Competitive Sorption of Nickel and Cadmium in Soils

Competing ions strongly affect heavy metal retention and release in soils. In this study, we evaluated the sorption of Ni and Cd in single and binary Ni-Cd systems in three different soils; Windsor sand, Olivier loam and Webster loam. Nickel and Cadmium sorption isotherms were obtained after 24 hours of reaction and exhibited L-type patterns. Cd adsorption on the two acidic soils (Windsor and Olivier) was larger than Ni. In contrast, for Webster soil with a neutral pH, Ni sorption was larger than Cd. The Freundlich model was utilized to describe Ni and Cd adsorption isotherms. The estimates for K-f indicated that Ni sorption by the soils was significantly inhibited by the presence of Cd. Similarly, the sorption of Cd was suppressed by the presence of Ni ions. It was also found that when the added metal concentration increased in Ni-Cd system, the suppressed effect of the metal ion competition increased. Moreover, kinetic batch experiments were carried out to study Ni retention behavior. Ni sorption increased with increasing reaction time for all soils and the rate of sorption varied among soils. Desorption of Ni was hysteretic in nature, which indicated a lack of equilibrium retention and/or irreversible or slowly reversible processes. Soil samples with highest Ni concentrations were analyzed through EXAFS. The EXAFS results provided evidence of irreversible reaction for Webster soil which exhibited strongest affinity for Ni.