BỘ GIÁO DỤC VÀ ĐÀO TẠO
TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI

.......................................

NGUYỄN VĂN CHÚC

NGHIÊN CỨU TỔNG HỢP TIO2 VÀ KHẢ NĂNG
ỨNG DỤNG ĐỂ XỬ LÝ SR6+ TRONG NƯỚC THẢI

LUẬN VĂN THẠC SĨ KHOA HỌC

NGƯỜI HƯỚNG DẪN : TS. NGUYỄN HỒNG LIÊN

HÀ NỘI – 2010


Research on synthesis of TiO2 and application for Cr6+treatment in wastewater

ACKNOWLEDGEMENT
I would like to express my gratitude to the people that support me in the completion of
my thesis.
In the first place, I owe a special thank to my dear teachers at the Department of organic
and petrochemical technology for their passionate teaching, generosity in dealing with
students and also enthusiastic inspiration during the past four years. Their devotion to
petrochemical technology study and teaching has highly motivated me to pursue my
future career as an expert in petrochemical by all my efforts and brain power.
To my supervisors, I would like to acknowledge and extend my heartfelt gratitude to
Doctor Nguyen Hong Lien and Associated Professor, Doctor Le Minh Thang,
lecturers of the Department of organic and petrochemical technology. Within their
petrochemical course, I luckily found my interest in petrochemical technology, and then

I.1.1. Water pollution.......................................................................................................... 8
I.1.2. Water pollution categories......................................................................................... 9
I.1.3. Causes of polluted water ......................................................................................... 10
I.1.4. Effects of polluted water ......................................................................................... 10
I.1.5. Control of water pollution ....................................................................................... 10
I.2. Cr(VI) treatment methods in wastewater ................................................................... 12
I.2.1. Ion exchange method............................................................................................... 12
I.2.2. Electrochemistry method......................................................................................... 13
I.2.3. Reduction-oxidation method and deposition method.............................................. 13
I.3. TiO2 review ................................................................................................................ 14
I.3.1. Occurrence............................................................................................................... 14
I.3.2. Physical and mechanical properties......................................................................... 14
I.3.3. Chemical properties................................................................................................. 16
I.3.4. Applications............................................................................................................. 16
I.4. Mechanism of titanium oxide photocatalytic reactions.............................................. 17
I.4.1. Band structure of semiconductors and band gap energy ......................................... 17
I.4.2. Energy structure of titanium oxide and photoeffect ................................................ 18
I.4.3. Effect of ultraviolet rays in activating titanium oxide............................................. 20
I.4.4. Decomposing power of titanium oxide photocatalyst ............................................. 21
1.4.5. The mechanism of photo-reduction of Cr(VI)........................................................ 23
I.5. Review of the used precursors in this thesis............................................................... 24
I.5.1. TiCl4 ........................................................................................................................ 24
I.5.2. Titanium isopropoxide (TTIP) ................................................................................ 28
I. 6. Literature review about using TiO2 as a photocatalyst for wastewater treatment ..... 29
I.6.1. In Vietnam ............................................................................................................... 29
I.6.2. In the world.............................................................................................................. 30
I.7. The importance and direction of thesis ...................................................................... 36
I.7.1. The importance of the thesis.................................................................................... 36
I.7.2. The direction of thesis ............................................................................................. 36
2

III.3.3. The influence of catalytic synthesis methods to the catalytic activity.................. 72
III.3.4. The influence calcination temperature of catalyst to the atalytic activity ............ 73
III.3.5. The influence of amount of TiO2 doped onto Al2O3 to the catalytic activity....... 74
III.3.5. The influence of concentration of ethanol to the catalytic activity....................... 74
CONCLUSIONS ............................................................................................................. 76
References........................................................................................................................ 77

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Organic and petrochemical Technology


Research on synthesis of TiO2 and application for Cr6+treatment in wastewater

TABLES IN THE THESIS
No

Name

Page

1

Typical physical and mechanical properties of titania

15

2


The amount of TiO2 loading onto Al2O3 of synthesized catalysts

40

8

The concentration of the metallic ions in wastewater

64

9

SSA of synthesized catalysts

66

10

Compositions of catalysts prepared by sol-gel, hydrolysis and
Impregnating method

69

11

The changes of concentration of Cr(VI) and COD

71

12

Titanium-oxide band structure

21

Crystal structures of titanium oxide

21

Electron structure of titanium oxide

22

Oxidation mechanism

22

Reduction mechanism

39

The Sol-gel method

40

Hydroysis method

41

Impregnating method



50

The reactor system
The calibration graph of Cr(VI) with 1,5 – diphenylcarbazide solution
Concentration of Cr(VI) in three reaction solutions
Concentration of Cr(VI) in reaction solution with and without
Flocculation

56
57
58

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Organic and petrochemical Technology


Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
18
19

The XRD patterns of the TiO2/Al2O3 samples
SEM images of the synthesized catalysts

56
68

20


Organic and petrochemical Technology


Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
INTRODUCTION
Water pollution has always been a major problem to the environment in the global
context. It has been supposed the leading worldwide cause of deaths and diseases
of human beings, organism in daily life. With the increasing growth of the
industrialization in major areas and urban cities, the water source just keeps
getting more polluted. In Vietnam, the water pollution is of the most serious
problems which draw the attention of all levels of society, especially,
governmental authorities, researchers, and scholars. The main reason which
caused the water pollution is suggested to originate from non-treated waterwaste
discharged from factories, and industrial plants. Therefore, wastewater treatment,
especially wastewater from coating companies containing a large amout of heavy
metal, is important task in Viet Nam in particular and in the world in general.
In this paper, the research topic focuses on the reduction of heavy metal
concentrations by using TiO2 as a photocatalyst. Cr(VI) is choosen as a poluted
agent and ethanol as a hole scavenger. The TiO2/Al2O3 system is synthesized by
variety methods and many effect agents are tested. The properties of the
sythesized catalysts are investigated by the physico-chemical method. The
catalytic activities of the sythesized catalysts are determined by the conversion of
Cr(VI) in water with the presence of ethanol.
The main content of the paper is divided into three parts. Part I discusses the
remark of water pollution and its impact to the human life, the wastewater
treatment methods, review of TiO2 and precursors, the researches about TiO2 as a
photocatalyst for wastewater treatment in the world and Viet Nam. Part II
introduces the synthesized catalytic methods, the methods to determine the
composition of plating chromium wastewater, the methods to investigate the


Photo I.1: Water polluted by garbage

Pollution affects organisms and plants that live in these water bodies and in almost all
cases the effect is damaging either to individual species and populations but also to the
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
natural biological communities; (iii) it occurs when pollutants are discharged directly or
indirectly into water bodies without adequate treatment to remove harmful constituents
[46].
I.1.2. Water pollution categories
Surface water and groundwater have often been studied and managed as separate
resources, although they are interrelated [40]. Sources of surface water pollution are
generally grouped into two categories based on their origin.
a. Point source pollution
Point source pollution refers to contaminants that enter a waterway through a discrete
conveyance, such as a pipe or ditch. Examples of sources in this category include
discharges from a sewage treatment plant, a factory, or a city storm drain.
b. Non-point source pollution
Non-point source (NPS) pollution refers to diffuse contamination that does not originate
from a single discrete source. NPS pollution is often accumulative effect of small
amounts of contaminants gathered from a large area. A typical example is that the
leaching out of nitrogen compounds from agricultural land which has been fertilized.
Nutrient runoff in storm water from "sheet flow" over an agricultural field or a forest are
also cited as examples of NPS pollution.

I.1.4. Effects of polluted water
There are various effects of water pollution.
• Spread of disease: Drinking polluted water can cause cholera or typhoid infections,
along with diarrhea.
• Affects aody organs: The consumption of highly contaminated water can cause injury
to the heart and kidneys.
• Harms the food chain: Toxins within water can harm aquatic organisms, thus
breaking a link in the food chain.
• Causes algae in water: Urea, animal manure and vegetable peelings are food for
algae. Algae grow according to how much waste is in a water source. Bacteria feed off
the algae, decreasing the amount of oxygen in the water. The decreased oxygen causes
harm to other organisms living in the water.
• Flooding: The erosion of soil into waterways causes flooding, especially with heavy
rainfall.
• Harms animals: Birds that get into oil-contaminated water die from exposure to cold
water and air due to feather damage. Other animals are affected when they eat dead fish
in contaminated streams.
• The effects of water pollution are not always immediate. They are not always seen at
the point of contamination. They are sometimes never known by the person responsible
for the pollution. However, water pollution has a huge impact on our lives.
I.1.5. Control of water pollution
a. Domestic sewage
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
In urban areas, domestic sewage is typically treated by centralized sewage treatment

atmospheric deposition. Farmers can develop and implement nutrient management plans
to reduce excess application of nutrients.

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
Pollution prevention practices include low impact development techniques, installation of
green roofs and improved chemical handling (e.g. management of motor fuels & oil,
fertilizers and pesticides) [44]. Runoff mitigation systems include infiltration basins,
bioretention systems, constructed wetlands, retention basins and similar devices.
d. Thermal pollution
Thermal pollution from runoff can be controlled by stormwater management facilities
that absorb the runoff or direct it into groundwater, such as bioretention systems and
infiltration basins. Retention basins tend to be less effective at reducing temperature, as
the water may be heated by the sun before being discharged to a receiving stream.
I.2. Cr(VI) treatment methods in wastewater
There are some methods to treat Cr(VI) in wastewater in the world.
I.2.1. Ion exchange method
Whenever an ion is removed out of an aqueous solution and is replaced by another ionic
species, this is what we generally refer to as “ion exchange”. There are synthetic
materials available that have been specially designed to enable ion exchange operations at
high performance levels. Among many other applications, these so called “ion
exchangers” can be used in processes of environmental protection such as purification,
decontamination, recycling or even for the design of new environment-friendly
production processes. Synthetic and industrially produced ion exchange resins consist of
small, porous beads that are insoluble in water and organic solvents. The most widely

Men+ (m>n≥0)

(m,n: oxidation number of metal Me)
I.2.3. Reduction-oxidation method and deposition method
Principles:
-The reduction-oxidation method used oxidation agents (Cl2, O2,…) or reduction agents
(Na2SO3, FeSO4,…) to change Cr(VI) and polutants to less poisonous forms.
-The deposition method used agents combining with metallic ions in wastewater to
deposition form at a suitable pH. The depositions were easly removed from wastewater
by decanting method.
I.2.4. Photocatalytic method
All of the extensive knowledge that was gained during the development of semiconductor
photoelectrochemistry during the 1970 and 1980s has greatly assisted the development of
photocatalysis. In particular, it turned out that TiO2 is excellent for photocatalytically
breaking down organic compounds. For example, if one puts catalytically active TiO2
powder into a shallow pool of polluted water and allows it to be illuminated with
sunlight, the water will gradually become purified. Ever since 1977, when Frank and
Bard first examined the possibilities of using TiO2 to decompose cyanide in water, there
has been increasing interest in environmental applications. These authors quite correctly
pointed out the implications of their result for the field of environmental purification.
Their prediction has indeed been borne out, as evidenced by the extensive global efforts
in this area.One of the most important aspects of environmental photocatalysis is the
availability of a material such as titanium dioxide, which is close to being an ideal
photocatalyst in several respects. For example, it is relatively inexpensive, highly stable

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0%

Modulus of Rupture

140MPa

Compressive Strength

680MPa

Poisson’s Ratio

0.27

Fracture Toughness

3.2 Mpa.m-1/2

Shear Modulus

90GPa

Modulus of Elasticity

230GPa

Microhardness (HV0.5)

880


Refractive
Index

Density
(g.cm-3)

Crystal
Structure

Anatase

2.49

3.84

Tetragonal

Rutile

2.903

4.26

Tetragonal

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TiO2

+ Na2CO3

= Na2TiO3

+

CO2

TiO2

+ 2K2S2O7

=

Ti(SO4)2 +

2K2SO4

TiO2 could be reduced to Ti2O3 by C at 870oC or with TiCl4 by H2 at 1400oC.
3TiO2

+ TiCl4 + 2 H2 =

2Ti2O3 +

4HCl

TiO2 was reduced to Ti3O5 by H2

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Organic and petrochemical Technology


Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
visible or UV light. The strong oxidative potential of the positive holes oxidizes water to
create hydroxyl radicals. It can also oxidize oxygen or organic materials directly.
Titanium dioxide is thus added to paints, cements, windows, tiles, or other products for
its sterilizing, deodorizing and anti-fouling properties and is used as a hydrolysis catalyst.
It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also
known as a Graetzel cell). The photocatalytic properties of titanium dioxide were
discovered by Akira Fujishima in 1967 and published in 1972. The process on the surface
of the titanium dioxide was called the Honda-Fujishima effect. Titanium dioxide has
potential for use in energy production as a photocatalyst.[48]
c. Oxygen Sensors
Even in mildly reducing atmospheres titania tends to lose oxygen and become sub
stoichiometric. In this form the material becomes a semiconductor and the electrical
resistivity of the material can be correlated to the oxygen content of the atmosphere to
which it is exposed. Hence titania can be used to sense the amount of oxygen (or
reducing species) present in an atmosphere. [47]
d. Antimicrobial Coatings
The photocatalytic activity of titania results in thin coatings of the material exhibiting self
cleaning and disinfecting properties under exposure to UV radiation. These properties
make the material a candidate for applications such as medical devices, food preparation
surfaces, air conditioning filters, and sanitaryware surfaces. [47]
I.4. Mechanism of titanium oxide photocatalytic reactions
I.4.1. Band structure of semiconductors and band gap energy
If the nucleus of an atom were the sun in our solar system, the electrons revolving
around the nucleus would be the orbiting planets. The path that an electron travels is

conduction band formation processes are complicated, but the principles involved are the
same. For example, it is known that the valence band of titanium oxide is comprised of
the 2p orbital of oxygen (O), while the conduction band is made up of the 3d orbital of
titanium (Ti). In a semiconductor with a large band gap, electrons in the valence band
cannot jump up to the conduction band. However, if energy is applied externally,
electrons in the valence band can rise (this is referred to as "excitation") to the conduction
band. Consequently, as many electron holes (holes left behind by the electrons moving up
to the conduction band) as the number of excited electrons are created in the valence
band. This is equivalent to the movement of electrons from the bonding orbital to the
antibonding orbital. In other words, the photoexcited state of a semiconductor is generally
unstable and can easily break down.
Titanium oxide, on the other hand, remains stable even when it is photoexcited. This is
one of the reasons that titanium oxide makes an excellent photocatalyst. The following
three factors pertaining to the band structure of semiconductors have the greatest effect
on photocatalytic reactions:
(1) Band gap energy
(2) Position of the lowest point in the conduction band
(3) Position of the highest point in the valence band
In photocatalytic reactions, the band gap energy principally determines which light
wavelength is most effective, and the position of the highest point in the valence band is
the main determinant of oxidative decomposing power of photocatalyst. [49]

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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater


.
Fig. I.2 Crystal structures of titanium oxide [49]
I.4.3. Effect of ultraviolet rays in activating titanium oxide
The band gap of anatase type titanium oxide is 3.2 eV, which is equivalent to a
wavelength of 388 nm. The absorption of ultraviolet rays shorter than this wavelength
promotes reactions. These ultraviolet rays are near-ultraviolet rays contained in the
sunlight reaching the earth and emitted by room lights, and they have a very limited range
of weak light throughout the spectrums of sunlight and room lights.
The development of a visible-light photocatalyst may be considered as a solution, but no
substance superior to titanium oxide as a material for photocatalysts has yet been
discovered. One major reason for this is that a semiconductor with a smaller band gap
than that of titanium oxide results in autolysis if it receives light in the presence of water.
In titanium oxide, the absorption of ultraviolet rays with a wavelength of 388 nm or
shorter promotes reactions; however, it is known that 254-nm rays having a greater
energy level, which are used in germicidal lamps, are absorbed by the DNA of living
organisms and form pyrimidine dimers, thereby damaging the DNA.
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
Titanium oxide photocatalyst does not require ultraviolet rays that have an energy level
as high as 254 nm and are hazardous to humans. It also allows reactions to be initiated by
the near-ultraviolet rays with relatively long wavelengths contained in sunlight and
emitted by fluorescent lamps. [49]
Table I.3. Ultraviolet rays in ordinary surroundings [49]

I.4.4. Decomposing power of titanium oxide photocatalyst

Fig. I.4. Oxidation mechanism[49]

Fig.I.5. Reduction mechanism[49]
As reduction tends to occur more easily in organic matter than in water, when the
concentration of organic matter becomes high, the possibility of positive holes being used
in the oxidative reactions with organic matter increases, thus reducing the rate of carrier
recombination. It is believed that, under conditions in which positive holes are
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Research on synthesis of TiO2 and application for Cr6+treatment in wastewater
sufficiently consumed, the process of electrons transferring to oxygen molecules on the
reduction side determines the reaction speed of the entire photocatalytic reaction. In other
words, by enabling easier transfer of electrons to oxygen molecules, the efficiency of
photocatalytic reactions can be improved. This can be achieved by allowing titanium
oxide to carry a metal as a support.[49]
1.4.5. The mechanism of photo-reduction of Cr(VI)
The photo-reduction of Cr(VI) toCr(III) can be achieved via a photocatalytic process with
a simplified mechanism as follows:
TiO2 +hν→ h+ +e− (1)
Cr2O72− +14H+ +6e− → 2Cr3+ +7H2O (2)
2H2O + 4h+ → O2 +4H+ (3)
H2O + h+ → •OH + H+ (4)
•

OH + Organics → ··· → CO2 +H2O (5)


photocatalyst and by introducing scavengers of holes and/or electrons in the solution.[60]
I.5. Review of the used precursors in this thesis
I.5.1. TiCl4
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an
important intermediate in the production of titanium metal and the pigment titanium
dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile. Upon
contact with humid air, it forms spectacular opaque clouds of titanium dioxide (TiO2) and
hydrogen chloride (HCl).
a. Properties and structure
Table I.4. The physical properties of TiCl4
Physical Properties
Molecular formula

TiCl4

Molar mass

189.71 g/mol

Appearance

Colourless fuming liquid

Density

1.726 g/cm3

Melting point

-24.8 °C