Heavy metal contamination of some leafy vegetables growing within Ibadan metropolis , South-western Nigeria

Over the years, many human diseases have been traced to pollution of the environment. Pollution is defined as the alteration or modification of the environment by the introduction of solid, liquid or gaseous substances at concentrations injurious to man, plants and animals (Chaudry et al. 1989). The combustion of fossil fuel and motor traffic exhaust is known to constitute a major source of pollution, particularly in the third world countries where leaded fuel is still being used. Nriagu (1989) reported that emission of Pb from gasoline combustion appears to be the world's most important and widespread heavy-metal emission. It has also been established that elevated levels of Cd and Pb in plants are closely associated with proximity to automobile traffic (Largerwerff and Specht 1970). Onianwa and Ajayi (1987) reported that urban activities such as large-scale refuse burning, welding and small metal works account for high levels of cadmium in highly populated urban centre. Thus, Pb and Cd are the heavy metals of great concern, as their presence in the food chain is known to have serious health implications.


Metropolis in Southwestern
The soils from different sampling sites contained different concentrations of Pb and Cd.These soils were also generally acidic with pH ranging from 4.6-6.6.The soil from high traffic density (HTD) area had a mean pH of 5.4, while that of the moderate traffic density (MTD) and low traffic density (LTD) areas had mean pH of 5.7 and 5.9, respectively (Table 1).
There were significant differences in the distribution of lead at each site.The same trend was observed for cadmium except for MTD area where Cd was relatively evenly distributed within the site (Table 1).

Lead and Cadmium Contents in the Vegetables
All vegetables samples contained different concentrations of Pb and Cd.In unwashed samples the highest lead (157.69pg/g) concentration was recorded in A. cruentus at the HTD area, whereas in washed samples the highest concentration (117.10 ug/g) was still recorded in A. cruentus but at the MTD area (Table 2).The highest cadmium in unwashed samples (138.76 ug/g) was recorded in A. cruentus at MTD area, but in washed samples the highest concentration (42.58 ug/g) was recorded in C. olitorius attheHTD area (Table 3).
The amount of these heavy-metals in each vegetable varied among sampling sites in the three traffic density areas, though specific trends could not be identified.In unwashed samples concentration of Pb and Cd followed the order HTD>MTD>LTD except in A. cruentus where the trend of Pb concentration was HTD>LTD>MTD (Tables 2 and  3).In washed samples of the three vegetable types, the concentrations of two heavy-metals did not follow a specific trend.
At the HTD area, sampling points were significantly (PO.05) different with regards to the     The low concentration of heavy metals in washed samples at the LTD implies that heavy metal deposition was low.Nasralla and Alii (1985) reported that heavy metal concentration of fruits was the result of traffic densities and distances of the farms or gardens from roadsides.They further reported that the primary source of Pb in leaves growing close to public roads was from leaf absorption of tetra-ethyl lead vapor and salts emitted from motor vehicles.
The low concentrations of heavy metals in the soil when compared to the plant would indicate that the availability of soil Cd and Pb to the plant was negligible.It could, therefore, be inferred that the source of Cd and Pb to the vegetables was from aerial deposition and foliar absorption.Various authors have reported that retention of particulate matter is the main source of contamination of roadside vegetation (Little 1973,Chamberlin 1997).
In unwashed samples the differences in heavy metal concentrations across various distances from edge of the road, though did not follow a specific trend, may reflect difference in the aerial deposition as aided by wind drift.Wind drift action may have aided the deposition of Pb since wind velocity affects and sometimes may increase deposition of atmospheric pollutants (Lewinska 1984).Fytiamos et al (1985) identified meteorology as one of the factors that affect the concentration of Pb to be found in the various environmental compartments bordering a roadway.The high concentrations of Pb in 20m and 30m distances from edge of the road may imply that fine grained Pb were airborne to a further distance before deposition.Hana and Al-Basam (1983) reported that 25% of the vehicular emitted Pb is coarse grained and deposited close to the road, while the remaining 75% is finer and may be airborne over a long distance.

In
the paved way, the Eleyele Water Works farm was about 45m from the paved road and the University of Ibadan vegetable farm was about 10m each from the Faculty of Agriculture/Forestry and Faculty of Education roads.Samples were collected in the dry season when most farmers practised'Fadama' irrigation planting.Sampling points were systematically located at 10 m, 20 m and 30 m distances from the edge of the road along transects laid across the farm.Samples (3 kg each) of Amaranthus cruentus, Celosia argentea and Corchorus olitorius were randomly collected at each sampling point.Each sample was divided into two sub-samples.One was thoroughly washed in distilled water while the other was unwashed.All samples were oven dried at 80° C for 48 hours, ground and sieved with a 1mm sieve.Soil samples were also collected from each of the sampling points.All samples were kept in labelled containers prior to digestion.The digestion of plant and soil samples were done following Allen etal (1976) and analysed for lead and cadmium using the atomic absorption spectrophotometer (BulkScientific200A Model).The data collected were subjected to 2-way analysis of variances (SAS 1987) and Duncan's multiple range test for separation of the means at 5% probability level.Percentage difference in the heavy metals between unwashed and washed samples were calculated the MTD area, samples taken at 10 m and 20 m distances from the edge of the farm did not differ significantly in their Pb contents.Also, the Pb concentration in Corchorus olitorius taken at 10 m and 30 m distances from the edge of the farm did not differ significantly at HTD, while samples taken at 10 m and 20 m distances from the edge of the farm at the LTD were not significantly different (Table 4).The vegetables contained different concentrations of cadmium.However, the occurrence of Cd in Celosia argentea and Corchorus olitorius at 10 m and 20 m distances from the farm edges at the LTD area, and in C. argentea at 20 m and 30 m distances from edge at the MTD area did not differ significantly (Table 5).Washing the vegetable samples reduced the concentrations of both Pb and Cd in the vegetables at the three traffic density areas.The amount of lead removed by washing ranged between 6.93% in C. argentea at the LTD area to 53.27% in the same C. argentea at the HTD area, whereas the amount of cadmium removed by washing ranged from 19.82% in C. olitorius at the LTD area to 72.64% in A. cruentus at the HTD area ( HTD area, heavy metal deposition on C. olitorus was far less than deposition on A. cruentus and C. argentea as indicated by the low percentage of heavy metal removed by washing. A. cruentus indicatively had highest surface deposition.The variation in concentrations of Pb and Cd among vegetable types (unwashed) may be explained by differences in leaf morphology.Leaf morphology has been implicated in the relative ability of plant species to absorb and retain heavy metals.The Amaranthus sp.used in this investigation had broader leaves than the other two vegetables.Also C. olitorius had shiny glabrous leaf surface that may not aid retention.Gradual retention and accumulation of surface Pb and other heavy metals have been demonstrated to be dependent on leaf shape and size, and on the general morphology of plants (Martin and Coughtrey 1981, Kingston etal 1988).Washing removed a considerable amount of heavy metals, as high as 72.64% of Cd in A cruentus at the HTD area.It has been reported that up to 80% of the total Pb content in leaves could be removed by rinsing with distilled water (Schodeller 1967, Breckle and Verche 1974).Also, reports of Hagemeyer et al (1986) and Lindeberg and Harris (1994) demonstrated the efficiency of washing in reducing the amount of Cd in plant leaves.This may further imply that the main source of Pb and Cd pollution in vegetables is aerial deposition from motor vehicle emissions and other athropogenic activities.Washing the vegetables in distilled water removed relatively more Cd than Pb, but the concentrations of these pollutants in the washed vegetables were still above internationally acceptable levels.Kloke (1974) recommended that maximum food content of Pb should be lOug/g.However, FAO/WHO (1978) recommended maximum tolerable daily Pb intake of 0.00043 ug/g or 7 ug/kg-body weight.Few of the vegetables samples had Cd level higher than the recommended tolerable level.WHO/UNEP (1976) recommended daily intake of57.14-71.43pg/kg-body weight.The relatively high concentrations of Pb and Cd in these common leafy vegetables raise some serious environmental concerns.Both lead and cadmium have been shown to be toxic to man.Cadmium, inhaled or ingested, is known to be a cumulative poison in man.The liver and kidney are known to contain half the total cadmium in the body and its bioaccumulation in the system has been related to hypertension and cardiovascular diseases (Underwood 1971).Acute effects of Pb poisoning could lead to constipation, vomiting, anaemia, swelling of the brain, paralysis and even death (Shapiro and Johnson 1989).Lead is also known to interfere with the function of the mitochondria, thereby impairing respiration (Chang 1992) Continuous consumption of heavy metalcontaminated vegetables could have serious health implication in the developing countries where leaded fuel is very much in use.Now that cities in Nigeria are crammed with poorly maintained heavy metalemitting vehicles, environmentalist have the responsibility of monitoring the level of pollutants being ingested by city dwellers.It is recommended that steps be taken to discourage the cultivation of leafy vegetables along roadside and especially, Environment risk,Vol.2, Toxins in Food.Plenum Press, New York, pp 61.Chaudry R, Ashish P and Mukerjik 1989 Effect of some atmospheric pollutant on micro organisms.In Frontiers in Applied Microbiology.Mukerji, K. Singh, V. and Garg, K. (Eds) Rastogi and co.India pp 84-89.

Means in a column with the same letter are not significantly different at 5% level of probability using Duncan's multiple range test. Table 2. Pooled mean of lead (fig/g) contents in.unwashed and washed samples of Celosia argentea, Amaranthus cruentus and Corchorus olitorius in three traffic density areas in Ibadan metropolis. (Values are means ± S.E; n=3). Table 3. Pooled mean of cadmium (ug/g) contents in unwashed and washed samples of Celosia argentea, Amaranthus cruentus and Corchorus olitorius in three traffic density areas in Ibadan metropolis. (Values are means ±S.E; n=3).
amount of Pb in the vegetable.However, for Amaranthus cruentus at

Table 6 ). On average, washing removed about 28.37% of Pb and 44.64% of cadmium from the vegetables. The concentrations of heavy metals removed from C. olitorius by washing were 25.03% and 37.52% for Table 4. Lead (ug/g) contents in unwashed and washed samples of Celosia argentea, Amaranthus cruentus and Corchorus olitorius at varying distances from the roadside in three traffic density areas in Ibadan metropolis. Distance from farm edge(m) Celosia argentea Amaranthus cruentus Corchorus olitorius
Means in a column of each vegetable type in each traffic density area with the same letter are not significantly different at 5% level of probability using Duncan's multiple range test.