Bromine Dioxide Research Articles

Significant roles of reactive bromine intermediates on organic co-contaminants degradation during bromate reduction in electrochemically activated (bi)sulfite process

(Bi)sulfite radical-based advanced reduction process (HSO3/SO3−-ARP) was an emerging technology to simultaneously reduce BrO3− and oxidize organic co-contaminants. However, the evolution of reactive bromine intermediates (RBrIs) and their role on degradation of organics has not been investigated. This study utilized an electrochemically activated (bi)sulfite process, a novel HSO3/SO3−-ARP for BrO3− reduction (EA-S (IV)-BrO3− process), to confirm the generation of RBrIs and elaborate the degradation mechanism of organic contaminants. Firstly, the generated RBrIs responsible for organic pollutants degradation were bromine dioxide (BrO2), bromine monoxide (BrO) and free bromine (HBrO/BrO−). Then, the species-specific second-order rate constants for RBrIs reacting with structurally diverse phenolic contaminants (PCs) (kBrO2, PCs = 107−1010 M−1 s−1 and kBrO, PCs = 106−1010 M−1 s−1) were obtained by fitting with the experimental data using a first-principle-based kinetic model of EA-S (IV)-BrO3− process in anaerobic condition. Notably, at pH 2 and 6, HSO3 (33–95 %) and BrO2 (5–66 %) dominated PCs degradation. Conversely, at pH 10, BrO (48–66 %), SO3− (27–42 %) and BrO2 (5–12 %) were the major radicals due to high reaction rates between RBrIs and dissociated PCs. The role of HBrO/BrO− was minimal (<1 %). Finally, negative linear relationships between kBrO2/BrO/HBrO, PCs and Hammett constants (Σσ+) indicated that PCs with electron-donating groups were more sensitive to RBrIs. BrO and BrO2 dominated oxidation of PCs via electron transfer, hydroxylation and ketonization, while the formation of brominated products attributed to HOBr/BrO− were negligible. This study advanced the understanding of BrO3− reduction process and the significant role of RBrIs on degradation of organic co-contaminants.

Electronic Structures and Spin Density Distributions of BrO2 and (HO)2BrO Radicals. Mechanisms for Avoidance of Hypervalency and for Spin Delocalization and Spin Polarization

The results are reported of an ab initio study of bromine dioxide BrO2, 1, and of the T-shaped trans- and cis-dihydroxides 2 and 3 of dihydrogen bromate (HO)2BrO. The thermochemistry has been explored of potential synthetic routes to (HO)2BrO involving water addition to BrO2, hydroxyl addition to bromous acid HOBrO, 4, protonation/reduction of bromic acid HOBrO2, 5, via tautomers 6-8 of protonated bromic acid, and by reduction/protonation of bromic acid via radical anion [HOBrO2](-), 9. The potential energy surface analyses were performed at the MP2(full)/6-311G* level (or better) and with the consideration of aqueous solvation at the SMD(MP2(full)/6-311G*) level (or better), and higher-level energies were computed at levels up to QCISD(full,T)/6-311++G(2df,2pd)//MP2. The addition of RO radical to bromous acid or bromite esters and the reduction of protonated bromic acid or protonated bromate esters are promising leads for possible synthetic exploration. Spin density distributions and molecular electrostatic potentials were computed at the QCISD(full)/6-311G*//MP2(full)/6-311G* level to characterize the electronic structures of 1-3. Both radicals employ maximally occupied (pseudo) π-systems to transfer electron density from bromine to the periphery. While the formation of the (3c-5e) π-system suffices to avoid hypervalency in 1, the formation of the (4c-7e) π-system in 2 or 3 still leaves the bromine formally hypervalent and (HO)2BrO requires delocalization of bromine density into σ*-SMOs over the trans O-Br-O moiety. Molecular orbital theory is employed to describe the mechanisms for the avoidance of hypervalency and for spin delocalization and spin polarization. The (4c-7e) π-system in 2 is truly remarkable in that it contains five π-symmetric spin molecular orbitals (SMO) with unique shapes.

Collective reaction behavior of an oscillating system coupled with an excitable reaction

The collective reaction behavior of limit cycles coupled to a nonoscillatory forcing is investigated experimentally and computationally. The coupled chemical system is constructed by adding 1,4-cyclohexanedione (CHD), a species which is capable of forming an oscillator with acidic bromate, into the cerium-catalyzed Belousov-Zhabotinsky reaction. Two levels of coupling exist in the system: (1) through autocatalytic reactions with bromine dioxide radicals and (2) via reactions with oxidized metal catalysts. Experiments illustrate that there is an optimum [1,4-CHD]/[cerium] ratio for inducing complex oscillations, whereas in the 1,4-CHD and malonic acid concentration phase plane two resonant ratios are observed for the onset of complex behavior. In addition, bromate, the oxidant for both suboscillators, also exhibits subtle influences on the complexity of the collective reaction behavior. The experimental observations are qualitatively reproduced with the Field-Koros-Noyes mechanism, modified to account for the coupling reactions with the 1,4-CHD-bromate system.

Kinetics and mechanisms of the ozone/bromite and ozone/chlorite reactions.

Ozone reactions with XO(2)(-) (X = Cl or Br) are studied by stopped-flow spectroscopy under pseudo-first-order conditions with excess XO(2)(-). The O(3)/XO(2)(-) reactions are first-order in [O(3)] and [XO(2)(-)], with rate constants k(1)(Cl) = 8.2(4) x 10(6) M(-1) s(-1) and k(1)(Br) = 8.9(3) x 10(4) M(-1) s(-1) at 25.0 degrees C and mu = 1.0 M. The proposed rate-determining step is an electron transfer from XO(2)(-) to O(3) to form XO(2) and O(3)(-). Subsequent rapid reactions of O(3)(-) with general acids produce O(2) and OH. The OH radical reacts rapidly with XO(2)(-) to form a second XO(2) and OH(-). In the O(3)/ClO(2)(-) reaction, ClO(2) and ClO(3)(-) are the final products due to competition between the OH/ClO(2)(-) reaction to form ClO(2) and the OH/ClO(2) reaction to form ClO(3)(-). Unlike ClO(2), BrO(2) is not a stable product due to its rapid disproportionation to form BrO(2)(-) and BrO(3)(-). However, kinetic spectra show that small but observable concentrations of BrO(2) form within the dead time of the stopped-flow instrument. Bromine dioxide is a transitory intermediate, and its observed rate of decay is equal to half the rate of the O(3)/BrO(2)(-) reaction. Ion chromatographic analysis shows that O(3) and BrO(2)(-) react in a 1/1 ratio to form BrO(3)(-) as the final product. Variation of k(1)(X) values with temperature gives Delta H(++)(Cl) = 29(2) kJ mol(-1), DeltaS(++)(Cl) = -14.6(7) J mol(-1) K(-1), Delta H(++)(Br) = 54.9(8) kJ mol(-1), and Delta S(++)(Br) = 34(3) J mol(-1) K(-1). The positive Delta S(++)(Br) value is attributed to the loss of coordinated H(2)O from BrO(2)(-) upon formation of an [O(3)BrO(2)(-)](++) activated complex.

HPLC analysis of complete BZ systems. Evolution of the chemical composition in cerium and ferroin catalysed batch oscillators: experiments and model calculations.

In the last few years many new reaction routes and intermediates have been discovered in the mechanism of the Belousov-Zhabotinsky (BZ) reaction with the aid of high performance liquid chromatography (HPLC). These previous HPLC studies, however, were limited to the Ce(4+)-organic substrate (malonic or bromomalonic acid) systems only. Very recently some measurements were made on a cerium catalysed full BZ system but only in its induction period. The present work follows the evolution of the main chemical components in a cerium and in a ferroin catalysed full BZ system from the start until the end of the oscillatory regime in a batch reactor. While recording the potential oscillations of a bromide selective electrode we measured from time to time the concentration of the following components: malonic and bromomalonic acids and bromate as main components; malonyl malonate, ethanetetracarboxylic and bromoethenetricarboxylic acids which are recombination products of organic free radicals; oxidized intermediates: tartronic, oxalic (OA) and mesoxalic (MOA) acids, and brominated products: dibromoacetic and tribromoacetic acids. Recombination products are generated in the intervals when the autocatalytic reaction is "switched off". In the course of the autocatalytic periods, however, the organic radicals react with the inorganic bromine dioxide radical mainly which leads to the formation of MOA and OA. Due to a very fast Ce(4+)-MOA reaction, MOA can be detected in the ferroin catalysed BZ system only. Our model calculations deal exclusively with the cerium catalysed system. The suggested new Marburg-Budapest-Missoula (MBM) model includes both negative feedback loops (bromous acid-bromide ion Oregonator type and bromine dioxide-organic free radicals Radicalator type feedback) and the recently discovered radical-radical recombination reactions. Comparison of the experimental data with the model calculations shows a good qualitative agreement but some open problems still remain. To overcome these problems oxygen atom transfer and other redox reactions are proposed.

Polymerization Coupled To Oscillating Reactions: (1) A Mechanistic Investigation of Acrylonitrile Polymerization in the Belousov−Zhabotinsky Reaction in a Batch Reactor

When acrylonitrile is added to the Belousov−Zhabotinsky reaction, periodic polymerization is observed. (Pojman, J. A.; Leard, D. C.; West, W. W. J. Am. Chem. Soc. 1992, 114, 8298). The polymer's structure and the amount of bromine in the polymer sample were determined by 13C NMR and elemental analysis, respectively. Carbon-13-labeled malonic acid indicates that polymerization of acrylonitrile is initiated by malonyl radical. The bromous radicals are believed to terminate the polymer leaving an alcohol group on the polymer end. Further analysis of the polymer sample confirms that the sample is partially brominated. Numerical simulations based on a modified FKN mechanism are found to be in good agreement with experimental findings. Simulations indicate that periodic termination of the polymer chain by bromine dioxide, not the periodic initiation by malonyl radical, is the dominant cause of the periodic polymerization. Similar results were found in the malonyl-controlled variant of the BZ reaction.

Bonding in bromine oxides: Isomers of BrO 2, Br 2O 2 and BrO 3

Several isomers of bromine dioxide BrO 2, dibromine dioxide Br 2O 2 and bromine trioxide BrO 3 are studied in ab initio correlated calculations. Optimizations are performed at the UMP2/AREP/TZ(2df) level and the resulting equilibrium geometries analyzed discussing general features of the bonding in these compounds. The torsional barrier of the stable BrOOBr peroxide is also presented and the structural distortion accompanying the internal rotation is considered in relation to the nature of the bonding. Enthalpies of formation at 0 and 298.15 K are estimated via isodesmic reactions with CCSD(T)/AREP/TZ(2df) eneries at the UMP2 geometries previously determined.

Laboratory Formation of OBrO and Its Reactivity toward Ozone at 298 K

Bromine dioxide, OBrO, has been formed in our laboratory using three methods in a discharge flow reactor: (1) O + Br2; (2) Br + O3; and (3) microwave discharge of a Br2/O2/He mixture. The OBrO radical was detected using a mass spectrometer at m/e = 111/113. A new mechanism is proposed to account for the formation of the OBrO in these methods. The key to this mechanism is the self-reaction of vibrationally excited BrO. At 298 K, ground-state OBrO does not notably react with ozone, and an upper limit rate constant for the reaction of the ground-state OBrO with ozone was estimated to be k13 ≤ 5 × 10-16 cm3 molecule-1 s-1. The vibrationally excited OBrO, on the other hand, is at least 3 orders of magnitude more reactive toward ozone than the ground-state OBrO, with a rate constant of k13a = (5.4 ± 2.7) × 10-13 cm3 molecule-1 s-1.

Ab initio calculations on bromine oxide and dioxides and their corresponding anions

Neutral bromine oxides and dioxides as well as their corresponding anions have been studied by means of ab initio molecular orbital calculations. To test the importance of static and dynamic correlation in these systems both single-configuration-based methods [MP2, QCISD, and QCISD(T)] and multiconfiguration-based methods (CASSCF and CASMP2) have been used. Equilibrium geometries and harmonic vibrational frequencies have been obtained for BrO and the two bromine dioxide isomers (OBrO and BrOO). For the corresponding anionic species, excellent agreement has been obtained for the predicted geometries at QCISD(T) and CASMP2 levels, while frequencies obtained at QCISD(T) agree to within 10 cm−1 with the available experimental data. An analysis of the charge density shows that the nature of the BrO bond is very different within OBrO and BrOO, and that the BrO charge density is reinforced in OBrO relative to BrO itself.

The rotational spectrum and molecular properties of bromine dioxide, OBrO

The rotational spectrum of the OBrO radical has been observed in the gas phase over the solid products of the O+Br2 reaction. Spectra have been measured for both O79BrO and O81BrO in their (000), (010), and (020) vibrational states in selected regions between 88 and 627 GHz spanning the quantum numbers 1⩽N⩽61 and 0⩽Ka⩽14. The spectra are well described by a Hamiltonian which includes centrifugal distortion effects for fine and hyperfine terms. The molecular structure, the dipole moment, and the harmonic force field have been derived, and they, as well as fine and hyperfine structure constants, are compared with data of related molecules and electron spin resonance data from OBrO isolated in cryogenic salt matrices.

Distribution of ammonium on sulphate and nitrate containing aerosols qualitative estimates based on measurements and model calculations

s 1113 DISTRIBUTION OF AMMONIUM ON SULPHATE AND NITRATE CONTAINING AEROSOLS QUALITATIVE ESTIMATES BASED ON MEASUREMENTS AND MODEL CALCULATIONS 0. Hertel,* E. Vignati, *tf H. Skov* and M. F. Hovmand* *National Environmental Research Institute, P.O. Box 358, Frederiksborgvej 399, 4000 Roskilde, Denmark ’ Rise National Laboratory, P.O. Box 49, Frederiksborgvej 399,400O Roskilde, Denmark r Joint Research Centre, Environmental Institute, TP 460, 21020 Ispra, Italy The deposition of ammonium is an important source of nitrogen to marine waters. Ammonium is to a great extent associated with sulphate containing aerosols, but this may change due to the reductions in sulphur emissions. A reduction in sulphate in the atmosphere may increase the relative importance of ammonium nitrate, which has a shorter lifetime than ammonium sulphate and ammonium bisulphate. Such a change will have considerable impact on the transport of ammonia, ammonium, nitric acid and nitrate in the atmosphere. In the present paper, a qualitative analysis of the distribution of ammonium on sulphate and nitrate containing aerosols is performed based on filter pack measurements from the Danish Nation-wide Monitoring Programme. Two scenarios are examined assuming that, respectively, 1.5 and 2 ammonium molecules are associated with each sulphate molecule. This analysis show no clear trend in the distribution of ammonium between the two types of aerosols. The results of this analysis are compared with calculations with the variable scale transport-chemistry model-ACDEP. The model reproduced the observed ammonium levels well. However, the comparison indicates that the model overestimates the fraction of ammonium associated with sulphate. HIGH-RESOLUTION FTIR SPECTROSCOPY OF OBrO M. S. Johnson* and B. Nelander’ * MAX-Lab Beamline for Infrared Spectroscopy, Lund University, S-221 00 Lund, Sweden + Center for Chemistry, Thermochemistry, Lund University, Box 124, S-221 00 Lund, Sweden The gas-phase infrared spectrum of OBrO has been measured for the first time. The vl and v3 symmetric and asymmetric stretching vibrations of bromine dioxide have been measured at high resolution using the White cell at the beamline for infrared spectroscopy at MAX-Lab. In addition, the vl + $12 transition is observed. Interest in halogen oxides has been stimulated by their role as catalysts in stratospheric ozone depletion. A great deal of effort has gone into understanding the mechanisms of ozone depletion processes which involve chlorine. There is evidence from direct observation of earth’s atmosphere that bromine species are also involved. Unfortunately, due to the difficulty of their synthesis and the limitations of instrumentation there are few laboratory studies concerning the structure and reactivity of the members of the BrO, family. A THREE-DIMENSIONAL GLOBAL MODEL STUDY OF CARBONYL SULFIDE IN THE TROPOSPHERE AND THE LOWER STRATOSPHERE

The OBrO C(2A2)←X(2B1) absorption spectrum

The highly structured visible absorption spectrum of the bromine dioxide radical, OBrO, has been observed in the 15 500–26 000 cm−1 region. The spectrum is dominated by a long progression in the Br–O symmetric stretching motion (ν1′) and a series of short progressions built on the bending mode (ν2′); there are no features associated with the excitation of the antisymmetric stretching mode (ν3′). The spectrum also contains numerous transitions originating from the (0,1,0) and (1,0,0) vibrational levels of the electronic ground state, X(2B1). A simultaneous fit to all of the observed vibronic features yielded the frequencies ν1″=799.4 cm−1, ν2″=317.5 cm−1, ω1′=641.5 cm−1, ω2′=210.7 cm−1, and a band origin T0=15 863 cm−1. Franck–Condon simulations combined with ab initio calculations of the four lowest OBrO doublet electronic states identify the spectrum as arising from the C(2A2)←X(2B1) electronic transition.

Effect of Electric Current in the Minimal Bromate Reaction

Experiments and calculations of two electrically coupled flow reactors are presented in which the minimal bromate (MB) reaction is carried out in its bistable region consisting of the thermodynamically controlled state which is characterized by a high Ce4+ concentration and the kinetically controlled state which shows low Ce4+ concentrations. Application of an electric current causes redox processes on the working electrodes. Inside the region of bistability a transition from the thermodynamic branch to the kinetic branch occurs in both the anodic and the cathodic reactors when the potential of the applied current exceeds certain threshold values. However, in the MB reaction a transition from the kinetic to the thermodynamic branch is not possible by the application of an electrical current. On the thermodynamic branch outside the bistability region, chemical oscillations are generated in both reactors when the electric current exceeds a threshold value. When the electric current is further increased, the oscillations give way to a steady state of a low Ce4+ concentration. After turning off the electric current the system returns to its original thermodynamic branch. On the kinetic branch at high flow rates the effect of the electric current on the redox potential is minimal. The experimental results are in good agreement with simulations using the NFT model provided that the effect of the electric current is specifically applied to the rate term for the bromine dioxide radical.

Oscillations and waves in metal-ion-catalyzed bromate oscillating reactions in highly oxidized states

A mathematical model of the Belousov-Zhabotinsky (BZ) oscillating reaction is presented along with experimental data showing that the catalyst can remain almost completely oxidized during a significant part of the oscillatory cycle in the ferroin-catalyzed BZ reaction. The model semiquantitatively describes oscillations and wave propagation in both the cerium- and ferroin-catalyzed systems when the phase of catalyst oxidation occupies a significant part of the oscillatory cycle. Such modes are of special interest because they give rise to enhanced instability of the bulk oscillations, enlargement of the number of local wave sources, and decremental wave propagation. Reversibility of the catalyst oxidation by bromine dioxide radicals is crucial for the dynamics of the system under these conditions

The role of radicals in the Belousov-Zhabotinsky reaction

A new theory of the Belousov-Zhabotinsky (BZ) reaction, the Radicalator model, is presented. This model is based on a negative feedback loop involving a fast reaction between malonyl and bromine dioxide radicals. Experimental evidence for the validity of the model is given for BZ systems in 3 M and 1 M sulfuric acid solution.

Raman spectrum and structure of bromine dioxide

The Raman spectrum of bromine dioxide is interpreted as indicating that it has the dimeric structure Br2O4 with a Br–Br bond.

The Reactions of Mobile Electrons in the Frozen Solution of Ethylene Glycol-Water at 77°K

Abstract The yield of trapped electrons produced in ethylene glycol-water glass at 77°K increased linearly with the gamma dose up to 1.5×1020 eV/g, and the G value was estimated to be 2.8±0.2. The relative reaction rates of mobile electrons with various inorganic ions at 77°K were determined by competition kinetics. The observed relative reaction rates with simple inorganic ions corresponded to those of hydrated electrons obtained at room temperature. For complex ions, however, the reactivities of the mobile electrons were larger than those of hydrated electrons. The reaction products, such as bromine dioxide, Cr(V) species, and the pentacyano Co(II) ion, all of which are unstable at room temperature, could be detected by optical and ESR spectroscopies at 77°K.