Hydroxyl radical carbonate ions

A recent study (1) has demonstrated that the electrochemical oxidation of hydroxide ion yields hydroxyl radical ( OH) and its anion (O"-). These species in turn are stabilized at glassy carbon electrodes by transition-metal ions via the formation of metal-oxygen covalent bonds (unpaired d electron with unpaired p electron of -OH and O- ). The coinage metals (Cu, Ag, and Au), which are used as oxygen activation catalysts for several industrial processes (e.g., Ag/02 for production of ethylene oxide) (2-10), have an unpaired electron (d10s1 or d9s2 valence-... 

A further use of the system is to mediate the reaction of adamantane with carbon monoxide and oxygen to form 1-adamantanecarboxylic acid . When long-wavelength light (>300 nm) is used, hydroperoxides efficiently generate hydroxyl radicals without the use of metal ions and would be an extremely useful source of hydroxyl radicals, particularly in the design of DNA-cleaving molecules . ... 

C) Hydrocarbon fuels when combusted under actual (nonideal) combustion conditions produce several intermediate products in addition to carbon dioxide and water and include the unbumed hydrocarbon, carbon monoxide, oxides of nitrogen, hydroxyl radicals, and the hydrogen ions. 

The Matrix TiOa photocatalytic treatment system is a technology that destroys dissolved organic contaminants in water in a continuous-flow process at ambient temperature. The technology uses ultraviolet (UV) light and a titanium dioxide (TiOa) semiconductor catalyst to break hydroxide ions (OH ) and water (H2O) into hydroxyl radicals (OH ). The radicals oxidize the organic contaminants to form carbon dioxide, water, and halide ions (if the contaminant was halogenated). 

A systematic dependence of reaction order on temperature and pH is not visible, n varies between one and two. Different experimental conditions and/or missing details about these conditions as well as different analytical methods make a comparison of these results impossible. Staehelin and Hoigne (1985) proposed a possible explanation for the second order reaction (n = 2). Since in clean water ozone not only reacts with the hydroxide ions but also with the intermittently produced hydroxyl radicals (see Chapter A 2), it behaves like a promoter and the decay rate increases with the square of the liquid ozone concentration. This is supported by the results obtained by Gottschalk (1997). She found a second order decay rate in deionized water, compared to a first order decay rate in Berlin tap water, which contains about 4 mg L DOC and 4 mmol LT1 total inorganic carbon. Staehelin and Hoigne (1982) also found first order in complex systems. 

The formation of scavenger substances can also retard removal efficiency and kinetic reaction rates. Scavengers are ions such as bicarbonate, carbonate, chloride, and humic acid, etc. These scavengers subsequently react with hydroxyl radicals produced during the degradation process. Therefore, the removal efficiency will be reduced significantly. In the presence of scaveng-... 

The first step is an homolytic cleavage of the Co-OH bond with the formation of the hydroxyl radical which is then scavenged by the carbonate ion. 

Organic radicals formed in these reactions may further react with oxygen (in an aerated medium as in water treatment) to yield organic peroxyl radicals that can eventually react with compounds present in the medium to release the superoxide ion radical (see route through 5 in Fig. 6 see also the work of von Sonntag and Schuchmann [122] for more details about peroxyl radical reactions). In these cases, compounds that react with the hydroxyl radical are known to be promoters of ozone decomposition because the superoxide ion radical consumes ozone at a fast rate [see reaction (63) above]. On the contrary, if the reaction between hydroxyl radical and compound M does yield inactive radicals, M is known as a scavenger or inhibitor of ozone decomposition (see route to 4 in Fig. 6). Many natural substances such as humic substances and carbonates are known to possess such a role [121]. However, the case of carbonate ion is rather special because it reacts with hydroxyl radicals to yield the carbonate ion radical ... 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

In accounting for the yield of carbon dioxide,which is equivalent to about 20% of the yield of hydroxyl radicals, it would be reasonable to suppose that the carboxyl group of the glucuronic-acid subunit is involved. If this is the case, some release of condensed counter ions might be expected following decarboxylation. Such a process would be expected to contribute to the conductivity changes observed in the pulse-radiolysis experiments described later. 

ROS produced by sugars and glycated protein autoxidation participate in oxidizing of already glycated proteins and affect other proteins (H24). These reactions are catalyzed by metal ions (W17). The presence of metal ions may, moreover, initiate the Fenton reaction and produce hydroxyl radicals. A carbon-centered 1-hydroxyalkyl radical was found during autoxidation of glyceraldehyde (T12). 

Thus, carbonate and bicarbonate ions are efficient scavengers of hydroxyl radicals in water due to their relatively low reduction potentials. 

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