Page 231 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 217

Soils that are rich in clay are also resistant to acidification through their ability to
adsorb the protons on clay mineral surfaces under release of metal ions, while sandy
soils are more sensitive to acidification. The sensitivity of an ecosystem towards
acidification can be described by its critical load—“A quantitative estimate of an
exposure to one or more pollutants below which significant harmful effects on
specified sensitive elements of the environment do not occur according to present
knowledge” (Nilsson and Grennfelt 1988). Critical loads are high in calcareous
regions like the Mediterranean and low in e.g. granite rock regions like most of
Scandinavia.
Incorporating the environmental mechanism described above, the impact path-
way of acidification is illustrated in Fig. 10.13.
Oceanic acidification is the process of dissolution of CO 2 into seawater leading
to a slight lowering of the pH in the open oceans as a consequence of increasing
concentrations of CO 2 in the atmosphere. Dissolution of CO 2 in water generates
carbonic acid, a rather weak acid (think soda water), which releases protons
according to

CO 2 þ H 2 O ! H 2 CO 3 ! HCO 3 þ H þ
The slightly lowered pH is deleterious to coral reefs, which should be included
in endpoint characterisation. CO 2 is the only important contributor to oceanic
acidification and inclusion of this impact category on midpoint level therefore offers
little additional information to the LCIA that already considers climate change, we
will hence not discuss it further here.

10.8.3 Emissions and Main Sources

Sulphur dioxides and nitrogen oxides are the man-made emissions that contribute
the most to acidification. Historically metal smelters of the mining industry have
been strong sources of local acidification with large localised emissions of sulphur
oxides. Today, the main sources of both SO x and NO x are combustion processes in
thermal power plants, combustion engines, waste incinerators and decentralised
furnaces. For sulphur oxides, the level of emissions depends on the sulphur content
of the fuels. Since nitrogen is abundant in the atmosphere and hence in all com-
bustion processes using air, emissions of nitrogen oxides are mainly determined by
conditions of the combustion process and possible treatment of the flue gases
through catalysers and filters. As response to the serious problems with acidification
in Europe and North America in previous times, regulation now ensures that sul-
phur content is removed from the fuels, that important combustion activities like
thermal power plants and waste incinerators have an efficient neutralisation of the
flue gases before they are released, and that combustion engines have catalysers
lowering the NO x content of the exhaust gases.