Phytoextraction potential of Prosopis juliflora (Sw.) DC. with specific reference to lead and cadmium.

Abstract

Root and shoot samples of Prosopis juliflora were assessed for their heavy metal content to evaluate the species as a green solution to decontaminate soils contaminated with lead and cadmium. The highest uptake of both the metals was observed in plants from industrial sites. Sites with more anthropogenic disturbance exhibited reduced chlorophyll levels, stunted growth, delayed and shortened reproductive phase. The ratios of lead and cadmium in leaves to lead and cadmium in soil were in the range of 0.62-1.46 and 0.55-1.71, respectively. Strong correlation between the degree of contamination and concentrations of lead and cadmium in plant samples identifies P. juliflora as an effective heavy metal remediator coupled with environmental stress.

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Bulletin of Environmental
Contamination and
Toxicology

ISSN 0007-4861
Volume 87
Number 1

Bull Environ Contam Toxicol
(2011) 87:45-49
DOI 10.1007/
s00128-011-0305-0
Phytoextraction Potential of Prosopis
juliflora (Sw.) DC. with Specific Reference
to Lead and Cadmium
Mayank Varun, Rohan D’Souza, João
Pratas & M. S. Paul

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123
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Phytoextraction Potential of Prosopis juliflora (Sw.)
DC. with Specific Reference to Lead and Cadmium
Mayank Varun•Rohan D’Souza•Joa ˜o Pratas•
M. S. Paul
Received: 31 December 2010/Accepted: 3 May 2011/Published online: 10 May 2011
? Springer Science+Business Media, LLC 2011
Abstract
were assessed for their heavy metal content to evaluate the
species as a green solution to decontaminate soils con-
taminated with lead and cadmium. The highest uptake of
both the metals was observed in plants from industrial sites.
Sites with more anthropogenic disturbance exhibited
reduced chlorophyll levels, stunted growth, delayed and
shortened reproductive phase. The ratios of lead and cad-
mium in leaves to lead and cadmium in soil were in the
range of 0.62–1.46 and 0.55–1.71, respectively. Strong
correlation between the degree of contamination and con-
centrations of lead and cadmium in plant samples identifies
P. juliflora as an effective heavy metal remediator coupled
with environmental stress.
Root and shoot samples of Prosopis juliflora
Keywords
Phytoremediation
Lead ? Cadmium ? Prosopis juliflora ?
Human activity has continuously increased the level of
heavy metals circulating in the environment. Heavy metal
pollution of the biosphere has accelerated rapidly and soil
toxicity poses major environmental problems. Lead (Pb)
and cadmium (Cd) are persistent environmental contami-
nants since they cannot be degraded or destroyed. Pb is
released into the environment during its mining and
smelting activities, from automobile exhausts, by com-
bustion of petro fuels with anti-knocking additives like
tetraethyl and tetra methyl lead, old lead paints, batteries,
etc. as well as from industrial and liquid domestic waste.
Cadmium is released as a byproduct of zinc and lead
refining (Duruibe et al. 2007) and from vehicular exhausts
and fertilizers. Excessive metal concentration in contami-
nated soils might result in decreased soil microbial activity
and soil fertility, yield loss and possible contamination of
the food chain. Agra is a major international tourist
attraction in the north-central region of India. The river
Yamuna located in the study area is also exposed to pol-
lution from commercial and domestic waste. The riverbed
and riverbank thus, act like a sink for the pollutants that the
river carries.
Several technologies exist for remediation of metal
contaminated soils and water, but the emphasis is on
‘green’ phytoremedial systems nowadays. Phytoremedia-
tion is the use of living plants to extract metals from the
soil (phytoextraction) or to render them harmless in situ by
binding them (phytostabilisation) (Lombi et al. 2001).
Many plants have been well documented as hyperaccu-
mulators as well as indicators of high metal concentrations
in soil, but these are slow growing, and not very wide-
spread. In most cases their biomass yield is also quite low.
The plants used in for successful phytoextraction should
have substantial biomass and a significant translocation
factor [TF]: the ratio of heavy metal concentration in the
shoot to that in the root of the plant (Luo et al. 2005). It has
been demonstrated that, wild native plants may be better
phytoremediators for waste lands than the known metal
bioaccumulators from literature. A range of native, well
adapted plants have been tested and used widely for heavy
metal bioindicator and phytoremedial purposes including
Lobelia chinensis, Solanum nigrum, Calotropis procera
M. Varun (&) ? R. D’Souza ? M. S. Paul
Department of Botany, St. John’s College, Agra 282002, India
e-mail: 30mayank@gmail.com
J. Pratas
Departmento de Cie ˆncias da Terra, Universidade de Coimbra,
3000-272 Coimbra, Portugal
123
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DOI 10.1007/s00128-011-0305-0
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and Crinum asiaticum (Peng et al. 2009; D’souza et al.
2010; Varun et al. 2010).
In the seasonal surveys carried out to identify the flora at
the sampling sites, Prosopis juliflora (Sw.) DC., a hardy
plant was seen growing well at nearly all sites. Other weeds
were seasonal and too short-lived for practical remediation.
P. juliflora (Honey Mesquite), a xerophyte well adapted to
many soil types under a wide range of moisture conditions.
The value of this tree lies in its exceptional tolerance of
drought and marginal soils. It can reach deep within the
soil and thus soil-root interaction is enhanced both in terms
of quality and magnitude. Thus, this plant appeared to be of
particular interest in this context and was studied.
Materials and Methods
Agra, on the banks of river Yamuna (27?100N 78?020E) is
located in North-central India. A part of the great northern
plains, the study region is considered a semi-arid zone as
two-thirds of its boundaries are surrounded by the Thar
Desert. Eight sites were selected in the target area from
different zones viz.—riverbank (sites 2, 3), roadside (sites
4, 5), industrial (sites 6, 7) and residential (sites 8, 9) zones.
Soil from the college’s botanical garden was taken as
control (site 1). A survey was carried out to evaluate the
existing levels of Pb and Cd in the soil. Root and leaf
samples of P. juliflora were collected to assess character-
istics of accumulation and tolerance of Pb and Cd in the
species.
At each site, soil samples were collected from a depth of
5–15 cm using a core sampler. Four random samples were
collected from each site to obtain a comprehensive profile
of Pb and Cd levels at each site for statistical analysis. The
samples were air dried, passed through 20 mesh sieve and
stored in clean ziplock polythene bags until further use.
Leaf and root samples were collected in similar bags from
plants growing on spots from where soil samples were
collected. In the lab, these were first washed with distilled
water to remove adhering mud. They were further washed
thoroughly 3–4 times with deionized water and allowed to
drip dry completely in a dust free chamber.
Soil samples were tested for various parameters to
obtain a clear profile of soils at the sites selected. Important
soil properties are shown in Table 1. Soil pH was measured
by a pH meter (Systronics lpH system 361) with a water
soil ratio of 1:2.5. Soil conductivity was measured by a
conductivity meter (Systronics) by preparing a 1:2.5 soil
suspension in water. Organic carbon was measured by
modifiedWalkley–Blackrapid
method (Piper 1966) where 10 mL 1 N potassium dichro-
mate (K2Cr2O7) and 20 mL conc. H2SO4were added to 5 g
soil. After 30 min. 200 mL distilled water was added along
with 10 mL 85% phosphoric acid and 1 mL DPA indicator.
This was titrated against N/2 ferrous ammonium sulphate.
The value obtained was multiplied by the factor 1.724 to
obtain organic matter content as reported. Available potash
was estimated by flame photometer (mediFlam), available
nitrogen by micro-kjeldahl and available phosphate was
also determined using digital spectrophotometer (Jackson
1973).
Soil, shoot and root samples from all sites were digested
using a microwave assisted wet digestion method with
3 mL HNO3? 9 mL HCl for 0.5 g soil, and 5 mL
HNO3? 2 mL H2O2for 0.5 g plant sample. The filtrate
was analyzed for each metal by flame atomic absorption
spectrophotometry or by graphite furnace atomic absorp-
tion spectrophotometry using a Solaar M2—Thermo
Unicam instrument. For control of the results certified
references (Virginia tobacco leaves CTA-VTL-2, Polish
Certified Reference Material and NIST 2709—San Joaquin
Soil) were used. The recovery rates and certified values for
Pb and Cd analysed are 69%; (18.9 ± 0.5) and 92%;
(0.38 ± 0.01), respectively. Limits of detection were 0.079
dichromateoxidation
Table 1 Physico-chemical profile of Soil
ZonesSitepH (1:2.5) Electrical
conductivity
(dS/m) (1:2.5)
Organic
matter (%)
Available
nitrogen
(kg/ha)
Available
phosphate (kg/ha)
Available
potash
(kg/ha)
Control Site 17.3 0.591.4 12612.2 268
RiverbankSite 27.20.330.4656.1545140
Site 37.570.420.6694.8 51 303
RoadsideSite 4 7.7 0.440.2439.4 14210
Site 5 8.4 0.48 0.34 53.62 383
Industrial Site 6 7.150.660.15 66.318 366
Site 7 7.660.860.21 87.3 30.2 640
ResidentialSite 87.71 0.320.682.2 29.3 553
Site 9 8.30.421.998.4 38.3794
46 Bull Environ Contam Toxicol (2011) 87:45–49
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and 0.016 mg/L for Pb and Cd, respectively (using impact
beads).
Chlorophyll content in P. juliflora leaf samples was
determined on fresh weight basis. 40 mg fresh leaves were
placed in 10 mL 80% acetone in a sealed, dark bottle in a
refrigerator. After 5 days optical density of the solution
was measured by a spectrophotometer at different wave-
lengths i.e. 480, 510, 630, 645, 652 and 665 nm and
chlorophyll content was calculated using relevant formulae
(Arnon 1949).
Pearson’s coefficient for correlation of the data was
statistically analyzed at a significance level of p\0.05 and
p\0.01. One way Analysis of Variance (ANOVA) was
calculated for data and means compared using Fisher’s
LSD method. For ANOVA, sites were jointly tested as
zones for better comparison.
Results and Discussion
The concentrations of Pb and Cd detected in soil, leaf and
root samples are recorded in Table 2. Maximum Pb and Cd
were found at site 7 and site 6 (urban industrial) with
134 mg/kg Pb and 4.1 mg/kg Cd, respectively. Levels of
Pb and Cd were each recorded in the order Indus-
trial[Roadside[Riverbank[Residential. 10–100 and
0.06–3 mg/kg have been reported as ranges for Pb and Cd
concentrations in soil, the lower value indicating normal
and the higher value toxic level, respectively (Liphadzi and
Kirkham 2005). The high level of Pb and Cd at sites 7 and
6 indicates pollution in the heavy industrial area. At river
bank, site 3 had higher Pb levels than site 2 as expected,
due to deposition of the heavy metal arising from heavy
traffic near it. Cd was not detected in all the sample
collected from site 8. Correlation analysis was performed
using univariate Pearson’s correlation coefficient, Pb and
Cd in soil showed significant correlation (r = 0.689) as did
Pb and Cd content in P. juliflora leaf (r = 0.793) and root
(r = 0.745) (p B 0.05).
Pb content values in P. juliflora leaf samples are within
the range of 5.6–83.1 mg/kg whereas in roots they are in the
lower range of 4.4–62.6 mg/kg. Levels of Pb in plant
samples increased in proportion to its concentration in soil
samples. Highest uptake values of Pb recorded were 83.1
and 62.6 mg/kg in P. juliflora leaf and root samples,
respectively, both from site 7 which had the highest Pb
levels of all sites. Cd concentrations in leaf and root samples
are in the range of 1.4–4.3 mg/kg and 0.98–3.1 mg/kg,
respectively. The sample plants accumulated higher levels
of Cd with an increase in Cdconcentration in soil at all sites,
as seen in the case of Pb. The highest Cd level in leaf and
root samples detected was 4.3 and 3.1 mg/kg respectively,
both from site 6. The uptake ranges are encouraging when
compared to reported toxic concentrations in plants as 30
and 0.1 mg/kg for Pb and Cd respectively (Liphadzi and
Kirkham 2005).
All plant samples tested showed higher Pb and Cd
accumulation in leaves compared to roots indicating greater
allocation of metal to leaves. Similar studies have shown
that leaves act as main sinks for heavy metals in hyperac-
cumulator plants (Psaras and Manetas 2001). This is attrib-
uted to the efficient translocation of heavy metals from roots
to shoots (Ku ¨per et al. 2000) and is considered an advanta-
geous strategy as the root system is the primary target in
heavy metal toxicity. Accumulation of potentially toxic
metals in leaves is also thought to be a plant’s defensive
strategy against herbivores (Liphadzi and Kirkham 2005).
Significantly, all plant samples analyzed except site 6 and 7
exhibited higher concentrations of Pb as compared to asso-
ciated soil samples. This indicates good uptake and accu-
mulation ofPbinP. juliflora.Similarly, Cdwas foundtooin
higher concentration in plant samples compared to soil
except site 2. Highly significant correlation was found
between Pb in soil and leaf (r = 0.972) and soil and root
(r = 0.976) (p B 0.01).
The ratios of Pb in P. juliflora leaves to Pb in soil are in
the range 1.46–0.62 while similar ratios of Cd are in the
range of 1.61–0.85. These ratios are an indication of good
accumulation of Pb and Cd in P. juliflora as a ratio greater
than 1 indicates higher accumulation of metals in the plant
(Barman et al. 2000). Pollutant bioavailability depends on
the chemical properties of the pollutant, soil properties,
environmental conditions and biological activity. Though
Pb levels were higher than Cd levels in all soil samples, it
is well documented that Cd is toxic to plants in far lower
concentrations than Pb. Plants at all the sites studied
showed a far higher uptake of Cd. This could be related to
Table 2 Pb & Cd content in soil and plant (P. juliflora) samples
ZonesSites Pb content (mg/kg)Cd content (mg/kg)
SoilLeafRoot Soil LeafRoot
ControlSite 1 5.4a
11.2b
22.3b
30.6c
23.4c
120.1d
134d
7.6a
10.1a
5.6a
16.4b
23.1b
34.2c
26.7c
76.4d
83.1d
9.2a
10.8a
4.4a
8.7b
14.6b
15.3c
17.6c
58.2d
62.6d
6.1a
8.3a
0.82a
2.7b
2.4b
3.2c
2.9c
4.1d
3.7d
1.4a
2.3b
2.6b
3.5c
3.0c
4.3d
3.9d
1.1a
1.6b
1.7b
2.1b
1.8b
3.1c
2.6c
RiverbankSite 2
Site 3
Roadside Site 4
Site 5
IndustrialSite 6
Site 7
ResidentialSite 8 ND
0.93a
ND
1.5a
ND
0.98d
Site 9
Different letter in the same coloumn denote significant statistical
difference (p\0.001) in mean Pb and Cd content at the zones
selected
Bull Environ Contam Toxicol (2011) 87:45–4947
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the fact that metals like Pb are mostly immobile in the soil,
which reduces their bioavailability and subsequent uptake
by the plant (Lombi et al. 2001). Cd on the other hand is
usually less adsorbed by soil and organic matter which
makes it more available to plants. Accumulation of Pb in
leaf showed highly significant positive correlation with Pb
content inroot(r = 0.976),
(p B 0.01). Similarly, Cd content in leaf also exhibited
positive correlation with Cd content in root (r = 0.884),
and soil (r = 0.857) (p B 0.05). Analysis of Variance
(ANOVA) showed significant difference (p\0.001) for
Pb and Cd content among zones (Control, riverbank, riv-
erbank/roadside, roadside, industrial and residential) and
medium (soil, leaf and root). Further, no significant dif-
ference was obtained in mean Pb and Cd content in soil,
leaf and root between control and residential zones.
In generalsoil pHseems to have the greatest effect of any
single factor on the availability of metals in soil (Ghosh and
Singh 2005). All sites had a soil pH in the range of 7–8
(Table 1). It has been demonstrated (Pierzyenski et al.
1994) that at pH levels between 6 and 11, Lead (Pb2?)
changes to form PbOH?(solid phase). This could also
account for lower uptake of Pb detected in plant samples in
contrast to Cd. Organic matter content is known to influence
the bioavailability of metals because some metals may form
complexes with organic matter (Yoo and James 2002), thus
reducing their bioavailability. Organic matter content was
quite low at all sites tested as they were wastelands where
only the hardiest of weeds were observed. So this parameter
was not of much practical importance in this study. These
parameters did not show any positive correlation with
concentrations of Pb and Cd in various samples.
Presence of metals in high concentration in soil show
potential toxic effects on overall growth and metabolism of
plants. Chlorophyll, essential for photosynthesis, is directly
influencedbyenvironmentalfactors.Chlorophyll‘a’content
was found greater than chlorophyll ‘b’ in P. juliflora leaves.
Similar findings have also been reported in C. procera
(D’souzaetal.2010).LowestChlorophyllcontent(Fig. 1)in
leafsampleswerereportedfromsite6and7(industrial)with
0.73 and 0.62 mg/g respectively, this could be due to pres-
ence Pb and Cd in high concentration in soil disrupting the
photosynthetic pigment synthesis in P. juliflora. Site 4 and 5
whichisonroadside,alsoshowsthereductioninchlorophyll
values as compared to control (site 1), drop in chlorophyll
levels couldbe attributedtothe additional stress theseplants
were exposed to in the form of aerial deposition of various
pollutantsfromvehicularemissionscombined withelevated
levels of Pb and Cd in the soil (Nabuloa et al. 2006). Plants
at riverbank (sites 2 and 3) and residential (sites 8 and 9), do
not exhibit marked variation from site 1 (control), due to
sufficientavailabilityofwater&lowmetalpollutioncontent
at these sites.
andsoil(r = 0.972)
The sites were surveyed on a bimonthly cycle for
2 years from November 2008–October 2010 to compare
P. juliflora plants at the sites with each other as well as
with the control plants in the college garden. Plants from
roadside and industrial sites exhibited delayed reproductive
phases and significantly poor overall health of the indi-
viduals. This could again be due to the deposition of
vehicular and industrial emissions on the plants combined
with elevated levels of heavy metals as sites 2 and 3 are
along the busiest national highways in the region. Plants
from riverbank and residential sites were not much affected
by the ambient levels of Pb and Cd in soil and their health
was comparable to plants present at site 1 i.e. control.
Phytoremediation in developing countries like India,
offers a feasible and economic alternative to achieve
remediation of contaminated soils. P. juliflora has a good
phytoextraction potential as shown by the accumulation
ratios under natural conditions. Luxuriant growth was
observed throughout the year at most of the sites selected. It
proliferates freely through seeds even in adverse conditions
with practically no agronomic inputs. The value of this tree
lies in its exceptional tolerance to drought, saline soils and
seasonal waterlogging. P. juliflora has been planted suc-
cessfully on soils with acid to alkaline reactions. Since it is
not consumed by humans or livestock, it is a safe and
effective choice to be used as a tool of phytoremediation.
Acknowledgments
Commission [F. no. 35-47/2008(SR)] is gratefully acknowledged.
Financialsupport fromUniversity Grants
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