and
durability of GO-Fe3O4 nanocomposites in Fenton-like process.
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4. Conclusions
Graphene oxide-magnetite (GO-Fe3O4) composites were successfully synthesized by
chemical precipitation of iron salts onto GO surface in the presence of ammonia.
FT-IR spectrum indicated that Fe3O4 particles integrated with GO via C-O-Fe
coordination bond. The loading of GO has a great impact on the morphology,
appearance and catalytic activities of as-synthesized GO-Fe3O4 composites. With GO
loading increased, less Fe3O4 particles were aggregated and smaller particle sizes of
Fe3O4 were observed in GO-Fe3O4 composites. Two GO-Fe3O4 structures could be
constructed at different GO loading. Single layer structure of GO-Fe3O4 composites
was obtained at low GO loading (i.e., 1/20, 2/20 and 3/20 for GO/Fe3O4), and
stacking structure was formed at high GO loading (i.e., 4/20 for GO/Fe3O4).
GO-Fe3O4 composites could stimulate the formation of hydroxyl radicals in the
presence of H2O2. Isatin was able to be degraded by these highly active GO-Fe3O4
composites, and 3/20 is the optimum ratio of GO-Fe3O4 composites for the
degradation of isatin, as the highest catalytic activity achieved. The suitable pH for
the isatin degradation by GO-Fe3O4 composites is 3.5. Evaluating temperature did not
show significant impacts on the removal efficiency when the reaction temperature is
long enough. The removal efficiency was hindered when the ionic strength is
increased. The high catalytic activity of the catalyst was mainly attributed to the
effective functional combination of GO and Fe3O4 nanoparticles. GO-Fe3O4
composites have great potential to be the high-performance catalysts for practical
applications.
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Acknowledgements
The authors gratefully acknowledge the funding for this research provided by the
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General Research Fund (17206714, 17212015) and Collaborative Research Fund
(C7044-14G) schemes of the Research Grants Council of Hong Kong, and HKU
Strategic Research Themes on Clear Energy and Earth as a Habitable Planet.
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Author information
* Corresponding Author：Dr. Minhua Su (mhsu@gzhu.edu.cn).
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Figure Captions
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Scheme 1. Synthesis of GO-Fe3O4 composites and the degradation of isatin by
GO-Fe3O4 composites.
Fig. 1. TEM images of Fe3O4 and GO-Fe3O4 composites as catalyst for isatin
degradation. (a) Catalyst 1 (Fe3O4), (b) Catalyst 2 (GO-Fe3O4, 1/20), (c) Catalyst 3
(GO-Fe3O4, 2/20), (d) Catalyst 4 (GO-Fe3O4, 3/20) and (e) Catalyst 5 (GO-Fe3O4,
4/20).
Fig. 2. TEM images of Catalyst 5 (GO-Fe3O4, 4/20). The arrows indicate the edge of
graphene oxide.
Fig. 3. XRD patterns of GO, Fe3O4 and GO-Fe3O4 composites.
Fig. 4. FT-IR spectra of GO, Fe3O4, and GO-Fe3O4 composites.
Fig. 5. Efficiency of isatin removal (Isatin concentration = 20 mg/L, initial H2O2
concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L, GO-Fe3O4 composites
suspension = 0.3 g/L in concentration of Fe3O4 and initial solution pH = 3.5).
Fig. 6. Efficiency of TOC removal (Isatin concentration = 20 mg/L, initial H2O2
concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L, GO-Fe3O4 composites
suspension = 0.3 g/L in concentration of Fe3O4, initial solution pH = 3.5 and T =
25 °C).
Fig. 7. The pH effect on isatin removal (Isatin concentration = 20 mg/L, initial H2O2
concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L, GO-Fe3O4 composites
suspension = 0.3 g/L in concentration of Fe3O4 and T = 25 °C).
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Fig. 8. The temperature effect on isatin removal (Isatin concentration = 20 mg/L,
initial H2O2 concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L, GO-Fe3O4
composites suspension = 0.3 g/L in concentration of Fe3O4 and initial solution pH =
3.5).
Fig. 9. The NaCl concentration effect on isatin removal (Isatin concentration = 20
mg/L, initial H2O2 concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L,
GO-Fe3O4 composites suspension = 0.3 g/L in concentration of Fe3O4 and initial
solution pH = 3.5).
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Scheme 1. Synthesis of GO-Fe3O4 composites and the degradation of isatin by
GO-Fe3O4 composites.
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Fig. 1. TEM images of Fe3O4 and GO-Fe3O4 composites as catalyst for isatin
degradation. (a) Catalyst 1 (Fe3O4), (b) Catalyst 2 (GO-Fe3O4, 1/20), (c) Catalyst 3
(GO-Fe3O4, 2/20), (d) Catalyst 4 (GO-Fe3O4, 3/20) and (e) Catalyst 5 (GO-Fe3O4,
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Fig. 2. TEM images of Catalyst 5 (GO-Fe3O4, 4/20). The arrows indicate the edge of
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Fig. 3. XRD patterns of GO, Fe3O4 and GO-Fe3O4 composites.
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Fig. 4. FT-IR spectra of GO, Fe3O4, and GO-Fe3O4 composites.
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Fig. 5. Efficiency of isatin removal (Isatin concentration = 20 mg/L, initial H2O2
concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L, GO-Fe3O4 composites
suspension = 0.3 g/L in concentration of Fe3O4 and initial solution pH = 3.5).
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Fig. 6. Efficiency of TOC removal (Isatin concentration = 20 mg/L, initial H2O2
concentration = 25 mmol/L, Fe3O4 suspension = 0.3 g/L, GO-Fe3O4 composites
suspension = 0.3 g/L in concentration of Fe3O4, initial solution pH = 3.5 and T =
25 °C).
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