Phytoremediation

Technical articles - NBSOIL - Soil regeneration

Depollution Decontamination Rhizofiltration NBSOIL

Phytoremediation process

Phytoremediation is a technology using plant metabolism to accumulate, transform, degrade, concentrate, stabilize or volatilize pollutants (organic and inorganic molecules, metals and radioactive elements) contained in contaminated soil or water. Other technologies use micro-organisms (bacteria, micro-algae), and are referred to as bioremediation. These are natural decontamination techniques that stand up to conventional methods.

Traditional techniques

The methods most commonly used today are mechanical and physico-chemical: excavation, use of solvents and/or incineration. They are only used on small, heavily contaminated surfaces, due to their high cost and impact on the landscape : they destructure the soil and greatly reduce its fertility and productivity. Their main advantage is their effectiveness over a treatment period of a few weeks to a few months.

Principles

There are four phytoremediation mechanisms:

• Phytovolatilization : Transformation and degradation of certain types of pollutants into less toxic volatile elements, which are then released into the atmosphere through plant transpiration (e.g. tobacco).
  • Pollutants concerned: some organic compounds and metals (selenium, mercury).
• Phytostabilization : Absorption and sequestration (or immobilization in the case of rhizofiltration) of pollutants at root level (rhizosphere). Objective: to reduce their dispersion by the wind or leaching by rainwater, and limit their migration and entry into the food chain or water tables (e.g. poplar trees).
  • Pollutants concerned: radioelements such as uranium.

• Phytodegradation : Absorption and decomposition of contaminants through the release of enzymes and oxidation and reduction processes. The degraded, less toxic pollutants are then incorporated into the plant or released back into the soil (e.g. weeping willow).
  • Pollutants concerned: organic compounds (hydrocarbons, pesticides, explosives, etc.).

• Phytoextraction : Extraction, transport and accumulation of pollutants in stems and leaves. Plants are known as accumulators. The leaves, or the whole plant, are harvested using agricultural techniques, then burned in factories. Pollutants are concentrated in the ashes and filters, which are then treated as high-level waste in the case of nuclear pollution (e.g. sunflowers).
  • Pollutants concerned: metals (copper, gold, etc.) and radioelements (caesium, strontium, etc.).

Phytoremediation process, 2022

Types of pollutants/molecules

• Organic compounds
  • Herbicides (Atrazine, Fluometuron, Metolachlor)
  • Trichloroethylene (TCE)

• Inorganic compounds
  • Heavy metals (Lead, Cadmium, Zinc, Nickel, Copper, Mercury)
  • Selenium
  • Radionuclides (Cesium 137, Uranium, Strontium 90)
  • Boron

In practice

• Decontamination of a site polluted by trichloroethylene (TCE) using hybrid poplars : TCE is a major contaminant of soil and groundwater, presenting carcinogenic risks. Traditional decontamination methods, such as charcoal absorption, are costly and can take several years. Laboratory experiments have shown that hybrid poplars can absorb, transform and volatilize TCE from soil. These trees were chosen for their rapid growth and extensive root system. The poplars succeeded in reducing TCE concentrations in the soil. Further research is needed to optimize the process under real-life conditions and assess the impact of environmental factors.

• Decontamination of Chernobyl radionuclide-contaminated water with sunflowers : Following the Chernobyl accident, surface water was contaminated with radionuclides. Rhizofiltration with sunflowers (Helianthus annuus) was used to absorb radionuclides, notably uranium, caesium and strontium. Sunflowers were able to accumulate radionuclides present in water. Further research is needed to assess the sustainability of this method and its impact on the environment.

• Village wastewater treatment using reeds : Wastewater from a village in the Savoie region of France was treated using an experimental wastewater treatment plant with macrophyte beds made up of reeds (Phragmites australis, Typha latifolia and Scirpus lacustris).
• Biological swimming pools : water is filtered by rhizofiltration, a natural process using plants to replace chlorine.
• Arabidopsis thaliana used to study cesium uptake and translocation.

Benefits

• Low treatment costs (10 to 100 times lower than conventional technologies).
• Suitable for large contaminated areas (tens of hectares).
• Recycling of pollutants.
• Valorization of residues : biomass can be converted into energy.
• Good social acceptability.
• Low disturbance to contaminated environment.

Limits

• Limited to surfaces that can be colonized by roots.
• Very long treatment time (minimum 3 years).
• Dependence on environmental conditions : soil type, meteorology, insect attacks, micro-organisms, etc.
• Requires large areas and shallow pollution (50 cm to 3 m ).
• Potential ecological risks  : The dissemination of contaminant-accumulating plants in the environment can pose risks for wildlife, notably via the food chain.
• Application for moderate contamination to ensure plant survival.

Research challenges

Scientists face five major challenges in improving phytoremediation processes :

• Reducing treatment times : Plants can take several years to clean up a site. Scientists are therefore looking for ways to speed up the process, for example by selecting fast-growing plants or genetically modifying plants to increase their capacity to absorb pollutants.

• Managing cases of multiple contamination : Contaminated sites are often polluted by several types of pollutant. Finding plants capable of effectively treating several pollutants at once is a major challenge.

• Take better account of different environmental parameters : The effectiveness of phytoremediation can be influenced by environmental factors such as rainfall, temperature and soil type. Scientists need to better understand how these factors interact with plants and pollutants to optimize processes.

• Making better use of biomass : After absorbing pollutants, plants need to be harvested and processed. Adding value to this contaminated biomass is a major challenge if phytoremediation is to become more profitable. Solutions such as producing energy by burning biomass in boilers equipped with filtration systems are currently being studied.
• Creating value from extracted metals : In the case of phytoextraction, metals extracted from soils by plants could be recovered and reused. This would create a new source of revenue and make phytoremediation more attractive.