Environmental Engineering Reference
In-Depth Information
Table 8.8
Maximum tolerable levels of metals/metalloids in plants in relation to maximum levels
tolerated by animals in forages (modified from Chaney
1983
)
Level in plant foliage
Maximum levels chronically tolerated
Soil-plant
barrier
Normal
Phytotoxic
Cattle
Sheep
Swine
Chicken
Metal(loid)
mg/kg
dw
mg/kg dry diet
Group 1
Cr
3+
yes
0.1-1
20
(3000)
(3000)
(3000)
3000
Group 2
As, inorg.
yes
0.01-1.0
3-10
50
50
50
50
Pb
yes
2-5
-
30
30
30
30
Group 3
B
yes
7-75
75
150
(150)
(150)
(150)
Cu
yes
3-20
25-40
100
25
250
300
Ni
yes
0.1-5
50-100
50
(50)
(100)
(300)
Zn
yes
15-150
500-1500
500
300
1000
1000
Group 4
Cd
Fails
0.1-1
5-700
0.5
0.5
0.5
0.5
Co
Fail?
0.01-0.3
25-100
10
10
10
10
Mo
Fails
0.1-3.0
100
10
10
20
100
Se
Fails
0.1-2
100
(2)
(2)
2
2
of such metals are Cr
3+
and also titanium, zirconium, tin, yttrium and silver. Group
2 comprises metals/metalloids that can be absorbed by roots, but are not readily
translocated to edible plant parts, like arsenic, mercury and lead. Since 1980, it
has become apparent that arsenic may be a food chain hazard in paddy rice pro-
duction systems (due to the redox-induced mobilization of arsenic as described in
Section
8.2.2
), but in urban residential scenarios, arsenic can be regarded as a low
hazard in terms of plant uptake. Group 3 comprises the elements boron, copper,
nickel and zinc, which are easily taken up by plants, but are phytotoxic to plants
before significant exposure to humans occurs. Group 4 are metals/metalloids which
most likely pose a food-chain risk, like cadmium, cobalt, molybdenum and sele-
nium, as plants can readily absorb and translocate these to edible portions, and they
are not highly phytotoxic.
It should be noted that this approach classifies elements on the basis of
transfer of contaminants via the soil-root-shoot/edible portion pathway. In urban
contaminated soils, aerial deposition of contaminants (Section
8.4.2
) provides a by-
pass of the 'soil-plant barrier', so that elements such as arsenic, lead and other
metals/metalloids may pose a risk to human or animal health where there is a
continuing source of metal/metalloid deposition to soil.
8.4.2 Vegetable Species
Vegetables vary widely in their ability to accumulate metals, either from soil or from
atmospheric deposition. Numerous studies have examined this in both glasshouse
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