The work describes the processes occurring in the natural waters from a kinetic perspective, highlighting the role of various components of the oxygen cycle and putting into evidence two water quality parameters that evaluate the contents in two intermediate products of oxygen reduction up to water, namely hydrogen peroxide and OH radicals. The flows of oxidative equivalents, hydrogen peroxide, and reducers that interact with it are evaluated by the redox state indicator. The pollution by OH “traps” is evaluated by measuring the inhibition capacity (Σki,OH[Si,OH]) parameter. In this work, the authors present the pollution assessment and water self-purification process supported by hydrogen peroxide and OH radicals during the years 2015-2019. The monitored objects were Nistru River in the Dubasari-Vadul-lui-Voda dam segment; its tributaries, Raut and Ichel, at the mouths of confluence with the river; and the Ghidighici and Danceni lakes, the surface water bodies in the Republic of Moldova.
TopBackground
Due to the sunlight, abiotic redox processes, and metabolic activity of hydrobionts in natural waters, the formation of oxygen and its intermediate reduced forms occur. These derivatives form the flow of main oxidative equivalents in aquatic systems. During photosynthesis, the simultaneous yielding or reception of four electrons takes place only in certain specific biological systems, more common being the processes of transfer of one or two electrons. As a result, intermediate products of oxygen reduction are generated, namely singlet oxygen (1O2), superoxide radicals (HO2•/O2•-), hydroxyl radicals (•OH), hydrogen peroxide (H2O2) (Sychev et al., 1983).
In natural waters, the action of sunlight causes the effective generation of singlet oxygen (1O2) (Hoigne et al., 1988; Sandvik et al., 2000; Marchisio et al., 2015). The stationary concentration of these particles constitutes 10-14-10-12 M. This value becomes even higher, the higher is the content of organic substances dissolved in water.
The first stage of oxygen activation is concluded by the formation of superoxide radicals (HO2•), which in natural waters can also be found in the dissociated form of superoxide radical anion (O2•-) (Petasne and Zika, 1988; Ernestova and Skurlatov, 1995; Fujii and Otani, 2017). Out of these two forms, the proton form has higher oxidizing properties. Following the various processes in natural waters, the stationary concentration of this particle varies between 10-8-10-9 mol/L. According to the redox and acid-base characteristics, the most stable intermediate product of O2 reduction is hydrogen peroxide (Sychev et al., 1983). In natural aquatic systems, the oxidant is formed predominantly in photochemical processes (Cooper, 1988; Shtamm, 1991; Ernestova and Skurlatov, 1995; Mostofa et al., 2013). A certain amount of H2O2 is formed as a result of the redox-catalytic processes with the participation of organic substances and oxygen (Duca, 1997, 2012; Duca et al., 1997, 2002), and another part is provided by biological processes (Batovskay et al., 1988; Shtamm, 1988). Like for the other biogenic substances, the concentration of H2O2 in natural waters is influenced by the ratio between the rate of its formation and decomposition. The multiple measurements on different aquatic bodies have shown that the stationary concentration of H2O2 in natural waters is of the order of 10-6-10-5 mol/L (Sinelnikov, 1971; Shtamm et al., 1991; Duca and Gladchi, 2000). Also, H2O2 is a form of preservation of hydroxyl radicals, as a dimer (OH)2. About 30% of hydrogen peroxide breaks down with the formation of OH radicals (Shtamm et al., 1991).