Geochemical partitioning model

Empirically determined retardation factors (either partition coefficients or breakthrough curve measurements, which are the change in solute concentration measured over time in laboratory or field experiments) have been widely used because of their inherent simplicity.162 Modeling of specific geochemical partition and transformation processes is not necessary if the retardation factor can be determined empirically. 

While over the past ten years, our ability to measure U-series disequilibria and interpret this data has improved significantly it is important to note that many questions still remain. In particular, because of uncertainties in the partition coefficients, fully quantitative constraints can only be obtained when more experimental data, as a function of P and T as well as source composition, become available. Furthermore, the robustness of the various melting models that are used to interpret the data needs to be established and 2D and 3D models need to be developed. However, full testing of these models will only be possible when more comprehensive data sets including all the geochemical parameters are available for more locations and settings. 

Lundstrom CC, Shaw H, Ryerson F, Phinney D, Gill J, Williams Q (1994) Compositional controls on on the partitioning of U, Th, Ba, Pb, Sr and Zr between clinopyroxene and haplobasaltic melts implications for uranium series disequilibria in basalts. Earth Planet Sci Lett 128 407-423 Lnndstrom CC (2003) Uranium-series disequilibria in mid-ocean ridge basalts observations and models of basalt genesis. Rev Mineral Geochem 52 175-214... 

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. 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

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). 

Nowosielski, B.E., Fein, J.B. (1998) Experimental study of octanol-water partition coefficients for 2,4,6-trichlorophenol and pentachlorophenol Derivation of an empirical model of chlorophenol partitioning behaviour. Appl. Geochem. 13, 893-904. 

In the second step of the modeling exercise, speciation and surface adsorption in the neutralized water were modeled using the geochemical code minteqa2. Analytical concentrations of Cd, Ni, Be, and U (5.5, 65,4.1, and 22 x 10-6 molL-1, respectively) were used as input concentrations. The surface properties and surface complexation constants were taken from Dzombak and Morel (1990). Due to the lack of experimental data for A1 hydroxides, we assume that all A1 hydroxides have the same sorptive properties as Fe hydroxide and that the mass of A1 hydroxide was added to the Fe hydroxide concentrations. The total sorbent concentration is 5.3 g/L as Fe(OH)3, which is calculated from the first step. minteqa2 was used to calculate the partitioning of these ions between the aqueous phase and ferric iron hydrous oxide (HFO) surfaces. 

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