Air Remediation Research Articles

Photocatalytic TiO2-based coatings for environmental applications

This review intends to provide a representative survey of nanostructured TiO2 coatings obtained by means of different deposition techniques, either starting from pre-formed TiO2 based nanostructures or directly synthetizing the nanostructured photocatalysts in situ on a surface, from suitable precursors. In particular, some among the most recent reports regarding a range of conventional as well as emergent techniques to obtain nanostructured TiO2 based coatings, are presented, discussing their characteristics and advantages, in the light of the experimental conditions and the supporting substrates. Selected examples of commercially available nanostructured TiO2 based products and real applications of nanostructured TiO2 based coatings for water and air remediation, self-cleaning, cultural heritage protection and bactericidal inactivation are described, including a very topical example of microbial inactivation represented by a TiO2 nanostructured coating against the spreading of SARS-CoV-2 on surfaces. Overall, the current coating technologies seems to be mature enough to hold a significant promise in view of the real scale applications of nanostructured TiO2 in environmental field.

Charge Carrier Processes and Optical Properties in TiO2 and TiO2-Based Heterojunction Photocatalysts: A Review.

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H2) production (e.g., via water splitting or photo-reforming of organic substrates), CO2 reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO2) is a “benchmark” photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO2. We describe the main characteristics and advantages of TiO2 as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO2 photoluminescence and on the effect of molecular oxygen (O2) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO2 limitations and possible ways to overcome them by discussing some of the “hottest” research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO2 without any cocatalysts. Discussed topics are highly-reduced “black TiO2”, grey and colored TiO2, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO2 is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO2 composites or heterostructures with metals (e.g., Pt-TiO2, Au-TiO2), with other metal oxides (e.g., Cu2O, NiO, etc.), direct Z-scheme heterojunctions with g-C3N4 (graphitic carbon nitride) and dye-sensitized TiO2.

The use of indoor plant as an alternative strategy to improve indoor air quality in Indonesia.

Indoor air pollution marked with decreased air quality below the set standard. The quality of indoor air is determined by ambient air quality as well as by a harmful substance resulting from the household activity. Indoor air pollution may cause several problems such as sick building syndrome, chronic obstructive pulmonary disease (COPD), asthma, lung cancer, and is responsible for nearly two million death in developing countries. One of the interesting research topics to overcome the indoor air pollution problem is the application of indoor plants. Although there are no established criteria to specify the best indoor plant, several studies have revealed the capability of a particular indoor plant to remove the harmful substances. This paper summarizes important information about indoor air pollution and provides the evidence-based insight of indoor plant usefulness as an alternative way for indoor air remediation.

Green synthesis of Ag@SnO2 nanocomposites for enhancing photocatalysis of nitrogen monoxide removal under solar light irradiation

Herein, Ag@SnO2 nanocomposite (NC) is prepared by UV-assisted depositing of Ag nanoparticles (NPs) onto hydrothermally synthesized SnO2 NPs surface without any additional assistance. To characterize the NC, transmission electron microscope (TEM) images, selected area electron diffraction (SAED), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) patterns are employed. The understanding of kinetics, photostability, and species lifespan in the NC are thoroughly studied with experiments and time-dependent electron spin resonance (ESR). The Ag@SnO2 exhibits an outstanding NO photocatalytic removal performance (70%) and low conversion of NO2 (4%) after 30 min under solar light. The Ag@SnO2 promises a potential application for air remediation.

Sansevieria trifasciata and Chlorophytum comosum botanical biofilter for cigarette smoke phytoremediation in a pilot-scale experiment—evaluation of multi-pollutant removal efficiency and CO2 emission

Botanical biofilters have been proposed as an effective technology for indoor air remediation. Plants, including Sansevieria trifasciata and Chlorophytum comosum, which remove VOCs effectively, can also reduce CO2 emission since S. trifasciata and C. comosum are CAM and C3 plant species, respectively. Therefore, a botanical biofilter using these plants together shows potential for use in contaminated sites. Herein, the potential of this mixed plant botanical biofilter was evaluated as a method of phytoremediation for multi-pollutants from cigarette smoke. The results showed that the combination of S. trifasciata and C. comosum in a botanical biofilter was highly effective in removing VOCs and PM2.5. In addition, this botanical biofilter can also successfully remove formaldehyde, acetone, benzene, and xylene, with low CO2 emission under indoor conditions of moderate light intensity (50 μmole PAR m−2 s−1). The system was also installed in a large volume room (24 m3) to test phytoremediation of multi-pollutants from cigarette smoke. The results showed that this mixed plant botanical biofilter can remediate indoor air pollution effectively under both light and dark conditions continuously for three cycles. The mixed plant botanical biofilter developed showed potential for use in real contamination sites.

Preparation and Characterization of Platinum Nanoparticles Supported by Non-woven Fabric for Formaldehyde Decomposition

Vacuum adsorption was employed to synthesize platinum/polyethylene terephthalate (PET) samples with great potential for indoor air quality improvement application. Pt nanoparticles (PNPS) were loaded onto surface of non-woven fabric composed of polyethylene terephthalate (PET) and with high air permeability. High activity was sought for the Pt/PET catalyst by investigating the effects of hexadecyl trimethyl ammonium bromide (CTAB) on PNP sizes and PET fiber surfaces. The prepared samples were characterized by XRD, SEM, TEM, TG, FTIR and XPS analysis. The results indicated that Pt/PET samples were successfully fabricated, and a formaldehyde conversation of 70.8 % was obtained within 150 min at 35 °C using Pt/PET with 0.6 wt% Pt loading. This research provided a facile method for depositing Pt onto PET fibers (non-woven fabric), and the catalytic activities of the resulting composite Pt/PET materials indicated them to be promising for applications to indoor air remediation.

Fabrication of Multifunctional Filters via Online Incorporating Nano‐TiO2 into Spun‐Bonded/Melt‐Blown Nonwovens for Air Filtration and Toluene Degradation

AbstractMultifunctional air filters with high filtration efficiency for aerosol and good degradation for toluene and bacteria are fabricated via online incorporation of TiO2/Ag nanoparticles into the composite nonwovens with spun‐bonded/melt‐blown (SM) hierarchical structure. Today, severe environmental pollution attributed to atmospheric pollutants, mainly including particulate materials, and gaseous volatile organic compounds, as well as bacterial and viral contaminants, has drawn worldwide attention. In this work, a novel online incorporation technique to fabricate a multifunctional air filter with hierarchical structure of SM nonwovens and TiO2 based nanoparticles is proposed. The in situ incorporation technique not only enables the SM nonwovens filters with four orders increment on toluene degradation rate, but also increases filtration quality factor Qf of the SM filters to 0.2251 Pa−1. This research shows promising applications of the multifunctional TiO2/SM nonwoven filters for indoor air remediation. In addition, this scale‐up fabrication method of fiber‐based filters will potentially promote engineering of macromolecular fiber materials.

Printing approaches to inorganic semiconductor photocatalyst fabrication

Printing approaches to inorganic semiconductor photocatalyst fabrication potentially offer a simplified path to realising designer photocatalytic systems tailored to applications including water and air remediation, chemical synthesis, and renewable energy.

Preparation of monodisperse charged droplets via electrohydrodynamic device for the removal of fine dust particles smaller than 10 μm

Electrohydrodynamic (EHD) device is utilized in various applications and could be useful for the suppression of particulate matter in ambient conditions. In this study, we focused on the ejection of charged droplets containing electrolytes in a microdripping mode and with high charge density. Several different electrolytes with different physical and electrical properties were tested for our EHD process in order to produce the charged droplets stably. Results from series images by high-speed camera represented that droplet size and frequency were dependent on the applied voltage and flow rate, and showed different behaviors in various EHD modes such as dripping, microdripping, mixed dripping, and unstable dripping. Consecutive experimental data for charge density showed that 15 wt% KCl solution was proper to obtain highly charged droplets with the size from 50 to 100 μm. For this solution, the suppression of the fine dust particle was tested for the removal of PM10, PM2.5, and PM1 at various applied voltages. The droplet formation in microdripping mode was effective for the removal of smaller dust particles and could be applicable for the air remediation in indoor or domestic environment.

Unexpectedly High Levels of Organic Compounds Released by Indoor Photocatalytic Paints.

Photocatalytic paints based on titanium dioxide (TiO2) nanoparticles represent a promising treatment technology for cleaning the air at our dwellings. A few studies have shown that instead of elimination of harmful indoor air pollutants the production of carbonyl compounds occurs from the photocatalytic paints. Herein, we report unexpectedly high concentrations of volatile organic compounds (VOCs) released upon irradiation of photocatalytic paints which are meant to clean the air at our dwellings. The concentrations of the VOCs were measured continuously and online by PTR-ToF-MS (Proton Transfer Reaction-Time of Flight-Mass Spectrometry) connected to a well-established flow tube photoreactor. The PTR-ToF-MS analysis revealed the presence of 52 ions in the mass range between 20 and 490 amu, among which 43 have been identified. In particular very high emission rates were estimated of two relevant indoor air pollutants, formaldehyde and acetaldehyde as 355 μg h-1 and 257 μg h-1 for 1 m2, respectively. We suggest a detailed reaction mechanism responsible for the production of these harmful indoor air pollutants (formaldehyde and acetaldehyde, among the others). The hydroxyl radicals (OH) formed upon activation of TiO2, react with the organic constituent (butyl acrylate and vinyl acetate) of the paint binder lead to generation of an important number of organic compounds. We demonstrate that the TiO2 quantity and the organic content of the binder is of paramount importance with respect to the formation of VOCs, which should be considered for future optimization of this air remediation technology based on TiO2 nanoparticles.

One-pot synthesis of catalytic molybdenum based nanocomposite nano-fiber membranes for aerosol air remediation

The development of fibrous air filters exhibiting high air filtration efficiency, low energy consumption, and self-cleaning properties is a critical challenge to generate the next generation of resilient air filtration systems. Nano-fibrous mats typically exhibit higher particle capture efficiency but may also lead to higher airflow resistance compared to macro-fibrous materials due to their tighter structure. In this paper, novel catalytic membranes mats were fabricated through a one-pot synthesis from ammonium tetrathiomolybdate (ATTM) doped poly(acrylonitrile) (PAN) nanofibers for sub-micron diameter aerosol particle removal. The presence of ATTM as a dopant in conjunction with a PAN polymeric matrix was found to not only enhance the air filtration performance by increasing aerosol particle removal down to 300 nm, but also increase the photocatalytic properties of the PAN material. The enhanced separation properties compared to bare polymeric PAN nanofibrous membranes were attributed to surface nanotexturation of the fibers, leading to protrusions and pores across the nano-fiber structures, thus leading to more permeable and lightweight membranes with higher particle capture capacities. The samples were benchmarked against commercial glass fiber air filters and found to offer higher filtration efficiency, lower pressure drop, and higher quality factor than the commercial filters. Specifically, the quality factors of the catalytic nano-fiber membranes were found to be up to four times higher than that of the benchmarked commercial air filters for PM2.5 particles, while two times higher for 300 nm sized contaminants. The presence of the ATTM across the PAN matrix was also found to enhance the photocatalytic activity of the membranes by up to 130% compared to the bare PAN reference nanofibers. This novel strategy opens avenues to engineering advanced multifunctional catalytic membranes, to capture toxic particulate matter from air while offering self-cleaning properties when exposed to sunlight.

Photocatalytic degradation of VOCs on various commercial titanium dioxides: Impact of operating parameters on removal efficiency and by-products generation

Ultraviolet photocatalytic oxidation (UVPCO) is regarded as one of the promising technologies for indoor air remediation. The present study examines the photocatalytic activity of four commercialized titanium dioxide photocatalysts (P25, PC500, UV100, and S5-300A) in a small-scale single-pass continuous flow reactor. Challenge compounds have been chosen from two prevalent VOC families in indoor environment: toluene (aromatics) and methyl ethyl ketone (ketones). The influence of key experimental conditions including concentration (100–1000 ppb), relative humidity (0–50% at 23 °C), light intensity (1.25–5 W m−2), and residence time (0.02–0.1 s) on removal efficiency are evaluated. Due to the fact that one of the main shortcomings of PCO air purifiers is by-products generation, a special emphasis is put on identification and quantification of gaseous by-products using HPLC and GC-MS methods. The obtained efficiencies on various photocatalysts are explained considering crystalinity, crystalline phase, crystal size, surface area, and population of surface hydroxyl groups. Despite possessing lower crystalinity, PC500, UV100 and S5-300A outperformed P25 in toluene and MEK removal efficiency, primarily owed to their larger surface area, smaller crystal size, and higher concentration of surface hydroxyl groups. Acetone, acetaldehyde, and formaldehyde were identified as the main gaseous by-products in PCO of toluene and MEK.

Large scale production of photocatalytic TiO<sub>2</sub> coating for volatile organic compound (VOC) air remediation

In this work, a pure TiO2 colloid was produced at pilot scale of 5 L and deposited on stainless steel with a pilot roll-to-roll line to produce photocatalytic coating for VOC degradation. The pure TiO2 colloid was synthesized with an aqueous sol–gel process, producing crystalline nanoparticles around 4–5 nm (mainly anatase phase) dispersed in water. The crystalline phases were produced at low temperature (<100 ℃) without calcination step. The crystalline coating produced with roll-to-roll process was very thin, around 50 nm. The photoactivity of this coating towards VOC destruction was evaluated on the degradation of acetaldehyde; the measured activity of the coating was 35 ± 5%. With the use of mass spectrometer, it was shown that acetaldehyde was mainly converted in CO2. The durability of the coating was assessed after 1, 2 and 3 weeks, and showed that the photoactivity stayed constant for this period.

Biomonitoring of Urban Pollution Using Silicon-Accumulating Species, Phyllostachys aureosulcata ‘Aureocaulis’

We investigated metal accumulation in bamboo leaves during three seasons at three urban locations differing in pollution levels. The higher content of Cu, Pb, and Zn in the leaves was in correlation with the highest bioavailable content of these elements in the soil at the most polluted location. The content of leaf trace elements was higher in summer and autumn compared to spring. Scanning electron microscopy with energy dispersive X-ray spectroscopy showed that Si accumulation in bamboo leaves was the highest in epidermis and vascular tissue, and was co-localized with trace metals. Analysis of phytoliths showed co-deposition of Al, C, and Si, implyingthe involvement of Si in metal detoxification. Compared to a common urban tree, linden, bamboo showed better capacity to maintain cellular redox homeostasis under deteriorated environmental conditions. The results suggest that bamboo can be efficiently used for biomonitoring of air and soil metal pollution and remediation in urban areas.

Phylloremediation of Air Pollutants: Exploiting the Potential of Plant Leaves and Leaf-Associated Microbes.

Air pollution is air contaminated by anthropogenic or naturally occurring substances in high concentrations for a prolonged time, resulting in adverse effects on human comfort and health as well as on ecosystems. Major air pollutants include particulate matters (PMs), ground-level ozone (O3), sulfur dioxide (SO2), nitrogen dioxides (NO2), and volatile organic compounds (VOCs). During the last three decades, air has become increasingly polluted in countries like China and India due to rapid economic growth accompanied by increased energy consumption. Various policies, regulations, and technologies have been brought together for remediation of air pollution, but the air still remains polluted. In this review, we direct attention to bioremediation of air pollutants by exploiting the potentials of plant leaves and leaf-associated microbes. The aerial surfaces of plants, particularly leaves, are estimated to sum up to 4 × 108 km2 on the earth and are also home for up to 1026 bacterial cells. Plant leaves are able to adsorb or absorb air pollutants, and habituated microbes on leaf surface and in leaves (endophytes) are reported to be able to biodegrade or transform pollutants into less or nontoxic molecules, but their potentials for air remediation has been largely unexplored. With advances in omics technologies, molecular mechanisms underlying plant leaves and leaf associated microbes in reduction of air pollutants will be deeply examined, which will provide theoretical bases for developing leaf-based remediation technologies or phylloremediation for mitigating pollutants in the air.

Polysulfone Hollow Porous Granules Prepared from Wastes of Ultrafiltration Membranes as Sustainable Adsorbent for Water and Air Remediation

Polysulfone hollow fibers (PS‐HF) are widely used in ultrafiltration modules for hemodialysis and drinking water purification for the removal of large colloids, microbes, and viruses. The preparation of commercial PS‐HF standard modules leads to the generation of PS‐HF scraps in amount higher than 5% of the total production (tons scale) that cannot be used for filtration purposes and represent large volume plastic wastes. Here, a simple approach to convert such high value waste into a new granular material with excellent adsorption properties toward organic compounds is introduced. Polysulfone hollow porous granules (PS‐HPG) are prepared by a mechanical grinding process and then exploited for the removal of selected organic compounds from water under flow and batch conditions. Volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons, and also selected emerging organic contaminants (EOCs), including two common drugs (carbamazepine and diclofenac), a plastic additive (bisphenol A), and a textile dye (Rhodamine B), are successfully adsorbed. Regeneration and reuse possibility of PS‐HPG are also demonstrated. Additionally, air remediation from VOCs and from airborne nanoparticles is successfully achieved by flowing an air stream through a filtration cartridge filled by PS‐HPG.

Application of green bioremediation technology for soil, water and air remediation

Application of green bioremediation technology for soil, water and air remediation

Adsorption and Thermal Desorption of Volatile Organic Compounds in a Fixed Bed – Experimental and Modeling

Adsorption and thermal desorption dynamics of acetone in fixed-bed silica gel were studied experimentally and theoretically. The effect of process factors on adsorption and desorption performances was established. Acetone adsorption from air stream was performed by the dynamic (flowing gas) method in a laboratory setup at two levels of air superficial velocity (0.7 and 1.7 cm s−1), temperature (30 and 40°C), and adsorbent particle diameter (0.21 and 0.54 cm). The values of saturation adsorption capacity (0.147–0.270 g g−1) increased up to 78% and 36%, respectively, with a decrease in air velocity and adsorption temperature. Acetone thermal desorption from spent silica gel was studied in a thermobalance at three levels of process temperature (60, 70, and 80°C) and two values of particle size (0.21 and 0.54 cm). Equilibrium desorption efficiency (63–81%) was up to 14% larger for finer particles and increased with the desorption temperature. Kinetic models with relevant parameters adjusted based on experimental data were adopted to predict the dynamics of acetone adsorption and thermal desorption. The models simulated well the real conditions and could be applied to scale up and operate the adsorption columns used for air remediation.

Discussion on “field study of air purification paving elements containing TiO2” by Folli et al. (2015)

In the study by Folli et al. (2015) photocatalytic pavement blocks were used on both sidewalks of a street canyon in Copenhagen (Denmark) for the purpose of air remediation of nitrogen oxides (NOx). Outstanding nitrogen monoxide (NO) degradation was observed with an average (day and night) reduction of 22% during summer months reaching values >45% at noontime. In contrast, for nitrogen dioxide (NO2) no significant improvement was obtained. Although these results would be of significant importance for many European urban environments usually suffering from high NOx levels, the results are highly unrealistic. Two simple back-to-the-envelope calculations show that the upper limit photocatalytic reduction of NO will be <1% for the investigated street canyon conditions. In addition, an alternative explanation of the experimental observations by the gas phase titration of NO by ozone (O3) is discussed.

Aerosol emission and collection in styrene-contaminated air remediation with a multi-stage plasma system

Abstract A multi-stage plasma reactor is developed for the remediation of styrene-contaminated air and the removal of plasma-generated aerosols. The plasma aerosol is fully positively charged and has a unimodal-like size distribution. More than 90% of the plasma aerosols are produced from styrene-related reaction processes. The use of an electrostatic precipitator not only leads to aerosol collection but also enhances styrene removal. Notably, below the onset corona voltage, an aerosol collection of up to 36% is observed. We also find that the collection of ultrafine aerosols (diameters