Ludwigia natans: a potential aquatic macrophyte for cadmium bioaccumulation and Phytoremediation

· Articles
Authors

D. Marbaniang* & S.S. Chaturvedi
Department of Environmental Studies
North-Eastern Hill University, Shillong-793022
sschaturvedinehu@gmail.com
*Corresponding author: D. Marbaniang; email:baphi66@gmail.com

Abstract

Aquatic macrophytes have tremendous potential for remediation of the heavy metal cadmium. A Laboratory experiment was conducted to evaluate the Cd bioaccumulation capacity of Ludwigia natans. The macrophyte was grown in the laboratory containing nutrient solution and working Cd standard solutions of different concentrations (1, 10, 50, and 100 mgL-1) and harvested at regular time interval of 5 and 10 days. The Cd accumulation by L. natans showed an increase with time and the maximum accumulation was on the 10th day at 50mgL-1 for both the roots and shoots. Cd accumulation in the plant parts was higher in the shoots as compared to the roots. The maximum bioconcentration factor values (7666) which indicate that the plant was a Cd hyperaccumulator and translocation factor values (2.8) which is >1 which points towards the suitability of L. natans for removing Cd from Cd-contaminated water.

Keywords Ludwigia natans, Cadmium, Accumulation, Bioconcentration Factor (BCF), Translocation Factor (TF)

Introduction

Heavy metals pollution in the aquatic environment has become a major cause of concern for many countries especially in India where it is too expensive for remediation process. However, with the rich diversity of aquatic macrophytes and their potential to hyperaccumulate many heavy metals and they can be used as an alternative way of remediating heavy metals from the aquatic system (Rai, 2008). Cadmium (Cd) is one of the potent and highly toxic metals among the other metals found in the earth’s crust and can easily contaminate the food chain (He et al., 2005). It is ranked seventh in the ASTDR (2011) priority list of hazardous substances. There are several sources of Cd in the environment which includes both from the natural and man-made sources. Notably, natural emissions and man-made industrial effluents emitted by ore processing industries and plants have immensely polluted water.

A variety of techniques which includes chemical, physical and biological technology have been used to remediate heavy metal contamination from soil or water. Toxic metals from industrial effluents have been remove by various other techniques such as precipitation, reduction, artificial membranes, and ion exchange, but however these techniques generate a huge amount of waste e.g., sludge, metal rich waste, etc which is difficult to dispose of and therefore, dangerous to the environment and they are also generally expensive, relatively inefficient (Rebhun and Galil, 1990). Phytoaccumulation, one of the biological indicators which indicate the degree of absorption of heavy metals in plants has lately gained its applicability because its cost-effectiveness, long-term and ecological aspect (Weiss et al., 2006). Aquatic macrophytes have received great attention and have shown to be one of the candidates in the aquatic system for pollutant uptake and biological indicators of heavy metal (Maine et al., 2001). An ideal hyperaccumulator plant would have shoots with a high capacity to accumulate heavy metals/metalloids, have a high biomass and rapid growth, and bioconcentration factor (BCF) and translocation factor (TF) values higher than 1 (Garbisu and Alkorta, 2001). Works on duckweed (Zayed et al., 1998; Wang et al., 2002), water hyacinth (Zhu et al., 1999; Wang et al., 2002), S. mucronatus, R. rotundifolia and M. intermedium (Marbaniang and Chaturvedi, 2014) were reported for Cd remediation.

The objective of the present study was to assess the uptake of Cd and phytoremediation potential of L. natans for Cd under laboratory conditions. The experiments were performed in a contained environmental set up inorder to eliminate all external environmental factors.

Materials and Methods

L. natans was collected from M/S Gills and Bills, Shillong, India and they transferred to the laboratory. Plants were washed several times with tap and distilled water in order to remove any adhering soils and plants of similar size, shape and height were selected and kept separately in a 40L capacity tank which contained half strength Hoagland’s solution of pH = 7 (Hoagland and Arnon, 1950) and kept for 15 days prior to experimentation for. After 15 days the acclimatized plants were transferred and maintained in 5% Hoagland’s solution containing working Cd standard solutions of different concentrations 1.0, 10.0, 50.0 and 100.0 mgL-1and then they were exposed to Cd concentrations at a time interval of 5 and 10 days. Cd of analytical grade, were supplied as CdCl2 (Himedia) were used as the source of Cd. Experiments were carried out under controlled temperature (24±10C) and light (3500 Lux) conditions. After each time interval the plants were collected and washed with deionised water to remove any metal adhering to its surface. The washed plant samples were carefully dried the adherent water using absorbent paper and then they are separated to roots and shoots. Samples were dried for 48h in an oven at 70±50C. The dried oven plant root and shoot was then chopped and finally powdered using a mortar and pestle to ensure homogeneity for facilitating organic matter digestion. One control plant groups were also set up where no Cd was added into the medium was not added.

For digestion, the plant samples were carried out according to Kara and Zeytunluoglu (2007). Atomic Absorption Spectrophotometer (AAS 3110, Perkin-Elmer) was used to determine the Cd contents in plant root and shoot parts. The bioconcentration factor (BCF) is a useful parameter and it provides the ability index of a plant to accumulate metals with respect to metal concentration in the medium and it was calculated on a dry weight basis (Zayed et al., 1998).

Translocation Factor (TF) is generally the translocation of heavy metal from roots to aerial part and indicates the internal metal transportation of the plant. The translocation factor is determined as a ratio of metal accumulated in the shoot to metal accumulated in the root (Deng et al., 2004).

Wherein, TF>1 indicates that the plant translocate metals effectively from the root to the shoot.

Statistics analyses

ANOVA and multiple linear regressions were performed for all the data to confirm their validity using SPSS 17.0. The data were all presented as mean ± standard error of three replicates. Fisher least significant difference (LSD) test was performed at p < 0.05 to check the significant difference between the means for different uptake at different Cd concentrations.

Results and Discussion

Accumulation of cadmium: Cadmium content in the roots and shoots L. natansshowed increases in metal accumulation in the roots and shoots if metal concentrations and time period are enhanced. At cadmium concentration of 1, 10, 50 and 100mgL-1, the cadmium content (Table 1) in L. natansroots increased to the maximum 2477, 5541, 7343 and 6670 \mugg-1 dry weight and in case of shoots it was 5188, 7540, 8520 and 8040 \mugg-1 dry weight at 10th day of harvesting and accumulation ranges from 836-7343 \mug/g dry weight in roots and 2346-8520 \mugg-1 dry weight in shoots (Figure 1). The maximum accumulation was on the 10th day at 50mg/L in both the roots and shoots and minimum was on the 5th day (1mgL-1) in roots and shoots. In L. natans, the accumulation of cadmium in the root and shoot increased significantly (p<0.05) from 5 to 10 days.

Hadad et al., (2010) reported that submerged plants have a greater surface area as compared to non-submerged plants and their shoots are in constant contact with the water medium which may help them to bioconcentrate more nutrients and metals which corroborates with the present study where it was observed that the considerable amount of cadmium accumulated by L. natanswas in the shoots as compared to the roots. The shoots of this submerged species also receives Cd form the roots but however due to their constant contact with the medium they may have also accumulate Cd directly from the medium to a greater extent which corroborates with the findings of Dunbabin and Bowmer (1992); Sivaci et al., (2004); Peng et al., (2008); Marbaniang and Chaturvedi, (2014).

The multiple regression model with all two predictors produced R² = .628, F(2, 21) = 17.7, p < .001 for L. natans. As can be seen in Table 2 the concentration of cadmium in the medium had significant positive regression weights, indicating with higher cadmium concentration in the medium and time were expected to have higher cadmium uptake in L. natans. Time i.e., number of days contribute to the multiple regression model and it have a significant regression weights, indicating that uptake of cadmium does depend on time period.

Table 1: Accumulation of cadmium (µgg-1 dry wt) in roots and shoots of L. natansat different exposure time

Metal concentration

(mgL-1)

Exposure time (days)

5 d

10 d

Roots

Shoots

Roots

Shoots

1.0

836±3.3a

2346±12a

2477±6.2a

5188±4.4a

10.0

2926±8.8b

5932±6.2b

5541±7.2b

7540±5.7ab

50.0

5623±12c

6666±8.8c

7343±8.8bc

8520±10b

100.0

5263±8.8d

6053±8.8cd

6670±11c

8040±17.3b

Control

BDL

BDL

BDL

BDL

 

 

 

 

 

 

Table 2: Summary statistics, correlations and results from the regression analysis

Variable

Mean

Std error

Correlation

with uptake

Multiple regression weights

B

?

Uptake

10871.25

1716.183

Time (in days)

1.50

1032.146

.505*

3918.000**

.505

Concentrations (mgL-1)

40.25

13.193

.610**

60.454***

.610

Bioconcentration factor (BCF) of cadmium

Bioconcentration factor (BCF) value indicates the ability of the plant to accumulate metal in their tissue parts. The BCF values at different cadmium concentrations (1,10, 50 and 100 mgL-1) were evaluated at 5 and 10 day. There was a general decrease in BCF with the increase in cadmium concentration at 5th and 10th day respectively. The BCF values of cadmium of the experimental plants increased with exposure periods. The maximum BCF value (7666) was obtained when treated with 1mgL-1 of cadmium at 10th day of harvesting (Table 3).

Plants which have the ability to accumulate heavy metal in the tissues are generally classified as a good accumulator. Generally it is considered that a plant useful for phytoremediation should have a BCF value greater then 1000 (Zayed et al., 1998). In the present study, the BCF value of L. natans was 7666, which suggest that it maybe be considered as a good accumulator of cadmium. The response of aquatic macrophytes to cadmium concentration varies from species to species in terms of metal accumulation and BCF. The Cd BCF have been reported in different species by Jain et al., (1990); Phetsombat et al., (2006) ; Abhilash et al., (2009); Ha et al., (2011); Das et al., (2014); Marbaniang and Chaturvedi, (2014 ). It has been found that in spite of significant increases of Cd concentration in the plant over the 5th and 10th day range of exposure concentrations, the relationship of Cd BCF to Cd exposure concentration was negatively correlated (Figure 2). The inverse BCF to exposure relationship may be due to the fact that, even though with the passages of time the concentration of Cd also increases, however, the internal accumulation of Cd does not rise as quickly as the external exposure levels. This indicates that the plants may have developed certain mechanism to control a significant degree over Cd accumulation. Thus, the BCF values decreases with the increase of exposure concentration of Cd in the medium.

Translocation Factor (TF) of cadmium

Translocation Factor (TF) in plants is the ratio of heavy metal accumulation in the shoots parts to the roots. Translocation of heavy metal in plants are generally dependent on plant species, type of heavy metals and various environmental factors like pH, redox potential (Eh), temperature, salinity (Fritioff et al., 2005). Yanqun et al., (2005) reported that a TF value greater than 1, the plants are considered as an accumulator species, whereas TF lesser than 1 is an excluder species. The TF>1 indicated that there is a transport of metal from root to leaf probably through an efficient metal transporter system (Zhao et al., 2002) metals sequestration in the leaf vacuoles and apoplast (Lasat et al., 2000).

The TF values for L. natans under different treatments are shown in Table 3. From the table it appeared that in most of the treatments the TF value was greater than one indicating that cadmium is translocate from the roots to shoots parts. In L. natans, there is a significant increase in cadmium translocation, but with the increase of cadmium concentration in the the medium there is a reverse effect on the cadmium translocation to the shoot part. It may be suggested that the cadmium translocation to the aerial parts of L. natans reached saturation or reduced or more or less static at a higher cadmium concentration in the medium. Cadmium ions are non-essential nutrients for many plant species but they are readily taken up by roots and translocate to the shoots (Demirezen and Aksoy, 2004). The high accumulation of cadmium in roots and shoots of submerged species of L. natans also corroborates with the findings by Marbaniang and Chaturvedi, (2014) in R. rotundifolia and M. intermedium. Translocation of ions is possible apoplastic in the phloem and acropetally in the xylem (Fritioff and Greger, 2007), therefore, L. natans being a submerged plant the roots and shoots are in direct contact with the cadmium concentrations in the medium and it may has the chance of direct cadmium accumulation via the shoots (apoplastic translocation) thereby, high cadmium accumulation was observed in the shoots of L. natans.

Table 3: Bioconcentration Factor (BCF) and Translocation Factor (TF) for cadmium in L. natans

Cd concentration (mgL-1)

Bioconcentration Factor

Translocation Factor

5d

10d

5d

10d

1

3183

7666

2.8

2.0

10

885

1308

2.0

1.3

50

245

317

1.1

1.1

100

113

147

1.1

1.5

Conclusion

In the present study, L. natans was found to accumulate Cd in both its root and shoot in a high degree. This plant has great potential for removing Cd from the surrounding water with the values of BCFs >103. The present study indicates that L. natans is suitable for the phytoremediation of Cd contamination from water. Therefore, L. natans could be useful for phytoremediation of Cd from contaminated water. Furthermore, field experiments are needed to carry out their Phytoremediation potentials of these plants for phytoremediation technique.

Acknowledgements

Authors would like to acknowledge UGC Govt. of India for providing financial support under Rajiv Gandhi National Fellowship Programme to carry out the study. The authors also would like to thank the Head, Department of Environmental Studies, North Eastern Hill University, for providing necessary laboratory facilities.

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