Delonix regia and Casuarina equisetifolia as passive biomonitors

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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 beenevaluated. Bark and leaf samples from these plant species were collected in June and July 2006 at five locations in Benin City. Four ofthe sampling sites were in areas of high traffic density and commercial activities, the fifth site is a remote site, selected to act as a controland also to provide background information for the metals. The plant samples were collected and processed using standard proceduresand trace metals were determined using atomic absorption spectrometer. The bark of the plants was able to bioaccumulate the tracemetals, especially Pb which originates from anthropogenic contributions in the city. The Pb range of 20.00–70.00 µg/g measured forthe 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. Thebark of the plants was observed to accumulate more metals compared to the leave, while D. regia was found to be slightly better thanC. equisetifolia in trace metal uptake efficiency. Spatial variations in the distributions of Pb and Zn were significant (p < 0.05), and thecontinuous 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 about130 million people (NPC, 1991). Airborne particulate mat-ter pollution in Nigeria is beginning to reach a disturbingmagnitude in certain cases. Some available data on thelevels of this pollutant and its associated trace metal loadinclude: particulate matter range of 424.86–618.22 µg/m3

measured in a market environment in Benin City (Ukpeboret al., 2006), dry season range of 39–9368 µg/m3 measuredaround cement factories in Nigeria (Akeredolu et al., 1994)and Pb, Zn, and Cu ranges of 2.30–9.40 µg/m3, 0.1–14.00µg/m3 and 0.10–0.90 µg/m3 obtained respectively withinand around the same cement industries.

Others include particulate matter distribution of 455.3–847.1 µg/m3 obtained at Obrikom in the Niger Deltaregion (Ideriah et al., 2001), and a range of 590–930µg/m3 measured around a vehicle spraying workshop anda battery charging workshop at Ile-Ife (Ikamaise et al.,2001). In all of the studies reported above, the particulatematter 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 situationin Nigeria is further worsened by the already obviousmanifestations at both rural and urban areas of the potentialimpacts of suspended particulate matter and trace metalpollution.

Some of the impacts of these pollutants that are alreadywidespread, include kidney and liver ailments, respiratorydistress and very high mortality rate (Ndiokwere, 2004).The average life expectancy in Nigeria is one of the lowestin the world and it is currently 44 years (Punch, 2006).

Despite these gloomy pictures of particulate matter(trace metals) pollution, intervention programmes are notavailable. Some of the factors responsible for the luke-warm attitude towards air pollution control in Nigeriaare ignorance, the poor economy and lack of trainedpersonnel in this area of study. However, something mustbe done and quickly if an imminent national tragedy is tobe avoided. A proactive measure would be to strengthenresearch in this area of study, so as to provide enhancedand adequate empirical data to attract International andNational intervention programmes. A sustainable approach

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1074 Emmanuel Ehiabhi Ukpebor et al. Vol. 22

to reduce the ambient levels of trace metals includesplanting of roadside trees such as bougainvillea, Elaeagnusangustitolia L. (Elaeagnaceae) and Robinia pseudo-acaciaL. (Fabaceae) which have been reported to ameliorate theimpact of suspended particulate matter pollution (Aksoyand Sahin, 1999; Aksoy et al., 2000).

Emissions from overloaded heavy duty diesel and petroltransport vehicles, industrial emissions and refuse incin-eration are some of the sources of atmospheric metallicburden in Benin City (Ukpebor et al., 2007). Consequently,this study was designed, firstly to assess the atmosphericlevels 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 speciesas biomonitors and as potential control measures foratmospheric metallic burden; and lastly, to ascertain thepart of these plants that is better suited for air pollutionassessment.

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 ofNigeria and has a population of about 1,137,770 (NPC,1991). It is commercial in nature with few petroleum andallied industries. The climate is typically humid, with hightropical ambient temperature and low wind speed. Thereare two distinct seasons (wet and dry) and an averagerainfall about 1898.0 mm with two distinguishing peaksin July (3313.0 mm) and September (3621.0 mm) (Iyamu,2004). The traffic density in the city is high all yearround because the city is a hub to the other parts of thecountry. The significance of air pollution to health has beenrecognized as a major public health challenge in this city(Edet, 2003).

In line with the objectives of the study, five monitoringsites were randomly selected to represent all the quarters ofthe city. Four of the sites were located in areas within thecity center with high air pollution sources (road junctionsand major roads) and the fifth site, was in a remoteregrowth forest location in the outskirts of the city to act asa control. Sampling location RR was sited at a distance of4 m from the city centre (ring road), with an average trafficvolume of about 1912 cars/hr. The second location NB wassited at New Benin, 3 m from the road with an averagetraffic volume of 1850 cars/hr. The third sampling site ARwas created 3 m from the airport road, with a mean trafficflow of 2524 cars/hr. The last sampling location MG wassited at University of Benin main gate (4 m from the road),with an average traffic flow of about 3126 cars/hr. Thecontrol station (PG) was at the University of Benin Parksand Garden, with a mean traffic flow of about 5 cars/hr.The sampling sites are shown in Fig. 1. The monitoringsites were geo-referenced by using GARMIN GPS MAP765 chart plotting receiver. Traffic census was taken over12 hour durations (7:00 am – 7:00 pm) daily for 7 days ateach 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 quitecommon and available for collection throughout the year.These trees are genetically homogenous, easily identifiableand ubiquitously distributed. D. regia has a slightly roughbark with a thin layer and compound leaves with an evenmargin (blade or lamina). C. equisetifolia is characterizedby a rough, porous, brittle and slightly thick bark, withtreadlike leaves that exhibit a thin stalk. The height of eachindividual 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 andall trees were roughly of the same age. The sampled D.regia trees had the following size ranges: height (13–26m), girth (1.67–2.42 m) and basal area (0.22–0.46 m2). ForC. equisetifolia trees the size ranges were: height (11.4–27.9 m), girth (1.11–2.42 m) and basal area (0.098–0.466m2).

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 withina distance of not more than 4 m from the motorwayswere sampled. Tree bark samples were carefully removedwith a clean stainless steel pen knife at a convenientheight of about 1.5–2.0 m above the ground, while theleaves samples were plucked from the branches of the treesclosest to the highway following the procedure describedby Backor et al. (2003). The samples were kept unwashedin labeled clean paper envelopes, securely sealed and im-mediately put into polythene bags. Triplicate samples fromeach tree were collected within two weeks intervals. Thecollected samples were thereafter taken to the laboratoryfor processing and analysis.

1.4 Sample analysis

Approximately 5 g each of the unwashed tree bark andunwashed leave samples were placed in crucibles and driedto constant weight in an oven at 105.3°C for 14 hr.

A thoroughly washed, rinsed and dried laboratory millwas used to grind samples. The powdered samples wasthen preserved in labeled clean polythene bags. Fromeach sample, 1 g was accurately weighed into labeledclean platinum crucibles, ashed at 450–500°C in a Mufflefurnace, and then left inside the furnace for 4 hr. Thecrucibles were removed from the furnace and then cooledto room temperature in a desiccator. Each ash of thesample was dissolved in 10 mL of 20% HNO3 and thewhole was carefully transferred through the filter paperand funnel into a 100 mL volumetric flask. The flaskwas made up to the mark with distilled water and shakento ensure homogeneity. Analysis of the samples in the36 flasks for their various metallic contents were carriedout in triplicates using an atomic absorption spectropho-tometer (Buck Scientific model 210/211 VGP (Variable


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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 byPorebska and Ostrowka (1999). Air-acetylene was used atthe wavelength slit with the light settings appropriate forthe metal being measured. The instrument was calibratedusing standard solutions prepared as required by stepwisedilution from the standard stock solutions. The stocksolutions were standards available for the instrument andwere 1000 mg/L of Pb, Zn, Cd and Cu. The standardsolutions were prepared by adding 5 mL of the stocksolutions to 1% HNO3 and then making them up to 50mL mark with distilled water. These were further dilutedstepwise as necessary.

1.5 Statistical analysis

The “Analysis Toolpak” available in the Microsoft Of-fice Excel 2007 provided the data analysis tool that wasused for the statistical analysis. A two-way analysis ofvariance (ANOVA) was implemented to test for significantdifferences in the trace metal uptake efficiency for the barksand leaves of each plant species, taking location and metalsaccumulation as the two treatments in the ANOVA. Alsothe barks and leaves of the plant species were comparedfor significant differences in the metal uptake efficienciesusing 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 thebark samples using the passive monitors except Cd whichwas found in trace amount and not detectable. Of all theheavy metals detected, Pb and Zn were the most prevalentand enriched (Table 1).

However, Pb was not detected in the bark samples takenfrom the control site. Within the other four urban locationswith high traffic density, a Pb concentration range of20.00–700.00 µg/g was obtained for D. regia and of 10.00–270.00 µg/g for C. equisetifolia. Pb concentration rangeof 0.2–20.0 µg/g in plants has been reported by Alloway(1995). The mean concentrations of Pb loading measuredat different sites, indicated an above normal range inbark 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 samplinglocations while the other experimental sites were withinthe expected ranges for an urban setting. Furthermore,spatial variations in the bioaccumulation of Pb by theseplants species were observed to be significant (p < 0.05).The highest Pb concentrations of 700.00 µg/g (D. regia)


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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 ± 2.00C. 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 ± 1.67C. equisetifolia 20.00 ± 1.50 35.00 ± 2.64 ND 4.00 ± 0.38

Main gate (MG) D. regia 700.00 ± 225.17 65.00 ± 7.53 ND 4.00 ± 0.75C. equisetifolia 270.00 ± 14.42 57.00 ± 1.32 ND 2.00 ± 0.38

Parks and gardens (PG) D. regia ND 24.00 ± 2.32 ND 8.00 ± 0.26Control 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 themain gate sampling location. Co-incidentally, this was alsothe site with the highest traffic density (3126 cars/hr).This observation pre-supposes a correlation between theairborne Pb particulate matter and traffic density. Themajor source of Pb in the atmosphere in Nigeria is throughthe organometallic compound –(C2H5)4Pb which is addedto fuel as an anti-knock. The lack of an effective regulatorypolicy in Nigeria requiring and enforcing strict compliancein vehicular emission control limits may have caused thehigh Pb concentration levels shown in this study.

Previous studies on Pb emissions from traffic in Nige-ria and elsewhere, reported positive correlations betweenPb 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 plantspecies 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. Inour study, the Pb concentration range from the two plantspecies of 10.0–700.0 µg/g exceeded the literature valuesof Pb levels. Possible factors responsible for the highlyenriched Pb levels in Benin City, may include the increaseduse of tetraethyl lead ((C2H5)4Pb) as fuel additives inNigeria, and the tremendously increased traffic volume andfrequency in the city in the last decade.

There was no apparent correlation between the barkZn levels and traffic density, although the main gate sitewhich was the location with the highest traffic densityshowed relatively higher concentrations of Zn (65.00 µg/gin D. regia; 57.00 µg/g in C. equisetifolia) in comparisonwith the other locations. The control site with small trafficdensity also showed fairly high values of Zn. It is pertinentto note that Zn concentrations from the control site mayrepresent the background level which is usually consideredto be the “natural” level (Conti and Cecchetti, 2001). Aslight or no dependence of Zn levels on traffic densityhas 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 inthe atmosphere include the use as a corrosion resistantcoating on steel, as a paint pigment, as an accelerating andactivating agent for hardening rubber products, particularlytyres and also as Zn dialkyldithiophosphate, which isoften added to lubricating oils. The levels of Zn rangesof 24.00–65.00 µg/g (D. regia) and 30.00–104.00 µg/g(C. equisetifolia) obtained in this study, were within thenormal plant Zn range of 1.0–400.0 µg/g (Alloway, 1995).Furthermore, the Zn values obtained in this study, arecomparable with the Zn range 26.8–102.7 µg/g obtainedpreviously in Benin City, Nigeria (Ademoroti, 1986) anda range of 8.2–102.7 µg/g measured in another part ofNigeria (Osibanjo and Ajayi, 1980). The Zn results ob-tained compare well with Zn bark ranges of 4.5–189.0µg/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 fora similar study done in Finland (Harju et al., 2002) near asteel industry.

Cadmium was not detected in any of the bark samples.This observation suggests that Cd was probably present ata very low concentration, lower than the detection limit of0.01 µg/g. Similar low Cd levels (0.04–3.80 µg/g) havebeen reported from previous studies (Ademoroti, 1986;Schulz et al., 1999; Harju et al., 2002; Mahmut, 2006).Relatively low Cu concentrations were obtained in thebark samples. The maximum Cu level of 16.00 µg/g inD. regia, at ring road, while the minimum of 2.00 µg/gwas obtained for bark samples of C. equisetifolia collectedfrom main gate and the control site. The Cu values reportedin our study were within the normal plant Cu concentrationrange of 5.0–20.0 µg/g (Alloway, 1995). Although spatialvariations in its distribution were statistically significant(p < 0.05), the differences between the background Cuvalues and the other sampling locations, were not large.In addition, there was no correlation between the bark Culevels and traffic density.

2.2 Concentration of heavy metals in leaf samples

The concentration of the trace metals from the leafsamples plucked from D. regia and C. equisetifolia are


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presented in Table 2. Zn and Cu were found in measureablequantities in the leaf samples. The only measurable level ofPb (10.00 µg/g) was obtained for the leaves of D. regiaat the ring road. The inability of these leaf samples toaccumulate the trace metals could be due to the natureand area of the leaf samples. Factors that have been shownto influence foliar deposition of particulate matter includenature 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 theenvironment (humidity wind velocity etc) are also knownto affect foliar deposition. The deposited particles maybe washed by rain into the soil, re-suspended or retainedon plant. The leaves from D. regia are compound leaveswith an even margin, while those of C. equisetifolia arethreadlike with a thin stalk. Furthermore, this study wascarried out in the wet season (June and July) which arecharacterized by heavy rainfall. The above explanationcould be responsible for the low metal uptake efficiencyby the leaf samples as shown in Table 3.

Spatial variations in zinc distribution in the leaf sampleswere significant (p < 0.05). The highest Zn concentrationof 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 NewBenin with a higher traffic density. The background Znvalues were also identical with the values from airportroad. This observation suggests the lack of a correlationbetween the leaf Zn values and traffic density. In a similarstudy in Kayseri (Turkey), Aksoy et al. (2000) reported aZn range of 19 µg/g (rural)–98 µg/g (industry) in the leavesof R. pseudo-acacia. These values are identical with thezinc ranges of 14.00–56.00 µg/g (D. regia) and 8.00–84.00µg/g (C. equisetifolia) obtained in our study.

Cadmium levels in the leaves samples from the two plantspecies were below the detection limit of the flame AAS.Other studies had similar results (Jaradat and Momani,1998). Relatively low Cu concentrations were reported forthe two plant species. The Cu values were also within thenormal 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 Cuvalues measured at airport road and New Benin samplingsites and were comparable with values obtained at ringroad and main gate sampling locations. Oncemore, the lowCu values reported in our study, are identical with low Cuvalues 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 ± 0.92C. equisetifolia ND 84.00 ± 10.40 ND 14.00 ± 3.78

NB D. regia ND 14.00 ± 3.00 ND 4.00 ± 2.00C. equisetifolia ND 8.00 ± 1.00 ND ND

AR D. regia ND 33.00 ± 1.75 ND 4.00 ± 0.36C. equisetifolia ND 29.00 ± 0.64 ND 1.00 ± 0.07

MG D. regia ND 56.00 ± 5.27 ND 10.00 ± 1.21C. equisetifolia ND 49.00 ± 1.45 ND 2.00 ± 0.48

PG D. regia ND 22.00 ± 7.64 ND 7.00 ± 0.36C. equisetifolia ND 30.00 ± 3.97 ND 2.00 ± 0.52

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 1.20NB 28.00 14.00 2.00 34.00 8.00 4.30AR 27.00 33.00 0.80 35.00 29.00 1.20MG 65.00 56.00 1.20 57.00 49.00 1.20PG 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 0.70NB 8.00 4.00 2.00 6.00 0.00 –AR 6.00 4.00 1.50 4.00 1.00 4.00MG 4.00 10.00 0.40 2.00 2.00 1.00PG 8.00 2.00 4.00 2.00 2.00 1.00


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

Element Location Bark LeaveD. 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 0.50NB 28.00 34.00 0.80 14.00 8.00 1.80AR 27.00 35.00 0.70 33.00 29.00 1.10MG 65.00 57.00 1.10 56.00 49.00 0.70PG 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 0.40NB 8.00 6.00 1.30 4.00 0.00 –AR 6.00 4.00 1.50 4.00 1.00 4.00MG 4.00 2.00 2.00 10.00 2.00 5.00PG 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 theplant species were statistically significant (p < 0.05).In the case of Pb, the bark of D. regia had a betteruptake efficiency than C. equisetifolia. A similar trendwas observed for Cu distribution in the bark of the treesamples. However, for Zn, there was no clear cut trend inthe accumulation in the trees. Except at main gate where D.regia bark accumulated more Zn, the C. equisetifolia barkaccumulated more of Zn at the other sampling locations.The differences in the trace metal uptake efficiency bythe bark samples, are mainly affected by the bark qualitysuch as surface structure, stand throughfall and stemflow(Hartel, 1982; Schulz et al., 1999). A coarse, rough surfacemore readily accumulates atmospheric pollutants than asmooth surface (Poikolainen, 2004). In general, the levelsof metals in the bark of deciduous trees are much higherthan 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 twoplant species. The only Pb dose accumulated was the10.00 µg/g for D. regia samples at main gate (Table 4).Consequently, it was impossible to indicate the better Pbaccumulators of the two plant (leaf) species. A similarobservation was noticed for Cd. Cadmium was not accu-mulated by any of the leaf samples.

Three (60%) of the sampling locations, indicated thatD. regia leaf accumulated more of the Zn. Similarly, forCu, higher Cu concentrations were measured in D. regialeaves at 4 out of the 5 sampling locations. Differences inthe Zn and Cu uptake efficiencies by the two species werestatistically significant (p < 0.05), with the D. regia leaf,performing slightly better.

Probable reasons for this observation had earlier beenprovided. However, by way of emphasis, Aydinalp andMarinova (2004), in their study inferred that horizontallyoriented, broad and sticky leaves, provide a much bettercollection opportunity for the atmosphere fallout. Thefeatures of the leaves of D. regia and C. equisetifoliais probably responsible for the slight differences in thetrace metals uptake efficiencies, have been provided undermethodology.

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 canbe inferred:

(a) Bark samples from the two plants were able tobioaccumulate significantly Pb, Zn and to a lesser ex-tent Cu. This observation supports their continuous useas biomonitors and possible control measures for thesemetallic burdens. However, Cd was found to be presentat trace levels, lower than the detection limit of the AASused.

(b) The leaves from D. regia and C. equisetifolia couldonly bioaccumulate Zn and Cu. Pb and Cu were present inthe leaf samples at levels lower than the detection limits ofAAS used.

(c) The plants bark accumulated more of the metals thanthe leaves, while D. regia bark was observed to be slightlymore effective than C. equisetifolia in the uptake of Pb, Zn,and Cu.

(d) The Pb concentrations reported in the barks of theplants studied, were found to be higher than most of thevalues reported in literature. However, Zn and Cu werefound to be within the normal plant concentration ranges.

(e) Spatial variations in the distributions of Pb and Znwere statistically significant, and the continuous use ofleaded fuel in Nigeria may be the source of the high levels


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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 efficienttechnical support.

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Delonix regia and Casuarina equisetifolia as passive biomonitors

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