Solar Photolysis Research Articles

The role of solar photolysis in the atmospheric removal of methacrolein oxide and the methacrolein oxide-water van-der Waals complex in pristine environments.

Biogenic hydrocarbons are emitted into the Earth's atmosphere by terrestrial vegetation as by-products of photosynthesis. Isoprene is one such hydrocarbon and is the second most abundant volatile organic compound emitted into the atmosphere (after methane). Reaction with ozone represents an important atmospheric sink for isoprene removal, forming carbonyl oxides (Criegee intermediates) with extended conjugation. In this manuscript, we argue that the extended conjugation of these Criegee intermediates enables electronic excitation of these compounds, on timescales that are competitive with their slow unimolecular decay and bimolecular chemistry. We show that the complexation of methacrolein oxide with water enhances the absorption cross section of the otherwise dark S1 state, potentially revealing a new avenue for forming lower volatility compounds via tropospherically relevant photochemistry.

Epoxiconazole degradation in water samples: a comparative study of Fenton, photo-Fenton, solar photo-Fenton, and solar photolysis processes.

Epoxiconazole (EPO) is classified as a persistent organic pollutant due to its ability to persist in the environment for prolonged periods. Its degradation is pivotal in mitigating its environmental impact. This investigation focuses on assessing the degradation of EPO using various methodologies, namely Fenton, photo-Fenton, solar photo-Fenton, and solar photolysis, conducted in both Milli-Q water and groundwater. These experiments encompassed evaluations at both the standard pH typically used in photo-Fenton reactions and the natural pH levels inherent to the respective aqueous environments. Additionally, EPO degradation products were analyzed after a 60-min reaction. Notably, in systems utilizing groundwater, the inclusion of additional iron was unnecessary, as the naturally occurring iron content in the groundwater facilitated the intended processes. Specifically, in Milli-Q water, solar photo-Fenton demonstrated an EPO degradation efficiency of 97%. Furthermore, the substitution of Milli-Q water with groundwater in Fenton-like processes did not significantly affect the efficacy of EPO degradation. These findings underscore the potential of solar photo-Fenton as an economically viable and environmentally sustainable strategy for EPO degradation.

Dissolved Organic Matter-Mediated Photosensitized Activation of Monochloramine for Micropollutant Abatement in Wastewater Effluent.

Utilizing solar light and water matrix components in situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. We herein propose a new strategy for micropollutant abatement through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH2Cl). Exposing the chlorinated wastewater effluent with residual NH2Cl to solar irradiation (solar/DOM/NH2Cl process) degrades six structurally diverse micropollutants at rate constants 1.26-34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH2Cl process are 1.13-4.32 times the summation of those by the solar/DOM and solar/NH2Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO·) from the photosensitized activation of NH2Cl. Triplet state-excited DOM (3DOM*) dominates the activation of NH2Cl, leading to the generation of RNS, while HO· is produced from the interactions between RNS and other photochemically produced reactive intermediates (e.g., O2·- and DOM·+/·-). The findings advance the knowledge of DOM-mediated photosensitization and offer a sustainable method for micropollutant abatement in wastewater effluents containing residual NH2Cl.

Is Natural Solar Photolysis Effective for the Removal of Antibiotics and Pathogens from Waste Stabilization Pond?

This study aimed to investigate the occurrence of photolysis of two relevant antibiotics, sulfamethoxazole and trimethoprim, when present in a Waste Stabilization Pond and its disinfection under tropical irradiance. The influence of pH and matrix components, and the generation of photoproducts during solar exposure were evaluated. The pH-dependent speciation of antibiotics had a greater influence on the photolysis of antibiotics than the water matrix, with higher photodegradation at pH 4.1 than at pH 7.2. Trimethoprim was more susceptible to indirect photolysis, indicated by the steeper influence of the photosensitizers nitrate and humic acid compared to sulfamethoxazole. The photoproducts generated were persistent to photolysis and remained present in the samples up to 70 h irradiation. Wastewater disinfection was achieved after 70 h of solar exposure. Nonetheless, the exposure time required to remove at least 50% of sulfamethoxazole was on average 29 h and of trimethoprim 83 h, at the natural pH of the matrix (pH 7.2). Given that the residence time in Waste Stabilization Pond is 41 h, which corresponds to a maximum of 26 h solar irradiation, the exposure time in Waste Stabilization Pond is not sufficient to completely photodegrade antibiotics or disinfect the effluent, demanding further treatment.

The electronic structure of 2(5H)-thiophenone investigated by vacuum ultraviolet synchrotron radiation and theoretical calculations

The electronic state spectroscopy of 2(5H)-thiophenone, C4H4OS, has been investigated by high-resolution vacuum ultraviolet photoabsorption in the 3.76–10.69 eV energy range using synchrotron radiation, together with novel quantum chemical calculations performed at the equation of motion coupled cluster singles and doubles (EOM-CCSD) level of theory. The major electronic transitions have been assigned to valence and Rydberg character, with relevant C=O, C=C and C–C stretching vibrations across the entire absorption spectrum. Photolysis lifetimes in the Earth’s atmosphere (0–50 km altitude) have been estimated from the absolute photoabsorption cross-sections, indicating that solar photolysis can be expected to be a strong sink mechanism.Graphical abstract

The overlooked role of Cr(VI) in micropollutant degradation under solar light irradiation

Hexavalent chromium (Cr(VI)) is ubiquitous in natural environments, whereas its role in the transformation of coexisting contaminants may have been overlooked. In this work, it was reported for the first time that the irradiation of Cr(VI) by solar light (solar light/Cr(VI) system) could effectively degrade various micropollutants with different structures. The removal efficiency of selected micropollutants was increased by 13.3–64.8% by the solar light/Cr(VI) system compared to that by direct solar photolysis. Meanwhile, the oxidation rates were enhanced by 2.2–21.5 folds, while they were negligible by Cr(VI) oxidation alone. Experiments by specific scavengers, probe compounds, fluorescence absorbance, and electron spin resonance analysis demonstrated that hydroxyl radical (•OH) was the major reactive species in the solar light/Cr(VI) system. Further experiments showed that the generation of •OH was closely related to the intermediate Cr(V) generated from Cr(VI) reduction, and Cr(V) could be re-oxidized back to Cr(VI). Increasing solution pH negatively affected model micropollutant (carbamazepine (CBZ)) degradation by the solar light/Cr(VI) system, mainly due to the decreased quantum yield of •OH at higher pH. Coexisting sulfate ions showed negligible effect on CBZ degradation in the solar light/Cr(VI) system, while the presence of bicarbonate, chloride, and humic acid inhibited CBZ degradation to varying degrees, owing to their diverse scavenging effects on •OH. Furthermore, moderate CBZ degradation was also achieved by natural solar light photolysis of Cr(VI). This study demonstrated the pivotal role of Cr(VI) in the transformation of micropollutants under solar irradiation, which advances the understanding of the fate of micropollutants in natural environments.

Unexpected trends for the formation of chlorate and bromate during the photolysis of chlorine in bromide-containing water

Solar photolysis of free chlorine (solar/chlorine) in bromide-containing water occurs under various scenarios, such as chlorinated reservoirs and outdoor swimming pools, and the formation of chlorate and bromate is an important issue in the system. We reported unexpected trends for the formation of chlorate and bromate in the solar/chlorine system. Excess chlorine inhibited the formation of bromate, i.e., increasing chlorine dosages from 50 to 100 μM reduced the bromate yield from 6.4 to 1.2 μM in solar/chlorine at 50 μM bromide and pH 7. The yield of bromate in solar/chlorine at 100 μM chlorine and 50 μM bromide in 240 min was 18.8% of that at 50 μM bromine only. The underlying mechanism was that HOCl can react with bromite (BrO2−) to form HOClOBrO−, whose multi-step transformation finally formed chlorate as the major product and bromate as the minor product. This reaction overwhelmed the oxidation of bromite to form bromate by reactive species, such as •OH, BrO• and ozone. On the other hand, the presence of bromide greatly enhanced the formation of chlorate. Increasing bromide concentrations from 0 to 50 μM enhanced the chlorate yields from 2.2 to 7.0 μM at 100 μM chlorine. The absorbance of bromine was higher than that of chlorine, thus the photolysis of bromine formed higher levels of bromite at higher bromide concentrations. Then, bromite rapidly reacted with HOCl to form HOClOBrO− and it further transformed to chlorate. Additionally, 1 mg L−1 NOM had a negligible effect on bromate yields in solar/chlorine at 50 μM bromide, 100 μM chlorine and pH 7. This study demonstrated a new pathway of chlorate and bromate formation in the solar/chlorine system with bromide.

Degradation of natural organic matter and disinfection byproducts formation by solar photolysis of free available chlorine

Environment disinfection effectively curbs transmission of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). However, elevated concentration of free available chlorine (FAC) in disinfectants can be discharged into surface water, generating toxic disinfection byproducts (DBPs). The impact of solar photolysis of FAC on natural organic matter (NOM) to form DBPs has not been well studied. In this work, solar photolysis of FAC was found to result in higher formation of DBPs, DBPs formation potential (DBPsFP), total organic chlorine (TOCl) and lower specific ultraviolet absorbance at 254 nm (SUVA254), compared to dark chlorination. In solar photolysis of FAC, formation of total DBPs was promoted by pH=8, but hindered by the addition of HCO3−, radical scavenger or deoxygenation, while addition of NO3−and NH4+both enhanced the formation of nitrogenous DBPs. Differences in the formation of DBPs in solar photolysis of FAC under various conditions were influenced by reactive species. The formation of trichloromethane (TCM) and haloacetic acids (HAAs) in solar photolysis of FAC positively correlated with the steady-state concentrations of ClO• and O3. The steady-state concentrations of •NO and •NH2 positively correlated with the formation of halonitromethanes (HNMs). HAAs and haloacetonitriles (HANs) mainly contributed to calculated cytotoxicity of DBPs. This study demonstrates that solar photolysis of FAC may significantly impact the formation of DBPs in surface water due to extensive use of disinfectants containing FAC during SARS-CoV-2 pandemic.

The impact of inorganic ions on the solar photolysis of chlorinated dissolved organic matter from different sources: Spectral characteristics, disinfection byproducts, and biotoxicities

Dissolved organic matter (DOM) from wastewater treatment plant (WWTP) effluent is chlorinated and then discharged into natural waters, where it is subject to solar irradiation. However, the impacts of inorganic ions in natural waters on the photochemical transformations of the chlorinated DOM (DOM-Cl) have not been studied comprehensively. In this study, variations in the spectral characteristics, disinfection byproducts (DBPs), and biotoxicities of DOM-Cl under solar irradiation at different pH values and in the presence of NO3- and HCO3- were revealed. Three sources of DOM, including DOM from a WWTP effluent, natural organic matter from the Suwannee River, and DOM from plant leaf leachate, were investigated. Solar irradiation resulted in the oxidation of the highly reactive aromatic structures and then reduced the amounts of chromophoric and fluorescent DOM, especially under alkaline conditions. Moreover, alkaline conditions significantly promoted the detected DBPs degradation and the biotoxicities attenuation, while NO3- and HCO3- generally impeded them (or did not work). Dehalogenation of the unknown halogenated DBPs and photolysis of the nonhalogenated organics were the main mechanisms for the DOM-Cl biotoxicity reductions. Hence, improving the ecological safety of WWTP effluents could be achieved through solar irradiation by removing the DBPs formed.

Photolysis of chlorite by solar light: An overlooked mitigation pathway for chlorite and micropollutants

Chlorite (ClO2−) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO2− but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO2− by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO2− could be efficiently decomposed to chloride (Cl−) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl− yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O3), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO2− system with the steady-state concentrations following the order of O3 (≈ 0.8 μΜ) > ClO• (≈ 4.4 × 10−6 μΜ)> •OH (≈ 1.1 × 10−7 μΜ)> Cl• (≈ 6.8 × 10−8 μΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO2− system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min−1 at pH 7.0, while most of them were negligibly degraded by SSL or ClO2− treatment alone. Kinetic modeling of BZF degradation by SSL/ClO2− at pHs 6.0 – 8.0 suggested that •OH contributed the most, followed by Cl•, O3, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO2− system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO2− and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO2− and micropollutants, which has significant implications for understanding their fate in natural environments.

The impact of sunlight on fouling behaviors and microbial communities in membrane bioreactors

Membrane fouling is a significant obstacle to applying membrane bioreactors (MBRs) for wastewater treatment. Here we report the impact of sunlight irradiation on membrane fouling, biopolymers, signal molecules, and microbial communities in MBRs. The degradation of signal molecules, which induce membrane biofouling, occurred through solar photolysis in batch tests. However, MBR sludge exposed to sunlight exhibited different biological behaviors creating more soluble microbial products (SMP) and signal molecules (particularly autoinducer-2). Cell lysis and deflocculation occurred when the MBR mixed liquor was exposed to sunlight. MBR fouling rates coincided with the temporal concentration profiles of SMP and signal molecules. Sunlight caused drastic changes in the MBR microbial community, stimulating the preferential growth of specific bacteria (e.g., Deinococcus Runella, Flavitalea, Glaiimonas, and Rurimicrobium). The nonmetric multidimensional scaling analysis of the MBR community structures with and without sunlight irradiation showed two distinct microbial community clusters and their reversibility. Network analysis based on Spearman's rank correlations revealed that with sunlight irradiation, fouling rates had significant positive connections with SMP proteins and the Flavitalea genus. The findings of this study demonstrate that sunlight is a considerable factor affecting membrane fouling and microbial ecology in MBRs, needing shade for fouling mitigation and sustainable operation.

Study of degradation of norfloxacin antibiotic and their intermediates by natural solar photolysis.

In this work, the photolysis of the antibiotic norfloxacin (NOR) and the formation of its photodegradation products were studied using UV and solar radiation. Their extraction was also assessed in Milli-Q water and secondary effluents from a wastewater treatment plant. The photolysis of NOR was chromatographically monitored. The structure of each degradation product is related to the reaction of NOR with reactive oxygen species (ROS), as confirmed using radical quenchers and mass spectrometry. Additionally, the feasibility of extracting NOR and its degradation products was assessed using a commercial solid phase extraction system. Photolysis results showed the formation of five degradation products, generated under exposure to both types of radiation. The decays in NOR concentrations for the solar and UV treatments were adjusted to pseudo first-order kinetics with apparent constant values of ksolar = 1.19 × 10-3s-1 and kUV = 3.84 × 10-5s-1. Furthermore, the superoxide radical was the main participant species in the formation of the degradation products P3, P4, and P5. Species P1 and P2 do not need this radical for their formation. The presence of NOR in water opens the possibility of its photolysis by solar radiation. This work contributes to the understanding of the mechanisms that mediate its photodegradation, in addition to studying potential options for its determination and its photodegradation products in the sample treatment.

Electronically excited states of formic acid investigated by theoretical and experimental methods

Absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements of gas-phase formic acid (HCOOH) in the photon energy range 4.7–10.8 eV (265–115 nm), together with quantum chemical calculations to provide vertical energies and oscillator strengths. The combination of experimental and theoretical methods has allowed a comprehensive assignment of the electronic transitions. The VUV spectrum reveals various vibronic features not previously reported in the literature, notably associated with (3pa′←10a′), (3p′a′←10a′), (3sa′←2a″) and (3pa′←2a″) Rydberg transitions. The assignment of vibrational features in the absorption bands reveal that the C=O stretching, v3′a′, the H′–O–C′ deformation, v5′a′, the C–O stretching, v6′a′, and the O=C–O′ deformation, v7′a′ modes are mainly active. The measured absolute photoabsorption cross sections have also been used to estimate the photolysis lifetime of HCOOH in the upper stratosphere (30–50 km), showing that solar photolysis is an important sink at altitudes above 30 km but not in the troposphere. Potential energy curves for the lowest-lying electronic excited states, as a function of the C=O coordinate, are obtained employing time dependent density functional theory (TD-DFT). These calculations have shown the relevance of internal conversion from Rydberg to valence character governing the nuclear dynamics, yielding clear evidence of the rather complex multidimensional nature of the potential energy surfaces involved.

Microcontaminant removal in solar pilot scale photoreactors with commercial iron nanoparticles obtained from olive mill wastewater

This study assessed removal of microcontaminants (MCs) from natural water (NW) using iron nanoparticles extracted from olive mill wastewater (ZVI-OMW), an agro-industrial waste product, combined with oxidants under natural solar radiation. The specific effects of corrosion, adsorption and solar photolysis, and of H2O2, S2O82-, carbonates and ZVI-OMW concentrations on treatment efficiency were also evaluated. Lab tests demonstrated that the combination of 1 mM S2O82- with NW and 1 mM ZVI-OMW was the best treatment option. Process performance was evaluated under these operating conditions in pilot-scale raceway pond reactors (RPR) at three liquid depths (5, 10 and 15 cm), as well as in a compound parabolic collector (CPC) photo-reactor. The time required to achieve 50% removal of a MC mixture (atrazine, carbendazim, imidacloprid and thiamethoxam, at 100 µg/L each) increased with liquid depth, resulting in 164, 224, and 250 min for 5, 10, and 15 cm, respectively. MC mass removal rate increased with liquid depth, achieving 50% MC removal in the CPC (1 mM ZVI-OMW) in 120 min. Doubling ZVI-OMW concentration was detrimental to process efficiency in both photoreactors due to light obstruction. Partial sedimentation of ZVI-OMW in the RPR suggests that the use of this type of reactor for heterogeneous photocatalysis could present problems for large-size operation.

Persistence of N-oxides transformation products of tertiary amine drugs at lab and field studies.

This work aimed at studying the formation and persistence of N-oxides transformation products (TPs) of tertiary amine drugs by combining laboratory and field studies relevant for surface water. A monitoring study using passive samplers was first achieved for assessing attenuation of selected pharmaceuticals and their related N-oxides and N-, O-dealkylated TPs (i.e., venlafaxine, tramadol, amisulpride and sulpiride) along a 1.7 km river stretch between two sampling sites. This study revealed the stability of tramadol-N-oxide, amisulpride-N-oxide and the fast dissipation of O-desmethylvenlafaxine-N-oxide, as well as the significance of N-oxidized TPs in comparison to N-dealkylated TPs and parent compounds in river. Lab-scale experiments were then implemented for a better understanding of their mechanisms of formation and degradation under aerobic water/sediment testing and under simulated solar photochemistry. N-oxidation reactions were always a minor transformation pathway under both degradation conditions with respect to N-and O-dealkylation reactions. The amount of generated N-oxides were similar for venlafaxine, tramadol and sulpiride and peaked in the 8.4–12.8% and <4% of their initial concentration (100 μg/L), during photodegradation and biodegradation experiments, respectively. Other transformation pathways such as hydroxylation and α-C-hydroxylation followed by oxidation to amide or dehydration were also identified. Investigated N-oxides TPs (except O-desmethylvenlafaxine-N-oxide) were found stable under solar photolysis and aerobic biodegradation with a very slight reverse reaction to parent compound observed for tramadol-N-oxide and amisulpride-N-oxide. Lab-scale degradation experiments were not able to anticipate the high occurrence levels of N-oxide compounds in the environment. This was most likely due to faster degradation kinetics and/or higher sorption to sediment of parent compounds and dealkylated TPs over N-oxide TPs, resulting in higher relative accumulation of the latter.

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates.

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH2 OO, CH3 CHOO, (CH3 )2 COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH2 OO and (CH3 )2 COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever-growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing 1 ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo-induced fission of the terminal O-O bond, and speculates about possible alternate decay processes that could occur following non-adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.

Solar-light driven photodegradation of antimicrobials, their transformation by-products and antibiotic resistance determinants in treated wastewater

This study aimed to assess the possibility of using solar light-driven photolysis and TiO2-based photocatalysis to remove (1) antibiotic residues, (2) their transformation products (TPs), (3) antibiotic resistance determinants, and (4) genes identifying the indicator bacteria in a treated wastewater (secondary effluent). 16 antimicrobials belonging to the different classes and 45 their transformation by-products were selected for the study. The most susceptible to photochemical decomposition was tetracycline, which was completely removed in the photocatalysis process and in more than 80% in the solar light-driven photolysis. 83.8% removal (on average) was observed using photolysis and 89.9% using photocatalysis in the case of the tested genes, among which the genes sul1, uidA, and intI1 showed the highest degree of removal by both methods. The study revealed that applied methods promisingly remove the tested antibiotics, their TPs and genes even in such a complex matrix including treated wastewater and photocatalysis process had a higher removal efficiency of antibiotics, TPs and genes tested. Moreover, the high percentage removal of the intI1 gene (>93%) indicates the possibilities of use of the solar light-driven photolysis and TiO2-based photocatalysis in minimizing the antibiotic resistance genes transfer by mobile genetic elements.

Low-lying electronic states of ethanol investigated by theoretical and synchrotron radiation methods

We investigate the ethanol absorption spectrum in the range 5.64–10.78 eV (220–115 nm), by combining ab initio theoretical and experimental methods in order to provide the most accurate and up-to-date information about the electronic state spectroscopy of ethanol. In particular, absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements. The present VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the Rydberg (ndσ, ndσ′ ← (3a′′/13a)), n ≥ 3 members of the Rydberg (ndπ′(a′) ← (3a′′/13a)) and n = 3 members of the Rydberg (npσ′ ← (10a′/12a)) transitions. The experimental absolute photoabsorption cross sections have subsequently been used to calculate the photolysis lifetime of ethanol in the Earth's atmosphere (0–50 km), showing that solar photolysis is expected to be a weak sink at altitudes lower than 40 km to ●OH radical reactions. Potential energy curves for the lowest-lying excited electronic states, as a function of the O–H and the C–OH coordinates, are also obtained employing the equation of motion coupled cluster single and doubles (EOM-CCSD) and time dependant density functional theory (TD-DFT) methods. These show clear dissociation character at inter-nuclear distances greater than 2.0 Å for the RO–H and 2.2 Å for the RC–OH bond lengths, indicating the rather complex multidimensional character of the potential energy surfaces involved.

Solar Heterogenous Photocatalytic Degradation of Methylthionine Chloride on a Flat Plate Reactor: Effect of pH and H2O2 Addition

Methylthionine chloride (MTC) is a compound with several applications both in the clinical and medical industries. Nevertheless, such compounds can become an environmental problem, as they are not properly treated by wastewater treatment plants. This objective of this work was to study MTC degradation in a flat plate reactor through solar photolysis and heterogeneous photocatalysis processes with TiO2 as a catalyst. In addition to the processes, three pH (3.5, 6.5, and 9) and the effect of H2O2 addition (no dose, 0.5, and 1 mM/L) were tested. The results show that acidic pH is the most appropriate for MTC degradation, which ranged between 56% and 68.7% for photolysis and between 76% and 86.7% in photocatalysis. The H2O2 addition resulted in lower degradation in all cases, leading the authors to conclude that the presence of peroxide actually hinders degradation in solar photolysis and photocatalysis processes. Statistical analysis showed that the constant rate reactions calculated for every process, under the same conditions of pH and H2O2 addition, are significantly different from one another, and the three factors considered for experimental design (process, pH, and H2O2) have a statistically significant effect on MTC degradation. The collector area per order confirmed higher efficiency for photocatalysis when compared to photolysis processes.

Modeling the Conformer-Dependent Electronic Absorption Spectra and Photolysis Rates of Methyl Vinyl Ketone Oxide and Methacrolein Oxide.

Criegee intermediates are important atmospheric oxidants, formed via the reaction of ozone with volatile alkenes emitted into the troposphere. Small Criegee intermediates (e.g., CH2OO and CH3CHOO) are highly reactive, and their removal via unimolecular decay or bimolecular chemistry dominates their atmospheric lifetimes. As the molecular complexity of Criegee intermediates increases, their electronic absorption spectra show a bathochromic shift within the solar spectrum relevant to the troposphere. In these cases, solar photolysis may become a competitive contributor to their atmospheric removal. In this article, we report the conformer-dependent simulated electronic absorption spectra of two four-carbon-centered Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). Both MVK-oxide and MACR-oxide contain four low-energy conformers, which are convoluted in the experimentally measured spectra. Here, we deconvolute each conformer and estimate contributions from each of the four conformers to the experimentally measured spectra. We also estimate the photolysis rates and predict that solar photolysis should be a more competitive removal process for MVK-oxide and MACR-oxide (cf. CH2OO and CH3CHOO).