Author, Trang 6 3 ABBREVIATION ABS Absorbance PMRs Photocatalytic membrane reactors MB Methylene blue XRD X-Ray Diffraction RC Regenerated Cellulose ICP-MS Inductively coupled plasma m
Literature Review
Introduction about 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);
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)
1.2.1 History of discovery and research about titanium dioxi de
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)
▪ White powder, turn into yellow at high temperatur e
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:
TiO2 + hv → h + + e - h + + e - → heat h + + (H2O/OH•)s ↔ OH•(aq) e - + O2 ↔ O2 -
Reactantsol+ S ↔ Reactant OH• + Reactant →Products
When electron in the balance band of the semiconductor absorbs a photon with an energy greater than the band gap (ΔE) of the semiconductor, th electron becomes at excited and jumps to the conduction band, leaving a positively charged hole in the valence band Besides the potentiality to recombin with the electron, the positively es charged hole can oxidize water molecules to form hyper-reactive hydroxyl free radicals (OH•) The resulting hydroxyl radicals are the main agent that attack the chemical pollutant molecules or microorganism cells to purify water The excited electron can react with dissolved oxygen molecule to form oxygen radical, which is
13 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 reduce high recombination rate of photogenerated electron-hole pairs to are the focus of the recent TiO2 photocatalysis research Several approaches for TiO2 modification have been proposed as follow:
▪ 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 et al., 2019)
1.3 Gold nanoparticle supported on titanium dioxide (Au/TiO2)
The term metal nanoparticle is used to describe nano-sized metals with dimensions (length, width or thickness) within the size range of 1 - 100 nm; they have different physical and chemical properties from bulk metals The main characteristics of metal nanoparticles are:
▪ Large surface area to volume ratio as compared to the bulk equivalents;
▪ Optical properties such as color;
▪ 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
15 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/TiO2)
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 activit of common semiconductors ies such as TiO2
Surface Plasmon Resonance (SPR) is crucial for assessing the adsorption capacity of materials on planar metal surfaces, particularly gold and silver, as well as on metal nanoparticles Noble metals like gold exhibit SPR at visible frequencies, with gold nanoparticles displaying a pronounced SPR peak that enhances light trapping This enhancement leads to photoexcitation of the SPR peak, generating a locally intensified electric field near the nanoparticles Consequently, SPR boosts solar energy conversion efficiency by extending light absorption to longer wavelengths and increasing light scattering The characteristics of SPR absorption and scattering are influenced by factors such as metal composition, nanoparticle size and shape, the dielectric properties of the surrounding medium, and inter-particle interactions.
The enhancement properties of long wavelength light significantly improve solar light absorption in semiconductors across the visible to near-infrared spectrum This enhancement occurs through the concentration of photon energy in plasmon oscillations, which arise from the substantial scattering cross-section linked to surface plasmon resonance (SPR) When metallic nanoparticles are incorporated on the surface or within a solar material or device, they scatter incident light and locally amplify the electromagnetic field, leading to an increased effective absorption cross-section.
16 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 gold (Au) loading in photocatalysts has been extensively conducted, primarily focusing on Au content ranging from 0 to 1 wt% Notably, Boccuzzi et al (2001) synthesized Au/TiO2 with a 1 wt% Au loading using the deposition precipitation method at pH 7 and a temperature of 343 K Additionally, Yu et al (2017) measured the atomic ratio of Au/TiO2 specimens, finding values of 0.36% and 0.51% through ICP-MS analysis.
Photocatalytic membrane reactor
Hybrid processes that integrate membrane separation and heterogeneous photocatalysis are revolutionizing water treatment by leveraging the strengths of both techniques while addressing their limitations This innovative approach leads to the development of Photocatalytic Membrane Reactors (PMRs), enhancing the efficiency and effectiveness of water purification systems.
Photocatalysts are primarily utilized as suspended powders in photocatalytic reactors, as slurries tend to offer greater efficiency than immobilized films However, a significant challenge arises in separating photocatalysts from treated water in slurries, hindering their reuse This separation and recovery process is a key engineering limitation in current photocatalytic applications.
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
Membrane separation processes have been proven effective in removing suspended solids, colloids, and microorganisms, making them a preferred choice for water treatment These processes require less floor space and provide more stable effluent quality compared to traditional technologies, leading to their growing adoption in various industrial applications Currently, it is estimated that membrane technology purifies around 60 million cubic meters of water daily As a mature and effective method for separation, integrating membrane processes with heterogeneous photocatalytic membrane reactors (PMRs) presents a promising advancement in water purification.
2.2 Research works on photocatalytic membrane reactor
Here is a rewritten paragraph that summarizes the content:The rapid development of Photocatalytic Membranes and Reactors (PMRs) in recent years has led to a surge in innovative configurations and applications, as evidenced by the growing body of literature Notably, research on PMRs has seen a four-fold increase over the past two decades, with approximately 70% of these studies focused on their application in water treatment, according to a 2015 study by Zheng et al.
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 and then coated/blended with a commercial membrane via an immobilization step
Photocatalyst coated on the membrane
The photocatalytic reaction and membrane separation take place in separate apparatuses
The photocatalytic reaction and membrane separation processes are merged in one apparatus
In photocatalytic membrane reactors (PMRs) with immobilized photocatalysts, both the membrane separation and heterogeneous photocatalysis occur within a single vessel Consequently, most immobilized PMR systems are designed with a feed tank and a single reaction tank Typically, UV or visible light sources are positioned above the membrane module to facilitate the process.
20 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 conducted a study on the effectiveness of titanium dioxide (TiO2) coated membranes in ultrafiltration (UF) under ultraviolet (UV) light The research focused on the membranes' ability to eliminate natural organic matter (NOM) and potentially decrease membrane fouling The experiments utilized heat-resistant ceramic discs for optimal performance.
Ultrafiltration (UF) membranes and humic acids serve as model substances for naturally occurring organic matter, with membrane sizes of 1, 15, and 50 kDa used to assess the impact of coating under ultraviolet (UV) irradiation A commercial humic solution underwent UF fractionation, and gel filtration chromatography was utilized to investigate how the molecular weight distribution of natural organic matter (NOM) affects UF membrane fouling Results indicate that TiO2-coated membranes illuminated with UV254 light experience greater flux decline compared to uncoated membranes, despite similar effluent quality While the UF membrane effectively removes a substantial amount of humic materials, the integration of photocatalysis leads to decreased permeate flux Under UV254 irradiation, TiO2-coated membranes modify the molecular weight distribution of humic materials to less than 1 kDa, smaller than the 1-kDa membrane size tested, resulting in no improved performance in removing natural organic matter or mitigating membrane fouling (Syafei, Lin, & Wu, 2008).
Immobilizing photocatalysts can enhance the effectiveness of purification technologies that combine photocatalysis with membrane filtration for practical applications In this integrated system, the membrane not only supports the photocatalyst but also serves as a selective barrier for the substances targeted for degradation (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
The limitations of current systems include unsatisfactory lifetimes due to potential catalyst deactivation and washout Therefore, it is essential to develop systems with optimal porosity and efficient catalyst particle dispersion to enhance performance and longevity.
Hairom et al investigated the efficacy of ZnO nanoparticles, self-synthesized through a precipitation method, as a photocatalyst for treating industrial dye wastewater in a suspended PMR system The study utilized both nanofiltration (NF) and ultrafiltration (UF) membranes, specifically the Polypiperazine amide NF membrane Trisep TS40 and the Polyamide UF membrane Trisep GMSP, to assess their performance in the treatment process.
The study evaluated the performance of GE Osmonics membranes in treating industrial wastewater from printing presses, highlighting the significance of pH in enhancing photocatalytic efficiency and reducing fouling in the PMR system Optimal results were achieved at a pH of 11 with a ZnO loading of 0.1 g L-1 Severe fouling occurred at pH levels of 2, 7, and 8 due to weak electrostatic repulsion on the membrane surface The NF membrane excelled in removing color (100%), reducing chemical oxygen demand (92%), turbidity (100%), and rejecting total suspended solids (100%) However, a 65% reduction in normalized flux was noted due to the accumulation of photocatalysts on the membrane In contrast, the UF membrane performed poorly due to the permeation of nanosized ZnO and dye molecules through its pores.
Table 2 Advantages and disadvantages of different configurations of PMR
• № need to separate and recycle the photocatalyst;
• Pollutants could be degraded either in feed or in permeate;
• Less membrane fouling due to enhanced hydrophilicity and degradation of organic pollutants that form the gel layer or filtration cake;
• Higher photocatalytic efficiency due to sufficient contact between photocatalyst and contaminants;
• Convenient to adjust the photocatalyst concentration to desired value;
• Membrane damage caused by UV light and generated hydroxyl radicals could be avoided
• Lower photocatalytic efficiency due to lower effective surface area of the photocatalyst;
• UV light and generated hydroxyl radicals may damage the polymer membranes;
• Impossible to adjust the 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
Researches and application of Au/TiO 2 in water treatment
The Au/TiO2 nanocomposite is highly effective in photocatalytic applications for the removal and decomposition of wastewater pollutants, particularly azo dyes Its superior photocatalytic activity has been demonstrated in the degradation of various dyes, including Methylene Blue, Methyl Orange, Orange 16, Acid Red 1, Acid Red 88, Acid Red G, Sulforhodamine-B, and Tartazine, under both visible and UV irradiation (Zhang et al., 2012; Tian et al., 2008; Oros Ruiz et al., 2012; Hsiao et al., 2011; Mrowetz et al., 2007; Kumar et al., 2008; Zhu et al., 2009; Rupa et al., 2009).
Integrating Au nanoparticles with TiO2 has led to a remarkable nine-fold enhancement in the photocatalytic decomposition rate of methyl orange (MeO) under visible light (Hou et al., 2011) Additionally, Li et al demonstrated that using 0.5% Au/TiO2 achieved complete degradation of 12 mg/L methyl blue (MeB) in just one hour, resulting in a 96% degradation rate (Li and Li, 2001) Furthermore, Li et al (2009) investigated the potential reaction mechanisms of Au/TiO2 for the degradation of azo dyes when exposed to UV light.
UV light irradiation of TiO2 generates photoexcited electron-hole pairs, with the electron migration to Au reducing the recombination rate and enhancing the production of superoxide and hydroxyl radicals, thus improving the decrease in chemical oxygen demand (COD) (Li et al., 2009) A laser flash photolysis study demonstrated a 40% increase in hole transfer efficiency for Au-capped TiO2 (Dawson and Kamat, 2001) Under visible light, Au is photoexcited via surface plasmon resonance, leading to electron injection into O2 adsorbed on TiO2, which enhances the reduction of dioxygen to superoxide radicals due to the local work function near oxygen adsorption sites (Tian et al., 2008) This process further accelerates the production of superoxide and hydroxyl radicals in the presence of O2, contributing to a more significant reduction in COD values (Hoffmann et al., 1995; Li et al., 2009) Additionally, phenols serve as effective bio reluctant probe molecules for detecting aromatic organic pollutants in water, highlighting the need to explore photocatalytic activity further.
Gold nanoparticles (Au-NPs) deposited on titanium dioxide (TiO2) have shown significant potential in the degradation of phenolic compounds Research by Wongwisate et al demonstrated that 0.05% and 0.1% Au/TiO2 effectively accelerated the breakdown of hydroquinone into hydroxy hydroquinone, serving as intermediates in the degradation process of 4-chlorophenol (4-CP) (Ayati et al., 2014) The high specific surface area of the photocatalyst enhances the adsorption of intermediates on its surface, further improving the degradation efficiency.
TiO2 modified with small amounts of Au nanoparticles (Au-NPs) has been effectively utilized for the photocatalytic degradation of methyl tert-butyl ether (MTBE) in dilute aqueous solutions using a fixed bed flow-through reactor Research indicates that the incorporation of Au NPs significantly enhances the photocatalytic performance of titania, with optimal Au loading resulting in a kinetic rate constant three times higher than that of unmodified titania Additionally, studies have explored the photocatalytic degradation of L-asparagine and L-glutamic acid over Au/TiO2 and TiO2 catalysts, revealing the formation of cyanide in both scenarios Notably, in the presence of Au/TiO2, cyanide contributes to the leaching of gold as [Au(CN)2]-, detectable in solution.
The study demonstrated that Au/TiO2 exhibits high photocatalytic efficiency for the decomposition of 3,4-dichlorophenylurea, a didemethylated product of diuron, under solar light Notably, the Au/TiO2 catalyst achieved a degradation rate 1.6 times faster than that of unmodified TiO2.
2011) Likewise, these catalysts promoted the 1,4-dioxane photocatalytic degradation which is almost two times higher than a commercial TiO2.
Organic pollutant status
Organic pollution arises when significant amounts of organic compounds are introduced into water bodies, serving as substrates for microorganisms This leads to accelerated decomposition, which can deplete dissolved oxygen levels faster than they can be restored, resulting in oxygen depletion that adversely affects aquatic life Additionally, organic effluents often carry other harmful substances.
25 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 combination s
The rapid socio-economic development in Vietnam has led to significant negative impacts on the environment and human health, with water pollution being a major concern, particularly in surface and groundwater Just a few years ago, residents in mountainous areas could safely drink groundwater or stream water, but this is no longer the case, especially in major cities like Hanoi and Ho Chi Minh City In Vietnam, most domestic water is sourced from rivers, and despite extensive treatment processes, organic matter often remains in the water, raising serious health concerns.
Figure 5 Annual average BOD 5 content in major rivers in Vietnam (2005-200 9)
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- periments ex
Experimental section
Experimental overview
The study included two main activities containing the evaluation of PMR system in two configurations: suspended and immobilized
Au/TiO2 powder was synthesized, followed by the determination of gold size and proportion The photocatalyst's efficiency in degrading methylene blue was tested under visible light irradiation to assess its performance Subsequently, the photocatalyst was applied in a PMR system for further evaluation.
In this study, Au/TiO2 powder was employed in a dead-end filtration mechanism for the suspended configuration Various factors, including membrane molecular weight, reaction time, and photocatalyst quantity, were tested to identify the optimal experimental 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
Evaluation of performance of TiO 2 and Au/TiO 2 under UV, Vis and non-irradiation
• Photocatalyst: Au/TiO 2 coated on membrane
• Amount of Au/TiO 2 : 5.5 mg
• Filtration mode: dead- end and cross-flow
To investigate the best filtration mode
Figure 6 Process of study experiment
▪ 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/TiO2 photocatalyst and Assessment of its characteristics
Evaluation of PMR with suspended photocatalyst Evaluation of PMR with immobilized photocatalyst
Evaluation of dead-end and continuous-flow filtration mode in PMR
Veryfing of treatment capacity of photocatalyst under VIS irradiation in batch reation
▪ Hot plate Airbrush pistols, Profi-Airbrush, Germany; ; Round-bottom flask
100 ml, 200 ml; Conical flask 500 ml; Pipet 50 ml, 20 ml, 5 ml, 1 ml; Porcelain funnel; Agate Mortar Oven;
Methods
2.1 Synthesis of Au/TiO2 photocatalyst
In this study, the synthesis of Au/TiO2 was based on the method developed by Ngo Anh Binh in his 2018 research on gold nanoparticles Chloroauric acid is transformed into Au(OH)3, which coats the surfaces of TiO2 crystals under neutral pH conditions Upon heating, this hydroxide decomposes into the unstable oxide Au2O3, which gradually converts into elemental gold (Au).
Preparation of Au TiO/ 2 followed by research of Priebe is et al., 2015:
▪ Step 1: Heating 50 mL HAuCl4ã3H2O 5mM solution into 70 o C
▪ Step 2: Dropping 10.5 mL NaOH 0.1M solution to adjust the pH of HAuCl4 solution to 7
▪ Step 3: Adding 96.8 mL distilled water into 10.8 mL of the solution above, s rti ring for 15 min
▪ Step 4: Adding 1.0 g of TiO2 into the solution Stirring solution for 1 h at 70°C and then for 1 h at 25°C
▪ Step 5: After stirring, filtrating the solution to obtain the solid part
▪ Step 6: Drying the solid part overnight at 100 o C
▪ Step 7: After drying, grinding the sample to powder in
Figure 7 Diagram of Au/TiO2 synthesis process
2.1.2 Au/TiO2 characteristic analysis method
After synthesis process, the characteristics of obtained catalysts, particularly amount of Au deposited in the TiO2, was determinedby using methods of Inductively coupled plasma mass spectrometry And size of gold nanoparticle was assessed by UV-VIS
The measurements were conducted at Leibniz Institute for Catalysis at the University of Rostock (LIKAT)
▪ Inductively coupled plasma mass spectrometry (ICP-MS)
96.8ml distilled water Stirring for 15 minutes
Filtrating to obtain solid part
Grinding into powder (Au/TiO 2 )
Inductively Coupled Plasma (ICP) mass spectrometry is a highly sensitive analytical technique that measures metal and non-metal concentrations down to one part per quadrillion (ppq) This process involves ionizing the sample with inductively coupled plasma and utilizing a mass spectrometer to effectively separate and quantify the resulting ions, allowing for precise detection of low-background isotopes.
Ultraviolet (UV) spectroscopy, a crucial technique in analytical chemistry, falls under the category of absorption spectroscopy It focuses on the small segment of the electromagnetic spectrum that includes ultraviolet and visible radiation, alongside other forms such as radio, infrared, cosmic, and X-rays This method involves the absorption of UV and visible light, which excites electrons in atoms and molecules, promoting them from lower to higher energy levels Due to the quantized nature of energy levels in matter, only light with specific energy amounts can induce these transitions, leading to absorption.
The Lambert-Beer law is a key principle that describes the linear relationship between the absorbance of light and the concentration of an absorbing substance This law highlights that the absorption of light is directly proportional to the number of absorbing molecules present Additionally, it emphasizes that the fraction of radiation absorbed remains constant, regardless of the intensity of the incoming radiation.
The law can be expressed as A = εcl, where A represents absorbance, ε is a constant unaffected by concentration and path length, and l denotes the path length of the solution Key information, including the wavelength of maximum absorption (λmax), is essential for accurate calculations By knowing ε and λmax, one can determine the concentration of the solution; however, it is crucial to account for potential interferences from the solvent or other components in the sample.
2.2 Coating photocatalyst into membrane surface
By using airbrush spray coating technique, photocatalyst powder was deposited on membrane surface which had been heated before
The airbrush functions by releasing a photocatalyst suspension through a fine nozzle, propelled by a fast-flowing gas carrier stream This gas stream is regulated using a lever, creating a finely dispersed droplet spray that targets the substrate surface (Gutkowski et al., 2014).
Figure 8 Spraying Au nanoparticles on membrane surface
This study utilized Regenerated Cellulose ultrafiltration membranes (RC) from Millipore Corporation, USA, which have a maximum recommended operating pressure of 70 Psi and a temperature limit of 121°C The original diameter of the RC membranes is 76 mm, but they were cut into smaller pieces to fit the requirements of PMR cells.
Before deposition, photocatalysts were finely ground with a pestle and mortar to prevent airbrush blockage and ensure uniform coatings The photocatalyst suspensions comprised 50 mg of finely ground catalyst powders mixed with 0.05 mL of H2O, 0.05 mL of C2H5OH, and 0.01 mL of Nafion, which serves as a binder for catalyst adhesion to the membrane surface A commercial airbrush was employed for the spray-coating of these suspensions, following the methodology outlined by Hollmann et al (2017).
The airbrush utilizes a fine nozzle to dispense a photocatalyst suspension, which is delivered from a solution reservoir via a rapid gas carrier stream connected to a nitrogen gas bulb To ensure optimal application, the membrane is heated to a temperature of 60°C during the airbrush deposition process.
To prevent solvent accumulation on the substrate and enhance the drying process of the powder catalyst film, a heating plate was utilized Au/TiO2 powders were applied using airbrush deposition from a distance of approximately 1 cm from the substrate Subsequently, the samples were heated at 80 °C for 5 minutes to eliminate ethanol and water The loading of the powder catalyst was assessed by weighing the membrane before and after the spray coating process.
2.3 Design and construction of a PMR reactor
Figure 9 Schematic diagram of a lab-scale PMR system
Here is a rewritten paragraph that captures the essence of the design idea while adhering to SEO rules:"The innovative design concept employs a cross-flow mode of operation, where the feed solution is pumped to the coated side of the photocatalytic membrane, flowing parallel to the membrane's surface This unique configuration enables the permeate flow to pass through the membrane in a perpendicular direction, while the retentate flow can be either discharged or recycled back into the feed tank, promoting efficient and effective membrane performance."
To select suitable catalysts and radiation sources, initial experiments are conducted using batch mode reactions with TiO2 and Au/TiO2 photocatalysts under varying irradiation conditions: no radiation, visible light, and UV light Following this, a continuous mode Photocatalytic Membrane Reactor (PMR) is employed with the chosen catalysts and irradiation to evaluate system performance under near-real operational conditions, utilizing two distinct PMR designs.
34 photocatalyst – suspended configuration and photocatalyst-immobilized configuration are applied, and the research implemented the assessment of effectiveness of these PMR types
The construction of system was conducted by Mr Peter Kumm in the Technical Workshop, Institute of Chemistry, University of Rostock
The constructed system, detailed in Annex 1-3, features a PMR cell with a volume of 5 mL, crafted from PTFE (Poly Tetra Fluorethylene) and measuring 80 mm in height, 60 mm in width, and 25 mm in length This cell is specifically designed for low flow rates and zero pressure conditions To ensure precise control of the inflow rate at low levels, the Mzr-Pump controller software is utilized, which is installed on a computer connected to the pump.
2.4 Evaluation of the system in removal of organic matter from feed water
This study evaluates the performance of catalysts in photo-reaction and PMR systems by analyzing the concentration changes of organic components in water before and after treatment Methylene blue (MB) is used as a contaminant to simulate pollution, with a 6 ppm concentration achieved by dissolving 3 mg of MB in 500 mL of distilled water at room temperature The degradation of methylene blue is monitored using a UV-visible spectrophotometer, measuring spectra in the range of 200-800 nm through UV-VIS spectroscopy.
Results and Discussion
Characteristics of photocatalyst
Determining the size of the gold nano-particle
The research successfully synthesized the Au/TiO2 photocatalyst, with the particle images available in Annex 4 Following the synthesis, the size of the Au/TiO2 particles was characterized using UV-Vis spectroscopy, employing the equation: d = λ/2.
▪ d is the diameter of spherical AuNPs (nm);
▪ is the wavelength at the peak of the SPR;
▪ , L1 and L2 are constant value, respectively 512 (nm), 6.53 and 0.0216
Figure 11 UV-VIS absorbance of Au/TiO 2 photocatalyst
The spectral analysis reveals minimal absorption at longer wavelengths, with a notable increase peaking at 550 nm Absorption decreases to approximately 75% of this maximum at 400 nm, before consistently rising in the ultraviolet range, reaching the measurement limit at 280 nm Consequently, the Surface Plasmon Resonance (SPR) of the catalyst sample was identified at λ = 550 nm, indicating that the gold nanoparticles are approximately 81.5 nm in size.
Proportion of gold nano-particle mass on generated photocatalyst
Table 4 exhibited the result of ICP measurement of Au/TiO2 photocatalyst This result showed the amount of gold nanoparticles deposited on TiO2 surface
Table 4 ICP result of Au/TiO 2 sample
Element 1 st measurement 2 nd measurement Average
The result met the expectation of author Percentage of gold in this research was also similar with Binh’s research (the amount of Au was 0.62%),
2 Coating Au/TiO2 membrane surface on
In Chapter 2, Section 2.2, it is detailed that six samples of RC 30KDa membrane were coated with Au/TiO2 on the glossy side using air brushing techniques The quantity of photocatalyst applied was calculated by measuring the mass of the film before and after the coating process.
Table 5 Mass of Au/TiO 2 ated in membrane surface co
(mg) mRC(after coating) (mg) m Au/TiO2 (mg)
All photocatalytic membrane samples were tested in PMR system The data presented in the following part of thesis are only the proceeded ones of sample №.3 with 5.5 mg Au/TiO2
3 Evaluation of photocatalyst in batch reaction
Figure 12 compares the performance of various doses of TiO2 and Au/TiO2 under different radiation conditions: no radiation, visible light, and UV radiation The results indicate that using radiation sources, particularly UVVIS, significantly enhances reaction effectiveness, with UVVIS-based photo-reactions achieving over 90% efficiency.
UV light operates at a faster rate than visible light, as noted by Zhu et al Under UV irradiation, the absorption leads to a significantly higher rate of electron transfer from the photocatalyst to oxygen molecules compared to visible light Electronegative elements effectively capture electrons from adsorbed organic molecules, neutralizing positive charges and resulting in the oxidation of these compounds Consequently, an increase in the number of positive charges enhances the reaction rate of photocatalytic oxidation (Zhu et al., 2009).
The results indicate that Au/TiO2 consistently outperforms TiO2 across all photocatalyst doses under similar radiation conditions This performance disparity is particularly pronounced in the absence of radiation and under visible (VIS) radiation As the quantity of photocatalysts increases, their effectiveness also rises Notably, the maximum performance of Au/TiO2 under VIS radiation is 63%, compared to 54% under non-radiation conditions.
Figure 12 Performance of photocatalyst in different concentrations
The study emphasizes the potential of using VIS light for drinking water purification, despite the strong performance of photocatalysis under UV illumination It was found that TiO2 showed limited oxidation activity under VIS light compared to Au/TiO2 Consequently, subsequent experiments concentrated on evaluating the effectiveness of the Au/TiO2 photocatalyst in converting MB solution when exposed to VIS irradiation.
This experiment utilized a PMR system with dead-end filtration to prevent the loss of suspended catalysts in the retentate flow The primary objective was to determine the optimal membrane molecular weight size, the ideal quantity of photocatalyst, and the best reaction time for enhanced performance.
R em ov al ra te o f M B c on ce nt ra tio n (%)
Concentration of photocatalyst (mg/mL)
TiO2-none.TiO2-VIS.TiO2-UVVISAu/TiO2-noneAu/TiO2-VISAu/TiO2-UVVIS
Figure 13 Performance of different membrane MW at different amount of Au/TiO 2 under VIS irradiation
Figure 13 illustrates the effectiveness of Au/TiO2-coated membranes with molecular weights of 10, 30, and 100 KDa over a 30-minute period, with photocatalyst concentrations varying from 0 mg/mL to 0.13 mg/L The performance of the membranes shows a clear distinction, with the 100 KDa membrane exhibiting the lowest effectiveness, ranging from 9% to 15% This reduced performance is likely due to its larger pore size, which allows contaminants to pass through more easily and quickly, resulting in insufficient time for the degradation of methylene blue (MB) In contrast, the 10 KDa membrane's efficiency decline may be attributed to structural breakdown during the heating process, compromising its filtration capability.
Based on the result above, next experiment chose amount of Au/TiO2at 0.1 mg/mL
(3 mg) and the membrane RC 30 KDa
R em ov al ra te o f M B c on ce nt ra tio n (% )
Amount of Au/TiO 2 (mg/mL)
Figure 14 Performance of during the reaction time with 3mg of Au/TiO 2
The graph illustrates the variation in reaction efficiency over a time span of 30 to 240 minutes, utilizing 3mg of photocatalyst (Au/TiO2) Notably, the efficiency of methylene blue (MB) removal significantly increases with the application of the photocatalyst, whereas membrane filtration alone achieves only a 34% reduction in MB concentration.
MB in the water, while photocatalyst made the performance over 50%
The stirring speed significantly influences metabolic performance, with a mixing speed of 700 rpm providing a more uniform and continuous disturbance of the catalyst compared to 500 rpm, resulting in higher performance Both speeds, however, demonstrated similar efficiency variations, peaking at 50% and 40%, respectively At low or high speeds around 500-700 rpm, the catalysts in the reaction cell tend to clump together, leading to uneven distribution.
TIME (MIN)Au/TiO2 - VIS Irradiation - 700r № Photocatalyst/VIS IrradiationAu/TiO2 - VIS Irradiation - 500r
The experiment revealed that the optimal reaction time for the degradation of MB by Au/TiO2 under visible light irradiation is 120 minutes, after which the reaction performance gradually declines.
The filtration performance of membranes without photocatalysts declines after reaching a peak percentage at 150 minutes, similar to the trends observed in two other cases Additionally, experiments with mixed photocatalysts revealed that Au/TiO2 gradually adhered to the stirrer, resulting in a quicker decrease in activity after just 120 minutes.
In Chapter 3, section 2, six photocatalytic membrane samples were created, with a focus on sample №.3, which contains 5.5 mg of Au/TiO2 This section details the results obtained from using this sample under visible light (VIS) irradiation.
Two modes of filtration were used in this step: dead-end and cross-flow mode at the same operation condition:
Figure 15 Performance of different filtration mode in continuous flow under VIS irradiation with 5.5 mg of Au/TiO 2
The result shows that MB degradation efficiency of dead-end and cross-flow filtrations are not much different after reaction time of 120 minutes, of which the
R em ov al ra te o f M B c on ce nt ra tio n (% )
Cross-flow filtration demonstrates higher efficiency, achieving performance rates of 57% and 63%, respectively This enhanced effectiveness is attributed to the discharge of some contaminants through the retentate flow, which reduces the concentration of methylene blue (MB) in the permeate Additionally, cross-flow filtration offers significant advantages over dead-end filtration by minimizing membrane fouling, leading to improved operational longevity and performance.
Evaluation of photocatalyst in batch reaction
Figure 12 compares the performance of various doses of TiO2 and Au/TiO2 under different radiation conditions: no radiation (TiO2-none, Au/TiO2-none), visible light (TiO2-VIS, Au/TiO2-VIS), and UV radiation (TiO2-UVVIS, Au/TiO2-UVVIS) The results clearly indicate that using radiation sources, particularly UVVIS, significantly enhances reaction effectiveness, with UVVIS-based photo-reactions achieving over 90% efficiency with both TiO2 and Au/TiO2.
UV light operates at a faster rate than visible light, as highlighted by Zhu et al The absorption of UV light significantly enhances electron transfer from the photocatalyst to oxygen molecules compared to visible light Electronegative elements effectively capture electrons from adsorbed organic molecules, neutralizing positive charges and facilitating the oxidation of these compounds Consequently, an increased number of positive charges correlates with a higher reaction rate in photocatalytic oxidation (Zhu et al., 2009).
In comparative studies of photocatalysts, Au/TiO2 consistently outperforms TiO2 across all tested doses This performance disparity is particularly pronounced under non-radiation and visible (VIS) radiation conditions, where increased photocatalyst amounts correlate with enhanced effectiveness Notably, the maximum performance of Au/TiO2 under VIS radiation reaches 63%, while its performance without radiation peaks at 54%.
Figure 12 Performance of photocatalyst in different concentrations
The research aims to develop an effective drinking water purification device utilizing visible (VIS) light, as photocatalysis has shown promising results under UV illumination However, TiO2 demonstrated limited oxidation activity under VIS light compared to the more efficient Au/TiO2 photocatalyst Consequently, subsequent experiments concentrated on evaluating the performance of the Au/TiO2 photocatalyst for the conversion of methylene blue (MB) solution when exposed to VIS irradiation.
Evaluation of suspended PMR
The experiment utilized a PMR system with dead-end filtration mode to prevent the loss of suspended catalysts in the retentate flow Its objective was to determine the optimal membrane molecular weight size, the ideal amount of photocatalyst, and the best reaction time for enhanced performance.
R em ov al ra te o f M B c on ce nt ra tio n (%)
Concentration of photocatalyst (mg/mL)
TiO2-none.TiO2-VIS.TiO2-UVVISAu/TiO2-noneAu/TiO2-VISAu/TiO2-UVVIS
Figure 13 Performance of different membrane MW at different amount of Au/TiO 2 under VIS irradiation
Figure 13 illustrates the effectiveness of Au/TiO2-coated membranes with varying molecular weights of 10, 30, and 100 KDa over a 30-minute duration, with photocatalyst concentrations ranging from 0 mg/mL to 0.13 mg/L The performance of the membranes varied significantly, with the 100 KDa membrane exhibiting the lowest efficiency, achieving only 9% to 15% performance This reduced effectiveness is likely due to its larger pore size, which allows contaminants to pass through too easily and quickly, insufficiently degrading methylene blue (MB) In contrast, the 10 KDa membrane's efficiency drop may be attributed to structural breakdown during the heating process, compromising its filtration capability.
Based on the result above, next experiment chose amount of Au/TiO2at 0.1 mg/mL
(3 mg) and the membrane RC 30 KDa
R em ov al ra te o f M B c on ce nt ra tio n (% )
Amount of Au/TiO 2 (mg/mL)
Figure 14 Performance of during the reaction time with 3mg of Au/TiO 2
The graph illustrates the variation in reaction efficiency over a period ranging from 30 to 240 minutes, utilizing a photocatalyst amount of 3mg It is evident that the removal efficiency of methylene blue (MB) significantly increases with the application of the Au/TiO2 photocatalyst In contrast, membrane filtration alone achieves only a 34% reduction in MB concentration without the catalyst.
MB in the water, while photocatalyst made the performance over 50%
The stirring speed significantly influences metabolic performance, with an optimal mixing speed of 700 rpm leading to a more uniform and continuous disturbance of the catalyst, resulting in higher performance compared to 500 rpm Both speeds demonstrated similar variations, achieving peak efficiency at 50% and 40%, respectively However, at lower or higher speeds around 500-700 rpm, the catalysts in the reaction cell become lumpy and unevenly distributed.
TIME (MIN)Au/TiO2 - VIS Irradiation - 700r № Photocatalyst/VIS IrradiationAu/TiO2 - VIS Irradiation - 500r
The experiment revealed that the optimal reaction time for the degradation of methylene blue (MB) using Au/TiO2 under visible light (VIS) irradiation is 120 minutes, after which the efficiency of the reaction progressively declines.
The filtration performance of membranes without photocatalysts decreased after reaching their peak efficiency at 150 minutes, indicating a similar trend observed in two other cases Furthermore, during experiments with mixed photocatalysts, the Au/TiO2 particles adhered to the stirrer over time, resulting in a decline in their activity after 120 minutes.
Evaluation of immobilized PMR
In Chapter 3, Section 2, six photocatalytic membrane samples were created, with a focus on sample №.3, which contains 5.5 mg of Au/TiO2 This section details the results obtained from the application of this sample under visible light (VIS) irradiation.
Two modes of filtration were used in this step: dead-end and cross-flow mode at the same operation condition:
Figure 15 Performance of different filtration mode in continuous flow under VIS irradiation with 5.5 mg of Au/TiO 2
The result shows that MB degradation efficiency of dead-end and cross-flow filtrations are not much different after reaction time of 120 minutes, of which the
R em ov al ra te o f M B c on ce nt ra tio n (% )
Cross-flow filtration demonstrates superior efficiency, achieving performance rates of 57% and 63% This enhanced efficiency is likely due to the removal of some contaminants through the retentate flow, which reduces the concentration of methylene blue (MB) in the permeate Additionally, cross-flow filtration offers significant advantages over dead-end filtration by minimizing membrane fouling, leading to improved operational longevity and effectiveness.
In an experiment involving two types of filtration, it was observed that after 10 minutes of reaction, catalyst particles detached from the membrane surface This detachment adversely affected the efficiency of the membrane, despite its overall good performance.
There were some solutions that were considered and tested in this research, however, all of them were not worked
Table 6 Solutions used to avoid photo catalysts peeling off the membrane
1 ▪ Increasing Nafion concentration in the photocatalyst mixture before coating
▪ Too much Nafion would block gold nanoparticles lead to decrease the photocatalyst active
2 ▪ Increasing temperature while heating coated membrane
▪ RC membrane would be burned at high temperature
3 ▪ Coating on the rough side of membrane
▪ Contrary to the principle of membrane operation
Nafion serves as an adhesive in the first solution, where a higher concentration in the spraying mixture diminishes the photocatalyst's adhesion to the membrane surface and obstructs nanoparticles, resulting in decreased activity.
For the second idea, it is not applicable since when increase heating temperature, the membrane become too dry and folding and might be burned
The application of Au/TiO2 coating on the rough side of the membrane demonstrated excellent adhesion, with no peeling observed, ensuring that the photocatalyst remains securely attached to the surface.
The membrane operation principle recommended by the producer is not being adhered to, which impacts performance Additionally, observations during testing in the PMR cell indicate minimal conversion of MB.
In summary, the research achieved following outputs:
The synthesized Au/TiO2 catalyst, featuring gold nanoparticles at 0.64% average weight and 81.5 nm size, exhibited impressive performance in degrading methylene blue (MB) under visible light The degradation efficiency was measured at 25%, 28%, 37%, and 63% for concentrations of 0.2, 0.5, 1.25, and 2.5 mg/mL, respectively.
The study successfully demonstrated the coating of a regenerated cellulose membrane with Au/TiO2 using the air-brushing coating method The findings indicate that the average amount of catalyst deposited on the membrane, which has a perimeter of 24 mm, is approximately 6.2 mg.
▪ Two configurations of PMR were evaluated:
- Before operating in PMR system, batch reaction was tested to verify the ability of Au/TiO2 in MB removal under VIS irradiation
A study was conducted using a suspended system with regenerated cellulose membranes of varying molecular weights (10 KDa, 30 KDa, and 100 KDa) in a continuous flow setup The optimal conditions were identified as a membrane molecular weight of 30 KDa, a photocatalyst concentration of 0.1 mg/mL (3 mg), and a reaction time of 120 minutes, achieving a maximum methylene blue degradation efficiency of 50%.
The 30 KDa membrane coated with Au/TiO2 was evaluated in both dead-end and cross-flow modes, achieving methylene blue (MB) removal rates of 63% and 57%, respectively However, the performance was adversely affected by the detachment of photocatalyst particles from the membrane surface over time, which also influenced the flow of the catalyst-mixed retentate.
Given by the opportunities and difficulties during the 6-month research, further studies are recommended to improve and develop PMR system:
▪ The impacts of operation condition of PMR system such as: flow rate, pressure and larger range of contaminant concentration should be assessed.
▪ For suspended PMR system, membrane fouling may occur, further studies are needed to address this problem
▪ For immobilized PMR system, photocatalyst peel off membrane problem need to be solved
▪ To use regenerated cellulose membrane, it is necessary to have more evaluation on impact of molecular weight of membrane, operation time, flow speed
▪ Coating method shows low effective then other method should be studied
▪ Other kind of membrane than regenerated cellulose can be tested to enhance the adhesion of catalyst on membrane surface
▪ Research on other solutions to reduce membrane failure to extend the system’s life
▪ Assessment of the PMR’s performance on other organic compounds
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Figure A2 Design of PMR cell in Auto-cad drawing
Figure A3 Whole PMR system in laboratory
Figure A4 Au/TiO 2 after synthesis
Figure A5 ICP result of Au/TiO 2 photocatalyst
Figure A6 Coated regenerated cellulose membrane
Figure A7 Standard line of MB 6ppm solution y = 0.1667x - 0.0182 R² = 0.9987
U V V IS A bs or ba nc e
Table A1 Data of experiment in batch reaction (Section 3, Chapter 3)
Conc of catalyst None VIS UV-VIS mg/mL ABS Conc Removal rate (%) ABS Conc Removal rate (%) ABS Conc Removal rate (%)
Table A2 Data of performance of different membrane MW at different amount of
Membrane Concentration of PC (mg/mL) ABS Concentration of
Table A3 Data of methylene (MB) removal performance during the reaction time
Naked 30RC VIS 30RC VIS 3mg
MB Removal rate (%) ABS Conc.of
MB Removal rate (%) ABS Conc.of
Table A4 Data of methylene (MB) removal performance in different filtration mode in continuous flow (Section 5, Chapter 3)
30RC 7mg on membrane VIS- dead end 30RC 7mg on membrane
MB Removal rate (%) ABS Conc.of