Introduction
Oxidation-reduction (redox) reactions are fundamental chemical processes that involve the transfer of electrons between two species. Redox reactions play a crucial role in many environmental processes, as they are often used to generate energy and they are key components of the atmosphere, soils and natural waters. Oxidation can take place in the presence of oxygen, as when metal rusts or when organic matter is oxidized to generate energy. The process of oxidation can also occur without oxygen, as when hydrogen is oxidized to form hydrogen sulfide or when metal is oxidized to form metal sulfide.
Atmospheric oxidation is an important factor in air pollution, as it affects the air quality of a given area and can result in accumulation of airborne particles. Oxidation also plays a crucial role in aquatic ecosystems, as it is responsible for the formation of inorganically bound organics (IBOs), which are toxic compounds involving metals and organic molecules. Oxidation also influences the rate of oxidation and degredation of organic matter in soils and sediments, as well as the nutrient availability.
Oxidation is a type of redox reaction in which the reactive oxygen species (ROS) oxidize an electron donor molecule and reduce an electron acceptor molecule (Figure 1). The oxidation of organic molecules occurs through a process called secondary oxidation, which involves the conversion of organic molecules into smaller, more reactive species. During secondary oxidation, oxygen molecules are activated by a process called activation of aryl- oxygen in which two oxygen molecules become activated and are oxidized by reducing agents, such as hydroxyl radicals or H2O2.
The most important secondary oxidants involved in the oxidation of organic molecules are hydroxyl radicals (OH*), superoxide anions (IO3*- ), and hydrogen peroxide (H2O2). Hydroxyl radicals are reactive species that contain an oxygen–hydrogen bond with a high reactivity level and can react with organic molecules to form a variety of secondary oxidants. Superoxide anions (IO3*– ) also exhibit high reactivity with organic molecules and can result in the formation of secondary oxidants, such as hydroperoxides and aldehydes. Hydrogen peroxide (H2O2) is a weak oxidant but can be converted to the more reactive form of hydroxyl radicals by catalysts, such as manganese dioxide (MnO2).
The formation of secondary oxidants can lead to the degradation of organic molecules, as well as the formation of toxic products. Secondary oxidation can also be catalysed by other oxidants, such as ozone and nitrogen dioxide, which can serve as oxidants and form less toxic, but still very toxic, ozone-derivatives. Oxidation reactions can also be catalyzed by metals and metal oxides, such as iron oxides, which can form more toxic oxidation products, such as hydroxyl radicals and hydroperoxides (Figure 2).
The formation of toxic oxidation products can be reduced through the use of electron donors, such as tertiary butanol (TBA), which can reduce the formation of hydroxyl radicals and hydroperoxides. Treated water can also reduce the formation of secondary oxidants, as the presence of biological organisms, such as nitrifying bacteria, can decrease the amount of reactive oxygen species and secondary oxidants in the water.
Conclusion
Oxidation-reduction (redox) reactions are fundamental chemical processes that involve the transfer of electrons between two species. Redox reactions are important components of the atmosphere, soils and natural waters, and they are involved in many environmental processes, such as the generation of energy and the formation of toxic compounds. Oxidation takes place in the presence of oxygen, as in metal rusting, or through the process of secondary oxidation without oxygen. Secondary oxidation is a type of redox reaction in which reactive oxygen species (ROS) oxidize an electron donor molecule and reduce an electron acceptor molecule. The formation of secondary oxidants and their reactions with organic molecules can lead to the formation of toxic and reactive oxidation products. Reducing the formation of these oxidation products involves the use of electron donors, as well as treated water, and can help to decrease the environmental impact of oxidation reactions.