Emerging advanced oxidation processes for the elimination of micro-pollutants

Emerging advanced oxidation processes for the elimination of micro-pollutant

Hydrothermal Carbon-Mediated Fenton-Like Reaction Mechanism in the Degradation of Alachlor: Direct Electron Transfer from Hydrothermal Carbon to Fe(III)

As Fenton systems suffer from the undesirable Fe­(III)/Fe­(II) cycle, great efforts were made to realize the effective reduction of Fe­(III) to Fe­(II). The effects of hydrothermal carbon (HTC) on the Fe­(III)/H₂O₂ Fenton-like reaction and the subsequent degradation of alachlor in water was systematically investigated, and the results indicated that HTC could enhance alachlor degradation in Fe­(III)/H₂O₂ by promoting the Fe­(III)/Fe­(II) cycle via electron transfer from HTC to Fe­(III) ions. The apparent alachlor degradation rate constant in the HTC-G/Fe­(III)/H₂O₂ system (7.02 × 10^–2 min^–1) was about 3 times higher than that in the Fe­(III)/H₂O₂ system (2.25 × 10^–2 min^–1). The electron spin resonance spectra analysis revealed that HTC consists of abundant carbon-centered persistent free radicals to act as the electron donor. Meanwhile, the hydroxyl groups on the surface of HTC also played an important role in the enhanced alachlor degradation because the decrease in the surface hydroxyl groups on HTC significantly decreased the degradation of alachlor. On the basis of these results, an Fe­(III) complex with surface hydroxyl groups on HTC was proposed to favor the electron transfer from the hydroxyl groups to Fe­(III), and then, the simultaneously produced Fe­(II) could accelerate the catalytic decomposition of H₂O₂ to facilitate alachlor degradation. These findings shed new light on the possible roles of carbon materials in a natural aquatic environment and provide a new pathway for environmental pollutant control and remediation of organic contaminants by HTC

Adsorption Mechanisms of Typical Carbonyl-Containing Volatile Organic Compounds on Anatase TiO₂ (001) Surface: A DFT Investigation

The carbonyl-containing compounds (CCs) are typical volatile organic compounds (VOCs) and ubiquitously present in the environment. Therefore, the adsorption structures and properties of typical CCs on the anatase TiO₂ (001) surface were investigated systematically with density functional theory (DFT) to understand their further catalytic degradation mechanisms. The adsorption mechanisms show that three selected typical CCs, acetaldehyde, acetone, and methyl acetate, can easily be trapped on the anatase TiO₂ (001) surface via the interaction between the carbonyl group with Ti_5c sites of catalyst surface. Especially for acetaldehyde with the bare carbonyl group and the strongest adsorption energy, it is the most stable on the surface, because the bare carbonyl group can interact with not only the Ti_5c atom, but also the O_2c atom of the surface. The substituent effect of different CCs has less impact on its adsorption models in this studied system and the bare carbonyl group is the key functional group within studied CCs. The Ti_5c atoms of anatase TiO₂ (001) surface are active sites to trap CCs. Our theoretical results are expected to provide insight into the adsorption mechanisms of these carbonyl-containing VOCs on TiO₂ catalyst and also to help understand the further catalytic degradation mechanisms of air pollutants at the molecular level

Spontaneous Iodide Activation at the Air–Water Interface of Aqueous Droplets

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air–water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3– formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir–Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols

Can Silica Particles Reduce Air Pollution by Facilitating the Reactions of Aliphatic Aldehyde and NO₂?

This study investigated the heterogeneous atmospheric reactions of acetaldehyde, propanal, and butanal with NO₂ onto silica (SiO₂) clusters using a theoretical approach. By analyzing spectral features and adsorption parameters, the formation of hydrogen bonds and negative adsorption energies provide evidence that an efficient spontaneous uptake of aliphatic aldehydes onto SiO₂ could occur. The atmospheric reaction mechanisms show that when aldehydes and NO₂ react on the surface model, the H atom abstraction reaction from the aldehydic molecule by NO₂ is an exclusive channel, forming nitrous acid and acyl radicals. This study included kinetics exploring the reaction of aldehydes with NO₂ using a canonical variational transition state theory. The reaction rate constants are increased in the presence of SiO₂ between the temperatures 217 and 298 K. This may explain how aldehydes can temporarily stay on mineral particles without chemical reactions. The results suggest that silica can depress the rate at which the studied aldehydes react with NO₂ and possibly reduce air pollution generated by these atmospheric reactions

Kinetics and Mechanism of ^•OH Mediated Degradation of Dimethyl Phthalate in Aqueous Solution: Experimental and Theoretical Studies

The hydroxyl radical (^•OH) is one of the main oxidative species in aqueous phase advanced oxidation processes, and its initial reactions with organic pollutants are important to understand the transformation and fate of organics in water environments. Insights into the kinetics and mechanism of ^•OH mediated degradation of the model environmental endocrine disruptor, dimethyl phthalate (DMP), have been obtained using radiolysis experiments and computational methods. The bimolecular rate constant for the ^•OH reaction with DMP was determined to be (3.2 ± 0.1) × 10⁹ M^–1s^–1. The possible reaction mechanisms of radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) were considered. By comparing the experimental absorption spectra with the computational results, it was concluded that the RAF and HAT were the dominant reaction pathways, and OH-adducts (^•DMPOH₁, ^•DMPOH₂) and methyl type radicals ^•DMP­(-H)­α were identified as dominated intermediates. Computational results confirmed the identification of transient species with maximum absorption around 260 nm as ^•DMPOH₁ and ^•DMP­(-H)­α, and these radical intermediates then converted to monohydroxylated dimethyl phthalates and monomethyl phthalates. Experimental and computational analyses which elucidated the mechanism of ^•OH-mediated degradation of DMP are discussed in detail

Relationship between the mutagenic ratio on TA98 without S9 activation and IR by SOS/<i>umu</i> test without S9 activation.

<p>Relationship between the mutagenic ratio on TA98 without S9 activation and IR by SOS/<i>umu</i> test without S9 activation.</p

Genotoxic activity of organic extracts in water samples detected by SOS/<i>umu</i> test from six sampling locations in Guangzhou drinking water source.

<p>Lower-case letters indicated pair-wise comparison in the dry season in different sampling regions at the same water level and upper-case letters indicated pair-wise comparison in the wet season in different sampling regions at the same water level. *indicated the significant difference from the wet season.</p

Synthesis and Characterization of Novel Plasmonic Ag/AgX-CNTs (X = Cl, Br, I) Nanocomposite Photocatalysts and Synergetic Degradation of Organic Pollutant under Visible Light

A series of novel well-defined Ag/AgX (X = Cl, Br, I) loaded carbon nanotubes (CNTs) composite photocatalysts (Ag/AgX-CNTs) were fabricated for the first time via a facile ultrasonic assistant deposition–precipitation method at the room temperature (25 ± 1 °C). X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption analysis, scanning electron microscopy, and ultraviolet–visible light absorption spectra analysis were used to characterize the structure, morphology, and optical properties of the as-prepared photocatalysts. Results confirmed the existence of the direct interfacial contact between Ag/AgX nanoparticles and CNTs, and Ag/AgX-CNTs nanocomposites exhibit superior absorbance in the visible light (VL) region owing to the surface plasmon resonance (SPR) of Ag nanoparticles. The fabricated composite photocatalysts were employed to remove 2,4,6-tribromophenol (TBP) in aqueous phase. A remarkably enhanced VL photocatalytic degradation efficiency of Ag/AgX-CNTs nanocomposites was observed when compared to that of pure AgX or CNTs. The photocatalytic activity enhancement of Ag/AgX-CNTs was due to the effective electron transfer from photoexcited AgX and plasmon-excited Ag(0) nanoparticles to CNTs. This can effectively decrease the recombination of electron–hole pairs, lead to a prolonged lifetime of the photoholes that promotes the degradation efficiency

Synthesis of Carbon Nanotube–Anatase TiO₂ Sub-micrometer-sized Sphere Composite Photocatalyst for Synergistic Degradation of Gaseous Styrene

The carbon nanotube (CNT)–sub-micrometer-sized anatase TiO₂ sphere composite photocatalysts were synthesized by a facile one-step hydrothermal method using titanium tetrafluoride as titanium source and CNTs as structure regulator. Various technologies including X-ray diffraction, UV–visible absorption spectra, N₂ adsorption–desorption, scanning electron microscopy, and transmission electron microscopy were employed to characterize the structure properties of the prepared composite photocatalysts. The results indicated that the composite photocatalysts consisted of CNTs wrapping around the sub-micrometer-sized anatase TiO₂ spheres with controllable crystal facets and that the aggregated particles with average diameter ranged from 200 to 600 nm. The fabricated composite photocatalysts were used to degrade gaseous styrene in this work. As expected, a synergistic effect that remarkably enhancing the photocatalytic degradation efficiency of gaseous styrene by the prepared composite photocatalysts was observed in comparison with that the degradation efficiency using pure anatase TiO₂ and the adsorption of CNTs. Similar results were also confirmed in the decolorization of liquid methyl orange. Further investigation demonstrated that the synergistic effect in the photocatalytic activity was related to the structure of the sub-micrometer-sized anatase TiO₂ spheres and the significant roles of CNTs in the composite photocatalysts. By controlling the content of CNTs, the content of TiO₂ or the temperature during the hydrothermal synthesis process, anatase TiO₂ spheres with controllable crystallite size and dominant crystal facets such as {001}, {101}, or polycrystalline could be obtained, which was beneficial for the increase in the synergistic effect and further enhancement of the photocatalytic efficiencies