Page 302 - Materials Chemistry, Second Edition

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Incineration of MSW 273
metal concentrations provide only a partial picture of the relative risk associated with heavy met-
als. Other methods are necessary to better understand the behavior of these metals in the biosphere,
for example, in a landfill, surface impoundment, or as fine dust attached to plant tissue.

9.6.3 LEACHABILITY OF METALS IN INCINERATOR ASH

In the high-temperature zone of the incinerator, metals are vaporized. For example, cadmium and
o
o
o
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mercury boil at 765 C (1412 F) and 355 C (674 F), respectively, temperatures that commonly occur
within the combustion chamber. As the combustion gases cool, these metals condense on to the sur-
face of fly ash particles. The concentrations of these condensed metals will increase with decreas-
ing ash particle size.
If ash containing metal-coated particulates comes into contact with ambient moisture, for exam-
ple, within the leachate of a landfill, some metals will become mobilized. The small particle size of
ash increases the surface area exposed to leaching, and the presence of metals at or near the surface
of such particles also enhances their leachability. Another factor contributing to metal leachability
in ash involves conversion into soluble salts. MSW contains significant amounts of chlorine from
plastics, bleached paper, and other sources. During incineration, the chloride ion combines with
metals to form metal chlorides:

−
M 2 2Cl → MCl 2 (aq) (9.20)
where M 2 represents the cationic metal species.
These salts are typically much more soluble in water than the original metallic form.
The leachability of metals in incinerator ash is one estimate of hazard, and relates specifically
to the potential for groundwater or surface-water contamination. Under federal law (40 CFR Part
261), leachability is assessed using the TCLP. To perform the test, a sample of ash is exposed to a
dilute acid solution for 16 to 20 h, and the concentrations of eight metals that leach into solution are
then measured. Most metals become more soluble under conditions of increasing acidity. If a min-
imum permissible limit of a metal in solution is exceeded, the ash (or other test material) must be
managed as a hazardous waste.
The slightly acidic conditions of the TCLP are designed to simulate conditions found in a typical
sanitary landfill. The majority of incinerator ash is disposed of in sanitary landfills along with unburned
MSW. As will be seen in Chapter 10, landfill leachate can become quite acidic as a result of microbial
decomposition processes. Metals in intimate contact with leachate will become solubilized to some
extent. Additionally, large amounts of incinerator ash are managed by even less-controlled means such
as being used as landfill cover, fill material in marshy areas, and deicing grit on winter roads.
The leachability of metals in incinerator ash is a function of numerous variables, for example,
the species of the metal. Lead occurring as PbCl is much more soluble in water than PbO or
2
Pb(OH) , for example (see Appendix). As mentioned above, a smaller ash particle size results in a
2
corresponding higher particle surface area. Metal occurring on a particle exterior will more rapidly
solubilize than the same metal occurring as a whole, intact particle, all other factors being equal.
The most important variable affecting metal mobility in ash, however, is the pH of the solution into
which the metal is leached. In leaching tests of ash from several U.S. incinerators, lead and cad-
mium leached at high levels, often in excess of regulatory standards defining a hazardous waste.
Such leaching occurred even when the tests were conducted using water rather than dilute acid
required in the TCLP (Vesilind et al., 2002).

9.6.4 ASH MANAGEMENT
As discussed above, ash is hazardous because it contains high levels of toxic metals and can also
contain chlorinated dibenzodioxins and polycyclic aromatic hydrocarbons. Ash mobility and toxic-
ity concerns have focused on the leachability of heavy metals in ash; in other words, how quickly