Ornamental Plant Growth in Different Culture Conditions and Fluoride and Chloride Removals with Constructed Wetlands

Abstract

Natural water resources often contain fluorides and chlorides due to wastewater discharge; however, excessive exposure to fluorides can pose health risks to humans. Elevated chloride levels can negatively affect aquatic fauna and disrupt the reproductive rates of plants. This study assessed constructed wetlands (CWs) featuring monocultures (including Canna hybrid, Alpinia purpurata, and Hedychium coronarium) and polycultures (combinations of species from the monoculture systems) of ornamental plants (OPs) to evaluate their efficiency in removing fluorides and chlorides. The results revealed that the ornamental plants flourished in the CW conditions without sustaining any physical damage. C. hybrid demonstrated the longest roots and the highest volume, as well as greater height compared to other species. However, this did not affect the ion removal efficiency. In polyculture systems, 42.2 ± 8.8% of fluoride was removed, a result that was not significantly different (p > 0.05) from the removal rates observed in monocultures of C. hybrid (42.5 ± 7.5%), H. coronarium (36.8 ± 7.0%), or A. purpurata (30.7 ± 7.9%). For chloride, a similar pattern emerged, with 32.4 ± 4.8% removed in constructed wetlands (CWs) using a polyculture of ornamental plants, a figure that was also not significantly different (p > 0.05) from the removal percentages in monocultures of C. hybrid (29.1 ± 5.3%), H. coronarium (28.1 ± 5.0%), or A. purpurata (32.0 ± 5.7%). Our results indicate that CWs with polyculture species contribute to pollutant removal at levels comparable to those found in monoculture systems. However, polyculture systems offer enhanced aesthetic appeal and biodiversity, incorporating various ornamental flowering plants. The use of this eco-technology for removing fluoride and chloride pollutants helps prevent river contamination and associated health issues.

1. Introduction

Fluoride and chloride are recognized as contaminants in drinking water by the World Health Organization (WHO) [1]. Fluoride is a naturally occurring element that is excreted through urine and feces, where it combines with wastewater, chlorides, and other pollutants. While fluoride is beneficial at optimal levels for the mineralization of bones and teeth [2], water fluoridation is employed in certain regions to help reduce tooth decay within the general population. The U.S. Food and Drug Administration (FDA) [3] stipulates that fluoride concentrations should not exceed 0.7 mg/L to effectively prevent tooth decay and minimize the risk of dental fluorosis. However, elevated fluoride levels can negatively impact tooth enamel, potentially leading to mild dental fluorosis at concentrations ranging from 0.8 to 1.7 mg/L. In extreme cases, excessive fluoride in drinking water—above 4 mg/L—can lead to paralysis and premature aging [4]. Recent research indicates that chronic fluoride toxicity can lead to negative health effects, including increased lipid peroxidation and myocardial damage [5,6,7].

Conversely, chloride in wastewater primarily originates from natural sources, such as domestic sewage and industrial effluents. At concentrations of 40 mg/L or higher, the impact of chloride ions (Cl⁻) in irrigation water on plant height and leaf count is detrimental [8]. Additionally, guidelines suggest that chloride concentrations nearing 100 mg/L can have toxic effects on plant growth [9,10].

According to the United Nations Organization (UNO), only 52% of wastewater is treated globally [11]. Rural areas are often overlooked in the implementation of wastewater treatment facilities, leading to significant hazards for residents, as untreated wastewater is frequently discharged into soils and rivers. Downstream water sources are then used for everyday activities and agricultural irrigation. Reports have indicated high concentrations of fluoride [12,13,14] and chloride [15,16] in both surface and subsurface water. Hence, investigating the removal of these contaminants through eco-technologies, such as constructed wetlands (CWs), is essential, especially in rural communities where conventional wastewater treatment plants are absent.

CWs can be categorized into free water surface (FWS) systems or subsurface constructed wetlands (SS-CWs), with HF-CWs being the most commonly utilized type [17,18,19,20,21]. These systems operate with simplicity, requiring low maintenance costs and minimal energy consumption from nonrenewable sources while demonstrating high efficiency in pollutant removal [22]. CWs have been extensively implemented worldwide to remove organic matter, nitrogen and phosphorus [23,24], and pathogens from wastewater [25]. There are few studies on removing fluoride and chloride [26,27,28], and even fewer studies using CWs with ornamental plants.

This study examines the use of constructed wetlands (CWs) with both monoculture and polyculture ornamental plants to remove fluoride and chloride from domestic wastewater generated by home CWs. The specific objectives of the study are as follows: (1) evaluate the growth characteristics of ornamental vegetation, (2) compare the fluoride and chloride removal performance between horizontal-flow CWs planted with ornamental plants, and (3) assess the effects of fluoride and chloride removal between monoculture and polyculture vegetation.

Based on these objectives, we developed the following hypotheses: 1. We expect that the growth characteristics of the ornamental plants will adapt to the conditions of the CWs and that the presence of contaminants will not adversely affect their growth. 2. Ornamental plants in horizontal-flow CWs will effectively remove fluoride and chloride ions. 3. Polyculture species in CWs will remove more fluoride and chloride ions than monoculture species.

2. Materials and Methods

2.1. Horizontal-Flow Domiciliary Constructed Wetland Design and Operation

This study was conducted in San José Pastorias, Actopan, located in the state of Veracruz, Mexico (coordinates: −96°57′08″ W and 19°55′83″ S). Eight constructed wetlands (CWs) were built with dimensions of 1.5 m in length, 0.23 m in width, and 0.6 m in depth. These CWs were filled with porous river rock (PR) sourced from the riverine area of the local river, which has an average porosity of 50% (see Figure 1). To prevent clogging, each CW was layered from bottom to top, starting with a 10 cm layer of PR with a diameter of 12 cm, followed by a 40 cm layer of a porous medium with an average diameter of 1–4 cm. The water level within the CWs was maintained 10 cm below the surface.

Among the eight CWs, two were constructed using monoculture species, with six plants of either Canna hybrid L.H. Bailey (CH), two plants of Alpinia purpurata K. Schum (AP), or two plants of Hedychium coronarium J. König (HC). Additionally, two mesocosms were categorized as polyculture CWs, incorporating two species of CH, two plants of AP, and two species of HC. To facilitate species adaptation, tap water was used for irrigation one month prior to the investigation instead of wastewater. The wastewater collected from a single-family residence for the investigation was stored in a 1100 L settling tank. This tank was positioned 0.35 m above the mesocosms to allow for gravity feeding. All constructed wetlands (CWs) had a hydraulic retention time of three days. The dimensions of each CW were definitely created on the available budget, the average household volume of wastewater produced, the available space in the back yard, and to ensure there was at least one replicate for each experiment (100 L of real volume of water in every CW).

2.2. Fluoride and Chloride Measurements

Fluoride and chloride analyses were performed using a Dionex ion chromatography system (Dionex, Sunnyvale, CA, USA) (for detailed methodological characteristics, refer to Marín-Muñiz et al. [21]). The percentage of contaminant removal (%R) was calculated using the formula [19,20]: %R = ((C1 − C2)/C1) × 100%, where C1 and C2 represent the average influent and effluent pollutant concentrations, respectively (mg/L).

2.3. Plant Growth

Data on plant height, stem diameter, and root length were collected every fifteen days. During planting, 2 random plant samples of every species were harvested and their height were recorded. These plants were rinsed and separated into roots and areal biomass, and dried at 40 °C to a constant weight within 0.001 g accuracy. At the end of the experiment, all plants were harvested and separated individually into roots, stems, and leaves; they were then washed and dried to a constant weight at 40 °C. The root volume for each species was measured using a water displacement method [29]. Initially, excess water on the surface of the washed roots was absorbed. The roots were then submerged in a container filled with water, equipped with an overflow pipe. Consequently, the volume of the roots was determined by the amount of water that overflowed. Data of

Table 1. Growth features of ornamental vegetation.

Ornamental Plant	Maximum Root Length (cm)	Maximum Root Volume (cm3)	Maximum Height (cm)	Wilting Degree a (# Plants)	D-P b	RGR (gg−1d−1)
C. hybrids	93.0 ±14.2 a	551 ± 33.2 a	59.2 ±16.2 a	w1	d1	0.009 ± 0.002 a
H. coronarium	49.0 ± 6.6 b	81 ± 15.08 b	40.8 ± 3.2 b	w2 (1)	d1	0.004 ± 0.002 b
A. purpurata	64.0 ± 11.9 b	97 ± 48.6 b	30.3 ± 6.7 b	w1	d1	0.006 ± 0.002 b

RGR = relative growth rate. ^a Wilting grade: w_1, not wilting; w_2, a little wilting. ^b D-P (Diseases and pests): d_1, no diseases and pests. n = 12 for every specie.

, Figure 4 and Figure 5 are analyzed according to the results observed in Marín-Muñiz et al. [21] and discussed with actualized information, laws and com-pared with new international studies.

Relative growth rate (RGR) were calculated as described by Youssef [30] using Equation (1):

RGR = (ln W₂ − ln W₁)/(t2 − t1)

(1)

In this equation, RGR represents the relative growth rate (g DW g⁻¹ d⁻¹), where W₁ and W₂ are the dry weights (g) of the total biomass (both below ground and above ground) recorded when the vegetation was planted in constructed wetlands (CWs). The time points (t1) and (t2) correspond to zero days and the last day of the experimental period (190 days), respectively.

2.4. Light Intensity and Temperature Measurements

Light intensity and temperature were recorded three times a day—at 09:00, 12:00, and 17:00—at the study site. Measurements were taken using a light meter (Steren Model: Her-410) and a static thermometer installed at the location.

2.5. Statistical Analysis

Statistical analyses were conducted using IBM SPSS 25. A one-way analysis of variance with repeated measures was employed to assess the impact of vegetation in both monoculture and polyculture CWs on growth characteristics, as well as fluoride and chloride removal. A significance level of 5% was established to identify differences across the experiments.

3. Results and Discussion

3.1. Variations of Temperature and Light Intensity During the Study

The average temperatures recorded throughout the study ranged from 18 to 34 °C (see Figure 2). Light intensity exhibited notable fluctuations (7000–110,000 lux) (refer to Figure 2). These conditions are typical of tropical regions and are ideal for plant growth, as the ornamental species utilized in the constructed wetlands thrive best when temperatures consistently exceed 15 °C. Three of the species can grow under full sunlight or in areas with partial sun and light shade [31,32,33].

3.2. Plant Growth Features

The plant growth characteristics of vegetation in systems treating fluoride and chloride pollutants from wastewater indicated that ornamental plants thrived in constructed wetlands. Both C. hybrids and A. purpurata exhibited no signs of wilting, whereas only one specimen of H. coronarium displayed features of wilting (see Table 1). Throughout the study period, all three species of ornamental plants showed no signs of disease or pest issues.

In terms of plant development, there were statistically significant differences (p ≥ 0.05) observed in maximum root length, root volume, height, and relative growth rates (p = 0.04, 0.001, 0.001, 0.001, respectively) among the various plant species (Table 1, Figure 3). The maximum root length ranked as follows: C. hybrids (93.0 ± 14.2 cm) > A. purpurata (64.0 ± 11.9 cm) > H. coronarium (49.0 ± 6.6 cm). A similar pattern was evident for maximum root volume: C. hybrids (551 ± 33.2 cm³⁾ > A. purpurata (81 ± 15.1 cm³⁾ > H. coronarium (97 ± 48.6 cm³), as well as for height: C. hybrids (59.2 ± 16.2 cm) > A. purpurata (30.3 ± 6.7 cm) > H. coronarium (40.8 ± 3.2 cm) (Table 1, Figure 3). The relative growth rate (RGR) was notably higher for C. hybrids compared to the other species (Table 1). This characteristic suggests that C. hybrids is a promising option for future constructed wetland designs. In addition, Karungamye [31] conducted a review study indicating that Canna species can thrive in environments with high salinity, elevated metal concentrations, and excess moisture. These characteristics make the species particularly suitable for CWs.

The growth and overall health of plants are closely linked to climatic conditions (see Figure 2), which is an important consideration when selecting vegetation for CWs. For instance, ornamental plants used in residential CWs have proven beneficial, as they enhance the aesthetics of the mesocosms and support biodiversity by attracting insects and birds, such as butterflies, hummingbirds, and dragonflies. This interaction fosters a new ecosystem and represents valuable features that can facilitate the installation and adoption of such sustainable systems in backyard gardens. Various authors have also noted the adaptability of Canna plants [31], A. purpurata [32], and H. coronarium [33,34,35] in the context of constructed wetland technology for treating different types of wastewaters.

3.3. Fluoride and Chloride Removals

3.3.1. Fluoride Removal

The concentrations of fluoride ions detected varied among the constructed wetlands and during different sampling stages, with values ranging from 0.2 to 11 mg/L (see Figure 4). The CWs featuring polyculture conditions with ornamental vegetation demonstrated an ability to eliminate approximately 43% of fluoride ions (Figure 5a). Notably, this removal rate was statistically comparable to that observed in monoculture mesocosms, where C. hybrids (42.5 ± 7.5%), H. coronarium (36.8 ± 7.0%), and A. purpurata (30.7 ± 7.9%) were used. The fluoride removal values recorded in this study align closely with those reported in other investigations utilizing submerged vegetation, such as Ceratophyllum demersum, Hydrilla verticillata, Potamogeton malaianus, Myriophyllum verticillatum, and Elodea nuttallii in CWs, where removal rates ranged from 20 to 60% [26]. Additionally, removals reported in vertical flow CWs planted with canna and calamus species and operated for four weeks indicated that fluoride was primarily adsorbed by the filter materials, achieving a removal rate of 14–37% [27].

In this investigation, C. hybrids demonstrated superior growth characteristics compared to A. purpurata and H. coronarium (see Table 1). While the removal rates were analogous across all monoculture mesocosms, we anticipated that the removal efficiency observed in this study was primarily due to adsorption on the filter material (a physical process) rather than plant absorption (a chemical process). In horizontal subsurface flow wetlands, which are used for the removal of organic matter and pathogens, Leiva et al. [28] compared a polyculture (Cyperus papyrus and Zantedeschia aethiopica) with a monoculture (Cyperus papyrus), finding no significant differences in removal efficiency, similar to our findings. Utilizing either polycultures or monocultures of ornamental plants for fluoride removal in domestically constructed wetlands represents an economical and ecological approach, offering an aesthetically pleasing landscape without harming the vegetation, as this study illustrates [36,37,38]. Wambu et al. [39] noted that options for water defluoridation can be categorized into chemical, membrane-based, physical, or adsorption-based methods. Many of these techniques are complex and costly, varying in scale, effectiveness, sustainability, affordability, and acceptability. Based on the findings of this study, adsorption-based methods and phytoremediation-utilizing constructed wetland biotechnology can effectively mitigate fluoride contamination in rivers and oceans, particularly where wastewater is discharged directly. This approach not only addresses contamination issues but also helps prevent health-related social complications and environmental damage.

Some authors have presented evidence showing the positive impact of vegetation on contaminant removal—both organic and inorganic—in constructed wetlands when compared to studies involving unplanted CWs [40,41,42]. Based on this information, we chose not to include an experimental design with unplanted mesocosms in our study. Instead, we focused exclusively on comparing the removal of fluoride and chloride under conditions of monoculture and polyculture of ornamental vegetation.

3.3.2. Chloride Removal

Chloride levels in surface water and groundwater arise from both natural and anthropogenic sources. These include runoff containing road de-icing salts, the application of inorganic fertilizers, landfill leachates, septic tank effluents, animal waste, industrial discharges, irrigation drainage, and seawater intrusion in coastal regions. In this study, in which the wastewater originated from a residential source, chloride concentrations ranged from 8 to 80 mg/L during the sampling periods. The mesocosms with polyculture species demonstrated a chloride removal efficiency of 32.4 ± 4.8%. This removal rate was statistically similar to the percentages observed in monocultures: C. hybrids (29.1 ± 5.3%), H. coronarium (28.1 ± 5.0%), and A. purpurata (32.0 ± 5.7%). The removal of chloride ions detected in domestic systems is beneficial because these eco-friendly treatments have lower costs for design, construction, operation, and maintenance compared to conventional treatment systems or chemical processes, such as reverse osmosis and electrodialysis, which are used to reduce chloride levels in wastewater [43,44]. The chloride removal results from this study were similar to those reported in constructed wetlands by Chairawiwut et al. [45] and Alsaiari and Tang et al. [46] (see Table 2), even when using a synthetic solution without ornamental vegetation. Importantly, the vegetation in this study showed no signs of damage from the wastewater (refer to Table 1). Several studies [8,45] indicate that high chloride concentrations (30–40 mg/L) in irrigation water can negatively affect plant development, evident within one month after transplantation. Therefore, reducing chloride ion concentrations in wastewater using constructed wetlands presents an opportunity for reusing treated water for crop irrigation, without the risk of negatively impacting crop yield.

3.4. Comparison of the Results with Other Similar Studies

Alternative treatments, such as hydroponics or dried biomass as sorbents, have also been employed for the removal of fluoride or chloride pollutants. These methods demonstrate removal percentages comparable to those observed in this study (see Table 1). However, our approach utilizes ornamental flowering plants—species that can enhance the design of constructed wetlands, while offering aesthetic and economic benefits [20]. Various substrates, such as activated carbon [47] or tepezyl [48,49], could further enhance removal efficiency through adsorption processes. When treatment systems are integrated, the quality of the water being treated can improve significantly. For instance, Lu et al. [50] reported a fluoride removal rate of 65% using hybrid-constructed wetland systems (refer to Table 1), which exceeds the results of our study. Therefore, it is recommended that after treating these contaminants with horizontal flow CWs, a second stage of polishing be implemented, potentially utilizing surface CWs.

This study did not assess the concentration of pollutants in the vegetation; however, it has been demonstrated that the pollutants in question are only present in the plant material at levels below 2% compared to the concentrations found in the influent [51]. One of the advantages of using ornamental flowering plants for phytoremediation lies in their ability to adapt and effectively remove pollutants. This process not only contributes to water purification but also enhances the aesthetic appeal of landscapes and generates commercially valuable products, such as ornamental plants and tissue for crafts [19,20,52,53].

It is important to note that the healthy plants observed in this study indicate that the ornamental vegetation used is suitable for single-family residential constructed wetlands. These systems not only provide aesthetic appeal but also avoid health issues while effectively removing fluoride, chloride, and other organic and inorganic pollutants from wastewater [51,52].

On the other hand, constructed wetlands utilizing a polyculture of ornamental plants have been shown to enhance biodiversity [53]. While the increase in biodiversity is a benefit of using polyculture, our results indicate that it did not significantly improve the removal of chloride and fluoride.

Table 2. Comparison of fluoride and chloride removal in wastewater.

Study Site	Removal Treatment	Vegetation	Fluoride Removal (%)	Chloride Removal (%)	Reference
Venezuela	Hydroponics	Vetiveria zizanioides	18–25		[54]
India	Sorbent-Dried biomass	Parthenium sp.		20–40	[42]
India	Sorbent-Dried biomass	Vetiveria zizanioides	40–90		[55]
China	CW	Ceratophyllum demersum, Hydrilla verticillata, Potamogeton malaianus, Myriophyllum verticillatum, Elodea nuttallii	10–40		[26]
India	CW	Eichhornia crassipes		30–80	[56]
China	CW	Cannas and calamus	14–37		[27]
Canada	Non-planted CW			40–50	[45]
USA	CW	Not mentioned	15–38	35–58	[46]
	Adsorption unit + CW	Not mentioned	65		[50]
Mexico	CW	Canna hybrid, Alpinia purpurata and Hedychium coronarium	30–43	29–32	This study

Table 2. Comparison of fluoride and chloride removal in wastewater.

Study Site	Removal Treatment	Vegetation	Fluoride Removal (%)	Chloride Removal (%)	Reference
Venezuela	Hydroponics	Vetiveria zizanioides	18–25		[54]
India	Sorbent-Dried biomass	Parthenium sp.		20–40	[42]
India	Sorbent-Dried biomass	Vetiveria zizanioides	40–90		[55]
China	CW	Ceratophyllum demersum, Hydrilla verticillata, Potamogeton malaianus, Myriophyllum verticillatum, Elodea nuttallii	10–40		[26]
India	CW	Eichhornia crassipes		30–80	[56]
China	CW	Cannas and calamus	14–37		[27]
Canada	Non-planted CW			40–50	[45]
USA	CW	Not mentioned	15–38	35–58	[46]
	Adsorption unit + CW	Not mentioned	65		[50]
Mexico	CW	Canna hybrid, Alpinia purpurata and Hedychium coronarium	30–43	29–32	This study

Considering a second system or hybrid constructed wetlands (CWs) may offer better wastewater treatment. Hybrid CWs combine different treatment methods, such as subsurface vertical or horizontal flow, or surface flow systems. These combinations have been proven effective in increasing pollutant removal [57,58,59,60].

On the other hand, it is important to mention that one factor not included in this study is evapotranspiration, a natural process of water loss through vegetation. This process can influence the calculations of removal efficiency in constructed wetlands (CWs) in terms of mass [61], making its measurement crucial. Milani et al. [62] reported evapotranspiration rates ranging from 7.35 to 17.31 mm/day in CWs with plants typical of natural wetlands.

Additionally, evaluating the potential of plants for the rhizofiltration of heavy metals is essential in this type of research. Some studies have shown the effective removal of cadmium and zinc using aquatic plants in CWs [63,64]. These findings highlight the importance of using CWs as multipurpose systems.

4. Conclusions

Monoculture and polyculture of ornamental species such as Canna, Hedychium coronarium, and Alpinia purpurata have been shown to tolerate fluoride and chloride pollutants found in domestic wastewater. This study provides evidence that fluoride and chloride ions can be effectively treated in subsurface constructed wetlands (CWs) using either monoculture or polyculture of these ornamental plants.

Utilizing constructed wetlands with Canna hybrids, Hedychium coronarium, and Alpinia purpurata offers various social, environmental, and economic benefits. These systems help remove fluoride and chloride pollutants, thereby reducing health risks for individuals who rely on water sources contaminated by untreated wastewater.

Moreover, this method should be implemented as a floral treatment planter in single-family homes or as a natural laboratory in rural schools. Despite the positive outcomes of both systems, polyculture systems present a superior option for addressing wastewater issues. They enhance biodiversity, improve the landscape, and exhibit greater resilience against environmental stresses, diseases, and pests. New studies including evapotranspiration and rhizofiltration measurements are necessary to complete the role of the CWs using monoculture and polyculture of ornamental plants.