Inorganic hydroxyl radical

Anbar, M. and Neta, P. (1967). A compilation of specific biomolecular rate constants for the reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds in aqueous solutions. Int. J. Appl. Radiat. Isot. 18, 493-497. 

The use of ultrasound in both the synthesis and crystallisation of a broad array of both organic and inorganic materials has been intensively researched and is well documented [61-64]. An application of ultrasound that has received relatively less attention however, is in the dissolution of colloidal particles. Prakash and Ghosh [65] reported on the dissolution of silver colloids under 1 MHz ultrasound irradiation, proposing that the silver is oxidised by sonochemically produced hydroxyl radicals. Sostaric et al. [66] investigated the dissolution of MnC>2 colloids in the presence of aliphatic alcohols at a lower frequency of 20 kHz. They found that... 

Advanced oxidation processes (AOPs) are a range of water treatments which involve the in situ formation of radicals, particularly hydroxyl radicals, in sufficient quantity to affect chemical or biological contaminants. These include ultrasonic and ultraviolet irradiation but they are sometimes ineffective for the remediation of water which contains a mixture of organic and inorganic compounds. Chemical oxidants can be used to add additional oxidising power to such processes and ozone in conjunction with ultrasound is one such option [31]. 

The hydroxyl radical is a powerful oxidant, having a reduction potential of 2.7 V in acidic solution. In neutral solution, where the free energy of neutralization of OH by H3O is not available, the reduction potential decreases to 1.9 V (Table 2). Several inorganic anions and low-valency transition metal ions readily undergo one-electron oxidation by reaction with OH, which is often represented as a simple electron transfer ... 

In order to fiicilitate the use of these symbolic types, it is advisable to become familiarised with the symbols of the following analogues of hydroxyl, in addition to those already given for inorganic compound radicals at p, 28, and for the monad and dyad organic radicals at p. 200... 

This comparison is only theoretical. In reality a high production of OH° can lead to a low reaction rate because the radicals recombine and are not useful for the oxidation process. Also not considerd are the effects of different inorganic and/or organic compounds in the water. Various models to calculate the actual OH-radical concentration can be found in the literature, some are described in Chapter B 5, Further information concerning the parameters which influence the concentration of hydroxyl radicals is given in Section B 4.4, as well as a short overview about the application of ozone in AOPs in Section B 6.2. 

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. 

Inorganic carbon can also influence the total reaction rate by acting as a scavenger for hydroxyl radicals, whereas ozone itself does not react with carbonate or bicarbonate (Hoigne, 1984). The reaction of OH° with inorganic carbon proceeds according to the following mechanisms ... 

Electron transfer is usually found in reactions between hydroxyl radicals and inorganic ions ... 

Because hydroxyl radicals have indiscriminate reactivity, they can react with almost all types of organic and inorganic compounds. Most aromatic compounds undergo radical attack on the aromatic ring in a manner similar to that of benzene systems. The products and the rate constants for hydroxyl radical attack on aromatic compounds are listed in Table 5.11. The data were obtained from the pulse radiolysis studies (Buxton et al., 1988). 

When hydroxyl radical is reacting with inorganic species, electron transfer will occur after aqueous solutions are irradiated with high energy electrons. For example, halogen ions (X-) will react with hydroxyl radical as follows ... 

Source includes data for other related compounds. For additional data on ozone, chlorine dioxide, and other inorganic radicals see (156). Data on hydroxyl radicals can be found in (157, 158) and chapters 14 and 15. 

Photochemical operations offer several routes of hydroxyl radical formation by UV irradiation. The formation of hydroxyl radicals by irradiation of samples doped with hydrogen peroxide or ozone is the state-of-the-art in water treatment. Two comprehensive reviews cover the historical development of the UV photo-oxidation technique as a pretreatment step in the inorganic analysis of natural waters, its principles and the equipment available, and its principal applications in the analytical field.3,4 They include tables summarizing the elements determined, the analytical techniques used, and the sample matrices studied. 

This is probably the most important radical species in aqueous solution. Oxidative reactions of the hydroxyl radical, ( OH), with inorganic and organic compounds have been well documented [8]. Compilations of bimolecular (second-order) rate constants have been published [3,8]. The OH- can undergo several types of reactions with species in aqueous solution, including addition, hydrogen abstraction, and electron transfer. 

A significant body of literature proposes that the photocatalytic oxidation of organic or inorganic solutes may occur by either indirect oxidation via a surface-bound hydroxyl radical (i.e., a trapped hole at the particle surface) or directly via the valence-band hole before it is trapped either within the particle or at the particle surface.Interfacial hole transfer from titanium dioxide to organic and inorganic solutes has been studied recently in [4f, 6c, 7]. An example of the latter paper is shown in Fig. 7.5. 

Fig. 2.1 Scheme of the electrochemical processes for the removal of organic compounds (R) (a) direct electrolysis (b) via hydroxyl radicals produced by the discharge of the water and (c) via inorganic mediators... 

They also found that, depending on the electrolyte composition, the organics were oxidized on both the electrode surface by reaction with hydroxyl radicals and in the bulk of the solution by inorganic oxidants electrogenerated on the BDD anodes, such as peroxodisulfuric acid from sulfuric acid oxidation ... 

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