Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
- Phytotechnology and Ecohydrology -
for the removal of organic contaminants
Prior to the 20th century, naturally occurring biodegradation
processes were largely adequate for recycling organic materials on the surface
of the Earth. A number of different biochemical processes were able to cope
with the organic substances that rarely accumulated in sufficient quantity and
concentration to cause environmental pollution problems. However, during the 20th
Century, humankind achieved the ability to synthesise and disseminate large
quantities of industrial chemicals. Simultaneously, the use of chemicals
per capita and the human population
increased, causing a dramatic increase in the over-all production and discharge
of industrial chemicals. The consequent environmental pollution problems have
become particularly prominent due to a concomitant improvement in analytical
chemistry, epidemiology, and toxicology that allowed an understanding of the
link between the chemicals the pollution problems. The rates of production and
dispersal of these, often toxic, industrial substances have completely out
paced naturally occurring biodegradation, and an unsteady state has resulted.
This unsteady state threatens both human health and ecosystem function. Thus,
the development of phytoremediation technologies becomes a pivotal
ecotechnological issue because the failure of ecosystem functions jeopardises
not only the ecosystem integrity, but also human health.
The typical process utilised in evaluating and implementing
phytoremediation technologies involves six steps:
the geographical distribution of the contaminant concentrations using standard
analytical chemistry, and the conduct of an environmental risk assessment based
upon analyses of the soil pore water.
testing to verify the applicability of bioremediation technologies.
often by appropriate modelling, the feasibility of the method
and producing in sufficient amounts the plants to be used for the
the phytotechnologies in situ,
managing ground water levels as required to prevent flooding and other
potentially negative impacts.
the results using a broad spectrum of analytical methods, including
conservative tracers, detection of intermediary metabolites, and respiration
A particularly important application of phytoremediation is in the
protection of ground water against toxic compound contamination. Contamination
of ground water is a serious concern for public health and environmental
quality. The problem is commonly manifested as a contaminant plume migrating in
the direction of ground water flow from a point source. In such cases,
containment of the contaminant plume is important in preventing further
migration. Plants, in some cases, can be used to
contain these plumes and remove toxic organic matter. Vegetation can
efficiently take up even moderately hydrophobic organic substances, with Kow
values of about 0.5 to 3.0. Hydrophobic substances with a Kow value
exceeding 3.0 bind so strongly to the soil and the roots that they are not
easily taken up by plants.
Plants can also enhance
microbiological decomposition rates because the roots pump oxygen into the root
zone, thereby creating a good environment for the microbiological oxidation of
organic matter. It may be necessary to add fertilisers to increase biomass of
the plants and, thereby, promote biological activity in the root zone, as
illustrated by Lin and Mendelssohn (1998). They described the successful removal
of crude oil from a marsh by fertilisation of
Spartina alterniflora and Spartina
patens communities. The rate of
oil degradation within the soil was significantly enhanced by the application
of fertiliser in conjunction with the presence of plants.
The productivity of the vegetation
may also be enhanced by the addition of ion exchange substrates to completely
depleted soils and barren sands (Soldatov et
al. 1997 suggest that a 1% addition is sufficient).
Soils contaminated by both heavy
metals and toxic organics are most difficult to remediate, because the presence
of heavy metals causes a distinct decline in the rate of organic matter
decomposition (see Zwolinski 1994). Thus, it generally is necessary to first
remove the heavy metals, and then to remove the toxic organic components.
N. Phytoremediation for the removal of heavy metals
Two, different phytoremediation techniques can be applied for the
removal of heavy metals from the environment; namely, the use of algae, and the
use of heavy metal-tolerant plants.
Both possibilities may be applied to reclaim contaminated soils or treat
polluted waters. Indeed, the techniques may be applied to waters polluted by
Both techniques are attractive alternatives to the traditional
environmental technologies based on physico-chemical processes, such as ion
exchange, liquid extraction, precipitation, and crystallization.
Phytoremediation, in most cases, is considerably less expensive than these
physico-chemical techniques, particularly if high concentrations of heavy
metals can be achieved within the vegetation. In addition, the recovery and
reuse of the metals is possible using phytoremediation methods. For example,
acid extraction could be used to recover heavy metals from plants. However, the
costs of the recovery processes often exceed the value of the metals.
Notwithsanding, all of the processes currently available for removing heavy
metals from water and soils, including those processes based on traditional
environmental technologies, only transfer the heavy metals into a more
concentrated and more accessible form that can facilitate disposal or recovery.
However, recovery or disposal is generally satisfactory.
Marine algae possess large quantities of
biopolymers - including polysaccharides, uronic acids, and sulphated
polysaccharides - that can bind heavy metals. Their metal uptake capacity is
consequently quite high. The marine alga, Sargassum
spp., can take up as much as 40% of
the algal dry weight in gold (Kuyucak and Volesky 1989). One advantage of using
such seaweeds as phytoremediation technologies is that these plants are large
enough to facilitate the immobilisation of the metals. These plants can, for
instance, be applied in packed columns without any pretreatment of the effluent
introduced into the columns for treatment.
Biosorption in algae has been attributed primarily to the cell wall,
where both electrostatic attraction and the formation of complexes play a role.
It is, therefore, important to know the cell wall characteristics of the
different groups of algae proposed to be used for bioremediation projects in
order to select the most promising species (Schiewer and Volesky 2000). The ion
exchange properties of the sulphated polysaccharides in seaweeds are also of
importance (Schiewer and Volesky 1995), and may explain a significant part of
the observed biosorption.
As with terrestrial plants, Langmuir adsorption isotherms can be used to
describe the uptake dynamics exhibited by algae (Holan and Volesky 1994):
M = B × K(M)/( 1 + K(M))
||M - the concentration of the heavy metal in the algal material
expressed in meq/g,
||B - the total number of binding
sites (dependent on the selected alga), and
||K(M) - the affinity of the alga for the metal, dependent
on the alga and the metal.
Soils and wastewaters may contain more than one type of heavy metal, and
various metals may compete for the same binding sites within the algae. A
multi-Langmuir adsorption isotherm can be applied in these cases. If a
two-component competition scenario is considered (metals M1 and M2), the
multi-Langmuir adsorption isotherm will take the following form:
M = B × K(M1)/( 1 + K(M1) + K(M2))
The binding capacity of the algae at low pH values is usually lower than
at higher pH values, as demonstrated in Figure 5.16. This property may be
utilised in the recovery of heavy metals removed from the environment by marine
algae. Desorption of the metals can be achieved by application of acid
The binding of metals to
cell surfaces is a rapid process. A large percentage of the binding is achieved
within a few minutes, while a complete equilibrium can be attained within a few
hours. The rate is generally proportional to the displacement from the
equilibrium condition, and to the external surface area of the alga. A more
detailed model of the kinetics of metal binding includes consideration of film
diffusion from the solution to the surface, pore diffusion through the
particles, and chemical binding processes. However, these processes are
beyond the scope of this application.
The practical application of marine algae in heavy metal removal may be
realised in situ by using either a
batch process or a packed bed column. In both cases, it is advantageous to
pretreat (reinforce and immobilise) the biosorbent particles. Reinforcement can
be achieved by chemical cross-linking with formaldehyde or gluralaldehyde,
which successfully reduces the swelling and leaching of biomolecules (Holan
et al. 1993). Immobilisation is possible
by treatment of the biosorbent with alginate, silica, and polyacrylamide
(Holbein 1990, Bedell and Darnell 1990).
Freshwater aquatic plants, like their
marine counterparts, have been used to remove heavy metals from water. For
example, Sharma and Gaur (1995) used duckweed in the removal of zinc, lead, and
nickel. Other aquatic plants, such as Salvinia and Spriodela,
have been used to remove chromium and nickel from
wastewaters. A significant removal efficiency, in the concentration range of 1
ppm to 8 ppm, although with fluctuations, has been reported by Srivastav
et al. (1994). Fungi, likewise, have recently been proposed for use in the
removal of heavy metals. The amounts of metal adsorbed per unit biomass are on
the order of several mg/g, for cadmium, copper, lead, and nickel (Kapoor and
Microbial biotraps, a variation of algal-based
phytotechnologies utilising both dead and living biomass in adsorption and ion
exchange processes (White et al.
1995), have also been used to remove heavy metals from water. This process has
been used in wastewater treatment systems, where more than 50% removal of heavy
metals has been shown to occur. The so-called Meander system, which passes
drainage water containing heavy metals through various channels containing
cyanobacteria and algae, can achieve a metals removal efficiency of greater
than 99% (Erlich and Brierley 1990). This process most likely utilises
precipitation and entrapment of particulate matter within the meanders, in
addition to the algal-mediated adsorption and ion exchange processes, since the
metals tend to be concentrated in the channel sediments.
Many microorganisms, including algae, produce extracellular polymers
composed mostly of polysaccharides and sulphated-polysaccharides. These
commonly occur in the form of a "slime" layer surrounding the cells. This layer
binds metal ions tightly in an ion exchange-like process. Hydrogen sulphide
produced by sulphate-reducing bacteria, when utilised in the removal of metals
from water by biota under anaerobic conditions, results in the formation of
metal sulphides having extremely low solubilities. This feature can be utilised
in the removal of toxic metals from water in the form of sulphides (Brierly
1995). Other precipitation processes, which may be utilised as
phytotechnologies, include the formation of insoluble uranium phosphate
complexes (Macaskie 1991), and copper phosphates and copper oxalates (Crusberg
et al. 1994).
It has been shown, recently, that a few plant species, such as the alpine
pennycress, can be used as bioaccumulators
(also known as hyperaccumulators). Hyperaccumulators are usually plants that
are slow growing and slowly remove heavy metals over a period of years. While
fast growing poplar trees are good phytoremediators because of their rapid
growth, they generally do not make good hyperaccumulators. When growing in
contaminated soils, bioaccumulators are able to concentrate metals up to 100
times the heavy metal concentrations in normal plant species. When these plants
are harvested, the corresponding amounts of heavy metals are removed from the
soils. If the soils are only slightly contaminated with heavy metals, this
method may be able to remediate the soils within just a few years, making this
method very attractive due to its low cost. For very contaminated soils,
however, the period needed for complete recovery of the soil may be too long.
Nevertheless, by watering the plants carefully with a diluted EDTA solution, it
may be possible to enhance the biological uptake of heavy metals. Several plant
species are known to naturally release chelating ligands and enzymes into the
soil which accelerate metal removal rates and reduce the toxicity of heavy
metal ions. Jørgensen (1993) reported removal rates (from an initial lead
concentration of 380 mg Pb/kg soil (dry matter)) of up to 11.5% lead per
harvest of plants watered with a 0.02 M EDTA solution, because the increased
solubility of the metal ions.