Volatile organic compounds (VOCs) removal by photocatalysts: A review

A novel nanocomposite for photocatalytic rhodamine B dye removal from wastewater using visible light

Providing safe access to water and addressing the impact of waterborne diseases, which claim over two million lives annually, is a major contribution to water purification. The study introduces a novel nanocomposite, Ch/Fe3O4/α-MoO3, which exhibits outstanding photocatalytic efficacy under visible light. An in-depth investigation of the nanocomposite's synthesis, characterization, and photodegradation mechanisms reveals its outstanding capabilities. Photocatalytic activity is influenced by the catalytic dose, pH, dye concentration, and reaction time, according to the study. A response surface method is used to determine the optimal conditions for Rhodamine B degradation, which results in 96.3% removal efficiency at pH 8.5, dye concentration 25 mg/L, nanocomposite dose at 22 mg/L, and reaction time 50 min. As a result of its high surface area, biocompatibility, availability, and magnetization with iron compounds, Chitosan is an excellent substrate for enhancing the photocatalytic properties of MoO3 nanoparticles. A nanocomposite with an energy band of 3.18 eV exhibits improved visible light absorption. This study confirms the nanocomposite's recyclability and stability, affirming its practicality. Besides dye removal, it offers hope for the global quest for clean water sources by addressing a broader range of waterborne contaminants. By combining molybdenum and magnetite, nanocomposite materials facilitate the degradation of pollutant and bacteria, contributing positively to society's quest for clean and safe water. It emphasizes the role nanotechnology plays in preserving human health and well-being in combating waterborne diseases.

Quantification of Byproduct Formation from Portable Air Cleaners Using a Proposed Standard Test Method

In response to the COVID-19 pandemic, air cleaning technologies were promoted as useful tools for disinfecting public spaces and combating airborne pathogen transmission. However, no standard method exists to assess the potentially harmful byproduct formation from air cleaners. Through a consensus standard development process, a draft standard test method to assess portable air cleaner performance was developed, and a suite of air cleaners employing seven different technologies was tested. The test method quantifies not only the removal efficiency of a challenge chemical suite and ultrafine particulate matter but also byproduct formation. Clean air delivery rates (CADRs) are used to quantify the chemical and particle removal efficiencies, and an emission rate framework is used to quantify the formation of formaldehyde, ozone, and other volatile organic compounds. We find that the tested photocatalytic oxidation and germicidal ultraviolet light (GUV) technologies produced the highest levels of aldehyde byproducts having emission rates of 202 and 243 μg h-1, respectively. Additionally, GUV using two different wavelengths, 222 and 254 nm, both produced ultrafine particulate matter.

Recent advances in catalytic oxidation of VOCs by two-dimensional ultra-thin nanomaterials

Catalytic oxidation, an end-of-pipe treatment technology for effectively purifying volatile organic compounds (VOCs), has received widespread attention. The crux of catalytic oxidation lies in the development of efficient catalysts, with their optimization necessitating a comprehensive analysis of the catalytic reaction mechanism. Two-dimensional (2D) ultra-thin nanomaterials offer significant advantages in exploring the catalytic oxidation mechanism of VOCs due to their unique structure and properties. This review classifies strategies for regulating catalytic properties and typical applications of 2D materials in VOCs catalytic oxidation, in addition to their characteristics and typical characterization techniques. Furthermore, the possible reaction mechanism of 2D Co-based and Mn-based oxides in the catalytic oxidation of VOCs is analyzed, with a special focus on the synergistic effect between oxygen and metal vacancies. The objective of this review is to provide valuable references for scholars in the field.

Synthesis of BiOX-Red Mud/Granulated Blast Furnace Slag Geopolymer Microspheres for Photocatalytic Degradation of Formaldehyde

Release of formaldehyde gas indoors is a serious threat to human health. The traditional adsorption method is not stable enough for formaldehyde removal. Photocatalytic degradation of formaldehyde is effective and rapid, but photocatalysts are generally expensive and not easy to recycle. In this paper, geopolymer microspheres were applied as matrix materials for photocatalysts loading to degrade formaldehyde. Geopolymer microspheres were prepared from red mud and granulated blast furnace slag as raw materials by alkali activation. When the red mud doping was 50%, the concentration of NaOH solution was 6 mol/L, and the additive amount was 30 mL, the prepared geopolymer microspheres possessed good morphological characteristics and a large specific surface area of 38.80 m2/g. With the loading of BiOX (X = Cl, Br, I) photocatalysts on the surface of geopolymer microspheres, 85.71% of formaldehyde gas were adsorbed within 60 min. The formaldehyde degradation rate of the geopolymer microspheres loaded with BiOI reached 87.46% within 180 min, which was 23.07% higher than that of the microspheres loaded with BiOBr, and 50.50% higher than that of the microspheres loaded with BiOCl. While ensuring the efficient degradation of formaldehyde, the BiOX (X = Cl, Br, I)-loaded geopolymer microspheres are easy to recycle and can save space. This work not only promotes the resource utilization of red mud and granulated blast furnace slag, but also provides a new idea on the formation of catalysts in the process of photocatalytic degradation of formaldehyde.

Data-driven autonomous operation of VOCs removal system

Removal of volatile organic compounds (VOCs) from the air has been an important issue in many industrial fields. Traditionally, the operation of VOCs removal systems has relied on fixed operating conditions determined by domain experts based on their expertise and intuition. In practice, this manual operation cannot respond immediately to changes in the system environment. To facilitate the autonomous operation of the system, the operating conditions should be optimized properly in real time to adapt to the changes in the system environment. Recently, optimization frameworks have been widely applied to real-world industrial systems across various domains using different approaches. The primary motivation for this study is the effective implementation of an optimization framework targeting a VOCs removal system. In this paper, we present a data-driven autonomous operation method for optimizing the operating conditions of a VOCs removal system to enhance the overall performance. An optimization problem is formulated with the decision variables denoting the parameters associated with the operating condition, the environmental variables representing the measurements for the system environment, the constraints specifying the control ranges of the parameters, and the objective function representing the system performance as determined by the operating conditions and environment. Using the previous operation data from the system, a neural network is trained to model the system performance as a function of the decision and environmental variables to approximate the objective function. For the current state of the system environment, the optimal operating condition is derived by solving the optimization problem. A case study of a targeted VOCs removal system demonstrates that the proposed method effectively optimizes the operating conditions for improved system performance without intervention from domain experts.

Combined association of urinary volatile organic compounds with chronic bronchitis and emphysema among adults in NHANES 2011-2014: The mediating role of inflammation

Evidence on the association of volatile organic compounds (VOCs) with chronic bronchitis (CB) and emphysema is spare and defective. To evaluate the relationship between urinary metabolites of VOCs (mVOCs) with CB and emphysema, and to identify the potential mVOC of paramount importance, data from NHANES 2011-2014 waves were utilized. Logistic regression was conducted to estimate the independent association of mVOCs with respiratory outcomes. Least absolute shrinkage and selection operator (LASSO) regression was performed to screen a parsimonious set of CB- and emphysema-relevant mVOCs that were used for further co-exposure analyses of weighted quantile sum (WQS) regression and Bayesian kernel machine regression (BKMR). Mediation analysis was employed to detect the mediating role of inflammatory makers in such associations. In single exposure analytic model, nine mVOCs were individually and positively associated with CB, while four mVOCs were with emphysema. In WQS regression, positive association between LASSO selected mVOCs and CB was identified (OR = 1.82, 95% CI: 1.25 to 2.69), and N-acetyl-S-(4-hydroxy-2-butenyl)-l-cysteine (MHBMA3) weighted the highest. Results from BKMR further validated such combined association and the significance of MHBMA3. As for emphysema, significantly positive overall trend of mVOCs was only observed in BKMR model and N-acetyl-S-(N-methylcarbamoyl)-l-cysteine (AMCC) contributed most to the mixed effect. White blood cell count (WBC) and lymphocyte number (LYM) were mediators in the positive pattern of mVOCs mixture with CB, while association between mVOCs mixture and emphysema was significantly mediated by LYM and segmented neutrophils num (NEO). This study demonstrated that exposure to VOCs was associated with CB and emphysema independently and combinedly, which might be partly speculated that VOCs were linked to activated inflammations. Our findings shed novel light on VOCs related respiratory illness, and provide a new basis for the contribution of certain VOCs to the risk of CB and emphysema, which has potential public health implications.

Comparative Analysis of Volatile Organic Compound Purification Techniques in Complex Cooking Emissions: Adsorption, Photocatalysis and Combined Systems

Volatile organic compounds (VOCs) are molecules present in our everyday life, and they can be positive, such as in the formation of odour and food flavour, or harmful to the environment and humans, and research is focusing on limiting their emissions. Various methods have been used to achieve this purpose. Firstly, we review three main degradation methods: activated carbon, photocatalysis and a synergetic system. We provide a general overview of the operative conditions and report the possibility of VOC abatement during cooking. Within the literature, none of these systems has ever been tested in the presence of complex matrices, such as during cooking processes. The aim of this study is to compare the three methods in order to understand the behaviour of filter systems in the case of realistically complex gas mixtures. Proton transfer reaction-mass spectrometry (PTR-MS) has been used in the real-time monitoring of volatilome. Due to the fact that VOC emissions are highly dependent on the composition of the food cooked, we evaluated the degradation capacity of the three systems for different burger types (meat, greens, and fish). We demonstrate the pros and cons of photocatalysis and adsorption and how a combined approach can mitigate the drawbacks of photocatalysis.

Application of Fe-MOFs in Photodegradation and Removal of Air and Water Pollutants: A Review

Photocatalytic technology has received increasing attention in recent years. A pivotal facet of photocatalytic technology lies in the development of photocatalysts. Porous metal-organic framework (MOF) materials, distinguished by their unique properties and structural characteristics, have emerged as a focal point of research in the field, finding widespread application in the photo-treatment and conversion of various substances. Fe-based MOFs have attained particular prominence. This review explores recent advances in the photocatalytic degradation of aqueous and gaseous substances. Furthermore, it delves into the interaction between the active sites of Fe-MOFs and pollutants, offering deeper insights into their mechanism of action. Fe-MOFs, as photocatalysts, predominantly facilitate pollutant removal through redox processes, interaction with acid sites, the formation of complexes with composite metal elements, binding to unsaturated metal ligands (CUSs), and hydrogen bonding to modulate their respiratory behavior. This review also highlights the focal points of future research, elucidating the challenges and opportunities that lie ahead in harnessing the characteristics and advantages of Fe-MOF composite catalysts. In essence, this review provides a comprehensive summary of research progress on Fe-MOF-based catalysts, aiming to serve as a guiding reference for other catalytic processes.

Stable Loading of TiO2 Catalysts on the Surface of Metal Substrate for Enhanced Photocatalytic Toluene Oxidation

To promote the practical application of TiO2 in photocatalytic toluene oxidation, the honeycomb aluminum plates were selected as the metal substrate for the loading of TiO2 powder. Surface-etching treatment was performed and titanium tetrachloride was selected as the binder to strengthen the loading stability. The loading stability and photocatalytic activity of the monolithic catalyst were further investigated, and the optimal surface treatment scheme (acid etching with 15.0 wt.% HNO3 solution for 15 min impregnation) was proposed. Therein, the optimal monolithic catalyst could achieve the loading efficiency of 42.4% and toluene degradation efficiencies of 76.2%. The mechanism for the stable loading of TiO2 was revealed by experiment and DFT calculation. The high surface roughness of metal substrate and the strong chemisorption between TiO2 and TiCl4 accounted for the high loading efficiency and photocatalytic activity. This work provides the pioneering exploration for the practical application of TiO2 catalysts loaded on the surface of metal substrate for VOCs removal, which is of significance for the large-scaled application of photocatalytic technology.

Technological solutions for NOx, SOx, and VOC abatement: recent breakthroughs and future directions

NOx, SOx, and carbonaceous volatile organic compounds (VOCs) are extremely harmful to the environment, and their concentrations must be within the limits prescribed by the region-specific pollution control boards. Thus, NOx, SOx, and VOC abatement is essential to safeguard the environment. Considering the importance of NOx, SOx, and VOC abatement, the discussion on selective catalytic reduction, oxidation, redox methods, and adsorption using noble metal and non-noble metal-based catalytic approaches were elaborated. This article covers different thermal treatment techniques, category of materials as catalysts, and its structure-property insights along with the advanced oxidation processes and adsorption. The defect engineered catalysts with lattice oxygen vacancies, bi- and tri-metallic noble metal catalysts and non-noble metal catalysts, modified metal organic frameworks, mixed-metal oxide supports, and their mechanisms have been thoroughly reviewed. The main hurdles and potential achievements in developing novel simultaneous NOx, SOx, and VOC removal technologies are critically discussed to envisage the future directions. This review highlights the removal of NOx, SOx, and VOC through material selection, properties, and mechanisms to further improve the existing abatement methods in an efficient way.

A Breakthrough in Photocatalytic Wastewater Treatment: The Incredible Potential of g-C3N4/Titanate Perovskite-Based Nanocomposites

Water pollution has emerged as a major global environmental crisis due to the massive contamination of water resources by the textile dyeing industry, organic waste, and agricultural residue. Since water is fundamental to life, this grave disregard puts lives at risk, making the protection of water resources a serious issue today. Recent research has shown great interest in improving the photocatalytic performance of graphitic carbon nitride (g-C3N4) for wastewater treatment. However, the photocatalytic removal activity of pure g-C3N4 is poor, owing to its minimal surface area, fast recombination of photo-generated electron-hole pairs, and poor light absorption. Recently, titanate perovskites (TNPs) have attracted significant attention in both environmental remediation and energy conversion due to their exceptional structural, optical, physiochemical, electrical, and thermal properties. Accordingly, TNPs can initiate a variety of surface catalytic reactions and are regarded as an emerging category of photocatalysts for sustainability and energy-related industries when exposed to illumination. Therefore, in this review article, we critically discuss the recent developments of extensively developed g-C3N4/TNPs that demonstrate photocatalytic applications for wastewater treatment. The different synthetic approaches and the chemical composition of g-C3N4/TNP composites are presented. Additionally, this review highlights the global research trends related to these materials. Furthermore, this review provides insight into the various photocatalytic mechanisms, including their potential impact and significance. Also, the challenges faced by such materials and their future scope are discussed.

BiVO4 As a Sustainable and Emerging Photocatalyst: Synthesis Methodologies, Engineering Properties, and Its Volatile Organic Compounds Degradation Efficiency

Bismuth vanadate (BiVO₄) is one of the best bismuth-based semiconducting materials because of its narrow band gap energy, good visible light absorption, unique physical and chemical characteristics, and non-toxic nature. In addition, BiVO₄ with different morphologies has been synthesized and exhibited excellent visible light photocatalytic efficiency in the degradation of various organic pollutants, including volatile organic compounds (VOCs). Nevertheless, the commercial scale utilization of BiVO₄ is significantly limited because of the poor separation (faster recombination rate) and transport ability of photogenerated electron-hole pairs. So, engineering/modifications of BiVO₄ materials are performed to enhance their structural, electronic, and morphological properties. Thus, this review article aims to provide a critical overview of advanced oxidation processes (AOPs), various semiconducting nanomaterials, BiVO₄ synthesis methodologies, engineering of BiVO₄ properties through making binary and ternary nanocomposites, and coupling with metals/non-metals and metal nanoparticles and the development of Z-scheme type nanocomposites, etc., and their visible light photocatalytic efficiency in VOCs degradation. In addition, future challenges and the way forward for improving the commercial-scale application of BiVO₄-based semiconducting nanomaterials are also discussed. Thus, we hope that this review is a valuable resource for designing BiVO₄-based nanocomposites with superior visible-light-driven photocatalytic efficiency in VOCs degradation.

Experimental and computational studies of photoelectrochemical degradation of atrazine by modified nanoporous titanium dioxide

The presence of herbicides like Atrazine (ATZ) in groundwater from non-target runoff of the agriculture industry becomes a big concern due to its potential negative impacts on the environment and human health. The use of advanced oxidative processes (AOP) to remove harmful contaminants has been shown to be effective for wastewater treatment. Herein, we report on an advanced photoelectrochemical (PEC) approach based on electrochemically modified nanoporous TiO₂ electrode for efficient degradation of ATZ. The electrochemical treated TiO₂ electrodes were shown to have a six-fold increase in the photo-current density over the untreated ones. This increase in PEC activity was attributed to the increase in Ti³⁺ sites after the electrochemical modification, which was corroborated by low-temperature electron paramagnetic resonance (EPR) studies. The removal of ATZ by the PEC process resulted in a rate constant of 1.91 × 10^-3 s^-1, compared to 3.12 × 10^-4 s^-1 obtained by a strictly photocatalytic process. Liquid-Chromatography Mass-Spectrometric measurements showed the modified TiO₂ electrodes highly effective at removing ATZ, with 96.1% removed after 10 h. Monitoring of the common degradation products desethyl atrazine (DEA), desisopropyl atrazine (DIA) and desethyl desisopropyl atrazine (DDA) revealed very low concentrations throughout the degradation process, indicating that further degradation was achieved. Quantum mechanical-based test for overall free radical scavenging activity (QM-ORSA) computational studies were performed and a mechanism for the N-dealkylation processes of ATZ has been proposed.

Advances in Hybrid Composites for Photocatalytic Applications: A Review

Heterogeneous photocatalysts have garnered extensive attention as a sustainable way for environmental remediation and energy storage process. Water splitting, solar energy conversion, and pollutant degradation are examples of nowadays applications where semiconductor-based photocatalysts represent a potentially disruptive technology. The exploitation of solar radiation for photocatalysis could generate a strong impact by decreasing the energy demand and simultaneously mitigating the impact of anthropogenic pollutants. However, most of the actual photocatalysts work only on energy radiation in the Near-UV region (<400 nm), and the studies and development of new photocatalysts with high efficiency in the visible range of the spectrum are required. In this regard, hybrid organic/inorganic photocatalysts have emerged as highly potential materials to drastically improve visible photocatalytic efficiency. In this review, we will analyze the state-of-art and the developments of hybrid photocatalysts for energy storage and energy conversion process as well as their application in pollutant degradation and water treatments.

Efficient degradation of VOCs using semi-coke activated carbon loaded ternary Z-scheme heterojunction photocatalyst BiVO4-BiPO4-g-C3N4 under visible light irradiation

The coal chemical industry generates large amounts of solid waste and volatile organic compounds (VOCs). In this study, the solid waste semi-coke powder obtained in the semi-coke production process was used as a raw material to prepare high-specific surface area semi-coke activated carbon (SAC) by a carbonization and activation process, and a ternary z-scheme heterojunction photocatalyst with high catalytic performance was loaded for synergistic treatment by adsorption and photodegradation to achieve waste treatment with waste. The prepared semi-coke activated carbon has a specific surface area of 619.27 m² g^-1, which can achieve effective adsorption of VOCs. The ternary z-scheme heterojunction photocatalyst BiPO₄-BiVO₄-g-C₃N₄ (PVCN) was supported on a semi-coke activated carbon substrate by a one-step sol-gel method. Based on the synergistic effect of adsorption and photocatalysis, the obtained PVCN/SAC material can degrade toluene by 85.6% within 130 minutes under simulated sunlight irradiation, which is 2.43 times that of pure photocatalyst. The rate of degrading toluene can be increased by 4.43 times. Capture experiments showed that superoxide radicals (_˙O₂^-) and hydroxyl radicals (_˙OH) were the key active species in the degradation pathway. Even after five cycles, the material maintained 81.6% of the degradation performance. In this work, we deeply investigate the mechanism of semi-coke activated carbon as a matrix for enhancing photocatalytic degradation performance. The findings of this work provide new insights into the efficient degradation of VOCs and provide a good theoretical basis for the development of high-performance photocatalysts.