Study On Photochemistry Of Manganese In Seawater

Manganese is one of essential micronutrients in seawater. Manganese is a biologically essential element with function roles in photosynthesis (acting as a redox catalyst in photosystem), in the detoxification of oxygen products produced by aerobic respiration and in the production of ATP. The element can only be taken up by biota in the soluble form. The thermodynamically stable form at a pH of 8.0 and a pE of 12.5 is Mn(IV) which is extremely insoluble in seawater. It was proved that in the surface layer of seawater, MnOx can be reduced via the dissolved organic substances in seawater through photochemical reaction in spite of being reduced via microorganism directly or indirectly. It is important for keeping manganese existing in soluble form in photic seawater and increasing the bioavailable manganese. Therefore, the study on the photochemical reduction of manganese is theoretically and practically significant.The photochemical reduction process of manganese on different conditions was studied in this paper. The concrete results are showed as follows:1. A new analysis method of the determination of Trace Mn(II) in waters was discussed and the influencing factors were studied. Chelating with 8-hydroxyquinoline, using octadecyl functional group bonded silica gel as sorbent and methanol as eluent, trace manganese in fresh water was determined by flame atomic absorption spectrometry. The result was fine and was hardly influenced by dissolved organic substances in waters. A significant linear relationship appeared as the concentration of manganese in water was between 0.50 10-6 mol/L and 5.00 10-6 mol/L with detection limit (3) of 0,03 10-6 mol/L.2. The influencing factors of the photoreduction of manganese were discussed and the result indicated that the photochemical reduction of manganese occurs through dissolved organic substances. Light irradiation, the species and concentration of the dissolved organic substances, aquatic mediate and pH could influence the reactive rate.a. For the three amino acids(L-Aspartic acid, Glycine, Alanine) in experiment, the reactive rate changed with the concentration of amino acids and for therest five hydroxycarboxylic acids(Tartaric acid, Malic acid, Malonic acid, Adipic acid, Succinic acid), the rule was not assured;b. When the concentration of the eight organic substances is low (1.00 10mol/L, 2.00 10-5mol/L), the reactive rate decreased by the below sequence: Tartaric acid> Malic acid > L-Aspartic acid > Malonic acid> Adipic acid Succinic acid Glycine Alanine. As the concentration increased to 5.00 10-5mol/L, the sequence was: L-Aspartic acid>Tartaric acid > Malonic acid > Alanine Glycine > Malic acid > Adipic acid > Succinic acid;c. Except Malic acid, the reactive rate of the other seven organic substances increased with the enhancing of light irradiation. The decrease of pH could also increase the reactive rate. As a result, the enhancing of light irradiation and the decrease of pH were helpful for the photoreaction of manganese;d. The reactive rate declined following the below sequence: deioned water > artificial sea water > natural sea water;e. As long as the system was even, the reactive rate was not affected by the rate of stir.3. The kinetic of the photoreduction of manganese was basely studied, and the result indicated that the reaction occurs by a zero-order reaction kinetic pattern. The rate constant changed according to the reactive rate.4. The process of photochemical reduction of manganese might happen via the following schemes when the concentration of organic substances is high: adsorption of the organic substances by MnOx, the photochemical reaction of the adsorbed organic substances, charge transfer reaction between MnOx and adsorbed organic substances, the reduction of MnOx; When the concentration of organic substances is low, there are another schemes besides the above schemes, they are: adsorption of a photon by the organic substances, (V produced from photo...