The Equilibrium between Spheres

An increase or decrease in the concentration of components or elements in ecosystems is of vital interest, but the observation of trends in global changes of concentrations might be even more important as they may cause changes in life conditions on earth.

Concentrations in the four spheres, atmosphere, lithosphère, hydrosphere and biosphere, are of importance in this context. They are determined by the transfer processes and the equilibrium concentrations among the four spheres. As shown in Part A of this chapter, the solubility of a gas at a given concentration in the atmosphere can be expressed by means of Henry's law which determines the distribution between the atmosphere and the hydrosphere.

If we consider only two components in the hydrosphere: a tracer/; and water, and we assume that Ch < < C„. we can replace Q with the concentration of water in water = 1000/18 = 55.56 mol/1. According to these approximations, we obtain the following equation:

C, He Ch ~ R T Cn where Ca is the molar concentration in the atmosphere of component h, expressed in (mol/1) and Ch is the concentration in the hydrosphere expressed also in (mol/1) and Cw is the (mol/1) of water (and other possible components).

The soil-water distribution may be expressed by one of the adsorption isotherms, presented in Part B of this chapter, for compounds of ecotoxicological interest, the exponent y in Freundlich's adsorption isotherm (3.36) is often close to 1 and for most environmental problems C is small. This implies that becomes a distribution coefficient, usually indicated by k. As shown in Section 3B.6 for 100% organic carbon, k is denoted by klK. may be estimated from kM. Several estimation equations have been published in the literature; see for instance J0rgensen et al. (1997a). The following log-log relationships between kiK (100% organic carbon presumed) and A:(m are typical examples (Brown and Flagg, 1981):

or (Leeuwen and Hermens, 1995):

Several other estimation equations of importance for ecotoxicological modelling can be found in Section 8.5.

In the case where the carbon fraction of organic carbon in soil is /, the distribution coefficient (kD) for the ratio of the concentration in soil and in water can be found as kD = kx ■ f. If the solid is activated sludge (from a biological treatment plant) instead of soil, Matter-Müller et al. (1980) have found the following relationship:

where FAS (fraction of the activated sludge) is the ratio between the equilibrium concentrations in activated sludge and in water.

km. can be found for many compounds in the literature, but if the solubility in water is known it is possible to estimate the partition coefficient /¡-octanol-water at room temperature by the use of a correlation between the water solubility (in^imol/l) and km„ A graph of this relationship is shown Fig. 8.10.

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