Humic Substances as an Important Factor of Metal Migration in Surface Waters of Ukraine

The results of long-term studies of the role of humic substances in the binding of a number of metals (Al, Fe, Cu, Mn, Cr, Zn, Pb, Mo, Cd, V, and Ti) in anionic complexes in various types of water bodies of Ukraine are summarized and discussed in the paper. It has been shown that the share of complex compounds of these metals increases significantly with increasing the concentration of humic substances. The water bodies and watercourses with a rather high content of humic substances include primarily the rivers of the Prypyat River basin and the reservoirs of the Dnieper cascade. The relative content of each metal in these water bodies (in % of the total concentration of M<sub>dis</sub>) on the average was as follows: Al - 63.3 - 83.0, Fe - 58.6 - 76.4, Cu - 63.0 - 78.2, Mn - 52.3 - 60.0, Cr - 44.6 - 76.5, Zn - 39.8 - 58.5, Pb - 55.8 - 73.4, Mo - 63.4 - 80.3, Cd - 57.6 - 70.0, V - ≈ 60.0, and Ti - 33.2 - 62.5. The share of anionic complexes decreases with a decrease in the concentration of humic substances in the water. Its lowest values are characteristic of water bodies in the urbanized areas subjected to anthropogenic load and characterized by a relatively low content of humic substances. It is emphasized that for some metals, the results of modeling and field studies of coexisting forms differ significantly. This depends largely on the values of the conditional stability constants of complexes with humic substances used in the calculations. The peculiarities of complexation of oxoanions (Mo<sup>2-</sup><sub>4</sub> and HVO<sup>2-</sup><sub>4</sub>) with humic substances are considered separately.