Physicochemical assessment of ammonium adsorption using a palm shell-based adsorbent activated with acetic acid: experimental and theoretical studies.

The adsorption of ammonium from water was studied on an activated carbon obtained using raw oil palm shell and activated with acetic acid. The performance of this adsorbent was tested at different operating conditions including the solution pH, adsorbent dosage, and initial ammonium concentration. Kinetic and equilibrium studies were carried out, and their results were analyzed with different models. For the adsorption kinetics, the pseudo-first order equation was the best model to correlate this system. Calculated adsorption rate constants ranged from 0.071 to 0.074 g/mg min. The ammonium removal was 70-80% at pH 6-8, and it was significantly affected by electrostatic interaction forces. Ammonium removal (%) increased with the adsorbent dosage, and neutral pH condition favored the adsorption of this pollutant. The best ammonium adsorption conditions were identified with a response surface methodology model where the maximum removal was 91.49% with 2.27 g/L of adsorbent at pH 8.11 for an initial ammonium concentration of 36.90 mg/L. The application of a physical monolayer model developed by statistical physics theory indicated that the removal mechanism of ammonium was multi-ionic and involved physical interactions with adsorption energy of 29 kJ/mol. This activated carbon treated with acetic acid is promising to depollute aqueous solutions containing ammonium.

Calcite Deposits Differentiate Cave from House-Farmed Edible Bird's Nest as shown by SEM-EDX, ATR-FTIR and Raman Microspectroscopy.

The difference between the swiftlet white edible bird's nest from limestone caves versus house-farmed ones, especially in response to high temperature and stewing time in water where the latter type would disintegrate readily, has been a puzzle for a long time. We show that edible bird's nests from the limestone caves have calcite deposits on the surface of the nest cement as compared to the house-farmed nests which are built by swiftlets on timber planks. The micron and sub-micron calcite particles are seen in SEM-EDX and further characterized by ATR-FTIR and Raman microspectroscopy. The calcite deposits make it possible for the cave nest to retain a gelatinous texture under the harsh retort conditions at 121 °C for 20 mins in commercial bottling. We show that house-farmed nests can be soaked in CaCl2 (aq) followed by rinsing with Na2 CO3 (aq) to grow the same calcite deposits on the nest cement with the same characteristic as cave nests. Therefore, there should no longer be a need to harvest cave nests, and we can better conserve the dwindling population and natural habitats of cave swiftlets.

Nitration of Tyrosine in the Mucin Glycoprotein of Edible Bird's Nest Changes Its Color from White to Red.

The edible bird's nest (EBN) of the swiftlet Aerodramus fuciphagus, a mucin glycoprotein, is usually white in color, but there also exist the more desirable red or "blood" EBN. The basis of the red color has been a puzzle for a long time. Here, we show that the nitration of the tyrosyl residue to the 3-nitrotyrosyl (3-NTyr) residue in the glycoprotein is the cause of the red color. Evidence for the 3-NTyr residue comes from (a) the quantitative analysis of 3-NTyr in EBN by enzyme-linked immunosorbent assay, (b) the ultraviolet-visible absorption spectra of red EBN as a function of pH being similar to 3-nitrotyrosine (3-NT), (c) the change in the color of red EBN from yellow at low pH to red at high pH just like 3-NT, and (d) strong Raman nitro bands at 1330 cm-1 (symmetric -NO2 stretch) and 825 cm-1 (-NO2 scissoring bend) for red EBN. The high concentrations of nitrite and nitrate in red EBN are also explained.

Full-Dimensional Quantum Dynamical Studies of the Cl + HOD → HCl/DCl + OD/OH Reaction: Bond Selectivity and Isotopic Branching Ratio.

Full-dimensional quantum dynamical calculations are carried out to study the mode specificity, bond selectivity, and isotopic branching ratio of the Cl + HOD reaction on an accurate global potential energy surface. Total reaction cross sections have been computed for several low-lying vibrational states of HOD. Our results confirm the experimental observed vibrationally promoted bond cleavage, in which the breaking of the OH(OD) bond is strongly enhanced by the OH(OD) excitation. These results are rationalized by the recently proposed sudden vector projection model. In addition, the OH/OD branching ratio as a function of energy is investigated and rationalized by a reorientation effect.

Six-dimensional and seven-dimensional quantum dynamics study of the OH + CH4 → H2O + CH3 reaction.

The reaction dynamics of hydroxyl radical with methane has been investigated using time-dependent wave packet approach within reduced six- and seven-dimensional models. Initial state-selected total reaction probabilities and integral cross sections for the hydrogen abstraction reaction have been computed on the empirical potential energy surface developed by Espinosa-García et al. [J. Chem. Phys. 112, 5731 (2000)]. Excitations of the CH stretching mode and/or the CH3 umbrella mode enhance the reaction. They are, however, both less efficient than translational energy in promoting the reaction, at least at low collision energies. Also, we studied the accuracy of two approximations: centrifugal sudden (CS) and J-shifting (JS), in the calculations of the integral cross sections by a comparison to coupled-channel (CC) calculations. The integral cross sections obtained indicated that the CS approximation works well over the whole energy range studied, and the JS approximation gives accurate cross sections at low collision energies, while noticeably overestimates them at relatively high collision energies. In addition, the OH radical acts as a good spectator as it has a negligible effect on the reaction.

Full-dimensional quantum calculations of the vibrational states of H5(+).

Full-dimensional quantum calculations of the vibrational states of H5(+) have been performed on the accurate potential energy surface developed by Xie et al. [J. Chem. Phys. 122, 224307 (2005)]. The zero point energies of H5(+), H4D(+), D4H(+), and D5(+) and their ground-state geometries are presented and compared with earlier theoretical results. The first 10 low-lying excited states of H5(+) are assigned to the fundamental, overtone, and combination of the H2-H3(+) stretch, the shared proton hopping and the out-of-plane torsion. The ground-state torsional tunneling splitting, the fundamental of the photon hopping mode and the first overtone of the torsion mode are 87.3 cm(-1), 354.4 cm(-1), and 444.0 cm(-1), respectively. All of these values agree well with the diffusion Monte Carlo and multi-configuration time-dependent Hartree results where available.

Theoretical studies of the CO2-N2O van der Waals complex: ab initio potential energy surface, intermolecular vibrations, and rotational transition frequencies.

Theoretical studies of the potential energy surface and bound states were performed for the CO(2)-N(2)O van der Waals complex. A four-dimensional intermolecular potential energy surface (PES) was constructed from 11,466 ab initio data points which were calculated at the coupled-cluster single double (triple) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to two equivalent isomers with a slipped parallel structure in which the O atom in N(2)O is near the C atom in CO(2) and a T-shaped isomer in which the terminal N atom in N(2)O is closest to the C atom in CO(2). The two slipped parallel isomers are energetically more stable than the T-shaped isomer by 178 cm(-1). Four fundamental vibrational excited states for the slipped parallel isomers and two fundamental vibrational excited states (torsion and disrotation) for the T-shaped isomer were assigned via bound states calculations based on this PES. The theoretical vibrational frequencies are in good agreement with the available experimental values for the slipped parallel isomers. Rotational excitations (J = 0-6) for the ground vibrational state of the slipped parallel structure were calculated and the accuracy of the PES in the vicinity of minima is validated by the good agreement between the theoretical and experimental transition frequencies and spectroscopic parameters.

Theoretical studies of the N2O van der Waals dimer: ab initio potential energy surface, intermolecular vibrations and rotational transition frequencies.

Theoretical studies of the potential energy surface and bound states were performed for the N(2)O dimer. A four-dimensional intermolecular potential energy surface (PES) was constructed at the CCSD(T) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to a nonpolar isomer with slipped-antiparallel planar structure and two equivalent polar isomers with slipped-parallel planar structures. The nonpolar isomer is energetically more stable than the polar ones by 162 cm(-1). To assign the fundamental vibrational frequencies for both isomers, more than 150 vibrational bound states were calculated based on this PES. The orientation of the nodal surface of the wave functions plays an important role in the assignment of disrotation and conrotation vibrational modes. The calculated vibrational frequencies are in good agreement with the available experimental data. We have also found a quantum tunneling effect between the two equivalent polar structures in the higher vibrational excited states. Rotational transition frequencies of the polar structure were also calculated. The accuracy of the PES is validated by the good agreement between theoretical and experimental results for the transition frequencies and spectroscopic parameters.

Depression of reactivity by the collision energy in the single barrier H + CD4 -> HD + CD3 reaction.

Crossed molecular beam experiments and accurate quantum scattering calculations have been carried out for the polyatomic H + CD(4) --> HD + CD(3) reaction. Unprecedented agreement has been achieved between theory and experiments on the energy dependence of the integral cross section in a wide collision energy region that first rises and then falls considerably as the collision energy increases far over the reaction barrier for this simple hydrogen abstraction reaction. Detailed theoretical analysis shows that at collision energies far above the barrier the incoming H-atom moves so quickly that the heavier D-atom on CD(4) cannot concertedly follow it to form the HD product, resulting in the decline of reactivity with the increase of collision energy. We propose that this is also the very mechanism, operating in many abstraction reactions, which causes the differential cross section in the backward direction to decrease substantially or even vanish at collision energies far above the barrier height.