Sorption-Desorption Behavior of Zn in Common Soil Diagnostic Horizons from Southwestern Iran.

A full understanding of the nature of sorption/desorption reactions of Zn in soils is crucial for the interpretation and prediction of its chemical behavior in soils. Therefore, to identify the kinetics of zinc (Zn) adsorption in selected common diagnostic horizons of soils in southwestern Iran, 30 ml of solutions containing 10 mg Zn/L was added to 3 g of each of mollic, calcic, and salic diagnostic horizons (in triplicate) and at specified times (5 to 2880 minutes) the residual concentration of Zn in the supernatant was measured by an atomic absorption spectrometer. The results indicated that the Zn adsorption process in the soils studied was a three-stage process. Accordingly, by end of the first step (120 minutes from the beginning of the experiment) 65.7, 57.8, and 26.7% of the total amount of Zn was adsorbed in mollic, calcic and salic horizons, respectively. The Zn adsorption capacity of mollic, calcic, and salic diagnostic horizons were 58.20, 63.57, and 79.50 mg g−1, respectively. Excellent agreement was found between the pseudo-second-order kinetic model and the experimental data (R2 ≈ 0.994–0.999). The Zn sorption/desorption isotherms were also derived with 0.1 M Ca (NO3)2 solutions containing varying concentrations of Zn (0 to 10 mg L−1) for 24 h. Accordingly, comparing with the Langmuir model (0.888 < R2 sorption < 0.956; 0.896 < R2 desorption < 0.984), Freundlich isotherm model (0.987 < R2 sorption < 0.999; 0.963 < R2 desorption < 0.994) could better explain Zn sorption/desorption data in the soils studied. The Zn adsorption data on all three soils followed an L-shaped isotherm. Besides, Zn adsorption isotherms showed greater values than desorption isotherms, suggesting that the Zn adsorption process is irreversible. Results obtained from hysteresis calculations indicated that the desorption of freshly sorbed Zn was less hysteretic in calcic and mollic horizons relative to salic horizon. This suggests low mobility and leaching potential of freshly sorbed Zn can be expected from these soils. [ABSTRACT FROM AUTHOR]