Environmental Engineering Reference
In-Depth Information
(both qualitative and quantitative) in seconds. The backscattered electrons in SEM
can be used to map the compositional contrast. Selected area electron diffraction in
the TEM can provide crystal structure information from a spot a few μm in diameter.
All of these techniques have been utilized to study waste forms.
Though many forms of microscopy are used to study the microstructure of a
waste form, SEM is the dominant one. This is because SEM sample preparation is
relatively easy; it provides a high depth of field compared to other techniques. SEM
has been widely used to examine the microstructure of S/S waste forms. Some SEM
representative photomicrographs of a waste form composed of lime and ASTM Class
C fly ash, containing an electroplating waste, with 8% by weight of Pb(NO
3
)
2
, are
shown in Figures 9.2 and 9.3. At very low magnification (Figure 9.2, magnified
108X) the waste form appears monolithic with some voids. Some of the voids are
due to trapped air, as these are spherical in shape. Cracks of various lengths are
common, some of them extending into the voids. Various lines of evidence suggest
that the cracks are original and not induced by the high vacuum of the SEM
observation chamber. Outlines of fly ash particles are seen in some areas. Some of
the voids are encrusted with very fine particles. Figure 9.3 (magnification 5300X)
shows one of the encrustations of one of the voids (below letter X in Figure 9.2).
The encrustation consists of very small equi-dimensional crystals, between 5 to 10
μm in diameter. Some of the crystal faces are visible. The morphology of the crystals
is unlike that of any of the common cement hydration phases. EDX from that spot
showed that these crystals are composed of Pb, and showed no other peak so the
anion could not be determined. This series of waste form contained Pb(NO
3
)
2
in
various amounts, 0%, 2%, 5%, and 8% by weight. The Pb precipitate could be
observed only when the amount of Pb(NO
3
)
2
reached 8%. In this experiment, micro-
structural observations were the key to understanding the behavior of Pb(NO
3
)
2
in
this system.
Phosphate-based waste forms have been proposed for both radioactive
4
and non-
radioactive wastes
5
as discussed in Chapter 6. Phosphate minerals apatite
FIGURE 9.2
Photomicrograph of electroplating waste with 8 wt% Pb(NO
3
)
2
stabilized in
lime-fly ash (108×).
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