Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 330

Table 8.3 Physical and chemical properties of the anionic metals/metalloids

Dominant species in soil solution

Dominant
species in
soil Element Symbol Valence pH 3.5-6.0 pH 6.0-8.5

As 3+ ,As 5+ H 2 AsO 4 - ,

H 3 AsO 3

HAsO 4 2-

Arsenic As - 3,0,3,5

B 3+

Boron B 3 B(OH) 3 B(OH) 3,

B(OH) 4 -

Cr 3+ ,CrO 4 2- Cr 3+ ,CrOH 2+ Cr(OH) 4 - Chromium Cr 2,3,6

Mo 6+ HMoO 4 - ,

MoO 4 2- ,
H 2 MoO 4

MoO 4 2- Molybdenum Mo 2,3,4,5,6

Se 0 ,Se 4+ ,
Se 6+

SeO 4 2- ,
HSeO 3 -

SeO 4 2- ,
SeO 3 2-

Selenium Se 0,2,3,4,6

and hence they may have relatively low K d values compared to the cationic metals.
Hence these elements are much more readily available for plant uptake from soil
compared to cationic metals with high K d values (e.g. lead). Exceptions to this
rule are those anions that can form strong bonds through ligand-exchange with soil
minerals, e.g. phosphate, fluoride, but there are few metals or metalloids with this
property. Arsenate is one of the few metalloids that may associate with soil minerals
through ligand exchange reactions, and hence have stronger binding than would be
suggested by consideration of charge alone.
In contrast to cationic metals, the oxyanions have higher K d values in acidic soils
compared to alkaline soils, and different redox species of the same metal/metalloid
mayhavewidelydiffering K d values (Bartlett and James 1988 ;Bowell 1994 ;
Goldberg 1997 ; Goldberg and Forster 1998 ; Nakamaru et al. 2005 ). Arsenic
is a good example, where As 5+ , usually present in soils as the arsenate ion
H x AsO 4 − (3 − x) generally has higher K d values in soil than As 3+ , present as H 3 AsO 3
(Bowell 1994 ).
Of the metals normally found at contaminated sites, chromium perhaps is unique
in that a valency change causes charge reversal, and this markedly affects not only
chromium partitioning (Bartlett and James 1988 ), but also toxicity (McGrath 1982 ),
with the Cr 6+ species being more available and toxic compared to Cr 3+ .

8.2.3 Effects of Soil Redox

As outlined above, changes in soil oxidation/reduction potential have the capability
to alter the valence state of metal/metalloids present in soil. However, soil redox has
a more important effect on metal chemistry in that the major components of soil
which are active in metal/metalloid retention, Fe and Mn oxyhydroxides, are also
redox sensitive principally through the Fe 2+ /Fe 3+ and Mn 2+ /Mn 4+ couples. Both
Fe 2+ and Mn 2+ species are much more soluble than their oxidised species, so that