Environmental Engineering Reference
In-Depth Information
complexation by dissolved OM reducing K d values (Sauvé et al. 2000 ). This is
particularly important for copper.
8.4.4.4 Salinity
Salinity may play a role in enhancing uptake of cadmium from soil by vegetables.
Chloro-complexation of Cd 2+ ions reduces the charge of the cadmium ion (CdCl + or
CdCl 2 0 ) and hence increases its mobility through soil (Hahne and Kroontje 1973 ).
Chloro-complexation also increases the diffusive flux of cadmium to root surfaces
(Degryse et al. 2006a ; Smolders and McLaughlin 1996a , 1996b ). As a result of
this increased mobility in soil and increased diffusive flux of cadmium to root
uptake sites, cadmium concentrations in vegetables are increased when soil salinity
increases, and this has been demonstrated in field-grown crops (Helal et al. 1998 ;
McLaughlin et al. 1994a , 1997 ).
Theoretically, soil salinity should also have a large effect on uptake of mer-
cury, but this has not been demonstrated in any uptake studies by vegetables in
soil. Effects of salinity on the uptake of other metals should be minimal, and for
the anionic metals/metalloids, the chloride ion may actually inhibit uptake of these
ions, although this has not been demonstrated in practice.
8.4.4.5 Redox Potential
Redox potential, through the effects on reductive dissolution of Fe and Mn oxides
(Section 8.2.2 ), can have a marked influence on the uptake of several metals by
plants. For example, uptake of arsenic by rice and uptake of cobalt by pasture plants
(Adams and Honeysett 1964 ) has been found to be significantly enhanced under
low redox conditions, again due to reductive dissolution of Fe/Mn oxides, and in the
case of cobalt, reductive dissolution of insoluble Co 3+ to soluble Co 2+ .
There is little information in the literature on effects of low redox on metal uptake
by vegetables. It should be noted that most plants are not tolerant of waterlogged
soil conditions which create low redox potentials, and generally die due to lack of
oxygen. Redox may therefore not be a major risk factor for affecting metal uptake
by vegetables in urban gardens.
8.4.4.6 Nutrient Status
It is well known that soil nutrient status can influence metal uptake by plants, and
the effects can operate in several ways. Low levels of addition of nitrogen and phos-
phorus in soil have been shown to enhance cadmium uptake by crops (Grant and
Sheppard 2008 ; Maier et al. 2002a, b ; Williams and David 1973 ). Higher rates
of addition of N fertiliser are likely to acidify soils and lead to enhanced uptake
of cationic metals (Eriksson 1990 ; Grant et al. 1996 ; Lorenz et al. 1994 ), while
high rates of P addition may reduce uptake of cationic metals through precipi-
tation reactions (McGowen et al. 2001 ). High P addition, however, may promote
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