On the removal of hexavalent chromium from a Class F fly ash

摘要

Coarse and fine samples of a Class F fly ash obtained from commercial combustion of Illinois bituminous coal have been exposed to two long-term leaching tests designed to simulate conditions in waste impoundments. ICP-AES analysis indicated that the coarse and fine fly ash samples contained 135 and 171 mg/kg Cr, respectively. Measurements by XAFS spectroscopy showed that the ash samples originally contained 5 and 8% of the chromium, respectively, in the hexavalent oxidation state, Cr(Ⅵ). After exposure to water for more than four months, the percentage of chromium as Cr(Ⅵ) in the fly-ash decreased significantly for the coarse and fine fly-ash in both tests. Combining the XAFS data with ICP-AES data on the concentration of chromium in the leachates indicated that after the nineteen-week-long, more aggressive, kinetic test on the coarse fly ash, approximately 60% of the Cr(Ⅵ) had been leached, 20% had been reduced to Cr(Ⅲ) and retained in the ash, and 20% remained as Cr(Ⅵ) in the ash. In contrast, during the six-month-long baseline test, very little Cr was actually leached from either the coarse or the fine fly-ash (＜0.1 mg/kg); rather, about 66% and 20%, respectively, of the original Cr(Ⅵ) in the coarse and fine fly-ash was retained in the ash in that form, while the remainder, 34% and 80%, respectively, was reduced and retained in the ash as Cr(Ⅲ). The results are interpreted as indicating that Cr(Ⅵ) present in Class F fly-ash can be reduced to Cr(Ⅲ) when in contact with water and that such chemical reduction can compete with physical removal of Cr(Ⅵ) from the ash by aqueous leaching.