Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 191

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Competitive Sorption and Transport
Over the last three decades, several studies were carried out to identify the
dominant mechanisms controlling the fate and overall behavior of heavy
metals in the soil-water environment. Most investigations focused on
describing the sorption and transport of heavy metals under field conditions
and in laboratory and greenhouse experiments. Adsorption-desorption on
the surface of solid minerals and organic matters is one of the dominant
reactions impacting the fate and transport of heavy metals in soils. Most
efforts focused on describing the transport and retention of one heavy metal
species only. Such an assumption implies that all other interactions that
occur in the soil do not greatly influence the behavior of the heavy metal
species under consideration. This simplification is unrealistic and does not
represent the soil environment, which contains many chemical species hav-
ing various interactions.
It is generally accepted that competing ions strongly affect heavy metal
retention and release in soils. Industrial waste and sewage sludge disposed
of on land often contain appreciable amounts of heavy metal such as Cu,
Zn, Cd, and Ni and thus create a risk for croplands, as well as animals and
humans. In most cases, soil contamination involves several heavy metals,
that is, a multiple-component system. In fact, competition among heavy
metal species present in the soil solution for available adsorption sites on
soil matrix surfaces is a commonly observed phenomenon (Murali and
Aylmore, 1983; Kretzschmar and Voegelin, 2001, among others). Competitive
adsorption and desorption processes of heavy metals in minerals and soil
organic matter have significant effects on their fate and mobility in soils and
aquifers. The selective sorption among competing heavy metal ions may
greatly impact their bioavailability in soils (Gomes, Fontes, and da Silva,
2001). Enhanced mobility as a result of competitive sorption has been often
observed for several trace metal contaminants in soils.
The adsorption of heavy metal by clay minerals, metal oxides, and organic
materials has generally been explained with two types of reaction mecha-
nisms: (1) ion exchange in the diffuse layer as a result of electrostatic force
and (2) surface complexation through the formation of strong covalent bonds
between heavy metal ions and specific reaction sites on the surfaces of min-
erals or organic matters. The ion exchange reaction is also referred to as
nonspecific sorption and the surface complexation is referred to as specific
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