Donboklang Marbaniang1 and S.S. Chaturvedi2
Department of Environmental Studies
North-Eastern Hill University, Shillong-793022
The bioaccumulation of Zinc (Zn) by Scirpus mucronatus, in Hoagland solution enriched with 2.0, 4.0, 8.0, 16.0 and 32 mg L-1 of Zn supplied as Zinc Sulphate (ZnSO4) for a period of 2, 4, 6, 8 and 10 days. The accumulation of Zinc in plant depending on time and concentration was measured by atomic absorption spectrophotometer (AAS Perkin Elmer Model 3110). The results showed that under experimental conditions (pH- 6.0±1, T- 24±1, Photoperiod- 16h), S. mucronatus (L.) are able to accumulated considerably amount of Zn. Removal of the metals from the solution reached the maximum on the 8th day. The accumulation of Zn increased with the increasing concentration. The highest accumulation was observed in the root then by shoot tissues. The bioconcentration factor (BCF) for the metal was found to be maximum on the 8th day exposure.
Keywords: Scripus mucronatus, Zinc, phytoaccumulation, Bioconcentration (BCF).
Due to its large industrial use (mining operations, agricultural industries and other anthropogenic activities), zinc (Zn) is considered as a serious environmental pollutant because of its non-degradability when discharged into a water body. When present at elevated concentrations in aquatic systems (lakes, ponds, aqueous streams, etc.), Zn causes a variety of environmental problems, including loss of vegetation, groundwater contamination and metal toxicity in the food chain. Remediation of heavy metal contamination can be use by a variety of technologies, viz., chemical, physical or biological. Methods such as precipitation, reduction, artificial membranes, and ion exchange are used to remove toxic metals from industrial effluents but they are expensive, relatively inefficient and in most cases they generate a great amount of by-products (sludge, metal-rich waste, etc.) which is difficult to dispose of and therefore dangerous to our environment (Markus and Kertes 1969; Rebhun and Galil 1990; Brewster and Passmore 1994).
In recent years, attention has been focused on the study of aquatic macrophytes as promising candidates for pollutant uptake and biological indicators of heavy metal in aquatic systems (Wolverton and Mc Donald 1979; Martin and Coughtrey 1982; Gersberg et al. 1986; Bishop and Eighmy 1989; Delgadoet al. 1993; Jenssen et al. 1993; Ozimek 1993; Sen and Bhattacharyya 1994; Aoi and Hayashi 1996; Maine et al. 1998, Maine et al.1999, Maine et al. 2001). Phytoaccumulation has recently gained importance because of its cost-effectiveness, long-term applicability and ecological aspect (Weiss et al. 2006; Rai 2008). This technology is based on the ability of plants to absorb and accumulate metal contaminants in their tissues and eliminate high amount of these elements from water or groundwater.
The objective of the present study was to assess the phytoaccumulation and bioremediation potential of S. mucronatus for Zn under laboratory conditions. The experiments were carried out under carefully controlled environmental conditions to eliminate the effects of all environmental factors.
Materials and Methods
S. mucronatus an emerging macrophyte was collected from two water bodies of Mawlai Umshing, (Lat 25036’36.76”N Long 91054’05.11”E) Meghalaya, India in the month of April 2011 and were transferred to the laboratory in polyethylene bags. Plants of similar size and height were selected and washed several times with tap and deionised water and kept separately in 40 litre glass containers containing deionised water and half strength Hoagland’s solution at pH = 7 (Hoagland and Arnon 1950) for a period of 15 days. The acclimatized plants were then transferred to different containers containing deionised water, 5% Hoagland’s solution and various concentrations of Zinc (2.0, 4.0, 8.0, 16.0 and 32.0 mgl-1) and exposed for 2, 4, 6, 8 and 10 days under controlled temperature (24 ± 10C) and light (3500Lux) conditions. ZnSO4 (AR, Himedia) was used as the source of Zn. Three sets of experiments were carried out separately for the selected macrophytes. At regular intervals (2, 4, 6, 8 and 10 days), plants were taken out and washed well with tap and deionised water. The washed samples were separated into roots and shoots (including leaves and floral parts) and adhering water was carefully removed using absorbent paper. Samples were then dried in an oven at 70 ± 50C for 48h. The oven-dried samples were chopped and finally ground to ensure homogeneity for facilitating organic matter digestion. One control group of plants was prepared for each sample where a metal ion solution was not added.
For digestion, the plant samples were carried out according to Kara and Zeytunluoglu (2007). Metal contents in plant samples were determined by using Atomic Absorption Spectrophotometer (AAS 3110, Perkin-Elmer). For analysis of Zn in the plant samples through AAS the wavelength is 213.9 nm, slit width 0.7 nm and the flame gases is air-acetylene are used.
The bioconcentration factor (BCF) is a useful parameter to evaluate plant’s potentiality to accumulate metal, it provides the ability index of a plant to accumulate metals with respect to metal concentration in the substrate and it was calculated on a dry weight basis (Zayed et al. 1998).
BCF = Trace elememnts concentration in plant tissue (µg/g-1)Initial concentration of the element in the external nutrient solution (mgL-1)
Translocation of heavy metal from roots to aerial part is generally expressed as translocation factor (TF) and it indicates the internal metal transportation of the plant. The translocation factor was determined as a ratio of metal accumulated in the shoot to metal accumulated in the root (Stoltz and Greger 2002; Deng et al. 2004). This evaluates the extent of metal translocation from roots to shoots.
Wherein, TF>1 indicates that the plant translocate metals effectively from the root to shoot.
One-way analysis of variance (ANOVA) was performed for all the data to confirm their validity using Statistica version 6. The data were all presented as mean ± standard error of three replicates. The significant difference between treatment means for different parameters was tested at P < 0.05 using a Fisher least significant difference (LSD) test.
Results and Discussion
The data presented in Fig 1 show the accumulation (µgg-1 dry weight) and BCF (table 1 and Fig 2) of Zn in S. mucronatus at different concentrations and treatment durations. Although Zn accumulation by plant was found concentration and duration dependent, the Zn content in the roots and shoot of S. mucronatus was analyzed at regular interval of 2, 4, 6, 8, and 10 days. The Zn content in shoots and roots at 2.0, 4.0, 8.0, 16.0 and 32.0 mg L?1 concentrations, were 2930, 3446.7, 4006.7, 4280 and 4510 µgg-1 dry weight in roots and 1353.3, 1720, 1873.3, 2253.3 and 2776.7 µgg-1 dry weight in shoot respectively after 10 days of exposure. A maximum accumulation of Zn in root and shoot were 4670.7 and 3683.3 µgg?1 at 32 mgL-1 after 8 day. In general, concentrations of metals are higher in roots compared to leaves and other aerial parts (Garcia et al. 1979; Sela et al. 1989).). The amount of Zn accumulated by plant tissues (roots >leaves) varied significantly (P < 0.05). To quantify metal accumulation in plant biomass, the bioconcentration factor (BCF) is more significant than the amount accumulated in plants since it provides an index of the ability of the plants to accumulate metal element with respect to the element concentration in water. Highest BCF value of 2705 was at 2 mgL-1 on 8th day for Zn. The BCF values decreased with increasing Zn concentration and which is similar to the finding by Jain et al. (1990) where the BCF value for Azolla pinnata and Lemna minor treated with Pb and Zn, gradually decreased with increasing metal concentration.
According to this study, S. mucronatus accumulate a considerable amount of Zn in its tissues when exposed to different concentrations and exposure time and is in accordance with the report found in Lemna minuta (Jafari and Akhavan 2011), S. polyrrhiza (Mishra and Tripathi 2008) L. polyrrhiza (Sharma and Gaur 1994), L. minor (Jain et al. 1989) and Spirodela intermedia and L. minor (Miretzky et al. 2004). According to Zayed et al. (1998), plant which is considered as a good accumulator, must have a BCF over 1000. Our results confirmed that S. mucronatus is a good accumulator of Zn (table 2) and has a potential for the remediation of Zn polluted water. Based on the BCF value it indicated that S. mucronatus is less effective than L. trisulca (Jafari and Akhavan 2011) but more effective than L. gibba (Khellaf and Zerdaou 2009) and L. polyrrhiza (Sharma and Gaur 1994). L. minor (Jain et al. 1989) and S. polyrrhiza (Mishra and Tripathi 2008) respectively. The bioconcentration of heavy metal by aquatic metal by aquatic plants such as Lemna has already demonstrated by Megateli et al. (2009) and Jalali-Rad et al. (2004).
The translocation factor (TF) of metals in shoots/roots ratio of S. mucronatus was < 1 for Zn (Table 2). Results of the present study, indicated that most of the metal accumulated in the plant was retained by the root and leading to less translocation to aerial parts. Roots of plants may act as a barrier against heavy metal translocation and this may be a potential tolerance mechanism operating in the roots (Ernst et al. 1992).
The study shows that S. mucronatus could efficiently reduce the Zn present in wastewater. The BCF value for Zn (> 1000) indicates that S. mucronatus can efficiently removed Zn from contaminated water. The increase of Zn concentration in the plant roots shows that the test metals are being poorly translocated to the aerial parts. Based on this study, S.mucronatus could be useful for phytoremediation of Zn from contaminated water.
In the present study, a laboratory experiment was carried out where all the external factors are controlled against Zn contamination in water. The results of the present study indicated that the experimental plant is suitable for the phytoremediation of Zn contamination from water under laboratory conditions. Therefore, S.mucronatus could be useful for phytoremediation of Zn from contaminated water. Furthermore, studies are needed to evaluate the on-site application of these plants for phytoremediation.
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