Polymers from Plant Oils Linked by Siloxane Bonds for Programmed Depolymerization.

The increased production of plastics is leading to the accumulation of plastic waste and depletion of limited fossil fuel resources. In this context, we report a strategy to create polymers that can undergo controlled depolymerization by linking renewable feedstocks with siloxane bonds. α,ω-Diesters and α,ω-diols containing siloxane bonds were synthesized from an alkenoic ester derived from castor oil and then polymerized with varied monomers, including related biobased monomers. In addition, cyclic monomers derived from this alkenoic ester and hydrosiloxanes were prepared and cyclized to form a 26-membered macrolactone containing a siloxane unit. Sequential ring-opening polymerization of this macrolactone and lactide afforded an ABA triblock copolymer. This set of polymers containing siloxanes underwent programmed depolymerization into monomers in protic solvents or with hexamethyldisiloxane and an acid catalyst. Monomers afforded by the depolymerization of polyesters containing siloxane linkages were repolymerized to demonstrate circularity in select polymers. Evaluation of the environmental stability of these polymers toward enzymatic degradation showed that they undergo enzymatic hydrolysis by a fungal cutinase from Fusarium solani. Evaluation of soil microbial metabolism of monomers selectively labeled with 13C revealed differential metabolism of the main chain and side chain organic groups by soil microbes.

Molecular Structure and Conformation of Biodegradable Water-Soluble Polymers Control Adsorption and Transport in Model Soil Mineral Systems.

Water-soluble polymers (WSPs) are used in diverse applications, including agricultural formulations, that can result in the release of WSPs to soils. WSP biodegradability in soils is desirable to prevent long-term accumulation and potential associated adverse effects. In this work, we assessed adsorption of five candidate biodegradable WSPs with varying chemistry, charge, and polarity characteristics (i.e., dextran, diethylaminoethyl dextran, carboxymethyl dextran, polyethylene glycol monomethyl ether, and poly-l-lysine) and of one nonbiodegradable WSP (poly(acrylic acid)) to sand and iron oxide-coated sand particles that represent important soil minerals. Combined adsorption studies using solution-depletion measurements, direct surface adsorption techniques, and column transport experiments over varying solution pH and ionic strengths revealed electrostatics dominating interactions of charged WSPs with the sorbents as well as WSP conformations and packing densities in the adsorbed states. Hydrogen bonding controls adsorption of noncharged WSPs. Under transport in columns, WSP adsorption exhibited fast and slow kinetic adsorption regimes with time scales of minutes to hours. Slow adsorption kinetics in soil may lead to enhanced transport but also shorter lifetimes of biodegradable WSPs, assuming more rapid biodegradation when dissolved than adsorbed. This work establishes a basis for understanding the coupled adsorption and biodegradation dynamics of biodegradable WSPs in agricultural soils.

Singlet Oxygen Quantum Yields in Environmental Waters.

Singlet oxygen (1O2) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent 1O2 quantum yields (ΦΔ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine ΦΔ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known ΦΔ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of ΦΔ values of environmental samples.

Global Corrections to Reference Irradiance Spectra for Non-Clear-Sky Conditions.

Photochemical reactions in surface waters play important roles in element cycling and in the removal of organic contaminants, among other processes. A central environmental variable affecting photochemical processes in surface waters is the incoming solar irradiance, as this initiates these processes. However, clear-sky incident irradiance spectra are often used when evaluating the fate of aquatic contaminants, leading to an overestimation of contaminant decay rates due to photochemical transformation. In this work, incident irradiance satellite data were used to develop global-scale non-clear-sky correction factors for commonly used reference irradiance spectra. Non-clear-sky conditions can decrease incident irradiance by over 90% depending on the geographic location and time of the year, with latitudes above 40°N being most heavily affected by seasons. The impact of non-clear-sky conditions on contaminant half-lives was illustrated in a case study of triclosan in lake Greifensee, which showed a 39% increase in the triclosan half-life over the course of a year under non-clear-sky conditions. A global annual average correction factor of 0.76 was determined as an approximate way to account for non-clear-sky conditions. The correction factors are developed at monthly and seasonal resolutions for every location on the globe between 70°N and 60°S at a 4 km spatial resolution and can be used by researchers, practitioners, and regulators who need improved estimates of incident irradiance.

Photochemical Production of Carbon Monoxide from Dissolved Organic Matter: Role of Lignin Methoxyarene Functional Groups.

Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO2, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH3) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH3 group, regardless of other structural features. We then confirmed that all ArOCH3-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO2 and that lignin photodegradation may represent a source of hydroxyl radicals.

Mechanistic Insights into Dissolved Organic Sulfur Photomineralization through the Study of Cysteine Sulfinic Acid.

Photochemical reactions convert dissolved organic matter (DOM) into inorganic and low-molecular-weight organic products, contributing to its cycling across environmental compartments. However, knowledge on the formation mechanisms of these products is still scarce. In this work, we investigate the triplet-sensitized photodegradation of cysteine sulfinic acid, a (photo)degradation product of cysteine, to sulfate (SO42-). We use kinetic analysis, targeted experiments, and previous literature from several fields of chemistry to explain the elementary steps that lead to the release of sulfate. Our analysis indicates that triplet sensitizers act as one-electron oxidants on the sulfinate S lone pair. The resulting radical undergoes C-S fragmentation to form SO2, which becomes hydrated to sulfite/bisulfite (S(IV)). S(IV) is further oxidized to SO42- in the presence of triplet sensitizers and oxygen. We point out that the reaction sequence SO2 ⇌ S(IV) → SO42- is valid independently of the chemical structure of the model compound and might represent a sulfate photoproduction mechanism with general validity for DOS. Our mechanistic investigation revealed that amino acids in general might also be photochemical precursors of CO2, ammonia, acetaldehyde, and H2O2 and that reaction byproducts can influence the rate and mechanism of S(IV) (photo)oxidation.

Substituent Effects on the Direct Photolysis of Benzotrifluoride Derivatives.

The chemical class of benzotrifluoride derivatives is widely used in active ingredients of various commercial products, such as pharmaceuticals, pesticides, herbicides, and crop protection agents. Past studies have shown that some benzotrifluorides are not stable under UV irradiation in water and convert into benzoic acids due to C-F bond hydrolysis. It was also observed, but never systematically studied, that the ring substituents play an important role on the direct photochemical reactivity of the CF3 moiety. In the present work, we explore the structure-reactivity relationship between ring substituent and direct photodefluorination for 16 different substituents, by determining fluoride production rates, quantum yields, and half-lives, and found that strong electron-donating groups enhance the reactivity toward hydrolysis. In addition, flufenamic acid, travoprost, dutasteride, cyflumetofen, flutoanil, and teriflunomide were also examined, finding that their direct photochemical reactivity could be qualitatively predicted based on their ring substituents. We provide here a tool to evaluate the environmental persistence of benzotrifluoride contaminants, as well as to design more photodegradable new active ingredients.

Linking Triclosan's Structural Features to Its Environmental Fate and Photoproducts.

Triclosan is a high-production volume chemical, which has become widely detected in environmental systems because of its widespread usage. Photodegradation has been identified as a major degradation pathway, but the identified photoproducts are also chemicals of concern. In this study, lower chlorinated derivatives of triclosan were synthesized to investigate the impact the chlorine substituents have on the photodegradation rate and the photoproducts produced. In addition, the photodegradation of two classes of photoproducts-dibenzo-p-dioxins (DDs) and 2,2'-dihydroxylated biphenyls-was also investigated. Degradation of triclosan in near-surface sunlit waters was relatively fast (t1/2 < 5 h). Calculated degradation rates were slower for DDs and faster for dihydroxylated biphenyls in comparison to that for triclosan. In addition, the 2'-Cl substituent was critical for the high quantum yield measured for triclosan and necessary for the photodegradation mechanism that forms DDs and dihydroxylated biphenyls. The 4-Cl substituent was responsible for higher rates of light absorption and the environmentally relevant pKa. Without either of these substituents, the environmental fate of triclosan would be markedly different.

Dissolved Organic Matter Singlet Oxygen Quantum Yields: Evaluation Using Time-Resolved Singlet Oxygen Phosphorescence.

Singlet oxygen (1O2) generation quantum yields from chromophoric dissolved organic matter (CDOM) have been reported for many samples over the past 4 decades. Yet even for standardized isolates such as those from the International Humic Substance Society (IHSS), wide-ranging values exist in the literature. In this manuscript, time-resolved 1O2 phosphorescence was used to determine the 1O2 quantum yields (ΦΔ) of a variety of dissolved organic matter (DOM) isolates and natural waters. In general, the 1O2 quantum yield values in this study are in the middle, although below the median of the range of past reported values (e.g., for Suwannee River Natural Organic Matter IHSS isolate: 1.8% vs 0.23-2.89%). Notably, hydrophobic neutral fractions of DOM isolates were found to possess the highest 1O2 quantum yields, an interesting result given that these fractions are not retained in typical humic and fulvic acid isolation procedures that use XAD resins. The excitation wavelength dependence of 1O2 generation from CDOM was also examined, and an approximate linear decrease with longer excitation wavelength was observed. This work advances the understanding of CDOM photoprocesses, especially in relation to wavelength-dependent 1O2 production, which is valuable for assessing real-world environmental behavior.

Quantification of Synthetic Polyesters from Biodegradable Mulch Films in Soils.

Soil biodegradable mulch films composed of the polyester polybutylene adipate-co-terephthalate (PBAT) are being increasingly used in agriculture. Analytical methods to quantify PBAT in field soils are needed to assess its soil occurrence and fate. Here, we report an analytical method for PBAT in soils that couples Soxhlet extraction or accelerated solvent extraction (ASE) with quantitative protonnuclear magnetic resonance (q-1H NMR) spectroscopy detection. The 1H NMR peak areas of aromatic PBAT protons increased linearly with PBAT concentrations dissolved in deuterated chloroform (CDCl3), demonstrating accurate quantitation of PBAT by q-1H NMR. Spike-recovery experiments involving PBAT addition to model sorbents and soils showed increased PBAT extraction efficiencies into chloroform (CHCl3) with methanol (MeOH) as cosolvent, consistent with MeOH competitively displacing PBAT from H-bond donating sites on mineral surfaces. Systematic variations in solvent composition and temperatures in ASE revealed quantitative PBAT extraction from soil with 90/10 volume % CHCl3/MeOH at 110-120 °C. Both Soxhlet extraction and ASE resulted in the complete recovery of PBAT added to a total of seven agricultural soils covering a range of physicochemical properties, independent of whether PBAT was added to soils dissolved in CHCl3, as film, or as particles. Recovery was also complete for PBAT added to soil in the form of a commercial soil biodegradable mulch film with coextractable polylactic acid (PLA). The presented analytical method enables accurate quantification and biodegradation monitoring of PBAT in agricultural field soils.

UV/Vis+ photochemistry database: Structure, content and applications.

The "science-softCon UV/Vis+ Photochemistry Database" (www.photochemistry.org) is a large and comprehensive collection of EUV-VUV-UV-Vis-NIR spectral data and other photochemical information assembled from published peer-reviewed papers. The database contains photochemical data including absorption, fluorescence, photoelectron, and circular and linear dichroism spectra, as well as quantum yields and photolysis related data that are critically needed in many scientific disciplines. This manuscript gives an outline regarding the structure and content of the "science-softCon UV/Vis+ Photochemistry Database". The accurate and reliable molecular level information provided in this database is fundamental in nature and helps in proceeding further to understand photon, electron and ion induced chemistry of molecules of interest not only in spectroscopy, astrochemistry, astrophysics, Earth and planetary sciences, environmental chemistry, plasma physics, combustion chemistry but also in applied fields such as medical diagnostics, pharmaceutical sciences, biochemistry, agriculture, and catalysis. In order to illustrate this, we illustrate the use of the UV/Vis+ Photochemistry Database in four different fields of scientific endeavor.

UVB-irradiated Laboratory-generated Secondary Organic Aerosol Extracts Have Increased Cloud Condensation Nuclei Abilities: Comparison with Dissolved Organic Matter and Implications for the Photomineralization Mechanism.

During their atmospheric lifetime, organic compounds within aerosols are exposed to sunlight and undergo photochemical processing. This atmospheric aging process changes the ability of organic aerosols to form cloud droplets and consequently impacts aerosol-cloud interactions. We recently reported changes in the cloud forming properties of aerosolized dissolved organic matter (DOM) due to a photomineralization mechanism, transforming high-molecular weight compounds in DOM into organic acids, CO and CO2. To strengthen the implications of this mechanism to atmospheric aerosols, we now extend our previous dataset and report identical cloud activation experiments with laboratory-generated secondary organic aerosol (SOA) extracts. The SOA was produced from the oxidation of α-pinene and naphthalene, a representative biogenic and anthropogenic source of SOA, respectively. Exposure of aqueous solutions of SOA to UVB irradiation increased the dried organic material's hygroscopicity and thus its ability to form cloud droplets, consistent with our previous observations for DOM. We propose that a photomineralization mechanism is also at play in these SOA extracts. These results help to bridge the gap between DOM and SOA photochemistry by submitting two differently-sourced organic matter materials to identical experimental conditions for optimal comparison.

Singlet Oxygen Photooxidation of Peptidic Oxazoles and Thiazoles.

Oxazoles and thiazoles are commonly found moieties in nonribosomal peptides (NRPs) and ribosomally synthesized post-translationally modified peptides (RiPPs), which are important biomolecules present in the environment and in natural waters. From previous studies, they seem susceptible to oxidation by singlet oxygen (1O2); therefore, we designed and synthesized model oxazole- and thiazole-peptides and measured their 1O2 bimolecular reaction rate constants, showing slow photooxidation under environmental conditions. We reasoned their stability through the electron-withdrawing effect of the carboxamide substituent. Reaction products were elucidated and support a reaction mechanism involving cycloaddition followed by a series of rearrangements. The first 1O2 bimolecular reaction rate constant for a RiPP, the thiazole-containing peptide Aerucyclamide A, was measured and found in good agreement with the model peptide's rate constant, highlighting the potential of using model peptides to study the transformations of other environmentally relevant NRPs and RiPPs.

Intramolecular [2 + 2] Photocycloaddition of Altrenogest: Confirmation of Product Structure, Theoretical Mechanistic Insight, and Bioactivity Assessment.

While studying the environmental fate of potent endocrine-active steroid hormones, we observed the formation of an intramolecular [2 + 2] photocycloaddition product (2) with a novel hexacyclic ring system following the photolysis of altrenogest (1). The structure and absolute configuration were established by X-ray diffraction analysis. Theoretical computations identified a barrierless two-step cyclization mechanism for the formation of 2 upon photoexcitation. 2 exhibited progesterone, estrogen, androgen, and pregnane X receptor activity, albeit generally with reduced potency relative to 1.

Development of N-Cyclopropylanilines to Probe the Oxidative Properties of Triplet-State Photosensitizers.

Anilines have been shown to be especially susceptible to single-electron oxidation by excited triplet-state photosensitizers (3sens*), and thus, are good potential candidates to probe the oxidative properties of triplet-state chromophoric dissolved organic matter (3CDOM*). However, steady-state experiments tend to underestimate their rate of oxidation by 3CDOM* due to radical cation quenching (i.e., aniline•+ → aniline) by antioxidant moieties present in DOM. We envisioned the potential utility of N-cyclopropylaniline (CPA) to overcome this limitation, as it is known to undergo spontaneous, irreversible cyclopropyl ring-opening after an initial single-electron oxidation. To test this, first a set of CPA analogs was synthesized and then paired with a model sensitizer and antioxidant, or various DOM isolates, to examine their reactivity and susceptibility to antioxidant quenching during steady-state photolysis experiments. Next, time-resolved measurements of CPA and CPA analog oxidation were obtained by laser flash photolysis through direct observation of 3sens* and radical cations of CPA and CPA analogs. Finally, CPA photolysis products were isolated by semi-preparative high-performance liquid chromatography and identified by nuclear magnetic resonance spectroscopy. Outcomes of this work, including oxidation bimolecular rate constants of CPA and CPA analogs (∼9 × 108 to 4 × 109 M-1 s-1), radical cation lifetimes of CPA and its analogs (140-580 ns), and identified ring-opened products, support the usefulness of cyclopropylanilines as single-electron transfer probes in photosensitized aqueous solutions.

Photodegradation of Fludioxonil and Other Pyrroles: The Importance of Indirect Photodegradation for Understanding Environmental Fate and Photoproduct Formation.

Fludioxonil is a pyrrole-containing pesticide whose registration as a plant protection product is currently under review in the United States and Europe. There are concerns over its potential persistence and toxicity in the aquatic environment; however, the pyrrole moiety represents a potential reaction site for indirect photodegradation. In this study, the direct and indirect photodegradation of fludioxonil, along with pyrrole, 3-cyanopyrrole, and 3-phenylpyrrole, were investigated. Results showed that pyrrole moieties are capable of undergoing direct photoionization and sensitized photooxidation to form radical cation species, which then likely deprotonate and react with dissolved oxygen. Additionally, pyrrole moieties can undergo reactions with singlet oxygen (1O2). Furthermore, the presence of electron-withdrawing or -donating substituents substantially impacted the reaction rate with 1O2 as well as the one-electron oxidation potential of the pyrrole that dictates reactions with triplet states of dissolved organic matter (3CDOM*). For fludioxonil, which can undergo both direct and indirect photodegradation, the reaction rate constant with 1O2 alone resulted in a predicted t1/2 < 2 days in waters under sunlit near-surface conditions, suggesting it will not be persistent in aquatic systems. These results are useful for evaluating the environmental fate of fludioxonil as well as other pyrrole compounds.

Sorbic Acid as a Triplet Probe: Triplet Energy and Reactivity with Triplet-State Dissolved Organic Matter via 1O2 Phosphorescence.

Sorbic acid (2,4-hexadienoic acid; HDA) is commonly used as a probe and quencher for triplet-excited chromophoric dissolved organic matter (3CDOM*), an important transient species in natural waters, yet much remains unknown about its reactivity with 3CDOM* and its triplet energy. To better understand the quenching behavior of HDA, we measured HDA quenching rate constants for various humic substance isolates and whole waters with singlet oxygen (1O2) phosphorescence and determined the triplet energy of HDA. Low-temperature phosphorescence measurements determined the triplet energy of HDA to be 217 kJ mol-1, whereas a complementary method based on triplet quenching kinetics found a triplet energy of 184 ± 7 kJ mol-1. Time-resolved 1O2 phosphorescence measurements yielded different HDA quenching rate constants depending on the fitting method. Using an approach that considered the reactivity of the entire triplet pool produced values of (∼1-10) × 108 M-1 s-1, while an approach that considered only the reactivity of the high-energy triplets output higher rate constants ((∼7-30) × 108 M-1 s-1). In addition, the model based on high-energy triplet reactivity found that ∼30-60% of 3CDOM* is not quenched by HDA. Findings from this study provide a more comprehensive view on the use of HDA as a probe for 3CDOM*.

Furan Carboxamides as Model Compounds To Study the Competition between Two Modes of Indirect Photochemistry.

Singlet oxygen (1O2) and triplet chromophoric dissolved organic matter (3CDOM*) are photochemically produced reactive intermediates responsible for the photodegradation of several micropollutants in the sunlit surface waters. However, elucidating the mechanism of reactions involving both 1O2 and 3CDOM* can be complicated by the deeply interconnected nature of these two reactive species. In this work, we synthesized a series of model compounds inspired by the chemical structure of fenfuram, a fungicide used in the 1980s, and used them to investigate structure-reactivity relationships in photodegradation reactions involving 1O2 and 3CDOM*. A combination of steady-state and time-resolved approaches was employed to successfully predict the extent of 1O2-induced degradation. Conversely, the prediction of triplet-induced reactivity was complicated by the presence of repair mechanisms whose extent and relative importance were difficult to predict. The results of our work indicate that bimolecular rate constants measured via time-resolved techniques alone are not sufficient to accurately predict environmental half-lives, as intrinsic differences in the reaction mechanism can amplify the importance of secondary degradation pathways.

Sorbic Acid as a Triplet Probe: Reactivity of Oxidizing Triplets in Dissolved Organic Matter by Direct Observation of Aromatic Amine Oxidation.

Sorbic acid (2,4-hexadienoic acid; HDA) isomerization is frequently used to probe triplet-state dissolved organic matter (3CDOM*) reactivity, but there remain open questions about the reaction kinetics of 3CDOM* with HDA due to the difficulties of directly measuring 3CDOM* quenching rate constants. Using our recently developed approach based on observing the radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) formed through oxidation of TMPD by 3CDOM*, we studied 3CDOM* quenching kinetics with HDA monitored via transient absorption spectroscopy. A competition kinetics-based approach utilizing formation yields of TMPD•+ was developed, validated with model sensitizers, and used to determine bimolecular rate constants between 3CDOM* oxidants and HDA for diverse DOM isolates and natural waters samples, yielding values in the range of (2.4-7.7) × 108 M-1 s-1. The unquenchable fraction of TMPD-oxidizing triplets showed that, on average, 41% of 3CDOM* oxidants cannot be quenched by HDA. Conversely, cycloheptatriene quenched nearly all TMPD•+-forming triplets in CDOM, suggesting that most 3CDOM* oxidants possess energies greater than 150 kJ mol-1. Comparing results with our companion study, we found slight, but noticeable differences in the 3CDOM* quenching rate constants by HDA and unquenchable triplet fractions determined by oxidation of TMPD and energy transfer to O2 (1O2 formation) methods.

Photochemical Production of Sulfate and Methanesulfonic Acid from Dissolved Organic Sulfur.

Photodegradation processes play an important role in releasing elements tied up in biologically refractory forms in the environment, and are increasingly being recognized as important contributors to biogeochemical cycles. While complete photo-oxidation of dissolved organic carbon (to CO2) and dissolved organic phosphorous (to PO43-) has been documented, the analogous photoproduction of sulfate from dissolved organic sulfur (DOS) has not yet been reported. Recent high-resolution mass spectrometry studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter, which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA), and methanesulfinic acid (MSIA) during photodegradation of DOM samples from a wide range of natural terrestrial environments. We show that photochemical production of sulfate is generally more efficient than the production of MSA and MSIA, as well as volatile S-containing compounds such as CS2 and COS. We also identify possible molecular precursors for sulfate and MSA, and we demonstrate that a wide range of relevant classes of DOS compounds (in terms of S oxidation state and molecular structure) can liberate sulfate upon photosensitized degradation. This work suggests that photochemistry may play a more significant role in the aquatic and atmospheric fate of DOS than currently believed.