TRENDS IN ELEMENTAL LEACHING FROM COAL COMBUSTION PRODUCTS FROM TWO COAL-FIRED POWER PLANTS

To assess how elements occur within and leach from coal combustion products (CCPs) and better understand possible risks from CCP use or disposal, CCPs were sampled from two bituminous-coal-fired power plants. One plant burns Pittsburgh coal from the central Appalachian Basin; the other, coal from the Colorado Plateau. From the former plant, the sampled CCPs were bottom ash (BA), economizer fly ash (EFA), and fly ash (FA); from the latter, BA, mixed EFA/FA, FA, and cyclone-separated coarse and fine fractions of FA. Samples of each ash were leached using the synthetic groundwater leaching procedure (SGLP, 60-day duration) or the toxicity characteristic leaching procedure (TCLP, 18-hour duration), for which the leachates are water and an acetic acid solution, respectively. Element concentrations (mass fractions) in the unleached ashes and the ash leachates were determined by ICP-AES and/or ICP-MS.

Qualitative trends in element concentrations in the ashes and in element leachabilities were similar between corresponding ashes from each plant, although some elements exhibited large quantitative differences between the plants (e.g. Cr, Ni). Element concentrations in the unleached ashes were lowest in BA and increased from EFA to FA for most trace elements analyzed, including As, Be, Cd, Cu, Ge, Li, Mo, Pb, Sr, Tl, U, V, and Zn, and for the major element Ca. This may reflect a temperature and/or particle-size dependence. No consistent changes in element concentrations with ash type were noted for Ba, Co, Cr, Ni, Rb, and Sc, and for Al, Fe, K, Mg, and Mn. All analyzed trace element concentrations also increased from the coarse to the fine FA fractions. For most elements, ash leachabilities were lowest in BA (least leachable) and increased from EFA to FA (most leachable). The opposite trend occurred with Cd, Fe, Ni, and Pb, which leached more from BA than FA under one or both leaching procedures. Leachability of most elements also increased as FA particle size decreased, possibly due in part to increasing specific surface area. Exceptions were Cd, Pb, and Zn, which leached more from coarse than fine FA fractions under SGLP and/or TCLP. If quantities of ash used (e.g. in construction, ice treatment on roads) or placed in landfills are known, these data allow estimation of the amounts of elements that may be leached into the environment.