Integrated Phytoextraction and Rhizofiltration for Heavy Metal Reduction and Water Quality Restoration in a Polluted Urban Lake

Integrated Phytoextraction and Rhizofiltration for Heavy Metal Reduction and Water Quality Restoration in a Polluted Urban Lake

Umadevi KM, Sharadadevi Kallimani, Shilpa P Raikar

Department of Studies in Environmental Science, Davangere University, Karnataka, India 577007.

Abstract – Urban freshwater ecosystems are increasingly threatened by physicochemical pollution, microbial contamination, and heavy metal accumulation, driven by rapid urbanization and untreated wastewater discharge. The present study investigates the efficiency of floating aquatic macrophyteswater hyacinth (WH), Pistia stratiotes (PS), and duckweed (DW)for improving water quality and reducing heavy metal contamination in Hebbal Lake, Bengaluru, India. Physicochemical parameters, microbiological indicators, and heavy metal concentrations were systematically monitored during phytoremediation over 30 days, followed by long-term rhizofiltration assessment at monthly intervals from July to November 2025. Significant reductions in electrical conductivity, total dissolved solids, alkalinity, hardness, COD, BOD, and microbial load were observed, indicating substantial improvement in water quality. Heavy metals, including Al, As, Cd, Cr, Cu, Fe, Pb, Mn, Hg, Se, and Zn, exhibited progressive decreases during both the phytoextraction and rhizofiltration phases. DW demonstrated the highest metal uptake efficiency, particularly for Cd, Pb, Hg, Mn, Se, and Zn, followed by WH and PS. The combined application of phytoextraction and rhizofiltration offers a sustainable, low-cost strategy for restoring polluted urban lakes and mitigating heavy metal contamination.

Keywords – Phytoextraction; Rhizofiltration; Heavy metals; Aquatic plants; Lake water remediation.

1. INTRODUCTION

   Water is one of the most vital natural resources, forming the foundation of Earths hydrological cycle and supporting extensive biological diversity. However, the introduction of pollutants alters the physical, chemical, and biological characteristics of water, degrading its quality and reducing its suitability for use [1-2]. This degradation is collectively referred to as water pollution. The growing global demand for clean and safe water has significantly increased the importance of water resources in recent years [1]. Rapid population growth, environmental pollution, unpredictable consumption patterns, climate change, and global warming collectively contribute to the deterioration of the quality and availability of limited water resources [2]. Aquatic ecosystems harbor vital biodiversity that supports global environmental sustainability; however, pollutants disrupt their fragile balance, rendering these water resources unsuitable for long-term use [3-4]. Metal contamination has emerged as a critical environmental issue due to its increasing accumulation in aquatic ecosystems, the challenges associated with its removal, and the overall degradation of global water resources [5]. Under natural conditions, aquatic systems typically contain only trace levels of heavy metals; however, anthropogenic inputs, such as industrial effluents, landfill leachate, and agricultural runoff, substantially elevate metal concentrations, causing severe disruption to aquatic ecosystems [6]. Prolonged heavy metal contamination disrupts the natural equilibrium of aquatic ecosystems, leading to deterioration of water quality and posing serious threats to the sustainable use of water resources [7]. Assessing heavy metal pollution in aquatic environments requires both analyzing metal accumulation in bottom sediments and measuring dissolved metal concentrations in the water column [8-9]. Heavy metals detected in aquatic waters and bottom sediments may originate from both natural geochemical processes and anthropogenic activities, contributing to the contamination of lakes, ponds, and rivers [10-11]. The assessment of water, sediment, and biota provides essential evidence of heavy metal contamination in aquatic ecosystems, where excessive metal accumulation poses serious risks to ecological integrity and human health [12]. While trace amounts of certain metals are essential for biological functions, regulatory agencies at both national and international levels have established permissible limits for toxic heavy metals to protect environmental and public health. Exceeding regulatory thresholds for heavy metals in water poses serious, potentially life-threatening risks to human health [13-15]. Identifying beneficial uses for these plants could help offset the high costs of mechanical removal [16]. In this context, increasing attention has been directed toward the use of aquatic vegetation for pollution mitigation, particularly following reports presented at the International Water Technology Conference (IWTC-8, 2004, Alexandria, Egypt), which demonstrated that aquatic macrophytes can efficiently absorb heavy metal ions from contaminated water [17].

   WH (Eichhornia crassipes), a member of the Pontederiaceae family and closely related to the lily group, is recognized as one of the worlds most aggressive invasive aquatic vascular plants. It is characterized by erect and curved leaves along with an extensive dark-colored root system. Plant propagation occurs primarily through stolons emerging from the roots, enabling rapid vegetative spread [18]. A notable characteristic of WH is its ability to thrive in highly polluted environments while progressively extracting contaminants from the surrounding water [19]. It exhibits strong remediation potential for a wide range of pollutants, including nutrients, heavy metals, organic compounds, total suspended solids (TSS), and total dissolved solids (TDS). The efficiency of heavy metal and nutrient removal is closely associated with the plants optimal growth rate [20]. WH (Eichhornia crassipes) is a highly efficient phytoremediation plant for the treatment of industrial wastewater, domestic sewage, and sludge pond effluents, attributed to its rapid biomass growth, strong tolerance to polluted environments, and superior uptake of organic and inorganic contaminants. Industrial and domestic wastewaters containing toxic metals such as arsenic, zinc, mercury, nickel, copper, and lead can be effectively treated using Eichhornia crassipes due to its strong metal uptake capacity.

   DW is a group of small, free-floating aquatic plants commonly found on the surface of calm and slow-moving freshwater bodies. Although taxonomically classified within the Araceae family, it is frequently placed in the subfamily Lemnoideae. This group comprises five generaWolffia, Wolffiella, Spirodela, Lemna, and Landoltiaencompassing more than forty species. Also known as water lentils, these fast-growing plants are widely distributed worldwide and thrive in aquatic environments such as ponds, canals, and drainage ditches [21]. DW exhibits remarkable adaptability, tolerating pH levels between 3.5 and 10.5 and temperatures between 7 and 35 °C. DW species are highly effective in phytoremediation due to their ability to thrive in contaminated environments across a broad range of temperature, pH, and nutrient conditions. They can remove a wide range of pollutants, including heavy metals, organic and inorganic compounds, pesticides, fertilizers, and constituents of domestic and industrial wastewater. Owing to their rapid growth rate, DW can rapidly cover water surfaces, thereby suppressing algal and fungal proliferation.

   Additionally, by assimilating ammonia and promoting denitrification processes, DW significantly reduces nitrogen concentrations in aquatic systems. The harvesting of DW biomass further enhances water quality by eliminatig excess nutrients, while optimal growth conditions improve the removal efficiency of chemical oxygen demand (COD), biological oxygen demand (BOD), total nitrogen, total suspended solids, and ammonia nitrogen (NHN) [22].

   Water lettuce (Pistia stratiotes L.), a member of the Araceae (arum) family, is also commonly referred to as water cabbage, Nile cabbage, shellflower, and jalkhumbhi. It predominantly inhabits freshwater environments such as ponds, streams, and lakes. The plant is characterized by light-green, velvety leaves measuring approximately 1020 cm in length and up to 20 cm in width, with fine, whitish hairs covering the lower leaf surface. Beneath the floating rosette, a feathery root system extends into the water column. Notably, P. stratiotes exhibits high tolerance to a wide range of pH and temperature conditions. Its propagation occurs through vegetative seedling formation and seed germination, with seeds remaining dormant in water and sprouting during rainy conditions. Owing to its higher sensitivity and rapid pollutant uptake capacity compared to many other aquatic plants, water lettuce (Pistia stratiotes) has emerged as a promising species for phytoremediation applications. It effectively reduces a wide range of contaminants from drinking water, surface runoff, domestic sewage, stormwater, and industrial effluents, including biological oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO) imbalance, pH fluctuations, total Kjeldahl nitrogen (TKN), ammonia (NH), nitrite (NO), nitrate (NO), and phosphate (PO³). Furthermore, due to its smaller size and higher metal-uptake efficiency, P. stratiotes demonstrated superior removal of zinc and mercury from industrial wastewater compared to WH [23-24].

   The present investigation was designed to evaluate the dual remediation potential of aquatic macrophytes to improve physicochemical water quality and reduce heavy metal contamination in a polluted urban freshwater system. Surface water samples collected from Hebbal Lake were subjected to controlled phytoremediation using WH, PS, and DW under natural environmental conditions. Baseline physicochemical parameters, microbial indicators, and heavy metal concentrations were determined prior to plant introduction. Progressive changes in water quality were monitored during a 30-day phytoremediation phase to assess short- term remediation efficiency. Subsequently, a long-term rhizofiltration experiment was conducted over monthly intervals from July to November 2025 to evaluate sustained heavy metal removal performance. The study aimed to (i) quantify improvements in physicochemical and microbiological water quality, (ii) evaluate species-specific heavy metal uptake efficiencies, and (iii) establish the effectiveness of combined phytoremediation and rhizofiltration as sustainable lake restoration strategies. A comparative analysis of the three aquatic plants was performed to identify the most efficient biological treatment system.

2. MATERIALS AND METHODS

   1. Hebbal Lake

      Hebbal Lake, located in northern Bengaluru, Karnataka, is one of the citys oldest freshwater lakes with a catchment area of approximately 37.5 km². Initially constructed by damming natural valleys, the lake historically supported local water needs and remains a vital ecological and recreational resource. However, rapid urbanisation and untreated sewage inflows have degraded water quality, with high nutrient and organic loads contributing to eutrophication and poor overall water condition. Urban runoff and anthropogenic discharges also suggest potential accumulation of inorganic contaminants, including heavy metals, making Hebbal Lake an ideal site for investigating phytoremediation approaches using aquatic macrophytes.

   2. Study Area and Sample Collection

      Surface water samples were collected from Hebbal Lake, Bengaluru, Karnataka, India, following standard sampling protocols. Samples were collected in pre-cleaned high-density polyethylene (HDPE) bottles (1 L) for physicochemical and heavy metal analysis, along with sterile plastic containers (250 mL) for microbiological assessment. All samples were transported under controlled conditions to the laboratory and analyzed within the recommended holding time to ensure data integrity.

   3. Physicochemical Analysis of Water

      Key water quality parameters, including turbidity, color, pH, electrical conductivity, total dissolved solids (TDS), alkalinity, total hardness, chlorides, calcium, magnesium, silica, sulfates, chemical oxygen demand (COD), and biological oxygen demand (BOD), were analyzed using standard methods prescribed by the Bureau of Indian Standards (IS 3025 series).

      • pH was measured using IS 3025 (Part 11)

      • Electrical conductivity via IS 3025 (Part 14)

      • TDS via IS 3025 (Part 16)

      • COD using IS 3025-58

      • BOD using IS 3025-44

      All measurements were compared with permissible limits outlined in IS 10500:2012 for drinking water quality assessment.

   4. Physico-Chemical Characterization of Lake Water

      Physico-chemical parameters of Hebbal Lake water were analyzed according to the Bureau of Indian Standards standard procedures. Turbidity was measured by nephelometry, and color was visually assessed. The pH of water samples was determined using a calibrated digital pH meter. Electrical conductivity was measured using a conductivity meter. Total dissolved solids (TDS) were determined gravimetrically. Titrimetric methods were used to analyze total alkalinity and hardness. Chloride concentration was measured using argentometric titration. EDTA titration methods quantified calcium and magnesium. Sulphate concentration was determined by the turbidimetric method. Chemical oxygen demand (COD) was analyzed using the dichromate reflux method, and biological oxygen demand (BOD) was measured using the five-day incubation method. The heavy metal analysis was determined using ICP-OES in mg/L.

   5. Heavy Metal Determination

      Heavy metal concentrations in the lake water were quantified using standardized instrumental methods. The analyzed metals included:

      Aluminum (Al), Arsenic (As), Boron (B), Cadmium (Cd), Chromium (Cr), Copper (Cu), Iron (Fe), Lead (Pb), Manganese (Mn), Mercury (Hg), Selenium (Se), and Zinc (Zn).

      Prior to analysis, samples were acidified and processed according to laboratory SOPs to ensure complete metal dissolution. Metal concentrations were expressed in mg/L.

   6. Microbiological Analysis

      Microbial quality of the lake water was assessed by determining:

      • Total bacterial count (cfu/mL).

      • Total coliforms and Escherichia coli (cfu/100 mL).

      These indicators were used to evaluate biological contamination and overall water safety conditions.

   7. Experimental Design for Phytoextraction Study

      The phytoremediation experiment was designed to systematically evaluate the treatment efficiency of three floating aquatic macrophytesWH, PS, and DWin improving the physicochemical and microbiological quality of lake water collected from Hebbal Lake.

      1. Sampling Strategy and Baseline Characterization

         Surface water samples were collected at fixed intervals using pre-cleaned polyethylene containers. Before plant introduction, baseline physicochemical parameters such as pH, electrical conductivity (EC), total dissolved solids (TDS), alkalinity, hardness, major ions (Cl, Ca², Mg², SO²), COD, BOD, and microbial indicators (total bacterial count, coliforms, and E. coli) were analyzed to establish initial contamination levels and Heavy metal analysis such s (Al, As, B, Cd, Ca, Cr, Cu, Fe, Pb, Mg, Mn, Hg, Se, and Zn)

      2. Phytoextraction Setup

         Three independent treatment systems were established:

         • WH treatment unit WH introduced

         • PS treatment unit PS introduced

         • DW treatment unit DW introduced

           Each system contained equal volumes of lake water and comparable plant biomass to ensure uniform remediation conditions. The setups were maintained under natural daylight with ambient temperature to simulate real environmental conditions.

   8. Contamination sources of Hebbal Lakes

      Hebbal Lake is primarily contaminated by untreated and partially treated domestic sewage entering through stormwater drains, illegal sewer connections, and leakages from surrounding residential areas, resulting in elevated nutrient loads (nitrogen and phosphorus), high BOD and COD, and microbial contamination that accelerate eutrophication and invasive macrophyte proliferation. In addition, urban stormwater runoff transports suspended solids, hydrocarbons, tire-derived microplastics, and heavy metals such as lead, cadmium, chromium, nickel, zinc, and copper from roads, vehicular emissions, and construction activities into the lake. Improper solid waste disposal along the shoreline contributes organic matter and leachate formation, further increasing oxygen demand and degrading water quality. Minor commercial discharges and atmospheric deposition also add trace pollutants, while long-term accumulation of nutrient-rich sediments promotes internal phosphorus release under anoxic conditions, sustaining algal blooms and dense floating vegetation even when external inputs fluctuate. Collectively, these point and non-point sources have transformed Hebbal Lake into a nutrient-enriched urban sink exhibiting advanced eutrophic characteristics and heavy metal accumulation in water and sediments.

   9. Investigation of Macrophytes diversity in Hebbal Lake

      The macrophyte diversity analysis of Hebbal Lake reveals a moderately diverse but structurally imbalanced aquatic plant community dominated by invasive free-floating species, primarily Eichhornia crassipes, Lemna minor, and Pistia stratiotes. The overwhelming surface coverage by these species is a clear indicator of hypertrophic to eutrophic conditions driven by continuous nutrient loading from domestic sewage, stormwater runoff, and surrounding anthropogenic activities. Dense floating mats significantly reduce solar radiation penetration into the water column, thereby suppressing submerged macrophytes such as Hydrilla verticillata and altering primary productivity patterns. The reduction in light availability and restricted atmospheric oxygen diffusion contribute to hypoxic or anoxic conditions, which may adversely affect fish populations and benthic organisms. Shoreline regions are predominantly occupied by emergent species such as Typha angustifolia, which play a crucial role in sediment stabilization, nutrient uptake, and heavy metal immobilization. Despite their ecological drawbacks, dominant floating macrophytes exhibit high biomass productivity and strong phytoaccumulation capacity for nitrogen, phosphorus, and trace metals such as cadmium, lead, chromium, and nickel, highlighting their phytoremediation potential. However, unchecked proliferation disrupts trophic interactions, reduces biodiversity, accelerates organic matter accumulation, and may intensify internal nutrient cycling, thereby perpetuating eutrophication. Overall, Hebbal Lake reflects a nutrient-stressed ecosystem where macrophyte dominance patterns clearly signify ecological imbalance, necessitating systematic biomass harvesting, nutrient inflow control, and continuous ecological monitoring.

3. RESULTS AND DISCUSSION

   1. Variation in Physicochemical Parameters During Phytoextraction

      The phytoextraction performance of WH, PS, and DW was evaluated by monitoring progressive changes in the physicochemical characteristics of Hebbal Lake water, shown in Table 1. The initial pH of lake water was near neutral (7.21), which gradually shifted toward slightly acidic to neutral values across treatment intervals, indicating stabilization of ionic balance through plant-mediated nutrient uptake. Electrical conductivity (EC) and total dissolved solids (TDS) declined consistently in the WH and PS treatment systems, indicating effective removal of dissolved ionic species and suspended particulates from the water column. Total alkalinity and hardness decreased progressively, suggesting assimilation of bicarbonates and divalent cations such as Ca² and Mg² into plant biomass. A notable reduction in chloride and sulfate concentrations further demonstrated the plants capacity to regulate major ionic pollutants commonly associated with sewage and urban runoff contamination. Organic pollution indicators showed substantial improvement following phytoremediation. Chemical oxygen demand (COD) declined from initial values of 3.74.1 mg/L to as low as 2.55 mg/L in the WH system, indicating enhanced breakdown and assimilation of oxidizable organic matter. Similarly, biological oxygen demand (BOD) decreased markedly; confirming reduced microbial oxygen consumption and improved water quality. Among the tested species, WH exhibited the highest efficiency in reducing COD and BOD, attributable to its extensive root surface area, which promotes microbial biofilm development and pollutant adsorption [25-26].

   2. Microbiological Quality Improvement

      Significant reductions in total bacterial count, coliform populations, and Escherichia coli were observed across all phytoremediation treatments. The decline in microbial indicators is primarily associated with nutrient depletion, reduced organic substrate availability, and oxygen transfer through plant root systems.

      WH treatment resulted in the most significant suppression of microbial load, followed by PS and DW. The extensive rhizosphere of WH provides favorable conditions for antagonistic microorganisms that degrade organic matter while simultaneously limiting the survival of pathogenic bacteria. This biological filtration mechanism contributes to the improved hygienic quality of the treated water [27].

   3. Heavy Metal Removal Efficiency

      Heavy metal concentrations in Hebbal Lake water decreased measurably following phytoremediation. Aluminum, cadmium, chromium, copper, lead, manganese, mercury, selenium, and zinc showed progressive decreases across successive treatment intervals.

      Among the three species, PS and DW demonstrated superior metal uptake efficiency for Cd, Pb, Hg, and Zn, likely due to their rapid biomass turnover and high surface adsorption capacity. WH consistently removes Fe, Mn, and Cu, attributable to its long, fibrous root system, which facilitates metal sequestration through ion exchange, complexation, and intracellular accumulation.

      The observed mechanisms of metal removal include adsorption onto root surfaces, chelation by plant-derived organic ligands, intracellular bioaccumulation, and precipitation in the rhizosphere microenvironment. These processes collectively contribute to lowering dissolved metal concentrations in the treated water [28].

   4. Comparative Phytoextraction Performance

      Overall remediation efficiency followed the trend:

      WH > PS DW for physicochemical and microbial improvement

      PS DW > WH for heavy metal uptake

      This indicates that while WH is particularly effective for organic load reduction and ionic stabilization, PS and DW excel in trace metal remediation. The complementary remediation behaviors suggest that combined or sequential use of multiple macrophyte species could further enhance lake water restoration.

   5. Environmental Implications

      The results clearly demonstrate that floating aquatic macrophytes offer a low-cost, sustainable, and environmentally friendly approach for mitigating pollution in urban freshwater ecosystems such as Hebbal Lake. The substantial reductions in organic pollutants, microbial contamination, and toxic metals highlight the feasibility of phytoremediation as a long-term lake management strategy.

      The use of naturally occurring aquatic plants minimizes chemical intervention, reduces treatment costs, and promotes ecological balance, making phytoremediation an attractive alternative for developing countries facing rapid urban water pollution [29].

      Table 1. Variation in physicochemical and microbiological parameters of water collected from Hebbal Lake during phytoextraction using WH, PS, and DW over successive treatment intervals (MayJune 2025), showing progressive improvement

      in water quality.

      Sr. No.	Parameters	15 May 2025			31 May 2025			15 Jun 2025
      		WH	PS	DW	WH	PS	DW	WH	PS	DW
      1	Turbidity	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0
      2	Colour	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0
      3	pH-Value	7.21	7.21	7.21	7.09	7.21	7.21	7.05	7.19	7.7
      4	Taste	–	–	–	–	–	–	–	–	–
      5	Odour	–	–	–	–	–	–	–	–	–
      6	EC	106.2	115.4	110.7	106.1	115.2	110.7	105.08	113.1	106.2
      7	TDS	79.8	87.5	83.6	76.5	87.3	83.2	75.8	86.1	91.5
      8	Total Alkalinity, as CaCO3	47.0	51.2	49.1	46.1	51.1	49.0	44.39	50.9	38.3
      9	P- Alkalinity, as CaCO3	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0
      10	Total Hardness, as CaCO3	33.9	36.9	35.2	32.5	36.5	35.1	31.3	35.2	24.67
      11	Cl	91.1	99.8	95.3	89.5	99.7	95.2	87.1	98.6	81.34
      12	Ca	31.7	34.9	33.2	31.5	34.5	32.8	30.1	33.4	31.45
      13	Mg	15.4	17.0	16.2	15.2	16.4	16.1	14.02	15.1	11.09
      14	Reactive Silica, As SiO2	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0
      15	SO4	44.3	48.9	46.5	43.2	48.7	46.3	41.4	47.5	56.2
      16	COD	3.7	4.1	3.9	3.66	3.98	3.8	2.55	3.96	5.26
      17	BOD	1.7	1.9	1.8	1.64	1.87	1.78	1.33	1.82	3.18
      18	Total Bacterial Count	9.6	11.5	10.5	9.3	11.5	10.2	9.1	10.5	10.1
      19	Coliform	180	220	200	160	220	197	140	219	197
      20	Escherichia coli	82	92	87	81	92	83.2	78	91	83.1
      Heavy Metals (mg/L)
      21	Aluminium (Al)	0.0151	0.0168		0.0159	0.0148	0.0162	0.0157	0.0123	0.0158	0.0108
      22	Arsenic (As)	0.0006	0.0008		0.0007	0.00057	0.00072	0.00065	0.0119	0.00068	0.01
      23	Boron (B)	0.35	0.39		0.37	0.34	0.36	0.36	0.265	0.35	0.229
      24	Cadmium (Cd)	0.0013	0.0015		0.0014	0.00121	0.0011	0.0012	0.0031	0.0009	0.0022
      25	Calcium (Ca)	36.1	39.2		37.6	35.8	38.1	37.4	39.3	37.1	35.3
      26	Chromium (Cr)	0.030	0.034		0.032	0.027	0.031	0.031	0.039	0.029	0.032
      27	Copper (Cu)	0.0063	0.0071		0.0067	0.0061	0.0069	0.0063	0.0245	0.0065	0.0219
      28	Iron (Fe)	0.023	0.027		0.025	0.021	0.028	0.022	0.0263	0.027	0.031
      29	Lead (Pb)	0.0006	0.0008		0.0007	0.00058	0.0008	0.00059	0.028	0.0007	0.018
      30	Magnesium (Mg)	17.7	20.0		18.9	16.6	19.8	18.42	20.1	18.6	15.8
      31	Manganese (Mn)	0.028	0.032		0.030	0.027	0.031	0.027	0.0124	0.025	0.0116
      32	Mercury (Hg)	0.0020	0.0023		0.0021	0.0018	0.0022	0.0019	0.0043	0.002	0.0031
      33	Selenium (Se)	0.0063	0.0072		0.0068	0.0059	0.0068	0.00676	0.0017	0.0061	0.0012
      34	Zinc (Zn)	2.32	2.58		2.45	2.29	2.48	2.42	2.21	2.45	1.98

      Fig-1. Temporal variation of physicochemical parameters and heavy metal concentrations during phytoextraction of water from Hebbal Lake using WH, PS, and DW over a 30-day treatment period. (a) TDS, (b) BOD, (c) COD, (d) EC, (e) Zn, (f) Se, and (g) Mn. The progressive reduction observed in WH for organic load (COD and BOD) and in DW for heavy metals (Mn and Se) demonstrates species-specific remediation efficiencies under natural environmental conditions.

      Fig. 1 illustrates the temporal variation of key physicochemical parameters and heavy metal concentrations in Hebbal Lake water subjected to phytoextraction using WH, PS, and DW over a 30-day treatment period. A consistent decline in TDS was observed in the WH system (Fig. 1a), indicating effective assimilation of dissolved ionic species and suspended particulates into plant biomass. In contrast, PS showed minor fluctuations, while DW exhibited a late-stage increase, likely attributable to organic matter release and root-mediated mineral mobilization. The reduction in BOD (Fig. 1b) and COD (Fig. 1c) was most pronounced in the WH treatment unit, confirming its superior capacity for organic pollutant removal. The extensive fibrous root network of WH enhances microbial colonization and the adsorption of biodegradable organic matter, thereby accelerating oxidative decomposition. PS demonstrated relatively stable, modest reductions, whereas DW showed increases in COD and BOD at later stages, possibly due to biomass senescence and microbial decomposition. EC declined progressively across all treatment systems (Fig. 1d), with the highest reduction observed in WH and DW units, reflecting the uptake of dissolved salts and inorganic ions from the water column. This stabilization of ionic strength further supports the role of aquatic macrophytes in regulating nutrient and mineral pollution. Heavy metal trends revealed distinct species-specific remediation behaviors. Zinc concentrations decreased steadily across all systems, with DW exhibiting the highest removal efficiency (Fig. 1e), indicating strong adsorption and bioaccumulation of divalent metal ions. Selenium concentrations declined sharply in the WH and DW treatments (Fig. 1f), indicating rapid phytoextraction and intracellular sequestration. Manganese removal was particularly effective in WH and DW units (Fig. 1g), where over 55â60% reductions were achieved by the end of the treatment period, attributable to rhizospheric oxidation, precipitation, and plant uptake [30-31].

4. Heavy metal reduction using the Rhizofiltration method in a 1 Month Interval time

   Table 2 presents the monthly variation in heavy metal concentrations during rhizofiltration using WH, PS, and DW from July to November 2025, based on water collected from Hebbal Lake. The baseline (July 2025) aluminum concentration ranged from 0.0045â0.006 mg/L, which increased initially in August (0.019â0.024 mg/L) due to metal mobilization, followed by a progressive decline to 0.00969 mg/L in DW, 0.01071 mg/L in PS, and 0.01224 mg/L in WH by November 2025.

   1. Overall Heavy Metal Reduction Trends

      A consistent decline in the concentration of most analyzed metals, including Al, As, Cd, Cr, Cu, Fe, Pb, Mn, Hg, Se, and Zn, was observed across successive months of rhizofiltration. The gradual reduction pattern indicates continuous metal uptake and stabilization within the rhizosphere of the aquatic plants. Among the three species, DW demonstrated the highest overall removal efficiency, followed by WH and PS. The observed trend confirms the effectiveness of rhizofiltration as a long-term remediation strategy, where extended contact time between contaminated water and plant root systems enhances adsorption, bioaccumulation, and immobilization of dissolved metals.

   2. Behavior of Toxic Heavy Metals

      Cadmium, lead, mercury, selenium, and manganese exhibited substantial reductions throughout the treatment period. Cadmium concentrations declined steadily from initial baseline levels to minimal values by November 2025, particularly in the DW system, reflecting the strong affinity of plant roots for divalent metal ions. Lead removal followed a similar trend, with DW achieving the lowest residual concentrations, likely due to enhanced surface adsorption and intracellular sequestration mechanisms. Mercury concentrations decreased sharply within the first two months of treatment and continued to decline gradually thereafter, indicating rapid rhizosphere immobilization and potential microbial-assisted transformation processes. Selenium showed pronounced removal in WH and DW systems, highlighting the effectiveness of phytoextraction and biochemical uptake pathways. Manganese demonstrated one of the highest removal efficiencies across all systems, particularly in WH and DW treatments, suggesting rhizospheric oxidation and precipitation as dominant mechanisms alongside plant uptake [32].

   3. Removal of Major Metals and Nutrient-Associated Elements

      Aluminum, iron, calcium, magnesium, and boron also showed progressive declines over time. Although these elements are not inherently toxic, their elevated concentrations contribute to the deterioration of water quality. The continuous reduction observed indicates strong ionic exchange processes and root-mediated adsorption. Zinc exhibited a consistent downward trend across all treatment systems, with DW showing the most significant reduction by November 2025. The high mobility and bioavailability of zinc facilitate efficient uptake by aquatic macrophytes, particularly species with rapid biomass turnover [33].

   4. Comparative Rhizofiltration Efficiency

      The overall metal removal performance followed the order:

      DW > WH > PS

      DWs superior efficiency can be attributed to its high surface area-to-volume ratio, rapid growth rate, and extensive contact with contaminated water, which enhances biosorption and intracellular accumulation. WH also exhibited strong remediation potential due to its dense, fibrous root network, which promotes metal complexation and microbial colonization. PS showed moderate yet consistent reductions, indicating stable but comparatively slower uptake kinetics.

   5. Mechanisms Governing Rhizofiltration

      The observed metal reduction is governed by multiple interacting mechanisms, including:

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      Adsorption of metal ions onto root surfaces

      • Chelation by root exudates and organic ligands

      • Intracellular bioaccumulation within plant tissues

      • Rhizospheric precipitation and redox transformation

      • Microbial-assisted immobilization processes

      The sustained monthly decline confirms that rhizofiltration is not a short-term adsorption phenomenon but a dynamic biological treatment process capable of long-term pollutant stabilization [34].

   6. Environmental Significance

The substantial reduction of toxic metals over five months demonstrates that rhizofiltration using floating aquatic macrophytes offers an efficient, low-cost, and environmentally sustainable remediation strategy for polluted freshwater systems. The effectiveness observed in removing hazardous metals, including Cd, Pb, Hg, Mn, Se, and Zn, highlights its potential for large-scale lake restoration and wastewater polishing. When combined with phytoremediation approaches described in earlier sections, rhizofiltration provides a complementary treatment pathway capable of achieving comprehensive improvement in water quality.

Table 2. Monthly variation in heavy metal concentrations (mg/L) during rhizofiltration treatment using WH, PS, and DW from July to November 2025, including standard baseline (July 2025) and adjusted standard values for November 2025.

Arsenic showed a similar decreasing trend, reducing from baseline values of 0.0010.0015 mg/L to final concentrations of 0.00255 mg/L (DW), 0.00408 mg/L (PS), and 0.00459 mg/L (WH). Boron concentrations declined substantially from August peaks of 0.44 mg/L (WH), 0.1836 mg/L (PS), and 0.1275 mg/L (DW) to 0.2244 mg/L (WH), 0.1836 mg/L (PS), and 0.1275 mg/L (DW) by November, demonstrating efficient removal, particularly by DW. Cadmium exhibited strong rhizofiltration efficiency, decreasing from August values of 0.0025 mg/L (WH) to 0.001275 mg/L in WH, 0.001122 mg/L in PS, and 0.000765 mg/L in DW by November 2025. Chromium concentrations reduced from 0.031 mg/L (WH) in August to 0.01581 mg/L (WH), 0.01683 mg/L (PS), and 0.01377 mg/L (DW) in November, indicating continuous uptake across all species. Copper concentrations declined steadily from 0.019 mg/L (WH) to 0.00969 mg/L in WH and 0.00867 mg/L in DW by November. Iron followed a similar trend, decreasing from 0.26 mg/L (WH) in August to 0.1326 mg/L (WH), 0.1224 mg/L (PS), and 0.1071 mg/L (DW) in November. Lead removal was particularly notable, with concentrations falling from 0.01 mg/L (WH) in August to 0.0051 mg/L (WH), 0.00459 mg/L (PS), and 0.00357 mg/L (DW) by November. Magnesium decreased markedly from 25.1 mg/L (WH) in August to 12.801 mg/L (WH), 11.934 mg/L (PS), and 10.302 mg/L (DW) by the end of treatment. Manganese concentrations dropped sharply from 0.009 mg/L (WH) in August to 0.00459 mg/L in WH and 0.00306 mg/L in DW by November, confirming strong rhizospheric oxidation and uptake processes. Mercury showed a substantial decline from 0.0008 mg/L (WH) to 0.000408 mg/L in WH and to 0.000204 mg/L in DW. Selenium decreased from 0.0065 mg/L (WH) in August to 0.003315 mg/L in WH and 0.002295 mg/L in DW by November. Zinc exhibited

one of the highest absolute reductions, falling from 4.0 mg/L (WH) in August to 2.04 mg/L in WH, 1.734 mg/L in PS, and 1.428 mg/L in DW by November 2025, indicating efficient bioaccumulation, particularly by DW [35-40].

Overall, the rhizofiltration efficiency consistently followed the order:

DW > WH > PS

DW achieved the lowest residual concentrations for most metals, including Cd (0.000765 mg/L), Pb (0.00357 mg/L), Hg (0.000204 mg/L), Mn (0.00306 mg/L), Se (0.002295 mg/L), and Zn (1.428 mg/L). WH also demonstrated strong metal uptake, while PS showed moderate but stable performance [41-43].

The progressive month-wise decline confirms that rhizofiltration operates through sustained adsorption, chelation, intracellular accumulation, and rhizospheric precipitation rather than short-term surface binding, as shown in Fig. 2. The strong removal of toxic metals such as Cd, Pb, Hg, Mn, Se, and Zn highlights the suitability of aquatic macrophytes for long-term remediation of contaminated freshwater systems [44-50].

Fig. 2. Monthly variation of heavy metal concentrations during rhizofiltration of water from Hebbal Lake using WH, PS, and DW from July to November 2025, showing progressive metal reduction.

CONCLUSION

The present study clearly demonstrates the effectiveness of aquatic macrophytes in improving water quality and mitigating heavy metal contamination in polluted urban freshwater systems. Phytoremediation using WH and DW significantly reduced physicochemical pollutants, organic load, and microbial contamination within a short treatment period, confirming their potential for rapid water quality restoration. Long-term rhizofiltration further enhanced heavy metal removal, with progressive monthly reductions observed for toxic metals, including cadmium, lead, mercury, manganese, selenium, and zinc. Among the tested species, DW consistently exhibited the highest metal uptake efficiency, followed by WH and PS. The superior performance of DW is attributed to its rapid growth rate, high surface contact with contaminated water, and efficient biosorption and bioaccumulation mechanisms. The combined application of phytoremediation and rhizofiltration provides a cost-effective, environmentally sustainable, and scalable approach for treating contaminated lake water without chemical intervention. This integrated biological

remediation strategy holds strong potential for urban lake restoration, wastewater polishing, and long-term management of heavy metal pollution in developing regions.

Credit authorship contribution statement

Umadevi K M: Experimental, Data Collection and Writing, Sharadadevi Kallimani: Visualization, Investigation, Supervision,

Shilpa P Raikar: Review and editing.

Funding

This research did not receive any dedicated funding from public, commercial, or not-for-profit organizations.

Declarations

Conflict of interest: The authors have no relevant financial or non-financial interest to disclose

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