Supported titanium oxide as photocatalyst in water decontamination: State of the art

Novel eco-friendly and easily recoverable bismuth-based materials for capturing and removing polyphenols from water

Olive oil production is one of the most developed Europe's sectors, producing olive oil and undesirable by-products, such as olive mill wastewater (OMWW) and organic waste. OMWW, containing large amounts of compounds (mainly polyphenols, phenols, and tannins), represents a problem. In fact, polyphenols have dual nature: i) antioxidant beneficial properties, useful in many industrial fields, ii) biorefractory character making them harmful in high concentrations. If not properly treated, polyphenols can harm biodiversity, disrupt ecological balance, and degrade water quality, posing risks to both environment and human health. From a circular economy viewpoint, capturing large quantities of polyphenols to reuse and removing their residuals from water is an open challenge. This study proposes, for the first time, a new path beyond the state-of-the-art, combining adsorption and degradation technologies by novel, eco-friendly and easily recoverable bismuth-based materials to capture large amounts of two model polyphenols (gallic acid and 3,4,5-trimethoxybenzoic acid), which are difficult to remove by traditional processes, and photodegrade them under solar light. The coupled process gave rise to collect 98% polyphenols, and to rapidly and effectively photodegrade the remaining portion from water.

The spinning disc reactor for photocatalytic degradation: A systematic review

In recent years, the use of a horizontal spinning disc reactor (SDR) as a photocatalytic reactor for the degradation of various pollutants in aqueous solutions has increased. This study was searched based on the PRISMA method. Two autonomous researchers carried out for the relevant studies using Scopus, Web of Science (WOS), and Science Direct databases. The search terms expanded focusing on the performance of horizontal spinning disc photocatalytic reactor (SDPR). In this review article, the main objective of the effect of operational factors on the efficiency of the degradation of pollutants with changes in the type of light source (range of visible light and UV radiation), disc rotational speed, flow rate, initial concentration of pollutants, pH, type of disc structure and flow regime are considered. Current challenges in SDPR include issues such as limited mass transfer, uneven light distribution, and difficulties in scaling up. To overcome these challenges, improvements can be made by optimizing reactor design for better mass transfer, enhancing light distribution through advanced light sources or reactor configurations, and developing scalable models that maintain efficiency at larger scales. Additionally, the use of innovative materials and coatings could improve the overall performance of SDPR.

Imine-Decorated Copper-Based Metal-Organic Framework for the Photodegradation of Methylene Blue

A low cost imine-decorated linker, 4,4'-(hydrazine-1,2-diylidenedimethylylidene)dibenzoic acid was utilized for the preparation of copper-based metal-organic framework (MOF) denoted as Cu-L via a solvothermal technique. The synthesized MOF material has been fully characterized by different analytical techniques such as Fourier-transform infrared (FT-IR) spectroscopy, powder X-Ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-Ray spectroscopy (EDX), nitrogen adsorption-desorption isotherm analysis, and thermogravimetric analysis (TGA). It has been found that the coordination of Cu2+ with L considerably reduced the band gap of the L of nearly about 1 eV, which is approximately 26% decline in total. Notably, a narrow band gap of the photocatalyst is an essential requirement for the proficient photodegradation of organic contaminants. An excellent optical properties and narrow band gap of (2.8 eV) of Cu-L ensure their suitability as a photocatalyst for the degradation of methylene blue (MB) dye. In the presence of Cu-L photocatalyst, 84.22% degradation of MB dye was observed after 150 min under sunlight exposure. It is the first time that imine-functionalized MOF was utilized for the degradation of MB dye under sunlight irradiation. For understanding the photodegradation of MB dye by the Cu-L photocatalyst, all the plausible mechanistic studies have been carried out in detail. Both theoretical (with the help of density functional theory (DFT) calculations) as well as experimental studies have been conducted to justify the possible mechanisms for the photodegradation of MB dye by Cu-L. The current work may open a new opportunity to construct a cheap MOF-based photocatalysts for fast degradation of dye contaminants.

Preparation and Characterization of TiO2-Coated Hollow Glass Beads for Functionalization of Deproteinized Natural Rubber Latex via UVA-Activated Photocatalytic Degradation

The photochemical degradation of natural rubber (NR) is a prevalent method used to modify its inherent properties. Natural rubber, predominantly derived from the Hevea Brasiliensis tree, exhibits an exceptionally high molecular weight (MW), often reaching a million daltons (Da). This high MW restricts its solubility in various solvents and its reactivity with polar compounds, thereby constraining its versatile applications. In our previous work, we employed TiO2 in its powdered form as a photocatalyst for the functionalization of NR latex. However, the post-process separation and reuse of this powder present substantial challenges. In this present study, we aimed to functionalize deproteinized NR (DPNR) latex. We systematically reduced its MW via photochemical degradation under UVA irradiation facilitated by H2O2. To enhance the efficiency of the degradation process, we introduced TiO2-coated hollow glass beads (TiO2-HGBs) as photocatalysts. This approach offers the advantage of easy collection and repeated reuse. The modified DPNR showed a reduction in its number-average MW from 9.48 × 105 to 0.28 × 105 Da and incorporated functional groups, including hydroxyl, carbonyl, and epoxide. Remarkably, the TiO2-HGBs maintained their performance over seven cycles of reuse. Due to their superior efficacy, TiO2-HGBs stand out as promising photocatalysts for the advanced functionalization of NR across various practical applications.

Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation

Azo dyes are synthetic organic dyes used in the textile, leather, and paper industries. They pose environmental problems due to their toxic and persistent nature. The toxicity is due to the presence of azo groups in the dye molecule that can break down into aromatic amines, which are highly toxic to aquatic organisms and humans. Various treatment methods have been developed to remove azo dyes from wastewater. Conventional wastewater treatments have some drawbacks, such as high operating costs, long processing times, generation of sludge, and the formation of toxic by-products. For these reasons, a valid alternative is constituted by advanced oxidation processes. Good results have been obtained using heterogeneous photocatalysis and supercritical water oxidation. In the former method, a photocatalyst is in contact with wastewater, a suitable light activates the catalyst, and generated reactive oxygen species that react with pollutants through oxidative reactions to their complete mineralization; the latter involves pressurizing and heating wastewater to supercritical conditions in a reactor vessel, adding an oxidizing agent to the supercritical water, and allowing the mixture to react. In this review paper, works in the literature that deal with processing wastewater containing azo dyes through photocatalysts immobilized on macroscopic supports (structured photocatalysts) and the supercritical water oxidation technique have been critically analyzed. In particular, advancement in the formulation of structured photocatalysts for the degradation of azo dyes has been shown, underlying different important features, such as the type of support for the photoactive phase, reactor configuration, and photocatalytic efficiency in terms of dye degradation and photocatalyst stability. In the case of supercritical water oxidation, the main results regarding COD and TOC removal from wastewater containing azo dyes have been reported, taking into account the reactor type, operating pressure, and temperature, as well as the reaction time.

Olive Mill Wastewater Remediation: From Conventional Approaches to Photocatalytic Processes by Easily Recoverable Materials

Olive oil production in Mediterranean countries represents a crucial market, especially for Spain, Italy, and Greece. However, although this sector plays a significant role in the European economy, it also leads to dramatic environmental consequences. Waste generated from olive oil production processes can be divided into solid waste and olive mill wastewaters (OMWW). These latter are characterized by high levels of organic compounds (i.e., polyphenols) that have been efficiently removed because of their hazardous environmental effects. Over the years, in this regard, several strategies have been primarily investigated, but all of them are characterized by advantages and weaknesses, which need to be overcome. Moreover, in recent years, each country has developed national legislation to regulate this type of waste, in line with the EU legislation. In this scenario, the present review provides an insight into the different methods used for treating olive mill wastewaters paying particular attention to the recent advances related to the development of more efficient photocatalytic approaches. In this regard, the most advanced photocatalysts should also be easily recoverable and considered valid alternatives to the currently used conventional systems. In this context, the optimization of innovative systems is today’s object of hard work by the research community due to the profound potential they can offer in real applications. This review provides an overview of OMWW treatment methods, highlighting advantages and disadvantages and discussing the still unresolved critical issues.

Engineering Biomimetic Extracellular Matrix with Silica Nanofibers: From 1D Material to 3D Network

Biomaterials at nanoscale is a fast-expanding research field with which extensive studies have been conducted on understanding the interactions between cells and their surrounding microenvironments as well as intracellular communications. Among many kinds of nanoscale biomaterials, mesoporous fibrous structures are especially attractive as a promising approach to mimic the natural extracellular matrix (ECM) for cell and tissue research. Silica is a well-studied biocompatible, natural inorganic material that can be synthesized as morpho-genetically active scaffolds by various methods. This review compares silica nanofibers (SNFs) to other ECM materials such as hydrogel, polymers, and decellularized natural ECM, summarizes fabrication techniques for SNFs, and discusses different strategies of constructing ECM using SNFs. In addition, the latest progress on SNFs synthesis and biomimetic ECM substrates fabrication is summarized and highlighted. Lastly, we look at the wide use of SNF-based ECM scaffolds in biological applications, including stem cell regulation, tissue engineering, drug release, and environmental applications.

Supported TiO2 in Ceramic Materials for the Photocatalytic Degradation of Contaminants of Emerging Concern in Liquid Effluents: A Review

The application of TiO₂ as a slurry catalyst for the degradation of contaminants of emerging concern (CEC) in liquid effluents has some drawbacks due to the difficulties in the catalyst reutilization. Thus, sophisticated and expensive separation methods are required after the reaction step. Alternatively, several types of materials have been used to support powder catalysts, so that fixed or fluidized bed reactors may be used. In this context, the objective of this work is to systematize and analyze the results of research inherent to the application of ceramic materials as support of TiO₂ in the photocatalytic CEC removal from liquid effluents. Firstly, an overview is given about the treatment processes able to degrade CEC. In particular, the photocatalysts supported in ceramic materials are analyzed, namely the immobilization techniques applied to support TiO₂ in these materials. Finally, a critical review of the literature dedicated to photocatalysis with supported TiO₂ is presented, where the performance of the catalyst is considered as well as the main drivers and barriers for implementing this process. A focal point in the future is to investigate the possibility of depurating effluents and promote water reuse in safe conditions, and the supported TiO₂ in ceramic materials may play a role in this scope.

Engineering and modeling perspectives on photocatalytic reactors for water treatment

The debate on whether photocatalysis can reach full maturity at commercial level as an effective and economical process for treatment and purification of water and wastewater has recently intensified. Despite a bloom of scientific investigations in the last 30 years, particularly with regards to innovative photocatalytic materials, photocatalysis has so far seen a few industrial applications. Regardless of the points of view, it has been realized that research on reactor design and modeling are now equally urgent to match the extensive research carried out on innovative photocatalytic materials. In reality, the development of photocatalytic reactors has advanced steadily in terms of modeling and reactor design over the last two decades, though this topic has captured a smaller specialized audience. In this critical review, we introduce the latest developments on photocatalytic reactors for water treatment from an engineering perspective. The focus is on the modeling and design of photocatalytic reactors for water treatment at pilot- or at greater scale. Photocatalytic reactors utilizing both natural sunlight and UV irradiation sources are comprehensively discussed. The most promising photoreactor designs and models are examined giving key design guidelines. Other engineering considerations, such as operation, cost analysis, patents, and several industrial applications of photocatalytic reactors for water treatment are also presented. The dissemination of key photocatalytic reactor design principles among the scientific community and the water industry is currently one of the greatest obstacles in translating PWT research into widespread real-world application.

Self-cleaning, photocatalytic films on aluminum plates for multi-pollutant air remediation: promoting adhesion and activity by SiO2interlayers

In recent years, nanoparticles have come under close scrutiny for their possible health and environmental issues, making them less attractive for photocatalytic applications in air or water purification. Replacing free nano-powders with active and stable films is thus a fundamental step towards developing effective photocatalytic devices. Aluminum represents a cheap and technologically-relevant substrate, but its photocatalytic applications have been hampered by adhesion issues and metal ion diffusion within the photocatalytic layer. In this work, the use of silica interlayers is investigated as a strategy to promote adhesion, efficiency and reusability of TiO₂films deposited on aluminum plates. Films were prepared from stable titania sols to avoid the use of nano-powders. Aluminum substrates with different surface morphology were investigated and the role of the silica interlayer thickness was studied. Films were extensively characterized, studying their structure, morphology, optical properties, adhesion and hardness. Self-cleaning properties were studied with respect to their superhydrophilicity and ability to resist fouling via alkylsilanes. Photocatalytic degradation tests were carried out using both volatile organic compounds and NO_x, also in recycle tests. The presence of the silica interlayer proved crucial to promote the film robustness and photocatalytic activity. The substrate morphology determined the optimal interlayer thickness, especially in terms of the film reusability.

Development of Heavy Metal-Free Photocatalytic RhB Decomposition System Using a Biodegradable Plastic Substrate

The heavy-metal-free photocatalytic system, in which carbon nitride is coated on polylactic acid (PLA) as biodegradable plastic through a simple dip coating method, was used for dye decomposition under visible light irradiation. Solvent selection, solvent concentration, and the number of coatings for dip coating were investigated to optimize the conditions for loading carbon nitride on PLA. Carbon nitride cannot be coated on PLA in water, but it can be strongly coated by decomposing the surface of PLA with ethanol or chlorobenzene to promote physical adsorption and activate surface. The number of dip coatings also affected the photocatalytic decomposition ability. The photocatalytic system was able to decompose the dye continuously in the flow method, and dye (rhodamine B) was decomposed by about 50% at a residence time of 12 min (flow rate 0.350 mL/min) for 30 h.

TiO2-Based Nanocomposites Thin Film Having Boosted Photocatalytic Activity for Xenobiotics Water Pollution Remediation

Photocatalytic remediation represents a potential sustainable solution to the abatement of xenobiotic pollutants released within the water environment. Aeroxide^® P25 titanium dioxide nanoparticles (TiO₂ NPs) are well-known as one of the most efficient photocatalysts in several applications, and have also been investigated in water remediation as suspended powder. Recently, their application in the form of thin films has been revealed as a potential alternative to avoid time-consuming filtration processes. Polymers represent suitable substrates to immobilize TiO₂ NPs, allowing further production of thin films that can be exploited as a photoactive coating for environmental remediation. Nevertheless, the methods adopted to immobilize TiO₂ NPs on polymer matrix involve time-consuming procedures and the use of several reactants. Here, titanium dioxide-based nanocomposites (NCx) were obtained through a new approach based on Methyl Methacrylate in situ bulk polymerization and were compared with a blended mixture (BL). Their morphology and chemical–physical properties were investigated through Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), UV–Vis, and Raman spectroscopies. It was revealed that the in situ approach deeply influences the chemical–physical interactions between the polymer matrix and TiO₂ NPs. Photocatalytic experiments revealed the boosted photodegradation activity of NCx thin films, induced by the in situ approach. The photodegradation of paraquat and acetaminophen was also ascertained.

Method for attachment of TiO2 using design of experiments: application to the photocatalysis of a model pollutant methylene blue dye

The preparation of titanium dioxide (TiO₂) supported on a glass plate by heat attachment method is presented. With the use of response surface methodology based on a central composite design we investigated the influence of the experiment parameters of the TiO₂ deposition (temperature of calcination (T), time of calcination (ts) and the concentration of TiO₂ ((TiO₂))) on photocatalytic activity of the semiconductor for the degradation of a model pollutant: methylene blue. The analysis of variance results showed that the selected quadratic model with interaction (R² = 0.9802) was statistically significant. The experimental results showed that the degradation quantity of methylene blue increased when the ts value increased and T decreased. We have evaluated the photocatalytic activity of this supported catalyst (TiO₂-GP) with a laboratory reactor under natural condition; the maximum removal (96.03%) was obtained at ts = 331 min, T = 559 °C and (TiO₂) = 2.38 g/l. The method of desirability function was used to obtain the best combination of factor settings for achieving the maximum of degradation quantity ((TiO₂) = 2.6 (g/l), T = 600 (°C) and ts = 240 (min)). The additional tests on the catalyst plates confirmed that the deposits keep their catalytic activity for several cycles of use.

Photocatalytic hydrogels for removal of organic contaminants from aqueous solution in continuous flow reactors

We present soft-lithography patterned photocatalyst-embedded hydrogel reactors with tunable material properties for removal of organic contaminants from wastewater.

Removing of carmoisine dye pollutant from contaminated waters by photocatalytic method using a thin film fixed bed reactor

Recently, the advanced oxidation processes have been used to treat wastewater containing various dye pollutants via various methods of purification. In this work, the removal of the Carmoisine dye from contaminated waters by a photocatalytic method in a recirculating thin-film fixed bed reactor (TFFBR) was investigated and was compared with photolysis. By the surface response method via Minitab software, the experiments were designed in such a way that the effects of different parameters on the process were studied. The effect of various parameters, for instance, pH of solution, reaction time, initial dye concentration, flow rate, hydrogen peroxide concentration on the dye removal efficiency has been investigated. By the screening and optimization stages in the response surface method, the optimal conditions for performing the process were as follows: 45 min of time, pH 2, 13.33 mM of H₂O₂ concentration, the flow rate of 0.25 Lmin^-1, and 10 mg/L of dye concentration. The proficiency of removing color by advanced oxidation is more than 97% under optimum conditions.

A fluidized-bed reactor for the photocatalytic mineralization of phenol on TiO2-coated silica gel

TiO₂ photocatalysis represents a promising class of oxidation techniques that are intended to be both supplementary and complementary to the conventional approaches for the removal of refractory and trace organic contaminants in water and air. Powdered TiO₂ dispersion systems employed in most studies require an additional separation step to recover the catalyst from the effluent water, which represents a major drawback for large scale applications. The optimization of photocatalytic treatment systems involves merging the benefits of catalyst immobilization on a retainable support, thus eliminating the need for downstream catalyst separation, maximization of photon-exposed catalyst area, and continuous operation.

Aiming to integrate such conditions into a single system, a bench-scale annular photo-reactor with concentric UV-C lamp was built to study the photocatalytic mineralization of phenol on fluidized silica gel beads coated with sol-gel-synthetized TiO₂. Reactor efficiency was investigated for different silica particle diameters (224, 357 and 461 μm), fluidized-bed concentrations in the bulk liquid (5, 10, 20 and 30 g L⁻¹), initial phenol concentrations in the aqueous solution (0.25 mmol L⁻¹ to 4.0 mmol L⁻¹), and single and multiple sol-gel depositions. Then, the resulting optimum reactor configuration was compared to that of the same process on suspended Degussa P25 TiO₂ nanoparticles under similar experimental conditions. The latter is expected to be more efficient, but post-treatment catalyst recovery, being an energy intensive process, represents a major limitation for large scale applications. Process efficiency was measured as a function of the accumulated energy necessary for the mineralization of 50% of the initial dissolved chemical oxygen demand (COD), or, Q_0.5. Results showed that for any given mass of fluidized bed material, photo-oxidation efficiency increases with decreasing particle size (even for bed concentrations with similar equivalent surface area), decreasing initial phenol concentrations, and increasing number of sol-gel coatings. It was found that, for any given particle size and contaminant mass, there is an optimum bed concentration of 20 g L⁻¹ for which Q_0.5 reaches a minimum. Finally, under the optimum configuration, the fluidized-bed reactor efficiency is only 30% lower than that of photocatalysis on suspended TiO₂ nanopowder, thus making the proposed fluidized system a viable alternative to slurry-TiO₂ reactors.

Liquid-Repellent Metal Oxide Photocatalysts

Metal oxide photocatalysts (MOPCs) decompose organic molecules under illumination. However, the application of MOPCs in industry and research is currently limited by their intrinsic hydrophilicity because MOPCs can be wetted by most liquids. To achieve liquid repellency, the surface needs to possess a low surface energy, but most organic molecules with low surface energy are degraded by photocatalytic activity. Herein, current methods to achieve liquid repellency on MOPCs, while preventing degradation of hydrophobic coatings, are reviewed. Classically, composite materials containing MOPCs and hydrophobic organic compounds possess good liquid repellency. However, composites normally form irregular coatings and are hard to prepare on surfaces such as those that are mesoporous or nanostructured. In addition, the adhesion of composites to substrates is often weak, resulting in delamination. Recent studies have shown that the direct grafting reaction of polydimethylsiloxane (PDMS) from silicone oil (methyl-terminated PDMS) under illumination results in a stable polymer brush. This easy and simple grafting method allows us to create stable liquid-repellent surfaces on MOPCs of various types, structures, and sizes. In particular, super-liquid-repellent drops with an underlying air layer can be created on PDMS-grafted nano-/microstructured MOPCs. Potential applications of surfaces combining liquid repellency and photocatalytic activity are also discussed; thus offering new ways of using MOPCs in a wider range of applications.

The problem of drinking water access: A review of disinfection technologies with an emphasis on solar treatment methods

The lack of access to safe drinking water is one of the biggest challenges facing humanity in the 21st century. Despite the collective global effort that has been made, the drinking water sources of at least 2 billion people are faecally contaminated, resulting in more than half a million diarrhoeal deaths each year, with the majority occurring in developing countries. Technologies for the inactivation of pathogenic microorganisms in water are therefore of great significance for human health and well-being. However, conventional technologies to provide drinking water, although effective, present limitations that impede their global application. These treatment methods often have high energy and chemical demands, which limits their application for the prevention of waterborne diseases in the most vulnerable regions. These shortcomings have led to rapid research and development of advanced alternative technologies. One of these alternative methods is solar disinfection, which is recognised by the World Health Organization as one of the most appropriate methods for producing drinkable water in developing countries. This study reviews conventional technologies that are being applied at medium to large scales to purify water and emerging technologies currently in development. In addition, this paper describes the merits, demerits, and limitations of these technologies. Finally, the review focuses on solar disinfection, including a novel technology recently developed in this field.

Crystal violet and toxicity removal by adsorption and simultaneous photocatalysis in a continuous flow micro-reactor

A continuous flow micro-reactor irradiated by UV-LEDs was employed to treat coloured wastewater by adsorption and simultaneous photocatalysis. Zinc oxide (ZnO) immobilized on commercial zeolites pellets in spherical shape (ZEO) was used as catalytic material in a micro-reactor maximizing the photocatalyst exposition to light sources, irradiating uniformly the entire solution volume and improving the mass transfer phenomena. Experimental tests were carried out on crystal violet dye (CV) as one of the main dying agent present in textile wastewater. The comparison between adsorption and adsorption/photocatalytic tests showed that UV irradiation can achieve a steady state CV concentration value corresponding to an equilibrium condition between adsorption and photocatalytic oxidation. The higher removal efficiency (i.e. 93%) was observed with a liquid flow rate of 1.1 mL/min (contact time = 4.7 min; CV = 10 mg/L) under UV light irradiation. In the steady state, CV removal remained constant for the overall testing time. Bioassays evidenced that toxicity was not completely removed (i.e. final effluent ranked as "slight acute toxic") from wastewater suggesting its suitability for sewage collection discharge. A Dubinin Radushkevich (D-R) isotherm model was applied for studying the adsorption behaviour of ZnO/ZEO sample. CV adsorption constants were evaluated from experimental data carried out in dark conditions in a batch system. Kinetic expression of CV removal and the D-R adsorption were incorporated in the CV mass balance estimating the kinetic parameter. The model was validated comparing the calculated CV conversion with the experimental tests collected at different CV inlet concentration.

Binder-free immobilization of TiO2 photocatalyst on steel mesh via electrospraying and hot-pressing and its application for organic micropollutant removal and disinfection

An immobilized photocatalyst was prepared by thermally treating TiO₂-coated steel mesh (TiO₂-IS) in a laboratory hot-press with no binder. TiO₂ coating was performed by electrospraying a 1 mg/mL methanol dispersion of Evonik P25 powder. The thermal treatment conditions at 350 °C, 100 Mpa, and 1 h were found to be the optimum conditions. Scanning electron microscopic images displayed a robust and adherent TiO₂ layer on steel mesh. X-ray photoelectron spectroscopy and elemental mapping studies confirmed that the Fe₃O₄ interface formed during thermal treatment strongly bound the TiO₂ on steel mesh. The XRD patterns of TiO₂-IS indicated the preservation of crystalline structure of Evonik P25 (anatase and rutile mixture) and the existence of iron oxide interface. Under UVA irradiation, 10 μM of methylene blue was completely decolorized within 40 min using an immobilized photocatalyst with 2.120 mg of TiO₂ per 2.5 × 5.0 cm² and showed stable efficacy in 25 consecutive photocatalytic runs. Furthermore, this sample degraded the organic micropollutants (e.g., pharmaceuticals) such as carbamazepine, ranitidine, acetaminophen, and trimethoprim at the rates of 0.041, 0.165, 0.089, and 0.079 min^-1, respectively. Under UVA irradiation, it exhibited high photocatalytic disinfection activity for Escherichia coli and MS2 coliphage.

Photocatalytic Inactivation of Enterobacter cloacae and Escherichia coli Using Titanium Dioxide Supported on Two Substrates

The antibacterial photocatalytic activity of TiO2 supported over two types of substrates, borosilicate glass tubes (TiO2/SiO2-borosilicate glass tubes (BGT)) and low-density polyethylene pellets (TiO2-LDPE pellets), which were placed in a compound parabolic collectors (CPC) reactor, was evaluated against Enterobacter cloacae and Escherichia coli under sunlight. Three solar photocatalytic systems were assessed, suspended TiO2, TiO2/SiO2-BGT and TiO2-LDPE pellets, at three initial bacterial concentrations, 1 × 105; 1 × 103; 1 × 101 CFU/mL of E. coli and total bacteria (E. cloacae and E. coli). The solar photo-inactivation of E. coli was achieved after two hours with 7.2 kJ/L of UV-A, while total bacteria required four hours and 16.5 kJ/L of UV-A. Inactivation order of E. coli was determined, as follows, suspended TiO2/sunlight (50 mg/L) > TiO2-LDPE pellets/sunlight (52 mg/L) > TiO2/SiO2-BGT/sunlight (59 mg/L), the best E. coli. inactivation rate was obtained with TiO2-LDPE pellets/sunlight, within 4.5 kJ/L and 90 min. The highest total bacteria inactivation rate was found for TiO2/sunlight (50 mg/L) and TiO2-LDPE pellets/sunlight (52 mg/L), within 11.2 kJ/L and 180 min. TiO2 deposited over LDPE pellets was the most effective material, which can be successfully used for water disinfection applications. Bacterial regrowth was assessed 24 h after all photocatalytic treatments, none of those microorganisms showed any recovery above the detection limit (2 CFU/mL).

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

Immobilized TiO2 on glass spheres applied to heterogeneous photocatalysis: photoactivity, leaching and regeneration process

Heterogeneous photocatalysis using titanium dioxide as catalyst is an attractive advanced oxidation process due to its high chemical stability, good performance and low cost. When immobilized in a supporting material, additional benefits are achieved in the treatment. The purpose of this study was to develop a simple protocol for impregnation of TiO₂-P25 on borosilicate glass spheres and evaluate its efficiency in the photocatalytic degradation using an oxidizable substrate (methylene blue), in a Compound Parabolic Concentrator (CPC) reactor. The assays were conducted at lab-scale using radiation, which simulated the solar spectrum. TiO₂ leaching from the glass and the catalyst regeneration were both demonstrated. A very low leaching ratio (0.03%) was observed after 24 h of treatment, suggesting that deposition of TiO₂ resulted in good adhesion and stability of the photocatalyst on the surface of borosilicate. This deposition was successfully achieved after calcination of the photocatalyst at 400 °C (TiO₂-400 °C). The TiO₂ film was immobilized on glass spheres and the powder was characterized by scanning electron microscopy (SEM), X-ray diffraction and BET. This characterization suggested that thermal treatment did not introduce substantial changes in the measured microstructural characteristics of the photocatalyst. The immobilized photocatalyst degraded more than 96% of the MB in up to 90 min of reaction. The photocatalytic activity decreased after four photocatalytic cycles, but it was recovered by the removal of contaminants adsorbed on the active sites after washing in water under UV-Vis irradiation. Based on these results, the TiO₂-400 °C coated on glass spheres is potentially a very attractive option for removal of persistent contaminants present in the environment.

Thin films containing oxalate-capped iron oxide nanomaterials deposited on glass substrate for fast Fenton degradation of some micropollutants

The main goal of the study was to evaluate the catalytic activity of two hybrid nanocatalysts consisting in Fe₃O₄ nanoparticles modified with either chitosan (CS) or polyethylene glycol (PEG)/ferrous oxalate (FO), and further deposited on solid substrate as thin films. X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM) were employed for the structural and morphological characterizations of the heterogeneous catalysts. The degradation kinetic studies of two reactive azo dye (Reactive Black 5 (RB5) and Reactive Yellow 84 (RY84)) as well as Bisphenol A (BPA) solutions were carried out using Fenton-like oxidation, in the presence of different concentrations of H₂O₂, at initial near-neutral pH and room temperature. The results indicated that a low amount of catalytic material (0.15 g/L), deposited as thin film, was able to efficiently trigger dye degradation in solution in the presence of 6.5 mmol/L H₂O₂ for RB5 and of only 1.6 mmol/L H₂O₂ in the case of BPA and RY84. In the presence of complex matrices such as WWTP waters, the removal of BPA was low (only 24% for effluent samples). Our findings recommend the studied immobilized nanocatalysts as promising economical tools for the pre-treatment of wastewaters using advanced oxidation processes (AOPs).

Photocatalytic reusable membranes for the effective degradation of tartrazine with a solar photoreactor

Recalcitrant dyes present in effluents constitute a major environmental concern due to their hazardous properties that may cause deleterious effects on aquatic organisms. Tartrazine is a widely-used dye, and it is known to be resistant to biological and chemical degradation processes and by its carcinogenic and mutagenic nature. This study presents the use of TiO₂ (P25) nanoparticles immobilized into a poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) membrane to assess the photocatalytic degradation of this dye in a solar photoreactor. The nanocomposite morphological properties were analyzed, confirming an interconnected porous microstructure and the homogeneous distribution of the TiO₂ nanoparticles within the membrane pores. It is shown that the nanocomposite with 8wt% TiO₂ exhibits a remarkable sunlight photocatalytic activity over five hours, with 78% of the pollutant being degraded. It was also demonstrated that the degradation follows pseudo-first-order kinetics model at low initial tartrazine concentration. Finally, the effective reusability of the produced nanocomposite was also assessed.

Detailed modeling of oxalic acid degradation by UV-TiO2 nanoparticles: Importance of light scattering and photoreactor scale-up

A detailed computational fluid dynamics model is presented that integrates reactor hydrodynamics with advanced light models and UV-TiO₂ advanced oxidation kinetics to yield the degradation of oxalic acid in a dispersed-phase photoreactor. Model predictions were first compared against experimental data obtained from the literature and subsequently used in a parametric study for investigating scale-up effects associated with both process and photoreactor variables. Investigated variables included: TiO₂ concentration (5-400 mg L^-1), initial oxalic acid concentration (0.9-32 mg L^-1), lamp irradiance (100-10,000 W m^-2), background fluid absorbance (0-30 m^-1), reactor size (1/4-4 as relative scaling factor), lamp orientation (0-360°) and flowrate (2.5-10 m³ h^-1). The analysis revealed that an optimum in oxalic acid degradation is observed when the TiO₂ concentration was controlled in the 20-40 mg L^-1 range (depending on lamp irradiance). While lamp orientation showed minimal impact, reactor size and flowrate emerged as key variables for photoreactor design. Moreover, an increase in initial oxalic acid concentration substantially reduced oxalic acid degradation performance observed at high loadings. Also, TiO₂ activation and photoreactor degradation performance were impacted negatively by light competition with background fluid absorbance.

Recent advancements in supporting materials for immobilised photocatalytic applications in waste water treatment

The aim of this paper is to provide a review on the usage of different anchoring media (supports) for immobilising commonly employed photocatalysts for degradation of organic pollutants. The immobilisation of nano-sized photocatalysts can eliminate costly and impractical post-treatment recovery of spent photocatalysts in largescale operations. Some commonly employed immobilisation aids such as glass, carbonaceous substances, zeolites, clay and ceramics, polymers, cellulosic materials and metallic agents that have been previously discussed by various research groups have been reviewed. The study revealed that factors such as high durability, ease of availability, low density, chemical inertness and mechanical stability are primary factors responsible for the selection of suitable supports for catalysts. Common techniques for immobilisation namely, dip coating, cold plasma discharge, polymer assisted hydrothermal decomposition, RF magnetron sputtering, photoetching, solvent casting, electrophoretic deposition and spray pyrolysis have been discussed in detail. Finally, some common techniques adopted for the characterisation of the catalyst particles and their uses are also discussed.

Evaluation of flow-rate dynamics in the simultaneous photocatalytic treatment of multichlorinated substituted phenols in continuous-flow systems

The project investigated the simultaneous degradation of chlorophenols dissolved in aqueous systems. The photocatalysis advanced oxidation process was the technology applied to achieve treatment. Chemical behavioural tracking was performed using the chloride dehalogenation sequence dynamics. The study recorded reductive dehalogenation chemical transformation kinetics of multi-substituted chlorinated phenolics in continuous flow reactor systems. This was performed by manipulation of liquid flow-rates in the photocatalytic oxidations process using suspended and immobilised catalyst applications. A modified Langmuir-Hinshelwood kinetic model was proposed that explained the oxidation transformational behaviour of the dehalogenation process derived intermediates. Complementary photocatalytic performance matrices were established for each flow regime; model parameters were calculated and estimated for behavioural profiles of all compounds under scrutiny.

OH radical generation in a photocatalytic reactor using TiO2 nanotube plates

In order to use TiO2 nanotubes grown on a Ti plate as a photocatalyst, self-organized oxide nanotube layers were grown by anodization in a glycerol based electrolyte. The ultimate conditions for the synthesis of the TiO2 nanotube array on the Ti plate were investigated by comparing the morphology, length, and inner diameter of the nanotubes. They were significantly affected by the applied anodic voltage, anodization time, and composition of the electrolyte such as the water and fluoride ion concentration. The crystallographic structures of TiO2 nanotubes before and after annealing were compared. The photocatalytic reactor used in this study consisted of two parallel and closely spaced TiO2 nanotube plates. The plates were squares while a UV lamp was inserted perpendicularly to them. OH radical generation in the photocatalytic reactor was monitored by using a probe compound, parachlorobenzoate (pCBA). The steady state OH radical concentration was compared depending on the length of nanotubes and crystallographic structure. The longer the nanotubes, the higher the steady state OH radical concentration.

Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment

Continuous-flow photochemistry in microreactors receives a lot of attention from researchers in academia and industry as this technology provides reduced reaction times, higher selectivities, straightforward scalability, and the possibility to safely use hazardous intermediates and gaseous reactants. In this review, an up-to-date overview is given of photochemical transformations in continuous-flow reactors, including applications in organic synthesis, material science, and water treatment. In addition, the advantages of continuous-flow photochemistry are pointed out and a thorough comparison with batch processing is presented.

Comparative efficiency of TiO2 nanoparticles in suspension vs. immobilization into P(VDF–TrFE) porous membranes

Optimized photocatalytic membranes based on TiO₂ and NaY immobilized into P(VDF–TrFE) have been developed showing a degradation efficiency of 96%.

Bioinspired Design of an Immobilization Interface for Highly Stable, Recyclable Nanosized Catalysts

Immobilization of nanometer-sized metal catalysts into porous substrates can stabilize the catalysts and allow their recycled uses, while immobilization often sacrifices the active surface of catalysts and degenerates the local microenvironments, resulting in the reduction of the catalytic activity. To maintain a high activity of immobilized nanocatalysts, it is critically important to design an interface that minimizes the contact area and favors reaction chemistry. Here we report on the application of mussel-inspired adhesion chemistry to the formation of catalytic metal nanocrystal-polydopamine hybrid materials that exhibit a high catalytic efficiency during recycled uses. Electrospun polymer nanofibers are used as a template for in situ formation and immobilization of gold nanoparticles via polydopamine-induced reduction of ionic precursors. The prepared hybrid nanostructures exhibit a recyclable catalytic activity for the reduction of 4-nitrophenol with a turnover frequency of 3.2-5.1 μmol g(-1) min(-1). Repeated uses of the hybrid nanostructures do not significantly alter their morphology, indicating the excellent structural stability of the hybrid nanostructures. We expect that the polydopamine chemistry combined with the on-surface synthesis of catalytic nanocrystals is a promising route to the immobilization of various colloidal nanosized catalysts on supporting substrates for long-term catalysis without the physical instability problem.

Photocatalytic applications of paper-like poly(vinylidene fluoride)-titanium dioxide hybrids fabricated using a combination of electrospinning and electrospraying

A paper-like photocatalyst was fabricated by electrospraying an N,N'-dimethylformamide (DMF) dispersion of titanium dioxide (TiO2) nanoparticles (NPs) on a poly(vinylidene fluoride) nanofiber (PVDF NF) mat prepared by electrospinning. Morphological studies revealed that the TiO2 NPs uniformly deposited as clusters on the surface of the PVDF NF mat. The immobilized amount of TiO2 was found to be 2.08, 2.44, 3.80, and 4.73 mg per 45 cm(2) of PVDF-TiO2 hybrids for the electrospraying of 10, 20, 40, and 60 ml of TiO2-DMF, respectively. The hybrid photocatalysts were effective in degrading bisphenol A (BPA), 4-chlorophenol (4-CP), and cimetidine (CMT), which dissolved in both deionized water and secondary wastewater effluents, with activity being proportional to the quantity of TiO2 NPs immobilized. For the highest loading amount of TiO2, BPA, 4-CP, and CMT degraded completely within 100, 100, and 40 min of UV irradiation, respectively. Stable photo-oxidation of CMT was maintained through 10 repeated cycles. During these cycles, it was confirmed that there was no loss of TiO2 NPs by inductively coupled plasma optical emission spectrometry. Our results suggest that effective and stable PVDF-TiO2 hybrid photocatalysts can be fabricated on a large scale by combining electrospinning and electrospraying techniques.

Floating photocatalysts based on loading Bi/N-doped TiO2 on expanded graphite C/C (EGC) composites for the visible light degradation of diesel

Enhanced visible-light photocatalytic activity for degradation of diesel based on photocatalyst easy recycle.

Synthesis and characterization of core–shell–shell magnetic (CoFe2O4–SiO2–TiO2) nanocomposites and TiO2nanoparticles for the evaluation of photocatalytic activity under UV and visible irradiation

CoFe₂O₄–SiO₂–TiO₂core–shell–shell magnetic nanocomposites have been prepared by coating cobalt ferrite nanoparticles with double layers of SiO₂–TiO₂.