Page 246 - Materials Chemistry, Second Edition

232 R.K. Rosenbaum et al.

characterisation modelling this leads to the use of fate models assuming steady-state
conditions.
The fate model predicts the chemical behaviour/distribution in the environment
accounting for multimedia (i.e. between environmental media and compartments)
and spatial (i.e. between different zones but within the same compartment or
medium) transport between environmental compartments (e.g. air, water, soil). This
is accomplished via modelling of (thermodynamic) exchange processes such as
partitioning, diffusion, sorption, advection, convection—represented as arrows in
Fig. 10.18—as well as biotic and abiotic degradation (e.g. biodegradation,
hydrolysis or photolysis), or burial in sediments. Degradation is an important loss
process for most organic substances, but may also lead to toxic breakdown com-
pounds. The rate by which the degradation occurs can be derived from the half-life
of the substance in the medium and it depends both on the properties of the
substance and on environmental conditions such as temperature, insolation or
presence of reaction partners (e.g. OH radicals for atmospheric degradation). The
basic principle underlying a fate model is a mass balance for each compartment
leading to a system of differential equations which are solved simultaneously,
which can be done for steady-state or dynamic conditions. A life cycle inventory
typically reports emissions as masses emitted into an environmental compartment
for a given functional unit. The mathematical relationship between the steady-state
solution for a continuous emission and the time-integrated solution for a mass of
chemical released into the environment has been demonstrated (Heijungs 1995;
Mackay and Seth 1999).
Figure 10.18 shows the overall nested structure of the USEtox model which is a
widely used global scientific consensus model for characterisation modelling of
human and ecotoxic impacts in LCA. Further details on fate modelling principles in
the USEtox model can be found in Henderson et al. (2011) and Rosenbaum et al.
(2008).
Exposure is the contact between a target organism and a pollutant over an
exposure boundary for a specific duration and frequency. The exposure model
accounts for the fact that not necessarily the total (‘bulk’) chemical concentration
present in the environment is available for exposure of organisms. Several factors
and processes such as sorption, dissolution, dissociation and speciation may
influence (i.e. reduce) the amount of chemical available for ecosystem exposure.
Such phenomena can be defined as bioavailability (“freely available to cross an
organism’s cellular membrane from the medium the organism inhabits at a given
time”), and bioaccessibility (“what is actually bioavailable now plus what is
potentially bioavailable”).
The effect model characterises the fraction of species within an ecosystem that
will be affected by a certain chemical exposure. Effects are described quantitatively
by lab-test derived concentration-response curves relating the concentration of a
chemical to the fraction of a test group that is affected (e.g. when using the EC50—
the Effect Concentration affecting 50% of a group of individuals of the same test
species compared to a control situation). Affected can mean various things, such as
increased mortality, reduced mobility, reduced growth or reproduction rate,