Performance evaluation of vetiver and pampas plants in reducing the hazardous ions of treated municipal wastewater for agricultural irrigation water use

Today, climate change towards global warming and reducing precipitation has caused a water crisis, especially in arid and semi-arid regions of the world, including Iran. On the other hand, an increase in human population along with the expansion of agricultural and industrial activities are other significant factors in reducing freshwater resources and increasing their pollution. Therefore, the use of unconventional water, including the treated wastewater of urban and industrial sectors for agricultural purposes, has been proposed in many countries as a solution to prevent severe droughts (Al Salem 1996; Asano & Levine 1996). However, due to the hygiene problems of these resources and their significant amounts of organic and mineral materials, long-term use of wastewater in the agricultural sector is faced with challenges such as threatening the health of farmers and food consumers, soil salinization, and groundwater pollution (Mojiri 2011). Excessive concentrations of sodium, chloride, carbonate, and bicarbonate are among the essential factors in reducing the quality of unconventional water resources and their non-approval for agricultural use. For example, high concentrations of carbonate and bicarbonate in wastewater undesirably affect the uptake of high-consumption elements such as manganese, iron, magnesium, zinc, potassium, and phosphorus in plants which ultimately lead to reduced plant root development (Valdez & Reed 2007; Yang et al. 2009). Since bicarbonates negatively affect the uptake of iron by plants and thus prevent the synthesis of chlorophyll in the chloroplast of plant cells, iron chlorosis or yellowing of young leaves is another problem due to the presence of bicarbonates in wastewater (Lucena 2000). In addition, the reaction of carbonate and bicarbonate with other elements in wastewater such as calcium and magnesium results in the formation of calcium carbonate, magnesium carbonate, and calcium bicarbonate deposits. This process is one of the most important reasons for clogging in pipes and drippers of the drip irrigation system and, as a result, is one of the most severe problems of using wastewater in agriculture (Pitts et al. 1990).

Although several physical and chemical methods can be used to treat urban and industrial wastewaters further, they are expensive and require specialized human resources to exploit their advanced facility and instruments. Phytoremediation is a new and cost-effective method with no adverse environmental effects due to its biological nature (Lu 2009). It is a nonintrusive, aesthetically soothing, and socially recognized technology to biodegrade contaminated environments (Malik et al. 2022).

It also can be applied in various climate conditions with the right choice of plants (Rahman et al. 2022). In this method, plant roots are used to eliminate or reduce the concentration of organic and inorganic pollutants as well as heavy metals in soil and water (Mustafa & Hayder 2020). It does not involve complex operations, expensive technologies and many highly skilled labors, which is definitely popular in countries facing financial and energy supply problems. Promising results from various studies have shown that the phytoremediation of unconventional waters using plants such as common reeds (Phragmites australis) (Bragato et al. 2009; Bello et al. 2018), water hyacinth (Eichhornia crasspies) (Dar et al. 2011; Gupta et al. 2012), water lettuce (Pistia stratiotes) (Dipu et al. 2011; Qin et al. 2016), bulrush (Typha) (Salhani et al. 2003; Rahman & Hasegawa 2011), duckweed (Lemna) (Mkandawire et al. 2004; Saha et al. 2015), pampas grass (Cortaderia selloana) (Tafazoli et al. 2014; Aydin et al. 2017), and vetiver grass (Chrysopogon zizanoioides) (Girija et al. 2011; Gupta et al. 2012) can be used as a complementary method for wastewater treatment plants .

Selecting suitable plant species with a high ability to absorb organic and inorganic pollutants is essential in implementing biological treatment systems. Moreover, plant species should easily grow in polluted water and soil environments (Mashauri et al. 2000). According to the literature, even in one genus, the capability of pollutant uptake varies from species to species (Roongtanakiat et al. 2007). The selected species should also be well adapted to the region's climatic conditions and the culture medium for efficient refining performance. Besides, some studies on phytoremediation show that its capacity can be significantly increased through the simultaneous application of minerals (Otunola et al. 2022).

Vetiver is a drought, salinity, and cold-resistant plant with an excellent ability to grow in various climates and areas with an annual rainfall of 450–500 mm (Xia & Shu 2001; Maffei 2002; Girija et al. 2011). The origin of this plant is in southern India, and it has a bulky root system that can reach a soil depth of 3–4 m in one year (Truong 2000). Vetiver exerts its maximum growth at 25 °C (Zhang 1992) and can withstand extremely cold conditions up to −22 °C and resumes its growth as soon as suitable weather is provided. Therefore, vetiver can resist temperature stresses in the range of −22 and 60 °C (Truong & Hart 2001). Vetiver grass can absorb soluble nutrients, such as nitrogen, phosphorus (Truong & Hart 2001), and heavy metals, e.g., zinc, copper, nickel, chromium, and lead (Roongtanakiat et al. 2003). Today, many countries use vetiver for soil conservation in agricultural lands, stabilizing unstable river slopes, watershed management, rehabilitating dams, mines, contaminated lands, and salt marshes, as well as wastewater treatment (Gupta et al. 2012).

Pampas grass is a stress-tolerant, leafy, and evergreen plant that belongs to the Cortaderia genus and Poaceae family. It has long, slender leaves with sharp edges, the length of which reaches 1–2 m, and it can be found along the coastlines, plains, and urban areas (Bacheta 2009). Pampas grass can absorb remarkable amounts of pollution and tolerate abiotic stresses, such as cold, heat, intense light, and nutrient deficiency (Saura & Lloret 2005).

The favorable characteristics reported for vetiver grass and pampas grass have led many studies to use these two plant species in recent years to eliminate or reduce pollutants in urban or industrial wastewaters. Some of these studies are summarized in Table 1.

Despite the successful experiments and proving the efficiency of phytoremediation of urban and industrial wastewater by two plants of vetiver grass and pampas grass, the use of these two plant species to remove undesirable ions such as chloride, sodium, carbonate, and bicarbonate and, in general, the performance of these two plants to reduce the obstacles to the progress of using unconventional water in the agricultural sector as well as the drip irrigation system is rare studied in previous research. Therefore, the purpose of this study was to evaluate the potential of vetiver grass and pampas grass to remove unfavorable content and improve the physicochemical properties of the treated municipal wastewater, for the agricultural irrigation water use, especially in drip irrigation systems. Considering that both vetiver grass and pampas grass can grow and develop in the climate of most regions, if the refinability of these two plant species is proven, they can be used as complementary units of the treatment plants and irrigation filtration systems around the world.

This study was carried out at Gorgan University of Agricultural Sciences and Natural Resources (54° 25′ E, 36° 50′ N) with an altitude of 170 m above sea level. Gorgan is the capital of Golestan province. It is located in the south of this province with an area of about 40 km². The region's climate is very diverse and due to its location near the Caspian Sea on one hand, and surrounded by Alborz mountain range in the south and south-east, on the other hand, Gorgan has a moderate and humid climate with hot-humid summers and cool-wet winters and this city is firmly productive for agricultural activities. The average annual temperature varies between 12.6 °C and 25 °C. The coldest month of the year is January and the warmest is July. The average annual rainfall is 477.8 mm and reaches its peak in April and January.

Gorgan is a non-industrial city, and industrial factories are mainly located in specific industrial towns on the outskirts, so the wastewater of these towns does not enter the Gorgan wastewater treatment plant.

This study aimed to evaluate the efficiency of vetiver grass and pampas grass in improving the quality of urban wastewater to suggest new and cost-effective complementary units in the refinery systems and for the natural purification of water required by farms. For this purpose, evaluating the ability to improve the quality of treated wastewater is more critical and better justified than raw wastewater. The relative elimination of contamination such as suspended solids in the process of the Gorgan treatment plant was another reason for not investigating raw wastewater in this study. Therefore, the samples were obtained from the final outlet of the Gorgan wastewater treatment plant located at km 2 of the Gorgan–Aq-Qala road. Table 2 shows the properties of the samples (i.e., treated wastewater) used in this study.

In August 2018, two plant species, i.e., vetiver grass and pampas grass, were prepared as seedlings in pots for studies. In the first stage, to eliminate the chemical effect of the soil, as well as the adaptation of plants and their growth in the hydroponic environment, the seedlings were transferred to containers containing urban water after being removed from the pots and thoroughly washed with distilled water. After two weeks, leaf growth and rooting of both species began, showing adaptation to the studied environmental conditions. In early September 2018, the plants were transferred to the main pots located in a space equipped with a shelter. The shelter was used to prevent the effect of precipitation and change the amount of parameters studied.

This study was conducted in the form of a factorial experiment in a completely randomized design with three replications. The first factor was the plant species (vetiver grass, pampas grass, and without plant, i.e., control). The second factor was the residence time (at five levels: 3, 6, 9, 12, and 15 days). Two seedlings with the same height of about 60 cm and with the same root volume were placed in each pot, and 10-cm plastic foams were used on the pots for proper placement of seedlings in the pot and eliminating the contact of plant roots. A total of 8 L of treated urban wastewater was added to each pot. According to the studied residence times, wastewater samples were transferred from the pots to the laboratory for further analysis on days 3, 6, 9, 12, and 15.

Various physical and chemical properties such as sodium adsorption ratio (SAR), Langelier saturation index (LSI), total hardness (TH), total dissolved solids (TDS), electrical conductivity (EC), pH, and concentrations of calcium (Ca⁺), magnesium (Mg⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), carbonate (CO³⁻), and bicarbonate (HCO³⁻) ions were analyzed at 3-day intervals during the experiment to investigate the qualitative changes resulting from phytoremediation.The methods and equipment used to measure the parameters are listed in Table 3.

The data obtained from the experiments were analyzed using analysis of variance and Duncan's multiple range test using SPSS software (ver. 25.0).

Table 4 shows the results of the analysis of variance on the effects of plant type and residence time on wastewater phytoremediation. The table reveals that the individual effects of both residence time and plant type were significant (P<0.01). Moreover, their interaction was also significant on all wastewater chemical properties at 1% probability, except for CO₃, EC, TDS, and HCO₃, which were significant at the 5% level.

Since the volume of the reservoir used in phytoremediation technology is directly related to the residence time, using plant species with a short residence time can lead to lower operating costs. Of course, the residence time should also be technically large enough for the purification process to be performed sufficiently and adequately. Figure 1 shows the results of experiments conducted to determine the optimal residence time of vetiver grass and pampas grass. As the results show, the highest percentage of removal of potassium, chloride, and carbonate ions occurred during the first 3 days of the study, while it took 6 days to reduce the EC and the TDS. After this period, the process of reducing the contamination slowed down. The results also revealed that the maximum reduction of the concentration of calcium, magnesium, and sodium ions, as well as the TH, required a minimum of 9 days, while the decreasing trend of pH and bicarbonate concentration lasted until the 12th day. LSI was also the parameter that decreased to an acceptable level just during the first 6 days of the study. In general, with exceptions, a 9-day period can be considered as the optimal hydraulic residence time. Although, selecting an appropriate residence time depends on the phytoremediation purposes in different industries and agriculture. For example, in cases where the only goal is to reduce the electrical conductivity, carbonate, TDS, LSI and, in general, salinity and hardness indexes, a 3-day to 6-day residence time is sufficient for both vetiver grass and pampas grass as a complementary process unit in treatment plants and agricultural irrigation water use. The most important reason for the slow decreasing trend of the contaminations or becoming constant can be explained by the phenomenon of saturation of plant tissues and their relative deterioration due to prolonged residence time.

The literature shows that despite the use of advanced and expensive technologies and equipment in urban wastewater treatment, the use of treated wastewater in agriculture, and especially in low-pressure irrigation systems, can clog the drippers and causes irreparable damage to farmers and environment, as well as costing the former expenses as they use sulfuric acid and hydrochloric acid to remove deposits in the pipelines. Therefore, the most crucial purpose of phytoremediation should be reducing the concentration of compounds that current wastewater treatment methods cannot or can at high costs. The high concentrations of these compounds in the treated wastewater that is the source of irrigation of agricultural lands reduce the quality of products and pollute surface water and groundwater resources. The long-term use of treated wastewater in agriculture ultimately leads to the payment of exorbitant costs to compensate for the damage to the environment. Therefore, it is also necessary to meet strict standards in producing agricultural products and upgrading the treatment plants. Phytoremediation not only reduces the concentration of unfavorable elements and compounds in water resources required for irrigation of agricultural products but also reduces the costs of constructing and operating the treatment plants (Maharjan & Pradhanang 2017).

As shown in Figure 2, most of the parameters studied in the control group (without using plants) did not change significantly over time except for HCO₃, CO₃, Cl, and TH. Therefore, all the changes observed in vetiver grass or pampas grass treatments are due to the phytoremediation process.

Figure 2 shows that vetiver grass and pampas grass reduced potassium concentrations from 14.61 mg/L to 2.25 and 2.94 mg/L, respectively, over 15 days, while no significant change was observed in the control samples. Also, the chloride concentration in the wastewater decreased from 130.16 mg/L on the first day of treatment to 35.67 and 34.72 mg/L on the 15th day due to treatment with vetiver grass and pampas grass, respectively. This value reached 121.15 mg/L in the control samples, which did not change remarkably. On the other hand, both plants showed a similar ability to purify carbonate from 115 mg/L to 15 mg/L after 15 days. This value was equal to 95 mg/L for the control, indicating the high performance of phytoremediation in pollution reduction. The ability of vetiver grass to remove potassium and chloride in this study is consistent with the results reported by Deva et al. (2019) and Gholipour M et al. (2020).

As described before and can be seen in Figure 1, the percentage of removal of potassium, chloride, and carbonate by vetiver grass on day 15 of the study was 84.6%, 72.6%, and 86.96%, respectively. Using Pampas grass, these values were equal to 79.88, 73.33, and 86.96%, respectively, after 15 days. According to Dhanya et al. (2022) the average removal efficiency of vetiver was determined as 58.41% for potassium and 49.67% for chloride after 15 days. However, in the present study, the highest reduction of these elements occurred in the first 3 days, so that both plant species can be used as a supplementary unit in treatment plants or agricultural water filtration sections with a residence time of 72 h. Under these conditions, the removal percentages of potassium, chloride, and carbonate are expected to be at least 70, 60, and 75%, respectively.

It is worth noting that when the amount of chloride in the water increases, it begins to deposit in plant tissues and causes severe damage to them. Excessive concentrations of this ion cause root dysfunction, reduced crop yield, plant deformation, and even plant death, making it one of the most critical environmental problems (Eaton 1942; Tavakkoli et al. 2010; Kafkafi 2011).

According to the results, pampas grass reduced both electrical conductivity and TDS by more than 18% in only three days, while this value for vetiver grass was 15.67% during the whole period (15 days). Also, the highest rate of change of these 2 parameters was observed in the first 6 days of the study. Hemamalini et al. (2019) showed the high ability of vetiver grass to improve the TDS in 3 days by 74.91%. Aregu (2022) also reported that vetiver reduced the TDS in industrial wastewater by 90.8% within 9 days. However, in the present study, Pampas grass showed more potential than vetiver grass to reduce the TDS in 15 days. High levels of electrical conductivity and TDS in irrigation water reduce water uptake by plant roots and ultimately lead to disruption of vital metabolism and plant growth process (Hussein et al. 2002). It also leads to major issues in drip irrigation systems. Therefore, phytoremediation by these two plants to further purify irrigation water can increase the quality of crops and also solve the problems related to the clogging of drippers in drip irrigation systems. These results are consistent with the findings of Keshtkar et al. (2016), Deva et al. (2019).

Calcium was another element that both vetiver grass and pampas grass could reduce its concentration from 67.33 mg/L to 18.97 and 19.04 mg/L, respectively, exerting a percentage reduction of about 71% after a 15-day period. However, no significant changes were observed in the control samples during 15 days. Gholipour M et al. (2020) stated that calcium was reduced by 25% when treated by vetiver after 18 days. Dhanya et al. (2022) reported that vetiver reduced the calcium by 60.48% within 15 days.

Both plants showed lower efficacy in removing magnesium from municipal wastewater than calcium. Pampas grass, with an advantage over vetiver grass, reduced the magnesium concentration by 25.68% in 15 days. Almost similar results were found in the study findings reported by Gholipour M et al. (2020), which revealed low ability of vetiver in removing magnesium and found that the removal efficiency was 10% during 18 days. Although high-consumption elements of calcium and magnesium increase the quality of the product, in some areas, high amounts of these elements precipitate with carbonates and bicarbonates, and increasing the concentration of these salts in soil and water causes problems such as soil calcification, soil structural damage, sedimentations, and clogging of the nozzles in drip systems.

The performance of vetiver grass and pampas grass to reduce the sodium concentration of the treated wastewater was calculated to be 38.32 and 42.34%, respectively: the sodium concentration of 8 mg/L reached 5.23 and 4.89 mg/L, respectively. Except for the sodium of the samples treated with pampas grass, the slope of calcium, magnesium, and sodium removal for both plants showed a significant increase until the ninth day. This study is supported by the study done by Dhanya et al. (2022). In that study, sodium was reduced by 60.78% efficiently within 15 days. The high amounts of sodium in wastewater, in the long run, cause salinization and alkalinity of agricultural soil and the formation of a heavy layer that prevents soil water infiltration, being one of the reasons for reducing the yield. Therefore, fully managed measures should be taken before using such waters in the agricultural sector.

TH was another parameter that decreased the most during the first 9 days of the study and then did not change remarkably. Both vetiver grass and pampas grass could improve this parameter up to 38.52 and 41.21% in a 15-day period, respectively. These findings were consistent with the results of Keshtkar et al. (2016) and Maharjan & Pradhanang (2017) that showed a reduction of approximately 46% when treated with Vetiver. Besides, significant changes were not observed in the control samples. According to the obtained data, this value decreased from 502.75 to 482.26 (4.1%) after 15 days.

According to the results, improving the pH and reducing the bicarbonate concentration to an acceptable level required a minimum period of 12 days of phytoremediation by either vetiver grass or pampas grass. During 15 days, both vetiver grass and pampas grass were able to reduce the pH of the treated wastewater from 8.4 to about 7, exerting a percentage reduction of approximately 16%. Reducing the pH of domestic and industrial wastewaters using vetiver grass as a phytoremediation method has been proven in several studies (Truong & Hart 2001; Seroja et al. 2018; Davamani et al. 2021). Jayashree et al. (2011) showed that the decomposition of organic matter by plant roots leads to the production of organic carbon and acid, and ultimately the pH value as the indicator of free ions in water is reduced. The occurrence of such reactions is likely due to the intrinsic characteristics and physiological responses of the plant.

The bicarbonate concentration in the treatment with Pampas grass reduced from 55.93 mg/L on the first day to 15.25 mg/L on the 15th day (a 72.73% reduction). However, this value on the 15th day was equal to 20.34 (63.64%) and 45.75 mg/L (18.2%) in the vetiver grass treatment and control, respectively. However, according to Gholipour M et al. (2020) the average bicarbonate removal efficiency of vetiver was determined as 4% after 18 days.

In general, high concentrations of carbonate and bicarbonate in wastewater can cause many environmental problems in humans, plants, and soil. By interfering with the absorption of high-consumption elements such as manganese, iron, magnesium, zinc, potassium, and phosphorus, it prevents plant root development and undesirably affects plant growth performance (Yang et al. 2009). In other words, carbonates and bicarbonates are the main causes of alkalinity that affect the water pH and disrupt plant growth directly and indirectly (Valdez & Reed 2007).

Although in accordance with the decrease in sodium concentration in the group treated with pampas grass, the SAR values decreased by 48.84% in this group in the first three days of the study, it showed a further decrease during 12 days. Finally, after the 15-day period, in the vetiver grass and pampas grass treatments, it reached 0.19 and 0.18 mEq/L, respectively, showing 51.85 and 56.42% reduction compared to the initial concentration (0.41 mEq/L).

Mirzaee et al. (2021) reported that using pampas and vetiver as the phytoremediation plants over a period of 3 days reduced SAR by 48.78 and 34.15%, respectively, which is consistent with the current study.

Exchangeable sodium tends to disperse in the soil and reduce the rate of soil water infiltration. This particle dispersion causes the formation of a soil crust and prevents seed germination, so reducing the amount of exchangeable sodium, and in other words, the SAR parameter by phytoremediation can significantly reduce its adverse effects in agriculture. On the other hand, salinity and SAR are the first factors in determining the physicochemical quality of water used in irrigation systems, so that high SAR values increase the risk of dripper clogging. The results showed that using pampas grass and vetiver grass reduced the SAR, and therefore, the obtained water can be used in drip systems to reduce the effects of dripper clogging.

In terms of LSI reduction, the best efficiency was observed in the pampas grass treatment, and the reduction percentage was equal to 160.31% during 15 days. Among the studied parameters, LSI was the only parameter that exerted a constant decrease during the study period.

If LSI has a positive value, it indicates that there is a potential for calcium carbonate deposition in water, and if it is negative, it tends to dissolve and corrode in drip irrigation systems (Alizadeh 2001). Therefore, LSI values close to zero and even slightly negative are more acceptable. In the present experiment, the optimal LSI value occurred on the sixth and ninth days. These values were equal to 0.34 and −0.07 in vetiver and 0.073 and −0.06 in pampas treatments. However, due to limitations such as providing a large reservoir to store wastewater and a large area for phytoremediation pool for longer residence time on one hand, and excessive reduction of TH and thus the negative values of the LSI and the possibility of pipe corrosion, on the other hand, a 3-day residence time with a 46.71 and 88.16% reduction in LSI using vetiver grass and pampas grass can be also a reliable residence time in phytoremediation technique. In general, the significant reduction in the two essential parameters of SAR and LSI that have an important impacts on soil quality, as well as clogging of drippers indicated the high ability of the studied plant species in improving the quality of urban wastewater. Therefore, using these plants can increase the capacity of the treated wastewater for use in the agricultural sector especially in drip irrigation systems.

According to the observed results, in most of the studied parameters, pampas grass showed better performance than vetiver grass (Figure 3). In addition, it reduced sodium, bicarbonate, LSI, and SAR factors more rapidly than its competitor. In general, although LSI had the highest reduction in both treated groups, vetiver grass and pampas grass showed the lowest ability in reducing the electrical conductivity EC and pH of the treated wastewater, respectively.

This study aimed to evaluate the efficiency of vetiver grass and pampas grass in improving the quality of municipal wastewater to suggest new and cost-effective complementary units in the refinery systems and for the natural purification of water required by farms. Therefore, various physical and chemical properties such as SAR, LSI, TH, TDS, EC, pH, and concentrations of calcium (Ca⁺), magnesium (Mg⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), carbonate (CO³⁻), and bicarbonate (HCO³⁻) ions were analyzed at 3-day intervals during the experiment.

Due to its adaptability to the climate of Iran and in addition to the ability to grow in a hydroponic environment, pampas grass can reduce the concentration of unfavorable compounds in treated wastewater used for agriculture and irrigation systems, especially to remove chloride, carbonate, and bicarbonate. According to the results, pampas grass reduced chloride, carbonate, and bicarbonate ions by 73.33, 86.96, and 72.73% at the end of the study period, respectively. Besides, due to its ability to reduce TH, TDS and corrosion and sedimentation index or LSI, this plant can be used in the supplementary unit of wastewater treatment plants and filtration of urban wastewater before entering the irrigation systems. This plant decreased calcium (by 71.88%) and magnesium (by 25.68%) ions over 15 days, resulting in a significant reduction in water TH. Therefore, the use of this plant can improve the quality of treated wastewater used in agriculture, solve the problems caused by clogging the pipes of the drip irrigation systems, and reduce the cost of construction, operation, and maintenance of wastewater treatment plants.

In addition, it should be noted that proper disposal of biomass generated by the phytoremediation process is one of the major environmental issues, but specific measures could be taken to alleviate this problem. Combustion and gas production through a series of chemical changes (pyrolysis process) is the most important method for generating thermal energy and electricity from contaminated plants. Recycling this energy from biomass will have economic benefits, because it could not be used as forage or fertilizer. Researches have shown that incineration of hazardous wastes outdoors is not appropriate, because gases released into the environment can be harmful, as this only reduces the volume of biomass and wastes the generated thermal energy. The pyrolysis method decomposes the material under anaerobic conditions with no release into the air, so heavy metals and other hazardous materials remain in the coke that can be used in the melting furnace. Given this situation, it is recommended that certain steps should be taken immediately to ensure that all these processes will not adversely affect environment and human health.

All relevant data are included in the paper or its Supplementary Information.