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.
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.
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.