Which Plants are Useful in Phytoremediation?
In response, it was decided that a concerted effort to reduce soil contamination through the use of beneficial plants would be undertaken. This process, known as phytoremediation, began immediately, and used various plants to take up specific contaminants—two brassica varieties to remove chromium, lead, copper and nickel, maize to take up lead (various researchers have demonstrated the remarkable lead-uptake capability of this important crop), and more recently, sunflower and hemp.
Sunflower plantings began in 1996 subsequent to the development of a variety that promised hitherto unheard-of efficiency of decontamination; hemp plantings soon followed, in 1998. Slavik Dushenkov, a research scientist with Phytotech, one of the organisations behind the hemp plantings, stated that “hemp is proving to be one of the best phyto-remediative plants we have been able to find”.
Much rural land in neighbouring Belarus was affected by the explosion, and authorities there are also pursuing the use of hemp as a decontaminate. The harvest produced will be turned into ethanol, as increased production of biofuel is a key target for increasing the overall economic and environmental health of the region.
The Chernobyl exclusion zone, site of the world’s worst nuclear disaster, is slowly being returned to health as plants and animals begin to reclaim the land.
Differences in Metal Uptake From Soil
In 2012, a Romanian study investigated the nutritional safety of hemp seed produced from plants grown in soils rich in calcium, magnesium, potassium and iron. The study determined that five distinct Romanian hemp strains developed different nutritional profiles according to uptake of the various metals in the soil. For example, the Zenit strain exhibited highest rates of calcium uptake, while the Armanca absorbed least calcium; the Diana, Denise and Silvana strains absorbed magnesium at the highest rates, and the Zenit variety showed the highest concentrations of iron.
Despite the differences, the seeds and oil of all five strains exhibited high levels of magnesium, calcium, iron, manganese, zinc and potassium, all highly beneficial dietary metals. However, all varieties also tested above the safe legal limit for cadmium, a toxic heavy metal that may cause various health complications—despite the soil being within the safe limit for cadmium concentration. The Armanca and Silvana strains showed particularly high cadmium levels.
Hemp and Cadmium Absorption
Excessive consumption of foodstuffs high in cadmium can lead to deformities in joints and bones, respiratory illness, anaemia, and kidney failure. In areas where cadmium is present in the soil, in order to be safe for human or animal consumption, poor cadmium uptake varieties of hemp should be selected.
According to a study into Chinese hemp strains conducted in 2011, many hemp strains have the ability to absorb and accumulate even large quantities of cadmium in soil without detriment to the plant itself. While this does throw up various implications for selection of sites for cultivation of food-safe hemp, it also indicates that cadmium-contaminated sites will particularly benefit from phytoremediation schemes that make primary or exclusive use of hemp. Furthermore, even if hemp used to decontaminate soil is unsafe for consumption, it can still be used in a number of industrial applications, such as for biofuel.
As vegetation slowly reclaims formerly inhabited areas, adding species known to be effective extractors of soil-based heavy metals can aid in ecosystem rejuvenation.
Hemp is Mostly Unaffected by Heavy Metals in Soil
Hemp’s resilience to contaminants in soil is well-documented. Even as early as 1975, a study published in the Agronomy Journal described how soil characteristics influenced elemental uptake and could even affect final cannabinoid profile in psychoactive strains. To illustrate this, fifteen sites with varying soil profiles were planted with the same strain of Afghan cannabis, and their harvests tested for metal content. Researchers concluded that differences could be used to determine geographic origin of cannabis through foliar analysis.
In 1995, the Polish Institute of Natural Fibres released a study demonstrating that tested varieties were able to withstand high levels of heavy metals in soil without impacting plant growth, yield or fibre quality. However, little research has been done into the safety of using fibres in clothing or other forms of industry, and this issue must be investigated fully in order to establish the possible uses for hemp grown in such conditions.
As a proven, valuable tool in the fight to repair human-inflicted damage to our soils and ecosystems, hemp could potentially benefit hundreds of thousands of sites across the globe—it is estimated that in the USA alone there are 30,000 sites requiring remediation. As is so often the case, US restrictions on hemp cultivation preclude any large-scale operations from being implemented, and the contaminated sites are largely left un-remediated, through lack of both funding and interest on the part of the government, although with the legalisation of cannabis in a handful of states there is optimism for the future.