Page 279 - Materials Chemistry, Second Edition

4.5 Impact Categories, Impact Indicators and Characterisation Factors 263

4.5.2.6.2 Indicator and Characterisation Factor The impact indicator for aquatic
eutrophication is the undesired formation of biomass in limnic (lakes, ponds and
rivers) and marine ecosystems (estuaries, brackish water seas and open ocean) by
an entry of fertilising substances into the environment.
A specification of a relative fertilisation effect of P and N is at the centre of
the definition of an equivalent or characterisation model. In the table of CML 256)
(Table 4.15) equivalence factors similar to those of the AP are made according to
simple stoichiometric calculations. Derivations of factors are based on an average
composition of algae biomass according to the relation C : N : P = 106 : 16 : 1,
named Redfield-relation 257) due to its revelation by Alfred Redfield (1890–1983),
valid to a surprising accuracy in Deep Oceans:
C 106 H 263 O 110 N P
16
A question arises with respect to the contribution of a nutritive substance (X)
to the formation of algae biomass by photo synthesis if it contains the limiting
element and if all other elements in biologically available form are presumed to be
abundantly present.
X + substrate, trace elements,ℎ →
(algae biomass = C H O N P) (4.22)
106 263 110 16
With this definition of EP values of all P- and N-containing compounds can
unambiguously be stoichiometrically calculated. This directness is the charm
of the method which disregards all local restrictions and derives a potential
impact – similar to an AP – from the chemical formula alone. Of course it has
to be reassessed with scientific prudence whether the regarded compound that
can actually provide the nutritive element, is biologically available! There is no
taking into account of regional composition of the water bodies or of observed
discrepancies in the Redfield-relation.

Sample Calculation

If X = P (a molecule or an ion with a bioavailable P-atom), = 1, thus 1 mol of
−1
P(M = 31 g mol ) causes the formation of 1 mol alga biomass of an average
−1
composition C H O N P(M = 3550 g mol ).
106 263 110 16
A molar ratio n (P)/n (algae biomass) = 1/1 results to
m(P)× M(algae biomass)
m(algae biomass)=
M(P)
For 1 kg P thus 114.5 kg algae biomass is calculated.

256) Heijungs et al. (1992), Kl¨ opffer and Renner (1995), Hauschild and Wenzel (1998) and Guin´ ee
et al. (2002).
257) Redfield (1934), Redfield, Ketchum and Richards (1993) and Samuelsson (1993);
http://de.wikipedia.org/wiki/Redfield-relation