Abstract
The leaching (release) of large amounts of oxyanions, such as those of
arsenic and selenium, is an major environmental problem when it comes to
the disposal or use of coal fly ash. To predict environmentally safe
conditions for the disposal or use of fly ash in, for example, construction
materials, it is necessary to
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know the underlying leaching processes. The
processes which cause leaching of arsenic and selenium from coal fly ash
are studied in this thesis. Although combustion residues, such as coal fly
ash, show a high variability in elemental composition, the residues show a
systematic leaching behaviour. It is, therefore, also possible to use the
results of this thesis to obtain a better understanding of the leaching of
other combustion residues.
Arsenic and selenium can be present in different valency states in the
environment. The valency state influences the toxicity and mobility of the
oxyanions in the natural environment. Studies concerning the valency
state of As and Se in fly ashes are reviewed in Chapter 2. Mter testing
determination methods, the valency state was determined in leachates of
five different coal fly ashes at different pH values. The results showed that
As and Se leach preferentially as the oxyanions arsenate, As(V), and
selenite, Se(N). However, a relatively large amount of selenate, Se(Vl),
was found in the leachates of brown coal fly ash originating from a
fluidized bed boiler.
The leaching process is not only is controlled by the speciation in solution,
but also by the speciation on the solid (e.g. adsorbed species and minerals).
The leaching of major elements can be generally described by
precipitation/dissolution reactions. Trace element leaching is generally not
controlled by these reactions but is expected to be controlled by sorption
reactions. The possible controlling sorbents for As and Se are studied in
Chapter 3 by comparing the leaching of As and Se from an acidic fly ash
and an alkaline fly ash with the sorption on their oxyanions the minerals
hematite, mullite, portlandite and ettringite, which are generally present
in the fly ash matrix. Hematite is one of the principal iron bearing
minerals in fly ash. Mullite is an important crystalline aluminium silicate.
Portlandite (calcium hydroxide) and ettringite (calcium-aluminium
hydroxysulphate) may be formed during leaching of alkaline fly ashes. It was shown that the leaching of As and Se from acidic fly ash can be
described by sorption on an iron (hydr)oxide, such as hematite, and the
leaching from a fly ash by sorption on an alkaline calcium phase.
Following the results of Chapter 3, an attempt is made to model the
leaching of As and Se from acidic fly ash by sorption on iron (hydr)oxide in
Chapter 4. Extractions of the crystalline and amorphous iron (hydr)oxide
show that latter mineral is likely to dominate in controlling the leaching of
As and Se. The amount of As, Se and Fe involved in the sorption process,
are obtained from hydroxylamine extraction of amorphous iron (hydr)oxide
an from isotopic exchange. It is shown that the leaching of As and Se can
be described with an overall surface complexation model for the sorption of
As and Se on amorphous iron (hydr)oxide.
When sorption on iron (hydr)oxides controls the leaching of As and Se from
acidic fly ash the actual release of these elements is also dependent on the
reversibility of the sorption process. The sorption reversibility is studied on
different mineral phases in chapter 2. In Chapter 5, the reversibility of
arsenate, selenite and molybdate on amorphous iron (hydr)oxide and
hematite is investigated. Arsenate and molybdate sorb less reversible on
the iron (hydr)oxides while selenite sorbs reversible with respect to
changes in the pH. The pH at which the oxyanion is sorbed is crucial in
controlling its sorption reversibility on iron (hydr)oxides. Arsenate is less
reversibly sorbed than selenite on all the minerals studied. This behaviour
may well be the reason for the general observation that the availability of
As for leaching is less than that of Se.
Chapter 6 describes an investigation of the processes controlling As and
Se leaching from fly ash under natural conditions using mechanistic
insights developed in the earlier chapters of this thesis. For this purpose
samples of solids and pore water were taken form an 11 year old, naturally
weathered, fly ash at different depth in a lysimeter and were studied both
experimentally and by geochemical modelling. The potentially important
sorbents for As and Se, amorphous iron (hydr)oxide and alkaline Camineral
ettringite, are shown to be present and likely to be in equilibrium
with the pore waters. Lysimeter profiles, pH dependent leaching experiments and extraction of As, Se, and Fe in the weathered fly ash, as
well as the modelling results, confirm the findings in the earlier chapters;
As and Se leaching from the fly ash, especially in the upper section of the
lysimeters, is likely to be controlled by sorption on amorphous iron
(hydr)oxide. In the lower section of the lysimeter interaction with alkaline
Ca-minerals such as ettringite may contribute in controlling the leaching
process.
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