Effect of hydrodynamic condition on algae control based on montmorillonite modified lime-ceramic sand-lake sediments

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.

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.

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).

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.

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.

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).

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.

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).

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.

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.

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.

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.

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.