INTRODUCTION
Clean water issue
Water covers 70% of our planet, yet only 2.5% is fresh water, which is insufficient to meet the needs of a growing global population projected to increase water demand by 400% by 2050 Currently, 1.1 billion people lack access to clean water, and over 2.7 billion experience water scarcity at least once a year Pollution and climate change are exacerbating water shortages, leading to drying rivers and wetlands, with agriculture being the largest consumer of water In Vietnam, saline intrusion in the Mekong Delta is threatening agricultural production and the living conditions of millions To combat water scarcity, strategies such as enhancing water filtration systems and promoting conservation are essential Notably, the development of freshwater processing technology from seawater has gained attention for its potential to improve efficiency and reduce energy consumption.
Desalination is a widely used method for converting seawater into freshwater, employing various technologies such as distillation, ion exchange, and membrane filtering However, these methods face challenges, including high costs, significant material usage, and reduced efficiency due to salt precipitation and seawater corrosion Recently, the use of solar energy for producing clean water from seawater has gained significant interest, offering a promising solution that is environmentally sustainable, cost-effective, and high-performing.
Figure 1.1 (a) Distribution of water on the Earth [6], (b) saline intrusion warning in the Mekong Delta [30].
Solar steam generation
Solar energy is a renewable and virtually limitless resource, with the Earth receiving approximately 200,000 times the total global daily electricity production from the sun each day Despite its abundance and the fact that solar energy is free, the significant costs associated with its collection, conversion, and storage hinder its widespread adoption Nevertheless, solar energy can be effectively transformed into thermal or electrical energy, offering practical applications for everyday life.
Vietnam boasts one of the highest sunshine hours globally, providing significant advantages for solar energy applications This abundant sunlight makes solar steam production a vital method for water desalination, effectively harnessing solar energy to generate clean water The process involves passing steam through a condenser, resulting in purified water.
The SSG system offers numerous advantages, such as being eco-friendly with zero CO2 emissions, easy installation, and low cost In regions like southern Vietnam, where there is an average of 6-7 hours of sunlight daily, a typical SSG appliance can produce between 15 to 30 liters of water per hour, meeting the minimum daily water needs of a household.
1.2.1 Design of solar steam generation (SSG)
Solar steam generation (SSG) is an innovative technology that harnesses the photo-thermal effect to convert seawater into freshwater, making it a viable solution for desalination, wastewater purification, and photothermal steam sterilization An SSG system comprises three essential components: photothermal material, a water supply device, and a freshwater storage container The core principle involves converting solar energy into thermal energy, where photothermal materials absorb photons and generate the heat necessary for water to evaporate into high-purity vapor, which is then condensed and stored Continuous water transfer to the photothermal surface is facilitated by mechanisms like capillary action As a sustainable approach, SSG addresses the global clean water crisis exacerbated by climate change, utilizing CO2-free photothermal materials that efficiently convert sunlight into heat Various research teams have developed advanced photothermal materials, including metal nanoparticles, semiconductors, and polymers, achieving over 90% sunlight absorption to enhance freshwater production in SSG systems.
To effectively convert sunlight into heat, photothermal nanomaterials for solar energy harvesting must possess a high broad-spectrum absorption capacity across solar wavelengths from 300 to 2500 nm These materials should efficiently transform solar energy into heat while minimizing heat loss to the surrounding environment Additionally, the advantageous physical properties of nanomaterials, including their small size and large specific surface area, enhance their competitiveness in the light-to-heat conversion process.
Photothermal materials can be categorized into four main types: (1) metal nanoparticles, (2) semiconductors, (3) polymer materials, and (4) natural materials primarily composed of cellulose.
Metal nanoparticles are pioneering materials for photothermal applications due to their ability to efficiently scatter, trap, and absorb light through plasmonic resonance When exposed to light, the free electrons in these nanoparticles oscillate within the metal lattice, creating a phenomenon known as localized surface plasmon resonance (LSPR), which occurs at specific frequencies of the incident light Various metals, such as gold, silver, and copper, are utilized in nanostructured absorber layers Notably, Zhenhui has developed a self-assembled film of gold nanoparticles in an SG system that harnesses capillary action to pump water to the evaporator surface, where localized heating from plasmonic effects facilitates evaporation.
The innovative plasmonic film, composed of gold nanoparticles (AuNPs), effectively floats on water and converts absorbed optical light into heat, generating bubbles that burst at the water-air interface, enhancing heat generation This thermal localization boosts the efficiency of heat-to-evaporation conversion by minimizing heat loss to the surrounding water Self-assembled membranes demonstrate a solar thermal efficiency of 40% under standard sunlight conditions Additionally, Fang and colleagues developed a solar-powered SG system by chemically coating silver/diatomite and integrating it with filter paper and foam-coated air-buffer paper, achieving an impressive evaporation rate of 1.39 kg m² h⁻¹ and an evaporation efficiency of 92.2% at 1 sun.
Non-radiative semiconductor relaxation represents a significant category of photothermal materials, where light absorption in semiconductor materials facilitates electron transitions from the valence band to the conduction band This process leads to an out-of-order expansion of electrons in the conduction band, resulting in an increase in the material's temperature and promoting evaporation In 2018, Hu's group introduced a high-performance SSG application film, known as black wood films, which feature a narrow band semiconductor (0.45 eV) combined with CuFeSe2 nanoparticles When irradiated at 5 kWm -2, these decorative wood films exhibit impressive solar thermal characteristics of 86.2%, alongside a neutral structure, low density, localized heat generation, low thermal conductivity, high hydrophilicity, and affordability.
Figure 1.3 CuFeSe2 NPs - decorated wood membrane for solar steam generation [19]
Polymers, such as polydopamine and polyacrylamide, are emerging as effective photothermal materials due to their affordability, biodegradability, and eco-friendliness A research team led by Jia developed a novel bilayer photothermal material using attapulgite/polyacrylamide (APAC) composites, achieving a high vapor ratio of 1.2 kg m -2 h -1 under one sun, which translates to an impressive solar vapor efficiency of 85% Additionally, Liu introduced a biomass-geopolymer-carbon-neutral synthesis (GBMCC) device designed for solar energy harvesting, demonstrating water evaporation rates of 1.58 and 2.71 kg m -2 h -1 under sunlight intensities of 1 and 3, respectively, with solar heat conversion efficiencies of 84.95% and 67.6%.
Photothermal materials exhibit exceptional hydrophilic properties, a porous structure, and a capillary mechanism that enables rapid water evaporation, alongside a low heat transfer coefficient that minimizes heat loss to the environment These characteristics contribute to the high evaporation rates of SSG systems under standard sunlight conditions However, the large-scale implementation of SSG systems using these materials faces significant challenges, including complex fabrication processes that are difficult to scale, high costs, low reliability in transport and practical applications, and potential environmental pollution from metal and polymer materials.
Bacterial cellulose (BC) is a highly versatile material characterized by cellulose fibers measuring 100–200 nm, forming a three-dimensional scaffold with exceptional properties such as high Young's modulus, significant water uptake capacity, and impressive crystallinity Its notable attributes include great porosity and a large total surface area, making it biocompatible and suitable for sustainable fabrication processes and biomedical applications Additionally, BC's low thermal conductivity and hydrophilic nature enhance its effectiveness in photothermal materials, allowing for rapid water transfer through its 3D network Research has demonstrated the use of BC in innovative evaporator devices for seawater filtration, such as the bi-layered BC biofoam evaporator developed by Zhang et al (2020), which achieves an optimal evaporation rate of 1.44 kg m^-2 h^-1 and an efficiency of 83.5% Furthermore, the combination of bacterial nanocellulose (BNC) and polydopamine (PDA) showcases high light absorption and effective water transport, yielding evaporation efficiencies of up to 78% Under standard sunlight conditions, these photothermal materials can achieve evaporation efficiencies exceeding 78-90% in steam evaporation systems.
The current photothermal materials achieve efficiencies between 1.2 and 1.6 kg m -2 h -1, but their complex fabrication processes pose significant challenges for large-scale production This highlights the urgent need for innovative photothermal materials that are not only easy to manufacture but also utilize eco-friendly input materials.
Purpose of thesis
Recent research on natural photothermal conversion materials like pomelo, fingered citron, and corn straw has demonstrated their effectiveness in water evaporation systems, but their size limitations restrict practical applications In contrast, bacterial cellulose (BC) is a versatile material that can be easily cultured and adjusted in size based on environmental conditions This thesis explores the creation of SSG structures using BC for photothermal applications, enhanced by an iron-tannic complex that improves solar absorption The study employs thermal imaging to evaluate the energy conversion efficiency from solar radiation to heat on the material's surface Additionally, the water evaporation rate is measured in both solar simulators and real-world scenarios to determine the efficiency of the SSG devices, while the purity of the water produced is assessed using SSG instruments in practical conditions.
The master's thesis aims at:
- Fabricating of photothermal materials from BC materials using a chemical process
- Studying the characteristics of the materials
- Evaluating of the evaporation efficiency of SSG system based on the photothermal material under laboratory condition
- Evaluating of the durability of the materials
- Demonstration of seawater desalination capacity of SSG system in real conditions
CHAPTER 2: EXPERIMENTAL METHOD
2.1.1 Fabrication of bacterial cellulose (BC)
To prepare kombucha, green tea was steeped in boiling water for 20 minutes, after which the tea bags were removed and sugar was dissolved in the mixture The tea was cooled to 37°C before adding the SCOBY and covered with a cotton cloth, then stored at 28°C in a dark cabinet Once the kombucha biofilm (BC) reached the desired thickness, it was collected and treated with a 1.0 M NaOH solution at 90°C for one hour, followed by immersion in sodium hypochlorite (NaOCl) at room temperature for two hours The BC was then rinsed six times with deionized water to eliminate any residual cleaning agents and stored at 4 °C in deionized water for future research.
Figure 2.1 Bacterial cellulose fabrication process
Figure 2.2 Bacterial cellulose fabrication process
- Chemicals used to fabricate photothermal materials included tannic acid (C76H52O46; GHTECH; China) and Iron (III) chloride hexahydrate (FeCl3.6H2O; GHTECH; China)
- To make the solutions, ultra-pure water from the Water distill D4000 (VJU) was utilized
0.8 g tannic acid was dissolved in 200 ml distilled water to make a tannic acid (TA) solution with a concentration of 0.0023M 0.4 g FeCl3 was dissolved in 200 ml distilled water to make the 0.012M FeCl3 solution The blackening process was procedured through 2 main steps as shown in figure 2.2 First step: BC (5 mm in thickness, 4.5 cm diameter) was immersed in TA solution and stir for 20 hours Then, the samples were rinsed with distilled water to remove free TA particles Second step: BC-TA is dipped in Fe 3+ solution and stirred for 2 hours After that, samples were rinsed with distilled water to obtain photothermal material BC-TA-Fe 3+
Figure 2.3 Preparation of photothermal material
At the Nanotechnology Laboratory of Vietnam Japan University and Hanoi University of Science (HUS), researchers conducted a detailed examination of photothermal materials, utilizing advanced equipment from VNU As illustrated in Figure 2.3, these tools were instrumental in analyzing the structural and morphological characteristics of the photo-thermal conversion materials.
The surface structure of BC pellicles was examined using Scanning Electron Microscopy (SEM) at an accelerated voltage of 10 kV Spectral measurements, including scattering, transmittance, and reflectance, were conducted from 300 nm to 2500 nm with an infrared spectrometer equipped with an integrating sphere, allowing for the calculation of absorbance (A) using the formula A = 1 - R - T Additionally, the chemical structure of BC pellicles and photothermal materials was analyzed through Fourier-transform infrared spectroscopy (FTIR), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD).
Figure 2.4 Some instruments in this study (a) JSM-IT100 InTouchScopeTM Scanning Electron Microscope (b) Oriel® Sol1ATM Solar Simulators (c) FLIR C2 Camera (d) X-ray Diffraction (XRD Mini Flex 600) (e) UV-VIS Lambda 950
Figure 2.5 Some instruments in this study (a) JSM-IT100 InTouchScopeTM Scanning Electron Microscope (b) Oriel® Sol1ATM Solar Simulators (c) FLIR C2 Camera (d) X-ray Diffraction (XRD Mini Flex 600) (e) UV-VIS Lambda 950
The BC photothermal material was applied to commercial polystyrene foam, measuring 15 mm in thickness, and covered with a cotton gauze of 0.2–0.3 mm thickness and 2 mm × 2 mm mesh size The samples were placed in a quartz beaker with a diameter of 4.5 mm filled with water Research on water evaporation from SSG devices was conducted at the Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, using an Oriel® Sol1ATM solar simulator under 1 sun irradiation (1 kW.m -2) Thermal imaging, captured with a FLIR C2 camera, provided infrared photographs of the samples and enabled the analysis of their temperature distributions.
Figure 2.6 The structure of steam generation part of SSG system
The SSG system's steam generator features a structural model and an actual image depicted in Fig 2.4 Its water supply system consists of a cup-filled reservoir and polystyrene foam wrapped in gauze The gauze facilitates water distribution to the photothermal material's surface, while the polystyrene foam serves as an insulator, minimizing heat loss from the photothermal material, which is applied directly on top of the foam.
2.3.2 Evaluate the water evaporation ability of SSG system
An experiment was conducted to measure the evaporation rate of the SSG device, as illustrated in Figure 2.5 The SSG system was placed on an electronic scale linked to a computer, which recorded the changes in water mass over time The evaporation rate of the water was then calculated based on these measurements.
12 the drop in water in the SSG system, and the mass change was observed at equal intervals and plotted on a computer
In the experiment depicted in Figure 2.6, a 100 cm² beaker was positioned within a glass flask and subjected to natural sunlight The material's remarkable capacity to transform light into heat facilitated the rise of saltwater through capillary channels, leading to its conversion into vapor.
Figure 2.23 Evaluating the steam evaporation index of SSG in the laboratory condition
Figure 2.16 The experiment of collecting purified from seawater of SSG system
Figure 2.17 Evaluating the steam evaporation index of SSG in the laboratory condition.Figure 2.18 The experiment of collecting purified from seawater of
Figure 2.19 Evaluating the steam evaporation index of SSG in the laboratory condition Figure 2.20 The experiment of collecting purified from seawater of
The roof's slope facilitates the condensation of water vapor in the upper glass cabinet, allowing it to flow down into the water tank To enhance the efficiency of this process, a dry ice cooling system has been installed adjacent to the glass cabinet, accelerating the vapor-to-liquid phase transition As a result, fresh water is collected and drained from the bottom of the glass container.
CHAPTER 3: RESULTS AND DISCUSSION
Chapter 3 will focus on discussing the characteristics of photothermal material BC-TA-
Fe 3+ (BTF) was synthesized using chemical methods outlined in Chapter 2, and its properties as a photothermal material were analyzed, highlighting its exceptional performance in the SSG system Key attributes include efficient water transport and effective light-to-heat conversion The water evaporation system's efficiency was assessed under various conditions, and the quality of fresh water produced by the SSG system will be evaluated against potable water standards.
3.1 The surface morphologies of BTF materials
Figure 3.1 depicts the growth and development of the BC membrane over time in 5 days
On the first day of observation, the culture solution remained relatively unchanged However, by the second day, noticeable color changes occurred, accompanied by the appearance of white spots at the air-liquid interface A thin white film developed on the third day, with a higher density of white spots in the center compared to the edges By the fourth day, the growth of bacterial cellulose (BC) membranes became more uniform By the fifth day, the thickness of the BC film continued to increase, indicating significant growth during the culture period.
Figure 3.1 Images of BC growth in 5 days
By increasing the interface area, it is feasible to produce large-area BC films As a result, photothermal materials derived from BC utilized in the SSG system offer enhanced flexibility in terms of shape, thickness, and surface area.
The fabrication of photothermal materials involves the production of bacterial cellulose (BC) through a fermentation process using yeast and bacteria in a solution of green tea and sucrose at 28 °C for 5–10 days The thickness and area of the BC can be controlled by adjusting the growth time, resulting in a BC membrane that is 5 mm thick and 4.5 cm in diameter The process begins with the BC being submerged in tannic acid (TA) solution, which causes a color change to light yellow as TA particles bond with the cellulose through hydrogen interactions Subsequent treatment with Fe 3+ solution results in a black material due to the formation of nanocomplexes of Fe 3+ and TA on the cellulose surface This black coloration is significant as it enhances the material's ability to absorb electromagnetic radiation, aligning with the principles of black body radiation Freeze-drying is then employed to analyze the structural and chemical properties of the final product Notably, Fe 3+ is preferred for complexing with tannic acid due to its ability to form stable black complexes, unlike other metal ions.
Figure 3.3 SEM images of BC and BTF materials
The SEM images in Figure 3.3 (a, b) reveal the surface structure of bacterial cellulose (BC) after the freeze-drying process, highlighting its composition of cellulose nanofibers with a radius of 50–100 nm BC features a highly open, microporous 3D network formed by intertwined cellulose fibers produced by bacteria at the air/medium interface This initial layer of fibers guides the formation of subsequent bacterial nanocellulose (BNC) layers, creating a dense, three-dimensional network with a large specific surface area of 28.3 m²/g, as indicated by BET analysis in Figure 3.4 The 3D network enhances the mechanical durability of BC, crucial for its application in photothermal materials Additionally, the microporous structure facilitates efficient water transfer from the bulk to the air-water interface, promoting evaporation Following functionalization with tannic acid (TA) and Fe³⁺ solutions, the integrity of the layer structure and 3D network of BC was preserved, as shown in the SEM images of the BTF material in Figure 3.3 (c, d).
The surface of cellulose nanofibers displayed 17 nano-sized structures measuring 200–300 nm, formed through the interaction of OH groups with Fe 3+ ions to create TA-Fe 3+ nanocomplexes These nanostructures adhered to both the exterior and interior nanofibers of bacterial cellulose (BC), contributing to the dark coloration observed in the black fermented tea (BFT).
3.2 The surface structure of BTF materials
Figure 3.5 illustrates the FT-IR spectra of BC and BTF materials, highlighting the differences in their peak characteristics The FTIR spectrum of BC displays notable peaks at 3344 cm⁻¹, 2895 cm⁻¹, and 1610 cm⁻¹, within the wave number range of 400 to 4000 cm⁻¹.
The spectral analysis revealed significant peaks at 3344 cm⁻¹, attributed to the intrinsic OH stretching vibration of hydroxyl groups present in cellulose Additionally, peaks within the 1610-1725 cm⁻¹ range indicated the C=O stretching vibration of the carbonyl group and the C=C bond of the aromatic ring Characteristic peaks at 1107 and 1030 cm⁻¹ corresponded to C-O bonds These findings highlight the functionalization of BC in the TA process.
The attachment of Fe 3+ solution to biochar (BC) was facilitated by tannic acid (TA), which interacted with the hydroxyl groups of phenolic compounds and the benzene rings This interaction was evidenced by the clear detection of characteristic peaks around 1610 cm -1 in the spectra of the BTF material, indicating the presence of numerous hydrophilic functional groups These groups enhance the water transmission properties of BTF A contact angle measurement further confirmed the hydrophilic nature of the BTF material, revealing a water contact angle of approximately zero.
The XRD analysis results of BC and BTF materials were shown in Figure 3.6 The XRD pattern of the BC exhibited three diffraction peaks at 2θ = 14,17 and 22.35º, which had
The XRD analysis revealed diffraction contributions from the I𝛼 and I𝛽 phases in the BC structure, with the BTF material showing a slight rightward shift in its diffraction peaks compared to the BC spectrum, attributed to the presence of amorphous TA molecules Notably, the XRD pattern of the TA-Fe 3+ nanocomplexes lacked discernible crystalline peaks, aligning with previous findings Elemental composition analysis via EDS demonstrated that BC comprised a compound with a mixing ratio of C (44.73%) to O (55.27%) Additionally, the BTF material contained 0.38% Fe, indicating that Fe 3+ ions were successfully chelated to the surface of BC.
TA and Fe 3+ , which was formed on the BC
Figure 3.6 XRD spectra of BC and BTF materials show slightly changes in the diffraction peaks
Table 3.1 Composition of BC, BC-TA and BTF
The interaction between tannic acid (TA) and the cellulose structure of bacterial cellulose (BC) is crucial for preserving its functional layers This interaction occurs through the formation of hydrogen bonds between the hydroxyl (OH) groups of TA and those of cellulose, enhancing the material's properties.
Tannic acid (TA) forms three types of hydrogen bonds with cellulose, as illustrated in Figure 3.7 First, hydrogen bonds are created between the oxygen of TA's carbonyl (C=O) groups and the hydrogen of cellulose's hydroxyl (OH) groups Second, the hydroxyl groups of TA bond with the oxygen from the OH groups of cellulose Lastly, hydrogen bonds are formed between the oxygen of TA's hydroxyl groups and the hydrogen from cellulose's OH groups.
Figure 3.7 Schematic representation of cellulose with hydrogen bonded tannic acid
Figure 3.8 The possible complexation mechanism of TA with Fe 3+
FeCl3 was utilized to create a layer of Fe(III)-tannin complexes on natural porous materials after the application of tannic acid (TA) Tannic acid, composed of one glucose molecule and up to five digallic acids, facilitates the formation of these complexes by allowing ferric ions to displace hydrogen atoms from the phenol hydroxyl groups, resulting in chelation The resulting material exhibits a dark color due to the strong light absorption properties of the iron-tannic complexes and the rough texture of the surface.
The light absorption capacity of photothermal materials is crucial for enhancing the performance of SSG systems Analysis of the scattering, reflection, and transmission spectra of BC and BTF materials revealed their absorption characteristics across wavelengths of 300–2500 nm, as illustrated in Figure 3.9 Notably, the average absorption of BC materials ranged from approximately 40% to 50% within these wavelength regions.
The freeze-dried BTF material demonstrates a high absorption rate of approximately 98% within the wavelength range of 300–900 nm, although its stability decreases in the 900–2500 nm range, where absorption varies between 60% and 95% In wet conditions, BTF maintains over 93% light absorption across 300–2500 nm The exceptional absorption characteristics of BTF, derived from biochar (BC), can be attributed to three primary factors: the nano-sized structures of ferric tannates facilitating ligand-to-metal charge transfer, the surface roughness of BTF, and its 3D network that effectively traps light Notably, absorption drops beyond 750 nm due to a decrease in iron-tannic acid complex absorption past 700 nm The absorption mechanism is primarily based on ligand-to-metal charge transfer (LMCT), which involves excited-state charge transfer in transition metal complexes, though the minimal absorption cross-section of transition metal elements poses challenges for direct excitation.
22 result, highly absorbent organic ligands are employed to coordinate with these metal ions, transferring energy from the light-collecting organic ligands' excited state to the metal center [21]
Figure 3.9 Absorption spectra of the BC and BTF in dry/wet condition
Figure 3.10 Thermal conductivity of BTF y = 0.7425x - 1.1626 R² = 0.9949