4.1 The effect of plants on water purification
According to the 2019 China Ecological Environment Bulletin, among the 107 lakes (reservoirs) monitored for water quality, oligotrophic status was found in 9.3%, mesotrophic status in 62.6%, mild eutrophication status in 22.4%, and moderate eutrophication status in 5.6%. Livestock and poultry manure, domestic garbage and domestic wastewater discharge are important factors in water pollution (Wang et al. 2018b, Wang et al. 2015a, Wang et al. 2018c). In particular, high loads of N and P can cause severe eutrophication of water bodies (Beusen et al. 2019; Aleksandra 2019). Phytoremediation technology is considered to be a promising method to address this issue (Yu et al. 2019). Plants absorb nutrients such as nitrogen and phosphorus through growth, and at the same time promote their own growth, so as to achieve the purpose of purifying water bodies (Vymazal 2013). Because different plants have different growth capabilities, root types, and nitrogen and phosphorus accumulation capabilities, they differ in their purification of wastewater (Zhu et al. 2011; Samal et al. 2017; Deane et al. 2018).
Aquatic plants can use their roots to absorb any form of soluble N and most forms of P from the wastewater. Aquatic plants transport oxygen to the roots through photosynthesis, and form an oxidative microenvironment in the root area, which further promotes nitrification (Li et al. 2020). In addition, the surface area of plant roots provides a place for microorganisms to attach and creates a microenvironment for the growth of various microorganisms (Chen et al. 2015; Li et al. 2020). Generally, the more developed the root system, the better the plant can treat wastewater. Wetland plants such as Phragmites australis, Typha orientalis Presl, Iris pseudacorus L., I. ensata and Canna indica L. are widely used because of their good purification effects (Choudhury et al. 2019). In particular, I. pseudacorus and I. ensata, which belong to the Iris family, show strong resistance to pollution and are widely used due to their adaptability to the environment, light and humidity tolerance and developed root systems. At present, there are many studies on these plants as CWs plants, and the purification effect is obvious (Li et al. 2021; Zhang et al. 2020; Chance et al. 2020). As an ecological treatment method of surface water, EFB should not only consider the purification capacity of plant, but also consider the landscape effect. Since emergent plants and submerged plants are distributed in different parts of water bodies, their combination not only improves water purification, but also has an obvious effect of the sense of landscape. Therefore, both emergent plants and submerged plants are important to research for use in EFBs. In this experiment, we used I. ensata and P. malaianus, and both emergent plants and submerged plants were taken into consideration.
Judging from the change in TN concentration, I. ensata and P. malaianus can remove nitrogen from wastewater, but the removal effect of the two plants is very different, even under the same concentration treatment. In the high-pollutant concentration water treatment, the TN removal rate of I. ensata increased significantly in the first 12 days of the experiment, reached more than 55% on the 12th day, and increased slowly after the 12th day. The absorption of nitrogen by I. ensata is mainly concentrated in the early stage, and is gradually reduced in the later period due to the slow growth of the plant (Yuan et al. 2018; Peng et al. 2020). In the high-pollutant concentration water treatment, the nitrogen removal of P. malaianus gradually stabilized on the sixth day, increased slightly by the twelfth day, and decreased in the later period. The P. malaianus had a low demand for nitrogen during growth (Liu et al. 2016). The nitrogen purification time of aquatic plants is related to the growth demand of plants (Su et al. 2019). The growth of I. ensata has a large demand for nitrogen, a long removal cycle and a high removal rate.
Under the conditions of medium-pollutant concentration water, the removal trend of I. ensata for TN was the same as that of the high-pollutant concentration water group, but its removal rate reached 51% on the sixth day, maintained an increasing trend, and was about 80% at the end of the experiment. This may be due to an insufficient nutrient supply under this pollutant concentration water treatment, and an excess supply under the high-pollutant concentration water treatment, so the purification effect of medium-pollutant concentration water environmental conditions is best (Dai and Li 2016). Compared with that of high-pollutant concentration water, the removal rate of TN under medium-pollutant concentration water conditions gradually increased, and the purification effect under these conditions was significantly better. Under low-pollutant concentration water treatment conditions, the nitrogen removal ability of I. ensata was much lower than that of the medium- and high-pollutant concentration water experimental groups. Although the removal rate gradually increased, the trend was relatively slow, which may be because the low concentration of nitrogen and phosphorus could not meet the needs of plant growth. Under low-pollutant concentration water conditions, P. malaianus showed a better removal effect (Zhou et al. 2018). The above results also show that the photosynthesis of submerged plants is significantly less than that of emergent plants, and the nitrogen demand of submerged plants is significantly smaller (Short et al. 2016). Compared with that of the control group, the removal rate of I. ensata was higher. The difference in purification was most obvious under medium-pollutant concentration water conditions, followed by high-pollutant concentration water conditions, which indicated that I. ensata had a higher demand for TN. Compared with that of the control group, the removal rate of TN in planted treatment with P. malaianus under low-pollutant concentration water conditions was significantly higher, while under high-pollutant concentration water conditions it was lower. Across the conditions of low, medium- and high-pollutant concentration water, the removal rate of TN showed a gradual decrease, which indicates that TN was released under the high-concentration simulated wastewater conditions. The main reason for this is that under high-pollutant concentration water conditions, plants grow and renew quickly, resulting in the release of TN from dead plant residues (Wang et al. 2011; Sun et al. 2012).
In terms of the change in TP concentration, I. ensata and P. malaianus differed greatly from their removal of TN. Under high-pollutant concentration water conditions, the TP removal rate of I. ensata was lower. However, P. malaianus did not remove any TP, but instead increased the concentration of TP, presumably due to the release of residues of plant growth. It has been confirmed by the study of water purification in CWs that plant residues can lead to the concentration of pollutants increases (Wang et al. 2020a). Under medium-pollutant concentration water conditions, the TP removal rate of I. ensata and P. malaianus increased over time, but the removal rate of I. ensata was higher compared with that of P. malaianus. In the low-pollutant concentration water treatment, I. ensata and P. malaianus both showed better purification effects. The purification effect of I. ensata was higher than that of the control under the conditions of medium and low-pollutant concentration water, while the purifying effect of P. malaianus was higher than that of the control only under low-pollutant concentration water conditions. Under high-pollutant concentration water conditions, the purification effects of the two plants were lower than that of the control. This shows that I. ensata is more suitable for growth in medium-pollutant concentration water, while P. malaianus is more suitable for low-pollutant concentration water treatment. Wastewater with high concentration of TP is not suitable for direct purification by aquatic plants. Under the low concentration condition of TP, more plants can be selected for water purification. The combination of emergent plants and submerged plants can significantly improve the purification of TP in water (Chen et al. 2020).
As shown in Table 3, each aquatic plant can basically achieve the optimal removal effect of TN and TP in the range of 20-28 days. No matter it is emergent plant or submerged plant, the removal rates of TN and TP are different under different concentrations. In general, the TP removal rate of most aquatic plants decreased with the increase of TP concentration in wastewater, and which indicates that the demand of aquatic plants for TP is limited. Only a small number of aquatic plants (Juncus effusus, P. orientale, I. pseudacorus and T. Orientalis) increased the removal rate of TN in wastewater with the increase of TN concentration. The removal of TN by most aquatic plants has no correlation with the change of TN concentration in wastewater, and which indicates that the removal of total nitrogen in wastewater by most aquatic plants is affected by external conditions. The highest removal rate of TN (60 mg/L) by P. orientale was more than 95%, and the highest removal rate of TN (6.22 mg/L) by Pistia stratiotes L. was more than 97%. The I. ensita has advantages in treating high concentration TN wastewater, and P. malaianus has advantages in treating low concentration TN wastewater. The I. ensata and P. malaianus have advantages in treating low concentration TP wastewater. The research on the removal of TN and TP in water by multiple plant cultivation needs to be strengthened.
4.2 Effect of pollutant concentration on plants in water purification
Nitrogen and phosphorus are essential nutrient elements for plant growth. Suitable concentrations of nitrogen and phosphorus can promote plant growth. The low concentration of nitrogen and phosphorus in wastewater will lead to the nutrient deficiency of aquatic plants, and then affect the growth of aquatic plants. However, high concentrations of nitrogen and phosphorus also inhibit the growth of aquatic plants (Li et al. 2017). The growth of P. crispus in the low nutrient environment was better than that in the eutrophic state (Qian et al. 2014). The high nutrient concentration limits the growth of P. crispus. Under the low biomass condition of aquatic plants, the inhibitory effect of ammonia nitrogen on plant growth is more obvious (Markou et al. 2014). Different plants have different adsorption of nitrogen and phosphorus due to their specific characteristics. When nitrogen and phosphorus in water exceed plant growth requirements, a stress effect on the growth of the plant will occur, resulting in a decrease in water purification. In plants, the nutrient tolerance range is usually greater than that suitable for growth. The nutrient tolerance range differs according to plant species (Table 4).
In this study, the growth status of I. ensata (an emergent plant) in the presence of high-pollutant concentration water was better than in the low-pollutant concentration water group, while the growth status of P. malaianus (a submerged plant) in the presence of low-pollutant concentration water was better than in the high-pollutant concentration water group. High-pollutant concentration water provided sufficient nutrients for the growth of I. ensata, and it grew vigorously throughout the experiment. On the contrary, the growth of P. malaianus was inhibited by the high-pollutant concentration water. In particular, plant leaf withering increased with the concentration of pollutants. In the low-pollutant concentration water experimental group, the growth of P. malaianus was good, and the removal rates of nitrogen and phosphorus were high. The results showed that the nutrient content in the presence of high-pollutant concentration water exceeded the tolerance of P. malaianus. The removal rates of nitrogen and phosphorus in low-pollutant concentration water were relatively high, which indicated that the contents of nitrogen and phosphorus under these conditions are suitable for the growth of P. malaianus. The Chl.a/b ratio of I. ensata is significantly higher than that of P. malaianus, which indicated that the growth ability of I. ensata was higher than that of P. malaianus (Table 2). I. ensata has strong tolerance to high concentration of nitrogen and phosphorus. The Chl.a/b ratio of P. malaianus was significantly lower at high concentration, which indicated that P. malaianus was not suitable for growth at high concentration.
Compared with the background value, the removal rates of TN at various nutrient concentrations in the I. ensata groups were higher, while the removal rate of TP was only higher than the background value in the medium- and low-pollutant concentration water experimental groups. This indicated that the tolerance range of I. ensata to TN was greater than the experimental value (> 29.89 mg·L-1), while the tolerance range to TP was less than the experimental value (< 7.87 mg·L-1). The tolerance ranges of I. ensata to TP and TN was significantly lower than E. crassipes, Iris sibirica L., I. pseudacorus, Thalia dealbata Fraser, Oenanthe javanica (Blume) DC and T. orientalis (Feng et al. 2020; Xu et al. 2012). The removal rates of TN and TP by P. malaianus were only higher than the background value under the low-pollutant concentration water treatment, and this indicated that the tolerance range of P. malaianus to TN and TP was less than the experimental value (< 2.06 mg·L-1 and < 0.43 mg·L-1). The purification effect of Chara inconnexa in high eutrophic water (TN = 15, TP = 2) was lower than that in low eutrophic water (TN = 1.5, TP = 0.2), which showed high concentration inhibition (Wei et al. 2012). On the whole, the tolerance concentration of emergent plants to pollutants is higher than that of submerged plants, which may be related to the fact that the tolerance of submerged plants leaves to pollutants is lower than that of emergent plant roots. Because the demand of aquatic plants for nitrogen is higher than that for phosphorus (Xing et al. 2020), in addition, the rapid release of phosphorus from roots and leaves to the water during the decay process also reduces the reduction of phosphorus concentration in polluted water (Wang et al. 2011). Therefore, the tolerance range of aquatic plants to TN is higher than that to TP. These results provide an important basis for plant selection for wastewater treatment, but it is recommended that further research be conducted on the nutrient tolerance ranges of other plant species.
4.3 Economic analysis
I. ensata and P. malaianus not only have the function of water purification, but also have high economic value. The economic value of I. ensata and P. malaianus was different. I. ensata has tender leaves, which is the food of goats (Zhao et al. 2020). I. ensata also has high ornamental value and is favored by more citizens (Zhao et al. 2008; Zhu et al. 2013). After being harvested, P. malaianus can be used as food for livestock and fish. In the indoor water ornamental culture, P. malaianus also has ornamental value. The ornamental value of I. ensata is higher than that of P. malaianus, and P. malaianus is more valuable as food than I. ensata. I. ensata and P. malaianus can also be used as raw materials for biogas fermentation (Zhou et al. 2016). I. ensata and P. malaianus also have good carbon fixation (Xiong et al. 2018; Wang et al. 2015b). Further studies are needed on the economy of I. ensata and P. malaianus.