Process identification and model development of contaminant transport in MSWI bottom ash

In this work we investigate to what extent we are able to predict experimental data on column leaching of heavy metals from municipal solid waste incinerator (MSWI) bottom ash, using the current knowledge on processes controlling aqueous heavy metal concentrations in combination with a multicomponent reactive transport computer model. Heavy metal concentrations were modelled with a surface complexation model for metal sorption to (hydr)oxide minerals in the bottom ash matrix. For transport modelling it was necessary to simplify the sorption modelling approach. Therefore, we determined a minimal set of components and species that still provided an adequate description of the pH dependent heavy metal behaviour. The concentration levels of the heavy metals are generally predicted to within one order of magnitude. Discrepancies between the model and the data are caused by uncertainty in modelling parameters and a still insufficient description of the dynamics of macroelement leaching and pH. In general, the simulated leaching curves show much more abrupt changes than the measurements. This observation might be an indication of non-equilibrium. Processes that have to be taken into account for further model development are the influence of non-equilibrium effects and the facilitated transport of heavy metals by dissolved organic matter.