Abstract: In industrial areas of the world the disposal of heavy metals such as mercury, cadmium, lead, copper and arsenic evokes great concern because of their increasing concentrations in many waters. This study was conducted to evaluate the metal content of the two common species of aquatic macrophytes (Polygonum sp. and Ludwigia sp.) in relation to the metal content of the sediments in Makwaye and Kubanni Lakes in the Nigerian Northern Guinea Savanna. Analysis of the elemental content of the samples was done using the Energy dispersive X-ray fluorescence Spectroscopy (XRF) method. Iron (Fe) was present at highest concentration, of 8 metals evaluated in both lakes. Titanium (Ti), Manganese (Mn), Nickel (Ni), Copper (Cu), Zinc (Zn) and Arsenic (As) followed in that order. Significantly, lead (Pb) was not recorded in the sediments of any of the two lakes. Concentrations of all the metals in the sediments of both lakes, as well as plant shoots were higher than levels internationally regarded as normal. Accumulation patterns for the trace metals in plants were in the order Fe>Ti>Mn>Cu. Ni, Zn, As and Pb were not detected in the shoot tissues of either species, although all (except Pb) were present in the sediments. Between 45-100% Mn found in the sediments of the two lakes was found in the shoots of the two plant species, while for Cu this was 31-34%, Ti (15-42%) and Fe (7-21%). On the whole accumulation patterns of metals in plant shoots suggested a trend that reflects the level of toxicity of the metals to plant tissues, with Fe, Ti, Mn and Cu following in that order. Implications of these findings in relation to biocontamination of the food chain and possibilities for phytoremediation are discussed.
INTRODUCTION
In industrial areas of the world disposal of heavy metals such as mercury, cadmium, lead, copper and arsenic evokes great concern because of their increasing concentrations in the environment. Typical examples of the effects of their accumulation have been reported from Japan where hundreds of fishermen were killed by eating fish containing too much mercury (Minamata disease, Takenchi, 1972) or cadmium (Itai-Itai disease, Kobayshi, 1971). Concentrations of many heavy metals in the environment have exceeded acceptable levels. Industrial and municipal effluents, fertilizer and pesticide applications, automobile exhausts among others are sources of metal pollution of the environment (Bako et al., 2006a, b). Particularly serious cases are mercury, copper, cadmium and lead. In Africa, large quantities of animal waste are applied to agricultural fields every year. Livestock manure may contain high concentrations of metals that originate from feed and medicines provided to the animals (Liphadzi and Kirkham, 2005). In most instances these metals along with other plant nutrients end up in water bodies through surface run off and precipitation (Bako et al., 2006a).
Development of aquatic macrophyte communities in lentic habitats is a function of events that modify physical and chemical characteristics of the habitat such as water temperature, organic matter, soil depth, etc. These factors influence the occurrence, establishment, survival and reproduction of macrophytes (Gbem et al., 1999; Bako and Oniye, 2004; Bako et al., 2005b). Emergent macrophytes obtain their nutrients directly from the sediments. The major plant nutrients and trace elements are taken up by similar mechanisms. While some of the trace elements (also referred to heavy metals) play important physiological roles at the appropriate concentrations, they may become toxic when accumulated to certain levels (Bako et al., 2005a, 2006a). Large stands of emergent macrophytes (including species of Polygonum and Ludwigia) are commonly found in lentic habitats in Nigeria. Some of these macrophytes are used as sources of food, fodder or medicine in rural communities. In particular, Ludwigia sp is used as an ingredient in soups in the Yelwa area of Kebbi State in Nigeria, while Polygonum sp is used in medicinal preparations for rheumatism (Ita, 1993). Generally, emergent macrophytes serve as a reserve source of browse material for livestock during the dry season in the Savanna eco-region of Nigeria. There is therefore a clear risk of bio-contamination of the food chain. The 2004 congress on environmental health highlighted that environmental metal poisoning is becoming a major public health burden in many African countries due to rapid globalisation and industrialization (Carnie, 2004). A prevalence of chronic ailments such as heart and kidney diseases, skin cancer and anaemia has been reportedly linked to prolonged exposure to metal polluted areas (Liphadzi and Kirkham, 2005).
Furthermore, plants growing in contaminated soils accumulate higher levels of metals than those growing in normal soils (Chang et al., 1992) The rate at which this is done depends on several factors that include soil conditions, age, species, physiological adaptations etc. (Bako et al., 2005a). Therefore, plants that are tolerant or resistant to high levels of metals employ one or several of many mechanisms to deal with toxic levels of metals and thus have potentials as species that can be used for phtytoremediation of contaminated environments (Bako et al., 2005a; Liphadzi and Kirkham, 2005). In Nigeria however, very little is known and even less has been done about the risk of metal poisoning and the possibility of using plants for phytoremediation of metal contamination.
In this regard, this research was conducted to evaluate the metal content of the two common species of macrophytes in relation to the metal content of the sediments in Makwaye and Kubanni Lakes in the Nigerian Northern Guinea Savanna.
MATERIALS AND METHODS
Study Area
The study sites were Kubanni Lake (Ahmadu Bello University Dam) and Makwaye
Lake (University Farm Dam) in Samaru, Zaria. Zaria is located on a plain at
11° 3`N and 7°42`E. It is 686 m above sea level in the Northern Guinea
Savanna eco-region of Nigeria. The soils of Samaru are generally ferruginous,
with relatively high Cation Exchange Capacity (CEC).
Makwaye Lake is an unprotected natural lake ecosystem around the catchments of which, a lot of irrigation farming and animal grazing goes on. Kubanni Lake on the other hand was formed by the impoundment of the Kubanni stream that receives a significant level of municipal waste from the surrounding University campus and neighbouring Samaru village. Although in the past there have been limited irrigated farming and grazing activity around the catchments of this lake, it is now (over the past five years) relatively well protected from these activities. A preliminary survey of the study area was conducted. Vegetation studies were conducted during which the composition of the macrophyte community was observed. Two species of the dominant emergent macrophytes (Polygonum sp. and Ludwigia sp.) that occurred in the two lakes were selected for study.
Sampling
The study sites were mapped and each of the four cardinal points of the
two lakes served as sampling sites. Random sampling was assumed and sampling
was done in triplicates in five by five meter quadrats to obtain a good representation
of the marshland vegetation. Samples of the plants at each of the sampling points
as well as the soil on which the plants grew were collected, put in labelled
polythene bags and taken to the laboratory. Samples were collected (in the dry
season of 2005, shortly before the commencement of the rains) put in well-labelled
polythene bags and taken to the herbarium of the Department of Biological Sciences
ABU Zaria, where the plant species were identified and sorted.
Sample Preparation
In the laboratory plant samples were washed with tap water and then with
distilled water to remove debris and surface contamination. Samples were then
bulked and sun dried for two days to remove excess moisture. They were then
oven dried at a temperature of 65°C for three days. Similarly, samples of
the soil sediments were collected then bulked and sun dried for three days.
Dried plant and soil samples were pounded using a porcelain mortar and pestle and sieved to attain a uniform particle size. Each sample was put in a small transparent polythene bag and labelled. These samples were subjected to analysis. Analysis of the elemental content of the samples was done at the Centre for Energy Research and Training (CERT) Zaria, using the Energy dispersive X-ray fluorescence Spectroscopy (XRF) method (Funtua, 1999).
Metal Analysis
The samples were ground manually to powder with an agate mortar and pestle
to grain size of less than 125 μm. Pellets of 19 mm diameter were prepared
from 0.3-0.5 kg powder mixed with three drops of organic liquid binder and pressed
at 10 tons of pressure in a hydraulic press.
Measurements were performed using an annular 25 mCi 109Cd as the excitation source, that emits Ag-K rays (22.1 KeV) in which case all elements with lower characteristic excitation energies were accessible for detection in the samples. The system consists furthermore of Si (Li) detector, with a resolution of 170eV for the 5.90 KeV line, coupled to a computer controlled ADC-card.
Quantitative analysis of the sample was carried out using the Emission Transmission (E-T) method and it involved the use of pure target material (Mo) to measure the absorption factors in the sample.
The Mo target served as a source of monochromatic X-rays, which are excited through the sample by primary radiation and then penetrate the samples on the way to the detector. In this way, the absorption factor is experimentally determined which the program uses in the quantification of concentration of the elements. In addition, the contribution to the Mo-K peak intensity by the Zr-K is subtracted for each sample.
| Table 1: | Precision of measurement ranges for elements (ppm) in IAEA-V-10 (hay powder) and IAEA-359 (Cabbage) compared with certified values |
| LOD = Limit of detection | |
Sensitivity calibration of the system was performed using thick pure metal foils (Ti, Fe, Co, N, Zn, Nb, Zr, Mo, Sn, Ta and Pb) and stable chemical compounds (K2CO3), CaCO3, Ce2O3), WO3, ThO2, U3O3). The spectra for the samples were collected for 3000 sec with the 109Cd source and the spectra were then evaluated using the AXIL-QXAS program (Bernasconi, 1996). 109Cd source was used for the analysis of K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ta, W, Ga, As, Se, Pb, Br, Sr, Th, Y, Zr, Nb and Mo. The International Atomic Energy Agency distributed this package. Other details of the measurement are as described by Funtua (1999).
The accuracy and precision of the measurements were confirmed though an analysis of IAEA-V10 (hay powder) and IAEA-359 (cabbage) certified reference material, distributed by International Atomic Energy Agency (IAEA). There was a general good agreement between measured and certified values (Table 1). Uncertainties in the measurement included errors from counting statistics, calibration error and uncertainty of the absorption correction factor. Results obtained were subjected to statistical analyses according to Parker (1980).
RESULTS
The trace metal contents in sediments of Makwaye and Kubanni Lakes are presented in Table 2. Iron (Fe) was present at highest concentration, of 8 metals evaluated in both lakes. Titanium (Ti), Manganese (Mn), Nickel (Ni), Copper (Cu), Zinc (Zn) and Arsenic (As) followed in that order. Significantly, lead was not detected in the sediments of any of the two lakes. Concentrations of Mn, Fe, Ni and Zn tended to be higher in Kubanni than Makwaye Lake, while for Ti the reverse was the case. The levels of Cu and As were somewhat similar for the two lakes. Concentrations of all the metals in the sediments of both lakes were higher than levels internationally regarded as normal (Table 3).
Table 4 presents the trace metal contents in shoots of Polygonum sp. and Ludwigia sp collected from the two lakes. Accumulation patterns for the trace metals were in the order Fe>Ti>Mn>Cu. Ni, Zn, As and Pb were not detected in the shoot tissues of either species, although all (except Pb) were present in the sediments. Concentration of all the metals in the shoots of both plant species were higher than levels internationally regarded as normal (Table 3).
| Table 2: | Trace metal contents (ppm) in sediments of Makwaye and Kubanni lakes |
| F = *9.72 for trace metal content, * = Significant at p≥0.05 | |
| Table 3: | Normal and toxic total concentrations (μg g-1/ppm) heavy metals in the soil and plants (Kirkham, 1975; Alloway, 1995; Fageria et al., 2002) |
| Table 4: | Trace metal contents (ppm) in shoots of Polygonum sp. and Ludwigia sp. from Makwaye and Kubanni lakes |
| ND = Not Detected, F = *16.38 for trace metal content, * = Significant at p>0.05 | |
| Table 5: | Trace metal content in (ppm) shoots of Polygonum sp. and Ludwigia sp. expressed as percentage of metal contents in sediments of Makwaye and Kubanni lakes |
| ND = Not Detected, F = *17.93 for trace metal content, * = Significant at p>0.05 | |
Table 5 shows the metal content of the two plant species expressed as a percentage of the metal content of the sediments of the two lakes. Between 45-100% Mn found in the sediments of the two lakes was found in the shoots of the two plant species, while for Cu this was 31-34%, Ti (15-42%) and Fe (7-21%). However, almost twice the concentration of Mn present in the sediment of Makwaye Lake was taken up by the plants as compared to the situation in Kubanni Lake (Table 5).
DISCUSSION
The fact that metal contents in sediments and plant shoots exceeded the internationally acceptable limits in both lakes, emphasizes the need for regulation of discharge of municipal waste into Kubanni, as well as regulation of agricultural activities and grazing around the catchments of Makwaye. For Cu, Ni and Zn, toxic levels were recorded in lake sediments, while for plant shoots this was true for Cu, Mn and Fe. The presence of these metals in the sediments and plant tissues has several and varied implications for the health of plants, animals and humans.
Copper, is an important pollutant that originates from sludge and manure application (Alloway, 1990; Panou-Filotheou, 2001) It causes reduced growth of stem and root, water/nutrient uptake and leaf chlorosis in plants, as well as gastrointestinal distress, liver and kidney damage in humans (US EPA, 2002) and damage to plasma membrane in both plants and animals (Demidchik et al., 2001; Hall, 2002).
Manganese toxicity is common in waterlogged areas (Liphadzi and Kirkham, 2005). The effects of Mn toxicity on animal and humans are essentially not known (Liphadzi and Kirkham, 2005). In plants it has been reported to cause dark brown spots on older leaves, interfere with absorption, translocation and utilisation of elements like Ca, Mg, Fe and P and alter activities of enzymes and hormones (Hopkins, 1995; Horst and Fecht, 1999; Wang et al., 2002).
Nickel is a pollutant that originates primarily from traffic or refinery emissions and industrial or municipal waste (Barbafeiri, 2000). In plants, it inhibits cell division in meristems of root and root expansion, inteferes with the translocation of Mn, Fe, Cu and Zn. In animals and humans, its toxicity inhibits spermatogenesis, amylase enzymes, insulin formation, kidney function and causes respiratory and renal disorders (Srivastava and Gupta, 1996).
Industrial waste, sewage sludge and farm manures have been identified as sources of zinc pollution (Liphadzi and Kirkham, 2005). Its toxicity causes malformation of the nucleus and nucleolus of meristematic cells of plant roots. Chlorophyll content and root length are reduced (Bekiaroglou and Karataglis, 2002). Further, there was a reported reduction in biomass, seed number, seed weight and soluble proteins of sunflower plants (Khurama and Chatterjee, 2001). However effects of Zn toxicity on humans and animals are unclear (Liphadzi and Kirkham, 2005).
Iron and Titanium were the metals that were recorded at the upper concentration ranges in both the sediments and plant tissues. The high concentration of Fe could have originated from the ferruginous nature of the soils in this area.
The high accumulation pattern of Fe observed in plant shoots may be because this metal is known to be of low toxicity in plants (Liphadzi and Kirkham, 2005). Significantly, this finding suggests that the two plant species in this study could be evaluated as species that may be used for phytoremediation of Fe in contaminated soils. However, in excess quantities, Fe promotes formation of reactive oxygen (free radicals) species that damage vital cellular constituents due to lipid peroxidation. Coalesced tissues, bronzing or necrosis, flaccidity and blackening of roots have all been reported to result from above optimal Fe concentrations. No serious human diseases have been linked directly to excessive Fe concentration (Liphadzi and Kirkham, 2005).
On the whole accumulation patterns of metals in plant shoots suggested a trend that reflects the level of toxicity of the metals to plant tissues, with Fe, Ti, Mn and Cu following in that order. While Fe, Mn and Cu are micronutrients normally useful to the plant, other metals that were found in the sediments but not detected in the plant shoots (Ni, Zn, As and Pb) are metals of high toxicity that have little or no physiological relevance. Such metals (except probably Pb) may have been excluded from the plant shoots either because they have been neutralized by phytostabilization in the soil (e.g., Ni and Zn) or by phytovolatization (e.g., As). These are two of several known mechanisms by which plants deal with toxic levels of heavy metals (Liphadzi and Kirkham, 2005).
Generally, Mn appeared to have been most readily absorbed by the plants, with 45-100% of soil Mn being found in the plant shoots. Cu was the next most readily absorbed, with Fe being the least readily absorbed, though the final actual concentration of this latter metal was the highest of all the metals.
There is usually a differential in the rate at which plants absorb metals, that depends on the bioavailability of such metals. Bioavailability is a function of changes that control concentration and free metal activity in the soil. These changes are influenced by soil properties such as pH, redox potential, organic matter content and soil mineralogy (Calace et al., 2002). In addition, there are plant dependent factors that aid in metal acquisition by plants. Such include the presence, or absence of mechanisms for detoxification/sequestration at the cellular and sub-cellular levels that may be operational for different species of plants or metals (Liphadzi and Kirkham, 2005).