Page 229 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 215

some areas due to the extensive use of coal-fired power generation using
sulphur-rich coal.

10.8.2 Environmental Mechanism

Acidification of soil or aquatic ecosystems can be defined as an impact which leads
to a fall in the system’s acid neutralising capacity (ANC), i.e. a reduction in the
quantity of substances in the system which are able to neutralise hydrogen ions
added to the system.
ANC can be reduced by:
1. Addition of hydrogen ions, which displace other cations which can then be
leached out of the system
2. Uptake of cations in plants or other biomass which is collected and removed
from the system
Particularly the former is relevant for acidification impacts in LCA. Acidification
occurs naturally over time, but it is greatly increased by man-made input of
hydrogen ions to soil and vegetation. The main source is air-borne emissions of
gases that release hydrogen when they are degraded in the atmosphere or after
deposition to soil, vegetation or water. Deposition is increased during precipitation
events where the gases are dissolved in water and come down with rain, which can
be rather acidic with pH values down to 3–4 in cases of strong air pollution (“acid
rain”).
The most important acidifying man-made compounds are:
Sulphur oxides,SO 2 and SO 3 (or jointly SO x ), the acidic anhydrides of sulphurous
acid H 2 SO 3 and sulphuric acid H 2 SO 4 , respectively, meaning that upon absorption
of water from the atmosphere they form these very strong acids which both release
two hydrogen ions when deposited:
þ 2
SO 2 þ H 2 O ! H 2 SO 3 ! 2H þ SO 3
þ 2
SO 3 þ H 2 O ! H 2 SO 4 ! 2H þ SO 4
Nitrogen oxides,NOand NO 2 (or jointly NO x ) that are also acidic anhydrides as
they can be converted to nitric and nitrous acids by oxidation in the troposphere.
NO is oxidised to NO 2 primarily by reaction with ozone (see Sect. 10.10):

NO þ O 3 ! NO 2 þ O 2

NO 2 can be oxidised to nitric acid, HNO 3 or HONO 2 :