Environmental Engineering Reference
In-Depth Information
PGEs occur in nickel (Ni), copper (Cu), and marcasite ore beds (Bradford 1988).
They belong to the noble metal group, and their reactivity is low. PGEs occur in
nature as primary alloys consisting mainly of Pt. The specific features of these ele-
ments, which make them widely applicable, are as follows (Bernardis et al. 2005):
high resistance to chemical corrosion over a wide range of temperatures, high melt-
ing point, high mechanical resistance, and high plasticity.
3
Production and Application of PGEs
Presently, PGMs are produced from appropriate ores that are mined in South
America, Siberia, and Sudbury (Ontario, Canada); their production has been stead-
ily increasing, since 1970, as a result of growing worldwide demand (World Health
Organization, WHO 1991). Total world demand in 2000 and 2006 reached 166.7 ×
10
3
and 215.0 × 10
3
kg for Pt, 260.5 × 10
3
and 206.1 × 10
3
kg for Pd, and 25.3 ×
10
3
and 28.6 × 10
3
kg for Rh, respectively (Johnson Matthey 2007). Figure 1
presents world demand by use category for the yr 2000 and 2006.
PGEs have exceptional catalytic qualities, which renders them particularly useful
in industrial catalytic devices. Moreover, these metals have found application in
various other sectors of industry, such as (Pyrzyñska 1998; Resano et al. 2007;
Brook 2006) chemicals, petrochemicals, electrical, and electronics, glass produc-
tion, jewelry production, the medical sector (e.g., components of anticancer drugs,
medical silicone gels, and elastomers), and in dentistry.
4
Environmental Emission Sources of PGEs
The broad application of PGEs in various industries has increased emissions of
these metals to the environment; in this regard, emissions from vehicle catalytic
converters and hospital wastewater discharges are particularly significant.
4.1
Emission of PGEs from Vehicle Exhaust Catalysts
Catalytic converters were first fitted to cars in the USA and Japan in the mid-1970s (in
response to new emission standards, such as the US Clean Air Act Amendment of
1970), and in Europe at the beginning of the 1980s (Benemann et al. 2005). These
catalysts primarily utilize a mix of Pt and Pd, and have the function of catalytically
converting carbon monoxide (CO) and hydrocarbons (HCs) to carbon dioxide (CO
2
)
and water. Because oxidation catalysts have little effect on NO
x
s, the new air standards
resulted in the development and introduction of “three-way catalysts.” Such catalysts
simultaneously oxidize CO and HC while reducing NO
x
s to nitrogen (Twigg 2007).
The most common three-way catalysts fitted to cars in the 1980s contained Pt and Rb
in a 5:1 ratio, Rb playing an important role in promoting the reduction of NO
x
s.
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