Sorption Mechanisms Determining Ni(II) Retention by a Calcareous Soil

The objective of the study was to determine the mechanisms of Ni retention in a calcareous soil by comparing the sorption kinetics and adsorption-desorption isotherms of the whole soil and of some of its fractions. The study was performed using diffiractometric, microscopic, and thermogravimetric techniques to corroborate the mechanisms hypothesized from the equilibrium and kinetic studies. The sorption kinetics and the adsorption-desorption isotherms show that in a calcareous soil, Ni(II) is also retained by carbonates. In addition to the well known insolubilization process as a hydroxide caused by the high pH value induced by the presence of carbonates, calcium carbonate also contributes to Ni retention through the formation of a strong complex probably ascribable to a co-precipitation process that involves Ni/Ca carbonate double salt or mixed Ni/Al hydroxides and carbonates formation. The study also demonstrates that montmorillonite and illite clays may contribute to the adsorption of Ni with a nonspecific ion exchange mechanism, but as the Ni retention in the calcareous soil is totally specific, it can be presumed that this clay sorption mechanism is totally overshadowed by the double effect (Ni precipitation and complexation) caused by CaCO3.