Miscanthus Giganteus is suitable for phytoremediation of contaminated land

By: Vijayalaxmi Kinhal

As the popularity of Miscanthus Giganteus increases, and it is promoted as the obvious choice for bio-fuel, the area needed to grow it becomes an important consideration. Land-use becomes an important criteria to ascertain sustainability of Miscanthus Giganteus, in addition to the other usual considerations of carbon sequestration, emissions in water and air, water use and impact on biodiversity.

Land-use change

Demand for land to grow Miscanthus Giganteus should not compete with land for food production, which leads to food production shifting to new areas where high carbon stocks, like forests, peatlands and permanent grasslands, are cleared causing ‘indirect land-use change'. Direct land use change occurs when high carbon stocks are cleared to grow other crops. To make Miscanthus Giganteus cultivation more sustainable and carbon neutral, it can be grown on marginal, degraded or contaminated land. This raises the question; do these lands meet the growth requirements necessary to maintain the potentially high yields that make this bioenergy crop popular? (1).

Contaminated land

There are 2.5 million potentially contaminated sites across Europe, of which approximately 14 % (340,000 sites) have already been confirmed as being contaminated. Of these, only 14% have been remediated (2). In the UK, there are 157,000 acres of brown fields, of which 89,000 acres are still derelict (3) Municipal and industrial wastes disposal and treatment is responsible for contaminating 38% of soil, followed by 34% from the industrial and commercial sector (mining, oil extraction and production, petrol stations, military camps, and nuclear power plants). The European Union (EU) Thematic Strategy for Soil Protection, has identified soil contamination as one of eight important concerns. Of the 3 billion solid wastes produced in Europe every year, 90 million tonnes are hazardous. 60% of soil contamination is caused by mineral oil and heavy metals. Lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), chromium (Cr), copper (Cu), selenium (Se), nickel (Ni), silver (Ag), and zinc (Zn), and to a lesser extent aluminium (Al), cesium (Cs), cobalt (Co), manganese (Mn), molybdenum (Mo), strontium (Sr), and uranium (U) are the metal contaminants (4). A cost of 6.5 billion Euros is incurred every year by private firms to remedy the soils. However, there are environmental and health costs borne by the public too (2).

Health and environmental risks of soil contamination

Different contaminants have differing effects on human health and the environment, depending on their dispersion, bioavailability and carcinogenicity etc. They can bio-accumulate and are carcinogenic for animals and humans; in people a variety of cancers such as lymphoma, leukemia, and liver and breast cancers are caused by these contaminants (4). Land used for waste disposal treatment or irrigated by raw waste, is contaminated by organic matter, human remains, heavy metals, pesticides, medical waste, micro-organisms, pharmaceuticals, inorganic and organic compounds and personal care products. Large scale use of mineral oil, as a consequence of petroleum transport, storage and refining, or accidents leads to environmental contamination. The concentration of metals and pollutants makes the soil toxic. It can also cause shifts in abundance and diversity of beneficial bacteria and fungi in the soil. This leads to loss of soil fertility and reduction in crop production (4).

Phytoremediation

Conventional remediation involves excavating the contaminated soil and disposing of it as hazardous wastes to another location. These methods are energy intensive, and involve high resource utilization. On the other hand, phytoremediation is relatively cheap, ecologically friendly and energy passive, and is effective for areas with small and medium concentration of contaminants (3). Wider environmental benefits include improved aesthetic value of land, creation of habitat for wildlife, and land with more ecological value than those of intensively farmed agricultural land. It also meets the economic, environmental and social criteria of sustainable development (3). Phytoremediation by energy crops, works by either extracting the contaminants from the soil little by little each year or by stabilising the contamination. Extraction, can result in long-term reduction of metal concentrations in the soil (3). This process though, is not suitable for mining wastes where inorganic contamination levels are very high. Soil phytostabilisation which prevents dispersion of pollutants by growing metal-tolerant crops is a better solution. The contaminants are stabilised by immobilising them, through accumulation in roots or by changing soil chemistry. So the spread of contamination is prevented and the contaminated land is cleaned. Moreover the pollutants do not enter the food-chain (5).

How effective is Miscanthus Giganteus for phytoremediation?

Miscanthus Giganteus has been proven to be one of the best crops for phytoremediation. It has been tested successfully to stabilise soil near closed coal, lead, zinc and cadmium mines; and clean up land used for municipal treatment, ship building yards, iron and steel works, and oil refinery. It has even been used to rehabilitate land in Chernobyl after the nuclear catastrophe in Ukraine (3,4,5,6,7).

Physiology

Miscanthus Giganteus is tolerant to contamination. It doesn't suffer from stress caused by the high concentrations of metal, nor is its physiology affected. In an experiment in UK, a comparison of Miscanthus Giganteus with other energy crops like willow and reed canary-grass showed that it had the least uptake of metals (Zn and Cd), making it the most suitable crop for biomass production on contaminated land (3). Miscanthus Giganteus can be effectively used for lead, arsenic and antimony remediation too (9). One of the features that makes it particularly suited for phytoremediation is the fact that there is low transfer of pollutants to its aerial parts. Most of the metals and compounds absorbed remains in the rhizome and root-system, so the plant material can be harvested for biomass productivity, as it does not have high levels of contamination (7). However this pattern can defer from metal to metal. In Poland, Miscanthus Giganteus showed little cobalt concentrations in aerial tissue, but copper was equally distributed in all plant parts. Moreover, lands suffering from similar levels of concentrations of metal can transfer varying amounts to the plant based on the bioavailability of the metals (8,9). As no biomass is usually harvested in the first year, test for subsequent years' yield found that concentrations of metal in plant tissue start to decrease from the third year on-wards (8,9).

Increases soil micro-flora and micro-fauna

In a recent study from Paris, Miscanthus Giganteus was used to remediate an area irrigated by raw waste for a century. Within three years, there was increase in the populations of beneficial bacteria and fungi between 1.5 to 3 times. Since these were the bacteria and fungi species that are involved in symbiosis, recycling various nutrients and organic matter mineralisation, soil fertility and thereby crop production was improved. The senescent leaves at the end of each growing season added to organic matter too. Miscanthus Giganteus has also been reported to increase the numbers of many micro-fauna like detritivores, which decompose organic matter and are therefore involved in nutrient recycling. Collombola that fragment organic matter and indirectly help in this process, was another group whose numbers benefited from Miscanthus Giganteus crops (6).

Maintains productivity

In comparison to other energy crops like willow, switchgrass, eastern cordgrass and canary-grass in Poland, Miscanthus Giganteus had the best yield (9). Yields ranging from 7.3 tonnes/acre/year or even 10.41 tonnes/acre/year after three years have been reported, and are impressive even if they are lower than the usual 13 tonnes /acre/ year when cultivated in ideal conditions (5). In case, the pollutants' concentration is very high and the crop cannot be used as biomass, Miscanthus Giganteus can still be used to stabilize soil contamination in old mine tailings to prevent wind erosion, soil erosion and contamination of runoff, thereby reducing risks to humans, livestocks and wildlife (5).

Harvesting Miscanthus Giganteus from contaminated land

Even though, concentration of heavy metals in tissues is low in Miscanthus Giganteus grown on contaminated land, some care is advisable in its harvest and use. It is best used as bio-fuel, and not as bedding or for textiles. If the concentrations of pollutants is higher than permissible levels, it should be mixed with material from clean fields or with wood biomass, to bring down levels of metals in the bio-fuel. Care should also be taken that combustion of biomass from contaminated land occurs in power plants where the metals oxides can be captured. Ash from the combustion, can contain high levels of heavy metals, and should be treated and disposed as hazardous material. It cannot be used for fertilization, nor can the sludge from anaerobic processing of these biomass (8,9). The additional care in the end-use of biomass from contaminated land is offset by the advantage of simultaneously remediating millions of acres in Europe, while reducing GHG emissions by using carbon neutral energy crops, to replace polluting technology like fossil fuels and coal. This option kills two birds with one stone.

Sources

1. D. Mortimer. 2013. North Sustainable Liquid Biofuels from Biomass Biorefining (SUNLIBB). Work Package 8: Sustainability Assessment . Supporting Sustainability Criteria: Miscanthus. North Energy Associates Ltd (Partner 7) – United Kingdom (UK)
2. http://www.eea.europa.eu/highlights/soil-contamination-widespread-in-europe
3. Lord R, et al. 2008. Biomass, Remediation, re-generation (BioReGen Life Project): Re-using brownfield sites for renewable ebergy crops.Proceedings CD, ConSoil: 3-6. ISBN: 978-3-00-024598-4
4. Panagos et al. 2013.Contaminated Sites in Europe: Review of the Current Situation Based on Data Collected through a European Network. Journal of Environmental and Public Health. Volume 2013. http://dx.doi.org/10.1155/2013/158764
5. Wanat N et al. 2013. Potentials of Miscanthus×giganteus grown on highly contaminated Technosols. Journal of Geochemical Exploration 126: 78–84.
6. Bourgeois E et al. 2015. Miscanthus bioenergy crop stimulates nutrient-cycler bacteria and fungi in wastewater-contaminated agricultural soil. Environmental Chemistry Letters:1-9. Doi:10.1007/s10311-015-0532-4 http://link.springer.com/article/10.1007/s10311-015-0532-4
7. Pidlisnyuk V et al. 2014. Sustainable Land Management: Growing Miscanthus in Soils Contaminated with Heavy Metals. Journal of Environmental Protection, 2014, 5, 723-730.http://dx.doi.org/10.4236/jep.2014.58073
8. www.iaea.org/inis/collection/NCLCollectionStore/_…/41131268.pdf
9. Pogrzeba M et al. 2013. Hazards related to Miscanthus x giganteus cultivation on heavy metal contaminated soils. E3S Web of Conferences DOI: 10.1051/C (http://www.e3s-conferences.org or http://dx.doi.org/10.1051/e3sconf/20130129006)