The aim of the research/research questions/why the research, background etc.:
The mobilization of heavy metals by man (through mining from ores and processing for
different applications) has led to the contamination of different environmental segments by
these elements. By contaminating food chain, these elements pose a risk to environmental
and human health. As a result of their release and presence in the ecosystems, these pollutants
are accumulated by living organisms in their bodies and subsequently biomagnified as they
pass from one trophic level to the next. Since man also is at the top of food chain, he is
vulnerable to heavy metal pollution.
Regarding the role of heavy metals in living systems, they are divided into two classes:
essential and non-essential. Essential heavy metals are those, which are needed by living
organisms for their growth, development and physiological functions like Mn, Fe, Ni, Cu and
Zn (Gohre and Paszkowski, 2006) while non-essential heavy metals are those, which are not
needed by living organisms for any physiological functions like Cd, Pb, Hg and As (Mertz,
Phytoremediation of heavy metals by Trifolium alexandrinum
Hazrat Ali, Muhammad Naseer and Muhammad Anwar Sajad
International Journal of Environmental Sciences Volume 2 No.3, 2012
1460
1981; Suzuki and Sano, 2001; Bidar et al., 2006; Peng et al., 2009). Higher levels of heavy
metals disturb the normal physiology and biochemistry of living systems. The most
dangerous heavy metals are Pb, Hg, As, Cd, Sn, Cr, Zn and Cu (Wright, 2007; Gosh, 2010).
Among these, Cd and Pb are the most dangerous metals for human health (Sekara et al.,
2005). Cd is considered as the most serious pollutant of the modern age (Singh et al., 2009).
Cd concentrations above the threshold limit values have been found to be carcinogenic,
mutagenic and teratogenic for a large number of animal species (Degraeve, 1981). Cd has
also been implicated as an endocrine disruptor (Awofolu, 2005). Pb has been found to be
responsible for quite a number of ailments in humans such as chronic neurological disorders
especially in fetus and children. This eventually results in behavioral and attitude changes
with progressive retardation (Awofolu, 2005). Pb-poisoning in children causes neurological
damage leading to reduced intelligence, loss of short-term memory, learning disabilities and
coordination problems (Padmavathiamma and Li, 2007).
The concentrations of heavy metals increase in the environment from year to year
(Govindasamy, 2011). Therefore decontamination of heavy metal-contaminated soils is very
important for maintenance of environmental health and ecological restoration. Different
physical and chemical methods are used for this purpose with each method having its merits
and demerits. However, physical and chemical methods are generally considered as
destructive, expensive, labor-intensive and causing secondary problems (Padmavathiamma
and Li, 2007; Wu et al., 2010). In comparison, phytoremediation is a novel, less expensive,
efficient, environment- and eco-friendly remediation strategy with good public acceptance
(Turan and Esringu, 2007; Singh et al., 2009; Saier and Trevors, 2010; Revathi et al., 2011).
Phytoremediation is the use of green plants for decontamination of polluted sites (soils and
waters). It can be used for heavy metals and radionuclides as well as for organic pollutants.
Phytoremediation technology is a recent technology. Most research efforts are focused in this
field since the last two decades (1990 onwards). Different techniques of phytoremediation
include phytoextraction, phytofiltration, phytostabilization, phytovolatilization and
phytodegradation (Alkorta et al., 2004). Phytoextraction is the uptake of elements (heavy
metals) or compounds (xenobiotics) from growth media by plant roots and their translocation
to the aerial parts (stem and leaves). It is the best approach to remove contaminants primarily
from soil and isolate them without destroying the soil structure and fertility (Gosh and Singh,
2005). An additional advantage of phytoextraction is that during this practice, plants cover
the soil and thus, erosion and leaching are minimized (Jadia and Fulekar, 2009). For
phytoremediation, two approaches are used: (1) The application of hyperaccumulators (such
as Thlaspi caerulescens or Alyssum bertolonii) producing a relatively low amount of above�ground biomass but accumulating high amounts of one or more elements (2) The application
of high biomass producing plants characterized by lower ability to accumulate target
elements where total uptake of elements is comparable to that of hyperaccumulators due to
high yield of above-ground biomass (Tlustos et al., 2006). Hyperaccumulators are defined as
“plant species whose shoots contain > 100 mg Cd kg−1
, > 1000 mg Ni, Pb and Cu kg−1 or >
10000 mg Zn and Mn kg−1 (dry weight) when grown on metal rich soils” (Baker and Brooks,
1989). As of 2010, more than 400 plant species have been identified as metal
hyperaccumulators (Wu et al., 2010; Mudgal et al., 2010). Grasses have been more preferable
in use for phytoaccumulation (phytoextraction) than shrubs or trees because of high growth
rate, more adaptability to stress environment and high biomass (Malik et al., 2010). Screening
hyperaccumulators and accumulators is a key step in the phytoremediation of soils
contaminated by heavy metals (Zhang et al., 2010). The use of hyperaccumulators to
decontaminate polluted soils might result in production of a bio-ore of some commercial
value to recoup some of the costs of soil remediation (Brooks et al., 1998). Phytoremediation
Phytoremediation of heavy metals by Trifolium alexandrinum
Hazrat Ali, Muhammad Naseer and Muhammad Anwar Sajad
International Journal of Environmental Sciences Volume 2 No.3, 2012
1461
technology is interdisciplinary in nature and requires input of knowledge from chemistry,
botany and soil microbiology. The present study was designed to investigate the
phytoextraction of four selected heavy metals (Cd, Pb, Cu and Zn) from simulated polluted
soil by Trifolium alexandrinum. T. alexandrinum is an herb belonging to the family Fabaceae.
It is cultivated as a fodder crop for cattle. It was selected because it is fast-growing, resistant
to pollution loads, produces high biomass and above all offers multiple harvests in a single
growth period.
Research Questions:
1. What is the potential of Trifolium alexandrinum in phytoextraction of Cd, Pb, Cu, and Zn from polluted soil?
2. How does the growth of Trifolium alexandrinum in polluted soil affect the concentration of heavy metals in the plant tissues?
3. Can Trifolium alexandrinum be considered as a potential hyperaccumulator for heavy metals?
4. What are the factors influencing the efficiency of Trifolium alexandrinum in phytoextraction of heavy metals from contaminated soil?
5. What are the implications of using Trifolium alexandrinum for phytoremediation on a larger scale in contaminated sites?
6. How does the phytoremediation process using Trifolium alexandrinum compare to other physical and chemical remediation methods in terms of efficiency and cost-effectiveness?