Study on the development of photocatalytic membrane reactor for the treatment of organics in water

In order to develop a purifier for drinking water with small contaminant concentration at household scale, the study approached a PMR system using photocatalyst activated by visible ligh

ỨNG DỤNG GIẢI PHÁP SỐ TRONG VIỆC XÁC ĐỊNH

BỘ THÔNG SỐ LÀM VIỆC PHÙ HỢP CHO HỆ THỐNG

VÍT TẢI CẤP LIỆU DẠNG RỜI

LUẬN VĂN THẠC SĨ KHOA HỌC

KỸ THUẬT CƠ ĐIỆN TỬ

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

LUẬN VĂN THẠC SĨ KHOA HỌC

KỸ THUẬT CƠ ĐIỆN TỬ

NGƯỜI HƯỚNG DẪN KHOA HỌC :

TS Bùi Tuấn Anh

Hà Nội – Năm 2019

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

Độc lập – Tự do – Hạnh phúc

BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ

Họ và tên tác giả luận văn: TRẦN VĂN THIỆN

Đề tài luận văn: ứng dụng giải pháp số trong việc xác định bộ thông

số làm việc phù hợp cho hệ thống vít tải cấp liệu dạng rời

Chuyên ngành: Kỹ thuật Cơ Điện Tử

Mã số HV: CA170370

Tác giả, Người hướng dẫn khoa học và Hội đồng chấm luận văn xác nhận tác giả đã sửa chữa, bổ sung luận văn theo biên bản họp Hội đồng ngày với các nội dung sau:

- Đã sửa các lỗi chính tả theo góp ý của hội đồng

- Đã trích dẫn tài liệu các công thức chương 2

- Trang 10 đã phân tích thêm ưu, nhược điểm của từng loại vít

- Viết tại lời mở đầu, tình hình vít tải Việt Nam, kết luận chung

- Chỉnh sửa sai lệch đơn vị

Ngày tháng năm

CHỦ TỊCH HỘI ĐỒNG

DECLARATION IN LIEU OF OATH

By

“Duong Nguyen Thi Thuy”

This is to confirm my Master Thesis was independently composed/authored by myself using solely the referred sources and support

I additionally assert that this Thesis has not been part of another examination process

in Germany Dr Ly Bich Thuy is a person always providing me valuable comments and feedback with her high responsibility and expertise on my work from time to time

to help me carry out the scientific work with high efficiency

I am also grateful to faculty of School of the Environmental Science and Technology

- Hanoi University of Science and Technology who enthusiastically guided, taught and helped me during the process of studying, researching and completing this thesis

I also would like to thank Prof Dr Le Minh Thang and ROHAN Project - Rostock - Hanoi DAAD SDG Graduate School and all colleagues at Technical Chemistry Group, University of Rostock for supporting and creating favourable conditions for

me to attend the Master Research Exchange Program at the Institute for Chemistry, University Rostock, Germany

Finally, my great gratitude is for my family and my friends who always believed and encouraged me so much during my years of study

This achievement would not have been possible without them

Author,

Duong Nguyen Thi Thuy

TABLE OF CONTENTS

DECLARATION IN LIEU OF OATH 1

ACKNOWLEDGEMENT 2

ABBREVIATION 3

TABLE OF CONTENTS 4

TABLE OF FIGURES 6

TABLE OF TABLES 7

INTRODUCTION 8

Chapter 1 Literature Review 10

1 Introduction about photocatalyst 10

1.1 Overview of photocatalyst 10

1.2 Introduction about titanium dioxide 11

1.3 Gold nanoparticle supported on titanium dioxide (Au/TiO2) 14

2 Photocatalytic membrane reactor 17

2.1 Definition 17

2.2 Research works on photocatalytic membrane reactor 18

3 Researches and application of Au/TiO2 in water treatment 23

4 Organic pollutant status 24

Chapter 2 Experimental section 26

1 Experimental overview 26

1.1 Experiment process 26

1.2 Chemical and materials 28

2 Methods 29

2.1 Synthesis of Au/TiO2 photocatalyst 29

2.1.1 Synthesis method 29

2.1.2 Au/TiO2 characteristic analysis method 30

2.2 Coating photocatalyst into membrane surface 31

2.3 Design and construction of a PMR reactor 33

2.4 Evaluation of the system in removal of organic matter from feed water 34

2.4.1 Evaluation of batch reaction 34

2.4.2 Evaluation of suspended reaction 35

2.4.3 Evaluation of immobilized system 36

Chapter 3 Results and Discussion 37

1 Characteristics of photocatalyst: 37

2 Coating Au/TiO2 on membrane surface 38

3 Evaluation of photocatalyst in batch reaction 39

4 Evaluation of suspended PMR 40

5 Evaluation of immobilized PMR 43

CONCLUSIONS 46

RECOMMENDATION 47

REFERENCES 48

ANNEX 51

TABLE OF FIGURES

Figure 1 Crystal structure of titanium dioxide phases of rutile, brookite and anatase

(Janzeer, 2013) 12

Figure 2 Mechanism of photocatalytic reaction (Xu, Rangaiah, & Zhao, 2014) 13

Figure 3 Comparison between TiO2 and Au/TiO2 reaction 16

Figure 4 The number of publications on the topic of PMR and PMR for water treatment (Zheng et al., 2015) 19

Figure 5 Annual average BOD5 content in major rivers in Vietnam (2005-2009) 25

Figure 6 Process of study experiment 28

Figure 7 Diagram of Au/TiO2 synthesis process 30

Figure 8 Spraying Au nanoparticles on membrane surface 32

Figure 9 Schematic diagram of a lab-scale PMR system 33

Figure 10 Schematic diagram of (a) tangential (cross) flow filtration and (b) dead– end filtration (El–Safty & Hoa, 2012) 36

Figure 11 UV-VIS absorbance of Au/TiO2 photocatalyst 37

Figure 12 Performance of Photocatalyst in different concentration 40

Figure 13 Performance of different membrane MW at different amount of Au/TiO2 41

Figure 14 Performance during the reaction time 42

Figure 15 Performance of different filtration mode in continuous flow 43

TABLE OF TABLES

Table 1 Classification of PMRs configuration 19

Table 2 Advantages and disadvantages of different configurations of PMR 21

Table 3 Experiments description 26

Table 4 ICP result of Au/TiO2 sample 38

Table 5 Mass of Au/TiO2 coated in membrane surface 38

Table 6 Solutions used to avoid photo catalysts peeling off the membrane 44

INTRODUCTION

Water is a unique natural resource playing an essential role in maintaining global ecosystems and quality of life It is estimated that 97% water on earth is saline while the freshwater makes up only 3% but it is unevenly distributed over the world in different forms of rivers, lakes, streams or glaciers, etc Despite very small proportion, however, many freshwater bodies nowadays face increasingly serious pollution caused by human activities in various regions Additionally, freshwater scarcity has happened commonly over the decades The causes of water shortage are related to climate change, rapid growth of population, increase in water demand for economic sectors ineffective water usage, and mostly quality degradation In the modern era, water pollution and contamination are regarded as one of the most substantial and worrisome problems that demands an immediate and practical solution According to United Nation World Water Development (UNWWD) report, around 748 million people do not have access to pure drinking water around the world, and the water demand for industrial manufacturing will increase to 400 percent

by 2050

Under this context, the importance of development water treatment technologies is largely realized by researchers Among wide array of water purification methods, photocatalysis has emerged as very promising solution because it requires low cost while showing great efficiency in pollutant removal without dangerous by-products The photocatalyst-based method itself has demonstrated its effectiveness in treating wide range of pollutants including organic and inorganic compounds from aqueous

or gas phase systems clean-up Besides, in order to prevent photocatalyst from being mixed with output flow, a separation process is required Membrane technology is well-known as an effective and mature method for a wide range of separation applications such as removal of small suspended solids or micro-organism Thus, it

is reasonable to consider coupling the membrane process with photocatalysis to collect and reuse photocatalysts in the photocatalytic system to create Photocatalysis Membrane Reactor (PMR) This technique has paved the way for researchers to

discover and achieve an applicable environmental-friendly solution to address water pollution issues

In order to develop a purifier for drinking water with small contaminant concentration

at household scale, the study approached a PMR system using photocatalyst activated

by visible light In which, the Au/TiO2 photocatalyst is selected to be applied TiO2

is known as high and stable activity catalyst besides its large commercial potential and non-toxic characteristics However, TiO2 is only effective under UV light due to its large band gap (~3.2eV) Deposition of Au nano-particles on surface of the photocatalyst for purpose of take advantage of Surface Plasmon Resonance (SPR) is chosen to improve the effect of the photocatalytic system under visible irradiation

This thesis aims to develop a simple configuration of an equipment of photocatalytic membrane for organic pollutant degradation in water In order to achieve the main objective, the following activities were carried out:

▪ Synthesis of Au/TiO2 photocatalyst

▪ Preparation of photocatalyst coated on regenerated cellulose membrane

▪ Evaluation of two PRM configuration in removal of organic matter (Methylene blue) from feed water: suspended system and immobilized system

In the purpose of develop a real device which consists of a combined PMR/pure MR reactor for purification of drinking water, this study conducted laboratory experiments in degradation of Methylene Blue The research synthesized and evaluated photocatalyst of AuTiO2 under visible light to approach application of photocatalysis to drinking water treatment in practical condition

Entire process of experiments was carried out at the laboratory at Interdisciplinary Faculty Life, Light and Matter, Institute of Chemistry, University of Rostock

Chapter 1 Literature Review

1 Introduction about photocatalyst

1.1 Overview of photocatalyst

1.1.1 Definition of photocatalyst

As part of catalysis - and more precisely of heterogeneous catalysis - heterogeneous photocatalysis is an area of chemistry impacting many reactions as varied as oxidation reactions, dehydrogenation reactions, metal deposition, hydrogen transfers, etc Heterogeneous photocatalysis can be described as the acceleration of photoreaction

in the presence of a catalyst Basically, photocatalysis differentiates from conventional catalysis by the activation of the catalytic solid because it is activated

by adsorbing a photon and is capable of accelerating a reaction without being consumed This photonic activation thus requires the use of a semiconductor material

as catalyst, provided that the radiation wavelengths are greater than its band gap, which corresponds to the energy gap between both conduct ion and valence bands of the semiconductor Generally, the photocatalysis discipline exists through the ability

of a material, usually is semiconductor, to simultaneously interact with light and reactants, through both absorption and adsorption phenomena, respectively There is

a wide array of photocatalyst such as TiO2 (3,2 eV); SrTiO3 (3,4 eV), Fe2O3 (2,2 eV); CdS (2,5 eV); WO3 (2,8 eV); ZnS (3,6 eV); FeTiO3 (2,8 eV); ZrO2 (5 eV); V2O5 (2,8 eV);

Nb2O5 (3,4 eV); SnO2 (3,5 eV)…

1.1.2 Applications of photocatalyst

Photocatalyst has been proven as an ideal method which can be used for various purposes such as degradation of different organic pollutants in wastewater, purification of air, and antibacterial activity When compared with other methods, photocatalysis is rapidly growing and gaining more attention from the researchers due to its several advantages such as low cost, non-toxicity and attractive efficiency (Saravanan, Gracia, & Stephen, 2017)

1.2 Introduction about titanium dioxide

1.2.1 History of discovery and research about titanium dioxide

Among different kind of photocatalyst, TiO2 has been most common and widely studied Photocatalyst has found its way since in 1972 by Fujishima and Honda when they had discovered Titanium Dioxide (TiO2) During that time, the purpose of TiO2

was for water splitting into hydrogen and oxygen in a photo-5 electrochemical cell This discovery has propagated researchers to explore the usage of TiO2 in many areas especially in photocatalysis One of the earlier works on photocatalysis for wastewater treatment was conducted by Bahnemann in 1991 using TiO2 suspensions They reported on the influence of light intensity, temperature and pH on the degradation rate of halogenated hydrocarbons using TiO2 photocatalyst suspensions They have concluded that this technology has a bright potential for wastewater treatment applications and detailed study are needed to further develop this technology Inspired from this, a lot of research works have been conducted by using TiO2 as photocatalyst for many applications because of its characters: chemical stability, non-toxicity, low cost, strong oxidizing abilities for the decomposition of organic pollutants, superhydrophilicity, long durability, nontoxicity, and transparency to visible light (Nakata & Fujishima, 2012)

1.2.2 Characteristics of titanium oxide

Physical characteristics of TiO2:

▪ White powder, turn into yellow at high temperature

▪ Stiffness, melting point is 18700C

▪ M = 79.88 g/mol

▪ D = 4.13 - 4.25 g/cm3

TiO2 have many crystal structures including three main structures which are anatase, brookite and rutile

Figure 1 Crystal structure of titanium dioxide phases of rutile, brookite and

anatase (Janzeer, 2013)

1.2.3 Mechanism of photocatalytic reaction of titanium dioxide

The most commonly assumed photodegradation mechanism of TiO2 is based on Langmuir-Hinshelwood kinetic model:

also active toward organic pollutants Figure 2 visually illustrates the mechanism of the photocatalytic reaction

Figure 2 Mechanism of photocatalytic reaction (Xu, Rangaiah, & Zhao, 2014)

1.2.4 Researches on titanium dioxide application

There has been much research effort to find effective photocatalysts as well as a more efficient reactor design The major drawback of TiO2 as photocatalyst is the wide band gap The band gap of this material limits the absorption of only small portion of the solar spectrum (UV region) One possible development for photocatalytic water

purification is to utilize sunlight as the light source However, Yu et al (2011) studied

the degradation of bacteria and found that TiO2 is a good photocatalyst but in the condition of lacking the visible light utilization that cause low quantum yield Absorption in normal sunlight is limited due to the large and wide bandgap (3.2 eV) Larger band gap would require higher energy to activate the photocatalyst while TiO2

only offer the absorption wavelength of less than 400 nm (Yahya, 2018) On the other hand, some researches revealed that the band gap of TiO2 corresponds to the

ultraviolet wavelength, which is only a small fraction of 5% of solar radiation (Xu et

al., 2014) The method to improve the photocatalytic activity of TiO2 in the visible region and to reduce high recombination rate of photogenerated electron-hole pairs are the focus of the recent TiO2 photocatalysis research Several approaches for TiO2

modification have been proposed as follow:

▪ metal-ion implanted TiO2;

▪ non-metal doped-TiO2;

▪ composites of TiO2 with semiconductor having lower band gap energy Particularly, noble metal like Gold (Au) have been attracting more attention because they have a wide range of absorption in the visible region and can act as electron traps Further information on gold titanium dioxide is illustrated in next section (Kang

▪ Large surface area to volume ratio as compared to the bulk equivalents;

▪ Large surface energies;

▪ Optical properties such as color;

▪ Plasmon excitation;

▪ Quantum confinement;

▪ Short range ordering;

▪ Increased number of kinks;

▪ A large number of coordination sites such as corners and edges, consequently specific chemical properties and the ability to store excess electrons

Many of the unique properties of metallic nanoparticles are determined not only by their finite size but also by their shape, defined by the crystallographic orientation of the surface facets (Wang., 2000) Size and shape of gold nanoparticles are extremely important features as they substantially affect the physical and chemical properties of

a particular composition of nanomaterials By using different types of reducing agents

or by changing temperature, it is possible to synthesize nanoparticles with a wide variety in shape

1.3.2 Gold nanoparticle supported on titanium dioxide (Au/TiO 2 )

An impressive application of gold nanoparticles is catalysis In 1989, Haruta and his co-workers reported that gold nanoparticles supported on CO3O4, Fe2O3 or TiO2 were highly active catalysts for CO and H2 oxidation, NO reduction, the water-gas shift reaction, CO2 reduction and the oxidation of organic compound (Seghetti, 2016) Additionally, in the recent years, gold nanoparticles have been tested as dopants or surface modifiers to increase the photocatalytic activities of common semiconductors such as TiO2

An important basis for determining adsorption capacity of material onto planar metal (typically gold or silver) surfaces or onto the surface of metal nanoparticles is Surface Plasmon Resonance (SPR) The resonance lies at visible frequencies for the noble metal like Au In particular, metal particle of Au having an intense surface plasmon resonance (SPR) peak can act as a receptor for light trapping, resulting in photoexcitation of the SPR peak to form a locally enhanced electric field in the proximity of metal nanoparticles On the other words, SPR improves the solar-energy-conversion efficiency by: (1) extending light absorption to longer wavelengths, and (2) increasing light scattering The frequency and cross-section of SPR absorption and scattering is dependent on the metal composition, nanoparticle size and shape, dielectric properties of the surrounding medium/substrate and presence of inter-particle interactions (El–Safty & Hoa, 2012)

As mentioned above, the properties of enhancement of long wavelength light enables the improvement of absorption of solar light in the semiconductor throughout the visible to near-infrared light range This process concentrates the incident photon energy in plasmon oscillations The latter process originates from the large scattering cross-section associated with SPR Metallic nanoparticles will scatter incident light and locally amplify the electromagnetic field when placed on the surface or inside a solar material/device This results in an enhancement of the effective absorption cross

section and an increase in the effective optical path length inside the semiconductor Therefore, gold nanoparticles can compensate the disadvantage of TiO2 of weak optical response on visible spectral range which accounts for up to 45% sun’s energy

to facilitate the practical applications of the catalyst material

Moreover, because the Fermi levels of these noble metals are lower than that of TiO2, photoexcited electrons can be transferred from the conduction band of TiO2 to metal particles deposited on the surface of TiO2, while photogenerated holes in the valence band remain on TiO2 This greatly reduces the possibility of electron-hole recombination, resulting in efficient separation and higher photocatalytic activity (Seghetti, 2016)

Figure 3 Comparison between TiO 2 and Au/TiO 2 reaction (Seghetti, 2016)

Coupling TiO2 and gold nanoparticles, therefore, enables leverage the applicability and performance of photocatalysis in decontamination

Research on Au loading on generated photocatalyst has been widely studying and most of them conducted the experiment with range of 0 to 1wt% Au content Of

which, Boccuzzi et al., 2001 synthesized Au/TiO2 with the Au loading of 1wt% by

the deposition–precipitation method at pH 7 and at 343 K; Yu et al., 2017, the atomic

ratio of Au/TiO2 was measured by ICP-MS to be 0.36% and 0.51%, for the specimens

of Branched-Au-NW and Branched-NW-Au, respectively In 2018, in Ngo Anh Binh’s Master thesis, he also conducted a research on gold nano-particle with the amount of Au was 0.62% (Binh, 2018) In the research on synthesis of TiO2/Au

Nanocomposites via Sol-Gel Process for Photooxidation of Methanol, Ismail et al

showed that smaller gold particles induce more negative Quasi-Fermi level shift than the bigger particles The negative shift in the Quasi-Fermi level is an indication of better charge separation and more reductive power for the photocatalyst Thus, the catalyst with smaller gold nanoparticles is more photocatalytically active than that with larger gold particle And also Au nanoparticles possess the property of storing electrons in a quantized fashion (Ismail & Institut, 2009)

2 Photocatalytic membrane reactor

2.1 Definition

In field of water treatment, hybrid processes combining membrane separation and heterogeneous photocatalysis represent an exciting technology because each technique complements the advantages and overcomes the challenges of the other This combination gives plants set-up named Photocatalytic Membrane Reactors (PMRs)

Photocatalysts are mostly used as suspended powders in the photocatalytic reactors, since the utilization of slurries usually presents higher efficiency with respect to the use of immobilized films However, it is difficult to separate the photocatalysts in slurries from the treated water for reuse This has been identified as the major obstacle among the current engineering limitations of photocatalytic processes Thus, a separation/recovery step is required to realize the reuse of photocatalysts

Membrane technology was first applied to water treatment processes in 1960s Since then, it has been widely employed for the physical separation process of pollutants in water treatment plants The most frequently used membrane technologies in the water treatment field are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), in descending order of membrane pore sizes It has been

widely proved in practice that the membrane separation process can remove the majority of the suspended solids, colloids and microorganisms effectively In addition, it requires smaller floor space and sustains a more stable effluent quality than traditional water treatment technologies, attracting an increasing number of industrial applications To date, it is estimated that approximately 60 million m3 of water is purified by membrane process every day Generally, membrane technology

is an effective and mature method for a wide range of separation applications Thus,

it is reasonable to consider coupling the membrane process with heterogeneous photocatalytic membrane reactors (PMRs) (Zheng, Wang, Chen, Wang, & Cheng, 2015)

2.2 Research works on photocatalytic membrane reactor

Taking into consideration that PMRs have developed rapidly in the last few years, many novel configurations and new applications have been described in the literature

According to Zheng et al (2015), the number of researches on PMR has increased 4

times within 20 years from 1996 to 2016, of which about 70% of those are studies on PMR application on water treatment

Figure 4 The number of publications on the topic of PMR and PMR for water

treatment (Zheng et al., 2015)

PMRs have two configurations are PMRs with immobilized photocatalyst and PMRs with suspended photocatalyst It can be classified as follow:

Table 1 Classification of PMRs configuration

The membrane itself is manufactured with a pure photocatalyst

Photocatalyst blended with the membrane

The photocatalysts are usually fabricated

immobilization step Photocatalyst coated on

membrane separation take place in separate apparatuses

Integrative

membrane separation processes are merged in one apparatus

2.2.1 Immobilized configuration

In PMRs with immobilized photocatalysts, the membrane separation process and heterogeneous photocatalysis take place in the same vessel Thereby most of the immobilized PMR systems consist of a feed tank and only one reaction tank The light sources are usually placed above the membrane module for UV/visible light

irradiation The PMR system could be operated in either dead-end mode or cross flow mode (Zheng et al., 2015)

In 2018, Syafei et al investigated the performance of ultrafiltration (UF) by

membranes coated with titanium dioxide (TiO2) photocatalyst under ultraviolet (UV) illumination in removing natural organic matter (NOM) and possibly in reducing membrane fouling Experiments were carried out using heat-resistant ceramic disc

UF membranes and humic acids as model substances representing naturally occurring organic matter Membrane sizes of 1, 15, and 50 kDa were used to examine the effects

of coating under ultraviolet irradiation A commercial humic solution was subjected

to UF fractionation (batch process); gel filtration chromatography was applied to study the effects of molecular weight distribution of NOM on UF membrane fouling When compared to naked membranes, UV254 (ultraviolet light of λ = 254 nm) illumination of TiO2-coated membranes exhibits more flux decline with similar effluent quality Although the UF membrane is able to remove a significant amount

of humic materials, the incorporated photocatalysis results in poor performance in terms of permeate flux The TiO2-coated membrane under UV254 irradiation alters the molecular weight (MW) distribution of humic materials, reducing them to <1 kDa, which is smaller than the smallest (1-kDa) membrane in this study Thus, TiO2-coated membranes under UV254 irradiation do not perform any better in removing natural organic matter and reducing membrane fouling (Syafei, Lin, & Wu, 2008)

Photocatalyst immobilization could improve the performance of purification technology based on coupling photocatalysis and membrane filtration in view of real applications In such a system, the membrane has the simultaneous task of supporting the photocatalyst as well as acting as a selective barrier for the species to be degraded (Molinari, Lavorato, & Argurio, 2017)

However, some drawbacks of PMs are: (i) moderate loss of photoactivity also related

to the low photocatalyst availability to irradiation; (ii) necessity to irradiate the surface of the membrane, resulting in technical difficulties and in possible membrane photodegradation; (iii) restricted processing capacities owing to mass transfer

limitations and (iv) unsatisfactory system lifetime owing to the possible catalyst deactivation and wash out Then it is fundamental to manufacture systems with

opportune porosity and effective dispersion of the catalyst particles (Molinari et al.,

2017)

2.2.2 Suspended configuration

A different photocatalyst, ZnO nanoparticles self-synthesized via precipitation

method, was tested by Hairom et al in a suspended PMR for industrial dye

wastewater treatment Both NF and ultrafiltration (UF) membranes (Polypiperazine amide NF membrane Trisep TS40 and Polyamide UF membrane Trisep GMSP, from

GE Osmonics, USA) were tested and their performances in obtaining cleaner water production and ZnO nanoparticles retaining were compared The results evidenced that an industrial wastewater from printing presses was successfully treated in the proposed system Operating pH played a significant role in controlling the photocatalytic efficiency and fouling behavior in the PMR system pH of 11 and 0.1

g L-1 of ZnO loading was the optimum operational condition A severe fouling was observed at pH of 2, 7 and 8 because of the weak electrostatic repulsion between the membrane surface The NF membrane gave the best performance in terms of color removal (100%), chemical oxygen demand (92%), turbidity reduction (100%) and total suspended solid rejection (100%) A 65% normalized flux reduction was obtained, because of photocatalyst accumulation on membrane surface Worse performance was obtained by using the UF membrane owing to the permeation of nanosized ZnO and dye molecules across the membrane pores (Nur Hanis Hayati Hairom, Mohammad, Ng, & Kadhum, 2013)

Table 2 Advantages and disadvantages of different configurations of PMR

PMR with Immobilized Photocatalyst

• Membrane damage caused

by UV light and generated hydroxyl radicals could be avoided

Disadvantages

• Lower photocatalytic efficiency due to lower effective surface area of the photocatalyst;

• UV light and generated hydroxyl

polymer membranes;

photocatalyst loading according

to the composition of waste water

• Higher operating cost and requires additional process

to separate photocatalyst;

• Membrane fouling caused

by photocatalyst and/or pollutants

3 Researches and application of Au/TiO 2 in water treatment

The Au/TiO2 nanocomposite has several applications in the photocatalytic removal and decomposition of wastewater pollutant Most of these applications are related to their high photocatalytic activities in the removal of azo dyes, such as Methylene Blue

(MeB) (Zhang et al., 2012), Methyl Orange (MeO) (Tian et al., 2008; Oros Ruiz et

al., 2012), Orange 16 (Hsiao et al., 2011), Acid Red 1 (Mrowetz et al., 2007), Acid

Red 88 (Kumar et al., 2008), Acid Red G (Zhang et al., 2012), Sulforhodamine-B (Zhu et al., 2009), and Tartazine (Rupa et al., 2009) under visible or UV irradiation

By integrating Au nanoparticles with TiO2, it has been observed a 9- fold improvement in the photocatalytic decomposition rate of MeO under visible

illumination (Hou et al., 2011) Li et al applied visible light to complete degradation

of 12 mg/L MeB (165 mL solution) in 1 h with 0.5%Au/TiO2, and 96% degradation with 0.5%Au/TiO2 (Li and Li, 2001) Li et al (2009) researched on possible reaction

mechanism of Au/TiO2 for degradation of azo dyes under UV light irradiation Under

UV light irradiation, TiO2 can be photoexcited and it produces h+ and e- pair The recombination rate of e- and h+ is restrained as the electrons migrate to Au So, the superoxide and hydroxyl radical production are enhanced and the decreasing rate of

chemical oxygen demand (COD) value is improved (Li et al., 2009) Laser flash

photolysis study of Au-capped TiO2 confirmed a 40% enhancement in the hole transfer efficiency (Dawson and Kamat, 2001) On the other hand, under visible light irradiation, the possible reaction mechanism of azo dye degradation on Au–TiO2 is that Au is photoexcited due to the surface plasmon resonance effect Then, photogenerated electrons are injected into O2 adsorbed on TiO2 The local work function of titania near to interfacial oxygen adsorption site enhances the reduction

of dioxygen to yield superoxide radicals (Tian et al., 2008) Therefore, the production

of superoxide and hydroxyl radicals in the presence of O2 accelerates, which

improves the decreasing rate of COD value (Hoffmann et al., 1995; Li et al., 2009)

Phenols are the most widely used bio reluctant probe molecule for aromatic organic pollutants present in water So, there is interest to find the photocatalytic activity of

Au-NPs deposited on TiO2 for decomposition of phenols Wongwisate et al indicated

that the presence of 0.05% and 0.1% Au/TiO2 rapidly degraded hydroquinone to

hydroxy hydroquinone as intermediates from 4-CP degradation (Ayati et al., 2014)

Because of high specific surface area of photocatalyst, intermediate adsorption on the photocatalyst surface increases

TiO2 modified with very small amounts of Au-NPs have been applied to the photocatalytic degradation of an important pollutant, methyl tert-butyl ether (MTBE),

in the dilute aqueous solution using a fixed bed flow-through photocatalytic reactor

(Orlov et al., 2006) The reported results have shown that Au NPs enhanced the photocatalytic performance of titania for the degradation of MTBE Orlov et al

indicated that at optimum Au loading, the kinetic rate constant was three times greater

than that of unmodified titania (Orlov et al., 2007) Also, the photocatalytic

degradation of L-asparagine and L-glutamic acid over Au/TiO2 and TiO2 catalysts was investigated However, cyanide was formed in the presence and absence of gold particles on the TiO2 surface and ammonium was observed in the reaction In the presence of Au/TiO2, the cyanide leads to leaching of gold in the form of [Au(CN)2]-

which can be detected in solution (Dolamic and Burgi, 2011)

The high photocatalytic efficiency for decomposition of 3,4-dichlorophenylurea, as a didemethylated product of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] (Tixier

et al., 2000), using Au/TiO2 was obtained under solar light The use of Au/TiO2 gave 1.6-fold faster degradation rate compared to unmodified TiO2 (Chusaksri et al.,

2011) Likewise, these catalysts promoted the 1,4-dioxane photocatalytic degradation which is almost two times higher than a commercial TiO2

4 Organic pollutant status

Organic pollution occurs when large quantities of organic compounds, which act as substrates for microorganisms, are released into water sources During the decomposition process the dissolved oxygen in the receiving water may be used up

at a greater rate than it can be replenished, causing oxygen depletion and having severe consequences for the stream biota Organic effluents also frequently contain

large quantities of suspended solids which reduce the light available to photosynthetic

organisms and, on settling out, alter the characteristics of the riverbed, rendering it an

unsuitable habitat for many invertebrates Toxic ammonia is often present

Organic pollutants consist of proteins, carbohydrates, fats and nucleic acids in a

multiplicity of combinations

Nowadays, the rapid social-economic development brings about negative impacts on

environment and human health, of which water pollution is one of most considerable

problem, specially, surface water and ground water pollution A few year ago, in the

mountain part of Vietnam, people could drink ground water or water form stream

directly, but now it is not safe to do the same thing, especially in big city like

Hanoi-the capital of country, or Ho Chi Minh city In Vietnam, almost domestic water is

supplied from river and here is one of surface water treatment process There are a

lot of treatment stage, however, organic matter could still in the water

Figure 5 Annual average BOD 5 content in major rivers in Vietnam (2005-2009)

Organic dyes are one of the chemicals that contaminate aquatic habitats that seriously

contaminate water sources and affect human health if not handled properly

Therefore, in this study, methylene blue, representing organic dye pollutant, was

selected as the input wastewater with concentration at 6 ppm according to pre-experiments

Chapter 2 Experimental section

For the suspended configuration, Au/TiO2 powder was used in both filtration mechanism: dead-end Different molecular weight of membrane, reaction time, amount of photocatalyst are tested to determine best experiment conditions for the research

For the immobilized configuration, photocatalytic membrane was prepared by using air-brushing technique, then applying both dead-end and cross-flow mode in the system

Table 3 Experiments description

Batch reaction

• Photocatalyst: TiO 2 , Au/TiO 2

• Catalyst concentration: 0.2, 0.5, 1.25, 2.5 mg/mL

• MB concentration: around 6 ppm

• Stirring rate: 300 r/min

• Reaction time: 30 min

• Radiation source: UV, Vis

Evaluation of performance of TiO 2 and Au/TiO 2

under UV, Vis and non-irradiation

Experiment Condition Purpose

• Membrane MW: 10 KDa,

30 KDa, 100 KDa

• Filtration mode: dead-end

• Stirring rate: 500 r/min,

700 r/min

To investigate optimal:

• Catalyst concentration

Figure 6 Process of study experiment

1.2 Chemical and materials

▪ TiO2 P25 ≥ 99.5%; CAS 13463 – 67 – 7; Evonik, Germany

▪ HAuCl4.3H2O; CAS 16961 – 25 – 1; Sigma Aldrich, United States

▪ NaOH 99.1%; CAS 1310 – 73 – 2; Fisher Chemical, Germany

▪ Methylene Blue powder; CAS 61 – 73 - 4; S3 Chemicals, Germany

▪ Nafion 5 wt%, Sigma- Aldrich, USA

▪ Regenerated Cellulose ultrafiltration membrane 10 Kda, 30 Kda, 100 Kda; Millipore Corporation, USA

Synthesis of Au/TiO 2 photocatalyst and Assessment of its characteristics

Evaluation of PMR with

suspended photocatalyst

Evaluation of PMR with immobilized photocatalyst

Preparation of photocatalytic membrane

Evaluation of dead-end and continuous-flow filtration mode in PMR