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Synthesis and characterization of nanostructured hematite for wastewater treatment
Nadezhda Angelova*, Savina Koleva, Georgi Yordanov
Faculty of Chemistry and Pharmacy, Sofia University St. Kliment Ohridski, Sofia, Bulgaria
nangelovaa@gmail.com
Abstract: Removal of heavy metals ions from water sources still represents a challenge and different materials are being developed in order
to overcome it. Iron oxide nanomaterials receive a lot of attention because of their small size, high surface area, biocompatibility and low
cost. However, most of the reported synthesis methods are multi-step and time-consuming. We investigated the co-precipitation method for
the synthesis of nanostructured hematite prepared at different temperatures and different calcination times. The obtained materials were
characterized by X-ray powder diffraction, scanning electron microscopy and infrared spectroscopy. The adsorption capacity for Pb(II) ions
was found to be about 6–12 mg of adsorbed Pb(II) per gram of adsorbent.
Keywords: hematite, nanoparticles, nanocrystals, water treatment, microstructure
1. Introduction
Nanostructured iron oxides have been found suitable for
applications in environmental nanotechnology, particularly in
wastewater treatment as catalysts, coagulants in flocculation
processes and efficient adsorbents for organic wastes and heavy
metal ions [1]. Among the various nanostructured iron oxides,
hematite (α-Fe2O3) nanoparticles have been used for removal of
metal ions [2-5], Cr(VI) [6-8], acid dyes [9] and organic carbon
[10]. Nanostructured hematite has also been applied for catalytic
degradation of organics wastes [6,11-13] and in coagulation and
flocculation processes of surface water [14]. These applications are
favored by the low toxicity of hematite and the relatively cheap and
easy preparation procedures. However, further studies on the
methods for preparation of nanostructured hematite are needed in
order to find the optimal conditions for production of materials with
high adsorption capacity.
Nanostructured hematite is usually prepared by using various
modifications of the so-called co-precipitation method [2,5,13,15].
This approach involves precipitation of Fe(III) by alkaline aqueous
solutions, followed by washing of the obtained precipitate and its
calcination at high temperatures, up to 400–500 °C. Variations in
the preparation conditions can result in significant variations in the
quality of the final product. In this article we report on the
preparation of nanostructured hematite by a modification of the co-
precipitation synthesis [2] and studied the effects of calcination
temperature and time on the crystallite size and adsorption capacity
for Pb(II) ions.
2. Experimental part
Synthesis of hematite. Solution of ammonia (2 M; ~138 ml)
was added dropwise to 250 ml 0.1 M ferric chloride (prepared from
6.75 g FeCl3.6H2O) up to pH 10 upon magnetic stirring (600 rpm)
at room temperature. All reagents were of analytical grade. The
obtained dispersion was stirred for 2 hours at room temperature and
the precipitate was centrifuged (3000 rpm for 10 min), triplicate
washed with distilled water to remove soluble salts and let to air
dry. The dried substance was calcined in a furnace at different
temperatures (200–500 °C) for different time periods (1–4 hours).
The calcinated samples were grinded in a mortar before further use.
Characterization of materials. Scanning electron microscopy
was performed with Hitachi TM4000 microscope. Samples for
SEM were deposited on. double sided duct tape and coated with
gold before observation. X-ray powder diffraction (XRD) data were
recorded by using a X-ray diffractometer Empyrean (PANalytical)
with CuKα radiation. Fourier transform infrared (FTIR) spectra
were measured by using an infrared spectrometer Nicolet 6700
(Thermo Scientific). Samples for FTIR analyses were prepared as
KBr tablets.
Adsorption of Pb(II). The tested adsorbent (50 mg) was mixed
for 10 minutes with 10 ml 3 mM solution of lead(II) nitrate. The
suspension was then filtered through a syringe filter (0.22 µm). The
concentrations of Pb(II) in all samples before and after adsorption
were determined by standard complexometric titration with 1 mM
Na2EDTA at pH 6 (acetate buffer) and xylenol orange indicator.
The adsorbance capacity was expressed as milligrams of adsorbed
Pb(II) per gram of adsorbent.
3. Results and discussion
It is known that ferric ions from aqueous solutions are
precipitated in alkaline medium as amorphous ferric hydroxide
and/or β-ferric oxyhydroxide [16]. Preliminary studies indicated
that in the precipitation step of our experiments ferric
oxyhydroxide, Fe(O)OH, was obtained with quite small crystallite
size of about 3–4 nm. Heating of the dried precipitate at high
temperatures (higher than 200 °C) for few hours resulted in the
formation of hematite (α-Fe2O3) with crystallite sizes of 15–22 nm.
Observation by scanning electron microscopy (SEM) showed a
powdered material that contained particles of various sizes, from
submicron to few micrometers (Fig. 1).
Fig. 1 SEM image of a hematite sample heated at 400 °C for 2 hours.
Analysis by infrared (FTIR) spectroscopy showed absorbance
peaks for Fe–O bonds at 461 and 536 cm-1, which are characteristic
for hematite. Similar FTIR of hematite nanoparticles prepared by
the chemical precipitation process were previously obtained,
although with a slight different peak positions [17].
XRD diffraction patterns of the samples prepared by heating for
2 hours at different temperatures are shown in Fig. 2. The sample
prepared at 200 °C appeared practically amorphous, while those
prepared at higher temperatures all contained hematite with average
size of crystallites about 15–22 nm. The samples contained also a
small amount of amorphous substance. The size of crystallites did
not appear to depend significantly on the heating temperature in this
particular interval (300–500 °C). Another series of experiments was
performed at a constant heating temperature of 400 °C and various
heating times. The XRD patterns showed almost identical structure
of the obtained hematite phase (Fig. 3).
The experiments on adsorption capacity for Pb(II) ions were
performed at relatively high Pb(II) to adsorbent ratio in order to
evaluate the maximal adsorption capacity (when the surface of the
adsorbent is almost saturated with adsorbed species). The
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adsorption capacity of the samples heated for 1–3 hours at 400 °C
was about 6–8 mg of Pb(II) per gram adsorbent and did not depend
significantly on the heating time (Fig. 4a). The sample heated for 4
hours showed slightly higher adsorption capacity of 12 mg/g. The
sample heated for 2 hours at 200 °C showed a similar adsorption
capacity of 12 mg/g, while using higher heating temperatures
resulted in slightly reduced adsorption capacities of about 8 mg/g
(Fig. 4b).
Fig. 2 XRD patterns of hematite samples prepared by heating at different
temperatures for 2 hours.
Fig. 3 XRD patterns of hematite samples prepared by heating at 400 °C
for different time.
Fig. 4 Adsorption capacity for Pb(II) of hematite samples prepared by: a)
heating at 400 °C for different time; b) heating at different temperatures for
2 hours.
4. Conclusions
Powder samples of nanostructured hematite were prepared by
the co-precipitation, followed by heating at various temperatures
(200–500 °C) and time periods (1–4 hours). All samples prepared at
temperatures higher than 200 °C contained crystallites of average
size about 15–22 nm. The crystallite size and phase composition did
not depend significantly on heating time. The obtained materials
appeared to be promising candidates for application as adsorbents in
wastewater treatment with adsorption capacity for Pb(II) ions within
the ranges of 6–12 mg of adsorbed Pb(II) per gram of adsorbent.
.
5. Acknowledgements
The authors are grateful to the Scientific Research Fund of
Sofia University, project № 80-10-138/20.05.2022.
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