Phytoremediation works by one or a combination of the following mechanisms:

a) degradation through metabolism by plants or enhanced microbial action;
b) vaporization as the plant transpires;
c) extraction—accumulation, then collection, recycling or disposal;
d) containment by adsorption or otherwise reducing movement or availability.

METALS
The mechanisms of metal phytoremediation.

PHYTOEXTRACTION—high biomass metal-accumulating plants and appropriate soil amendments are used to transport and concentrate metals from the soil to the roots and/or above-ground shoots, which are then harvested. These plant parts are dried, incinerated or composted. Metals are recycled (perhaps for a net profit) or disposed of as hazardous waste. Soil amendments, such as metal chelators, facilitate uptake by releasing metals such as lead which are bound to soil components.

Examples:

Brake fern (Pteris vittata) hyperaccumulates arsenic in its above-ground shoots to concentrations 200 times higher than those found in the soil. Other target metals include lead, cadmium, chromium, nickel, zinc, and various radionuclides.

RHIZOFILTRATION—Plant roots grown in aerated water precipitate and concentrate heavy metals from polluted effluents.

Example: Hydroponically-grown sunflowers treat for lead, copper, uranium, strontium, cesium, cobalt and zinc. Native wetland plants in Georgia are being evaluated for their ability to remediate acidic contaminated runoff.

PHYTOSTABILIZATION—Plants control movement of toxins from the site either by controlling erosion or movement by water, or by binding them tightly to their roots, rendering them unavailable and thus harmless. The contaminants are not removed from the site.

Example: Poplars.

PHYTOVOLATILIZATION—Plants extract volatile metals from the soil and volatize them from the foliage.

Example: Mercury and selenium can be removed in this way.
Less research has been conducted in the areas of phytostabilization and phytovolatilization than on other methods.

EXCESS NUTRIENTS = POLLUTANTS = ESSENTIAL NUTRIENTS
Phosphates and nitrates are essential for plants, but become water pollutants in excess, and can also be treated by phytoremediation.

ORGANIC CONTAMINANTS

The mechanisms of organic contaminant phytoremediation.

PHYTODEGRADATION—The plants absorb hydrocarbons and other complex organic molecules, then metabolize or mineralize them in chemical reactions energized by sunlight.

Example: Some enzymes break down ammunition wastes (explosives), chlorinated solvents or herbicides.

PHYTOVOLATILIZATION—Plants extract contaminants from the soil and release the contaminant or a modified form thereof into the atmosphere via evapotranspiration from foliage.

Examples: Targets of this technology include PCBs (polychlorinated biphenyls), TCEs (trichloroethylenes), PAHs (polyaromatic hydrocarbons), pesticide residues and various explosives. Poplar trees have been shown to volatilize 90% of the TCE taken up.

POLLUTED GROUNDWATER

The mechanisms of groundwater phytoremediation

Controls the spread of contaminants by controlling the movement of groundwater
PHREATOPHYTES (“ground-water plants”)—Take up large volumes of water, treating deep groundwater by working as hydraulic pumps.

TRICKS
Plants deprived of a nutrient may take up an element/molecule of similar “shape,”specifically with the same number of electrons in the valence shell. This mechanism may prove to be a design strategy.