Phytoremediation of Zinc-Contaminated Industrial Effluents with Phragmites australis and Typha latifolia in Constructed Wetlands

Abstract

This study evaluated the ability of two plants, Phragmites australis and Typha latifolia, to bioaccumulate zinc from industrial effluents in constructed wetlands using ceramsite as a support medium. Two types of experiments were conducted: one with real industrial effluent containing 9.4 mg/L of Zn and another with synthetic effluent containing Zn at concentrations ranging from 10 to 100 mg/L. Zinc uptake in plant segments, ceramsite, and its concentration in wastewater were determined using ICP-OES. Both plants removed 97–99% of zinc ions from the industrial effluent, with the highest metal uptake occurring in the roots. In the case of synthetic solutions, Typha latifolia demonstrated higher zinc removal efficiency (95–99% removal) compared to Phragmites australis (74–90%). Typha latifolia also accumulated significantly higher levels of Zn, primarily in the roots. Transfer factor values were calculated to assess zinc translocation within plant tissues. No visual signs of toxicity were observed during the experiment. This phytoremediation approach could represent a sustainable and environmentally friendly method for treating industrial effluents.

1. Introduction

In many countries, particularly in developing ones, wastewater of various origins is often discharged into the aquatic environment without prior treatment, adversely affecting ecosystems and posing public health risks [1]. Consequently, proper wastewater management and the adoption of eco-friendly remediation methods are essential [2,3]. Among these methods, constructed wetlands are already considered an economically viable, natural, low-cost and environmentally friendly wastewater treatment technology worldwide [1,4]. Over the past three decades, extensive research has led to reliable and efficient wetland designs, primarily for treating domestic and municipal wastewater [1,4,5,6,7]. Although domestic/municipal wastewater research is still ongoing, the current challenge lies in the effective treatment of industrial wastewater, which has a more complex composition, containing a wider range of pollutants, depending on the industrial processes involved [8].

Among pollutants, heavy metals deserve special attention because they persist in the environment without degrading and can cause mutagenesis, carcinogenesis, and severe risks to living organisms [2,3]. Although it is difficult to completely remove heavy metals from the environment, they can be effectively converted into less toxic forms or neutralized [2,9].

Constructed wetlands consist of three key components: porous filter media, microorganisms and vegetation [10]. Various plant species have the capacity to immobilize, uptake, detoxify, stabilize, or degrade heavy metals [11]. Trace elements, Zn, Mn, Fe, and Cu are essential for plant physiology at certain concentrations. However, excessive concentrations of these metals and/or presence of highly toxic metals (As, Cd, Hg) in the environment can disrupt ion regulation and metabolic processes [12,13]. Certain species from the Typha, Scirpus (Schoenoplectus), Phragmites, Juncus, and Eleocharis genera are recognized as metal hyperaccumulators, capable of tolerating and storing high metal concentrations in their tissues [14]. The most commonly used species include Typha latifolia, Phragmites australis, Typha angustifolia, Juncus effusus, Scirpus lacustris, Scirpus californicus, and Phalaris arundinacea. These species are particularly valued for their dense root systems, high biomass production, and efficient detoxification mechanisms [9,15,16].

Plants in constructed wetlands are cultivated in an environment optimized for metal uptake through specialized substrates or chelating agents that enhance metal bioavailability. Substrates (soil, sand, rocks, limestone, gravel, bentonite, kaolin, ceramsite, biochar, etc.) provide a high surface area for microorganisms’ colonization, promoting macrophyte growth, while simultaneously acting as filtration and adsorption media for waterborne contaminants [9,10,17,18].

Zinc (Zn), one the most abundant elements in Earth’s crust, is an essential trace metal for living organisms. As a structural component of numerous proteins, Zn serves as a cofactor for over 300 enzymes, playing critical roles in enzyme activation and regulation, DNA synthesis, cellular metabolism, antioxidant defense by scavenging free radicals and reactive oxygen species [19,20]. A sufficient zinc intake plays an important role in the prevention of various diseases. Thus, it can help boost immunity and prevent respiratory infections, prevent or reduce the occurrence of acne, dermatitis and other skin-related conditions. Additionally, zinc is involved in the synthesis of collagen, which is essential for maintaining healthy skin [21]. Despite its biological importance, Zn exhibits dual characteristics—both essentiality and toxicity—depending on concentration [20]. In humans, high Zn concentrations can compromise the immune system, and cause iron-deficiency anemia, copper deficiency, metallic taste, nausea, vomiting, abdominal cramps, diarrhea, lethargy, and reduction in plasma high-density lipoprotein [22].

Major anthropogenic sources of Zn include mining and metallurgical operations, agriculture, chemical manufacturing, landfill leachates, electroplating and metal finishing, and battery production [23,24,25].

The main part of the literature report is devoted to the application of wetlands for the treatment of acid mine drainage containing Zn along with other metal ions. For example, Machemer et al. [26] reported Zn removal from acid mine drainage in an experimental constructed wetland in the Idaho Springs-Central City mining district of Colorado. Concurrent studies were performed in the field on the waters flowing from the wetland and in the laboratory on the wetland substrate. Zinc was completely removed from the wastewater due to its precipitation as zinc sulfide. Singh and Chakraborty [27] investigated Zn removal from the high acidity sulfate-rich wastewater in horizontal constructed wetland planted with Typha latifolia. Only 1.2% of Zn was removed by the plants, while the majority of Zn retention occurred within the wet-land media. A vertical subsurface flow constructed wetlands planted with Juncus effusus was used for removal of Cr, As, Zn, Fe and Al from acid mine drainage at the Ouixane abandoned mine site in Morocco [28]. In a study by Scholes et al. [29], urban runoff containing Zn, Pb, Ni, Cu, Cr, and Cd was treated in a constructed wetland planted with Phragmites australis and a natural wetland colonised by Typha latifolia. However, to our knowledge, there is no information available on the application of constructed wetlands for the treatment of industrial effluents generated by galvanic units.

The aim of this study was to evaluate the potential of Phragmites australis and Typha latifolia for zinc removal from real and synthetic industrial effluents generated by galvanic units in constructed wetlands using ceramsite as a support medium. Ceramsite, a lightweight and porous material, offers several advantages as a substrate in constructed wetlands for wastewater treatment, including enhanced pollutant removal, improved microbial activity, and greater resistance to clogging compared to traditional substrates like gravel [30,31,32].

2. Materials and Methods

2.1. Materials

All chemicals used were of analytical grade (Sigma-Aldrich, Darmstadt, Germany). Phragmites australis and Typha latifolia were obtained from Kamyshi-Pitomnik (Istra, Moscow Region, Russia). Industrial effluent containing zinc was collected from the galvanic unit of the company “Atom” (Dubna, Russia). The effluent had a zinc concentration of 9.54 mg/L and a pH of 6.0. The concentrations of other elements in the effluent were below the ICP-OES detection limit.

2.2. Experiment with Industrial Effluent

The experiments were conducted under controlled conditions simulating vertical subsurface flow (VSSF) CWs. Phragmites australis and Typha latifolia specimens were planted in 15 L plastic containers (height: 40 cm; upper diameter: 30 cm; lower diameter: 26 cm) equipped with bottom drainage outlets. The containers were filled with ceramsite as the substrate. In each container, three plants were planted. The effluent was applied to the top of each container, allowing it to percolate vertically through the substrate layer before draining through a bottom tap when required, thereby simulating VSSF CW conditions. Plants were maintained at room temperature (approximately 22 °C) under natural growth conditions. The experiment lasted for two weeks, and Zn concentration in the effluent was measured weekly.

When Zn concentrations in the effluent fell below permissible discharge limits for industrial and agricultural wastewater, plant tissues (roots, leaves, and stems for Phragmites australis and roots, leaves for Typha latifolia) and ceramsite were collected for further analysis.

2.3. Batch Experiment

Given the rapid zinc removal observed in industrial wastewater (within two weeks), additional batch experiments were conducted using synthetic effluents with higher Zn concentrations (10, 20, 50, and 100 mg/L). These solutions were designed to simulate industrial wastewater under various contamination scenarios. The design of the experiment was similar to that used industrial effluent. The experiment lasted for four weeks, with Zn concentration in the effluent measured weekly. As a control for both experiments, the wetland was used (with plants and ceramsite) in which distilled water was applied instead of wastewater. All experiments were performed in triplicate.

2.4. Sample Analysis

Prior to analysis, plants from each experimental container were harvested and thoroughly rinsed with distilled water to remove surface contaminants. Plant tissues (roots, leaves, and stems), and ceramsite were separately pulverized using a planetary ball mill (PULVERISETTE 6, Fritsch GmbH, Idar-Oberstein, Germany) equipped with agate grinding balls at 400 rpm, then dried to constant weight. For digestion, 0.5 g of plant material was treated with 5 mL concentrated HNO₃ and 2 mL H₂O₂, while 0.1 g ceramsite was digested with 3 mL HNO₃ and 1 mL HF, using a MARS 6 microwave digestion system (CEM Corporation, Stallings, NC, USA). Water samples were acid-preserved with two drops of concentrated HNO₃ prior to analysis. Zinc concentrations were quantified using an ICP-OES spectrometer (PlasmaQuant PQ 9000 Elite, Analytik Jena AG, Jena, Germany) with appropriate quality control measures, including method blanks and certified reference materials.

2.5. Data Processing

The efficiency of CWs for treatment of Zn-containing industrial effluents was evaluated by calculating the removal efficiency (RE, %) using Equation (1):

$$\mathrm{R}\mathrm{E}={\displaystyle \frac{({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}})}{{\mathrm{C}}_{\mathrm{i}}}}\times 100$$

(1)

where C_i is the initial concentration of Zn, C_f is the concentration of Zn after 7 days of retention time, and RE is the removal efficiency of each parameter after 7 days of retention time (in percentage).

Plant metal translocation capacity was assessed using the translocation factor (TF), calculated using Equation (2):

TF: C_l/C_r, C_st/C_r, and C_l/C_st

(2)

where C_l is Zn content in leaves, C_st is Zn content in leaves and C_r is Zn content in roots, in mg/kg.

A TF value of less than 1 indicates that metals are mainly immobilized in the roots, preventing their translocation to aerial parts. In contrast, a TF value greater than 1 indicates that Zn is effectively translocated from the roots to the stems and leaves [33].

2.6. Statistical Analysis

All measurements were performed in triplicate, with results expressed as mean ± standard error. Statistical significance of observed differences was determined using one-way ANOVA followed by non-parametric Wilcoxon tests (p < 0.05). Data normality was verified prior to parametric testing.

3. Results and Discussion

3.1. Zn Removal from Galvanic Industrial Effluent

Zinc removal from galvanic effluent was a rapid process, with 97–99% of the element recovered during the two-week experiment (

Table 1. Zinc concentration in industrial effluent before and after treatment, along with the removal efficiency.

Plant		1st Week		2nd Week
	Initial Zn Concentration mg/L	Zn Concentration, mg/L	RE, %	Zn Concentration, mg/L	RE, %
Phragmites australis	9.54	0.18	98.0	0.09	99.0
Typha latifolia	9.54	0.26	97.0	0.12	98.7

).

The U.S. EPA [34] recommends a maximum permissible discharge limit of 2 mg/L for Zn. Throughout the study, the Zn concentration in the effluent remained well below this regulatory limit. For comparison, approximately 65% of Zn ions were removed from the effluent at the A-01 National Pollution Discharge Elimination System outfall at the Savannah River Site (Aiken, SC) and retained in the sediments [35].

The observed decrease in Zn concentration in the effluent correlated with increased Zn accumulation in both ceramsite and plant tissues. Zn content in ceramsite increased by 1.7-fold compared to the control in experiments with Phragmites australis (Figure 1a) and 2.5-fold in experiments with Typha latifolia (Figure 1b). Zinc was mainly accumulated in plant’s roots: 430 mg/kg for Phragmites australis and 440 mg/kg for Typha latifolia.

Analysis of Zn distribution in plants indicated that Typha latifolia was more effective than Phragmites australis in accumulating metals in subaerial tissues and translocating them to aerial parts (Figure 1). In Phragmites australis, Zn accumulation was similar in stems (31.2 mg/kg) and leaves (30.6 mg/kg). In contrast, Typha latifolia exhibited significantly higher (p < 0.05) Zn accumulation in stems (130 mg/kg). This aligns with Scholes et al. [29] who showed higher accumulation of Zn by Typha latifolia compared to Phragmites australis.

No visual toxicity symptoms were observed during the experiment. The TF values for Phragmites australis were as follows: 0.06 (for stem to root system), 0.08 (for leaf to root system), and 1.2 (for leaf to stem system). In the case of Typha latifolia, the translocation of the Zn from roots to leaves was low, with a TF value of 0.3.

3.2. Zn Removal from Synthetic Effluents

The obtained results showed that the Zn concentration in wastewater used in the pilot system was not toxic for the tested plants. To evaluate the effect of higher concentrations, synthetic solutions with Zn concentrations of 10, 20, 50, and 100 mg/L were prepared.

For Typha latifolia (Figure 2a), almost complete Zn removal (99%) was achieved within two weeks at metal concentrations 10 and 20 mg/L. At higher concentrations (50 and 100 mg/L), removal efficiencies reached 98% and 95%, respectively, after four weeks (Figure 2a). For Phragmites australis, the highest removal efficiency (99%) was observed at 10 mg/L after four weeks. Removal decreased with increasing Zn concentrations: 90% (20 mg/L), 79% (50 mg/L), and 74% (100 mg/L) (Figure 2b). Constructed wetlands with gravel and straw removed up to 78% of Zn from wastewater containing 1–10 mg/L of metal [36]. In a gravel-bed constructed wetland, 97% removal was achieved at a much lower Zn concentration (0.2 mg/L) [37]. These results confirm that removal efficiency decreases with rising Zn concentrations and suggest a saturation threshold above 50 mg/L.

Zinc uptake by both plant species increased with increasing its concentrations in synthetic effluent (Figure 3). At the same time, Zn content in Typha latifolia segments was statistically higher (p < 0.05) compared to Phragmites australis. Unpolluted plants exhibited much lower Zn content in roots, stems, and leaves, i.e., 48 mg/kg, 30.8 mg/kg, and 30.6 mg/kg, respectively, for Phragmites australis and 43 mg/kg and 27.7 mg/kg, respectively, for Typha latifolia. In contaminated conditions, Zn accumulation was the highest in roots of Typha latifolia (447–4917 mg/kg), while much lower levels were determined in leaves (150–1217 mg/kg). Higher accumulation of Zn in subaerial organs of Typha latifolia compared to aerial organs was shown by [38]. A similar pattern was observed for Phragmites australis, with the highest Zn uptake in roots (481–1153 mg/kg), followed by leaves (39–221 mg/kg), and stems (36–176 mg/kg).

Numerous studies have documented predominant zinc accumulation in plant roots [37,39,40]. According to Jiang and Wang [40], zinc in Phragmites australis roots was primarily concentrated in the intercellular space, followed by the cell wall, vacuole, and cytoplasm.

The pH of effluents in the performed experiments varied from neutral to slightly alkaline. Zn²⁺ is the predominating form of Zn at a neutral pH (below 7.7) that facilitates its uptake by plants [41]. At the same time, higher Zn content in roots compared to other segments indicates plants’ strong metal extraction capability from the surrounding medium. In contrast, low Zn content in stems and leaves suggests limited translocation ability, which is considered a defence mechanism against metal toxicity. This pattern highlights plants’ phytostabilization potential [27].

The TF values for both plants support this observation (

Table 2. The translocation factor values of Zn in CWs with Phragmites australis and Typha latifolia.

Zn Concentration, mg/L	Phragmites australis			Typha latifolia
	TL/S	TSt/R	TL/R	TL/R
10	0.91	0.08	0.07	0.34
20	1.85	0.09	0.16	0.16
50	1.57	0.13	0.20	0.17
100	1.25	0.14	0.18	0.26

).

In both species, Zn transfer from roots to leaves was significantly below 1.0. Values exceeding 1.0 were only observed in the leaf–stem system of Phragmites australis. This suggests more effective translocation of Zn from stems to leaves in this species, potentially indicating a more active internal transport mechanism or species-specific detoxification strategy. In contrast, Typha latifolia appears to retain zinc primarily in the roots and lower shoot tissues, limiting its distribution to aerial parts. Similar results were reported in other studies. Thus, Naghoum et al. [28] showed low Zn translocation in Juncus effusus shoots (TF < 1) and suggested that the metal was mainly phytostabilized in the roots. The Zn translocation from roots to stems was 0.16 in Phragmites australis and 0.69 in Typha latifolia, while translocation from roots to leaves was 0.49 in Phragmites australis and 0.43 in Typha latifolia [42].

The limited translocation of Zn from roots to stems and leaves can be considered a tolerance mechanism against metal toxicity. Additional adaptive strategies include immobilization of metals in the cell wall, compartmentalization in vacuoles, metal complexation with soluble compounds (e.g., organic acids, amino acids), or binding to specific proteins [40]. Obtained results suggest that both plants sequester Zn in metabolically less active areas, maintaining low cytosolic Zn concentrations and minimizing its toxic effects [40].

The content of Zn adsorbed on ceramsite was 17-fold higher compared to the control for Typha latifolia and 8-fold higher for Phragmites australis. It is known that CWs consist of porous-filter media, microorganisms and vegetation. Since in the present study, only uptake of Zn in plants and its adsorption on ceramsite were assessed, the differences in Zn uptake can also be explained by the involvement of microbial communities in metal uptake. It is an important aspect, which needs to be investigated in future studies.

4. Conclusions

A constructed wetland technology using ceramsite as a support medium and plants (Phragmites australis and Typha latifolia) was proposed for the treatment of Zn-containing industrial effluents. The system efficiently removed zinc from real industrial effluent containing 9.54 mg Zn/L, with both plants demonstrating high metal removal efficiency (97–99%). At higher Zn concentrations in the effluent, Typha latifolia exhibited greater suitability, as reflected by its higher Zn removal from effluent (95–99%) and uptake in plant tissues. In both species, Zn was primarily accumulated in the roots, with limited translocation to stems and leaves. Constructed wetlands—particularly those planted with Typha latifolia—can be considered an effective and sustainable approach for treating industrial effluents from galvanization processes. Due to the high accumulation capacity of the plants, in further studies, it is recommended to reduce the time of treatment. A limitation of the study can be regarded as the absence of the investigation of microorganisms’ involvement in Zn uptake, which needs to be solved in further studies.