Application of a Monte Carlo Approach to Surface Complexation Modelling
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APPLICATION OF A MONTE CARLO APPROACH TO SURFACE COMPLEXATION MODELLING M.M. Askarieh1, T.G. Heath2 and W.M. Tearle3 United Kingdom Nirex Limited, Curie Avenue, Harwell, Didcot, Oxfordshire OX11 0RH, UK. 2 Serco Assurance, B150 Harwell, Didcot, Oxfordshire OX11 0QJ, UK. 3 formerly Serco Assurance, present address Ghyll Mount, Gillan Way, Penrith 40 Business Park, Penrith, Cumbria CA11 9BP, UK.
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ABSTRACT A Monte Carlo-based approach has been adopted for development of a chemical thermodynamic model to describe the goethite surface in contact with sodium nitrate solutions. The technique involves the calculation of the goethite surface properties for the chemical conditions corresponding to each experimental data point. The representation of the surface was based on a set of model parameters, each of which was either fixed or was randomly sampled from a specified range of values. Thousands of such model representations were generated for different selected sets of parameter values with the use of the standard geochemical speciation computer program, HARPHRQ. The method allowed many combinations of parameter values to be sampled that might not be achieved with a simple least-squares fitting approach. It also allowed the dependence of the quality of fit on each parameter to be analysed. The Monte Carlo approach is most appropriate in the development of complex models involving the fitting of several datasets with several fitting parameters. Introduction of selenate surface complexes allowed the model to be extended to represent selenate ion sorption, selenium being an important radioelement in evaluation of the long-term safety of ILW disposal. The sorption model gave good agreement with a wide range of experimental sorption datasets for selenate.
INTRODUCTION The role of United Kingdom Nirex Limited is to provide safe, environmentally sound and publicly acceptable options for the long-term management of radioactive materials. One option developed is the Nirex Phased Geological Disposal Concept, which represents a stepwise and reversible approach to the disposal of radioactive waste within a deep geological repository. It involves the immobilisation of wastes in stainless steel containers that eventually would be surrounded by a cementitious backfill, if and when society decides, after their emplacement. Part of the Nirex research programme is concerned with developing models for radioelement sorption both onto repository and natural materials, including a model for sorption onto goethite. Sorption provides an important retardation mechanism for the migration of some radioelements. Surface complexation models (SCMs) have been widely applied to the interpretation of sorption experiments for radionuclides and for inactive elements. These thermodynamic sorption models can be combined with aqueous equilibria in chemical speciation codes such as HARPHRQ [1]. In such modelling approaches, suitable sorption equilibria are selected, and the associated equilibrium constants are derived by fitting one or more sets of
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