MOBILITY OF HEAVY-METALS FROM COAL FLY-ASH

The mobility of Cd, Co, Cu, Ni, Pb, Sb, and Zn from six different coal -fired power plant fly ashes that show a wide compositional range was examined using a sequential extraction procedure in order to assess th eir mobility when these wastes are ponded or landfilled. The extractio n sequence was as follows: (1) water extractable, (2) cation exchangea ble (CH3COONH4 at pH 7),(3) surface oxide-bound cations (CH3COONH4 at pH 5), (4) Fe oxide-bound cations (HONH3Cl), and (5) residual (HF, HCl , HNO3, 2: 1 : 1). The heavy metal contents in the extraction solution s were determined by anodic (Cd, Cu, Pb, Sb, and Zn) and cathodic (Ni and Co) stripping voltammetry. The results reveal differences in the t otal contents of the selected trace elements among the fly ash samples , which must be related to differences in coal composition and combust ion technology. The extractable fraction under natural conditions rang es from 1.5 to 36.4 percent of the total element content. Cadmium, Co, Cu, and Zn show the highest extractable fraction (10.8-18.9 percent o n average). Cadmium is the most easily water-extractable element, whil e Co, Cu, and Zn increase their mobility as the severity of the extrac tion increases. Cobalt, Ni, Pb, and Zn are mainly associated with the surface oxide-bound and Fe oxide-bound fractions. Nickel, Pb, and Sb h ave ow mobility potentials (5.3-6.6 percent as extractable fraction), but Sb presents a relatively high water-extractable fraction.