Ultraviolet-Light-emitting-diode activated monochloramine for the degradation of carbamazepine: Kinetics, mechanisms, by-product formation, and toxicity

Degradation behaviors of Nabumetone and its metabolite during UV/monochloramine process: Experimental and theoretical study

This study investigated degradation behaviors of a nonsteroidal anti-inflammatory drug Nabumetone (NMT) and its major metabolite 6-methoxy-2-naphthylacetic acid (MNA) in the coupling process of ultraviolet and monochloramine (UV/NH2Cl). The second-order rate constants of the contaminants reacting with reactive radicals (HO•, Cl•, Cl2•⁻, and CO3•⁻) were determined by laser flash photolysis experiments. HO• and Cl• contributed predominantly with 52.3% and 21.7% for NMT degradation and 60.8% and 22.3% for MNA degradation. The presence of chlorides retarded the degradation of NMT, while promoted the destruction of MNA, which was ascribed to the photosensitization effects of MNA under UV irradiation. Density functional theory (DFT) calculations revealed that radical adduct formation (RAF) was dominant pathway for both HO• and Cl• reacting with the contaminants, and hydrogen atom transfer (HAT) preferred to occur on side chains of NMT and MNA. NMT reacted with NO2• through single electron transfer (SET) with the second-order rate constant calculated to be 5.35 × 107 (mol/L)-1 sec-1, and the contribution of NO2• was predicted to be 13.0% of the total rate constant of NMT in pure water, which indicated that NO2• played a non-negligible role in the degradation of NMT. The acute toxicity and developmental toxicity of NMT were enhanced after UV/NH2Cl treatment, while those of MNA were alleviated. The transformation products of both NMT and MNA exhibited higher mutagenicity than their parent compounds. This study provides a deep understanding of the mechanism of radical degradation of NMT and MNA in the treatment of UV/NH2Cl.

Insight into the generation of toxic by-products during UV/H2O2 degradation of carbamazepine: Mechanisms, N-transformation and toxicity

Carbamazepine (CBZ) is a widely used anticonvulsant drug that has been detected in aquatic environments. This study investigated the toxicity of its by-products (CBZ-BPs), which may surpass CBZ. Unlike the previous studies, this study offered a more systematic approach to identifying toxic BPs and inferring degradation pathways. Furthermore, quadrupole time-of-flight (QTOF) and density functional theory (DFT) calculations were employed to analyze CBZ-BP structures and degradation pathways. Evaluation of total organic carbon (TOC) and total nitrogen (TN) mineralization rates, revealed carbon (C) greater susceptibility to mineralization compared with nitrogen (N). Furthermore, three rules were established for CBZ decarbonization and N removal during degradation, observing the transformation of aromatic compounds into aliphatic hydrocarbons and stable N-containing organic matter over time. Five potentially highly toxic BPs were screened from 14 identified BPs, with toxicity predictions guiding the selection of commercial standards for quantification and true toxicity testing. Additionally, BP207 emerged as the most toxic, supported by the predictive toxicity accumulation model (PTAM). Notably, highly toxic BPs feature an acridine structure, indicating its significant contribution to toxicity. These findings offered valuable insights into the degradation mechanisms of emerging contaminants and the biosafety of aquatic environments during deep oxidation.

A critical review on the pathways of carbamazepine transformation products in oxidative wastewater treatment processes

Carbamazepine (CBZ) is an anticonvulsant drug, released in domestic and hospital wastewater, and one of the drugs most commonly detected in surface water. Conventional secondary processes do a very poor job of removing it (<25 %), but its concentrations are significantly reduced by polishing oxidation processes. However, there are still many unknowns regarding the transformation products generated and their fate. This review first presents the journey of CBZ and its transformation products (TPs) in wastewater, from human consumption to discharge in water bodies. It then goes on to detail the diversity of mechanisms responsible for CBZ degradation and the generation of multiple TPs, laying the emphasis on the different types of advanced oxidation processes (AOP). 135 TPs were reported and a map describing their formation/degradation pathways was drawn up. This work highlights the wide range of physicochemical properties and toxicity effects of TPs on aquatic organisms and provides information about TPs of interest for future research. Finally, this review concludes on the importance of quantifying TPs and of determining kinetic characteristics to produce more accurate reaction schemes and computer-based fate predictions.

Toxicological aspect of water treated by chlorine-based advanced oxidation processes: A review

As well established in the literature, residual toxicity is an important parameter for evaluating the sanitary and environmental safety of water treatment processes, and this parameter becomes even more crucial when chlorine-based processes are applied for water treatment. Eliminating initial toxicity or preventing its increase after water treatment remains a huge challenge mainly due to the formation of highly toxic disinfection by-products (DBPs) that stem from the degradation of organic contaminants or the interaction of the chlorine-based oxidants with different matrix components. In this review, we present a comprehensive discussion regarding the toxicological aspects of water treated using chlorine-based advanced oxidation processes (AOPs) and the recent findings related to the factors influencing toxicity, and provide directions for future research in the area. The review begins by shedding light on the advances made in the application of free chlorine AOPs and the findings from studies conducted using electrochemical technologies based on free chlorine generation. We then delve into the insights and contributions brought to the fore regarding the application of NH₂Cl- and ClO₂-based treatment processes. Finally, we broaden our discussion by evaluating the toxicological assays and predictive models employed in the study of residual toxicity and provide an overview of the findings reported to date on this subject matter, while giving useful insights and directions for future research on the topic.

EU need to protect its environment from toxic per- and polyfluoroalkyl substances

The Environmental Protection Agencies (EPAs) of Denmark, Sweden, Norway, Germany and the Netherlands submitted a proposal to the European Chemical Agency (ECHA) in February 2023 calling for a ban in the use of toxic industrial chemicals per- and polyfluoroalkyl substances (PFAS). These chemicals are highly toxic causing elevated cholesterol, immune suppression, reproductive failure, cancer and neuro-endocrine disruption in humans and wildlife being a significant threat to biodiversity and human health. The main reason for the submitted proposal is recent findings of significant flaws in the transition to PFAS replacements that is leading to a widespread pollution. Denmark was the first country banning PFAS, and now other EU countries support the restrictions of these carcinogenic, endocrine disruptive and immunotoxic chemicals. The proposed plan is among the most extensive received by the ECHA for 50 years. Denmark is now the first EU country to initiate the establishment of groundwater parks to try and protect its drinking water. These parks are areas free of agricultural activities and nutritious sewage sludge to secure drinking water free of xenobiotic including PFAS. The PFAS pollution also reflects the lack of comprehensive spatial and temporal environmental monitoring programs in the EU. Such monitoring programs should include key indicator species across ecosystems of livestock, fish and wildlife, to facilitate detection of early ecological warning signals and sustain public health. Simultaneously with inferring a total PFAS ban, the EU should also push for more persistent, bioaccumulative and toxic (PBT) PFAS substances to be listed on the Stockholm Convention (SC) Annex A such as PFOS (perfluorooctane sulfonic acid) that is currently listed on the SCs Annex B. The combination of these regulative restrictions combined with groundwater parks and pan-European biomonitoring programs, would pave the way forward for a cleaner environment to sustain health across the EU.

Study on the operation performance and floc adhesion mechanism of dissolved air flotation equipment

As a critical air dissolving system, the performance of air flotation equipment directly determines the adhesion efficiency and pollutant removal efficiency of air flotation processes. The factors affecting the performance of air flotation equipment and the relationships between equipment performance and pollution removal efficiency were studied. The results show that when the dissolved gas pressure was 0.4 MPa and the air intake rate was 24 mL/min, the dissolved gas efficiency of the equipment reached its highest value of 55%, the average particle size of bubbles was maintained at 24 µm, and the dissolved oxygen (DO) content significantly increased. When the dissolved gas pressure was 0.4 MPa, the air intake rate was 24 mL/min, and the coagulant dose was 6 mg/L; the removal rates for turbidity, chlorophyll-a, total organic carbon (TOC), and UV absorbance at 254 nm (UV₂₅₄) reached 95.76%, 96.41%, 34.21%, and 65.96%, respectively. The degree of pollutant removal was positively correlated with changes to the equipment performance parameters. Microbubbles (MBs) showed good removal of high-molecular weight, strongly hydrophobic organic matter and showed some removal of the trihalomethane formation potential (THMFP) of the water. The removal mechanism mainly depended on the hydrophobic interactions of the MBs with algae and organic matter. The flocs and MBs collided and adhered to form air-entrained flocs. The separation of air-entrained flocs depended on the relationship between the surface load and the rising velocity. The surface load has to be lower than the rising velocity of the minimum air-entrained flocs to ensure good effluent outcomes.

Insight into the degradation of ciprofloxacin by medium-pressure UV-activated monochloramine process

The degradation efficiency and mechanisms of ciprofloxacin (CIP), a typical antibiotic, by a medium-pressure ultraviolet/chloramine (MPUV/NH₂Cl) treatment were investigated. The results showed that CIP degradation by MPUV/NH₂Cl was significantly higher than that by NH₂Cl oxidation and MPUV photolysis, and that this degradation processes were consistent with pseudo-first-order kinetics. The initial CIP concentration (7.5-30.2 μM) and the presence of HCO₃^- (0.5-10 mM) significantly inhibited CIP degradation with k_obs,CIP 0.0090-0.0069 and 0.0078-0.0048 cm²/mJ. In contrast, NO₃^- (50-500 μM) and Br^- (0.5-10 mM) significantly promoted the degradation with k_obs,CIP 0.0078-0.0102 and 0.0078-0.0124 cm²/mJ. The effect of Cl^- (0.5-10 mM) and natural organic matter (1-5 mg/L) were negligible. The NH₂Cl dosage (30-60 μM) presented a dual effect, in which its increase within the optimal concentration range (30-40 μM) accelerated CIP degradation due to the formation of reactive radicals, whereas an excessive increase (40-60 μM) quenched the free radicals, ultimately quenching the free radicals and inhibiting the degradation. The optimum pH for CIP degradation under MPUV/NH₂Cl treatment was 7.0. The contribution of reactive halogen species (i.e., reactive chlorine species and reactive nitrogen species) to CIP degradation was substantially greater than that of hydroxyl radicals under acidic or neutral conditions. We identified the degradation products of CIP and proposed degradation pathways, which included defluorination and cracking of the piperazine ring, with the latter being dominant. Compared to haloacetic acid (HAA) and nitrogenous disinfection byproducts (N-DBPs), MPUV/NH₂Cl significantly reduced trihalomethane (THM) production and theoretical cytotoxicity by 80.1% and 78.4% respectively, compared to the background experiment in natural water at a UV dose of 300 mJ/cm².