Transport, Mobility, and Partitioning of Pollutants in Soils

A given pollutant may penetrate in soil down to a specific depth, and therefore transport calculations need individual depth data. Owing to mass transport restrictions, residence times of many pollutants in soils are (unfortunately) much longer than those in the gas or liquid phases. In addition, partitioning effects in soils can be dramatic a case in point is the concentration effect that occurs with uranium, which sometimes reaches levels up to 104 times higher than its concentration in water with which the soil is in equilibrium. Biota plays a key role in the transport and mobilization of pollutants from soil, because for example, many of them bioaccumulate in vegetation and cattle (see Section 9.2). 

Partitioning and mobility of metal ions, metal complexes, and ligands in soils or sediments are affected by their adsorption onto a variety of substrates. As mentioned earlier (see Section 6.3.1), natural oxides offer suitable adsorption sites for some of these species and may even undergo dissolution as a result. Here, an understanding of the bonding phenomena is crucial. For example, the adsorption of [Co(III)EDTA] (here written as [ML]-) on hydrated aluminum oxide surfaces (written as =A10H) can be represented as  

It involves an outer-sphere mechanism (see Section 3.1.4) because the four carboxylate groups in the monovalent [Co(III)EDTA] complex are coordinated to the metal center, whereas in the divalent [Co(II)EDTA]2- complex, one carboxylate group is free to coordinate with the surface and thus it adsorbs through an entirely different path. 

Polyphosphates also play a role in the chemistry of natural metal oxides. For example, if the concentration of polyphosphates is smaller than that needed to saturate the available sites of a solid oxide surface, their presence enhances the sorption of surrounding metal ions by contrast, if their concentration exceeds that needed to saturate the surface, their presence decreases metal ion sorption due to two effects  

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