Photocatalytic efficiency toward volatile organic compounds (VOCs) decomposition has crucially relied on the nature of their stereochemical structures, in which the complicated decomposition mechanism has not been unveiled. As typical cases of VOCs pollutants, m-, p-, and o-xylene isomers share the identical molecular formula with discrepant methyl positions at the benzene ring. The essential contribution of the methyl position to the decomposition mechanism of xylene isomers, especially the rate-determining step for benzene ring-opening, is unraveled in this work. It is identified that the decomposition rate of xylene isomers on the SnO2 catalyst is decreased in the order of o-xylene > m-xylene ≈ p-xylene. The durability of SnO2 photocatalyst is also accomplished for a superior o-xylene decomposition performance. By combining the experimental and theoretical investigation, it is manifested that the regulation of methyl positions in the ortho-sites is an appealing route for reducing the ring-opening energy barriers and guiding the complete mineralization of the hazardous xylene. This work could provide insights into unraveling the unique role of the stereochemical structure of xylene on ring-opening barriers for efficient and stable VOC decomposition.
The massive use of antibiotics has led to their omnipresence in aquatic environments, and the photodegradation was found to be the dominant transformation process for antibiotics in the natural river system. Herein, we investigated the photodegradation kinetics of 77 antibiotics in 7 classes in water under simulated sunlight. Using the quantum chemical descriptors predicted by the density functional theory calculation, the quantitative structure-activity relationship (QSAR) models were established to explore the main chemical descriptors determining the photodegradation of antibiotics. The results showed that the photodegradation kinetics of antibiotics conformed to the pseudo-first order kinetic model. The photodegradation rate constants of different antibiotics varied 4 orders of magnitude, and the photodegradation rate constants of quinolones were significantly higher than those for other classes of antibiotics due to the F atoms in their molecular structures. The developed QSAR models revealed that the energy gap (Egap) between ELUMO and EHOMO was the main chemical descriptor determining the photodegradation of antibiotics, and it was negatively correlated with lgk. In addition, the number of F atom was also included in the QSAR models due to the great contribution of F atom to the direct photolysis of quinolones. This study ordered the photodegradation rate constants of 77 antibiotics, and revealed the major chemical descriptors determining the photodegradation of antibiotics. The results provide the basic information for the photolysis of antibiotics, which is significant for predicting the environmental behaviors and evaluating the ecological risks of antibiotics in aquatic environments.
Though metal resistance genes (MRGs) are of global concern in aquatic ecosystems, the underlying factors responsible for MRGs dissemination, especially in urban rivers on the vulnerable Qinghai-Tibet Plateau, are rarely known. Here, we collected 64 samples including water and sediments during the wet and dry seasons and effluents from six wastewater treatment plants (WWTPs) during the dry season and measured 50 metal(loid)s, 60 bacterial phyla, and 259 MRGs. We observed the distinct difference of metal(loid)s, bacterial communities, and MRGs between water and sediments and the great seasonal changes in metal(loid)s and bacterial communities instead of MRGs. Thirty-one metal(loid)s were detectable in the water, with relatively low concentrations and no significant effects on the planktonic bacterial communities and MRGs. Interestingly, the WWTPs effluent partially promoted the prevalence of dissolved metal(loid)s, bacterial communities, and MRGs along the river. In the sediments, the average concentrations of 17 metal(loid)s exceeded their corresponding background levels in this region and strongly influenced the bacterial communities and the MRGs. Sedimentary Hg and Cd, mainly sourced from the intensive animal husbandry, were the major pollutants causing ecological risks and largely shaped their corresponding resistomes. Moreover, we found that bacterial communities predominantly determined the variation of MRGs in both water and sediments. Metagenome-assembled genomes further reveals the widespread co-occurrence of MRGs and antibiotic resistance genes (ARGs) in MRG hosts. Our study highlighted the concern of effluents discharged from WWTPs and emphasized the importance of controlling the anthropogenic inputs of sedimentary metal(loid)s in the plateau river ecosystems.
Vacuum triodes have been scaled down to the microscale on a chip by microfabrication technologies to be vacuum transistors. Most of the reported devices are based on field electron emission, which suffer from the problems of unstable electron emission, poor uniformity, and high requirement for operating vacuum. Here, to overcome these problems, a vacuum transistor based on Field-Assisted thermionic emission from individual carbon nanotubes is proposed and fabricated using microfabrication technologies. The carbon nanotube vacuum transistor exhibits an ON/OFF current ratio as high as 104 and a subthreshold slope of 4 V·dec−1. The gate controllability is found to be strongly dependent on the distance between the collector electrodes and electron emitter, and a device with the distance of 1.5 μm shows a better gate controllability than that with the distance of 0.5 μm. Benefiting from Field-Assisted thermionic emission mechanism, electric field required in our devices is about one order of magnitude smaller than that in the devices based on field electron emission, and the surface of the emitters shows much less gas molecule absorption than cold field emitters. These are expected to be helpful for improving the stability and uniformity of the devices.
In this study, vanadium trioxide (V2O3) was adopted to activate PMS via a Fenton-like reaction to degrade metronidazole (MNZ). The V2O3-PMS system can almost completely degrade MNZ at 30 min with 42.4% TOC removal. Comparative tests reveal that V2O3 stands out among a variety of heterogeneous catalysts, including metallic oxides and carbon materials. Sulfate radicals (SO4•−) and hydroxyl radicals (•OH) derived from PMS decomposition are major reactive oxygen species, based on quenching tests, electron spin resonance (ESR) analysis, the steady-state concentrations of radicals ([SO4•−]ss = 5.1 × 10-13 M and [•OH]ss = 4.0 × 10-14 M), and kinetics model. The process of stepwise electron transfer from vanadium species to PMS to produce reactive radicals was proved by small-molecule simulation experiments and pickling experiments of vanadium oxides. Possible pathways of MNZ degradation were proposed based on the results of LC-MS and Fukui function, including two stages of the hydroxylation and bond cleavage of nitro and the subsequent ring-opening. This study reveals the high reusability and practicability of the V2O3-PMS system over a relatively wide pH range, which puts forward a new vision on V2O3 induced Fenton-like reactions and a new reference method for the removal of medical organic contaminants in water.
Design of high-efficiency visible light photocatalysts is critical in the degradation of antibiotic pollutants in water, a key step towards environmental remediation. In the present study, Mo-doped BiOBr nanocomposites are prepared hydrothermally at different feed ratios, and display remarkable visible light photocatalytic activity towards the degradation of sulfanilamide, a common antibacterial drug. Among the series, the sample with 2% Mo dopants exhibits the best photocatalytic activity, with a performance 2.3 times better that of undoped BiOBr. This is attributed to Mo doping that narrows the band gap of BiOBr and enhances absorption in the visible region. Additional contributions arise from the unique materials morphology, where the highly exposed (102) crystal planes enrich the photocatalytic active sites, and facilitate the adsorption of sulfanilamide molecules and their eventual attack by free radicals. The reaction mechanism and pathways are then unraveled based on theoretical calculations of the Fukui index and liquid chromatography/mass spectrometry measurements of the reaction intermediates and products. Results from this study indicate that deliberate structural engineering based on heteroatom doping and morphological control may serve as an effective strategy in the design of highly active photocatalysts towards antibiotic degradation.
Development of low-cost, high-performance photocatalysts for the effective degradation of antibiotics in wastewater is critical for environmental remediation. In this work, titanium dioxide/zirconium dioxide/graphitic carbon nitride (TiO2/ZrO2/g-C3N4) ternary composites are fabricated via a facile hydrothermal procedure, and photocatalytically active towards the degradation of berberine hydrochloride under visible light illumination. The performance is found to increase with the Ti:Zr atomic ratio in the nanocomposites, and obviously enhanced in comparison to that of the binary TiO2/g-C3N4 counterpart, due to the formation of type I/II heterojunctions that help separate the photogenerated electron-hole pairs and produce superoxide and hydroxy radicals. The mechanistic pathways are unraveled by a deliberate integration of liquid chromatography-mass spectrometry measurements with theoretical calculations of the condensed Fukui index. Furthermore, the ecotoxicity of the reaction intermediates is examined by utilizing the Toxicity Estimation Software Tool (TEST) and quantitative structure activity relationship calculations (QSAR).
Volatile methyl siloxanes (VMS) are ubiquitous in indoor environments due to their use in personal care products. This paper builds on previous work identifying sources of VMS by synthesizing time-resolved proton-transfer reaction time-of-flight mass spectrometer VMS concentration measurements from four multiweek indoor air campaigns to elucidate emission sources and removal processes. Temporal patterns of VMS emissions display both continuous and episodic behavior, with the relative importance varying among species. We find that the cyclic siloxane D5 is consistently the most abundant VMS species, mainly attributable to personal care product use. Two other cyclic siloxanes, D3 and D4, are emitted from oven and personal care product use, with continuous sources also apparent. Two linear siloxanes, L4 and L5, are also emitted from personal care product use, with apparent additional continuous sources. We report measurements for three other organosilicon compounds found in personal care products. The primary air removal pathway of the species examined in this paper is ventilation to the outdoors, which has implications for atmospheric chemistry. The net removal rate is slower for linear siloxanes, which persist for days indoors after episodic release events. This work highlights the diversity in sources of organosilicon species and their persistence indoors.
Rapid atmospheric warming since the mid-twentieth century has increased temperature-dependent erosion and sediment-transport processes in cold environments, affecting food, energy and water security. In this Review, we summarize landscape changes in cold environments and provide a global inventory of increases in erosion and sediment yield driven by cryosphere degradation. Anthropogenic climate change, deglaciation, and thermokarst disturbances are causing increased sediment mobilization and transport processes in glacierized and periglacierized basins. With continuous cryosphere degradation, sediment transport will continue to increase until reaching a maximum (peak sediment). Thereafter, transport is likely to shift from a temperature-dependent regime toward a rainfall-dependent regime roughly between 2100–2200. The timing of the regime shift would be regulated by changes in meltwater, erosive rainfall and landscape erodibility, and complicated by geomorphic feedbacks and connectivity. Further progress in integrating multisource sediment observations, developing physics-based sediment-transport models, and enhancing interdisciplinary and international scientific collaboration is needed to predict sediment dynamics in a warming world.
This paper proposes a Bayesian multilevel spatio-temporal model with a time-varying spatial autoregressive coefficient to estimate temporally heterogeneous network interdependence. To tackle the classic reflection problem, we use multiple factors to control for confounding caused by latent homophily and common exposures. We develop a Markov Chain Monte Carlo algorithm to estimate parameters and adopt Bayesian shrinkage to determine the number of factors. Tests on simulated and empirical data show that the proposed model improves identification of network interdependence and is robust to misspecifcation. Our method is applicable to various types of networks and provides a simpler and more flexible alternative to coevolution models.