Speciation models PHREEQE

The computer program PHREEQE (3) was used for chemical equilibrium and reaction path modeling. PHREEQE does speciation and mass transfer calculations to find the distribution of aqueous complexes and the saturation indices of potential mineral phases present. 

Uranium solubility is increased even more in the nitrate microcosms, and one possible explanation is the conversion of acetate to CO2, coupled with nitrate reduction, which would give higher dissolved carbonate concentrations in the nitrate microcosms. However, PHREEQE modelling showed that the higher CO concentration would not greatly affect uranium speciation in solution and is therefore unlikely to account for the enhanced solubility. Alternatively, as nitrate is reduced to ammonium (NH/), which promotes cation exchange, this could lead to displacement of U02 from surface complexes, which are the predominant uranyl species on mineral surfaces. " ... 

Limitations and problems additional to those of the speciation codes The models do not consider solid-solution mass transfer and provide only limited information on ion exchange/adsorption mass transfer. All the programs except PHREEQE, PHRQPITZ, and MINTEQA2 keep track of water mass. Except for EQ3/6 and the Geochemist s Workbench, rate laws for mass transfer kinetics cannot be specified. Convergence problems occur more often than for the speciation codes. 

Nevertheless the combination of a reactive model with an optimization software is easier to obtain with the model approach presented above, than with standard geochemical software, like PHREEQE. By purpose the number of chemical components was kept as small as possible with respect to the regarded processes. By purpose the details of biochemistry, i.e. bacterial speciation (cf. Schafer et al., 1998), were not included. Moreover a high number of relevant (or irrelevant) parameters is surely an obstacle to understand complex (and easy) systems. 

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