Phytoremediation

Phytoremediation or phytoremediation is a sub-area of biological remediation techniques and generally describes the remediation of polluted and contaminated soils or groundwater with the help of plants. This is a so-called in-situ process, as the treatment of the soil or water takes place on site. Phytoremediation is continuously being developed. A distinction is made between different procedures:

Phytoextraction

Phytoextraction is a remediation process in which plant cultivars are used that increasingly absorb pollutants from the soil and enrich them in their biomass in high concentrations (so-called hyperaccumulators). The pollutants can be stored in the roots as well as in the aboveground biomass. These parts of the plant are then removed during harvest after the vegetation period (i.e. also with removal of the roots as the case may be) and, depending on the pollution, disposed of in a suitable manner; Incineration is just one of the possible disposal methods, along with other methods. The greatest advantage of phytoextraction is over v. a. the soil exchange (clearing) with deposit in a landfill ( dig and dump ) as well as acid washing so that the soil functions are preserved. This is particularly important for the remediation of arable land (e.g. when contaminated with heavy metals from sewage sludge ).

Up until then, however, it was problematic that many examined hyperaccumulators such as B. thale cress in Central Europe are very small or grow very slowly. This has been demonstrated in recent series of tests that have already lasted several years. B. circumvented on zinc-contaminated and not too acidic surfaces by using specially selected cultivars from Central European crops (including neophytes such as tobacco and sunflowers ) with high biomass, even if only medium-heavy metal enrichment. Overall, these cultivars then extract more net than hyper-accumulators with relatively low biomass. In addition, environmentally friendly disposal is much easier, especially if the extracted substances are essential substances in animals and humans (such as zinc).

It is important to consider that the soil properties (such as the pH value), the amount of soil cover, the microclimate conditions and the type of fertilization have a significant influence on the success of a phytoextraction. Alkaline soils are less suitable than slightly acidic soils. Correctly carried out phytoremediation not only has a pollutant-reducing effect, but also stabilizes the pH value relationships and promotes the desired crumb formation.

The aim of phytoremediation does not necessarily have to be to remove excessively high total levels, e.g. B. of other naturally existing substances in the soil such as zinc in spoil heaps. It can also only serve to remove excessively high soluble contents and at the same time prevent their subsequent delivery from the total (mainly poorly soluble) pollutant content in the soil. This replaces an otherwise much more expensive and ecologically not always advantageous soil replacement. In the sense of “intelligent” remediation of contaminated sites, phytoremediation is not the panacea for all soil pollution , but rather a supplement to other soil remediation processes. In general, in accumulator cultivars above certain pollutant concentrations, the rate of accumulation decreases sharply due to symptoms of poisoning (necrosis). Whether phytoremediation can be used successfully therefore always requires a careful clarification of the specific conditions on site by specialists before their use.

The cultivation of thick-stemmed water hyacinths can also lower the content of cyanides and copper in waste water from gold mining .

Phytodegradation

During phytodegradation, the (mostly organic) pollutants that the plant absorbs are not accumulated, as is the case with phytoextraction, but chemically changed and thus inactivated. Processes in which the pollutants are not absorbed by the plant, but rather the plants only stimulate the breakdown of the pollutants by microorganisms , are also part of phytodegradation. This stimulation occurs on the one hand through the better assimilate supply of the microorganisms due to the rooting of the soil (rhizodeposition) and on the other hand also through certain substances that are excreted by the plant roots (root exudates).

Phytomining

The extraction of metals with the help of plants is called phytomining. In contrast to phytoextraction , this process only applies to metals. These metals can be recovered from combustion residues, for example. So far, this method has only been suitable for reducing the costs of phytoremediation (or phytoremediation) somewhat by recovering the extracted metals. However, research is being carried out - on the model plant Hallersche foam cress , for example - into using phytomining for ore mining - for example to extract rare earths .

Phytovolatilization

During phytovolatilization, the plant absorbs the pollutants with its roots and releases them into the air through its aboveground organs. This can lead to biochemical conversions in the plant into volatile forms of the pollutants. In the case of mercury , this occurs through the methylation of the mercury to methylmercury by the plant. Methylmercury is volatile and can therefore be released into the air by the plant.

Rhizofiltration

Rhizofiltration is a process in which the plants are not directly involved in the remediation, but rather, through their root system, contribute to increasing the microbial activity in the soil. The pollutants are then broken down by microorganisms. In relation to groundwater remediation, rhizofiltration also generally means absorption and condensation on the plant roots and / or uptake and accumulation in the roots.

Phytostabilization

Phytostabilization is not a remediation process, but only serves to secure the soil. In general, the mobility of pollutants is reduced. A thick grass cover protects the soil from erosion . The superficial removal of pollutants by the wind and the rain is thus prevented. Pollutants can adsorb on the fine roots, they are thus kept in the soil and their leaching into the groundwater is reduced. The transpiration of the plants leads to a largely upwardly directed water flow, which can also prevent the pollutants from being leached into the groundwater. In addition, certain substances excreted by the plant roots can lead to an immobilization of the pollutants (e.g. precipitation of heavy metals).

Bioaugmentation

→ Main article bioaugmentation

literature

Matthias Kästner, Bernd Mahro, Reinhard Wienberg: Biological degradation of pollutants in contaminated soils with special consideration of the polycyclic aromatic hydrocarbons . Economica Verlag, Bonn 1993, ISBN 3-87081-142-0 .
Terry Norman, Banuelos Gary: Phytoremediation of Contaminated Soil and Water , CRC Press LLC., 2000.
Puschenreiter & Wenzel (2003): Plants as metal swallowers , in: Ländlicher Raum, online specialist newspaper of the Federal Ministry for Agriculture, Forestry, Environment and Water Management, edition 01/2003.