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The Atmosphere and Atmospheric Chemistry
Published in Stanley E. Manahan, Environmental Chemistry, 2022
The hydroperoxyl radical HOO• is an intermediate in some important chemical reactions. In addition to its production by the reactions discussed previously, in polluted atmospheres, hydroperoxyl radical is made by the following two reactions, starting with photolytic dissociation of formaldehyde to produce a reactive formyl radical: CHO+hν→H+HC•OHC•O+O2→CO+HOO•
The Atmosphere and Atmospheric Chemistry
Published in Stanley Manahan, Environmental Chemistry, 2017
The hydroperoxyl radical HOO• is an intermediate in some important chemical reactions. In addition to its production by the reactions discussed above, in polluted atmospheres, hydroperoxyl radical is made by the following two reactions, starting with photolytic dissociation of formaldehyde to produce a reactive formyl radical: HCHO+hν→H+HC•OHC•O+O2→CO+HOO•
Inactivation of Waterborne Pathogens in Municipal Wastewater Using Ozone
Published in Iqbal M. Mujtaba, Thokozani Majozi, Mutiu Kolade Amosa, Water Management, 2018
Achisa C. Mecha, Maurice S. Onyango, Aoyi Ochieng, Maggy N.B. Momba
Ozone and · OH radicals are strong chemical oxidants, and both are involved in the destruction of microorganisms as well as the oxidation of a wide range of organic and inorganic compounds (Gray 2014). The degradation of pollutants in water using ozonation occurs through two mechanisms. The first is direct oxidation by molecular ozone, which occurs at low pH and involves highly selective reactions, such as electrophilic, nucleophilic or dipolar addition reactions with low reaction rates (Hoigne 1998). The other is an indirect mechanism, which occurs at basic pH through the ozone decomposition to produce · OH radicals, which are non-selective and highly reactive. The formation of ·OH radicals occurs in a number of reaction steps (Table 14.3). The first step of this process is the decomposition of ozone by hydroxide ions (Equation 14.1) to form superoxide radicals (O2·–) and hydroperoxyl radicals (HO2·). The formed hydroperoxyl radical is in an equilibrium state (Equation 14.2). The superoxide anion radical and ozone then react to form ozonide anion radical (Equation 14.3), which then immediately decomposes into oxygen and an ·OH radical (Equation 14.4) via hydrogen trioxide (HO3·) (Gottschalk et al. 2010). The ·OH radical reacts with ozone to form oxygen and a hydroperoxyl radical (Equation 14.5) thus completing the chain reaction and starting it anew. Overall, three ozone molecules theoretically produce two ·OH radicals. Finally, the reaction is terminated (Equation 14.6).
Photochemical impacts on the toxicity of PM2.5
Published in Critical Reviews in Environmental Science and Technology, 2022
Jialin Xu, Wenxin Hu, Donghai Liang, Peng Gao
Atmospheric photochemical reactions occur when molecules, atoms, free radicals, or ions in the atmosphere absorb solar radiation (Bao et al., 2018). Under normal circumstances, the absorbent in the atmosphere is in an excited state due to the absorption of photons, which leads to a photochemical reaction with other substances (Bao et al., 2020). Many free radicals in the atmosphere, such as hydroxyl (HO•), hydroperoxyl (HO2•), alkoxy (RO•), and alkyl peroxyl (RO2•) are formed by photochemical reactions among ozone, nitrous acid, and water. At the same time, free radicals in the atmosphere can react with nitrogen oxides and VOCs to generate a series of secondary pollutants (Calvert & Lindberg, 2005). Currently, most studies based on atmospheric photochemistry focus on photochemical smog. Photochemical smog is formed by the photochemical reactions of NOx and VOCs in the atmosphere under stable weather conditions, such as strong sunlight, low wind speed, and low humidity (Louka et al., 2003). In contrast, few studies have linked atmospheric aerosols and photochemical reactions. Moreover, most of the existing studies have reported that atmospheric aerosols affect ozone generation by attenuating ultraviolet radiation and affecting photochemical pollution; however, they have rarely discussed the photochemical impact on hazardous pollutants in the case of PM2.5 (Castro et al., 2001).
Impact of Ozone Treatment on Seed Germination – A Systematic Review
Published in Ozone: Science & Engineering, 2020
R. Pandiselvam, V.P. Mayookha, Anjineyulu Kothakota, L. Sharmila, S.V. Ramesh, C.P. Bharathi, K. Gomathy, V. Srikanth
Ozone dissolved in water has many practical applications including water disinfection and purification. Also, the rate of decomposition of ozone in water is high compared to its dissolution in oxygen or air. At pH below 7 it dissolves in water, does not react with water and stay in molecule form. An increase in pH especially above 7.5 leads to spontaneous ozone decomposition that generates free radicals, like hydroxyl radicals, oxygen and hydroxide ions, which are highly reactive (Pandiselvam et al. 2017a). The oxidation potential for hydroxyl radicals is higher (2.80 V) than for ozone (2.07 V). Within 10 min at pH 8, nearly half of the applied ozone is decomposed into oxygen and different intermediate forms (Manousaridis et al. 2005; Pirani 2010). The decay of ozone increases the rate of production of radicals such as hydroxyl (°OH), superoxide (°O2−) and hydroperoxyl (°HO2), thereby possesses very high oxidizing power. Stability of ozone is analyzed through the quality of water and its purity as ozone get decomposed within a few seconds to hours. In pure water, the rate of ozone decomposition to oxygen is lesser than in impure solutions (Hoigné 1998; Manousaridis et al. 2005; Miller 2005). Hill et al. (1982) concluded that ozone could decompose by 50% within 20 min at 20°C in distilled and tap water, however, it takes 85 min to degrade by 10% at 20°C in double-distilled water.
A theoretical study on the kinetics of multichannel Multiwell reaction of H2S(1A1) with HO2(2A′′)
Published in Molecular Physics, 2020
Marziyeh Sadat Masoumpour, Seyed Hosein Mousavipour
The atmosphere consists of different oxidising species. Hydroperoxyl radical (HO2) is amongst the most common HOx radicals in the atmosphere. Hydroperoxyl radical is one of the important oxidising agents in the Earth's atmosphere, biological systems, and combustion processes. Many environmentally important trace gases are removed from the atmosphere mainly by oxidation. A key process for the formation of HO2 in the troposphere is the reaction of OH with CO in the presence of O2 [4]. It is not only the simplest and most important peroxy radical in the chemistry of Earth's atmosphere [5,6] but also a key transient intermediate in the combustion chemistry [7,8], atmospheric photolysis cycles [9], and biochemical processes [10,11].