Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 725

Table 16.1 Summary of the general characteristics that determine the kinetics of uptake and
depuration of organic contaminants, metals, and inorganic metal contaminants in biota

Organics Metals and metalloids

Tissue uptake is most commonly a blood flow
limited process, with linear partitioning into
tissues.

Metals and their complexes are often
ionized, with tissue uptake (membrane
transport), having greater potential to
be diffusion-limited or to use
specialized transport processes.

Metabolism is generally extensive and often
species specific.

Metabolism is usually limited to oxidation
state transitions and
alkylation/de-alkylation reactions.

Persistence in body fat is common, because of
lipid solubility (not capacity-limited).

Often sequestered , bound to specific
plasma or tissue proteins (intrinsically
capacity-limited), or deposited in inert
forms like waste nodules, mineral
concretions and granules.

May be eliminated by excretion in urine after
biotransformation from lipophilic forms to
hydrophilic forms, due to complex
metabolism.

Predominantly eliminated by excretion
because metal contaminants are
generally small molecules and are
hydrophilic. Thereupon excretion of
inert species.

Generally substance-specific homeostatic
mechanisms a are not available.

Essential metals have homeostatic
mechanisms that maintain optimum
tissue concentrations over a range of
exposure rates.

Interactions with other structurally similar
contaminants may occur, especially during
metabolism.

Interactions among metals and between
metals and organics are numerous and
occur commonly during the processes
of absorption, excretion, and
sequestration.

Taken from Campbell et al. ( 2006 )
a Homeostatic mechanisms are physiological mechanisms by which organisms maintain concen-
trations of essential elements inside their body within specific fixed levels. These fixes levels are
often termed the “window of essentiality”

16.3.1 Metals and Metalloids

For inorganic contaminants present as cations the bioavailability tends to decrease
from acid to neutral through alkaline conditions. Typically, metals are more soluble
under acidic conditions. However, over time, a highly acid soil is likely to loose
solution cations through leaching. As a consequence, in the longer term the con-
centration of bioavailable contaminants can be reduced by acidic conditions, unless
there is a significant source of contaminants to replenish those lost, for example the
gradual dissolution of metal-bearing materials. Loss of cations through leaching can
be offset by exchange processes. Cations held on exchange sites are normally con-
sidered to readily become bioavailable. As pH increases from acid to alkali, metal
ions are more likely to displace protons from exchange sites and be held in the soil in
a form that allows rapid replenishment of the reservoir of bioavailable contaminants