Hysteresis in the individual and competitive sorption of cadmium, copper, and lead by various soil horizons.

Investigations of the sorption of heavy metals by soils often involve a fast ion-exchange process followed by a more sorbate-specific slow process. Desorption and sorption isotherms often fail to coincide, thus showing real or artifactual hysteresis, and failure to conform to conventional models generally prevents meaningful comparison of the hysteretic behavior of different sorbates in different soils. In the work described here, the irreversibility of the sorption of Cd, Cu, and Pb from single-metal and multimetal solutions by samples of 20 soil horizons was evaluated in terms of a hysteresis index, defined as the ratio between the values obtained in desorption and sorption experiments for an affinity measure defined in previous work, K(r). Sorption of these metals from both types of solution was more irreversible in the more basic soils, and its other chief determinants were CIC(e), Mn oxides content, and, except for Cd, organic matter content. The least sorbed metal was invariably Cd.