الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 144 |  |  |
| | | from | 144 | | |
Abstract
A number of rice straw ash (RSA) – based composite catalytic systems
were designed in disc-shaped forms, through compression at certain
pressures. Specialized components were included in presence of a
recycled-polymer foam binding component. The highly stable composite
discs contained titania (TiO
2, 10 – 70 %), copper phthalocyanine complex
(CuPc, 1.0%) and TiO
2-CuPc combination. For comparative study, the
composites of RSA-TiO2 were also prepared in powder form.
Physicochemical characterization of the as-synthesized composites was
performed by adopting XRD, N2-physisorption, FTIR, EDX, TEM, and
SEM techniques. The various synthesized composites were applied for
removal of hazardous dyes (represented by methylene blue, MB), in dark
and photo-catalytically by using highly desirable visible light. They were
applied also for removal of toxic heavy metal ions (e.g., Cd
2+
, Pb
2+
, Cu
2+
).
The obtained data were analyzed in view of different kinetic models and
the different variables affecting the rates of removal processes were
considered. The most significant conclusions drawn are given as follows:
- The used composite discs had the following parameters: O.D. = 40
mm, thickness = 2.5 mm, compressive strength = 9.27 N/mm
2
and
compressive load = 11.647 KN.
- In titania-contained composites (e.g., RSA-TiO2), the main
amorphous silica was observed together with some silica
nanoparticles, RSS NPs ( of 50 nm) and TiO2 NPs (of 16-20 nm)
as separate phases The incorporated titania, homogeneously
dispersed in RSA matrix, showed some interaction with (RSS),
while carbon was redistributed in more ordered profile along the
entire surface of the ash system. Presence of silica inhibited the
transformation of titania to rutile form; only a single anatase phase
SUMMARY & CONCLUSIONS
CHAPTER IV
110
was indicated. Mesoporosity was enhanced by increasing TiO2
content up to 50 % in RSA.
- In CuPc-contained composites (e.g., RSA-TiO2-CuPc), addition of
CuPc could lead to some decrease in surface area and pore
parameters by interaction with both silica and titania phases, while
mixing with TiO2 developed the mesoporous nature of the
composites, which reflected positively on their adsorption
behavior.
- In removal of MB over TiO2-contained composites in powder
form, RSA-TiO2
(50 %) composite showed the highest
photocatalytic activity, owing to distinguished high surface area
and pore dimensions.
- Using the different disc-shaped composites, the rate of adsorption
process of MB (in dark, at pH = 5.85) decreased according to the
second order model in the order:
RSA-TiO2-CuPc > RSA-TiO2 > RSA > RSA-CuPc. TiO2 or TiO2-CuPc combination enhanced the rate of MB adsorption due to
development of the surface characteristics.
- The rate of visible-photodegradation of MB, decreased according
to pseudo-first order kinetic model, in the order: RSA ≈ RSA-TiO2
> RSA-CuPc > RSA-TiO2-CuPc. TiO2, by generated hydroxyl
radicals (
.
OH) and superoxide radicals (
.
O2
-) and CuPc complex,
by generated singlet oxygen (
1
O2) through photosensitization role,
could activate the rice straw ash toward MB photo-degradation in
aqueous solutions. Some retardation in presence of hybrids from
CuPc and TiO2 was attributed to the recombination of most of
electrons donated by the bound CuPc with positive holes of titania.
SUMMARY & CONCLUSIONS CHAPTER IV
111
- The extent of photo-degradation decreased by increasing the dye
concentration (in the range, 5 – 35 ppm) on RSA and RSA-TiO2
composite systems, which could be attributed to the less number of
photons that could reach the catalyst surface leading hence to less
•
OH radicals produced.
- Absorption peaks at 290 and 664 nm decreased continually with
irradiation up to 21 h in presence of the composite discs, without
detectable shift. The reduction extent of TOC followed the order:
RSA > RSA-CuPc > RSA-TiO2 > RSA-TiO2-CuPc, after photodegradation. Mineralization rate was far below the de-colorization
rate, which confirmed the formation of some intermediates during
the photo-catalytic process.
- Among the studied composites, RSA-binder disc seemed more
resistant, bearable up to 4 reuse cycles; nearly constant %
degradation was displayed. CuPc-contained discs showed less
durable catalytic systems.
- The rate of removal (adsorption) of Cd
2+
ions at neutral pH on the
used catalytic discs decreased, according to the pseudo-second
order kinetic model, in the same order of the MB dye adsorption,
viz., RSA-TiO2-CuPc RSA-TiO2 RSA-CuPc ≥ RSA. This
seemed also to be due to development of the surface
characteristics, implying most likely that the process belongs to
activated sorption (chemisorptions).
- The intra-particle diffusion rates of Cd
2+
removal showed the same
pseudo-second order kinetic model: RSA-TiO2-CuPc RSA-TiO2
RSA-CuPc ≈ RSA. The rate onto the neat RSA increased by
presence of other components, as in RSA-TiO2-CuPc of higher
intra-particle diffusion profile, i.e., becoming rate determining.
SUMMARY & CONCLUSIONS
CHAPTER IV
112
This composite exhibited also the smallest RSS particle size, 46.8
nm (with TiO2
particle size, 23 nm), among the studied systems
and the highest surface area (31.0 m
2
g
-1
); smaller particles with
higher surface areas produce shorter times to equilibration. The
analysis according to Elovitch model followed pseudo-first order
kinetics, suggesting that the adsorption of Cd
2+
ions depends on the
boundary conditions of studied components.
- Based on the obtained results in removal of Cd
2+
, the RSA-binder
disc could solely be used effectively in removal of heavy metals
ions in aqueous solutions.
- The best initial concentration of Pb
2+
was 100 ppm, where the
removal process proceeded in smoother style, optimally at 30
o
C at
neutral pH and in much higher removal % than Cd
2+
. The values of
k and R
2
confirmed the validity of pseudo-first order adsorption
kinetic behavior with 100 ppm. The intra-particle diffusion rate of
Pb
2+
ions became little lower in these conditions due crowded
species on surface, damping the diffusion process.
- The analysis of kinetic data of Pb
2+
ions adsorption according to
Elovitch model suggested that the adsorption of Pb
2+
ions depends
also on the boundary conditions of the studied composite system
(silica, carbon nanoparticles and organic porous fibers).
- The optimum (highest) adsorption rate of Cu
2+
was achieved by
using 400 ppm initial concentration at 30
o
C. The highest extent of
adsorption was achieved with initial concentration of 400 ppm at
pH = 1.8, (the lowest extent was shown at pH = 6.2).