Evaluation of Phytoremediation Potential of Vetiver Grass (Chrysopogon zizanioides (L.) Roberty) for Wastewater Treatment

Water pollution is one of the most critical global issues. Meanwhile, the problem of water pollution of rivers especially in Iran is rising due to expansion of agricultural and industrial applications. Due to a large number of sewer catchments, there are some dam reservoirs like ZHAVE in Iranian state of Kurdistan that have not been able to collect significant amount of water since last 10 years. Removal of heavy metals as contaminants from runoffs and recycling of water is a necessity and a vital issue in the world. Various methods and standards are invented and used to isolate and remove all types of pollutants. (is study focuses on the purification and removal of contaminants in water sources using the phytoremediation method by introducing Vetiver grass species in the case of floating treatment wetland (FTW). (is study’s preliminary purpose is to investigate a practical remedial solution and improvement methodology for the water quality of reservoirs and rivers by growing the floating Vetiver island method. (e results show that following parameters such as COD by 97%, TN by 90%, phosphorus by 66%, TDS by 26%, and evapotranspiration by 40% were reduced. (erefore, we concluded that for a wastewater with varying neutrient concentrations such as in ZHAVE dam, concentration of nutrients N and P was controlled and consequently inhibition and prevention of the eutrophication of water resources in the medium and long term became possible due to reduction in the rate of evaporation from reservoirs.


Introduction
Floating treatment wetland (FTW) is a suitable wastewater treatment system for developing countries in the tropics, which requires access to land and high temperatures for biodegradation [1]. ese wetlands can treat municipal, domestic, industrial wastewater, aquaculture wastewater, and so on [2][3][4]. In particular, many practical studies and various purification methods have been conducted to investigate the possibility of reusing wastewater to irrigate agricultural lands [5][6][7][8][9]. Plant-based water treatment methods are effective, cost-effective, and sustainable [10]. One of the crucial factors in applying the phytoremediation method is choosing the right plant [11]. e use of aquatic plants in phytoremediation is beneficial because they have an extraordinary capacity to absorb and destroy pollutants [12]. For example, removal efficiency of NO − 3 -N, NH 4 + -N, and P in Typha Angustifolia are 51.6%, 56.5%, and 9.1%, respectively [13]. In particular, Vetiver has been studied and used by various researchers to treat different types of wastewater [13][14][15][16][17][18]. e Vetiver system (VS), as a new phytoremediation method based on Vetiver grass, was recognized for the first time in 1995 due to its "super absorbent" properties suitable for leachate and effluent produced disposal from landfills and wastewater treatment plants in Australia [19].
Vetiver is a hydrophilic plant with physiological properties such as absorbing soluble nutrients such as N and P, reducing BOD, COD, TSS, heavy metals, and high resistance to pesticides [20]. In the research study done by Kabata, the concentration of lead in Vetiver is considered toxic to the plant if roots and leaves exceed 30 ppm [21].
According to Ash and Truong, the results show that Vetiver has the highest percentage of nitrogen and phosphorus removal than other plants. Vetiver can remove 13,688 kg/ha of nitrogen per year in hydroponic environments, which is even 6 times better than potted conditions and 12 times better than conditions planted in dry soil. It is also 6 times more efficient than the other two media in terms of nitrogen removal in hydroponic environments [22]. Plant growth depends on physical properties such as temperature, soil texture, soil moisture, and chemical properties such as pH, salinity, and heavy metals concentration. Still, this plant can grow in harsh environmental conditions and tolerate these conditions [23,24].
Although Vetiver has a high tolerance range, it is susceptible to lack of light and does not tolerate shade. However, if the shade is removed from the plant, it returns to its normal state and begins to grow and take root. However, the plant dries out and dies if the lack of light continues [25]. Vetiver grows relatively fast. Its crown is 1 to 2 meters high, and its root has a depth of 3 to 4 meters, which is branched and intertwined and can penetrate different layers of soil and weave them together [26]. Wang tested three species of Vetiver plants with different genotypes at different temperatures. In the presented results, the Vetiver shows the best growth rate at a temperature between 25 and 35 degrees Celsius. Research has shown that the plant has the fastest root growth rate of 4 cm per day at 25°C soil. In other words, in 75 days, the Vetiver's root can reach a length of 3 meters in the best situation [27]. e leaves and stems of Vetiver can be dried to a dark brown color. Its compost can be used. is compost is very nutritious and beneficial for the soil. Leaves are also used for livestock, but it is better to use them in combination because the crude protein in these leaves is less than other plants [24].
Alsghayer et al. investigated PAHs' removal efficiency from wastewater using a constructed wetland where Vetiver was planted. Removal efficiencies of phenanthrene and benzo and pyrene have been reported to be 67%, 66%, and 73%, respectively [28].
Koupai et al. investigated the efficiency of phytoremediation using Vetiver in the Isfahan landfill's leachate quality. According to the results obtained in this study, the removal efficiencies of COD, BOD 5 , phosphate, and nitrate using Vetiver after 21 days were 68, 60, 82, and 83%, respectively [29]. Ky et al. evaluated the efficiency of nutrient removal by floating treatment wetland using Vetiver some studies, by using the floating treatment islands system in containers. Darajeh et al. investigated the treatment of palm oil plant effluent (POME), a pollutant produced by the palm oil industry, using vetiver system (VST) technology. is technology was first used to treat POME to reduce biochemical oxygen demand (BOD) and chemical oxygen demand (COD). e results showed that vetiver was able to reduce BOD up to 90 at low POME concentration and 60 at high POME concentration, while control sets (plant-free) were only able to reduce 15 BODs. e reduction in COD was 94% at low POME concentrations and 39% at high concentrations, while the control showed only 12% reduction [30,31]. RoyChowdhury et al. (2021) used the vetiver plant to treat wastewater containing insensitive ammunition such as 3-nitro-1,2,4-triazol-5-one (NTO). Various NTO treatment technologies are being developed for wastewater treatment of industrial ammunition facilities. is is the first study to discover the potential plant treatment of industrial NTO wastewater using Vetiver grass (Chrysopogon zizanioides L.). e results showed that vetiver can effectively remove NTO from wastewater and also transfer NTO from the root to the stem [32,33].
Panja et al. reviewed the removal efficiency of antibiotics and nutrients from the secondary effluent. Removal efficiencies of antibiotics, nitrate, phosphate, TOC, and COD from secondary effluents using Vetiver have been reported to be more than 90, 40, 60, 50, and 40%, respectively [34]. Maharjan and Pradhanang investigated the ability of Vetiver to treat wastewater. e results showed that, after one month, the removal efficiencies of BOD 5 , chloride, nitrate, phosphate, hardness, and alkalinity were reported to be 71.03%, 42.9%, 93.93%, 88.4%, 46.4%, and 22.2%, respectively. Based on these results, Vetiver seems to be very effective in wastewater treatment with high phosphate and nitrate amounts [35]. Mathew et al. investigated the efficiency of Vetiver in wastewater treatment of an organizational kitchen. In this study, the removal efficiencies of BOD 5 , COD, and total coliform were reported to be 85%, 90%, and 85%, respectively [36]. e present study focuses on the purification and removal of contaminants in water sources using the phytoremediation method by introducing Vetiver grass species in the case of floating treatment wetland (FTW). is study's preliminary purpose is to investigate a practical remedial solution and improvement methodology for the water quality of reservoirs and rivers by growing the floating Vetiver island method.

Pilot Preparation.
e pilot was made using three polyethylene containers of 126 liters. e container perimeter was covered with black plastic to reduce the penetration of sunlight as much as possible and to prevent the production of moss and algae that cause errors in the test results. e first container contains 12 floating plants with the addition of dilute and nonaerated sewage. e second container has 12 floating plants with dilute and aerated wastewater equipped with an aeration pipe. e tube is placed in a spiral with a distance of 5 cm from the container's bottom. Holes in the pipe's surface were drilled at intervals of 5 to 8 cm to completely disrupt the wastewater and perform adsorption and treatment operations at a higher speed.
e third container has 12 floating plants with concentrated wastewater � (5 times dilute wastewater) and without aeration. e floating surface was made of compressed ionolite with 5 cm thickness and a 25 density to have more strength and continue to float on the water's surface, taking into account the plant's weight. e ionolithic plate containing Vetiver seedlings with the same root and stem length was placed floating on the aqueous medium so that more than 99% of the water surface was covered.

Preparation of Artificial Sewage.
ree 126 liter containers containing 100 liters of synthetic wastewater were selected in the laboratory to evaluate the purification power of Vetiver in ambient air conditions. According to Table 1, synthetic materials were chosen separately, and 100 liters were added to each container. Preliminary data after adding artificial wastewater are shown in Table 1 and Figure 1.
According to Figure 1, all three pots have the same physical and visual conditions (similar number of plants, container, ionolite, cover, and so on) and also the height of taller plants using cutting operations is considered equal to 30 cm, which all plants should be 30 cm high. e amount of the adsorbed (mg) per unit mass of Vetiver (g) q e was obtained by mass balance using the following equation [37]: where C i and C e are the initial and equilibrium concentrations of the nitrogen and phosphate (mg/L), m is the mass of Vetiver (g), and V is the volume of the solution (L).

Results and Discussion
Seven tests have been performed on each container during one month, in which a total of 21 wastewater samples were tested. Table 2 shows the preliminary results of qualitative experiments obtained from the cultivation of Vetiver. e changes of T, pH, EC, TN, TP, and COD parameters in 30 days are presented in Table 2.

Evaporation and Transpiration.
e percentage of water consumption (total evaporation and transpiration) in the samples of Vetiver culture medium in Figure 2 is considerable. Accordingly, the aerated container has the most evaporation due to the rising air pressure. While in the sample of wastewater with higher concentration, it has the least evaporation. Calculations show that during a week in one square meter of free water surface in September, approximately 48 liters of evaporation have been done.

COD Results.
Floating islands were very effective in reducing COD. At the end of the one month, COD decreased about 33% in a container containing concentrated wastewater, even though the plant had almost given up and lost its efficiency. is reduction is very significant in the other two containers, which include more dilute wastewater.
It turns out that in a container containing dilute nonaerated wastewater, after about three weeks, we observed 0.90% removal rate according to the numbers and graphs related to COD removal ( Figure 3). However, in a container containing dilute wastewater with aeration, the system quickly began to reduce COD so that after ten days, the rate of COD reduction reached more than 0.90%. Furthermore, in both cases, one month after the start of the test, the removal percentages of 0.93% and 0.97% were obtained for nonaerated and aerated containers, respectively. erefore, these results demonstrate very desirable effect of the system and justify using of this method to reduce COD in polluted waters ( Figure 3). Table 3, parameters A, C, and D are effective in the study response. e correlation coefficient (R 2 ) was 0.99, which indicates that the fitting model follows the data very well. e equation of the first-order polynomial model resulting from the analysis of the test results is as follows:

Results of Statistical Analysis with RSM for the First Response (COD). According to
(2) In the above equation, the parameters A, B, C, and D are temperature, pH, EC, and time, respectively. Figure 4 shows the interaction of independent parameters affecting the COD response from data analysis. According to the figure, the COD value decreases with increasing temperature at pH 7.5. e lowest COD value was obtained at 26.2°C and EC � 500 cm/μs over 31 days.

TN Results.
Many researchers have found that nitrogen removal efficiency in treatment using plants is always higher than without plants in studying plants' role [38,39]. In the TN removal test, except for the container containing concentrated wastewater, we saw a 65% increase in nitrogen sources. As shown in Figure 4, in the following two samples, we see a favorable reduction of total nitrogen from the wastewater source.
In a container containing dilute nonaerated wastewater, 50% of the total nitrogen was removed from the beginning of the experiment until the tenth day. However, after that, the reduction operation was slower, and finally the removal percentage reached 55 at the end of the experiments, which is nonsignificant compared with the first days of the investigation. In the aerated container, the nitrogen removal operation had a better process so that on the seventh day, more than 50% of the total nitrogen was removed by the system. In the second week, the removal percentage reached 80%, but after that the graph's slope gradually decreased, and the rate of nitrogen removal decreased (Figure 4).   As shown in Figure 4, this removal process continued until the last day, and at the end of the month, we achieved a 90% removal rate. However, the removal percentage is not expected to increase significantly with the continuation of the experiment.

Result of Statistical Analysis with RSM for the Second
Response (TN). Parameters C and D are effective in the study response, according to Table 4. In the above equation, the parameters A, B, C, and D are temperature, pH, EC, and time, respectively. Figure 5 shows the interaction of independent parameters affecting the TN

TP Results.
According to the study of Wang et al., the decomposition of phosphorus by microorganisms in treatment using plants is more than without plants [40]. It should be noted that in addition to adsorption, phosphorus can also be removed by chemical precipitation [41]. In phosphorus-related experiments, we eventually encountered a decrease in total phosphorus in both containers over one month. In a container containing dilute, aerated wastewater, aerobic bacteria begin to absorb and remove phosphorus, and the trend is a decreasing graph as shown in the figure. Phosphorus value decreases by up to 50% within 10 days, and by the end of one month, this reduction reaches 66%. Phosphorus values increase by 80% after a week in an anaerobic container and then decrease so that at the end of one month, the total phosphorus rate drops to 42%. e reason for the initial increase is that the polyphosphates are not detected in the experiments, and anaerobic-friendly polyphosphate bacteria begin to break down these polyphosphates because the anaerobic state occurs in this sample and mineral phosphorus is released as a result of their activity. In this time, mineral phosphorus is determined in experiments. is problem is the reason for this increase in phosphorus in Figure 5. In the next step, phosphorophilic bacteria absorb and consume the produced phosphates, and they chart a downward trend from this point. e floating system with Vetiver works well as it is known, and the total phosphorus is reduced in wastewater with this concentration.

Results of Statistical Analysis with RSM for the ird Response (TP)
. Parameters A, B, C, and D are effective in the investigated response, according to Table 5. e correlation coefficient of R 2 was 0.99, which indicates that the model fits well.
e first-order polynomial model equation resulting from the experimental results' analysis is given in the following equation: In the above equation, the parameters A, B, C, and D are temperature, pH, EC, and time. Figures 6-9 show the interaction of independent parameters affecting COD, DO, TN, and TP responses resulting from data analysis. According to the figures, the TP value decreases with increasing temperature at pH 7. e lowest TP value was obtained at 36.5°C and EC � 500 cm/μs over 31 days.

Plant Root Growth Results.
e presence of plants increases the biofilms formed around the roots, which play an essential role in oxidation, absorption, and so on. In floating treatment wetland with higher biomass growth rates, better nitrogen transfer has been observed [41]. A set of   [42,43]. In this study, the root growth diagram shows that the container with lower contaminants concentrations has grown more without aeration. e root material has taken a diverse and thicker shape. is difference may be due to the plant trying to gain more nutrients. In an aerated container, the roots easily reach these sources and grow naturally because the movement of food is done by the pressure of air bubbles.
ey are mixed continuously, and they do not precipitate. However, in the case of aeration, the roots have to go deep to reach food sources.
As shown in Figures 10-12, the roots are still growing normally after 2 weeks because the nutrients are watersoluble, and there is no competition. However, the roots quickly start to grow vertically and do not branch after absorbing these substances. is problem is beneficial for using in water reservoirs of dams with a low or medium concentration of nutrients because nutrients are absorbed by root growth. e rate of root growth, as it is known, during 30 days of research for the Vetiver plant in an unaerated container with diluted wastewater and aerated container with diluted wastewater is on average equal to 25 and 20 cm, respectively.
However, various researchers have conducted several studies on the effectiveness of Vetiver, for example, Kirti et al. compared the phytoremediation efficiency of vetiver and lemongrass in pots against compared simulation of Ni-Cd battery electrolyte waste (EW). ey observed that the accumulation of nickel (μgg-1) in shoots (36.8) and roots (252.9) of Vetiver was higher than that of lemongrass 12.5 and 7.79, respectively. erefore, Vetiver was more resistant to EW toxicity than lemongrass because it had fewer morphological parameters and lower rise in TBARS against the doses of EW. Also Vetiver roots had higher Niel accumulation than Cd in EW-contaminated soil in comparison with lemongrass [44].
In another study, Siyar et al. evaluated the use of Vetiver grass in plating real contaminated soils around a smelter. e studied parameters including voltage gradient (1-2 V/ cm), different electric field (AC-DC), saturated and unsaturated state, and Eh-pH changes for plating Vetiver performance were investigated. Vetiver grows for 21 days. en, three different voltage gradients (1, 2DC V/cm and 2AC V/ cm) were applied to the soil for 8 hours a day for 21 days and were compared with control cells without electric field (PR).
e results showed that the use of DC current significantly changes the values of Eh-pH, and this is in contrast to the use of AC current, which causes minor changes. Absorption of total metals in the stems of plants treated with AC with BCF > 1 was much higher than PR and DC. Due to the speed of movement and plant health, if AC current is applied for a long time, it can have better results in electrical treatment of Vetiver plant through the plant extraction process [45]. Ng et al. evaluated the growth performance, accumulation process, and adsorption skills of Vetiveria zizanioides (Linn.) Nash. In both single and mixed heavy metals (Cd, Pb, Cu, and Zn)-disodium EDTA-enhanced contaminated soil. Results showed that all accumulation of heavy metals were significantly higher (p < 0.05) in both lower and upper roots and tillers of Vetiver grass for Cd + Pb + Cu + Zn + EDTA A : T e m p r a t u r e       [47]. In another study, Datta et al. investigated the potential of using Vetiver as a plant purifier to remove tetracycline (TC) from aqueous media. Vetiver plants were grown for 60 days in a greenhouse in a TC-contaminated hydroponic system. Preliminary results indicate that complete elimination of tetracycline occurred within 40 days in all TC treatments. e initial concentration of TC had a significant effect (p < 0.0001) on the removal kinetics of tetracycline in roots and stem tissues, which confirmed root uptake and transfer to the stem. In general,  A : T e m p r a t u r e past and present studies prove the effectiveness of plant phytoremediation, especially Vetiver in water and wastewater treatment of various pollutants [48]. ese man-made made structures of floating islands could be further improved in their filtration efficiencies and survival when a chitosan-based filter was added which could reduce the overall heavy metal ion concentration of polluted water [49].

Conclusion
According to studies, the use of wastewater treatment by floating islands in developed countries receives more attention. As mentioned, this method is considered more than others, with many benefits, especially its economics. It is better to pay more attention to such practices in undeveloped countries, following the developing and developed countries. erefore, research has been done to increase its efficiency and apply them to optimize artificial floating islands. We will now briefly review the highlights of this research. e highest percentage of COD removal was related to the container containing dilute aerated wastewater (97%); the lowest reduction rate was related to the container containing dilute unaerated sewage (93%). e highest TN removal percentage was related to the container containing dilute aerated waste (90%). e lowest rate of reduction was related to the container containing dilute unaerated wastewater (55%). In total phosphorus, the highest percentage of removal was related to the container containing dilute aerated wastewater (66%). e lowest rate of reduction (42%) was related to the container containing dilute unaerated wastewater. e highest evaporation percentage is related to the container containing dilute aerated wastewater (40%); the lowest evaporation percentage is associated with the container containing dilute unaerated sewage (31%). In general, the efficiency of this project is high and worth the investment. Since the leaves and roots of this plant have economic value, it compensates its initial capital investment and starts to create profit after a short time. Due to the above results, the aerated system had a higher efficiency in other experiments, except for the evaporation section. However, it is recommended to use an unaerated system on a large scale since both the energy cost of the aerators and evaporation are removed and reduced, respectively.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.