chemistry journal of moldova

Gh. Duca et al./Chem.J. Mold.. 2008, 3 (1), 10-21came up after comparing the data obtained by several researchers. Thus, the values for the H initiation reactionobtained at the irradiation of arctic waters which contain 10 -12 M·s -1 ODS, are comparable with the values obtained byMooper and Zhou in Gulf Stream waters - around 14,7·10 -12 M·s -1 , and ocean waters - 15,9·10 -12 M·s -1 [12, 15]. Again,Mooper and Zhou, have measured the photochemical production in natural water submitted to irradiation and observedfrom 2,8·10 -12 M·s -1 – in open ocean water, up to 420·10 -12 M·s -1 for surface waters. Values of O H initiation rates of10 -11 M·s -1 have resulted in investigations of the river Volga (Rusia), rivers Prut and Nistru (Moldova) [14, 23].Measurements made by Vaughan and Blough (1998) regarding the generation of O H by fulvoacids (at 320nm), resulted in values of 2·10 -11 M·s -1 (or 5 ·10 -12 M·s -1 ppm -1 C) [13]. After the extrapolation of their results for 320 nm,Grannas A. M. et al. (2006), obtained the value of 2·10 -12 M·s -1 ppm -1 C [15].Besides the photochemical reactions, O H radicals can be formed in processes with the participation oftransition metals ions and reducing compounds with complexes’ properties [20-22]:CuD + H 2O 2 Cu 2+ (OH - ) +O H +DThe catalytic destruction of H 2O 2in the presence of copper (I) ions is accompanied by the formation of OHradicals:Cu++ H 2O 2 (CuO) + H 2O Cu 2+ (OH - ) + O HIn the case of water solubilization of ozone (O 3aq, with a concentration in surface waters of around 10 -8 M) andits subsequent interaction with the dissociated form of H 2O 2, its destruction takes place according to a radical chainmechanism [17, 19]:O 3aq+ HO 2- O H + O 2Also, OH-radicals are formed during the interaction of reducing substances with ozone:O 3+ DH - O H + D + O 2The formation of free OH radicals inevitably leads to the generation of secondary radicals. Mill et al. in a rangeof photochemical studies on natural water samples, identified, besides the OH radicals, concentrations of 310 -9 M ofalkyl peroxide radicals [18]:RH + O H R + H 2OR + O 2 O2The constants that describe the interaction of R O radicals with many organic substances are, commonly,2smaller than 1 M -1 s -1 . Due to the oxidation with peroxide radicals, the lifetime of chemical reactive species, such asphenols, aromatic amines, hydroxylamines, poliphenols, constitutes several days.According to the data obtained by Zepp et al. under the influence of sunlight the effective generation of singletoxygen takes place in natural waters, with a stationary concentration of 10 -14 -10 -12 M [16]. This value is proportional tothe content of dissolved humic compounds in water.S h1 S 3 S 3S + O 21 O 2+ SSinglet oxygen ( 1 O 2) is an effective oxidant for compounds which contain double bonds with high electronicdensity or readily oxidized functional groups (alkyl sulfide, thiol, phenolate ions etc.). These compounds have lifetimesof hours.Besides the singlet oxygen, humic compounds photolysis results in free electrons. In the natural aquaticenvironment, solvated electrons ( aq) interact with dissolved oxygen, forming the superoxide anion radical ( O2),which is the precursor of an effective oxidant – hydrogen peroxide (H 2O 2): + O 2O2; 2O 2+ 2H + H 2O 2+ O 2The general photochemical scheme of DOS transformation under the influence of sunlight can be drawn as:S hS 3 S O2S + 1 O 2 O2 ; 2HO 2 H 2O 2+ O 2R S+ +S + O+ aq2R O 2 , R O , ROOH O 2O 14