METALS - Marine Pollution: What Everyone Needs to Know - page 98

Wetland sediments accumulate contaminants, and there are
many cases in which wetland plants can remove pollutants,
including metals. The use of plants to remove or stabilize con-
taminants is referred to as phytoremediation, and there are dif-
ferent mechanisms that can be utilized. Phytoremediation is a
relatively recent technology and is perceived as cost-effective,
efficient, and ecofriendly, with good public acceptance. It is an
area of active current research. The approach in salt marshes
is generally one of phytostabilization, where the plants are
used to immobilize the metals and store them below ground
in roots or soil, in contrast to phytoextraction, in which cer-
tain plants that can accumulate very high concentrations in
aboveground tissues (hyperaccumulators) are used to remove
metals from the soil and concentrate them above ground.
Hyperaccumulators must in turn be harvested and disposed
of to prevent recycling of the accumulated high concentra-
tions of metals once they die and decompose. This is done
frequently in terrestrial sites. However, wetland plants gener-
ally do not hyperaccumulate and, in any case, the mechanical
aspects of harvesting plants would be destructive to wetlands
with rooted plants.
Wetland sediments are generally considered a “sink” for
metals and may contain very high concentrations of metals in a
reduced state, especially in sediments with low oxygen. In such
areas the bioavailability of the metals is very low compared to
terrestrial systems, which have oxidized soils. Different forms
of metals have different availability: water-soluble metals are
the most available, while metals precipitated as inorganic com-
pounds, or combined with humic materials are potentially
available, and metals precipitated as insoluble sulfides (e.g.,
AVS) or bound within the structure of minerals are essentially
unavailable. In estuaries, much of the metals are tightly bound
to sulfides (e.g AVS) in anoxic sediments. However, some wet-
land plants (e.g., cordgrass, Spartina alterniflora) ) can oxidize
the sediments near their roots by moving oxygen downward,
and this oxidation can remobilize the metals, thus increasing