Coupled chemical speciation-solute transport model for prediction of solute transport in clay buffers

摘要

Modelling of the physico-chemical processes which occur in the buffer over a long time period is an invaluable tool as it is often not possible to conduct experiments over sufficiently long time scales to observe the long term leaching behaviour of waste. A coupled solute transport and chemical equilibrium speciation model has been developed. The model has the capability for simulating transport of multiple components of radioactive substances, thermodynamically reacting chemical species as they move, through a clay barrier system. It consists of two main modules, a finite difference transport module and an equilibrium geochemistry module which is a modified version of MINTEQA2. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient model to simulate the simultaneous processes of advective-dispersive transport (advection, diffusion, osmotic and ion restriction effect) and geochemical reactions (complexation, exchange, precipitation, adsorption and desorption) under different temperatures and pressures. In this research montmorillonite clay is employed as a buffer and strontium as a radioactive component for the simulation study. Two levels of temperature were chosen for simulation (25 ℃ and 100 ℃). The results indicated that the distribution and diffusion coefficients of Sr~(2+) are time-, space- and temperature-dependent and may not be considered as constant values.