Effects of calcium peroxide on growth of Pontederia cordata and water quality

Abstract

Combined chemical-biological methods play an important role in ecological restoration. However, it is still unknown whether the addition of calcium peroxide (CaO₂) can promote the growth of Pontederia cordata (P. cordata) and the improvement of water quality. Effects of different levels of added CaO₂ (0, 0.2, 0.4, 0.6, 0.8, 1, and 2 kg/m²) on P. Cordata growth and water quality were investigated. CaO₂ promoted germination and tillering and increased the leaf number (p < 0.05) of P. cordata, but reduced leaf length. The plant height and leaf width increased only when the added CaO₂ levels were 0.6 kg/m² and 1 kg/m², respectively. Adding 0.4–0.6 kg/m² CaO₂ had a cumulative effect on the biomass. High levels of added CaO₂ (2 kg/m²) affected the physiology of P. cordata resulting in the most serious damage to cell membrane lipids, the lowest catalase activity, and the highest chlorophyll content. Adding CaO₂ did not cause DO and pH of the water to be too high for a long time, but excessive CaO₂ made TN and COD_Mn too high. Thus, 1 kg/m² CaO₂ was most conducive to the growth of P. cordata and the improvement of water quality.

Keywords:

• calcium peroxide
• Pontederia cordata
• growth
• water quality

1. Introduction

Eutrophication is the main problem in current aquatic environments. Methods for controlling eutrophication include physical, chemical, and biological approaches (Shi et al. 2022). Although chemical reagents have a negative impact on aquatic organisms and cause secondary pollution (Fang et al. 2020), they are generally not recommended. However, due to its rapid treatment effect and convenient application, it also has a certain value in application especially in emergency treatment (Chen et al. 2023; Cao et al. 2024). Chemical reagents commonly used in ecological restoration include calcium peroxide (CaO₂), copper sulfate, hydrogen peroxide (H₂O₂), magnesium peroxide, and calcium carbonate (Gu et al. 2023; Isaac et al. 2023; Zhou et al. 2023). CaO₂ can slowly release oxygen (O₂) when exposed to water (Wang et al. 2016). Compared to other chemical reagents, it is easy to obtain, convenient to transport, low cost, good treatment effect. And it is non-toxic and does not pollute the environment, so it has been widely used in the water, soil, and silt restoration (Wang et al. 2018; Xu et al. 2020; Taillard et al. 2023). Adding CaO₂ to water can not only inhibit phosphorus (P) release from sediments (Xia et al. 2020), increase the dissolved oxygen (DO) and solve the problem of water hypoxia (Wang et al. 2019; Li et al. 2020), which plays an important role in treating black-odorous water (Wang et al. 2019), but also effectively inhibits cyanobacterial blooms, kills pathogenic bacteria (Gu et al. 2023), and changes the species dominance of phytoplankton communities (Zhang et al. 2022; Ma et al. 2023). Nykänen et al. (2012) used granular CaO₂ as a slow-release agent to increase O₂ content of lakes in Finland in view of the hypoxia caused by an increase in nutrient load. At the same time, due to the excellent oxidation ability of CaO₂, it can oxidize various organic pollutants (Liu et al. 2006; Yang et al. 2021).

However, it is often difficult to achieve the desired results using a single method, and in practical repair applications, CaO₂ also has defects such as short timelines. So it is more effective to combine multiple methods for joint repair. Gu et al. (2015) found the combination of Pontederia cordata (P. cordata) and 15 L min⁻¹ aeration had the best effect on improving the water quality of urban polluted river. When CaO₂ combined with sponge iron to treat eutrophic water, it can control the eutrophication of water effectively, reduce the content of total nitrogen (TN) and strengthen the stability of phosphorus (P) in sediments (Zhu et al. 2022). However, gaps remain in the knowledge of using CaO₂ combined with aquatic macrophytes to repair water. P. cordata is a perennial emergent macrophyte of the genus Pontederia, which is native to North America. It is widely used in aquatic ecological restoration because of its strong adaptability, fast diffusion, easy formation of a dominant population per plant, and its ornamental value (Gu et al. 2015; Zhang et al. 2016). P. cordata can inhibit algae (Qian et al. 2017) and has strong cadmium tolerance (Mei et al. 2020; Xin et al. 2022), which has a significant effect on the elimination of toxic heavy metals from water (Xin et al. 2020), and has an advantage in the treatment of organic and farmland wastewater (He et al. 2021; Li et al. 2022).

Combining chemical methods and aquatic macrophytes provides an important means of restoration. Both CaO₂ and P. cordata have the function of repairing water; however, factors affecting the growth of emergent macrophytes include temperature, nutrients, pH, and salinity (Hadad et al. 2017; Xiao et al. 2019). The effect of CaO₂ decomposition on the growth of P. cordata and water quality improvement remains to be further studied. Thus, through greenhouse experiments, this study considered the emergent macrophyte P. cordata as the research object and investigated the effect of CaO₂ on the growth of P. cordata and water quality. We hypothesized: (1) The addition of CaO₂ can promote the growth of P. cordata because CaO₂ reacts with water to provide O₂. However, excessive CaO₂ will lead to high pH which causes morphological and physiological damage to P. cordata. (2) The addition of CaO₂ releases O₂ and increases DO, which is conducive to the degradation of pollutants and the improvement of water quality. But the addition of CaO₂ to cause high pH may also bring side effects to water quality. This study can provide a scientific reference for the rational use of chemical reagents and aquatic macrophytes in ecological restoration.

2. Materials and methods

2.1. Experimental design

This experiment was carried out in the outdoor greenhouse of the Aquatic Environment Ecological Restoration Laboratory of Hubei Normal University (115°3′19′′E, 30°14′6 ′′E). The experiment began on March 6, 2019 and ended on April 23, 2019. The average temperature during the day and night of the experiment was 21.4 °C and 12.3 °C, respectively. The water temperature can be seen in Table S1.

The sediment of Qingshan Lake was used as the substrate (total phosphorus (TP):1201.77 ± 106.85 mg/kg, TN: 620.74 ± 171.75 mg/kg, organic matter (OM):71.32 ± 6.36 g/kg). After digging up the sediments from Qingshan Lake, a sieve was used to screen out the stones, animal and plant debris, and other debris. After sufficient mixing for 20 min, we placed the sediment in each experimental group to reduce the inhomogeneity of sediment in each treatment group. Each planting box (50 cm length, 40 cm width, 30 cm height) was laid with a 15 cm sediment matrix, and six uniform and healthy P. cordata rhizomes with well-preserved roots and similar root number and length (weight: 53.5 ± 0.33g) were planted. Then lake water from Qingshan Lake (TP:0.45 ± 0.05 mg/L, TN:2.9 ± 0.21 mg/L) approximately 10 cm high was added (Figure 1). We used a small container to hold water and then added the water slowly to prevent the disturbance of the sediment. After the water was clear, CaO₂ particles were added to the six treatment groups at concentrations of 0.2 (SY1), 0.4 (SY2), 0.6 (SY3), 0.8 (SY4), 1 (SY5), and 2 (SY6) kg/m². At the same time, a group of treatment groups without adding CaO₂ was set as the control group (CK). Each treatment group was comprised of three replicates.

Figure 1. Experimental design.

2.2. Index determination

2.2.1. Growth indexes

The germination rate, plant height, total tiller number, leaf number, leaf length and width of each treatment group were measured every five days. Finally, the plants were harvested, gently rooted, and washed with water. The aboveground and underground biomass were weighed using an electronic balance, and the root-shoot ratio (R/S) was calculated as follows: (1) $$\frac{R}{S}=\frac{w_1}{w_2}$$(1) where w₁ was aboveground biomass, g, w₂ was underground biomass, g.

2.2.2. Physiological indexes

After the plants were harvested, the malondialdehyde (MDA), catalase (CAT) and chlorophyll (Chl) were determined according to the steps described in the ‘Plant Physiology Experiment Tutorial’ (Wang 2017). MDA, CAT, and Chl were determined by the thiobarbituric acid method, ultraviolet spectrophotometry and 95% ethanol-spectrophotometry, respectively. The absorbance was measured at wavelengths of 665 nm and 649 nm, and the contents of chlorophyll a (Chla) and chlorophyll b (Chlb) were calculated according to the formula. (2) $$\mathit{\text{Chla}}=13.95\times A_{665}-6.88\times A_{649}$$(2) (3) $$\mathit{\text{Chlb}}=24.96\times A_{649}-7.32\times A_{665}$$(3) (4) $$\mathit{\text{Chl}}=\mathit{\text{Chla}}+\mathit{\text{Chlb}}$$(4)

A was the value of absorbance.

2.2.3. Water quality indexes

During the growth of P. cordata, DO and pH of the water were measured in the first three days and then every six days. DO was determined in situ using a portable water quality monitor, EXO (YSI, USA) in the morning, and pH was determined using a precision test paper (Shanghai Sanaisi Reagent Co., Ltd.). In the middle (Day 24) and at the end (Day 48) of the experiment, TP, TN, permanganate index (COD_Mn), and chlorophyll a (Chla) in the water were measured by potassium persulfate oxidation ultraviolet spectrophotometry, alkaline potassium persulfate digestion, acid titration, and the 90% acetone method, respectively. All determination methods were based on the ‘Water and Wastewater Monitoring and Analysis Method (4th edition)’ (2002).

2.3. Data analysis

The experimental data were processed using Microsoft Excel 2010. One-way ANOVA was used to test for significant differences in growth indexs, physiological indexes, and water quality among the different treatment groups. Before One-way ANOVA, we used the Shapiro-Wilk test and Levene’s test to determine whether the data met the assumptions of normality and homogeneity of variance (p > 0.05). Data that did not meet assumptions of normality were transformed. Turky method was used for multiple comparisons. The significance level was set as p < 0.05. Origin 2021 was used for the drawing.

3. Results

3.1. Growth indexes

3.1.1. Germination rate

The addition of CaO₂ promoted the germination of P. cordata to some extent (Figure 2). Except for SY4, the germination rates of treatment groups added CaO₂ were higher than that of CK (the control group without CaO₂ added) (p > 0.05), which reached 83.33% on Day 8. The germination rate of CK was the lowest, even on Day 23, with a germination rate of only 72.22%, but there were no significant differences in the germination rate among all treatment groups (p > 0.05).

Figure 2. The germination rate of Pontederia cordata under different levels of CaO₂. CK represented the treatment group without CaO₂, SY1, SY2, SY3, SY4, SY5, and SY6 represented the treatment groups added CaO₂ with 0.2, 0.4, 0.6, 0.8, 1, and 2 kg/m², respectively. Error bars represent standard errors and different lowercase letters represent the difference between the treatment groups on the same experimental day (p < 0.05).

3.1.2. Plant height

The effect of added CaO₂ on the plant height of P. cordata varied over time (Figure 3). In the first 38 d, the plant heights of each treatment group were significantly different (p < 0.05). In the first 28 days, SY4 had the lowest plant height which was significantly lower than those of other treatment groups (p < 0.05), while from the 33rd day, SY6 had the minimum plant height. At the end of the experiment, adding CaO₂ had a significant effect on the plant height of P. cordata (p < 0.05). The plant height of SY3 alone was higher than that of CK (p > 0.05), while SY6 was the lowest, only 57.5 cm.

Figure 3. The plant height of Pontederia cordata under different levels of CaO₂.

3.1.3. Total tiller number

The addition of CaO₂ promoted the tiller of P. cordata (Figure 4). On Day 18, the number of total tillers in the CK reached 11, which was higher than that in the treatment groups with CaO₂. On Day 23, the total tiller number of treatment groups adding CaO₂ began to increase and exceeded that of CK on Day 33. SY6 grew slowly on Days 33–43 with individual deaths. On Day 48, the number of total tillers in the treatment groups with CaO₂ was higher than that in the CK (only 12 tillers).

Figure 4. Total tiller number of Pontederia cordata under different levels of CaO₂.

3.1.4. Leaf number

CaO₂ significantly increased the number of leaves of P. cordata (p < 0.05) (Figure 5). During the entire growth period of P. cordata, the leaf number in CK was the lowest. At the end of the experiment, the leaf number of SY5 reached 98.67 pieces/plant, which was 1.62 times that of the CK (p < 0.05).

Figure 5. The leaf number of Pontederia cordata under different levels of CaO₂.

3.1.5. Leaf length and leaf width

The addition of CaO₂ reduced the leaf length of P. cordata (p > 0.05) (Figure 6(a)). Except on Day 43, the leaf length of the CK was higher, which was significantly higher than that of SY6 (p < 0.05). At the end of the experiment, the leaf length of CK was 18.32 cm, whereas that of SY6 was only 14.21 cm.

Figure 6. The leaf length (a) and leaf width (b) of Pontederia cordata under different levels of CaO₂.

The effect of CaO₂ on the leaf width of P. cordata varied over time (Figure 6(b)). On Day 33–38, the leaf width increased slowly. After Day 38, the growth of leaf width accelerated, the maximum value of leaf width was recorded in SY5, and the minimum value was still recorded in SY6 (p < 0.05). At the end of the experiment, adding CaO₂ had a significant effect on the plant height of P. cordata (p < 0.05), but only SY5 had a larger leaf width than CK (p > 0.05).

3.1.6. Biomass and R/S

The addition of CaO₂ had no significant effect on aboveground biomass (p > 0.05), but had significant effects on underground biomass and total biomass (p < 0.05). With increasing levels of CaO₂, aboveground and underground biomass first increased and then decreased (p > 0.05) (Figure 7(a,b)). Aboveground and underground biomass reached maximum of 142.50 g in SY3 and 70.14 g in SY4, respectively (p > 0.05). However, the maximum total biomass appeared in SY2 (756.67 g), which was significantly higher than that of SY6 (p < 0.05) (Figure 7(c)).

Figure 7. Biomass (a–c) and R/S (d) of Pontederia cordata under different levels of CaO₂.

In the range of 0–0.6 kg/m² added CaO₂, the R/S increased with increasing CaO₂ (Figure 7(d)). The R/S of SY5 was significantly higher than that of CK, SY4 and SY6 (p < 0.05). The R/S of SY6 was the lowest (0.26).

3.2. Physiological indexes

3.2.1. Malondialdehyde

The addition of CaO₂ significantly affected the MDA content of P. cordata (p < 0.05). The MDA content in the CK was 0.018 mg/g. Except for SY1 and SY4, the MDA contents of SY2, SY3, SY5, and SY6 were higher than that of CK (Figure 8(a)) and increased with increasing CaO₂. The maximum content was 0.034 mg/g, which was 2.16 times that of SY1 (p < 0.05).

Figure 8. Malondialdehyde (a), catalase (b) and chlorophyll (c) in Pontederia cordata under different levels of CaO₂.

3.2.2. Catalase

Adding different doses of CaO₂ had a significant effect on CAT of P. cordata (p < 0.05). The CAT of SY6 was 0.69, which was significantly lower than that of CK (p < 0.05). Other treatment groups were 2.84, 1.32, 1.32, 2.87, and 3.72 times of CK, respectively (p < 0.05) (Figure 8(b)).

3.2.3. Chlorophyll

With increasing CaO₂, the Chl contents first decreased and then increased (p < 0.05) (Figure 8(c)), reaching a maximum of 3.10 mg/g at SY6, which was significantly higher than that in CK but not significant (p > 0.05). The Chl contents of the other groups were lower than that of CK. SY4 and SY5 had the lowest Chl content, only 20.5% and 23.32% of SY6, respectively (p < 0.05).

3.3. Physical and chemical indexes of water

3.3.1. Physical indexes

The initial value of DO in each treatment group was 9.32–10.17 (p > 0.05). DO of the CK increased continuously until Day 15, and then it began to decrease (Figure 9(a)). The DO values of the treatment groups with added CaO₂ were lower than the initial value in the first three days, decreased first, and then increased with increasing CaO₂. During Days 27–39, DO of SY1–4 were higher than that of CK, but after Day 45 was the opposite. In addition, during Days 1–3, DO of SY6, which had the highest CaO_2, was the highest. However, after Day 9, DO of SY6 was the lowest. At the end of the experiment, DO of CK was significantly higher than those of the other treatment groups and the initial value (p < 0.05), whereas those of the other treatment groups were opposite.

Figure 9. DO (a) and pH (b) in water under different levels of CaO₂.

The initial pH of the treatment groups were 7.27–7.47 (p > 0.05) (Figure 9(b)). During Days 1–39, pH increased with increasing CaO₂. During Days 15–39, pH of SY1-SY3 was lower than that of the previous nine days, while pH of SY4-SY6 was the opposite (p < 0.05). At the end of the experiment, pH did not increase with increasing CaO₂ but decreased significantly, even lower than the initial value, and then began to increase. And pH of the treatment groups with added CaO₂ (SY1–SY5) were lower than that of CK (p > 0.05), which was insignificant difference from the initial value (p > 0.05).

3.3.2. Chemical indexes

The addition of CaO₂ significantly affected TN, TP, and COD_Mn (p < 0.05), but had no significant effect on Chla (Figure 10). As the experiment progressed, TP and TN in the water decreased. In the middle of the experiment (Day 24), TP increased and then decreased with increasing CaO₂ (Figure 10(a)), reaching a maximum at the end of the experiment (Day48) in SY4. Only TP of SY6 was lower than that of CK. TN of CK, SY1, and SY2 were always significantly lower than SY3–SY6 (p < 0.05) (Figure 10(b)). Compared to that of CK, TN of SY1 and SY2 were lower (p < 0.05). At the end of the experiment, Chla in SY3, SY4, and SY5 increased sharply, whereas in other treatments, it decreased compared to Day 24, and Chla in SY6 was the lowest (Figure 10(c)). COD_Mn increased with increasing CaO₂ (Figure 10(d)) and ranged in 11.11–16.14 mg/L in Day 24 (p > 0.05). At the end of the experiment, COD_Mn began to increase in SY3–SY6, and were significantly higher than those in Day 24 (p < 0.05). SY6 reached the maximum value of 74.18 mg/L (p < 0.05).

Figure 10. TP (a), TN (b), Chla (c) and COD_Mn (d) in the water under different levels of CaO₂ in the middle and end of the experiment.

4. Discussion

4.1. Effect of added CaO₂ on the growth of Pontederia cordata

CaO₂ reacted with water to produce Ca(OH)₂ and O₂, and sufficient O₂ promoted the germination of P. cordata to a certain extent, making P. cordata reached 77.77% on Day 8 of the experiment (except in SY4). Mei et al. (2017) found that CaO₂ provided O₂ during rice seed germination, thereby promoting rice germination and seedling growth. In the first 18 days of our experiment, the observed greater number of total tillers in the control group (CK) without CaO₂ may be related to the higher DO and lower pH of the water during this period. Sufficient DO provides energy for plant growth and produces sufficient tillers. At the end of our experiment, the number of tillers in the treatment groups with added CaO₂ were higher that in CK indicating that the addition of CaO₂ was beneficial to the tillering of P. cordata, but the mechanism needed to be further explored in the future. Leaf numbers was closely related to tillers. The more tillers, the more leaves. Thus, fewer tillers led to the lowest leaf number in CK. Chl is essential for plant photosynthesis, and the Chl content reflects the physiological activity of plants. Ma et al. (2023) found that the decrease in Chla content was related to damage caused by CaO₂ to the photosynthetic system because H₂O₂ produced by CaO₂ inhibited the photosynthetic yield (Drábková et al. 2007). In the present experiment, except for SY6, the Chl content of other treatment groups were lower than those of the CK, which indicated that under the addition of 0.2–1 kg/m² CaO₂, H₂O₂ produced by CaO₂ could not inhibit the photosynthetic yield. Thus, only when the amount added reached 2 kg/m² did it have a cumulative effect on the Chl content of P. cordata.

Plants exhibit their regulatory functions in response to changes in the external environment. However, when environmental factors cause stress in plants, their regulatory functions will be disrupted. There are many protective enzymes in plants that can alleviate or reduce the damage caused by reactive oxygen radicals to plants (Liu et al. 2023). As one of the main cellular protective enzymes against reactive oxygen radical damage, CAT decomposes H₂O₂ into H₂O and O₂, and plays an important role in scavenging free radicals and peroxides (Liu et al. 2019; Liang et al. 2022). Xin et al. (2018) found that with the increase of cadmium concentration and the prolongation of stress time, the scavenging ability of CAT decreased, and the accumulation of reactive oxygen radicals such as H₂O₂ in cells increased, thus accelerating the senescence of P. cordata leaves. In the present study, the CAT of the treatment group with 0–1 kg/m² CaO₂ was higher than that of the CK indicating that P. cordata had a specific adaptability to 0–1 kg/m² CaO₂. While its adaptability changed to stress when 2 kg/m² CaO₂ was added, which made P. cordata in SY6 shorter with more yellow leaves. This may be due to the low concentration of H₂O₂ released by low-dose CaO₂ and the short action time, so P. cordata has a specific adaptability. When the addition of CaO₂ is excessive, the concentration of H₂O₂ is high and the action time is prolonged (Liu et al. 2017). Moreover, the excessive addition of CaO₂ led to the high pH, and the combination of H₂O₂ and pH caused stress to P. cordata. The MDA content was the highest when the amount of CaO₂ added was 2 kg/m², which also indicated that the addition of excessive CaO₂ would destroy the metabolic activity of P. cordata, causing the accumulation of superoxide anion free radicals, H₂O₂, and other reactive oxygen free radicals in cells. The damage to the cell membrane was more serious and caused P. cordata to suffer great harm. Thus, the addition of excessive CaO₂ causes physiological stress in P. cordata. Although the MDA content of SY5 was also higher, second only to SY6, its CAT activity was the strongest to resist reactive oxygen species, so the growth indexes of the SY5 treatment group were better than those of SY6.

The stress effect of excessive added CaO₂ on P. cordata was also reflected in the growth of the plants. The addition of CaO₂ had no significant effect on the plant height, and the leaf length in each treatment group was shorter than that in the CK. Plant height and leaf length in the 2 kg/m² CaO₂ treatment were the lowest, which may be related to the reaction of CaO₂ with water to generate OH^-, resulting in an increase in the pH of the water (Wang et al. 2016). When pH is less than 3 or greater than 9, the protoplasts of most vascular plant root cells are severely damaged (Akçin et al. 1993). Yang (2014) also found that when the pH exceeded 9, the growth of Hydrilla verticillata was stressed. However, Typha orientalis is stressed when grown at very low pH values because very low pH values reduce the uptake of cations by the plants (Hadad et al. 2017). In the present study, the treatment group with 2 kg/m² CaO₂ (SY6) had the highest pH, and its plants showed greater differences in growth compared to other groups. On Day 18 of the experiment, the plant heights of SY5 and SY6 were the highest, and the biomass of P. cordata first increased and then decreased, indicating that P. cordata had a specific tolerance to the addition of CaO₂. However, at the end of the experiment, the plant height and biomass of SY6 were the lowest. The plants in SY6 were easy to brittle, the leaves were yellow, many white crystals were attached to the stems and leaves, the number of tillers was low, and some tillers died. These phenomena did not occur in the other treatment groups, further indicating that when the amount of CaO₂ added was 2 kg/m², the growth of P. cordata was inhibited. Thus the optimal range for addition of CaO₂ for the growth of P. cordata was 1 kg/m².

4.2. Effect of added CaO₂ on water quality

As a chemical agent, CaO₂ plays an important role in improving water quality. It can stabilize water at a mesotrophic level and low turbidity (Zhu et al. 2022), and significantly improve the transparency of water (Wang et al. 2019; Ma et al. 2023). In our study, after adding different doses of CaO₂, pH of each treatment group increased, especially during Days 15–39. Because CaO₂ reacts with water to generate Ca(OH)₂, which is dissolved in water, and ionized OH⁻, which will make water alkaline (Diao et al. 2022). However, in our experiment, CaO₂ did not cause the pH of water to be too high for a long time, but rather, pH gradually recovered after an initial pH increase. This was because CaO₂ had a certain timeliness. After 48 days, the consumption of CaO₂ and water reduces the amount of CaO₂, so OH⁻ will not continue to increase. In addition, photosynthesis during the growth of P. cordata and the decay of some litters also affect the pH of the water (Hu et al. 2023). Ma et al. (2023) and Li et al. (2020) also found that the addition of CaO₂ not make the pH of water too high for a long time.

DO can provide available O₂ for aquatic organisms, which is essential for the survival of aquatic organisms (Meng 2023; Chen et al. 2024). CaO₂ continuously releases O₂ and H₂O₂ by reacting with water, and H₂O₂ further decomposes to produce O₂, thereby inhibiting the formation of an anoxic environment (Wang et al. 2016; Ma et al. 2023). Nykänen et al. (2012) also showed that the addition of CaO₂ increased the concentration of DO in the lake. In the early stage (Day 1–2), DO of SY6 was higher than those of other treatment groups with added CaO₂, but after Day 9, DO of SY6 was at a low level for a long time. This may be because, after adding CaO₂, white crystals formed on the surface of the water and of P. cordata, which have hindered the photosynthesis of the plant and the supply of air to DO of the water. In addition, the biomass of SY6 was low; therefore, O₂ supply to the water was also low, resulting in lower DO in the water. However, after Day 45, DO of the treatment groups with added CaO₂ were lower than that of the CK. This may be due to the continuous consumption of CaO₂ with water over time, coupled with the vigorous growth of P. cordata. The demand for O₂ was high, resulting in a decrease in the level of DO in the water. In addition, DO reflects the degree of water pollution, especially organic pollution. At the middle and end of the experiment, DO in CK was significantly higher than that of the treatment group with CaO₂, and higher DO was beneficial for the degradation of pollutants and accelerated water purification. So COD_Mn in CK was the lowest. Similarly, Li et al. (2020) found that the dosage of CaO₂, the greater the increase in COD in the overlying water. Although the catabolism of phytoplankton can also cause an increase in COD_Mn (Wang et al. 2022), however, no algal bloom was observed in our experiment.

The effect of pH on the P removal efficiency of CaO₂ is significant (Xiong et al. 2015). OH^- released from CaO₂ causes the pH of the water to be weakly alkaline, and the formed Ca(OH)₂ can form a physical barrier on the surface of sediments and effectively inhibit the release of P in the sediment and eutrophic water (Xia et al. 2020; Chen et al. 2023). In this study, TP first increasing and then decreased with increasing levels of CaO₂. It was related to the pH of water. When pH is too high, CaCO₃, Ca(OH)₂, and other precipitates are easily formed to reduce the concentration of Ca²⁺ in the solution, which is not conducive to the crystallization reaction; thus, the rate of removal of P by CaO₂ is reduced. However, when the pH of water is greater than 9.5, Ca²⁺ reacts with HPO₄^2- to form a stable hydroxyl calcium phosphate precipitate, thereby reducing the concentration of P in the water (Madsen et al. 1995; Cho and Lee 2002; Zhang et al. 2009). In the middle of the experiment, pH of SY5 and SY6 was greater than 9.5, which may have been due to the formation of stable hydroxyl calcium phosphate precipitation by Ca²⁺ and HPO₄^2-, resulting in low P content in the water. The amount of CaO₂ added was less than 0.4 kg/m², and the combination of P. cordata and CaO₂ had a better removal effect on TN, but when the amount of CaO₂ added reached 0.6 kg/m², TN did not decrease but increased, which may be related to the fact that CaO₂ accelerated the release of TN in the sediment (Yang et al. 2021). Because CaO₂ has strong oxidizing properties, it promotes the decomposition of sediment OM and the conversion of ammonia nitrogen to nitrate nitrogen (Yuan 2019). At the end of the experiment, P. cordata entered the rapid growth stage, and the absorption capacity of N was enhanced, which made TN lower than that in the middle of the experiment. However, because of the relatively poor growth of P. cordata in SY6, the absorption capacity of N was also weak; thus, TN of SY6 remained high. Ca(OH)₂ and H₂O₂ produced by the reaction of CaO₂ with water reduces the concentration of nutrients, kills algae, and maintains Chla at a low level (Wang et al. 2022). The flocculation of H₂O₂ also reduces the concentration of Chla (Zhang et al. 2022). Consequently, Chla in each treatment group with CaO₂ in the middle of experiment was lower than that in the CK. Cho and Lee (2002) also found that the growth of Microcystis aeruginosa was severely inhibited with CaO₂, resulting in a sharp decrease in Chla. However, due to the fast decomposition rate of H₂O₂, its residence time in water is short, which will lead to the failure of mitigation of algal blooms (Wang et al. 2019). Therefore, at the end of the experiment, due to the decomposition of H₂O₂, the flocculation and sterilization were reduced, and Chla in some treatment groups with CaO₂ was higher than that in the middle of the experiment.

4.3. Implication for the application of CaO₂ in aquatic macrophytes growth and water purification

Although CaO₂ promoted germination, tillering, and leaf length in P. cordata, it is necessary to control the amount of CaO₂ added. Excessive CaO₂ led to the accumulation of a large amount of reactive oxygen radicals, damage to P. cordata, and inhibition of enzyme activity, resulting in dwarf plants and yellow leaves. Previous studies have generally suggested that the use of CaO₂ to repair water leads to a sharp increase of pH in the water, and then increases the release rate of O₂, thereby increasing the loss of ineffective oxygen (Zhang et al. 2014). However, in our study, CaO₂ did not cause water to remain in a high-pH state for a long time and ultimately only made pH slightly higher than the initial value, which was consistent with Ma et al. (2023). In addition, in field open waters, the water is mobile and has a good buffering capacity against pH increases caused by CaO₂ (Nykänen et al. 2012). Li et al. (2020) showed that the pH of water could be controlled at 7.5–8 when the addition amount of CaO₂ was 3–5 g/L, which met the water quality standards. Thus, in practice, controlling the amount of CaO₂ or adding buffer substances in time will not lead to an excessively high pH (Arghavan et al. 2022). However, repairing eutrophic water by adding CaO₂ has a time limit, which needs to be combined with biotechnological methods.

5. Conclusions

There were knowledge gaps in the effects of chemical reagents on the growth of emergent plants and the purification of water. By adding different amounts of CaO₂ to the water where P. cordata was grown experimentally, we explored the effects of CaO₂ on the growth of P. cordata and water quality. Adding CaO₂ promoted germination (p > 0.05) and tillering, and increased leaf number (p < 0.05), but reduced leaf length. Only when the addition amount were 0.6 kg/m² and 1 kg/m², it can promote the plant height and leaf width of P. cordata, respectively. Addition of a low amount of CaO₂ had a cumulative effect on the biomass of P. cordata. But adding excessive CaO₂ (2 kg/m²) caused the most serious damage to P. cordata, with the lowest CAT and highest Chl. Adding CaO₂ did not cause DO and pH of the water to be too high for a long time, but excessive CaO₂ made TN and COD_Mn too high.

In practice, adding 1 kg/m² CaO₂ is beneficial to the growth of P. cordata and the purification of water quality.

Disclosure statement

The authors declare no conflict of interest.

Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Additional information

Funding

This research was funded by the Open Foundation of Hubei Key Laboratory of Pollutant Analysis & Reuse Technology (Hubei Normal University) (PA220103), the Open Foundation of Research Center for Transformation and Development of Resource depletion Cities (Hubei Normal University) (KF2023Z01), the Open Project Funding of Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakes, Ministry of Education, Hubei University of Technology(HGKFYBP03), graduate innovative research project construction of Hubei Normal University (20220454), and the College Students’ Innovative Entrepreneurial Training Plan Program (202310513013, S202310513064).