Stabilization of air pollution control residues by coprecipitation with ferrous iron and subsequent thermal treatment
(at 600 and 900 °C) has been examined as a means to
reduce heavy metal leaching and to improve product stability.
Changes in mineralogy and metal binding were analyzed
using various analytical and environmental techniques.
Ferrihydrite was formed initially but transformed upon thermal
treatment to more stable and crystalline iron oxides
(maghemite and hematite). For some metals leaching studies
showed more substantial binding after thermal treatment,
while other metals either volatilized or destabilized with
respect to leaching. Pb, in particular, exhibited increased
reactivity following the formation of an ordered iron
oxide structure at 900 °C. The thermal treatment had a
positive effect on Cr release, which was reduced significantly
at 900 °C in the presence of organic matter. Thermal
treatment of the stabilized residues produced structures
with an inherently better iron oxide stability. However, the
concentration of metals in the leachate generally increased
as a consequence of the decreased solubility of metals in
the more stable iron oxide structure.
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