G.C. Silva et al. / Materials Research Bulletin 49 (2014) 544–551
545
reaction time (24 h). Furthermore, the combined effect of
oxidation/sorption is not investigated.
mass was also investigated (25 and 100 mg of Mnmag). The tests
were conducted in duplicate. Absorbance ranging from 0.3 to 1.0 is
less susceptible to stray light and noise problems and hence
becomes the preferred absorbance range for UV–vis analyses.
Therefore, the MB concentration was chosen to obtain initial
absorbance of 1.0.
Given that context, the present work addresses the develop-
ment a simplified synthesis method (temperature, reagents, time)
of magnetic composites with a manganese oxide of high standard
reducing potential, such as Mn3O4. Moreover, the investigation of
their combined oxidation/sorption properties in stirred, solid-
aqueous environmental systems to remove both inorganic and
organic contaminants is a subject of relevance. Here, we report the
2.4. Analysis instruments
application in methylene blue degradation of
composite based on Mn3O4 synthesized at room temperature
and using air as an oxidant.
a
magnetic
Raman spectroscopic and X-ray diffraction (XRD) analyses were
carried out for solid identification. Raman spectra were collected
on a Horiba Jobin Yvon LABRAM-HR 800 spectrograph, equipped
with a 633 nm helium–neon laser, 20 mW of power, attached to an
Olympus BHX microscope equipped with 10ꢃ, 50ꢃ, and 100ꢃ
lenses. Raman-scattered radiation was collected with 600 g/mm
grating, in a 1808 backscattering configuration. The spectra were
collected in a frequency range of 100 to at least 1100 cmꢄ1 with a
step size of 1.1 cmꢄ1. A N2 cooled charge couple device (CCD)
detector was used to suppress extra noise and obtain sufficiently
accurate results. To reduce noise ratio, spectra were acquired at
acquisition times of 3 min twenty times.
2. Materials and methods
All chemicals were of analytical grade and used without further
purification. All solutions were prepared with deionized water
with a conductivity of 18.2
mS/cm obtained with a Milli-Q water
purification system (Millipore). To remove contaminants that had
been potentially adsorbed onto the glass and plastic walls, all
vessels and instruments were cleaned by soaking in detergent
solution, then in 1.0 mol/L HNO3 solution, and subsequently in
deionized water, in each case for at least 24 h. All parts of the
spectroscopic equipment used to extract and fill the sample were
cleaned and rinsed properly with acetone. The pH electrode (713
pH Meter, Metrohm) was calibrated prior to use with three pH
buffers (pH 4.0, 7.0, and 10.0).
The diffractograms were obtained on a Shimadzu 7000 X-ray
diffractometer, using a copper anode (Cu Ka1 radiation) and
graphite crystal monochromator. Analyses were run by step-
scanning from 48 to 908 2u, increments of 0.02 2u and count time of
3 s. The Rietveld refinement was performed using the GSAS 2001
software and the EXPGUI interface. The function of the pseudo-
Voigt profile of Thompson-Cox-Hastings was used and the
background was adjusted by the Chebyschev polynomial. The
scale factor, unit cell, background, asymmetric profile, parameters
of the full width at half-height from the instrumental broadening
(obtained with a standard atomic position), isotropic atomic
displacements and occupation factors of cations were refined. The
difference between the theoretical (calculated profile) and
experimental (mineral phase) diffractograms was good, as
demonstrated by the residual curves and by the small values of
x2 (GOF-values). The values are: x2 = 1.440% for sample mag,
x2 = 1.265% for sample Mnmag and x2 = 2.224% for sample Mn3O4.
The crystallite size was estimated using the crystallite size/lattice
strain calculation (Williamson–Hall method) software for Shi-
madzu X-ray diffractometer. The Williamson–Hall equation is as
follows (Eq. (1)):
2.1. Synthesis of magnetite nanoparticles (mag)
Magnetite particles were prepared by a chemical precipitation
route. An aqueous solution prepared with 400 mL of deionized
water, 9.61 g of Fe2(SO4)3.5H2O (97%, Aldrich), 7.13 g of FeS-
O4.7H2O (RegentPlusTM, ꢁ99% – Aldrich) and 100 mL of a 5.0 mol/L
KOH (Sigma–Aldrich) solution was stirred under N2 atmosphere at
70 8C for 2 h. The black suspension was filtered, washed with water
several times, and finally dried in an oven at 45 8C [15–17].
2.2. Synthesis of magnetic Mn3O4 composite (Mnmag)
The composite was prepared by placing 1000 mL of deionized
water in contact with 0.5 g of previously synthesized magnetite
nanoparticles and 45 mL of 1.0 mol/L MnCl2ꢂ4H2O (Sigma–
Aldrich) solution at pH 12 (1.0 mol/L KOH – Sigma–Aldrich) in
a 2000 mL Pyrex beaker under stirring (mechanical stirrer,
Fisatom 713 D) and constant air input (aquarium pump power
500) for 30 min. The same reaction was also carried out in the
absence of magnetite nanoparticles for comparison. A brown
colored solid was separated from solution by a neodymium
b
cos
u
1 þ 2
h
sinu
l
¼
(1)
l
e
where
u
is the Bragg angle,
b
is the expanse of the diffraction line
width (integral width),
the wavelength of the X-ray and
particle size in the direction perpendicular to the (h k l) plane is
Dh k l = K , where K is the Scherrer’s constant which is different
depending on the crystal (1.05 is typical). Using these equations,
the average crystallite size, dXRD, is calculated. The value to be
e
is the average size of crystal particles,
l is
h
is the grating distortion. The
´
magnet (180 mm ꢃ 100 mm ꢃ 35 mm, Imatec Produtos Magne-
e
ticos Ltda), washed with deionized water solution and dried in a
dessicator at room temperature.
b
substituted for this Equation is the value obtained by correcting the
integral width according to the integral width curve using the
‘‘Jones Correction’’ method by which the inherent expansion of the
system is corrected.
A Perkin Elmer, Paragon 1000 spectrometer was used for FTIR
spectra collection. The sample scans ranged from 400 to 4000 cmꢄ1
with 4.0 cmꢄ1 resolution and were obtained as 128 scans. IR
spectra were obtained as dry samples mixed with KBr correspond-
ing to 8 mg of sample in approximately 40 mg of spectral grade
KBr.
2.3. Methylene blue (MB) degradation
Decolorization experiments were conducted in a glass beaker,
typically containing 68 mL of 1.4 ꢃ 10ꢄ5 mol/L MB dye solution
and 50 mg of Mn3O4 nanoparticles and Mn3O4 magnetic compos-
ite. The mixture was allowed to react at room temperature under
stirring. The progress of decolorization was assessed by UV–vis
spectroscopic measurements of the mixture at different time
intervals (5 min to 3 h). Some aliquots were taken, centrifuged and
filtered before spectrometric analysis. To investigate the effect of
pH on the decolorization process, the experiment was performed
at pH 3.0, 4.0 and 6.0 keeping the amount of Mn3O4 and the dye
constant. The effect of duplicating and reducing the composite
Mo¨ssbauer spectroscopy data were collected on a conventional
constant acceleration Mo¨ssbauer spectrometer (Halder) in trans-
mission mode with a 57Co (Rh) source to identify the composite’s
magnetic phase. An iron metal sheet with a thickness of 25
mm was