Geochemical partitioning distribution

The partitioning of As in the aquifer solid-water interface can best be explained with the distribution coefficient, Kd (a ratio of solute adsorbed in sediment to that of dissolved in groundwater). Due to being simplistic in nature, Kd has long been well appreciated as well as applied by geochemical modelers. 

Surface complexation model A computer code or geochemical model that provides an explanation and attempts to predict the partitioning of a chemical species between the surface of an adsorbent and the associated solvent. The models consider a number of factors, including pH and ionic strength (see (Langmuir, 1997), 369-395 for details compare with charge distribution multisite complexation model). 

The ionic radius criterion for interpreting geochemical distributions of trace elements was given a boost in the early 1970 s when correlations were shown to exist between ionic radii and partition coefficients of some trace elements (Onuma et al., 1968 Higuchi and Nagasawa, 1969 Jensen, 1973). The influence of cation radius and charge on trace element distribution patterns was demonstrated by measurements of the distribution coefficient, >, defined by... 

Seyler P. and Boaventura G. (2002) Distribution and partition of trace elements in the Amazon basin. In Hydrological Processes, Special Issue of International symposium on Hydrological and Geochemical Processes in Large Scale River Basins, Nov. 15-19, 1999, Manaus, Bresil. 

Shallow geochemical environments consist of solid, aqueous, and air reservoirs and their interfaces. Hydrocarbon compounds partition into these various reservoirs in a manner determined by the stmcture and physical properties of the compounds and the media, and the mechanism of hydrocarbon release. The structure and physical properties of the compounds and media understandably impact their sorption, solubility, volatility, and decomposition behavior (e.g., Schwarzenbach et al., 1993). In addition, hydrocarbon partitioning in real systems is holistically a disequilibrium process hence, the distribution of hydrocarbons depends as much on the pathway taken as on the final physical state of the system (e.g., Schwarzenbach et al., 1993 Luthy et al., 1997). Shallow aquatic systems may tend towards some equilibrium distribution (Figure 10), but this is seldom, if ever, truly attained. 

For geochemical purposes, the dependence of isotope fractionation factors on temperature is the most important property. In principle, fractionation factors for isotope exchange reactions are also slightly pressure-dependent, but experimental studies have shown the pressure dependence to be of no importance within the outer earth environments (Hoefs 2004). Occasionally, the fractionation factors can be calculated by means of partition functions derivable from statistical mechanics. However, the interpretation of observed variations of the isotope distribution in nature is largely empirical and relies on observations in natural environments or experimental results obtained in laboratory studies. A brief summary of the theory of isotope exchange reactions is given by Hoefs (2004). 

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