Evidence of Al-Cr interactions affecting Cr-leaching from waste incineration ashes

Leaching tests on flue gas ashes from waste incineration showed low leaching of Cr from ashes that under moist anaerobic conditions also produced hydrogen gas. In some cases, also the redox levels (aerobic/anaerobic conditions) during the leaching test affected Cr leaching. Aerobic ashes tested in an open batch leaching test leached Cr up to 2-3 orders of magnitude more than the ashes kept moist under anaerobic conditions and tested in a closed leaching test. Model experiments showed that metallic Al could reduce Cr and at the same time produce H(2). The hydrogen gas per se could not reduce Cr. Laboratory experiments with ashes provided evidence that metallic Al, present naturally in the ashes or amended, could reduce Cr under moist anaerobic, but not under aerobic storage. Significant Cr reduction was linked to, but not caused by hydrogen formation. The reduced Cr seemed to be partially reoxidizable upon aeration or drying. The observations presented provide a basis for understanding the complexity of Cr leaching from waste incineration ashes, as for example, why some chemical stabilization methods increase Cr leaching, and point out the need for standardizing leaching test conditions with respect to Cr.