Defect-modified reduced graphitic carbon nitride (RCN) enhanced oxidation performance for photocatalytic degradation of diclofenac.

Hydroxyl radicals (OH) have robust non-selective oxidizing properties to effectively degrade organic pollutants. However, graphitic carbon nitride (g-C3N4) is restricted to directly generate OH due to its intrinsic valence band. In this study, we report a facile environmental-friendly self-modification strategy to synthesize reduced graphitic carbon nitride (RCN), with nitrogen vacancies and CN functional groups. The incorporation of CN enabled to downshift the valence band level, which endowed RCN with the capacity to directly generate OH via h+. Experimental and instrumental analyses revealed the critical roles of nitrogen vacancies and CN groups in the modification of the RCN band structure to improve its visible light absorption and oxidizing capacity. With these superior properties, the RCN was significantly enhanced for the photocatalytic degradation of DCF under visible light irradiation. The self-modification strategy articulated in this study has strong potential for the creation of customized g-C3N4 band structures with enhanced oxidation performance.

A novel concept for the biodegradation mechanism of dianionic catechol with homoprotocatechuate 2,3-dioxygenase: A non-proton-assisted process.

The theory of "proton-assisted process" can well explain the catalytic mechanism of homoprotocatechuate 2,3-dioxygenase (2,3-HPCD) with a monoanionic substrate (homoprotocatechuate, HPCA). Here a "non-proton-assisted process" is presented to interpret catalytic mechanism of 2,3-HPCD with a dianionic substrate (4-nitrocatechol, 4NC). The ONIOM calculation is performed to investigate the reaction pathway of a wild-type 2,3-HPCD with 4NC (H200H-4NC system). The catalytic reaction is comprised of four steps: (1) A dioxygen attacks the aromatic ring to produce an alkylperoxo species. (2) O-O bond cleavage and the formation of an epoxide species occur. (3) A seven-membered O-heterocyclic compound is generated by the extinction of the epoxy structure. (4) The seven-membered ring undergoes ring opening to form the final product (C2-C3 cleavage product). The effective free energy barrier of the catalytic reaction of the H200H-4NC system is 26.2 kcal mol-1, which is much higher than that of the H200H-HPCA system. Furthermore, two calculated electronic configurations (Fe(III)-O2•- and Fe(III)-SQ•) have a high similarity to previously detected ones, which demonstrates that the Asn200 variant (H200N-4NC variant system) employs a C4 (para-carbon) pathway to produce a C4-C5 cleavage product. Our findings provide an in-depth understanding of the catalytic mechanisms of dianionic catechol and its derivatives.

Fluorescence regional integration combined with parallel factor analysis to quantify fluorescencent spectra for dissolved organic matter released from manure biochars.

Dissolved Organic Matter (DOM) in biochars is important to carbon dynamics and contaminant transport in soils. Fluorescence excitation-emission matrices (EEMs) have been widely used to characterize dissolved organic matter (DOM). In this study, fluorescence regional integration (FRI) and parallel factor analysis (PARAFAC) applied to EEM allows good quantitative assessment of the composition of DOM derived from manure biochars. Manure biochars were produced using four types of manure, chicken, pig, cow, and sheep manure under various pyrolysis temperatures (300-600 °C) and holding times (0-120 min). The results from the determination of dissolved organic carbon (DOC), SUVA254, and humification index (HIX) reflected that high pyrolysis temperature and long holding time led to a significant decrease in DOM quantity, aromaticity and humification. The FRI result showed that the pyrolysis process of DOM released from manure biochars included three changes, aromatic protein-like substance and microbial by-product-like substance generation (300-600 °C), fulvic-acid like substance decomposition (300-500 °C) and humic acid-like substance decomposition (600 °C). The PARAFAC modeling result showed that the pyrolysis process of DOM released from manure biochars contained two changes: three high molecular-weight humic-like substances decomposition and a low molecular-weight humic-like substance generation. The effect of the holding time on biochar-DOM is more significant at higher pyrolysis temperatures than lower pyrolysis temperatures. The correlation analysis result revealed that the generation of aromatic proteins, microbial by-products and fulvic acid came from the decomposition of humic-like substances including marine humic-like, UVA humic-like, and UVC + UVA humic-like substances. The results obtained in this study would be beneficial to guide the rational production and application of manure biochars.

Experimental and theoretical investigation on photodegradation mechanisms of naproxen and its photoproducts.

The photochemical degradation of the pharmaceuticals and personal care products (PPCPs) has attracted increasing attention. In this study, a deep inspection of the photolysis mechanisms of naproxen and its photoproducts has been performed by employing experimental and theoretical methods. Contributions of different reactive oxygen species (ROS, such as OH, 1O2, and O2-) in the photolysis reaction also have been clarified. Based on the detected intermediates and DFT calculations, several photodegradation pathways of naproxen and its photoproducts have been proved. Furthermore, the deprotonated form of naproxen has been confirmed to be more reactive than the protonated one, and the lowest triplet state of naproxen is the reactive state. The decarboxylation mechanism of naproxen has been fully discussed. Meanwhile, the free energy barriers of OH-induced photolysis reactions (ΔG‡eff(1a) = 7.6 kcal mol-1, ΔG‡eff(4a) = 7.0 kcal mol-1) are much lower than the free energy barriers induced by O2- and 1O2. It proves that OH is the most favourable one among the three ROS. The similar inhabition rates and free energy barriers of reactions induced by O2- and 1O2, respectively, have demonstrated that O2- and 1O2 equally contribute to the degradation. Additionally, the computational results are coincident with the observed experimental findings. Hence, this work has verified a part of naproxen photodegradation mechanism under UV irradiation and brought about a rational way to investigate contributions of different ROS in the complex photochemical system of PPCPs.