Novel highly stable β-cyclodextrin fullerene mixed valent Fe-metal framework for quick Fenton degradation of alizarin

Article abstract of DOI:10.1039/c7ra06447k

β-Cyclodextrin (β-CD) supported magnetic nanoscaled fullerene/Fe₃O₄ (CDFMNPs) and fullerene/Fe₃O₄ (FMNPs) composites were prepared and characterized. These composites can be utilized as heterogeneous catalysts for the Fenton oxidation reaction to degrade alizarin in aqueous solutions. The saturation magnetization (M_s) value of quasi-spherical CDFMNPs was found to be 13.16 emu g^-1 and their diameter was in the range of 25-30 nm. The catalytic activities of the prepared materials were tested with varying conditions of pH, amount used and the concentration of H₂O₂ for degradation of alizarin at room temperature. The exceptionally high degradation efficiency of CDFMNPs was observed for alizarin at pH 3 with 2.0 g L^-1 catalyst and 25 mM of H₂O₂. The increased oxidative degradation efficiency is attributed mainly to the formation of active hydroxyl radicals (OH) on the surface of the catalyst, which are generated by the active decomposition of H₂O₂ by the solid heterogeneous catalyst and the promoting effect of β-CD. CDFMNPs can be magnetically separated and the catalyst was found to be reusable and stable for five successive runs with no significant loss of catalytic activity. In the magnetic environment of Fe₃O₄ nanoparticles, fullerene has a crucial role to enhance the activity by increasing the stability with nominal iron leaching. Based on mass analysis of alizarin degradation, the formation of aliphatic acids and monocyclic compounds through phthalic anhydride and di-methyl phthalate established the proposed degradation path.

Full text of DOI:10.1039/c7ra06447k

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PAPER
Novel highly stable b-cyclodextrin fullerene mixed
valent Fe-metal framework for quick Fenton
degradation of alizarin†
Cite this: RSC Adv., 2017, 7, 40371
Aniruddha Gogoi,a Madhukar Navgire,b Kanak Chandra Sarmaa
cd
*
and Parikshit Gogoi
b-Cyclodextrin (b-CD) supported magnetic nanoscaled fullerene/Fe3O4 (CDFMNPs) and fullerene/Fe3O4
(FMNPs) composites were prepared and characterized. These composites can be utilized as
heterogeneous catalysts for the Fenton oxidation reaction to degrade alizarin in aqueous solutions. The
saturation magnetization (Ms) value of quasi-spherical CDFMNPs was found to be 13.16 emu gꢀ1 and
their diameter was in the range of 25–30 nm. The catalytic activities of the prepared materials were
tested with varying conditions of pH, amount used and the concentration of H2O2 for degradation of
alizarin at room temperature. The exceptionally high degradation efficiency of CDFMNPs was observed
for alizarin at pH 3 with 2.0 g Lꢀ1 catalyst and 25 mM of H2O2. The increased oxidative degradation
efficiency is attributed mainly to the formation of active hydroxyl radicals (cOH) on the surface of the
catalyst, which are generated by the active decomposition of H2O2 by the solid heterogeneous catalyst
and the promoting effect of b-CD. CDFMNPs can be magnetically separated and the catalyst was found
to be reusable and stable for five successive runs with no significant loss of catalytic activity. In the
magnetic environment of Fe3O4 nanoparticles, fullerene has a crucial role to enhance the activity by
increasing the stability with nominal iron leaching. Based on mass analysis of alizarin degradation, the
formation of aliphatic acids and monocyclic compounds through phthalic anhydride and di-methyl
phthalate established the proposed degradation path.
Received 8th June 2017
Accepted 8th August 2017
DOI: 10.1039/c7ra06447k
rsc.li/rsc-advances
and advanced oxidation process (AOP) viz. Fenton oxidation,
etc. have been adopted to eliminate these dyes and stable
organic pollutants.2,4–8 In spite of these methods, there is an
1. Introduction
The ever increasing pollution of water and soil by different
industries, agricultural and domestic activities has been
a major concern with respect public health due to recalcitrant
toxic organic pollutants including dyes. Effluents from these
activities have a high toxic organic content, color strength and
resistant to degradation by light, chemicals and biological
treatment.1,2 Most of these compounds are responsible for
numerous diseases, including cancer, due to their toxicity as
mentioned by the European community and US Environmental
protection agency.3 Therefore, wastewater treatment is essential
before discharge into the environment. Numerous methods
based on adsorption, photocatalytic degradation, ion exchange
urgent environmental need to develop more effective catalytic
systems for degradation of pollutants. Fenton oxidation
processes consisting of iron (Fe2+/Fe3+) in heterogeneous phase
and H2O2 would be the better choice. Unlike other oxidation
processes, the generation of hydroxyl radicals in the Fenton
process is from ferrous ions and hydrogen peroxide in aqueous
solution. Besides this fact, the catalytic activities of heteroge-
neous Fenton systems depend on numbers of other parameters;
mainly solution pH, catalyst dosage, substrate concentration,
H2O2 concentration and reaction time.9–11
In the recent years, magnetic nanoparticles with low iron
dissolution have gained attention as a catalyst for the degra-
dation of organic dyes due to their remarkable features such as
a large specic surface area and easy recovery of catalyst.12 The
application of magnetic Fe3O4 particles are limited in different
media as it has less number of surface active functional groups,
tend to aggregate, formation of clusters and iron leaching which
reduce their activity and stability.13 Thus, surface modication
of Fe3O4 is essential and choosing the right candidate to
enhanced stability of the Fe3O4 is a challenge, depending on the
type of application. To overcome these limitations, some
aDepartment of Instrumentation & USIC, Gauhati University, Guwahati 781014,
Assam, India
bDepartment of Chemistry, Jijamata College of Science & Arts, Bhende, Ahmadnagar,
Maharastra, India
cDepartment of Chemistry, Nowgong College, Nagaon 782001, Assam, India. E-mail:
parikk100@gmail.com
dSchool of Chemical and Biomolecular Engineering, Renewable Bioproducts Institute,
Georgia Institute of Technology, Atlanta, Georgia, 30318, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra06447k
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studies have reported the use of support materials such as clays, method and used as a Fenton catalyst for the degradation of
Naon, activated carbon, zeolite, fullerene, carbon nanotubes, alizarin dye from aqueous solution. Alizarin, also called 1,2-
silica, polymers, metal oxides etc.11,13–20 To keep iron in hetero- dihydroxyanthraquinone belong to madder plant with formula
geneous phase, use of carbon based materials are the advan-
C
14H8O4 is a major pigment in textile industries.37–39 To the
tageous due to its unique catalytic properties including the knowledge of the authors, no studies have been reported till
stability in acidic medium and higher temperature.21 Jaafarza- now on the heterogeneous Fenton degradation of alizarin from
deh et al. reported the Fenton oxidation of tetracycline, cata- aqueous solutions using b-CD coated fullerene/Fe3O4 nano-
lyzed by powder activated carbon/Fe3O4 hybrid composite with composite at optimum mild operating conditions.
good stability of the catalyst within 4 h.22 Nidheesh et al. found
that iron loaded on activated carbon can effectively degrade
2. Experimental section
2.1 Preparation of fullerene/Fe3O4
rhodamin B in aqueous medium by Fenton process.23 Degra-
dation of phenol by carbon supported Fe catalyst24 and orange II
removal by Fe supported on carbon nanotubes25 showed good
Fullerene/Fe3O4 nanoparticles (MFNPs) were prepared by
catalytic activities, but with less stability of the catalyst in terms
of the high amount of Fe leaching in the former case aer the
reaction. Despite some progresses have been made so far, the
industrial application of heterogeneous Fenton catalyst is still
limited. Our strategy is to develop a magnetically recoverable,
decently stable catalytic system with high catalytic activity
towards dye degradation in the absence of light irradiation.
Fullerene (C60) is a hollow sphere of carbon, which has
a stable cage structure with high electron affinity (can accept up
to six electrons) and resistant to oxidation. It has unique
properties to form composites with various materials, ability to
trap substances inside the cage and to act as a photo catalyst
etc.26–28 Encapsulation of fullerene to metal oxide provides
ability to oxide surface by resisting air oxidation and maintains
good magnetic properties. Synthetic strategies to produce
nanocomposites of fullerene and magnetic iron oxide nano-
particles have been reported recently. Iron oxide forms strong
colloidal assembly with fullerene at various pH on air–water
interface. Zerovalent iron–fullerene nanocomposite system was
reported to produce reactive oxygen species and O2, both in the
presence and absence of UV light at near neutral pH for
bacterial inactivation.8,29,30 It is expected that binding of
fullerene with iron oxide likely to minimize iron precipitation
conventional co-precipitation method. The reaction mixture
was obtained by dissolving 0.86 g of FeCl2$4H2O (Merck), 2.63 g
of FeCl3$6H2O (Merck) and ne powdered fullerene (1 wt%)
(C60, Sigma-Aldrich, 99.5%) in mL of de-ionized water with
constant stirring. NH4OH solution was added dropwise to
maintain the pH ¼ 8. The mixture was stirred for 12 h for
complete precipitation and then kept 12 h for aging. The
resultant precipitate was magnetically separated, washed
ꢁ
several times with deionized water and dried in oven at 90 C
for 12 h.
2.2 Preparation of b-cyclodextrin-fullerene/Fe3O4
The b-CD fullerene/Fe3O4 nanoparticles (CDMFNPs) were also
prepared by simple method. The synthesized MFNPs (1.8 g)
were mixed with (0.5 g) of b-cyclodextrin (Himedia) in 75 mL of
deionized water with constant stirring. The mixture was kept
under constant stirring for 24 h and then kept another 24 h for
aging. The resultant particles were separated magnetically from
residual solution and washed several times with deionized
ꢁ
water and dried in oven at 90 C for 12 h.
2.3 Characterization of b-cyclodextrin-fullerene/Fe3O4
through co-ordination. In this context, we hypothesized that The formation of fullerene/Fe3O4 (FMNPs) and b-cyclodextrin
doping of fullerene to magnetic nanoparticles will enhance the coated fullerene/Fe3O (CDFMNPs) were investigated systemati-
stability and generate reactive oxygen species (ROS) in the cally by different sophisticated analytical techniques.
presence of H2O2 to act as a heterogeneous catalyst without UV
Powder X-ray diffraction (XRD) measurements were observed
light. Further, for surface modication and to increase the by using a Bruker/D2 phaser, Germany; diffractometer (Cu Ka
˚
activity of the catalyst, cyclodextrin was found to be one of the radiation source, l ¼ 1.542 A, operates at 30 kV and 10 mA). The
suitable material.31 b-Cyclodexrtin (b-CD) is a cyclic oligosac- sample scan was done over a diffraction angle of 2q in the range
charide with 7 membered cyclic D-glucose units connected of 20–80ꢁ in 0.02ꢁ step scan with exposure time of 2 s.
through a-(1 / 4) glucosidic linkages to form a cone shaped
cylindrical cavity. Cyclodextrin can form self-assembled ments were performed with a Perkin Elmer, USA/Spectrum Two;
organic–inorganic framework with metal oxide surface by spectrometer using KBr pellet over a range of 4000–400 cmꢀ1
Fourier transform infrared spectroscopy (FTIR) measure-
.
forming hydrogen bond through hydroxyl groups of the cyclo- Same setup is used to perform pyridine-FTIR study of the
dextrin. It also forms host–guest inclusion complexes selectively synthesized samples.
with different organic molecules through their hydrophobic
Magnetic measurements were carried out using a vibrating
cavity. The host–guest interactions are mainly hydrogen sample magnetometer (VSM; Lakeshore 7410).
bonding and van der Waals forces between b-CD and organic
Field Emission Scanning Electron Microscopy (FESEM) study
molecules.32–36 Therefore, the b-CD and fullerene/Fe3O4 ex- was performed on Carl Zeiss-Sigma instrument. Transmission
pected to simplify the stability and activity issues related to Electron Microscopy (TEM) and High Resolution TEM (HRTEM)
degradation of dyes in Fenton system.
characterization was done on JEM-2010 (JEOL), instrument.
Thermogravimetric analysis (TGA) was performed by using
In the present study, highly stable b-CD coated fullerene/
Fe3O4 nanostructures were prepared via a facile co-precipitation a Mettler Toledo (TGA/SDTA 581e) thermogravimetric analyzer
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from 30–700 ꢁC with a heating rate of 10 ꢁC minꢀ1 under 4.6 ꢂ 100 mm). The solvent used as mobile phase was a mixture
nitrogen atmosphere.
of acetonitrile water with a ow rate of 3 mL minꢀ1. A volume of
Electron Paramagnetic Resonance (EPR) studies were per- 20 mL samples was loaded with a syringe to inject through
formed by using a JEOL (JES-FA200) EPR spectrometer. capillary column. Nitrogen gas used as nebulizer gas and
UV-vis DRS (Diffuse Reectance Spectra) measurements were collision cell gas (nitrogen) was 99.999% pure. The other
performed by Perkin Elmer (Lambda 750 UV/Vis/NIR) spectro- parameters were set as standard for the system. The degrada-
photometer using a diffuse reectance attachment. BaSO4 tion products identied by LC-MS studies were veried by
matrix was used as a reference to record the spectra in the range standard mass spectra database, literature and comparing with
of 200–800 nm.
MS spectra of those of known chemicals.
The absorption spectra were measured using a Shimadzu/
UV-1800 UV-vis spectrophotometer to determine band gap
energy and degradation study.
3. Results and discussions
3.1 Characterization of magnetic nanoparticles
3.1.1. XRD analysis. The structural characterization of the
synthesized materials was carried out using X-ray diffraction
(XRD). Fig. 1 shows the XRD patterns of FMNPs and CDFMNPs.
It has all the major diffraction peaks corresponding to the
lattice planes of (220), (311), (400), (422), (511) and (440), which
conrm the presence of Fe3O4 in the composite (Fig. S1 of ESI,†
for comparison). The values are narrowly matched with stan-
dard data (JCPDS le PDF no. 65-3107). Both the XRD patterns
clearly show two prominent diffraction peaks for fullerene at 2q
values of 17.6 and 21 corresponding to the lattice plane of (220)
and (311), respectively.28,42 The peaks at 2q of 29.44, 39.0, 47.3,
52.8 and 57.5 corresponding to the crystal planes (421), (532),
(642), (654) and (761), indicate the existence of fullerene or
carbon substrate in cubic crystalline symmetry. All the values
for fullerene are closely matched with the JCPDS card no. 79-
1715. These peaks conrm the presence of fullerene in Fe3O4
nanoparticles. Usually b-CD does not give any diffraction peaks
due to the amorphous nature or ne dispersion of it onto metal
oxide surfaces.43 The XRD patterns of FMNPs and CDFMNPs in
Fig. 1 are nearly same which indicating that no deformation of
crystal structure of FMNPs aer incorporation of b-CD. It was
also observed that aer incorporating b-CD in the FMNPs
2.4 Catalytic test study
The performances of the prepared catalyst were tested by
carrying out batched degradation reaction of alizarin (Sigma-
Aldrich, 97%) at room temperature. Typically, samples for
alizarin degradation system were prepared by adding required
amount of catalyst to a solution containing known amount of
dyes (10 mM) in a conical ask (100 mL), ultra-sonicated to
make a homogeneous suspension followed by constant stirring.
To initiate the reactions a known dosage of H2O2 was added to
each reaction mixture. The catalyst amount, H2O2 and pH
concentrations were varied systematically from 0.25 g Lꢀ1
,
5–25 mM and pH 3–10 respectively at an alizarin concentration
of 10 mM. The various pH conditions were maintained by using
1 M NaOH or H2SO4. The progress of the reaction was moni-
tored and determined by measuring absorbance at a regular
interval by taking aliquots of the reaction mixture to evaluate
the catalytic activity in the range of 300–800 nm. The change in
absorbance with respect to time of alizarin degradation (lmax at
497–526 nm) was measured for all sets of catalyst. The experi-
ments were performed in triplicate to check the reproducibility
of results and the values are presented as means and standard
deviation. The leaching of iron from the catalyst surface at the
optimum reaction conditions during the process of degradation
of alizarin was studied.
The Fe2+ ion concentration in solution was measured using
1,10-phenanthroline method by spectrophotometrically at
510 nm. The total dissolved iron was also measured by same
method using hydroxylamine hydrochloride as a reducing agent
for the experiment.18,40,41
The degraded sample was ltered with 0.02 mm lter paper
aer the reaction and Liquid Chromatography Mass Spec-
trometry (LC-MS) analysis was carried out to determine the
intermediate and nal products of alizarin degradation. Control
and degraded sample of alizarin was analyzed by the Agilent
6520 series Accurate-Mass Q-TOF LC/MS spectrometer. In this
LC/MS system, Liquid Chromatography (LC) part is coupled
with High Resolution Mass Spectrometer (HRMS). Following
were the operating conditions for the system: rst the instru-
ment was calibrated with a standard calibrated solution for ESI-
(+) mode in the mass range of 50–500 m/z to record mass spectra
within this range. The system was operated on electrospray
ionization (ESI), positive (+) mode. The column used for sepa-
Fig. 1 XRD spectrum of (a) fullerene/Fe3O4 (FMNPs) and (b) b-CD-
ration of the products was C18 (Agilent Eclipse plus, 3.5 mm, fullerene/Fe3O4 (CDFMNPs) nanocomposite.
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composite the intensity of the XRD peaks decreased, indicating
the presence of b-CD in the composite. The decrease in the
intensity of the XRD peaks may be due to decreases in the
crystalline nature of CDFMNPs. Signicantly no detectable
peaks were observed for b-CD in the XRD pattern of CDFMNPs
but the small changes in the peak position of FMNPs with
addition of b-CD conrms the complete incorporation of the
b-CD in the CDFMNPs. Therefore, the prepared catalysts are
mesoporous structure and simultaneously have the properties
of iron oxide, fullerene and b-CD.
3.1.2. FT-IR analysis. The coating of b-CD over FMNPs is
conrmed by the FTIR study. Fig. 2 shows the FTIR spectra of
the synthesized b-CD, FMNPs and CDFMNPs. Peaks at 3153
cmꢀ1 and 1640 cmꢀ1 can be ascribed to bending and stretching
vibration of the adsorbed water molecules. Two absorption
bands at 629 cmꢀ1 and 562 cmꢀ1 are produced from the split-
ting of the characteristic stretching vibration of Fe–O bond of
Fe3O4 at around 574 cmꢀ1 for both the catalysts.18 Other peaks
at 1405 cmꢀ1 and 1004 cmꢀ1 can be assigned as –C]C– and –C–
Fig. 3 Magnetic hysteresis of (a) Fe3O4 (b) fullerene/Fe3O4 (FMNPs) (c)
b-CD-fullerene/Fe3O4 (CDFMNPs) nanocomposite and separation of
catalyst using an external magnet (inset).
R
stretching vibrations, respectively. For CDFMNPs, the
absorption bands in the range of 1200–900 cmꢀ1 with new
peaks are characteristic for b-CD. The peaks at 1140 cmꢀ1 and
1084 cmꢀ1 can be assigned to coupled n (C–C/C–O) vibration
and antisymmetric glycosidic n (C–O–C) vibration, respec-
tively.33,36 Some absorption bands of FMNPs also present in
CDFMNPs with a little shi in position, suggested the
successful incorporation of b-CD on the surface of the FMNPs.
3.1.3. Magnetic properties measurement. Magnetic
measurements for the synthesized materials were carried out
using a vibrating sample magnetometer at room temperature
are shown in Fig. 3. The presence of (MH) loops indicates
ferromagnetic nature of the synthesized materials at room
temperature. The magnetic–hysteresis loops reveal negligible
coercivity for Fe3O4, FMNPs and CDFMNPs with saturation
magnetization value (Ms) of 58.0, 21.49 and 13.16 emu gꢀ1
respectively. The differences in Ms values for all the compounds
are attributed to the presence of fullerene and formation of
heterostructure with nonmagnetic organic b-CD to exist as
magneto crystalline anisotropy. The moderate magnetic prop-
erty of both FMNPs and CDFMNPs indicates their excellent
catalytic behavior in terms of magnetic separation and reus-
ability aer reaction. The lower magnetization value of
CDFMNPs is due to the higher percentage of non-magnetic
carbon and b-CD as compared to FMNPS composite. Though
the Ms value is low for the CDFMNPs in comparison to bare
Fe3O4 nanoparticle, the CDFMNPs can be easily separated with
strong external magnet for reuse which is one of the important
property of the reported catalyst. Fig. 3 (inset) shows the
magnetic response of the CDFMNPs.
3.1.4. FE-SEM studies. The surface morphology of the
prepared catalysts was studied by scanning electron micros-
copy. Fig. 4 shows the FESEM low and high resolutions images
of the prepared FMNPs (Fig. 4(a and b)) and CDFMNPs (Fig. 4(c
and d)). The gures clearly indicate crystalline structure of both
the composites. From the Fig. 4(a and b) it can be seen that
particle surface is smooth and assemblies are created because
of the magnetic nature. On the other hand, Fig. 4(c and d)
clearly indicates the surface modication aer incorporating b-
CD over FMNPs. Most of the particles are nearly spherical in
shape with rough surface having crystalline nature. These
surface morphology results are consistent with the above XRD
results, indicating the presence of physical interactions
between the fullerene and magnetite with surface active sites
and inconformity with Rodriguez et al. who reported magnetic
fullerene and activated carbon adsorbents for selective
adsorption of azo dyes from aqueous solutions.27
3.1.5. TEM analysis. TEM and HRTEM studies were per-
formed to understand the size and morphology of the synthe-
sized sample. Bright eld image of CDFMNPs in Fig. 5(a) shows
the good dispersion of the semispherical nanoparticles in the
size range of 25–30 nm. Fullerene and Fe3O4 binary composites
Fig. 2 FT-IR spectra of (a) b-CD (b) b-CD-fullerene/Fe3O4 (CDFMNPs)
and (c) fullerene/Fe3O4 (FMNPs) nanocomposite.
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Fig. 4 FESEM images of (a and b) b-CD-fullerene/Fe3O4 (CDFMNPs) and (c and d) fullerene/Fe3O4 (FMNPs) nanocomposite.
can be clearly seen in the TEM and HRTEM images in Fig. 5(b rings in the electron diffraction patterns is found to be 0.29 nm
and c). The coating of b-CD is also clearly seen on the outer and 0.25 nm corresponds to the (220) and (311) crystal plane of
surface of the nanoparticles as shown in Fig. 5(c and d). From Fe3O4 respectively.43 These results of TEM characterization are
the IFFT pattern Fig. 5(e1), the lattice spacing was calculated as consistent with the earlier reports of characterization of nano-
0.25 nm corresponding to (311) plane of the crystalline Fe3O4 particles.5 Further, TEM and HRTEM analysis indicates that
nanoparticle. Detailed information about the crystal plane can fullerene likely to inuence the morphology of the CDFMNPs
be obtained from the selected area electron diffraction (SAED) thereby effect the reactivity towards the formation of hydroxyl
pattern as in Fig. 5(f). The lattice spacing calculated from the radicals and overall stability of the system.
Fig. 5 TEM images (a) and (b); HRTEM images (c) HRTEM of single particle (d); HRTEM (e) and IFFT pattern (inset of e1); SEAD pattern (f) of b-CD-
fullerene/Fe3O4 nanocomposite.
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where, A is a constant; a, h and n are the absorption coefficient,
Plank constant and frequency respectively. By plotting the
(ahn)2 versus hn and extrapolation of the linear part of the curve
to the (ahn)2 ¼ 0, the Ebg for both the samples were calculated.
Fig. S4(b & d) (ESI†) shows the Tauc plot for FMNPs and
CDFMNPs. The band gap energies calculated form the Tauc plot
were found to be 4.10 eV and 3.35 eV for FMNPs and CDFMNPs
respectively. Both values are higher than the reported values for
magnetite Fe3O4 nanoparticles (0–3 eV)45 which were mainly
due to the composite formation between Fe3O4 and fullerene for
FMNPs and a combination of fullerene and b-CD for CDFMNPs
respectively. This could be attributed to the structural change
occurs within the lattice site due to defect or oxygen vacancy.
3.1.8. Electron paramagnetic resonance (EPR) study. Elec-
tron paramagnetic resonance (EPR) lines were measured for
FMNPs and CDFMNPs at X-band (9.44 GHz) and shown in
Fig. S5 (ESI†). For both the samples broad and strong asym-
metry single signals were obtained around the resonance
magnetic eld (Hres) of 261 and 282 Gauss for FMNPs and
CDFMNPs respectively. At room temperature, the observed
intensity for FMNPs is more than three times the intensity of the
CDFMNPs. The difference in intensity and signal width could be
due to the magnetic loss aer coating of b-CD over FMNPs.
Additionally; spin disorder could be the reason for resonance
line broadening which arises from the antiferromagnetic
interaction of neighboring spins of the magnetic particles.46
3.1.9. Pyridine FTIR studies. The FTIR spectra of pyridine
adsorbed on the surface of the FMNPs and CDFMNPs
composites were shown in the Fig. S6 (ESI†). The FTIR spectra
show the Bronsted and Lewis acid site absorption bands for the
composites. The characteristic absorption band for pyridine
adsorption on the surface is shown at 1640 cmꢀ1 and 1455 cmꢀ1
corresponding to Bronsted and Lewis acidic sites respectively.
The absorption peak at 1490 cmꢀ1 can be attributed to the
presence of both Bronsted and Lewis acidic sites.43
3.1.6. TGA study. The conjugation of b-CD on the surface of
nanoparticles and presence of fullerene in CDFMNPs was
conrmed by TGA study. The TGA thermograms for both the
composites FMNPs and CDFMNPs are shown in Fig. S2 (ESI†). It
can be seen from the graph that both the curves have similar
pattern, which indicate that the catalysts have similar thermal
stability. CDFMNPs is slightly unstable than FMNPs up to
ꢁ
650 C and thereaer both are thermally stable. The TGA ther-
mogram of CDFMNPs exhibits three step weight loss between
ꢁ
300–650 C, leaving Fe3O4 nanoparticles and residual carbon.
The rst 11% weight loss up to 120 ꢁC is due to evaporation of
surface adsorbed water. The second weight loss starts from
200 ꢁC, it increases with increasing temperature and attains
maximum weight loss of 54% at 450 ꢁC. This is due to the
decomposition of remaining organic residue and decomposi-
tion of oxygen containing functional groups. While aer 450 ꢁC
weight loss was due to decomposition of b-CD present in
material. The nal weight loss between 550–640 ꢁC is mainly
due to the progressive degradation of fullerene. Comparing
both the curves, it was observed that the degradation of
fullerene started little early in the CDFMNPs, which is due to the
presence of b-CD in the composite. Thereaer the composite is
stable.
ꢁ
For FMNPs, the 5% weight loss up to 120 C is due to the
surface bonded water molecules, second 35% weight loss
ꢁ
between 200–450 C is due to the decomposition of remaining
organic residue, while aer 450 ꢁC due to the decomposition of
oxygen containing functional groups. The nal weight loss of
16% in the range of 580–650 ꢁC is due to the degradation of
fullerene. Thereaer, the composite is stable; it conrms strong
binding between the fullerene and Fe3O4 present in FMNPs.
3.1.7. Optical properties study
UV-vis DRS analysis. To understand the optical properties of
synthesized FMNPs and CDFMNPs, UV-visible DRS spectra were
taken and the observed results are displayed in Fig. S3 (ESI†).
The absorption intensities of FMNPs were higher than
CDFMNPs. For FMNPs two absorption bands were observed at
482 nm and 542 nm whereas for CDFMNPs only maximum
absorption band appeared around 542 nm, clearly indicates the
change. The lower intensity of CDFMNPs band could be due to
the change of crystalline size caused by the incorporation of b-
CD.
UV-vis spectroscopy. The optical absorption spectral study is
used to understand the electronic structure associated with the
band gap of a material as electronic transition within the
molecule is responsible for absorption to occur. Fig. S4(a & c)
(ESI†) shows the observed optical absorption spectra in the
range of 200–800 nm for FMNPs and CDFMNPs. For FMNPs,
observed spectra have two distinct absorption bands, one at
301 nm of the UV region and another at 414 nm of the visible
region. CDFMNPs spectra shows only one well dened absorp-
tion peak at 303 nm, which clearly indicates that the change
occurred, is due to the addition of b-CD. Using Tauc equation
band gap energy (Ebg) can be calculated for both the catalyst.
The equation is:
3.2 Catalytic activity of b-cyclodextrin coated fullerene/Fe3O4
nanocomposite
The catalytic activity of the CDFMNPs was investigated for the
degradation of alizarin at a planned reaction conditions. Fig. 6
shows the typical UV-vis degradation spectra of alizarin in the
presence of H2O2 (lmax at 497–526 nm) for 1 h. Addition of
CDFMNPs results in a progressive decrease of the typical aliz-
arin absorption band with slight change in adsorption position,
suggested the degradation of alizarin. Reaction with control
experimental conditions showed that CDFMNPs without H2O2
or H2O2 without catalyst do not give alizarin degradation. Fig. 7
shows the alizarin degradation by Fe3O4, FMNPs and CDFMNPs
with 2 g Lꢀ1 catalyst, H2O2 (25 mM) at pH 3 where, alizarin
concentration was 10 mM at room temperature. It was found
that CDFMNPs degrades alizarin almost completely (about
93%) within 1 h and at same reaction condition FMNPs and
Fe3O4 nanoparticles degrades only about 64% and 57% of the
alizarin concentration, respectively. The higher degradation
efficiency of the composite was due to the higher number of
(ahn)2 ¼ A(hn ꢀ Ebg
)
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Fig. 8 Effect of H2O2 on degradation of alizarin by CDFMNPs, reaction
conditions: catalyst ¼ 2.0 g Lꢀ1, pH ¼ 3, [alizarin] ¼ 10 mM at room
temperature. The experiments were performed in triplicate and error
bars show the standard deviation for the analyses.
Fig. 6 UV-vis spectra showing the degradation of alizarin (lmax at
497–526 nm) by b-CD-fullarene/Fe3O4 (CDFMNPs), reaction condi-
tions: [H2O2] ¼ 25 mM, catalyst ¼ 2.0 g Lꢀ1, pH ¼ 3, [alizarin] ¼ 10 mM
at room temperature.
formed during the electron transfer process of Fe2+ and Fe3+.47
Fe2+ initiates the reaction faster as compared to Fe3+ and series
of reactions takes place quickly. Moreover, Fe3+ reacts with H2O2
to regenerate Fe2+ to reuse it in the Fenton reaction again.5,48 We
have performed the reactions at pH 3, as in acidic conditions
H2O2 is more stable. In basic conditions, the oxidative ability of
H2O2 decreases because of its decomposition to produce water
and oxygen.11 The use of low H2O2 concentration for the alizarin
degradation makes it promising for the system.
As the pH of the reaction plays an important role, varying pH
of 3, 4, 6, 8 and 10 were used to study the effect of pH on
degradation of alizarin. Hydrogen peroxide and Fe2+ show
Fig. 7 Comparative degradation of alizarin by Fe3O4, FMNPs and
CDFMNPs, reaction conditions: [H2O2] ¼ 25 mM, catalyst ¼ 2.0 g Lꢀ1
,
pH ¼ 3, [alizarin] ¼ 10 mM at room temperature. The experiments were
performed in triplicate and error bars show the standard deviation for
the analyses.
surface active groups and more effective surface area provided
by the addition of b-CD.
Reaction was carried out with varying amount of H2O2
concentration viz. 5 mM, 10 mM, 15 mM, 20 mM and 25 mM to
know the effect of H2O2 on Fenton reaction. Fig. 8 & S7 (ESI†)
shows that the 25 mM H2O2 has the height degradation effi-
ciency for 10 mM of alizarin indicating that it directly depends
on the concentration of hydroxyl radical produced from the
reaction of H2O2 and Fe2+. In Fenton or Fenton like reactions
the active species for degradation is hydroxyl radical and is
Fig. 9 Effect of pH on degradation of alizarin by CDFMNPs, reaction
conditions: [H2O2] ¼ 25 mM, catalyst ¼ 2.0 g Lꢀ1, [alizarin] ¼ 10 mM at
room temperature. The experiments were performed in triplicate and
error bars show the standard deviation for the analyses.
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acted as peroxidase like catalyst to generate hydroxyl radicals
from H2O2. The excess of catalyst loading decreases the
production of hydroxyl radicals by adsorbing H2O2 on the
surface of the catalyst thereby decreasing the degradation rate
of 4-chlorophenol at optimum H2O2 concentrations by
Fe3O4@b-CD.48 Moreover, increasing catalyst loading decreases
the density of surface adsorbed H2O2 for hydroxyl radical
production since the degradation reaction believed to be
occurring on the surface of the catalyst. Again, agglomeration of
nanoparticles due to excessive catalyst loading is another
reason of decreasing hydroxy radical production in most of
heterogeneous Fenton reactions.48–50
The metal components in the catalyst framework are
considered to play the vital role in catalytic reactions.5 Similarly,
in our work also the degradation reaction depends on the metal
sites in the catalyst framework, so it is necessary to analyze the
stability of the catalysts in liquid-phase oxidation. The leaching
of Fe2+ and total dissolved iron in solution was determined for
the CMFMNPs and results were shown in the Fig. 11(a). It is
important to determine the iron dissolution, as it is directly
related to the catalytic activity of the catalyst, in acidic condition
as it favours the process in comparison to neutral or basic
conditions.51 The leaching experiments were carried out in
optimum Fenton oxidation reaction conditions of the alizarin
degradation for 2.0 g Lꢀ1 catalyst. Other reaction parameters
were: alizarin concentration 10 mM, H2O2 25 mM, pH 3 at room
temperature. From the Fig. 11(a), it can be seen that leaching of
ferrous ion steadily increased up to 3.8 mg Lꢀ1 at 30 min,
suggested the faster Fenton process and then decreased to
2.4 mg Lꢀ1 at 100 min. At this stage the oxidative process is
further enhanced by the hydroxylated form of fullerene called
fullerol by producing more ROS.52 Simultaneously, the total
dissolved iron in the solution increases continuously and
attained the value of 7.1 mg Lꢀ1 at 100 min. The continuously
increased dissolved iron value was due to the combined disso-
lution of ferrous and ferric irons from the CDFMNPs catalyst.
The observed values were less than the previously reported
values of our iron leaching for Fe3O4–CeO2 composite,18 where
Fe2+ ions were oxidized to Fe3+ ions and form intermediate
Fe(III) complexes. It could be expected that the initial release of
Fig. 10 Effect of catalyst amount on degradation of alizarin by
CDFMNPs, reaction conditions: [H2O2] ¼ 25 mM, pH ¼ 3, [alizarin] ¼
10 mM at room temperature. The experiments were performed in
triplicate and error bars show the standard deviation for the analyses.
better stability at pH lower than 3.5, consequently, give more
efficiency at lower pH.21,47 Moreover, stability of Fe2+ ions
decreases above pH 4 and converted to Fe3+ and can form
intermediate hydroxo complexes.11 It was found from Fig. 9 & S8
(ESI†) that at pH value of 3 degradation efficiency was
maximum, decrease from pH 4 to 8 and considerably decreases
aer pH 8. The order of decreasing efficiency was: pH 3 < pH 4 <
pH 6 < pH 8 < pH 10. The higher degradation efficiency at pH 3
is due to the production of more number of reactive cOH radical
in acidic conditions than other pH range, conrms the effec-
tiveness of Fenton reaction at lower pH range.
Similarly, the Fig. 10 shows the effect of varied amount of
catalyst from 0.25–2.0 g Lꢀ1 on the degradation of alizarin and it
was found that the 2.0 g Lꢀ1 catalyst shows the maximum
degradation under above mentioned reaction conditions at
room temperature. The trend of increasing the degradation
with catalyst loading is due to the increase in active sites which
Fig. 11 (a) Concentration of Fe2+ ion and total dissolved iron, reaction conditions: catalyst ¼ 2.0 g Lꢀ1, [alizarin] ¼ 10 mM, [H2O2] ¼ 25 mM, pH ¼
3 at room temperature. (b) Alizarin degradation efficiency catalyzed by Fe leaching. The experiments were performed in triplicate and error bars
show the standard deviation for the analyses.
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ferrous ion from CDFMNPs was due to the oxidation by H2O2 a homogeneous reaction to occur, the CDFMNPs catalyst was
and decreasing trend was due to the conversion of ferrous to stirred in a reaction mixture and removed by an external magnet
ferric ion, which results the increases of Fe2+ and Fe3+ (total aer 100 min. Then the alizarin was added into the reaction
dissolved iron).
mixture, containing leached iron, with 200 mL of H2O2 to initiate
The homogeneous phase reaction was also performed aer the reaction. From Fig. 11(b), it can be seen that only 21% of
the heterogeneous process optimization was done. For alizarin was degraded within 1 h because of the iron leaching in
Fig. 12 (a and b) LC-MS [ESI (+) ve mode] spectra of degraded alizarin by b-CD-fullerene/Fe3O4 (CDFMNPs).
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a homogenous manner, value increased a bit to 28% aer species. These structural features make it efficient for degrading
100 min of reaction time. So, the degradation of alizarin is dyes from aqueous solutions by using the electrogeneration of
predominantly due to the heterogeneous Fenton process with Fenton catalysts.44
a minor contribution from homogeneous process. The feasi-
bility of heterogeneous process was supported by stabilizing
effect of fullerene and enhanced activity of b-CD. The aggrega-
3.3 ESI mass analysis of alizarin
tion or incorporation of fullerene onto iron oxide in solution
greatly effects the morphology of the composite thereby impli-
cations on reactivity and enhance the stability of the system.
The interaction of fullerene with iron oxide nanoparticles
produces ordered assemblies along with amorphous aggregates
with short range periodic structures during immobilization.8 In
magnetite Fe cations occupy the tetrahedral and octahedral
sites within the cubic inverse spinal structure of it. The Fe2+ and
Fe3+ both occupy the octahedral sites and retaining the crystal
structure during the reversible oxidation and reduction of Fe
Liquid chromatography mass spectroscopic (LCMS) study in
positive ion mode was performed to know the product formed
in degradation reaction by analyzing the alizarin solution aer
the reaction. The molecular ion peak of alizarin at m/z 240 was
not observed aer degradation by CDFMNPs, instead we
observed a number of different peaks in different interval of
time as shown in the Fig. 12(a and b) and S9 (ESI†). The
oxidation of alizarin followed a complex path and started with
the reactive cOH radical attack. Upon oxidation with H2O2 and
catalyst it loses its two hydroxyl groups one by one to produce
Fig. 13 Proposed reaction scheme for the degradation of alizarin, based on the LCMS data, to phthalic anhydride and di methyl phthalate, further
oxidation to aliphatic acids and monocyclic products. Finally, after many oxidation steps to aliphatic hydrocarbons, H2O and CO2.
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Table 1 Reusability of CDFMNPs catalyst on degradation of alizarina
Number of cycle
First
Second
Third
Forth
Fih
Degradation (%) of alizarin
Retained activity (%)
93.2 ꢃ 1.5
91.4 ꢃ 2.1
98 ꢃ 1.2
88.1 ꢃ 1.8
94 ꢃ 2.0
85.3 ꢃ 2.1
91 ꢃ 1.9
80.2 ꢃ 3.2
86 ꢃ 2.2
100
Reaction conditions: [H2O2] ¼ 25 mM, [alizarin] ¼ 10 mM, catalyst ¼ 2.0 g Lꢀ1, pH ¼ 3 at room temperature. The experiments were performed in
a
triplicate and the values are presented as means and standard deviation.
anthracene-9,10-dione (208 amu) and produced [M + 1]+ peak at and deionized water to remove any organic products and
m/z 209, intake its parent ring structure. In the next step ring unreacted precursors. Then it was used for ve consecutive
cleavage took place to produce monocyclic aromatic product viz. runs. Table 1 shows the degradation efficiency and retained
dimethyl phthalate (194 amu) and phthalic anhydride (148 activity of the catalyst for alizarin. It was found that catalyst
amu) corresponds to [M + 1]+ peak with m/z at 195 and [M]+ peak shown remarkable stability aer a h run from about 93.2% to
with m/z at 148 respectively. Further oxidation of the products 80.2% degradation. The insignicant decrease of alizarin
give [M]+ peak at m/z 116 and at m/z 130, [M+ + Na] peaks at m/z degradation and minor loss of catalytic activity is due to the
113 corresponds to maleic acid (116 amu), glutaconic acid (130 deactivation of the catalyst by Fe(III) complex formation,
amu) and oxalic acid (90 amu) respectively. Additionally, other nominal iron leaching from the catalyst surface and the catalyst
degraded products of alizarin with monocyclic ring are, m/z at lost during recovery process. Overall, catalyst has shown
207 for 2,4-di-tert-butyl phenol (206 amu), m/z at 123 for benzoic a signicant activity under the studied experimental reaction
acid (123 amu). Benzoic acid appeared as [M]+ peak and other conditions.
two products appeared in mass spectra as [M + 1]+ peak.
Another [M + 1]+ peak with m/z at 109 can be identied as 2-
4. Conclusions
methyl phenol (108 amu). The peak at m/z 113 for oxalic acid
aer combination with Na+ to appear as [M+]+ ion peak, as in
We have presented here the synthesis of FMNPs and CDFMNPs
with a simple method. The CDFMNPs show excellent activity
towards alizarin degradation at optimum reaction conditions.
The addition of fullerene has enhanced the activity of the
magnetic composite with nominal iron leaching whereas
coating of b-CD alters the crystalline nature, magnetic property
and most importantly the surface activity of the catalyst. The
oxidative degradation process predominantly occurs in hetero-
geneous manner as conrmed by quantication of insignicant
amount of leached iron in solution. Because of these combined
effects of fullerene and b-CD with magnetic nature of Fe3O4, this
catalyst can effectively serve as a heterogeneous Fenton catalyst
for the alizarin degradation reaction. Moreover, the catalyst can
be separated magnetically and reused without any signicant
loss of catalytic activity up to ve successive uses. These prop-
erties make the catalyst more attractive and provide the
knowledge-base for future applications in the eld of catalysis
to understand the degradation mechanism of a wide range of
structurally similar textile dyes to alizarin.
ESI-(+)ve mode formation of sodium adduct is very common. At
18.6 s we observed only three prominent peaks in the mass
spectra (Fig. S9 of ESI†) for benzoic acid (m/z ¼ 123), dimethyl
phthalate (m/z ¼ 195) and compound with m/z 133, which could
be a radical intermediate with M.W. 132. Since the degradation
of alizarin in the presence of H2O2 by CDFMNPs is an oxidative
process, it produces a few degraded products which could not
be identied and may be appeared because of the produced
from solvent or contamination. However, it is proved from the
observed data that aromatic ring structure of the alizarin was
destroyed or reduced by CDFMNPs and it is expected that aer
a number of oxidative steps all aromatic rings opened up to
form small molecules like H2O and CO2 and aliphatic hydro-
carbons, which could not be identied by LC-MS study.53
Probable degradation pathway of alizarin by Fenton system
of CDFMNPs/H2O2 is depicted in Fig. 13, though the mecha-
nism of heterogeneous Fenton process is not certain.54 During
the multistep oxidative destruction process, cOH radical rst
produced dimethyl phthalate and phthalic anhydride aer
removal of two hydroxyl groups from alizarin. Further, oxida-
tion executes the complete decolourization of alizarin and
believed that aer many oxidative steps cleavage of the aromatic
ring take place to produce acids and different small molecules.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
3.4 Reusability and recovery of the catalyst
Authors are highly thankful to NEHU, Shillong and Gauhati
University, Guwahati for providing instrumental support.
It is important for a solid catalyst to be recovered aer reaction
and reused for a fresh set of experiment. The CDFMNPs can be
easily recovered aer every reaction with an external magnet
(Fig. 3, inset) which was conrmed from its magnetization value
as shown in VSM study. Aer every set of experiment catalyst
was magnetically recovered, washed several times with ethanol
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