Predicting chemical speciation and computer simulation

In recent years with the proliferation of PCs several computer models have become widely available and are used for a variety of applications. Some of the commonly used models include GEOCHEM (Mattigod and Sposito, 1979) and its updated version SOILCHEM (Sposito and Coves, 1988) HYDRAQL (Papelis et al, 1988), ECOSAT (Keizer, 1991) and MINTEQA2 (Allison et al., 1991). A comprehensive discussion on many of the earlier models is given by Nordstrom and Ball (1984). 

In this chapter some of the theoretical concepts used in these models will be outlined. In particular, emphasis will be given to the chemical thermodynamic principles that can be used to predict the stable forms of a given element. Such chemical principles provide the theoretical foundation of the commonly used chemical models. These models can be used to predict the final extent of reaction but not the rate. It is probably fair to say that these laws as basic principles are indisputable scientific fact however, problems arise when we try to apply them to ill-defined complex natural media such as soils and soil solutions where some reactions are kinetically slow and practically irreversible. However inadequate our chemical models are in relation to real-world situations they are the best we have and can be used to give valuable insight and meaning into the processes we observe. 

Many models include features allowing calculations involving the speciation of soluble complexes, the precipitation and dissolution of solid mineral phases and the adsorption of ions from solution on to surfaces. In addition, conditions of oxidation-reduction and partial pressures of gas phases can be superimposed on the calculation. Throughout the chapter some simple examples of chemical modelling calculations will be given. 

It is also worth mentioning that the results of the calculations are only as good as the data supplied, and much care is needed in selecting appropriate chemical 

Most speciation modelling is based on the assumption of thermodynamic equilibrium between phases, so it is necessary to describe the various equations that are used to quantify these chemical reactions. 

---

Lumsdon, D. G., and Evans, L. J. (1995). Predicting chemical speciation and computer simulation. In Chemical Speciation in the Environment, ed. Ure, A. M., and Davidson, C. M., Blackie Academic Professional, London, 86-134. 

---