Download - Delonix regia and Casuarina equisetifolia as passive biomonitors and as bioaccumulators of atmospheric trace metals

Journal of Environmental Sciences 2010, 22(7) 1073–1079

Delonix regia and Casuarina equisetifolia as passive biomonitors and asbioaccumulators of atmospheric trace metals

Emmanuel Ehiabhi Ukpebor1,∗, Justina Ebehirieme Ukpebor2, Emmanuel Aigbokhan3,Idris Goji4, Alex Okiemute Onojeghuo2, Anthony Chinedum Okonkwo1

1. Air Pollution Research Group, Department of Chemistry, University of Benin, Nigeria. E-mail: [email protected]. Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK

3. Department of Botany, University of Benin, Benin City, Nigeria4. Federal Ministry of Environment and Housing, Abuja, Nigeria

Received 18 August 2009; revised 22 October 2009; accepted 17 November 2009

AbstractThe suitability of two common and ubiquitously distributed and exotic ornamental plant species in Nigeria–Delonix regia and

Casuarina equisetifolia as biomonitors and as effective bioaccumulators of atmospheric trace metals (Cd, Pb, Zn and Cu) has been

evaluated. Bark and leaf samples from these plant species were collected in June and July 2006 at five locations in Benin City. Four of

the sampling sites were in areas of high traffic density and commercial activities, the fifth site is a remote site, selected to act as a control

and also to provide background information for the metals. The plant samples were collected and processed using standard procedures

and trace metals were determined using atomic absorption spectrometer. The bark of the plants was able to bioaccumulate the trace

metals, especially Pb which originates from anthropogenic contributions in the city. The Pb range of 20.00–70.00 μg/g measured for

the bark samples of D. regia, exceeded the normal plant Pb concentration of 0.2–20.0 μg/g and most Pb data available in literature. The

bark of the plants was observed to accumulate more metals compared to the leave, while D. regia was found to be slightly better than

C. equisetifolia in trace metal uptake efficiency. Spatial variations in the distributions of Pb and Zn were significant (p < 0.05), and the

continuous use of leaded fuel in Nigeria was identified as the predominant source of Pb in the atmosphere.

Key words: passive biomonitors; trace metals; Delonix regia; Casuarina equisetifolia

DOI: 10.1016/S1001-0742(09)60219-9

Introduction

Nigeria is a tropical country with a population of about

130 million people (NPC, 1991). Airborne particulate mat-

ter pollution in Nigeria is beginning to reach a disturbing

magnitude in certain cases. Some available data on the

levels of this pollutant and its associated trace metal load

include: particulate matter range of 424.86–618.22 μg/m3

measured in a market environment in Benin City (Ukpebor

et al., 2006), dry season range of 39–9368 μg/m3 measured

around cement factories in Nigeria (Akeredolu et al., 1994)

and Pb, Zn, and Cu ranges of 2.30–9.40 μg/m3, 0.1–14.00

μg/m3 and 0.10–0.90 μg/m3 obtained respectively within

and around the same cement industries.

Others include particulate matter distribution of 455.3–

847.1 μg/m3 obtained at Obrikom in the Niger Delta

region (Ideriah et al., 2001), and a range of 590–930

μg/m3 measured around a vehicle spraying workshop and

a battery charging workshop at Ile-Ife (Ikamaise et al.,

2001). In all of the studies reported above, the particulate

matter regulatory limit prescribed by the Federal Ministry

* Corresponding author. E-mail: [email protected]

of Environment (FMENV) Nigeria (250 μg/m3) (FEPA,

1991) and the World Health Organization (WHO) (150–

230 μg/m3) (WHO, 2000) were exceeded. The situation

in Nigeria is further worsened by the already obvious

manifestations at both rural and urban areas of the potential

impacts of suspended particulate matter and trace metal

pollution.

Some of the impacts of these pollutants that are already

widespread, include kidney and liver ailments, respiratory

distress and very high mortality rate (Ndiokwere, 2004).

The average life expectancy in Nigeria is one of the lowest

in the world and it is currently 44 years (Punch, 2006).

Despite these gloomy pictures of particulate matter

(trace metals) pollution, intervention programmes are not

available. Some of the factors responsible for the luke-

warm attitude towards air pollution control in Nigeria

are ignorance, the poor economy and lack of trained

personnel in this area of study. However, something must

be done and quickly if an imminent national tragedy is to

be avoided. A proactive measure would be to strengthen

research in this area of study, so as to provide enhanced

and adequate empirical data to attract International and

National intervention programmes. A sustainable approach

1074 Emmanuel Ehiabhi Ukpebor et al. Vol. 22

to reduce the ambient levels of trace metals includes

planting of roadside trees such as bougainvillea, Elaeagnusangustitolia L. (Elaeagnaceae) and Robinia pseudo-acaciaL. (Fabaceae) which have been reported to ameliorate the

impact of suspended particulate matter pollution (Aksoy

and Sahin, 1999; Aksoy et al., 2000).

Emissions from overloaded heavy duty diesel and petrol

transport vehicles, industrial emissions and refuse incin-

eration are some of the sources of atmospheric metallic

burden in Benin City (Ukpebor et al., 2007). Consequently,

this study was designed, firstly to assess the atmospheric

levels of trace metals in Benin City by using two com-

monly cultivated exotic ornamental plants: Delonix regia(Bojer) Raf. and Casuarina equisetifolia L. as biomonitors;

secondly, to assess the suitability of these plant species

as biomonitors and as potential control measures for

atmospheric metallic burden; and lastly, to ascertain the

part of these plants that is better suited for air pollution

assessment.

1 Materials and methods

1.1 Study area/sitesThe study area is Benin City, the capital of Edo State,

Nigeria. The city is located in the southwestern part of

Nigeria and has a population of about 1,137,770 (NPC,

1991). It is commercial in nature with few petroleum and

allied industries. The climate is typically humid, with high

tropical ambient temperature and low wind speed. There

are two distinct seasons (wet and dry) and an average

rainfall about 1898.0 mm with two distinguishing peaks

in July (3313.0 mm) and September (3621.0 mm) (Iyamu,

2004). The traffic density in the city is high all year

round because the city is a hub to the other parts of the

country. The significance of air pollution to health has been

recognized as a major public health challenge in this city

(Edet, 2003).

In line with the objectives of the study, five monitoring

sites were randomly selected to represent all the quarters of

the city. Four of the sites were located in areas within the

city center with high air pollution sources (road junctions

and major roads) and the fifth site, was in a remote

regrowth forest location in the outskirts of the city to act as

a control. Sampling location RR was sited at a distance of

4 m from the city centre (ring road), with an average traffic

volume of about 1912 cars/hr. The second location NB was

sited at New Benin, 3 m from the road with an average

traffic volume of 1850 cars/hr. The third sampling site AR

was created 3 m from the airport road, with a mean traffic

flow of 2524 cars/hr. The last sampling location MG was

sited at University of Benin main gate (4 m from the road),

with an average traffic flow of about 3126 cars/hr. The

control station (PG) was at the University of Benin Parks

and Garden, with a mean traffic flow of about 5 cars/hr.

The sampling sites are shown in Fig. 1. The monitoring

sites were geo-referenced by using GARMIN GPS MAP

765 chart plotting receiver. Traffic census was taken over

12 hour durations (7:00 am – 7:00 pm) daily for 7 days at

each of the monitoring sites.

1.2 Sampling and analytical procedure

1.2.1 Trees investigatedD. regia (Flamboyant tree) and C. equisetifolia

(Whistling pine) were selected because they are quite

common and available for collection throughout the year.

These trees are genetically homogenous, easily identifiable

and ubiquitously distributed. D. regia has a slightly rough

bark with a thin layer and compound leaves with an even

margin (blade or lamina). C. equisetifolia is characterized

by a rough, porous, brittle and slightly thick bark, with

treadlike leaves that exhibit a thin stalk. The height of each

individual tree was determined using a HAGA altimeter.

A girth measurement was obtained using a girthing tape.

Only trees older than 10 years were used in this study and

all trees were roughly of the same age. The sampled D.regia trees had the following size ranges: height (13–26

m), girth (1.67–2.42 m) and basal area (0.22–0.46 m2). For

C. equisetifolia trees the size ranges were: height (11.4–

27.9 m), girth (1.11–2.42 m) and basal area (0.098–0.466

m2).

1.3 Sampling procedures

Bark and leaf samples from D. regia and C. equisetifoliawere collected during June and July 2006 from five sam-

pling locations in Benin City. Only trees located within

a distance of not more than 4 m from the motorways

were sampled. Tree bark samples were carefully removed

with a clean stainless steel pen knife at a convenient

height of about 1.5–2.0 m above the ground, while the

leaves samples were plucked from the branches of the trees

closest to the highway following the procedure described

by Backor et al. (2003). The samples were kept unwashed

in labeled clean paper envelopes, securely sealed and im-

mediately put into polythene bags. Triplicate samples from

each tree were collected within two weeks intervals. The

collected samples were thereafter taken to the laboratory

for processing and analysis.

1.4 Sample analysis

Approximately 5 g each of the unwashed tree bark and

unwashed leave samples were placed in crucibles and dried

to constant weight in an oven at 105.3°C for 14 hr.

A thoroughly washed, rinsed and dried laboratory mill

was used to grind samples. The powdered samples was

then preserved in labeled clean polythene bags. From

each sample, 1 g was accurately weighed into labeled

clean platinum crucibles, ashed at 450–500°C in a Muffle

furnace, and then left inside the furnace for 4 hr. The

crucibles were removed from the furnace and then cooled

to room temperature in a desiccator. Each ash of the

sample was dissolved in 10 mL of 20% HNO3 and the

whole was carefully transferred through the filter paper

and funnel into a 100 mL volumetric flask. The flask

was made up to the mark with distilled water and shaken

to ensure homogeneity. Analysis of the samples in the

36 flasks for their various metallic contents were carried

out in triplicates using an atomic absorption spectropho-

tometer (Buck Scientific model 210/211 VGP (Variable

No. 7 Delonix regia and Casuarina equisetifolia as passive biomonitors and as bioaccumulators of atmospheric trace metals 1075

Fig. 1 Map of sampling locations in Benin City, Nigeria.

Giant Pulse), USA) following procedures described by

Porebska and Ostrowka (1999). Air-acetylene was used at

the wavelength slit with the light settings appropriate for

the metal being measured. The instrument was calibrated

using standard solutions prepared as required by stepwise

dilution from the standard stock solutions. The stock

solutions were standards available for the instrument and

were 1000 mg/L of Pb, Zn, Cd and Cu. The standard

solutions were prepared by adding 5 mL of the stock

solutions to 1% HNO3 and then making them up to 50

mL mark with distilled water. These were further diluted

stepwise as necessary.

1.5 Statistical analysis

The “Analysis Toolpak” available in the Microsoft Of-

fice Excel 2007 provided the data analysis tool that was

used for the statistical analysis. A two-way analysis of

variance (ANOVA) was implemented to test for significant

differences in the trace metal uptake efficiency for the barks

and leaves of each plant species, taking location and metals

accumulation as the two treatments in the ANOVA. Also

the barks and leaves of the plant species were compared

for significant differences in the metal uptake efficiencies

using a two-way ANOVA.

2 Results and discussion

2.1 Concentration of heavy metals in the bark samples

Measurable levels of Pb, Zn, and Cu were detected in the

bark samples using the passive monitors except Cd which

was found in trace amount and not detectable. Of all the

heavy metals detected, Pb and Zn were the most prevalent

and enriched (Table 1).

However, Pb was not detected in the bark samples taken

from the control site. Within the other four urban locations

with high traffic density, a Pb concentration range of

20.00–700.00 μg/g was obtained for D. regia and of 10.00–

270.00 μg/g for C. equisetifolia. Pb concentration range

of 0.2–20.0 μg/g in plants has been reported by Alloway

(1995). The mean concentrations of Pb loading measured

at different sites, indicated an above normal range in

bark samples from D. regia at all the experimental sites.

However, for bark samples from C. equisetifolia, this rangewas exceeded only at airport road and main gate sampling

locations while the other experimental sites were within

the expected ranges for an urban setting. Furthermore,

spatial variations in the bioaccumulation of Pb by these

plants species were observed to be significant (p < 0.05).

The highest Pb concentrations of 700.00 μg/g (D. regia)


Table 1 Concentrations of heavy metals in bark samples of the plants

Sampling location Plant species Mean heavy metals concentration (μg/g dw)

Pb Zn Cd Cu

Ring road (RR) D. regia 20.00 ± 1.10 51.00 ± 11.17 ND 16.00 ±1.40C. equisetifolia 10.00 ± 3.00 104.00 ± 6.14 ND 10.00 ±1.52

New Benin (NB) D. regia 23.00 ± 5.00 28.00 ± 6.00 ND 8.00 ± 2.00

C. equisetifolia 11.00 ± 3.50 34.00 ± 7.00 ND 6.00 ± 1.00

Airport road (AR) D. regia 43.00 ± 3.82 27.00 ± 3.12 ND 6.00 ± 1.67


Main gate (MG) D. regia 700.00 ± 225.17 65.00 ± 7.53 ND 4.00 ± 0.75


Parks and gardens (PG) D. regia ND 24.00 ± 2.32 ND 8.00 ± 0.26

Control site C. equisetifolia ND 30.00 ± 10.44 ND 2.00 ± 1.98

ND: not detected.

Detection limits: Cu 0.005, Pb 0.08, Zn 0.005, Cd 0.01 μg/g. Data are expressed as mean ± standard error (n = 3).

and 270.00 μg/g (C. equisetifolia) were measured at the

main gate sampling location. Co-incidentally, this was also

the site with the highest traffic density (3126 cars/hr).

This observation pre-supposes a correlation between the

airborne Pb particulate matter and traffic density. The

major source of Pb in the atmosphere in Nigeria is through

the organometallic compound –(C2H5)4Pb which is added

to fuel as an anti-knock. The lack of an effective regulatory

policy in Nigeria requiring and enforcing strict compliance

in vehicular emission control limits may have caused the

high Pb concentration levels shown in this study.

Previous studies on Pb emissions from traffic in Nige-

ria and elsewhere, reported positive correlations between

Pb concentrations on bark samples and traffic density

(Laaksovirta et al., 1976; Osibanjo and Ajayi, 1980; Jara-

dat and Momani, 1998; Aydinalp and Marinova, 2004).

More specifically, Pb in bark samples for different plant

species was studied in Nigerian cities (Benin City, Oyo,

Abeokuta) and in Turkey, Finland, Germany, the report-

ed Pb concentrations were 58.3–143.5 μg/g (Ademoroti,

1986), 40.0–140.0 μg/g (Osibanjo and Ajayi, 1980), 1.9–

159.8 μg Pb/g (Odukoya et al., 2000), 0.6–82.2 μg/g

(Mahmut, 2006), 2.10–298.0 μg/g (Harju et al., 2002)

and 1.7–61.2 μg/g (Schulz et al., 1999), respectively. In

our study, the Pb concentration range from the two plant

species of 10.0–700.0 μg/g exceeded the literature values

of Pb levels. Possible factors responsible for the highly

enriched Pb levels in Benin City, may include the increased

use of tetraethyl lead ((C2H5)4Pb) as fuel additives in

Nigeria, and the tremendously increased traffic volume and

frequency in the city in the last decade.

There was no apparent correlation between the bark

Zn levels and traffic density, although the main gate site

which was the location with the highest traffic density

showed relatively higher concentrations of Zn (65.00 μg/g

in D. regia; 57.00 μg/g in C. equisetifolia) in comparison

with the other locations. The control site with small traffic

density also showed fairly high values of Zn. It is pertinent

to note that Zn concentrations from the control site may

represent the background level which is usually considered

to be the “natural” level (Conti and Cecchetti, 2001). A

slight or no dependence of Zn levels on traffic density

has been reported (Osibanjo and Ajayi, 1980; Ademoroti,

1986; Mahmut, 2006). The spatial variations in the Zn

distributions in the bark samples was significant (p <0.05), which could be due to other anthropogenic factors,

but a vehicular emission. Anthropogenic sources of Zn in

the atmosphere include the use as a corrosion resistant

coating on steel, as a paint pigment, as an accelerating and

activating agent for hardening rubber products, particularly

tyres and also as Zn dialkyldithiophosphate, which is

often added to lubricating oils. The levels of Zn ranges

of 24.00–65.00 μg/g (D. regia) and 30.00–104.00 μg/g

(C. equisetifolia) obtained in this study, were within the

normal plant Zn range of 1.0–400.0 μg/g (Alloway, 1995).

Furthermore, the Zn values obtained in this study, are

comparable with the Zn range 26.8–102.7 μg/g obtained

previously in Benin City, Nigeria (Ademoroti, 1986) and

a range of 8.2–102.7 μg/g measured in another part of

Nigeria (Osibanjo and Ajayi, 1980). The Zn results ob-

tained compare well with Zn bark ranges of 4.5–189.0

μg/g measured in Germany (Schulz et al., 1999) and 6.30–

53.7 μg/g measured in Turkey (Mahmut, 2006). However,

much higher Zn values (13.6–2713 μg/g) was reported for

a similar study done in Finland (Harju et al., 2002) near a

steel industry.

Cadmium was not detected in any of the bark samples.

This observation suggests that Cd was probably present at

a very low concentration, lower than the detection limit of

0.01 μg/g. Similar low Cd levels (0.04–3.80 μg/g) have

been reported from previous studies (Ademoroti, 1986;

Schulz et al., 1999; Harju et al., 2002; Mahmut, 2006).

Relatively low Cu concentrations were obtained in the

bark samples. The maximum Cu level of 16.00 μg/g in

D. regia, at ring road, while the minimum of 2.00 μg/g

was obtained for bark samples of C. equisetifolia collected

frommain gate and the control site. The Cu values reported

in our study were within the normal plant Cu concentration

range of 5.0–20.0 μg/g (Alloway, 1995). Although spatial

variations in its distribution were statistically significant

(p < 0.05), the differences between the background Cu

values and the other sampling locations, were not large.

In addition, there was no correlation between the bark Cu

levels and traffic density.

2.2 Concentration of heavy metals in leaf samples

The concentration of the trace metals from the leaf

samples plucked from D. regia and C. equisetifolia are


presented in Table 2. Zn and Cu were found in measureable

quantities in the leaf samples. The only measurable level of

Pb (10.00 μg/g) was obtained for the leaves of D. regiaat the ring road. The inability of these leaf samples to

accumulate the trace metals could be due to the nature

and area of the leaf samples. Factors that have been shown

to influence foliar deposition of particulate matter include

nature and area of leaf surface as well as leaf orientation

(Rao and Dubey, 1992: Bache et al., 1991).

Specific physical properties of particulate matter (par-

ticle size and mass) and ambient conditions of the

environment (humidity wind velocity etc) are also known

to affect foliar deposition. The deposited particles may

be washed by rain into the soil, re-suspended or retained

on plant. The leaves from D. regia are compound leaves

with an even margin, while those of C. equisetifolia are

threadlike with a thin stalk. Furthermore, this study was

carried out in the wet season (June and July) which are

characterized by heavy rainfall. The above explanation

could be responsible for the low metal uptake efficiency

by the leaf samples as shown in Table 3.

Spatial variations in zinc distribution in the leaf samples

were significant (p < 0.05). The highest Zn concentration

of 84.00 μg/g was measured in C. equisetifolia leaf sam-

ples at ring road. The background Zn concentrations of

22.00 μg/g (D. regia) and 30.00 μg/g (C. equisetifolia)were slightly higher than the Zn values measured at New

Benin with a higher traffic density. The background Zn

values were also identical with the values from airport

road. This observation suggests the lack of a correlation

between the leaf Zn values and traffic density. In a similar

study in Kayseri (Turkey), Aksoy et al. (2000) reported a

Zn range of 19 μg/g (rural)–98 μg/g (industry) in the leaves

of R. pseudo-acacia. These values are identical with the

zinc ranges of 14.00–56.00 μg/g (D. regia) and 8.00–84.00

μg/g (C. equisetifolia) obtained in our study.

Cadmium levels in the leaves samples from the two plant

species were below the detection limit of the flame AAS.

Other studies had similar results (Jaradat and Momani,

1998). Relatively low Cu concentrations were reported for

the two plant species. The Cu values were also within the

normal plant Cu range of 5.0–20.0 μg/g (Alloway, 1995).

The background mean values for Cu 7.00 μg/g (D. regia)and 2.00 μg/g (C. equisetifolia) were higher than the Cu

values measured at airport road and New Benin sampling

sites and were comparable with values obtained at ring

road and main gate sampling locations. Oncemore, the low

Cu values reported in our study, are identical with low Cu

values of 7.32–14.04 μg/g reported by Aksoy et al. ( 2000)

in their study on the test of the leaves of R. pseudo-acacia

Table 2 Heavy metal concentrations in leaves of the plants

Sampling location Plant species Mean heavy metals concentration (μg/g dw)

Pb Zn Cd Cu

RR D. regia 10.00 ± 1.51 39.00 ± 3.23 ND 5.00 ± 0.92

C. equisetifolia ND 84.00 ± 10.40 ND 14.00 ± 3.78

NB D. regia ND 14.00 ± 3.00 ND 4.00 ± 2.00

C. equisetifolia ND 8.00 ± 1.00 ND ND

AR D. regia ND 33.00 ± 1.75 ND 4.00 ± 0.36


MG D. regia ND 56.00 ± 5.27 ND 10.00 ± 1.21


PG D. regia ND 22.00 ± 7.64 ND 7.00 ± 0.36


ND: not detected.

Detection limits: Cu 0.005, Pb 0.08, Zn 0.005, Cd 0.01 μg/g. Data are expressed as mean ± standard error n = 3.

Table 3 Comparison of the accumulation capability of the bark/leaf samples

Element Location Bark D. regia Leaf Bark/Leaf Bark C. equisetifolia leaf Bark/leaf

Pb RR 20.00 10.00 2.00 10.00 0.00 –

NB 23.00 0.00 – 11.00 0.00 –

AR 43.00 0.00 – 20.00 0.00 –

MG 700.00 0.00 – 270.00 0.00 –

PG 0.00 0.00 – 0.00 0.00 –

Zn RR 51.00 39.00 1.30 104.00 84.00 1.20

NB 28.00 14.00 2.00 34.00 8.00 4.30

AR 27.00 33.00 0.80 35.00 29.00 1.20

MG 65.00 56.00 1.20 57.00 49.00 1.20

PG 24.00 22.00 1.10 30.00 30.00 1.00

Cd RR 0.00 0.00 – 0.00 0.00 –

NB 0.00 0.00 – 0.00 0.00 –

MG 0.00 0.00 – 0.00 0.00 –

PG 0.00 0.00 – 0.00 0.00 –

Cu RR 16.00 5.00 3.20 10.00 14.00 0.70

NB 8.00 4.00 2.00 6.00 0.00 –

AR 6.00 4.00 1.50 4.00 1.00 4.00

MG 4.00 10.00 0.40 2.00 2.00 1.00

PG 8.00 2.00 4.00 2.00 2.00 1.00


Table 4 Comparison of the accumulation capability of the different barks and leaf samples

Element Location Bark Leave

D. regia bark C. equisetifolia bark D. regia/ D. regia leaf C. equisetifolia leaf D. regia/C. equisetifolia C. equisetifolia

Pb RR 20.00 10.00 2.00 10.00 0.00 –

NB 23.00 11.00 2.10 0.00 0.00 –

AR 43.00 0.00 – 0.00 0.00 –

MG 700.00 0.00 – 0.00 0.00 –

PG 0.00 0.00 – 0.00 0.00 –

Zn RR 51.00 104.00 0.50 39.00 84.00 0.50

NB 28.00 34.00 0.80 14.00 8.00 1.80

AR 27.00 35.00 0.70 33.00 29.00 1.10

MG 65.00 57.00 1.10 56.00 49.00 0.70

PG 24.00 30.00 0.80 22.00 30.00 1.00

Cd RR 0.00 0.00 – 0.00 0.00 –

NB 0.00 0.00 – 0.00 0.00 –

MG 0.00 0.00 – 0.00 0.00 –

PG 0.00 0.00 – 0.00 0.00 –

Cu RR 16.00 10.00 1.60 5.00 14.00 0.40

NB 8.00 6.00 1.30 4.00 0.00 –

AR 6.00 4.00 1.50 4.00 1.00 4.00

MG 4.00 2.00 2.00 10.00 2.00 5.00

PG 8.00 2.00 4.00 2.00 2.00 3.50

as a possible biomonitor in Kayseri (Turkey).

2.3 Trace metal uptake efficiency of D. regia versus C.equiselifolia in bark

Different amounts of trace metals accumulated by the

plant species were statistically significant (p < 0.05).

In the case of Pb, the bark of D. regia had a better

uptake efficiency than C. equisetifolia. A similar trend

was observed for Cu distribution in the bark of the tree

samples. However, for Zn, there was no clear cut trend in

the accumulation in the trees. Except at main gate where D.regia bark accumulated more Zn, the C. equisetifolia bark

accumulated more of Zn at the other sampling locations.

The differences in the trace metal uptake efficiency by

the bark samples, are mainly affected by the bark quality

such as surface structure, stand throughfall and stemflow

(Hartel, 1982; Schulz et al., 1999). A coarse, rough surface

more readily accumulates atmospheric pollutants than a

smooth surface (Poikolainen, 2004). In general, the levels

of metals in the bark of deciduous trees are much higher

than those in coniferous bark (Rasmussen, 1978).

2.4 Trace metal uptake efficiency of D. regia leaf versusC. equisetifolia leaf

Lead was poorly accumulated by the leaves of the two

plant species. The only Pb dose accumulated was the

10.00 μg/g for D. regia samples at main gate (Table 4).

Consequently, it was impossible to indicate the better Pb

accumulators of the two plant (leaf) species. A similar

observation was noticed for Cd. Cadmium was not accu-

mulated by any of the leaf samples.

Three (60%) of the sampling locations, indicated that

D. regia leaf accumulated more of the Zn. Similarly, for

Cu, higher Cu concentrations were measured in D. regialeaves at 4 out of the 5 sampling locations. Differences in

the Zn and Cu uptake efficiencies by the two species were

statistically significant (p < 0.05), with the D. regia leaf,

performing slightly better.

Probable reasons for this observation had earlier been

provided. However, by way of emphasis, Aydinalp and

Marinova (2004), in their study inferred that horizontally

oriented, broad and sticky leaves, provide a much better

collection opportunity for the atmosphere fallout. The

features of the leaves of D. regia and C. equisetifoliais probably responsible for the slight differences in the

trace metals uptake efficiencies, have been provided under

methodology.

3 Conclusions

From this study on the suitability of D. regia and C.equisetifolia as passive biomonitors and as effective bioac-

cumulators of atmospheric trace metals, the following can

be inferred:

(a) Bark samples from the two plants were able to

bioaccumulate significantly Pb, Zn and to a lesser ex-

tent Cu. This observation supports their continuous use

as biomonitors and possible control measures for these

metallic burdens. However, Cd was found to be present

at trace levels, lower than the detection limit of the AAS

used.

(b) The leaves from D. regia and C. equisetifolia could

only bioaccumulate Zn and Cu. Pb and Cu were present in

the leaf samples at levels lower than the detection limits of

AAS used.

(c) The plants bark accumulated more of the metals than

the leaves, while D. regia bark was observed to be slightly

more effective than C. equisetifolia in the uptake of Pb, Zn,

and Cu.

(d) The Pb concentrations reported in the barks of the

plants studied, were found to be higher than most of the

values reported in literature. However, Zn and Cu were

found to be within the normal plant concentration ranges.

(e) Spatial variations in the distributions of Pb and Zn

were statistically significant, and the continuous use of

leaded fuel in Nigeria may be the source of the high levels


of Pb found in the environment.

Acknowledgments

We are grateful to the management of Edo State Min-

istry of Environment for providing the laboratory facilities,

and to Mr. Harrison Igene for providing a most efficient

technical support.

References

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