technical guidance documents - Institute for Health and Consumer ...

In a natural soil or sediment system, metals can be distributed over the following fractions:
• dissolved in the pore water,
• reversibly or irreversibly bound to soil or sediment particles,
• reversibly or irreversibly bound to organic ligands,
• encapsuled in secondary clay minerals and metal(hydr)oxides,
• encapsuled in the primary minerals.
APPENDIX VIII
It is recognised that for various organisms, only the metal species present in the aqueous phase
(pore water) are potentially available for direct uptake by biota and thus mainly responsible for
effects on biota. Other uptake routes may also be important, especially for metals with high Kp
values, but at the moment little is known on how to treat these processes quantitatively in the
risk assessment. Processes determining the availability of metals for direct uptake by biota from
the aqueous phase include precipitation, dissolution, adsorption, desorption and complexation.
All processes mentioned are not only pH-dependent (adsorption of metal cations for instance
increases with pH), but are also strongly influenced by competition for adsorption sites and to all
complexation reactions likely to increase the solubility of the metal.
At the moment most Kp values are expressed in terms of total concentrations present in both the
aqueous and the solid phase. As can be derived from the possible distribution sites for metals
mentioned above, availability of metals for uptake by biota can differ from site to site and, due to
amongst others weathering and (de)sorption processes, may change over time. At this stage it is
of importance to realise that in general the bioavailability of metals in test systems (expressed as
the fraction of the total amount of metal present in the system) may be higher than the
bioavailability under field conditions.
When performing risk assessment it is of utmost importance that both PEC and PNEC are based
on similar levels of availability. What is required is that for both exposure and effect assessment,
Kp values are expressed in terms of concentrations available for uptake by biota in both the
aqueous and the solid phase:
K p =
total available concentration in solid phase
concentration in aqueous phase
It is of importance to be aware that equation 1 differs from the commonly used expressions for
Kp in the sense that instead of total concentrations in both the solid and liquid phase, available
concentrations are to be used. Reason for this is that part of the metal present in the solid phase
may be incorporated in the mineral fraction and is therefore not available. Several experimental
extraction techniques have been developed to determine available concentrations of metals, thus
enabling the calculation of Kp values according to equation (1). However, up till now the
underlying concepts for a standardised approach towards partition coefficients representing
availability have not yet been sufficiently worked out.
Finally, with regard to availability of metals it should be noted that apart from the general
processes denoted above, under certain environmental conditions additional complexation and
precipitation processes may take place that may strongly diminish aqueous metal concentrations.
An example of such a process is the formation of insoluble metalsulphides under anaerobic
conditions (the so-called Acid Volatile Sulphide, or AVS-concept).
(1)
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