Pollutants adsorption onto soils

A residual CP concentration is often observed after soil bioremediation. The leveling-off of degradation is not due to decreased microbial activity, since freshly added CPs are rapidly degraded (Harmsen, 1993 Salkinoja-Salonen et al., 1989). The residual concentrations are explained by the gradual diffusion of pollutants deep into micropores, as well as by their adsorption onto soil organic matter (Harmsen, 1993). Lagas (1988) observed that the nonextractable fraction of CPs in sterile soil increased according to the square root of time as a consequence of diffusion into humic material. 

Lastly, non-ionic organic pollutants can also adsorb onto soil from water. This constitutes a major process in pollutant transport/mobility in subsurface environments. Both the organic fraction of soils and the clay minerals can play key roles as adsorption sites. 

Illustration 6.5 Adsorption of a Pollutant from Groundwater onto Soil... 

This isotherm finds use mainly in the study of the adsorption of gases on solids however, it can be useful in the study of adsorption of pollutants from aqueous systems, particularly onto solid phases. The heterogeneous nature of a solid surface (i. e., soils, sediments, suspended solids) would obviously invalidate the first assumption (i.e., a, above) used in developing the relationship. The third assumption (i. e., c, above) also would be invalid in a situation where one is dealing with multi-layer adsorption. 

As a contaminant moves through soil and groundwater, chemical processes will affect both contaminant concentration and overall hydrogeochemistry (Schoonen, 1998) of the system. Different adsorption mechanisms cause pollutants to adsorb onto the soil, volatilize, precipitate, and be part of the oxidation-reduction processes. Adsorption is loosely described as a process in which chemicals partition from a solution phase into or onto the surfaces of solid-phase materials. Adsorption at particle surfaces tends to retard contaminant movement in soil and groundwater. 

The adsorption behavior of colloidal material onto river particles can play a vital role in the transport and fate of pollutants. FFF methods provide a means to evaluate the relative importance of different fractions in adsorption of contaminants in soils and sediments. 

Adsorption of ionizable organic pollutants onto hydrous metal oxide surfaces in soils, sediments, and aquifers can have an important impact on pathways and rates of chemical transformations. In some instances, a particular degradative pathway can only occur at the oxide/water interface, for example, because of the low solubility of these higher-valent metals in most natural waters. In other instances, the unique chemical microenvironment of the oxide/water interface may catalyze transformations that otherwise would have occurred in solution. Hydrolysis of two carboxylic acid esters catalyzed by hydrous metal oxides is discussed. A detailed understanding of adsorption phenomena provides the basis for assessing the nature and importance of surface chemical transformations. 

Adsorption of ions onto alumina is a problem of great interest in very different fields. As can be seen from the journals mentioned in the reference list, it is important in geochemistry as well as in limnology, in pollution control as well as in catalyst preparation or soil chemistry, and sometimes even in unexpected disciplines such as nuclear techniques. The different applications lead to different questions and experimental conditions. For example, the "normal" concentrations are low in limnology and the pH is restricted to nearly neutral conditions. In contrast, in catalysis there are rather high concentrations and the pH, in principle, is variable, though sometimes not even discussed, probably as a consequence of the use of older technical impregnation techniques in which nonequilibrium conditions were applied. 

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