· Scientific Context · 

The phytoremediation

As defined by Cunningham et al. (1995), the phytoremediation is the use of plants to remove, contain, or degrade environmental contaminants in water, in soil or in air. The natural processes involved in the phytoremediation and their interactions are quite complexes, and the research effort on this area should be supported by the willingness of the governments, in order to develop the full potential of the phytoremediation at the industrial scale.
The term “phytoremediation” is a general term which designates in fact several natural processes/green technologies; to learn more on the phytoremediation, see for example McCutcheon and Schnoor, 2003.
For polluted soils (see Fig.1)

• Phytostabilization (1): For soils polluted by inorganic contaminants (such as metals), resistant plants can be used on site to simply immobilized pollutants in the soil, onto and inside the root system of the plant.
• Phytoextraction (2): Some specific plants species called “hyperaccumulatrices” are not only resistant to pollutant but they can also accumulate and transfer large amounts of pollutant in their above-ground parts. In that case, the pollutant is extracted progressively from the soil. Generally the hyperaccumulators are plants which produced a weak biomass.
• Rhizodegradation (3): In the case of soils polluted by organic contaminants, the root system of the plant greatly promotes the microorganism activity (up to 100 times more as compared to the same soil without plant; Macek et al., 2000). The root system and its associated microorganisms exudate numerous chemical compounds in the soil solution which are able to degrade organic pollutants.
• Phytodegradation (4): These latter pollutants can be also absorb by the roots and metabolized within the plant.
• Phytovolatilization (5): At last some pollutants can be metabolized in plant and released in the air by the shoots under gaseous form, which is generally less harmful for the environment that the primary chemical form of the contaminant.

Figure 1: Schema of the main natural processes underlying the phytoremediation.

For waste waters

• Rhizofiltration: for the treatment of waters, a phytoremediation technique consists in immersing the root system of aquatic plants directly in the water to treat. The contaminants are chemically adsorbed onto the root and they are also absorbed by the plant; the fate of the contaminant in the plant (phytodegradation/phytovolatilization or phytoextraction) will depend on its nature, and it will also depend on the plant used. Terrestrial plant can be also used for rhizofiltration if the water to treat is well aerated and supplied with nutrients (similar to hydroponics).
• The constructed wetland (CW) is the most widespread technology using the phytoremediation for the water treatment in France (mainly for domestic waters), and probably in the world. To simplify, the CW can be regarded as an artificial marsh which are generally impervious in order to control the outlet water of the device. In the case of the most elaborate CWs, the basin which constitutes the bed of the CWs is filled with several layers of selected filtration materials (the granular size of the materials ranges from gravels to sand); The macrophytes (i.e. aquatic plants) are, obviously planted in the upper part of the filter; Common reed (Phragmites australis) is a macrophyte which is often used in the CW.
  
  There are several kinds of CWs which can be classified according mainly on 3 criteria (Vimazal 2011): (i) the flow path of the inlet water (vertical or horizontal), (ii) the way of the water input (open-water surface flow or subsurface flow) and the types of macrophytes used (emergent, submerged or free-floating). Several kinds of CW can be combined with each other depending on the treatment effectiveness expected. For more details on the CWs the reader can refer to the review of Vimazal (2011).
  
  The CWs act obviously as a physical filter, and generally the role attributed to the plant in the treatment is regarded as minor: basically, they improve the aeration of the upper part of the filter due to the motion of the stem with the wind, and they reduce the risks of clogging thanks to the development of the root system in the filtration material. However, phytoremediation processes, and more especially the rhizodegradation, also contribute to the efficiency of this technology. Indeed, the particles constituting the filtration materials serve as a support for the establishment of the microorganism colonies and the root system promote their activity (see above). Depending on the nature of the pollutant and on the plant used, other phytoremediation processes, such as the phytodegradation, the phytostabilization and the phytoextraction can also occur.

Why using bamboos?

There are approximately 1200 species of bamboo all around the world (Bystriakova and Kapos, 2006). Compared to other plants used in phytoremediation, the bamboo is a good candidate for phytoremediation applications on many points:

• Bamboos are resistant to environmental stresses (lack or excess of water, temporary flooding, frozen, wind…), and also to numerous contaminants; as a most striking example, bamboo survived the Hiroshima atomic blast closer to ground zero and it has been the first plant species which re-installed a vegetal cover after the blast (environmental bamboo foundation ; http://www.bamboocentral.org/).
  
  
• Bamboos are among the most productive terrestrial plants in the world.  
  
  The biomass yield depends obviously on the bamboo species, the climate, the agronomic value of the soil, and also on the age of the bamboo forest. Under temperate climate a mature bamboo forest (more than 5-7 years) can produce between 20 and 40 T of Dry Matter (DM)/ha/year. This production of biomass can be achieved in only 2 years under conducive conditions. The biomass of an exploited mature bamboo forest can reach up to 287 T of DM/ha (Bois 2010).
  
  
• A bamboo forest is an efficient carbon trap.
  
  
• Bamboos generate high evapotranspiration flows, which can considerably reduce the volume of water outing from the phytoremediation device.
  
  
• The wood and fiber of bamboo have interesting properties, which find many commercial applications (construction material, paper manufactory, energy production…).
  
  
• Bamboos species exist in almost all over the world.
  
  
• The bamboos have a rhizomatic root system (rhizome = underground stem) with a very dense root running network (Fig. 2). This specific root architecture of bamboos considerably favors the rhizodegradation process, and also reduces the risks of clogging.

 

Figure 2: The rhizomatic root system of bamboo.

• For a phytoremediation purpose, the development of bamboos can be easily controlled. Indeed, bamboos have a vegetative reproduction (that is say that a bamboo forest extends via the growth of the rhizomes in the soil) contrary to the most part of higher plants which a have a sexual reproduction. The sexual reproduction occurs also for bamboos, but actually it is a minor way of reproduction (the bamboos flower at long intervals, strangely, some species have never been observed flowering). So, if the growth of the rhizomes is properly confined inside the area to remediate, bamboo is not an invasive plant.
  
  
• Among the bamboo species which can be used for the phytoremediation, there is a wide variety of colors and shapes available to select the stem that gives an undeniable aesthetic aspect to the treated site (Fig.3).
  
  
• A bamboo forest can live more than one century.

Figure 3: Some Bambou-Assainissement® sites.

References cited:

• Bois 2010, final report of the research program “RUN INNOVATION II”.
• Bystriakova and Kapos, 2006. Biodiversity 6 (4):12-16.
• Cunningham, Berti, and Huang, 1995. Trends in Biotechnol. 13: 393-397.
• Macek T., Macková M, Kás J., 2000. Biotechnology Advances 18 (2000) 23–34.
• McCutcheon and Schnoor, 2003. PHYTOREMEDIATION Transformation and Control of Contaminants.
• Vimazal, 2011. Environ. Sci. Technol., 45 : 61–69.
• Environmental Bamboo Foundation; http://www.bamboocentral.org