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 population expected to increase water demand by 400% by 2050 Currently, 1.1 billion people lack access to clean water, while 2.7 billion experience water scarcity for at least one month each year Pollution is causing rivers, wetlands, and aquifers to dry up, with half of the world’s wetlands lost Agriculture is the largest consumer of water, suffering from significant inefficiencies Climate change is disrupting global weather patterns, leading to water shortages and floods In Vietnam, where two-thirds of the population lives near three major river basins, saline intrusion in the Mekong Delta is worsening, impacting agriculture and water supply for millions Addressing these challenges requires strategies to combat rising water scarcity and protect our climate, biodiversity, and communities.
To alleviate water shortage, various solutions have been suggested, such as improving water filtration systems, encouraging water conservation
In recent decades, freshwater processing technology derived from seawater has garnered significant attention due to its ability to enhance metabolic efficiency while simultaneously reducing 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 caused by salt precipitation and seawater corrosion Recently, the development of solar energy-based technology for producing clean water from seawater has gained significant interest, offering the promise of an environmentally sustainable, cost-effective, and high-performance solution.
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 abundant resource, with the Earth receiving approximately 200,000 times the total daily electricity production globally in solar energy each day However, the high costs associated with its collection, conversion, and storage hinder its widespread adoption Despite being a free energy source, these financial barriers limit the practical application of solar energy in everyday life, where it can be transformed into thermal or electrical energy.
Vietnam boasts one of the highest sunshine hours globally, presenting significant advantages for solar energy installations Solar steam production has emerged as a vital method for water desalination, harnessing the country's abundant solar resources to generate clean water This process involves passing steam through a condenser, effectively providing access to purified water.
The SSG system offers numerous advantages, such as being eco-friendly with zero CO2 emissions, easy installation, and low costs In regions like southern Vietnam, where there is 6-7 hours of sunlight daily, a typical appliance can produce between 15 to 30 liters of water per hour, meeting a household's minimum daily water needs.
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, with applications in seawater desalination, wastewater purification, and photothermal steam sterilization An SSG system consists of three essential components: photothermal material, a water supply device, and a container for the collected freshwater The system operates on the principle of converting solar energy into thermal energy, where the photothermal material absorbs sunlight and generates the heat necessary for water to evaporate into high-purity vapor This vapor is then condensed and stored for use Continuous water transfer to the photothermal surface is facilitated by mechanisms such as capillary action SSG technology offers a promising solution to the clean water scarcity exacerbated by global warming, as it utilizes solar energy without CO2 emissions Ongoing research has led to the development of various photothermal materials, including metal nanoparticles, semiconductors, and polymers, which can absorb over 90% of sunlight, significantly enhancing freshwater production.
To efficiently convert sunlight into heat, photothermal nanomaterials for solar steam generation (SSG) must possess high broad-spectrum absorption capacity across the solar spectrum (300 - 2500 nm), ensuring effective solar energy conversion with minimal heat loss to the environment Additionally, the small size and large specific surface area of these nanomaterials provide a competitive edge in enhancing light-to-heat conversion efficiency.
Many different types of materials were used to fabricate photothermal materials that are generally classified into four categories as follows: photothermal materials from
(1) metal nanoparticles, (2) semiconductors, (3) polymer materials, (4) natural materials that the main component is cellulose.
Metal nanoparticles are pioneering photothermal materials, utilizing plasmonic resonance to effectively scatter, trap, and absorb light across a broad spectrum When exposed to light, the free electrons in these nanoparticles oscillate within the metal lattice, resonating at specific frequencies of the incident light, a phenomenon known as localized surface plasmon resonance (LSPR) Various metals, including gold, silver, and copper, are used to create nanostructured absorber layers Recently, Zhenhui developed a self-assembled gold nanoparticle film in an SG system that harnesses capillary action to transport water to the evaporator surface, where plasmonic heating 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 This thermal localization enhances heat-to-evaporation conversion efficiency by minimizing heat loss, achieving a solar thermal efficiency of 40% at 1 sun Fang and his team developed a solar-powered SG system by chemically coating silver/diatomite on filter paper, which is then affixed to foam-coated air-buffer polystyrene This system demonstrates 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 key category of photothermal materials, where light absorption facilitates the transition of electrons from the valence band to the conduction band These electrons undergo disordered expansion in the conduction band before returning to the valence band, resulting in an increase in material temperature that promotes evaporation In 2018, Hu's group introduced a high-performance SSG application film, known as black wood films, which feature a narrow band semiconductor with a bandgap of 0.45 eV, enhanced with CuFeSe2 nanoparticles.
Irradiated at 5 kWm -2, CuFeSe 2 nanoparticles in decorative wood films exhibit impressive solar thermal performance, achieving an efficiency of 86.2% These films feature a neutral structure, low density, localized heating, low thermal conductivity, high hydrophilicity, and cost-effectiveness.
Figure 1.3 CuFeSe 2 NPs - decorated wood membrane for solar steam generation [19].
Polymers represent a significant category of photothermal materials, with widely used options like polydopamine and polyacrylamide serving as effective sunlight absorbers due to their affordability, biodegradability, and eco-friendliness A research team led by Jia has successfully developed a novel bilayer photothermal material utilizing attapulgite and polyacrylamide, showcasing a cost-effective, scalable, and high-performance solution.
The dual-layer APAC composites exhibit a high vapor ratio of 1.2 kg m -2 h -1 under one sun exposure, translating to an impressive solar vapor efficiency of 85% Additionally, Liu's innovative biomass-geopolymer-carbon-neutral synthesis (GBMCC) device features a neutral and macroscopic structure designed for effective solar energy harvesting This device demonstrates remarkable water evaporation rates of 1.58 kg m -2 h -1 and 2.71 kg m -2 h -1 under sunlight intensities of 1 and 3, respectively, achieving solar heat conversion efficiencies of 84.95% and 67.6%.
Photothermal materials exhibit exceptional hydrophilic properties, a porous structure, and a capillary mechanism that facilitates rapid water evaporation while maintaining a low heat transfer coefficient to minimize heat loss These characteristics contribute to the high evaporation rates of SSG systems under one sun conditions However, the large-scale implementation of these systems faces significant challenges, including complex fabrication processes that are difficult to scale, high costs, low material reliability for transportation and practical use, and environmental pollution risks associated with metal and polymer materials.
Bacterial cellulose (BC) is a 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 great crystallinity Its notable attributes include high porosity and a large total surface area, making it biocompatible and suitable for sustainable fabrication processes and biomedical applications Additionally, BC offers advantages for photothermal materials due to its low thermal conductivity and hydrophilic nature, which facilitates rapid water transfer through its 3D network Research has demonstrated the use of BC in photothermal applications, such as a bi-layered BC biofoam evaporator developed by Zhang et al (2020), which incorporates a CuS/BC composite for photothermal conversion and a regenerated BC biofoam for water transport and thermal insulation, achieving an optimal evaporation rate of 1.44 kg m⁻².
The combination of bacterial nanocellulose (BNC) and polydopamine (PDA) demonstrates impressive light absorption and photothermal conversion capabilities, achieving a water evaporation efficiency of up to 78% Under optimal conditions (1 kW/m²), these photothermal materials can reach evaporation efficiencies exceeding 78-90% in steam evaporation systems, translating to rates of 1.2-1.6 kg/m²/h However, the complex fabrication processes associated with these materials pose significant challenges for large-scale production Consequently, there is a pressing need for innovative photothermal materials that are easier to manufacture and utilize environmentally friendly input materials.
Purpose of thesis
Recent studies on natural photothermal conversion materials like pomelo, fingered citron, and corn straw have shown promise for water evaporation systems, but their size adjustability limits practical applications In contrast, bacterial cellulose (BC) is a versatile material that can be easily cultured and modified in size based on environmental factors This thesis details the creation of SSG structures utilizing BC to develop effective photothermal materials By chemically functionalizing the natural porous content with an iron-tannic complex, enhanced solar absorption is achieved Thermal imaging is employed to evaluate the conversion of solar energy into heat on the material's surface The efficiency of the SSG devices is assessed through water evaporation rates measured in both solar simulator and real-world conditions, while the purity of water produced is analyzed using SSG instruments to evaluate the portability of fresh water.
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)
Figure 2.1 Bacterial cellulose fabrication process.
To prepare kombucha, green tea was steeped in boiling water for 20 minutes, after which sugar was added and the mixture cooled to 37°C before incorporating the SCOBY The sweetened tea was then covered with a cotton cloth and maintained at 28°C in a dark cabinet until the kombucha biofilm (BC) thickened Once ready, the BC was collected and treated in a 1.0 M NaOH solution at 90°C for one hour, followed by immersion in sodium hypochlorite (NaOCl) for two hours at room temperature Afterward, the BC was rinsed six times with deionized water to eliminate any residual cleaning agents and stored at 4°C in deionized water for future research.
- Chemicals used to fabricate photothermal materials included tannic acid (C 76 H 52 O 46 ; GHTECH; China) and Iron (III) chloride hexahydrate (FeCl 3 6H 2 O; GHTECH; China).
- To make the solutions, ultra-pure water from the Water distill D4000 (VJU) was utilized.
Figure 2.3 Preparation of photothermal material.
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 FeCl 3 was dissolved in 200 ml distilled water to make the 0.012M FeCl 3 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+
At the Nanotechnology Laboratory of Vietnam Japan University and Hanoi University of Science (HUS), researchers investigated photothermal materials and their structures using advanced equipment As shown in Figure 2.3, these tools were essential for analyzing the structural and morphological characteristics of the photo-thermal conversion materials.
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)
The surface structure of the BC pellicle was examined using Scanning Electron Microscopy (SEM) at an accelerated voltage of 10kV Additionally, the material's scattering, transmittance, and reflectance spectra were measured from 300 nm to 2500 nm using an infrared spectrometer with an integrating sphere Absorbance was calculated with the formula A = 1 - R - T To analyze the chemical structure of the BC pellicles and photothermal materials, Fourier-transform infrared spectroscopy (FTIR), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD) techniques were employed.
The BC photothermal material was affixed to a 15 mm thick commercial polystyrene foam, which was then covered with a cotton gauze measuring 0.2–0.3 mm in thickness and a mesh size of 2 mm × 2 mm These samples were immersed in a quartz beaker with a diameter of 4.5 mm, filled with water At VNU University of Engineering and Technology, the water evaporation capabilities of SSG devices were analyzed using an Oriel® solar simulator.
Under a solar irradiation of 1 kW/m², the Sol1ATM samples were analyzed using a FLIR C2 thermal imaging camera, which captured infrared photographs to determine the temperature distributions of the samples.
Figure 2.6 The structure of steam generation part of SSG system.
The SSG system's steam generator features a structural model and actual image, depicted in Fig 2.4 Its water supply system consists of a cup-filled reservoir and a gauze-covered polystyrene foam, which ensures water effectively reaches the photothermal material's surface The polystyrene foam serves as insulation, minimizing heat loss from the photothermal material, which is directly applied 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 water mass change over time The evaporation rate was determined by monitoring the decrease in water levels at consistent intervals, with the results plotted on a computer for analysis.
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.
In the experiment, a 100 cm² beaker was placed inside a glass flask and exposed to natural sunlight, allowing the material's efficient light-to-heat conversion to bring saltwater to the surface via capillary channels, where it was transformed into vapor The inclined roof facilitated the condensation of water vapor in the upper glass cabinet, directing it down to the water tank To enhance the condensation efficiency of the evaporated water, a dry ice cooling system was installed next to the glass cabinet, accelerating the vapor-liquid phase transition, with fresh water ultimately draining from the bottom of the glass container.
CHAPTER 3: RESULTS AND DISCUSSION
Chapter 3 will focus on discussing the characteristics of photothermal material BC-
The synthesis of TA-Fe 3+ (BTF) through chemical methods, as outlined in Chapter 2, highlights its exceptional properties as a photothermal material in the SSG system Key characteristics include its effective water transport and its ability to convert light into heat Additionally, the efficiency of the water evaporation system is assessed under various conditions, and the quality of fresh water produced is compared against potable water standards.
3.1 The surface morphologies of BTF materials
Over a period of five days, the growth and development of the BC membrane were observed Initially, on the first day, the culture solution remained largely unchanged However, by the second day, noticeable changes occurred as the solution's color altered and white spots emerged at the liquid-air interface On the third day, a thin white film formed, with a denser concentration of white spots in the center compared to the edges By the fourth day, the BC membranes exhibited more uniform growth Finally, on the fifth day, the thickness of the BC film continued to increase, signifying substantial 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 This enhancement allows photothermal materials derived from BC to offer greater flexibility in shape, thickness, and surface area when utilized in the SSG system.
Figure 3.2 The change of materials during the fabrication of photothermal materials The
Bacterial cellulose (BC) was produced through a fermentation process involving yeast and bacteria in a solution of green tea and sucrose at room temperature (28 °C) over 5–10 days, resulting in a BC membrane that measured 5 mm in thickness and 4.5 cm in diameter The fabrication process of BC-based materials, as illustrated in Figure 3.2, began with the white BC being immersed in a tannic acid (TA) solution, which caused the surface to turn light yellow due to hydrogen bonding between TA and cellulose molecules Subsequently, treatment with Fe 3+ solution transformed the BC-TA material to a black color, attributed to the formation of nanocomplexes of Fe 3+ and TA on the cellulose nanofibers This black surface is significant as it enhances the material's ability to absorb electromagnetic radiation, in line with the principles of black body radiation The final product was freeze-dried for further analysis of its structural and chemical properties.
Figure 3.3 SEM images of BC and BTF materials.
Figure 3.3 (a, b) were SEM images of the BC surface structure after freeze-drying process.
Bacterial cellulose (BC) is composed of cellulose nanofibers with a radius of 50–100 nm, forming a highly open 3D microporous network This structure is created as bacteria produce cellulose fibers on the air/medium surface, which wind together to form layers that guide the formation of additional layers, resulting in a dense, parallel orientation of cellulose fibers As a result, BC has a large specific surface area of 28.3 m²/g, enhancing its mechanical durability, which is crucial for photothermal applications The microporous architecture facilitates efficient water transfer from the bulk to the air-water interface, promoting evaporation in BTF materials Following functionalization with tannic acid (TA) and Fe³⁺ solutions, the integrity of the BC's layer structure and 3D network was preserved, while nano-sized structures of 200–300 nm emerged on the cellulose nanofibers due to the interaction between the OH groups and Fe³⁺ ions These nanostructures contributed to the black coloration of the BTF.
3.2 The surface structure of BTF materials
The FT-IR spectra analysis of BC and BTF materials, as shown in Figure 3.5, reveals distinct differences in their peak characteristics The spectrum for BC displays notable peaks at 3344 cm⁻¹, highlighting the unique chemical composition of the sample within the wave number range of 400 to 4000 cm⁻¹.
The analysis revealed key absorption peaks at 2895, 1610-1725, 1313, 1107, 1030, and 612 cm -1 Notably, the peak at 3344 cm -1 corresponds to the OH stretching vibration of hydroxyl groups present in cellulose The peaks within the 1610-1725 cm -1 range indicate C = O stretching vibrations of carbonyl groups and C = C bonds in aromatic rings Additionally, peaks around 1107 and 1030 cm -1 are characteristic of C-O bonds Following the functionalization of BC in TA and Fe 3+ solutions, TA, along with its phenolic OH functional groups and benzene, was successfully attached to the BC, highlighting the presence of aromatic ring characteristics.
The spectra of the BTF material clearly showed a peak at 1610 cm-1, indicating the presence of numerous hydrophilic functional groups that facilitate water transmission To further assess the hydrophilic properties of the BTF material, a contact angle measurement revealed that the water contact angle of the BTF sample was nearly zero.
The XRD analysis of BC and BTF materials revealed distinct patterns, with BC showing three diffraction peaks at 2θ = 14, 17, and 22.35º, indicating contributions from I and I phases in its structure In contrast, the BTF material's diffraction peaks shifted slightly to the right, attributed to the presence of TA molecules with amorphous structures Notably, the XRD pattern did not display any crystalline peaks for the TA-Fe 3+ nanocomplexes, aligning with previous studies Additionally, EDS analysis indicated that BC comprised a composition of C (44.73%) and O (55.27%), while BTF contained 0.38% Fe, suggesting that Fe 3+ ions were bonded to the BC surface through chelation with TA.
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 is crucial for preserving the functional layers of bacterial cellulose (BC) when it is modified with TA This interaction is facilitated by the formation of hydrogen bonds between the hydroxyl (OH) groups of tannic acid and those of cellulose.
Tannic acid (TA) forms three distinct types of hydrogen bonds with cellulose, as illustrated in Figure 3.7 Firstly, hydrogen bonds are established between the oxygen atoms of TA's carbonyl (C=O) groups and the hydrogen atoms of cellulose's hydroxyl (OH) groups Secondly, additional hydrogen bonds arise from the interaction between the hydrogen atoms of TA's hydroxyl groups and the oxygen atoms of cellulose's OH groups Lastly, hydrogen bonds are formed between the oxygen in TA's hydroxyl groups and the hydrogen in 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+
FeCl 3 was employed to generate a layer of Fe(III)-tannin complexes after TA was applied to natural porous materials The production of Fe(III)-tannin complexes follows the diagram in Figure 3.8 Tannic acid consists of one glucose molecule combined with up to five of digallic acids Tannic acid is made up of one glucose molecule and up to five different digallic acids By displacing the hydrogen atoms of the phenol hydroxyl groups, ferric ions can form chelates with tannic acid [26] The dark color of the changed material is further explained by the iron-tannic complex's significant light absorption and the rough surface of the material [9].
The light absorption capacity of photothermal materials is crucial for enhancing the performance of SSG systems Measurements of the BC and BTF materials' absorption across wavelengths of 300–2500 nm reveal that BC exhibits an average absorption of about 40–50%, while freeze-dried BTF achieves approximately 98% absorption in the 300–900 nm range, though its stability varies between 60% and 95% in the 900–2500 nm range Under wet conditions, BTF maintains over 93% absorption across the entire wavelength spectrum The high absorption of BTF, derived from BC, can be attributed to three 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 However, BTF's absorption decreases beyond 750 nm due to the reduced absorption of iron-tannic acid complexes in that spectral region The absorption mechanism is based on ligand-to-metal charge transfer (LMCT), which involves excited-state charge transfer in transition metal complexes, where organic ligands enhance light absorption by transferring energy to the metal center.
Figure 3.9 Absorption spectra of the BC and BTF in dry/wet condition.
Figure 3.10 Thermal conductivity of BTF.
Understanding the thermal conductivity of the BTF layer in its wet state is crucial for solar steam generation We utilize an infrared camera to assess the thermal conductivity of the BNC layer when wet The infrared imaging reveals a temperature gradient across the thickness of the BTF layer, which is sandwiched between two slides at varying temperatures The thermal conductivity of the BNC layer is measured at 0.6641 W m -1 K -1, indicating it is only marginally higher than that of water, which is 0.6 W m -1 K -1.
The low thermal conductivity of BTF material effectively reduces heat transfer at the evaporator surface, thereby minimizing heat loss into the water mass and enhancing the efficiency of solar steam generation.
Figure 3.11 Contact angle of BTF.