Oxidation by OH radical

The majority of systems studied have been aqueous solutions of either aromatic compounds or halogenated hydrocarbons. Such materials represent models for the major classes of organic pollutants in waste and ground water. The primary products resulting from the sonochemical treatment of phenol at 541 kHz (27 °C with bubbled air) are hydroquinone and catechol [22]. These compounds are easy to monitor and are clearly seen to be intermediates which disappear as the reaction progresses (Fig. 4.1). Similarly the sonolysis of aqueous 4-chlorophenol leads to products mainly characteristic of oxidation by OH radical e. g. 4-chlorocatechol but in both cases the final organic products are CO, CO2 and HCOOH (Scheme 4.2) [22-25]. 

The titanium-mediated photocatalytic oxidation of a pyridine solution was conducted by Low et al. (1991). They proposed that the reaction of OH radicals with pyridine was initiated by the addition of a OH radical forming the 3-hydro-3-hydroxypyridine radical followed by rapid addition of oxygen forming 2,3-dihydro-2-peroxy-3-hydroxypyridine radical. This was followed by the opening of the ring to give At-(formylimino)-2-butenal which decomposes to a dialdehyde and formamide. The dialdehyde is oxidized by OH radicals yielding carbon dioxide and water. Formamide is unstable in water and decomposes to ammonia and formic acid. Final products also included ammonium, carbonate, and nitrate ions. 

The main sink for methane in the atmosphere is oxidation by OH radical to carbon monoxide, which in turn is oxidized to carbon dioxide according to the reaction scheme first proposed by Levy (1971) and McConnell et al. (1971) ... 

Indeed, radical trapping in the gas phase performed by HX is bound to increase production of CO which would otherwise be oxidized by OH radicals. Furthermore, restriction of oxidation increases the amount of nonoxidized products which may condense into droplets or particles when they leave the flame, increasing the optical density of the smoke. Finally HX and metal halides are highly corrosive. The ensuing threat to people, structures, and goods involved in the fire may discourage the use of these fire retardants in spite of their high effectiveness and versatility which... 

As demonstrated by papers presented at this Symposium, the increase of our knowledge about acid deposition in recent years has been enormous. The SURE project (Hidy, This Volume) demonstrated that huge field projects can be conducted with good quality control of samples, analyses and data. Kok, Tanner (This Volume) and others have developed highly sophisticated systems for measuring concentrations of many species, including the very important H2O2, in clouds and clear air with aircraft. In the area of mechanisms, we know that oxidation by -OH radicals is the dominant gas-phase reaction in the conversion of S02 to and sulfate (5). Furthermore, we know that oxidation in solution by is rapid and that by 03 and O9 (the latter catalyzed by metal ions or carbon soot) canoe important under some conditions (6 Schwartz, This Volume). 

Peroxynitrous acid, ONOOH, forms in another photochemical channel at shorter wavelengths but is absent at k > 300 nm. The O-atoms generated in reaction 99 may react with 02([02 Water 0.3 mM) via reaction 100 or, preferably, with nitrate via reaction 101. Nitrite (smax = 22.5 M 1 cm1 at 360 nm) will undergo secondary photolysis, reaction 102, and oxidation by OH radicals, reaction 103 ... 

S02 is oxidized to sulfuric acid both by homogeneous gas-phase reactions and by multiphase processes when a precursor gas is dissolved in water and then subsequently oxidized. The routes to atmospheric sulfuric acid production have been studied extensively for decades. The most important gas-phase mechanism is oxidation by OH radicals ... 

In the batch mode, M wiU be oxidized by OH radicals to M (Fig. 8-2, situation A). Because initially M is in a very large excess, the OH radicals will preferentially be scavenged by M. Therefore, the concentration of M rises and subsequently it competes efficiently with M for OH radicals leading slowly to the formation of M . If most of M is oxidized the concentration of M reaches its maximum and it wiU preferably react with OH radicals leading to a decrease in its concentration. Finally, the concentration of M increases. Therefore, the batch photoreactor configuration seems to be best suited for the partial oxidation of hazardous pollutants in water. The accumulation of oxidation products M must be taken into account with respect to their toxicity and their environmental impact... 

Thus, a steady-state (SS) analysis (Bolton et al., 2001a) is based on Eq. 8-5 that describes the rate of substrate oxidation by OH radicals under steady-state conditions, in which the concentration of OH radicals is unknown. However, the total rate of OH radical formation and consumption under steady-state conditions can be described by Eq. (8-6), which is derived by adding up the rate expressions of Eqs. 8-2 to 8-4. This term is set to zero with the assumptions of extremely low OH radical concentration and a zero rate of OH radical formation in the steady-state. 

U = 0.74 d based on rate constants for oxidation by OH radical in the atmosphere at 25°C (Mill 1982) loss rate k = (3.4 1.7) x 10 min" in outdoor Teflon chambers in the dark (Grosjean 1985) calculated lifetimes of 11 h andl8 d for reactions with OH radical, NO3 radical, respectively (Atkinson 2000). Surface water ... 

All the experimental results for alkali and alkaline-earth metals can be interpreted by considering that soot precursors P (e.g., polyacetylenes in the combustion of aliphatic hydrocarbons (10)) are both oxidized by OH radicals with the rate Vi and are turned into small soot particles with the rate V2. Depending on the kind of metal and the experimental conditions, one or the other of these reactions will be promoted, with the rates becoming Vi and V2, respectively. 

Oxidation by OH Radicals The amount of OH radicals available for oxidation of a solute M depends on the amount of OH formed and the relative rate with which it reacts with M when compared with the rate at which it is consumed by all other solutes, in accordance with the following scheme ... 

Ammonia is also oxidized by OH radicals, and this has been proposed as a source and a sink for NO , (Logan, 1983), although it is a minor sink for NH3. The reaction sequence for this oxidation begins as follows ... 

The ozonation of dyes at various initial pH levels (4, 7 and 10) were also examined, and is shown in Figure 18. The pseudo first-order rate constants of ozonation of all selected dyes varied from 5.2 x 10 4 to 1.2 x 10 s 1 at pH 4, 1.5 x KT to 5.1 x KT4 s 1 at pH 7, and l.l x l O 4 to 6.3 x I0"4 s 1 at pH 10, respectively, as shown in Figure 18. The decay rates of azo dyes were found to increase with the increment of the pH, and the solution pH decreased during ozonation. In general, ozone oxidation pathways include direct oxidation by ozone or radical oxidation by OH radical. Direct oxidation is more selective and predominates under acidic conditions, while radical oxidation is less selective and predominates under basic conditions. Since the oxidation potential of hydroxyl radicals is much higher than that of the ozone molecule, direct oxidation is slower than radical oxidation. 

In this scheme two molecules of ozone are generated for each molecule of methane consumed. The subsequent photodissociation of formaldehyde produces equivalent amounts of carbon monoxide, which also undergoes oxidation by OH radicals ... 

Potential S02 oxidants are the radicals OH, H02, R02, and Criegee intermediates. S02 oxidation by OH radicals is now well understood to proceed via an addition product ... 

Derwent, R. G. (1982). On the comparison of global, hemispheric, one-dimensional model formulations of halocarbon oxidation by OH radicals in the troposphere. Atmos. Environ. 

The consequences of local peculiarities for global atmospheric chemistry are still uncertain but if the levels of hydroxyl radical would be decreased, many pollutants that are normally oxidized by OH radical could build up, leading to significant relevant... 

In a pure water matrix, a species having a similar spectrum was obtained (Figure 7, Curve I). However, in neutral 0.01M chloride (not containing alcohol), a double humped spectrum forms (Figure 7, Curve II). The peak near 410 n.m. corresponds to the one in pure water or alkali owing to reduction by hydrated electrons and that near 325 n.m. to oxidation by OH radicals. 

Tury et al. (2003) have investigated the atmospheric fate and potential environmental effects of TDI and diphenylmethane-4,4 -diisocyanate MDl. TDI degrades in the atmosphere by oxidation by -OH radicals with an estimated half-life of one day. The compound does not react with water in the gas phase at a significant rate. Typical emission losses are 25 g/t of TDI used in making slabstock foam. 

The lifetime of CH4 with respect to oxidation by OH radicals is around 10 years in today s atmosphere. In the early atmosphere OH levels would be so low that only the photolysis remains as a sink, resulting in a residence time of CH4 in the order of 10 years (Kasting and Siefert 2002). It is assumed that methane remained until formation of the oxygenic environment in air (2.2-2.7 Gyr ago) at relatively high concentration (ppm level) to maintain a warming potential (a greenhouse effect ). Therefore, CH4 must be produced from the crust at rates compensating its atmos-... 

In a much less sophisticated system hydrocarbons are being oxidized by OH radicals at the cathode This occurs by simultaneous reduction of Fe(III) and oxygen at cpe. The OH radicals are generated by a Fenton reaction from Fe(II) and cathodically formed hydrogen peroxide. Linear alkanes from C5 to C o are being oxidized to ketones as the only products. The yields decrease with increasing number of carbon atoms and with the Fe(III) concentration. 

The alkyl radical formed in the initial oxidation reactions subsequently react with O2 resulting in the formation of alkylperoxy radical. The alkylperoxy radical is the key intermediate in the oxidation of VOCs by OH radical and further reacts with HO2, NO2 and NO in the atmosphere. This intermediate can also undergo self-reaction and epoxidation reactions. These reactions lead to the formation of hydroperoxide adducts, alkoxy radical and O3, and also OH radical regeneration. The alkoxy radical is the second key intermediate in the VOCs oxidation by OH radical. The alkoxy radical undergoes prompt decomposition resulting in the oxidation of NO. The characterization of these secondary reactions is studied extensively using theoretical methods, but only very few experimental studies are available for such reactions. 

Phosphorus-containing substances are first converted in the flame into phosphorus oxides with an uneven number of electrons. Anions formed in the alkali reaction by the addition of an electron are oxidized by OH radicals. The electrons added are released and produce a signal current. 

---