Evaluation of nitrate effects in the aqueous photodegradability of selected phenolic pollutants

Charged polystyrene microplastics inhibit uptake and transformation of 14C-triclosan in hydroponics-cabbage system

INTRODUCTION

Since the outbreak of COVID-19, microplastics (MPs) and triclosan in pharmaceuticals and personal care products (PPCPs) are markedly rising. MPs and triclosan are co-present in the environment, but their interactions and subsequent implications on the fate of triclosan in plants are not well understood.

OBJECTIVE

This study aimed to investigate effects of charged polystyrene microplastics (PS-MPs) on the fate of triclosan in cabbage plants under a hydroponic system.

METHODS

14C-labeling method and liquid chromatography coupled with quadrupole/time-of-flight mass spectrometry (LC-QTOF-MS) analysis were applied to clarify the bioaccumulation, distribution, and metabolism of triclosan in hydroponics-cabbage system. The distribution of differentially charged PS-MPs in cabbage was investigated by confocal laser scanning microscopy and scanning electron microscopy.

RESULTS

The results showed that MPs had a significant impact on bioaccumulation and metabolism of triclosan in hydroponics-cabbage system. PS-COO-, PS, and PS-NH3+ MPs decreased the bioaccumulation of triclosan in cabbage by 69.1 %, 81.5 %, and 87.7 %, respectively, in comparison with the non-MP treatment (control). PS-MPs also reduced the translocation of triclosan from the roots to the shoots in cabbage, with a reduction rate of 15.6 %, 28.3 %, and 65.8 % for PS-COO-, PS, and PS-NH3+, respectively. In addition, PS-NH3+ profoundly inhibited the triclosan metabolism pathways such as sulfonation, nitration, and nitrosation in the hydroponics-cabbage system. The above findings might be linked to strong adsorption between PS-NH3+ and triclosan, and PS-NH3+ may also potentially inhibit the growth of cabbage. Specially, the amount of triclosan adsorbed on PS-NH3+ was significantly greater than that on PS and PS-COO-. The cabbage biomass was reduced by 76.9 % in PS-NH3+ groups, in comparison with the control.

CONCLUSION

The uptake and transformation of triclosan in hydroponics-cabbage system were significantly inhibited by charged PS-MPs, especially PS-NH3+. This provides new insights into the fate of triclosan and other PPCPs coexisted with microplastics for potential risk assessments.

Design of a multi-electrode dielectric barrier discharge reactor and experimental study on the degradation of atrazine in water

Atrazine (ATZ) is widely used in agriculture as a triazine herbicide, and its long-term use can cause serious environmental pollution. This paper independently designed a multi-electrode reactor, explored the output power and energy utilization efficiency of the dielectric barrier discharge reactor, and used the dielectric barrier discharge reactor to treat ATZ solution. The results showed that the degradation efficiency of ATZ was 96.39% at 30 min at an initial ATZ concentration of 14 mg/L, an input voltage of 34 kV, an input current of 1.38 mA, an aeration rate of 100 L/h, and a treatment water volume of 150 mL. The degradation of ATZ was significantly increased by the addition of persulfate (PS), Fe2+, and H2O2. After adding radical quenchers (EtOH, p-BQ, and FFA), the degradation efficiency of ATZ decreased, indicating that free radicals (•OH, •O2-, and 1O2) played a key role in the degradation process of ATZ.

Effect of bicarbonate on nitrate-induced photosensitive degradation of sulfamethoxazole under UV irradiation

In this study, the influence of HCO3- on NO3--induced photosensitive degradation of sulfamethoxazole (SMX) under UV irradiation was investigated. It was found that the removal of SMX by UV in the presence of NO3- improved significantly compared to its photolysis, which was confirmed to be due to the role of hydroxyl radical (HO•) formed through UV-activated NO3-. However, the addition of HCO3- in UV/NO3- system could further enhance SMX degradation, which was verified to be ascribed to the formed carbonate radical (CO3•-) through the reaction of HCO3- with HO•. The second-order rate constant of CO3•- with SMX was determined to be 2.58 × 108 M-1 s-1. In UV/NO3-/HCO3- system, the reactive species for SMX removal were HO• and CO3•-, and the contribution of CO3•- to SMX degradation might be much higher than that of HO•. The concentration of NO3- was almost unchanged after reaction in UV/NO3- and UV/NO3-/HCO3- systems because of its regeneration. Based on the detected four transformation products, the possible degradation pathways of SMX in UV/NO3-/HCO3- system were proposed including hydroxylation, amino-oxidation and bond cleavage.

Direct and indirect photodegradation of bisphenol A in the presence of natural water components

The impacts and mechanisms of natural water constituents, such as humic acid (HA), nitrates, iron and chloride ions, to the photodegradation of bisphenol A (BPA) were investigated in aqueous media under UV light irradiation. Due to the contributions of ·OH, 1O2, O2- and BPA* to BPA photodegradation in pure water in 13.4, 7.7, 22.9 and 47.9%, respectively, BPA was attenuated through the reaction pathway of direct photodegradation more than self-sensitized photodegradation. About indirect photodegradation, BPA photolysis through inhibitory effect from HA was mainly by light screening effect and quenching effect was insignificant. NO- 3 and NO- 2 both showed inhibitory effect but due to different reactive oxidization species (ROS). The photodegradation of BPA was significantly enhanced by the addition of iron from the formation of ·OH and H2O2, due to iron-assisted indirect photolysis for the degradation process. A dual effect of chloride depending on the different concentration levels involved quenching and promotion effect on reactive photo-induced species (RPS). A simple linear model revealed that BPA photodegradation was significantly impacted by the interaction of the above factors. In natural water, the decreased photolytic rate of BPA was mainly attributed to triple-excited dissolved organic matter (3DOM*), indicating that indirect photolysis was the primary transformation pathway of BPA. The detected photolysis products, such as nitrate and chlorinated products, suggest that there might be potential ecological risk of BPA photodegradation.

Uptake and metabolism of 14C-triclosan in celery under hydroponic system

As triclosan is used extensively as an antimicrobial agent, it inevitably enters agroecosystems, when sewage and treated wastewater are applied to agricultural fields. As a result, triclosan can be accumulated into crops and vegetables. Currently, limited information is available on the metabolism of triclosan in vegetables. In this study, the fate of ¹⁴C-triclosan in celery under a hydroponic system was investigated in a 30-day laboratory test. Most (97.7 %) of the ¹⁴C-triclosan accumulated in celery. The bioconcentration factors of triclosan were up to 3140 L kg^-1 at day 30. The concentration of ¹⁴C-triclosan in roots (17.8 mg kg^-1) was 57- and 127-fold higher than that in stems (0.31 mg kg^-1) and leaves (0.14 mg kg^-1), respectively, at day 30, suggesting a higher accumulation of triclosan in celery roots and negligible transport to stems and leaves. Moreover, triclosan, as well as its eight metabolites, was detected and identified in celery tissues and the growth medium using ¹⁴C-labelling and LC-Q-TOF-MS analysis methods. Phase I metabolites in the growth medium were from hydroxylation, dechlorination, nitration, and nitrosylation. Phase II metabolism was the major pathway in celery tissues. Monosaccharide, disaccharide, and sulfate conjugates of triclosan were putatively identified. The results represent an important step toward a better evaluation of the behavior of triclosan in vegetables, with notable implications for environmental and human risk assessments of triclosan.

Triclosan uptake and transformation by the green algae Euglena gracilis strain Z

Triclosan is an antimicrobial chemical present in consumer products that is frequently detected in aquatic environments. In this research, we investigated the role of a common freshwater microalgae species, Euglena gracilis for triclosan uptake and transformation in open-water treatment wetlands. Lab-scale wetland bioreactors were created under various conditions of light (i.e., continuous (white) light, red light, and in the dark), media (i.e., wetland, autoclaved wetland, Milli-Q, and growth media water), and presence or absence of algae. Triclosan and its potential transformation products were identified in the water and algae phases. Triclosan transformation occurred most rapidly with reactors that received continuous (white) light, with pseudo first-order rate constants, k, ranging from 0.035 to 0.292 day^-1. This indicates that phototransformation played a major role in triclosan transformation during the day, despite light screening by algae. Algae contributed to the uptake and transformation of triclosan in all reactors, and algae and bacteria both contributed to triclosan biotransformation under dark conditions, representative of nighttime conditions. Some transformation products were formed and further transformed, e.g., triclosan-O-sulfate, methoxy and diglucosyl conjugate of hydroxylated triclosan, and dimethoxy and glucosyl conjugate of 2,4-dichlorophenol, suggesting their minimal accumulation over the 25 days of the experiments. This study shows that the combined action of light, microbes, and algae allows the safe transfer and transformation of triclosan in open-water treatment wetlands.

Emerging investigator series: contributions of reactive nitrogen species to transformations of organic compounds in water: a critical review

Reactive nitrogen species (RNS) pose a potential risk to drinking water quality because they react with organic compounds to form toxic byproducts. Since the discovery of RNS formation in sunlit surface waters, these reactive intermediates have been detected in numerous sunlit natural waters and engineered water treatment systems. This critical review summarizes what is known regarding RNS, including their formation, contributions to contaminant transformation, and products resulting from RNS reactions. Reaction mechanisms and rate constants have been described for nitrogen dioxide (˙NO₂) reacting with phenolic compounds. However, significant knowledge gaps remain regarding reactions of RNS with other types of organic compounds. Promising methods to quantify RNS concentrations and reaction rates include the use of selective quenchers and probe compounds as well as electron paramagnetic resonance spectroscopy. Additionally, high resolution mass spectrometry methods have enabled the identification of nitr(os)ated byproducts that form via RNS reactions in sunlit surface waters, UV-based treatment systems, treatment systems that employ chemical oxidants such as chlorine and ozone, and certain types of biological treatment processes. Recommendations are provided for future research to increase understanding of RNS reactions and products, and the implications for drinking water toxicity.

A critical review on advanced oxidation processes for the removal of trace organic contaminants: A voyage from individual to integrated processes

Advanced oxidation processes (AOPs), such as photolysis, photocatalysis, ozonation, Fenton process, anodic oxidation, sonolysis, and wet air oxidation, have been investigated extensively for the removal of a wide range of trace organic contaminants (TrOCs). A standalone AOP may not achieve complete removal of a broad group of TrOCs. When combined, AOPs produce more hydroxyl radicals, thus performing better degradation of the TrOCs. A number of studies have reported significant improvement in TrOC degradation efficiency by using a combination of AOPs. This review briefly discusses the individual AOPs and their limitations towards the degradation of TrOCs containing different functional groups. It also classifies integrated AOPs and comprehensively explains their effectiveness for the degradation of a wide range of TrOCs. Integrated AOPs are categorized as UV irradiation based AOPs, ozonation/Fenton process-based AOPs, and electrochemical AOPs. Under appropriate conditions, combined AOPs not only initiate degradation but may also lead to complete mineralization. Various factors can affect the efficiency of integrated processes including water chemistry, the molecular structure of TrCOs, and ions co-occurring in water. For example, the presence of organic ions (e.g., humic acid and fulvic acid) and inorganic ions (e.g., halide, carbonate, and nitrate ions) in water can have a significant impact. In general, these ions either convert to high redox potential radicals upon collision with other reactive species and increase the reaction rates, or may act as radical scavengers and decrease the process efficiency.

Can UV-C laser pulsed irradiation be used for the removal of organic micropollutants from water? Case study with ibuprofen

A novel approach based on the direct pulsed irradiation of UV-C light onto ibuprofen (IBP) solutions was evaluated in this work, as proof of concept for the direct removal of micropollutants. The experiments confirmed that laser irradiation is able to completely degrade IBP in 15 min in distilled water, with a DOC depletion of ca. 25% and with transformation products (TPs) remaining in solution and estimated to represent ca. 10% of the initial IBP concentration. In wastewater spiked samples, removal efficiency is slightly lower but still significant (ca. 5% IBP remaining after 15 min). Hence, this work suggests that low power solid state pulsed lasers, emitting at 266 nm wavelength, show promise for the removal of these type of micropollutants from water. These results open new opportunities towards the development of chemical-free water treatment methods based on direct, selective irradiation using state of the art, miniaturized laser devices.

Electrochemical oxidation of 4-chlorophenol for wastewater treatment using highly active UV treated TiO2 nanotubes

In the present work, we report on a facile UV treatment approach for enhancing the electrocatalytic activity of TiO₂ nanotubes. The TiO₂ nanotubes were prepared using an anodization oxidation method by applying a voltage of 40 V for 8 h in a DMSO + 2% HF solution, and further treated under UV light irradiation. Compared with Pt and untreated TiO₂ nanotubes, the UV treated electrode exhibited a superior electrocatalytic activity toward the oxidation of 4-chlorophenol (4-ClPh). The effects of current density and temperature on the electrochemical oxidation of the 4-ClPh were also systematically investigated. The high electrocatalytic activity of the UV treated TiO₂ nanotubes was further confirmed by the electrochemical oxidation of other persistent organic pollutants including phenol, 2-, 3-, 4-nitrophenol, and 4-aminophenol. The total organic carbon (TOC) analysis revealed that over 90% 4-ClPh was removed when the UV treated TiO₂ electrode was employed and the rate constant was 16 times faster than that of the untreated TiO₂ electrode; whereas only 60% 4-ClPh was eliminated at the Pt electrode under the same conditions. This dramatically improved electrocatalytic activity might be attributed to the enhanced donor density, conductivity, and high overpotential for oxygen evolution. Our results demonstrated that the application of the UV treatment to the TiO₂ nanotubes enhanced their electrochemical activity and energy consumption efficiency significantly, which is highly desirable for the abatement of persistent organic pollutants.