OH Production Rates Research Articles

Synergistic Interaction between Electricigens and Natural Pyrrhotite to Produce Active Oxygen Radicals

In this work we demonstrated that the active oxygen radicals could be produced by the synergistic interaction between electricigens and natural pyrrhotite. The identification of such an interactive pathway was conducted by using a fuel cell-type design, in which the electricigen-attached carbon felt electrode was used as the anode and the pyrrhotite-coated graphite electrode was used as the corresponding cathode. Current density, polarization and power density curves obtained at different treatments demonstrated the synergistic effects of electricigens and pyrrhotite improved the electrons transfer rate between them. Cyclic voltammetry (CV) analysis showed the reductive peaks of O2/H2O2 at 0.88 V (vs. SCE, saturated calomel electrode) and ionic and structural Fe(III)/Fe(II) at 0.31 V (vs. SCE) and 0 V (vs. SCE), respectively. The electrochemical results indicated the electricigen-assisted pyrrhotite photoelectrochemical reactions gave rise to Fenton's reagents: Fe2+ and H2O2, which underwent a further reaction to generate active oxygen radical ·OH. By using N, N-dimethyl-p-nitrosoaniline discoloration as a model reaction, the ·OH production rate at the pyrrhotite-cathode was found to follow the first-order kinetics. Practical application of the synergistic interaction between the electricigen and natural pyrrhotite to a real old-aged landfill leachate degradation resulted in 78% chemical oxygen demand (COD) removal and 77% decolourization efficiency. The current generation lasted more than 45 days verified the validity of such system in long-term operation. The proposed interactive pathway would be expected as an alternative cost-effective technology for future wastewater treatment.

Seasonal observations of OH and HO<sub>2</sub> in the remote tropical marine boundary layer

Abstract. Field measurements of the hydroxyl radical, OH, are crucial for our understanding of tropospheric chemistry. However, observations of this key atmospheric species in the tropical marine boundary layer, where the warm, humid conditions and high solar irradiance lend themselves favourably to production, are sparse. The Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009 allowed, for the first time, seasonal measurements of both OH and HO2 in a clean (i.e. low NOx), tropical marine environment. It was found that concentrations of OH and HO2 were typically higher in the summer months (June, September), with maximum daytime concentrations of ~9 × 106 and 4 × 108 molecule cm−3, respectively – almost double the values in winter (late February, early March). HO2 was observed to persist at ~107 molecule cm−3 through the night, but there was no strong evidence of nighttime OH, consistent with previous measurements at the site in 2007. HO2 was shown to have excellent correlations (R2 ~ 0.90) with both the photolysis rate of ozone, J(O1D), and the primary production rate of OH, P(OH), from the reaction of O(1D) with water vapour. The analogous relations of OH were not so strong (R2 ~ 0.6), but the coefficients of the linear correlation with J(O1D) in this study were close to those yielded from previous works in this region, suggesting that the chemical regimes have similar impacts on the concentration of OH. Analysis of the variance of OH and HO2 across the Seasonal Oxidant Study suggested that ~70% of the total variance could be explained by diurnal behaviour, with ~30% of the total variance being due to changes in air mass.

Kinetics and Mechanism of OH Oxidation of Small Organic Dicarboxylic Acids in Ice: Comparison to Behavior in Aqueous Solution

To better understand the oxidation of organic species by OH within frozen solutions, experiments were conducted with a simplified model system consisting of succinic acid (2.5, 5, and 10 mM), using H(2)O(2) (30 mM) as a condensed-phase OH precursor. Frozen (-20 °C) and aqueous (0 °C and room temperature) solutions were irradiated using a xenon arc lamp to study the differences between frozen and nonfrozen reaction media. Formation of products and decay of reactants were measured using ion chromatography (IC) and gas chromatography-mass spectrometry (GC-MS). Malonic acid was observed as the dominant product, with malic acid formed in significantly smaller amounts, in both frozen and unfrozen substrates, suggesting a common mechanism independent of phase. Notably, a large decrease was observed in the succinic acid first-order decay rate constant when moving from aqueous to frozen samples. This is due not to temperature-dependent second-order kinetics or different OH production rates between the samples, but instead probably arises from physical separation of the succinic acid and H(2)O(2) upon freezing or precipitation of succinic acid upon freeze concentration. The effect is not nearly as pronounced for the decay kinetics of a much more soluble species, namely, malonic acid. From an environmental perspective, this work is the first experimental demonstration that dicarboxylic acids present in the cryosphere might be subject to photochemical degradation into smaller organic acids and carbon dioxide.

Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest

Abstract. The lifetime of methane is controlled to a very large extent by the abundance of the OH radical. The tropics are a key region for methane removal, with oxidation in the lower tropical troposphere dominating the global methane removal budget (Bloss et al., 2005). In tropical forested environments where biogenic VOC emissions are high and NOx concentrations are low, OH concentrations are assumed to be low due to rapid reactions with sink species such as isoprene. New, simultaneous measurements of OH concentrations and OH reactivity, k'OH, in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ~2.5×106 molecule cm−3 (10 min average) observed around solar noon. Whilst j(O1D) and humidity were high, low O3 concentrations limited the OH production from O3 photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s−1, corresponding to an OH lifetime of only 34 ms. To maintain the observed OH concentration given the measured OH reactivity requires a rate of OH production approximately 10 times greater than calculated using all measured OH sources. A test of our current understanding of the chemistry within a tropical rainforest was made using a detailed zero-dimensional model to compare with measurements. The model over-predicted the observed HO2 concentrations and significantly under-predicted OH concentrations. Inclusion of an additional OH source formed as a recycled product of OH initiated isoprene oxidation improved the modelled OH agreement but only served to worsen the HO2 model/measurement agreement. To replicate levels of both OH and HO2, a process that recycles HO2 to OH is required; equivalent to the OH recycling effect of 0.74 ppbv of NO. This recycling step increases OH concentrations by 88 % at noon and has wide implications, leading to much higher predicted OH over tropical forests, with a concomitant reduction in the CH4 lifetime and increase in the rate of VOC degradation.

Key modeling issues in prediction of minor species in diluted-preheated combustion conditions

Abstract A numerical investigation of a jet in hot coflow (JHC) burner emulating Moderate or Intense Low-oxygen Dilution (MILD) combustion is carried out in order to understand key modeling issues for such three stream problem. Favre-averaged Navier–Stokes (FANS) equations were solved in a finite volume scheme with the Eddy Dissipation Concept (EDC) model for the turbulence-chemistry interaction and the skeletal KEE mechanism (consisting 17 species and 58 reactions) for the combustion reactions. It was found that mixing in shear layers has a significant impact on the temperature, species and flow fields and could be predicted only with a proper selection of the turbulence levels at the inlet boundaries. Analyzing the minor species reaction path together with the mean and fluctuating temperature fields elucidated the great influence of temperature fluctuations on the net rate of CO and OH production in MILD combustion regime.

Quantitative characterization of hydroxyl radicals produced by various photocatalysts

Hydroxyl radicals ( OH) have been deemed to be the major active species during the photocatalytic oxidation reaction. In this study, OH produced on various semiconductor photocatalysts in aqueous solution under Xenon lamp irradiation was quantitatively investigated by the photoluminescence (PL) technique using coumarin (COU) as a probe molecule. The results indicated that the formation rate of OH on the surface of irradiated commercial Degussa P25 (P25) was much higher than that of other semiconductor. The pH values of the solution and phase structure of TiO 2 significantly influenced the production rate of OH. The acidic pH environment of the solutions and bi-phase structure (anatase and rutile) of TiO 2 were beneficial to enhancing the formation rate of OH. In addition, the formation rate of OH on anatase TiO 2 and P25 was much faster than that of OH on the other semiconductors (such as rutile TiO 2, ZnO, WO 3, CdS, Bi 2WO 4 and BiOCl, etc.). A new concept “OH-index” was first proposed to compare photocatalytic activity of photocatalysts, which would provide new insight into the investigation of semiconductor photocatalysts.

SWIFT-UVOT GRISM SPECTROSCOPY OF COMETS: A FIRST APPLICATION TO C/2007 N3 (LULIN)

We observed comet C/2007 N3 (Lulin) twice on UT 28 January 2009, using the UV grism of the Ultraviolet and Optical Telescope (UVOT) on board the Swift Gamma Ray Burst space observatory. Grism spectroscopy provides spatially resolved spectroscopy over large apertures for faint objects. We developed a novel methodology to analyze grism observations of comets, and applied a Haser comet model to extract production rates of OH, CS, NH, CN, C3, C2, and dust. The water production rates retrieved from two visits on this date were $6.7 \pm 0.7$ and 7.9 $\pm$ 0.7 x 1E28 molecules s-1, respectively. Jets were sought (but not found) in the white-light and `OH' images reported here, suggesting that the jets reported by Knight and Schleicher (2009) are unique to CN. Based on the abundances of its carbon-bearing species, comet Lulin is `typical' (i.e., not `depleted') in its composition.

Observations of OH and HO<sub>2</sub> radicals over West Africa

Abstract. The hydroxyl radical (OH) plays a key role in the oxidation of trace gases in the troposphere. However, observations of OH and the closely related hydroperoxy radical (HO2) have been sparse, especially in the tropics. Based on a low-pressure laser-induced fluorescence technique (FAGE – Fluorescence Assay by Gas Expansion), an instrument has been developed to measure OH and HO2 aboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft. During the African Monsoon Multidisciplinary Analyses (AMMA) campaign, observations of OH and HO2 (HOx) were made in the boundary layer and free troposphere over West Africa on 13 flights during July and August 2006. Mixing ratios of both OH and HO2 were found to be highly variable, but followed a diurnal cycle: OH varied from 1.3 pptv to below the instrumental limit of detection, with a median mixing ratio of 0.17 pptv. HO2 varied from 42.7 pptv to below the limit of detection, with a median mixing ratio of 8.0 pptv. A median HO2/OH ratio of 95 was observed. Daytime OH observations were compared with the primary production rate of OH from ozone photolysis in the presence of water vapour. Daytime HO2 observations were generally reproduced by a simple steady-state HOx calculation, where HOx was assumed to be formed from the primary production of OH and lost through HO2 self-reaction. Deviations between the observations and this simple model were found to be grouped into a number of specific cases: (a) within cloud, (b) in the presence of high levels of isoprene in the boundary layer and (c) within a biomass burning plume. HO2 was sampled in and around cloud, with significant short-lived reductions of HO2 observed. Up to 9 pptv of HO2 was observed at night, with HO2 above 6 pptv observed at altitudes above 6 km. In the forested boundary layer, HO2 was underestimated by a steady state calculation at altitudes below 500 m but overestimated between 500 m and 2 km. In a biomass burning plume, observed HO2 concentrations were significantly below those calculated.

Release of Gas-Phase Halogens by Photolytic Generation of OH in Frozen Halide−Nitrate Solutions: An Active Halogen Formation Mechanism?

To better define the mechanisms by which condensed-phase halides may be oxidized to form gas-phase halogens under polar conditions, experiments have been conducted whereby frozen solutions containing chloride (1 M), bromide (1.6 x 10(-3) to 5 x 10(-2) M), iodide (<1 x 10(-5) M), and nitrate (0.01 to 1 M) have been illuminated by ultraviolet light in a continually flushed cell. Gas-phase products are quantified using chemical ionization mass spectrometry, and experiments were conducted at both 248 and 263 K. Br(2) was the dominant product, along with smaller yields of IBr and trace BrCl and I(2). The Br(2) yields were largely independent of the Br(-)/Cl(-) ratio of the frozen solution, down to seawater composition. However, the yields of halogens were strongly dependent on the levels of NO(3)(-) and acidity in solution, consistent with a mechanism whereby NO(3)(-) photolysis yields OH that oxidizes the condensed-phase halides. In support, we observed the formation of gas-phase NO(2), formed simultaneously with OH. Gas-phase HONO was also observed, suggesting that halide oxidation by HONO in the condensed phase may also occur to some degree. By measuring the production rate of condensed-phase OH, using benzoic acid as a radical trap, we determine that the molar yield of Br(2) formation relative to OH generation is 0.6, consistent with each OH being involved in halide oxidation. These studies suggest that gas-phase halogen formation should occur simultaneously with NO(x) release from frozen sea ice and snow surfaces that contain sufficient halides and deposited nitrate.

One-pot hydrothermal synthesis and enhanced photocatalytic activity of trifluoroacetic acid modified TiO 2 hollow microspheres

Trifluoroacetic acid (TFA) modified TiO 2 (TFA-TiO 2) hollow microspheres were prepared by one-pot hydrothermal treatment of Ti(SO 4) 2 in the presence of TFA at 180 °C for 12 h. The prepared samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N 2 adsorption–desorption isotherms and Fourier transform infrared. The production rates of OH on the surface of UV-illuminated TiO 2 were detected by a photoluminescence (PL) method using terephthalic acid (TA) as probe molecule. The photocatalytic activity was evaluated by the photocatalytic decomposition of acetone in air under UV light illumination. The results show that TFA not only induces the formation of hollow microspheres, but also enhances their crystallization. The molar ratios of TFA to Ti(SO 4) 2 ( R) have a great influence on photocatalytic activity. When R is in the range of 0.5–5, the photocatalytic activity of the samples is higher than that of commercial Degussa P25 TiO 2 powders (P25) and pure TiO 2 samples. At R = 2, the photocatalytic activity of the sample reaches the highest and exceeds that of P25 by a factor of more than two times. This is ascribed to the fact that the former has hollow interior and bimodal mesoporous shells enhancing harvesting of light and the transfer and transport of reactant and product molecules, also, the surface-adsorbed TFA can reduce the recombination of photo-generated electrons and holes.

Seasonal dependence of the oxidation capacity of the city of Santiago de Chile

The oxidation capacity of the highly polluted urban area of Santiago de Chile has been evaluated during a winter measurement campaign from May 25 to June 07, 2005, with the results compared and contrasted with those previously evaluated during a summer campaign from March 8 to 20, 2005. The OH radical budget was evaluated in both campaigns employing a simple quasi-photostationary state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O3, NO, NO2, j(O1D), j(NO2), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used for the analysis of the radical budgets and concentrations of OH, HO2 and RO2. Besides the above parameters, the MCM model has been constrained by the measured CO and other volatile organic compounds (VOCs) including alkanes and aromatics. Total production and destruction rates of OH and HO2 in winter are about two times lower than that during summer. Simulated OH levels by both PSS and MCM models are similar during the daytime for both winter and summer indicating that the primary OH sources and sinks included in the simple PSS model are predominant. On a 24 h basis, HONO photolysis was shown to be the most important primary OH radical source comprising 81% and 52% of the OH initiation rate during winter and summer, respectively followed by alkene ozonolysis (12.5% and 29%), photolysis of HCHO (6.1% and 15%), and photolysis of O3 (<1% and 4%), respectively. During both winter and summer, there was a balance between the OH secondary production (HO2 + NO) and destruction (OH + VOCs) showing that initiation sources of RO2 and HO2 are no net OH initiation sources. This result was found to be fulfilled also for all other studies investigated. Seasonal impacts on the radical budgets are also discussed.

Photo-Fenton Reaction at Near Neutral pH

The photo-Fenton reaction, oxidation of photoproduced ferrous iron by hydrogen peroxide, produces reactive oxidants that may be important to degradation of biologically and chemically recalcitrant organic compounds in surface waters at circum-neutral pH. Sufficient Fe(II) for the photo-Fenton reaction was produced in situ at two field sites (pH 6.1-7.1) and during irradiations of model systems at pH 7.0 with Suwannee River fulvic acid (SRFA). Amorphous iron oxyhydroxide preparations were much more easily photoreduced than ferrihydrite (Ferr-90). The rate of the photo-Fenton reaction was measured as the difference in the H2O2 accumulation rate in irradiations without and with iron in model systems. Use of benzene as a probe for hydroxyl radical (OH) and nitrate photolysis as the OH source indicated that the yield of phenol from the reaction of benzene with OH is reduced in the presence of iron and SRFA. Even when this reduction in yield was accounted for, the rate of OH production from the Fenton reaction in the model systems was much smaller than the rate of the photo-Fenton reaction. These results indicate that OH is not the only oxidant produced by the photo-Fenton reaction in circum-neutral natural waters; however the photo-Fenton reaction could still be significant for contaminant degradation.

Induction of hydroxyl radical production in Trametes versicolor to degrade recalcitrant chlorinated hydrocarbons

Extracellular hydroxyl radical ( OH) production via quinone redox cycling in Trametes versicolor, grown in a chemically defined medium, was investigated to degrade trichloroethylene (TCE), perchloroethylene (PCE), 1,2,4- and 1,3,5-trichlorobenzene (TCB). The activity of the enzymes catalyzing the quinone redox cycle, quinone reductase and laccase, as well as the rate of OH production, estimated as the formation of thiobarbituric acid reactive substances (TBARS) from 2-deoxyribose, increased rapidly during the first 2–3 days and then remained at relatively constant levels. Under quinone redox cycling conditions, TCE degradation was concomitant to TBARS production and chloride release, reaching a plateau after 6 h of incubation. Similar results were obtained in PCE, 1,2,4- and 1,3,5-TCB time course degradation experiments. The mole balance of chloride release and 1,2,4-TCB and TCE degraded suggests that these chemicals were almost completely dechlorinated. Experiments using [ 13C]-TCE confirmed unequivocal transformation of TCE to 13CO 2. These results are of particular interest because PCE and 1,3,5-TCB degradation in aerobic conditions has been rarely reported to date in bacterial or fungal systems.

Induction of Extracellular Hydroxyl Radical Production by White-Rot Fungi through Quinone Redox Cycling

A simple strategy for the induction of extracellular hydroxyl radical (OH) production by white-rot fungi is presented. It involves the incubation of mycelium with quinones and Fe(3+)-EDTA. Succinctly, it is based on the establishment of a quinone redox cycle catalyzed by cell-bound dehydrogenase activities and the ligninolytic enzymes (laccase and peroxidases). The semiquinone intermediate produced by the ligninolytic enzymes drives OH production by a Fenton reaction (H(2)O(2) + Fe(2+) --> OH + OH(-) + Fe(3+)). H(2)O(2) production, Fe(3+) reduction, and OH generation were initially demonstrated with two Pleurotus eryngii mycelia (one producing laccase and versatile peroxidase and the other producing just laccase) and four quinones, 1,4-benzoquinone (BQ), 2-methoxy-1,4-benzoquinone (MBQ), 2,6-dimethoxy-1,4-benzoquinone (DBQ), and 2-methyl-1,4-naphthoquinone (menadione [MD]). In all cases, OH radicals were linearly produced, with the highest rate obtained with MD, followed by DBQ, MBQ, and BQ. These rates correlated with both H(2)O(2) levels and Fe(3+) reduction rates observed with the four quinones. Between the two P. eryngii mycelia used, the best results were obtained with the one producing only laccase, showing higher OH production rates with added purified enzyme. The strategy was then validated in Bjerkandera adusta, Phanerochaete chrysosporium, Phlebia radiata, Pycnoporus cinnabarinus, and Trametes versicolor, also showing good correlation between OH production rates and the kinds and levels of the ligninolytic enzymes expressed by these fungi. We propose this strategy as a useful tool to study the effects of OH radicals on lignin and organopollutant degradation, as well as to improve the bioremediation potential of white-rot fungi.

Oxidation capacity of the city air of Santiago, Chile

Abstract. The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during a summer measurement campaign carried out from 8–20 March 2005. The hydroxyl (OH) radical budget was evaluated employing a simple quasi-photostationary-state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O3, NO, NO2, j(O1D), j(NO2), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used to estimate production rates and total free radical budgets, including OH, HO2 and RO2. Besides the above parameters, the MCM model has been constrained by the measured CO and volatile organic compounds (VOCs) including alkanes and aromatics. Both models simulate the same OH concentration during daytime indicating that the primary OH sources and sinks included in the simple PSS model predominate. Mixing ratios of the main OH radical precursors were found to be in the range 0.8–7 ppbv (HONO), 0.9–11 ppbv (HCHO) and 0–125 ppbv (O3). The alkenes average mixing ratio was ~58 ppbC accounting for ~12% of the total identified non-methane hydrocarbons (NMHCs). During the daytime (08:00 h–19:00 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone ~55% of the total initial production rate, followed by alkene ozonolysis (~24%) and photolysis of HCHO (~16%) and O3 (~5%). The calculated average and maximum daytime OH production rates from HONO photolysis was 1.7 ppbv h−1 and 3.1 ppbv h−1, respectively. Based on the experimental results a strong photochemical daytime source of HONO is proposed. A detailed analysis of the sources of OH radical precursors has also been carried out.

A new approach to quantify the degradation kinetics of linuron with UV, ozonation and UV/O 3 processes

The degradation of linuron, one of phenylurea herbicides, was investigated for its reaction kinetics by different treatment processes including ultraviolet irradiation (UV), ozonation (O 3), and UV/O 3. The decay rate of linuron by UV/O 3 process was found to be around 3.5 times and 2.5 times faster than sole-UV and ozone-alone, respectively. Experimental results also indicate overall rate constants increased exponentially with pH above 9.0 while the increase of rate constants with pH below 9 is insignificant in O 3 system. All dominant parameters involved in the three processes were determined in the assistant of proposed linear models in this study. The approach was found useful in predicting the process performances through the quantification of quantum yield ( Φ LNR ) , k OOH (rate constant for the formation of free radical HOO − from ozone decomposition at high pH), rate constant of linuron with ozone ( k O 3 ,LNR ), rate constant of linuron with hydroxyl radical ( k OH,LNR), and α (the ratio of the production rate of OH and the decay rate of ozone in UV/O 3 system).

Placing the Deep Impact Mission into context: Two decades of observations of 9P/Tempel 1 from McDonald Observatory

We report on low-spectral resolution observations of Comet 9P/Tempel 1 from 1983, 1989, 1994 and 2005 using the 2.7 m Harlan J. Smith telescope of McDonald Observatory. This comet was the target of NASA's Deep Impact mission and our observations allowed us to characterize the comet prior to the impact. We found that the comet showed a decrease in gas production from 1983 to 2005, with the decrease being different factors for different species. OH decreased by a factor 2.7, NH by 1.7, CN by 1.6, C 3 by 1.8, CH by 1.4 and C 2 by 1.3. Despite the decrease in overall gas production and these slightly different decrease factors, we find that the gas production rates of OH, NH, C 3, CH and C 2 ratioed to that of CN were constant over all of the apparitions. We saw no change in the production rate ratios after the impact. We found that the peak gas production occurred about two months prior to perihelion. Comet Tempel 1 is a “normal” comet.

Sonoelectro-Fenton process: A novel hybrid technique for the destruction of organic pollutants in water

Here we present the results obtained by applying a novel hybrid advanced oxidation technique, which we have called sonoelectro-Fenton (sono-EF) process, for the degradation of organic pollutants in aqueous medium. Its high performance arises from the coupling between ultrasound irradiation and the in situ electrogeneration of Fenton’s reagent. An undivided electrolytic cell with a Pt anode and a three-dimensional carbon-felt cathode has been used to carry out the very effective electro-Fenton (EF) process at constant current, which allows the production of great amounts of hydroxyl radicals ( OH). The synergistic action of sonication in the sono-EF process has been studied at low and high frequency within a range of output power. We demonstrate that the destruction of the herbicides 4,6-dinitro- o-cresol (DNOC) and 2,4-dichlorophenoxyacetic acid (2,4-D) is significantly accelerated, whereas no improvement is observed for the degradation of the dye azobenzene (AB), suggesting that the nature of the organic structure plays an important role. A pseudo-first order kinetics is found in all cases. Similar results are reached for sono-EF at low and high frequency, but the lowest ultrasounds power (i.e., 20 W) is shown to be much better. We conclude that the improvement yielded by sono-EF is due to various contributions, which in order of relevance are: (i) the enhanced mass transfer rate of both reactants (Fe 3+ and O 2) towards the cathode for the electrochemical generation of Fenton’s reagent (Fe 2+ + H 2O 2) and its transfer into the solution in order to enhance Fenton’s reaction kinetics thus increasing the OH production rate and accelerating the destruction process, (ii) the additional generation of OH by sonolysis, and (iii) pyrolysis of organics due to cavitation generated by ultrasound irradiation.

Soluble Amyloid β1−28−Copper(I)/Copper(II)/Iron(II) Complexes Are Potent Antioxidants in Cell-Free Systems

Amyloid beta (Abeta) is a central characteristic of Alzheimer's disease (AD). Currently, there is a long-standing dispute regarding the role of Abeta-metal ion (Zn, Cu, and Fe) complexes in AD pathogenesis. Here, we aim to decipher the connection between oxidative damage implicated in AD and Abeta-metal ion complexes. For this purpose we study, using ESR, the modulation of Cu/Fe-induced H 2O 2 decomposition by Abeta 1-28 (Abeta 28), a soluble model of Abeta 40/42. The addition of H 2O 2 to 0.6 nM-360 microM Abeta 28 solutions containing 100 microM Cu(II)/Cu(I)/Fe(II) at pH 6.6 results in a concentration-dependent sigmoidal decay of [*OH] with IC 50 values of 61, 59, and 84 microM, respectively. Furthermore, Abeta 28 reduces 90% of *OH production rate in the Cu(I)-H 2O 2 system in 5 min. Unlike soluble Abeta 28, Abeta 28-Cu aggregates exhibit poor antioxidant activity. The mode of antioxidant activity of soluble Abeta 28 is twofold. The primary (rapid) mechanism involves metal chelation, whereas the secondary (slow) mechanism involves (*)OH scavenging and oxidation of Cu(Fe)-coordinating ligands. On the basis of our findings, we propose that soluble Abeta may play a protective role in the early stages of AD, but not in healthy individuals, where Abeta's concentration is nanomolar. Yet, when Abeta-metal ion complexes undergo aggregation, they significantly lose their protective function and allow oxidative damage to occur.

Photolysis of surface O3 and production potential of OH radicals in the atmosphere over the Tibetan Plateau

With an area of 2.5 million km2 and an average altitude of about 4000 m above sea level, the Tibetan Plateau is the largest and highest plateau in the mid‐latitudes of the Northern Hemisphere. The high altitude, low latitude, and large snow coverage create strong surface solar radiation on the plateau. It is therefore likely that the atmosphere over the plateau is very photochemically active, with a high production potential for OH radicals by ozone photolysis. We used the Tropospheric Ultraviolet‐Visible Radiation (TUV) model to calculate the photolysis rate of ozone in the Mt. Qomolangma (Everest) region under clear sky conditions. Field measurement data were used to calculate the production rate and concentration of OH radicals in the atmosphere. In the summer noon, over non‐snow‐covered area, surface ozone photolysis rates J(O1D) were 4–5 × 10−5 s−1, the OH radical production rate range was 1–4 × 106 s−1, and the OH concentrations range was 106 to 107 cm−3. Over areas covered with ice and snow, the J(O1D) and OH production rates could be as high as 1.3 × 10−4 s−1 and 5.5 × 106 cm−3 s−1, respectively, because of strong snow albedo. We also calculated the spatial distributions of J(O1D) and OH production over the whole Tibetan Plateau. Because of the large area covered with snow, the O3 photolysis rates in winter could be equal to or even higher than the rates in summer and might create as photochemically active an atmosphere as in summer.