Hydrogen Peroxide Photolysis in Acidic Aqueous Solutions Containing Chloride Ions. I. Chemical Mechanism

Radiation Induced Degradation of Organic Pollutants in Waters and Wastewaters

In water treatment by ionizing radiation, and also in other advanced oxidation processes, the main goal is to destroy, or at least to deactivate harmful water contaminants: pharmaceutical compounds, pesticides, surfactants, health-care products, etc. The chemical transformations are mainly initiated by hydroxyl radicals, and the reactions of the formed carbon centered radicals with dissolved oxygen basically determine the rate of oxidation. The concentration of the target compounds is generally very low as compared to the concentration of such natural 'impurities' as chloride and carbonate/bicarbonate ions or the dissolved humic substances (generally referred to as dissolved organic carbon), which consume the majority of the hydroxyl radicals. The different constituents compete for reacting with radicals initiating the degradation. This manuscript discusses the radiation chemistry of this complex system. It includes the reactions of the primary water radiolysis intermediates (hydroxyl radical, hydrated electron/hydrogen atom), the reactions of radicals that form in radical transfer reactions (dichloride-, carbonate- and sulfate radical anions) and also the contribution to the degradation of organic compounds of such additives as hydrogen peroxide, ozone or persulfate.

Photochemical oxidation of chloride ion by ozone in acid aqueous solution

The experimental investigation of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in the bulk of acid aqueous solution at pH 0-2 has been performed. Processes of chloride oxidation in these conditions are the same as the chemical reactions in the system O3 - OH - Cl(-)(aq). Despite its importance in the environment and for ozone-based water treatment, this reaction system has not been previously investigated in the bulk solution. The end products are chlorate ion ClO3(-) and molecular chlorine Cl2. The ions of trivalent iron have been shown to be catalysts of Cl(-) oxidation. The dependencies of the products formation rates on the concentrations of O3 and H(+) have been studied. The chemical mechanism of Cl(-) oxidation and Cl2 emission and ClO3(-) formation has been proposed. According to the mechanism, the dominant primary process of chloride oxidation represents the complex interaction with hydroxyl radical OH with the formation of Cl2(-) anion-radical intermediate. OH radical is generated on ozone photolysis in aqueous solution. The key subsequent processes are the reactions Cl2(-) + O3 → ClO + O2 + Cl(-) and ClO + H2O2 → HOCl + HO2. Until the present time, they have not been taken into consideration on mechanistic description and modelling of Cl(-) oxidation. The final products are formed via the reactions 2ClO → Cl2O2, Cl2O2 + H2O → 2H(+) + Cl(-) + ClO3(-) and HOCl + H(+) + Cl(-) ⇄ H2O + Cl2. Some portion of chloride is oxidized directly by O3 molecule with the formation of molecular chlorine in the end.

Degradation of carbon tetrachloride in aqueous solution in the thermally activated persulfate system

Thermal activation of persulfate (PS) has been identified to be effective in the destruction of organic pollutants. The feasibility of carbon tetrachloride (CT) degradation in the thermally activated PS system was evaluated. The experimental results showed that CT could be readily degraded at 50 °C with a PS concentration of 0.5M, and CT degradation and PS consumption followed the pseudo-first order kinetic model. Superoxide radical anion (O2(*-)) was the predominant radical species responsible for CT degradation and the split of CCl was proposed as the possible reaction pathways for CT degradation. The process of CT degradation was accelerated by higher PS dose and lower initial CT concentration. No obvious effect of the initial pH on the degradation of CT was observed in the thermally activated PS system. Cl(*-), HCO3(*-), and humic acid (HA) had negative effects on CT degradation. In addition, the degradation of CT in the thermally activated PS system could be significantly promoted by the solvents addition to the solution. In conclusion, the thermally activated PS process is a promising option in in-situ chemical oxidation/reduction remediation for degrading highly oxidized organic contaminants such as CT that is widely detected in contaminated sites.

Degradation of trichloroethylene in aqueous solution by calcium peroxide activated with ferrous ion

The application of calcium peroxide (CaO2) activated with ferrous ion to stimulate the degradation of trichloroethylene (TCE) was investigated. The experimental results showed that TCE could be completely degraded in 5 min at a CaO2/Fe(II)/TCE molar ratio of 4/8/1. Probe compound tests demonstrated the presence of reactive oxygen species HO· and O2(-·) in CaO2/Fe(II) system, while scavenging tests indicated that HO· was the dominant active species responsible for TCE removal, and O2(-·) could promote TCE degradation in CaO2/Fe(II) system. In addition, the influences of initial solution pH and solution matrix were evaluated. It suggested that the elevation of initial solution pH suppressed TCE degradation. Cl(-) had significant scavenging effect on TCE removal, whereas HCO3(-) of high concentration showed favorable function. The influences of NO3(-) and SO4(2-) could be negligible, while natural organic matter (NOM) had a negative effect on TCE removal at a relatively high concentration. The results demonstrated that the technique of CaO2 activated with ferrous ion is a highly promising technique in in situ chemical oxidation (ISCO) remediation in TCE contaminated sites.

Degradation of multi-DNAPLs by a UV/persulphate/ethanol system with the additional injection of a base solution

This study was conducted to investigate the inhibited influences on and solution to the degradation of four types of dense non-aqueous phase liquids (DNAPLs) (i.e. perchloroethylene [PCE], trichloroethylene [TCE], chloroform [CF], and carbon tetrachloride [CT]) all at the same instance in groundwater (GW). Degradations of DNAPLs in de-ionized water (DW) and GW were carried out by applying an ultraviolet radiation-activated persulphate (UV/PS) system. PCE and TCE were degraded by over 90% and CT was only degraded by 25% in both DW and GW. However, CF was degraded by over 90% in DW, while it was only degraded by 50% in GW. First of all, degradations with an inorganic anion (either Cl- or HCO3-) indicated that the lower degradation of CF in GW was caused by the existence of the chloride ion. Moreover, the low CF degradation in GW was overcome by the additional injection of a base solution (sodium hydroxide [NaOH]) into the UV/PS system. The results showed that PCE, TCE, and CF were degraded by over 90%, respectively, when a molar ratio of [base]0:[PS]0 was larger than 0.5:1, but CT was still not effectively degraded in the UV/PS system. To achieve effective CT degradation, UV/PS with the ethanol (EtOH) system was evaluated and it was found that it degraded CT over 90%. However, at this time, CF was not effectively degraded in the UV/PS/EtOH system. Finally, degradations of DNAPLs in the UV/PS/EtOH system with the additional injection of a base solution were conducted and it showed that multi-DNAPLs were degraded by over 90%, respectively, when the molar ratio of [PS]0:[EtOH]0:[base]0 was 1:1:3.

A novel pre-oxidation method for elemental mercury removal utilizing a complex vaporized absorbent

A novel semi-dry integrative method for elemental mercury (Hg(0)) removal has been proposed in this paper, in which Hg(0) was initially pre-oxidized by a vaporized liquid-phase complex absorbent (LCA) composed of a Fenton reagent, peracetic acid (CH3COOOH) and sodium chloride (NaCl), after which Hg(2+) was absorbed by the resultant Ca(OH)2. The experimental results indicated that CH3COOOH and NaCl were the best additives for Hg(0) oxidation. Among the influencing factors, the pH of the LCA and the adding rate of the LCA significantly affected the Hg(0) removal. The coexisting gases, SO2 and NO, were characterized as either increasing or inhibiting in the removal process, depending on their concentrations. Under optimal reaction conditions, the efficiency for the single removal of Hg(0) was 91%. Under identical conditions, the efficiencies of the simultaneous removal of SO2, NO and Hg(0) were 100%, 79.5% and 80.4%, respectively. Finally, the reaction mechanism for the simultaneous removal of SO2, NO and Hg(0) was proposed based on the characteristics of the removal products as determined by X-ray diffraction (XRD), atomic fluorescence spectrometry (AFS), the analysis of the electrode potentials, and through data from related research references.

Comparative study of the degradation of carbamazepine in water by advanced oxidation processes

Degradation of carbamazepine (CBZ) using ultraviolet (UV), UV/H2O2, Fenton, UV/Fenton and photocatalytic oxidation with TiO2 (UV/TiO2) was studied in deionized water. The five different oxidation processes were compared for the removal kinetics of CBZ. The results showed that all the processes followed pseudo-first-order kinetics. The direct photolysis (UV alone) was found to be less effective than UV/H2O2 oxidation for the degradation of CBZ. An approximate 20% increase in the CBZ removal efficiency occurred with the UV/Fenton reaction as compared with the Fenton oxidation. In the UV/TiO2 system, the kinetics of CBZ degradation in the presence of different concentrations of TiO2 followed the pseudo-first order degradation, which was consistent with the Langmuir-Hinshelwood (L-H) model. On a time basis, the degradation efficiencies ofCBZ were in the following order: UV/Fenton (86.9% +/- 1.7%) > UV/TiO2 (70.4% +/- 4.2%) > Fenton (67.8% +/- 2.6%) > UV/H2O2 (40.65 +/- 5.1%) > UV (12.2% +/- 1.4%). However, the lowest cost was obtained with the Fenton process.

Influence of chloride and carbonates on the reactivity of activated persulfate

Chloride and carbonates have the potential to impact pathway, kinetics, and efficiency of oxidation reactions, both as radical scavengers and as metal complexing agents. Traditionally, it is assumed that they have an overall negative impact on the activated persulfate performance. This study investigated the influence of carbonates and chloride on the reactivity of persulfate for three different activation techniques to produce reactive free sulfate radicals; heat, alkaline and iron activation. By using p-nitrosodimethylaniline as model target compound, it was demonstrated that iron activation at neutral pH was not affected by Cl(-) or HCO(3)(-), alkaline activation was enhanced by Cl(-) and even more by CO(3)(2-), and heat activation was enhanced by Cl(-), and no effect from HCO(3)(-) was observed. At pH 2 destruction of perchloroethylene by iron activated persulfate was significantly affected by chloride. Reaction rates decreased, but the overall oxidation efficiency was unaffected up to 28 mM Cl(-). The effect of chloride and carbonates is caused by direct attack of produced reactive chlorine, or carbonate species or by catalysis of the propagation reactions resulting in more sulfate radicals. These results show that carbonate and chloride might play an important role in activated persulfate applications and should not strictly be considered as scavengers.

Formation of Pb(III) intermediates in the electrochemically controlled Pb(II)/PbO₂ system

The formation of lead dioxide PbO(2), an important corrosion product in drinking water distribution systems with lead-bearing plumbing materials, has been hypothesized to involve Pb(III) intermediates, but their nature and formation mechanisms remain unexplored. This study employed the electrochemical (EC) method of rotating ring disk electrode (RRDE) and quantum chemical (QC) simulations to examine the generation of intermediates produced during the oxidation of Pb(II) to PbO(2). RRDE data demonstrate that PbO(2) deposition and reduction involves at least two intermediates. One of them is a soluble Pb(III) species that undergoes further transformations to yield immobilized PbO(2) nanoparticles. The formation of this intermediate in EC system is mediated by hydroxyl radicals (OH(•)), as was evidenced by the suppression of intermediates formation in the presence of the OH(•) scavenger para-chlorobenzoic acid. QC simulations confirmed that the oxidation of Pb(II) by OH(•) proceeds via Pb(III) species. These results show that Pb(III) intermediates play an important role in the reactions determining transitions between Pb(II) and Pb(IV) species and could impact lead release in drinking water.

Standard Gibbs free energies of reactions of ozone with free radicals in aqueous solution: quantum-chemical calculations

Free radicals are common intermediates in the chemistry of ozone in aqueous solution. Their reactions with ozone have been probed by calculating the standard Gibbs free energies of such reactions using density functional theory (Jaguar 7.6 program). O(2) reacts fast and irreversibly only with simple carbon-centered radicals. In contrast, ozone also reacts irreversibly with conjugated carbon-centered radicals such as bisallylic (hydroxycylohexadienyl) radicals, with conjugated carbon/oxygen-centered radicals such as phenoxyl radicals, and even with nitrogen- oxygen-, sulfur-, and halogen-centered radicals. In these reactions, further ozone-reactive radicals are generated. Chain reactions may destroy ozone without giving rise to products other than O(2). This may be of importance when ozonation is used in pollution control, and reactions of free radicals with ozone have to be taken into account in modeling such processes.

Tropospheric aqueous-phase free-radical chemistry: radical sources, spectra, reaction kinetics and prediction tools

The most important radicals which need to be considered for the description of chemical conversion processes in tropospheric aqueous systems are the hydroxyl radical (OH), the nitrate radical (NO(3)) and sulphur-containing radicals such as the sulphate radical (SO(4)(-)). For each of the three radicals their generation and their properties are discussed first in the corresponding sections. The main focus herein is to summarize newly published aqueous-phase kinetic data on OH, NO(3) and SO(4)(-) radical reactions relevant for the description of multiphase tropospheric chemistry. The data compilation builds up on earlier datasets published in the literature. Since the last review in 2003 (H. Herrmann, Chem. Rev. 2003, 103, 4691-4716) more than hundred new rate constants are available from literature. In case of larger discrepancies between novel and already published rate constants the available kinetic data for these reactions are discussed and recommendations are provided when possible. As many OH kinetic data are obtained by means of the thiocyanate (SCN(-)) system in competition kinetic measurements of OH radical reactions this system is reviewed in a subchapter of this review. Available rate constants for the reaction sequence following the reaction of OH+SCN(-) are summarized. Newly published data since 2003 have been considered and averaged rate constants are calculated. Applying competition kinetics measurements usually the formation of the radical anion (SCN)(2)(-) is monitored directly by absorption measurements. Within this subchapter available absorption spectra of the (SCN)(2)(-) radical anion from the last five decades are presented. Based on these spectra an averaged (SCN)(2)(-) spectrum was calculated. In the last years different estimation methods for aqueous phase kinetic data of radical reactions have been developed and published. Such methods are often essential to estimate kinetic data which are not accessible from the literature. Approaches for rate constant prediction include empirical correlations as well as structure activity relationships (SAR) either with or without the usage of quantum chemical descriptors. Recently published estimation methods for OH, NO(3) and SO(4)(-) radical reactions in aqueous solution are finally summarized, compared and discussed.

Involvements of chloride ion in decolorization of Acid Orange 7 by activated peroxydisulfate or peroxymonosulfate oxidation

The effects of chloride anion (Cl-) (up to 1.0 mol/L) on the decolorization of a model compound, azo dye Acid Orange 7 (AO7), by sulfate radical (SO4-*) based-peroxydisulfate (PS) or peroxymonosulfate (PMS) oxidation under various activated conditions (UV254 nm/PS, Thermal (70 degrees C/PS, UV254 nm/PMS, Co2+/PMS) were investigated. Methanol and NH4+ were used as quenching reagents to determine the contributions of active chlorine species (dichloride radical (Cl2-*) and hypochlorous acid (HClO)). The results indicated that the effects of Cl- on the reaction mechanism were different under various activated conditions. For UV/PS and Thermal/PS, the inhibition tendency became more clear as the Cl- concentration increased, probably due to the reaction between Cl- and SO4-* and the generation of Cl2-* or HCIO. For UV/PMS, Cl- did not exhibit inhibition when the concentration was below 0.1 mol/L. As Cl- concentration reached to 1.0 mol/L, the decolorization rate of AO7 was, however, accelerated, possibly because PMS directly reacts with Cl- to form HClO. For Co2+/PMS, Cl- exhibited a significant inhibiting effect even at low concentration (< or = 0.01 mol/L). When Cl- concentration exceeded 0.1 mol/L, the activation of PMS by Co2+ was almost completely inhibited. Under this condition, HClO maybe played a major role in decolorization of AO7. The results implicated that chloride ion is an important factor in SO4(-*) -based degradation of organic contamination in chloride-containing water.

Chemical and microbial decontamination of pool water using activated potassium peroxymonosulfate

Potassium peroxymonosulfate activation leads to the formation of highly reactive species, mainly the sulfate radicals. Activated potassium peroxymonosulfate (from now on peroxymonosulfate) was tested against specific pollutants such as ammonium ion, creatinine, chlorinated creatinine products, arginine and Escherichia coli (E. coli), all constituents or derivatives of human discharges. The objective was to assess whether activated peroxymonosulfate can be a viable treatment reagent in recreational water applications. It was found that organic molecules such as creatinine, chlorinated creatinine products and arginine could be effectively treated with activated peroxymonosulfate. Ammonium ion was oxidized only by chlorine species and only in de-ionized water. Chlorine species were formed from the reaction of sulfate radicals with chloride ions. In pool water, the reaction of sulfate radicals with chloride ions and the subsequent ammonium ion oxidation were scavenged by the presence of bicarbonate ions. The Co/Peroxymonosulfate system was also shown to be an effective disinfection reagent, since 99.99% (4-log) kill of E. coli was achieved in 60 min of treatment. At the concentrations tested here, however, it is still not efficacious enough to qualify as an EPA-registered sanitizer for swimming pools (requires 6-log kill of E. coli, ATCC 11229, and Enterococcus faecium, ATCC 6569, in 30s).

Polychromatic UV photon irradiance measurements using chemical actinometers based on NO3- and H2O2 excitation: applications for industrial photoreactors

Ultraviolet light (UV) is a proven technology for disinfecting and decontaminating drinking water. For this purpose, both low-pressure monochromatic and medium-pressure polychromatic mercury arc lamps are commonly being used. Methods for photon irradiance measurements of monochromatic light are not necessarily appropriate for polychromatic light, and there is a need for the latter especially when large volumes of water are involved. The present manuscript reports a comparative study of polychromatic UV photolysis of aqueous NO3- containing RH (RH = HCO2-, CH3OH, or C2H5OH) and H2O2 containing CH3OH, all in aerated buffered aqueous solutions. The UV photolysis of NO3- generates ONOO-, *OH, and *NO2 intermediates and the stable NO2- ion. The maximum yield of NO2- is obtained in the presence of RH. The UV photolysis of H2O2 generates *OH radicals, which in the presence of CH3OH form formaldehyde. The H2O2/CH3OH actinometer is limited to low and moderate light intensities because the reaction mechanism involves competition between second- and first-order processes. Therefore, the NO3- actinometer is preferable at high photon irradiance despite the relatively low quantum yield of NO2- and its dependence on the excitation wavelength. The two actinometers are compared to radiometry and to iodide/iodate actinometer. The latter is limited to the 200-280 nm range due to the absorption of the photoproduct I3-. The NO3-/C2H5OH actinometer is particularly useful for large volumes of water in industrial high-intensity UV photoreactors as the actinometer solution can be safely disposed.

Abatement of the inhibitory effect of chloride anions on the photo-Fenton process

The inhibition of the photo-Fenton (Fe2+/Fe3+, H2O2, UV light) degradation of synthetic phenol wastewater solutions by chloride ions is shown to affect primarily the photochemical step of the process, having only a slight effect on the thermal or Fenton step. Kinetic studies of the reactions of oxoiron (IV) (FeO2+) with phenol indicate that, if FeO2+ is formed in the photo-Fenton degradation, its role is probably minor. Finally, it is shown that, for both a synthetic phenol wastewater and an aqueous extract of Brazilian gasoline, the inhibition of the photo-Fenton degradation of the organic material in the presence of chloride ion can be circumvented by maintaining the pH of the medium at or slightly above 3 throughout the process, even in the presence of significant amounts of added chloride ion (0.5 M).

Photochemistry of Peroxoborates: Borate Inhibition of the Photodecomposition of Hydrogen Peroxide

The UV absorbance and photochemical decomposition kinetics of hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. At higher pH borate/boric acid inhibits the photodecomposition of hydrogen peroxide (molar absorptivity and quantum yield of H(2)O(2) and HO(2) (-), (19.0+/-0.3) M(-1) cm(-1) and 1, and (237+/-7) M(-1) cm(-1) and 0.8+/-0.1, respectively). The results are consistent with the equilibrium formation of the anions monoperoxoborate, K(BOOH)=[H(+)][HOOB(OH)(3) (-)]/([B(OH)(3)][H(2)O(2)]), 2.0 x 10(-8), R. Pizer, C. Tihal, Inorg. Chem. 1987, 26, 3639-3642, and monoperoxodiborate, K(BOOB)=[BOOB(2-)]/([B(OH)(4) (-)][HOOB(OH)(3) (-)]), 1.0+/-0.3 or 4.3+/-0.9, depending upon the conditions, with molar absorptivity, (19+/-1) M(-1) cm(-1) and (86+/-15) M(-1) cm(-1), respectively, and respective quantum yields, 1.1+/-0.1 and 0.04+/-0.04. The low quantum yield of monoperoxodiborate is discussed in terms of the slower diffusion apart of incipient (.)OB(OH)(3) (-) radicals than may be possible for (.)OH radicals, or a possible oxygen-bridged cyclic structure of the monoperoxodiborate.

Influences of carbonate and chloride ions on persulfate oxidation of trichloroethylene at 20 degrees C

Application of in situ chemical oxidation (ISCO) involves application of oxidants to contaminants such as trichloroethylene (TCE) in soil or groundwater in place. Successful application of ISCO at a hazardous waste site requires understanding the scavenging reactions that could take place at the site to better optimize the oxidation of target contaminants and identification of site conditions where ISCO using persulfate may not be applicable. Additionally, estimation of the oxidant dose at a site would need identification of groundwater constituents such as alkalinity and chlorides that may scavenge radicals and therefore use up the oxidant that is targeted for the contaminant(s). The objective of this study was to investigate the influence of various levels of chloride and carbonates on persulfate oxidation of TCE at 20 degrees C under controlled conditions in a laboratory. Based on the results of the laboratory experiments, both chloride and alkalinity were shown to have scavenging effects on the rate of oxidation of TCE. It was found that at a neutral pH, persulfate oxidation of TCE was not affected by the presence of bicarbonate/carbonate concentrations within the range of 0-9.20 mM. However, the TCE degradation rate was seen to reduce with an increase in the level of carbonate species and at elevated pHs. TCE degradation in the presence of chlorides revealed no effect on the degradation rate especially at chloride levels below 0.2 M. However, at chloride levels greater than 0.2 M, TCE degradation rate was seen to reduce with an increase in the chloride ion concentration. Prior to application of persulfate as an oxidant, a site should be screened for the presence of scavengers to evaluate the potential of meeting target cleanup goals within a desirable timeframe at the site.

Ultrasound-Induced Aqueous Removal of Nitric Oxide from Flue Gases: Effects of Sulfur Dioxide, Chloride, and Chemical Oxidant

The effects of sulfur dioxide (SO(2)), sodium chloride (NaCl), and peroxymonosulfate or oxone (2KHSO(5).KHSO(4).K(2)SO(4) with active ingredient, HSO(5)(-)) on the sonochemical removal of nitric oxide (NO) have been studied in a bubble column reactor. The initial concentration of NO studied ranged from about 500 to 1040 ppm. NaCl in the concentration range of 0.01-0.5 M was used as the electrolyte to study the effect of ionic strength. At the low NaCl concentration (0.01 M), the percent fractional removal of NO with initial concentration of 1040 ppm was enhanced significantly, while as the NaCl concentration increased, the positive effects were less pronounced. The presence of approximately 2520 ppm SO(2) in combination with 0.01 M NaCl further enhanced NO removal. However, with a NO initial concentration of 490 ppm, the addition of NaCl was detrimental to NO removal at all NaCl concentration levels. The combinative effect of sonication and chemical oxidation using 0.005-0.05 M oxone was also studied. While the lower concentrations of HSO(5)(-) enhanced NO removal efficiency, higher concentrations were detrimental depending on the initial concentration of NO. It was also demonstrated that in the presence of ultrasound, the smallest concentration of oxone was needed to obtain optimal fractional conversion of NO.

Laser Flash Photolysis Study of the Photocatalytic Step of the Photo-Fenton Reaction in Saline Solution†

The photo-Fenton reaction (Fe2+/Fe3+, H2O2, UV light) is strongly inhibited by high concentrations of added chloride ion. In this work, the effect of added chloride ion on the photocatalytic step that converts Fe(III) back to Fe(II) is studied by nanosecond laser flash photolysis over a wide range of pH (1.0-3.3) and concentrations of Fe(III) (0.1-1.0 mM) and chloride ion (0.05-0.75 M). An explicit mechanistic model based on the preferential formation of the less-reactive Cl2*- radical anion via two routes (competitive photolysis of the iron(III)-chloride complex to chlorine atoms instead of the desired hydroxyl radical and pH-dependent scavenging of the hydroxyl radical by chloride ion) is proposed. This model, which fits the laser flash photolysis data for the production and decay of Cl2*- over the entire range of conditions investigated, suggests that inhibition of the photocatalytic step of the photo-Fenton process in the presence of chloride ion can be circumvented by maintaining the pH of the medium at or slightly above 3.0 throughout the reaction.

Kinetics Studies of Aqueous Phase Reactions of Cl Atoms and Cl2- Radicals with Organic Sulfur Compounds of Atmospheric Interest

A laser flash photolysis-long path UV-visible absorption technique has been employed to investigate the kinetics of aqueous phase reactions of chlorine atoms (Cl) and dichloride radicals (Cl2(-)) with four organic sulfur compounds of atmospheric interest, dimethyl sulfoxide (DMSO; CH3S(O)CH3), dimethyl sulfone (DMSO2; CH3(O)S(O)CH3), methanesulfinate (MSI; CH3S(O)O-), and methanesulfonate (MS; CH3(O)S(O)O-). Measured rate coefficients at T = 295 +/- 1 K (in units of M(-1) s(-1)) are as follows: Cl + DMSO, (6.3 +/- 0.6) x 10(9); Cl2(-) + DMSO, (1.6 +/- 0.8) x 10(7); Cl + DMSO2, (8.2 +/- 1.6) x 10(5); Cl2(-) + DMSO2, (8.2 +/- 5.5) x 10(3); Cl2(-) + MSI, (8.0 +/- 1.0) x 10(8); Cl + MS, (4.9 +/- 0.6) x 10(5); Cl2(-) + MS, (3.9 +/- 0.7) x 10(3). Reported uncertainties are estimates of accuracy at the 95% confidence level and the rate coefficients for MSI and MS reactions with Cl2(-) are corrected to the zero ionic strength limit. The absorption spectrum of the DMSO-Cl adduct is reported; peak absorbance is observed at 390 nm and the peak extinction coefficient is found to be 5760 M(-1) cm(-1) with a 2sigma uncertainty of +/-30%. Some implications of the new kinetics results for understanding the atmospheric sulfur cycle are discussed.

Efficient dehalogenation of polyhalomethanes and production of strong acids in aqueous environments: Water-catalyzed O–H-insertion and HI-elimination reactions of isodiiodomethane (CH2I–I) with water

A combined experimental and theoretical study of the ultraviolet photolysis of CH2I2 in water is reported. Ultraviolet photolysis of low concentrations of CH2I2 in water was experimentally observed to lead to almost complete conversion into CH2(OH)2 and 2HI products. Picosecond time-resolved resonance Raman spectroscopy experiments in mixed water/acetonitrile solvents (25%-75% water) showed that appreciable amounts of isodiiodomethane (CH2I-I) were formed within several picoseconds and the decay of the CH2I-I species became substantially shorter with increasing water concentration, suggesting that CH2I-I may be reacting with water. Ab initio calculations demonstrate the CH2I-I species is able to react readily with water via a water-catalyzed O--H-insertion and HI-elimination reaction followed by its CH2I(OH) product undergoing a further water-catalyzed HI-elimination reaction to make a H2C=O product. These HI-elimination reactions produce the two HI leaving groups observed experimentally and the H2C=O product further reacts with water to produce the other final CH2(OH)2 product observed in the photochemistry experiments. These results suggest that CH2I-I is the species that reacts with water to produce the CH2(OH)2 and 2HI products seen in the photochemistry experiments. The present study demonstrates that ultraviolet photolysis of CH2I2 at low concentration leads to efficient dehalogenation and release of multiple strong acid (HI) leaving groups. Some possible ramifications for the decomposition of polyhalomethanes and halomethanols in aqueous environments as well as the photochemistry of polyhalomethanes in the natural environment are briefly discussed.

Direct observation of an isopolyhalomethane O–H insertion reaction with water: Picosecond time-resolved resonance Raman (ps-TR3) study of the isobromoform reaction with water to produce a CHBr2OH product

Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy was used to obtain the first definitive spectroscopic observation of an isopolyhalomethane O-H insertion reaction with water. The ps-TR3 spectra show that isobromoform is produced within several picoseconds after photolysis of CHBr3 and then reacts on the hundreds of picosecond time scale with water to produce a CHBr2OH reaction product. Photolysis of low concentrations of bromoform in aqueous solution resulted in noticeable formation of HBr strong acid. Ab initio calculations show that isobromoform can react with water to produce a CHBr2(OH) O-H insertion reaction product and a HBr leaving group. This is consistent with both the ps-TR3 experiments that observe the reaction of isobromoform with water to form a CHBr2(OH) product and photolysis experiments that show HBr acid formation. We briefly discuss the implications of these results for the phase dependent behavior of polyhalomethane photochemistry in the gas phase versus water solvated environments.

Comparison of the dehalogenation of polyhalomethanes and production of strong acids in aqueous and salt (NaCl) water environments: Ultraviolet photolysis of CH[sub 2]I[sub 2]

The ultraviolet photolysis of CH(2)I(2) was studied in water and salt water solutions using photochemistry and picosecond time-resolved resonance Raman spectroscopy. Photolysis in both types of environments produces mainly CH(2)(OH)(2) and HI products. However, photolysis of CH(2)I(2) in salt water leads to the formation of different products/intermediates (CH(2)ICl and Cl(2) (-)) not observed in the absence of salt in aqueous solutions. The amount of CH(2)(OH)(2) and HI products appears to decrease after photolysis of CH(2)I(2) in salt water compared to pure water. We briefly discuss possible implications of these results for photolysis of CH(2)I(2) and other polyhalomethanes in sea water and other salt aqueous environments compared to nonsalt water solvated environments.