Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 218

The OH -1 band (3643.8 cm -1 ) from calcium hydroxide is very strong in the
lime-fly ash mixture. The OH -1 peak persists in the presence of sludge without any
copper nitrate and even when 2% and 5% by weight of copper nitrate has been added.
Many strong bands from calcite (e.g., 1432 cm -1 and 874 cm -1 ) are present in
the LFA mixture. In the presence of the sludge, with or without copper nitrate, the
carbonate band around 1400 cm -1 is broader. It is the broadest with 8% by weight
copper nitrate. Vaterite has a very wide band in this region, and it appears that the
presence of the waste is aiding vaterite formation. Since copper is a significant
component of the system, formation of copper carbonate phases, such as malachite
or azurite, is a possibility. FTIR did not show any match with the spectra of these
minerals, and the green color of malachite, commonly found on copper objects due
to weathering, was not observed in the waste form. The peaks around 2400 cm -1
are from the atmospheric carbon dioxide in the sample compartment.
FTIR is a widely used complementary analytical tool for waste form character-
ization. Mollah et al. 41 observed that curing of a waste form in a carbon dioxide-
rich atmosphere polymerized the silicate matrix and also altered the nature of the
waste species. Van Jaarsveld and van Deventer 42 used FTIR to show that Cu and Pb
are incorporated in the silicate (geopolymer) binder of a waste form. Jing et al. 43
used FTIR to monitor the fate of adsorbed arsenic on iron hydroxide sludge. Arsenic
was adsorbed on the sludge before treatment, but precipitated as calcium arsenate
when mixed with portland cement. The FTIR band for arsenic moved from 830 cm -1
to 860 cm -1 .

9.6 EXAMPLES OF SOLIDIFICATION/STABILIZATION

The application of several complementary techniques for characterization of a waste
form is much more powerful than any technique alone, and some typical examples
are discussed in this section.

9.6.1 B EHAVIOR OF C HROMIUM IN A S IMULATED W ASTE F ORM

Omotoso et al. and others 44-46 studied the behavior of chromium, in different valence
states, in a simple matrix (tricalcium silicate). The premise for choosing tricalcium
silicate is that it is the most important component of portland cement. A range of
analytical techniques, including quantitative XRD, SEM, TEM, and EDX, were used
to monitor the hydration process of tricalcium silicate in the presence of Cr III and
Cr VI solutions.
Contrary to some earlier reports, they found that Cr III was precipitating as some
Ca-Cr complexes. These complexes recrystallized into calcium hydroxide and a
different Ca-Cr complex when abundant moisture was present. Otherwise, it
remained unaltered. Leaching of this model waste form led to the removal of large
quantities of Ca but little Si or Cr. Apparently, the high pH of the waste form
converted the Cr to an oxide or hydroxide form, which kept the Cr solubility low.
The behavior of Cr VI was very different. With its mixing with tricalcium silicate,
a calcium chromate hydrate phase (Ca 2 CrO 5 .3H 2 O) precipitated. The calcium chro-
mate hydrate phase is highly soluble, and most of it leaches out easily. FTIR indicated