Oxidation of nauseous sulfur compounds by photocatalysis or photosensitization

Management of methyl mercaptan contained in waste gases - an overview

Methyl mercaptan is a typical volatile organosulfur pollutant contained in many gases emitted by urban waste treatment, various industries, natural gas handling, refining processes, and energy production. This work is a comprehensive overview of the scientific and practical aspects related to the management of methyl mercaptan pollution. The main techniques, including absorption, adsorption, oxidation, and biological treatments, are examined in detail. For each method, its capability as well as the technical advantages and drawbacks have been highlighted. The emerging methods developed for the removal of methyl mercaptan from natural gas are also reviewed. These methods are based on the catalytic conversion of CH3SH to hydrocarbons and H2S.

Experimental and theoretical investigation on degradation of dimethyl trisulfide by ultraviolet/peroxymonosulfate: Reaction mechanism and influencing factors

With a large amount of domestic sewage and industrial wastewater discharged into the water bodies, sulfur-containing organic matter in wastewater produced volatile organic sulfide, such as dimethyl trisulfide (DMTS) through microorganisms, caused the potential danger of drinking water safety and human health. At present, there is still a lack of technology on the removal of DMTS. In this study, the ultraviolet/peroxymonosulfate (UV/PMS) advanced oxidation processes was used to explore the degradation of DMTS. More than 90% of DMTS (30 µg/L) was removed under the conditions of the concentration ratio of DMTS to PMS was 3:40, the temperature (T) was 25 ± 2℃, and 10 min of irradiation by a 200 W mercury lamp (365 nm). The kinetics rate constant k of DMTS reacting with hydroxyl radical (HO·) was determined to be 0.2477 min^-1. Mn²⁺, Cu²⁺ and NO₃^- promoted the degradation of DMTS, whereas humic acid and Cl^- in high concentrations inhibited the degradation process. Gas chromatography-mass spectrometry was used to analyze the degradation products and the degradation intermediates were dimethyl disulfide and methanethiol. Density functional theory was used to predict the possible degradation mechanism according to the frontier orbital theory and the bond breaking mechanism of organic compounds. The results showed that the SS, CS and CH bonds in DMTS molecular structure were prone to fracture in the presence of free radicals, resulting in the formation of alkyl radicals and sulfur-containing radicals, which randomly combined to generate a variety of degradation products.

Effectiveness of an air cleaner device in reducing aerosol numbers and airborne bacteria from an enclosed type dairy barn

There is growing pressure to find technically feasible and economically viable solutions in reducing emissions of pollutants from various occupational settings to minimise environmental pollution. Hence, it is essential to develop and test methods for controlling pollutants from occupational backgrounds. We have tested an air cleaner device in reducing aerosol numbers by filtration and airborne bacteria by photocatalysis from an enclosed type dairy barn. Here, we had shown a significant reduction of larger size aerosol numbers (2.0-10.0 µm) and airborne total aerobic bacteria and Staphylococcus aureus (S. aureus) and complete clearance of Escherichia coli (E. coli) in the exhaust air of the air cleaner device. A greater 8.05% and 61.56% reduction of 5.0-10.0 µm aerosol numbers and airborne E. coli, respectively, were observed in the instantly treated central air of the dairy barn. We had found an increasing trend of aerosol numbers and airborne bacteria concentrations in the central air of the dairy barn after stopping the air cleaner device. We also had observed increased bacterial load in the filter paper of the air treatment chamber of the air cleaner device with the advancement of cleaning time. These findings are essential to validate air cleanings from various types of dairy microenvironments.

Degradation of odorous sulfide compounds by different oxidation processes in drinking water: Performance, reaction kinetics and mechanism

Swampy/septic odor caused by various sulfides is one of the most frequently encountered odor problems in drinking water. However, even though it is much more offensive, few studies have specifically focused on swampy/septic odor compared to the extensively studied musty/earthy problems. In this work, four sulfide odorants, diamyl sulfide (DAS), dipropyl sulfide (DPS), dimethyl disulfide (DMDS) and diethyl disulfide (DEDS), were selected to evaluate the treatment performance of different oxidation processes in drinking water. The results demonstrated that DMDS, DEDS, DPS and DAS could be oxidized effectively by KMnO_4, NaClO and ClO₂. The oxidation processes could be well described by the second-order kinetic model, in which k values of selected sulfides followed the order DMDS≈DEDS ≪ DPS≈DAS. As for the three oxidants, the order of reactivity was KMnO₄ ≪ ClO₂ < NaClO, which was also verified in raw water. The results of oxidation treatability, reaction kinetics and mechanisms confirmed that the characteristics of the central sulfur atom rather than the side chain is the decisive factor in controlling the oxidation rate and transformation pathway of sulfides. The transformation products and pathways were significantly different for the three oxidants. Sulfones (DPSO, DASO) were always formed by cycloaddition reactions during KMnO₄ oxidation, yet recombination reactions proceeded during ClO₂ oxidation and formed more products, such as MADS, DADS and EADS. Density functional theory (DFT) calculations confirmed that the differences in transformation pathways were caused by the variations in the activity of the oxidants and sulfides. Finally, NaClO was certified as the most effective oxidant for controlling sulfide odorants in drinking water treatment.

Photocatalytic degradation of H2S in the gas-phase using a continuous flow reactor coated with TiO2-based acrylic paint

For the photocatalytic degradation of the hydrogen sulphide (H₂S) in the gas-phase it was developed a rectangular reactor, coated with acrylic paint supported on fiber cement material. The surface formed by the paint coverage was characterized structural and morphologically by scanning electron microscopy with energy dispersive X-ray and X-ray diffraction analysis. The flow rate and the inlet concentration of H₂S were evaluated as operational performance parameters of the reactor. Removal efficiencies of up to 94% were obtained at a flow rate of 2 L min^-1 (residence time of 115 s) and inlet concentration of 31 ppm of H₂S. In addition, the H₂S degradation kinetics was modelled according to the Langmuir-Hinshelwood (L-H) model for the inlet concentrations of 8-23 ppm of H₂S. The results suggest that flow rate has a more important influence on photocatalytic degradation than the inlet concentration. It is assumed that H₂S has been oxidized to SO42- , a condition that led to a deactivation of the photocatalyst after 193 min of semi-continuous use.

Investigation of reactive oxygen species produced by microwave electrodeless discharge lamp on oxidation of dimethyl sulfide

Microwave electrodeless discharge lamp (MEDL) has been regarded as a powerful light source of photoreaction. Four kinds of chemicals, nitrogen (N₂), oxygen (O₂), water (H₂O) and dimethyl sulfide (DMS), were used as molecular probes to explore the generation process of reactive oxygen species (ROS) and their photo-oxidation performances on the photodegradation of organic pollutants with application of an exterior MEDL system. ROS such as O (³P), O₃, O (¹D) and ¹O₂ were generated via irradiation of O₂ and H₂O except dry N₂ by MEDL. They were transformed to other ROS including ·OH and H₂O₂ with increase of relative humidity. The ROS productivity was inhibited evidently by humidity and ·OH became the major active species at high humidity. An optimal mineralization rate of 23.6% for DMS photodegradation was reached in dry air compared with 8.74% at high humidity, which indicated that O (¹D) and ¹O₂ were more powerful oxidants than O₃ and OH. The results showed that the higher mineralization rate of organic pollutants was obtained by increasing the generation efficiency of ROS of O (¹D) and ¹O₂. Furthermore, the results provided an alternative to develop intensification technology on photodegadation of organic pollutants with MEDL system and an optimal operation process including photocatalyst and humidity.

Photocatalytic materials and technologies for air purification

Since there is increasing concern for the impact of air quality on human health, the present work surveys the materials and technologies for air purification using photocatalytic materials. The coverage includes (1) current photocatalytic materials for the decomposition of chemical contaminants and disinfection of pathogens present in air and (2) photocatalytic air purification systems that are used currently and under development. The present work focuses on five main themes. First, the mechanisms of photodegradation and photodisinfection are explained. Second, system designs for photocatalytic air purification are surveyed. Third, the photocatalytic materials used for air purification and their characteristics are considered, including both conventional and more recently developed photocatalysts. Fourth, the methods used to fabricate these materials are discussed. Fifth, the most significant coverage is devoted to materials design strategies aimed at improving the performance of photocatalysts for air purification. The review concludes with a brief consideration of promising future directions for materials research in photocatalysis.

The use of Yucca schidigera and microbial preparation for poultry manure deodorization and hygienization

The aim of this study was to determine the effectiveness of microbial preparation and Yucca schidigera in the removal of odorous volatile compounds from poultry manure as well as to evaluate antimicrobial properties of these amendments. It was demonstrated that the combined treatment of poultry manure (PM) with the microbial preparation and Y. schidigera extract can reduce the concentration of odorants by 58%-73%, depending on the tested compound. When Y. schidigera extract and the microbial preparation were applied at a time interval of 48 h, the deodorization efficiency was improved by 6-24%. Furthermore, Y. schidigera extract has antimicrobial properties, which affect poultry manure hygienization. It was found that when the microbial preparation was enriched with Lactobacillus plantarum, it became insensitive to the antimicrobial properties of Y. schidigera.

Materials for selective photo-oxygenation vs. photocatalysis: preparation, properties and applications in environmental and health fields

Photosensitizing materials made of organic dyes embedded in various supports are compared to usual supported TiO₂-based photocatalysts.

Single-Step Synthesis of SnS₂ Nanosheet-Decorated TiO₂ Anatase Nanofibers as Efficient Photocatalysts for the Degradation of Gas-Phase Diethylsulfide

We report on a facile one-step soft hydrothermal process for synthesizing 1D anatase TiO2 nanofibers decorated with ultrathin SnS2 nanosheets. H-titanate nanofibers were used as preshaped Ti precursor. Under controlled conditions, the H-titanate structure was transformed into anatase maintaining the fibril morphology, while at the same time SnS2 nanosheets were grown in situ on the surface of the nanofibers. The successful formation of SnS2 nanosheets on the TiO2 nanofibers was confirmed by high-resolution TEM, and together with XPS spectroscopy, the tight interface formed between the SnS2 and the anatase TiO2 nanofibers was verified. The 1D SnS2/TiO2 hierarchical nanostructures with semiconductor heterojunction were proven to be very efficient under artificial solar irradiation in the photocatalytic degradation of gaseous diethylsulfide as simulant for live yperite chemical warfare agent as well as model substrate for malodorous organosulfide volatile organic compounds. SnS2 did not operate as a visible light sensitizer for TiO2 but rather as an oxidizing agent and charge-carrier separator. The semiconductor ratio in the heterostructure controlled the photoactivity. Samples with no or high content of SnS2 were less active than those with moderate SnS2 content. Enhanced reactivity was ascribed to an efficient separation of the photogenerated charge carriers driven by the differences in band edge positions and favored by the tight interface within the coupled heterostructure.

Potential of electric discharge plasma methods in abatement of volatile organic compounds originating from the food industry

Increased volatile organic compounds emissions and commensurate tightening of applicable legislation mean that the development and application of effective, cost-efficient abatement methods are areas of growing concern. This paper reviews the last two decades' publications on organic vapour emissions from food processing, their sources, impacts and treatment methods. An overview of the latest developments in conventional air treatment methods is presented, followed by the main focus of the paper, non-thermal plasma technology. The results of the review suggest that non-thermal plasma technology, in its pulsed corona discharge configuration, is an emerging treatment method with potential for low-cost, effective abatement of a wide spectrum of organic air pollutants. It is found that the combination of plasma treatment with catalysis is a development trend that demonstrates considerable potential. The as yet relatively small number of plasma treatment applications is considered to be due to the novelty of pulsed electric discharge techniques and a lack of reliable pulse generators and reactors. Other issues acting as barriers to widespread adoption of the technique include the possible formation of stable oxidation by-products, residual ozone and nitrogen oxides, and sensitivity towards air humidity.

Selective oxidation with nanoporous silica supported sensitizers: an environment friendly process using air and visible light

Transparent and porous silica xerogels containing various grafted photosensitizers (PSs) such as anthraquinone derivatives, Neutral Red, Acridine Yellow and a laboratory-made dicyano aromatics (DBTP) were prepared. In most cases, the xerogels were shown to be mainly microporous by porosimetry. The PSs were characterized in the powdered monoliths (form, aggregation, concentration) by electronic spectroscopy which also proved to be a useful tool for monitoring the material evolution after irradiation. These nanoporous xerogels were used as microreactors for gas/solid solvent-free photo-oxygenation of dimethylsulfide (DMS) using visible light and air as the sole reactant. All these PSs containing monoliths were efficient for gas-solid DMS oxidation, leading to sulfoxide and sulfone in varying ratios. As these polar oxidation products remained strongly adsorbed on the silica matrix, the gaseous flow at the outlet of the reactor was totally free of sulfide and odorless. The best results in term of yield and initial rate of degradation of DMS were obtained with DBTP containing xerogels. Moreover, as these materials were reusable without loss of efficiency and sensitizer photobleaching after a washing regeneration step, the concept of recyclable sensitizing materials was approved, opening the way to green process.

Effects of TiO2 based photocatalytic paint on concentrations and emissions of pollutants and on animal performance in a swine weaning unit

The beneficial effect of a TiO2-based photocatalytic treatment on the indoor air purification of a swine farm has been evaluated in a trial performed in two identical mechanically ventilated traditional weaning units, with 391 animals lodged in each of them. One unit was used as reference, whereas the walls of the second unit (260 m2) were coated with ca. 70 g m(-2) of TiO2 and irradiated with ten UV-A lamps. The environmental parameters (i.e. the ventilation rate, the internal and external temperature and relative humidity), together with NH3, CH4, CO2 and N2O concentrations in the exhaust ducts and PM10 emissions, were monitored in the two units throughout all of the production cycle (75 days). Significant decreases in CH4 concentration (ca. 27%, P < 0.05) and PM10 emission (ca. 17%, P < 0.01) were observed, together with an increase of the piglets' productive performance in the treated unit with respect to the reference one. Indeed, the ADG (Average Daily Gain of piglets) was 424 g vs. 414 g for the piglets lodged in the two units, respectively, with a significantly better feed conversion ratio (FCR, ratio between the food ingested by the animals and their weight gain) of 2.18 vs. 2.44 (P < 0.001). Therefore, the photocatalytic treatment with TiO2 coating had positive effects not only on methane concentration and particulate matter concentration and emission, but also significantly improved the feed conversion ratio of growing piglets, very likely due to the increased quality of indoor air, with positive economic repercussions for the farmer. Internal photocatalytic treatment in swine husbandry could thus be considered as a potential Best Available Technology (BAT).

Applicability of a continuous-flow system inner-coated with S-doped titania for the photocatalysis of dimethyl sulfide at low concentrations

The present study investigated the photocatalytic activity of an S-doped TiO(2) photocatalyst with regards to dimethyl sulfide degradation under visible-light irradiation, along with its deactivation and reactivation. The dimethyl sulfide conversion was between 85% and 93% for the lowest relative humidity range (10-20%) and close to 100% for the two higher relative humidity ranges (45-55% and 80-90%). The conversion was also close to 100% for the two lowest input concentrations (0.039 and 0.195 ppm), while it was between 91% and 96% at 3.9 ppm and between 85% and 90% at 7.9 ppm. In contrast to the input concentration dependences on conversion, the calculated degradation rates increased as input concentrations increased. The dimethyl sulfide conversion at low concentrations (<or=0.39 ppm), which are associated with non-occupational inn occurring. However, catalyst deactivations were observed during the photocatalytic process whdoor air quality issues, was up to nearly 100% for long time periods (at least 603 h), without any significant catalyst deactivatioen higher concentrations (3.9 and 7.8 ppm) were used. The photocatalyst, reactivated by using two types of air (dried and humidified) under visible-light irradiation, did not regain all of its initial activities. Sulfate groups were qualitatively identified as the reaction products on the photocatalyst surface. In addition, gaseous byproducts, quantitatively determined, included dimethyl disulfide, methanol, and SO(2). It is noteworthy that the peak concentration of dimethyl disulfide (0.79 ppm = 790 ppb), generated over the photocatalytic process with the highest dimethyl sulfide input concentration, exceeded the odor threshold value of 0.1-3.6 ppb for dimethyl disulfide.

Visible-light-activated photocatalysis of malodorous dimethyl disulphide using nitrogen-enhanced TiO2

This study evaluated the feasibility of applying a visible-light-activated photocatalytic technique to cleanse air dimethyl disulphide (DMDS) at low concentration conditions (0.027-5.4 ppm), by using nitrogen-enhanced TiO2. In addition, the applicability of a backup adsorption unit for the secondary control of DMDS exiting from the photocatalytic oxidation (PCO) unit was investigated. The PCO unit functioned effectively for the control of DMDS at low concentration levels ( < or = 0.027 ppm) for long-time periods (at least 603 h). However, rapid photocatalyst deactivation levels were observed during photocatalytic processes with a higher DMDS input concentration (IC) (2.7 ppm). The photocatalyst reactivated with humidified or dried air, under visible-light irradiation, did not regain all its initial activities. The photocatalytic degradation efficiencies (PDEs) for DMDS were close to 100% for the relative humidity (RH) range of 45-55%, whereas they were between 86% and 91% and between 78% and 82% regarding the RH ranges of 10-20% and 80-90%, respectively. The PDEs via the PCO alone were close to 100% during this time period for the lowest IC conditions (0.027 ppm), whereas they decreased gradually for the other ICs. The FTIR spectra of the photocatalysts, as well as a solid-liquid extraction method, suggested the formation of sulphate groups on the catalyst surface during a photocatalytic process. Methanol was identified as a gaseous by-product. In addition, the backup adsorption unit could be effectively utilized to remove methanol, under a broad indoor pollution level (0.027-5.4 ppm), as well as DMDS exiting from the PCO units.

Solvent-free production of singlet oxygen at the gas-solid interface: visible light activated organic-inorganic hybrid microreactors including new cyanoaromatic photosensitizers

We synthesized new cyanoaromatics, benzo[b]triphenylene-9,14-dicarbonitrile (DBTP) 1a and a graftable derivative, 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid (DBTP-COOH) 1b, easily prepared from commercial reagents. Their photosensitizing properties were investigated. Hybrid porous silica monoliths loaded with encapsulated 1a or grafted 1b were prepared, and their adsorption, spectroscopic and photosensitizing properties, as well as stability, were compared. Solvent-free, efficient oxidation of dimethylsulfide (DMS) was observed at the gas-solid interface under visible light irradiation. Quantum yields of formation of 1O2 inside the porous monoliths are comparable to those of phenalenone. Singlet oxygen lifetimes (approximately 25 micros) were found to be longer in silica monoliths than in usual polar solvents such as methanol or ethanol. This new class of hybrid materials work as porous, transparent, and highly efficient microreactors for oxidation reactions under visible light.