Abstract: The increase of urban and industrial activities has led to pollution and deterioration of Asa river. A study aimed at ascertaining heavy metal concentrations and allocation in Phragmites karka growing in polluted sites of Asa River with the prospect of using Phragmites karka in phytoextraction of zinc, copper, cadmium and lead was carried out. Phragmites karka was separated into (roots, stem and leaves), water and sediment samples were collected from three sampling points in Asa River were analysed using Flame atomic absorption Spectrophotometer. The results of the translocation ability were in the order Cu>Zn>Cd and Pb. The bioconcentration factor for Zn, Cu, Cd and Pb in the root, stem and leaf had low values thus, a limited transportability of heavy metals from the sediment to the plant. Heavy metal accumulation in the roots was greater than those of the shoots. The quantities of accumulation in the roots were in the order Zn>Cu>Pb>Cd. Translocation values>1 were found for Zn and Cu while Translocation values for Cd and Pb<1. It can be deduced from the result that these metals (Zn, Cu, Cd and Pb) have limited translocation to the aerial parts of plant. Cu and Zn accumulations are transported to shoots while Cd and Pb are stored in the roots. Enrichment coefficient of Zn, Cu, Pb and Cd was <1.0. This study suggests that Phragmites karka is less suitable for sequestering of these studied heavy metals.
INTRODUCTION
The increase in industrial activities has intensified environmental pollutions and the deterioration of several aquatic ecosystems with the accumulation of metals in biota and flora (Spinoza-Quinones et al., 2005). Aquatic plants sequester large quantities of metals (Mays and Edwards, 2001; Stoltz and Greger, 2002; Baldantoni et al., 2004) from the environment by storing them in the roots and or shoots.
There are extensive studies on metal accumulation by aquatic plants. The aquatic plants include floating plants, such as Salvinia herzogii (Maine et al., 2004), water hyacinth (Eichhornia crassipes) (Mishra et al., 2008), duckweed (including Lemna polyrrhiza L., Lemna minor and Spirodela polyrrhiza W. Koch) (John et al., 2008; Mishra and Tripathi, 2008), mosquito fern (Azolla pinnata R. Brown) (Mishra et al., 2008) and water lettuce (Pistia stratiotes) (Maine et al., 2004; Mishra et al., 2008), emergent plants such as common cattail (Typha latifolia) (Das and Maiti, 2008) and submerged plants, such as pondweed (Potamogeton pectinatus or Potamogeton crispus) (Badr and Fawzy, 2008; Mishra et al., 2008), Hydrilla (Hydrilla verticillata) (Bunluesin et al., 2004; Mishra et al., 2008) and coontail (Ceratophyllum demersum L.) (Badr and Fawzy, 2008; Bunluesin et al., 2004).
The bulk of water for domestic and other uses in Ilorin comes from Asa River North Central Nigeria. However, soap and detergent industry; a beverages industry and hospital empty their waste discharge into this river thus leading to environmental pollution. This river may be temporarily or permanently impaired in quality as a result of these industrial and domestic activities. The utilization of aquatic plants as natural filters for the abatement of pollutants transported by water in rivers or lakes is considered to be an effective, low cost, cleanup option to ameliorate the quality of surface waters.
According to Gopal (2003), aquatic plants have been extensively utilized to clean polluted water. Copper, cadmium, lead and zinc were chosen as the metals for the study because the presence of cadmium above trace levels in the environment is an indicator of contamination; lead is a common pollutant from road runoff. Zinc is a common metal present in variable amounts and if found in appreciable amounts can be an indicator of industrial pollution. While copper is also an indicator of industrial contamination of urban waters (Cardwell et al., 2002). The aim of the study was to determine the potential use of Phragmites karka, a dominant aquatic macrophyte as a suitable monitor to assess the heavy metals present in the study area.
MATERIALS AND METHODS
Asa river is located in Ilorin, North Central Nigeria with a catchment area of about of 104 km2. It lies between latitude 8°24’ and 8°36’ N and longitude 4°10’ and 4°36’ E. (Olayemi et al., 1990). It is a source of water for domestic and industrial uses and agricultural activities. The dominant aquatic macrophyte is Phragmites karka.
Water, plant and sediment sampling: Water, sediment and aquatic plant samples were carried out monthly between January and June, 2008 in three sampling points. Water samples were collected directly with a 1 L plastic container, washed with nitric acid and rinsed with distilled water.
Phragmites karka plants were harvested from the sampling points using the modified method of Allen (1989). The collected plant samples were separated into roots, stem and leaves, subsequently washed with river water and rinsed with distilled water and then air dried. At least three plants of the same species were collected at each site. The samples were later packed in polyethylene bags and transported to the laboratory in an ice box at 4°C. Samples were separated into portions of roots, stems and leaves samples. Plant were then cut into smaller portions and washed with distilled water and then double rinsed with deionised water. The samples were air dried and then homogenized into fine grained fractions in a grinding mill. Dried samples of 1.2 g were transferred into 250 mL Pyrex beakers, 20 mL of 65% HNO3 added and left overnight before heating them on a hot plate. An open-beaker digestion was performed at 250°C hot plate attained gradually until the mixture was heated to near dryness. Then 5.0 mL of 30% hydrogen peroxide was added to complete the digestion and the resulting mixture heated again to near dryness. The residues were not completely dissolved. The beaker walls were washed with 2.5 mL of deionised water and the digest heated till boiling. The digest liquor was allowed to cool and later transferred into 25 mL standard flasks which were filled with deionised water to the mark.
The sediment samples were collected in each of the sampling points from the sediment surface using Ekman grab sampler, air dried, homogenized and were analysed as described by Sekabira et al. (2011). Heavy metals were then analyzed by direct aspiration of the sample solution into a Perkin-Elmer model 2380 Flame Atomic Absorption Spectrophotometer.
The movement of the heavy metal from the polluted sediments into the roots of the plant and the ability to translocate the metals from roots to aerial parts were assessed correspondingly by means of Bioconcentration Factor (BCF) and the Translocation Factor (TF). Bioconcentration factor is an index of the ability of plant to accumulate a particular metal with respect to its concentration in the sediment (Ghosh and Singh, 2005). Bioconcentration factor (BCF) was calculated as a ratio of concentration of heavy metal in plant roots to that of soil (Yoon et al., 2006):
The higher the BCF value the more suitable is the plant for phytoextraction (Blaylock et al., 1997). BCF values >2 will be regarded as high values.
Translocation coefficient (TF) was described as the ratio of heavy metal concentration in plant shoot to that in plant root (Yanqun et al., 2005). This ratio is an indication of the ability of the plant to translocate metals from the roots to the aerial parts of the plant (Marchiol et al., 2004):
Metals that are accumulated by plants and largely stored in the roots of plants are indicated by TF values <1 indicating that the metals are stored in the stems and leaves.
According to Yanqun et al. (2005), Enrichment Factor (EF) was defined as the concentration of heavy metals in plant shoots divided by the heavy metal concentration in soil:
RESULTS AND DISCUSSION
The elemental concentrations were higher in sediment when compared to water. Zinc was higher both in sediment (78.15 ppm) and water (0.42), followed by copper with concentration of 29.31 ppm in sediments and 0.14 ppm in water, respectively. Other metals (Cd and Pb) showed a decreasing trend. Sediment had higher values of heavy metals compared to the water. According to Fleeger et al. (2003), heavy metals have direct toxicity when discharged into the aquatic environment and thus sediment constitutes the sink for these pollutants. It was also corroborated by Horsfall et al. (1999), who reported that sediment act as carriers and sources of heavy metal pollution which can be released to the water column as a result of changes in environmental condition. The values of the ratio between concentrations of metals in sediment and water were highest for copper (209.36); it was followed by zinc (186.07). Cadmium was the lowest (Table 1). The mean concentrations of Cu and Zn in this study were higher than the average mean concentration of the metals for Cu (5.84 mg kg-1) and Zn (67.5 mg kg-1) reported by Adekola and Eletta (2007).
| Table 1: | Mean concentrations in sediment and water and ratios between the concentrations in the sediments and that in the water |
| Table 2: | Mean concentration found in the sediment and maximum permissible metal content in sediment |
| *Kabata-Pendias and Pendias (2001) | |
This high level of anthropogenic input into the river could be as a result of application of agro chemicals during farming activities, domestic wastes and industrial effluents thrown into the river. The comparison of maximum levels of the various heavy metals in the sediment from the studied sites to acceptable standards is shown in Table 2. Pb, Cu and Zn were above the stipulated standard while Cd was within the stipulated standard.
The mean accumulation of metals in the plant parts of Phragmites karka from Asa river varied with Zn 39.99 mg kg-1 root, 25.09 mg kg-1 stem and 26.32 mg kg-1 leaf. Cu ranged from 6.89 mg kg-1 for root, 4.73 mg kg-1 stem and 5.13 mg kg-1 leaf. Cd was 0.02 mg kg-1 for root, 0.01 mg kg-1 and leaf 0.02 mg kg-1. Pb varied with 0.03 mg kg-1 in root, 0.01 mg kg-1 in stem and 0.02 mg kg-1 in leaf. Heavy metal accumulation in the roots was greater than those of the shoots. The quantities of accumulation were in the order Zn>Cu>Cd>Pb in the roots while in the stem Zn>Cu>Cd and Pb and in the leaves Zn>Cu>Pb>Cd (Table 3). Heavy metal concentration in plant parts was in this succession root>leaves >stem. Sekabira et al. (2011) reported that there is low utilization of heavy metals from sediments through roots, stems and leaves and tolerance mechanism developed by plants to accumulate heavy metals in roots.
Table 4 shows the Bioconcentration Factor (BCF) movement of each metal from sediment to Phragmites karka plants. BCF values of zero imply limited movement from the sediment to the plant. BCF value at the root was highest for Zn followed by Cd, Pb and Cu while the stem and leaf values were in the order, Zn>Pb>Cu>Cd and Zn> Cd>Cu>Pb, respectively. The bioconcentration factor for Zn, Cu, Cd and Pb in the root, stem and leaf showed low values <1 (Table 2) thus, there was limited movement of metals from the sediment to the plant. According to Ghosh and Singh (2005) bioconcentration factor is an index of the ability of plant to accumulate a particular metal with respect to its concentration in the sediment. The higher the BCF values the more suitable is the plant for phytoextraction (Blaylock et al., 1997). BCF values greater than 2 were regarded as high values.
Translocation factor: The translocation ability of these heavy metals were in the order Cu (1.43)>Zn (1.29)>Cd (1.0); Pb (1.0). Metals that are accumulated by plants and mostly stored in the roots of plants are indicated by TF values <1. It implies that translocation values greater than one shows translocation to the shoots of the plant. Translocation values>1 were found for Zn and Cu while translocation values for Cd and Pb<1 (Table 5). Since, translocation factor is an indication of the ability of the plant to translocate metals form roots to the aerial parts (Marchiol et al., 2004), it can be deduced from the result in Table 5 these metals (Zn, Cu, Cd and Pb) have limited translocation to the aerial parts of plant.
| Table 3: | Heavy metal accumulation in the root, stem and leaf of Phragmites karka |
| Table 4: | Bioconcentration factor of each metal |
| Values>2 will be regarded as high values | |
| Table 5: | Translocation factor of the studied heavy metals |
| Values>1 are regarded as high | |
| Table 6: | Enrichment coefficient of the studied heavy metals |
Cu and Zn accumulations are transported to shoots while Cd and Pb are stored in the roots. Yanqun et al. (2005) reported that TLFs higher than 1.0 were determined in metal accumulator species, whereas TLFs was typically lower than 1.0 in metal excluder species. The TLFs higher than 1.0 indicated an efficient ability to transport metal from root to leaf, most likely due to efficient metal transporter system of plants (Zhao et al., 2002) and probably sequestration of metals in leaf vacuoles and apoplast (Lasat et al., 2000). Plants with both bioconcentration factors and translocation factors greater than one (TF and BCF>1) have the potential to be used in phytoextraction. Besides, plants with bioconcentration factor greater than one and translocation factor less than one (BCF>1 and TF<1) have the potential for phytostabilization (Yoon et al., 2006). From the result Phragmites karka is not suitable for both phytoextraction and phytostabilization of Zn, Cu, Pb and Cd. Enrichment coefficients are very important factor which indicate phytoremediation of a given species (Zhao et al., 2003). Both EF and TF have to be considered while evaluating whether a particular plant is a metal hyperaccumulator (Ma et al., 2001). Therefore, a hyperaccumulator plant should have EF >1 and TF >1, as well as total accumulation> 1000 mg kg-1 of Cu, Co, Cr, Ni or Pb or >10000 mg kg-1 of Fe, Mn or Zn. In this study, the enrichment coefficient of Zn, Cu, Pb and Cd was <1.0 (Table 6) as such Phragmites karka cannot be regarded as a hyper accumulator of these metals.
CONCLUSION
This study attempted to determine the phytoremediation potential by the aquatic macrophyte Phragmites karka. The study has suggested that the phytoextraction ability of the studied plant is very low. The result presented here could be very useful for environmental monitoring and assessment of the water body. Thus this study can form part of the sustainable development of the ecosystem and pollution.