Agriculture Reference
In-Depth Information
Soil Equilibria
Equilibration of Organisms
with Their Environment
Redistribution within
Organisms
Environmental Availability
Environmental Bioavailability
Toxicological Bioavailability
C(eq)
K1
Circulation
Target(s)
Solid Phase
pH, %Clay, etc.
Storage
Organs(s)
C(0)
Liquid Phase
time
K
P
Toxicokinetic Parameters
Toxicodynamic Parameters
FIGURE 17.1
The three major processes in availability: (1) chemical availability of the compound bound to
mineral or organic soil material or present in the soil pore water; (2) biological uptake processes into an
organism (including exoenzymes of microorganisms); and (3) internal transport and processing (storage,
degradation, or effective entry in a target organ). (Adapted from Posthuma personal communication.)
Class A
Borderline
Class B
Period
2
3
4
Li
Na
K
Be
Mg
Ca
Cr
Mn
Fe
Co
Ni
Cu
Zn
Cu
5
6
Rb
Cs
Sr
Cd
Sn
Ag
Pb
Au
Hg
Pb
FIGURE 17.2
ClassiÝcation of metals according to biological working mechanisms (arranged according to
periodic table). Class A: Macronutrients forming O bonds; class B: forming N and S bonds. (From Nieboer
and Richardson 1980.)
roles in the overall effects. Nieboer and Richardson (1980) distinguished two groups of metals:
one that tends to bind metals to oxygen sites in biological ligands and act as macronutrients and
a second that forms strong bonds with N or S and exerts toxicity on earthworms by affecting protein
structure and enzyme function (Figure 17.2).
As a consequence, some heavy metals are regulated dynamically after uptake by earthworms
(e.g., Zn) whereas others such as Cd are accumulated continuously and become tightly bound and
immobilized. This accumulation is reinforced because of the low toxicity of Cd to earthworms.
This type of continuous accumulation of heavy metals that remain in earthworms after a transfer
to clean soil was observed for Cd by Van Gestel et al. (1993a). The metals in the group, which act
as macronutrients, however, may become toxic under certain doses, conditions, and end points. In
addition, Fischer and Molnar (1997) observed that Na, K, and Ba all affected earthworm cocoon
production negatively, possibly by inÞuencing soil/cocoon osmotic relations.
Toxic metals can interact with trace metals as well as metal macronutrients. Substitution
between these forms is an important process in this context, as demonstrated for Ca-Cd and Ca-
Pb interactions by Ma (1993). In general, invertebrates with a high Ca requirement actively
accumulate heavy metals. Janssen et al. (1998) showed that potassium affects the uptake of
Cs
but not of Na, whereas Morgan and OÔReilly (2002) and Oste et al. (2002) reported a competitive
inhibition of heavy metal accumulation by Mn.
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