Photochemical fate of medetomidine in coastal and marine environments

Photochemical fate of β-blocker pindolol in riverine and its downstream coastal waters

Pindolol (PIN) is a commonly used β-blocker drug and has been frequently detected in various natural waters. Comprehensive understanding of its environmental photochemical transformation is necessary to assess its environmental risk. In this study, the photodegradation kinetics and mechanisms of PIN in both freshwater and coastal water were investigated for the first time. The photodegradation experiments were carried out by steady-state photochemical experiment under simulated sunlight irradiation. The results showed that the photodegradation rate of PIN in the freshwater of the Pearl River estuary was significantly faster than that in its downstream coastal water. In river water, PIN can undergo both direct photolysis and indirect photolysis induced by riverine dissolved organic matter (DOM) mainly through excited triplet-state of DOM and singlet oxygen, while direct photolysis dominated its degradation in coastal water. The promotion effect was found to be much greater for Suwannee River Natural Organic Matter (SRNOM) than that of the sampled riverine DOM, due to its high steady-state concentrations of reactive species. Interestingly, coastal DOM in northern and southern China were found to have similar promotion effects on PIN photodegradation for the first time, but both less than that of riverine DOM. A total of seven degradation products of PIN resulting from hydroxylation, hydrogen abstraction and cleavage of ether bond were identified. Biological toxicity of one products were found to be higher than that of PIN. These results are of significance for knowing the persistence and ecological risk of PIN in natural waters.

Wavelength-dependent direct and indirect photochemical transformations of organic pollutants

Sunlight-induced photochemical transformations greatly affect the persistence of organic pollutants in natural environment. Whereas sunlight intensity is well-known to affect pollutant phototransformation rates, the reliance of pollutant phototransformation kinetics on sunlight spectrum remains poorly understood, which may greatly vary under different spatial-temporal, water matrix, and climatic conditions. Here, we systematically assessed the wavelength-dependent direct and indirect phototransformations of 12 organic pollutants. Their phototransformation rates dramatically decreased with light wavelength increasing from 375 to 632 nm, with direct photolysis displaying higher wavelength-dependence than indirect photolysis. Remarkably, UV light dominated both direct (90.4-99.5 %) and indirect (64.6-98.7 %) photochemical transformations of all investigated organic pollutants, despite its minor portion in sunlight spectrum (e.g., 6.5 % on March 20 at the equator). Based on wavelength-dependent rate constant spectrum, the predicted phototransformation rate of chloramphenicol (4.5 ± 0.7 × 10-4 s-1) agreed well with the observed rate under outdoor sunlight irradiation (4.3 ± 0.0 × 10-4 s-1), and there is no significant difference between the predicted rate and the observed rate (p-value = 0.132). Moreover, rate constant and quantum yield coefficient (QYC) spectrum could be applied for facilely investigate the influence of spectral changes on the phototransformation of pollutants under varying spatial-temporal (e.g., season, latitude) and climatic conditions (e.g., cloud cover). Our study highlights the wavelength-dependence of both direct and indirect phototransformation of pollutants, and the UV part of natural sunlight plays a decisive role in the phototransformation of pollutants.

Singlet oxygen: Properties, generation, detection, and environmental applications

Singlet oxygen (1O2) is molecular oxygen in the excited state with high energy and electrophilic properties. It is widely found in nature, and its important role is gradually extending from chemical syntheses and medical techniques to environmental remediation. However, there exist ambiguities and controversies regarding detection methods, generation pathways, and reaction mechanisms which have hindered the understanding and applications of 1O2. For example, the inaccurate detection of 1O2 has led to an overestimation of its role in pollutant degradation. The difficulty in detecting multiple intermediate species obscures the mechanism of 1O2 production. The applications of 1O2 in environmental remediation have also not been comprehensively commented on. To fill these knowledge gaps, this paper systematically discussed the properties and generation of 1O2, reviewed the state-of-the-art detection methods for 1O2 and long-standing controversies in the catalytic systems. Future opportunities and challenges were also discussed regarding the applications of 1O2 in the degradation of pollutants dissolved in water and volatilized in the atmosphere, the disinfection of drinking water, the gas/solid sterilization, and the self-cleaning of filter membranes. This review is expected to provide a better understanding of 1O2-based advanced oxidation processes and practical applications in the environmental protection of 1O2.

DOM optical parameters as a tool to understand degradation of phenolic contaminants of emerging concern

Composition and source of dissolved organic matter (DOM) in water influence the rate of production of reactive intermediates (RIs), affecting the photodegradation of phenolic contaminants of emerging concern (PhCECs). However, this relationship has not been fully quantified. Here, for the first time, we propose a mechanism for photodegradation of a surrogate of PhCECs, p-cresol, in different DOM standard solutions under simulated sunlight irradiation. More importantly, the correlation of DOM optical parameters and p-cresol photodegradation kinetic parameters was determined by Pearson correlation. Results showed that indirect photodegradation was the only degradation pathway for p-cresol, mainly through reaction with excited triplet state of dissolved organic matter (3DOM*). Singlet oxygen (1O2) and hydroxyl radical (•OH) hindered degradation of p-cresol by decreasing the steady state concentration of 3DOM*. Moreover, less aromatic and smaller molecular size DOM showed higher steady-state concentration and quantum yield of 1O2, and 3DOM*, resulting in faster p-cresol photodegradation. Finally, 7 out of 8 optical parameters showed strong correlation with the p-cresol photodegradation rate constant. The mechanism and correlations found are a potential tool to predict PhCECs photodegradation in water using DOM optical parameters.

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.

Photodegradation of propranolol in surface waters: An important role of carbonate radical and enhancing toxicity phenomenon

Antihypertensive propranolol (PRO) is frequently detected in surface waters and has adverse effects on aquatic organisms. In this study, its photochemical fate in surface water with the aspect of kinetics, products and toxicity were investigated employing steady-state photochemistry experiments and ecotoxicity tests. The results showed that photodegradation of PRO was enhanced in river water than that in phosphate buffer where dissolved organic matter (DOM), NO₃^-, and HCO₃^- played important roles. DOM accelerated the photodegradation mainly through generation of excited triplet-state DOM while NO₃^- played dual roles in the photodegradation. The reaction between excited triplet-state PRO and HCO₃^- can generate carbonate radical (CO₃^·-) to promote the photodegradation. The second-order reaction rate constant between PRO and CO₃^·- was determined to be (3.4 ± 0.8) × 10⁸ M^-1 s^-1. Eight photodegradation products were identified in the studied river water sample. Finally, the toxicity evaluated by Vibrio fischeri increased after photodegradation and three photodegradation products were responsible for the increasing toxicity, which was concluded from the significant correlation between toxicity parameters and quantity of the photodegradation products.

Metagenomics reveals bacterioplankton community adaptation to long-term thermal pollution through the strategy of functional regulation in a subtropical bay

Thermal effluents from coastal nuclear power plants have led to undesirable pollution and subsequent ecological impacts on local marine ecosystems. However, despite the ecological importance, we know little about the impacts on functionality of bacterioplankton subjected in systems with long-term thermal pollution. We used metagenomic sequencing to study of the effect of thermal pollution on bacterioplankton community metagenomics in summer in a subtropical bay located on the northern coast of the South China Sea. Thermal pollution (>15 y), which resulted in an increase in the summer seawater temperature around 8°C and caused seawater temperature up to approximate 39°C, significantly decreased bacterioplankton metabolic potentials in photosynthesis, organic carbon synthesis, and energy production. The bacterioplankton community metagenomics underwent a significant change in its structure from Synechococcus-dominant autotrophy to Alteromonas, Vibrio, and Pseudoalteromonas-dominated heterotrophy, and significantly up-regulated genes involved in organic compound degradation and dissimilatory nitrate reduction for the matter and energy acquisition under thermal pollution. Moreover, the bacterioplankton community metagenomics showed an up-regulation with heating of genes involved in DNA repair systems, heat shock responsive chaperones and proteins, and proteins involved in other biological processes, such as biofilm formation and the biosynthesis of unsaturated fatty acids and glycan, to adapt to the thermal environment. Collectively, it indicates a functional regulation of bacterioplankton adaptation to high-temperature stress, which might advance the understanding of the molecular mechanisms of community adaptation to global extreme warming in aquatic ecosystems.

Biocides in antifouling paint formulations currently registered for use

Antifouling paints incorporate biocides in their composition seeking to avoid or minimize the settlement and growing of undesirable fouling organisms. Therefore, biocides are released into the aquatic environments also affecting several nontarget organisms and, thus, compromising ecosystems. Despite global efforts to investigate the environmental occurrence and toxicity of biocides currently used in antifouling paints, the specific active ingredients that have been used in commercial products are poorly known. Thus, the present study assessed the frequencies of occurrence and relative concentrations of biocides in antifouling paint formulations registered for marketing worldwide. The main data were obtained from databases of governmental agencies, business associations, and safety data sheets from paint manufacturers around the world. The results pointed out for 25 active ingredients currently used as biocides, where up to six biocides have been simultaneously used in the examined formulations. Cuprous oxide, copper pyrithione, zinc pyrithione, zineb, DCOIT, and cuprous thiocyanate were the most frequent ones, with mean relative concentrations of 35.9 ± 12.8%, 2.9 ± 1.6%, 4.0 ± 5.3%, 5.4 ± 2.0%, 1.9 ± 1.9%, and 18.1 ± 8.0% (w/w) of respective biocide present in the antifouling paint formulations. Surprisingly, antifouling paints containing TBT as an active ingredient are still being registered for commercialization nowadays. These results can be applied as a proxy of biocides that are possibly being used by antifouling systems and, consequently, released into the aquatic environment, which can help to prioritize the active ingredients that should be addressed in future studies.

Core-shell P-laden biochar/ZnO/g-C3N4 composite for enhanced photocatalytic degradation of atrazine and improved P slow-release performance

Technologies that can effectively address the environmental issues arisen from the use of agrochemicals and P fertilizers are needed for the development of green agriculture. Here, we reporta new core-shell P-laden biochar/ZnO/g-C₃N₄ composite (Pbi-ZnO-g-C₃N₄) used both as an efficient photocatalyst for degrading atrazine and a promising slow-release fertilizer for improving the P utilization efficiency. In comparison with P-laden biochar/ZnO (Pbi-ZnO), Pbi-ZnO-g-C₃N₄ exhibits enhanced photocatalytic activity with the maximum atrazine degradation efficiency of 85.3% after 260 min. Pbi-ZnO-g-C₃N₄ also shows superior P slow-release performance with the cumulative P release concentration of 216.40 g/L in 260 min. Besides, it is found that the coating of g-C₃N₄ on the surface of Pbi-ZnO improves the utilization of visible light and separation of photoinduced electron-hole pairs, producing more radicals (•OH and •O₂^-) under visible light irradiation. The mechanistic study reveals that Z-shaped heterojunction is formed between ZnO and g-C₃N₄ in Pbi-ZnO-g-C₃N₄, and biochar serves as an electron-transfer bridge that promotes the separation of electron-hole pairs. Finally, pot experiments reveal that the P utilization efficiency for pepper seedlings fertilized by Pbi-ZnO-g-C₃N₄ is higher than that by Pbi-ZnO. The application of Pbi-ZnO-g-C₃N₄ is beneficial for the growth of native soil microorganism.

Emerging investigator series: quantifying the impact of cloud cover on solar irradiance and environmental photodegradation

Environmental photodegradation is dependent on the solar irradiance that reaches the Earth's surface, and photodegradation half-lives of contaminants are typically estimated assuming clear sky (i.e., cloudless) conditions. In this work, the effect of cloud cover on solar irradiance was investigated. Data from the National Renewable Energy Laboratory (NREL), which spanned 3 years of observations (10/2017 to 12/2020), were used to train two machine learning models to predict irradiance based on three inputs - day of year, time of day, and percentage of the sky that was cloudy. Results showed a non-linear relationship between cloud cover and irradiance. Solar irradiance was minimally impacted up to ≈50% cloud cover but decreased by ≈67% at 100% cloud cover. Both random forest and artificial neural network models performed well with relative root mean squared errors of 26-31%, which varied depending on the source of cloud cover data and the spectral region being modeled. Daily irradiance values for a whole year were predicted for varying cloud conditions using the machine learning models; this result was approximated using a quadratic fit of y = 1 - 0.00243x - (4.24 × 10^-5)x² where y is the fraction of clear sky irradiance expected and x is the percentage of cloud cover in the sky. In addition, the model results supported that there was no wavelength dependence for the effect of cloud cover. Therefore, decreases in both direct and indirect photodegradation rates should be proportional to the decrease in irradiance, which has a non-linear dependence on cloud cover.