Most of the Cr content in initial S1 was found in F3 (24.5%) and F5 (46.8%), while those in F1 and F2 were lower (9.9%). It is important to note that the toxicity of Cr depends on its existential valence state, whereas Cr(VI) is more toxic than Cr (III). Therefore, if Cr is mainly in the form of Cr(VI) in the leachate, the residual FA solids may have a significant negative impact on the environment (Quina et al., 2009). The bioavailable F1 and F2 fractions of Ni were the dominant fractions (44.8%), which indi- cated that Ni was easily leached out from initial S1 under suitable pH and redox conditions (Sundaray et al., 2011).