Phytoremediation refers to the use of plants and their associated microorganisms to stabilise, extract, or reduce contaminants in various environmental media, such as soil, sediment, and water. It includes several subtypes: phytoextraction for removing metals or organics into plant biomass; phytodegradation and rhizodegradation for breaking down contaminants; rhizofiltration for removing pollutants from water using plant roots; phytostabilisation to immobilise contaminants in soils; and phytovolatilisation, where pollutants are transformed and released into the air. These processes can also support site reuse, ecosystem restoration, and biomass generation for energy or material applications.
This remediation strategy is favoured for large, soft-use sites where contamination is diffuse and not immediately hazardous. It is considered a “gentle” and sustainable solution, requiring low input and offering long-term risk management. Phytoremediation is particularly suitable where soil functionality and ecological services are to be preserved or restored, such as in agricultural land, parks, or marginal lands. However, it is not ideal for sites needing rapid redevelopment, heavily built-up areas, or where regulations require rapid reductions in total contaminant levels.
The advantages of phytoremediation include its potential for biomass recovery, sustainability benefits, low carbon intensity, and long-term containment or breakdown of both organic and inorganic pollutants. It can manage a wide range of contaminants, including PAHs, PCBs, pesticides, PFAS, and heavy metals. Yet, limitations include its confinement to the rooting zone, seasonal dependency, the need for long-term site management, and challenges with biomass reuse due to contaminant content. Additionally, regulatory concerns may arise around pollutant volatilisation and ecological impacts from non-native species.
Implementation requires appropriate site access, security, and environmental monitoring. The method aligns well with other remediation approaches such as in situ stabilisation and bioremediation. Although genetically modified plants offer promising enhancements, their use is currently hindered by public and political opposition. Overall, phytoremediation offers a flexible, ecologically conscious option for risk management and land restoration, especially where long-term environmental and social benefits are prioritised.
Phytocontainment
Phytocontainment involves hydraulic control of contaminant plumes through plant transpiration and root water uptake USGS. This mechanism uses the natural water absorption capacity of plants, particularly deep-rooted trees, to control the movement of contaminated groundwater. Trees remove contaminated water from aquifers through their root systems, followed by biological alteration of contaminants within the trees or transpiration and volatilisation of contaminants into the atmosphere USGS. Long-rooted trees operate as pumps, drawing vast amounts of water from the subsurface water table, and contaminants in the water are absorbed along with the water during this process PubMed Central.
The approach provides hydraulic containment of groundwater plumes and is particularly effective with high-transpiring tree species such as poplars and willows. Trees can also promote microbial reductive dechlorination of dissolved contaminants within originally aerobic aquifers through root zone processes USGS. This makes phytocontainment suitable for managing contaminated groundwater, particularly with volatile organic compounds (VOCs) such as trichloroethylene (TCE), and preventing migration of contaminant plumes.
Phytodegradation
Phytodegradation involves the degradation of organic contaminants directly through the release of enzymes from roots or through metabolic activities within plant tissues Nature. This process breaks down contaminants into less toxic or non-toxic forms. Organic contaminants are taken up by roots and metabolised in plant tissues to less toxic substances Nature, with plants producing enzymes that catalyse the breakdown of complex organic molecules. The process may involve either the volatilisation of constituents or their incorporation into the soil matrix RSC Publishing.
Phytodegradation has been particularly successful with hydrophobic organic contaminants Nature, and can degrade petroleum hydrocarbons, PCBs, PAHs, pesticides, and explosives. Related to this is phytostimulation (rhizodegradation), which enhances soil microbial activity through plant root exudates of carbohydrates and acids, resulting in biodegradation of organic contaminants Wikipedia. Poplar trees (Populus spp.) have been used successfully in phytodegradation of toxic and recalcitrant organic compounds Nature, whilst other species including tobacco and various grasses are effective for different contaminant types.
Phytoextraction
Phytoextraction involves plants uptaking pollutants from soil, water, or sediments by their roots and transferring them to aboveground biomass where they accumulate, such as in shoots or other harvestable parts PubMed Central. This is also known as phytoaccumulation, and represents one of the most studied phytoremediation approaches. Hyperaccumulators sequester high concentrations of heavy metals in aerial parts through enhanced active metal transport ScienceDirect. The underlying process is complex: it involves enhanced translocation across biological membranes, including transport through cytoplasmic membranes and uploading/unloading of xylem vessels PubMed Central.
There is rapid and active translocation of heavy metals to the shoot via the xylem, which is upregulated by transpiration, and metals are detoxified through complexation with amino acids, organic acids, or metal-binding peptides and sequestered into vacuoles ScienceDirect. Hyperaccumulating plants can contain more than 1% of a metal in their dry biomass Nature, and the contaminants are physically removed from the environment when plants are harvested. It is even possible to extract metals from harvested biomass in a process termed phytomining Nature. However, the main limitation is low biomass production of many natural hyperaccumulators PubMed Central.
Suitable species vary by target metal. Alpine Pennycress (Thlaspi caerulescens) is effective for Zn, Cd, and Ni hyperaccumulation, whilst Indian mustard (Brassica juncea) can accumulate Cd, Pb, Cu, Cr(VI), Zn, Ni, and Se ScienceDirect. For arsenic contamination, Pteris vittata is notable as a hyperaccumulator, capable of accumulating up to 20 mg per gram of dry weight ScienceDirect. The technique is primarily used for heavy metals including Cd, Zn, Ni, Cu, Pb, As, and Se.
Phytostabilisation
Phytostabilisation involves the reduction of the mobility of heavy metals in soil through immobilisation, which can be accomplished by decreasing wind-blown dust, minimising soil erosion, and reducing contaminant solubility or bioavailability to the food chain University of Hawaii. Unlike phytoextraction, the objective is not to remove contaminants but to immobilise them in place. The mobility of contaminants is reduced by the accumulation of contaminants by plant roots, absorption onto roots, or precipitation within the root zone University of Hawaii.
Plant roots stabilise heavy metals, ultimately reducing their mobility and bioavailability, and established vegetation cover on degraded soil surfaces effectively mitigates soil erosion ResearchGate. In some cases, plants can excrete substances that produce chemical reactions, converting heavy metal pollutants into less toxic forms Wikipedia. The advantages of phytostabilisation include effective rapid immobilisation and no need for biomass disposal, though the major disadvantage is that pollutants remain in the soil or root system, generally in the rhizosphere Nature.
Enhanced results can often be achieved through "aided phytostabilisation", where soil amendments such as organic matter, phosphates, alkalising agents, and biosolids are added to decrease solubility of metals University of Hawaii. The technique has proved useful for treatment of Pb, As, Cd, Cr, Cu, and Zn contaminated soils Nature, and is particularly attractive when other methods to remediate large-scale areas having low contamination are not feasible University of Hawaii. Suitable plant species include grasses such as Festuca rubra (red fescue), Miscanthus × giganteus, and other metal-tolerant species that accumulate metals primarily in roots rather than shoots.
Phytovolatilisation
Phytovolatilisation involves the uptake of contaminants by plant roots and their conversion to a gaseous state, with release into the atmosphere, driven by the evapotranspiration of plants Nature. The mechanism involves contaminants partitioning into air spaces within a plant and subsequently diffusing into ambient air ACS Publications. More specifically, plants absorb contaminants from soil, then convert these harmful substances into less hazardous volatile forms, releasing them into the surrounding environment through the leaf or foliage mechanism of transpiration RSC Publishing.
Selenium (Se) and Mercury (Hg) are often removed from soil through phytovolatilisation Wikipedia. For selenium, the metal can be volatilised through conversion into dimethylselenide Nature, whilst mercury, which exists in liquid state at room temperature with high tendency to vaporise, is absorbed as methylmercury and transformed into ionic mercury, then into less hazardous elemental form released as gas RSC Publishing. The technique also works for organic contaminants: volatile organic compounds (VOCs) are passively volatilised, and hybrid poplar trees have been used to volatilise trichloroethylene (TCE) by converting it to chlorinated acetates and CO₂ Nature.
Plants with high evapotranspiration rates are sought after for phytovolatilisation Nature, with poplar trees amongst the most successful plants for removing VOCs due to high transpiration rate Wikipedia. The Brassicaceae family, specifically Brassica juncea, effectively volatilises Se RSC Publishing. Whilst this method has the advantage of not requiring plant harvesting and removal RSC Publishing, there are important environmental considerations. The process does not result in complete elimination of contaminants from the environment; it merely transfers harmful substances from soil to atmosphere RSC Publishing. Toxic volatile components can contaminate the environment through air or even return to soil through precipitation ResearchGate, and regulatory concerns exist around pollutant volatilisation and potential air quality impacts.