Uptake of Heavy Metals by Plant Leaves around Industrial and Agricultural Areas of Enugu State, Southeastern, Nigeria

This research was carried out to determine the quantity of uptake of trace elements by the leaf of plants growing around industrial and agricultural areas of Enugu State. A total of twenty-two (22) plant leaf samples which consist of 12 different species such as S. pyramidalis, Py. polystachyos, Se. sesban, C. odorata, P. foetida, H. suaveolens, T. platycarpa, C. mucunoids, C. digitalis, D. aegypticum, Sc. dulcis and L. oryza sativa were collected from dumpsites, industrial sites, wetlands, mining sites and sewage site, across the study area. The samples were analyzed for heavy metals using standard laboratory methods. Results obtained in the study area revealed that the plant leaf samples obtained from the study area contained the following mean values: Fe (25.69 ± 21.04 mg/kg), Mn (11.03 ± 0.64 mg/kg), As (4.60 ± 1.56 mg/kg), Pb (3.38 ± 1.46 mg/kg), Zn (21.93 ± 11.02 mg/kg), Cd (0.79 ± 0.03 mg/kg), Cr (2.86 ± 4.84 mg/kg), Ni (0.53 ± 4.27 mg/kg) and Cu (12.74 ± 2.89 mg/kg). The mean concentrations of iron, arsenic, lead, cadmium and chromium were above the recommendable limits as stipulated by FAO/WHO for edible plants. Principal component analyses of the variables revealed strong relationship among the variables indicating same source of enrichment. This study is limited to plant leaves; hence, further research is recommended in the study area to evaluate the uptake of heavy metals by the roots and stems of the same plants.


Introduction
Heavy metals are toxic to both plants and animals (Escarre et al., 2000). Traces amount of heavy metals can be found in our daily food intake, water we drink, air we breathe and soil we cultivate (Boularbah et al., 2006). Soils contaminated with toxic heavy metals from point sources have been considered as potential exposure routes for plant and animal populations (Carrizales et al., 2006). The uptake of heavy metals by plants can be from both natural and human activities (Smith and Bradshaw, 1979). The major induced heavy metals contamination by human activities are mining, industrial activities, agricultural activities, municipal solid waste disposal and sludge application (Kumar et al., 1995). Content of metals is a useful indicator for the assessment of plant pollution as excluders and as accumulators. Excluders are plants that possess low values of metals in their shoot or restrict the levels of heavy metal translocation within them over a wide range of soil concentrations (Baker 1981). Whereas, accumulators concentrate heavy metals in their shoots at both low and high soil metal concentrations and are utilized in extracting heavy metals from contaminated soils (Rotkittikhun et al., 2006). High accumulation of heavy metals in soil affects the quality of plants and crops. The enrichment of heavy metals in edible metal-tolerant plants and crops is creating hazard to animal and human health.
Research have showed that industrial and agricultural activities can lead to significant pollution of soils, water and plants. However, heavy metals like As, Cr, Cd, Pb, Mn, Fe 3+ , Cu, Zn, and nickel (Ni) are toxic for plants and humans even at minimal amounts (Khan et al., 2008). Cadmium and lead play a key role in plant pollution especially when its value exceed certain level. Ni, Cu, and Zn are vital elements for plant growth. Nickel is an essential element of the enzyme urease, but when Ni absorptions in leaf tissue of plants exceed 50 mg/kg, the plants may suffer from excess Ni and reveal toxicity symptoms (Adriano 1986;McGrath and Smith, 1990). Once absorbed, Cu visibly cumulates in roots, even when the roots have been damaged by toxicity (Adriano 1986). Zinc phytotoxicity is accounted for acid and heavily sludge soils (Kabata-Pendias et al., 1993).
The toxicity of heavy metals in plants, particularly root and leafy plants, grown in heavy metal contaminated soils having higher concentrations of metals has led to considerable research to find out alternative strategies to minimize or remove the heavy metals from the environment. However, several studies have been also carried out the accumulating capacity of plants that grew naturally on industrial and on agricultural contaminated soils. It has been discovered from such reviews that some of the plant species accumulate high levels of heavy metals more than the normal levels encountered generally in plants ( Kidd and Monterroso, 2005;Yanqun et al., 2004;Walter et al., 2003;Bunzl et al., 2001) thereby suggesting that such plants can be used as decontaminants of heavy metal polluted soils. In the study area, there is scarcity of information about such heavy metals' tolerant plants locally. Therefore, the aim of this research is to determine the quantity of uptake of trace elements by plants around industrial and agricultural areas of Enugu State.

Study Area
The study area lies between latitudes 6°00ˈ and 7° 00ˈ N and longitudes 7° 00ˈ and 8° 00ˈ E with total areal extent of 7,161km 2 (2,764,9 sq.m) and population density of 5,590,513 according to National population commission data (2006). The topography of the area is mainly lowlands, escarpment, plateau and plains. The relief of the study area ranges from 50 -450m above mean sea level. Two main rivers, Abonyi and Anambra, drain the entire area, along with their tributaries. Whereas, the Abonyi River drains the eastern portion, the Anambra River drains the western portion of the study area. The predominant drainage pattern is dendritic. The area under this study is part of the tropical rainforest vegetation of Nigeria. Enugu area is characterized by two distinct climatic seasons, namely; wet and dry seasons respectively. The wet season which commence from April ends in October with average annual rainfall of about 2,000 millimeters. While dry season starts from November to March with average temperature of about 27ºC.

Geology and Hydrogeology
The study area is located in the Anambra Basin and some parts of Southern Benue Trough and Niger Delta Basin. Several authors (Reyment, 1965;Murat, 1972;Nwachukwu, 1972;Benkhelil, 1982;Nwajide and Reijers, 1996;Obi, 2000) have studied the lithostratigraphic framework for the Early Cretaceous-Paleocene strata in southeastern Nigeria. The major geological formations found in study area are (from oldest to youngest) Asu River Group (Albian), Ezeaku Group (Touranian), Awgu Group (Coniacian), Nkporo Group (Campanian), Mamu Formation (Maastrichtian), Ajali Formation (Upper Maastrichtian), Nsukka Formation (Danian) and Imo Group (Paleocene) (Fig. 1). These formations consisst mainly of shale, shelly limestone, siltstone, claystone, and sandstone. The general strike direction of the beds is NW-SE with dip amount between 3 o and 20 o NE/SW. Generally, aquifer distribution in the area is categorized by perched (semi -confine), shallow (unconfined) and deep (confined and unconfined) aquifer systems (Ezeigbo and Ozoko, 1989). The perched aquifers occurred in the Nsukka Formation while shallow unconfined aquifer systems occurred in the fractured and weathered horizon of the Abakaliki, Ezeaku, Awgu, Enugu, Mamu and Imo Formations. This aquifer is tapped by several hand dug wells and shallow boreholes at depth between 10 -20m in several area of the study. The confined aquifer occurs at depths of between 50 -350m.

Materials and Methods
A total of twenty-two (22) plant leaf samples consisting 12 different species were collected from dumpsites (2), industrial sites (10), wetlands (3), mining sites (3) and sewage site (4), across the study area ( Table 1). The choice of plant species collected was based on close distance to the source of contamination. Each of the plant samples were collected fresh from the field in required amounts, wrapped in polyethylene bags, labelled and transported to laboratory for analysis. All the collected plant samples were air dried and placed in oven for drying at 70ºC and then ground to powder to pass through a 2 mm sieve for chemical analysis. 1 gm of finely ground plant sample was weighed into a 100 ml Kjeldahl flask and digested with 10 ml conc. HNO3 and 2 ml conc. The digested plant samples were analysed for heavy metals, e.g., Fe, Cd, As, Cr, Cu, Mn, Ni, Pb, and Zn by atomic absorption spectrophotometer (AAS) at the Department of Crop Science, University of Nigeria, Nsukka. Similarly, plant samples used for identification were also collected separately and kept in a brown envelope. The plants identification was done at Department of Botany, same University.

RESULTS AND DISCUSSION
The results obtained during the undertaken study is presented in Tables 2. It was discovered that the heavy metals composition of the plant leaves was highly different in every areas of the undertaken study. Note that all the parameters are measured in mg/kg; PLS represents plant leaf samples  (Fig. 2b). The FAO/WHO standard is 8 mg/kg for edible plants. Our observed values were within this limit except at dumpsite samples. The S. pyramidalis leaf growing in industrial sites had the highest amount (11.03 mg/kg) of Mn, which support the idea that this plant has high absorption rate of trace heavy metals from the soil where it grows while the Se. sesban and Ca. Cajan leaves growing in wetlands contained the least contents (0.07 mg/kg) of Mn. The concentration of arsenic in plant leaf samples was found exceptional higher in some locations at the study area. The maximum and minimum concentrations of arsenic were 3.09 and 1.56 mg/kg (Fig. 2c). Concentration of arsenic in some of the locations exceeded the permissible level of arsenic in plant leaf samples which is set at 2 mg/kg (FAO/WHO, 1996). The C. odorata, H. suaveolens, Py. polystachyos and S. pyramidalis leaf growing in sewage, industrial, mining and dumpsites showed mean value higher than the acceptable limit for arsenic, while I. Cylindrical, L. oryza sativa, Cal. mucunoides and D. aegypticum leaves growing in wetlands contained the least mean values below the recommendable limit.
The mean concentration of Pb in plant leaf samples varied from 2.52 to 1.46 mg/kg (Fig. 2d) (Fig. 3b). Cd concentration at all the sampling locations were above the WHO/FAO (1996) permissible limit of 0.02 mg/kg.
Chromium concentration in plant samples was high at the study area. The average concentration of chromium fluctuates between from 4.84 to 0.58 mg/kg (Fig. 3c)

Principal Component Analysis
Principal component analysis, conducted using centroid approach, aid to observe various interesting relationships among the analyzed variables (Table 4).

Conclusion
The study was carried out to evaluate the uptake of heavy metals by plant leaves at different locations of Enugu State industrial and agricultural sites. The aim of the research is to determine toxicity of heavy metals in different species of plant leaves in different locations. The results revealed that all plants absorb water and mineral from soil through the ascent of sap; as a result, the heavy metals uptake by these species of plants also occurs in the same way. The mean concentrations of iron, arsenic, lead, cadmium and chromium were above the recommendable limits as stipulated by FAO/WHO for edible plants.
Principal component analyses of the variables revealed strong relationship among the variables indicating same source of enrichment.