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J Environ Health Sustain Dev 2025, 10(2): 2643-2655 |
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Hossain M S, Parveen M, Rajib M, Rahman S M. Phytoremediation Capacity of Egeria Densa for Heavy Metal Contaminated Wastewater and Its Applicability for Constructed Wetland. J Environ Health Sustain Dev 2025; 10 (2) :2643-2655
URL: http://jehsd.ssu.ac.ir/article-1-917-en.html
URL: http://jehsd.ssu.ac.ir/article-1-917-en.html
Department of Environmental Science and Disaster Management, Daffodil International University, Dhaka 1216, Bangladesh
Keywords: Environmental Pollution, Wastewater, Water Purification, Phytoremediation, Wetlands, Plants.
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Figure 1: Pictorial plates of employed media (a), Experimental setup (b).
Sediment preparation
The study utilized processed silica from river-born sand with a SiO2 presence of approximately 94%, along with mixed gravel sand, which is traditionally used for construction purposes. The sand was used in two layers: the bottom layer comprised silica sand, whereas the subsequent layer located above it was composed of mixed sand containing a mixture of gravel and sand. Both sands were used in equal amounts, weighing 250 g each; a total of 500 g of sand was used. Before use, the sand was cleaned and made dirt-free.
Macrophyte preparation
Submerged macrophytes Egeria densa were collected from a meticulously maintained aquatic nursery in Kataban, Dhaka, Bangladesh. The collected plants were cultured for 45 days in the Environmental Science and Disaster Management (ESDM) laboratory at the Daffodil International University (DIU). Before cultivation, the plants were rinsed with distilled water, and the necessary nutrients were added to the aquarium water. After 45 days, selected mature plants were collected from an aquarium and cut at 11 cm along the tip. The plants were thoroughly rinsed with distilled water before being placed in experimental beakers.
Experimental analysis
Laboratory analyses were conducted at the ESDM laboratory at DIU. Before E. densa plantation on the medium (day 1), the morphological parameters of E. densa, such as plant length, weight, branch length, and root length, were analyzed. Furthermore, the physicochemical parameters (pH, EC, TDS, and COD) and heavy metals (Cr, Cu, Fe, and Zn) of synthetic wastewater were analyzed. On the last day of the experiment (day 14), the plants were uprooted from the experimental medium and the same parameters measured on day 1 were measured. Water quality parameters were measured using standard methods 18. pH, EC, TDS, and temperature were recorded using a Hanna HI9814 meter, and COD was analyzed using a Hanna HI-83399 multiparameter photometer. Chlorophyll concentration was measured using a HACH DR 6000 UV-VIS spectrophotometer. All data were statistically analyzed using SPSS and Microsoft Excel software. Equations 1 and 2 were used to determine the shoot growth rate (SGR) and root growth rate (RGR), and to assess the shoot weight growth rate (SWGR), respectively 19,20.
Table 2: Treatment efficiency on physicochemical and heavy metal parameters across different wastewater samples
Figure 2: Heavy metal removal efficiencies of Egeria densa across wastewater samples with
varying contamination levels
.JPG)
Figure 3: MI value in the water during the initial and final phases of the study with removal efficiency
Figure 4: plant (a), SGR (b) SWGR, (c) RGR, and (d) Total chlorophyll across all the samples
Table 3 presents the growth rate (%) of E. densa in the wastewater samples. The control group displayed the highest overall growth rates, with SGR, SWGR, and RGR values of 8.15%, 19.13%, and 373.33%, respectively, indicating optimal growth conditions. Sample 1 showed the most severe decline in growth, with negative values across all parameters (SGR: -47.48%, SWGR: -53.70%, and RGR: 27.78%), indicating a substantial inhibitory effect. Sample 2 exhibited mixed results; SGR slightly decreased (-2.85%), SWGR increased to 7.79%, and moderate improvements in RGR (200%) were observed. Sample 3 demonstrated improved growth compared to the control, with SGR (7.30%) and SWGR (26.88%), all showing positive increases, although RGR (173.33%) was lower than that of the control group.
State of chlorophyll content
After 14 days of experimentation, the total chlorophyll levels in the stems of E. densa are illustrated in Figure. 4d. The total chlorophyll concentration varied in all samples. In the control sample, the concentration of total chlorophyll was the highest at 6.65 mg/g FW, whereas in sample 1, the concentration was the lowest at 3.63 mg/g FW. The total chlorophyll concentrations in samples 1 and 3 were 4.45 mg/g FW and 6.13 mg/g FW, respectively. Based on the observed results, it can be concluded that in wastewater with low levels of heavy metal contamination, as in sample 3, the plants effectively balanced the total chlorophyll content. Sample 2 had a slightly balanced concentration of total chlorophyll. Compared to the control and sample 3 plants, the total chlorophyll concentration was relatively low in sample 2.
Correlation between heavy metal concentration and plant health
The correlations between heavy metal concentrations and plant health parameters were assessed to evaluate the impact of heavy metal stress. As shown in Figure. 5 a–d, MI inversely correlated with SGR, SWGR, RGR, and total chlorophyll content. The linear regression equations indicated similar slopes and intercepts for all parameters, with coefficient of determination values ranging from -0.8943 to -0.9995, highlighting consistent patterns of decline. Among the parameters, MI showed the strongest correlation coefficient with SWGR ( −0.9995) and the weakest with total chlorophyll content (−0.8943).
Figure 5: Correlation between (a) MI and SGR, (b) MI and SWGR, (c) MI and RGR, (d) MI and chlorophyll
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Phytoremediation Capacity of Egeria Densa for Heavy Metal Contaminated Wastewater and Its Applicability for Constructed Wetland
Md. Swadhin Hossain 1, Mahfuza Parveen 1*, Mohammad Rajib 1,2, S.M Mahmudur Rahman 1
1 Department of Environmental Science and Disaster Management, Daffodil International University, Dhaka 1216, Bangladesh.
2 Institute of Nuclear Minerals, Atomic Energy Research Establishment, Bangladesh Atomic Energy Commission, Dhaka 1349, Bangladesh.
Md. Swadhin Hossain 1, Mahfuza Parveen 1*, Mohammad Rajib 1,2, S.M Mahmudur Rahman 1
1 Department of Environmental Science and Disaster Management, Daffodil International University, Dhaka 1216, Bangladesh.
2 Institute of Nuclear Minerals, Atomic Energy Research Establishment, Bangladesh Atomic Energy Commission, Dhaka 1349, Bangladesh.
| A R T I C L E I N F O | ABSTRACT | |
| ORIGINAL ARTICLE | Introduction: Phytoremediation is a process for restoring the environment that utilizes plant physiology and metabolism. The study was performed on a laboratory scale to assess the heavy metal removal efficiency of Egeria densa, its response to contaminated wastewater, and its potential for application in constructed wetlands. Materials and Methods: Synthetic wastewater samples with low, moderate, and high concentrations of Cr, Cu, Fe, and Zn were prepared. E. densa was cultivated in these samples for 14 days under controlled conditions, and parameters such as heavy metal removal efficiency, growth rates, chlorophyll content, and water quality indices were analyzed. Results: Results showed that in low heavy metal-contaminated wastewater, the removal efficiency of the E. densa for Cr, Cu, Fe, and Zn was 93.7%, 91.20%, 87.2%, and 100%, respectively. In highly heavy metal-contaminated wastewater, the heavy metal removal efficiency of the E. densa for Cr, Cu, Fe, and Zn was 36%, 54%, 35.2%, and 75%, respectively. Therefore, the study revealed that elevated heavy metal concentrations adversely affected the plant growth rate. Conclusion: E. densa is a viable option for the constructed wetland system in scenarios where wastewater ranges from low to moderately contaminated respecting heavy metals. These findings can serve as a reference for future studies and provide valuable insights for researchers looking to implement E. densa as constructed wetland vegetation. |
|
Article History: Received: 18 February 2025 Accepted: 20 April 2025 |
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*Corresponding Author: Mahfuza Parveen Email: mahfuza.esdm@diu.edu.bd Tel: +88 017 98145583 |
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Keywords: Environmental Pollution, Wastewater, Water Purification, Phytoremediation, Wetlands, Plants. |
Citation: Hossain S, Parveen M, Rajib M, et al. Phytoremediation Capacity of Egeria Densa for Heavy Metal Contaminated Wastewater and Its Applicability for Constructed Wetland. J Environ Health Sustain Dev. 2025; 10(2): 2643-55.
Introduction
Water contamination and scarcity have posed significant environmental challenges. Rapid urbanization, industrialization, and agricultural activities over the past few decades have increased the discharge of polluted wastewater1. Wastewater carries an immense quantity of pollutants that are exceedingly noxious to aquatic ecosystems and human health. Moreover, certain water pollutants, such as heavy metals, are hazardous and harmful, affecting ecosystems and human health2. Many heavy metals, such as chromium (Cr), lead (Pb), mercury (Hg), and metalloids, can be hazardous even at low concentrations 3. Wastewater reclamation is a critical approach to safeguarding the environment and human health.
This issue can be addressed using various methods. These techniques can be classified into various categories, such as physical, chemical, biological, and engineering procedures 4. Engineering and physical treatment methods have successfully enhanced the quality of water and sediment to a certain degree. However, these methods also have negative consequences, including disruption of natural ecosystems and financial burden due to expensive initial investment and ongoing maintenance expenses 5. Chemical techniques offer a rapid solution for purifying contaminated water. However, these solutions are only temporary and can result in the generation of additional waste, leading to potential environmental and health risks6. At low concentrations, most of these methods are expensive and inefficient. The drawbacks of these methods necessitate the development of alternative, low-cost water treatment systems. Consequently, phytoremediation is the preferred approach for removing heavy metal contaminants from the environment. This technology utilizes raw or genetically modified plant species to remediate contaminated water sources, effectively extracting or eliminating pollutants from wastewater 7. It represents a cost-effective, energy-efficient, and environmentally friendly approach to extensive water treatment applications.
Modern phytoremediation methods are successfully utilized in constructed wetland technology for wastewater treatment. Several studies have used macrophyte plants in constructed wetland technology to remove pollutants and heavy metals from wastewater 8,9. Choosing a suitable plant according to the pollutants and climatic conditions is essential in constructed wetland technology to achieve optimal pollutant and heavy metal removal efficiency 10. The potential of multiple submerged aquatic macrophytes for wastewater remediation has been studied. For instance, studies have shown that the submerged aquatic plants Typha angustifolia, Erianthus arundinaceus, and Phragmites australis are effective in treating wastewater from the pulp and paper industry, which contains heavy metals 11. The aquatic plant Ceratophyllum demersum L. accumulates As from contaminated wastewater 12. A previous study demonstrated that Canna indica L. effectively eliminated organic pollutants from wastewater 13. The positive outcomes of phytoremediation using aquatic plants have attracted significant interest from researchers and scientists.
Among various species, E. densa, a submerged aquatic macrophyte, holds a distinct position because of its high adoption capability and tolerance to a wide range of temperatures 14. It is fast-growing, cost-effective, and capable of thriving in varied hydrological and climatic conditions, making it suitable for tropical countries such as Bangladesh. Extensive research has been conducted on various macrophytes to determine their pollutant and heavy metal removal capacities. Despite this growing body of research, one common and widely available macrophyte, Egeria densa, remains underexplored. Previous studies have mostly focused on other species, such as Typha, Phragmites, and Ceratophyllum, while there is a lack of detailed investigation on E. densa under varied heavy metal concentrations relevant to South Asian River systems. This gap in research not only limits our understanding of the potential of E. densa for heavy metal removal from wastewater but also prevents ecologists, engineers, and scientists from utilizing it in constructed wetland systems. This gap in knowledge suggests a potential opportunity to examine the heavy metal removal capacity of E. densa from wastewater and its potential utilization in constructed wetlands. This study represents the first assessment of E. densa applicability in constructed wetlands in Bangladesh. Its novelty lies in demonstrating the plant’s effective heavy metal removal capabilities, while examining previously underexplored physiological and morphological responses. This study aimed to quantify the heavy metal (Cr, Cu, Fe, Zn) removal efficiency of E. densa, establish its tolerance thresholds for heavy metal exposure, and evaluate its suitability for treating contaminated wastewater in constructed wetlands.
Materials and Methods
Experimental setup
The study used three distant synthetic wastewater samples, referred to as sample 1 (S1), sample 2 (S2), and sample 3 (S3), and each sample was represented by three replicate points to ensure the accuracy and reliability of the experiment, referred to as R1, R2, and R3. Furthermore, a control sample was used and designated as (C). The study used 1000 ml of synthetic wastewater per beaker, each containing three branchless E. densa plants. The evaporation rate of water from the beaker was 8.17 ml per day. The distilled water occupied the evaporation gap, providing 16.34 ml of water over 2 days. The experimental setup was subjected to 10 h of continuous natural light at an average temperature of 27 °C. E. densa was planted on March 28, 2024, followed by uprooting on April 12, 2024, for subsequent analysis of its height, weight, roots, and branches. The experiment was conducted over 14 days. Figure. Figure 1 a-b illustrates a pictorial representation of the media employed and the experimental setup of the study.
Water contamination and scarcity have posed significant environmental challenges. Rapid urbanization, industrialization, and agricultural activities over the past few decades have increased the discharge of polluted wastewater1. Wastewater carries an immense quantity of pollutants that are exceedingly noxious to aquatic ecosystems and human health. Moreover, certain water pollutants, such as heavy metals, are hazardous and harmful, affecting ecosystems and human health2. Many heavy metals, such as chromium (Cr), lead (Pb), mercury (Hg), and metalloids, can be hazardous even at low concentrations 3. Wastewater reclamation is a critical approach to safeguarding the environment and human health.
This issue can be addressed using various methods. These techniques can be classified into various categories, such as physical, chemical, biological, and engineering procedures 4. Engineering and physical treatment methods have successfully enhanced the quality of water and sediment to a certain degree. However, these methods also have negative consequences, including disruption of natural ecosystems and financial burden due to expensive initial investment and ongoing maintenance expenses 5. Chemical techniques offer a rapid solution for purifying contaminated water. However, these solutions are only temporary and can result in the generation of additional waste, leading to potential environmental and health risks6. At low concentrations, most of these methods are expensive and inefficient. The drawbacks of these methods necessitate the development of alternative, low-cost water treatment systems. Consequently, phytoremediation is the preferred approach for removing heavy metal contaminants from the environment. This technology utilizes raw or genetically modified plant species to remediate contaminated water sources, effectively extracting or eliminating pollutants from wastewater 7. It represents a cost-effective, energy-efficient, and environmentally friendly approach to extensive water treatment applications.
Modern phytoremediation methods are successfully utilized in constructed wetland technology for wastewater treatment. Several studies have used macrophyte plants in constructed wetland technology to remove pollutants and heavy metals from wastewater 8,9. Choosing a suitable plant according to the pollutants and climatic conditions is essential in constructed wetland technology to achieve optimal pollutant and heavy metal removal efficiency 10. The potential of multiple submerged aquatic macrophytes for wastewater remediation has been studied. For instance, studies have shown that the submerged aquatic plants Typha angustifolia, Erianthus arundinaceus, and Phragmites australis are effective in treating wastewater from the pulp and paper industry, which contains heavy metals 11. The aquatic plant Ceratophyllum demersum L. accumulates As from contaminated wastewater 12. A previous study demonstrated that Canna indica L. effectively eliminated organic pollutants from wastewater 13. The positive outcomes of phytoremediation using aquatic plants have attracted significant interest from researchers and scientists.
Among various species, E. densa, a submerged aquatic macrophyte, holds a distinct position because of its high adoption capability and tolerance to a wide range of temperatures 14. It is fast-growing, cost-effective, and capable of thriving in varied hydrological and climatic conditions, making it suitable for tropical countries such as Bangladesh. Extensive research has been conducted on various macrophytes to determine their pollutant and heavy metal removal capacities. Despite this growing body of research, one common and widely available macrophyte, Egeria densa, remains underexplored. Previous studies have mostly focused on other species, such as Typha, Phragmites, and Ceratophyllum, while there is a lack of detailed investigation on E. densa under varied heavy metal concentrations relevant to South Asian River systems. This gap in research not only limits our understanding of the potential of E. densa for heavy metal removal from wastewater but also prevents ecologists, engineers, and scientists from utilizing it in constructed wetland systems. This gap in knowledge suggests a potential opportunity to examine the heavy metal removal capacity of E. densa from wastewater and its potential utilization in constructed wetlands. This study represents the first assessment of E. densa applicability in constructed wetlands in Bangladesh. Its novelty lies in demonstrating the plant’s effective heavy metal removal capabilities, while examining previously underexplored physiological and morphological responses. This study aimed to quantify the heavy metal (Cr, Cu, Fe, Zn) removal efficiency of E. densa, establish its tolerance thresholds for heavy metal exposure, and evaluate its suitability for treating contaminated wastewater in constructed wetlands.
Materials and Methods
Experimental setup
The study used three distant synthetic wastewater samples, referred to as sample 1 (S1), sample 2 (S2), and sample 3 (S3), and each sample was represented by three replicate points to ensure the accuracy and reliability of the experiment, referred to as R1, R2, and R3. Furthermore, a control sample was used and designated as (C). The study used 1000 ml of synthetic wastewater per beaker, each containing three branchless E. densa plants. The evaporation rate of water from the beaker was 8.17 ml per day. The distilled water occupied the evaporation gap, providing 16.34 ml of water over 2 days. The experimental setup was subjected to 10 h of continuous natural light at an average temperature of 27 °C. E. densa was planted on March 28, 2024, followed by uprooting on April 12, 2024, for subsequent analysis of its height, weight, roots, and branches. The experiment was conducted over 14 days. Figure. Figure 1 a-b illustrates a pictorial representation of the media employed and the experimental setup of the study.
Figure 1: Pictorial plates of employed media (a), Experimental setup (b).
Wastewater preparation
The chemical composition of heavy metal-contaminated wastewater varies considerably across different sites. This study involved the preparation of synthetic wastewater reflecting the heavy metal concentrations observed in various rivers across Bangladesh 15,16,17. This study employed three distinct concentrations of synthetic wastewater samples. Required amounts of K2Cr2O7, CuSO4(H2O)5, ZnSO4(H2O)7, FeSO4.7H2O powders, and H2SO4 solution were introduced into distilled water to prepare synthetic wastewater with the pollutant concentrations listed in Table 1. Sample 1 showed significant heavy metal contamination, sample 2 showed moderate contamination, and sample 3 showed minimal contamination.
Table 1: Heavy metals concentration of synthetic wastewater samples
The chemical composition of heavy metal-contaminated wastewater varies considerably across different sites. This study involved the preparation of synthetic wastewater reflecting the heavy metal concentrations observed in various rivers across Bangladesh 15,16,17. This study employed three distinct concentrations of synthetic wastewater samples. Required amounts of K2Cr2O7, CuSO4(H2O)5, ZnSO4(H2O)7, FeSO4.7H2O powders, and H2SO4 solution were introduced into distilled water to prepare synthetic wastewater with the pollutant concentrations listed in Table 1. Sample 1 showed significant heavy metal contamination, sample 2 showed moderate contamination, and sample 3 showed minimal contamination.
Table 1: Heavy metals concentration of synthetic wastewater samples
| Parameter (mg/l) | Sample 1 | Sample 2 | Sample 3 |
| Cr | 0.5 | 0.25 | 0.125 |
| Cu | 0.5 | 0.25 | 0.125 |
| Fe | 5.0 | 2.5 | 1.25 |
| Zn | 0.20 | 0.10 | 0.05 |
Sediment preparation
The study utilized processed silica from river-born sand with a SiO2 presence of approximately 94%, along with mixed gravel sand, which is traditionally used for construction purposes. The sand was used in two layers: the bottom layer comprised silica sand, whereas the subsequent layer located above it was composed of mixed sand containing a mixture of gravel and sand. Both sands were used in equal amounts, weighing 250 g each; a total of 500 g of sand was used. Before use, the sand was cleaned and made dirt-free.
Macrophyte preparation
Submerged macrophytes Egeria densa were collected from a meticulously maintained aquatic nursery in Kataban, Dhaka, Bangladesh. The collected plants were cultured for 45 days in the Environmental Science and Disaster Management (ESDM) laboratory at the Daffodil International University (DIU). Before cultivation, the plants were rinsed with distilled water, and the necessary nutrients were added to the aquarium water. After 45 days, selected mature plants were collected from an aquarium and cut at 11 cm along the tip. The plants were thoroughly rinsed with distilled water before being placed in experimental beakers.
Experimental analysis
Laboratory analyses were conducted at the ESDM laboratory at DIU. Before E. densa plantation on the medium (day 1), the morphological parameters of E. densa, such as plant length, weight, branch length, and root length, were analyzed. Furthermore, the physicochemical parameters (pH, EC, TDS, and COD) and heavy metals (Cr, Cu, Fe, and Zn) of synthetic wastewater were analyzed. On the last day of the experiment (day 14), the plants were uprooted from the experimental medium and the same parameters measured on day 1 were measured. Water quality parameters were measured using standard methods 18. pH, EC, TDS, and temperature were recorded using a Hanna HI9814 meter, and COD was analyzed using a Hanna HI-83399 multiparameter photometer. Chlorophyll concentration was measured using a HACH DR 6000 UV-VIS spectrophotometer. All data were statistically analyzed using SPSS and Microsoft Excel software. Equations 1 and 2 were used to determine the shoot growth rate (SGR) and root growth rate (RGR), and to assess the shoot weight growth rate (SWGR), respectively 19,20.
| (1) | |
| (2) |
Determined metal index (MI)
This study adopted the Metal Index to determine the level of heavy metal contamination in wastewater 21. The MI reflects different heavy metal concentrations in a particular sample and allocates one value representing the total pollution level. Equation 3 represents the MI Formula.
This study adopted the Metal Index to determine the level of heavy metal contamination in wastewater 21. The MI reflects different heavy metal concentrations in a particular sample and allocates one value representing the total pollution level. Equation 3 represents the MI Formula.
| (3) |
Where Ci denotes the ith heavy metal concentration in the sample analysis, and the maximum allowed concentration of the ith heavy metal is referred to as MAC. The MI value was categorized into six distinct classes: “seriously affected (> 6), strongly affected (4-6), moderately affected (2-4), slightly affected (1-2), pure (0.3-1.0), and very pure (< 0.3)” 22. For this study, the MAC value was obtained from the Bangladesh drinking water quality standard 23.
Determination of the chlorophyll content
The first step involved extracting chlorophyll from the stems and leaves of the plant to ascertain the chlorophyll content. The researchers then thoroughly washed and sterilized the test tubes and inserted the measured leaves and plant stems into the test tubes with appropriate labeling. The test tube was then filled with 10 ml of 80% acetone, and the test tube was enclosed by placing a layer of aluminum foil over it. For chlorophyll extraction, the test tube stand was placed in a dark place for twenty-four hours. The chlorophyll samples were extracted, and the amount of chlorophyll in each sample was determined. The chlorophyll content was evaluated using three specific wavelengths (470, 646, and 663 nm) according to Equations 4-6 24.
Determination of the chlorophyll content
The first step involved extracting chlorophyll from the stems and leaves of the plant to ascertain the chlorophyll content. The researchers then thoroughly washed and sterilized the test tubes and inserted the measured leaves and plant stems into the test tubes with appropriate labeling. The test tube was then filled with 10 ml of 80% acetone, and the test tube was enclosed by placing a layer of aluminum foil over it. For chlorophyll extraction, the test tube stand was placed in a dark place for twenty-four hours. The chlorophyll samples were extracted, and the amount of chlorophyll in each sample was determined. The chlorophyll content was evaluated using three specific wavelengths (470, 646, and 663 nm) according to Equations 4-6 24.
| Chlorophyll a = 12 (Abs663) – 3.11 (Abs646) | (4) |
| Chlorophyll b = 20.78 (Abs646) – 4.88 (Abs663) | (5) |
| Chlorophyll (a + b) = 17.67 (Abs646) + 7.12 (Abs663) | (6) |
The equations have been used in several studies related to estimating the plant's chlorophyll
content 25,26.
Results
The phytoremediation capacity of Egeria densa
Table 2 presents the treatment efficacy for various physicochemical parameters of water samples, including the control, high heavy metal-contaminated wastewater in sample 1, moderate heavy metal-contaminated wastewater in sample 2, and low heavy metal-contaminated wastewater in sample 3.
content 25,26.
Results
The phytoremediation capacity of Egeria densa
Table 2 presents the treatment efficacy for various physicochemical parameters of water samples, including the control, high heavy metal-contaminated wastewater in sample 1, moderate heavy metal-contaminated wastewater in sample 2, and low heavy metal-contaminated wastewater in sample 3.
Table 2: Treatment efficiency on physicochemical and heavy metal parameters across different wastewater samples
| Parameter | Control | Sample 1 | Sample 2 | Sample 3 | ||||
| Initial | final | Initial | Final | Initial | Final | Initial | Final | |
| pH | 6.98 | 7.02 | 6.59 | 6.97 | 6.88 | 7.12 | 6.77 | 7.15 |
| EC (mS/cm) | 0.05 | 0.05 | 0.47 | 0.50 | 0.26 | 0.19 | 0.16 | 0.08 |
| TDS (ppm) | 40 | 43 | 360 | 387 | 200 | 147 | 120 | 60 |
| COD (mg/l) | 0 | 0 | 40 | 25 | 21 | 7.67 | 9 | 0.67 |
| Cr (mg/l) | 0 | 0 | 0.50 | 0.32 | 0.25 | 0.09 | 0.125 | .0087 |
| Cu (mg/l) | 0 | 0 | 0.50 | 0.23 | 0.25 | 0.083 | 0.125 | 0.011 |
| Fe (mg/l) | 0 | 0 | 5 | 3.24 | 2.5 | 0.95 | 1.25 | 0.16 |
| Zn (mg/l) | 0 | 0 | 0.20 | 0.050 | 0.10 | 0.027 | 0.05 | 0.00 |
Changes in pH, EC, TDS, COD, and heavy metal concentrations were observed pre- and post-treatment, indicating a distinct impact on water quality. The initial pH levels of all samples were within the slightly acidic to neutral range, with minor increases after treatment. The control sample showed minimal change (6.98–7.02), whereas sample 1 increased from 6.59 to 6.97, sample 2 from 6.88 to 7.12, and sample 3 from 6.77 to 7.15. These findings suggest that the treatment process contributed to pH stabilization. Electrical Conductivity (EC) remained stable in the control (0.05 mS/cm) but varied in the treated samples. Sample 1 showed a minor increase (0.47 to 0.50 mS/cm), whereas samples 2 and 3 demonstrated decreases (0.26 to 0.19 mS/cm and 0.16 to 0.08 mS/cm, respectively), suggesting the effective removal of ionic elements. Total Dissolved Solids (TDS) increased slightly in the control (40–43 ppm) and sample 1 (360–387 ppm) groups, likely due to the decay of dead plants. However, it decreased significantly in sample 2 (200 to 147 ppm) and sample 3 (120 to 60 ppm), indicating the efficient removal of dissolved pollutants. Chemical Oxygen Demand (COD) was considerably reduced in all samples. Sample 1 COD declined from 40 to 25 mg/L, sample 2 from 21 to 7.67 mg/L, and sample 3 from 9 to 0.67 mg/L, respectively. These reductions suggest effective organic matter removal across the samples, with the most significant COD reduction observed in Sample 3.
Heavy metals, including chromium (Cr), copper (Cu), iron (Fe), and zinc (Zn), also exhibited substantial reductions in their concentrations. Cr decreased from 0.50 mg/L to 0.32 mg/L in sample 1, 0.25 mg/L to 0.09 mg/L in sample 2, and 0.125 mg/L to 0.0087 mg/L in sample 3. Cu was reduced from 0.50 to 0.23 mg/L in sample 1, 0.25 to 0.083 mg/L in sample 2, and 0.125 to 0.011 mg/L in sample 3, respectively. Fe levels fell from 5 to 3.24 mg/L in sample 1, 2.5 to 0.95 mg/L in sample 2, and 1.25 to 0.16 mg/L in sample 3, while Zn was near zero in sample 3 (0.05 to 0.00 mg/L).
Comparison of heavy metal removal efficiencies across three wastewater samples
The heavy metal removal efficiencies of E. densa were evaluated across three wastewater samples, as shown in Figure. 2. The variation in removal efficiency highlights the adaptability and effectiveness of E. densa at different contamination levels.
Heavy metals, including chromium (Cr), copper (Cu), iron (Fe), and zinc (Zn), also exhibited substantial reductions in their concentrations. Cr decreased from 0.50 mg/L to 0.32 mg/L in sample 1, 0.25 mg/L to 0.09 mg/L in sample 2, and 0.125 mg/L to 0.0087 mg/L in sample 3. Cu was reduced from 0.50 to 0.23 mg/L in sample 1, 0.25 to 0.083 mg/L in sample 2, and 0.125 to 0.011 mg/L in sample 3, respectively. Fe levels fell from 5 to 3.24 mg/L in sample 1, 2.5 to 0.95 mg/L in sample 2, and 1.25 to 0.16 mg/L in sample 3, while Zn was near zero in sample 3 (0.05 to 0.00 mg/L).
Comparison of heavy metal removal efficiencies across three wastewater samples
The heavy metal removal efficiencies of E. densa were evaluated across three wastewater samples, as shown in Figure. 2. The variation in removal efficiency highlights the adaptability and effectiveness of E. densa at different contamination levels.
Figure 2: Heavy metal removal efficiencies of Egeria densa across wastewater samples with
varying contamination levels
For Cr, the removal efficiency varied significantly, with the lowest efficiency observed in sample 1 at 36%, increasing to 64% in sample 2 and reaching 93.07% in sample 3. A similar trend was observed for Cu, where the efficiency improved from 54% in sample 1 to 66% in sample 2 and peaked at 91.2% in sample 3. The removal of Fe exhibited a progressive increase, with efficiencies of 35.2%, 62%, and 87.2% for samples 1, 2, and 3, respectively. Zn removal efficiency was notably high across all samples, with 75% for sample 1, 73% for sample 2, and a perfect 100% for sample 3.
Metal Index (MI)
MI measures the levels of various metals present in water and compares them to the maximum allowable concentration (MAC) set for each metal. Figure 3 shows the MI values in water at the initial and final phases of the study.
Metal Index (MI)
MI measures the levels of various metals present in water and compares them to the maximum allowable concentration (MAC) set for each metal. Figure 3 shows the MI values in water at the initial and final phases of the study.
Figure 3: MI value in the water during the initial and final phases of the study with removal efficiency
The effectiveness of the applied treatment method was assessed by comparing the MI values before and after treatment across three water samples with varying levels of contamination. Sample 1, with the highest initial MI value of 15.54, was classified as being seriously affected. After treatment, the MI value decreased to 9.88, yet it remained within the seriously affected classification, reflecting a 36.42% removal efficiency. This suggests that although some pollutants were removed, the treatment was not fully effective in significantly improving water quality in samples with severe contamination. In contrast, Sample 2, which also began as seriously affected with an MI value of 7.77, showed a more substantial reduction after treatment, reaching an MI value of 2.83 and achieving reclassification to moderately affected. The removal efficiency for this sample was 63.47%, indicating that the treatment was notably more effective for samples with moderate contamination levels. Sample 3, with an initial MI value of 3.88, classified as moderately affected, demonstrated the most significant improvement post-treatment. Its MI value dropped to 0.34, achieving a 91.12% removal efficiency, and reclassifying it as pure. This result highlights the high effectiveness of the treatment for samples with lower initial contamination.
Heavy metal tolerance level of Egeria densa
The heavy metal tolerance of E. densa was assessed using synthetic wastewater samples containing varying concentrations of Cr, Cu, Fe, and Zn. The experiment demonstrated that E. densa has a defined tolerance range for each heavy metal, beyond which the survival of the plant was compromised. The results revealed that E. densa tolerated Cr concentrations of up to 0.25 mg/L. In sample 2 (Cr: 0.25 mg/L), the plants exhibited normal growth, whereas in sample 1 (Cr: 0.5 mg/L), signs of stress and mortality were observed, indicating that concentrations above 0.25 mg/L are lethal for E. densa. A similar trend was observed for Cu. The plant remained viable at Cu concentrations of up to 0.25 mg/L in sample 2 but failed to survive at 0.5 mg/L in sample 1, confirming a tolerance limit of 0–0.25 mg/L. For Fe, the plants demonstrated greater resilience, with a tolerance threshold of up to 2.5 mg/L. In sample 3 (Fe: 1.25 mg/L), E. densa maintained healthy growth, but its survival declined in sample 1, where Fe levels were elevated to 5.0 mg/L. A similar pattern was observed for Zn, where the plant thrived at concentrations of 0.05 mg/L in sample 3 and 0.10 mg/L in sample 2 but exhibited severe stress and mortality at 0.20 mg/L in sample 1. Based on these observations, the tolerance thresholds were established as follows: Cr (0–0.25 mg/L), Cu (0–0.25 mg/L), Fe (0–2.5 mg/L), and Zn (0–0.10 mg/L). Beyond these concentrations, E. densa exhibited complete mortality (Cr/Cu>0.25 mg/L; Zn>0.10 mg/L), indicating a narrow tolerance range for most of the tested metals.
Heavy metal contaminated wastewater impact on Egeria densa
Shoot growth rate (SGR)
Figure 4a shows the SGR of the plants across all samples. This study evaluated the mean plant growth across all water samples. The control sample had the maximum growth rate of 0.0640 cm/day, whereas sample 1 displayed the lowest growth rate of -0.3730 cm/day. The subsequent sample growth rates were as follows: sample 2: -0.0223 cm/day; sample 3:0.0573 cm/day. A decline in SGR was noted in samples 1 and 2. The growth rates of the control and sample 3 plant shoots were compaable.
Heavy metal tolerance level of Egeria densa
The heavy metal tolerance of E. densa was assessed using synthetic wastewater samples containing varying concentrations of Cr, Cu, Fe, and Zn. The experiment demonstrated that E. densa has a defined tolerance range for each heavy metal, beyond which the survival of the plant was compromised. The results revealed that E. densa tolerated Cr concentrations of up to 0.25 mg/L. In sample 2 (Cr: 0.25 mg/L), the plants exhibited normal growth, whereas in sample 1 (Cr: 0.5 mg/L), signs of stress and mortality were observed, indicating that concentrations above 0.25 mg/L are lethal for E. densa. A similar trend was observed for Cu. The plant remained viable at Cu concentrations of up to 0.25 mg/L in sample 2 but failed to survive at 0.5 mg/L in sample 1, confirming a tolerance limit of 0–0.25 mg/L. For Fe, the plants demonstrated greater resilience, with a tolerance threshold of up to 2.5 mg/L. In sample 3 (Fe: 1.25 mg/L), E. densa maintained healthy growth, but its survival declined in sample 1, where Fe levels were elevated to 5.0 mg/L. A similar pattern was observed for Zn, where the plant thrived at concentrations of 0.05 mg/L in sample 3 and 0.10 mg/L in sample 2 but exhibited severe stress and mortality at 0.20 mg/L in sample 1. Based on these observations, the tolerance thresholds were established as follows: Cr (0–0.25 mg/L), Cu (0–0.25 mg/L), Fe (0–2.5 mg/L), and Zn (0–0.10 mg/L). Beyond these concentrations, E. densa exhibited complete mortality (Cr/Cu>0.25 mg/L; Zn>0.10 mg/L), indicating a narrow tolerance range for most of the tested metals.
Heavy metal contaminated wastewater impact on Egeria densa
Shoot growth rate (SGR)
Figure 4a shows the SGR of the plants across all samples. This study evaluated the mean plant growth across all water samples. The control sample had the maximum growth rate of 0.0640 cm/day, whereas sample 1 displayed the lowest growth rate of -0.3730 cm/day. The subsequent sample growth rates were as follows: sample 2: -0.0223 cm/day; sample 3:0.0573 cm/day. A decline in SGR was noted in samples 1 and 2. The growth rates of the control and sample 3 plant shoots were compaable.
Figure 4: plant (a), SGR (b) SWGR, (c) RGR, and (d) Total chlorophyll across all the samples
Shoot weight growth rate (SWGR)
Figure 4b illustrates the growth rate of the plant shoot weight for all samples. The control group showed a positive SWGR, indicating normal growth in the control group. In contrast, Sample 1 exhibited a significantly negative SWGR, suggesting that this treatment severely inhibited shoot growth; Sample 2 displayed a near-zero SWGR, indicating minimal or stagnant growth; and Sample 3 showed an increased SWGR compared to the control, implying that this treatment enhanced shoot biomass accumulation.
Root growth rate (RGR)
Figure 4c demonstrates that after 14 days of experimentation, the highest RGR was found in the control sample (0.26 cm/D), while the lowest plant RGR was identified in sample 1 (0.0198 cm/D). The growth rate of E. densa in sample 2 was 0.143 cm/D mm per day, while in sample 3, it was 0.123 cm/D. High heavy metal concentrations could be responsible for the low RGR of E. densa in sample 1.
Table 3: Growth rate percentage of Egeria densa across wastewater samples
Figure 4b illustrates the growth rate of the plant shoot weight for all samples. The control group showed a positive SWGR, indicating normal growth in the control group. In contrast, Sample 1 exhibited a significantly negative SWGR, suggesting that this treatment severely inhibited shoot growth; Sample 2 displayed a near-zero SWGR, indicating minimal or stagnant growth; and Sample 3 showed an increased SWGR compared to the control, implying that this treatment enhanced shoot biomass accumulation.
Root growth rate (RGR)
Figure 4c demonstrates that after 14 days of experimentation, the highest RGR was found in the control sample (0.26 cm/D), while the lowest plant RGR was identified in sample 1 (0.0198 cm/D). The growth rate of E. densa in sample 2 was 0.143 cm/D mm per day, while in sample 3, it was 0.123 cm/D. High heavy metal concentrations could be responsible for the low RGR of E. densa in sample 1.
Table 3: Growth rate percentage of Egeria densa across wastewater samples
| Sample | Growth rate percentage (%) | ||
| SGR | SWGR | RGR | |
| Control | 8.15 | 19.13 | 373.33 |
| Sample 1 | -47.48 | -53.70 | 27.78 |
| Sample 2 | -2.85 | 7.79 | 200 |
| Sample 3 | 7.30 | 26.88 | 173.33 |
Table 3 presents the growth rate (%) of E. densa in the wastewater samples. The control group displayed the highest overall growth rates, with SGR, SWGR, and RGR values of 8.15%, 19.13%, and 373.33%, respectively, indicating optimal growth conditions. Sample 1 showed the most severe decline in growth, with negative values across all parameters (SGR: -47.48%, SWGR: -53.70%, and RGR: 27.78%), indicating a substantial inhibitory effect. Sample 2 exhibited mixed results; SGR slightly decreased (-2.85%), SWGR increased to 7.79%, and moderate improvements in RGR (200%) were observed. Sample 3 demonstrated improved growth compared to the control, with SGR (7.30%) and SWGR (26.88%), all showing positive increases, although RGR (173.33%) was lower than that of the control group.
State of chlorophyll content
After 14 days of experimentation, the total chlorophyll levels in the stems of E. densa are illustrated in Figure. 4d. The total chlorophyll concentration varied in all samples. In the control sample, the concentration of total chlorophyll was the highest at 6.65 mg/g FW, whereas in sample 1, the concentration was the lowest at 3.63 mg/g FW. The total chlorophyll concentrations in samples 1 and 3 were 4.45 mg/g FW and 6.13 mg/g FW, respectively. Based on the observed results, it can be concluded that in wastewater with low levels of heavy metal contamination, as in sample 3, the plants effectively balanced the total chlorophyll content. Sample 2 had a slightly balanced concentration of total chlorophyll. Compared to the control and sample 3 plants, the total chlorophyll concentration was relatively low in sample 2.
Correlation between heavy metal concentration and plant health
The correlations between heavy metal concentrations and plant health parameters were assessed to evaluate the impact of heavy metal stress. As shown in Figure. 5 a–d, MI inversely correlated with SGR, SWGR, RGR, and total chlorophyll content. The linear regression equations indicated similar slopes and intercepts for all parameters, with coefficient of determination values ranging from -0.8943 to -0.9995, highlighting consistent patterns of decline. Among the parameters, MI showed the strongest correlation coefficient with SWGR ( −0.9995) and the weakest with total chlorophyll content (−0.8943).
Figure 5: Correlation between (a) MI and SGR, (b) MI and SWGR, (c) MI and RGR, (d) MI and chlorophyll
Figure 5 illustrates the correlation between the metal index (MI) and shoot growth rate (SGR). A consistent and significant negative relationship was observed for all measured parameters.
Discussion
The findings of this study show that Egeria densa effectively reduces heavy metal concentrations in contaminated water through phytoremediation. The results implied that Sample 3 achieved the highest removal efficiency. This is likely because this sample contained a lower concentration of heavy metals, which enhanced the phytoremediation capacity of E. Densa. Furthermore, previous studies have reported similar findings, demonstrating that submerged aquatic macrophytes are effective in removing heavy metals from wastewater 27,28,29,30.
The trend in the removal efficiencies of Cr, Cu, Fe, and Zn demonstrated a clear relationship between the contamination levels and removal performance. Removal efficiencies improved as contamination levels decreased; E. densa removed up to 100% Zn and over 90% Cr and Cu in sample 3. This suggests that E. densa performs better in wastewater with lower contamination levels, possibly because of favorable uptake conditions in such environments. These consistently higher removal rates in samples 2 and 3 indicate the potential of E. densa as a sustainable tool for bioremediation, particularly in moderately to lightly contaminated systems. Similar studies have reported that other submerged aquatic macrophytes, such as Pontederia cordata, Lemna minor, and Eichhornia crassipes, also exhibit promising heavy metal removal capacities, but their tolerance thresholds are often higher or their adaptation is more limited in tropical regions than that of E. densa 31,32. In contrast, the present study demonstrated that E. densa can achieve up to 100% removal of Zn and over 90% for Cr and Cu at lower contamination levels, surpassing the performance of Typha angustifolia under similar conditions 33. This highlights the high suitability of E. densa for use in constructed wetlands, especially in South Asian contexts with variable contamination profiles and climatic conditions.
The effectiveness of the treatment method was further assessed using the Metal Index (MI). The findings clearly show an inverse relationship between the initial contamination level and treatment effectiveness. While the process reduced pollutants in all samples, it was most effective at moderate to low contamination levels. This pattern suggests that although E. densa is effective, it may not be sufficient alone for treating highly polluted wastewater. Additional or enhanced treatment strategies may be needed for highly polluted water, such as sample 1, as demonstrated by previous studies on phytoremediation limitations in heavily contaminated systems 34.
The results indicate that E. densa is highly sensitive to elevated concentrations of Cr, Cu, and Zn, whereas it tolerates moderate levels of Fe, likely due to its essential role in plant metabolism. The narrow tolerance range for Cr and Cu suggests a limited capacity for the detoxification or exclusion of these toxic metals. Although Zn is also a micronutrient, its threshold remains low, possibly due to its bioavailability, chemical form, or physiological sensitivity. Notably, these thresholds are substantially lower than those reported in a previous study, where E. densa tolerated a Zn concentration of 20 mg/L under 7-day exposure conditions 35. Variations in exposure duration, growth conditions (e.g., light and nutrients), and genetic adaptations may contribute to differential tolerance. The observed Zn tolerance threshold of E. densa (0.10 mg/L) is significantly lower than the mean Zn concentration reported in groundwater from the Chittagong industrial area (0.49 mg/L) 36. This disparity suggests that E. densa alone may be insufficient for effective remediation in highly contaminated environments without the support of complementary pretreatment systems. These findings highlight the potential of E. densa for phytoremediation within the specified concentration limits, while underscoring its vulnerability to excessive heavy metal contamination.
Regarding Wastewater with low levels of heavy metal contamination (sample 3), the plants effectively balanced the total chlorophyll content. Sample 2 slightly balanced the total chlorophyll concentration. Compared to the control and sample 3 plants, the total chlorophyll concentration was relatively low in sample 2. The findings also indicate that wastewater with high levels of heavy metal contamination (sample 1) significantly decreased the total chlorophyll concentration compared to the control sample. This reduction aligns with the response of Ceratophyllum demersum to multi-metal stress 37. Several previous studies demonstrated that heavy metal concentration negatively impacts plant growth and chlorophyll concentration 38,39,40. A consistent and significant negative relationship was observed across all measured parameters. The sharp decline in SGR, SWGR, and RGR reflects impaired growth processes, whereas the reduction in chlorophyll content highlights disruptions in photosynthesis under heavy metal exposure. These findings revealed that increasing heavy metal concentrations led to severe plant growth inhibition and physiological dysfunction. The strong MI-SWGR correlation (r = -0.9995) mirrors phytotoxicity models for submerged macrophytes 37.
This study suggests that E. densa is suitable for constructed wetland systems when wastewater is contaminated with heavy metals from low to moderate levels. The sustainable growth of E. densa requires moderate temperatures, sufficient light, nutrient-rich water, and a slightly acidic to neutral pH 41. Furthermore, E. densa can survive across a wide range of temperatures and can grow even under unfavorable environmental conditions 42,43,44. In Bangladesh, all conditions are conducive to the sustainable growth of Egeria densa. It could be utilized in several types of constructed wetlands in Bangladesh when the effluent contains low-to-moderate concentrations of heavy metals. It can be used in horizontal-flow constructed wetlands, vertical-flow constructed wetlands, free-water-surface constructed wetlands, or hybrid constructed wetlands. These findings may serve as a reference for future research and offer useful insights for scholars intending to utilize E. densa as vegetation in constructed wetlands.
Conclusion
Egeria densa effectively removed heavy metals (Cr, Cu, Fe, and Zn) from low heavy metal contaminated river water, with removal rates ranging from 87.2% to 100%. When treating moderately heavy metal-contaminated water, the efficiency of E. densa in removing these metals can range from 62% to 73%. Finally, E. densa is unsuitable for treating river water with high concentrations of heavy metals. The heavy metal tolerance levels of E. densa are as follows: Cr (0–0.25 mg/L), Cu (0–0.25 mg/L), Fe (0–2.5 mg/L), and Zn (0–0.10 mg/l). Water with low heavy metal contamination supports increased SGR and SWGR in E. densa while maintaining balanced chlorophyll concentrations. In water with moderate heavy metal contamination, plants can survive but exhibit reduced SGR, SWGR, and chlorophyll concentration. However, highly contaminated water is unsuitable for their survival. The study revealed a strong negative correlation between MI and SGR, demonstrating that higher heavy metal concentrations lead to a decline in the growth rate. These findings suggest that E. densa is well-suited for constructed wetland systems that treat effluents with low to moderate heavy metal levels.
Acknowledgements
The authors would like to thank the Department of Environmental Science and Disaster Management, Daffodil International University, for their kind support throughout this study.
Conflict of Interest
The authors declare no conflicts of interest.
Funding information
This research was funded by the International Atomic Energy Agency (IAEA)'s Coordinated Research Project (CRP)#F22076 through Research Contract#26702 entitled Determination of radionuclide sorption capacity of constructed wetland materials for hydrodynamics study.
Ethical Considerations
The present study was approved by the Ethics Committee of the Faculty of Health and Life Sciences, Daffodil International University, Bangladesh (ethical code: BD.DIU.REC.2024.128).
Code of Ethics
The authors adhered to the ethical guidelines and regulations throughout the study, ensuring the integrity and reliability of the research process. Data collection, analysis, and reporting were conducted transparently with due regard to scientific and ethical standards.
Data availability
Data are available from the corresponding author upon reasonable request.
Author contributions
Md. Swadhin Hossain was involved in the conceptualization, methodology, investigation, formal analysis, writing-original draft preparation, and visualization. Mahfuza Parveen was involved in supervision, methodology, validation, writing-review and editing. Mohammad Rajib ensured supervision, writing-review and editing, funding acquisition; and S.M Mahmudur Rahman carried out the investigation.
This is an Open-Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt, and build upon this work for commercial use.
Reference
Discussion
The findings of this study show that Egeria densa effectively reduces heavy metal concentrations in contaminated water through phytoremediation. The results implied that Sample 3 achieved the highest removal efficiency. This is likely because this sample contained a lower concentration of heavy metals, which enhanced the phytoremediation capacity of E. Densa. Furthermore, previous studies have reported similar findings, demonstrating that submerged aquatic macrophytes are effective in removing heavy metals from wastewater 27,28,29,30.
The trend in the removal efficiencies of Cr, Cu, Fe, and Zn demonstrated a clear relationship between the contamination levels and removal performance. Removal efficiencies improved as contamination levels decreased; E. densa removed up to 100% Zn and over 90% Cr and Cu in sample 3. This suggests that E. densa performs better in wastewater with lower contamination levels, possibly because of favorable uptake conditions in such environments. These consistently higher removal rates in samples 2 and 3 indicate the potential of E. densa as a sustainable tool for bioremediation, particularly in moderately to lightly contaminated systems. Similar studies have reported that other submerged aquatic macrophytes, such as Pontederia cordata, Lemna minor, and Eichhornia crassipes, also exhibit promising heavy metal removal capacities, but their tolerance thresholds are often higher or their adaptation is more limited in tropical regions than that of E. densa 31,32. In contrast, the present study demonstrated that E. densa can achieve up to 100% removal of Zn and over 90% for Cr and Cu at lower contamination levels, surpassing the performance of Typha angustifolia under similar conditions 33. This highlights the high suitability of E. densa for use in constructed wetlands, especially in South Asian contexts with variable contamination profiles and climatic conditions.
The effectiveness of the treatment method was further assessed using the Metal Index (MI). The findings clearly show an inverse relationship between the initial contamination level and treatment effectiveness. While the process reduced pollutants in all samples, it was most effective at moderate to low contamination levels. This pattern suggests that although E. densa is effective, it may not be sufficient alone for treating highly polluted wastewater. Additional or enhanced treatment strategies may be needed for highly polluted water, such as sample 1, as demonstrated by previous studies on phytoremediation limitations in heavily contaminated systems 34.
The results indicate that E. densa is highly sensitive to elevated concentrations of Cr, Cu, and Zn, whereas it tolerates moderate levels of Fe, likely due to its essential role in plant metabolism. The narrow tolerance range for Cr and Cu suggests a limited capacity for the detoxification or exclusion of these toxic metals. Although Zn is also a micronutrient, its threshold remains low, possibly due to its bioavailability, chemical form, or physiological sensitivity. Notably, these thresholds are substantially lower than those reported in a previous study, where E. densa tolerated a Zn concentration of 20 mg/L under 7-day exposure conditions 35. Variations in exposure duration, growth conditions (e.g., light and nutrients), and genetic adaptations may contribute to differential tolerance. The observed Zn tolerance threshold of E. densa (0.10 mg/L) is significantly lower than the mean Zn concentration reported in groundwater from the Chittagong industrial area (0.49 mg/L) 36. This disparity suggests that E. densa alone may be insufficient for effective remediation in highly contaminated environments without the support of complementary pretreatment systems. These findings highlight the potential of E. densa for phytoremediation within the specified concentration limits, while underscoring its vulnerability to excessive heavy metal contamination.
Regarding Wastewater with low levels of heavy metal contamination (sample 3), the plants effectively balanced the total chlorophyll content. Sample 2 slightly balanced the total chlorophyll concentration. Compared to the control and sample 3 plants, the total chlorophyll concentration was relatively low in sample 2. The findings also indicate that wastewater with high levels of heavy metal contamination (sample 1) significantly decreased the total chlorophyll concentration compared to the control sample. This reduction aligns with the response of Ceratophyllum demersum to multi-metal stress 37. Several previous studies demonstrated that heavy metal concentration negatively impacts plant growth and chlorophyll concentration 38,39,40. A consistent and significant negative relationship was observed across all measured parameters. The sharp decline in SGR, SWGR, and RGR reflects impaired growth processes, whereas the reduction in chlorophyll content highlights disruptions in photosynthesis under heavy metal exposure. These findings revealed that increasing heavy metal concentrations led to severe plant growth inhibition and physiological dysfunction. The strong MI-SWGR correlation (r = -0.9995) mirrors phytotoxicity models for submerged macrophytes 37.
This study suggests that E. densa is suitable for constructed wetland systems when wastewater is contaminated with heavy metals from low to moderate levels. The sustainable growth of E. densa requires moderate temperatures, sufficient light, nutrient-rich water, and a slightly acidic to neutral pH 41. Furthermore, E. densa can survive across a wide range of temperatures and can grow even under unfavorable environmental conditions 42,43,44. In Bangladesh, all conditions are conducive to the sustainable growth of Egeria densa. It could be utilized in several types of constructed wetlands in Bangladesh when the effluent contains low-to-moderate concentrations of heavy metals. It can be used in horizontal-flow constructed wetlands, vertical-flow constructed wetlands, free-water-surface constructed wetlands, or hybrid constructed wetlands. These findings may serve as a reference for future research and offer useful insights for scholars intending to utilize E. densa as vegetation in constructed wetlands.
Conclusion
Egeria densa effectively removed heavy metals (Cr, Cu, Fe, and Zn) from low heavy metal contaminated river water, with removal rates ranging from 87.2% to 100%. When treating moderately heavy metal-contaminated water, the efficiency of E. densa in removing these metals can range from 62% to 73%. Finally, E. densa is unsuitable for treating river water with high concentrations of heavy metals. The heavy metal tolerance levels of E. densa are as follows: Cr (0–0.25 mg/L), Cu (0–0.25 mg/L), Fe (0–2.5 mg/L), and Zn (0–0.10 mg/l). Water with low heavy metal contamination supports increased SGR and SWGR in E. densa while maintaining balanced chlorophyll concentrations. In water with moderate heavy metal contamination, plants can survive but exhibit reduced SGR, SWGR, and chlorophyll concentration. However, highly contaminated water is unsuitable for their survival. The study revealed a strong negative correlation between MI and SGR, demonstrating that higher heavy metal concentrations lead to a decline in the growth rate. These findings suggest that E. densa is well-suited for constructed wetland systems that treat effluents with low to moderate heavy metal levels.
Acknowledgements
The authors would like to thank the Department of Environmental Science and Disaster Management, Daffodil International University, for their kind support throughout this study.
Conflict of Interest
The authors declare no conflicts of interest.
Funding information
This research was funded by the International Atomic Energy Agency (IAEA)'s Coordinated Research Project (CRP)#F22076 through Research Contract#26702 entitled Determination of radionuclide sorption capacity of constructed wetland materials for hydrodynamics study.
Ethical Considerations
The present study was approved by the Ethics Committee of the Faculty of Health and Life Sciences, Daffodil International University, Bangladesh (ethical code: BD.DIU.REC.2024.128).
Code of Ethics
The authors adhered to the ethical guidelines and regulations throughout the study, ensuring the integrity and reliability of the research process. Data collection, analysis, and reporting were conducted transparently with due regard to scientific and ethical standards.
Data availability
Data are available from the corresponding author upon reasonable request.
Author contributions
Md. Swadhin Hossain was involved in the conceptualization, methodology, investigation, formal analysis, writing-original draft preparation, and visualization. Mahfuza Parveen was involved in supervision, methodology, validation, writing-review and editing. Mohammad Rajib ensured supervision, writing-review and editing, funding acquisition; and S.M Mahmudur Rahman carried out the investigation.
This is an Open-Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt, and build upon this work for commercial use.
Reference
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Type of Study: Original articles |
Subject:
Water quality and wastewater treatment and reuse
Received: 2025/02/18 | Accepted: 2025/04/20 | Published: 2025/06/29
Received: 2025/02/18 | Accepted: 2025/04/20 | Published: 2025/06/29
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