Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 358

tubers grown in field soils, and again found that consideration of free ion activ-
ities did not improve prediction of metal accumulation in tubers. This is not to
say that knowledge of elemental speciation is not important in explaining metal
accumulation by plants, but the analytical effort of separating and determining free
metal ion activities in pore water may not be justified in terms of improving pre-
dictions of metal uptake by plants. This is more fully explained in Nolan et al.
( 2003 ).
8.5.1.4 Biotic Ligand Model
Originally the BLM described metal uptake and toxicity as a function of bind-
ing to certain biotic ligands present on surfaces of aquatic organisms (Di Toro
et al. 2001 ). The applicability of the BLM has been studied for a range of aquatic
organisms and heavy metals (lead, copper, zinc, nickel) and has shown potential
to be used to define water quality standards (Comber et al. 2008 ). Toxicity has
been described for soils using a BLM approach for copper and nickel toxicity to
barley (Thakali et al. 2006 ), or nickel toxicity to oat (Weng et al. 2003 ; 2004 ).
However zinc uptake by algae could not be predicted by surface-bound zinc or
solution zinc chemistry, probably due to internal regulation of zinc (Hassler and
Wilkinson 2003 ). Also the competition between different cations in a BLM could
not improve the prediction of cadmium and zinc uptake by Lolium perenne (Hough
et al. 2005 ), or describe the competition between copper and lead and zinc on algae
(Hassler et al. 2004 ).
A problem of the FIAM and the BLM is that if these models are used to pre-
dict effects of various cations on metal uptake by roots (Cheng and Allen 2001 ),
one still needs to describe the translocation of the metal from root to shoot or fruit
with a translocation coefficient, as was done by Cheng and Allen ( 2001 ) for copper.
However, translocation of metals from root to shoot is not constant, especially not
for essential metals such as copper and zinc (Kalis et al. 2006 ).
8.5.1.5 Physiological Models
Uptake of any contaminants from soil is related to the water uptake and, for some
contaminants also the concentration or free ion activity in the pore water (Ingwersen
and Streck 2006 ; Peijnenburg et al. 2000 ). In models for neutral organic xenobiotics
it is often assumed that uptake is a passive process related to water transpiration
(see Chapter 9 by Trapp and Legind, this topic). Ultimately, the metal content of
plant tissues is a function of plant growth, water transpiration, metal concentra-
tion or speciation in the pore water, selectivity/ion competition effects at the root
surface and diffusional limitations to uptake. An advantage of such models is that
they can explain differences in metal uptake between years based on differences in
weather. In practice this still seems to be rather complex, because other parameters
(e.g. root depth) also vary between years (Ingwersen and Streck 2006 ). The phys-
iological model can be extended to include competitive effects of other cations,