Sunlight-induced degradation of COVID-19 antivirals arbidol in natural aquatic environments: Mechanisms, pathways and toxicity

Dissociation-dependent kinetics and distinct pathways for direct photolysis and •OH/SO4•- radical dominated photodegradation of ionizable antiviral drugs in aquatic systems

Advanced oxidation processes (AOPs), such as UV, UV/H2O2 and UV/persulfate, are widely used to remove emerging organic contaminants from wastewater streams. However, knowledge on chemical degradation pathways, reaction kinetics as well as formation and toxicity of key degradates is limited. We investigated the direct photolysis and •OH/SO4•- dominated kinetics, intermediates and toxicity evolution of three ionizable antiviral drugs (ATVs): tenofovir (TFV), didanosine (DDI), and nevirapine (NVP). Their transformation kinetics were found to depend on the dominant protonated states. Under UV-Vis irradiation (λ > 290 nm), TFV and DDI photolyzed the fastest in the cationic forms (H2TFV+ and H2DDI+), whereas NVP exhibited the fastest photodegradation in the anionic forms (NVP-). The anionic forms (TFV- and NVP-) demonstrated the highest reactivities towards •OH in most cases, while the cationic forms (H2DDI+ and H2NVP+) reacted the fastest with SO4•- for most of the ATVs. The dissociation-dependent kinetics can be attributed to the discrepancies in deprotonation degrees, quantum yields, electron densities and coulombic repulsion with SO4•- in their dissociated forms. Based on the key product identification via HPLC-MS/MS, the pathways involved hydroxylation, dehydroxylation, oxidation, reduction, cyclopropyl cleavage, C-N breaking, elimination, cyclization and deamidation reactions, which can be prioritized based on the specific compound and the photochemical process. Furthermore, a bioassay showed the photomodified toxicity of the ATVs to Vibrio fischeri (bioluminescent bacteria) during the three processes, which was also demonstrated by ECOSAR model assessment. Nearly half of the chemical intermediates were demonstrably more toxic than their respective parent ATVs. These results provide new insights into understanding the persistence, fate and hazards associated with applying the UV-assisted AOPs to treat wastewater containing ATVs.

Heat-activated peroxodisulfate oxidation of sulfapyridine: kinetics, transformation pathways, and nitrated byproducts

ABSTRACTThe widespread existence of sulfapyridine (SPD, a typical representative of sulfonamide) in natural environment has raised increasing interest because its potential to cause antibiotic-resistant genes. In this work, the degradation of SPD during heat-activated peroxodisulfate (heat/PDS) oxidation process was explored. The pseudo-first-order rate constant (kobs) of SPD was 0.0149 min-1 with 0.5 mM PDS at pH 7. The kobs values were increased obviously with increasing PDS concentration. Such degradation was ascribed to the oxidation of sulfate radical (SO4•-) primarily according to radical quenching test. A total of 16 transformation products derived from hydroxylation, aniline moiety oxidation, and SO2 extrusion & rearrangement pathways were identified by high-resolution mass spectrometry (HRMS) and theoretical calculations. Of which, the production of nitrated byproducts was attributed to the oxidation of aniline moiety in SPD molecule. The existence of natural organic matter (NOM) obviously reduced the degradation efficiency of SPD, while the effects of coexisting anions (i.e., NO3-, CO32-, and Cl-) were negligible. These findings illustrated that SPD can be effectively degraded but cause the nitrated byproducts generation during the heat/PDS oxidation process, which should be paid attention to when SR-AOPs is applied.

Toxic mechanisms of the antiviral drug arbidol on microalgae in algal bloom water at transcriptomic level

Increasing antivirals in surface water caused by their excessive consumption pose serious threats to aquatic organisms. Our recent research found that the input of antiviral drug arbidol to algal bloom water can induce acute toxicity to the growth and metabolism of Microcystis aeruginosa, resulting in growth inhibition, as well as decrease in chlorophyll and ATP contents. However, the toxic mechanisms involved remained obscure, which were further investigated through transcriptomic analysis in this study. The results indicated that 885-1248 genes in algae were differentially expressed after exposure to 0.01-10.0 mg/L of arbidol, with the majority being down-regulated. Analysis of commonly down-regulated genes found that the cellular response to oxidative stress and damaged DNA bonding were affected, implying that the stress defense system and DNA repair function of algae might be damaged. The down-regulation of genes in porphyrin metabolism, photosynthesis, carbon fixation, glycolysis, tricarboxylic acid cycle, and oxidative phosphorylation might inhibit chlorophyll synthesis, photosynthesis, and ATP supply, thereby hindering the growth and metabolism of algae. Moreover, the down-regulation of genes related to nucleotide metabolism and DNA replication might influence the reproduction of algae. These findings provided effective strategies to elucidate toxic mechanisms of contaminants on algae in algal bloom water.

The environmental risks of antiviral drug arbidol in eutrophic lake: Interactions with Microcystis aeruginosa

The environmental risks resulting from the increasing antivirals in water are largely unknown, especially in eutrophic lakes, where the complex interactions between algae and drugs would alter hazards. Herein, the environmental risks of the antiviral drug arbidol towards the growth and metabolism of Microcystis aeruginosa were comprehensively investigated, as well as its biotransformation mechanism by algae. The results indicated that arbidol was toxic to Microcystis aeruginosa within 48 h, which decreased the cell density, chlorophyll-a, and ATP content. The activation of oxidative stress increased the levels of reactive oxygen species, which caused lipid peroxidation and membrane damage. Additionally, the synthesis and release of microcystins were promoted by arbidol. Fortunately, arbidol can be effectively removed by Microcystis aeruginosa mainly through biodegradation (50.5% at 48 h for 1.0 mg/L arbidol), whereas the roles of bioadsorption and bioaccumulation were limited. The biodegradation of arbidol was dominated by algal intracellular P450 enzymes via loss of thiophenol and oxidation, and a higher arbidol concentration facilitated the degradation rate. Interestingly, the toxicity of arbidol was reduced after algal biodegradation, and most of the degradation products exhibited lower toxicity than arbidol. This study revealed the environmental risks and transformation behavior of arbidol in algal bloom waters.