Zinc Sorption in Calcareous Soils of the Kurdistan Region of Iraq

Abstract

Zinc (Zn) sorption was studied in four calcareous soils of the Kurdistan Region of Iraq. The experiment was conducted by equilibrating 5 g of soil in 50 mL of a 0.01 M CaCl₂ solution containing different levels of Zn. Suspensions were centrifuged and filtered, and the concentrations of Zn in the filtered solutions were determined. The sorption of Zn was evaluated using four adsorption isotherms (Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich); the data were best fit by a linearized Langmuir model as evidenced by the higher coefficient of determination (R²) values and lowest standard error (SE) values. For the Langmuir and Freundlich isotherms, the separation factor (R_L) was 0 < R_L < 1, and the adsorption intensity (n) was 1 < n < 10, respectively, indicating a favorable (spontaneous) Zn sorption process in the soil. The Dubinin-Radushkevich isotherm indicated a physisorption process of Zn in the soil (E < 8 kJ mol⁻¹). Thermodynamic parameters ΔG^o, ΔH^o, and ΔS^o were also determined at three different temperatures. The ΔG^o values were negative and ranged from −7.45 to −7.80, −9.56 to −10.28, and −11.36 to −12.85 kJ mol⁻¹, at 278, 298, and 318 K, respectively, where negative values of ΔG^o indicate the Zn sorption was endothermic. The values of ΔH^o were positive and ranged from 17.23 to 29.25 kJ mol⁻¹, and the values of ΔS^o were positive and ranged from 89.87 to 132.15 J mol⁻¹ K⁻¹. The positive values of ΔH^o and ΔS^o indicate that the sorption of Zn in the soil is an endothermic process. The results indicate that the studied soils had a high capacity of Zn sorption, which leads to decline in the availability of Zn.