**5. Conclusions**

In the present study, two BA samples characterized by a different mineralogical and chemical composition and with a different ANC were investigated applying some standardized extractions/leaching tests (CH3COOH extraction, ammonium-EDTA extraction, and pHstat leaching test). A high extractability of trace elements (except As and Mo) was observed in the CH3COOH extraction which also resulted in the lowest pH among the experiments. pHstat leaching allows investigating the kinetics of element release under acidic conditions and can be linked to the solid-phase characteristics of some selected trace elements. Nickel and Co are occluded in stable phases such as Fe-alloys and Fe-oxides while Zn might be associated with amorphous or poorly crystalline phases. Moreover, Zn may also exist in S-rich phases. Based on the results obtained in this study, some preliminary treatment to remove trace elements (e.g., by washing) or immobilize trace elements from the ash should be applied to the studied BAs before landfilling. This is necessary to avoid contamination of the environment, both as consequence of the high total concentrations and because of the relatively high mobility (as deduced from single extractions) of Cu-Mo (in AS1) and Pb (in AS2). Additionally, the potential recovery of some metals should be explored. The extractions and leaching tests applied in this study, which are generally used for preliminary screening in assessment of elemental mobility, could also be used for a first estimate of the potential recovery of valuable elements from waste materials. The approach used in this study is helpful for the development of appropriate waste managemen<sup>t</sup> options, especially in countries facing inefficient waste and waste water treatment technologies, resulting in residual waste materials with considerable concentrations of valuable elements.

**Author Contributions:** T.T.T.D. designed and performed the experiments. T.T.T.D. interpreted the results with significant contribution of Valérie Cappuyns. T.T.T.D. wrote the paper and significant contribution to writing was made by V.C. and R.S.E.V. performed ICP-OES analysis.

**Funding:** This research was funded by [BTC] gran<sup>t</sup> number [10VIE/0015].

**Acknowledgments:** We acknowledge the members of Geology Division, Department of Earth and Environmental Sciences, KU Leuven for their support. We also thank the personnel at hazardous waste treatment plants for providing samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
