UV-induced photodegradation of emerging para-phenylenediamine quinones in aqueous environment: Kinetics, products identification and toxicity assessments

Substituted para-phenylenediamine quinones (PPD-quinones) are a class of emerging contaminants frequently detected in the aqueous environment. One of them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), was found to cause acute toxicities to aquatic species at extremely low environmental levels. The ubiquitousness and ecotoxicity of such pollutants underscore the importance of their transformation and elimination. In this work, we demonstrated effective removals of five PPD-quinones in aqueous environments under UV irradiation, with up to 94% of 6PPD-Q eliminated after a 40-min treatment. By applying high-resolution mass spectrometry (HRMS) non-targeted screening in combination with isotope labeling strategies, a total of 22 transformation products (TPs) were identified. Coupling with the time-based dynamic patterns, potential transformation mechanisms were identified as an •OH-induced photocatalysis reaction involving bond cleavage, hydroxylation, and oxidation. Computational toxicity assessment predicted lower aquatic toxicity of the TPs than their parent PPD-quinones. Our results in parallel evidenced an obvious reduction of PPD-quinones accompanied by the presence of their TPs in the effluent after UV disinfection in real municipal wastewater. This work builds a comprehensive understanding of the fate, transformation products, and related toxicological characteristics of emerging PPD-quinone contaminants in the aqueous environment.

Reductive degradation mechanism of perfluorooctanoic acid (PFOA) during vacuum ultraviolet (VUV) reactions combining with sulfite and iodide

In this study, PFOA removal and defluorination were examined during vacuum ultraviolet (VUV) photolysis in the presence of sulfite and sulfite/iodide conditions. PFOA (24 μM) degradation rate constant (kobs) and defluorination amount in VUV photolysis, and VUV/sulfite, and VUV/sulfite/iodide reactions under nitrogen-purging condition were 5.50 × 10-3, 7.26 × 10-2, 1.60 × 10-1 min-1, and 34.6, 72.7, 73.9% in 6 h, respectively. When tert-butanol (t-BuOH), NO2-, and NO3- ions were added as radical scavengers, hydrated electrons (eaq-) was confirmed as the main species responsible for degrading PFOA and mediating defluorination in VUV-based reactions. While, during VUV photolysis, short-chain perfluoroalkyl carboxylic acids (PFCAs), such as PFHpA, PFHxA, PFPeA, and PFBA, were mainly produced as transformation products (TPs) by the chain-shortening mechanism, additional 14 and 15 TPs were identified in the VUV/sulfite and VUV/sulfite/iodide reactions by LC-QTOF/MS, respectively. The main degradation mechanisms in these reactions are H-F exchange (e.g., TP395 (m/z = 394.9739) and TP377 (m/z = 376.9838)), •SO3--F exchange (TP474, m/z = 474.9323), carbon double bond formation by defluorination (e.g., TP392 (m/z = 392.9455), TP410 (m/z = 410.9355), and TP436 (m/z = 436.9347)), and H-F exchange followed by hydration reaction (TP393, m/z = 392.9773), respectively. PFOA degradation pathways were proposed for these VUV-based reactions based on the identified TPs, their time profiles, and the density functional theory (DFT). Finally, the toxicity of PFOA and its TPs produced during three reactions were assessed using ECOSAR simulation.

Critical assessment of the Kendrick mass defect analysis as an innovative approach to process high resolution mass spectrometry data for environmental applications

The growing application of high resolution mass spectrometry (HRMS) over the last decades has dramatically improved our knowledge about the occurrence of environmental contaminants. However, most of the compounds detected remain unknown and the large volume of data generated requires specific processing approaches. Therefore, this study presents the concepts of mass defect (MD), Kendrick mass (KM) and Kendrick mass defect (KMD) to the expert and non-expert reader along with relevant examples of applications in environmental HRMS data processing. A preliminary bibliometric overview indicates that the potential benefits of KMD analysis are rather overlooked in environmental science. In practice, a simple calculation allows transforming a mass from the IUPAC system (normalized so that the mass of ¹²C is exactly 12) to its corresponding KM normalized on a specific moiety such as CH₂ (the mass of CH₂ is exactly 14). Then, plotting the KMD according to the nominal KM allows revealing groups of compounds that differ only by their number of CH₂ moieties. For instance, data processing using KM and KMD was proven particularly useful to characterize natural organic matter in a sample, to reveal the occurrence of polymers as well as poly/perfluorinated alkylated substances (PFASs), and to search for transformation products (TPs) of a given chemical.

Degradation of iopromide during the UV-LED/chlorine reaction: Effect of wavelength, radical contribution, transformation products, and toxicity

Three different UV-LED wavelengths (265, 310, and 365 nm) were used in the UV-LED/chlorine reaction to investigate the degradation mechanism of iopromide (IPM) at different wavelengths, a representative iodinated contrast media compound. The degradation rate (k'_IPM) increased from pH 6-8 at 265 nm, but, decreased as the pH increased up to 9 at 310 nm and 365 nm. Radical scavenging experiments showed that reactive chlorine species (RCS) are the dominant radical species at all wavelengths, but a higher contribution of OH• was observed at lower pH and longer wavelengths. The contribution of RCS decreased but the contribution of OH• increased as the wavelength increased. Among RCS, the largest contribution was found to be ClO•. Total nine transformation products (TPs) were identified by LC-QTOF-MS during the UV-LED/chlorine reaction at 265 nm. Based on the identified TPs and their time profiles, we proposed a degradation pathway of IPM during UV-LED/chlorine reaction. The Microtox test using V. fischeri showed that no significant increase in toxicity was observed at all wavelengths. The synergistic effect of UV-LED and chlorine was greater at a higher wavelength by the electrical efficiency per order (EEO) calculation.

Analysis, occurrence, toxicity and environmental health risks of synthetic phenolic antioxidants: A review

The continuous improvement of living standards is related to higher requirements for the freshness and taste of food. For example, synthetic phenolic antioxidants (SPAs) are added to fats and fried foods as food additives to minimize the oxidative rancidity of oils and fats. Hence, the global use of SPAs is increasing year by year. Dibutyl hydroxytoluene is one of the widely used SPAs, often in combination with butyl hydroxyanisole or gallate SPAs. The extensive use of these compounds makes them and their transformation products to be widespread in various environmental matrices, including indoor dust, wastewater, river water, sewage sludge, and sediment, as well as human samples, such as nails and urine, at concentrations varying from nanogram per gram (ng/g) to microgram per gram (μg/g). Animal experiments have shown that high-dose SPA exposure is toxic, which may lead to DNA damage and mismatches and the development of cancerous tumors. Since the biosphere shares the same set of genetic codes, humans and animals have many identical or similar feedback mechanisms and information pathways. Therefore, the damage of SPAs to animals may also threaten human health. This review discusses the properties, occurrence, analysis, and environmental health risks of typical SPAs, including butyl hydroxyanisole, dibutyl hydroxytoluene, tert-butylhydroquinone, propyl gallate, octyl gallate, and lauryl gallate, used as food additives. In addition, AO2246, which is used in food packaging bags, is also considered. Future research directions on SPAs and their transformation products (TPs) are identified and discussed.

Degradation of cyclophosphamide during UV/chlorine reaction: Kinetics, byproducts, and their toxicity

Cyclophosphamide (CP) is a widely used anticancer drug and an immunosuppressant. Since CP is nonbiodegradable, it is hardly removed by the conventional wastewater treatment processes, resulting in continuous detection in surface water. In this study, the degradation of CP during the UV-B/chlorine reaction was investigated. CP was not degraded by UV-B photolysis and chlorination only but was effectively degraded in the UV-B/chlorine reaction with pseudo-first-order kinetics. Acidic pH conditions in the UV-B/chlorine reaction showed the most effective removal of CP. More than 56% of the CP was mineralized within 8 h of the reaction. Seven organic transformation products (TPs) (m/z = 141.01, 192.10, 198.03, 212.01, 258.01, 274.00, and 276.02, respectively) and four inorganic byproducts (NH₄⁺, NO₃^-, HCOO^-, and PO₄^3-) were identified using LC-qTOF/MS and ion chromatography, respectively. Microtox test based on bioluminescence inhibition showed that the toxicity inhibition increased to 88% as the reaction proceeded during the UV/chlorine reaction, probably due to the production of TPs, especially TP 258 (m/z = 258.01). The results of this study imply that the toxicity of TPs needs to be reduced when applying a UV-B/chlorination process to treat CP in water.