Gh. Duca et al./Chem.J. Mold.. 2008, 3 (1), 10-21One very important specific <strong>of</strong> natural waters is the presence <strong>of</strong> micro-quantities <strong>of</strong> transition metals in it. Thesecomponents can participate in reactions with intermediate free radicals, hydrogen peroxide, and molecular oxygen.Metals can form various complexes with natural waters components which possess ligand properties, catalytic propertiesin redox transformations or which participate in photochemical processes [10, 35, 36].The results obtained in model and natural conditions have demonstrated that in the natural aquatic environmentand a pH value between 5 and 9 (characteristic for surface waters), almost all metals with variable valence are foundpredominantly as colloids <strong>of</strong> oxide or hydroxide or as soluble complexes, with natural ligands [35, 36]. Given thatsurface waters are oxidative environments in normal conditions, transition metals ions are found mainly in oxidizedform (M 2+ ). Only a small quantity <strong>of</strong> metals is in reduced form (M + ), and it participates in the activation <strong>of</strong> O 2andH 2O 2. Among all transition metals in surface waters, especially important for redox transformations are copper, ironand manganese. Only copper ions exist in neutral environment in homogenic form, both in oxidized ([Cu 2+ = 310 -7mol/l]), and in reduced form ([Cu + = 10 -8 mol/l]). Iron ions are found in aerobic conditions mainly in oxidized form, asmicro-colloids <strong>of</strong> hydroxide (FeOOH), at concentrations <strong>of</strong> 10 -5 mol/l. The highest concentration <strong>of</strong> the solvated form<strong>of</strong> the compound FeOOH is <strong>of</strong> 210 -7 mol/l [24]. The soluble forms <strong>of</strong> iron (III) are represented especially by anionichydroxy-fulvic complexes [35, 36]. Manganese ions are found either as free ions Mn 2+ , or as micro-colloids <strong>of</strong> Mn (IV)hydroxide, which is formed in natural conditions at biological oxidation <strong>of</strong> Mn (II) [25].A characteristic <strong>of</strong> copper ions is their participation in reactions <strong>of</strong> one- or two-electrons transfer, leading tothe formation <strong>of</strong> free O H radicals and superoxide radical O2 . As to iron and manganese ions, they participate incatalytic reactions <strong>of</strong> H 2O 2decomposition with a two-electrons transfer [26].Besides oxidants and catalysts (transition metals), in the oxidation-reduction processes also participate variousreducing compounds, which can be formed directly in the aquatic environment (autochthonous compounds, metabolicproducts and those <strong>of</strong> biochemical destruction) or infiltrate in it with precipitations, floods or waste waters (alochthonoussubstances). Organic compounds are present in surface waters in relatively small concentrations, especially 0,1 mg/lor 10 -5 M [36].Modeling <strong>of</strong> redox transformations in the natural aquatic environmentFrom the chemical-kinetic point <strong>of</strong> view, natural waters form an open oxidation-reduction system; oxidizing andreducing compounds continuously infiltrate in it.According to the database accumulated by various researchers, the chemical composition <strong>of</strong> the abiotic component<strong>of</strong> natural waters can be represented as follows:S – A – O 2,aq– H 2O 2– {R x}– Me n+ /Me (n-1)+ – L – Sedwhere O 2,aq– H 2O 2– {R x}– dissolved molecular oxygen and its activation products; S – substrates <strong>of</strong> chemicaltransformations, including anthropogenic pollutants, A – active particles acceptors, which form secondary, less activeradicals, Me n+ /Me (n-1)+ – ions and complexes <strong>of</strong> transition metals in oxidized/reduced form; L – components with ligandproperties; Sed – suspensions with various dispersion rate, which form the heterogeneous phase.The oxidation-reduction processes exist in natural waters only due to the presence <strong>of</strong> oxidative equivalents(M n+ , O 2, H 2O 2, O H ), reducing agents (DH 2) and catalysts (M (n-1)+ /M n+ ). It should be mentioned that the redox model<strong>of</strong> natural waters doesn’t include analysis <strong>of</strong> redox reactions that take place due to other oxidants that can infiltrateinto waters as a result <strong>of</strong> anthropogenic pollution or chemical-biological transformations, but molecular oxygen andhydrogen peroxide. It is considered that these processes can be described by the laws established for catalytic oxygenperoxidesystems.In order to involve O 2in oxidation processes chemical energy is necessary to activate it. This can happen onthe account <strong>of</strong> sunlight absorption or interaction with compounds with significant reducing properties (metal ions inreducing forms, free radicals).Hydrogen peroxide, the product <strong>of</strong> bi-electronic reduction <strong>of</strong> O 2, is considered the carrier <strong>of</strong> oxidative equivalents<strong>of</strong> oxygen in natural waters. As opposed to O 2, H 2O 2can exhibit reducing and oxidizing properties in neutral environment,depending on its redox partner; thus it can interact with reduced and oxidized metal ions.Among the transition metals present in natural waters, the most important for redox transformations are copper,iron and manganese ions.Qualitative kinetic characteristics allow the prognostication <strong>of</strong> possible concentrations <strong>of</strong> compounds in waterswith various loads, the estimation <strong>of</strong> maximal quantity <strong>of</strong> substance which can be discharged in water without disturbingthe self-purification processes within the reservoir. The kinetic characteristic <strong>of</strong> processes is the effective rate constant,which is a function <strong>of</strong> the environment’s parameters.15
Gh. Duca et al./Chem.J. Mold.. 2008, 3 (1), 10-21Much data exist nowadays regarding the chemical composition <strong>of</strong> natural waters, but very little is known aboutthe kinetic laws that describe the interactions between these compounds [10, 39, 40]. A number <strong>of</strong> systems, such asM 2+ /M + – L – S – O 2/H 2O 2(where, M 2+ /M + variable valence metal in oxidized/reduced form, L – ligand, S – substrate)have been studied. There is a lot <strong>of</strong> data on thermodynamic properties <strong>of</strong> metal complexes, reactivity <strong>of</strong> intermediatefree radicals, mechanisms <strong>of</strong> activation <strong>of</strong> O 2and H 2O 2; however very little is known about catalytic oxidation <strong>of</strong> severalcompounds <strong>of</strong> significant ecological importance.Among all reducing agents which model the most successfully natural reducing compounds, most extensivelystudied are the ascorbic acid [20], thiolic compounds [27, 28], hydrochinone [27, 29], oxy and hydroxyacids [21, 22].The ascorbic acid is a compound that regulates the intracellular redox state, having a significant medical-biological role.Oxy- and hydroxyl acids form the metabolic cycle <strong>of</strong> dicarboxylic acids and take part in the exchange <strong>of</strong> substances<strong>of</strong> living organisms with the environment. Hydroquinone is one <strong>of</strong> the poliphenols very <strong>of</strong>ten encountered in surfacewaters, being the precursors <strong>of</strong> natural humus. Thiolic compounds participate in metabolic processes <strong>of</strong> microorganisms,are used in the leather industry, in fertilizers technology etc.On the basis <strong>of</strong> ascorbic acid the redox mechanism <strong>of</strong> processes within the aquatic environment has beenestablished, with the participation <strong>of</strong> ions and complexes <strong>of</strong> copper when the pollutant (P) possesses ligand propertiesor significant reducing properties [20]. During the interaction between the metal and P, a compound with partial chargetransfer is formed (CuDH + ):Cu 2+ (OH - ) + DH - CuDH + + OH -This complex plays the role <strong>of</strong> a one-electron donator, reducing the peroxide to O H radicals, whereas themetal ion doesn’t change its oxidation state:CuDH + + H 2O 2 Cu 2+ + D + O H + H 2OAnd this lead to the initiation <strong>of</strong> processes <strong>of</strong> conjugated oxidation <strong>of</strong> pollutants in natural water.In the case <strong>of</strong> small copper ions concentrations, the complex can decompose in products with one-electrontransfer:CuDH + Cu + + DH· ( D + H + )And in the case <strong>of</strong> high metal ions concentrations, it can interact with the second copper ion, thus oxidizing thedonor by the two-electron pathway:CuDH + + CuOH + 2Cu + + D + H +Given the copper ions concentration in natural waters, which is around 10 -7 M, the one-electron mechanismprevails in these conditions.The discussed system is characterized by a specificity regarding the formation <strong>of</strong> OH radicals. As opposed to thePhenton system, the generation <strong>of</strong> O H radicals doesn’t interfere with the valence <strong>of</strong> the metal, this one remaining inits oxidized form. Copper (I) ions appear in this system as secondary particles, formed as a result <strong>of</strong> H 2O 2destruction:O H + H 2O 2H O 2H O 2 + Cu 2+ Cu + + O 2+ H +The impact <strong>of</strong> compounds with don’t exhibit ligand properties in the natural aquatic environment (such ashydroquinone, glyoxalic acid) [28, 44] can be positive or negative, depending on the substrate’s concentration. Duringthe interaction <strong>of</strong> copper (I) ions with H 2O 2, the particle CuO + results (which can be treated as the hydrolyzed Cu 3+ ion),being the precursor <strong>of</strong> the O H radical [34]:Cu + + H 2O 2 CuO + + H 2OIn the absence <strong>of</strong> reactive substrates, this particle oxidizes water, forming OH radicals:CuO + + H 2O (CuOH) + + O HGiven the fact that it is a two-electrons acceptor towards substrates, in the presence <strong>of</strong> H donors it oxidizes the donoraccording to the two-electrons pathway:CuO + + QH 2 Cu + + Q + H 2OThus, depending on the donors concentration in the environment, processes <strong>of</strong> inhibition or generation <strong>of</strong> OH radicalscan take place.Considering all stated above, the presence <strong>of</strong> reducing agents with no ligand properties has two roles: at smallconcentrations, substrates will nor essentially interfere the process <strong>of</strong> OH radicals generation, as a result <strong>of</strong> wateroxidation by CuO + (i.e. CuO + will oxidize intensely the waters molecules and will be less consumed for the QH 2oxidation), at high concentrations, these substances will act as inhibitors, trapping the CuO + particle, which is theprecursor <strong>of</strong> OH radicals. This is <strong>of</strong> significant importance for the realization <strong>of</strong> chemical self-purification processes <strong>of</strong>natural waters, due to the possibility <strong>of</strong> conjugated oxidation.16
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