In general, 7 types of Phytoremediation are identified.
Phytoextraction is a remediation method that uses plant cultivars that absorb pollutants from the soil more strongly and accumulate them in their biomass in high concentrations. These phytoremediative plants are often referred to as so-called hyperaccumulators. The pollutants and contaminants contained in the soil can be stored both in the roots and in the above-ground biomass, such as the leaves and stems. Often, accumulator and/or hyperaccumulator plants are used that are able to tolerate and accumulate heavy metals and metallic trace elements; TMEs. It is possible to improve extraction by adding chelates to the soil.
These plant parts are then removed during harvesting after the vegetation period, and depending on the case, the roots will also be removed. Depending on the contamination the plants are then sent for suitable disposal using a variety of processes. In most cases, the plants are incinerated, the ashes are either stored in a suitable storage facility, such as Composite Elevated Storage Tanks (CET), or transformed to recover the accumulated metals. The process of recovering the stored metals is called Phytomineralisation or Phytomining. Also see our dedicated page on Phytomining.
2. Phytotransformation and Phytodegradation or Rhizodegradation
In Phytotransformation, also referred to as Phytodegradation, the pollutants taken up by the phytoremediative plants are not accumulated, as is the case with phytoextraction. Instead, phytoremediative plants that emply phytotransformation chemically alter and thus inactivate mostly organic pollutants. Some plants produce enzymes, such as dehalogenase and oxygenase, that catalyse the degradation of absorbed or adsorbed substances. These pollutants are transformed into less toxic or non-toxic substances by the metabolisation of the contaminants in the plant tissues
Similar processes in which the pollutants are not taken up by the plant, but the plants only stimulate the degradation of the pollutants by microorganisms, count as phytodegradation. This stimulation takes place on the one hand through the improved assimilate supply of the microorganisms due to the rooting of the soil, kknwon as rhizodeposition, and on the other hand also through certain substances that are excreted by the plant roots; the root exudates. This phytoremediation process by rhizosphere organisms fed and maintained by the plant is knwon as rhizodegradation, or the degradation by the rhizosphere.
3. Phytofiltration or Rhizofiltration
Phytofiltration or Rhizofiltration is a process in which the plants are not directly involved in the remediation, but contribute through their root system to increase the microbial activity in the soil. The pollutants are then degraded by microorganisms.
This is often used for the bioremediation and biorestoration of surface and groundwater. In terms of groundwater remediation, rhizofiltration also generally means absorption and condensation at the plant roots and/or uptake and accumulation in the roots in a wet environment. Rhizofiltration or rhizo-purification for water purification can be done in hydroponic systems or in situ on floating islands built with floating cushions of synthetic material that can be crossed by roots.
In Phytovolatilisation, the plant absorbs pollutants in the water containing organic contaminants and other toxic products from the lithosphere with its roots. In this process, biochemical transformations into volatile forms of the pollutants can occur in the plant. They then release the elements into the atmosphere via their above-ground organs such as their leaves. In the case of mercury, this occurs through methylation of mercury to methylmercury by the plant. Methylmercury is volatile and can therefore be released into the air by the plant.
Phytovolatilization is not always satisfactory, as it decontaminates soils and sometimes releases toxic substances into the atmosphere. In other, more satisfactory cases, the pollutants are broken down into less - or non-toxic components before being released.
Phytostabilisation integrates the control of what is commonly called hydraulic control, or Phytohydro-regulation.4 Hydraulic pumping, (literally translated from English) may take place when the roots reach the groundwater, removing large volumes of water and controlling the hydraulic gradient and lateral migration of pollutants in the aquifer. In short, this involves using plants with high evapotranspiration to reduce the movement of pollutants from runoff (lateral or deep). Another practice is to stop pollutant compounds by reacting them chemically. Plants adsorb pollutants from soil, water or air, retaining them locally (hence the use of the term "adsorption" rather than absorption) and reducing their bioavailability. The process can sometimes be amplified and accelerated by the addition of natural or artificial organic or mineral compounds. It is an effective method to prevent the dispersion of pollutants in surface or groundwater.
This technique consists of the complete restoration of contaminated soils to a state close to the functioning of a natural soil (Bradshaw, 1997). This subdivision of phytoremediation uses plants native to the area exposed to the phytoremediation work. The aim is to achieve full re-establishment of the original natural ecosystems. As pointed out by Peer et al. (2005), compared to other phytoremediation techniques, phytoremediation highlights the issue of the level of decontamination necessary and sufficient. There is a big difference between a soil decontaminated to achieve its satisfactory adaptation to a law and restoring the space to make it fully usable so that it returns to pre-pollution conditions. When we refer to phytorestoration of wastewater, we speak of a process related to the use of plants to achieve decontamination (Dabouineau et al., 2005). Used in this sense, it becomes synonymous with phytopurification. This particular type of process involves the purification of water by macrophytes. In this case, it is the bacteria living in the root zone of the macrophytes that are the guarantors of remediation, the plants are simply a substrate for the growth of micro-organisms (see Honfleur station).
Phytostimulation is mainly located in the rhizosphere, it is the stimulation by plants of microbial activities favourable to the degradation of pollutants. This aspect, when studied, was found in all hyperaccumulating plants.
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