Environmental Engineering Reference
In-Depth Information
dictate the potential rate and extent of hydrocar-
bon removal.
Biodegradability potential depends on func-
tion of hydrocarbon type, size, structure, and
concentration. Polycyclic hydrocarbon concen-
trations must be within specific ranges. If con-
centrations are too low, indigenous microbes
may not use PHCs as a primary source of organic
carbon in preference to dissolved organic carbon;
however, PHCs may be inhibitory if concentra-
tions are too high. The availability of biodegrad-
able PHCs, microbial viability is controlled by
a variety of factors including oxygen, inorganic
nutrients, osmotic/hydrostatic pressure, tempera-
ture, and pH.
Indigenous microbes use ambient inorganic
nutrients and organic carbon to maintain cell tis-
sue and increase biomass. Consequently, inorgan-
ic nutrient availability is reflected in microbial
population densities within contaminant plumes
in which intrinsic biodegradation is occurring.
Although other factors that influence microbial
viability are directly related to population density
as inorganic nutrient and organic carbon avail-
ability. Population density is an indicator of am-
bient organic carbon and inorganic nutrient avail-
ability. According to USEPA (
1987
), groundwa-
ter samples collected from background locations
hydraulically up-gradient/side-gradient of petro-
leum contaminant plumes typically contain total
population densities of about 102-103 colony
forming units per milliliter (cfu/ml). Microbial
population densities within petroleum contami-
nant plumes typically increase in response to
supplemental organic carbon supplied by dis-
solved/adsorbed-phase PHCs. Hence, there is a
positive correlation between population densi-
ties and PHC concentrations within contaminant
plumes under conditions in which intrinsic bio-
degradation is occurring. This correlation indi-
cates that indigenous heterotrophs are stimulated
to metabolize PHCs, and that ambient inorganic
nutrient levels are not limiting biodegradation in
situ. Other potential limiting factors include hy-
drostatic pressure, temperature, and pH, however,
these factors are frequently within the range of
microbial viability and typically do not limit in-
trinsic biodegradation, with the possible excep-
tion of pH.
Researchers determined the effects on bio-
degradation kinetics of a number of factors, in-
cluding (i) intrinsic soil properties (particle size,
carbon content, water holding capacity), (ii) soil
contaminants (petroleum hydrocarbons, heavy
metals), (iii) controllable conditions (tempera-
ture, nitrogen, and phosphorous content), and
(iv) inoculation with hydrocarbon-degrading mi-
croorganisms. The hydrocarbon-degrading soil
microfloras of polar regions are limited by N
and P, as are such microflora in warmer regions.
Addition of nitrogen and phosphorous stimulate
hydrocarbon degradation.
1.8
Phytoremediation
Phytoremediation, the use of plants for environ-
mental restoration is an emerging cleanup tech-
nology to exploit plant potential to remediate soil
and water contaminated with a variety of com-
pounds, several technological subsets have been
proposed. Phytoextraction is the use of higher
plants to remove inorganic contaminants, primar-
ily metals, from polluted soil. In this approach,
plants capable of accumulating high levels of
metals are grown in contaminated soil. At ma-
turity, metal-enriched above-ground biomass is
harvested and a fraction of soil-metal contamina-
tion is removed. Plants have a natural propensity
to take up metals. Some, such as Cu, Co, Fe, Mo,
Mn, Ni, and Zn, are essential mineral nutrients.
Others, however, such as Cd and Pb, have no
known physiological activity. Perhaps, not sur-
prisingly, phytoremediation as an environmental
cleanup technology was initially proposed for
the remediation of metal-contaminated soil. The
general use of plants to remediate environmental
media through in-situ processes which includes
rhizofiltration (absorption, concentration, and
precipitation of heavy metals by plant roots),
phytoextraction (extraction and accumulation of
contaminants in harvestable plant tissues such as
roots and shoots), phytotransformation (degra-
dation of complex organic molecules to simple
molecules which are incorporated into plant
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