coppice willow photo courtesy of http://woodlands.co.uk
Some metals, such as zinc (Zn) and copper (Cu) are
micronutrients needed in small amounts by plants, animals and humans alike, for
optimum health. Others, such as cadmium (Cd), aluminium (Al) and lead (Pb) are
not needed and can be toxic to humans, animals and ecosystems. Thankfully,
there are plants called hyperaccumulators that can remove these metals from the
soil and store them, usually in the vacuole, out of the way. The accumulated
metal serves no apparent purposes to the plant, but some believe (e.g. Rascio
and Navari-Izzo 2011) it serves as a defence against natural enemies, such as
herbivores.
Much research has been carried out and a few plant species
have been discovered that have been used for phytoremediation or phytodegradation, which is the use
of higher plants and their rhizosphere microorganisms to remediate soils
contaminated with toxic metals by industrial activities. Toxic metals have to be removed from soils to prevent
contamination of surface and ground water.
The phytoremediation technique has been shown to be
effective for decontamination of soils in several laboratories and field
studies (Lou et al. 2013; Tian et al. 2012). Since the first studies on the
subject over 20 years ago, several plant species have been found that can
remove not only heavy metals but also other pollutants from the environment,
e.g. arsenic, and this has become a very useful green technology used to remove
all sorts of pollutants, including organic compounds and radionuclides from the
environment, with some plant species being better than others at the job.
Plant species researched for their ability to remove metals
from soils include: willow (Slycken et al. 2013), who looked into metal uptake
and extraction potentials of eight willow clones in soil contaminated with Cd
and Zn at concentrations of 6.5 ± 0.8 and 377 ± 69 mg/kg soil, respectively;
maize (Tian et al. 2012), who investigate the phytoremediation potential of the
bioenergy crop maize in Cd-contaminated soil; and Thlaspi
caerulescens v maize (Lombi et al. 2001), who compared removals of Zn and Cd in
agricultural soils by the two plants.
In Slycken et al. (2013) the study of short rotation
coppice (SRC) evaluated growth, metal uptake and extraction potentials of eight
willow clones (Belders, Belgisch Rood, Christina, Inger, Jorr, Loden, Tora and
Zwarte Driebast) on a metal-contaminated agricultural soil, with total Cd and
Zn concentrations of 6.5 and 377 mg/kg soil, respectively. Although, during the
first cycle, generally low productivity levels (3.7 ton DM (dry
matter)/ha/year) were obtained on the sandy soil, certain clones exhibited quite
acceptable productivity levels (e.g. Zwarte Driebast 12.5 ton DM ha/year). Even at
low biomass productivity levels, SRC of willow showed promising removal
potentials of 72 g Cd and 2.0 kg Zn ha/year, which is much higher than e.g. energy
maize or rapeseed grown on the same soil, the authors reported. Cd and Zn removal can be increased by
40% if leaves are harvested as well. Nevertheless, nowadays the wood price
remains the most critical factor in order to implement SRC as an acceptable,
economically feasible alternative crop on metal-contaminated agricultural
soils, added the authors.
Metals that hyperaccumulator plants remove from soils can
be recovered from the plant tissue in a process called phytomining (growing
plants to harvest the metals). There are many plant species also being used to produce biofuels; combine the two and we would have multi-tasking plants, i.e. the same plants being used to clean up pollution, recover metals from contaminated soils and for producing biofuels? Well, various studies are being carried
out to find these multi-tasking plants.
Sorghum bicolor
L., which is normally an important crop widely used as food, feed and energy
crop has also been shown to immobilize heavy metals in contaminated soils. In addition, sorghum appears promising for
bioethanol production (Epelde et al.,
2009).
Castor bean (Ricinus
communis L.), which was investigated by Olivares et al. (2013). They looked at the potential of castor bean to remediate sites polluted with mine
tailings containing high concentrations of Cu, Zn, manganese (Mn), Pb and Cd and as an energy crop.
Willow
and maize are also being used as a biofuel source. Biomass from SRC willow is already used for heat
and power, but it also has potential as a source of lignocellulose for liquid
transport biofuels, as shown in a study by Brereton et al (2010), which showed
that SRC willow has strong potential as a source of bioethanol and that there
may be opportunities to improve the breeding programs for willows, for
increasing enzymatic saccharification yields and biofuel production.
Resources conservation and sustainability have become important
goals; hence the prospect of using plants for cleaning environmental pollution,
while also serving as biofuel source (multi-tasking) is an attractive prospect.
References
Brereton, N. J., Pitre, F. E. Hanley, S. J., Ray, M., Karp,
A. & Murphy, R. J. (2010) Mapping of enzymatic saccharification in short
rotation coppice willow and its independence from biomass yield. Bioenergy Research 3, 251-261. DOI:
10.1007/s12155-010-9077-3.
Epelde, L.; Mijangos, I.; Becerril, J. M.; Garbisu, C.;
Jones, D.; Killham, K.; van Hees, P. (2009)
Soil microbial community as bioindicator of the recovery of soil
functioning derived from metal phytoextraction with sorghum. Soil Biology & Biochemistry,
41(9):1788-1794.
Lombi, E.; Zhao, F.J.; Dunham, S.J.; McGrath, S.P. (2001). "Phytoremediation of Heavy
Metal, Contaminated Soils, Natural Hyperaccumulation versus Chemically Enhanced
Phytoextraction". Journal of
Environmental Quality 30 (6):1919–26.
Lou YanHong, L.; Luo HongJi; Hu Tao; Li HuiYing; Fu
JinMin (2013). Toxic effects, uptake,
and translocation of Cd and Pb in perennial ryegrass. Ecotoxicology, 22(2):207-214.
Olivares,
A. R.; Carrillo-González, R.; González-Chávez, M. del C. A.; Soto Hernández, R.
M. (2013). Potential of castor bean (Ricinus
communis L.) for phytoremediation of mine tailings and oil production. Journal of Environmental Management, 114:316-323.
DOI 10.1016/j.jenvman.2012.10.023.
Rascio N, Navari-Izzo F. (2011) Heavy metal
hyperaccumulating plants: how and why do they do it? And what makes them so
interesting? Plant Sci. 2011
Feb;180(2):169-8. doi: 10.1016/j.plantsci.2010.08.016.
Slycken, S. van; Witters, N.; Meiresonne, L.; Meers, E.;
Ruttens, A.; Peteghem, P. van; Weyens, N.; Tack, F. M. G.; Vangronsveld,
J. (2013). Field evaluation of willow
under short rotation coppice for phytomanagement of metal-polluted agricultural
soils. International Journal of Phytoremediation,
Vol. 15(7): 677-689. DOI 10.1080/15226514.2012.723070.
Tian YongLan; Zhang HuaYong; Guo Wei; Chen ZhongShan; Wei
XiaoFeng; Zhang LuYi; Han, L.; Dai, L. M. (2012). Assessment of the
phytoremediation potential in the bioenergy crop maize (Zea mays) in soil
contaminated by cadmium: morphology, photosynthesis and accumulation. Fresenius Environmental Bulletin,
21(11c):3575-3581. URL: http://www.psp-parlar.de.