Hydroxyl Radical Formation Rate Research Articles

Effects of urea on the microstructure and photocatalytic activity of bimodal mesoporous titania microspheres

Bimodal mesoporous TiO 2 microspheres with high photocatalytic activity were prepared by a hydrothermal method using titanium sulfate as precursor in the presence of urea. The results indicate that all prepared samples show bimodal pore-size distributions in the mesoporous region: smaller intra-aggregated pores with peak pore diameter of ca. 2 nm and larger inter-aggregated pores with peak pore diameter of ca. 12.5 nm. The molar ratio of urea to Ti(SO 4) 2 ( R u) has an obvious influence on the morphology, microstructure and photocatalytic activity of TiO 2. With increasing R u, specific surface areas and porosity increase, contrarily, the crystallite size and relative anatase crystallinity decrease. The photocatalytic activity first increases with R u. At R u = 2.0, the photocatalytic activity reaches the highest and is obviously higher than that of Degussa P25. With further increasing R u, the photocatalytic activity decreases. The formation rate of hydroxyl radicals during photocatalysis has a positive correlation with the photocatalytic activity.

4-chlorophenol degradation by pulsed high voltage discharge coupling internal electrolysis

A system of pulsed high voltage discharge coupling internal electrolysis (PHVDCIE) in water has been developed to remove 4-chlorophenol (4-CP). The hydrogen peroxide formed by discharge was little utilized by 4-CP degradation in the sole discharge system, but most of the hydrogen peroxide could be utilized in the PHVDCIE system to form hydroxyl radicals for the production of Fe 2+. The Fe 2+ was generated in the cell reaction and was reacted with the hydrogen peroxide through the Fenton's reaction. The formation rate of hydroxyl radical was increased in the PHVDCIE system. It was 5.28 × 10 −7 mol L −1 s −1 with bubbling oxygen but it was 1.49 × 10 −7 mol L −1 s −1 in the sole discharge system. With increase in the yields of hydroxyl radical, the 4-CP removal was sped up. The removal efficiency of 4-CP was improved to more than 90% correspondingly by 36 min discharge in the PHVDCIE system. With the promotion of 4-CP degradation, more intermediate products such as formic, acetic and oxalic acid were produced.

Effect of pH on Fenton process using estimation of hydroxyl radical with salicylic acid as trapping reagent

This study estimates the yield of hydroxyl radical using salicylic acid as the trapping reagent and investigates the relationship between hydroxyl radical and pH value. The formation and variation of hydroxyl radical under different pH values were evaluated using reaction products, 2,3-DHBA, 2,5-DHBA, and catechol. The formation rate of hydroxyl radical was dependent on the ratio of ferrous ion to hydrogen peroxide and pH values. The difference between various pH values was explored. The kinetics and mechanisms of hydroxyl radical reactions were established in the Fenton process. Experimental results showed that the best reaction conditions were 8.5 mM H(2)O(2), 1.25 mM Fe(2 + ), Fe(2 + )/H(2)O(2) = 0.147 at pH 3 and the formation rate constant of hydroxyl radical was 1.12 x 10(11) M(-1) s(-1).

Enhanced degradation of p-chlorophenol in a novel pulsed high voltage discharge reactor

The yields of active specie such as ozone, hydrogen peroxide and hydroxyl radical were all enhanced in a novel discharge reactor. In the reactor, the original formation rate of hydroxyl radical was 2.27 × 10 −7 mol L −1 s −1, which was about three times than that in the contrast reactor. Ozone was formed in gas-phase and was transferred into the liquid. The characteristic of mass transfer was better in the novel reactor than that in the contrast reactor, which caused much higher ozone concentration in liquid. The dissociation of hydrogen peroxide was more evident in the former, which promoted the formations of hydroxyl radical. The p-chlorophenol (4-CP) degradation was also enhanced. Most of the ozone transferred into the liquid and hydrogen peroxide generated by discharge could be utilized by the degradation process of 4-CP. About 97% 4-CP was removed in 36 min discharge in the novel reactor. Organic acids such as formic, acetic, oxalic, propanoic and maleic acid were generated and free chloride ions were released in the degradation process. With the formation of organic acid, the pH was decreased and the conductivity was increased.

Superoxide Dismutase Enhanced the Formation of Hydroxyl Radicals in a Reaction Mixture Containing Xanthone under UVA Irradiation

To clarify the effect of superoxide dismutase (SOD) on the formation of hydroxyl radical in a standard reaction mixture containing 15 microM of xanthone, 0.1 M of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and 45 mM of phosphate buffer (pH 7.4) under UVA irradiation, electron paramagnetic resonance (EPR) measurements were performed. SOD enhanced the formation of hydroxyl radicals. The formation of hydroxyl radicals was inhibited on the addition of catalase. The rate of hydroxyl radical formation also slowed down under a reduced oxygen concentration, whereas it was stimulated by disodium ethylenediaminetetraacetate (EDTA) and diethyleneaminepentaacetic acid (DETAPAC). Above findings suggest that O(2), H(2)O(2), and iron ions participate in the reaction. SOD possibly enhances the formation of the hydroxyl radical in reaction mixtures of photosensitizers that can produce O(2)(-.).

Contribution of the Photo-Fenton Reaction to Hydroxyl Radical Formation Rates in River and Rain Water Samples

The hydroxyl radical (OH radical) formation rates from the photo-Fenton reaction in river and rain water samples were determined by using deferoxamine mesylate (DFOM), which makes a stable and strong complex with Fe(III), resulting in a suppression of the photo-Fenton reaction. The difference between the OH radical formation rates with and without added DFOM denotes the rate from the photo-Fenton reaction. The photoformation rates from the photo-Fenton reaction were in the range of 0.7 - 45.8 x 10(-12) and 2.7 - 32.3 x 10(-12) M s(-1) in river and rain water samples, respectively. A strong positive correlation between the OH radical formation rate from the photo-Fenton reaction and the amount of fluorescent matter in river water suggests that fluorescent matter, such as humic substances, plays an important role in the photo-Fenton reaction. In rain water, direct photolysis of hydrogen peroxide was an important source of OH radicals as well as the photo-Fenton reaction. The contributions of the photo-Fenton reaction to the OH radical photoformation rates in river and rain water samples were in the ranges of 2 - 29 and 5 - 38%, respectively. Taking into account the photo-Fenton reaction, 33 - 110 (mean: 80) and 42 - 110 (mean: 84)% of OH radical sources in river and rain water samples, respectively, collected in Hiroshima prefecture were elucidated.

An electrohydraulic discharge system of salt-resistance for p-chlorophenol degradation

An electrohydraulic discharge system of salt-resistance has been developed and analyzed for p-chlorophenol (4-CP) degradation. The discharge electrodes used in the system was consisted of four steel acupuncture needles (Ø 0.25 mm) which were covered by capillaries through which gas was supplied to the discharge region, forming gas bubbles. In the comparing system, the aeration capillaries were encapsulated and the bubbling was cancelled. It was confirmed that introducing gas into the discharge zone could reduce the affection of salt content in the pulsed high-voltage electrohydraulic discharge system. In the non-bubbling system, the formation of active species and 4-CP degradation was sharply influenced by increasing salt content. The formation rate of hydroxyl radical and hydrogen peroxide was decreased almost to zero and the 4-CP was hardly removed as the NaCl concentration was higher than 5.0 × 10 −3 mol L −1. But in the developed system, the formation rates were changed little with increasing the salt concentration. Increasing the NaCl concentration high to 0.1 mol L −1, the hydroxyl radical formation rate was 4.43 × 10 −7 mol L −1 s −1, and the removal rate of 4-CP was still high, where more than 90% was removed in 18 min.

Sources and Sinks of Hydroxyl Radicals upon Irradiation of Natural Water Samples

Hydroxyl radical formation rates, steady-state concentration, and overall scavenging rate constant were measured by irradiation of surface lake water samples from Piedmont (NW Italy) and nitrate-rich groundwater samples from Moldova (NE Romania). Dissolved organic matter (DOM) was the main source and sink of *OH upon lake water irradiation, with [*OH] being independent of DOM amount. Water oxidation by photoexcited DOM is a likely *OH source in the presence of very low levels of nitrate and dissolved iron. Under different circumstances it is not possible to exclude other processes, e.g., DOM-enhanced photo-Fenton reactions. Under the hypotheses of no interaction and absence of mutual screening of radiation, nitrate would prevail over DOM as *OH source for a NO3-/DOM ratio higher than 3.3 x 10(-5) (mol NO3-) (mg C)(-1), DOM prevailing for lower values. Substantial DOM photolability was observed upon irradiation of nitrate-rich groundwater, mainly due to the elevated *OH generation rate. For the first time to our knowledge, evidence was also obtained of the photoformation of potentially toxic and/or mutagenic nitroaromatic compounds upon irradiation of natural lake water and groundwater samples, proportionally to the nitrate levels.

河川水における溶存化学成分からのヒドロキシルラジカルの光化学的生成に関する研究

The hydroxyl radial, which is one of the most reactive free radicals in the environment, plays an important role in the photochemical degradation of natural organic matter and organic pollutants. The photochemical formation rates of hydroxyl radicals in 41 water samples collected from 6 rivers in Hiroshima Prefecture were determined and the sources of hydroxyl radicals were discussed. The hydroxyl radical photochemical formation rates in the Kurose and Seno rivers were much higher than those in the Ohta and Oze rivers. River water samples containing high nitrite concentrations were found to show high hydroxyl radical formation rates. In the samples which had the higher hydroxyl radical formation rates, nitrite was found to be the most important source of hydroxyl radicals. On the other hand, with decreasing photochemical formation rate of hydroxyl radicals, the percent contribution of unknown sources other than nitrate, nitrite and hydrogen peroxide increased.

DNA degradation by water extract of green tea in the presence of copper ions: implications for anticancer properties.

In recent years a number of reports have documented the chemopreventive effect of green tea consumption on various types of cancers such as those of bladder, prostate, esophagus and stomach. This property is attributed to the presence in green tea of polyphenols known as catechins. These include epigallocatechin-3-gallate, epigallocatechin and epicatechin. In addition to their antioxidant properties plant derived polyphenolics are also capable of oxidative DNA damage particularly in the presence of transition metal ions. We have recently proposed a mechanism for cytotoxic action of plant-derived polyphenols against cancer cells that involves mobilization of endogenous copper and consequent prooxidant action. In partial support of the idea, in the present paper we show that water extract of green tea is considerably more efficient than black tea extract in DNA cleavage in the presence of copper ions. Green tea extract also shows a higher rate of Cu(II) reduction and consequent hydroxyl radical formation. Cu(II) reduction is presumably accompanied by the formation of 'oxidized species' of tea polyphenols, which in turn also appear to catalyze the reduction of Cu(II) leading to redox cycling of copper ions. The results are discussed in relation to the structural differences between polyphenols of green and black tea.

Quantitation of hydroxyl radical during Fenton oxidation following a single addition of iron and peroxide

Chemical probes were used to study the formation of hydroxyl radical in aqueous iron–hydrogen peroxide reaction. Hydroxyl radical formation rate and time dependent concentration were determined in pure water, in aqueous fulvic acid (FA) and humic acid (HA) solutions, and in natural surface waters. Indirect determinations of hydroxyl radical were made by quantitating hydroxyl radical reactions with probe compounds under controlled conditions. High probe concentrations were used to determine radical formation rates and low probe concentrations were used to determine time dependent radical concentration. Two independent probes were used for intercomparison: benzoic acid and 1-propanol. Good agreement between the two probes was observed. Natural water matrices resulted in lower radical formation rates and lower hydroxyl radical concentrations, with observed formation rate and yield in natural waters up to four times lower than in pure water. HA and FA also reduced hydroxyl radical formation under most conditions, although increased radical formation was observed with FA at certain pH values. Hydroxyl radical formation increased linearly with hydrogen peroxide concentration.

Iron-mediated cardiotoxicity develops independently of extracellular hydroxyl radicals in isolated rat hearts.

Myocardial iron toxicity is often attributed to free radical damage. Present studies examine the role of extracellular hydroxyl radical formation in this process. In vitro reactions examined the rate of hydroxyl radical formation using salicylate trapping with high-pressure liquid chromatography separation and electrochemical detection of 2,3- and 2,5- dihydroxybenzoic acid. Isolated rat hearts were perfused by the Langendorff technique under the same buffer conditions to determine changes in myocardial contractility, release of tissue lactate dehydrogenase activity, and formation of lipid peroxidation products when iron was added to the perfusate with or without the formation of extracellular radicals. In vitro reactions, performed in Krebs buffer alone or with addition of iron (25 microM), produced levels of hydroxyl radicals that were nondetectable with salicylate trapping. Addition of iron/ascorbate (FeSO4 = 25 microM, ascorbate = 1 mM), or iron/ascorbate/histidine (FeSO4 = 25 microM, ascorbate = 1 mM, histidine = 15 mM) produced significant and equivalent accumulation of hydroxyl radicals. Isolated rat hearts were perfused under the same 4 conditions. Control heart contractile function was stable with little release of lactate dehydrogenase activity and low levels of thiobarbituric acid reactive substances (TBARS). There was significant and equal injury to contractile function, release of lactate dehydrogenase activity, and accumulation of TBARS in hearts in the presence (iron/ascorbate) and absence (iron alone) of extracellular hydroxyl radicals. In addition, there was significant reduction in injury with iron/ascorbate/histidine, where the formation of extracellular hydroxyl radicals was equal to those observed with iron/ascorbate alone. Additional control hearts, perfused with histidine alone, showed stable heart function. These findings indicate that the extracellular formation of hydroxyl radicals is not responsible for iron-mediated cardiotoxicity.

Stability of Lactate Dehydrogenase in Metal‐Catalyzed Oxidation Solutions Containing Chelated Metals

AbstractThe role of metal chelators in altering the reactivity of metal‐catalyzed oxidation solutions containing iron or copper, ascorbate, and hydrogen peroxide was evaluated by measuring the loss of enzymatic activity of lactate dehydrogenase (LDH) and the formation of hydroxyl radicals. The chelators used were iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, bicinchoninic acid, dihydrobenzoic acid, desferroxalamine mesylate, and phytate. All of the chelators tested at a 10:1 chelator:metal ratio afforded some protection to LDH compared to the unchelated system. Selected chelators that afforded only partial protection to LDH were able to provide nearly complete protection at higher chelator:metal ratios. This protection is likely attributable to the chelators' ability to remove surface‐bound metals and, thereby, reduce “site‐specific” oxidation. Many metal−chelator pairs generated hydroxyl radicals. The rate and amount of hydroxyl radicals formed varied depending on the metal−chelator pair. Although large amounts of hydroxyl radicals did not always result in increased protein inactivation, the absence of radicals was required for complete protein protection. The chelator to metal ratio required to eliminate hydroxyl radical formation varied widely with the metal−chelator pair and was unusually high in some cases. The results suggest that metal−chelator pairs (of various ratios) can be screened for hydroxyl radical formation and that those which show no radical formation are expected to protect the protein. No definitive conclusions can be drawn concerning the stability of LDH in solutions capable of producing hydroxyl radicals as the rate and extent of hydroxyl radical formation was not correlated simply to LDH inactivation under the conditions studied.

Photooxidation of porphyrin in Mg-substituted horseradish peroxidase.

Upon photoirradiation under aerobic conditions, the porphyrin prosthetic group in Mg-substituted horseradish peroxidase was oxidized to a mixture of its pi-cation radical and an oxidized product with an absorption band at 448 nm. The 448 nm compound was then converted to a 489 nm compound in the dark and the activation energy for the conversion was 19.3 kcal/mol. About 1 mol of O2 was consumed per mol of the 448 nm compound formed and no O2 consumption was seen in the dark reaction. The substitution of ethyl groups (meso) and hydroxyethyl groups (hemato) for the vinyl groups in protoporphyrin IX did not have an effect on the result. Under anaerobic conditions and in the presence of a suitable electron acceptor, the only photooxidation product of porphyrin was its pi-cation radical. The formation of hydroxyl radicals during irradiation under aerobic conditions was confirmed by the spin-trapping method. The formation of the above two radicals could be followed by ESR spectroscopy separately at a fixed magnetic field which was set to maximize each ESR signal. The rate of hydroxyl radical formation depended linearly on the concentration of Mg peroxidase. The photooxidation of porphyrin was slow and gave nonspecific product(s) when Mg protoporphyrin IX was present in the heme crevice of apomyoglobin or free in solution.