Chlorination of methotrexate in water revisited: Deciphering the kinetics, novel reaction mechanisms, and unexpected microbial risks

Should Transformation Products Change the Way We Manage Chemicals?

When chemical pollutants enter the environment, they can undergo diverse transformation processes, forming a wide range of transformation products (TPs), some of them benign and others more harmful than their precursors. To date, the majority of TPs remain largely unrecognized and unregulated, particularly as TPs are generally not part of routine chemical risk or hazard assessment. Since many TPs formed from oxidative processes are more polar than their precursors, they may be especially relevant in the context of persistent, mobile, and toxic (PMT) and very persistent and very mobile (vPvM) substances, which are two new hazard classes that have recently been established on a European level. We highlight herein that as a result, TPs deserve more attention in research, chemicals regulation, and chemicals management. This perspective summarizes the main challenges preventing a better integration of TPs in these areas: (1) the lack of reliable high-throughput TP identification methods, (2) uncertainties in TP prediction, (3) inadequately considered TP formation during (advanced) water treatment, and (4) insufficient integration and harmonization of TPs in most regulatory frameworks. A way forward to tackle these challenges and integrate TPs into chemical management is proposed.

Chlorination of antiviral drug ribavirin: Kinetics, nontargeted identification, and concomitant toxicity evolution

Residual antiviral drugs in wastewater may increase the risk of generating transformation products (TPs) during wastewater treatment. Therefore, chlorination behavior and toxicity evolution are essential to understand the secondary ecological risk associated with their TPs. Herein, chlorination kinetics, transformation pathways, and secondary risks of ribavirin (RBV), one of the most commonly used broad-spectrum antivirals, were investigated. The pH-dependent second-order rate constants k increased from 0.18 M-1·s-1 (pH 5.8) to 1.53 M-1·s-1 (pH 8.0) due to neutral RBV and ClO- as dominant species. 12 TPs were identified using high-resolution mass spectrometry in a nontargeted approach, of which 6 TPs were reported for the first time, and their chlorination pathways were elucidated. The luminescence inhibition rate of Vibrio fischeri exposed to chlorinated RBV solution was positively correlated with initial free active chlorine, probably due to the accumulation of toxic TPs. Quantitative structure-activity relationship prediction identified 7 TPs with elevated toxicity, concentrating on developmental toxicity and bioconcentration factors, which explained the increased toxicity of chlorinated RBV. Overall, this study highlights the urgent need to minimize the discharge of toxic chlorinated TPs into aquatic environments and contributes to environmental risk control in future pandemics and regions with high consumption of antivirals.

The adsorption and its mechanism of venlafaxine by original and aged polypropylene microplastic and the changes of joint toxicity

Conjugation with the increment of consumption of polypropylene (PP) masks and antidepressants during pandemic, PP microplastics (MPs) and Venlafaxine (VEN) widely co-existed in surface waters. However, their environmental fate and the combined toxicity were unclear. Hence, we investigated the adsorption behaviors, and associated mechanisms of PP MPs for VEN. The impact factors including pH, salinity, and MPs aging were estimated. The results indicated PP MPs could adsorb amount of VEN within 24 h. The pseudo second-order kinetic model (R2 = 0.97) and Dubinin-Radushkevich model (R2 = 0.89) fitted well with the adsorption capacity of PP MPs for VEN, implying that chemical adsorption accompanied by electrostatic interaction might be the predominant mode for the interactions between PP MPs and VEN. Meanwhile, the adsorption capacity of PP MPs declined from pH of 2.5-4.5 and then increased from 4.5 to 9.5. The increased salinity (5-35 ppt) significantly suppressed the adsorption capacity. Aging by sunlight and UV triggered the formation of new functional group (carbonyl) on MPs, and then enhanced the adsorption capacity for VEN. Gaussian Model analysis further evidenced the electrostatic adsorption occurring in PP MPs and VEN. The combined exposure to PP MPs and VEN showed significantly antagonistic toxicity on Daphnia magna. The adsorption of VEN by PP MPs mitigated the lethal effects and behavioral function impairment posed by VEN on animals, implying the potential protective effects on zooplankton by PP MPs. This study for the first time provides perspective for assessing the environmental fate of MPs and antidepressants in aquatic system.

Environmental fate and risk evolution of calcium channel blockers from chlorine-based disinfection to sunlit surface waters

Organic micropollutants present in disinfected wastewater and discharged to sunlit surface waters may be transformed by multiple processes, such as chlorination due to the presence of chlorine residuals, solar irradiation as well as solar-irradiated chlorine residues. This study reports, for the first time, the multi-scenario degradation kinetics, transformation products, and risk evolution of calcium channel blockers (CCBs), a class of emerging pharmaceutical contaminants with worldwide prevalence in natural waters and wastewater. It was found that the chlorination of the studied CCBs (amlodipine (AML) and verapamil (VER)) was dominated by the reaction of HOCl with their neutral species, with second-order rate constants of 6.15×104 M-1 s-1 (AML) and 7.93×103 M-1 s-1 (VER) at pH 5.0-11.0. Bromination is much faster than chlorination, with the measured kapp,HOBr values of 2.94×105 M-1 s-1 and 6.58×103 M-1 s-1 for AML and VER, respectively, at pH 7.0. Furthermore, both CCBs would undergo photolytic attenuations with hydroxyl and carbonate radicals as the dominant reactive species in water. Notably, free chlorine mainly contributed to their abatement during the solar/chlorine treatment. Additionally, the halogen addition on the aromatic ring was observed during chlorination and bromination of the two CCBs. Cyclization was observed under solar irradiation only, while the aromatic ring was opened in the solar/chlorine system. Some products generated by the three transformation processes exhibited non-negligible risks of high biodegradation recalcitrance and toxicity, potentially threatening the aquatic environment and public health. Overall, this study elucidated the environmental fate of typical CCBs under different transformation processes to better understand the resulting ecological risks in these environmental scenarios.

Photodegradation of typical pharmaceuticals changes toxicity to algae in estuarine water: A metabolomic insight

The ubiquitous existence of various pharmaceuticals in the marine environment has received global attention for their risk assessment. However, rather little is known thus far regarding the natural attenuation (e.g., photolysis)-induced product/mixture toxicity of these pharmaceuticals on marine organisms. In this study, the photodegradation behavior, product formation, and risks of two representative pharmaceuticals (i.e., ciprofloxacin, CIP; diclofenac, DCF) were explored in the simulated estuary water. It was noted that both pharmaceuticals can be completely photolyzed within 1 h, and five products of CIP and three products of DCF were identified by a high-resolution liquid chromatography-mass spectrometer. Accordingly, their photodecomposition pathways were tentatively proposed. The in silico prediction suggested that the formed transformation products maintained the persistence, bioaccumulation potential, and multi-endpoint toxic effects such as genotoxicity, developmental toxicity, and acute/chronic toxicity on different aquatic species. Particularly, the non-targeted metabolomics first elucidated that DCF and its photolytic mixtures can significantly affect the antioxidant status of marine algae (Heterosigma akashiwo), triggering oxidative stress and damage to cellular components. It is very alarming that the complete photolyzed DCF sample induced more serious oxidative stress than DCF itself, which called for more concern about the photolysis-driven ecological risks. Overall, this investigation first uncovered the overlooked but serious toxicity of the transformation products of prevalent pharmaceuticals during natural attenuation on marine species.

Degradation of methotrexate by unactivated and solar-activated peroxymonosulfate in water: Moiety-specific reaction kinetics and transformation product-associated risks

Anticancer drugs have raised worldwide concern owing to their ubiquitous occurrence and ecological risks, necessitating the development of efficient removal strategies in water and wastewater treatment. Although peroxymonosulfate (PMS) is known to be a promising chemical in water decontamination, limited information is available regarding the removal efficiency of anticancer drugs by PMS and solar/PMS systems. This study first reports the moiety-specific reaction kinetics and mechanisms of methotrexate (MTX), an anticancer drug with widespread attention, by PMS (unactivated) and solar-activated PMS in water. It was found that MTX abatement by the direct PMS oxidation followed second-order kinetics, and the pH-dependent rate constants increased from 0.4 M-1 s-1 (pH 5.0) to 1.3 M-1 s-1 (pH 8.0), with a slight decrease to 1.1 M-1 s-1 at pH 9.0. The presence of chloride and bromide exerted no obvious influence on the removal of MTX by PMS. Furthermore, the chemical reactivity of MTX and its seven substructures with different reactive species was evaluated, and the degradation contributions of the reactive species involved were quantitatively analyzed in the solar/PMS system. The product analysis suggested similar reaction pathways of MTX by PMS and solar/PMS systems. The persistence, bioaccumulation, and toxicity of the transformation products were investigated, indicating treatment-driven risks. Notably, MTX can be removed efficiently from both municipal and hospital wastewater effluents by the solar/PMS system, suggesting its great potential in wastewater treatment applications. Overall, this study systematically evaluated the elimination of MTX by the unactivated PMS and solar/PMS treatment processes in water. The obtained findings may have implications for the mechanistic understanding and development of PMS-based processes for the degradation of such micropollutants in wastewater.

Elucidating sulfate radical-induced oxidizing mechanisms of solid-phase pharmaceuticals: Comparison with liquid-phase reactions

As a class of organic micropollutants of global concern, pharmaceuticals have prevalent distributions in the aqueous environment (e.g., groundwater and surface water) and solid matrices (e.g., soil, sediments, and dried sludge). Their contamination levels have been further aggravated by the annually increased production of expired drugs as emerging harmful wastes worldwide. Sulfate radicals (SO4•-)-based oxidation has attracted increasing attention for abating pharmaceuticals in the environment, whereas the transformation mechanisms of solid-phase pharmaceuticals remain unknown thus far. This investigation presented for the first time that SO4•-, individually produced by mechanical force-activated and heat-activated persulfate treatments, could effectively oxidize three model pharmaceuticals (i.e., methotrexate, sitagliptin, and salbutamol) in both solid and liquid phases. The high-resolution mass spectrometric analysis suggested their distinct transformation products formed by different phases of SO4•- oxidation. Accordingly, the SO4•--mediated mechanistic differences between the solid-phase and liquid-phase pharmaceuticals were proposed. It is noteworthy that the products from both systems were predicted with the remaining persistence, bioaccumulation, and multi-endpoint toxicity. Therefore, some post-treatment strategies need to be considered during practical applications of SO4•--based technologies in remediating different phases of micropollutants. This work has environmental implications for understanding the comparative transformation mechanisms of pharmaceuticals by SO4•- oxidation in remediating the contaminated solid and aqueous matrices.

Amino antioxidants: A review of their environmental behavior, human exposure, and aquatic toxicity

Amino antioxidants (AAOs), a suite of emerging organic contaminants, have been widely used in numerous industrial and commercial products to inhibit oxidation and corrosion. Recently, their environmental ubiquity, health risks, bioaccumulative and toxic potential have led to mounting public concern. This review summarizes the current state of knowledge on the production and usage, environmental occurrence, bioavailability, human exposure, and aquatic toxicity of representative AAOs, and provides suggestions for future research directions. Previous studies have revealed widespread distribution of many AAOs in various environmental matrixes, including air, water, sediment, dust, and biota. In addition to parent compounds, their degradation products, such as 2-anilino-5-(1,3-dimethylbutylamino)-1,4-benzoquinone (6PPD-Q) and 4-nitrodiphenylamine (4-NO₂-DPA), have also been detected at high levels in multiple compartments. Dust ingestion and air inhalation are the two most well-investigated routes for human exposure to AAOs and their transformation products, while studies on other pathways (e.g., skin contact and dietary intake) still remain extremely limited. Moreover, AAO burdens in human tissue have been poorly documented. Toxicological data have shown that a few AAOs may cause teratogenic, developmental, reproductive, endocrinic, neuronic, and genetic toxicity to aquatic organisms, and the toxic capacities of degradation products differ from their precursors. Future studies should focus on elucidating AAO exposure for humans and associated health risks. Additionally, more attention should be given to AAO transformation products (particularly those quinoid derivatives possessing substantial affinity with DNA) and to the effects of complex mixtures of these chemicals.