Computer speciation simulation models

A second obvious line of research for the future must be that related to the development and improvement of computer-based simulation of long-term environmental behaviour of radionuclides. Most currently available models are still comparatively simple compared with the physical, chemical and biological complexity of environments they purport to represent but, as noted in Section 13.5, our ability to construct ever more complex conceptual models for predicting the future behaviour of radionuclides is improving. However, the more complex the model, the more demands it places on the basic thermodynamic data and knowledge of likely speciation. The challenge for the future will therefore be to produce high-quality data for model construction and to devise realistic ways to validate those models once produced. 

Although the emphasis in this article has been on the discussion of toihniques and methods that can be used in the laboratory for the identification of species, increasing importance is being attached to computer simulation of trace element speciation. The reason for this increased interest could be attributed in part to the availability of relevant experimental data which could be used in developing the required models. However, computer simulation comes into its own when the species are so imstable that separation techniques cannot be applied and/or the detection systems do not have the required sensitivity. 

Computer simulation is now used extensively as a tool to help to understand and predict the transport of radionuclides through environmental systems. Most models relate to waste disposal and are based on measured parameters such as water movements, salinity, suspended load and the radionuclide concentration in the solute, suspended particulate matter and bottom deposits. Comparatively few attempts appear to have been made to include chemical speciation into this type of model, presumably because of the added complexity involved. Some modellers have attempted to take into account the characteristics of the major chemical phases such as those present in different particles or coatings (e.g. Martinez-Aquirre et al., 1994). Others have noted the importance of including details of particular chemical species present in industrial waste releases when constructing models to predict dispersion (Abril and Fraga, 1996). 

For experimental studies, a chemical thermodynamic modelling approach could theoretically reduce unnecessary experimental effort and hence the overall cost of a research project. Once experiments are underway, the computer simulation should also offer valuable assistance in the interpretation of results. Modelling techniques with particular reference to radionuclide speciation have been discussed by Cross and Day (1986) who pointed out that models are only as good as the thermodynamic data upon which they are based. For example, formation constants (a prerequisite for chemical modelling) are invariably generated in idealised laboratory conditions and their use seldom reflects the natural environment... 

The difficulties of experimentally determining the speciation of actinides present at very low concentrations in natural waters have encouraged the use of computer simulations, based on thermodynamic data, as a means of predicting their speciation and hence their environmental behaviour. The use of modelling techniques to describe the speciation, sorption, solubility and kinetics of inorganic systems in aqueous media has been reviewed in the papers given at an international conference in 1978. Both chemical equilibrium models, exemplified by computer programs such as MINEQL and SOLMNQ, and dynamic reaction path models, exemplified by EQ6, have been developed. Application of the equilibrium models to radioactive waste disposal... 

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