Environmental Engineering Reference
In-Depth Information
The sludge is a synthetic one, prepared by adding calcium hydroxide to a mixture
of nitrates of the metals nickel, cadmium, chromium, and mercury.
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None of the
calcium hydroxide could be presently detected in the XRD pattern of the sludge,
whereas several calcium carbonate peaks could be found. Thus, any remaining
calcium hydroxide had carbonated on exposure to air. The remaining peaks could
not be matched with simple hydroxides of the metals in the sludge. A careful analysis
of the XRD pattern in the region 10° to 13° 2θ shows that there are really two highly
overlapping peaks. These peaks, corresponding to 0.877 and 0.807 nm d-spacings,
are very broad. Below 1 μm grain size, the peak width increases with decreasing
grain size.
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The widths of these two peaks suggest that these particles are much
less than 1 μm across. Considering the chemistry of the sludge, these peaks appear
to be from the very fine-grained or poorly crystalline heavy metal precipitates in the
sludge. The very small grain size of the precipitates would indicate that they are
probably quite unstable and would also dissolve easily.
The XRD pattern of the control sample (0% zinc nitrate) indicates the presence
of calcium hydroxide, gypsum, ettringite, quartz, calcium carbonate, and iron oxide.
In addition, there is a broad peak corresponding to 0.789 nm d-spacing, and another
unidentified peak corresponding to 1.054 nm. Calcium hydroxide came from the
binder, and ettringite is a reaction product between the Class C fly ash and calcium
hydroxide. Quartz, gypsum, and iron oxide were present in the fly ash. A comparison
of the XRD patterns with increasing zinc nitrate content shows some systematic
changes. No calcium hydroxide is observed when 5% or more zinc nitrate is present.
Ettringite is present irrespective of zinc nitrate concentration. Zinc nitrate reacted
with calcium hydroxide, but no simple zinc hydroxide could be detected in the XRD
patterns. The broad sludge peak could be seen in the 0% and 2% zinc nitrate samples
but not at higher concentrations. A stronger, sharper peak corresponding to 0.812
nm is seen in 5% and 8% samples, but it is difficult to assign this peak to any
particular phase. The peak at 1.054 nm, found in the control, is diminished in the
5% sample and is absent in the 8% sample. This peak could be from a calcium
silicate hydrate type phase from the binder.
The above discussion shows that the XRD patterns of waste forms can be very
complex. A waste form is very likely to be a multi-phase material. When cementitious
materials are present, poorly crystalline phases, impure phases, and solid solutions
between phases can make phase identification difficult. Peak overlap of various
degrees makes this task even more difficult. Some prior chemical knowledge about
the binder can make phase identification somewhat easier. Ideally, three or more
peaks of a phase need to be identified in a pattern for its definite detection. Prior
knowledge may allow identification of a phase with one peak. For example, very
often one and at the most two peaks of ettringite are found in the XRD pattern of
a cementitious material. Since ettringite is expected in such a system, its identifica-
tion by one or two peaks is easier.
Even from the complex XRD patterns in Figure 9.10, where several peaks remain
unidentified, important conclusions can be reached. The absence of calcium hydrox-
ide at zinc nitrate concentration 5% and higher indicates that the pH value of the
waste form was lowered by its presence. The pH, however, remained alkaline as
seen by the presence of ettringite. The presence of ettringite also indicates that the
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