Leaching behaviour of incineration bottom ash in a reuse scenario: 12years-field data vs. lab test results

Several types of standardized laboratory leaching tests have been developed during the past few decades to evaluate the leaching behaviour of waste materials as a function of different parameters, such as the pH of the eluate and the liquid to solid ratio. However, the link between the results of these tests and leaching data collected from the field (e.g. in disposal or reuse scenarios) is not always straightforward. In this work, we compare data obtained from an on-going large scale field trial, in which municipal solid waste incineration bottom ash is being tested as road sub-base material, with the results obtained from percolation column and pH-dependence laboratory leaching tests carried out on the bottom ash at the beginning of the test. The comparisons reported in this paper show that for soluble substances (e.g. Cl, K and SO4), percolation column tests can provide a good indication of the release expected in the field with deviations usually within a factor of 3. For metals characterized by a solubility-controlled release, i.e. that depends more on eluate pH than the liquid to solid ratio applied, the results of pH-dependence tests describe more accurately the eluate concentration trends observed in the field with deviations that in most cases (around 80%) are within one order of magnitude (see e.g. Al and Cd). The differences between field and lab-scale data might be in part ascribed to the occurrence in the field of weathering reactions (e.g. carbonation) but also to microbial decomposition of organic matter that modifying leachate pH affect the solubility of several constituents (e.g. Ca, Ba and Cr). Besides, weathering reactions can result in enhanced adsorption of fulvic acids to iron/aluminum (hydr)oxides, leading to a decrease in the leaching of fulvic acids and hence of elements such as Cu, Ni and Pb that strongly depend on DOC leaching. Overall, this comparison shows that percolation column tests and pH-dependence tests can represent a reliable screening tool to derive data that could be employed in risk-based analysis or life cycle assessment (LCA) frameworks for evaluating potential environmental impacts deriving from specific disposal/reuse options for waste materials.