Color pollution in water
Sources of color pollution
Color in water or wastewater can be due to the presence of metals, organic acids, microbiological agents, and/or industrial wastes
Industrial wastewater, particularly from sectors that utilize dyes, such as textiles, cosmetics, and plastics, is a significant source of color pollution in water Additionally, aqueous wastes like landfill leachate and food waste contribute to this issue The textile industry, known for its high water consumption, generates substantial volumes of wastewater, exacerbating the problem of water pollution.
The textile and garment industry encompasses various manufacturing stages, starting from the development of raw materials like cotton fibers to processes such as spinning, weaving, dyeing, and sewing Water usage in this industry varies based on fiber types, with the dyeing and finishing processes being the primary contributors to water consumption According to documentation for cleaner production in the textile and dyeing sector, Table 1.1 illustrates the water consumption associated with textile wastewater.
Table 1.1 Water comsumption of textile wastewater
Types Water consumption (m 3 /ton product)
Wastewater generated during the dyeing and finishing processes is highly variable, influenced by the types of raw materials, dyes, chemicals, and technological methods used Dye molecules, primarily composed of chromogenic groups, absorb visible light, with nitrogen dyes being the most notable category Examples include Orange G, Acid Orange 7, New Coccine, Acid Black 1, Tartrazine, Acid Yellow 17, and Congo Red, all characterized by at least one azo group (N=N) The complex aromatic structures of these nitrogenous dyes hinder their effective degradation through conventional biological treatments, necessitating alternative methods for discoloring colored wastewater Consequently, textile sewage significantly contributes to color pollution in water bodies globally, including in Vietnam.
Color pollutants in water are primarily influenced by human activities, although natural factors like rainfall and flooding can also contribute by dispersing these pollutants into surface water and groundwater The rapid urbanization and industrialization lead to increased wastewater generation, with many sources being discharged untreated due to inadequate environmental management Additionally, poor infrastructure, a lack of prioritization for environmental protection in some developing countries, and limited human resources exacerbate the issue of water pollution.
Status of color pollution in water
In 2019, the Vietnam General Statistics Office reported that the textile and garment industry contributed 16% to the country's GDP, generating $39 billion in export revenue However, despite this significant economic impact, the industry is responsible for discharging large volumes of wastewater containing high pollutant concentrations into the environment annually This wastewater is often released untreated or inadequately treated, failing to meet acceptable environmental standards.
Figure 1.1 Textile wastewater pollution at La Khe Panel, Duong Noi, Ha Dong
In Vietnam, numerous weaving and dyeing craft villages, such as Phuong La weaving village in Thai Phuong commune, are grappling with severe environmental pollution due to untreated wastewater The living conditions for residents are deteriorating as most production facilities lack effective wastewater treatment systems, often resorting to discharging waste directly into sewers This issue is exacerbated by the diminishing drainage systems connected to rivers and canals, as highlighted by the Vietnam Environment Administration in 2019.
Figure 1.2 Textille and dyeing wastewater of craft villages Phuong La (Hung
Ha, Thai Binh) is discharged into the environment
The textile and apparel industry generates significant wastewater, particularly during the dyeing and finishing stages This wastewater is characterized by its unstable composition, which varies based on the specific types of materials and processes used.
The textile industry utilizes various dyeing equipment, raw dyed materials, and chemicals, which can lead to significant environmental concerns Sewage generated from this sector often contains heavy metals and toxic substances that pose a threat to aquatic life and can adversely affect human health.
Sewage from the textile industry is characterized by elevated pH levels, high temperatures, and significant concentrations of organic matter, non-biodegradable substances, toxic compounds, and suspended or dissolved solids, particularly with a pronounced color parameter According to Centema (2010), Table 1.2 illustrates the concentration of wastewater generated from various sources within the textile sector.
Table 1.2 Components of textile wastewater
Dyed wastewater is notoriously challenging to treat due to its fundamental properties and unstable composition, particularly concerning color indicators The need to achieve specific color ratios often leads to increased processing costs, which directly impacts production expenses and significantly affects the competitiveness of businesses.
Effects of color pollution in water on environment and human health
While the textile and garment industry significantly boosts the country's economic development and enhances social security, its production activities also have detrimental effects on the environment and ecological systems.
Excessive dye contamination in wastewater significantly restricts light penetration in water bodies, adversely impacting photosynthetic activity This disruption can lead to imbalances in the natural ecosystems of aquatic flora and fauna, as the high coloration affects the overall health of receiving streams.
High levels of organic pollutants significantly reduce dissolved oxygen in water, harming aquatic life Among various pollutants, dyes stand out as they are visible even at low concentrations The impact of dyes on health can vary based on their type, concentration, and duration of exposure, leading to acute or chronic issues These effects may include skin irritation, respiratory problems, mental disorders, vomiting, and in some cases, dyes can be carcinogenic and mutagenic.
Therefore, the handling of textile and dyeing wastewater in particular, or the tackling of the source of color pollution in general, is a necessary and urgent mission.
Treatments for color removal
Chemical/Physical processes
Coagulation/flocculation is a widely utilized chemical-physical process for effectively removing coloring substances from wastewater, comprising three main phases: Coagulation, Flocculation, and Sedimentation The coagulation phase destabilizes colloidal suspensions by neutralizing their charges, while flocculation groups smaller particles for easier removal In the final sedimentation phase, these particles are separated from the water, achieving color removal efficiencies of up to 99% A significant advantage of this method is its ability to completely separate dyes from sewage through physical means, rather than just partially removing them as seen in some chemical methods However, a notable drawback is the substantial amount of chemical sludge generated during the process.
Adsorption onto activated carbon is a widely utilized technique for dye extraction, particularly effective for cationic, mordant, and acid dyes, while showing lesser efficacy for dispersed, direct, and reactive dyes The effectiveness of this method is influenced by the type of activated carbon and the characteristics of the wastewater being treated For example, a study demonstrated that 90% of the color from synthetic water dyed with Congo red could be removed using 1 g/L of activated carbon Alternative materials like clay, peat, coconut shells, zeolite, silica beads, sawdust, and wood chips are also effective for dye extraction due to their availability and low cost Sawdust, particularly from Tectona grandis, has shown removal rates exceeding 94% for crystal violet after 180 minutes at pH 7.5 However, using wood chips may require longer contact times for effective dye removal, though they can be incinerated after use, adding to their cost-effectiveness Despite these advantages, the inherent chemical properties of certain dyes can pose challenges in the adsorption process.
10 their incineration means the possible formation of toxic substances such as polycyclic aromatic hydrocarbons (PAHs) [10]
Membrane filtration is an effective physical method for color extraction from wastewater, offering advantages such as high temperature resistance, durability in harsh chemical environments, and significant color removal capabilities However, it also presents challenges, including high disposal costs for concentrated residues, elevated management expenses, and reduced efficiency at high flow rates Research indicates that ultrafiltration (UF) can achieve color removal rates of up to 90% from textile wastewater, while other studies report even higher efficiencies of 98% to 100% using UF and nanofiltration (NF) membranes.
Chemical oxidation processes
In recent years, studies have increasingly focused on advanced oxidation processes (AOPs) to address color problems in wastewater treatment Originally introduced in the 1980s for drinking water treatment, AOPs have since been adapted to tackle colored wastewater, driven by stricter regulations on the removal of persistent organic compounds like dyes and pesticides AOPs utilize highly reactive radical species, such as hydroxyl radicals (OH•) and sulfate radicals (SO4•-), which possess significant redox potentials (OH•: 2.80 V; SO4•-: 2.60 V) compared to traditional oxidants like ozone (2.08 V) and chlorine (1.36 V) This high reactivity enables AOPs to effectively and rapidly decompose various pollutants, including chromophores, ensuring cleaner water.
Ozone (O3), along with hydrogen peroxide and chlorine compounds, is a notable agent of chemical oxidation With a high redox potential of 2.08 V, ozone acts as a powerful oxidant, selectively reacting with ionized and dissociated organic compounds However, it is an unstable gas with a limited lifespan Efforts are underway to enhance ozone production efficiency and reduce costs Factors such as pH, contact time, and dye concentration significantly impact color removal, with longer contact times leading to more effective decolorization Additionally, both basic and acidic pH conditions facilitate complete water decolorization in a shorter timeframe.
Ozone treatment of wastewater under neutral pH conditions can effectively reduce contaminants; however, a significant drawback is the potential formation of toxic by-products (OBPs) Notably, these by-products include bromate (BrO3-) and nitrosamines, such as N-nitrosodimethylamine (NDMA), which are considered possible human carcinogens.
Another chemical oxidant can also be used to discolor the wastewater, namely hydrogen peroxide At room temperature, it tends to dissociate into water and oxygen
Hydrogen peroxide (H2O2) has a high redox potential of 1.78 V, making it an effective oxidizing agent for organic compounds, including dyes However, it may not be sufficient alone to decolorize heavily pigmented wastewater To enhance its effectiveness, various combinations have been explored, such as incorporating biological enzymes like peroxidases to catalyze the dissociation reaction Factors such as temperature, pH, and peroxidase concentration significantly influence the efficiency of the treatment process.
Chlorine compounds can effectively discolor wastewater by targeting the amino groups in dye molecules, facilitating the removal of nitrogen bridges However, their efficiency is limited when dealing with reactive dyes and metal complexes, often requiring prolonged contact times and resulting in residual coloration Additionally, reactions between chlorine compounds and organic substances can produce toxic by-products, such as N-nitrosodimethylamine and trihalomethanes like chloroform and bromoform, which have mutagenic and carcinogenic properties Consequently, strict regulations have been established to limit the use of chlorine in industrial applications and during the final stages of wastewater treatment.
The advanced processes consist of some main categories such as O 3 -based, H 2 O 2 - based, fenton-related, wet air oxidation, TiO 2 /UV, sulfate radical-based [3]
Ozone-based process exploit the combination of O 3 and hydroxyl radical OH • [17]
Hydroxyl radicals (OH•) are known to target both ionic and neutral forms of organic substances These radicals can be generated through the addition of catalysts, which may include metal oxides and ions like MnO2, TiO2-Me, Fe2+, and Fe3+ Additionally, ultraviolet irradiation can also facilitate the production of these reactive species.
26]; adjusting pH [27], combining peroxone [22] or inserting H 2 O 2 to the process of
The direct photolysis of hydrogen peroxide (H2O2) generates hydroxyl radicals upon absorbing ultraviolet radiation, particularly at a wavelength of 254 nm This H2O2/UV process is effective for the degradation of dyes, chlorophenols, and other chlorinated compounds, showcasing its strong disinfecting capabilities However, the operational costs of this treatment method vary based on the type and concentration of the dye, the wastewater flow rate, and the required removal efficiency.
The Fenton process utilizes the Fenton reagent, a powerful oxidizing agent that generates hydroxyl radicals through the combination of ferrous ions (Fe²⁺) and hydrogen peroxide (H₂O₂) This rapid reaction is facilitated by the regeneration of ferrous ions from the reduction of ferric ions (Fe³⁺) by hydrogen peroxide The use of Fe²⁺/H₂O₂ as a bleaching agent for wastewater is advantageous due to four key reasons: iron is abundant, it produces no toxic by-products, hydrogen peroxide is user-friendly, and it poses minimal environmental risks However, this process results in the formation of ferric hydroxide, which must be disposed of as chemical sludge, leading to increased costs In the Photo-Fenton process, ultraviolet (UV) rays further enhance the generation of hydroxyl radicals and facilitate the conversion of Fe³⁺ back to Fe²⁺, enabling the rapid degradation of chromophores and organic matter.
The conventional wet air oxidation (WAO) process can be carried out on wastewater
In the presence of air or pure oxygen at specific temperatures and pressures, wastewater containing chromophoric substances requires treatment for effective color removal The degradation process is complex, with the rate of color removal primarily influenced by temperature, oxygen levels, and the characteristics of the chromophore Catalytic Wet Air Oxidation (CWAO) has been utilized, employing catalytic complexes like Cu/CNFs (Carbon Nanofibers) and activated carbon to enhance the efficiency of removing stubborn dyes during this process.
Photocatalysis using TiO2 and UV radiation has gained significant attention in recent years for its effectiveness in wastewater color removal, a key process in advanced oxidation processes (AOPs) The treatment initiates when UV-generated photons, possessing sufficient energy to surpass the TiO2 bandgap, interact with the catalytic surface This interaction promotes the transfer of electrons from the valence band to the conduction band, creating electron-hole pairs However, a significant challenge arises as many of these electrons quickly recombine, releasing thermal energy and limiting the overall efficiency of the photocatalyst.
In recent years, titanium dioxide (TiO2) has been synthesized in two main forms: as particles or in immobilized systems Each of these approaches presents unique advantages and disadvantages Researchers have focused on the challenges of photocatalyst separation post-treatment and have explored alternative synthesis methods for TiO2.
Besides, the sulfate radical-based process was known as using SO 4 •- to create OH •- to increase the efficiency of S 2 O 8 2- , which is a powerful oxidant.
Electrochemical processes
Electrochemical processes have shown effectiveness in decolorization while minimizing the use of oxidants The combination of TiO2, UV light, and electrical bias highlights the potential of integrating photocatalysis with electrochemical methods This electro-photocatalytic approach reduces the recombination of photogenerated electron spaces, enhancing efficiency Furthermore, the electro-Fenton process is gaining traction for its capability to electrochemically degrade organic pollutants, including chromophores found in wastewater.
Anodic oxidation offers several advantages as an alternative to traditional processes, including minimal need for chemical oxidants, no requirement for specialized equipment, and the absence of harmful by-products However, its low stability and high operating costs currently limit its economic viability, despite its effectiveness in removing staining and organic substances Meanwhile, research on electrocoagulation is still in progress, and the number of trials focused on dye removal from wastewater remains limited.
Biological processes
Microbial biomass reactors (RBM) represent a promising method for the removal of colorants, particularly azo dyes, from wastewater Currently under research, these systems have shown that sequences of anaerobic-aerobic treatment can effectively reduce dye concentrations, yielding satisfactory results in the decolorization process.
Promising microbial cultures for dye removal from wastewater include Pseudomonas, Aeromonas jandaei, and Bacillus firmus Key factors influencing wastewater discoloration are the type and concentration of dye, contact time, availability of alternative electron acceptors, biomass concentration, and dye toxicity Additionally, fungal treatments are effective even against dyes that resist typical microbial degradation Certain fungi, such as Trametes versicolor, P chrysosporium, and Hirschioporus larincinus, utilize enzymes like lignin peroxidase and manganese peroxidase to degrade dyes efficiently.
Color removal from water by nanomaterial-cellulose membranes
Basis for methods of color removal in water
Various methods have been proposed for the removal of color from industrial wastewater, with no single ideal solution available The most effective treatment for a specific dye should be determined on a case-by-case basis Recently, there has been a growing trend towards combining different treatment methods, which has shown promising results in terms of discoloration and efficiency This approach is expected to gain even more popularity in the future.
This research explores the application of membrane filtration combined with photocatalysis for effective color removal in water Recent advancements in membrane technology have enhanced its role in water and wastewater treatment, offering benefits such as reduced equipment size, lower energy consumption, and decreased investment costs As a sustainable solution, membrane technology minimizes or eliminates chemical usage, making it an accessible and eco-friendly option for wastewater treatment Consequently, it has become an increasingly attractive choice for achieving cost-effectiveness and sustainability in this field.
Membranes are classified into two main categories based on their structure: organic and inorganic Organic membranes, primarily composed of synthetic polymers like polyethylene, polytetrafluoroethylene, polypropylene, and cellulose acetate, are predominantly used in pressure-driven separation processes such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis In contrast, inorganic membranes are made from materials including ceramics, metals, zeolites, or silica, offering stability under various chemical and thermal conditions These inorganic membranes find extensive applications in industrial processes, particularly in hydrogen separation, ultrafiltration, and microfiltration.
Pressure-driven membrane filtration technologies, including microfiltration (MF) for large particle separation and reverse osmosis (RO) for salt ion removal, are among the most energy-efficient methods for water purification The relationship between pore size and pressure in various types of pressure-driven membranes is depicted in Figure 1.4.
Figure 1.4 Classification of pressure-driven membrane filtration, and their pore size and pressure relationship
Dye molecules, characterized by their chromogenic or aromatic structures, can absorb visible light, making them visible to the naked eye There are twelve known classes of chromogenic groups, with nitrogen dyes being the most prominent, including examples such as orange G, acid orange 7, new coccine, acid black 1, tartrazine, acid yellow 17, and Congo red, all of which contain at least one azo group (N=N double bond) The complex aromatic structures of these nitrogen-containing dyes pose challenges for degradation through conventional biological treatment methods, necessitating alternative approaches for the effective decolorization of colored wastewater.
Discoloration in wastewater refers to the loss of color, indicating a change in appearance, while the water may still contain significant organic matter In contrast, degradation involves the breakdown of substances, highlighting the distinction between mere color change and the actual decline in water quality.
This study explores the role of membranes as carriers for catalysts that can absorb and degrade pollutants under radiation conditions Through advanced oxidation processes, free radicals generated by photocatalytic activity engage in redox reactions, transforming complex pollutants into simpler, less toxic substances.
Mechanism for color treatment in water
The process of separation in water purification is based on the natural movement of impurities, such as molecules or metal ions, from areas of high concentration to low concentration When external pressure is applied, these impurities can then flow from low to high concentration zones, facilitating purification Generally, membranes with smaller pore sizes necessitate higher pressure for effective operation.
Membranes can be classified into two main categories: porous membranes, which include microfiltration (MF) and ultrafiltration (UF) with average pore sizes greater than 1 nm, and non-porous membranes, which encompass nanofiltration (NF) and reverse osmosis (RO) with theoretical pore sizes equal to or smaller than 1 nm The primary removal mechanism for MF and UF membranes is size exclusion, allowing the filtration process to operate independently of pressure and contaminant concentration In contrast, NF and RO membranes rely on solubility and diffusivity differences for contaminant removal, making their operation dependent on pressure and solute concentration.
Current membrane technology is the most efficient method for removing contaminants from water, but it relies on costly synthetic materials, which are unsustainable Therefore, developing cost-effective and sustainable water treatment technologies is crucial This article explores the use of cellulose, derived from agricultural residues and underutilized biomass waste, as a sustainable solution for water treatment Cellulose membranes can significantly reduce separation costs while effectively removing various pollutants through size exclusion and adsorption These membranes offer a promising low-cost platform for pressure-driven filtration techniques, including microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.
Cellulose materials present a sustainable alternative to carbon nanotubes, offering significant advantages in various aspects A comparison of cellulose nanomaterials and carbon nanotubes reveals differences in material properties and life cycle assessments, including manufacturing costs, applications, ecotoxicity, and end-of-life disposal While cellulose nanomaterials are priced between $1 and $5 per gram, carbon nanotubes range from $8 to $280 per gram Additionally, research indicates that cellulose materials can effectively replace products derived from coal, highlighting their potential for environmentally friendly applications.
Cellulose nanomaterials offer numerous biodegradation pathways and low toxicity, making them an attractive alternative to carbon nanotubes However, significant differences exist between these materials, limiting their complete substitution Unlike carbon nanotubes, unmodified cellulose nanomaterials lack electrical conductivity and photocatalytic properties Additionally, while the tensile strength of cellulose nanomaterials has improved, it remains over ten times lower than that of carbon nanotubes Despite these challenges, the potential for substituting carbon nanotubes with cellulose nanomaterials hinges on advancements in modification techniques and the design of cellulose nanomaterials-based composites.
Cellulose membranes are effective in separating pollutants but do not possess degradation capabilities Therefore, integrating the membrane process with heterogeneous photocatalysis is a practical approach to prevent membrane fouling, leveraging the strong oxidative power of photocatalysts.
Titanium dioxide (TiO2) is the most widely researched and utilized photocatalyst, featuring multiple crystal structures The three primary forms of TiO2 are anatase, brookite, and rutile, each contributing to its diverse applications in photocatalysis.
Figure 1.5 Crystal structure of titanium dioxide phases of rutile, brookite and anatase [46]
TiO2 exists in three phases: anatase, rutile, and brookite, with anatase and rutile being the preferred photocatalysts for UV light irradiation in the wavelength range of 250 nm to 380 nm When exposed to UV light, photons with energy equal to or exceeding the band gap energy (Eg) excite electrons from the valence band to the conduction band, creating positive holes in the valence band The band gap energy for TiO2 anatase is 3.2 eV, while for rutile, it is 3.0 eV The resulting photogenerated electron-hole pairs migrate to the surface of TiO2, where they engage in redox reactions.
The most frequently adopted photodegradation mechanism relies on the Langmuir- Hinshelwood kinetic model:
TiO 2 + hv → h + + e - h + + e - → heat h + + (H 2 O/OH•)s ↔ OH• (aq) e - + O 2 ↔ O 2 - Reactant sol + S ↔ Reactant OH• + Reactant →Products When the electron in the balance band of the semiconductor absorbs a photon that has an energy greater than the band gap (ΔE) of the semiconductor, the electron becomes excited and jumps to the conduction band, leaving a positively charged hole in the valence band In addition to recombining with the electron, the positively charged hole can oxidize water molecules to form hyper-reactive hydroxyl free radicals (OH•) The formed hydroxyl radicals are the main agent that attack the chemical pollutant molecules or dyes to eliminate color from water The excited electron can react with dissolved oxygen molecule to produce an oxygen radical, which is also active toward organic pollutants or dyes Figure 1.6 visually represents the mechanism of the photocatalytic reaction
Figure 1.6 Mechanism of photocatalytic reaction [48]
Titanium dioxide (TiO2) is primarily active under UV light, which constitutes less than 5% of solar energy To harness visible light for TiO2 photocatalysis, various strategies have been explored, including the successful modification of TiO2 surfaces with metal nanoparticles to improve its photocatalytic efficiency.
Metal nanoparticles refer to nano-sized metals that range from 1 to 100 nm in dimensions, exhibiting distinct physical and chemical properties compared to their bulk counterparts.
Metal nanoparticles exhibit 19 key characteristics, including a high surface area to volume ratio compared to bulk materials, significant surface energies, and unique optical properties that manifest as color variations They also demonstrate plasmon excitation and quantum confinement effects, along with short-range ordering and an increased number of kinks Additionally, metal nanoparticles possess a large number of coordination sites, such as corners and edges, which contribute to their specific chemical properties and capacity to store excess electrons.
Gold nanoparticles have emerged as a remarkable catalyst, particularly in enhancing the photocatalytic activity of widely used semiconductors like TiO2 Recent studies have explored their role as dopants or surface modifiers, demonstrating significant improvements in photocatalytic performance.
Surface Plasmon Resonance (SPR) is a crucial technique for assessing the adsorption of materials on planar metal surfaces, particularly gold and silver, as well as on metal nanoparticles SPR occurs at visible frequencies for noble metals like gold, where gold nanoparticles exhibit a strong SPR peak that enhances light trapping and leads to photoexcitation, creating a locally intensified electric field nearby This phenomenon significantly boosts solar energy conversion efficiency by broadening light absorption into longer wavelengths and enhancing light scattering The characteristics of SPR absorption and scattering, including frequency and cross-section, are influenced by factors such as metal composition, nanoparticle size and shape, the dielectric properties of the surrounding medium, and inter-particle interactions.
The ability to amplify long-wavelength light significantly enhances the absorption of sunlight in semiconductors across the visible to near-infrared spectrum This enhancement occurs as the energy from incident photons is transformed into plasmon oscillations, a process facilitated by the large scattering cross-section associated with surface plasmon resonance (SPR) When metallic nanoparticles are integrated into solar materials or devices, they scatter incoming light and locally boost the electromagnetic field, resulting in an increased effective absorption cross-section and a longer optical path length within the semiconductor Consequently, gold nanoparticles effectively address the limitations of TiO2, which has a weak optical response in the visible range—responsible for nearly 45% of solar energy—thereby enhancing the practical applications of catalyst materials.
Researches on nanomaterial-cellulose membranes
Researches on nanomaterial-cellulose membranes fabrication
Addressing the challenges of finite resources, increasing demand, and the need for sustainable production and consumption is crucial for humanity Biomass plays a vital role in sustainable development, as plant-based biomass and waste can be utilized for various purposes, including feed, energy, and industrial applications While much of the biomass is typically combusted to produce biogas, bioethanol, and biodiesel, selectively separating its main components—cellulose, hemicellulose, and lignin—allows for their direct use without the need for chemical modifications or combustion.
Cellulose is a crucial biopolymer derived from biomass, known for its environmentally friendly properties, low cost, excellent hydrophilicity, non-toxicity, and biodegradability Due to these attributes, it finds extensive applications across various industries, including manufacturing, membrane filtration technologies, and biomedical fields Additionally, cellulose is increasingly utilized in innovative areas such as magneto-responsive composites, bio-imaging materials, and as a support for catalysts.
Recently, we demonstrated a straight forward alternative towards regeneration of cellulose using non-toxic chemicals [55]
Cellulose is the primary source of biomass globally, predominantly produced by plants, and serves as the main structural component of plant cells and tissues This natural polymer is biosynthesized from long chains of anhydroglucose units (AGU), with each cellulose molecule containing three hydroxyl groups per AGU, except for the terminal unit.
Cellulose molecules are characterized by their average degree of polymerization, highlighting their unique properties as a highly insoluble substance This includes being insoluble in water and most organic solvents, with only partial solubility in select solvents The limited solubility of cellulose is primarily due to the robust inter- and intramolecular hydrogen bonds that exist between its individual chains.
Cellulose, due to its inert properties, is increasingly used as a matrix in various applications, including batteries and photocatalytic membranes, where transparency and permeability are crucial The photocatalytic process, which harnesses solar energy, is gaining traction as an effective method for water treatment, particularly in developing countries facing rising demands for high-quality water amid population growth and strict health regulations This has prompted scientists to innovate suitable separation solutions, particularly through the integration of photocatalysis with membranes, facilitating the efficient separation of contaminants from reaction solutions Consequently, research is focused on developing advanced photocatalytic membrane reactors that significantly enhance the removal and degradation of hazardous substances from wastewater.
A novel and eco-friendly approach was developed to incorporate TiO2 into regenerated cellulose membranes through a cellulose dissolution-regeneration process These TiO2-infused membranes demonstrated excellent photocatalytic activity and distinct absorbance capabilities This research highlights a promising application for energy-efficient dye removal systems, utilizing two concurrent methods: absorption and photocatalytic decomposition.
Cellulose-TiO2 nanocomposites have been effectively synthesized through non-solvent induced phase separation from cellulose acetate solutions, followed by deacetylation This process successfully immobilized photocatalysts within porous cellulose acetate, showcasing strong catalytic activity Various nanocomposite cellulose membranes with distinct structures and properties were produced; however, a slight reduction in catalytic performance was noted after cellulose generation.
Another study investigates the fabrication of TiO 2 -cellulose hybrid nanocomposite and demonstrates that obtained material can be a potential candidate for an inexpensive, flexible and disposable glucose biosensor [58]
This study investigates the structure and properties of cellulose ultrafiltration membranes enhanced by the incorporation of TiO2 nanoparticles into the cellulose matrix The addition of titanium dioxide particles is a promising approach to developing ultrafiltration membranes that exhibit superior operational performance.
TiO2-P25, a combination of anatase and rutile phases of titanium dioxide, is recognized for its excellent properties, including high chemical stability, low toxicity, and cost-effectiveness However, pure nano-TiO2 primarily absorbs UV light, which constitutes less than 5% of solar energy To harness visible light, researchers have explored metal doping as a strategy to enhance TiO2's photocatalytic performance By incorporating various noble metals such as Ag and Pt, the recombination of electron-hole pairs can be slowed down, thereby improving the absorption of visible light and boosting overall photocatalytic efficiency.
Gold-doped TiO2 demonstrates superior photocatalytic performance compared to other noble metal-doped TiO2 This enhanced activity is attributed to surface plasmon resonance, which allows the photo-excitation of gold nanoparticles to release electrons under visible light These electrons are then transferred to the conduction band of TiO2, facilitating oxidation-reduction reactions In contrast, TiO2 without noble metal doping shows minimal photocatalytic activity in the visible spectrum.
Research indicates that doping TiO2 thin films with pure and gold (Au) nanoparticles in varying Au/Ti ratios significantly alters their structural, morphological, and optical properties.
A prepared thin film supported on bacterial cellulose under the presence of the Au NPs and TiO 2 was produced and characterized [62]
Recently, carbon nitrides have known as a fastinating semiconductor that shows many advantages as high chemical stability, strong thermal stability, free-pollution
[50] So called sol-gel derived carbon nitride (sg-C 3 N 4 ) was employed in a wide range applications in decomposition of organic pollutants [63-65] to water splitting
[66] due to its abundance and out-standing visible light absorption However,
Despite its low cost and band gap of approximately 2.7 eV, the photocatalytic performance of sg-C3N4 is hindered by the recombination of photo-generated electron-hole pairs This challenge, similar to that faced by TiO2, can be mitigated by incorporating noble metals like gold Notably, carbon nitride semiconductors have been recognized for their ability to function as visible light-active semiconductors even in the absence of metal doping.
Various techniques are utilized to integrate nanomaterials into membrane matrices, including phase separation, hydrothermal methods, dip-coating, and in situ techniques Research indicates two primary methods for creating nanomaterial cellulose membranes The first method involves modifying the membrane surface with nanoparticle layers, achievable through dip-coating, electrospraying, or gas phase photocatalyst deposition However, this method poses a significant risk of photocatalyst leaching.
A novel method for membrane fabrication involves incorporating a photocatalyst directly into the cellulose matrix This process begins with the preparation of a cellulose solution by dissolving pure cellulose powder in ionic liquids, followed by coagulation using organic carbonates Propylene carbonate, a type of organic carbonate, acts as a promoter to facilitate rapid precipitation, enhancing the overall membrane formation.
To the best of our knowledge, this technique was not reported beforehand for the synthesis of cellulose-nanomaterial composites
This research aims to integrate a novel methodology for cellulose regeneration with the application of organic and inorganic photocatalysts We focus on developing transparent cellulose membranes from dissolved cellulose, as well as homogeneous photocatalyst-cellulose membranes infused with photoactive Au/TiO2 and Au/sg-C3N4 The creation of nanomaterial-cellulose blended membranes is identified as the most suitable approach for this study.
The Deposition-Precipitation (DP) method was used to prepare Au/TiO 2 and Au/sg-
Application of nanomaterial-cellulose membranes
Nanomaterial-cellulose membranes in water treatment technologies was reported In
In 2003, S.-J Yao developed a spherical TiO2-densified cellulose composite matrix for EBA applications J Zhou successfully created composite films from cellulose and two types of nanocrystalline TiO2 particles using a NaOH/urea aqueous solution, demonstrating a method for producing functional composite materials without compromising the structure and properties of the nanoparticles Additionally, a review highlighted the advantages of cellulose nanomaterials in environmental remediation and water filtration membranes, emphasizing their high surface area-to-volume ratio, low environmental impact, strength, functionality, and sustainability This research compared cellulose nanomaterials to carbon nanotubes regarding their physical and chemical properties, production costs, and disposal methods, illustrating cellulose's potential as a sustainable alternative in water treatment technologies Furthermore, studies on cellulose-gold nanoparticle composites discussed various preparation methods and applications, noting their effectiveness as catalysts for reducing 4-nitrophenol to 4-amino-phenol in the presence of NaBH4.
Conventional photocatalytic reactions typically occur in batch reactors using suspended photocatalyst powders, which complicates the recovery process due to the need for separation from treated water By immobilizing photocatalysts within a membrane matrix, a continuous reactor system can be established, eliminating the need for separation and thereby reducing operational costs This approach not only enhances the effectiveness and controllability of membrane filtration processes but also improves the thermal and mechanical stability of the photocatalysts Furthermore, nanomaterial cellulose membranes are recognized for their potential applications beyond water treatment, such as in glucose biosensors and as cancer markers.
In the early 1990s, researchers began utilizing ceramic and cellulose membranes to immobilize TiO2, marking the inception of photocatalytic membrane reactors (PMRs) This innovative combination of photocatalysts and cellulose membranes has led to a significant rise in publications related to PMRs, particularly in the context of water treatment By 2016, there were 132 publications on PMRs, showcasing a substantial increase compared to earlier decades PMR configurations for water treatment can be primarily categorized based on the deployment state of the photocatalysts.
PMRs with photocatalyst suspended in feed solution; PMRs with photocatalyst immobilized in the membrane
This research explores the effectiveness of composite films in the photodegradation of colored solutions using a bath reactor and Photocatalytic Membrane Reactors (PMRs) PMRs offer several advantages over traditional photocatalytic reactors, including the confinement of photocatalysts through membrane technology, enabling continuous processes that separate photocatalysts and products, and facilitating the removal of photocatalysts from treated water These features enhance process controllability, stability, and efficiency, while also allowing for the easy recovery and reuse of photocatalysts in future runs Additionally, PMRs contribute to energy savings and reduce installation size, eliminating the need for additional operations like coagulation, flocculation, and sedimentation required in conventional systems to maintain effluent quality.
2 RESEARCH METHODS AND EXPERIMMENTAL PROCESSES
Chemicals and apparatus
Chemicals
- Tetrabutylphosphonium hydroxide (TBPH) containing 40 wt.% in water was purchased from Acros
- Microcrystalline cellulose (MCC) with the size 20 àm used in this study were purchased from Sigma Aldrich
- Propylene carbonate (PC, 99.7 % purity), dimethyl sulfoxide (DMSO, Sigma Aldrich), HAuCl 4 3H 2 O were obtained from Sigma Aldrich NaOH was used for adjusting the pH
- TiO 2 P25 (anatase/rutile = 85:15, Evonik, S BET = 50 m 2 /g) sg-C 3 N 4 was provided from Department of Chemistry, Technical University Berlin
- Polyethylene glycol (PEG) with mollecular weight in the range as follows: PEG 8000, PEG 35000, PEO 200000, PEO 400000) were provided from Sigma Aldrich
- Methylene blue (MB), Thymol blue (TB), Eosin Y disodium salt (EY) were obtained from Sigma Aldrich
Apparatus
- Casting machine (TQC Automatic Film Applicators)
- PMR system, casting knife, glass plate, coagulation bath were designed by
Mr Peter Kumm in the Technical Workshop, Institute of Chemistry, University of Rostock
- Xe-Lamp 300W (LOT), Ikamag RCT basic magnetic stirrer with heater (IKA), Performance Liquid Chromatography (HPLC) system (Knauer)
- Volumatric flasks, glass vials, analytical balance (VWR).
Research methods
Research methods of fabrication of nanomaterial-cellulose membranes:
Inherit and develop the research that is currently underway on the RoHan project to create a effectively applicable material in PMRs
A preliminary investigation into the fabrication of nanomaterial-cellulose membranes reveals critical factors such as the choice of solvents for cellulose dissolution, the solvent-to-cellulose ratio, and the sequence of substance addition These conditions play a significant role in optimizing the membrane production process.
The Au/TiO2 catalyst is synthesized in batches using the Deposition-Precipitation (DP) method, which involves two key processes The first process is precipitation, where a solid is formed from bulk solutions or pore fluids, relying on gravity to settle the solid particles effectively.
The deposition-precipitation technique involves the interaction of precipitate particles with a support surface, relying on processes such as supersaturation, nucleation, growth, and settling This method begins with the formation of slurries using powders or particles of the desired salt in sufficient quantities to achieve the target loading, followed by the addition of an alkali solution to induce precipitation.
This research utilized the deposition-precipitation (DP) method, which involved heating an aqueous solution of HAuCl4·3H2O and subsequently adding TiO2 support A comparable approach was employed for the preparation of Au/sg-C3N4, with detailed procedures outlined in section 2.3.1.2.
Nanomaterial-cellulose membranes was fabricated by blending photocatalysts (Au/TiO 2 , Au/sg-C 3 N 4 ) into regenerated cellulose solution Detailed procedures were illustrated in the section 2.3.1.3
The catalyst-free cellulose membrane was also carried out in parallel with samples containing the catalyst Each fabrication of 3-4 membranes to select the best films for analysis and application
Research methods of application of nanomaterial-cellulose membranes:
Preliminary tests for treating methylene blue are conducted in both batch and continuous reactions (PMRs), with detailed procedures outlined in section 2.3.2 of the experimental processes.
Experimental processes
Experimental processes of nanomaterial-cellulose membranes fabrication
2.3.1.1 Investigation of optimal reaction conditions
To optimize the process, microcrystalline cellulose (20 wt.%) is pre-dissolved in TBPH (50 wt.%, 200 mg) with a co-solvent added to reduce viscosity An experiment was conducted at room temperature (23°C) for 30 minutes, comparing DMSO (0.1:1 ml ratio with TBPH) to water as solvents for cellulose dissolution Following this, 5 wt.% of Au/TiO2 or Au/sg-C3N4 was added to the cellulose suspensions After thorough mixing, the resulting Au/TiO2 or Au/sg-C3N4 cellulose suspensions were cast onto a glass plate to a thickness of 500 µm and then quickly immersed.
DMSO as a solvent Ratios of DMSO:TBPH Coagulation condition in PC The order of addition of substances
Au/TiO 2 cellulose membranes (CellP) Au/sg-C 3 N 4 cellulose membranes (CellCN) Cellulose membranes (Cell)
Batch reator Photocatalytic membrane reactor (PMR)
TiO 2 , Au/TiO 2 , sg-C 3 N 4 , Au/sg-C 3 N 4
29 propylene carbonate bath for coagulation and washed with deionized water The results showed that DMSO is essential for the formation of transparent films
The study investigated the impact of varying DMSO:TBPH ratios (0.1:1, 0.2:1, 0.35:1, 0.6:1, and 1.1:1) on membrane surface characteristics A 5 wt.% concentration of photocatalysts (either Au/TiO2 or Au/sg-C3N4) was incorporated into a cellulose solution After thorough mixing, the resulting cellulose suspensions were cast onto a glass plate to a thickness of 500 µm The cast liquid was then rapidly immersed in a propylene carbonate bath for coagulation and subsequently washed with deionized water The optimal DMSO ratio was identified based on the homogeneity of the membranes and the distribution of photocatalysts.
The investigation explored the optimal order of addition for various substances in the reaction process Initially, microcrystalline cellulose (20 wt.%, 200 mg) was dissolved in TBPH (50 wt.% in 1 ml of water), with the ideal DMSO:TBPH ratio determined to be 0.35:1 The study examined three methods of adding the substances: (i) incorporating Au/TiO2 powder directly into the cellulose solution, (ii) introducing a suspension of Au/TiO2 with 0.1 ml DMSO into the cellulose solution, and (iii) mixing the cellulose solution into a suspension of Au/TiO2 with 0.1 ml DMSO The results indicated that the order of addition significantly influenced the reaction efficiency.
In the experiment, varying concentrations of Au/TiO2 (10 wt.%, 15 wt.%, and 30 wt.%) were incorporated into the mixture Once the photocatalyst was thoroughly dispersed, the Au/TiO2 cellulose suspensions were cast onto a glass plate using a casting knife to achieve a thickness of 500 µm The cast liquid was then rapidly immersed in a propylene carbonate bath for coagulation and subsequently washed with deionized water The detailed results of this process are presented in section 3.1.
2.3.1.2 Nanomaterial preparation (Au/TiO 2 and Au/sg-C 3 N 4 )
Gold nanoparticles were doped into TiO2 using the deposition-precipitation method Initially, a 5 mM solution of HAuCl4 was heated to 70 ºC and adjusted to pH 7 with NaOH This solution was then diluted with distilled water and stirred for 15 minutes at the same temperature Following this, 1g of P25 support or 1g of sg-C3N4 was added, and the suspension was stirred for an additional hour at 70 ºC and one hour at 25 ºC The resulting catalysts were obtained through filtration and subsequently dried.
12 hours at 100 ºC and grinded to powder
2.3.1.3 Synthesis of nanomaterial-cellulose membranes
To create a uniform solution, cellulose powder must be dissolved in a 50% TBPH solution This higher concentration of TBPH, initially at 40 wt.%, is achieved through rotary evaporation at 60 mbar and temperatures below 40 °C The concentration of TBPH is verified using Karl Fischer Titration.
Microcrystalline cellulose (200 mg, 20 wt.%) was dissolved at 23 °C in a solution of TBPH (1 mL of 50 wt.% in water) and DMSO (0.35 mL) After stirring for 30 minutes at 300 rpm, varying weight ratios of Au/TiO2 or Au/sg-C3N4 (0 wt.%, 5 wt.%, 10 wt.%, 15 wt., and 30 wt.%) were added and mixed thoroughly The resulting nanomaterial-cellulose suspension was cast onto a glass plate to a thickness of 500 µm using a TQC Automatic Film Applicator The cast liquid was then quickly immersed in a PC bath for coagulation and subsequently washed with deionized water to yield homogeneous films.
Figure 2.2 Steps of CellP and CellCN synthesis
The Au/TiO 2 cellulosefilms and Au/sg-C 3 N 4 cellulose films are named as CellP and CellCN, followed by the amount of catalyst, respectively (Figure 2.2 and Figure 2.3)
Figure 2.3 Procedure of CellP and CellCN fabrication
Investigation on photocatalytic performance of nanomaterial-cellulose
The photocatalytic performance of nanomaterial-cellulose membranes was evaluated for the degradation of methylene blue at a concentration of 6 ppm in both batch and continuous reactors To prepare the dye solution, 3 mg of methylene blue was dissolved in a 500 ml volumetric flask and diluted to the mark with deionized water.
2.3.2.1 Photocatalytic measurements in batch reactor
Pure photocatalysts as well as nanomaterial-cellulose membranes were tested as powders in a batch reactor via photodegradation of methylene blue (concentration
At 23°C, experiments were conducted using a batch reactor with dried and ground nanomaterial-cellulose membranes, utilizing a Xe-lamp (LOT Oriel, 300W) as the light source The lamp operated without a filter for UV-Vis irradiation and included a 420 nm cut-off filter for specific light wavelength control The concentration was measured at 6 ppm.
The study involved a 30 wt.% concentration of nanomaterial-cellulose mixture in 20 ml of methylene blue (MB) under stirring conditions Additional factors affecting degradation, such as light exposure without a catalyst and absorbance in nanomaterial-cellulose membranes without light, were also examined The amount of the nanomaterial-cellulose mixture and pure photocatalysts was calculated to achieve a concentration of 0.2 mg photocatalyst per ml of MB After a 30-minute reaction period, the concentration of the MB solution was assessed using a UV-Vis Spectrometer, following the separation of the photocatalysts through centrifugation.
Figure 2.4 Photocatalytic performmance in batch reator
2.3.2.2 Photocatalytic measurements in continuous reactor (PMRs)
The PMR system is illustrated in Figure 2.5 and Figure 2.6 The PMR cell (Figure 2.7), which is made from poly tetra fluoroethylene (PTFE) (60 mm x 25 mm x 80
The PMR cell, measuring 32 mm and with a total volume of 5 ml, features two separate compartments divided by a central membrane The front compartment is designed with a glass surface to allow light to penetrate the composite membrane, thereby activating the photocatalytic process In this study, a feed solution of methylene blue at a concentration of 12 ppm was pumped into the first compartment of the PMR cell at a flow rate of 1.5 ml/min, corresponding to a pressure of approximately 0.65 bar For the photoreactions, a UV-Vis light source (Xe-Lamp 300W, without a filter) was utilized.
The membrane functions as a selective barrier that effectively removes contaminants, while the photocatalyst facilitates photocatalytic oxidation Subsequently, the absorbance of the permeate is analyzed using UV-Vis spectroscopy.
Figure 2.5 Schematic diagram of lab scale PMR system
Figure 2.6 PMR system in laboratory
Mr Peter Kumm conducted the construction of the system at the Technical Workshop of the Institute of Chemistry at the University of Rostock, where the permeate solution was analyzed using UV-Vis spectroscopy The absorbance measurements were taken every minute as the solution passed through a UV cell (Figure 2.8).
Influence of irradiation on photocatalytic performance in PMRs
This study examines the impact of irradiation on the treatment efficiency of methylene blue Initially, the reaction was conducted under the illumination of a Xe-Lamp, followed by turning off the lamp to assess how the absence of irradiation affects the results.
Measurements and analysis methods
Measurements and analysis methods of fabricated materials
Inductively coupled plasma - optical emission spectrometry (ICP-OES)
ICP-OES analysis, utilizing the Varian 715-ES ICP emission spectrometer from Agilent (Germany), is employed to determine the elemental composition of catalysts This technique specifically measures the deposited gold content in Au/TiO2, Au/sg-C3N4, and nanomaterial-cellulose composite membranes.
Fourier Transform Infrared Spectroscopy (FT-IR)
Infrared spectra of samples were recorded with a Bruker FT-IR Alpha II spectrometer in ATR mode The specimens were measured directly with a scan range from 500 cm -1 to 4000 cm -1
X-ray diffraction (XRD) is utilized for identifying the crystalline phases in materials The XRD powder patterns were obtained using a Panalytical X'Pert diffractometer with an Xcelerator detector and automatic divergence slits Measurements were conducted with a data collection time of 100 seconds per step, employing Cu radiation.
Ultraviolet Visible Spectra (UV-Vis)
The UV-Vis transmission spectra were obtained using a fiber optic system that included the AvaLight-DH-S-BAL Balanced Power Light Source and the AvaSpec-ULS2048 StarLine Versatile Fiber-optic Spectrometer from Avantes, enabling real-time detection of the dyes.
35 was performed using homemade filter equipment connected to a micro-flow cell (Avantes)
Transmission electron microscopy TEM – HAADF Spectra
TEM detected the formation of Au nanoparticles on the surface of TiO 2 and sg-
C 3 N 4 , and perfomed the distribution and particle size of Au nanoparticles as well.
A series of polyethylene glycol polymers were used as molecular probes For preparation of start solution 2000 ppm, 0.2 g of polyethylene glycol was taken in a
A 100 ml volumetric flask was prepared and filled to the mark with deionized water Subsequently, 5 ml of the filtrate was collected using a homemade filtration system, maintaining a feed flow rate of 1.5 ml per minute.
Figure 2.9 Home-made filtration system
The concentration was determined by automatic injecting samples into High Performance Liquid Chromatography (HPLC) system using the apparatus shown in Figure 2.10
Figure 2.10 Apparatus to measure PEG concentration after going through nanomaterial-cellulose membranes
In this apparatus, a pump provided a flow rate (0.1 ml min -1 ) of distilled water with a
20 àl sample loop was used to load the PEG solution after going through
36 membranes The signal is detected by RI detector with retention time (10 min) For each sample an average value of 3 injections was used to determine the average PEG concentration
PEG retention was calculated by following equation:
C s - The concentration of PEG solution after filtration (ppm)
Water flux (WF) refers to the flow rate through a membrane, measured in gallons per square foot of membrane filter surface area per day This rate can be calculated using a specific equation.
V - The volume of water goes through a membrane (L)
S - Surface area of the membrane (m 2 ) t - Time to take the volume of water (h)
The measurement was performed at a temperature of 23 °C, utilizing membranes with a diameter of 26 mm inserted into the filter The feed flow rate was systematically controlled by the pump at various levels, including 1 ml/min, 1.5 ml/min, 2 ml/min, 2.5 ml/min, 3 ml/min, 3.5 ml/min, 4 ml/min, 4.5 ml/min, and 5 ml/min.
The permeability (P) of a membrane is the rate of passive diffusion of molecules through the membrane
Photocatalytic measurement of fabricated materials
Photocatalytic measurement in bath reactor
The initial concentration of the methylene blue solution is 6 ppm After 30 minutes of light irradiation, a 2 ml sample was extracted and analyzed using UV-Vis Spectroscopy at a wavelength of 664 nanometers, compared to a reference sample or blank.
Photocatalytic measurement in continuous reactor (PMRs)
The online detection of dyes was conducted using a micro-flow cell (Avantes) and absorbance measurements were taken with Avantes spectrometers under UV-Vis conditions Initially, a starting solution with a concentration of 6 ppm was pumped through the system without a membrane, establishing a baseline absorbance Subsequently, the system was thoroughly cleaned by washing it three times with ethanol to remove any residual substances.
The nanomaterial-cellulose membrane is placed in the PMR cell after being dried with compressed air and cleaned of contaminants The methylene blue solution is pumped at a flow rate of 1.5 ml/min, and the absorbance of the solution passing through the membrane is continuously measured By analyzing the data on adsorbed concentration alongside the initial solution concentration, the treatment efficiency of the PMR system can be calculated over time.
Survey for optimal conditions and fabricated materials
The production of nanomaterial-cellulose membranes was achieved through a modified method, demonstrating that transparent cellulose films can be created via coagulation with organic carbonates, particularly utilizing DMSO for transparency This advancement allows for the integration of light-active nanoparticles, resulting in photoactive membranes Notably, visible light-active catalysts, such as inorganic gold on titanium oxide and carbon nitride photocatalysts, were synthesized using the Deposition-Precipitation method established in earlier studies.
Section 2.3.1.1 outlines the optimal reaction conditions for membrane preparation After immersion in PC for coagulation, the membranes detached effortlessly from the glass plate, requiring no additional stress for removal, allowing for easy peeling in water These films demonstrate stability and can be stored in deionized water for several weeks without degradation The films containing Au/TiO2 and Au/sg-C3N4 are designated as CellP and CellCN, respectively, with their naming followed by the catalyst amount, while pure cellulose films without nanoparticles are referred to as Cell.
The study successfully developed photocatalyst-infused regenerated cellulose membranes, characterized by smooth surfaces and uniform photocatalyst distribution The membranes displayed distinct colors, with Au/TiO2 membranes appearing purple and Au/sg-C3N4 membranes yellow, with color intensity increasing alongside photocatalyst quantity Additionally, incorporating the co-catalyst DMSO enhanced the films' flexibility and surface smoothness, facilitating easier and faster separation from glass panels Overall, the findings indicate that the photocatalysts are effectively integrated into the regenerated cellulose membranes for potential applications in PMR systems.
Recent advancements in membrane technology have led to the development of membranes with exceptional properties that surpass previous studies This research introduces regenerated cellulose films as a novel substrate for both inorganic and organic semiconductors modified with precious metals like gold (Au) These innovative materials efficiently harness visible light from solar energy to facilitate the photodegradation of water pollutants Additionally, the photocatalysts integrated into these membranes do not require separation from the treatment stream, resulting in significant cost savings Notably, with only 1% gold deposited in the membranes, the durability and remarkable photocatalytic efficiency of Au highlight its potential for reuse in environmental applications.
Systematic studies have shown that incorporating DMSO is crucial for achieving a uniform distribution of photocatalytic nanoparticles (NPs) and ensuring high-quality films The absence of DMSO leads to an uneven distribution of NPs, likely due to the high viscosity of the suspension.
Figure 3.1 NP-cellulose membranes of TiO 2 and CN with co-solvent
The absence of DMSO in a 20 wt.% cellulose solution in TBPH resulted in a viscous mixture comparable to honey, making it difficult to fully dissolve the cellulose powder This high viscosity hindered the effective distribution of photocatalysts within the solution Therefore, incorporating an appropriate amount of DMSO is crucial for producing regenerated cellulose membranes DMSO not only reduces the viscosity of the cellulose solution but also facilitates the mixing and dissolution of cellulose powder With DMSO, the membrane formation process during PC coagulation is expedited, resulting in films that are more flexible and have a smoother surface compared to those made without DMSO.
Figure 3.2 CellP with various ratios of DMSO:TBPH (ml:ml) 0.1:1 (A); 0.2:1
Figure 3.3 CellCN with various ratios of DMSO:TBPH (ml:ml) 0.1:1 (A); 0.2:1
The ratio of DMSO is crucial for achieving a uniform surface in membrane production Higher amounts of DMSO enhance membrane flexibility, yet ratios exceeding 0.35:1 (DMSO:TBPH) lead to uneven surface structures Excessive DMSO can create challenges, such as low viscosity that results in wave-like structures and lower film quality Additionally, maintaining a film thickness of 500 µm becomes difficult due to the overly liquid casting solution This surplus of DMSO also causes uneven distribution of photocatalysts and can deform the membrane during the quick immersion of the casting solution with photocatalysts Figures 3.2 and 3.3 illustrate cellulose membranes of Au/TiO2 and Au/sg-C3N4 with varying DMSO amounts.
The order of addition of substances is crucial for achieving an optimal quantitative ratio of photocatalysts In this study, the transfer of cellulose solution or Au/TiO2 suspension between clear glass vials was performed using needles, which introduced potential errors.
In the experiment, various methods for combining Au/TiO2 and cellulose solutions were explored Figure 3.4 illustrates three approaches: (A) introducing a suspension of Au/TiO2 mixed with 0.1 ml of DMSO into the cellulose solution, (B) incorporating the cellulose solution into the Au/TiO2 suspension with 0.1 ml of DMSO, and (C) directly adding Au/TiO2 powder into the cellulose solution Each method aims to optimize the integration of these materials for enhanced performance.
The transfer of viscous cellulose solution or catalyst suspension in cases (A) and (B) results in material loss due to adherence to glass vials or needles However, by directly adding Au/TiO2 powder into the cellulose solution, homogeneous membranes can be achieved without losing catalysts or cellulose It is essential for the nanoparticles to be dispersed directly within the cellulose solution The optimized preparation procedure for the photocatalyst-NP-Cellulose films is illustrated in Figure 3.5.
Figure 3.5 CellP, CellCN with different weight ratios of photocatalysts and Cell
Cellulose membranes typically appear white and uniform in color without catalysts However, when infused with Au/TiO2, the membranes exhibit a distinct purple hue, which intensifies with increasing amounts of Au/TiO2 Both CellP and CellCN demonstrate excellent surface distribution For experimental purposes, the membranes were precisely cut into 26 mm diameter circles to fit the PMR cell.