Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 156

Figure7.2 Diagrammaticmacrophytetreatmentsystem(MaTS)
held within a containment layer, within which the macrophytes have been estab-
lished. Less commonly, pre-existing natural features have been used. Although
wetlands have an innate ability to accumulate various unwanted chemicals, the
concept of deliberately polluting a habitat by using it as a treatment system is
one with which few feel comfortable today. A gentle hydraulic flow is estab-
lished, which encourages the incoming wastewater to travel slowly through the
system. The relatively long retention period that results allows adequate time for
processes of settlement, contaminant uptake, biodegradation and phytotransfor-
mation to take place.
The mechanisms of pollutant removal are essentially the same, irrespective
of whether the particular treatment system is a natural wetland, a constructed
monoculture or polyculture and independent of whether the macrophytes in
question are submerged, floating or emergent species. Both biotic and abiotic
methods are involved. The main biological mechanisms are direct uptake and
accumulation, performed in much the same manner as terrestrial plants. The
remainder of the effect is brought about by chemical and physical reactions,
principally at the interfaces of the water and sediment, the sediment and the
root or the plant body and the water. In general, it is possible to characterise
the primary processes within the MaTS as the uptake and transformation of
contaminants by micro-organisms and plants and their subsequent biodegra-
dation and biotransformation; the absorption, adsorption and ion exchange on
the surfaces of plants and the sediment; the filtration and chemical precipita-
tion of pollutants via sediment contact; the settlement of suspended solids; the
chemical transformation of contaminants. It has been suggested that although
settlement inevitably causes the accrual of metals, in particular, within the sed-
iment, the plants themselves do not tend to accumulate them within their tis-
sues. While this appears to be borne out, particularly by original studies of
natural wetlands used for the discharge of mine washings (Hutchinson 1975),
this does not form any basis on which to disregard the contribution the plants
make to water treatment. For one thing, planting densities in engineered sys-
tems are typically high and the species involved tend to be included solely for
their desired phytoremediation properties, both circumstances seldom repeated