Phytoextraction (Phytoaccumulation)

Phytoextraction involves plants absorbing contaminants (mainly heavy metals) through their roots and transporting them to above-ground tissues. These contaminants accumulate in leaves and stems, which can then be harvested and removed from the site.

Key points:

• Plants use natural metal-transport proteins to move contaminants from roots to shoots.

• Works best for contaminants that plants can tolerate without toxicity symptoms.

• Over multiple growth cycles, pollutant concentrations in soil can be significantly reduced.

• The harvested biomass can be:

  • Safely disposed of,

  • Incinerated, or

  • Used for “phytomining” (extracting valuable metals like nickel or gold).

Phytostabilisation

Instead of removing contaminants, phytostabilisation uses plants to lock pollutants in place, preventing their movement into groundwater, air, or the food chain.

Function and mechanisms:

• Roots act as a living barrier, trapping contaminants in the rhizosphere.

• Plants release root exudates that alter pH or oxidation–reduction conditions, reducing contaminant solubility.

• Dense vegetation reduces soil erosion and dust migration.

Most effective for:

• Heavy metals (lead, cadmium, arsenic)

• Mine tailings

• Radionuclides

• Acidic or highly erodible soils

This strategy is especially useful when removing contaminants is either impossible or too dangerous.

Rhizodegradation (Phytostimulation)

Rhizodegradation enhances the natural microbial activity around plant roots to break down contaminants. It is microbe-driven, but the plant creates the conditions that make it possible.

Mechanisms:

• Plants release carbon-rich exudates (sugars, amino acids, organic acids) that feed soil microorganisms.

• Microbial populations increase and become more effective at degrading pollutants.

• Works especially well for organic contaminants that microbes can metabolise.

Key points:

• The rhizosphere can host microbial communities thousands of times more active than bulk soil.

• This strategy is widely used in oil-contaminated soils.

• It does not remove metals, but it is highly effective for hydrocarbons and solvents.

Phytovolatilisation

In phytovolatilisation, plants take up contaminants and release them into the atmosphere in a transformed, often less toxic form.

Process overview:

• Contaminants are absorbed by roots.

• Plants transform them (e.g., into elemental mercury or volatile selenium).

• The transformed compounds move through stems and leaves.

• They are released as gas into the air.

Common targets:

• Mercury

• Selenium

• Some organic pollutants, such as chlorinated solvents

Although this reduces soil contamination, it can raise concerns about transferring pollutants to the atmosphere, so it must be used carefully.

Phytodegradation (Phytotransformation)

Phytodegradation occurs when plants use enzymes to chemically break down organic pollutants into less toxic or harmless substances.

What happens during phytodegradation?

• Plants produce enzymes such as:

  • Dehalogenases (remove halogens)

  • Peroxidases

  • Oxidoreductases

  • Laccases

• These enzymes transform contaminants within plant tissues or at the root surface.

• Organic pollutants are often converted into carbon dioxide, water, or simple organic molecules.

Best for:

• Pesticides, herbicides

• Chlorinated solvents

• Explosives (TNT, RDX)

• Petroleum hydrocarbons

Poplar and willow trees are especially well-known for this process due to their high metabolic activity and rapid growth.

Rhizofiltration uses plant roots, either hydroponic or soil-grown, to filter contaminants from water rather than soil. It is effective for polluted ponds, streams, irrigation water, or industrial effluent.

How it works:

• Roots absorb, bind, or precipitate contaminants directly from water.

• Plants can be grown hydroponically and introduced to contaminated water bodies.

• Once roots become saturated with contaminants, the plants are harvested and replaced.

Best for:

• Heavy metals (lead, chromium, copper, cadmium)

• Radionuclides

• Nutrient pollution in water (nitrate, phosphate)

Phytohydraulics (Phytohydraulic Control)

Phytohydraulics refers to the use of deep-rooted plants to control the movement of groundwater. Instead of removing contaminants directly, these plants act as biological pumps, taking up large volumes of water and thereby slowing or redirecting the flow of contaminated groundwater plumes.

What it does:

• Reduces groundwater velocity by lowering the local water table.

• Creates a hydraulic barrier that prevents contaminants from spreading.

• Helps contain polluted zones while other phytoremediation processes (like rhizodegradation or phytodegradation) act in parallel.

• Particularly suited for solvent-contaminated aquifers or sites where groundwater transport is the main risk pathway.

Key points:

• This technique does not inherently remove or transform contaminants.

• Its purpose is containment, not clean-up.

• Species with high transpiration rates—such as poplar, willow, and eucalyptus—are most effective.

Important disclaimer:
Phytohydraulics is not universally included as one of the “main” phytoremediation strategies. Some scientific frameworks recognise six core categories and treat phytohydraulic control as a supplementary or hybrid mechanism. Others formally include it as the seventh strategy because it addresses groundwater behaviour, which is critical to contaminant management.
Additionally, it is sometimes grouped conceptually with rhizofiltration because both involve water, but they are not interchangeable:

• Rhizofiltration removes contaminants from water,

• Phytohydraulics controls water movement to limit contaminant spread.