Layered Double Hydroxide–Carbon Dot Composite: High-Performance Adsorbent for Removal of Anionic Organic Dye

It would be of significance to design a green composite for efficient removal of contaminants. Herein, we fabricated a facile and environmentally friendly composite via direct assembly of surface passivated carbon dots with abundant oxygen-containing functional groups on the surface of the positively charged layered double hydroxide (LDH). The resulting LDH–carbon dot composites were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and N₂ adsorption–desorption technique. The adsorption performances of the resulting LDH–carbon dot composites were evaluated for the removal of anionic methyl blue dye. Taking advantage of the combined benefits of LDH and carbon dots, the as-prepared composites exhibited high uptake capability of methyl blue (185 mg/g). The adsorption behavior of this new adsorbent fitted well with Langmuir isotherm and the pseudo-second-order kinetic model. The reasons for the excellent adsorption capacity of methyl blue on the surface of the LDH–carbon dot hybrid were further discussed. A probable mechanism was speculated to involve the cooperative contributions of hydrogen bonding between methyl blue and carbon dots and electrostatic attraction between methyl blue and LDH, in the adsorption process. This work is anticipated to open up new possibilities in fabricating LDH–carbon dot materials in dealing with anionic dye pollutants

Theoretical Investigation on Mechanistic and Kinetic Transformation of 2,2′,4,4′,5-Pentabromodiphenyl Ether

This study investigates the decomposition of 2,2′,4,4′,5-pentabrominated diphenyl ether (BDE99), a commonly detected pollutant in the environment. Debromination channels yielding tetrabrominated diphenyl ethers and hydrogen abstracting aromatic bromine atom formations play significant roles in the reaction of BDE99 + H, in which the former absolutely predominates bimolecular reactions. Polybrominated dibenzo-<i>p</i>-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) can be produced during BDE99 pyrolysis, especially for PBDFs under inert conditions. The expected dominant pathways in a closed system are debromination products and PBDF formations. The bimolecular reaction with hydroxyl radical mainly leads to hydroxylated BDE99s rather than hydroxylated tetrabrominated diphenyl ethers. PBDDs are then generated from <i>ortho</i>-hydroxylated PBDEs. HO₂ radical reactions rarely proceed. The total rate constants for the BDE99 reaction with hydrogen atoms and hydroxyl radicals exhibit positive dependence on temperature with values of 1.86 × 10^–14 and 5.24 × 10^–14 cm³ molecule^–1 s^–1 at 298.15 K, respectively

Microscopic Evaluation of Trace Metals in Cloud Droplets in an Acid Precipitation Region

Mass concentrations of soluble trace metals and size, number, and mixing properties of nanometal particles in clouds determine their toxicity to ecosystems. Cloud water was found to be acidic, with a pH of 3.52, at Mt. Lu (elevation 1,165 m) in an acid precipitation region in South China. A combination of Inductively Coupled Plasma Mass Spectrometry (ICPMS) and Transmission Electron Microscopy (TEM) for the first time demonstrates that the soluble metal concentrations and solid metal particle number are surprisingly high in acid clouds at Mt. Lu, where daily concentrations of SO₂, NO₂, and PM₁₀ are 18 μg m^–3, 7 μg m^–3, and 22 μg m^–3. The soluble metals in cloudwater with the highest concentrations were zinc (Zn, 200 μg L^–1), iron (Fe, 88 μg L^–1), and lead (Pb, 77 μg L^–1). TEM reveals that 76% of cloud residues include metal particles that range from 50 nm to 1 μm diameter with a median diameter of 250 nm. Four major metal-associated particle types are Pb-rich (35%), fly ash (27%), Fe-rich (23%), and Zn-rich (15%). Elemental mapping shows that minor soluble metals are distributed within sulfates of cloud residues. Emissions of fine metal particles from large, nonferrous industries and coal-fired power plants with tall stacks were transported upward to this high elevation. Our results suggest that the abundant trace metals in clouds aggravate the impacts of acid clouds or associated precipitation on the ecosystem and human health

Atmospheric Chemical Reactions of 2,3,7,8-Tetrachlorinated Dibenzofuran Initiated by an OH Radical: Mechanism and Kinetics Study

Reactions with the OH radical are expected to be the dominant removal processes for gas-phase polychlorinated dibenzo-<i>p</i>-dioxins and dibenzofuran (PCDD/Fs). The OH-initiated atmospheric chemical reaction mechanism and kinetics of 2,3,7,8-tetrachlorinated dibenzofuran (TCDF) are researched using the density functional theory and canonical variational transition state theory. The reaction mechanism of TCDF with the OH radical and ensuing reactions including bond cleavage of furan ring, O₂ addition or abstraction, dechlorination process, bimolecular reaction of TCDF-OH-O₂ peroxy radical with NO, and reaction of carbonyl free radicals TCDF-OH-O with H₂O are investigated. In the subsequent reactions of TCDF-OH, O₂ abstraction and dechlorination are most likely to predominate the process. As the main products, the HO₂ radical and the Cl atom are active and may play important roles in the atmospheric oxidation processes. The rate constants of TCDF with the OH radical are calculated, which are consistent with the reported data

Microscopic Observation of Metal-Containing Particles from Chinese Continental Outflow Observed from a Non-Industrial Site

Atmospheric metal-containing particles adversely affect human health because of their physiological toxicity. Mixing state, size, phase, aspect ratio, and sphericity of individual metal-containing particles collected in Hong Kong air in winter are examined through transmission electron microscopy (TEM). Eighteen percent of the sulfate particles have one or more tiny metal inclusions. Size distributions of metal and fly ash particles (or inclusions) with diameters from 15 nm to 2.7 μm show the same peak at 210 nm. The major metal particles were classified as Fe-rich (e.g., hematite), Zn-rich (e.g., zinc sulfate and zinc oxide), Pb-rich (e.g., anglesite), Mn-rich, and As-rich, which were likely emitted from industries and coal-fired power plants at high temperatures in mainland China. Compared to fly ash and S-rich particles, metal particles display a lower sphericity of 0.51 and a higher aspect ratio of 1.47, which means their shapes are poorly defined. The elemental mapping of individual particles reveal that sulfate areas without metal inclusions also contain minor Fe, Mn, or Zn. Therefore, the internal mixing of metals and acidic constituents likely solubilize metals and modify metal inclusion shapes. Solubilization of metals in airborne particles can extend their toxicity into nontoxicity parts in the particles. The structure of the metal-containing particles may provide important information for assessing health effects of fine sulfate and nitrate particles with metal inclusions in urban areas

AFM topographic images for the different states of the device on Au(111) surface

<p><b>Copyright information:</b></p><p>Taken from "Alternating-electric-field-enhanced reversible switching of DNA nanocontainers with pH"</p><p></p><p>Nucleic Acids Research 2007;35(5):e33-e33.</p><p>Published online 31 Jan 2007</p><p>PMCID:PMC1865044.</p><p>© 2007 The Author(s).</p> () Initial closed state at pH 4.5 after self-assembly shows 1.5 ± 0.5 nm surface roughness. () Open state at pH 8.0 shows 6.0 ± 1.0 nm surface roughness. () Repeated closed state at pH 4.5 restores the surface roughness to 1.5 ± 0.5 nm. Height scales for all images are adjusted to a uniform range of 15 nm. The color mapping to height is indicated by the height scale bar. The poly-dA spacer length of DNA motif is 10 bp. () The height analysis of the horizontal cross sections along the middle dashed white lines shown in (). (D) corresponds to (A), (E) to (B), and (F) to (C), respectively

Gas-Phase Oxidation of Allyl Acetate by O₃, OH, Cl, and NO₃: Reaction Kinetics and Mechanism

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O₃, OH, Cl, and NO₃) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm³ molecule^–1 s^–1) are (1.8 ± 0.3) × 10^–18, (3.1 ± 0.7) × 10^–11, (2.5 ± 0.5) × 10^–10, and (1.1 ± 0.4) × 10^–14, for reactions with O₃, OH, Cl, and NO_3, respectively. While results for reactions with O₃, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO₃ is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τ_O₃ = 9 days, τ_OH = 5 h, τ_Cl = 5 days, τ_NO₃ = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime

Computational Evidence for the Enzymatic Transformation of 2‑Hydroxypropylphosphonate to Methylphosphonate

Understanding the origins of greenhouse gas methane in the ocean is of great environmental importance, especially for global climate change and the flow of carbon within the earth surface system. A mutant (E176H) of 2-hydroxyethylphosphonate dioxygenase (HEPD) has been reported to catalyze the transformation of 2-hydroxypropylphosphonate (2-HEP) to methylphosphonate (MPn), a compound that can be easily transformed to methane by C–P lyase in a marine microbe. Here, the HEPD E176H-catalyzed transformation of 2-HEP to MPn was investigated at the molecular level using the quantum mechanics/molecular mechanics method. The results evidenced the feasibility of the transformation of 2-HEP to MPn and highlighted that the transformation contains five elementary steps: H abstraction, O–O bond cleavage, H transfer, C–C bond cleavage, and MPn formation. H abstraction was found to be the rate-determining step with an energy barrier of 17.8 kcal/mol, which is in reasonable accordance with the experimentally determined rate constant (0.38 s^–1, corresponding to 18.0 kcal/mol). Three intersystem crossing events were involved in H-abstraction, H-transfer, and MPn-formation steps. Residue electrostatic analysis on the rate-determining step suggests that proper mutation of Tyr174 may improve the enzymatic efficiency

Role of Water Molecule in the Gas-Phase Formation Process of Nitrated Polycyclic Aromatic Hydrocarbons in the Atmosphere: A Computational Study

Nitro-PAHs are globally worrisome air pollutants because their high direct-acting mutagenicity and carcinogenicity. A mechanistic understanding of their formation is of crucial importance for successful prevention of their atmospheric pollution. Here, the formation of nitro-PAHs arising from the OH-initiated and NO₃-initiated atmospheric reactions of PAHs was investigated by using quantum chemical calculations. It is widely assumed that OH or NO₃ radicals attack on the C atoms of the aromatic rings in the PAH molecule, followed by the addition of NO₂ to the OH–PAH or NO₃–PAH adducts at the <i>ortho</i> position and the loss of water or nitric acid to form nitro-PAHs. However, calculations show that the direct loss of water from the OH–NO₂–PAH adducts via the unimolecular decomposition is energetically unfavorable. This study reveals for the first time that water molecule plays an important catalytic effect on the loss of water from the OH–NO₂–PAH adducts and promotes the formation of nitro-PAHs. In addition, the introduction of water unwraps new formation pathway through the addition of NO₂ to the OH–PAH or NO₃–PAH adduct at the <i>para</i> position. The individual and overall rate constants for the addition reactions of PAHs with OH and NO₃ radicals were deduced by using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory

Double-Edged Role of VOCs Reduction in Nitrate Formation: Insights from Observations during the China International Import Expo 2018

Aerosol nitrate (NO3–) constitutes a significant component of fine particles in China. Prioritizing the control of volatile organic compounds (VOCs) is a crucial step toward achieving clean air, yet its impact on NO3– pollution remains inadequately understood. Here, we examined the role of VOCs in NO3– formation by combining comprehensive field measurements conducted during the China International Import Expo (CIIE) in Shanghai (from 10 October to 22 November 2018) and multiphase chemical modeling. Despite a decline in primary pollutants during the CIIE, NO3– levels increased compared to pre-CIIE and post-CIIENO3– concentrations decreased in the daytime (by −10 and −26%) while increasing in the nighttime (by 8 and 30%). Analysis of the observations and backward trajectory indicates that the diurnal variation in NO3– was mainly attributed to local chemistry rather than meteorological conditions. Decreasing VOCs lowered the daytime NO3– production by reducing the hydroxyl radical level, whereas the greater VOCs reduction at night than that in the daytime increased the nitrate radical level, thereby promoting the nocturnal NO3– production. These results reveal the double-edged role of VOCs in NO3– formation, underscoring the need for transferring large VOC-emitting enterprises from the daytime to the nighttime, which should be considered in formulating corresponding policies