Adsorption kinetic and cadmium fractions in two calcareous soils affected by zinc and different moisture regimes

This study has been conducted to ascertain the impact of zinc (Zn) on cadmium (Cd) fractions in two types of calcareous soils of the Fars Province in Iran at the waterlogged and field capacity conditions; also we investigated the adsorption of Cd in soils throughout different shaking times. We examined the effect of Zn on Cd fractions due to their similar characteristics. Treatments were three levels of Cd (0, 30 and 60 mg kg −1 of soil as CdSO 4 ·8H 2 O), three levels of Zn (0, 5 and 10 mg kg −1 of soil as ZnSO 4 ·7 H 2 O) three-level incubation times (2, 4 and 8 weeks), two types of soils (clay and sandy clay loam) and two moisture regimes (waterlogged and field capacity). The randomized completed block design (RCBD) was used for this experiment. After 2, 4 and 8 weeks of treatments, the sequential extraction technique was used to determine the Cd concentration in (WsEx), (Fe–MnOx), (Car), (OM) and (Res) fractions. In the waterlogged condition, adding Zn as zinc sulphate reduced the concentration of Cd in Fe–MnOx, Car and OM fractions but had no significant effect on the WsEx and Res fractions. Changes in the chemical fractions of Cd under the influence of zinc sulphate in both soils followed a similar trend. In the field capacity condition, adding Zn as zinc sulphate reduced the concentration of Cd in WsEx, Car and OM fractions and increased the concentration of Cd in the Fe–MnOx and Res fractions in the clay soil. In the sandy clay loam soil, adding Zn decreased the Cd concentration in Car and OM fractions and increased the Fe–MnOx fraction while has no significant effect on WsEx and Res fractions. It can be concluded that the changing in the Cd concentration in its chemical fractions caused by Zn addition might be from the competitive transport and adsorption interactions between these two ions. The presence of Zn reduces the concentration of Cd in those fractions that are easily released into the soil solution from where they can be absorbed by plants. The results showed that the best-fitted model which can describe the Cd adsorption was the power function model. It seems that clay and organic mineral are the dominant parts which control the Cd adsorption in soils. The rate of Cd adsorption in almost all shaking times was high which shows that Cd has more ability to occupy the adsorption sites.