Mechanisms for the Reaction of Thiophene and Methylthiophene with Singlet and Triplet Molecular Oxygen

Mechanisms for the reaction of thiophene and 2-methylthiophene with molecular oxygen on both the triplet and singlet potential energy surfaces (PESs) have been investigated using ab initio methods. Geometries of various stationary points involved in the complex reaction routes are optimized at the MP2/6-311++G­(d, p) level. The barriers and energies of reaction for all product channels were refined using single-point calculations at the G4MP2 level of theory. For thiophene, CCSD­(T) single point energies were also determined for comparison with the G4MP2 energies. Thiophene and 2-methylthiophene were shown to react with O₂ via two types of mechanisms, namely, direct hydrogen abstraction and addition/elimination. The barriers for reaction with triplet oxygen are all significantly large (i.e., >30 kcal mol^–1), indicating that the direct oxidation of thiophene by ground state oxygen might be important only in high temperature processes. Reaction of thiophene with singlet oxygen via a 2 + 4 cycloaddition leading to endoperoxides is the most favorable channel. Moreover, it was found that alkylation of the thiophene ring (i.e., methyl-substituted thiophene) is capable of lowering the barrier height for the addition pathway. The implication of the current theoretical results may shed new light on the initiation mechanisms for combustion of asphaltenes

Pollutant Exposure for Chinese Wetland Birds: Ecotoxicological Endpoints and Biovectors

Levels of heavy metals and organic contaminants in main waters from China were reviewed from literature data to assess the ecological risks of pollutants for wetland birds and the biotransport of pollutants mediated by migratory wetland birds. Cr, Cu, and Pb and polycyclic aromatic hydrocarbons (PAHs) dominated in sediments, with higher concentrations in rivers and estuaries than in lakes and seas. Plants are the main dietary sources of less hydrophobic organic pollutants, while sediment is the primary source of more hydrophobic PAHs in birds. The hazard index (HI) for birds was mainly contributed by mercury (Hg) and polybrominated diphenyl ethers (PBDEs) and ranked as piscivore > omnivore > herbivore. Pollutant exposure risks to birds depend on the biomagnification potential of pollutants, food items of birds, and pollution levels in habitats. Migratory birds are important biovectors of persistent and bioaccumulative pollutants that may serve as a vital geochemical cycling process in addition to atmospheric deposition. This study provided a comprehensive overview of water environment pollution in China and the potential risks for high trophic level wetland birds in aquatic ecosystems. The results also identified the pollution hotspots of wetland birds and habitats, which provide new insights into bird conservation and biodiversity protection

Pollutant Exposure for Chinese Wetland Birds: Ecotoxicological Endpoints and Biovectors

Levels of heavy metals and organic contaminants in main waters from China were reviewed from literature data to assess the ecological risks of pollutants for wetland birds and the biotransport of pollutants mediated by migratory wetland birds. Cr, Cu, and Pb and polycyclic aromatic hydrocarbons (PAHs) dominated in sediments, with higher concentrations in rivers and estuaries than in lakes and seas. Plants are the main dietary sources of less hydrophobic organic pollutants, while sediment is the primary source of more hydrophobic PAHs in birds. The hazard index (HI) for birds was mainly contributed by mercury (Hg) and polybrominated diphenyl ethers (PBDEs) and ranked as piscivore > omnivore > herbivore. Pollutant exposure risks to birds depend on the biomagnification potential of pollutants, food items of birds, and pollution levels in habitats. Migratory birds are important biovectors of persistent and bioaccumulative pollutants that may serve as a vital geochemical cycling process in addition to atmospheric deposition. This study provided a comprehensive overview of water environment pollution in China and the potential risks for high trophic level wetland birds in aquatic ecosystems. The results also identified the pollution hotspots of wetland birds and habitats, which provide new insights into bird conservation and biodiversity protection