ABSTRACT
Another class of models disregards the chemical thermodynamic relationships and focuses primarily on empirical approaches in which lumped parameters quantify the fate of sulfate in soils. Such lumped-parameters approaches can be looked at it terms of equilibrium, fully reversible kinetic approaches, and irreversible kinetic reactions. Hodges and Johnson (1987) tested the validity of several models for describing sulfate adsorption and desorption in a Cecil soil. Their test included first-order kinetics, and Elovich and other diffusion equation types. First-order reactions provided only adequate descriptions of results. They also found that sulfate desorption results were very different in terms of relations with soil solution in comparison to adsorption data. Such behavior is commonly referred to as “hysteresis,” and may be a result of irreversible reactions as suggested by Hodges and Johnson (1987). Lack of irreversible retention of sulfate has been observed by Gobran et al. (1998b). This observation has also been noted in soils by Harrison et al. (1989) and on oxide mineral surfaces by Turner and Kramer (1992). It should also be noted that deviations between sorption and desorption could also be related to kinetic retention behavior (Gobran et al., 1998a, 1998b). Irreversible sorption/desorption and the extent of kinetic reactions has also been quantified using the pressure-jump relaxation method on goethite by Zhang and Sparks (1990), who found that sulfate adsorption occurs rapidly at initial reaction stages whereas desorption is a slow one and may be considered as a limiting step. Such findings have been observed for other inorganic and heavy metal species present in the soil solution (Selim, 1992).
