This chapter presents a synthesis of the typical mechanisms and patterns of catalytic redox processes that take place in the natural aquatic environment with the participation of oxygen, its reactive species, hydrogen peroxide, OH radicals, and various forms of copper and iron transition metals. There are presented the redox subsystems (slow or fast) which form the redox character of the environment; the mechanisms and conditions for activating oxygen for its involvement in redox processes; biotic and abiotic sources and pathways of the hydrogen peroxide degradation, as well as its role in the formation, maintenance, and regulation of the environment redox state; and formation and peculiarities of hydroxyl radicals' participation in the processes inside the water basins.
TopIntroduction: Redox Components In The Natural Waters
From a chemical point of view, natural water is defined as an open catalytic and photochemical oxidation-reduction system (Sychev et al., 1983; Sychev et al., 2002; Duca et al., 2003). Oxygen, hydrogen peroxide, ions and complexes of transition metals, reducing substances of autochthonous and allochthonous nature, and solar radiation lead to the development of catalytic redox processes in natural waters. The concentration of dissolved oxygen in natural waters varies within the range of values between 0 and 14 mg/l and is determined by two processes: photosynthesis and biotic and abiotic oxygen consumption (Alekhin, 1970; Nicanorov, 1989).
Following the fluorine, oxygen is the next strong oxidant which, being reduced to water, removes about 491 kJ/mol (2.73 V) of free energy. Although, from a thermodynamic point of view, oxygen is a strong oxidant, from a kinetic point of view, it is quite inert. The inertness of oxygen is explained by the particularities of the electronic structure: under normal conditions, O2 has a triplet structure. Since, under the same conditions, the organic substances are in the singlet state, the interaction with oxygen cannot take place due to the spin prohibition (Sychev et al. 1983; Sychev et al., 2002; Duca et al., 2003).
Involvement of dissolved oxygen in pollutants oxidation processes requires its activation, which can be achieved through interaction with substances with reducing properties (metal ions in reduced form, free radicals) or as a result of sunlight absorption. The reactivity of oxygen in natural waters is actually determined not so much by its molecular form, as by the intermediate products of its biogeochemical cycle (1O2, 
H2O2) (Fig. 1).
Figure 1. Biogheochemic cycle of Oxigen
Based on multiple field measurements (Sinelnicov et al., 1974; Zepp et al., 1977; Petterson et al., 1973; Mill et al., 1980; Li et al., 2019; Gladchi, 2018; Wang et al., 2020; Duca et al., 2020; Lis et al., 2021; Garcia et al., 2020; Yon et al., 2021; Ossola et al., 2021; Duca et al., 2002; Bunduchi et al., 2006; Takeda et al., 2004; Vione et al., 2006), the concentrations of these particles were evaluated in the following ranges: