The Cannabis sativa plant has long been recognized for its potential role in removing contaminants from soil in a process known as phytoremediation. However, the use of constructed wetlands and biomass plants as phytoremediants are not new. So how does hemp stack up against what we already know? What can you do with hemp that has been used for phytoremediation?
First, a little bit about plant biology. Different species of plants have differing tolerance and sensitivity to accumulating metals from the soil. Generally, this is done through the roots where the metals are found in some readily absorbable and water-soluble form (like as a salt). Plants, thus, fall into one of three categories: excluders, indicators & accumulators. The majority of agricultural plants are indicators and the metal content of these plants generally increases as soil concentration of metals increase. Accumulators concentrate higher amounts of metals in plant tissue than indicators, and “hyperaccumulators” are those plants that can commonly grow on metal-bearing soils without symptoms of toxicity. Plants can store metals in either below ground (roots) and above ground (shoots, leaves, seeds) parts. Leaves tend to hold more metals than seeds, so in general, vegetables will extract more metals into above ground parts than cereal grains.
The process of phytoremediation can remove organic and/or inorganic pollutants (metals, pesticides, persistent organic pollutants) from contaminated soil, sludge, sediments and water. Many small communities in the US have used aquatic plants and constructed wetlands to treat wastewater. To be truly useful, a plant should be native, grow quickly, have extensive roots, high biomass yield and be able to adapt to various habitats and possess the ability to accumulate pollutants in aboveground parts.
EPA has noted that maize, sorghum, tobacco and lucerne are known metal accumulat ors and plants from the famililes Caryophyllaceae, Brassicaceae, Cyperaceae, Poaceae, Fabaceae and Chenopodioceae. Fast-growing trees (especially willows) are very promising phytoremediants due to their large aboveground biomass and effective use of cadmium and zinc. Maize has been noted as perhaps an ineffective species for cadmium and lead due to large amount of metal accumulation in roots (below ground biomass); however, some herbaceous species are good for lead removal (such as Indian mustard, rye grass, sunflower or smallwing sedge).
In 2006, a research group from the Czech Republic compared how different biomass crops (white sweetclover, red clover, safflower, curled mallow and hemp) performed in both container and field experiments to remove arsenic, cadmium, lead and zinc from soil. Safflower was the most efficient at removing cadmium and zinc from soil in containers and in field experiments. None of the plants tested removed appreciable amounts of arsenic or lead. Overall, safflower and curled mallow showed the greatest effectiveness. Cannabis sativa was not found to be competitive with these plants in field experiments for the removal of cadmium, lead and zinc.
A Chinese research group in 2009 looked a cadmium removal rates in a number of potential biomass/energy crops, including hemp. Hemp, flax, castor and peanuts were found more tolerant to cadmium than soybean sunflower, safflower and rapeseed. In terms of total cadmium uptake from soil, peanut removed roughly 5x more cadmium than hemp. Hemp is perhaps a poor candidate for removing cadmium as it mostly collects this metal in its roots, while safflower and field mustard (Brassica rapa spp.) collected much higher quantities of cadmium in above ground parts (shoots). Plants that collect more metals in the roots as opposed to shoots are less effective for phytoextraction and more useful for phytostabilization. Hemp appears to be an outstanding candidate for phytostabilization of cadmium in soil.
This is but one small example of the type of analysis that would be needed on individual metals to determine to best phytoextraction crop for that metal. While it has been noted that hemp is a hyperaccumulator of metals in popular press accounts, this is clearly not the case when looks at individual compounds.
Now, we turn our attention to the uses of hemp roots and shoots that have been used for phytoremediation applications. Most heavy metals found as contaminants in soil are not desired as inclusions in any product that will be consumed by humans or animals. In others words, hemp as a “mop crop” to pull metals out of soil (i.e. phytoextraction) cannot be repurposed as human food or animal feed. This is not just an issue for hemp grown on contaminated soils, though. Why?
In 2012, a Romanian research group examined the metal content of hemp seed and oil, which can include essential elements for human biology (such as magnesium, phosphorus, iron and manganese) as well as undesired metals (zinc and cadmium). This work showed that significant amount of iron, manganese, zinc and cadmium can result in seeds grown in light to moderate areas of contamination and the authors concluded than hemp may be useful for phytoremediation of iron, zinc and cadmium.
Clearly, additional careful research is needed to ascertain the ability of the hemp plant to extract specific metals from soils into specific parts of the plant. If metal contents in seed are below levels of health concern, then it may be possible to use that crop for human food or animal feed. It may be the case that hemp does not extract certain metals into above ground parts at appreciable levels, which could potentially be the case for arsenic and lead.
Sources used:
Mihoc, M., Pop, G., Alexa, E., & Radulov, I. (2012). Nutritive quality of romanian hemp varieties (Cannabis sativa L.) with special focus on oil and metal contents of seeds. Chemistry Central Journal, 6(1), 122.
Rezania, S., Taib, S. M., Md Din, M. F., Dahalan, F. A., & Kamyab, H. (2016). Comprehensive review on phytotechnology: Heavy metals removal by diverse aquatic plants species from wastewater. Journal of Hazardous Materials, 318, 587–599. http://doi.org/http://dx.doi.org/10.1016/j.jhazmat.2016.07.053
Shi, G., & Cai, Q. (2009). Cadmium tolerance and accumulation in eight potential energy crops. Biotechnology Advances, 27(5), 555–561.
Tlustoš, P., Száková, J., Hrubý, J., Hartman, I., Najmanová, J., Nedělník, J., … Batysta, M. (2006). Removal of As, Cd, Pb, and Zn from contaminated soil by high biomass producing plants. Plant, Soil and Environment, 52(9), 413–423.