Synergetic mechanism of defective g-C3N4 activated persulfate on removal of antibiotics and resistant bacteria: ROSs transformation, electron transfer and noncovalent interaction.

The environmental hazards of antibiotics and the resulting antibiotic-resistant bacteria (ARB) have attracted more and more attention. In this study, an efficient synergistic system constructed by vacancy g-C3N4 (CNV0.8) and persulfate (PS) showed excellent oxidation performance to degrade aztreonam (AZT) and Escherichia coli (E. coli) screened from wastewater treatment plant (WWTP), as the typical ß-lactam antibiotic and ARB. As the recombination of electron and hole was effectively inhibited and the interaction with PS was enhanced after the introduction of defects, CNV0.8 showed superior PS activation ability compared with bulk-g-C3N4 (BCN). The synergistic mechanism was systematically analyzed at three levels step by step. Firstly, the conversion of reactive oxygen radicals (ROSs) was studied using electron spin resonance (ESR) and quenching experiments. Then based on the DFT simulation, the enhancement of adsorption energy between catalysts and PS from -8.924 eV (BCN) to -11.190 eV (CNV0.8) and the elongation of O-O bond in PS (from 1.496 Å to 1.505 Å) indicated CNV0.8 had better activation performance for PS compared with BCN. The electron transfer results observed by deformation charge density showed that more electrons could be transferred from the CNV0.8 layer to the surrounding of PS for its own activation in the synergistic mechanism. Thirdly, the noncovalent interaction of PS/CNV0.8 belonged to the region of van der Waals force which was defined by the reduced density gradient (RDG) analysis. The intermediate products in the degradation of AZT were first studied in detail using Fukui function calculations and HPLC-QTOF-MS analysis. Subsequently, the environmental practicability of the oxidation system was investigated through wastewater simulation. This research provides a possible strategy for the effective removal of micropollutants and promotes the development of the sulfate radical-advanced oxidation processes (SR-AOPs) in the field of wastewater treatment.

Effect of carbon nitride synthesized by different modification strategies on the performance of carbon nitride/PVDF photocatalytic composite membranes.

Carbon nitride (CN)/polyvinylidene fluoride (PVDF) photocatalytic composite membrane (PCM) is considered as a promising candidate to improve the anti-fouling characteristic of conventional PVDF membrane and overcome the difficulty encountered during recovery of powder catalyst simultaneously. However, the effects of differently-modified CN on PCM and its mechanism are still unclear. In this study, bulk-CN (BCN), carbon defects CN (CCN), nitrogen defect CN (DCN), mesoporous CN (MCN), and nitrogen-rich CN (NCN) were incorporated into PVDF by phase inversion method. The influence of changes in the physical and chemical properties of CN, including hydrophilic groups, photocatalytic activity, and particle size, on the permeability, anti-fouling characteristic, and photocatalytic self-cleaning activity of CN/PVDF was systematically analyzed. The mechanism of excellent performance of PCM was revealed by experimental test and theoretical calculation. The flux of PCM was significantly improved by increasing the hydrophilic group on modified CN. However, the differences in particle size and interaction between different types of modified CN and PVDF chains endowed the CN/PVDF with different porosity. DCN/PVDF showed high porosity and hydrophilicity, leading to high water flux and rejection rate of 293.6 L (m2 h)-1 and 90.1%, respectively. Compared to pure PVDF, the flux recovery rate of DCN30/PVDF increased by 27.6%, and the irreversible fouling decreased from 36.9% to 9.2%. The modified CN/PVDF showed excellent photocatalytic activity for the removal of cefotaxime (CFX) and E. coli. Owing to the narrow band gap of DCN, large specific surface area, and low photogenerated carrier recombination rate, the CFX removal rate reached 99% in 2 h, and E. coli inactivation achieved 3.7 log within 4 h via DCN30/PVDF.

Origins of selective differential oxidation of ß-lactam antibiotics with different structure in an efficient visible-light driving mesoporous g-C3N4 activated persulfate synergistic mechanism.

In the study, an efficient '1 + 1 > 2' synergistic coupling system driven by visible light consisting of mesoporous g-C3N4 (MCN) and persulfate (PS)was constructed. The free radical transformation, electron transfer and non-covalent interaction between the MCN layer and PS in the system were explored via experiments and DFT calculations. The similarity for the fate of the seven ß-lactam antibiotics with typical structures in the oxidation system was studied systematically in depth. First, the consistencies and differences of the seven antibiotics were summarized from three aspects: three-dimensional structures, electron cloud distributions, and the vulnerable sites. Notably, the selective differential degradation of ß-lactam antibiotics in the MCN/PS system was speculated to be related with the molecular ionization potential (MIP), as a key index to describe the difficulty of oxidation. The distribution relationship between MIP and the oxidation kinetic constant (K) was explored and showed the following trend: a higher MIP indicates a weaker ability to provide electrons, and this leads to a greater resistance to oxidative degradation. In total, four main oxidation pathways of ß-lactam antibiotics were systematically summarized combining HPLC-QTOF-MS and the simplified Fukui function calculation. The toxicity assessment of intermediate products provided by the T.E.S.T software of USEPA also shows a decreasing trend in the oxidation process. In the end, the superior practicability and stability of the MCN/PS system was verified by complex environment simulation and cyclic test. This research clarified the selective differential degradation mechanism of ß-lactam antibiotics and provided a possible idea for the effective removal of refractory organic pollutants in water.

A porous g-C3N4 nanosheets containing nitrogen defects for enhanced photocatalytic removal meropenem: Mechanism, degradation pathway and DFT calculation.

A porous g-C3N4 nanosheet containing nitrogen defects (D-g-C3N4) was synthesized by using a one-step polymerization process in an atmosphere produced via the decomposition of ammonium persulfate. The photocatalytic removal rate of D3-g-C3N4 for meropenem (MER) is 7.45-fold higher than the one of a conventional g-C3N4 sample. The sample mineralization increases from 27% to 52% when the defects are generated. The position of the N defects was inferred via XPS, element analysis and ESR. The introduction of the N2C defects leads to the formation of a midgap state that suppresses the photoexcited carrier recombination. In addition, several environmental factors were simulated during the MER degradation including the initial concentration of MER, of humic acid (HA), and of the common anions and cations. The analysis of the Fukui function combined with LC-Q-TOF-MS predicted the probable degradation path of MER. Its main channel includes the breaking of the ß-lactam ring and of the C-S bond, and the shedding of the carboxyl group and the amino group. Moreover, the toxicity of the intermediates was acquired via USEPA.

Enhanced photocatalytic removal of amoxicillin with Ag/TiO2/mesoporous g-C3N4 under visible light: property and mechanistic studies.

In present study, an efficient ternary Ag/TiO2/mesoporous g-C3N4 (M-g-C3N4) photocatalyst was successfully synthesized through depositing Ag nanoparticles (NPs) on the surface of TiO2/M-g-C3N4 heterojunction. Ag/TiO2/M-g-C3N4 nanocomposite displayed the highest degradation efficiency for amoxicillin (AMX) compared to TiO2/M-g-C3N4 heterojunction, M-g-C3N4, and bulk-g-C3N4 (B-g-C3N4). The removal efficiency of AMX in real situation, surface water (SW), hospital wastewater (HW), and waste water treatment plant (WWTP) also were studied to illustrate the effectiveness of Ag/TiO2/M-g-C3N4 photocatalysts. The vulnerable atoms in AMX structure were revealed through DFT calculation. Additionally, the dominating active groups produced in time of the photocatalytic procedure were determined on account of free radical trapping experiments and ESR spectra. The mechanism of photocatalytic degradation was proposed and verified. The transfer of the electrons and the inhibition of the recombination of photogenerated electron-holes were enhanced effectively under the synergistic effect of the Ag NPs and TiO2. As a consequence, the catalytic activity of the composite was improved under visible light.