Photodegradation Of Triclosan Research Articles

Photodegradation of Triclosan on the Kaolinite Surface: Kinetic, Mechanistic, and Molecular Modeling Approach.

The photodegradation of triclosan (TCS) was investigated on the kaolinite surface. The quantum yield was evaluated, and the photoproducts were identified by HPLC/MS (LC/Q-TOF), showing that the phototransformation is completely different from that reported in aqueous solutions. In particular, the formation of dioxin derivatives was fostered and occurred with a higher efficiency when compared to aqueous solutions. This suggests that TCS has specific interactions with the clay that clearly modifies its photochemical behavior. Moreover, it has also been shown that higher concentrations of TCS, namely, higher than 1.0 μmol g-1 of kaolinite, lead to a significant decrease of the photodegradation rate constant and enhance the formation yield of dimer-type photoproducts. This suggests that the distribution of TCS is clearly not homogeneous at the clay surface and the formation of aggregates is more likely occurring. To get a better insight into this specific interaction, a molecular dynamic modeling of TCS adsorption at the surface of kaolinite was carried out. This clearly shows that when equilibrium is reached, TCS binds to the kaolinite surface by hydrogen bonds involving the phenol function of TCS and the hydroxyl groups of the kaolinite surface. Such behavior confers a particular conformation to the adsorbed TCS that is different from that obtained in water and which could be a key step to partially explain the specific photochemical reactivity in both media. In addition, several TCS molecules appear to interact with each other through the π-stacking (aromatic stacking) process while retaining this hydrogen bond with the kaolinite surface. This is clearly in favor of cluster formation on the clay surface and promotes dimer-type photoproducts.

Day-night alternation and effect of sulfate ions on photodegradation of triclosan in water

Photodegradation is an essential process for the in-stream elimination of triclosan (TCS). There are still some knowledge gaps concerning reaction kinetics, pathways, and transformation products, which are important for understanding the environmental fate of TCS. For example, the effects of ionic strength and day-night alternation on TCS photodegradation have been barely considered in previous studies. To fill these gaps, this study assesses the effect of sulfate ions (SO42−) on the photodegradation of TCS under day-night shifts. Compound-specific carbon and chlorine isotope analysis were applied to characterize different bond-cleavage pathways. Moreover, variations in TCS reaction products distribution were analyzed by using Fourier transform ion cyclotron resonance mass spectrometry. A numerical model was developed to describe the concentration and stable carbon isotopic evolution of TCS under different conditions. Based on the observed experimental data, we show that SO42− can affect the kinetics and product distribution during the photodegradation of TCS. In particular, sulfate ions (100 mM) inhibited the yield of the toxic product, 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD). Furthermore, day-night alternations lead to different predominant bond-cleavage pathways depending on sulfate concentration. By so doing, we propose a realistic framework for the direct photodegradation of TCS in sunlit natural surface waters.

Influence of nitrate/nitrite on the degradation and transformation of triclosan in the UV based disinfection

This study investigated the influence of nitrate/nitrite on the degradation and transformation pathway of triclosan (TCS) in UV, UV/peracetic acid (PAA) and UV/HClO processes. The results indicated that the function of nitrate/nitrite significantly depended on the UV source and wavelength, especially nitrate. Generally, the presence of nitrate decreased the direct photo-degradation of TCS in the UV based disinfection. In the LED-UV and LED-UV/HClO processes, the presence of nitrate improved the radical oxidation, and transformation pathway of TCS was varied accordingly. However, nitrate more played a role of photo-competitor in the UV/PAA process, and the reactive nitrogen species (RNS) was difficult to participant in the degradation of TCS due to low redox potential. Compared to nitrate, the presence of nitrite decreased the degradation of TCS in three different UV based disinfection processes. Under UV irradiation, nitrite primarily acted as an irradiation competitor and radical scavenger. Thus, the indirect photo-degradation of TCS was reduced. Noticeably, nitrate/nitrite were the improtant precersors of nitrogenous products in the UV base disinfection. Many new nitrogenous products were identified. But RNS preferentially reacted with the intermediates by –NO2 addition compared to directly reacted with TCS.

Formation of environmentally persistent free radicals from photodegradation of triclosan by metal oxides/silica suspensions and particles

Metal oxides play an essential role in the photocatalysis of contaminants and substantially increase in the environment by the engineering production. However, whether emerging contaminants will be produced during photocatalysis of contaminants remains unclear. Here, triclosan (TCS) photodegradation in metal oxides/silica suspensions and particles, simulated as the states of metal oxides in water and soil environments, were studied. The photodegradation results confirmed that metal oxides exhibited a double-effect. They promoted TCS photodegradation by generating reactive oxidizing species (ROS) in metal oxides/silica suspensions and inhibited the photodegradation by competing with TCS for irradiation in metal oxides/silica particles. In this study, the critical discovery was the formation of emerging contaminants, environmentally persistent free radicals (EPFRs), and EPFRs yields were promoted by metal oxides (Al2O3, ZnO, TiO2). They were more stable in metal oxides than silica, and the half-lives ranged from 6.7 h to 90.9 d. Although CuO did not increase EPFRs yields compared to silica, the half-lives of EPFRs were also longer. In addition, this study found that EPFRs yields were dependent on the metal oxides concentrations. Our results provided a new insight into the negative environmental impacts of metal oxides and improved our understanding of the formation and fate of EPFRs by metal oxides in soil and aquatic environments.

Selective removal of pharmaceuticals and personal care products from water by titanium incorporated hierarchical diatoms in the presence of natural organic matter

Natural organic matter (NOM), such as humic acids, fulvic acids, and tannic acids, is ubiquitous in water bodies and hinders the photodegradation of pharmaceuticals and personal care products (PPCPs). We prepared titanium incorporated hierarchical diatoms as a novel photocatalyst to selectively remove PPCPs (triclosan, bisphenol A or BPA, and N, N-Diethyl-meta-toluamide or DEET) in the presence of NOM (humic acid). Diatom (Stephanodiscus hantzschii) grown in a titanium(IV) bis(ammonium lactato) dihydroxide solution integrated 7.2% ± 1.4% (mass fraction) of titanium in their cell wall and formed silica-titania frustules. The photodegradation of triclosan, BPA, and DEET by both silica-titania frustules and titania nanopowder (a control photocatalyst) follows pseudo-first-order kinetics. Under ultraviolent light irradiation, the titanium-content-normalized pseudo-first-order removal rate constants of triclosan, BPA, and DEET by silica-titania frustules were 3, 4, and 4-times those by titania nanopowder, respectively, at a humic acid concentration of 10 mg•L−1. Incorporation of titanium did not alter the morphology and hierarchical nano/microstructures of the diatom. The silica-titania frustules were rich in nanopores with a diameter of 20 ± 4 nm (mean ± standard deviation), allowing PPCPs with a small molecular weight (typically < 600 g•mol−1) to pass through while efficiently rejecting NOM with high molecular weights. The silica-titania frustules with hierarchical nano/microstructures served as a prefiltration unit by selectively allowing PPCPs to pass through the nanopores and are therefore promising for photodegradation and environmental remediation applications.

Highly efficient photocatalytic hyperbranched polyethyleneimine/bismuth vanadate membranes for the degradation of triclosan

Worldwide, pharmaceutical and personal care products are increasingly being used and produced and their presence in various water bodies has attracted significant attention amongst the public and scientists. Classified as emerging pollutants, these pharmaceutical and personal care products ultimately accumulate in and contaminate natural water bodies through several pathways. In this study, a photocatalytic membrane consisting of bismuth vanadate nanoparticles and hyperbranched polyethyleneimine, blended in polyethersulphone, was used in the degradation of triclosan. Bismuth vanadate was characterised using X-ray diffraction and Raman spectroscopy. These analyses revealed that there was successful synthesis of the monoclinic phase of bismuth vanadate, as well as a monoclinic-to-tetragonal (M → T) phase transformation upon the addition of hyperbranched polyethyleneimine to the nanoparticles. The modified membranes showed improved water flux and hydrophilicity (contact angles of between 71° and 56°) as compared to that of the bare polyethersulphone (75°) upon the addition of hyperbranched polyethylene amine and bismuth vanadate. The modified membranes were effective in the photodegradation of triclosan (up to 86%), and the generation of chloride ions was observed.

Eco-friendly rapid removal of triclosan from seawater using biomass of a microalgal species: Kinetic and equilibrium studies.

Triclosan is an important emerging pollutant. It has become ubiquitous due to its incomplete elimination in municipal wastewater treatment plants causing serious environmental problems. Biomass from microorganisms as sorbent of pollutants can be an eco-friendly alternative for triclosan removal. In this work, the elimination of triclosan using biomass (dead and living) of the marine microalga Phaeodactylum tricornutum was characterized in cultures exposed to light and in a complex solution (seawater). Maximum removal capacity, isotherms, kinetics, FTIR characterization, pH effect and reuse were evaluated and discussed. Photodegradation of triclosan was also evaluated. Both biomasses showed similar effectiveness; around 100% of pollutant was eliminated when its concentration was 1 mg L-1 in only 3 h using a biomass concentration of 0.4 g L-1. A pseudo-second order model guided the biosorption process. Considering the photodegradation as a first-order process, the whole process (photodegradation + biosorption) was suitably modelled with pseudo-third order and Elovich kinetics. Biosorption increased with the decrease in pH. Temkin isotherm showed the best fit for the experimental data. Both biomasses showed good reuse after five cycles, losing only 7% in efficiency. P. tricornutum biomass is an attractive eco-material for triclosan elimination with low-cost and easy handling than other sorbents.

Effects of inorganic anions on the photolysis of triclosan under UV irradiation.

Triclosan is a widely used antimicrobial agent and may pose health risks to many aquatic organisms. Photodegradation is an important transformation pathway for triclosan, but studies on the effects of inorganic anions on photodegradation of triclosan are limited. In the present study, the single and combined effects of NO3 -, Cl- and HCO3 - on the photolysis of triclosan in aqueous solutions under UV irradiation was evaluated. The results showed that photodegradation of triclosan was inhibited by NO3 - and promoted by HCO3 -, while no significant effect was observed with Cl-. When Cl- was added to NO3 -, no effect was observed, but the addition of Cl- hindered the promotion effect of HCO3 -. The coexistence of NO3 -, Cl- and HCO3 - inhibited the photolysis of triclosan. These results showed the complex effects of inorganic anions in the photolysis of triclosan and provide useful information for an accurate ecological risk assessment of triclosan in natural waters. It will also help to develop appropriate treatment ways of triclosan.

Degradation of triclosan in the presence of p-aminobenzoic acid under simulated sunlight irradiation.

This study aimed to investigate the degradation of triclosan (TCS) in the presence of p-aminobenzoic acid (PABA) under simulated sunlight irradiation (λ≥290nm). The effect of PABA concentration, pH, dissolved organic matter (DOM), and DOM-hydrolytic Fe(III) species complexes on the photodegradation of TCS in the presence of PABA (TCS-PABA) was also studied. The photolysis of TCS-PABA obeyed pseudo-first-order kinetics well, and the degradation of TCS-PABA enhanced with increasing solution pH (from 3.0 to 11.0). The presence of PABA inhibited the degradation of TCS-PABA, and the weakest inhibitory effect was observed while the concentration of PABA was 5mgL-1. The addition of DOM (Suwannee River fulvic acid standard I [SRFA], Suwannee River HA standard II [SRHA], and Suwannee River natural organic matter [SRNOM]) showed different inhibition effects on TCS-PABA degradation. However, higher Fe(III) concentration at the DOM concentration of 5mgL-1 could favor the formation of DOM-hydrolytic Fe(III) species complexes, further accelerating the degradation of TCS-PABA. In comparison with deionized water (DI water), TCS-PABA could be better photodegraded in river water nearby the effluent of a wastewater treatment plant. This study provides useful information for understanding the natural behavior of TCS in the presence of other organic contaminants.

Interactions between the antimicrobial agent triclosan and the bloom-forming cyanobacteria Microcystis aeruginosa

Cyanobacteria can co-exist in eutrophic waters with chemicals or other substances derived from personal care products discharged in wastewater. In this work, we investigate the interactions between the antimicrobial agent triclosan (TCS) and the bloom-forming cyanobacteria Microcystis aeruginosa. M. aeruginosa was very sensitive to TCS with the 96h lowest observed effect concentration of 1.0 and 10μg/L for inhibition of growth and photosynthetic activity, respectively. Exposure to TCS at environmentally relevant levels (0.1–2.0μg/L) also affected the activities of superoxide dismutase (SOD) and the generation of reduced glutathione (GSH), while microcystin production was not affected. Transmission electron microscope (TEM) examination showed the destruction of M. aeruginosa cell ultrastructure during TCS exposure. TCS however, can be biotransformed by M. aeruginosa with methylation as a major biotransformation pathway. Furthermore, the presence of M. aeruginosa in solution promoted the photodegradation of TCS. Overall, our results demonstrate that M. aeruginosa plays an important role in the dissipation of TCS in aquatic environments but high residual TCS can exert toxic effects on M. aeruginosa.

Determination of 2,8-dichlorodibenzo-p-dioxin in toothpaste and mouthwash consumer products using GC-MS.

A gas chromatography-mass spectrometry (GC-MS) method was developed for the identification and quantification of 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) in toothpaste and mouthwash consumer products. Liquid-liquid extraction and solid-phase extraction were used in the sample preparation. The limit of detection was 0.96 ng/g in toothpaste and 0.83 ng/g in mouthwash. The accuracy represented by relative errors was less than 12.5%. The intra-day and inter-day precision, which are represented by the relative standard deviation values, were within 11.2 and 10.6%, respectively. The method was successfully applied to analyze 2,8-DCDD in toothpaste and mouthwash products, as well as that from the photo-degradation of triclosan spiked in both sample matrix.

Faster photodegradation rate and higher dioxin yield of triclosan induced by cationic surfactant CTAB

Triclosan has received extensive attention as it has been frequently detected in the aquatic environment. Photolysis was found to be a major pathway governing the fate of triclosan in the aquatic environment. However, the effects of surfactants that usually coexist with triclosan, on the photodegradation of triclosan, are largely unknown. In this study, the effects of selected surfactants on the photodegradation of triclosan were investigated experimentally. The results show that anionic sodium dodecyl benzene sulfonate, sodium dodecyl sulfate and neutral polyoxyethylene (20) sorbitan monooleate inhibit the photolysis of triclosan, whereas cationic cetyltrimethylammonium bromide (CTAB) significantly accelerates the photodegradation rate of triclosan. The interactions between the hydrophilic group of CTAB and anionic triclosan lead to the apparent decrease of pKa of triclosan from 8.4 to 6.1, which increase the fraction of anionic triclosan from 4% to 89% in neutral solution. A red shift in the UV–VIS absorption spectrum is exhibited, thus leading to the increased photodegradation rate of triclosan. The accelerations caused by CTAB were observed under xenon lamp and Hg lamp irradiances, as well as under natural sunlight. Effect of CTAB demonstrated pH dependence with significantly enhancement under pH 5∼9 and inhibition at pH=3. The presence of CTAB also increased the yield of 2,8-dichlorodibenzo-p-dioxin from the photolysis of triclosan about 7 times at pH=7.

Photolytic degradation of triclosan in freshwater and seawater

A 12-days photolysis experiment of triclosan, a widely used bactericide, was performed in freshwater and seawater under a low intensity artificial white light source. Photodegradation of triclosan was observed in both the freshwater and seawater samples. Assuming a first-order reaction, half-lives of triclosan in the freshwater and seawater were approximately 8 and 4 days, respectively. 2,8-Dichlorodibenzo- p-dioxin (DCDD) was detected in both the freshwater and seawater samples after 3 days of irradiation. The photodegradation of triclosan and the production of DCDD suggest that triclosan could be less stable and DCDD might be more stable in seawater than freshwater. As a result, DCDD produced from triclosan survives for a longer time in seawater.

Monitoring the photochemical degradation of triclosan in wastewater by UV light and sunlight using solid-phase microextraction

Photo solid-phase microextraction (photo-SPME) is applied for the first time to study the photochemical behavior of an emerging pollutant, triclosan, in real contaminated wastewater samples using a solar simulator. In this study, water samples are extracted by SPME and then, the fiber coating is irradiated for a selected time. This on-fiber procedure, so-called photo-SPME, followed by gas chromatography–mass spectrometry makes it possible to study photodegradation kinetics and the generation of byproducts. Several photoproducts were identified in the real samples including the 2,8-dichlorodibenzo- p-dioxin, dichlorophenols and a compound tentatively identified as other DCDD congener or a dichlorohydroxydibenzofuran. Accordingly, it was possible to postulate main photodegradation mechanisms. Photo-SPME demonstrated slower kinetics in wastewater than in spiked ultrapure water probably due to the presence of dissolved organic matter. This technique was extensively compared with conventional aqueous photodegradation showing high similarity. The influence of pH on the triclosan photolysis and on the triclosan-dioxin conversion was also investigated in wastewater. Photodegradation of triclosan and formation of 2,8-DCDD occurred independently of sample pH. This study represents an advance in the use of photo-SPME to understand the photochemical fate of environmental organic pollutants and demonstrates its clear advantages with real samples.

Confirmation of the formation of dichlorodibenzo-p-dioxin in the photodegradation of triclosan by photo-SPME

Photodegradation is a possible way to eliminate organic pollutants from the environment but, at the same time, can be a source of toxic byproducts. The photochemical conversion of triclosan, a common pollutant in continental waters, into dichlorodibenzo-p-dioxin (DCDD) has been confirmed in our preliminary experiments employing photo-SPME (photo-solid-phase microextraction) using 18-W UV irradiation at 254-nm wavelength. Under these conditions, triclosan is rapidly photodegraded (70% of triclosan was degraded in 2 min); the most important novel aspect of this work is the conversion of triclosan to DCDD directly on the polydimethylsiloxane coating of the SPME fiber. Moreover, this conversion is also confirmed in non-buffered aqueous photodegradation experiments using SPME as the extraction technique. In all the experiments of this study, analysis was carried out by gas chromatography-electronic impact mass spectrometry (GC-EI/MS).