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Photochemical Reaction Mechanism of Environmental Pollutants

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Photochemistry".

Deadline for manuscript submissions: closed (15 February 2023) | Viewed by 11062

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Special Issue Editor

Prof. Dr. Wanhong Ma

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Guest Editor

Key Laboratory of Photochemistry Institute of Chemistry, Chinese Academy of Sciences, Beijing 100084, China
Interests: photodegradation; photocatalysis; selective oxidation
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The degradation and control of persistent, highly toxic organic pollutants, especially those accumulated in different waters, are pivotal to global ecological security. Advanced oxidation technologies (AOTs) are playing a more and more important role in both source treatment (high concentration) and extensive watershed treatment (low concentration) of these organic pollutants, because they aim to convert organic pollutants into CO₂, H₂O and mineralized inorganic ions. However, for many years, AOTs have remained difficult to achieve application on a large scale; the main obstacles are high energy consumption and high cost. As a result, photocatalytic degradation and photooxidative degradation by utilizing solar energy are the only feasible ways to thoroughly solve the bottleneck of AOT. To this end, researchers focus on designing more efficient photocatalysts through varying composition, structure, morphology and so on. Among them, the reaction mechanism between various photocatalysts (or sensitizers), reactive sites or reactive species and pollutant molecules is of vital importance during the process of photo-induced degradations. In addition, there are many chemical reactions of environmental pollutants in the natural world, including soil, water, atmosphere and even plant media forced by solar irradiation. Therefore, the in-depth understanding of mechanisms of environmental pollutants transformation, degradation and mineralization under light irradiation is critical to the design and development of high-efficiency and high-performance photo-induced degradation systems.

The mechanism of photochemical reaction for current AOTs eliminating organic pollutants in water mainly includes the following aspects: (1) the processes of organic pollutant molecules being oxidized, reduced, hydrolyzed and finally broken into CO₂, H₂O and inorganic ion X^-; (2) photo-induced catalyst (or sensitizer) activates molecular oxygen O₂, O₃, H₂O₂, S₂O₈^2-, Cl₂, ClO₂ etc. and the influence factors of the solution; (3) generation pathway and characterization of various species of reactive oxygen species and photo-generated holes/electron pairs in homogeneous or heterogeneous systems; (4) analysis and characterization of intermediate products in degradation of organic pollutants; (5) relaxation, lifetime and reaction path of photo-excited states in homogeneous and heterogeneous system. The mechanisms of photocatalysis, sensitization and direct photolysis are very complex and involve multidisciplinary intersections, especially the spectral characterization of steady and transient states. However, this Special Issue only encompasses five aforementioned aspects and mainly focuses on the studies of homogeneous and heterogeneous photodegradation mechanisms of environmental organic pollutants.

You may choose our Joint Special Issue in Photochem.

Prof. Dr. Wanhong Ma
Guest Editor

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Keywords

Photo-degradation of organic pollutants in waters
Homogeneous photo-catalytized or sensitized mechanisms
Hetergenous photo-catalytized mechanisms
Intermediate fragments and intermediate process
Reactive oxygen species (ROS)

Published Papers (5 papers)

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Research

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15 pages, 2268 KiB

Open AccessArticle

Computational Study of Photodegradation Process and Conversion Products of the Antidepressant Citalopram in Water

by Yifan Shen, Se Wang, Ying Lu, Kai Chen, Li Luo and Ce Hao

Molecules 2023, 28(12), 4620; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28124620 - 07 Jun 2023

Cited by 1 | Viewed by 996

Abstract

Citalopram (CIT) is a commonly prescribed medication for depression. However, the photodegradation mechanism of CIT has not yet been fully analyzed. Therefore, the photodegradation process of CIT in water is studied by density functional theory and time-dependent density functional theory. The calculated results show that during the indirect photodegradation process, the indirect photodegradation of CIT with ·OH occurs via OH-addition and F-substitution. The minimum activation energy of C10 site was 0.4 kcal/mol. All OH-addition and F-substitution reactions are exothermic. The reaction of ¹O₂ with CIT includes the substitution of ¹O₂ for F and an addition reaction at the C14 site. The E_a value of this process is 1.7 kcal/mol, which is the lowest activation energy required for the reaction of ¹O₂ with CIT. C–C/C–N/C–F cleavage is involved in the direct photodegradation process. In the direct photodegradation of CIT, the activation energy of the C7-C16 cleavage reaction was the lowest, which was 12.5 kcal/mol. Analysis of the E_a values found that OH-addition and F-substitution, the substitution of ¹O₂ for F and addition at the C14 site, as well as the cleavage reactions of C6–F/C7–C16/C17–C18/C18–N/C19–N/C20–N are the main pathways of photodegradation of CIT. Full article

(This article belongs to the Special Issue Photochemical Reaction Mechanism of Environmental Pollutants)

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18 pages, 2688 KiB

Open AccessArticle

Removal of Mn(II) from Acidic Wastewaters Using Graphene Oxide–ZnO Nanocomposites

by Eduardo Leiva, Camila Tapia and Carolina Rodríguez

Molecules 2021, 26(9), 2713; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26092713 - 05 May 2021

Cited by 11 | Viewed by 2080

Abstract

Pollution due to acidic and metal-enriched waters affects the quality of surface and groundwater resources, limiting their uses for various purposes. Particularly, manganese pollution has attracted attention due to its impact on human health and its negative effects on ecosystems. Applications of nanomaterials such as graphene oxide (GO) have emerged as potential candidates for removing complex contaminants. In this study, we present the preliminary results of the removal of Mn(II) ions from acidic waters by using GO functionalized with zinc oxide nanoparticles (ZnO). Batch adsorption experiments were performed under two different acidity conditions (pH1 = 5.0 and pH2 = 4.0), in order to evaluate the impact of acid pH on the adsorption capacity. We observed that the adsorption of Mn(II) was independent of the pH_PZC value of the nanoadsorbents. The qmax with GO/ZnO nanocomposites was 5.6 mg/g (34.1% removal) at pH = 5.0, while with more acidic conditions (pH = 4.0) it reached 12.6 mg/g (61.2% removal). In turn, the results show that GO/ZnO nanocomposites were more efficient to remove Mn(II) compared with non-functionalized GO under the pH2 condition (pH2 = 4.0). Both Langmuir and Freundlich models fit well with the adsorption process, suggesting that both mechanisms are involved in the removal of Mn(II) with GO and GO/ZnO nanocomposites. Furthermore, adsorption isotherms were efficiently modeled with the pseudo-second-order kinetic model. These results indicate that the removal of Mn(II) by GO/ZnO is strongly influenced by the pH of the solution, and the decoration with ZnO significantly increases the adsorption capacity of Mn(II) ions. These findings can provide valuable information for optimizing the design and configuration of wastewater treatment technologies based on GO nanomaterials for the removal of Mn(II) from natural and industrial waters. Full article

(This article belongs to the Special Issue Photochemical Reaction Mechanism of Environmental Pollutants)

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20 pages, 4526 KiB

Open AccessArticle

LDPE Transformation by Exposure to Sequential Low-Pressure Plasma and TiO₂/UV Photocatalysis

by Luis D. Gómez-Méndez, Luis C. Jiménez-Borrego, Alejandro Pérez-Flórez, Raúl A. Poutou-Piñales, Aura M. Pedroza-Rodríguez, Juan C. Salcedo-Reyes, Andrés Vargas and Johan M. Bogoya

Molecules 2021, 26(9), 2513; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26092513 - 26 Apr 2021

Cited by 3 | Viewed by 2516

Abstract

Low-density polyethylene (LDPE) sheets (3.0 ± 0.1 cm) received sequential treatment, first by the action of direct-current low-pressure plasma (DC-LPP) with a 100% oxygen partial pressure, 3.0 × 10⁻² mbar pressure, 600 V DC tension, 5.6 cm distance, 6-min treatment. Then, sheets were submitted to TiO₂ photocatalysis at UV radiation at 254 nm (TiO₂/UV) with a pH value of 4.5 ± 0.2 and a TiO₂ concentration of 1 gL⁻¹. We achieved a complementary effect on the transformation of LDPE films. With the first treatment, ablation was generated, which increased hydrophilicity. With the second treatment, the cavities appeared. The changes in the LDPE sheets’ hydrophobicity were measured using the static contact angle (SCA) technique. The photocatalytic degradation curve at 400 h revealed that the DC-LPP photocatalysis sequential process decreased SCA by 82°. This was achieved by the incorporation of polar groups, which increased hydrophilicity, roughness, and rigidity by 12 and 38%, respectively. These sequential processes could be employed for LDPE and other material biodegradation pretreatment. Full article

(This article belongs to the Special Issue Photochemical Reaction Mechanism of Environmental Pollutants)

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17 pages, 2706 KiB

Open AccessArticle

Phototransformation of Three Psychoactive Drugs in Presence of Sedimental Water Extractable Organic Matter

by Cristina Jiménez-Holgado, Vasilios Sakkas and Claire Richard

Molecules 2021, 26(9), 2466; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26092466 - 23 Apr 2021

Cited by 7 | Viewed by 1757

Abstract

Psychoactive drugs are classified as contaminants of emerging concern but there is limited information on their fate in surface waters. Here, we studied the photodegradation of three psychoactive drugs (sertraline, clozapine, and citalopram) in the presence of organic matter (WEOM) extracted under mild conditions from sediment of Lake Pamvotis, Greece. Spectral characterization of WEOM confirmed its humic-like nature. Preliminary experiments using chemical probes showed that WEOM was able to produce oxidant triplet excited state (³WEOM*), singlet oxygen (¹O₂), and hydroxyl radicals under irradiation with simulated solar light. Then, WEOM at 5 mgC L⁻¹ was irradiated in the presence of the three drugs. It enhanced their phototransformation by a factor of 2, 4.2, and 16 for sertraline, clozapine, and citalopram, respectively. The drastic inhibiting effect of 2-propanol (5 × 10⁻³ M) on the reactions demonstrated that hydroxyl radical was the key intermediate responsible for drugs photodegradation. A series of photoproducts were identified by ultra-high performance liquid chromatography (UHPLC) coupled to high resolution mass spectrometry (HR-MS). The photodegradation of the three drugs proceeded through several pathways, in particular oxidations of the rings with or without O atom inclusion, N elimination, and substitution of the halogen by OH. The formation of halogenated aromatics was observed for sertraline. To conclude, sedimental natural organic matter can significantly phototransform the studied antidepressant drugs and these reactions need to be more investigated. Finally, ecotoxicity was estimated for the three target analytes and their photoproducts, using the Ecological Structure Activity Relationships (ECOSAR) computer program. Full article

(This article belongs to the Special Issue Photochemical Reaction Mechanism of Environmental Pollutants)

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Review

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27 pages, 3712 KiB

Open AccessReview

Applications of Heterogeneous Photocatalysis to the Degradation of Oxytetracycline in Water: A Review

by Renato Pelosato, Isabella Bolognino, Francesca Fontana and Isabella Natali Sora

Molecules 2022, 27(9), 2743; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules27092743 - 24 Apr 2022

Cited by 27 | Viewed by 2962

Abstract

Photocatalytic processes are being studied extensively as potential advanced wastewater treatments for the removal of pharmaceuticals, pesticides and other recalcitrant micropollutants from the effluents of conventional wastewater treatment plants (WWTPs). Oxytetracycline (OTC) is a widespread antibiotic which is frequently detected in surface water bodies as a recalcitrant and persistent micropollutant. This review provides an update on advances in heterogeneous photocatalysis for the degradation of OTC in water under UV light, sunlight and visible-light irradiation. Photocatalysts based on pure semiconducting oxides are rarely used, due to the problem of rapid recombination of electron–hole pairs. To overcome this issue, a good strategy could be the coupling of two different semiconducting compounds with different conduction and valence bands. Several methods are described to enhance the performances of catalysts, such as doping of the oxide with metal and/or non-metal elements, surface functionalization, composites and nano-heterojunction. Furthermore, a discussion on non-oxidic photocatalysts is briefly provided, focusing on the application of graphene-based nanocomposites for the effective treatment of OTC. Full article

(This article belongs to the Special Issue Photochemical Reaction Mechanism of Environmental Pollutants)

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