Exchange Between Organic Substances And Compounds Of Metals In Bottom Sediments – Water System In Conditions Of Model Experiment
EXCHANGE BETWEEN ORGANIC SUBSTANCES AND COMPOUNDS OF METALS IN "BOTTOM SEDIMENTS – WATER" SYSTEM IN CONDITIONS OF MODEL EXPERIMENT
T.A. Vasilchuk, P.N. Linnik
Department of Hydrochemistry, Institute of Hydrobiology,Ukrainian National Academy of Sciences, Geroev Stalingrada avenue 12, 04210 Kyiv, Ukraine,
e-mail:mvasilchuk@mail.ru
Accumulation of heavy metals and organic substances in bottom sediments and their destruction are the important mechanisms that provides the content of these compounds affecting the productivity of the water ecosystems and water quality to regulate. On the one hand, the storage of the toxic substances in sediments leads to the self-purification of water body and, on the other hand, sediments are a potentional source of the secondary water pollution The processes of exchange between organic compounds and metals in a "bottom sediments – water" system were investigated under the conditions of a model experiment. The pore solutions are a primary source of substances exchange between bottom sediments and adjacent water. Manganese and iron were chosen to study these processes, because the content of both metals in the pore solutions considerably surpasses that in the water medium, and hence concentration gradient is reasonably high. The exchange depends not only on the total content of metals, but also on metal speciation. The forms of the studied metals differ themselves. The main part of manganese is nothing else than the free (hydrated) ions, iron represents itself the more complex substances containing the organic compounds (Table 1). The concentration gradients of compounds and their diffusivities depend on the molecular weight of diffusing substances so affecting the rate of molecular diffusion. The diffusion coefficient decreases with increasing the molecular weight of a compound. These phenomena are mainly due to a decrease of pH and redox-potential in the surface layer of the bottom sediments and adjacent water.
Ion-exchange chromatography (DEAE, CM-cellulose) was used to find component composition of dissolved organic matter (DOM) and metal
complex compounds. The molecular weight distribution of these substances was established by use of gel-chromatography (Sephadex G-25, G-50, G-75). The pore solution has high concentrations of organic substances in comparison with the control water (Corg=22,82 mg/l). Fulvic acids (FA) are the main part of DOM (about 70 %). Data on the molecular weight distribution of these substances showed that the low molecular substances prevail.
Table1.Content and forms of existence iron and manganese in the pore solution
Metal
Total content,
mg/l
Complexes with DOM
Free (hydrated) ions
mg/l
% of total
mg/l
% of total
Iron
2,69
2,60
96,6
0,09
3,4
Manganese
2,20
0,20
9,0
2,0
91,0
A study of the chemical nature of metals being in a dissolved state showed that iron in the pore solution is bounded with humus-like substances. The creation of anaerobic condition leads to an increase of FA concentration (from 19,42 to 35,51 mg/l) in adjacent water and its enrichment by the compounds with the low molecular weight (
Because iron and manganese have different complexing capability, an increase in content of these metals occurs in a different way. Under the aerobic and anaerobic conditions the maximum manganese concentration in the bulk water increases about 30 and 150 times respectively. Exchange between the pore solution and adjacent water is due to the diffusion of Mn2+ -ions. A sharp increase in manganese concentration occurs only at the simultaneous decrease of pH to 5-6. At the same time an increase in the dissolved iron concentration is not essential (2,5-3,5 times). As mentioned above, iron in the pore solution is nothing else than its complexes with organic substances. Migration of these compounds is much lower than that of the free ions. The molecular weight distribution of complex compounds of metals in the water phase is alike that of the pore solution. But it should be noted, under the anaerobic conditions a part of the low molecular weight complexes of iron and manganese increases.
Thus, the creation of anaerobic zones and the decrease of pH promote the more intense flow of studied compounds from the sediments into water.
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