Photodegradation of azithromycin in various aqueous systems under simulated and natural solar radiation: kinetics and identification of photoproducts

This article describes the photolysis of azithromycin, a macrolide antibiotic with reported occurrence in environmental waters, under simulated solar radiation. The photodegradation followed first-order reaction kinetics in five matrices examined. In HPLC water, the degradation rate was the slowest (half-life: 20 h), whereas in artificial freshwater supplemented with nitrate (5 mg L⁻¹) or humic acids (0.5 mg L⁻¹) the degradation of azithromycin was enhanced by factors of 5 and 16, respectively, which indicated the role of indirect photolysis involving the formation of highly reactive species. Following chromatographic separation on a UPLC system, the characterization of the transformation products was accomplished using high-resolution QqToF-MS analysis. The presence of seven photoproducts was observed and their formation was postulated to originate from (bis)-N-demethylation in the desosamine sugar, O-demethylation in the cladinose sugar, combinations thereof, as well as from hydrolytic cleavages of the desosamine and/or cladinose residue. Two of these photoproducts could also be detected in natural photodegradation process in river water which was spiked with azithromycin.     

Pathway of anthracene modification under simulated solar radiation

Exposure of polycyclic aromatic hydrocarbons (PAHs) to sunlight results in rapid structural photomodification generally via oxidation reactions. These PAH modification products are in many cases more toxic than their parent compounds. In this study, anthracene (ANT), a rapidly photooxidized PAH, was irradiated with simulated solar radiation (SSR, 100 μmol m⁻² s⁻¹) in aqueous solution to examine the photomodification pathway. The photoproducts formed were identified by HPLC. The ANT product profile after 9 h in SSR was very complex, with more than 20 compounds detected. The photoproducts formed were anthraquinones, benzoic acids, benzaldehydes and phenols showing the process to be oxidative in nature. Some of the anthraquinones were themselves subject to photooxidation, and were thus intermediates in the product pathway. The kinetics of ANT photooxidation revealed a pseudo first-order reaction with a half-life of 2 h under the SSR source used. The kinetics of product formation allowed deduction of a probable photomodification pathway. This study indicates that PAH photooxidation products are likely to exist as complex, dynamically changing mixtures in PAH contaminated aquatic environments.     

Photodegradation of sulfonamides and their N 4-acetylated metabolites in water by simulated sunlight irradiation: kinetics and identification of photoproducts

Once released into the aquatic environment, pharmaceuticals may undergo different degradation processes. Photodegradation, for example, might be an important elimination process for light-sensitive pharmaceuticals, such as antibiotics. In this study, the fate of sulfonamides (sulfamethazine, sulfadiazine, and sulfamethoxazole) and their N ⁴-acetylated metabolites (N ⁴-acetylsulfadiazine, N ⁴-acetylsulfamethazine, and N ⁴-acetylsulfamethoxazole) under simulated sunlight irradiation was investigated. The irradiation resulted in total or almost total degradation (88 to 98 %) of the pharmaceuticals tested, except for sulfamethazine (52 %), during 24 h of irradiation. The photoproducts of all investigated pharmaceuticals have been analyzed using high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry. Structure elucidation performed from photodegradation products of both, sulfonamides and their N ⁴-acetylated metabolites, clearly showed two major formation pathways. These were cleavage of the sulfonamide bond as well as SO₂ extrusion. In total, nine photoproducts were elucidated. Among these photoproducts, the tautomers of sulfamethoxazole and desulfonated products of sulfadiazine and sulfamethazine were also present. Tautomers of sulfadiazine and sulfamethazine have been characterized here for the first time as well as some photoproducts of sulfadiazine, sulfametoxazole, and their metabolites N ⁴-acetylsulfadiazine and N ⁴-acetylsulfametoxazole. The obtained results are an important piece in the complex puzzle for assessing the environmental fate of sulfonamides and their metabolites in the environment.     

Photodegradation of the novel fungicide fluopyram in aqueous solution: kinetics,transformation products,and toxicity evolvement

The aqueous photodegradation of fluopyram was investigated under UV light (λ?≥?200 nm) and simulated sunlight irradiation (λ?≥?290 nm). The effect of solution pH, fulvic acids (FA), nitrate (NO₃ ^?), Fe (III) ions, and titanium dioxide (TiO₂) on direct photolysis of fluopyram was explored. The results showed that fluopyram photodegradation was faster in neutral solution than that in acidic and alkaline solutions. The presence of FA, NO₃ ^?, Fe (III), and TiO₂ slightly affected the photodegradation of fluopyram under UV irradiation, whereas the photodegradation rates of fluopyram with 5 mg L^?1 Fe (III) and 500 mg L^?1 TiO₂ were about 7-fold and 13-fold faster than that without Fe (III) and TiO₂ under simulated sunlight irradiation, respectively. Three typical products for direct photolysis of fluopyram have been isolated and characterized by liquid chromatography tandem mass spectrometry. These products resulted from the intramolecular elimination of HCl, hydroxyl-substitution, and hydrogen extraction. Based on the identified transformation products and evolution profile, a plausible degradation pathway for the direct photolysis of fluopyram in aqueous solution was proposed. In addition, acute toxicity assays using the Vibrio fischeri bacteria test indicated that the transformation products were more toxic than the parent compound.     

Photolysis of Enrofloxacin in aqueous systems under simulated sunlight irradiation: Kinetics, mechanism and toxicity of photolysis products

Photolysis of Enro in water was investigated under simulated sunlight irradiation using a Xenon lamp. The results showed that Enro photolysis followed apparent first-order kinetics. Increasing Enro concentration from 5.0 to 40.0 mg L⁻¹ led to the decrease of the photolysis rate constant from 1.6 × 10⁻² to 3.0 × 10⁻³ min⁻¹. Compared with the acidic and basic conditions, the photolysis rate was faster at neutral condition. Both of nitrate and humic acid can markedly decrease the photolysis rate of Enro because they can competitively absorb photons with Enro. The electron spin resonance and reactive oxygen species scavenging experiments indicated that Enro underwent self-sensitized photooxidation via OH and ¹O₂. After irradiation for 90 min, only 13.1% reduction of TOC occurred in spite of fast photolysis of 58.9% of Enro, indicating that Enro was transformed into intermediates without complete mineralization. The photolysis of Enro involved three main pathways: decarboxylation, defluorination, and piperazinyl N⁴-dealkylation. The bioluminescence inhibition rate using Vibrio fischeri increased to 67.2% at 60 min and then decreased to 56.9% at 90 min, indicative of the generation of some more toxic intermediates than Enro and then the degradation of the intermediates. The results will help us understand fundamental mechanisms of Enro photolysis and provide insight into the potential fate and transformation of Enro in surface waters.     

Photodegradation of bisphenol A in simulated lake water containing algae, humic acid and ferric ions

The photodegradation of bisphenol A (BPA), a suspected endocrine disruptor (ED), in simulated lake water containing algae, humic acid and Fe3+ ions was investigated. Algae, humic acid and Fe3+ ions enhanced the photodegradation of BPA. Photodegradation efficiency of BPA was 36% after 4h irradiation in the presence of 6.5 x 10(9) cells L(-1) raw Chlorella vulgaris, 4 mg L(-1) humic acid and 20 micromol L(-1) Fe3+. The photodegradation efficiency of BPA was higher in the presence of algae treated with ultrasonic than that without ultrasonic. The photodegradation efficiency of BPA in the water only containing algae treated with ultrasonic was 37% after 4h irradiation. The algae treated with heating can also enhance the photodegradation of BPA. This work helps environmental scientists to understand the photochemical behavior of BPA in lake water.     

模拟日光下铌酸盐/钛酸纳米片催化降解水中环丙沙星

通过一步水热法制备新型铌酸盐/钛酸纳米片(Niobate/titanate nanoflakes,Nb-TiNFs)复合材料,采用XRD、XPS、FT-IR、SEM、TEM等分析手段对其形貌和结构进行了表征,并探究其在模拟日光下对目标污染物环丙沙星(ciprofloxacin,CIP)的降解性能和内在反应机理。结果表明,Nb-TiNFs可高效、快速光催化降解水中CIP。溶液pH可通过影响静电作用以及羟基自由基的形成而影响CIP的降解,在pH为6时,光催化剂(0.1 g·L⁻¹)对水中CIP(10 mg·L⁻¹)的降解率最大,即180 min内达到96.2%;常规离子Na⁺和Fe³⁺对CIP的降解无明显影响,但Ca²⁺的存在对其产生一定抑制作用。降解过程中,超氧自由基(·O₂⁻)是主要的活性物质,材料内部形成的异质结导致带隙偏移,促进电子转移,抑制电子-空穴对的复合,从而促进了太阳光驱动的光催化活性。Nb-TiNFs合成方法简单、高效、稳定且对环境友好,在光催化去除水中新兴污染物领域具有一定的应用前景。     

Photodegradation of the antimicrobial triclocarban in aqueous systems under ultraviolet radiation

This work aimed to investigate the effectiveness of ultraviolet (UV) radiation on the degradation of the antimicrobial triclocarban (TCC). We investigated the effects of several operational parameters, including solution pH, initial TCC concentration, photocatalyst TiO₂ loading, presence of natural organic matter, and most common anions in surface waters (e.g., bicarbonate, nitrate, and sulfate). The results showed that UV radiation was very effective for TCC photodegradation and that the photolysis followed pseudo-first-order kinetics. The TCC photolysis rate was pH dependent and favored at high pH. A higher TCC photolysis rate was observed by direct photolysis than TiO₂ photocatalysis. The presence of the inorganic ions bicarbonate, nitrate, and sulfate hindered TCC photolysis. Negative effects on TCC photolysis were also observed by the addition of humic acid due to competitive UV absorbance. The main degradation products of TCC were tentatively identified by gas chromatograph with mass spectrometer, and a possible degradation pathway of TCC was also proposed.     

Photodegradation and adsorption of hexazinone in aqueous solutions: removal efficiencies,kinetics, and mechanisms

Environmental Science and Pollution Research - Hexazinone, a globally applied broad-spectrum triazine herbicide, has not been mechanistically investigated previously under advanced oxidation...     

Photodegradation of malachite green under natural sunlight irradiation: kinetic and toxicity of the transformation products

This article describes the photolytic degradation of malachite green (MG), a cationic triphenylmethane dye used worldwide as a fungicide and antiseptic in the aquaculture industry. Photolysis experiments were performed by direct exposure of a solution of MG in water to natural sunlight. The main transformation products (TPs) generated during the process were identified by liquid chromatography time-of-flight mass spectrometry (LC–TOF-MS) and gas chromatography mass spectrometry (GC–MS). The 28 TPs identified with this strategy indicate that MG undergoes three main reactions, N-demethylation, hydroxylation and cleavage of the conjugated structure forming benzophenone derivatives. These processes involve hydroxyl radical attack on the phenyl ring, the N,N-dimethylamine group and the central carbon atom. The Vibrio fischeri acute toxicity test showed that the solution remains toxic after MG has completely disappeared. This toxicity could be assigned, at least in part, to the formation of 4-(dimethylamine)benzophenone, which has an EC_50,30 min of 0.061 mg l⁻¹, and is considered “very toxic to aquatic organisms” by current EU legislation.     

Photodegradation of phosphonates in water

Phosphonates are widely used as chelating agents, e.g., in water cooling systems, in bleaching baths or as scale inhibitors in deflocculation agents. They are considered to be difficult to degrade and produce aminomethylphosphonic acid (AMPA) as a metabolite. As the fate of phosphonates in the environment is not very well known the present work aims at simulating the time dependent photodegradation of four selected phosphonates: nitrilotris-methylenephosphonic acid (NTMP), ethylenediamine-tetra-methylenephosphonic acid (EDTMP), diethylenetriaminepenta-methylenephosphonic acid (DTPMP) and hexaethylenediamine-tetra-methylenephosphonic acid (HDTMP), at concentrations of 1 mg/l (i.e. 3.2 microM NTMP, 2.3 microM EDTMP, 1.7 microM DTPMP and 2.0 microM HDTMP) irradiated by a middle pressure mercury lamp emitting between 190 and 600 nm. The influence of iron under different pH ranges (3, 5-6 and 10) are tested. The degradation of phosphonates is measured by the release of orthophosphates (PO4-P) and aminomethylphosphonic acid (AMPA). This study shows that phosphonates are substances that undergo UV light conversion, which is enhanced in the presence of iron. The half-life without iron is between 15 and 35 min at pH 3, between 10 and 35 min at pH 5-6 and between 50 and 75 min at pH 10. The half-life in the presence of 3.6 microM iron is between 5 and 10 min at pH 3, between 5 and 15 min at pH 5-6 and between 35 and 60 min at pH 10. The individual substances do not significantly influence the reaction rates whereas the presence of iron and the pH have significant effects. The total conversion of phosphonates after 90 min is 75-100% for pH values of 3 and 5-6 and 55-75% for a pH of 10 dependent on the presence of iron. In the environment longer degradation times are to be expected since natural light is weaker by a factor between 125 and 300 in the UVB, a factor between 3 and 8 in the UVA and of the same intensity in the visible range than the light in our study. Although orthophosphates are the major products, AMPA is also shown to be a by-product of the photodegradation of phosphonates that is later converted into orthophosphate.     

Photodegradation of fluoroquinolones in aqueous solution under light conditions relevant to surface waters,toxicity assessment of photoproduct mixtures

Environmental Science and Pollution Research - Photochemical degradation of fluoroquinolones ciprofloxacin, enrofloxacin and norfloxacin in aqueous solution under light conditions relevant to...     

Gadolinium nanoparticle-decorated multiwalled carbon nanotube/titania nanocomposites for degradation of methylene blue in water under simulated solar light

Gadolinium oxide nanoparticles of diameters <5 nm were uniformly decorated on the surfaces of multiwalled carbon nanotubes which were subsequently used as templates to fabricate gadolinium oxide nanoparticle-decorated multiwalled carbon nanotube/titania nanocomposites. The prepared nanocomposites were evaluated for the photocatalytic degradation of methylene blue under simulated solar light irradiation. Higher photocatalytic activity was observed for the gadolinium oxide-decorated multiwalled carbon nanotube-based nanocomposites compared to the neat multiwalled carbon nanotube/titania nanocomposite and commercial titania. This improvement in photocatalytic activity was ascribed to the gadolinium oxide nanoparticles supported at the interface of the carbon nanotubes and titania resulting in efficient electron transfer between the two components of the composite. Total organic carbon (TOC) analysis revealed a higher degree of complete mineralisation of methylene blue (80.0 % TOC removal) which minimise the possible formation of toxic by-products. The photocatalyst could be re-used for five times, reaching a maximum degradation efficiency of 85.9 % after the five cycles. The proposed photocatalytic degradation mechanism is outlined herein.     

Photodegradation of halobenzenes in water ice

Results from the photolysis of o, p-dichlorobenzene, bromobenzene, and p-dibromobenzene in water ice are reported. All phototransformations appeared to be based on dehalogenation, coupling, and rearrangement reactions in ice cavities. No photosolvolysis products, i.e. products from intermolecular reactions between organic and water molecules, were found. Many of the products were very toxic substances of a high environmental risk, such as PCBs. The results support our model, in which secondary, very toxic, pollutants can be formed in ice, snow, and atmospheric ice particles from primary pollutants through the action of solar irradiation. The photoproducts may be released to the environment by ice melting and evaporation.     

Photodegradation of haloacetic acids in water

The global distribution and high stability of some haloacetic acids (HAAs) has prompted concern that they will tend to accumulate in surface waters and pose threats to humans and the ecosystem. It is important to study the degradation pathways of HAAs in aqueous systems to understand their ecotoxicological effects. Previous studies involving thermal degradation reactions show relatively long lifetimes for HAAs in the natural environment. Photolysis and photocatalytic dissociation are potentially efficient routes for the degradation of HAAs such as trichloroacetic acid to hydrochloric acid, carbon dioxide and chloroform, although such processes are poorly understood in surface waters. In our present study, we have used light to degrade the HAAs in the presence of titanium dioxide suspensions. All chloro and bromo HAAs degrade in photocatalysis experiments and the rate of degradation is directly proportional to the number of halogen atoms in the acid molecule. The half-lives of the HAAs from the photodegradation at 15 degrees C in the presence of suspended titanium dioxide photocatalyst are 8, 14, 83 days for the tri-, di- and mono-bromoacetic acids. Tri-, di- and mono-chloroacectic acids have half-lives of 6, 10 and 42 days respectively. The mixed bromochloro and chlorodifluoroacetic acids degrade with half-lives of 18 and 42 days respectively. Our results therefore suggest that the photocatalytic process can provide an additional degradation pathway for the HAAs in natural waters.     

Biodegradation kinetics and toxicity of vegetable oil triacylglycerols under aerobic conditions

The aerobic biodegradation of five triacylglycerols (TAGs), three liquids [triolein (OOO), trilinolein (LLL), and trilinolenin (LnLnLn)] and two solids [tripalmitin (PPP) and tristearin (SSS)] was studied in water. Respirometry tests were designed and conducted to determine the biochemical oxygen demand (BOD) parameters of the compounds. In the case of the solid lipids, the degradation process was limited by their extremely non-polar nature. When added to water, PPP and SSS formed irregular clumps or gumballs, not a fine and uniform suspension required for the lipase activity. After 30 days, appreciable mineralization was not achieved; therefore, first-order biodegradation coefficients could not be determined. The bioavailability of the liquid TAGs was restricted due to the presence of double bonds in the fatty acids (FAs). An autoxidation process occurred in the allylic chains, resulting in the production of hydroperoxides. These compounds polymerized and became non-biodegradable. Nevertheless, the non-oxidized fractions were readily mineralized, and BOD rate constants were estimated by non-linear regression: LLL (k = 0.0061 h⁻¹) and LnLnLn (k = 0.0071 h⁻¹) were degraded more rapidly than OOO (k = 0.0025 h⁻¹). Lipids strongly partitioned to the biomass and, therefore, Microtox^® toxicity was not observed in the water column. However, EC₅₀ values (<15% sample volume) were measured in the solid phase.     

Photodegradation of sulfonamide antibiotics in simulated and natural sunlight: Implications for their environmental fate

The photochemical fate of seven sulfonamides was investigated in matrices representative of natural water bodies under various light sources. Fundamental photolysis parameters such as molar absorption coefficient, quantum yield (QY) and first-order rate constants were determined. The photolysis decay rate was dependent on the protonation state of the molecule, pH of the water sample and dissolved organic matter. Natural organic matter was the most significant factor in the indirect photolysis of sulfonamides. Half-lives were in the range of minutes at 254 nm to days under natural sunlight. Under natural sunlight, all sulfonamides showed higher removal rates in natural waters implying that indirect photolysis is the predominant mechanism.     

Removal of propham from water by using electro-Fenton technology: kinetics and mechanism

The removal of a carbamate herbicide, propham, from aqueous solution has been carried out by the electro-Fenton process. Hydroxyl radical, a strong oxidizing agent, was generated catalytically and used for the oxidation of propham aqueous solutions. The degradation kinetics of propham evidenced a pseudo-first order degradation. The absolute rate constant of second order reaction kinetics between propham and ()OH was determined as (2.2+/-0.10)x10(9)m(-1)s(-1). The mineralization of propham was followed by the organic carbon (TOC) removal. The optimal Fe(3+) concentration was found as 0.5mM at 300mA. The 94% of initial TOC of 0.25mM propham solution was removed in 8h at the optimal conditions by using the cathode area to solution volume ratio of 3.33dm(-1). The maximum mineralization current efficiency values were obtained at 60mA in the presence of 0.5mM Fe(3+). During the electro-Fenton treatment, several degradation products were formed. These intermediates were identified by using high performance liquid chromatography, liquid chromatography-mass spectrometry, gas chromatography-mass spectrometry and ion chromatography analysis. The identified by-products allowed proposing a pathway for the propham mineralization.     

Removal of faecal indicator pathogens from waters and wastewaters by photoelectrocatalytic oxidation on TiO2/Ti films under simulated solar radiation

Purpose

The disinfection efficiency of water and secondary treated wastewater by means of photoelectrocatalytic oxidation (PEC) using reference strains of Enterococcus faecalis and Escherichia coli as faecal indicators was evaluated. Operating parameters such as applied potential (2?C10?V), initial bacterial concentration (10³?C10⁷?CFU/mL), treatment time (up to 90?min) and aqueous matrix (pure water and treated effluent) were assessed concerning their impact on disinfection.

Methods

PEC experiments were carried out using a TiO₂/Ti film anode and a zirconium cathode in the presence of simulated solar radiation. Bacterial inactivation was monitored by the culture method and real-time SYBR green PCR.

Results

A 6.2 log reduction in E. faecalis population was achieved after 15?min of PEC treatment in water at 10?V of applied potential and an initial concentration of 10⁷?CFU/mL; pure photocatalysis (PC) led to only about 4.3 log reduction, whilst negligible inactivation was recorded when the respective electrochemical oxidation process was applied (i.e. without radiation). PEC efficiency was generally improved increasing the applied potential and decreasing initial bacterial concentration. Regarding real wastewater, E. coli was more susceptible than E. faecalis during treatment at a potential of 5?V. Wastewater disinfection was affected by its complex composition and the contained mixed bacterial populations, yielding lower inactivation rates compared to water treatment. Screening the results obtained from both applied techniques (culture method and real-time PCR), there was a discrepancy regarding the recorded time periods of total bacterial inactivation, with qPCR revealing longer periods for complete bacterial reduction.

Conclusions

PEC is superior to PC in terms of E. faecalis inactivation presumably due to a more efficient separation and utilization of the photogenerated charge carriers, and it is mainly affected by the applied potential, initial bacterial concentration and the aqueous matrix.