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Article

Elucidating the Potential of Vertical Flow-Constructed Wetlands Vegetated with Different Wetland Plant Species for the Remediation of Chromium-Contaminated Water

by
Fazila Younas
1,
Irshad Bibi
1,*,
Muhammad Afzal
2,
Nabeel Khan Niazi
1 and
Zubair Aslam
3
1
Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan
2
Soil and Environmental Biotechnology Division, National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad 38000, Pakistan
3
Department of Agronomy, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(9), 5230; https://doi.org/10.3390/su14095230
Submission received: 25 March 2022 / Revised: 16 April 2022 / Accepted: 23 April 2022 / Published: 26 April 2022
(This article belongs to the Special Issue Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural)
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Abstract

:
Water scarcity is one of the key global challenges affecting food safety, food security, and human health. Constructed wetlands (CWs) provide a sustainable tool to remediate wastewater. Here we explored the potential of vertical flow-CWs (VF-CWs) vegetated with ten indigenous wetland plant species to treat chromium (Cr)-contaminated water. The wetland plants were vegetated to develop VF-CWs to treat Cr-contaminated water in a batch mode. Results revealed that the Cr removal potential of VF-CWs vegetated with different wetland plants ranged from 47% to 92% at low (15 mg L−1) Cr levels and 36% to 92% at high (30 mg L−1) Cr levels, with the maximum (92%) Cr removal exhibited by VF-CWs vegetated with Leptochloa fusca. Hexavalent Cr (Cr(VI)) was reduced to trivalent Cr (Cr(III)) in treated water (96–99 %) of all VF-CWs. All the wetland plants accumulated Cr in the shoot (1.9–34 mg kg−1 dry weight (DW)), although Cr content was higher in the roots (74–698 mg kg−1 DW) than in the shoots. Brachiaria mutica showed the highest Cr accumulation in the roots and shoots (698 and 45 mg kg−1 DW, respectively), followed by Leptochloa fusca. The high Cr level significantly (p < 0.05) decreased the stress tolerance index (STI) percentage of the plant species. Our data provide strong evidence to support the application of VF-CWs vegetated with different indigenous wetland plants as a sustainable Cr-contaminated water treatment technology such as tannery wastewater.

1. Introduction

The freshwater resources are diminishing globally due to uncontrolled human consumption of clean water and untreated wastewater being discharged into water reservoirs and waterways [1,2]. As a result, the world population faces water security, food security, and food safety challenges. The industrial sector is one of the main water contamination sources that generate copious amounts of contaminants in the environment [3,4]. Among the various contaminants, chromium (Cr) is of huge concern due to its mutagenic, carcinogenic, and teratogenic effects on human health [5]. In the aquatic and terrestrial systems, Cr exists in trivalent Cr (Cr(III)) and hexavalent Cr ((Cr(VI)), of which Cr(VI) is highly toxic and mobile [6,7]. Chromium(VI) toxicity is well-reported; it can impair plant growth, cause ultrastructural modifications of the cell membrane and chloroplast, persuade chlorosis in leaves, damage root cells, reduce pigment contents, disturb water relations and mineral nutrition, and alter enzymatic activities [6]. Various methods have been employed to remediate Cr-contaminated wastewater, including chemical reduction, membrane separation, ion exchange, and adsorption [8,9]. However, all these Cr removal technologies have some disadvantages in terms of their high application and maintenance cost, secondary contamination, and difficult operational procedure [10,11,12]. Therefore, it is imperative to deploy low-cost, sustainable, eco-friendly, and effective remediation technologies to treat Cr-contaminated water such as tannery wastewater. Constructed wetlands (CWs) can provide low-cost, less energy-consuming, environmentally-friendly, and sustainable solutions for the removal and detoxification of Cr from water, and also fulfill the criteria of UN-Sustainable Development Goals 2030 [7,13,14].
Vertical flow-CW (VF-CW) is one of the types of CWs that involves the vertical flow of wastewater in bedding media. This type of CW became popular after understanding the disadvantages of other subsurface flow CW systems in terms of nitrification capacity of wastewater. Despite its denitrification efficiency, the VF-CW system is characterized by a high oxygen transfer rate and degree of removal and nitrification of organic substances [15]. Constructed wetland with vertical flow is considered an efficient treatment technology that can withstand faults and variable quality of influence and temperature changes [16]. In CWs, many mechanisms such as biosorption, uptake by microbes and plants, adsorption on bedding media, biodegradation, co-precipitation, and sedimentation occur [17,18,19]. However, it remains a challenge to select suitable wetland plant species which are useful for removing specific toxic metal ions from water [20].
A wide range of wetland plant species, such as Brachiaria decumbens, Canna indica, Iris pseudacorus, Pennisetum purpureum, Phragmites australis Juncus effusus, Schoenoplectus americanus, Typha latifolia, Cyperus rotundus, have been used in CWs to treat industrial wastewater [21]. In this regard, the most used wetland plant was P. australis for tannery wastewater treatment [22]. Various wetland plant species have different capacities for taking up various metals [23]. Therefore, the selection of wetland plant species is the most promising aspect of CWs to remove a specific metal from contaminated water [24].
Schück and Greger [25] tested thirty-four wetland plant species in CWs for their tolerance to Cd, Cu, Zn, and Pb. Their data showed a large variation in capacity and metal removal rate between the investigated plant species. The authors found that C. pseudocyperus and C. riparia were the most promising plants to remove all the four metals studied from wastewater. Both metal type and levels in wastewater affect the performance and metal removal potential of wetland plant species. For example, Zn is generally accumulated to a higher extent than Cu, followed by Cd and Pb [23]. Therefore, the selection of wetland plant species is the most promising part of CWs to remove any specific metal from contaminated water because of variation in specificity and capacity between species for metals accumulation. Hence, appropriate wetland plants may enhance the removal ability of specific metals from wastewater [24].
The objective of this study was to evaluate the performance of VF-CWs vegetated with ten indigenous wetland plant species (Phragmites australis, Leptochloa fusca, Brachiaria mutica, Typha domingensis, Canna indica, Cyperus laevigatus, Cymbopogon citratus, Cynodon dactylon, Pennisetum purpureum, and Paspalum dilatatum) for the remediation of Cr-contaminated water. The survival and growth of wetland plants exposed to Cr-contaminated water were determined. The Cr removal efficiency of VF-CWs vegetated with ten wetland plant species was evaluated, and speciation of Cr in water and plants tissues was studied. Moreover, the bioaccumulation of Cr in plants and their stress tolerance index (%) to different levels of Cr was measured.

2. Materials and Methods

2.1. Chemicals

Chemicals such as hydrogen peroxide (30% w/v) and nitric acid (assay 68–70% w/v) were used to digest plants. A solution of potassium dichromate (K2Cr2O7) was used to prepare artificial Cr-contaminated water (15 and 30 mg L−1). These two Cr levels were selected because of the average concentration of Cr in the tannery wastewater of Kasur in Punjab, Pakistan, which ranged from 15–49 mg L−1 in the tannery wastewater. All the chemicals were of analytical grade.

2.2. Experimental Setup

The wetland plant species used to develop VF-CWs were obtained from the nursery of the National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad, Pakistan. These wetland plant species were selected based on their adaptability to local climate and flooded conditions, easy accessibility, rapid growth, and tolerance to the harsh environment (e.g., extreme heat and cold). These plants are perennial grasses that pose no other environmental issues, such as invasive plants [21,26]. These ten wetland plant species (P. australis, L. fusca, B. mutica, T. domingensis, C. indica, C. laevigatus, C. citratus, C. dactylon, P. purpureum, and P. dilatatum) can grow in almost all areas of the country [7].
Cobbles, small and medium-size gravels, and sand were used to fill the VF-CWs that help vegetate the local wetland plants upon it [27]. The VF-CWs system was established in the greenhouse of the NIBGE for the treatment of Cr-contaminated water. A total of ninety similar VF-CWs mesocosms, each with a length of 3 m, width of 2.5 m, and height of 3 m, were constructed to evaluate the Cr-removal efficiency of wetland plant species (Figure 1). The total water storage capacity of each VF-CWs unit was 3 L. Each VF-CW unit was filled with a 7.62 cm layer of cobbles, a 5.08 cm layer of medium-sized gravels, and a 2.54 cm layer of small gravels and sand (from bottom to top in the same order) (Figure 1). Cobbles were used at the bottom of the CWs system near the outlet, which provides support to avoid clogging and facilitate water distribution. The total period of plant growth in CWs was from April–September 2021. Before the experiment, the wetland plants were grown and acclimatized in tap water from April–August and then exposed to Cr stress from August–September. In Pakistan, summer is long and harsh compared to winter, so we selected this duration for our study.
In the VF-CWs, the plantation process was performed based on the United States Environmental Protection Agency (USEPA) recommendation (USEPA, 1993). Wetland plants were planted with a hand-keeping initial spacing of 0.50 to 0.10 cm, and rhizome/root or stem cuttings material was placed in the porous media at a depth equal to the operational water level (Figure 1). An individual rhizome/root or stem cuttings material with a growing shoot at least 0.2 m in length was planted (Figure 2).
A few dead wetland plants were replaced with new ones during adaptation periods. The wetland plant species were planted on the VF-CWs media in triplicate and fed with tap water until it was adapted to VF-CWs. When all selected wetland plants were completely adapted to the VF-CWs, the units were irrigated with tap water (control; Cr 0 mg L−1) and Cr-contaminated water (15 and 30 mg L−1) from low to high Cr concentrations to improve the plants’ resilience under Cr stress. The system was operated in batch mode.

2.3. Maintenance and Operations of VF-CW Treatment Systems

The VF-CWs treatment systems were checked daily for maintenance to ensure proper functioning. The main reason for these inspections was to attend to the circulation and distribution of water from the outlet to the inlet zone. Daily, distilled water was supplied to all the VF-CW units to replace the reduction of water volume due to evapotranspiration [21]. Distilled water was maintaining the water level in CWs without affecting Cr levels.

2.4. Water Sampling and Analysis

Water sampling and analysis were conducted at various intervals (1, 15, and 30 days) during the experimental period (30 days). This time interval was selected on the base of plants growth. The first interval was selected after one day of Cr exposure, the second at the plants’ maximum growth stage, and the last interval before harvesting when plants started to die. The pH and redox potential (Eh) of the water were measured at the sampling site using pH (ST 300, Ohaus, Parsippany, NJ, USA) and redox meters (Model 8424, Hanna-USA), respectively. Water samples were collected from the outlet of CWs at various intervals (1, 15, and 30 days) during the experiment to determine the total Cr, Cr(VI), and Cr(III).

2.5. Morphological and Chemical Parameters of Wetland Plant Species

After one month of Cr exposure, wetland plants were carefully separated from the bedding medium of VF-CWs to observe the effect of Cr-contaminated water on the growth and biomass of all the wetland plant species. After harvesting, the wetland plants were separated into roots and shoots. The root and shoot length of all wetland plant species was measured with a measuring tape. The dry weight of root and shoot was measured after drying in an oven at 65 °C. The oven-dried wetland plant samples were powdered (<1 mm) and digested in a mixture of HNO3 and H2O2 (1:1 ratio) at 120 °C [28]. Digested samples were then filtered and stored in a refrigerator at 4 °C

2.6. Estimation of Chromium and Nutrient Elements in Roots and Shoots

Total Cr concentration in the root and shoot was determined using a flame atomic absorption spectrometer (F-AAS, Thermo-AA®, Solar Series, Waltham, MA, USA). The Cr(VI) concentration in wetland plants tissues was determined using 1,5-diphenylcarbazide method at 540 nm on a UV-Visible spectrophotometer (NovAA® 800 series, Analytik Jena, Germany) (APHA 2005). The concentration of Cr(III) was calculated by the difference between total Cr and Cr(VI) in plant tissues. The Mn, Fe, and Zn concentrations were analyzed using an F-AAS. The concentrations of Ca, Na, and K in wetland plant samples were analyzed using a flame photometer (BWB Model BWB-XP, 5 Channel Flame Photometer, Newbury, England. All the metals and nutrient elements analyses were replicated thrice, and reagent blanks were included for quality assurance.

2.7. Bioaccumulation and Translocation Factors

The wetland plant species efficiency for Cr remediation was calculated by translocation factor (TF) and bioaccumulation factor (BAF) [26].
The translocation factor provides an index of the wetland plants’ ability to transfer Cr from root to shoot (Equation (1)):
TF   = C A / C U
where CA is the concentration of Cr in the shoot (mg kg−1 dry weight (DW)) and CU is the concentration of Cr in the root (mg kg−1 DW). The TF > 1 indicates an efficient translocation of metals such as Cr from root to shoot.
The bioaccumulation factor (BAF) of Cr was determined by Equation (2):
BAF   = C P / C W
whereas CP is the concentration of Cr in wetland plant species shoot (mg kg−1 DW), and CW is the concentration of Cr in the water (mg L−1). Wetland plants with BAF > 1 are classified as bioaccumulators.

2.8. Stress Tolerance Index (STI)

The stress tolerance index is an important parameter for measuring the high biomass production and stress tolerance potential of plant species. Stress tolerance indexes for various growth parameters were calculated using the following formulae [29].
Shoot length STI (SLSTI) = (shoot length of stressed plant/shoot length of control plant) × 100
Root length STI (RLSTI) = (root length of stressed plant/root length of control plant) × 100
Shoot fresh weight STI (SFSTI) = (shoot fresh weight of stressed plant/shoot fresh weight of control plant) × 100
Root fresh weight STI (RFSTI) = (fresh root weight of stressed plant/root fresh weight of control plant) × 100
Shoot dry weight STI (SDSTI) = (shoot dry weight of stressed plant/shoot dry weight of control plant) × 100
Root dry weight STI (RDSTI) = (root dry weight of stressed plant/root dry weight of control plant) × 100

2.9. Quality Assurance and Quality Control of Chromium Analysis

Appropriate quality assurance precautions and procedures were followed to ensure data reliability and accuracy. The F-AAS was calibrated after every five readings using drift and blank reagents. After every 12 samples, a reference sample with known Cr(VI)) concentration was run on a spectrophotometer for quality control and analytical accuracy. Three reagent blanks were also included with each batch of plant samples during the acid digestion to assure quality.

2.10. Statistical Analysis

Data collected from the screening experiment was analyzed using Sigma Plot (version10). Significant statistical differences (p < 0.05) among wetland plant species for Cr removal were determined by two-way analysis of variance (ANOVA). Variation in Cr reduction efficacy among tested wetland plant species was determined by Tukey’s HSD test at p < 0.05. For Pearson correlation and principal component analysis (PCA), XLSTAT 2018 software was used [30].

3. Results and Discussion

3.1. Redox Potential and pH of the VF-CWs Medium

During the experiment, a change over time in the redox potential (Eh) and pH of the VF-CWs medium was observed depending on wetland plants species in the VF-CWs (Table S1, Supplementary Information). The CWs were mainly operating under reducing conditions with Eh values spanning in the negative region (−62 to −122 mV) in all the VF-CWs having various wetland plant species. Water pH in the CWs fed with Cr-contaminated and un-contaminated water showed some fluctuation with a trend for a slight increase (pH 7.8–8.9) at various intervals of water sampling (1, 15, and 30 days). However, a decrease (9.1 to 8.0) in the pH of the water treated by VF-CWs vegetated with B. mutica was observed at both levels (15 and 30 mg L−1) of Cr-contamination and control. It may be because the roots of these wetland plants release organic acids, which can decrease the pH of the media [31].

3.2. Chromium Removal Efficacy of VF-CWs

The concentration of total Cr, Cr(VI), and Cr(III) in the water treated by VF-CWs is shown in Table 1. The total Cr removal efficiencies of the VF-CWs spanned 37–92% and 47–92% at an initial concentration of 15 and 30 mg Cr L−1, respectively. After 30 days, at low (15 mg L−1) initial Cr concentration, L. fusca, C. laevigatus, P. australis, B. mutica, C. indica, P. purpureum, C. citratus, C. dactylon, T. domingensis, and P. dilatatum removed 92%, 88%, 80%, 75%, 61%, 53%, 53%, 50%, 48% and 47% of total Cr from the water, respectively. At high (30 mg L−1) initial Cr level, L. fusca, C. laevigatus, P. australis, B. mutica, T. domingensis, C. dactylon, C. indica, C. citratus, P. purpureum, and P. dilatatum, removed 92%, 87%, 83%, 74%, 58%, 50%, 47%, 46%, 44%, and 37% total Cr, respectively, after 30 days.

3.3. Removal of Chromium from the Water

The analysis of water samples collected from VF-CWs planted with L. fusca showed that there was the lowest (0.01 mg L−1) Cr(VI) concentration in the water with 15 mg L−1 Cr treatment (Table 1). The highest Cr(VI) concentration (2.13 mg L−1) was observed in treated water collected from VF-CWs vegetated with C. laevigatus having a high level of Cr (30 mg L−1). The water samples analysis indicated there was a significant reduction in Cr(VI) to Cr(III) (96% and 91% at low and high levels of Cr contamination, respectively) in VF-CWs planted with T. domingensis.
The Cr(VI) uptake by the plants uses necessary anionic carriers such as sulfates. In wetland plant roots, Cr(VI) can be reduced to Cr(III), which may bind with the extracellular cells of roots [32]. Also, wetland plant roots release root exudates that can decrease Cr(VI) concentration in water by making chelating compounds with organic acids [7]. The minimum Cr(VI) reduction into Cr(III) occurred in VF-CWs planted with C. indica, 40–46% at both levels of Cr. Therefore, Cr(VI) significantly (p < 0.05) reduced to Cr(III) in water samples of all VF-CWs throughout the experiment because negative Eh (reduced conditions) could provide a highly suitable environment for the reduction of Cr(VI) to Cr(III) [7].
However, at the end of the experiment, water samples analysis showed some difference in Cr speciation. Data showed maximum Cr(III) concentration (18.9 mg L−1) in VF-CWs planted with P. dilatatum with Cr at 30 mg L−1. The minimum Cr(III) concentration (1.5 mg L−1) was observed in VF-CWs planted with C. laevigatus with a high level of Cr. The lowest Cr(VI) concentrations (0.01 mg L−1) and total Cr (1.1 and 2.3 mg L−1) were observed in VF-CWs planted with L. fusca at both levels of Cr, respectively.
Table
Table 1. Total chromium (Cr), Cr(VI), and Cr(III) concentration in treated Cr-contaminated water collected from the outlet of VF-CWs planted with 10 different wetland plant species. ND: not detected. Values are presented as mean ± standard error of three replicates (n = 3).
Table 1. Total chromium (Cr), Cr(VI), and Cr(III) concentration in treated Cr-contaminated water collected from the outlet of VF-CWs planted with 10 different wetland plant species. ND: not detected. Values are presented as mean ± standard error of three replicates (n = 3).
1 Day15 Day30 Day
Wetland Plant SpeciesCr Treatment
(mg L−1)		Cr(III)Cr(VI)Total CrCr(III)Cr(VI)Total CrCr(III)Cr(VI)Total Cr
Paspalum dilatatumCr0NDNDNDNDNDNDNDNDND
Cr1511.27 ± 0.42.36 ± 0.213.63 ± 0.38.81 ± 0.80.18 ± 0.79.00 ± 0.47.79 ± 0.50.09 ± 0.027.89 ± 0.8
Cr3022.18 ± 0.413.63 ± 0.126.09 ± 0.319.78 ± 0.50.21 ± 0.720.00 ± 0.418.9 ± 0.50.04 ± 0.0318.9 ± 0.5
Phragmites australisCr0NDNDNDNDNDNDNDNDND
Cr1510.45 ± 0.91.27 ± 0.211.72 ± 0.42.68 ± 0.50.31 ± 0.63.00 ± 0.32.86 ± 0.50.03 ± 0.022.9 ± 0.5
Cr3024.82 ± 0.61.58 ± 0.226.41 ± 0.46.19 ± 0.70.86 ± 0.87.00 ± 0.34.78 ± 0.40.08 ± 0.064.87 ± 0.5
Cyperus laevigatusCr0NDNDNDNDNDNDNDNDND
Cr1510.35 ± 0.40.32 ± 0.0710.67 ± 0.41.72 ± 0.80.27 ± 0.72.00 ± 0.31.59 ± 0.40.08 ± 0.061.68 ± 0.5
Cr3014.78 ± 0.35.60 ± 0.220.39 ± 0.41.52 ± 0.63.47 ± 1.85.00 ± 0.51.50 ± 0.42.13 ± 0.13.68 ± 0.8
Typha domingensisCr0NDNDNDNDNDNDNDNDND
Cr1513.56 ± 0.30.21 ± 0.0313.78 ± 0.28.86 ± 0.40.13 ± 0.79.00 ± 0.57.58 ± 0.60.11 ± 0.057.7 ± 0.5
Cr3027.58 ± 0.40.20 ± 0.0427.79 ± 0.414.86 ± 0.40.13 ± 0.715.00 ± 0.412.2 ± 0.40.11 ± 0.0212.34 ± 0.7
Canna indicaCr0NDNDNDNDNDNDNDNDND
Cr156.00 ± 0.33.49 ± 0.39.50 ± 0.35.88 ± 0.50.12 ± 0.76.00 ± 0.55.72 ± 0.40.06 ± 0.035.78 ± 0.6
Cr3014.17 ± 0.36.41 ± 0.220.58 ± 0.316.92 ± 0.40.08 ± 0.0217.00 ± 0.415.6 ± 0.40.08 ± 0.0615.76 ± 0.4
Cymbopogon citratusCr0NDNDNDNDNDNDNDNDND
Cr157.10 ± 0.32.94 ± 0.0210.0 ± 0.27.80 ± 0.40.19 ± 0.38.00 ± 0.36.89 ± 0.70.1 ± 0.047.00 ± 0.7
Cr3019.84 ± 0.24.07 ± 0.123.92 ± 0.418.72 ± 0.50.27 ± 0.419.00 ± 0.415.8 ± 0.50.16 ± 0.116.0 ± 0.7
Leptochloa fuscaCr0NDNDNDNDNDNDNDNDND
Cr159.69 ± 0.20.15 ± 0.099.84 ± 0.32.43 ± 0.70.06 ± 0.12.50 ± 0.31.08 ± 0.80.01 ± 0.021.1 ± 0.5
Cr3016.19 ± 0.32.90 ± 0.219.09 ± 0.36.73 ± 0.50.06 ± 0.16.80 ± 0.42.2 ± 0.40.01 ± 0.052.3 ± 0.7
Cynodon dactylonCr0NDNDNDNDNDNDNDNDND
Cr1511.67 ± 0.20.16 ± 0.0811.8 ± 0.59.42 ± 0.70.08 ± 0.19.50 ± 0.37.43 ± 0.60.06 ± 0.027.5 ± 0.8
Cr3025.65 ± 0.21.67 ± 0.227.32 ± 0.716.90 ± 0.50.09 ± 0.117.00 ± 0.514.9 ± 0.40.07 ± 0.0315 ± 0.7
Brachiaria muticaCr0NDNDNDNDNDNDNDNDND
Cr1510.33 ± 0.40.33 ± 0.0810.6 ± 0.33.86 ± 0.40.13 ± 0.24.00 ± 0.23.88 ± 0.60.09 ± 0.023.98 ± 0.7
Cr3021.02 ± 0.30.48 ± 0.221.51 ± 0.49.38 ± 0.40.11 ± 0.19.50 ± 0.27.6 ± 0.70.10 ± 0.17.78 ± 0.6
Pennisetum purpureumCr0NDNDNDNDNDNDNDNDND
Cr159.83 ± 0.21.08 ± 0.210.9 ± 0.58.84 ± 0.50.06 ± 0.18.90 ± 0.26.99 ± 0.50.01 ± 0.017.00 ± 0.5
Cr3017.42 ± 0.45.28 ± 0.222.71 ± 0.416.92 ± 0.50.08 ± 0.117.00 ± 0.316.6 ± 0.80.07 ± 0.0116.7 ± 0.8
At both levels of Cr (15 and 30 mg L−1), L. fusca showed maximum Cr removal efficiency from contaminated water, which was 92% Cr removal of total Cr. The minimum removal efficiency, 47%, and 37%, of total Cr was observed in CWs planted with P. dilatatum at Cr 15 and 30 mg L−1, respectively. L. fusca and C. dactylon showed 92% and 50%removal efficiency at both levels of Cr in water, respectively. The Cr removal from water in all VF-CWs throughout the experiment may be due to plant uptake, adsorption with organic matter or root exudates, and microbial reduction [33,34,35]. In this study, most of the Cr(VI) present in the contaminated water was reduced to Cr(III). Similar findings were also observed earlier [36], where more than 90% of Cr(VI) was reduced to Cr(III) by wetland plant species.

3.4. Morphological Parameters of Wetland Plants

Results revealed that Cr-contamination significantly (p < 0.05) reduced the length of root and shoot dry and fresh weight of all the wetland plants compared to plants grown in uncontaminated water (control) (Table 2). There were some visible toxicity symptoms, including leaves necrosis, drying, and shedding in C. indica, T. domingensis, C. citratus, C. dactylon, P. purpureum, and P. dilatatum. Among all the wetland plant species, L. fusca, C. laevigatus, P. australis, and B. mutica exhibited the highest survival at both levels of Cr without any significant growth difference compared to control.
The determination of dry and fresh biomass at the end of the experiment showed that the fresh weight of C. indica shoots was higher at 15 mg L−1 Cr level (25.4 g), but a significant (p < 0.05) reduction (12.1 g) occurred at a high Cr level (30 mg L−1). The highest reduction of dry root biomass was observed in C. dactylon (85.6%) and the lowest in L. fusca (7.4%) at a high level of Cr compared to control. The lowest reduction (11.4%) of shoot weight was observed in L. fusca at 30 mg L−1 of Cr. Some wetland plants under these conditions showed a continuous survival at both levels of Cr without significant reduction in growth, which is a crucial parameter of phytoremediation ability. In an earlier study, it was observed that L. fusca could tolerate a high level of Cr (247 mg L−1) without any significant change in its growth compared to control treatment [27]. Chromium affects various processes in plants, such as seed germination, plant growth, and biomass production, as well as various plant physiological processes such as photosynthesis reduction and oxidative and nutrient imbalance [37]. In this study, the growth behavior and morphological parameters of wetland plants revealed differences among plant species exposed to Cr-contaminated water. This difference in growth parameters of wetland plant species over time may be due to their intrinsic nature [33].
In this study, the use of L. fusca, C. laevigatus, B. mutica, and P. australis in VF-CWs to treat Cr-contaminated water proved better in terms of rapid plant growth and Cr removal ability than other plant species due to their high Cr removal efficiency [38,39]. P. dilatatum, C. indica, and C. citratus were less active in removing Cr than other plant species because of their slow growth and low biomass yield at a high level of Cr-contamination. This study revealed that Cr reduction from contaminated water in VF-CWs vegetated with L. fusca and B. mutica was attributed to their extensive plant root growth, which served as an active zone for inorganic contaminants sequestration by microbial population. Wetland plant species having extensive root growth in Cr-contaminated water are more active in removing contaminants, as observed in earlier studies [40,41,42,43]. In the CWs, the extensive root system of wetland plant species enhances the oxygen transfer efficiency, which increases the contribution of microbial biomass towards contaminant degradation and treatment [7,44].

3.5. Phytoaccumulation of Cr in Wetland Plants

In this study, there was a significant difference in Cr accumulation in the shoots and roots of each wetland plant species (Figure 3). Considering all the wetland plants, B. mutica showed the highest total Cr accumulation in the roots and shoots (698 and 45 mg kg−1 DW, respectively), followed by L. fusca at both levels of Cr (Figure 3b). This may be due to their rapid growth and extensive root system in the CWs fed with Cr-contaminated water (Table 2). The lowest Cr accumulation was observed in the roots and shoots of C. dactylon (115 and 4.46 mg kg−1 DW, respectively) compared to other plants in VF-CWs.
Among all the wetland plant species, B. mutica showed the highest Cr(III) accumulation in the root (297 and 693 mg kg−1 DW) and shoot (32 and 41 mg kg−1 DW) at 15 and 30 mg L−1 Cr levels, respectively, followed by L. fusca (Figure 3c,d; Table 3). The lowest Cr(VI) concentration was observed in the roots of P. dilatatum (1.19 mg kg−1 DW) at a Cr level of 15 mg L−1, and the highest Cr(VI) accumulation was found in the shoot of C. citratus (6.31 mg kg−1 DW) at 30 mg L−1 of Cr (Figure 3). Tadese and Seyoum (2015) reported that up to 83% of Cr taken up by wetland plants remained in root cells. In this study, Cr accumulation in the roots is consistent with prior research showing that wetland plants accumulated various metals mainly in their root tissue followed by the shoot [26,45]. This is because of the adsorption of Cr at the extracellular negatively charged sites (e.g., -COO-) of the cell walls of the roots. So, the immobilization of Cr likely occurred in the vacuoles of root cells [46].

3.6. Bioaccumulation Factor (BAF) and Translocation Factor (TF)

In this study, the BAF of the wetland plants significantly (p < 0.05) decreased (0.5–41%) with the increase in applied Cr levels (0, 15, and 30 mg L−1) in the water, except for B. mutica that possessed 11% greater BAF at high Cr level (30 mg L−1) compared to control (Table S2, Supplementary Information). L. fusca showed significantly (p < 0.05) higher BAF (29) at a low level of Cr, while B. mutica had significantly (p < 0.05) higher BAF (24) at 30 mg L−1 Cr. At both Cr levels, C. dactylon and P. dilatatum possessed the lowest BAF (3.9–5.1 and 1.3–5.2, respectively). Wetland plants with a BAF greater than 1.0 are considered bioaccumulators [47,48]. Therefore, the wetland plants, L. fusca and B. mutica, having BAF higher than 20, are the best candidates for accumulating Cr from contaminated water.
At both levels of Cr-contamination, all the wetland plant species had varying TF (0.02–0.11) (Table S2, Supplementary Information). At a high level of Cr, C. indica showed the highest TF (0.08). Phytoremediation of metals, including Cr, differs between the plant species types [49]. Some wetland plants have TF > 1 and show high Cr translocation from the root to the shoots of the plants. According to an earlier study [50], high TF of heavy metals, including Cr, indicated metals phytoextraction. The TF of Cr in some wetland plants had values < 1, indicating less translocation from roots to shoots (Table 3). Contaminants translocation from roots to aerial parts of plants is based on the contaminants types, wetland plant species, and environmental conditions [51].
The Cr partitioning in plants depends on how plants control and manage contaminants in the roots to prevent hazardous effects on the site of photosynthesis, leaves, and other metabolic activities [52]. Moreover, Cr(III) is not an essential element for the plants, so the plants did not develop any specific mechanisms to translocate Cr(III) from root to shoot [53].

3.7. Chromium Tolerance of the Wetland Plant Species

In this study, L. fusca, C. laevigatus, P. australis, and B. mutica did not show visible toxicity symptoms compared to other wetland plants. However, in C. indica, T. domingensis, C. citratus, C. dactylon, P. purpureum, and P. dilatatum, leaves and some stem parts died at a high concentration of Cr (30 mg L−1). The high Cr level significantly (p < 0.05) decreased the stress tolerance index (STI; %) of wetland plant species (Table S3, Supplementary Information).
The lowest value (33%) of shoot length stress tolerance index (SLSTI) was recorded at 30 mg L−1 for C. indica and the highest value (98%) at 15 mg L−1 for L. fusca. Similarly, the highest RLSTI (98%) was measured for B. mutica at 15 mg L−1 and the lowest (33%) for C. citratus at 30 mg L−1. The SFSTI and RFSTI of all wetland plant species significantly (p < 0.05) decreased as the Cr level increased in VF-CWs. The maximum SDSTI value (97%) was recorded for C. laevigatus at 15 mg L−1 and the minimum (36%) for P. dilatatum at 30 mg L−1. Similarly, the RDSTI of all wetland plant species decreased as Cr concentration increased in VF-CWs. The excessive metals translocation into old leaves before their shedding and detoxification by plant roots may also be considered a mechanism of plant tolerance to metals [54]. A study on stress tolerance revealed that the tolerance mechanism helps plants maintain growth even in the presence of toxic metals concentration [29].

3.8. Mineral Nutrients Content in Root and Shoot of Wetland Plants

Various mineral nutrients (Fe, Zn, Mn, Ca, K and Na) were also analyzed in all wetland plant species (Figures S1 and S2, Supplementary Information). Across all plant species, shoots nutrient elements data indicated that Fe, Zn, Mn, Ca, K, and Na contents decreased significantly (p < 0.05) (20–100%, 19–100%, 1.4–100%, 5.5–75%, 5.4–55%, and 22–100%, respectively) compared to their controls. In the case of root nutrient concentration of plants, Fe, Zn, Mn, Ca, K, and Na contents decreased significantly (p < 0.05) (31–100%, 20–100%, 34–100%, 7.5–67%, 1.4–56%, and 27–100%) at both Cr levels compared to controls.
The nutrient concentration (Fe, Zn, Mn, Ca, K, and Na) in root and shoot of all the wetland plants decreased with an increase in Cr level from 15 to 30 mg/L (Figures S1 and S2, Supplementary Information). Wetland plants do not have any specific mechanism for Cr uptake, so plants uptake Cr using different pathways. The uptake of Cr is through passive and active mechanisms performed by carriers to uptake various essential elements like sulfate [55]. Due to the structural similarity of Cr(VI) with sulfate and phosphate, its uptake by roots involves sulfate or phosphate transporters that interfere with the nutrients uptake and translocation mechanisms of plants [6]. The gradual decrease in the essential nutrients uptake or translocation could be due to competitive binding Cr potential with carrier channels and reduced plasma membrane H+ATPase activity [56]. Chromium exposure may remove the essential nutrients from the binding sites in the plant body. However, Cr is reported to play an antagonistic role in the translocation and uptake of essential nutrients [57].
Chromium also competes with S, P, and Fe for carrier binding during uptake and reduces the uptake of essential elements [37,58]. Due to the similar ionic form, Cr(VI) also inhibits the absorption of certain essential elements such as Fe, K, Mg, Mn, P, and Ca by plants and decreases their uptake [59]. High Cr concentration reduced the uptake of essential minerals like Fe, Mg, P, and Ca by binding on the sorption sites and forming insoluble complexes with minerals [37].

3.9. Principal Component Analysis (PCA)

Recently, multivariate analysis has attained major importance in finding out possible relationships and trends among data variables [60]. The principal component analysis considers the correlation and variance of various response attributes concerning inputs [61]. In this experiment, multivariate analysis divided response variables (wetland plants) into various groups for Cr remediation (Figure S3, Supplementary Information). The metal concentration in wetland plants was grouped to explain a similar trend in response to Cr stress. However, total Cr content was grouped with their species, explaining an enhancement in their activities with Cr treatment. The other variables’ responses (pH and Eh) were not grouped in the PCA because of their different responses concerning Cr treatments. However, in roots and shoots, Cr contents clustered together, which may be due to their similar effects under Cr stress. Many previous studies have also reported the PCA correlation to trace various metal concentrations in plant parts [62,63,64]. The relationship between various wetland plant response attributes and applied Cr contamination was found in some attributes using the Pearson correlation matrix (Table S4, Supplementary Information).
According to multivariate analysis, the attributes which combined provided a moderate to a strong linear correlation relationship. This evaluation showed that the overall impact of various Cr treatments on wetland plant responses varied from each other and from control treatment. This also showed that under specific conditions, other simple statistical analyses might not explain any significant difference between various treatments. Still, the PCA and correlation analysis separates them depending on their overall effect on various response attributes. Therefore, this analysis may be preferred over other statistical analyses under specific conditions where huge variation in the data set and complicity in treatments do not show clear linear trends.

3.10. Significance in Environmental Risk Reduction of Chromium

The use of Cr-contaminated wastewater has raised health and environmental concerns. Specifically, these concerns relate to treating Cr-contaminated water using CWs because of its cost-effectiveness and simplicity of operation. The utilization of contaminated water has increased because of the global water scarcity, and around half of the world population is likely to experience water stress by 2030 [65]. More than 70% of water in the world is used for agricultural irrigation purposes. Therefore, the application of treated wastewater for irrigation has a huge potential to relieve water resources pressure. Our study showed that the wetland plant species could reduce the environmental risk of Cr on land and irrigation water in crops by reducing the concentration of contaminated water. Among the plant species studied, L. fusca has the highest ability to reduce total Cr concentration from 30 mg L−1 to 2.3 mg L−1 after one month of Cr stress (Table 1). If used for more than one month, Cr levels would likely reduce and meet the criteria of national and international agencies’ recommended limits for the safe reuse of Cr-contaminated water for irrigation purposes.
L. fusca reduced Cr(VI) concentration to 0.01 mg L−1, which is less than the safe limit of Cr(VI) in wastewater set by WHO, USEPA, and NEQS (0.05, 0.05, and 0.25 mg L−1, respectively) (Table S5, Supplementary Information) [21,66,67]. In this study, L. fusca removed 92% of Cr from water within 30 days of Cr exposure. Therefore, the data in this study indicated that the use of the wetland plants, especially L. fusca, for a long period (about 33 days), may remove 100% of Cr and make the Cr-contaminated water safe for agriculture purposes and reduce the environmental risk of Cr accumulation in soil, water, food, and humans.

3.11. The Fate of Wetland Plants Used for Cr Remediation

The challenge of proper management and disposal of wetland plants biomass used in CWs for the Cr remediation is an important aspect that should be considered with caution because its mishandling may cause secondary contamination. As a result, the plant waste material with high total metals content will generally need to be disposed of in a confined and controlled manner [68]. The other alternative is the incineration of used wetland plants and disposal of the ash safely in specialized dumps or may be used for Cr recovery and reuse in the relevant industrial sector. The incineration is feasible, environmentally sound, and economically acceptable, and can be used to reduce the secondary pollution burden from Cr-containing plant biomass [26].

4. Conclusions

At low Cr levels, the wetland plant species removed 47% to 92% of total Cr from the water, and at a high Cr level, it spanned 36% to 92%, with the maximum Cr removal (92%) potential of L. fusca. At both levels of Cr, the BAF of B. mutica (22 and 24, respectively) and L. fusca (29 and 22, respectively) were significantly higher than the other wetland plants. The TF of Cr in the studies of wetland plants had values < 1 suggesting lower translocation from roots to shoots. High Cr level significantly (p < 0.05) decreased the STI of all the wetland plants, with the maximum STI obtained for L. fusca (97%). Leptochloa fusca showed the highest Cr removal (92%) coupled reduction of Cr(VI) to Cr(III), followed by Brachiaria mutica. The findings of this study show that VF-CWs with suitable plant species (especially L. fusca) is a suitable option for the remediation of Cr-contaminated water.
Future research should be focused on understanding the secondary pollution risk of wetland plants after harvesting. There is a need to assess the life cycle of VF-CWs to ensure the total time required of VF-CW systems and investigate the life cycle costs of CWs on a large scale for wastewater treatment. The emission of greenhouse gases during Cr-contaminated wastewater treatment in VF-CWs needs to be monitored, and how to overcome this problem for the sustainable use of CWs under changing climate situations must be determined. Also, future research on engineering parameters (hydraulic residence time and flow rate) should be considered when testing the Cr removal efficiency of CWs.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su14095230/s1. Table S1: Effect of chromium treatments on pH and redox potential of water in constructed wetlands planted with ten wetland plant species. Values are presented as mean ± standard deviation of three replicates (n = 3). Table S2: Chromium bioaccumulation factor (BAF) and translocation factor (TF) of wetland plant species. Table S3: Stress tolerance index (%) of wetland plant species growing in CWs fed with Cr-contaminated water. Table S4: Pearson correlation relation of various attributes. Table S5: Comparison of Cr concentration present in CWs vegetated with ten wetland plant species and its safe limits given by various national and international agencies for environmental risk of Cr. Figure S1: Iron, Na, and K concentrations (a, c, e) root, and (b, d, f) shoots of ten wetland plant species grown in VF-CWs containing Cr-contaminated water (Cr 0, 15, and 30 mg L-1 in three replicates). Error bars show ± S.E of means of three replicates (n = 3). Similar bars (filled with similar color) labeled with different alphabets are significantly different from each other (Tukey’s test; p < 0.05). Figure S2: Calcium, Mn, and Zn concentrations in (a, c, e) roots, and (b, d, f) shoots of ten wetland plant species grown in VF-CWs containing Cr-contaminated water (Cr 0, 15, and 30 mg/L in three replicates). Error bars show ± S.E of means of three replicates (n = 3). Similar bars (filled with similar color) labeled with different alphabet letters are significantly different from each other (Tukey; p < 0.05). Figure S3: Principle component analysis of water and pant samples taken from CWs vegetated with ten wetland plant species.

Author Contributions

Conceptualization, reading, editing and finalizing the paper: I.B., N.K.N., M.A. and F.Y.; Experimental work: F.Y. under supervision of I.B., M.A. and N.K.N.; Funds and Resources, I.B. and N.K.N.; Z.A. read and improved the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Higher Education Commission (Project Nos. 6425/Punjab/NRPU/R&D/HEC/2016 and 6396/Punjab/NRPU/R&D/HEC/2016), Pakistan.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are thankful to Higher Education Commission (Project Nos. 6425/Punjab/NRPU/R&D/HEC/2016 and 6396/Punjab/NRPU/R&D/HEC/2016), Pakistan, for providing financial support. Irshad Bibi acknowledges the support from COMSTEQ-TWAS research grant 2018 (18-268 RG/EAS/AS_C).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Layout of vertical flow constructed wetlands used in this study.
Figure 1. Layout of vertical flow constructed wetlands used in this study.
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Figure 2. Schematic diagram of VF-CWs and illustration of mechanisms involved in removal of chromium (Cr) from wastewater (modified from Younas et al. [7]).
Figure 2. Schematic diagram of VF-CWs and illustration of mechanisms involved in removal of chromium (Cr) from wastewater (modified from Younas et al. [7]).
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Figure 3. (a,c,e) Total chromium (Cr), Cr(III) and Cr(VI) concentrations in roots and shoots (T Cr: (b), Cr(III): (d), Cr(VI): (f)) of ten various wetland plant species grown in vertical flow-constructed wetlands (VF-CWs) containing Cr-contaminated water (0, 15 and 30 mg L−1). Error bars show ± standard error of mean of three replicates (n = 3). Similar bars (filled with similar color) labeled with different alphabets are significantly different from each other (Tukey’s test; p < 0.05).
Figure 3. (a,c,e) Total chromium (Cr), Cr(III) and Cr(VI) concentrations in roots and shoots (T Cr: (b), Cr(III): (d), Cr(VI): (f)) of ten various wetland plant species grown in vertical flow-constructed wetlands (VF-CWs) containing Cr-contaminated water (0, 15 and 30 mg L−1). Error bars show ± standard error of mean of three replicates (n = 3). Similar bars (filled with similar color) labeled with different alphabets are significantly different from each other (Tukey’s test; p < 0.05).
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Table
Table 2. Impact of Cr treatments on morphological parameters of ten wetland plant species planted in vertical flow-constructed wetlands (VF-CWs). Values are presented as mean ± standard deviation of three replicates (n = 3). Values with different alphabets are significantly different from each other (Tukey’s test at p < 0.05).
Table 2. Impact of Cr treatments on morphological parameters of ten wetland plant species planted in vertical flow-constructed wetlands (VF-CWs). Values are presented as mean ± standard deviation of three replicates (n = 3). Values with different alphabets are significantly different from each other (Tukey’s test at p < 0.05).
Wetland Plant SpeciesCr Treatments
(mg/L)		Shoot Length (cm)Root Length (cm)Shoot Fresh Weight (g)Shoot Dry Weight (g)Root Fresh Weight (g)Root Dry Weight (g)
Paspalum dilatatumCr027 ± 1 gk30 ± 1 abc5.32 ± 0.5 klm0.56 ± 0.05 jk15.79 ± 1.6 fg2.09 ± 0.2 i–n
Cr1522 ± 1 ijk25 ± 1 d–g4.56 ± 0.4 k–n0.25 ± 0.02 kl8.61 ± 0.9 i–l1.82 ± 0.2 j–m
Cr3019 ± 1 jk17 ± 1 h–k1.14 ± 0.1 n0.20 ± 0.02 kl4.07 ± 0.1 m0.76 ± 0.1 lmn
Phragmites australisCr086 ± 3 ab20 ± 1 f–i14.45 ± 1.3 def1.24 ± 0.1 ghi19.74 ± 2.0 efg3.52 ± 0.3 f–j
Cr1578 ± 2 ab17 ± 1 g–j11.79 ± 1.0 fgh0.94 ± 0.06 hij14.35 ± 1.4 ghi2.99 ± 0.3 g–k
Cr3073 ± 2 bc17 ± 1 h–k9.41 ± 1.8 g–j0.75 ± 0.08 ij13.33 ± 0.9 hij2.53 ± 0.2 h–k
Cyperus laevigatusCr091 ± 3 a35 ± 1 ab9.52 ± 2.5 b1.09 ± 0.09 ghi27.27 ± 2.7 abc16.99 ± 1.0 a
Cr1581 ± 2 ab35 ± 1 abc8.39 ± 1.7 c1.05 ± 0.1 ghi24.76 ± 2.5 b–e14.59 ± 0.4 b
Cr3078 ± 2 ab35 ± 1 bcd6.16 ± 1.4 cde0.81 ± 0.1 ij21.63 ± 2.4 b–e13.98 ± 1.6 b
Typha domingensisCr055 ± 1 de20 ± 1 f–i6.46 ± 0.6 i–l2.54 ± 0.2 ab22.79 ± 2.3 cde5.17 ± 0.5 ef
Cr1547 ± 2 d–g20 ± 2 g–j6.27 ± 0.5 i–l2.40 ± 0.2 abc22.25 ± 2.2 cde4.45 ± 0.4 fg
Cr3040 ± 2 d–h17 ± 1 g–j5.70 ± 0.5 j–m2.08 ± 0.1 cd18.17 ± 2.1 def4.25 ± 0.9 f–i
Canna indicaCr058 ± 2 cd10 ± 1 l–o39.92 ± 3.5 a1.39 ± 0.1 fj26.91 ± 2.7 a–d2.22 ± 0.2 h–l
Cr1543 ± 1 d–g7 ± 1 mno29.47 ± 2.6 b1.14 ± 0.1 ghi19.74 ± 2.0 efg1.96 ± 0.2 i–n
Cr3019 ± 1 f–j7 ± 1 no12.12 ± 1.5 cd1.12 ± 0.1 ghi8.35 ± 1.4 ghi1.13 ± 0.2 i–n
Cymbopogon citratusCr043 ± 1 d–h12 ± 1 j–m8.36 ± 0.7 h–k2.31 ± 0.2 bc8.97 ± 0.9 h–k4.87 ± 0.4 f
Cr1525 ± 2 h–k7 ± 1 no5.32 ± 0.5 klm1.88 ± 0.1 de6.46 ± 0.6 j–m2.19 ± 0.2 h–l
Cr3025 ± 1 i–k5 ± 1 o2.28 ± 0.2 mn0.96 ± 0.08 hi2.87 ± 0.3 lm1.57 ± 0.4 k–n
Leptochloa fuscaCr091 ± 3 a27 ± 1 cde13.31 ± 1.2 d–g1.85 ± 0.07 a26.91 ± 2.7 a–d7.16 ± 0.6 cd
Cr1588 ± 3 ab25 ± 1 c–f13.13 ± 0.7 ghi1.76 ± 0.09 ef23.92 ± 2.4 b–e6.75 ± 0.6 de
Cr3086 ± 3 ab22 ± 1 e–h12.60 ± 0.9 ijk1.64 ± 0.03 fgh22.73 ± 2.3 cde6.63 ± 0.6 de
Cynodon dactylonCr038 ± 1 e–i17 ± 1 g–j4.94 ± 0.4 k–n0.14 ± 0.01 l14.71 ± 1.5 gh3.84 ± 0.3 fgh
Cr1525 ± 1 h–k15 ± 1 i–l2.94 ± 0.4 k–n0.06 ± 0.01 l4.31 ± 0.4 j–m2.25 ± 0.2 h–l
Cr3025 ± 1 i–k15 ± 1 i–l2.66 ± 0.2 lmn0.08 ± 0.01 l3.23 ± 0.3 klm0.55 ± 0.05 lmn
Brachiaria muticaCr083 ± 2 ab35 ± 1 a16.92 ± 1.5 cd1.77 ± 0.1 de32.96 ± 3.3 a8.62 ± 0.8 c
Cr1581 ± 2 ab35 ± 1 ab16.25 ± 1.2 def1.38 ± 0.1 fg30.86 ±3.1 a8.16 ± 0.7 cd
Cr3081 ± 2 ab33 ± 1 ab15.73 ± 1.1 efg1.30 ± 0.1 fgh28.98 ± 2.9 ab7.59 ± 0.7 cd
Pennisetum purpureumCr045 ± 1 def20 ± 1 f–i7.60 ± 0.7 ijk0.14 ± 0.01 l2.51 ± 0.3 m0.64 ± 0.1 lmn
Cr1520 ± 1 jk15 ± 1 i–l5.70 ± 0.5 j–m0.12 ± 0.01 l2.15 ± 0.2 m0.29 ± 0.03 mn
Cr3015 ± 1 k10 ± 1 k–n2.70 ± 0.5 j–m0.07 ± 0.01 l0.97 ± 0.2 m0.45 ± 0.04 n
Table
Table 3. Root and shoot uptake of chromium (Cr) by wetland plant species grown in vertical flow-constructed wetlands (VF-CWs).
Table 3. Root and shoot uptake of chromium (Cr) by wetland plant species grown in vertical flow-constructed wetlands (VF-CWs).
Wetland PlantsCr Treatment (mg/L)Shoot Cr Uptake (mg/CW)Root Cr Uptake (mg/CW)
Paspalum dilatatumCr300.002 ± 0.0010.10 ± 0.01
Phragmites australisCr300.023 ± 0.0071.17 ± 0.04
Cyperus laevigatusCr300.010 ± 0.0032.49 ± 0.09
Typha domingensisCr300.026 ± 0.0080.74 ± 0.02
Canna indicaCr300.009 ± 0.0020.19 ± 0.07
Cymbopogon citratusCr300.010 ± 0.0030.29 ± 0.01
Leptochloa fuscaCr300.056 ± 0.0174.26 ± 0.15
Cynodon dactylonCr300.0003 ± 0.00010.06 ± 0.002
Brachiaria muticaCr300.058 ± 0.0175.29 ± 0.19
Pennisetum purpureumCr300.0007 ± 0.0020.06 ± 0.002
CW; Constructed wetland, Cr30: 30 mg L−1.
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Younas, F.; Bibi, I.; Afzal, M.; Niazi, N.K.; Aslam, Z. Elucidating the Potential of Vertical Flow-Constructed Wetlands Vegetated with Different Wetland Plant Species for the Remediation of Chromium-Contaminated Water. Sustainability 2022, 14, 5230. https://doi.org/10.3390/su14095230

AMA Style

Younas F, Bibi I, Afzal M, Niazi NK, Aslam Z. Elucidating the Potential of Vertical Flow-Constructed Wetlands Vegetated with Different Wetland Plant Species for the Remediation of Chromium-Contaminated Water. Sustainability. 2022; 14(9):5230. https://doi.org/10.3390/su14095230

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Younas, Fazila, Irshad Bibi, Muhammad Afzal, Nabeel Khan Niazi, and Zubair Aslam. 2022. "Elucidating the Potential of Vertical Flow-Constructed Wetlands Vegetated with Different Wetland Plant Species for the Remediation of Chromium-Contaminated Water" Sustainability 14, no. 9: 5230. https://doi.org/10.3390/su14095230

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