Reduction of contaminant leaching from MSWI bottom ash by addition of sorbing components

The potential for contaminant leaching often necessitates the application of extra safety measures when municipal solid waste incinerator (MSWI) bottom ash is used in (road) construction. The aim of this study was to try and develop a technique to improve the quality of bottom ash with respect to the leaching of, in particular, copper and molybdenum, to the extent that this waste material can be used in construction with lower risks of adverse effects on the surrounding environment. Based on fundamental knowledge of the processes controlling contaminant leaching from bottom ash, a strategy was chosen based on stimulation of contaminant binding by increasing the adsorptive properties of the bottom ash matrix. In addition, the pH of the bottom ash is lowered from its original value of about 10–11 to approximately pH 7–8, in order to enable simultaneous adsorption to the matrix of both metals such as Cu and oxyanions such as Mo. A number of potential sorbent minerals have been added to up to 5% m/m, as (precipitated) solids, to bottom ash suspensions in batch systems and the leaching of Cu and Mo has been measured as a function of pH. The most promising sorbents were selected for treatment of a larger amount of bottom ash and further testing by means of dynamic column tests. It is shown that treatment of bottom ash by addition of Fe(III) and Al(III) salts and in situ precipitation of the metal(hydr)oxides constitutes a very promising technique to reduce the leaching of both metals and oxyanions in bottom ash to below regulatory limits. It is also shown that durable reduction of the bottom ash pH to near-neutral values is the major factor controlling the final emissions of Cu and particularly Mo from the treated ash.