Environmental contextUrea is an important component of dissolved organic nitrogen in rainfall and aerosols, but the sources and the mechanisms of its production are not well understood. This computational study explores the effects of urea and water on the hydrolysis of NO2 and urea nitrate production. The results will aid our interpretation of the role of urea in the formation of atmospheric secondary nitrogen contaminants and aerosols. AbstractThe effects of urea on the hydrolysis reaction 2NO2 + mH2O (m = 1–3) have been investigated by theoretical calculations. The energy barrier (−2.67 kcal mol−1) of the urea-promoted reaction is lower than the naked reaction by 14.37 kcal mol−1. Urea also has a better catalytic effect on the reaction than methylamine and ammonia. Urea acts as a catalyst and proton transfer medium in this process, and the produced HONO may serve as a source of atmospheric nitrous acid. In addition, the subsequent reactions include clusters of nitrite, urea, and nitric acid. Then urea nitrate (UN), which is a typical HNO3 aerosol, can be formed in the subsequent reactions. The production of the acid-base complex (UN-2) is more favourable with an energy barrier of 0.10 kcal mol−1, which is 3.88 kcal mol−1 lower than that of the zwitterions NH2CONH3+NO3− (UN-1). The formation of zwitterions and the hydrolysis reaction are affected by humidity. The multi water-promoted hydrolysis reactions exhibit better thermodynamic stability when the humidity is increased. The extra water molecules act as solvent molecules to reduce the energy barrier. The natural bond orbital (NBO) analysis is employed to describe the donor-acceptor interactions of the complexes. The hydrogen bond interaction between the urea carbonyl and nitric acid of UN-2 is the strongest. The potential distribution maps of the urea nitrate and hydrate are examined, and the result shows that they tend to form zwitterions.

Fe 3 O 4 nanoparticles hybridized with carbonaceous materials, such as pinecone and graphene, were successfully synthesized by a facile hydrothermal method, which could be applied for Cr(VI) removal in aqueous solution. The nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N 2 adsorption-desorption isotherms. Due to the combination of pinecone and graphene, both the surface properties and morphologies of Fe 3 O 4 were modified. Fe 3 O 4 spherical particles were distributed and firmly anchored on the loose surface of pinecone or wrinkled graphene layers. The specific surface area increased from 23.85 to 27.86 and 121.17 m 2 g À1 for Fe 3 O 4 /P and Fe 3 O 4 / G, respectively. It enhanced the adsorption capacity for Cr(VI) of Fe 3 O 4 /P (62.5 mg g À1 ) and Fe 3 O 4 /G (78.5 mg g À1 ). Study of the kinetics and isotherms showed that the pseudo-second-order kinetic and Langmuir isotherm models fitted the adsorption data well. There were three steps in the adsorption process, namely an instantaneous adsorption step, intraparticle diffusion and a final equilibrium stage. The reaction rate decreased along with temperature increasing, which indicated that Cr(VI) adsorption was an exothermic process. The E a values were 34.39, 25.77 and 34.92 kJ mol À1 for Fe 3 O 4 , Fe 3 O 4 /P and Fe 3 O 4 /G, respectively, which illustrated that the adsorption of Cr(VI) onto the surface of the nanocomposites was a physical process. In no more than 5 h, about 92.6% and 94% Cr(VI) were desorbed from the surface of Fe 3 O 4 /P and Fe 3 O 4 /G, respectively, which indicated that the adsorptiondesorption process for Cr(VI) was reversible. The results demonstrated that Fe 3 O 4 /P and Fe 3 O 4 /G

In this study, the mechanistic and kinetic analysis for reactions of CF3OCH(CF3)2 and CF3OCF2CF2H with OH radicals and Cl atoms have been performed at the CCSD(T)//B3LYP/6-311++G(d,p) level. Kinetic isotope effects for reactions CF3OCH(CF3)2/CF3OCD(CF3)2 and CF3OCF2CF2H/CF3OCF2CF2D with OH and Cl were estimated so as to provide the theoretical estimation for future laboratory investigation. All rate constants, computed by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT), are in reasonable agreement with the limited experimental data. Standard enthalpies of formation for the species were also calculated. Atmospheric lifetime and global warming potentials (GWPs) of the reaction species were estimated, the large lifetimes and GWPs show that the environmental impact of them cannot be ignored. The organic nitrates can be produced by the further oxidation of CF3OC(•)(CF3)2 and CF3OCF2CF2• in the presence of O2 and NO. The subsequent decomposition pathways of CF3OC(O•)(CF3)2 and CF3OCF2CF2O• radicals were studied in detail. The derived Arrhenius expressions for the rate coefficients over 230–350 K are: k T(1) = 5.00 × 10−24T3.57 exp(−849.73/T), k T(2) = 1.79 × 10−24T4.84 exp(−4262.65/T), kT(3) = 1.94 × 10−24 T4.18 exp(−884.26/T), and k T(4) = 9.44 × 10−28T5.25 exp(−913.45/T) cm3 molecule−1 s−1.

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