A SEQUENTIAL CHEMICAL EXTRACTION STUDY OF THE LEACHABILITY OF TRACE ELEMENTS FROM UNTREATED AND TREATED COAL FLY ASH

More than half of the electricity generated in the US comes from coal-fired electric power plants which produce large volumes of fly ash and other coal combustion products. While approximately 41% of the by-products are reused, the majority are disposed of in surface impoundments and landfills located throughout the US. Fly ash contains leachable, hazardous trace elements such as As, B, Cr, Mo, Ni, Se, Sr and V. The potential release of trace elements into the environment is of concern, especially at older, unlined ash disposal facilities and ash impoundments. Simple and effective treatment techniques are therefore needed to stabilize coal fly ash in the disposal environment and to increase its reuse.

The goal of this study is to understand the potential for leaching of trace elements into the environment by using a 7-step sequential chemical extraction procedure (SCEP) to determine the distribution of trace elements in untreated and ferrous sulfate (FS) treated coal fly ash. The SCEP used in this study gives valuable information on the concentrations of trace elements associated with the following fly ash fractions: 1) water soluble, 2) exchangeable, 3) acid soluble, 4) easily reducible, 5) moderately reducible, 6) poorly reducible and 7) oxidizable. In general, the mobility of elements associated with these fractions decreases with each successive extraction step.

The SCEP data showed that FS treatment at S:L=1:30 successfully transferred significant quantities of As, Cr, Mo, Se and V from water soluble, exchangeable and acid-soluble fractions to more stable, reducible fractions in acidic fly ash. Large quantities of water soluble B in untreated fly ash (mainly alkaline) indicates its high mobility in the environment. FS treatment mobilized large quantities of B that did not get transferred to other stable fractions. FS treatment resulted in higher concentrations of Ni in the water soluble and exchangeable fractions of fly ash indicating increased mobility. The 1:30 FS treatment released large quantities of Sr from fly ash resulting in its higher mobility after treatment. The results of this study suggest that 1:30 FS treatment could significantly decrease the leachability of oxyanionic trace elements and therefore increase fly ash utilization for useful purposes.