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Potential Applications of Cannabis sativa in Environmental Bioremediation. A Review.
Alufasi, Richwell1, Bagar Tanja2, Chingwaru, Walter1§
1Biological Sciences Department, Bindura University of Science Education, P. Bag 1020 Bindura,
Zimbabwe.
2ICANNA, Slovenia
§Address correspondence to Walter Chingwaru, Biological Sciences Department, Faculty of Science,
Bindura University Science Education, P. Bag 1020, Bindura, Zimbabwe; Phone: +263 7777 666 06;
Email: wchingwaru@yahoo.co.uk.
Abstract
Hemp / marijuana, scientifically referred to as Cannabis sativa, is a controversial herb in all spheres
of society. While the herb is praised for its novel therapeutic and perhaps prophylactic properties
against a number of ailments, including cancer, diabetes, hypertension and a number of neuronal
diseases, the herb is popular for its psychoactive properties, hence a major drug of abuse. Besides
these uses, hemp is also a major source of materials of industrial importance. In the field of
environmental protection, hemp has also found a place in bioremediation, with applications that
include ridding environments of biological and chemical contaminants, particularly in wastewater and
solid waste management. The following account appraises the known and potential applications of
hemp in environmental remediation. The specific uses and mechanisms that hemp employs in the
bioremediation processes include: (i) phytoexraction, (ii) rhizofiltration, (iii) phytodegradation and
(iv) phytovolatisation. Based on the novelty of applications of hemp in bioremediation, further
research is urged to unravel the full potential of the herb in all spheres of environmental management.
Keywords
Solid waste; phyoremediation; Cannabis sativa; hemp; heavy metals; leachate
Introduction
Hemp / marijuana, scientifically referred to as Cannabis sativa L., is a controversial herb in all
spheres of society that has been cultivated for over 6000 years (Vaverkova et al., 2017). While the
herb is praised for its novel therapeutic and perhaps prophylactic properties against a number of
ailments, including cancer, diabetes, hypertension and a number of neuronal diseases, the herb is
popular for its psychoactive properties, hence a major drug of abuse. Besides these uses, hemp is also
a major source of materials of industrial importance. It has a wide range of application including its
uses as a source of seed oil, industrial fibre, livestock feed, food as well as for recreation, religious
and spiritual practices (Kumar et al., 2017), paper-making, cosmetics, personal care and
pharmaceutical product manufacturing (Vaverkova et al., 2017). Lately hemp has found a place in
the field of environmental protection, and studies are now focused on the potential use of the plant in
bioremediation of sites contaminated with toxic or hazardous anthropogenic wastes. Hemp is an
annual plant that has a high biomass, high tolerance to drought and heavy metal stress as well as high
metals accumulating capability (Ahmad et al., 2015), which qualify it for use in phytoremediation.
Phytoremediation, a technology that is based on the use of green plants to extract, sequester and/or
detoxify pollutants, has been gaining wide interest, and is considered a sustainable approach for
remediation of sites contaminated with toxic or hazardous substances (Gomes, 2012; Kumar et al.,
2017). The technology is widely regarded cost-effective and reliable, and can be used to remove a
wide range of pollutants like inorganic chemicals including heavy metals and metalloids, many
organic substances including persistent organic pollutants and radioactive elements from
contaminated environments (Pandey et al., 2016, Yao, 2017, Vaverkova et al., 2017).
Several studies explored the potential use of various plants in environmental protection through
phytoremediation. Indications are that plants can be used sustainably to decontaminate polluted
environments. Of interest in the field of environmental protection, hemp has also found a place in
bioremediation, with applications that include ridding environments of biological and chemical
contaminants, particularly in wastewater and solid waste management. The following sections
appraise the known and potential applications of hemp in environmental remediation.
Hemp and remediation of heavy metal contaminated soils
Pollution of the environment with heavy metals has dramatically accelerated during the last century
(Barazani et al., 2004) as humans began to engage in mining, smelting, manufacturing, disposal of
municipal waste (Ayers, 1992). Soil contamination by heavy metals is a major problem to the world
today (Ahmad et al., 2015). Heavy metals are known to persist in the environment since they are not
chemically or biologically degradable (Barazani et al., 2004). Several studies have explored the use of
C. sativa in the remediation of heavy metal contaminated soils. Uptake and accumulation of a variety
of heavy metals including nickel (Ni), lead (Pb), cadmium (Cd), zinc (Zn), copper (Cu) and chromium
(Cr) in hemp tissues have been reported (Linger et al., 2002; Kos et al., 2003; Piotrowsk-Cyplik and
Czarnecki, 2003; Tlustoš et al., 2006; Ahmad et al., 2015). A study by Linger et al., (2002), in
Germany, examined the capability of C. sativa to decontaminate heavy metal polluted soils. Field
based experiments using soil polluted with sewage sludge containing Cd, Ni and Pb revealed that
hemp can take up the heavy metals and distribute them throughout the tissues (seeds, leaves, fibres
and hurds) of the plant. However the concentrations of these metals differed between tissues with the
highest concentration in leaves. With regards to cadmium, the study showed that C. cannabis
extracted approximately 120 g Cd per hectare (ha) over a period of 3-4 months. A similar study was
conducted Ahmad et al., (2015) in Pakistan, focusing on phytoextraction of Cu, Cd and Ni by hemp
growing on heavy metal contaminated soil. Heavy metals accumulation rates of 1530 mg kg-1 Cu, 151
mg kg-1 Cd and 123 mg kg-1 Ni were recorded, making the plant a suitable candidate for remediation
of soils contaminated with these metals. The study however reports that increasing concentrations of
metals in soils leads to an increase in heavy metal transfer from roots to leaves and shoots of the hemp
plants. On the contrary, Tlustoš et al., (2006) reports that increase in the concentration of heavy metals
in the soil increases plant growth inhibition due to element toxicity. Highly contaminated soils reduces
plant growth and hence biomass and this affects the rate of metal removal by the plants. However
some plant species have high tolerance to heavy metals with some heavy metal genes ( GSR and
PLDα) having been identified in hemp cultivars (Ahmad et al., 2015).
The tolerance and accumulation potential to cadmium of 18 hemp cultivars was also assessed by Shi
et al., (2012) using pot tests in a greenhouse at the Huaibei Normal University in China. Cd
accumulation rates and distribution in root and shoot tissues of the plant were shown to be
significantly different (p < 0.001). Hemp roots were shown to accumulate high Cd concentrations
(217–481 mg kg−1) compared to the shoots (11.4–24.9 mg kg−1). This is in contrast to earlier reports
by Linger et al., (2002) that high concentrations of Cd accumulate in leaves. However the
concentrations of Cd may have differed due to differences in initial metal concentration in the soil and
hemp biomass. Regardless of the discrepancies in distribution of Cd in plant tissues, almost all hemp
cultivars, except USO-31, Shenyang, Shengmu, and Yangcheng, could tolerate 25 mg Cd kg−1 soil
stress, accompanied with biomass reductions of less than 50% - a dose even higher than that in
heavily polluted soils (Shi et al., 2012). This further confirms the suitability of the various cultivars
suitable for phytoremediation of heavy metal contaminated soils.
In 2005 Linger et al., (2005) investigated the effects of different cadmium concentrations on C. sativa
growth (i.e. on roots, stem and leaves) and on photosynthesis. Study reports high tolerance to
cadmium ( 800 mg of Cd kg˃-1(d.m)) of roots and no major effect on C. sativa. However Cd
concentrations of 50-100 mgkg-1(d.m) had a strong effect on the viability and vitality of leaves and
stems. The study showed that Cd affected chlorophyll synthesis as well the photosynthesis machinery
thus affecting both photosynthesis and overall growth of hemp. However moderate concentrations of
cadmium (17mgkg-1 soil) preserved both growth and photosynthesis apparatus in hemp (Linger et al.,
2005).
Heavy metal removal from the soil is reportedly enhanced by biodegradable chelating agents that
increase bioavailability of metal elements (Malhotra et al., 2014). A study conducted by Kos et al.,
(2003) in Slovenia investigated the effects of chelates ethylenediamine-tetracetic acid (EDTA) and
ethylenediamine-disuccinic acid (EDDS) on phytoextraction of Pb, Zn and Cd by fourteen plant
species. EDDS significantly improved phytoextration in C. sativa but was generally less effective in
other tested plants. In the case of Pb, phytoextraction potential 26.3 kg/ha were recorded for C. sativa,
which was much higher than 126 g/ha as reported by Linger et al., (2002). This improves the
prospects of hemp as a remediation agent. Further investigations are therefore needed on this aspect to
improve phytoextraction of heavy metals even in the management of solid waste and wastewater.
Use of hemp in remediation of landfill leachate
Landfills are considered a convenient and cost effective method for solid waste management in many
countries across the globe. Of note is the fact that solid waste materials in a landfill undergo physical,
chemical and biological transformation which produces leachates (Zloch et al., 2017). The leachate,
which is a major source of pollution, commonly contains large amounts of organic matter,
ammonium, heavy metals, and chlorinated organic and inorganic salts, which in turn are a major
threat to soils and water sources in the vicinity of the landfill (Vaverková1 et al., 2017).
Hemp have reportedly been used in the treatment of landfill leachate. Studies indicates that leachate
can induce both positive and negative responses in the plants (Mor et al., 2013). In 2017 Vaverkova et
al., (2017) evaluated the potential of C. sativa for toxicity removal from landfill leachate. Laboratory
based hydroponic experiments were carried out using raw leachate collected from the pond of
untreated leachate at a sanitary landfill in Czech Republic to investigate effects of different
concentrations of leachate on seed germination and seedling growth of three hemp cultivars. Study
results indicate that leachate can severely inhibit plant growth particularly concentrations greater than
90%. However leachate concentrations lower than 25% stimulated growth. Furthermore the response
to leachate toxicity differed. The toxic effect of leachate on plants depends on several factors
including the plant species and the composition of the leachate. Leachate contains a wide range of
inorganic and xenobiotic organic (XOCs) compounds like hydrophobic, volatile, aromatic and
aliphatic organic substances, (Vaverkova et al., (2017). No studies to show the toxicity of the
individual components on hemp plants were found.
A recent field based study by Zloch et al., 2017 investigated reaction of two C. sativa varieties
(Bialobrzeskie and Monoicaon) to leachate irrigations. Comparisons were made in terms of growth
between plants that were irrigated with leachate and those with rainwater, the controls. Study results
indicate that Bialobrzeskie and Monoicaon varieties watered with rainwater grew 26% and 34% taller
on average respectively, than plants watered with leachate. This result supports earlier results by
Vaverkova et al., (2017) indicating that leachate inhibit growth of C. sativa. However growth
inhibition and or toxicity may not be the same in seeds, seedling and older plants. Further
investigations are therefore necessary on the toxicity of leachate on hemp plants, that is, in terms
which cultivar can tolerate leachate toxicity and toxic substance removal from leachate. Although
results indicate high levels of inhibition to growth of hemp, other studies conducted revealed that it
can accumulate a considerable amount of heavy metals making it a good candidate for remediation.
Conclusion
Phytoremediation is fast developing field and metals uptake by plants seems to be an economic and
sustainable way to remediate contaminated environment. Evidence from the studies above indicates
that C. sativa (hemp) can tolerate heavy metals thus can grow in heavy metal contaminated soils
removing metal contaminants from soils and landfill leachate. The improved uptake of metals due to
application of chelating agents and the presence of metal tolerance genes affirms the suitability of the
species for phytoremediation. These genes can be instrumental in the transformation of other varieties
or species, through genetic engineering, into effective bio-accumulators. Furthermore hemp can
accumulate significant amounts of heavy metals in its tissues due to its high biomass and deep roots.
This makes it a good candidate for phytoremediation. Although little research have been done in the
application of phytoremediation of landfill leachate, the potential of hemp in cleaning up
contaminants from hemp is promising. Based on the novelty of applications of hemp in
bioremediation, further research is urged to unravel the full potential of the herb in all spheres of
environmental management.
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