Cadmium adsorption isotherms onto clay separates of a yermic cambisol soil from central Jordan valley

Abstract

Soils of central and southern Jordan Valley (JV) are partially irrigated with waste waters relatively enriched with heavy metals, such as Pb and Cd. A composite surface soil sample (0–25 cm) was collected from a central JV field of Deir‐Alla. The clay fraction (reference clay: <2 μm primarily of carbonate, mica, and kaolinite minerals) was fractionated from a subsample and employed in studying Cd adsorption isotherms (10, 20, or 30° C) from 0.05 M NaCl (Cd 2+ and CdCl+ chemical species) or NaClO4(Cd 2+ species only) background solutions. Twelve Cd initial concentrations ranging between 0.4 and 10 mg L ‐1 were employed in the adsorption study. Cadmium adsorption onto OM‐free or CO3‐free clay suspensions in 0.05 M NaClO4 background solutions was also studied. Equilibrium Cd concentrations were determined using Differential Pulse Anodic Stripping Voltammetry. For the reference clay suspensions in NaClO4 or NaCl solutions. Cd2+ adsorption isotherms conformed to linear Langmuir plot. Maximum adsorption capacity (B) from the ClO4 background solutions increased slightly with increasing temperature from 1020 (at 10° C) to 1099 μg g‐1 clay (at 30° C). This was accompanied by a respective increase in the Langmuir's affinity constant (K) from 16.0 to 30.3. Changing the background solution to NaCl resulted in a marked increase in B values and decrease in K values with increasing temperature. This was attributed to concurrent adsorption of CdCl+ and Cd2+ ionic species from the Cl solutions. Adsorption of CdCl+ species decreased from 367 to 254 μg g‐1 with increasing temperature from 20 to 30° C due to decreasing stability of that complex ion. Removal of organic matter slightly increased the B values of Cd2+ due to the exposure of internal surfaces as a result of deflocculation of clay particles. To the contrary, acid removal of CaCO3 caused a substantial reduction in the adsorption capacity of Cd2+ due to flocculation of clay particles by Ca2+.