Abstract: Moss samples Barbula lambarenensis (P. Vard) were taken along major and minor roads of Ile-Ife, in the South Western part of Nigeria and were analysed after triple acid digestion for the trace metals: Lead, Zinc and Copper by Graphite Furnace Atomic Absorption spectrophotometry. The triplicate field data were averaged to yield mean concentrations at each sample location. The lead and zinc levels ranged from 196.8 (0.04)-108.5 (0.03) μg g-1 and from 593.1 (0.02)-35.0 (0.10) μg g-1 dry weight, respectively in the moss samples. These results from the analysis of the moss samples show that Pb and Zn levels were generally higher in areas with relatively high traffic density than low traffic density areas. Similar studies on Copper gave lower values (47.0 (0.01)-10.1 (0.01) μg Cu g-1) dry weight in the samples which did not show any correlation with traffic density. Analysis of standard plant reference materials (BCR:CRM 062) gave results Pb 26.7 (0.10); Zn 17.4 (0.46) and Cu 45.8 (0.40) which are found to be in good agreement with the certified values for Pb 25.0 (1.5); Zn 16.0 (0.7) and Cu 46.6 (1.8). The average traffic volumes of the study area are 990 vehicles per hour and 520 vehicles per hour for high and low traffic density areas, respectively.
INTRODUCTION
Pollution of vegetation by metallic elements has been a subject of active investigation over the years. Lower plants, especially mosses and Lichens, are often used as indicators of atmospheric pollution[1-3]. It has been shown that vegetation is an important sink for atmospheric pollutants and its use in atmospheric pollution monitoring has been found to be simple and cheap[4]. As a result, many biological materials such as tree leaves, tree barks and tree rings have been used as bioindicators of atmospheric heavy metal pollution[5-8]. The measured levels of metals in these materials serve as index of the extent of pollution and as a basis for which explanations for many ecological phenomena can be formulated.
Motor vehicles introduce a number of toxic metals into the atmosphere which are later deposited on roadsides[9-11]. For example, lead emission from motor vehicles produces high concentrations of lead in road side dust[12] and vegetation adjacent to roadsides[13-15]. The release of lead from vehicle exhaust has been established and attributed to the addition of lead alkyl as an antiknock additive to gasoline to increase its octane number[16]. The implication of this is that high levels of lead and other heavy metals are emitted to the atmosphere along motor ways. This may lead to serious health consequences like impairment of circulatory, reproductive and nervous systems as well as kidney problems, more so that a large number of the populace in Nigeria are susceptible to traffic pollution problems because their daily activities bring them near the roadside. In this regards, school children, traffic warders, livestock and street hawkers are most directly susceptible[16-18]. Furthermore, Zinc is emitted into the environment as a result of wear from tires of automobiles and motor vehicles emission due to Zinc-containing additives in lubricating oils. Copper on the other hand is an essential component of motor vehicle parts such as break linings. It is also emitted into the environment as a result of wear and tear[6,7].
Present study reported the levels of Lead, Zinc and Copper in Barbula lambarenensis taken close to some major and minor roads of Ile-Ife town, Nigeria using Graphite Furnace Atomic absorption spectrophotometry. Barbula lambarenensis is found widely grown naturally on different kinds of substrate e.g. concrete wall, ground surfaces etc. of Ile-Ife. The moss samples analysed serve as indicators of trace metals concentrations in the environment of this ancient city, for which no such previous data have been reported. This plant was considered in this study for aerial deposition of metals because of their large area cation-exchange properties and the fact that they derive their water and nutrients almost exclusively from air. They usually form an extensively dense green patch or a soft velvet-like carpet[19].
MATERIALS AND METHODS
Sample and sample collection: Palace road, Enuwa is the central part of Ile-Ife. It is a tourist centre; hence traffic density is high on the road. Ondo road and Ibadan/Ilesa road are the two major busy roads of Ile-Ife apart from the palace road and represent areas of heavy traffic density. NITEL road sample serve as the pollution free reference point being remote from traffic flow. Thus the sampling points designated (X1-X6) and (X7-X10) are locations of high traffic density and low traffic density, respectively.
Moss samples (Barbula lambarenensis) were collected along some major and minor roads of Ile-Ife. Samples were collected close to the road and were carefully scraped from the substrate with a treated stainless steel pen-knife[7]. The moss samples were kept in labelled polythene bags and taken to the laboratory avoiding contamination during sampling, transport and storage.
Analysis: The moss samples were placed in a pre-treated Teflon beaker and dried in the oven (Gallenkamp, England) at about 80°C for about 24 h to constant weight and was ground to uniform size in a mortar to pass through 80-mesh sieve. All reagents used for digestion were of analytical grade. About 0.5 g of the sieved moss sample was accurately weighed into teflon beaker [previously soaked in 10% nitric acid (Hopkins and Williams Chadwell Heat, Essex England) for 24 h and later rinsed with deionized water]. This was placed on hot plate maintained at 100°C and the samples were digested in a triple acid system viz. 2 cm3 perchloric acid (Koch-Light Laboratories Haverhill Suffolk England), 10 cm3 hydrofluoric acid (Hopkins and Williams Chadwell Heat, Essex England) and 20 cm3 nitric acid. The residue left was further digested with 2 cm3 of conc. HCl (May and Baker, Limited, Dagenharm England) on the hot plate. Furthermore, the final residue obtained was dissolved in a 4 cm3 2.5M HCl and the concentrate brought up to the mark in a 50 cm3 volumetric flask with de-ionized water[20].
The worked-up moss samples were analysed for the Pb, Zn and Cu contents by Graphite Furnace Atomic Absorption spectrophotometry (Chemtech Analytical UK) at the Centre for Energy Research and Development of the Obafemi Awolowo University, Ile-Ife, Nigeria. Measurements were made at 217.0, 213.9 and 324.7 nm for lead, zinc and copper, respectively. The slit widths were 2cm for Zn, 3cm for Pb and 2cm for Cu. The instrument was operated as per the instrument's handbook and data were acquired on a Gateway 2,000 PC system using the Alphaster software. The instrument was calibrated using mixed calibration standard solutions obtained by stepwise dilution of stock standard solution prepared from the pure analytical grade salt of the metal. As control, a blank experiment was carried out following the same procedure described above, omitting the sample. The background correction was carried out by subtracting concentrations of Pb, Zn and Cu obtained from AAS analysis of the blank from those for the samples.
Determination of precision and accuracy : The analytical quality control included determination of limit of detection of the technique-AAS for each of the element-Pb, Zn and Cu. The quantity of the standard analytes aspirated into the equipment, gave rise to a reading equal to twice the standard deviation of six measurements of 5 mg L-1 analyte[21]. The accuracy of the analytical technique (AAS) was evaluated by analysing a certified standard reference material BCR: CRM 062, trace elements in plant Olea europaea [Commission of the European communities Bureaus of reference Brussels] after the triple acid system digestion of the plant following the procedure described above.
Data analysis: The standard deviation for each element from replicate measurements was determined. The mean values were used for the subsequent general evaluation of data. The student t-test[22] was used to demonstrate significant difference between two means of elements (i.e. Pb, Zn, Cu) from high and low traffic density sites using the equation:
RESULTS AND DISCUSSION
The detection limits and precision found for lead, zinc and copper using AAS-technique are presented in Table 1. The detection limit ranged from 0.001-0.009 ppm. Precision of the results can be adjudged satisfactory for all the elements from the low values of relative standard deviations obtained which varied from 1.50-2.86%. Comparison between the elemental values observed in this study with certified actual values for the reference material Olea europaea BCR: CRM 062 are presented in Table 1. The results are in good agreement with the certified values. Good levels of precision were also obtained with a range of elemental standard deviation of 0.10-0.46 compared with certified values of 0.7-1.8.
The results of the chemical analysis for elemental concentrations in moss Barbula lambarenensis of Ile-Ife are presented in Table 2. The results are for the two series of heavy and low traffic pollution areas.
| Table 1: | Results of control analyses of certified reference materials CRM 062 (trace elements in plant Olea europaea) and the detection limits of the AAS technique |
| Note: Concentrations are in μg g-1 of triplicate analyses. Values in parenthesis are standard deviation, R.S.D = Relative Standard Deviation | |
| Table 2: | Levels of lead, zinc and copper in Nigerian Barbula lambarenensis at high traffic and low traffic density sites of Ile-Ife |
| *Concentrations are means of triplicate analysis. Values in parenthesis are standard deviations**Obafemi Awolowo University (OAU) | |
| Table 3: | Statistical analysis of lead, zinc and copper contents of Nigerian Barbula lambarenensis collected from two areas of different traffic density levels of Ile-Ife, with traffic volume |
| The t-critical = 2.365 at 8 degrees of freedom and 95% probability level (P<0.05) | |
It is obvious from Table 2 that there is a marked variation of concentration of Pb, Zn and Cu with traffic volume. Moss samples of relatively high traffic density such as Ibadan road, Ilesa road and Ondo road (X1-X6) generally have higher values of Pb and Zn for each sample analysed compared to those of low traffic density sites (X7-X10) which is an indication of an enhanced level in the surrounding atmosphere. The values of lead obtained for high traffic density areas ranged from 132.5 (0.01) to 196.8 (0.04) μg Pb g-1, whilst lead in samples from low traffic density areas range from 14.7 (0.04) to 121.0 (0.06) μg Pb g-1 (Table 2). The same trend observed for Pb was also observed for Zn in moss samples of Ile-Ife (Table 2). It is perhaps not surprising that Zn level obtained from X4 moss samples taken from Lexy's Place Ondo road was the highest (593.1 (0.02) μg Zn g-1) for all samples analysed. This can be attributed to the presence of road side motor workshops located close to the sampling site which may be an additional source of this contaminant apart from the high traffic density along this busy road. It is a common practice for road side mechanics to dispose off their wastes/effluent, particularly lubricating oils directly into the environment untreated and these lubricating oils contain Zinc dialkyl dithiophosphate[6]. Expectedly NITEL road sample X10 (a low density residential area) where the traffic density is very low had the least concentration of the three heavy metals compared to other areas already mentioned above (Table 2).
From Table 2, the values of Zn and Pb are consistently higher than the values of Copper. Similar observation had been reported in road side tree barks[7]. These authors reported consistently low values of Cu (16.0-51.2 μg Cu g-1) compared to that of Zn (33.5-68.9 μg Zn g-1) for tree barks of a polluted area. Besides the generally low values of Cu compared to Pb and Zn in all the samples, the Cu levels follow no specific trend from low to high traffic density areas. The low values of Copper reported in this study compared to those of Pb and Zn and the non-dependence of the Cu concentration on traffic density suggest different sources for Pb and Zn on the one hand and Cu on the other hand. The sources of Copper are probably varied and may be from other sources besides motor traffic. Soil and depositions as a result of refuse and tyre burning which is commonly practiced in the study area are probable sources of Copper in the moss samples.
Table 3 shows the mean concentration of Pb, Zn and Cu in the moss samples of the two groups of high traffic density and low traffic density sites with their coefficient of variation. The average traffic volume of the site with high traffic density was 990 vehicles h-1 while that of low traffic density was 520 vehicles h-1. In this Table, there is an increasing trend in the mean concentration of Pb and Zn in moss samples from low to high traffic density. The student t-test demonstrates significant difference between the two means of lead, Zinc and Copper from both groups of traffic sites which shows the difference between the samples taken from high and low polluted sites.
The suitability of moss Barbula lambarenensis as indicator of aerial Pb and Zn burdens has been demonstrated in this study. The source of aerial Pb and Zn appears to be from motor traffic while copper probably has natural source. The results of this study may therefore allay the fear of copper poisoning especially for the livestock during grazing on grass and pecking of associated soil along the highway which is a common practice in the study area, but high levels of Pb and Zinc reported may give cause for concern.
ACKNOWLEDGMENT
The authors are grateful to Dr. A.M. Makinde of Botany Department of the Obafemi Awolowo University Ile-Ife Nigeria for identifying the moss samples.