Characteristics of heavy metals leaching from MSWI fly ashes in sequential scrubbing processes

Two types of fly ashes, termed ESP and APC, according to their collection points from two successive processes, i.e., electrostatic precipitation followed by air pollution control in the incineration plant, were subject to ethylenediaminetetraacetic acid (EDTA) leaching or EDTA thermal leaching tests in batches. The removal efficiency was metal-specific and also a function of the chemical concentration, time, and pH relevant to mineral dissolution. Increased dosage of EDTA enhanced removal of Cu (from 27.83 ± 5.18 to 88.34 ± 4.43%), Zn (from 14.05 ± 1.81 to 55.95 ± 8.55%), and As (from 0.94 ± 0.06 to 29.45 ± 2.97%) in ESP, in contrast to trivial influences on most metals in APC. EDTA thermal leaching suggested a limited capacity, whereas significant inhibitions were noticed for Cu, Pb, and Zn from ESP, while Cr from APC. Sequential extraction was performed with fly ashes under various treatments. Most metals present in APC (51.7–99.8%) were complexed with amorphous Fe–Mn oxides showing highly stable. By contrast, metal distributions in ESP (the precursor of APC) were highly heterogeneous across different fractions. X-ray diffraction and scanning electron microscopy were applied to investigate the mineral distribution in fly ashes, suggesting more complicated mineral precipitation in air pollution control process, whereby metals were well fixed.