Montmorillonite modified lime-ceramic sand-lake sediment (LC-sediments) was synthesized and its algae removal efficiency was investigated in this study. Montmorillonite not only improved the morphology and surface area of original LC-sediments, but also promoted the algal removal rate due to its inherent properties such as accumulating an electric charge, acting as a flocculant, and displaying a local bridging effect. Based on parameter optimization including the ratio of raw materials, agent dosage, initial algae density, pH and a determination of overlying water, the effect of hydrodynamic conditions on the algal removal process was researched. Under the optimal condition, the removal rates of turbidity, algae density and chlorophyll a could reach 86, 88 and 68%, respectively. As verified with a response surface model, it was shown that low disturbance (stirring) of the algae could promote algal removal by montmorillonite modified LC-sediment. Furthermore, a water column was utilized to approximatively simulate the flocculation and algae control in shallow lakes. This study solved the problem of reducing the dosage of lake sediment and improving the removal efficiency of algae without causing secondary pollution to the environment. It was expected to provide a certain theoretical basis for clay flocculation-based algae control in a real environment.

  • Hydrodynamic conditions on algal removal process were studied.

  • Montmorillonite improved algal control efficiency of modified lake sediment.

  • Water column was utilized to simulate the state of natural lakes.

  • The removal rates of turbidity and algae reached 86 and 88%, respectively.

Graphical Abstract

Graphical Abstract
Graphical Abstract

In recent years, algal blooms have already become a notorious and serious environmental problem (Paerl & Paul 2012). Overgrowth of algae consumes oxygen in the water, leading to a decrease in dissolved oxygen (Atkins et al. 2001; Xu et al. 2021). As the physical-chemical environment of the original water has been changed, the turbidity of water increases. It not only pollutes the drinking water source and perturbs fisheries production, but also produces a pungent smell and affects the landscape water quality. Therefore, the effective removal of cyanobacteria is a critically urgent problem to be solved.

At present, common technologies including physical methods, chemical methods (Gustafsson et al. 2009), and biological methods (Marcoval et al. 2013) have been used to control harmful algal blooms. As a physical-chemical technology, algae removal by clays utilizes the collision of the clay particles and algae cells to coagulate floc, which can effectively purify the algae-laden water. Clay is cheap and readily available. In particular, it is safe and non-toxic. As a natural clay, lake sediment is a significant part of the water ecosystem. After proper modification, it has a certain adsorption effect on the pollutants in water without secondary pollution. It was reported that lake sediments modified by hexadecyltrimethylammonium bromide (CTAB) and ZnSO4 could effectively remove phosphorus and inhibit alkaline phosphatase activity (APA) (Liu et al. 2019a). Liu et al. oxidized the sediments of a real black-odorous river by metal ions. The synthetic material showed excellent removal effect on nitrogen and phosphorus (Liu et al. 2019b). In addition, the previous research of our team also proved the modified lake sediments played a certain role in promoting the removal of algae in water; however, the removing speed is purely based on the modified lake sediments and still needs to be improved. The added dosage was very large even with the help of coagulant aids PAM and potassium ferrate for pre-oxidation (Xia et al. 2021).

Unlike lake sediments, the clay flocculant have an algae control ability based on the mechanism of bridging action and electrical neutralization (Qiu et al. 2020). Luo et al. found that clay flocculant was benificial to improve the structure characteristics and sedimentation performance of floc (Luo et al. 2007). Gu et al. demonstrated that the photocatalytic activity and adsorption flocculation ability of ZnO were significantly improved after being modified with montmorillonite, which further promoted the removal of Microcystis aeruginosa (Gu et al. 2015a). Alshahri et al. improved the performance of coagulation-flocculation-sedimentation technology (CFS, a pretreatment process) with montmorillonite and kaolin. It effectively removed turbidity, organic carbon, and algal cells while reducing chemical consumption and sludge production (Alshahri et al. 2021).

On the other hand, relevant studies (Fang et al. 2014) have shown that disturbance of water flow had a certain effect on energy metabolism and nutrient absorption of algae cells. The internal circulation of water body was intensified when the water flow rate was fast. High shear stress of water destroyed the morphology of algae cells and inhibited the growth of algae (Song et al. 2018). Generally, hydrodynamic action affects not only the substance exchange at the sediment-water interface (Nelson & Mohseni 2020) but also the distribution and accumulation of algae (Xiao et al. 2016). Zezulka et al. utilized high-pressure jet-induced hydrodynamic cavitation as a pre-treatment step to avoid cyanobacterial contamination during water purification (Zezulka et al. 2020). Zhu et al. investigated the influence of wind field on algal blooms in Taihu Lake from 2011 to 2017. They found that the area of algal blooms would gradually decrease when the wind speed was greater than 4 m/s (Zhu et al. 2019). In addition, technologies for water treatment were also influenced by hydrodynamic conditions (Lee et al. 2001; Minase et al. 2019). It can be seen that hydrodynamic conditions have a huge impact on the growth of algae in water and determine the parameter settings in algae control technology. However, the related research is still lack of in-depth discussion.

In this study, a montmorillonite and lime-ceramic sand modified lake sediments (LC-sediments) was synthesized and its algal removal efficiency was investigated. Based on the parameter optimization including the ratio of raw materials, agent dosage, initial algae density, pH and effect of hydrodynamic conditions on algal removal process, these were researched under different agal disturbance (stirring) intensity. Combined with a simulation experiment utilizing a water column, the flocculation and algae control in real lakes was investigated. This research is expected to provide a certain theoretical basis for clay flocculation-based algae control in a real environment.

Materials

The algal species selected for the experiment was Microcystis aeruginosa 469 (M. aeruginosa), which was obtained from Jialing Lake in Suzhou (E120 °35′34.44360″, N31 °35′29.97960″). Quick lime was purchased from Nanjing Chemical Reagent Co., Ltd. Montmorillonite (K-10, calcium-based) with specific surface area 240 m2/g was produced by Shanghai Aladdin Biochemical Technology Co., Ltd. Ceramic sand was provided by the local market. Sepiolite powder (200 mesh), diatomite, talc powder and kaolin were bought from Shanghai McLean Biochemical Technology Co., Ltd.

Cultivation and pretreatment of algae

M. aeruginosa culture was grown in BG-11 medium at 25 ± 1 °C in the MGC-259BP light incubator (Shanghai Jiecheng Experimental Instrument Co., Ltd) under intensity of 2000 lx and the ratio of light to dark was set as 12:12 h. Algal liquid which was in log phase after 12 days of culture was added to the collected water samples, followed by the determination of absorbance at 680 nm (OD680). When OD680 = 0.1, the number of algal cells was measured at about 3.825 × 107cells/L, which was close to the number of algal cells in freshwater algal blooms (Qin et al. 2015) (pH was adjusted to 7.3–7.5).

Preparation of LC-sediment

The lake sediment was dried by a freeze-drying box (LGJ-12E type) for 48 hours (Stutter et al. 2013; Boulard et al. 2020). The pretreated sediment was mixed with ceramic sand according to the mass ratio of 1 : 3, and then milled through a 180 mesh sieve. The undersized mixture was poured into 2% lime water (the mass ratio of sediment-ceramic sand mixture to lime water was 2 : 5) until it was completely submerged. The solution was heated in a 70 °C water bath for 90 min, and dried in a 80 °C oven for 24 h. Then the dried solids were cooled to room temperature before being ground and screened. The above procedure resulted in the synthesis of LC-sediment.

Instrument

The obtained compounds were air dried under natural conditions. The compounds were observed by SEM (SUPRA 55) at 15 kV after spraying with gold for 90 s. An X-ray diffractometer (XRD-7000S/L) was used to analyze the crystal form of the compounds. A BET analyzer (Tristar-3020) was utilized to measure the specific surface area of the compounds. Turbidity was applied to characterize the level of suspended particles in water by WGZ-3B turbidity meter (Shanghai Xinrui Instruments Co., Ltd). The OD680 was measured at 3 cm below the liquid level.

Selection of clay flocculant

In an attempt to select the optimal clay flocculant to make a composite with LC-sediment, we investigated the algal removal ability of the composite synthesized by the sediment and different clay flocculants (kaolin, montmorillonite, sepiolite powder, diatomite, talc), respectively. 0.7 g clay flocculant and the equivalent amount of LC-sediments were added into a 1,000 mL algae solution. The solution was stirred at 250 r/min for 15 min, followed by sitting for 30 min. We proceeded to calculate the cyanobacteria removal rates and associated turbidities (NTU) as follows.
formula
formula

Optimum ratio of montmorillonite and LC-sediments under different disturbance intensity

100 mL pretreated algae solution was diluted to 1,000 mL (pH 7.1). Different ratios of montmorillonite and LC-sediments (Table 1) were added into diluted solutions. Meanwhile, stirring was applied to simulate the real environment of shallow lakes (Reddy et al. 1996; Pettersson 2001; Rydin et al. 2011; Zhang et al. 2020) (the stirring speed and corresponding wind speed were listed in Table 2). The removal rates of turbidity, chlorophyll a and algae density of water were calculated and analyzed (the specific operations were described of the Paragraph S1-S4 in Supplementary Information).

Table 1

Ratio of montmorillonite and LC-sediments

Number123456
LC-sediments/mg 100 150 200 250 300 350 
Montmorillonite/mg 100 100 100 100 100 100 
Ratio 1:1 3:2 2:1 5:2 3:1 7:2 
Number123456
LC-sediments/mg 100 150 200 250 300 350 
Montmorillonite/mg 100 100 100 100 100 100 
Ratio 1:1 3:2 2:1 5:2 3:1 7:2 
Table 2

Relationship between stirring speed and corresponding wind speed

Disturbance levelWind-velocity rangeRange of disturbance
Low wind/low disturbance 2–4 m/s 60–100 r/min 
Medium wind/medium disturbance 4–6.5 m/s 120–150 r/min 
Strong wind/high disturbance 7 m/s 170–200 r/min 
Disturbance levelWind-velocity rangeRange of disturbance
Low wind/low disturbance 2–4 m/s 60–100 r/min 
Medium wind/medium disturbance 4–6.5 m/s 120–150 r/min 
Strong wind/high disturbance 7 m/s 170–200 r/min 

Simulation experiment by water column

In an effort to further analyze the influence of hydrodynamic factors on algae control, as well as the changes of algae density and nutrient salt concentration in water, a water column was utilized to simulate the state of natural lakes. As shown in Figure 1 (2.5 m height, 0.23 m bottom diameter, 1.5 m shading part), the upper and lower ends of the device were equipped with a water intake and a mud intake. The natural lake sediment was put into the bottom of the simulated water column. Afterwards, the algae water was poured into the water column along the pipe wall. After the mud and algal water formed layers, pH, turbidity, OD680 of water, TN and TP of natural sediments were measured under different disturbance intensity. A A7-ALC2000 submersible pump (Zhongshan Haiyi Electric Co., Ltd) with variable frequency speed regulation was placed 30 cm below the water surface to adjust the flow speed. Dynamic changes of overlying water and sediment indexes were analyzed after algae-settling for 25-day continuous monitoring.
Figure 1

Water column experimental device diagram.

Figure 1

Water column experimental device diagram.

Close modal

Measurement index and method

The turbidity was measured by a portable turbidimeter. Chlorophyll a was determined by spectrophotometry. The algal density was tested by the hemocytometer method. A linear relationship between OD680 and the number of algal cells was established.
formula

The total nitrogen of the sediment was tested by potassium persulfate digestion. The total phosphorus was obtained by sodium hydroxide alkali fusion-molybdenum antimony anti-spectrophotometry.

Characterization

In order to observe the surface morphology of different sediments, the samples were characterized by scanning electron microscopy (SEM). As shown in Figure 2, compared to the natural one (Figure 2(a)), the sediment particle after being modified by lime-ceramic sand (Figure 2(b)) displayed increased fragmentation and fragments had rougher surfaces. The calcium hydroxide solution adhered to the sediment surface, forming a layer of film during the lime modification process. The resulting loose and rough surface enhanced the adsorption capacity. With further introduction of montmorillonite, Al3+ improved the cation exchange capacity of modified sediments, which contributed to rapidly adsorbing the negatively charged algal cells in water (Figure 2(c)) (Gu et al. 2015b). Combined with the BET characterization results, the specific surface areas of the three sediment types were 32.4313 m2/g, 33.9650 m2/g and 67.6379 m2/g, respectively. This result demonstrates that the increased specific surface area was conducive in improving the adsorption property of sediments.
Figure 2

SEM photos of (a) unmodified sediment (b) LC-sediment (c) montmorillonite modified LC-sediment.

Figure 2

SEM photos of (a) unmodified sediment (b) LC-sediment (c) montmorillonite modified LC-sediment.

Close modal
Fig. S1 in Supplementary Information showed the XRD spectrum of the sediment. It was identified that there was no new characteristic peak after modification by lime-ceramic sand, indicating that the original crystal structure of lake sediments was unchanged. The increased peak intensity demonstrated the sediment particles became dispersed, which was consistent with the SEM results; however, a new characteristic peak appeared after the introduction of montmorillonite (Figure 3). This was due to the Al3+ in montmorillonite that reacted with LC-sediments to generate a new compound Al4Si, which directly demonstrated the successful synthesis of the composite (Najafi et al. 2021).
Figure 3

XRD characterization of montmorillonite modified LC-sediment.

Figure 3

XRD characterization of montmorillonite modified LC-sediment.

Close modal

Selection of clay flocculant

The algae removal performance was related to the selection of clay flocculant. Turbidity (NTU) and algae density were tested to characterize the algae removal performance of LC-sediments with different flocculants. Compared with those composited with sepiolite powder, diatomite, talc powder or kaolin, the composite with montmorillonite had the highest removal rates (Figure 4). Montmorillonite itself contained Al3+ and Mg2+, while algae in water were negatively charged. When montmorillonite was composited with LC-sediments, negative-charged algae cells in water were quickly adsorbed. The surface charge of particles was neutralized, which increased the condensability of algae and made algae easy to accumulate and settle (Gu et al. 2018). It was suggested that montmorillonite modified LC-sediments sequestered and wrapped the algae particles which formed a high density floc. Based on the mixed adsorption bridging effect in the process of algae sedimentation (Anirudhan & Ramachandran 2015), the effect of flocculation and algae removal was improved, through which removal rate of algae could reach 82%.
Figure 4

Algae control effect of LC-sediments composited with different clay flocculants.

Figure 4

Algae control effect of LC-sediments composited with different clay flocculants.

Close modal

Effect of hydrodynamic conditions on algal removal

With a view to ensure that the experimental environment was closer to the real disturbance of shallow lakes, the influence of hydrodynamic factors on the algae control effect needed to be considered. Based on the published research (Liwarska-Bizukojć & Olejnik 2020), the maximum, minimum and average wind speed were respectively 8.1 m/s, 0.5 m/s and 3.6 m/s during cyanobacterial blooms in Taihu Lake. The removal rates of turbidity, chlorophyll a and algae density were studied under different ratios of montmorillonite and LC-sediments (Table 1).

Compared to the undisturbed (Figure 5(a)), medium (Figure 5(c)) and high (Figure 5(d)) disturbances groups, the overall removal rates of turbidity, algae density and chlorophyll a under low disturbance conditions were the highest, which indicated that appropriate disturbance was beneficial to water purification. The sediments released nitrogen and phosphorus as the disturbance intensity increased, which promoted the reproduction of algae. Although excessive disturbance inhibited the growth of M. aeruginosa, it restrained the promotion of montmorillonite to the algal control of LC-sediments. Under low disturbance, the best removal rate appeared at the ratio of LC-sediment to montmorillonite of 3:2 (86%, 88%, 68% respectively). With the increase of the LC-sediment dosage, the excess LC-sediment particles suspended in the water resulted in a decrease of the removal rate of turbidity. Concurrently, the proportion of montmorillonite was reduced and its promotion effect on algae control was weakened (Gu et al. 2016).
Figure 5

Effect of hydrodynamic conditions on algal removal (a) undisturbed (b) low disturbance (c) medium disturbance (d) high disturbance.

Figure 5

Effect of hydrodynamic conditions on algal removal (a) undisturbed (b) low disturbance (c) medium disturbance (d) high disturbance.

Close modal
It was observed that the optimal ratios of LC-sediments and montmorillonite were diverse under various intensities of disturbance (3:2, 1:1, 1:1 for low, medium and high disturbance, respectively). Accordingly, the OD680 under different disturbance intensities were measured at the respective optimal ratios. As shown in Figure 6, the value under low disturbance was significantly lower than the others. This result demonstrated that low disturbance was more beneficial to the optimum contact and collision between flocculant and algae, promoting the removal of algae. The above illustrated that hydrodynamic condition might have an influence on the selection of the algaecide ratio, signifying that the synthesis process of the agent should be properly adjusted according to the real wind speed of the lake.
Figure 6

OD680 under different disturbance intensities.

Figure 6

OD680 under different disturbance intensities.

Close modal

TN and TP of overlying water

The effects of the algae removal process on N and P of overlying water by the optimal material was investigated under different disturbance intensities. As illustrated in Figure 7, the change of TN and TP was relatively gentle and maintained at a low value under low disturbance. On the one hand, low disturbance perturbed the concentration of TN and TP around the algal cells, which helped the cells to absorb nutrients (Zhu et al. 2016). Montmorillonite modified LC-sediments were fully mixed under low disturbance. The algal cells were trapped by the encapsulation, net capturing and flocculation of montmorillonite modified LC-sediment (Gu et al. 2016), which resulted in those sediments essentially absorbing N and P. The TN and TP of overlying water was increased under medium and high disturbances. The nutrients in the sediment were released to the overlying water with increased disturbance. The release of P was due to the lake sediment containing large amounts of phosphorus (Ren et al. 2021). It was worth noting that the TN under high disturbance was lower than that under medium disturbance. Owing to the increase of dissolved oxygen in the water under high disturbance, the absorption of N by algae was promoted (DeVore et al. 2019).
Figure 7

(a) TN (b) TP of overlying water.

Figure 7

(a) TN (b) TP of overlying water.

Close modal

Optimum

Under different disturbance intensities, the initial algae density (characterized by OD680) and pH of the solution were optimized (the operation process was shown in Paragraph S5-S6 of Supplementary Information). Figure 8(a) showed that the overall algae removal rate of the low-disturbed group was the highest. It indicated that the algae-removal process had strong adaptability (the ability of algicide to adapt to different concentrations of algal liquid under low disturbance) and impact resistance (algicide can resist the impact of water quality change on the effect of algae removal) under low disturbance, which was suitable for algae-containing water with a wide range of concentrations. Specifically, the algae removal rate at initial OD680 = 0.1 was the highest under different disturbance intensities. The algae density of OD680 = 0.1 corresponds to natural conditions when cyanobacteria emerge. Hence, montmorillonite modified LC-sediment algaecide could be used for algae removal in real flowing shallow lakes, particularly just before the cyanobacteria bloom emergence period.
Figure 8

Effect of initial (a) algal density (b) pH on algae control under different disturbance intensities.

Figure 8

Effect of initial (a) algal density (b) pH on algae control under different disturbance intensities.

Close modal

Initial pH also influenced the removal performance. As shown in Figure 8(b), an acidic environment was unfavorable for algae control. The removal rate of algae was the largest under the low disturbance state in neutral or alkaline conditions. The reason was that the calcium ions and aluminum ions in the agent underwent a neutralization reaction in the alkaline water. And the adsorption of cations to the cyanobacteria with negative charge would form bridging (Liu et al. 2021). Furthermore, the low disturbance accelerated the contact of ions and algal cells, which made them coagulate into flocs for settling. It was to be noted that excessive disturbance would destroy the flocs and make the particles float.

Response surface tests (Design Expert 8.05) were utilized to further optimize factors of algae removal. The LC-sediment dosage (X1), montmorillonite dosage (X2), initial pH (X3) and disturbance intensity (X4) were chosen as the investigation factors (the level coding was shown in Table S1 in Supplementary Information). The Box-Behnken Design model (BBD) was applied to fit the test results. From the experimental results of the response surface (Table S2 of Supplementary Information) and the model variance of the algae removal rate (Table S3 of Supplementary Information), P < 0.0001 and the coefficient of variation of R2 = 0.9056 was 2.25%, which indicated it had a good fitting level and small error. The influence level of each factor on the removal rate of algae ranged from high to low in the following order: disturbance intensity > LC-sediment dosage > initial pH > montmorillonite dosage. It was clear that hydrodynamic conditions played an important role during the whole process. As for the interaction level, the F of X1X2 was the largest. It manifested that the dosage of LC-sediment and montmorillonite had a significant interaction on the removal of algae. The quadratic multinomial regression model was as follows.
formula

The optimal conditions predicted by multiple BBD experiments and surface models (Fig. S2–S13 of Supplementary Information) were shown in Paragraph S7 of the Supplementary Information. Considering the algae precipitation effect of flocculants and the influence of hydrodynamics in the actual experiment, the optimal conditions of algae removal by montmorillonite modified LC-sediment were revised to 154 mg/L LC-sediments, 100 mg/L montmorillonite, with an initial PH of 7.5 and a disturbance intensity of 90 r/min. The imparity between the experimental value and the predicted one was less than 5%, demonstrating that the model fitted by this response surface test had a good prediction capability.

Simulation experiment by water column

In an effort to further investigate the influence of hydrodynamic conditions on algae control of montmorillonite modified LC-sediment, a water column was applied to simulate natural lakes. Various indicators of overlying water and sediment in the water column were analyzed based on 25-day continuous monitoring.

The water column simulation experiment was carried out in groups with different interference intensity. TN and TP results in sediments are shown in Figure 9(a). The natural lake sediments of the three groups absorbed nitrogen from the water during the initial period. After the 5th day, the sediments all began to release nitrogen into water, resulting in the gradual decrease of TN in the sediments. Compared to other groups, the low-disturbed group had the lowest TN after 75 min. The effect of hydrodynamics facilitated the nitrification reaction. As such, nitrogen originating from nitrate diffused from the overlying water to the interstitial one and was consequently removed by denitrification in the anaerobic layer of the sediment (Falahati-Marvast & Karimi-Jashni 2020). The final TN of the high-disturbed group was higher than that of the low-disturbed group. The greater the disturbance intensity, the easier it was for the denitrification reaction of nitrate to be removed in the anaerobic layer of the sediment and absorbed by the sediment.
Figure 9

Changes of (a) TN (b) TP in natural lake sediments.

Figure 9

Changes of (a) TN (b) TP in natural lake sediments.

Close modal

Unlike TN, natural sediment was relatively rich in phosphorus. The long-term importation of exogenous phosphorus and the deposition of aquatic biological residues made lake sediments form a phosphorus reservoir. Phosphorus was released into the water, resulting in eutrophication and degradation of the aquatic ecosystem. The sediments no longer absorbed phosphorus from the overlying water. Excess phosphorus would instead be absorbed by the algaecide (Figure 9(b)). The TP of the undisturbed group increased significantly, while the disturbed groups did not. The algae were gradually removed by the algicide. The residual algae settled to the surface of the sediment under static conditions, which brought the release of P; however, disturbance promoted the P at the sediment surface to the overlying water, which re-equilibrated the P concentration in the water column.

The pH, turbidity and OD680 of the overlying water was measured in the simulated lake water column. The pH of the three groups (Figure 10(a)) were generally stable. After a few days, the montmorillonoid modified LC-sediment was fully mixed with the algae fluid. Ca2+ and Al3+, combined with the negative ions in the algae fluid, made the pH of the two disturbance groups rise slightly. It demonstrated that the algae precipitation agent in the undisturbed group dissolved slowly, and the efficiency of contact and collision with the algae was lower. After the 3rd day, the turbidity of the undisturbed and low-disturbed group (Figure 10(b)) continued to decrease, while that of the high-disturbed group was higher. High disturbance made algae cells and nutrients in the sediments float again, which caused the turbidity of the overlying water to rise. Figure 10(c) presented that the algal density of the three groups declined significantly, and the low-disturbed group was ultimately better than the others. In terms of simulating the process of algal control in real shallow lakes, a low disturbance of the water column is more conducive to improving agal removal by montmorillonite modified LC-sediment.
Figure 10

Changes of (a) pH (b) turbidity (c) OD680 of overlying water.

Figure 10

Changes of (a) pH (b) turbidity (c) OD680 of overlying water.

Close modal

In this study, montmorillonite modified LC-sediment was synthesized to control M. aeruginosa algal blooms under different hydrodynamic conditions. Montmorillonite improved the composite morphology, and the surface area was twice as large with better pore structure. The reaction brought by montmorillonite promoted the algal removal rate due to the clay's inherent properties including an electric charge, flocculation capabilities and bridging effect. As verified with a response surface model, it was shown that hydrodynamic conditions played an important role during the whole process. Low disturbance could promote the algae removal. Under optimal conditions, pH of 7.5, initial OD680 of 0.1, a ratio of LC-sediments and montmorillonite of 3:2, removal rates of turbidity, algae density and chlorophyll a could reach 86, 88 and 68%, respectively. Furthermore, the simulation experiment utilizing a water column was carried out to mimic the flocculation and algae control in shallow lakes. In this investigation, algae removal is recommended to be avoided at high disturbance. These results are expected to provide a certain theoretical basis for clay flocculation-based algae control in the real environment.

Shallow water lakes in nature are always disturbed, so how to regulate the effect of hydrodynamics will become the subject of future research. Hydrodynamic factors will affect the lake ecosystem, the growth of algae and the migration of trace elements between water and mud. Further research focused on lake hydrodynamics regulation will greatly improve the efficiency of algae control.

This work was supported by the Key Research and Development Projects in Anhui Province (202004a06020030), China's National Water Pollution Control and Governance of Science and Technology Major Special (2018ZX07208-004), the National Natural Science Foundation of China (No. 22106070).

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

The authors declare there is no conflict.

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