Soil systems mechanisms

Fig. 14-4 Schematic representation of the transport of P through the terrestrial system. The dominant processes indicated are (1) mechanical and chemical weathering of rocks, (2) incorporation of P into terrestrial biomass and its return to the soil system through decomposition, (3) exchange reactions between soil interstitial waters and soil particles, (4) cycling in freshwater lakes, and (5) transport through the estuaries to the oceans of both particulate and dissolved P.

Given the deviation of n from unity, this isotherm confirms the mechanistically different behavior of diagenetically altered organic matter from that of softer soil organic matter. In soil systems comprised by admixtures of different types of soil organic matter, it is logical to expect overall sorption to be a composite process involving different combinations of sorption mechanisms. 

Mechanisms of retention are similar in either situation, but expression of the mechanism will vary. Practically, measurement of reaction rates in soil systems is usually limited to observing changes in reactant. This means that observations will be related to the most rate-limiting step, since this will control the amount of reactant observed at any given time. Unfortunately, diffusion rates are often the rate-limiting step, so observation of reaction kinetics often depends on the ability to negate or interpret diffusion processes. This ability differs, depending on the experimental technique chosen. 

Lastly, a wide selection of papers, focus the most recent achievements in the application of zeolitic compounds, in the use of oxide materials in the fuel cells technology and in the understanding of the complex mechanisms of oxide-based materials in the soil system. 

This relationship between Mo and S has practical significance for crop production, because it permits the manipulation of Mo concentrations in plant tissue through S application. This mechanism can also lead to the development of Mo deficiency in crops when sulfate-containing materials are added to the soil to correct S deficiency. Olsen (1972) reviewed the literature on Mo-S relations and concluded that in some situations fertilization with both Mo and S may be required. Thus it is useful to consider the conditions that influence the relationship between Mo and S in plant and soil systems. 

Electrophoresis Electrophoresis (also known as cataphoresis) is the transport of charged particles of colloidal size and bound contaminants due to the application of a low DC or voltage gradient relative to the stationary pore fluid. Compared with ionic migration and electroosmosis, mass transport by electrophoresis is negligible in low-permeabihty soil systems. However, mass transport by electrophoresis may become signiflcant in soil suspension systems, and it may also be a dominant transport mechanism for biocoUoids (i.e. bacteria) and micelles. 

As suggested by the above paragraph, other types of phenomena affect the previously described transport mechanisms of the contaminants toward the electrodes. These are the physical-chemical interactions, both between different compounds in the aqueous phase and between these aqueous species and the sohd phases of the soil system. Some of these interactions are precipitation, acid-base, complex formation and redox reactions, adsorption, and ion exchange and surface complexation reactions. 

This conceptual approach has been developed in the framework of the concept of "chemical time bombs" (a waste deposit or contaminated hot-spot which initially appears to be relatively harmless, but which can eventually have disastrous environmental effects as toxic contaminants are released). The effects of these time bombs are non-linear and delayed, e.g. toxic metals can "break through" once the specific buffering capacity of a sediment or soil system has been surpassed. To make the scientific objectives clearer, it is useful to distinguish between two different mechanisms (Stigliani 1992) the first is direct saturation, by which the capacity of a soil or sediment for toxic chemicals becomes exhausted. The second way to "trigger" a time bomb is through a fundamental change in a chemical property of the substrate that reduces its capacity to adsorb (or keep adsorbed) toxic materials. 

---