Hydroxyl radical production via the photo-Fenton reaction in the presence of fulvic acid

The photo-Fenton reaction, the reaction of photoproduced Fe(II) with H2O2 to form the highly reactive hydroxyl radical (OH*), could be an important source of OH* in sunlit natural waters. To determine if the photo-Fenton reaction is significant in mildly acidic surface waters, we conducted experiments simulating conditions representative of natural freshwaters using solutions of standard fulvic acid and amorphous iron oxide at pH 6.0. A probe method measuring 14CO2 produced by the reaction of 14C-labeled formate with OH* was used to detect small OH* production rates without otherwise influencing the chemical reactions occurring in the experiments. Net H2O2 accumulation was simultaneously measured using an acridinium ester chemiluminescence method. Measured losses of H2O2 by reaction with Fe(II) in dark experiments produced approximately the expected quantities of OH*. The difference between H2O2 accumulation in the presence and absence of Fe(III) in fulvic acid solutions exposed to light was interpreted as the loss of H2O2 by reaction with photoproduced Fe(II), consistent with measured OH* production rates. The Fe ligand desferrioxamine mesylate eliminated both OH* production and H2O2 photoloss induced by Fe. Our results imply that when Fe is a major sink of H2O2, the photo-Fenton reaction is likely to be the most important source of OH*, leading to a significant sink of organic compounds in a wide variety of sunlit freshwaters.     

Efficient degradation of TCE in groundwater using Pd and electro-generated H2 and O2: a shift in pathway from hydrodechlorination to oxidation in the presence of ferrous ions

Degradation of trichloroethylene (TCE) in simulated groundwater by Pd and electro-generated H(2) and O(2) is investigated in the absence and presence of Fe(II). In the absence of Fe(II), hydrodechlorination dominates TCE degradation, with accumulation of H(2)O(2) up to 17 mg/L. Under weak acidity, low concentrations of oxidizing ?OH radicals are detected due to decomposition of H(2)O(2), slightly contributing to TCE degradation via oxidation. In the presence of Fe(II), the degradation efficiency of TCE at 396 μM improves to 94.9% within 80 min. The product distribution proves that the degradation pathway shifts from 79% hydrodechlorination in the absence of Fe(II) to 84% ?OH oxidation in the presence of Fe(II). TCE degradation follows zeroth-order kinetics with rate constants increasing from 2.0 to 4.6 μM/min with increasing initial Fe(II) concentration from 0 to 27.3 mg/L at pH 4. A good correlation between TCE degradation rate constants and ?OH generation rate constants confirms that ?OH is the predominant reactive species for TCE oxidation. Presence of 10 mM Na(2)SO(4), NaCl, NaNO(3), NaHCO(3), K(2)SO(4), CaSO(4), and MgSO(4) does not significantly influence degradation, but sulfite and sulfide greatly enhance and slightly suppress degradation, respectively. A novel Pd-based electrochemical process is proposed for groundwater remediation.     

Preliminary investigation of the supply of chemical species to an aqueous solution using a hydrogen-oxygen flame

A new method of supplying radical species to aqueous solutions using a hydrogen-oxygen flame is investigated. When a hydrogen-oxygen flame is directed on the surface of an aqueous solution, hydroxyl radicals (*OH) produced in the flame are extracted into the aqueous phase. The presence of *OH in the aqueous solution was confirmed by electron paramagnetic resonance with spin trapping using 5,5-dimethyl-1-pyrroline-N-oxide. The extraction of *OH into the aqueous solution was monitored using a quantitative analysis of hydrogen peroxide (H2O2). The effects of the hydrogen and oxygen gas flow rates, hydrogen/oxygen ratio, and atmosphere on H2O2 formation were studied. When the hydrogen-oxygen flame blew on a phosphate buffer solution (pH 6.7) under an Ar atmosphere, the concentration of H2O2 increased with the blowing time of the flame and the flow rate of hydrogen gas. Under air, nitrate and nitrite ions were formed in the aqueous phase in addition to H2O2, and the H2O2 concentration was lower than that under argon. The application of this new method to an aqueous solution of Cu(II)-ethylenediaminetetraacetic acid (EDTA) caused a remarkable decrease in the concentration of Cu(II)-EDTA and total organic carbon.     

Mechanism for OH-initiated degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins in the presence of O2 and NO/H2O

The atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TeCDD) is investigated theoretically by high-accuracy molecular orbital calculation. The study shows that the OH radical can easily be added to the C atom adjacent to the O atom in dioxin ring to form OH radical adduct. The 2,3,7,8-TeCDD-OH adduct can immediately react with O(2) to form the 2,3,7,8-TeCDD-OH-O(2) adduct which can react with NO or H(2)O to complete the decomposition process. The degradation mechanism varies with the addition position of O(2) and the O-abstraction by NO. The OH radical can be reproduced through the H-abstraction of H(2)O and initiate a new round of degradation. The direct dynamic calculation is performed, and the rate constants is calculated over a temperature range of 200-1200 K, using the canonical variational transition state theory with small-curvature tunneling effect. The four-parameter formula of rate constants with the temperature is fitted and the lifetimes of the reaction species in the troposphere are estimated according to the rate constants, which is helpful for the atmospheric model study on the formation and degradation of dioxin.     

Development of an E-H2O2/TiO2 photoelectrocatalytic oxidation system for water and wastewater treatment

In this study, an innovative E-H2O2/TiO2 (E-H2O2 = electrogenerated hydrogen peroxide) photoelectrocatalytic (PEC) oxidation system was successfully developed for water and wastewater treatment. A TiO2/Ti mesh electrode was applied in this photoreactor as the anode to conduct PEC oxidation, and a reticulated vitreous carbon (RVC) electrode was used as the cathode to electrogenerate hydrogen peroxide simultaneously. The TiO2/Ti mesh electrode was prepared with a modified anodic oxidation process in a quadrielectrolyte (H2SO4-H3PO4-H2O2-HF) solution. The crystal structure, surface morphology, and film thickness of the TiO2/Ti mesh electrode were characterized by X-ray diffraction and scanning electron microscopy. The analytical results showed that a honeycomb-type anatase film with a thickness of 5 microm was formed. Photocatalytic oxidation (PC) and PEC oxidation of 2,4,6-trichlorophenol (TCP) in an aqueous solution were performed under various experimental conditions. Experimental results showed that the TiO2/Ti electrode, anodized in the H2SO4-H3PO4-H2O2-HF solution, had higher photocatalytic activity than the TiO2/Ti electrode anodized in the H2SO4 solution. It was found that the maximum applied potential would be around 2.5 V, corresponding to an optimum applied current density of 50 microA cm(-2) under UV-A illumination. The experiments confirmed that the E-H2O2 on the RVC electrode can significantly enhance the PEC oxidation of TCP in aqueous solution. The rate of TCP degradation in such an E-H2O2-assisted TiO2 PEC reaction was 5.0 times that of the TiO2 PC reaction and 2.3 times that of the TiO2 PEC reaction. The variation of pH during the E-H2O2-assisted TiO2 PEC reaction, affected by individual reactions, was also investigated. It was found that pH was well maintained during the TCP degradation in such an E-H2O2/TiO2 reaction system. This is beneficial to TCP degradation in an aqueous solution.     

Controlled OH radical production via ozone-alkene reactions for use in aerosol aging studies

We present a novel method for continuous, stable OH radical production for use in smog chamber studies, especially those focused on organic aerosol aging. Our source produces OH radicals from the reaction of 2,3-dimethyl-2-butene and ozone and is unique as a method that requires neither NOx nor UV photolysis of a radical precursor. Typical radical concentrations are in the range of (4-8) x 10(6) molec cm(-3) and are easily sustainable over experimental time scales of several hours. We discuss design considerations, radical production capability under different operating conditions, and the core source chemistry. As a proof of concept we present preliminary results from oxidation of n-hexacosane aerosol observed with an Aerodyne Aerosol Mass Spectrometer. The extent of hexacosane oxidation is sufficient to significantly change the organic aerosol mass spectrum by virtue of fast heterogeneous uptake of OH radicals at the particle surface, with a calculated uptake coefficient gamma = 1.04 +/-0.21.     

Hydroxyl radical generation mechanism during the redox cycling process of 1,4-naphthoquinone

Airborne quinones contribute to adverse health effects of ambient particles probably because of their ability to generate hydroxyl radicals (·OH) via redox cycling, but the mechanisms remain unclear. We examined the chemical mechanisms through which 1,4-naphthoquinone (1,4-NQ) induced ·OH, and the redox interactions between 1,4-NQ and ascorbate acid (AscH(2)). First, ·OH formation by 1,4-NQ was observed in cellular and acellular systems, and was enhanced by AscH(2). AscH(2) also exacerbated the cytotoxicity of 1,4-NQ in Ana-1 macrophages, at least partially due to enhanced ·OH generation. The detailed mechanism was studied in an AscH(2)/H(2)O(2) physiological system. The existence of a cyclic 1,4-NQ process was shown by detecting the corresponding semiquinone radical (NSQ·-) and hydroquinone (NQH(2)). 1,4-NQ was reduced primarily to NSQ·- by O2·- (which was from AscH(2) reacting with H(2)O(2)), not by AscH(2) as normally thought. At lower doses, 1,4-NQ consumed O2·- to suppress ·OH; however, at higher doses, 1,4-NQ presented a positive association with ·OH. The reaction of NSQ·- with H(2)O(2) to release ·OH was another important channel for OH radical formation except for Haber-Weiss reaction. As a reaction precursor for O2·-, the enhanced ·OH response to 1,4-NQ by AscH(2) was indirect. Reducing substrates were necessary to sustain the redox cycling of 1,4-NQ, leading to more ·OH and a deleterious end point.     

Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen

The corrosion of zero-valent iron (Fe0(s)) by oxygen (O2) can lead to the oxidation of organic compounds. To gain insight into the reaction mechanism and to assess the nature of the oxidant, the oxidation of methanol, ethanol, 2-propanol, and benzoic acid by the reaction of nanoparticulate zero-valent iron (nZVI) or ferrous iron (Fe[II]) with O2 in the absence of ligands was studied. At pH values below 5, Fe0(s) nanoparticles were oxidized by O2 within 30 min with a stoichiometry of approximately two Fe0(s) oxidized per O2 consumed. The yield of methanol and ethanol oxidation products increased from 1% at acidic pH to 6% at pH 7, relative to nZVI added. Product yields from 2-propanol and benzoic acid were highest under acidic conditions, with little oxidation observed at neutral pH. At pH values below 5, product formation was attributable to hydroxyl radical (OH.) production through the Fenton reaction, involving hydrogen peroxide and Fe(II) produced during nZVI oxidation. At higher pH values, the oxidation of Fe(II), the initial product of nZVI oxidation, by oxygen is responsible for most of the oxidant production. Product yields at circumneutral pH values were consistent with a different oxidant, such as the ferryl ion (Fe[IV]).     

Decomposition of hydrogen peroxide and organic compounds in the presence of dissolved iron and ferrihydrite

This work examines the contribution of solution phase reactions, especially those involving a chain reaction mechanism, to the decomposition of hydrogen peroxide (H2O2) and organic compounds in the presence of dissolved iron and ferrihydrite. In solutions at pH 4, where Fe was introduced as dissolved Fe(III), both H2O2 and 14C-labeled formic acid decomposed at measurable rates that agreed reasonably well with those predicted by a kinetic model of the chain reaction mechanism, using published rate constants extrapolated to pH 4. The ratio of the formic acid and H2O2 decomposition rates, as well as the dramatic effect of tert-butyl alcohol on these rates, confirmed that a solution chain reaction mechanism involving *OH controlled the decomposition kinetics of both compounds. In the presence of ferrihydrite as the iron source, the ratio of the rate of formic acid decomposition to that of H2O2 decomposition was significantly lower than that observed in the presence of only dissolved Fe. Moreover, neither rate diminished drastically upon addition of tert-butyl alcohol, indicating that the solution phase chain reaction is not a dominant decomposition pathway of H2O2 and formic acid. Relative decomposition rates of formic acid and a second *OH probe, benzoic acid, were consistent with oxidation of these compounds by *OH. These observations can be reproduced by a kinetic model including (a) decomposition of H2O2 at the iron oxide surface, producing *OH with lower yield than the reaction sequence with dissolved Fe, and (b) low concentrations of dissolved Fe in the presence of ferrihydrite, preventing propagation of the solution phase chain reaction.     

Efficient electrochemical oxidation of perfluorooctanoate using a Ti/SnO2-Sb-Bi anode

The electrochemical decomposition of persistent perfluorooctanoate (PFOA) with a Ti/SnO2-Sb-Bi electrode was demonstrated in this study. After 2 h electrolysis, over 99% of PFOA (25 mL of 50 mg·L(-1)) was degraded with a first-order kinetic constant of 1.93 h(-1). The intermediate products including short-chain perfluorocarboxyl anions (CF3COO-, C2F5COO-, C3F7COO-, C4F9COO-, C5F11COO-, and C6F13COO-) and F- were detected in the aqueous solution. The electrochemical oxidation mechanism was revealed, that PFOA decomposition first occurred through a direct one electron transfer from the carboxyl group in PFOA to the anode at the potential of 3.37 V (vs saturated calomel electrode, SCE). After that, the PFOA radical was decarboxylated to form perfluoroheptyl radical which allowed a defluorination reaction between perfluoroheptyl radical and hydroxyl radical/O2. Electrospray ionization (ESI) mass spectrum further confirmed that the oxidation of PFOA on the Ti/SnO2-Sb-Bi electrode proceeded from the carboxyl group in PFOA rather than C-C cleavage, and the decomposition processes followed the CF2 unzipping cycle. The electrochemical technique with the Ti/SnO2-Sb-Bi electrode provided a potential method for PFOA degradation in the aqueous solution.     

Atmospheric Chemical Reactions of 2,3,7,8-Tetrachlorinated Dibenzofuran Initiated by an OH Radical: Mechanism and Kinetics Study

Reactions with the OH radical are expected to be the dominant removal processes for gas-phase polychlorinated dibenzo-p-dioxins and dibenzofuran (PCDD/Fs). The OH-initiated atmospheric chemical reaction mechanism and kinetics of 2,3,7,8-tetrachlorinated dibenzofuran (TCDF) are researched using the density functional theory and canonical variational transition state theory. The reaction mechanism of TCDF with the OH radical and ensuing reactions including bond cleavage of furan ring, O(2) addition or abstraction, dechlorination process, bimolecular reaction of TCDF-OH-O(2) peroxy radical with NO, and reaction of carbonyl free radicals TCDF-OH-O with H(2)O are investigated. In the subsequent reactions of TCDF-OH, O(2) abstraction and dechlorination are most likely to predominate the process. As the main products, the HO(2) radical and the Cl atom are active and may play important roles in the atmospheric oxidation processes. The rate constants of TCDF with the OH radical are calculated, which are consistent with the reported data.     

Rates of hydroxyl radical generation and organic compound oxidation in mineral-catalyzed Fenton-like systems

The iron oxide-catalyzed production of hydroxyl radical (*OH) from hydrogen peroxide (H2O2) has been used to oxidize organic contaminants in soils and groundwater. The goals of this study are to determine which factors control the generation rate of *OH (VOH) and to show that if VOH and the rate constants of the reactions of *OH with the system's constituents are known, the oxidation rate of a dissolved organic compound can be predicted. Using 14C-labeled formic acid as a probe, we measured VOH in pH 4 slurries of H2O2 and either synthesized ferrihydrite, goethite, or hematite or a natural iron oxide-coated quartzitic aquifer sand. In all of our experiments, VOH was proportional to the product of the concentrations of surface area of the iron oxide and H2O2, although different solids produced *OH at different rates. We used these results to develop a model of the decomposition rate of formic acid as a function of the initial formic acid and hydrogen peroxide concentrations and of the type and quantity of iron oxide. Our model successfully predicted the VOH and organic compound oxidation rates observed in our aquifer sand experiment and in a number of other studies but overpredicted VOH and oxidation rates in other cases, possibly indicating that unknown reactants are either interfering with *OH production or consuming *OH in these systems.     

Photocatalytic oxidation of arsenite on TiO2: understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors

Previously we have reported that superoxide plays the primary role as oxidant of As(III) in the UV/TiO2 system, however, since then there has been a controversy over the true identity of the major As(III) oxidant. This study aims to establish a comprehensive understanding of the oxidative mechanism which satisfactorily explains all of the observed results during the photocatalytic oxidation (PCO) of As(III). The key step that has masked the true oxidative mechanism is related to the fact that the adsorbed As(III) on TiO2 serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by an electron transfer to make a null cycle. This was confirmed by the observation that the photoanodic current obtained with a TiO2 electrode immediately decreased upon spiking with As(III), portraying the superoxide-mediated PCO as the dominant pathway. The degradation of competitive substrates (benzoic acid and formic acid) was delayed until As(III) was fully converted into As(V) since the normal PCO mechanism that is based on the action of adsorbed OH radicals (or holes) is not working as long as As(III) is present on the TiO2 surface. However, the As(III) PCO mechanism is entirely altered when alternative electron acceptors (Ag+, Cu2+, polyoxometalate) are present. When these alternative electron acceptors are more efficient than 02 they are able to intercept the CB electron, impeding the recombination pathway and enabling an anoxic oxidation mechanism in which OH radicals and holes play the role of main As(III) oxidant. In the presence of polyoxometalate or Cu2+, the above-mentioned photoanodic current immediately increases upon spiking As(III), indicating that the PCO mechanism has changed in the presence of more efficient electron acceptors. Comprehensive mechanisms of As(III) PCO and experimental factors that alter the mechanism are discussed.     

Kinetic model for Fe(II) oxidation in seawater in the absence and presence of natural organic matter

A detailed kinetic model has been developed to describe the oxidation of Fe(II) in seawater in both the absence and the presence of natural organic material. Experimental data were collected using a luminol chemiluminescence-based method to measure Fe(II), assuming that both the inorganic and the organically complexed species were detected. In the absence of organic matter, the data were modeled based on the Haber-Weiss mechanism with the inclusion of a back-reaction of Fe(III) with superoxide and precipitation of Fe(OH)3. Both reactions were found to be significant using sensitivity analysis. When organic matter is present, the model was extended by organic complexation of Fe(II) and Fe(III) with the creation of a parallel oxidation pathway for Fe(II). Fe(II) oxidation at natural (nanomolar) concentrations was accurately predicted for a range of organic concentrations. The model also accounted for scavenging of superoxide by sub-nanomolar levels of dissolved copper and by organic matter when present. The presence of a relatively strong Fe(III) binding ligand was observed to significantly increase the rate of Fe(II) oxidation, while ultimately retaining most of the iron in the system in dissolved (organically complexed) form. The complexation reactions and reaction of inorganic and organically bound Fe(II) with oxygen were found to be critical reactions in the system, while Fe(III) hydrolysis became unimportant even at low organic concentrations. The superoxide radical was also observed to have a major role in the cycling of iron due to its ability to act as both an oxidant and a reductant. The model indicates that the rate constant for the reaction of Fe(II) with O2 has generally been underestimated in previous work and that the secondary oxidation of Fe(II) by H2O2 and subsequently OH* plays a relatively minor role in these systems.     

Hydroxyl radical formation upon oxidation of reduced humic acids by oxygen in the dark

Humic acids (HAs) accept and donate electrons in many biogeochemical redox reactions at oxic/anoxic interfaces. The products of oxidation of reduced HAs by O(2) are unknown but are expected to yield reactive oxygen species, potentially including hydroxyl radical (·OH). To quantify the formation of ·OH upon oxidation of reduced HAs by O(2), three HAs were reduced electrochemically to well-defined redox states and were subsequently oxidized by O(2) in the presence of the ·OH probe terephthalate. The formation of ·OH upon oxidation increased with increasing extent of HA reduction. The yield of ·OH ranged from 42 to 160 mmol per mole of electrons donated by the reduced HA. The intermediacy of hydrogen peroxide (H(2)O(2)) in the formation of ·OH was supported by enhancement of ·OH formation upon addition of exogenous H(2)O(2) sources and by the suppression of ·OH formation upon addition of catalase as a quencher of endogenous H(2)O(2). The formation of ·OH in the dark during oxidation of reduced HA represents a previously unknown source of ·OH formation at oxic/anoxic interfaces and may affect the biogeochemical and pollutant redox dynamics at these interfaces.     

Oxidation mechanism of As(III) in the UV/TiO2 system: evidence for a direct hole oxidation mechanism

Although it is well-known that As(III) is oxidized to As(V) in the UV/TiO2 system, the main oxidant for that reaction is still not clear. Accordingly, the present study aims at reinvestigating the TiO2-photocatalyzed oxidation mechanism of As(III). We performed a series of As(II) oxidation experiments by using UV-C/H2O2 and UV-A/TiO2, focusing on the effects of competing compounds. The experiment with UV-C/H2O2 indicated that HO2*/O2-* is not an effective oxidant of As(III) in the homogeneous phase. The effects of oxalate, formate, and Cu(II) on the photocatalytic oxidation of As(III) contradicted the controversial hypothesis that HO2*/ O2-* is the main oxidant of As(III) in the UV/TiO2 system. The effect of As(III) on the TiO2-photocatalyzed oxidations of benzoate, terephthalate, and formate was also incompatible with the superoxide-based As(II) oxidation mechanism. Instead, the experimental observations implied that OH* and/or the positive hole are largely responsible forthe oxidation of As(III) in the UV/TiO2 system. To determine which species plays a more significant role, the effects of methanol and iodide were tested. Since excess methanol did not retard the oxidation rate of As(III), OH* seems not to be the main oxidant. Therefore, the best rationale regarding the oxidation mechanism of As(III) in the UV/TiO2 system seems to be the direct electron transfer between As(III) and positive holes. Only with this mechanism, it was possible to explain the data of this study. Besides the mechanistic aspect, an application method for this technology was sought. The usage of UV/TiO2 for oxidizing As(II) requires a posttreatment in which both As(V) and TiO2 should be removed from water. For this objective, we applied FeCl3 and AIK(SO4)2 as coagulants, and the result implied that the combined usage of TiO2 and coagulation might be a feasible solution to treat arsenic contamination around the world.     

Photocatalytic oxidation of arsenite in TiO2 suspension: kinetics and mechanisms

Arsenite [As(III)] and arsenate [As(V)] are highly toxic aquatic contaminants. Since arsenite is more mobile in natural waters and less efficiently removed in adsorption/coagulation processes than arsenate, the oxidation of arsenite to arsenate is desirable in water treatment. We performed the photocatalytic oxidation of arsenite in aqueous TiO2 suspension and investigated the effects of pH, dissolved oxygen, humic acid (HA), and ferric ions on the kinetics and mechanisms of arsenite oxidation. Arsenite oxidation in UV-illuminated TiO2 suspension was highly efficient in the presence of dissolved oxygen. Homogeneous photooxidation of arsenite in the absence of TiO2 was negligibly slow. Since the addition of excess tert-butyl alcohol (OH radical scavenger) did not reduce the rate of arsenite oxidation, the OH radicals should not be responsible for As(III) oxidation. The addition of HA increased both arsenite oxidation and H2O2 production at pH 3 under illumination, which could be ascribed to the enhanced superoxide generation through sensitization. We propose that the superoxide is the main oxidant of arsenite in the TiO2/UV process. The addition of ferric ions also significantly enhanced the arsenite photooxidation. In this case, the addition of tert-butyl alcohol reduced the arsenite oxidation rate, which implied thatthe OH radical-mediated oxidation path was operative in the presence of ferric ions. Since both Fe3+ and HA that were often found with the arsenic in groundwater were beneficial to the photocatalytic oxidation of arsenite, the TiO2/UV process could be a viable pretreatment method. This can be as simple as exposing the arsenic-polluted water in a TiO2-coated trough to sunlight.     

Photocatalytic oxidation of gaseous 2-chloroethyl ethyl sulfide over TiO2

Photocatalytic oxidation of gaseous 2-chloroethyl ethyl sulfide (2-CEES, ClCH2CH2SCH2CH3) over TiO2 illuminated with UV light and maintained at 25 or 80 degrees C in air has been investigated. 2-CEES was found to suffer progressive oxidation to yield ethylene (CH2CH2), chloroethylene (ClCHCH2), ethanol (CH3CH2OH), acetaldehyde (CH3C(O)H), chloroacetaldehyde (ClCH2C(O)H), diethyl disulfide (CH3CH2S2CH2CH3), 2-chloroethyl ethyl disulfide (ClCH2CH2S2CH2CH3), and bis(2-chloroethyl) disulfide (ClCH2CH2S2CH2CH2Cl) as the main primary intermediates, and water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), surface sulfate ions (SO4(2-)), and hydrogen chloride (HCl) as the final products. Trace concentrations of gaseous 2-chloroethanol (ClCH2CH2OH), ethanesulfonyl chloride (CH3CH2SO2Cl), ethyl thioacetate (CH3CH2SC(O)CH3), and considerable amounts of acetic acid (CH3C(O)OH), crotonaldehyde (CH3CHCHC(O)H), methyl acetate (CH3C(O)OCH3), and methyl formate (CH3OC(O)H) were also detected in the gas phase during the photooxidation conducted at 80 degrees C. Increase in temperature from 25 to 80 degrees C accelerates formation of gaseous ethanol, acetaldehyde, chloroacetaldehyde, diethyl disulfide, 2-chloroethyl ethyl disulfide, and bis(2-chloroethyl) disulfide but suppresses ethylene and chloroethylene production at initial stages of the process. Some aspects of the possible reaction mechanism leading to this wide array of intermediates and final products are discussed.     

Mechanisms of hydrogen peroxide decomposition in soils

The rates and mechanisms of hydrogen peroxide (H2O2) decomposition were examined in a series of soil suspensions at H2O2 concentrations comparable to those found in rainwaters. The formation of hydroxyl radical (OH) as a possible decomposition intermediate was investigated using a new, highly sensitive method. In surface soils with higher organic matter or manganese content, H2O2 usually decayed rapidly, with disproportionation to water and dioxygen dominating the decomposition, whereas the formation of the hydroxyl radical (OH) represented <10% of the total H2O2 decomposed. In contrast, for soils with lower organic matter content, H2O2 usually decayed much more slowly, but OH was a major product of the H2O2 decomposed. The decomposition was principally associated with soil particles, not the soil supernatant. Different sterilization techniques indicated that decomposition of H2O2 was at least partly due to biological activity. Because the loss of H2O2 can largely be accommodated by the production of O2 and OH within these soils, our results suggest that disproportionation through a catalase-type mechanism and the production of OH through a Haber-Weiss mechanism represent the principal routes through which H2O2 is lost.     

Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction

The oxidation kinetics of As(III) with natural and technical oxidants is still notwell understood, despite its importance in understanding the behavior of arsenic in the environment and in arsenic removal procedures. We have studied the oxidation of 6.6 microM As(II) by dissolved oxygen and hydrogen peroxide in the presence of Fe(II,III) at pH 3.5-7.5, on a time scale of hours. As(III) was not measurably oxidized by O2, 20-100 microM H2O2, dissolved Fe(III), or iron(III) (hydr)-oxides as single oxidants, respectively. In contrast, As(III) was partially or completely oxidized in parallel to the oxidation of 20-90 microM Fe(II) by oxygen and by 20 microM H2O2 in aerated solutions. Addition of 2-propanol as an *OH-radical scavenger quenched the As(III) oxidation at low pH but had little effect at neutral pH. High bicarbonate concentrations (100 mM) lead to increased oxidation of As-(III). On the basis of these results, a reaction scheme is proposed in which H2O2 and Fe(II) form *OH radicals at low pH but a different oxidant, possibly an Fe(IV) species, at higher pH. With bicarbonate present, carbonate radicals might also be produced. The oxidant formed at neutral pH oxidizes As(III) and Fe(II) but does not react competitively with 2-propanol. Kinetic modeling of all data simultaneously explains the results quantitatively and provides estimates for reaction rate constants. The observation that As(III) is oxidized in parallel to the oxidation of Fe(II) by O2 and by H2O2 and that the As(III) oxidation is not inhibited by *OH-radical scavengers at neutral pH is significant for the understanding of arsenic redox reactions in the environment and in arsenic removal processes as well as for the understanding of Fenton reactions in general.