Environmental Engineering Reference
In-Depth Information
Certain organics are not solely degraded by bac-
teria, but instead utilize the enzymatic pathways
of other plant symbionts such as fungi. In addi-
tion to soluble exudates, the rapid decay of fine
root biomass can become an important addition
of organic carbon to soils which serves to re-
tard organic chemical transport. Microbial min-
eralization of atrazine is directly related to the
fraction of organic carbon in the soil (Zabloto-
wicz et al.
2006
). Rhizofiltration is effective and
economically utilized under low concentrations
of contaminants and large volumes of water,
therefore particularly applicable to radionuclide-
contaminated water. The cationic and anionic
radionuclide contaminants are substantially or
completely removed from water using selective
metal-accumulating plants under an optimized
rhizofiltration system, although mechanism of
uptake is not fully studied (Macaskie
1991
).
contaminants can also be confined in place by
phytostabilization, and result in significant risk
reduction under small half-lived contaminants.
Soil amendments such as phosphate, lime and
organic matter are sometimes needed to immo-
bilize toxic metals such as lead, cadmium, zinc
and arsenic. Cadmium is readily translocated to
leaves in many plants, which represents a risk to
the food chain, and this pathway may be the lim-
iting consideration in applying phytostabilization
at some metal-contaminated sites.
5.3.4 Phytoextraction
Phytoextraction refers to the use of metal-accumu-
lating plants that translocate and concentrate met-
als from the soil to roots and shoots or leaves. It has
been used effectively at brownfield sites with rela-
tively low-level lead and cadmium contamination
and proposed for extraction of radionuclides from
sites with mixed wastes (Mulligana et al.
2001
).
Phytoextraction offers significant cost advantages
over alternative schemes of soil excavation and
treatment or disposal. The issue that needs to be
considered in phytoextraction is whether the met-
als can be recovered economically from the plant
tissue or directly disposed as waste. Design con-
siderations include the accumulation factor (ratio
of metal in the plant tissue to that in the soil) and
the plant productivity (kilogram of dry matter that
is harvestable each season). In order to use phyto-
extraction as regular practice, one needs a vigor-
ously growing plant (> 3 tons dry matter/ha year)
accumulating large concentrations of metal in the
harvestable portion (> 1000 mg/kg metal) which is
easy to harvest. Metals like cadmium, nickel, zinc,
arsenic, selenium and copper are generally consid-
ered to be readily bioavailable for phytoextraction.
Moderately bioavailable metals are cobalt, manga-
nese and iron; while lead, chromium and uranium
are not readily bioavailable. Bioavailability of lead
can be enhanced by greatly adding ethylenediami-
netetraacetic acid (EDTA) to soils. The disadvan-
tage with this technology is longer time require-
ment than other technologies; thus many crops are
usually required to reduce all the contaminants to
the desired levels.
5.3.3 Phytostabilization
Phytostabilization refers to stabilization of heavy
metal contaminants in soil and sediments through
revegetation with metal-tolerant plant species.
Generally heavy metal-polluted soils lack veg-
etation cover due to the toxic effects of pollut-
ants, which makes such soil prone to erosion and
leaching, leading to the spread of pollutants in the
environment (Salt et al.
1995
). The rooted vegeta-
tion established at contaminated sites prevents
windblown dust, thus preventing human expo-
sure of hazardous waste. Migration of leachate
can be prevented through transpiration-mediated
hydraulic control of groundwater or receiving
waters. Phytostabilization is preferred for metal
contaminants at waste sites where confinement
of contaminants is required at a localized place.
Since metals do not ultimately degrade, the best
alternative is capturing them in situ at sites with
low contamination levels (below risk thresholds)
or vast contaminated areas where a large-scale
removal action. For phytostabilization, vigorous-
ly growing plants are necessary to exert hydrau-
lic control and immobilization at the site where
plants cannot die or removed during the phytosta-
bilization design period. Low-level radionuclide
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