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2.3.6.1 Hydrolysis
ENVIRONMENTAL EXPOSURE ASSESSMENT
Values for the half-life (DT50) of a hydrolysable substance can be converted to degradation rate
constants, which may be used in the models for calculating PEClocal and especially
PECregional. The results of a ready biodegradability study will show whether or not the
hydrolysis products are themselves biodegradable. Similarly, for substances where DT50 is less
than 12 hours, environmental effects are likely to be attributed to the hydrolysis products rather
than to the parent substance itself. These effects should also be assessed. QSAR methods are
available for certain groups of substances, e.g. the EPIWIN program (US EPA, 2002) and other
methods described in Chapter 4.
For many substances, the rate of hydrolysis will be heavily dependent on the specific
environmental pH and temperature and in the case of soil, also moisture content. For risk
assessment purposes for fresh water, sediment and soil, a pH of 7 and a temperature of 12°C
(285 K) will normally be established which conform to the standard environmental parameters of
Table 5. However, for some substances, it may be necessary to assume a different pH and
temperature to fully reflect the potential of the substance to cause adverse effects. This may be of
particular importance where the hydrolysis profile shows significantly different rates of
hydrolysis over the range pH 4 - 9 and the relevant toxicity is known to be specifically caused by
either the stable parent substance or a hydrolysis product.
Rates of hydrolysis always increase with increasing temperature. When hydrolysis half-lives
have been determined in standard tests, they should be recalculated to reflect an average EU
outdoor temperature by the equation:
DT50(
X
( 0.
08⋅(
T − X ))
° C)
= DT50(
t)
⋅ e
(25)
where X = 12°C for fresh water. When it is documented for a specific substance that the typical
pH of the environmental compartment to be assessed also affects the hydrolysis rate in addition
to temperature, the most relevant hydrolysis rate should be taken or extrapolated from the results
of the standard test in different pH values. Thereafter the temperature correction is to be applied,
where relevant.
When the use of an alternative pH will affect the environmental distribution and toxicity by
changing the nature of the soluble species, for example with ionisable substances, care should be
taken to ensure that this is fully taken into account when making a final PEC/PNEC comparison.
The half-life for hydrolysis (if known) can be converted to a pseudo first-order rate constant:
Explanation of symbols
ln 2
khydr water =
DT50 hydrwater
DT50hydrwater half-lifetime for hydrolysis in surface water [d] data set
khydrwater first order rate constant for hydrolysis in surface water [d -1 ]
(26)
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