Chapter Five - Transport and Retention of Heavy Metal in Soils: Competitive Sorption

Abstract

Appreciable amounts of various heavy metals (e.g. As, Cu, Ni, and Zn) are found in surface soils of lands as a result of accidental spills or industrial waste and sewage sludge applications. Such conditions often create environmental risks and potential contamination of soil, surface and groundwater resources. Competitive adsorption and desorption processes of heavy metals by the soil matrix have significant importance on their fate and mobility in soils. In this contribution, equilibrium and kinetic models governing competitive heavy-metal sorption and transport in soils were presented. Several examples were discussed to illustrate the impact of competing ions on the reactivities and mobility of heavy metals in the soil–water environment. The examples exhibited that competition among various heavy-metal species for available adsorption sites on soil matrix surfaces often results in the enhancement of the mobility of contaminants in the soil environment. Competitive sorption based on equilibrium Freundlich and Langmuir models were derived in order to account for competitive sorption of cations and anions in soils. Competitive models of the multiple reaction type including the two-site nonlinear equilibrium-kinetic models, the concurrent- and consecutive multireaction models were modified to describe kinetic adsorption–desorption of heavy metals behavior in soil. It was shown that equilibrium Langmuir and kinetic second-order models can be extended to simulate the competitive sorption and transport in soils. A drawback of Freundlich and Langmuir approaches is that their associated parameters are specific for each soil. Moreover, since predictions of the transport of heavy metals were sometime inadequate, improved competitive modeling approaches are needed. On the other hand, geochemical models, which are based on ion exchange and surface complexation concepts, are frequently utilized to quantify competitive behavior of several chemical species under a wide range of environmental conditions. However, further research is also needed since geochemical models are incapable of describing kinetic sorption–desorption of heavy metal ions in competitive systems.