Novel kinetic modeling of photocatalytic degradation of ethanol and acetaldehyde in air by commercial and reduced ZnO: Effect of oxygen vacancies and humidity

A comprehensive kinetic model has been developed to address the factors and processes governing the photocatalytic removal of gaseous ethanol by using ZnO loaded in a prototype air purifier. This model simultaneously tracks the concentrations of ethanol and acetaldehyde (as its primary oxidation product) in both gas phase and on the catalyst surface. It accounts for reversible adsorption of both compounds to assign kinetic reaction parameters for different degradation pathways. The effects of oxygen vacancies on the catalyst have been validated through the comparative assessment on the catalytic performance of commercial ZnO before and after the reduction pre-treatment (10% H2/Ar gas at 500 °C). The influence of humidity has also been assessed by partitioning the concentrations of water molecules across the gas phase and catalyst surface interface. Given the significant impact of adsorption on photocatalytic processes, the beginning phases of all experiments (15 min in the dark) are integrated into the model. Results showcase a notable decrease in the adsorption removal of ethanol and acetaldehyde with an increase in relative humidity from 5% to 75%. The estimated number of active sites, as determined by the model, increases from 7.34 10-6 in commercial ZnO to 8.86 10-6 mol gcat-1 in reduced ZnO. Furthermore, the model predicts that the reaction occurs predominantly on the catalyst surface while only 14% in the gas phase. By using quantum yield calculations, the optimal humidity level for photocatalytic degradation is identified as 25% with the highest quantum yield of 6.98 10-3 (commercial ZnO) and 10.41 10-3 molecules photon-1 (reduced ZnO) catalysts.

Application of different packing media for the biofiltration of gaseous effluents from waste composting

A pilot-scale experiment was implemented in a waste bioreactor with an inner capacity of 1 m3 in order to simulate a real-scale composting process. The waste underwent composting conditions that are typical of the initial bio-oxidation phase, characterised by a high production of volatile organic compounds (VOCs), hydrogen sulphide (H2S) and odorants. The waste bioreactor was fed with an intermittent airflow rate of 6 Nm3/h. The target of this study was to investigate the air treatment performance of three biofilters with the same size, but filled with different filtering media: (1) wood chips, (2) a two-layer combination of lava rock (50%) and peat (50%), and (3) peat only. The analyses on air samples taken upstream and downstream of the biofilters showed that the combination of lava rock and peat presents the best performance in terms of mean removal efficiency of odour (96%), total VOCs (95%) and H2S (77%) concentrations. Wood chips showed the worst abatement performance, with respective mean removal efficiencies of 90%, 88% and 62%. From the results obtained, it is possible to conclude that the combination of lava rock and peat can be considered as a promising choice for air pollution control in waste composting facilities.

Quartz Nonadherent and Clean Exfoliation of the Heteroatom-Doped Bulk Carbon Nanotubes Array

Vertically aligned carbon nanotubes array offers unique properties for various applications. Detaching them from the growth substrate, while preserving their vertical structure, is essential. Quartz, a cost-effective alternative to silicon wafers and metal-based substrates, can serve as both a reaction chamber and a growth substrate. However, the strong adhesive interaction with the quartz substrate remains an obstacle for further applications. Herein, we presented a simple and well-controlled exfoliation strategy assisted by the introduction of heteroatoms at root ends of a carbon nanotubes array. This strategy forms lower surface polarity of the carbon fragment to significantly reduce adhesion to the quartz substrate, which contributes to the effortless exfoliation. Furthermore, this scalable approach enables potential mass production on recyclable quartz substrates, enhancing the cost-effectiveness and efficiency. This work can establish a solid foundation for cost-competitive carbon nanotube-based technologies, offering a promising avenue for their widespread applications.

Large-scale evaluation of microorganism inactivation by bipolar ionization and photocatalytic devices

The COVID-19 pandemic has raised awareness in the spread of disease via airborne transmission. As a result, there has been increasing interest in technologies that claim to reduce concentrations of airborne pathogens in indoor environments. The efficacy of many of these emerging technologies is not fully understood, and the testing that has been done is often conducted at a small scale and not representative of applied settings. There is currently no standard test method for evaluating air treatment technologies, making it difficult to compare results across studies or technology types. Here, a consistent testing approach in an operational-scale test chamber with a mock recirculating heating, ventilation, and air conditioning (HVAC) system was used to evaluate the efficacy of bipolar ionization and photocatalytic devices against the non-enveloped bacteriophage MS2 in the air and on surfaces. Statistically significant differences between replicate sets of technology tests and control tests (without technologies active) are apparent after 1 h, ranging to a maximum of 0.88 log₁₀ reduction for the bipolar ionization tests and 1.8 log₁₀ reduction for the photocatalytic device tests. It should be noted that ozone concentrations were elevated above background concentrations in the test chamber during the photocatalytic device testing. No significant differences were observed between control and technology tests in terms of the amount of MS2 deposited or inactivated on surfaces during testing. A standardized, large-scale testing approach, with replicate testing and time-matched control conditions, is necessary for contextualizing laboratory efficacy results, translating them to real-world conditions, and for facilitating technology comparisons.

Efficacy of single pass UVC air treatment for the inactivation of coronavirus, MS2 coliphage and Staphylococcus aureus bioaerosols

There is strong evidence that SARS-CoV-2 is spread predominantly by airborne transmission, with high viral loads released into the air as respiratory droplets and aerosols from the infected subject. The spread and persistence of SARS-CoV-2 in diverse indoor environments reinforces the urgent need to supplement distancing and PPE based approaches with effective engineering measures for microbial decontamination - thereby addressing the significant risk posed by aerosols. We hypothesized that a portable, single-pass UVC air treatment device (air flow 1254 L/min) could effectively inactivate bioaerosols containing bacterial and viral indicator organisms, and coronavirus without reliance on filtration technology, at reasonable scale. Robust experiments demonstrated UVC dose dependent inactivation of Staphylococcus aureus (UV rate constant (k) = 0.098 m²/J) and bacteriophage MS2, with up to 6-log MS2 reduction achieved in a single pass through the system (k = 0.119 m²/J). The inclusion of a PTFE diffuse reflector increased the effective UVC dose by up to 34% in comparison to a standard Al foil reflector (with identical lamp output), resulting in significant additional pathogen inactivation (1-log S. aureus and MS2, p < 0.001). Complete inactivation of bovine coronavirus bioaerosols was demonstrated through tissue culture infectivity (2.4-log reduction) and RT-qPCR analysis - confirming single pass UVC treatment to effectively deactivate coronavirus to the limit of detection of the culture-based method. Scenario-based modelling was used to investigate the reduction in risk of airborne person-to-person transmission based upon a single infected subject within the small room. Use of the system providing 5 air changes per hour was shown to significantly reduce airborne viral load and maintain low numbers of RNA copies when the infected subject remained in the room, reducing the risk of airborne pathogen transmission to other room users. We conclude that the application of single-pass UVC systems (without reliance on HEPA filtration) could play a critical role in reducing the risk of airborne pathogen transfer, including SARS-CoV2, in locations where adequate fresh air ventilation cannot be implemented.

Continuous air purification by front flow photocatalytic reactor: Modelling of the influence of mass transfer step under simulated real conditions

In this work, a solution for the treatment of toxic gases based on a photocatalytic process using TiO₂ coated on a cellulosic support, has been investigated. Here, cyclohexane was chosen as the reference for testing its removal efficiency via a continuous front flow reactor as type A anti-gas filters. The photocatalytic support was firstly characterized by EDX, to confirm its elemental composition. Then, the experiments were carried out, starting with a batch reactor in order to evaluate the degradation efficiency of the photocatalytic media, as well as the monitoring of the photocatalytic process which allowed the establishing of a carbon mass balance corresponding to the stoichiometric number of our target pollutant. The transition to a continuous treatment with a front flow reactor aims to highlight the influence of the input concentration (0.29-1.78 mM m^-3) under different flow rates (12, 18 and 36 L min^-1). The relative humidity effect was also investigated (from 5 to 90% of humidity) where an optimum rate was obtained around 35-45%. In addition, the mineralization rate was monitored. The major rates obtained were for a cyclohexane input concentration of 0.29 mM m^-3 in wet condition (38%) at an air flow rate of 18 L min^-1, where the CO₂ selectivity reached 77% for an abatement of 62%. In order to understand the limiting steps of the photocatalytic process, a model considering the reactor geometry and the hydraulic flow was developed. The obtained results showed that the mass transfer must be considered in the photocatalytic process for a continuous treatment. The Langmuir-Hinshelwood bimolecular model was also developed to represent the influence of the humidity.

Pig barns ammonia and greenhouse gas emission mitigation by slurry aeration and acid scrubber

Livestock production is associated with several gaseous pollutant emissions to the environment. These emissions can degrade local and regional air quality, contribute to surface water eutrophication and acid rain, and contribute to the greenhouse gas footprint of the production sector. Modern production systems must balance animal welfare and environmental pollution potential with economic reality, which is a great challenge to maintain as global demand for animal protein increases. Accordingly, gaseous emission technologies were the main target for this research, in which mitigating gas emissions of ammonia, nitrous oxide, and methane from pig production facilities via slurry aeration system was tested. Five treatments with different airflow rates in the test room were examined continuously over a period of 6 weeks and the results were compared with the control room. Test results indicate that the highest mitigation potentials were 12, 57.6, and 10.4% for nitrous oxide, methane, and ammonia, respectively. Subsequently directing exhaust air into a sulfuric acid air scrubber at 3.0 pH further reduced total ammonia emissions by 80 to 87%.

A comparison between the four Geldart groups on the performance of a gas-phase annular fluidized bed photoreactor for volatile organic compound oxidation

Heterogeneous photocatalytic oxidation (PCO) is a widely studied alternative for the elimination of volatile organic compounds (VOC) in air. In this context, research on novel photoreactor arrangements to enhance PCO rates is desired. Annular fluidized bed photoreactors (AFBPR) have yielded prominent results when compared to conventional thin film reactors. However, very few works aimed at optimizing AFBPR operation. In this study, TiO₂ photocalytic agglomerates were synthesized and segregated in specific size distributions to behave as Geldart groups A, B, C, and D fluidization. The TiO₂ agglomerates were characterized by XRD, FTIR spectra, and N₂ adsorption. Photocatalyst performances were compared in a 10-mm gapped AFBPR for degrading the model pollutant methyl-ethyl-ketone (MEK), using a 254-nm radiation source. Geldart group C showed to be inadequate for AFBPR operation due to the short operation range between fluidization and elutriation. In all the cases, photocatalytic reaction rates were superior to sole UV photolysis. Group A and group B demonstrated the highest reaction rates. Considerations based on mass transfer suggested that the reasons were enhanced UV distribution within the bed at lower flow rates and superior catalyst surface area at higher flow rates. Results also revealed that groups A, B, and D perform equally per catalyst area within an AFBPR if the fluidization numbers (FN) are high enough.

Towards visible-light photocatalysis for environmental applications: band-gap engineering versus photons absorption-a review

A range of different studies has been performed in order to design and develop photocatalysts that work efficiently under visible (and near-infrared) irradiation as well as to improve photons absorption with improved reactor design. While there is consensus on the importance of photocatalysis for environmental applications and the necessity to utilized solar irradiation (or visible-light) as driving force for these processes, it is not yet clear how to get there. Discussion on the future steps towards visible-light photocatalysis for environmental application is of great interest to scientific and industrial communities and the present paper reviews and discusses the two main approaches, band-gap engineering for efficient solar-activated catalysts and reactor designs for improved photons absorption. Common misconceptions and drawbacks of each technology are also examined together with insights for future progress.

A modified biotrickling filter for nitrification-denitrification in the treatment of an ammonia-contaminated air stream

A conventional biotrickling filter for airborne ammonia nitrification has been modified, by converting the liquid sump into a biological denitrifying reactor. The biotrickling filter achieves an average ammonia removal efficiency of 92.4 %, with an empty bed retention time (EBRT) equal to 36 s and an average ammonia concentration of 54.7 mg Nm^-3 in the raw air stream. The denitrification reactor converts ammonia into inert gas N₂, in addition to other important advantages connected to the alkaline character of the biochemical pathway of the denitrifying bacteria. Firstly, the trickling water crossing the denitrification reactor underwent a notable pH increase from 7.3 to 8.0 which prevented the acidic inhibition of the nitrifying bacteria due to the buildup of nitric and nitrous acids. Secondly, the pH increase created the ideal conditions for the autotrophic nitrifying bacteria. The tests proved that an ammonia removal efficiency of above 90 % can be achieved with an EBRT greater than 30 s and a volumetric load lower than 200 g NH₃ m^-3 day^-1. The results of the biofilm observation by using a scanning confocal laser microscope are reported together with the identification of degrading bacteria genera in the biotrickling filter. The efficiency of the plant and its excellent operational stability highlight the effectiveness of the synergistic action between the denitrification reactor and the biotrickling filter in removing airborne ammonia.

Airborne toluene removal for minimizing occupational health exposure by means of a trickle-bed biofilter

The paper presents the experimental results on a biotrickling pilot plant, with a water scrubber as pre-treatment, finalized to the treatment of an airborne toluene stream in a working place. The air stream was characterized by a very high variability of the inlet concentrations of toluene (range: 4.35-68.20 mg Nm(-3)) with an average concentrations of 16.41 mg Nm(-3). The pilot plant has proved its effectiveness in toluene removal, along a 90-day experimentation period, in steady-state conditions. The scrubbing pre-treatment has achieved an average removal efficiency of 69.9 %, but in particular it has proven its suitability in the rough removal of the toluene peak concentrations, allowing a great stability to the following biological process. The biotrickling stage has achieved an additional average removal efficiency of 75.6 %, confirming the good biodegradability of toluene. The biofilm observation by a scanning confocal laser microscope has evidenced a biofilm thickness of 650 μm fully penetrated by toluene degrading bacteria. Among the micro-population Pseudomonas putida resulted the dominant specie. This bacterium can therefore be considered the responsible for most of the toluene degradation. The whole experimented process has determined an average 92.7 % for toluene removal efficiency. This result meets the most stringent limits and recommendations for occupational safety, given by authoritative organizations in the USA and EU; it also meets the odorous threshold concentration of 11.1 mg Nm(-3).

A biofilter for treating toluene vapors: performance evaluation and microbial counts behavior

A lab-scale biofilter packed with mixed packing materials was used for degradation of toluene. Different empty bed residence times, 148.3, 74.2 and 49.4 s, were tested for inlet concentration ranging from 0.2 to 1.2 g/m³. The maximum elimination capacity of 36.0 g/(m³ h) occurred at an inlet loading rate of 45.9 g/(m³ h). The contribution of the lower layer was higher than other layers and always had the highest elimination capacity. The carbon dioxide production rate and distribution of micro-organisms followed toluene elimination capacities. The results of this study indicated that mixed packing materials could be considered as a potential biofilter carrier, with low pressure drop (less than 84.9 Pa/m), for treating air streams containing VOCs.

Carbon dioxide capture utilizing zeolites synthesized with paper sludge and scrap-glass

The present work introduces the study of the CO2 capture process by zeolites synthesized from paper sludge and scrap glass. Zeolites ZSM-5, analcime and wairakite were produced by means of two types of Structure Directing Agents (SDA): tetrapropilamonium (TPA) and ethanol. On the one hand, zeolite ZSM-5 was synthesized using TPA; on the other hand, analcime and wairakite were produced with ethanol. The temperature programmed desorption (TPD) technique was performed for determining the CO2 sorption capacity of these zeolites at two sorption temperatures: 50 and 100 °C. CO2 sorption capacity of zeolite ZSM-5 synthesized at 50 °C was 0.683 mmol/g representing 38.2% of the value measured for a zeolite ZSM-5 commercial. Zeolite analcime showed a higher CO2 sorption capacity (1.698 mmol/g) at 50 °C and its regeneration temperature was relatively low. Zeolites synthesized in this study can be used in the purification of biogas and this will produce energy without increasing the atmospheric CO2 concentrations.

Effluents from MBT plants: plasma techniques for the treatment of VOCs

Mechanical-biological treatments (MBTs) of urban waste are growing in popularity in many European countries. Recent studies pointed out that their contribution in terms of volatile organic compounds (VOCs) and other air pollutants is not negligible. Compared to classical removal technologies, non-thermal plasmas (NTP) showed better performances and low energy consumption when applied to treat lowly concentrated streams. Therefore, to study the feasibility of the application of NTP to MBTs, a Dielectric Barrier Discharge reactor was applied to treat a mixture of air and methyl ethyl ketone (MEK), to simulate emissions from MBTs. The removal efficiency of MEK was linearly dependent upon time, power and specific input energy. Only 2-4% of MEK was converted to carbon dioxide (CO2), the remaining carbon being involved in the formation of byproducts (methyl nitrate and 2,3-butanedione, especially). For future development of pilot-scale reactors, acting on residence time, power, convective flow and catalysts will help finding a compromise between energy consumption, desired abatement and selectivity to CO2.