Photocatalytic Inactivation of Microcystis aeruginosa Using MoS 2 @Zeolitic Imidazolate Framework/TiO 2 : A Hybrid Catalyst for Efficient Control of Harmful Algal Blooms

Abstract

Lake eutrophication and harmful algal blooms (HABs) have emerged as paramount concerns in the global aquatic environment. Microcystis aeruginosa is a kind of common harmful algae in fresh water. In this study, the surface of TiO₂ was modified by incorporating molybdenum disulfide (MoS₂) and zeolitic imidazolate framework-67 (ZIF-67), resulting in the formation of a novel titanium dioxide (TiO₂) photocatalyst (MoS₂@ZIF-67/TiO₂). MoS₂@ZIF-67/TiO₂ photocatalyst was also used to investigate the inactivation of M. aeruginosa under visible light. After doping with MoS₂@ZIF-67, the energy bandgap of MoS₂@ZIF-67/TiO₂ was reduced to 2.4 eV, while exhibiting an increase in the average pore diameter. The inactivation efficiency of 10% MoS₂@ZIF-67/TiO₂ composite photocatalyst for M. aeruginosa was 86.08%. The content of photosynthetic pigment, soluble protein, and various antioxidant indexes also continued to decline during the experiment, dropping to the lowest level on the fifth day. Based on the comprehensive data analysis, it was evident that the 1030% MoS₂@ZIF-67/TiO₂ photocatalyst exhibited remarkable potential for advancements in the realm of photocatalytic algae removal and mitigation of HABs.

Keywords

harmful algal blooms (HABs)inactivation mechanism photocatalyst

Introduction

Lake eutrophication and associated harmful algal blooms (HABs), driven by climate change, have emerged as one of the most critical challenges in the global aquatic environment (Chen et al., 2020; Paerl and Otten, 2013). Nutrients from agricultural discharge and municipal sewage entered the water and stimulated the excessive growth of algae, forming algal blooms. HABs could change the structure of bacterial colonies and pose a serious threat to mariculture, human health, and ecological security (Zhang et al., 2016). Among them, there were many kinds of toxins produced by Cyanobacteria, and their modes of action and effects on humans were different. In recent years, the number of blooms caused by Microcystis aeruginosa has increased year by year (Lamas-Samanamud et al., 2022). M. aeruginosa was dominant in the bloom of Cyanobacteria. Microcystin produced by M. aeruginosa has strong hepatotoxicity (Colas et al., 2021; Fan et al., 2021a; Sharma et al., 2011; Zhou et al., 2014). Microcystin could not only cause fish poisoning and pollute water bodies but also endanger the health of people through contaminated aquatic products and drinking water (Paerl and Otten, 2013). For a long time, low-cost and efficient photocatalysis technology (Sato et al., 2017) has been considered an efficient method for water treatment (Fan et al., 2021b; Song et al., 2018b). Photocatalytic technology utilizes semiconductors as catalysts and light as energy to carry out oxidation–reduction reactions. The primary principle was to utilize light to excite semiconductor materials such as TiO₂ and utilize the electrons and holes to participate in oxidation–reduction reactions. When light with energy greater than or equal to the bandgap hits the semiconductor surface, the electrons in the valence band (VB) are excited, transitioning to the conduction band (Dhiman and Kondal, 2022). Relatively stable holes were left in the VB, forming electron–hole pairs (Wang et al., 2017). Due to the large number of defects and suspensions in materials, these defects and suspensions could trap electrons and prevent the recombination of electron–hole pairs. The photogenerated electrons and holes migrated to the material surface, generating potent redox potentials. Consequently, the resulting reactive oxygen species (ROS) degraded algal cellular structures during the photocatalytic process, leading to efficient algal inactivation (Song et al., 2018a).

Titanium dioxide (TiO₂) is a widely used traditional inorganic photocatalyst because of its low cost, environmental friendliness, and chemical stability (Liao et al., 2009). TiO₂ could remain stable in environments resistant to acid, alkali, light corrosion, and non-toxicity. However, the structure of graphitic carbon nitride (g-C₃N₄) material might be damaged under strong acid and strong oxidation conditions. At the same time, compared with TiO₂, the electron recombination rate of g-C₃N₄ was also higher. Based on comprehensive chemical stability, light absorption range, carrier mobility, and other properties, TiO₂ was selected as the modified material. TiO₂ was widely applied in the field of removing harmful algae and water treatment. For example, Lu et al. (2015) investigated the effects of nanosized TiO₂ on M. aeruginosa under ultraviolet light. In the process of photocatalysis, the cell surface was damaged, the biomolecules in the cell membrane and cell wall were degraded, and the membrane permeability was changed. Chen et al. (2018b) also investigated the photocatalytic effect of nanosized TiO₂ on M. aeruginosa. The experimental results confirmed that the activity of M. aeruginosa cells decreased due to the free radicals produced by TiO₂. The metabolic activity of the cells was inhibited. In addition to algal cells, algal toxins inside and outside the cells could also be removed by photocatalysis of TiO₂ (Pinho et al., 2015). However, in practical applications, the high bandgap width (3.2 eV) (Chen et al., 2018a; Nasir et al., 2022) and the fast recombination rate of electron–hole pairs became the defects that could not be ignored (Zhou et al., 2020). To reduce the bandgap of TiO₂ and the recombination rate of electron–hole pairs (Rathinavelu et al., 2013; Tatiya et al., 2023), scientists doped TiO₂ with different compounds, such as silver (Ag)–TiO₂ photocatalyst (Chang et al., 2015), nitrogen-doped TiO₂ (Jin et al., 2019; Zhou et al., 2020), TiO₂ with doped nitrogen and modified carbon (Zhang et al., 2020), Ag/AgCl@g-C₃N₄@UIO-66 (NH₂) (Fan et al., 2021b), and so on.

Molybdenum disulfide (MoS₂) could absorb visible light and possessed strong photocatalytic activity (Jia et al., 2019). The bandgap width of single-layer MoS₂ was ∼1.80 eV. At the same time, its unique “sandwich” structure with alternating sulfur layer and molybdenum layer also attracted the attention of researchers. Zeolitic imidazolate framework-67 (ZIF-67), a subclass of metal–organic frameworks, exhibited high structural stability and tunable porosity (Naghshbandi et al., 2023; Saeed et al., 2022). ZIF-67 showed excellent stability and durability in aqueous solution, strong alkaline environment, and high thermal environment (Bakar et al., 2024; Liu et al., 2018). The aperture size and distribution were uniform and controllable (Hu et al., 2023; Zhang et al., 2024). Furthermore, the inherent cage-like architecture of ZIF-67 facilitated its integration with various semiconductor materials, enabling the rational design of hollow and core–shell nanostructures. Consequently, ZIF-67 emerged as a highly promising material in the field of photocatalysis.

Therefore, MoS₂@ZIF-67 composite material was selected to modify TiO₂. In the previous study (Wei et al., 2024), three monomer materials (MoS₂, ZIF-67, and TiO₂), three binary composites (MoS₂@ZIF-67, TiO₂@MoS₂, and TiO₂@ZIF-67), and three proportional terpolymer composites (10%, 20%, and 30% MoS₂@ZIF-67/TiO₂) were synthesized by hydrothermal synthesis. A variety of characterization experiments confirmed the successful preparation of the expected material. Nine kinds of materials were applied to the removal experiment of Karenia mikimotoi. After 12 h of visible light irradiation, 10% MoS₂@ZIF-67/TiO₂ photocatalyst showed excellent photocatalytic performance. It was eventually able to remove 97% algal cells. After further investigation, it was found that 10% MoS₂@ZIF-67/TiO₂ material initially caused damage to the membrane system, resulting in lactate dehydrogenase (LDH) leakage. Next, the photosynthetic system of algal cells was also destroyed, and the chlorophyll content continued to decrease. The antioxidant system eventually broke down after the cell was completely destroyed, and many enzyme levels were reduced to the minimum. Therefore, the above materials were further applied to the removal of M. aeruginosa in this study.

Materials and Methods

Synthesis of various photocatalysts

MoS₂, ZIF-67, TiO₂, MoS₂@ZIF-67, TiO₂@MoS₂, TiO₂@ZIF-67, MoS₂@ZIF-67, TiO₂@MoS₂, and TiO₂@ZIF-67 were synthesized according to the methods reported in several literatures (Chen et al., 2019; Chu et al., 2020; Dong et al., 2016; Khay et al., 2016; Zhong et al., 2020). The synthesis process is shown in Figure 1.

FIG. 1.

Synthesis of various photocatalysts method.

Preparation of MoS₂

The first step was to add ammonium (NH₄)₆Mo₇O₂₄·4H₂O, CH₄N₂S, and C₂H₅NO₂ to 40 mL deionized water according to the ratio of 0.596:1.218:1. Materials were heated at 200°C for 12 h. After cooling to room temperature, it was removed and washed with anhydrous ethanol and deionized water three times, respectively. The washed sample was placed in a constant temperature oven at 60°C for 12 h to obtain black MoS₂ powder.

Preparation of ZIF-67

Co(NO₃)₂·6H₂O (0.717 g) solution and C₄H₆N₂ (1.622 g) solution were prepared with 10 mL deionized water, respectively. The two solutions were then thoroughly mixed and stirred at room temperature for 24 h. After mixing, the samples were washed with anhydrous ethanol and deionized water, respectively, and then the washed samples were dried in vacuum in the oven. The purple powder obtained after drying was ZIF-67.

Preparation of TiO₂

About 0.78 g CH₄N₂S was dissolved in 30 mL deionized water. After the thiourea was completely dissolved, 20 mL anhydrous ethanol and 2.5 mL trolamine were added. Then 0.05 mL phosphoric acid was added to the mixture, and finally, 5 mL of C₁₆H₃₆O₄Ti was added drop by drop and stirred for 0.5 h. After the stirring was completed, the mixed solution was heated to 210°C within 1.5 h and kept warm for 4 h. After cooling to room temperature, it was washed with anhydrous ethanol and deionized water three times, respectively. The washed sample was placed in the oven for vacuum drying. The white powder obtained after drying was TiO₂.

Preparation of MoS₂@ZIF-67

Before preparing MoS₂ material, a certain amount of ZIF-67 powder was dissolved in 40 mL deionized water. After ZIF-67 was fully dissolved, the drug product used in the preparation of MoS₂ was operated. The final material was MoS₂@ZIF-67. TiO₂@MoS₂, TiO₂@ZIF-67, and different proportions of MoS₂@ZIF-67/TiO₂ were synthesized by the same method.

The form was observed by scanning electron microscopy

The distribution of chemical elements in the catalyst was analyzed by energy-dispersive X-ray spectroscopy (EDS). Ultraviolet–visible (UV–Vis) absorbable spectroscopy was used to study the light absorption characteristics and calculate the bandgap width. The photoluminescence spectroscopy (PL) revealed the recombination efficiency of photogenerated electron–hole pairs. The crystal images of composite photocatalytic materials were analyzed by X-ray diffraction (XRD). The surface functional groups of the photocatalyst were detected by Fourier infrared spectroscopy (FTIR). The chemical valence states of the chemical elements were analyzed by X-ray photoelectron spectroscopy (XPS). The pore structure and pore size distribution were analyzed by the N₂ adsorption–desorption curve and the pore size distribution curve.

Photocatalytic inactivation of M. aeruginosa

M. aeruginosa (Algae No. FACHB-905) was obtained from the Freshwater Algae Species Bank of the Wuhan Institute of Aquatic Biology. The algal cells were cultured in BG-11 medium (1.7 g/L). The incubator was kept at 25°C, the light intensity was 2,200 lux, and the daily light time was 12 h. In subsequent experiments, the initial concentration of algal cells was 9.58 × 10⁵ cells/mL. Prior to the formal experiment, the photocatalyst was determined by a preliminary experiment. Initially, different photocatalyst concentrations were selected for growth inhibition experiments according to several literatures (Pinho et al., 2015; Wei et al., 2021). Finally, according to the pre-test results, the concentration of photocatalyst added was 0.3 g/L. In the formal experiment, the number of surviving algal cells was counted by the blood count method according to the specified time point, and the growth curve was drawn. According to the results of the growth inhibition experiment, TiO₂, TiO₂@MoS₂, and 30% MoS₂@ZIF-67/TiO₂ composite photocatalysts were selected to conduct the follow-up experiment. The control group and three groups of parallel experiments were setup to ensure the accuracy of the experimental results.

Determination of growth, metabolism, and antioxidant indexes of M. aeruginosa

The chlorophyll a (chl a) content of M. aeruginosa was determined under the action of photocatalyst using the following formula:

Ca (mg / L) = 13.95 \times D_{665} - 6.88 \times D_{649}

(Ca was the concentration of chl a)

The Bradford protein content of M. aeruginosa under the action of composite photocatalyst was determined by using the Biyuntian Bradford Protein Assay Kit.

According to the Biyuntian LDH Cytotoxicity Assay Kit, the LDH leakage rate of K. mikimotoi under the action of composite photocatalyst was determined.

Superoxide dismutase (SOD) enzyme activity of algal cells was detected with reference to the Biyuntian Total SOD Assay Kit WST-8.

The catalase (CAT) activity of algal cells was detected with reference to the Biyuntian Hydrogen Peroxide Assay Kit.

The peroxidase (POD) activity of algal cells was detected by using the POD Activity Assay Kit of Nanjing Jiancheng Bioengineering Institute.

The content of malondialdehyde (MDA) of algal cells was detected by using the MDA Assay Kit of Nanjing Jiancheng Bioengineering Institute.

Statistical analysis

SPSS software was used to complete the statistical analysis of the data. One-way analysis of variance was used to compare whether the difference between the experimental group and control group was significant.

Results and Discussions

Physical and chemical characterization of different photocatalysts

Scanning electron microscopy (SEM), EDS, UV–Vis, PL, XRD, FTIR, XPS spectra, N₂ adsorption–desorption curve, and pore size distribution curve were presented in previous studies and Table 1 (Wei et al., 2024). SEM, FTIR, and XRD spectra confirmed the successful preparation of the expected photocatalyst. The 10% MoS₂@ZIF-67/TiO₂ material showed changes in specific surface area, pore volume, and average pore size. Among them, the specific surface area and pore volume of MoS₂@ZIF-67/TiO₂ were lower than those of TiO₂. And the average aperture was increased. The electron–hole pair recombination rate and bandgap width of MoS₂@ZIF-67/TiO₂ materials were reduced. The photocatalyst was more likely to be excited by low-energy photons, and the carrier recombination rate was low. The photocatalytic performance was improved. Meanwhile, the doping of MoS₂@ZIF-67 expanded the light absorption range of TiO₂. Its photocatalytic activity was enhanced.

Table 1.

Properties of Zeolitic Imidazolate Framework-67, Molybdenum Disulfide, Titanium Disulfide, and 10% MoS₂@ZIF-67/TiO₂ Photocatalyst

Photocatalyst	BET surface area (m²/g)	Pore volume (cm³/g)	Pore size (nm)
ZIF-67	18.3892	0.014937	9.1315
MoS₂	6.4856	0.010239	12.9913
TiO₂	156.2416	0.289289	6.6531
10% MoS₂@ZIF-67/TiO₂	107.1256	0.195807	7.0798

BET, Brunauer–Emmett–Teller; MoS₂, molybdenum disulfide; TiO₂, titanium dioxide; ZIF-67, zeolitic imidazolate framework-67.

In Figure 2a-d, the morphological changes of M. aeruginosa cells were illustrated during the photocatalytic process at different stages. These images clearly demonstrated the process of cell membrane contraction. The shriveled cells exhibited surface depressions and serrated edges, accompanied by cytoplasmic concentration. This phenomenon occurred because, as the photocatalytic reaction progressed, an increase in ROS led to lipid peroxidation of the cell membrane. Consequently, the cell membrane was damaged, resulting in an imbalance in cellular osmotic pressure. Subsequently, cellular contents continuously leaked out, causing the cell membrane to adopt a wrinkled morphology.

FIG. 2.

(a–d) Scanning electron microscopy of algal cell morphology during photocatalysis. EHT, electron high tension; WD, working distance.

Inactivation experiment of M. aeruginosa by photocatalyst

To explore the photocatalytic performance of different photocatalysts, a photocatalytic inactivation experiment was carried out on M. aeruginosa. The experimental results are shown in Figure 3. Figure 3a showed the changes in the number of cyanobacteria cells in the case of different photocatalysts. To be more intuitive, the changes in the number of algal cells in TiO₂, TiO₂@MoS₂, and 10% MoS₂@ZIF-67/TiO₂ groups were shown in the form of 3D histogram (Fig. 3b). Here, the x-axis represented time, the y-axis represented the type of photocatalyst, and the z-axis represented the number of algal cells. The number of algal cells in the experimental group decreased significantly within 6.5 h. Combined with the subsequent changes in the number of algal cells and the specific surface area of the photocatalyst, it could be inferred that the decrease in the number of algal cells at 6.5 h was due to the adsorption of the photocatalyst to algal cells. From the third day, the number of algal cells in the TiO₂ group began to increase. From the fifth day, the number of algal cells in the 10% MoS₂@ZIF-67/TiO₂ group also increased slightly (Fig. 3b). In addition, the number of algal cells in the other experimental groups continued to decline. On the fifth day, the inactivation efficiency of TiO₂@MoS₂, TiO₂@ZIF-67, 10% MoS₂@ZIF-67/TiO₂, and 30% MoS₂@ZIF-67/TiO₂ groups were 95.65%, 88.7%, 86.08%, and 96.52%, respectively (Fig 3a). Therefore, TiO₂, TiO₂@MoS₂, and 10% MoS₂@ZIF-67/TiO₂ were selected for the subsequent determination of growth and metabolism indexes. To investigate the stability of photocatalyst inactivation, the experiment lasted for 8 days. As can be seen from the figure, the inactivation effect of 10% MoS₂@ZIF-67/TiO₂ composite photocatalyst was stable, and the number of algal cells still decreased slightly.

FIG. 3.

(a) Inactivation curve, (b) 3D histogram of Microcyctis aeruginosa inactivation under the action of photocatalysts.

Photosynthetic pigments participated in the process of absorption and transfer of light energy in photosynthesis and could cause primary photochemical reactions. Photosynthetic pigments mainly included chlorophyll, carotenoid, and phycobilin. Chlorophyll and phycobilin were the main pigments for photosynthesis in plants. Phycobilin was mainly found in Rhodophyta and Cyanobacteria. Phycobilin was often bound to phycobilin proteins, including phycocyanin (PC), p-phycoerythrin (PE), and allophycocyanin (APC). Therefore, chlorophyll a (chl a) and phycobilin were selected for content determination (Fig. 4). By observing the change in chl a content (Fig. 4a), the chl a content of the experimental groups decreased to varying degrees on the first day. But chl a content rose again the next day. Combined with the specific surface area, porosity, and average pore size of the photocatalyst, the change in chl a content on the first day might be caused by the adsorption function of the photocatalyst itself. The change in chl a content represented the deactivation of algal cells due to photocatalysis. The overall trend of TiO₂ group was first decreasing and then increasing. This was the same trend as the number of algal cells. At the same time, the chl a content of TiO₂@MoS₂ and 10% MoS₂@ZIF-67/TiO₂ experimental groups continued to decline after the second day. It was the lowest level on the fifth day. The contents of PC, PE, and APC were roughly the same as the contents of chl a (Fig. 4b-d). On the fifth day, the contents of PC, PE, and APC of 10% MoS₂@ZIF-67/TiO₂ group dropped to the lowest levels, which were 0.00145, 0.01931, and 0.00499 mg/L, respectively. At the same time, by comparing the content changes of different phycobilins, the effect of photocatalysis on different phycobilins was different. PC and APC were more affected. The above results showed that photocatalysis possessed significant effect on the decrease of photosynthetic pigments in algal cells. Comprehensive analysis showed that the photocatalyst affected photosynthetic pigments in algal cells through photocatalysis. The photosynthetic system of algal cells was further affected, resulting in the inactivation of algal cells.

FIG. 4.

Changes in (a) chl a, (b) PC, (c) APC, (d) PE content in algal cells for 5 days. APC, allophycocyanin; chl a, chlorophyll a; MoS₂, molybdenum disulfide; PC, phycocyanin; PE, p-phycoerythrin; TiO₂, titanium dioxide; ZIF-67, zeolitic imidazolate framework-67.

Figure 5a showed the change of soluble protein content. Soluble protein was an important osmoregulatory substance and nutrient. Their increase and accumulation could improve the water retention ability of cells and played a protective role in the life substances and biofilms of cells. Therefore, the content of soluble protein was often utilized as one of the indicators for screening resistance. The analysis of soluble protein content could reflect the degree of cell stress from the external environment. From Figure 5a , the content of soluble protein in the experimental group was continuously reduced. The content of soluble protein in each group decreased significantly on the third day. In particular, the change was most obvious in the 10% MoS₂@ZIF-67/TiO₂ group, reaching its lowest point on the fifth day. This change trend could reflect the gradual increase of environmental stress and the decrease of osmotic regulation ability and water retention ability of cells. The changes of intracellular MDA content are shown in Figure 5b. In the process of photocatalysis, oxygen free radical was produced due to the addition of photocatalyst. Oxygen radicals attacked the polyunsaturated fatty acids in the biofilm, triggering lipid peroxidation and thus forming lipid peroxides (MDA, etc.) (Li et al., 2021). Therefore, the content of MDA could reflect the degree of intracellular lipid peroxidation and indirectly reflect the degree of cell damage. According to Figure 5b , the change trend of MDA content could reflect that the cell membrane of each group was seriously damaged. And the damage degree increased with time. On the fifth day of reaction, MDA content in all groups reached the maximum. Therefore, it was reasonable to speculate that the mechanism of photocatalytic inactivation of algal cells included the damage of cell membrane. The algal cells were removed by membrane damage. Comparing the change of MDA content and the number of algal cells, the number of algal cells in TiO₂ group decreased significantly not only because of the adsorption function of the photocatalyst itself. Cell membrane damage caused by photocatalytic reactions was also one of the reasons.

FIG. 5.

Content changes of (a) soluble protein, (b) malondialdehyde, (c) catalase enzyme, (d) peroxidase enzyme, (e) superoxide dismutase enzyme, (f) reactive oxygen species in the photocatalytic process.

SOD was an important component of antioxidant enzymes in biological systems. It could catalyze the superoxide anion radical disproportionation to produce O₂ and H₂O₂, which played a crucial role in the balance of oxidation and antioxidant. CAT was an enzyme scavenger. The function of CAT was to promote the decomposition of H₂O₂ into molecular O and H₂O, and remove H₂O₂ from the body. Thus, cells were protected from H₂O₂ toxicity. CAT was one of the key enzymes in the biological defense system. Its enzymatic activity provided the antioxidant defense mechanism for the body. POD was a kind of oxidase widely existing in various animals, plants, and microorganisms. The functionality of POD was similar to that of CAT. Both could eliminate H₂O₂ inside the cell. POD was one of the key enzymes in the plant defense system under stress conditions. It worked synergistically with SOD and CAT to remove excess free radicals in the body, thereby improving plant stress resistance. It could be seen from Figure 5c-e that the change trend of the three enzymes was the same. As the reaction progresses, the amount of the three enzymes increased. The increase of antioxidant enzyme content could reflect the antioxidant system at work. The increase in enzyme content also indirectly reflected the increase in the stress degree of cells. The cells were getting more and more damaged. However, the effect of photocatalysis on the three enzymes was different. According to the analysis of SPSS results, the degree of change of the CAT enzyme was the smallest among the three enzymes. It could be reasonably inferred that the antioxidant activity of the cells was mainly through the increase of SOD and POD enzyme content.

ROS were highly reactive chemicals containing oxygen free radicals that were highly toxic to living cells and could cause cell damage. ROS were originally a product of normal aerobic metabolism in the body and were at low level in the body. As “oxidation–reduction reaction messenger,” it was involved in intracellular signaling and regulation and played an important role in cell cycle, gene expression, and maintenance of environmental homeostasis. However, when the body was stimulated, the ROS level would increase sharply, and the oxidation–antioxidant effect of the body was unbalanced, resulting in oxidative stress. As a result, lipid peroxidation occurred, the structure and function of the protein changed, the cell membrane was destroyed, and finally the body cell died. Therefore, ROS level was an important marker of cellular oxidative damage caused by normal physiological function and environmental factors. As shown from Figure 5f, ROS levels within the 3 days of the experiment were similar to those in the control group. But on the fourth day, ROS levels rose dramatically. Among them, the ROS levels of the TiO₂@MoS₂ and MoS₂@ZIF-67/TiO₂ group were higher. The intracellular ROS increased gradually during 3 days, but did not exceed its own clearance capacity. By the fourth day, the body was unable to clear all ROS on its own, so high levels of ROS were exposed. Analysis of ROS content changes showed that the photocatalytic activity of the composite material increased the ROS level in algal cells, resulting in changes in protein structure and function. The cell membrane was destroyed and eventually the algal cell died.

Experimental study on the cyclic removal of M. aeruginosa by photocatalyst

To explore the stability of 10% MoS₂@ZIF-67/TiO₂ photocatalyst in practical application, five-cycle experiments were carried out in this study (Fig. 6). After the first experiment, the photocatalyst was recycled. First, the photocatalyst was cleaned. It was then dried. After drying, 30% MoS₂@ZIF-67/TiO₂ photocatalyst was applied to the next cycle experiment. In the five cycles, the mass of the photocatalyst was 0.3, 0.276, 0.253, 0.249, and 0.231 g, respectively. The removal rates of algal cells were 95.56%, 95.83%, 95.57%, 95.04%, and 93.91%, respectively. In five experiments, the mass loss was 0.069 g. This might be due to the loss of a small amount of photocatalyst during the cleaning recovery process. The first reason for the decrease in the removal rate might be the reduction in the quality of the added photocatalyst. Second, the surface of the reused photocatalyst adsorbed algal cells. Therefore, the photocatalyst could no longer adsorb algal cells in large quantities. The photocatalytic performance was reduced. In general, the photocatalyst exhibited excellent stability.

FIG. 6.

Algal cell density in cyclic experiments.

The photocatalytic effects of MoS₂@ZIF-67/TiO₂ and other photocatalysts on M. aeruginosa are shown in Table 2.

Table 2.

Comparison of the Inactivation Efficiency of Molybdenum Disulfide@Zeolitic Imidazolate Framework/Titanium Dioxide and Other Photocatalysts

Algal type	Photocatalyst	The synthesis method	Effective inhibition	References
Microcystis aeruginosa	Zn–Fe LDHs	Coprecipitation	After 2.5 h, the chlorophyll removal rate was 80.6%.	(Gu et al., 2016)
	P25 TiO₂ powder	From Shenshi Chem	After 12 h, nano TiO₂ effectively degraded algal cells.	(Lu et al., 2015)
	rGO/BiOBr@LF	Hydrothermal synthesis	After 3 h, the total organic carbon (TOC) removal rate was 77%.	(Tu et al., 2021)
	AgBiO₃	Ion exchange method	After 96 h, the removal rate of algal cells was 92.09%.	(Yu et al., 2010)
	Ag/AgCl@g-C₃N₄@UIO-66(NH₂)	Hydrothermal and in-situ growth method	After 3 h, the chlorophyll removal rate was 99.9%.	(Fan et al., 2021b)
	Ag/AgCl@ZIF-8	Stirring method	After 6 h, the chlorophyll removal rate was 90%.	(Fan et al., 2020)
	Ag₂O@PG-0.4	Chemical deposition method	After 5 h, the removal rate of algal cells was 99.1%.	(Chen et al., 2022)
	Pd/g-C₃N₄	Typical thermal polymerization method	95.17% algal cells were removed in the Pd/g-C₃N₄ (5%) system within 10 min.	(Lu et al., 2022)
	MoS₂@ZIF-67/TiO₂	Hydrothermal synthesis	On the fifth day, the removal rate of algal cells was 96.52%	This study

LDH, lactate dehydrogenase.

The inactivation mechanism of cyanobacterium M. aeruginosa by photocatalysis

The possible mechanism of algal cell inactivation is shown in Figure 7. TiO₂ particles were attached to the surface of MoS₂@ZIF-67 nanoparticles. The complex nanoflower structure greatly improved the specific surface area of MoS₂@ZIF-67/TiO₂ material and its own adsorption capacity. Moreover, the bandgap width of 10% MoS₂@ZIF-67/TiO₂ composite photocatalyst was reduced (2.4 eV). The narrow bandgap width meant that the composite photocatalyst was more likely to be excited by low-energy photons, which made the photocatalytic reaction more likely to occur. In addition, 10% MoS₂@ZIF-67/TiO₂ material could also absorb visible light. This broke through the imprisonment, which TiO₂ only absorbed ultraviolet light. Combined with the SEM images of algal cells and the changes in MDA content, both confirmed that the cell membrane was damaged during photocatalysis. The change in ROS content explained the cause of cell membrane damage. It was the increase in ROS content that led to changes in the structure and function of membrane proteins, and the destruction of cell membranes. The changes in the contents of various antioxidant enzymes could also reflect the degree of body injury. As the reaction progressed, the degree of damage increased and reached its peak on the fifth day. Among the three antioxidant enzymes, SOD and POD played a major role in the antioxidant process.

FIG. 7.

Possible mechanism of algal cell inactivation.

Conclusions

In this study, three monomer materials, three binary composites, and three proportional MoS₂@ZIF-67/TiO₂ composites were successfully prepared. The microscopic morphology, valence state of chemical elements, light absorption characteristics, and pore size distribution of different materials were investigated. After doping MoS₂@ZIF-67, the advantages of the photocatalyst were wide light absorption range, large specific surface area, narrow bandgap, and so on. In the process of photocatalysis, the addition of photocatalyst caused algal cells to be stressed by the surrounding environment, and algal cells produced high ROS content. Excessive ROS content destroyed the structure and function of intracellular proteins, resulting in the loss of cell membranes. The result was the apoptosis of algal cells. Thirty percent MoS₂@ZIF-67/TiO₂ possessed an excellent algae removal effect. The inactivation rate for M. aeruginosa was 86.08% by 10% MoS₂@ZIF-67/TiO₂. This study would provide a new choice for the material aspect of photocatalysis.

Authors’ Contributions

R.W.: Conceptualization, software, investigation, writing—original draft, writing—review and editing, supervision, and data curation. Y.W.: Methodology, software, investigation, writing—original draft, writing—review and editing, supervision, and data curation. Q.W. and M.W.: Writing—review and editing, supervision, and data curation. J.C.: Conceptualization, methodology, software, investigation, writing—original draft, writing—review and editing, supervision, and data curation.

Footnotes

Author Disclosure Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

Funding Information

This work was financially supported by the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province (2022KJ182), the National Natural Science Foundation of China (no. 31971503), and the Shandong Postdoctoral Science Foundation (SDCX-ZG-202400174).

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Photocatalytic Inactivation of Microcystis aeruginosa Using MoS 2 @Zeolitic Imidazolate Framework/TiO 2 : A Hybrid Catalyst for Efficient Control of Harmful Algal Blooms

Abstract

Keywords

Introduction

Materials and Methods

Synthesis of various photocatalysts

Preparation of MoS2

Preparation of ZIF-67

Preparation of TiO2

Preparation of MoS2@ZIF-67

The form was observed by scanning electron microscopy

Photocatalytic inactivation of M. aeruginosa

Determination of growth, metabolism, and antioxidant indexes of M. aeruginosa

Statistical analysis

Results and Discussions

Physical and chemical characterization of different photocatalysts

Inactivation experiment of M. aeruginosa by photocatalyst

Experimental study on the cyclic removal of M. aeruginosa by photocatalyst

The inactivation mechanism of cyanobacterium M. aeruginosa by photocatalysis

Conclusions

Authors’ Contributions

Footnotes

Author Disclosure Statement

Funding Information

References

Preparation of MoS₂

Preparation of TiO₂

Preparation of MoS₂@ZIF-67