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Abstract:

The aim of the experiment was to determine how earlier Zn pollution or the acidification of the soil influenced Zn adsorption and desorption of adsorbed Zn. Previous Zn pollution levels were 0, 500 and 2500 mg kg^-1 Zn, while the shaking solutions used for adsorption analysis had concentrations of 0, 25, 50, 100, 250 and 500 mg dm^-3. Desorption analysis was performed using the hot water percolation (HWP) method, resulting in ten 100 cm³ fractions. The pH value was measured both for adsorption and desorption. Langmuir isotherms were fitted to the adsorption data and the maximum adsorbable Zn quantity and the value of the equilibrium constant were determined. As a modified Langmuir isotherm was applied, the quantity of Zn bound on the surface prior to adsorption analysis was also determined. Sorption properties of soil were changed by both reduction of soil pH and partial saturation of Zn binding sites. The change in pH (pH6-pH4) had the greatest influence on the soil buffer capacity and on the equilibrium constant of this process, while previous Zn pollution also reduced the adsorption capacity. Zn loads in excess of the sorption capacity of the soil resulted in a significant decrease in soil pH. A considerable part of the adsorbed zinc is bound to the soil in readily mobilisable form. More the pH of the soil differs from the original value the more pronounced this phenomenon becomes.