الفهرس 
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Abstract
Lanthanide elements (Ln: La-Lu; 57–71) belong to the rare-earth series of elements (Sc, Y, and Ln). They present specific chemical, optical, and magnetic properties that are a consequence of their peculiar electronic structure. Although used in small quantities (about 120,000 tons equivalent rare-earth oxides per year worldwide), they have become essential to almost all aspects of modern life, being the active cores in catalysts for oil cracking, lighting devices, high coercivity magnets used in motorization (electric cars, wind turbines, hard disk drives) or audio applications, lasers, telecommunications, biomedical analyses, imaging, and agriculture. They are classified as strategic materials by several governments. Therefore, REEs demand throughout the world is projected to increase. On the other hand, REEs are important in the context of the nuclear fuel cycle because they are produced during the fission of uranium and are hence present in radioactive wastes. The present work is mainly concerned with the synthesis of GO, MnO2 and GO-MnO2 composite as sorbents for the removal of Eu(III), Ce(III) and Nd(III) ions from aqueous solutions. characterization of the prepared sorbents using different tools and studying the sorption behavior of Eu(III), Ce(III) and Nd(III) ions onto these sorbents under different batch experimental conditions were performed. Also, modeling the equilibrium sorption data using different models was carried out with determining the sorption kinetics, isotherms and thermodynamics parameters governing the applied system. The chromatographic separation of Eu(III) and Ce(III) from GO-MnO2 composite packed column was studied using different eluents..
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The main components of thesis are introduction, experimental and
results and discussion.
1. Introduction
This chapter focuses on giving an overview on characteristics and
different applications of lanthanides. Also, it includes a brief review on
extraction and separation of lanthanides from naturally occurring ores and
other mixtures using different techniques. The chapter also includes some
information about carbon nanomaterials such as graphene oxide and their
properties. Also, it includes a short note about the investigated rare earth
elements; europium, cerium and neodymium.
2. Experimental
This chapter presents all chemicals, reagents and apparatus used in
this work and methods of preparation of different materials and
radiotracers used in this study. The chapter includes a brief description
for physical and chemical methods used in characterization of
synthesized sorbents. Experimental also involves a detailed description of
batch experiments for sorption of Eu(III), Ce(III) and Nd(III) ions onto
synthesized sorbents under investigation through different sorption
parameters including the effect of pH, contact time, ionic strength, initial
concentration, adsorbent dose and temperature. Also, this chapter
involves a brief description of column experiments for separation of
Eu(III) and Ce(III) from GO-MnO2 composite packed column.
3. Results and discussion
This chapter includes a discussion of the obtained data from
experimental work. The discussion covers: 1) characterization of
synthesized sorbents, 2) sorption behavior of Eu(III), Ce(III) and Nd(III)
onto synthesized sorbents, 3) modeling the equilibrium sorption data
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using different models and 4) the chromatographic separation of Eu(III)
and Ce(III) from GO-MnO2 composite packed column.
The physical and chemical characterization of synthesized sorbents
in the present work was performed using (BET) surface area
measurements, (FT–IR) spectroscopy, TG–DTA analysis, scanning
electron microscopy (SEM), X–Ray diffraction and Raman spectroscopy.
The surface area measurements showed that GO–MnO2 composite
have higher values of SBET, VP and DP compared with that of their
precursors GO and MnO2. These properties declared the promising
surface characteristics of the GO–MnO2 composite to be used as advancd
sorbents for removal of different metal ions from aqueous solution.
FT–IR spectroscopic analysis of synthesized sorbents showed
number of characteristic peaks of both GO, MnO2 and GO–MnO2
composite. This confirmed that synthesis of GO–MnO2 composites
having a variety of surface functionality.
XRD measurements showed that characteristic peaks of GO and
MnO2. While in case GO–MnO2 composite, the (001) reflection peak of
layered GO has almost weaken. This result correlated with the report that
the diffraction peaks of MnO2 become weakened or even disappears
whenever the regular stacks of GO are exfoliated. This confirmed, that a
composite from GO and MnO2 particles was synthesized successfully.
The average crystallite size of synthesized sorbents was estimated from
XRD patterns using Scherrer’s equation and the values demonstrated the
nano–sized structure of synthesized sorbents.
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TG–DTA analysis showed that MnO2 have thermal stability higher
than graphene oxide. Also, TGA analysis showed that addition of MnO2
to GO improved the thermal stability of GO.
Scanning electron microscope (SEM) images of GO and GO–
MnO2 exhibited similar lamellar wrinkled structures and MnO2 ultrathin
flakes are loosely assembled and closely anchored on both sides of GO
sheet, representing a multilayer flake structure, but the sheets of GO
stacked together due to its strong inter-planar interactions and GO sheets
have been exfoliated and decorated randomly with structure the MnO2,
while SEM image of MnO2 exhibited almost spherical morphology with
large aggregates and a typical flower structure of MnO2 with rough
agglomeration.
Finally, Raman spectroscopy showed that characteristic bands (G
and D bands) for GO, while these bands are shifted in case GO–MnO2
composite and appearance of another characteristic band which referred
to Mn–O vibration in GO–MnO2 composite. This band confirmed the
formation of a composite between GO and MnO2 particles.
Sorption of Eu(III), Ce(III) and Nd(III) ions onto synthesized
sorbents was performed under different experimental conditions to clarify
the main factors controlling the sorption process and optimize the
separation conditions. Contact time showed that the sorption started with
an initial rapid sorption rate followed by a slower uptake and the
equilibrium state was attained after almost 5 h. Further increase in contact
time beyond 5 h up to 48 h did not show any remarkable effects in uptake
of Eu(III), Ce(III) and Nd(III) ions which remained almost constant.
Accordingly, the equilibrium time was fixed at 24 h for the rest of batch
experiments.
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Sorption of Eu(III), Ce(III) and Nd(III) ions onto synthesized
sorbents was studied at different pH values ranged from 1 to 7. The
sorption was markedly influenced by solution pH and the uptake value
increased with increasing pH values. The synthesized sorbents exhibited a
high sorption affinity towards Eu(III), Ce(III) and Nd(III) ions at pH 3
that is far from the precipitation limit of studied REE. The results
illustrated that at pH value > 4, the sorption percentage continuously
increased and that could be attributed to the precipitation of REE as
hydroxides. Hence, pH 3 was chosen as the optimal pH in all experiments
of Eu(III), Ce(III) and Nd(III) sorption.
The maximum sorption capacities of synthesized sorbents towards
Eu(III), Ce(III) and Nd(III) ions were determined experimentally through
successive sorption process. The revealed data clarified that, The
synthesized sorbents exhibited a higher maximum sorption capacities
towards Eu(III), Ce(III) and Nd(III) ions compared with other sorbents
reported in previous studies. The synthesized GO–MnO2 composite
exhibited a higher sorption affinity towards Eu(III), Ce(III) and Nd(III)
ions with respect to GO and MnO2 sorbents. This revealed that the
synthesized GO–MnO2 composite are promising candidates can play an
important role in adsorption, separation and purification of lanthanide
elements.
The sorbent mass showed that the efficiency of removal of studied
metal ions from aqueous solutions increased with increasing the sorbent
mass up to a certain weight. The revealed data clarified that a maximum
removal of Eu(III), Ce(III) and Nd(III) ions at 0.01 g.
The ionic strength showed a slight influence on uptake of Eu(III),
Ce(III) and Nd(III) ions onto synthesized GO, MnO2 and GO–MnO2
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composite using NaCl as a background electrolyte. This confirmed the
specific tendency of these sorbents towards studied metal ions and
suggested that the sorption of Eu(III), Ce(III) and Nd(III) ions onto
synthesized sorbents was mainly dominated by inner–sphere surface
complexation.
The effect of initial concentration of Eu(III), Ce(III) and Nd(III)
ions on synthesized sorbents was studied using a series of initial
concentration from 25– 300 mg.L-1, The uptake percent of Eu(III), Ce(III)
and Nd(III) ions onto synthesized sorbents decreased with increasing the
initial concentrations of studied metal ions.
The temperature showed that the uptake percent of Eu(III), Ce(III)
and Nd(III) ions onto synthesized sorbents slightly increased with
increasing system temperature from 20 – 60 °C indicating an endothermic
nature of sorption process, where a better sorption was achieved at high
temperatures.
Desorption of Eu(III), Ce(III) and Nd(III) ions from loaded
sorbents was performed using some organic eluents as: Oxalic, Malonic,
Mandelic, Citric, Succinic acids and EDTA and inorganic eluents as:
H3PO4, HCl, MnCl2, FeCl3 and AlCl3). The maximum desorption of
Eu(III) and Nd(III) from synthesized sorbents was occurred using 0.01M
EDTA, while the maximum desorption of Ce(III) from synthesized
sorbents was occurred using 0.01M AlCl3 and 0.1M FeCl3.
The regeneration efficiency of synthesized sorbents was investigated
by successive sorption–desorption cycles of Eu(III), Ce(III) and Nd(III)
ions. The loaded sorbents were regenerated using 0.01M EDTA for
Eu(III) and Nd(III) ions, while 0.01M AlCl3 was used for Ce(III) ions as
eluent. The results revealed that the synthesized sorbents could be
regenerated and reused for repeated sorption–desorption regeneration
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cycles up to five cycles with relatively low decrease in the uptake
percents in case of GO–MnO2 composite compared with GO and MnO2
sorbents.
According to the kinetic modeling, the data revealed from sorption
of Eu(III), Ce(III) and Nd(III) ions onto synthesized sorbents were
mathematically analyzed using different kinetic models. The sorption
kinetics of Eu(III), Ce(III) and Nd(III) could be explained more favorably
by pseudo–second order model. So, the rate determining step in sorption
of europium, cerium and neodymium onto synthesized sorbents is a
chemisorption process depends on both initial concentration of metal ions
and number of active sites in sorbent surface.
Freundlich, Langmuir, D-R and Temkin isotherm models were
applied to analyze the experimental data. The correlation coefficient (R2)
values showed that the experimental data were well–fitted Langmuir
isotherm model than the other models. This indicated that the sorption
isotherms of Eu(III), Ce(III) and Nd(III) ions onto synthesized sorbents
could be explained through a chemisorption process and expected to be
occurred between the studied metal ions and sorbents’ surfaces. Also,
a monolayer of Eu(III), Ce(III) and Nd(III) ions could be predicted to
cover the surface of synthesized sorbents.
The thermodynamic parameters ΔSo, ΔHo and ΔGo corresponding to
Eu(III), Ce(III) and Nd(III) sorption onto synthesized sorbents were
assessed using Van’t Hoff linear equation. The values of thermodynamic
parameters indicated that the sorption is an endothermic process takes
place spontaneously with the possibility of strong bonding between
studied metal ions and sorbents surface.
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The sorption dynamic study was performed using a column
technique. Fixed bed columns were constructed to individually study the
sorption of Eu(III) and Ce(III) onto the GO–MnO2 composite packed
columns. The two rare elements [Eu(III) and Ce(III)] were firstly loaded
individually on two similar columns, then eluted using both AlCl3 and
EDTA. The breakthrough capacity (Q) of GO–MnO2 composite for
Eu(III) ions in studied column was found to be 47.07 mg.g–1. The total
recovery of Eu(III) from loaded GO–MnO2 composite column exhibited
the value 87.03 % using 105 mL of 0.01M EDTA. The elution percent of
Eu(III) from GO–MnO2 composite packed column attained the value
33.93 % using 105 mL of 0.01M AlCl3. On the other hand, the
breakthrough capacity of GO–MnO2 composite for Ce(III) ions was
found to have the value 41.61 mg.g–1. The total recovery of Ce(III) from
GO–MnO2 composite packed column exhibited the value 72.13 % using
233 mL of 0.01M AlCl3. The possibility of chromatographic separation of
Eu(III) from Ce(III) was evaluated by measuring the separation factor
between Eu(III) and Ce(III). The separation factor between Eu(III)
and Ce(III) was only 1.51. This value indicated that separation of Eu(III)
from Ce(III) was extremely difficult. The chromatographic separation of
Eu(III) and Ce(III) from GO–MnO2 composite packed column was
carried out by loading of both Eu(III) and Ce(III) together followed by
eluting them using both EDTA and AlCl3. The breakthrough capacity of
GO–MnO2 composite for Eu(III) ions in studied column was found to
have the value 29.66 mg.g–1, while that for Ce(III) ions was 25.66
mg.g–1. The total recovery of Eu(III) and Ce(III) from GO–MnO2
composite packed column exhibited the values 87.54 and 46.35 %,
respectively using 150 mL of 0.01M EDTA. The elution percentages of
Eu(III) and Ce(III) from GO–MnO2 composite loaded column were
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calculated and founded 27.55 and 42.81 %, respectively using 220 mL of
0.01M AlCl3.
Finally, the overall conclusion is summarizing as follow:
Grapheme oxide (GO) was successfully synthesized via modified
Hummers’ method by chemical oxidation method of graphite
powder. Manganese dioxide was synthesized via a coprecipitation
method. Manganese dioxide decorated graphene
oxide (GO-MnO2) was prepared via fixation of MnO2 on the
surface of GO.
The synthesized sorbent were successfully characterized using
different techniques as FT-IR, XRD, SEM, Thermal analysis (TGA
and DTA), (BET) surface area measurements and Raman
spectroscopy.
The composite formation between GO and MnO2 improves: 1) The
physical and mechanical properties of MnO2, 2) sorption capacity
and 3) The thermal stability of GO.
The synthesized sorbents highly removed Eu(III), Ce(III) and
Nd(III) ions from their aqueous solutions of pH 3. They exhibited
a higher sorption capacity towards Eu(III), Ce(III) and Nd(III)
ions compared with other sorbents.
The adsorption capacity is greatly influenced by solution pH,
initial concentration and temperature.
The ionic strength had a slight influence on uptake of Eu(III),
Ce(III) and Nd(III) ions onto synthesized sorbents using NaCl as
a background electrolyte.
Both AlCl3 and EDTA effectively eluted Eu(III), Ce(III) and
Nd(III) ions from synthesized sorbents.
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146
The synthesized sorbents could be reused repeatedly for sorption
of Eu(III), Ce(III) and Nd(III) ions from aqueous solutions.
Sorption of Eu(III), Ce(III) and Nd(III) ions onto synthesized
sorbents is a chemical process. A monolayer of Eu(III), Ce(III)
and Nd(III) ions could be predicted to cover the surface of
synthesized sorbents through a favorable sorption.
Sorption of Eu(III), Ce(III) and Nd(III) is an endothermic process
takes place spontaneously with the possibility of strong bonding
between studied metal ions and sorbents surface.
Eu(III) was partially separated from Ce(III) using GO–MnO2
composite as a solid phase by using EDTA and AlCl3 as an
eluents.
On the light of these data, the synthesized sorbents effectively and
quantitatively removed studied species from their aqueous solutions.
Therefore, they could be regarded as promising materials can play a vital
role in preconcentration and purification of REEs.