Redox processes involving hydrogen peroxide, as oxidizing agent, and compounds of sulfur as carrier of reducing equivalents form the quality of natural waters. The inflow of reductants interacting with H2O2 can lead to a toxic quasi-reductive state. Dynamic change of redox in a natural aqueous medium is pernicious for organisms with intensive water-exchange, such as larvae of fishes, despite the concentration of dissolved oxygen being normal. Favorable conditions for “flowering” of emerging toxins blue-green alga are formed. In reductive state, copper ions become biologically unavailable. Sewage after biological cleaning are the main anthropogenic source of the reductants, mainly hydrosulphide. The natural sources of reductants are blue-green alga and the bottom sediments. The ions of Cu(I) and Fe(II) form high-strength 1:1 complexes with reduced sulfur compounds that are stable to O2 but efficiently react with H2O2. The increased content of manganese can form mixed-valence manganese species Mn(III,IV) giving super-oxidizing state of the aquatic environment, which is also toxic.
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Most of natural redox processes in the environment use dioxygen O2, which transforms to water via intermediate formation of hydrogen peroxide H2O2. In the aerobic water environment and in living cell as the natural carrier of oxidizing equivalents the main role is played by the oxygen dissolved in water which content “normal” makes »2.10-4 M, varying in rather narrow limits depending on temperature and the content in water of the easily oxidized substances - carriers of reducing equivalents, DH2.
Typical reactions with participation of the molecular oxygen dissolved in water in the ground state (triplet) and in electron-excited (singlet) states are shown in Schemes 1, 2.
In the aqueous environment molecular oxygen is chemically inert, effectively it interacts mainly with organic free radicals •R with formation of peroxide-radicals ROO •, and also with the electronically excited molecules S* with formation or the superoxide radical •O2- – at transfer of an electron, or singlet oxygen - when electronic excitation is transferred (Duca Gh, Skurlatov Iu., Miziti A.,1994):
M+
О2 + {2Н} (DH2) ⇄ H2O2 (+ D)
M+
О2 + 2{2Н} ⇄ 2 H2O
M+
О2 + RH ⇄ OH + H2O
О2 + •R ⇄ ROO•
О2 + e- (S*) ⇄ •O2- (+ •S+)
О2 + hν (3S*) ⇄ 1O2 (+ S)
Scheme 1
Oxidation of H-donors by molecular oxygen is possible only at its activation in the coordination sphere of an ion of metals of a variable valence which is in the reduced form (M+) (Sychev et al., 1983).
As a rule, in the oxidase processes oxidation of H-donors is followed by the reducing of O2 to H2O2. In the processes of cellular respiration there is mainly four- electronic reduction of O2 to the water molecule, and in the oxygenase processes - introduction of the O atom of the O2 molecule into the C-H bond
Singlet oxygen in water either relaxes quickly to the ground state, or interacts either with donors of H with formation of H2O2, or with donors of an electron with formation of the superoxide radicals •O2-:
1O2 + H2O ⇄ O2 + H2O
1O2 + {2Н} (DH2) ⇄ H2O2 (+ D)
1O2 + e- (•D-) ⇄ •O2- (+D)