Geochemical modelling and identification of leaching processes in MSWI bottom ash : implications for the short-term and long-term release of contaminants

Municipal Solid Waste Incinerator (MSWI) bottom ash is the major residue that remains after the incineration of Municipal Solid Waste. The slag-like material is produced world-wide in very large and everincreasing quantities. In the past the bottom ash was usually disposed, nowadays it is increasingly being used as a construction material. Incineration concentrates the mineral components of the original waste, including elements of environmental concern such as heavy metals, oxyanions, and salts. To ensure an environmentally safe utilisation or disposal of MSWI bottom ash it is, therefore, necessary to investigate its leaching properties. It is in particular necessary to identify the mechanisms that control leaching, because only then reliable predictions of the long-term leaching can be made. This thesis focuses on the geochemical processes that control the shortterm and long-term leaching of major and minor elements from MSWI bottom ash. The research is based on (I) carefully controlled laboratory leaching experiments on MSWI bottom ash at different stages of weathering, (II) geochemical modelling of aqueous (in)organic complexation, precipitation/dissolution, and sorption processes, (III) specific experiments to obtain parameters for the modelling of sorption reactions and organic complexation reactions, (IV) spectroscopic analyses of bottom ash samples, and (V) a field investigation of a 20-year old disposal of MSWI residues

The leaching of trace elements from municipal solid waste incinerator bottom ash at different stages of weathering

For a proper assessment of the environmental impact of the utilisation and disposal of Municipal Solid Waste Incinerator (MSWI) bottom ash it is necessary to understand weathering processes and their effects on (trace) element leaching. The authors have investigated the processes that control the leaching of Cd, Pb, Zn, Cu, and Mo from 3 categories of bottom ash: (A) unweathered bottom ash (grate siftings and unquenched samples), (B) quenched/non-carbonated bottom ash (freshly quenched and 6-week-old samples), and (C) weathered bottom ash (1.5- and 12-year-old samples). Leaching experiments were performed in a pH-stat at a large range of pH values. The speciation code MINTEQA2 was used for subsequent modelling of precipitation/dissolution processes. The speciation of trace elements in weathered bottom ash was also investigated by microanalytical techniques. In A- and B-type bottom ash the general controlling processes are thought to be precipitation/dissolution of relatively soluble minerals or, in the case of Cu in particular, extensive complexation with dissolved organic C. At the 'natural' pH of the samples, the leaching of Cd, Pb, Cu, Zn and Mo is generally significantly lower from C-type bottom ash than from less weathered types of bottom ash. This reduction in leaching is due to the neutralisation of bottom ash pH and the formation of less soluble species of these elements as weathering continues. In the more weathered (C-type) bottom ash trace element leaching does not seem to be solubility-controlled; although slow precipitation reactions cannot be totally excluded, it is hypothesised that the controlling mechanism in those samples is sorption to neoformed minerals