Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 389

1.2

Fig. 9.7 Simulated
concentration of o-xylene in
soil and roots as a function of
time, with a degradation rate
of 0.11 d − 1

1.0

0.8

0.6

0.4

0.2

0.0

0 10 20 30 40

Time (d)

soil root

with low K OW . Volatilization to air depends on K AW (or K OA ). Therefore, polar and
volatile contaminants (low K OW ,high K AW ) show shorter residence time in soil.
The actual dissipation rate, however, cannot be predicted from physico-chemical
properties alone but is an input data usually derived from experiments.

9.5.5 Impact of pKa and pH on Uptake of Ionisable Contaminants

Ionisable contaminants, i.e. acids, bases, zwitterions or amphoters, may be present
in soils as neutral or ionic molecules. The neutral and the ionic molecules have
completely different physico-chemical properties. The ion is usually much more
polar and water soluble and has a very low vapor pressure compared to the neutral
species. Thus, the ion is a polar and non-volatile contaminant, and as such has a
high potential for accumulation in plants. On the other hand, cations have a strong
tendency to adsorb to soil organic matter and/or clay (Franco and Trapp 2008 ).
Besides, charged contaminants cross biomembranes slowly (Trapp 2004 ), which
limits their uptake.
A process that may lead to high accumulation of ionisable contaminants is the
ion trap. This principle is described in Fig. 9.8 , for an acid. If the pH outside in
the soil pore water is low, a weak acid is at least partly neutral. The uptake into
the cell is rapid (provided that the contaminant is not too hydrophilic). The pH in
cell sap (cytosol) is about 7 to 7.5, in xylem about 5.5, and in phloem about 8. In
particular in cytosol and phloem, weak acids dissociate and form the anion. The
membrane permeability of the anion is very low, and reverse diffusion is slow. As
a consequence, the anion is trapped inside the cytosol or the phloem. The same
process occurs for a base, when the cell sap is acid, compared to the outside
soil pore water, for example in the vacuoles of plant cells and in the xylem (pH
is 5.5).
Summarizing, the ion trap requires a gradient in pH between outside and inside
of the plant, so that the outside pH is lower (for bases: higher) than the inside pH.
Second, a pKa at or somewhat below (for bases: above) the outside pH is neces-
sary, so that a relevant fraction of contaminants is neutral outside, but most of it is