Environmental Engineering Reference
In-Depth Information
Table 8.7
A sensitivity analysis of predicted atmospheric contribution to plant metal con-
centrations, using current air quality conditions in rural and contaminated areas. Assumed air
accumulation factors (AAF: plant:air concentration ratio, m
3
/g
dw
) and soil-plant bioconcentra-
tion factors (BCF, dry weight based) for lettuce are based on experimental data of Harrison
and Chirgawi (
1989
). The 4 scenarios of soil and air concentrations are based on expert judge-
ment, reflecting current air concentrations in rural areas in EU and background soil concentration
for lowest values, and air concentrations near metal smelters and contaminated soils, for high
concentrations
Soil metal
conc.
mg/kg
dw
Predicted plant
metal conc.
(mg/kg
dw
)
Air conc.
(ng/m
3
)
Percent of
airborne
AAF
(m
3
/g)
BCF (-)
Cd
20
1.5
0.2
0.2
0.3
1
20
1.5
20
0.2
0.7
57
20
1.5
0.2
2
3.0
0.1
20
1.5
20
2
3.4
12
Pb
10
0.006
10
20
0.2
45
10
0.006
200
20
2.1
94
10
0.006
10
200
1.3
8
10
0.006
200
200
3.2
62
8.4 Integrating Factors Affecting Metal/Metalloid Accumulation
by Vegetables
There are a number of metal/metaloid-specific, soil and plant factors that can affect
the accumulation of metals/metalloids by vegetables in urban gardens (Chaney
1973
). Much of the information in this area is derived from studies of the use of
sewage biosolids on land, with only a few studies examining contaminant uptake
by vegetables in soils contaminated by industrial, mining or urban sources of con-
taminants. The most important factors controlling metal/metalloid accumulation by
vegetables are outlined in the sections below.
8.4.1 Type of Metal/Metalloid
From the preceding sections, it is evident that the behaviour of metals/metalloids in
urban/industrial soils and their uptake by plants are highly metal/metaloid specific.
Cationic elements are generally more firmly held by soils than anions, and plants
accumulate cations more readily than anions due to the negative electrical potential
across the root membrane. Combining these properties of metals/metalloids, Chaney
developed the 'soil-plant barrier concept' (Chaney
1980
) where metals/metalloids
were classified into groups for food-chain Risk Assessment. The classification of
metals/metalloids depends on their partitioning behaviour in soil, their propensity
for root uptake, and their propensity to accumulate in edible portions of plants in
relation to critical phytotoxicity concentrations
versus
critical food concentrations
for humans (Table
8.8
). Using this concept, metals/metalloids which are highly
insoluble, or are retained very strongly by plant roots, are in Group 1. Examples
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