Sorption of Cu2+ and Zn2+ Ions by Humic Acids of Tundra Peat Gley Soils (Histic Reductaquic Cryosols)

Sorption properties of humic acid (HA) preparations extracted from the peaty horizon of peat gley tundra soil (Histic Reductaquic Cryosols) are characterized. The kinetic and thermodynamic sorption parameters of Cu 2+ and Zn 2+ ions by HAs are calculated. The sorption of the study heavy metal ions by HA preparations is estimated to be truly approximated by the pseudo-first-order response equation. The negative values of the Gibbs energy change at temperatures of 298.0 К and 314.3 К for Сu 2+ (–26.2 and –24.5 kJ/mol, respectively) and Zn 2+ ions (–22.2 and –22.0 kJ/mol, respectively) attest to the possibility of self-sorption of the study ions within the considered temperature range. Changes in the sorption enthalpy values (–33.1 kJ/mol for Cu 2+ ions and –4.5 kJ/mol for Zn 2+ ions) characterize both reactions as exothermal reactions. As the temperature rises, the sorption equilibrium constant for Сu 2+ ions considerably decreases, whereas it does not change significantly for Zn 2+ ions. The decreasing sorption equilibrium constants with the rise in temperature point to the equilibrium shift towards desorption. This is a characteristic feature of physical sorption that indicates possible existence of two fixation mechanisms of ions on the surface of HA preparations, namely, physical sorption and сhemisorption.