Ultraviolet photolysis of metformin: mechanisms of environmental factors, identification of intermediates, and density functional theory calculations

Degradation of organic UV filters in the water environment: A concise review on the mechanism, toxicity, and technologies

Organic ultraviolet filters (OUVFs) have been used globally for the past 20 years. Given that OUVFs can be quickly released from sunscreens applied on human skins, they have been frequently detected in aquatic environments and organisms. Some byproducts of OUVFs might be more recalcitrant and toxic than their parent compounds. To further assess the toxicity and potential risk of OUVFs' byproducts, it is necessary to determine the fate of OUVFs and identify their transformation products. This review summarizes and analyzes pertinent literature and reports in the field of OUVFs research. These published research works majorly focus on the degradation mechanisms of OUVFs in aquatic environments, their intermediates/byproducts, and chlorination reaction. Photodegradation (direct photolysis, self-sensitive photolysis and indirect photolysis) and biodegradation are the main transformation pathways of OUVFs through natural degradation. To remove residual OUVFs' pollutants from aqueous environments, novel physicochemical and biological approaches have been developed in recent years. Advanced oxidation, ultrasound, and bio-based technologies have been proven to eliminate OUVFs from wastewaters. In addition, the disinfection mechanism and the byproducts (DBPs) of various OUVFs in swimming pools are discussed in this review. Besides, knowledge gaps and future research directions in this field of study are also mentioned.

Acetaminophen degradation in aqueous solution by the UV-LED-EC/Cl2 process

In this study, electrochemically generated free chlorine (EC/Cl2) was activated by UV irradiation with a light emitting diode (LED) lamp at 275 nm to degrade acetaminophen (AAP, 2 μM) in aqueous solution. The potential at a RuO2-IrO2/Ti plate anode was set at 1.5 V vs. the Ag/AgCl electrode. Chlorine was in situ generated in the presence of Cl at the anode and then it was transformed into various active species such as OH and reactive chlorine species (RCS) under UV-LED irradiation. The degradation of AAP was investigated using batch tests, evaluating the influence of different experimental conditions such as NaCl concentration, phosphate buffer saline concentration, irradiation time and solution pH, keeping constant the UV-LED power and temperature. Results show that AAP could be completely degraded by the hybrid process with a high mineralization ratio (73%), and the degradation process followed a pseudo-first-order kinetics. The value of the Electric Energy per Order (EEO) = 1.272 kWh m3 order?, which is lower than the energy consumption of some other UV-based processes for AAP degradation. Adding 1 mM HCO3 ions slightly decreased the rate of AAP degradation. Luminescent bacteria experiment revealed that the acute toxicity of the reacted solution could be greatly reduced and the ecological risk was effectively abated. The scavenging assay shows that RCS plays a key role in the AAP degradation. The intermediate products were identified, and possible degradation routes were proposed. The system can advantageously replace conventional UV mercury lamp based ones in the degradation of microorganic pollutants.

An efficient removal approach for degradation of metformin from aqueous solutions with sulfate radicals

Metformin consumption for diabetes treatment is increasing, leading to its presence in wastewater treatment plants where conventional methods cannot remove it. Therefore, this work aims to analyze the performance of advanced oxidation processes using sulfate radicals in the degradation of metformin from water. Experiments were performed in a photoreactor provided with a low-pressure Hg lamp, using K₂S₂O₈ as oxidant and varying the initial metformin concentration (C_A0), oxidant concentration (C_ox), temperature (T), and pH in a response surface experimental design. The degradation percentages ranged from 26.1 to 87.3%, while the mineralization percentages varied between 15.1 and 64%. Analysis of variance (ANOVA) showed that the output variables were more significantly affected by C_A0, C_ox, and T. Besides, a reduction of C_A0 and an increase of C_ox up to 5000 μM maximizes the metformin degradation since the generation of radicals and their interaction with metformin molecules are favored. For the greatest degradation percentage, the first order apparent rate constant achieved was 0.084 min^-1. Furthermore, while in acidic pH, temperature benefits metformin degradation, an opposite behavior is obtained in a basic medium because of recombination and inhibition reactions. Moreover, three degradation pathways were suggested based on the six products detected by HPLC-MS: N-cyanoguanidine m/z = 85; N,N-dimethylurea m/z = 89; N,N-dimethyl-cyanamide m/z = 71 N,N-dimethyl-formamide m/z = 74; glicolonitrilo m/z = 58; and guanidine m/z = 60. Finally, it was shown that in general the toxicity of the degradation byproducts was lower than the toxicity of metformin toward Chlamydomonas reinhardtii.

Metformin Contamination in Global Waters: Biotic and Abiotic Transformation, Byproduct Generation and Toxicity, and Evaluation as a Pharmaceutical Indicator

Metformin is the first-line antidiabetic drug and one of the most prescribed medications worldwide. Because of its ubiquitous occurrence in global waters and demonstrated ecotoxicity, metformin, as with other pharmaceuticals, has become a concerning emerging contaminant. Metformin is subject to transformation, producing numerous problematic transformation byproducts (TPs). The occurrence, removal, and toxicity of metformin have been continually reviewed; yet, a comprehensive analysis of its transformation pathways, byproduct generation, and the associated change in adverse effects is lacking. In this review, we provide a critical overview of the transformation fate of metformin during water treatments and natural processes and compile the 32 organic TPs generated from biotic and abiotic pathways. These TPs occur in aquatic systems worldwide along with metformin. Enhanced toxicity of several TPs compared to metformin has been demonstrated through organism tests and necessitates the development of complete mineralization techniques for metformin and more attention on TP monitoring. We also assess the potential of metformin to indicate overall contamination of pharmaceuticals in aquatic environments, and compared to the previously acknowledged ones, metformin is found to be a more robust or comparable indicator of such overall pharmaceutical contamination. In addition, we provide insightful avenues for future research on metformin.

Ultraviolet oxidative degradation of typical antidepressants: Pathway, product toxicity, and DFT theoretical calculation

The ubiquity of antidepressants in the environment has posed a potential threat to eco-systematic safety. In this study, six kinds of antidepressants including fluoxetine (FLU), paroxetine (PAR), sertraline (SER), fluvoxamine (FLX), citalopram (CTP), and venlafaxine (VEN) were selected to explore their degrading kinetics, transformation pathways, and the acute toxicity of the reaction solution during UV oxidation. The results showed that the order of the photodegradation rate was FLU > PAR > SER > CTP > FLX > VEN. The calculation results of density functional theory (DFT) and molecular orbital theory showed that it was positively correlated with the frontier electron density of drugs and negatively correlated with the HOMO-LUMO gap, respectively. Intermediates were identified with UHPLC-Q-TOF/MS/MS to propose the possible degradation pathways of the drugs and the most likely directions of the reactions were determined by the single point energy calculation. The results of toxicity tests indicated that the acute toxicity of the reaction solution of PAR did not change significantly. The photolysates toxicity of FLU, SER, and FLX decreased at the end of the reaction, while that of CTP and VEN was increased by 1.5 and 1.3 times compared with the parent compound, respectively. Toxicity predictions by the quantitative structure activity relationship (QSAR) model showed that except FLU-162, FLX-174, and VEN-230, other degradation products have developmental toxicity. The results revealed the transformation pathways of these drugs under the UV disinfection process in wastewater treatment plants, especially the formation of toxic by-products during the disinfection process.

Metformin as an emerging concern in wastewater: Occurrence, analysis and treatment methods

Metformin is a wonder drug used as an anti-hypoglycemic medication; it is also used as a cancer suppression medicament. Metformin is a first line of drug choice used by doctors for patients with type 2 diabetes. It is used worldwide where the drug's application varies from an anti-hypoglycemic medication to cancer oppression and as a weight loss treatment drug. Due to its wide range of usage, metformin and its byproducts are found in waste water and receiving aquatic environment. This leads to the accumulation of metformin in living beings and the environment where excess concentration levels can lead to ailments such as lactic acidosis or vitamin B12 deficiency. This drug could become of future water treatment concerns with its tons of production per year and vast usage. As a result of continuous occurrence of metformin has demanded the need of implementing and adopting different strategies to save the aquatic systems and the exposure to metformin. This review discuss the various methods for the elimination of metformin from wastewater. Along with that, the properties, occurrence, and health and environmental impacts of metformin are addressed. The different analytical methods for the detection of metformin are also explained. The main findings are discussed with respect to the management of metformin as an emerging contaminants and the major recommendations are discussed to understand the major research gaps.

Analysis of degradation and pathways of three common antihistamine drugs by NaClO, UV, and UV-NaClO methods

Antihistamines (ANTs) are medicines to treat allergic diseases. They have been frequently detected in the natural water environment, posing potential threats to the ecological environment and human health. In this study, the degradation of three common antihistamines, loratadine, fexofenadine, and cetirizine, was estimated under different oxidation methods (NaClO, UV, and UV-NaClO). The results showed that UV-NaClO had the highest degree of degradation on the drugs under most conditions: 100% degradation for fexofenadine within 20 s at pH 7 and 10. Under UV irradiation, the degradation efficiencies of the three drugs during 150 s were all above 77% at a pH of 7. The drugs' removal by NaClO was much lower than that of the previous two methods. In addition, this study explored the contribution rates of active oxygen species in the photolysis process. Among them, the contribution of ¹O₂ to the fexofenadine and cetirizine removal rate reached 70%. Different aqueous matrices (HCO₃^-, NO₃^-, and humic acid) had varying degrees of influence on the degradation. Acute toxicity tests and ultraviolet scans of the degradation products showed that the drugs were not completely mineralized, and the toxicities of the intermediates were even higher than those of the parent drugs. There were 9, 8, and 10 chloride oxidation products of loratadine, fexofenadine, and cetirizine, respectively, and 8 photolysis products of cetirizine were identified. For cetirizine, it was found that there were three identical intermediates produced by photodegradation and NaClO oxidation.

Ultraviolet photolysis of four typical cardiovascular drugs: mechanisms, influencing factors, degradation pathways, and toxicity trends

The cardiovascular drugs (CDDs), such as metoprolol (MET), atenolol (ATE), bezafibrate (BZB), and atorvastatin (ATO), have been frequently detected in the water environment. They can cause potential threats to the ecological environment and human health due to their "pseudo-persistence" effect. In this study, the photolysis kinetics, degradation mechanisms, by-products, influencing factors, and acute toxicity of these four typical CDDs under polychromatic ultraviolet irradiation (200-400 nm) were investigated. The results showed that the photolysis of ATE, BZB, MET, and ATO all followed pseudo-first-order kinetics, and their average photon quantum yields of the wavelength studied were 0.14×10^-2, 0.33×10^-3, 0.78×10^-4, and 0.24×10^-4 mol einstein^-1, respectively. Singlet oxygen (¹O₂), hydroxyl radical (·OH), and the triplet-excited state of the cardiovascular drug (³CDD*) were all involved in the photolysis while ¹O₂ was the dominator. The effects of NO₃^-, Cl^-, HCO₃^-, and humic acid (HA) on the photolysis were the combination of light-shielding, quenching, and excitation of reactive species. Seven, four, four, and nine photolysis products of ATO, BZB, ATE, and MET were identified, respectively, and their possible degradation pathways were proposed. The acute toxicity of ATE was basically unchanged during photolysis; however, ATO, BZB, and MET toxicity all increased due to the generation of ketonization and hydroxylation products.

Thermal plasma activation and UV/H2O2 oxidative degradation of pharmaceutical residues

The aquatic environment becomes increasingly contaminated by anthropogenic pollutants such as pharmaceutical residues. Due to poor biodegradation and continuous discharge of persistent compounds in sewage water samples, pharmaceutical residues might end up in surface waters when not removed. To minimize this pollution, onsite wastewater treatment techniques might complement conventional waste water treatment plants (WWTPs). Advanced oxidation processes are useful techniques, since reactive oxygen species (ROS) are used for the degradation of unwanted medicine residues. In this paper we have studied the advanced oxidation in a controlled laboratory setting using thermal plasma and UV/H₂O₂ treatment. Five different matrices, Milli-Q water, tap water, synthetic urine, diluted urine and synthetic sewage water were spiked with 14 pharmaceuticals with a concentration of 5 μg/L. All compounds were reduced or completely decomposed by both 150 W thermal plasma and UV/H₂O₂ treatment. Additionally, also hospital sewage water was tested. First the concentrations of 10 pharmaceutical residues were determined by liquid chromatography mass spectrometry (LC-MS/MS). The pharmaceutical concentration ranged from 0.08 up to 2400 μg/L. With the application of 150 W thermal plasma or UV/H₂O₂, it was found that overall pharmaceutical degradation in hospital sewage water were nearly equivalent to the results obtained in the synthetic sewage water. However, based on the chemical abatement kinetics it was demonstrated that the degree of degradation decreases with increasing matrix complexity. Since reactive oxygen and nitrogen species (RONS) are continuously produced, thermal plasma treatment has the advantage over UV/H₂O₂ treatment.