Batch equilibrium study on sorption, desorption, and immobilisation of cadmium in some semi-arid zone soils as affected by soil properties

Abstract

Little information is available for cadmium (Cd) sorption/desorption behaviour in soils with relatively variable CaCO3 content. The objectives of this study were to: (i) parameterise the sorption and desorption of Cd and the hysteretic behaviour of Cd sorbed to soils with varying CaCO3 content; and (ii) correlate sorption, desorption, and retention parameters with physicochemical characteristics of the soils. Twenty soil samples of different physico-chemical properties were taken from agricultural regions of Western Azerbijan province, Iran. A batch equilibrium experiment was conducted to construct sorption/desorption curves of Cd. The linear, Langmuir, and Freundlich isotherm equations were fitted to the experimental data of Cd sorption and retention, using either linear regression procedure or nonlinear least square optimisation (LSO). Both the Freundlich and Langmuir approaches described the Cd sorption and retention data well. A strong and irreversible binding of Cd in the soils was recorded, using a desorption approach. In the case of Cd sorption, a significant positive correlation (r = 0.38, P ≤ 0.05) was found between the Freundlich constant (Kf) and active CaCO3 equivalent (ACCE). The Freundlich n was positively correlated with cation exchange capacity (r = 0.49, P ≤ 0.05) and clay (r = 0.61, P ≤ 0.01) and negatively with ACCE (r = –0.60, P ≤ 0.01). The soil partition coefficient (KSD) showed a positive correlation with ACCE. The sorption maxima (b) were much less than the cation exchange capacity of soils. However, the relatively high pH (7.0–8.0) of the experimental soils and presence of relatively high values of free and active carbonate in the soils, along with the large sorption capacity of the soils, suggest the possibility of solid-phase precipitation as octavite (CdCO3). Parameter b, when fitted through LSO, showed a negative correlation with clay (r = –0.51, P ≤ 0.05) and a positive correlation with ACCE (r = 0.63, P ≤ 0.01). Langmuir K (Kl) showed a positive correlation with clay (r = 0.52, P ≤ 0.05) and a negative relationship with pH (r = –0.58, P ≤ 0.05) and ACCE (r = –0.65, P ≤ 0.01). Sorption of Cd showed a positive correlation (r ≥ 0.54, P ≤ 0.05) and its desorption a high negative correlation (r ≤ –0.61, P ≤ 0.05), with ACCE. It can be concluded that the ACCE is the important soil property controlling the sorption and retention of Cd in the studied soils.