Modeling the competitive sorption and transport of Ni(II) and Zn(II) in soils: Comparing two multicomponent approaches.

The mobility of contaminants in soil is controlled by sorption reactions which can be affected by the presence of other solutes that compete for sorption sites. The ability to model such effects is necessary for evaluating the environmental risk of a given contaminant. In this study, the competitive sorption and transport of nickel (Ni) and zinc (Zn) in Olivier and Windsor soils was investigated using batch equilibration and miscible displacement experiments. During batch experiments, the sorption of Ni and Zn was mutually reduced in multicomponent systems, indicating that the metal cations compete for sorption sites. When applied concurrently, the retardation of both ions decreased and peak effluent concentrations increased relative to single ion experiments, demonstrating that competition increased the mobility of both ions during miscible displacement experiments. A novel Freundlich-type multicomponent isotherm (CDI) and its kinetic analog (CDIT) were developed and compared to the commonly used SRS isotherm and SRS-based kinetic approach (SRST) in describing the experimental data. The CDI provided a superior description of the competitive batch data, especially at low surface coverage, and may therefore be more applicable to multicomponent sorption than the SRS. The Olivier and Windsor transport data were best described by the CDIT and SRST, respectively, however, both models generally described the data well. Since both approaches gave comparable descriptions of the transport data while the CDI outperformed the SRS in describing the batch data, the CDI/CDIT may be more generally applicable to multicomponent systems and warrants further study.