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.     

Trichloroethene degradation by UV/H2O2 advanced oxidation process: product study and kinetic modeling

The broadband UV irradiation of 1.1 mM trichloroethene (TCE) aqueous solution in the presence of 10.4 mM H2O2 resulted in formic, oxalic, dichloroacetic (DCA), and monochloroacetic (MCA) acids, as organic byproducts. The organic chlorine was converted completely to chloride ion as a final product. TCE and its degradation products were completely mineralized in 30 min, under a volume-averaged UV-C irradiant power of 35.7 W/L from a 1 kW medium-pressure mercury vapor arc lamp. TCE degraded primarily through hydroxyl radical-induced reactions and onlyto a low extentthrough direct UV photolysis and chlorine atom-induced chain reactions. The experimental patterns of TCE, H2O2, and detected reaction products combined with the literature information on radical reactions in the aqueous phase were used to postulate a degradation mechanism and to develop a kinetic model to predict the TCE decay, formation and degradation of byproducts, and pH and oxygen profiles. The agreement between the model calculations and the experimental data is satisfactory.     

Pd-catalytic in situ generation of H2O2 from H2 and O2 produced by water electrolysis for the efficient electro-fenton degradation of rhodamine B

A novel electro-Fenton process was developed for wastewater treatment using a modified divided electrolytic system in which H2O2 was generated in situ from electro-generated H2 and O2 in the presence of Pd/C catalyst. Appropriate pH conditions were obtained by the excessive H+ produced at the anode. The performance of the novel process was assessed by Rhodamine B (RhB) degradation in an aqueous solution. Experimental results showed that the accumulation of H2O2 occurred when the pH decreased and time elapsed. The maximum concentration of H2O2 reached 53.1 mg/L within 120 min at pH 2 and a current of 100 mA. Upon the formation of the Fenton reagent by the addition of Fe2+, RhB degraded completely within 30 min at pH 2 with a pseudo first order rate constant of 0.109 ± 0.009 min(-1). An insignificant decline in H2O2 generation and RhB degradation was found after six repetitions. RhB degradation was achieved by the chemisorption of H2O2 on the Pd/C surface, which subsequently decomposed into ?OH upon catalysis by Pd0 and Fe2+. The catalytic decomposition of H2O2 to ?OH by Fe2+ was more powerful than that by Pd0, which was responsible for the high efficiency of this novel electro-Fenton process.     

Pd-catalyzed TCE dechlorination in water: effect of [H2](aq) and H2-utilizing competitive solutes on the TCE dechlorination rate and product distribution

The aqueous-phase H2 concentration ([H2](aq)) and the presence of H2-utilizing competitive solutes affect TCE dechlorination efficiency in Pd-based in-well treatment reactors. The effect of [H2](aq) and H2-utilizing competing solutes (cis-DCE, trans-DCE, 1,1-DCE, dissolved oxygen (DO), nitrite, nitrate) on the TCE transformation rate and product distribution were evaluated using 100 mg/L of a powdered Pd-on-Al2O3 catalysts in batch reactors or 1.0 g of a 1.6-mm Pd-on-gamma-Al2O3 catalyst in column reactors. The TCE dechlorination rate constant decreased by 55% from 0.034 +/- 0.006 to 0.015 +/- 0.001 min-1 when the [H2](aq) decreased from 1000 to 100 microM and decreased sharply to 0.0007 +/- 0.0003 min-1 when the [H2](aq) decreased from 100 to 10 microM. Production of reactive chlorinated intermediates and C4-C6 radical coupling products increased with decreasing [H2](aq). At an [H2](aq) of 10 microM (P/Po = 0.01), DCE isomers and vinyl chloride accounted for as much as 9.8% of the TCE transformed at their maximum but disappeared thereafter, and C4-C6 radical coupling products accounted for as much as 18% of TCE transformed. The TCE transformation rate was unaffected by the presence of cis-DCE (202 microM), trans-DCE (89 microM), and 1,1-DCE (91 microM), indicating that these compounds do not compete with TCE for catalyst active sites. DO is twice as reactive as TCE but had no effect on TCE conversion in the column below a concentration of 370 microM (11.8 mg/L), indicating that DO and TCE will not compete for active catalyst sites at typical groundwater DO concentrations. TCE conversion in the column was reduced by as much as a factor of 10 at influent DO levels greater than 450 mM (14.3 mg/L) because the [H2](aq) fell below 100 microM due to H2 utilized in DO conversion. Nitrite reacts 2-5 times slower than TCE and reduced TCE conversion by less than 4% at a concentration of 6630 microM (305 mg/L). Nitrate was not reactive and did not effect TCE conversion at a concentration of 1290 microM (80 mg/L).     

Measurement of dissolved H2, O2, and CO2 in groundwater using passive samplers for gas chromatographic analyses

A simple in-situ passive dissolved gas groundwater sampler, comprised of a short length of silicone tubing attached to a gastight or other syringe, was adapted and tested for in-situ collection of equilibrium gas samples. Sampler retrieval after several days of immersion in groundwater allowed the direct injection of the sample onto a gas chromatograph (GC), simplifying field collection and sample handling over the commonly used "bubble stripping" method for H2 analyses. A GC was modified by sequencing a thermal conductivity (TC) detector followed by a reductive gas (RG) detector so that linear calibration of H2 over the range 0.2-200,000 ppmv was attained using a 0.5-mL gas sample; inclusion of the TC detector allowed the simultaneous quantification of other fixed gases (O2, CO2, He, and Ne) to which the RG detector was not responsive. Uptake kinetics for H2 and He indicated that the passive sampler reached equilibrium within 12 h of immersion in water. Field testing of these passive samplers revealed unusually large equilibrium gas-phase H2 concentrations in groundwater, ranging from 0.1 to 13.9%, by volume, in 11 monitoring wells surrounding four former radiological wastewater disposal ponds at the Y-12 plant in Oak Ridge, Tennessee.     

Photoassisted degradation of azodyes over FeOxH2x-3/Fe0 in the presence of H2O2 at neutral pH values

Fe0 was calcined in air at 200 degrees C and showed enhanced activity in three cycling runs for the degradation of acid red B (ARB) in the presence of H2O2 under UVA irradiation. Subsequently, the catalyst's activity was maintained effectively after 10 successive cycling experiments. Moreover, the catalyst was found to be highly effective for the degradation of nonbiodegradable azodyes ARB, reactive brilliant red X-3B, reactive red K-2G, cationic red X-GRL, and cationic blue X-GRL at neutral pH values. On the basis of characterization by X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectra, the surface layer of the catalyst was mainly composed of alpha-FeOOH and gamma-Fe2O3, and the core was Fe0 (FeOxH2x-3/ Fe0). Fe,OxH2x3/Fe0 was very easily recovered from the reaction system by magnetic separation. The degradation of azodyes came from the synergistic effect of the catalysis of galvanic cells and the oxidation of heterogeneous photo-Fenton reaction on the basis of all information obtained under different experimental conditions. By the total organic carbon and GC-MS analysis, the degradation process of ARB was shown to proceed with decolorization and naphthalene ring openings into CO2 and small organic acid.     

Visible-light-assisted degradation of dye pollutants over Fe(III)-loaded resin in the presence of H2O2 at neutral pH values

A novel catalyst was synthesized by direct exchange of ferric ions onto a cationic resin (Amberlite IRA200). Upon visible light irradiation (lambda > 420 nm) in the presence of H2O2, this catalyst was found to be highly effective for the degradation of nonbiodegradable cationic dyes, Malachite green, Rhodamine B, and Methylene blue, even at neutral pH values. It was also easy to separate from the degraded solution. By total organic carbon, FT-IR, and GC-MS analysis, the degradation process of Malachite green was shown to proceed with demethylation and phenyl ring openings into CO2 and small molecular compounds. EPR studies revealed that *OH radicals, other than *OOH/O2*-, were involved as the active species. A possible reaction mechanism is proposed on the basis of all the information obtained under various experimental conditions.     

Treatment of volatile organic chemicals on the EPA Contaminant Candidate List using ozonation and the O3/H2O2 advanced oxidation process

Seven volatile organic chemicals (VOCs) on the EPA Contaminant Candidate List together with 1,1-dichloropropane were studied for their reaction kinetics and mechanisms with ozone and OH radicals during ozonation and the ozone/ hydrogen peroxide advanced oxidation process (O3/H2O2 AOP) using batch reactors. The three aromatic VOCs demonstrated high reactivity during ozonation and were eliminated within minutes after ozone addition. The high reactivity is attributed to their fast, indirect OH radical reactions with k(OH,M) of (5.3-6.6) x 10(9) M(-1) s(-1). Rates of aromatic VOC degradation are in the order 1,2,4-trimethylbenzene > p-cymene > bromobenzene. This order is caused by the selectivity of the direct ozone reactions (k(O3,M) ranges from 0.16 to 304 M(-1) s(-1)) and appears to be related to the electron-donating or -withdrawing ability of the substituent groups on the aromatic ring. The removal rates for the five aliphatic VOCs are much lower and are in the order 1,1-dichloropropane > 1,3-dichloropropane > 1,1-dichloroethane > 2,2-dichloropropane > 1,1,2,2-tetrachloroethane. The second-order indirect rate constants for the aliphatic VOCs range from 0.52 x 10(8) to 5.5 x 10(8) M(-1) s(-1). The relative stability of the carbon-centered intermediates seems to be related to the relative reactivity of the aliphatic VOCs with OH radicals. Except for 1,3-dichloropropane, ozonation and the O3/H2O2 AOP are not effective for the removal of other aliphatic VOCs. Bromide formation during the ozonation of bromobenzene indicates that bromate can be formed, and thus, ozonation and O3/H2O2 AOP may not be suitable for the treatment of bromobenzene.     

Polyoxometalate-enhanced oxidation of organic compounds by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen

In the presence of oxygen, organic compounds can be oxidized by zerovalent iron or dissolved Fe(II). However, this process is not a very effective means of degrading contaminants because the yields of oxidants are usually low (i.e., typically less than 5% of the iron added is converted into oxidants capable of transforming organic compounds). The addition of polyoxometalate (POM) greatly increases the yield of oxidants in both systems. The mechanism of POM enhancement depends on the solution pH. Under acidic conditions, POM mediates the electron transfer from nanoparticulate zerovalent iron (nZVI) or Fe(II) to oxygen, increasing the production of hydrogen peroxide, which is subsequently converted to hydroxyl radical through the Fenton reaction. At neutral pH values, iron forms a complex with POM, preventing iron precipitation on the nZVI surface and in bulk solution. At pH 7, the yield of oxidant approaches the theoretical maximum in the nZVI/O2 and the Fe(II)/O2 systems when POM is present, suggesting that coordination of iron by POM alters the mechanism of the Fenton reaction by converting the active oxidant from ferryl ion to hydroxyl radical. Comparable enhancements in oxidant yields are also observed when nZVI or Fe(II) is exposed to oxygen in the presence of silica-immobilized POM.     

Efficient catalytic decomposition of CO2 to CO and O2 over Pd/ mixed-conducting oxide catalyst in an oxygen-permeable membrane reactor

The thermal decomposition of CO2 to CO and O2 is a potential route for the consumption and utilization of CO2. However, this reaction is limited by both the thermodynamic equilibrium and the kinetic barrier. In this study, we reported an innovative catalytic process to decompose CO2 in an oxygen-permeation membrane reactor packed with a mixed-conducting oxide supported noble metal catalyst, or Pd/SrCo0.4Fe0.5Zr0.1O3-delta (Pd/ SCFZ), which is of high activity in the decomposition of CO2 into CO and O2. Pd/SCFZ catalyst was prepared by incipient wetness impregnation of the SCFZ powders with an aqueous solution of PdCl2, and the CO2 sorption/desorption property was examined by in situ Fourier transform infrared spectroscopy and temperature-programmed desorption-mass spectrometry technologies. It was shown that there appeared a typical of bridged carbonyls (Pd-CO) on the surface of the Pd/SCFZ catalyst formed after CO2 decomposition. Both CO2 and CO could be detected in the species desorbed from Pd/SCFZ catalyst, which implied that the Pd/SCFZ catalyst could effectively activate the CO2 molecule. During the catalytic process, furthermore, the activity of the Pd/SCFZ catalyst can self-regenerate by removing the produced lattice oxygen through the dense oxygen-permeable ceramic membrane. At 900 degrees C, this catalytic process attains 100% of CO formation selectivity at 15.8% of CO2 conversions.     

In situ chemical reduction of Cr(VI) in groundwater using a combination of ferrous sulfate and sodium dithionite: a field investigation

A field study was conducted to evaluate the performance of a ferrous iron based in situ redox zone for the treatment of a dissolved phase Cr(VI) plume at a former industrial site. The ferrous iron based in situ redox zone was created by injecting a blend of 0.2 M ferrous sulfate and 0.2 M sodium dithionite into the path of a dissolved Cr(VI) plume within a shallow medium to fine sand unconfined aquifer formation. Monitoring data collected over a period of 1020 days after more than 100 m of linear groundwater flow through the treatment zone indicated sustained treatment of dissolved phase Cr(VI) from initial concentrations between 4 and 8 mg/L to less than 0.015 mg/L. Sustained treatment is assumed to be primarily due to the reduction of Cr(VI) to Cr(III) by ferrous iron adsorbed to, precipitated on, and/or incorporated into aquifer iron (hydr)oxide solid surfaces within the treatment zone. Precipitated phases likely include FeCO3 and FeS based on saturation index considerations and SEM/EDS analysis. The detection of solid phase sulfites and thiosulfates in aquifer sediments collected from the treatment zone more than 2 years following injection suggests dithionite decomposition products may also play a significant role in the long-term treatment of the dissolved phase Cr(VI).     

Comparison of H2O2/UV and heterogeneous photocatalytic processes for the degradation of dichloroacetic acid in water

A comparative study between two advanced oxidation technologies for pollutant degradation has been made. With the use of dichloroacetic acid (DCA) as the model pollutant, the reactions with hydrogen peroxide and UV radiation (H2O2/UV, 253.7 nm) and photocatalysis with titanium dioxide (TiO2/UV, 300-400 nm) are analyzed. Three criteria have been selected to compare the performances of both processes: (i) the percentage conversion of DCA and TOC (total organic carbon) at a fixed reaction time; (ii) the quantum efficiency, employing the true radiation absorption rates for both activated species (H2O2 and TiO2); (iii) the specific energy consumption to degrade 50% of the initial TOC. The optimal molar concentration ratio of H2O2/DCA and the optimal catalyst concentration have been employed in the experiments. The results indicate that, under the optimal operating conditions, the H2O2/UV process exhibits, by a large difference, the best performance taking into account the above-mentioned criteria. Nevertheless, both systems show similar values of specific energy consumption when a thinner reactor is employed. These results cannot be safely extrapolated to other contexts if (i) other compounds of different structure are degraded and (ii) a different catalyst is used. Moreover, they were obtained under optimized conditions, and typical, real-life situations may render quite different results due to the robustness of the titanium dioxide operation. They should serve as an indication that, under the studied conditions, a much-improved catalyst performance must be achieved to parallel, with a heterogeneous process, a yield similar to the one obtained with the homogeneous system.     

On the preparation of TiO2-sepiolite hybrid materials for the photocatalytic degradation of TCE: influence of TiO2 distribution in the mineralization

Hybrid structured photocatalysts based on sepiolite, an adsorbent, and TiO2 were prepared by extrusion of ceramic dough and conformed as plates. The influence of the photocatalyst configuration was studied either by including TiO2 in the extrusion process (incorporated materials) or by coating the sepiolite plates with a TiO2 film (coated materials). The influence of the OH- surface concentration in the photocatalytic performance was studied by treating the ceramic plates at different temperatures. The samples were characterized by N2 adsorption-desorption, MIP, SEM, XRD, and UV-vis-NIR spectroscopy and tested in the photocatalytic degradation of trichloroethylene (TCE) as a target VOC molecule. Most of the catalysts presented high photoactivity, but considerable differences were observed when the CO2 selectivity was analyzed. The results demonstrate that there is a significant effect of the catalyst configuration on the selectivity of the process. An intimate contact between the sepiolite fibers and TiO2 particles for incorporated materials with a corncob-like structure favored the migration of nondesirable reaction products such as COCl2 and dichloroacetyl chloride (DCAC) to the adsorbent, reacting with OH- groups of the adsorbent and favoring the TCE mimeralization.     

双氧水/硫脲氧化还原体系下羊毛低温染色机制

为了探讨羊毛织物在双氧水/硫脲体系下的低温染色机制,采用电子顺磁共振谱(ESR)对羊毛纤维、染料进行自由基的检测,采用气相色谱和紫外光谱等方法对氧化还原体系产物进行检测。结果显示:双氧水/硫脲体系在染液中能激发羊毛纤维和弱酸性染料红B产生自由基,促使纤维表面和染料分子高度活化;同时,双氧水/硫脲体系能持续释酸,维持染液稳定的pH值,促进羊毛纤维的溶胀,从而使弱酸性染料能在低温条件下对羊毛织物有较好的染色性能。     

Kinetics and modeling of the Fe(III)/H2O2 system in the presence of sulfate in acidic aqueous solutions

This work examined the effect of sulfate ions on the rate of decomposition of H2O2 by Fe(III) in homogeneous aqueous solutions. Experiments were carried out at 25 degrees C, pH < or = 3 and the concentrations of sulfate ranged from 0 to 200 mM ([Fe(III)]0 = 0.2 or 1 mM, [H2O2]0 = 10 or 50 mM). The spectrophometric study shows that addition of sulfate decreased the formation of iron(III)-peroxo complexes and that H2O2 does not form complexes with iron(III)-sulfato complexes. The rates of decomposition of H2O2 markedly decreased in the presence of sulfate. The measured rates were accurately predicted by a kinetic model based on reactions previously validated in NaClO4/HClO4 solutions and on additional reactions involving sulfate ions and sulfate radicals. At a fixed pH, the pseudo-first-order rate constants were found to decrease linearly with the molar fraction of Fe(II) complexed with sulfate. The model was also able to predict the rate of oxidation of a probe compound (atrazine) by Fe(III)/H2O2. Computer simulations indicate that the decrease of the rate of oxidation of organic solutes by Fe(III)/H2O2 can be mainly attributed to the complexation of Fe(III) by sulfate ions, while sulfate radicals play a minor role on the overall reaction rates.     

Monitoring oxidation of chlorinated ethenes by permanganate in groundwater using stable isotopes: laboratory and field studies

Permanganate injection is increasingly applied for in situ destruction of chlorinated ethenes in groundwater. This laboratory and field study demonstrates the roles that carbon isotope analysis can play in the assessment of oxidation of trichloroethene (TCE) by permanganate. In laboratory experiments a strong carbon isotope fractionation was observed during oxidation of TCE with similar isotopic enrichment factors (-25.1 to -26.8 per thousand) for initial KMnO4 concentrations between 67 and 1,250 mg/L. At the field site, a single permanganate injection episode was conducted in a sandy aquifer contaminated with TCE as dense nonaqueous liquid (DNAPL). After injection, enriched delta13C values of up to +204% and elevated Cl- concentrations were observed at distances of up to 4 m from the injection point. Farther away, the Cl- increased without any change in delta13C of TCE suggesting that Cl- was not produced locally but migrated to the sampling point Except for the closest sampling location to the injection point, the delta13C rebounded to the initial 613C again, likely due to dissolution of DNAPL Isotope mass balance calculations made it possible to identify zones where TCE oxidation continued to occur during the rebound phase. The study indicates that delta13C values can be used to assess the dynamics between TCE oxidation and dissolution and to locate zones of oxidation of chlorinated ethenes that cannot be identified from the Cl- distribution alone.     

Saccharin and other artificial sweeteners in soils: estimated inputs from agriculture and households, degradation, and leaching to groundwater

Artificial sweeteners are consumed in substantial quantities as sugar substitutes and were previously shown to be ubiquitously present in the aquatic environment. The sweetener saccharin is also registered as additive in piglet feed. Saccharin fed to piglets was largely excreted and, consequently, found in liquid manure at concentrations up to 12 mg/L, where it was stable during 2 months of storage. Saccharin may thus end up in soils in considerable quantities with manure. Furthermore, other studies showed that saccharin is a soil metabolite of certain sulfonylurea herbicides. Sweeteners may also get into soils via irrigation with wastewater-polluted surface water, fertilization with sewage sludge (1-43 μg/L), or through leaky sewers. In soil incubation experiments, cyclamate, saccharin, acesulfame, and sucralose were degraded with half-lives of 0.4-6 d, 3-12 d, 3-49 d, and 8-124 d, respectively. The relative importance of entry pathways to soils was compared and degradation and leaching to groundwater were evaluated with computer simulations. The data suggest that detection of saccharin in groundwater (observed concentrations, up to 0.26 μg/L) is most likely due to application of manure. However, elevated concentrations of acesulfame in groundwater (up to 5 μg/L) may result primarily from infiltration of wastewater-polluted surface water through stream beds.     

Influence of vitamin E on lipid and protein oxidation induced by H(2)O(2)-activated MetMb in microsomal membranes from turkey muscle

Lipid and protein oxidation generated by metmyoglobin+H₂O₂ were studied in microsomal membranes of turkey muscles. With a basal diet enriched with 6% soya oil and supplemented with vitamin E (30 ppm for control and 400 ppm for supplemented animals) oxidations were investigated by different methods. Lipid oxidation was estimated by TBARS and lipofuscins measurement and protein oxidation was measured by an estimation of carbonyl groups and free thiols. Supplementation of turkeys with -tocopheryl acetate increased the vitamin E content of microsomal membranes and had a protective (and significant) effect on lipid oxidation when measured by the two techniques. Vitamin E supplementation significantly protected free thiols from oxidation but had only a small effect (non significant) on carbonyl group formation. No vitamin E dietary supplementation effect was observed on radical formation, as measured by optical and ESR spectroscopy.     

Antioxidant activities of phlorotannins purified from Ecklonia cava on free radical scavenging using ESR and H2O2-mediated DNA damage

The potential antioxidant activities of three phlorotannins (phloroglucinol, eckol and dieckol) purified from Ecklonia cava collected in Jeju Island were investigated to evaluate their potential value as the natural products for foods or cosmetic application. In this study, antioxidant activities were measured by electron spin resonance spectrometry (ESR) technique for scavenging effects of free radicals such as 1,1-diphenyl-2-picrylhydrazyl (DPPH), alkyl, hydroxyl (HO^•) and superoxide anion radical (O₂ ^•−) and by comet assay for protecting effects against H₂O₂-mediated DNA damage. The results show that all the phlorotannins have the potential DPPH, alkyl, hydroxyl and superoxide radical scavenging activities. Especially, eckol samples scavenged around 93% of DPPH at 0.25, 0.5, 1 mg/mL of concentrations and were higher than the other phlorotannins, such as phloroglucinol and dieckol samples. Also, protecting effects of the phlorotannins against H₂O₂-mediated DNA damage increased with increased concentrations of the samples in the L5178 mouse T-cell lymphoma cell lines (L5178Y-R). In conclusion, these results suggest that the three phlorotannins purified from E. cava have the potential inhibitory effect on H₂O₂-mediated DNA damage and harmful free radicals and can be used as antioxidants in cosmetic, foods and drug industry.