1 Introduction
Owing to global climate change and dramatic increasing bioavailable nutrients in surface waters, blooms of cyanobacteria (blue-green algae) have occurred frequently worldwide in recent decades (
Paerl and Scott, 2010;
Gobler, 2020). As a dominant species in these blooms,
Microcystis aeruginosa can produce various harmful metabolites, including taste and odor compounds, and microcystins (MCs, a type of hepatotoxic cyanotoxins detrimental to animals and humans) (
Codd et al., 2005;
Zhang et al., 2013). In addition, the excessive growth of
M. aeruginosa cells leads to the release of algal organic matters (AOM), impairing water quality (
Jia et al., 2018). AOM may also negatively affect water treatment processes by reducing coagulation efficiency, causing serious membrane fouling, and acting as precursors of disinfection by-products (
Fang et al., 2010;
Xie et al., 2013;
Li et al., 2014;
Zhang et al., 2016).
Conventional water treatments including coagulation, sedimentation, and filtration are ineffective in cyanobacterial removal. Moreover, a combination of those methods is relatively complex and costly (
Ma et al., 2007;
Rajasekhar et al., 2012;
Zamyadi et al., 2012). Addition of oxidants (e.g., chlorine) can improve the subsequent removal of cyanobacteria, but it may also rupture cyanobacterial cells, leading to the release of intracellular toxins and AOM (
Huo et al., 2015;
Naceradska et al., 2017). Therefore, alternative techniques are required to efficiently remove cyanobacterial cells and minimize the release of harmful metabolites into surrounding waters.
Nanoscale zero-valent iron (Fe
0) has been emerged as a technique for mitigating cyanobacteria problems in recent years. However, its effectiveness in removing and inhibiting cyanobacteria is limited, probably because of the agglomeration and rapid-oxidation of Fe
0 in aqueous solutions (
Marsalek et al., 2012;
Lei et al., 2016;
Eljamal et al., 2018;
Nguyen et al., 2018;
Chen et al., 2021). Recently, Mg(OH)
2 coated Fe
0 nanoparticles (Fe
0@Mg(OH)
2), recognized as an efficient material, have attracted extensive attention. The Mg(OH)
2 shell can protect Fe
0 from rapid corrosion and reduce the magnetic attraction between Fe
0 particles (
Hu et al., 2018;
Chen et al., 2021). A study by
Fan et al. (2018a) has shown that the effectiveness of
M. aeruginosa removal via Fe
0@Mg(OH)
2 greatly increased to 99.9% after 3 h, in contrast to 39.5% by bare Fe
0. Although
M. aeruginosa cells remained intact during Fe
0@Mg(OH)
2 treatment, as observed by scanning electron microscopy (SEM), the quantitative impact on the release of MCs and AOM remains unknown, necessitating further investigation.
To date, reported
M. aeruginosa treatment by Fe
0@Mg(OH)
2 was conducted in the BG-11 medium, with a cell density of 2.5 × 10
6 cells/mL (
Fan et al., 2018a). However, the effectiveness of cyanobacteria treatment by various technologies are affected by multi-factors such as background water matrix, chemical dosages, and initial cell densities (
Zamyadi et al., 2012;
Zhang et al., 2015;
Jian et al., 2019). For examples, high concentrations of dissolved organic carbon and various ions in source waters can inhibit K
2FeO
4 oxidation in
M. aeruginosa inactivation (
Fan et al., 2018b).
Lin et al. (2009) demonstrated that the rate constants of
M. aeruginosa cell rupture by chlorination decreased from 180 to 110 mol/(L·s), when the initial cell density increased from 1.3 × 10
5 to 4.4 × 10
5 cells/mL. The influence of cell density is expected to be larger since
M. aeruginosa cell densities can vary from 10
4 to 10
9 cells/mL in natural blooms (
Ger et al., 2014;
Ma et al., 2015). Therefore, it is important to evaluate those factors on the effectiveness of Fe
0@Mg(OH)
2 in treating
M. aeruginosa, for ensuring its application under practical conditions.
Consequently, the aims of this study were to (1) assess the influence of various factors (initial cell densities, Fe0@Mg(OH)2 dosages, and water matrices) on the removal efficiency of M. aeruginosa by Fe0@Mg(OH)2 and understand the underlying mechanisms; (2) investigate the viability of M. aeruginosa cells in natural water by Fe0@Mg(OH)2 treatment; and (3) explore the concomitant release of MCs and AOM during the Fe0@Mg(OH)2 treatments.
2 Materials and methods
2.1 Water source
Raw water was collected from the Ming Hui Lake (MHL) (where cyanobacterial blooms occur frequently) located in Zhoushan, China. The sampled water was filtered through 0.45 μm and then 0.22 μm cellulose acetate filters (Xingya, China) to remove impurities and microorganisms. The filtered source water was then stored at 4 °C for the subsequent experiments, its qualities were presented in Table S1.
2.2 Cultures of M. aeruginosa
M. aeruginosa (strain FACHB-905, from the Institute of Hydrobiology, Chinese Academy of Sciences, China) was chosen as the target cyanobacterium.
M. aeruginosa was cultured in sterilized basal glucose (BG-11) medium and routinely sub-cultured (
Stanier et al., 1971). The strain was incubated under cool fluorescent light flux (25 μmol/(m
2·s), light-dark cycle = 12 h:12 h), at a constant temperature of 25 ± 1 °C to achieve healthy
M. aeruginosa cultures.
2.3 Synthesis of Fe0@Mg(OH)2
The Fe
0@Mg(OH)
2 particles were chemically prepared based on a rate-controlled precipitation method as per
Hu et al. (2019). In general, 70 mg Fe
0 was suspended into ethanol by ultrasonically dispersing under N
2 atmosphere at 25 ± 2 °C to avoid oxidation. Subsequently, a certain amount of MgCl
2/ethanol solution was added to the suspension to ensure the ration of Mg/Fe (wt%) was 1:2. The concentrations and dropping speed of the NaOH/ethanol solutions were controlled to achieve complete reaction. After aged for 1 h under ultrasonic conditions, the Fe
0@Mg(OH)
2 particles were subsequently washed using methanol and ethanol for three times in turns and dried with N
2 gas.
2.4 M. aeruginosa exposure to Fe0@Mg(OH)2
All experiments were performed at a room temperature of 20 ± 2 °C. Fe0@Mg(OH)2 was dispersed with deoxygenated deionized water under ultrasonic conditions to prepare nano-particle suspension. Then 5 mL of the suspension contained with certain amount of Fe0@Mg(OH)2 particles was mixed with 95 mL of M. aeruginosa culture in 100 mL conical flasks. Air-breathing membranes were used to seal the bottles ensuring gas exchange while preventing cross-contamination. Subsequently, various tests were applied to explore the efficiency of Fe0@Mg(OH)2 on M. aeruginosa cell removal, with all tests conducted in triplicates.
2.4.1 Factors on the removal efficiency of M. aeruginosa by Fe0@Mg(OH)2
To investigate the removal effects of Fe0@Mg(OH)2 on M. aeruginosa samples across varying cell densities, initial cell densities ranging from 1.0 × 105 to 8.0 × 106 cells/mL were prepared by dilution with BG-11 medium. A consistent Fe0@Mg(OH)2 (Fe) concentration of 50 mg/L was employed. Moreover, to explore the impacts of different Fe0@Mg(OH)2 concentrations on M. aeruginosa removal, specific volumes of Fe0@Mg(OH)2 suspension were added into M. aeruginosa samples (cell density = 1.0 × 106 cells/mL) to achieve the final Fe concentration from 10 to 80 mg/L. In addition, to explore the influences of water matrix on the removal efficiency of M. aeruginosa cells by Fe0@Mg(OH)2, experiments were conducted using BG-11 medium (with and without the released AOM) and MHL water for comparison. The initial cell density of M. aeruginosa was set at 1.0 × 106 cells/mL, with Fe0@Mg(OH)2 (Fe) concentration of 20 mg/L. The cell removal efficiency (R) was calculated as followed Eq. (1):
where Nc and Nt represent the M. aeruginosa cell density in the control and Fe0@Mg(OH)2 treated sample at a specific sampling time of t, respecively.
2.4.2 The impacts of Fe0@Mg(OH)2 on characteristics of M. aeruginosa cells
According to the results from 2.4.1, M. aeruginosa cultures with an initial cell density of 1.0 × 106 cells/mL (diluted with MHL water) were chosen for the experiments involving Fe0@Mg(OH)2 treatment at a Fe concentration of 20 mg/L. Prior to the test, the pH value of M. aeruginosa samples was controlled as 8.0 ± 0.1 with 0.1 mol/L NaOH or 0.1 mol/L HCl. Then, M. aeruginosa samples collected at pre-determined time intervals were prepared for further analysis. The evaluation of M. aeruginosa cell viability was conducted through the detection of Chlorophyll a (Chl-a), Fv/Fm indicating the maximum effective quantum yield of PSII, and potassium ion (K+) levels. Transmission electron microscopy (TEM, JEM-1230, JEOL, Japan) was utilized to characterize the ultrastructure of M. aeruginosa cells. The concentrations of extracellular microcystin-LR (MC-LR) and amounts of the released AOM were measured to assess the subsequent impacts on water quality. X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, USA) and X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) were conducted to investigate the mechanisms of M. aeruginosa removal by Fe0@Mg(OH)2.
2.5 Analytical methods
2.5.1 Cell counts
M. aeruginosa samples were collected from 0 to 10 h from the suspensions, and immediately treated with Lugol’s iodine. Subsequently, the cell densities were enumerated with a microscopy (ECLIPSE E100, Nikon, Japan) at 100× magnification using a Sedgwick Rafter chamber (Graticules Ltd, UK) (
Fan et al., 2013).
2.5.2 Characterization of flocs during exposure process
XRD was used to characterize crystal structure of Fe0@Mg(OH)2 after exposure to SO42– solutions (Fe:SO42– = 1:3, wt%). The XRD data were performed in an angular range 2θ = 10–80° with a step length of 0.02 °/min and a scanning speed of 5 °/min. The diffractograms were then compared to International Centre for Diffraction Data (ICDD) cards: brucite 83-0114, Fe0 87-0721, goethite 81-0462, hematite 85-0987. To analyze the changes in surface composition, the crystals of flocs were characterized by XPS before and after the reaction.
2.5.3 Cell viability
The concentration of Chl-a was estimated according to a previous study
Gao and Tam (2011). In brief, Chl-a was extracted from 10 mL
M. aeruginosa sample with 95% ethanol for 10 min at 60 °C. The extraction supernatant was measured subsequently at the wavelength of 665 and 652 nm, and then the amount of Chl-a was calculated based on these readings. The Fv/Fm values of
M. aeruginosa were quantified after dark-adaption for 15 min, using a Phyto-Pulse-Amplitude modulated fluorometer (Phyto-PAM, Walz, Germany) (
Wu et al., 2007).
To determine the integrity of
M. aeruginosa cells during Fe
0@Mg(OH)
2 treatment, the levels of K
+ and released AOM after treatments were assessed.
M. aeruginosa sample with a volume exceeding 20 mL (collected at 0.25, 0.75, 1.5, 3, 6, and 10 h, respectively) was successively filtrated through 0.45 and 0.22 μm glass-fiber filters (GF/F, Xingya, China), and then divided into two sub-samples. One filtrate (10 mL) was acidified with concentrated HNO
3 for immediate K
+ detection using an atomic absorption spectrophotometer (200 Series, Agilent Technologies, USA). Another 10 mL filtrated sample was used to analyze the fluorescence excitation-emission matrix (EEM) spectra of the released AOM, with a fluorescence spectrophotometer (RF-6000, Shimadzu, China). Data analysis was referred to a method recorded by
Tang et al. (2018). The EEM spectra were collected by scanning the emission wavelength (Em) of 250–550 nm at 2 nm increments, and excitation wavelength (Ex) of 200–450 nm at 5 nm increments.
2.5.4 Quantification of MC-LR
Due to the strong aggregation and binding between the
M. aeruginosa cells and Fe
0@Mg(OH)
2 particles, as reported by
Fan et al. (2018a), it appears to be challenging to separate the cells from the sediment. Consequently, only extracellular MC-LR was determined in this study. Following exposure to Fe
0@Mg(OH)
2 for 0.25, 0.75, 1.5, 3, 6, and 10 h, 100 mL
M. aeruginosa suspension was collected and subsequently filtered through 0.45 and 0.22 μm membrane filters to analyze the amount of extracellular toxin. The filtrates were then concentrated using C18 solid-phase (Waters, USA) extraction (
Nicholson et al., 1994). The concentrations of extracellular MCs were quantified using a high-performance liquid chromatography (LC-20ADCR, China) and the procedural details were adapted from a previous study (
Lin et al., 2020).
2.5.5 TEM imaging
M. aeruginosa samples, each with a volume of 10 mL, were harvested at specific times (0.25, 0.75, 1.5, 3, 6, and 10 h) through rapid centrifugation at 6000 r/min for 5 min. The resultant sediments were then subsequently treated as reported previously by
Wang et al. (2011). TEM was then used to obtain the ultrastructural alternation of
M. aeruginosa cells treated by Fe
0@Mg(OH)
2. Meanwhile, crystallographic change of Fe
0@Mg(OH)
2 were observed using a high-resolution transmission electron microscope (JEM-2100Plus, JEOL, Japan).
2.5.6 Statistical analysis
Figures were created with Origin 2018 (OriginLab, USA) and Prism 8.0 (GraphPad Software, USA). One-way ANOVA was used for determination of the statistical significance (P < 0.05) among different test groups.
3 Results
3.1 M. aeruginosa removal by Fe0@Mg(OH)2
M. aeruginosa cells rapidly aggregated as flocs within the first 0.75 h by Fe0@Mg(OH)2 treatment (Fig. S1). Hence, the cells in suspensions were efficiently removed via sedimentation. The influence of initial cell densities (1.0 × 105–8.0 × 106 cells/mL) on the removal of M. aeruginosa cells by 50 mg/L Fe0@Mg(OH)2 in BG-11 medium was investigated (Fig.1(a)). Cells were removed rapidly from the water column during 0.5 h in all groups. In general, the removal efficiency of M. aeruginosa by Fe0@Mg(OH)2 was enhanced with increasing initial cell densities. The cell removal efficiency improved from 70% to 97% within 0.5 h treatment, when the initial cell densities increased from 1.0 × 105 to 8.0 × 106 cells/mL. More than 98% of M. aeruginosa cells were removed after exposure to Fe0@Mg(OH)2 for 3 h, for the samples with cell densities exceeding 1.0 × 106 cells/mL.
The effect of Fe0@Mg(OH)2 dosages (10–80 mg/L) on the removal efficiency of M. aeruginosa cells (cell density = 1.0 × 106 cells/mL) in BG-11 medium was shown in Fig.1(b). Apparently, the removal efficiency of cyanobacterial cells increased with Fe0@Mg(OH)2 dosages increasing from 10 to 50 mg/L. However, no significant differences (P > 0.05) in removal efficiencies were observed at Fe0@Mg(OH)2 dosages > 50 mg/L. For example, 87% and 91% of M. aeruginosa cells were removed by 50 and 80 mg/L Fe0@Mg(OH)2 treatment for 1 h, respectively, while the R value was only 26% by 10 mg/L Fe0@Mg(OH)2. The removal efficiency of M. aeruginosa (1.0 × 106 cells/mL) by Fe0@Mg(OH)2 treatment in natural water (MHL water) was also assessed, it was lower than that in BG-11 medium initially (Fig.1(c)). For instance, 20 mg/L Fe0@Mg(OH)2 removed 39% of M. aeruginosa cells in BG-11 medium after 0.75 h, while it only removed 19% of cells in MHL water. However, the R value of M. aeruginosa by Fe0@Mg(OH)2 in MHL water was increased subsequently, and over 98% of cells were removed at a sampling time of 10 h.
3.2 XPS and XRD analysis
The chemical composition and the element change on the surface of
M. aeruginosa flocs before and after Fe
0@Mg(OH)
2 treatment were analyzed (Table S2). Fe contents in
M. aeruginosa flocs increased approximately by 30% and 130%, after dosing with Fe
0@Mg(OH)
2 for 1.5 and 6 h, respectively. The detailed XPS spectra from 700 to 740 eV presented relative abundances of different Fe species on the surface of
M. aeruginosa flocs, and the coexistence of Fe(II) and Fe(III) was observed (Fig.2). The peaks at 710.4 and 724.0 eV correspond to Fe
2+ 2p
3/2 and Fe
2+ 2p
1/2, respectively, and the peaks at 712.4 and 726.1 eV are assigned to Fe
3+ 2p
3/2 and Fe
3+ 2p
1/2 (
Zhou et al., 2020). Additionally, the signals of Fe
0 peaks were observed at 706.4 and 719.4 eV, and the peak located at 714.3 eV can be attributed to the Fe(III)-OH (
Hu et al., 2018).
Fig.3 presented the XRD plots of the Fe0@Mg(OH)2 particles before and after exposure to SO42– solutions. The typical peak at 2θ of 44.68° corresponded to the (110) plane of Fe0. Broad peaks at 2θ of 18.58°, 37.98°, and 58.61° were consistent with the (001), (011) and (110) directions of Mg(OH)2 respectively, which confirmed the formation of Mg(OH)2 shell. After exposure to SO42– solutions, new diffraction peaks appeared at 21.19° and 33.21°, which corresponded to the (110) and (121) directions of FeOOH and Fe2O3, respectively.
3.3 Cell photosynthetic capacity and the variation of K+
The content of Chl-a in the control sample remained almost constant during the 10 h experiment (Fig.4(a)). In contrast, with the application of 20 mg/L Fe0@Mg(OH)2, the Chl-a contents in the M. aeruginosa suspension gradually decreased to below the detection limit after 1.5 h (Fig.4(a)). The Fv/Fm value in samples treated with Fe0@Mg(OH)2 maintained at ~ 0.5 during the initial 1.5 h, but dropped to 0.35 after 10 h (Fig.4(a)). Following the treatment with Fe0@Mg(OH)2 for 0.25 h, the concentration of K+ in the suspension decreased from 12.6 to 11.8 mg/L (P < 0.05) (Fig.4(b)), and then returned to ~12.3 mg/L after 1.5 h.
3.4 Morphology of M. aeruginosa cells
The alternations in the ultrastructure of M. aeruginosa cells by Fe0@Mg(OH)2 treatment were captured in TEM images (Fig.5). Intact intracellular structures, including thylakoids, lipid droplets, gas vesicles and nucleoid regions were distinctly observed in the control samples (Fig.5(b)). Similar ultrastructure of M. aeruginosa cells were found after Fe0@Mg(OH)2 treatment for 1.5 h (Fig.5(c)). Only a small number of cells appeared irregular and rough after 6 and 10 h, yet the majority remained intact (Fig.5(d)–Fig.5(e)). The internalization of Fe0@Mg(OH)2 by M. aeruginosa was observed after 10 h of treatment (Fig.5(f)). Overall, despite being enveloped by Fe0@Mg(OH)2 particles during the treatment, incidents of cell collapse and lysis were rarely observed in M. aeruginosa samples (Fig.5(c)–Fig.5(f)).
3.5 Variations of MC-LR and AOM in M. aeruginosa samples
The
M. aeruginosa samples treated with 20 mg/L Fe
0@Mg(OH)
2 for 0–10 h were collected and measured for MCs (Fig.4(b)). The concentration of extracellular MC-LR in the control samples remained at around 1.8 μg/L. Following the treatment with Fe
0@Mg(OH)
2 for 0.25 h, the concentration of MC-LR decreased from 1.8 to 1.4 μg/L, and then remained relatively stable thereafter. Additionally, the main components of dissolved AOM in the
M. aeruginosa cells were also assessed using EEM fluorescence spectra (Fig.6). Only one main peak at Ex/Em of 275/320 nm was identified at each sampling time, indicating the presence of soluble microbial products as described by
Chen et al. (2003).
4 Discussion
4.1 Factors affecting the removal efficiency of M. aeruginosa by Fe0@Mg(OH)2 and related mechanisms
M. aeruginosa with cell densities varying from 1.0 × 10
5 cells/mL to 8.0 × 10
6 cells/mL, could be effectively removed from suspensions by Fe
0@Mg(OH)
2 (Fig.1(a) and S1). Besides simplifying the operation, Fe
0@Mg(OH)
2 can achieve higher efficiency in
M. aeruginosa removal, in comparison with traditional methods that combine pre-oxidation, coagulation, and sedimentation (
Takaara et al., 2010;
Qi et al., 2016). A previous study also indicated that Fe
0@Mg(OH)
2 could lead to efficient
M. aeruginosa removal at a cell density of 2.5 × 10
6 cells/mL, in which magnetic attraction between Fe
0@Mg(OH)
2 particles and cells was identified as the main removal mechanism (
Fan et al., 2018a). In the current study, Fe(III) ions were gradually generated during the Fe
0@Mg(OH)
2 treatment (Fig.2). These
in situ formed Fe(III) ions could act as potential cation bridges between
M. aeruginosa cells and Fe
0@Mg(OH)
2 particles, hence facilitating the aggregation of cells and increasing the size of cyanobacterial flocs (
Ma et al., 2012). Notably, the
R values of
M. aeruginosa by Fe
0@Mg(OH)
2 was found to increase with increasing cell densities, but decreased upon the removal of dissolved AOM (Fig.1(a)–1(c)). Both the surface charge of
M. aeruginosa cells and AOM are negative, in contrast to the positive charge of Fe
0@Mg(OH)
2 (
Henderson et al., 2008;
Ma et al., 2012;
Qu et al., 2012). Therefore, the formation and sedimentation of complexes comprising cells, Fe
0@Mg(OH)
2, and AOM could be enhanced by increasing the amounts of cells and associated AOM.
The pseudo-second-order equation was used to describe
M. aeruginosa removal after exposure to Fe
0@Mg(OH)
2, which is shown in the following Eq. (2) (
Hamadi et al., 2004).
where represents the amount of M. aeruginosa cells removed by Fe0@Mg(OH)2 at time t (cells/μg), represents the amount of M. aeruginosa cell removal at equilibrium (cells/μg), k represents the rate constant for cell removal (μg/(cells·h)) and k is the initial cell removal rate constant (cells/(μg·h)).
The removal rate constant (
k) remained almost consistent throughout the treatment period. This may be owing to the substantial removal of
M. aeruginosa cells within the first hour by 50 mg/L Fe
0@Mg(OH)
2. Therefore, the initial removal rates were calculated to evaluate the cell removal ability (Tab.1). The rate of cell removal was dependent on initial cell density, increasing from 10 to 16667 cells/(μg·h) as the initial cell densities increased. However, the initial removal rate of
M. aeruginosa increased slightly with increasing Fe
0@Mg(OH)
2 dosages (Tab.2). It only increased from 47 to 135 cells/(μg·h) when the Fe
0@Mg(OH)
2 dosages increased from 10 to 80 mg/L (Tab.2). Overall, Fe
0@Mg(OH)
2 demonstrated outstanding performance in the treatment of high concentrations of cyanobacteria, while some other nanocomposites (e.g., Fe-Cu-Mn oxides/peroxymonosulfate) are limited to treating cyanobacterial samples with low cell densities (
Sun et al., 2022;
Yang et al., 2023).
The efficient removal of
M. aeruginosa by Fe
0@Mg(OH)
2 in natural waters has also been confirmed in this study (Fig.1(c) and Fig.4(a)). Initially, within the first 1.5 h, there was a decrease in the removal efficiency of
M. aeruginosa (Fig.1(c)). The presence of a high concentration of SO
42– in MHL water (68.5 mg/L) may have accelerated the passivation of the core-shell structure of Fe
0@Mg(OH)
2 (
Xie and Cwiertny, 2012;
Pullin et al., 2017), which was verified by the observation of FeOOH transformed from Fe
0@Mg(OH)
2 (Fig.3 and Fig.5) (
Zhou et al., 2014). A study by Teixeira and Rosa (2007) has reported that the efficiency of Al
2O
3 in
M. aeruginosa removal was decreased in natural water, requiring higher Al
2O
3 dosages. However, the removal efficiency of
M. aeruginosa by Fe
0@Mg(OH)
2 could subsequently exceed 98% in MHL water after 1.5 h without increase in dosages (Fig.1(b)). Besides, no cyanobacterial regrowth was observed (Fig.1(d)). Therefore, Fe
0@Mg(OH)
2 demonstrates a high potential to efficiently mitigate cyanobacteria problems, even in complex aquatic environments.
4.2 The characteristics of M. aeruginosa samples after exposure to Fe0@Mg(OH)2
As a critical component to biosynthesize cytoplasmic membranes, the concentration of dissolved K
+ is usually used to assess the cell integrity of
M. aeruginosa (
Zhou et al., 2013;
2020). In this study, a decrease of K
+ was observed during the first 0.25 h, likely due to the adsorption by Fe
0@Mg(OH)
2 (Fig.4(b)). A similar phenomenon was reported by Li and Zhang (2007), indicating that metal cations could be adsorbed onto the surface of Fe
0. Although the adsorbed K
+ was subsequently desorbed, its concentration remained below the initial level during the 10-h treatment (Fig.4(b)). The steady trend of K
+ during the treatment suggested that the
M. aeruginosa cells probably remained intact. In addition, humic-like substances, decomposition products of dead cells, were not observed in the
M. aeruginosa samples throughout the treatment (Fig.6), indicating that Fe
0@Mg(OH)
2 did not induce additional damage to the cells (
Qu et al., 2012;
Xu et al., 2016).
Fv/Fm value, being biomass-independent, serves as a sensitive indicator for assessing the PSII function in photosynthetic organisms (
Maxwell and Johnson, 2000;
Drábková et al., 2007). The Fv/Fm values of
M. aeruginosa samples diminished after 1.5 h treatment with 20 mg/L Fe
0@Mg(OH)
2 (Fig.4), and the particles were found to attach on
M. aeruginosa cells (Fig.5). These findings suggest that the adhesion of Fe
0@Mg(OH)
2 on the cell surface may limit the accessibility of
M. aeruginosa to light, echoing the effects observed from other nanoparticles, thus hindering cell regrowth (Fig.1(d)) (
Wang et al., 2011;
Long et al., 2014). Furthermore, similarly to the impact of other nanoparticles, the internalization of Fe
0@Mg(OH)
2 by
M. aeruginosa cells may also impede their photosynthesis (Fig.5(f)) (
Wang et al., 2011).
In general, the levels of extracellular MC-LR in the
M. aeruginosa samples treated with Fe
0@Mg(OH)
2 were comparable to those in the control, with a notable decrease observed within the first 0.25 h (Fig.4(b)). Given the minimal oxidation capacity of Fe
0@Mg(OH)
2, this reduction in MC-LR levels could be attributed to the adsorption capacity of FeOOH (
Pivokonsky et al., 2012), which was formed from Fe
0@Mg(OH)
2 process (Fig.3). This indicates that Fe
0@Mg(OH)
2 presents an effective method to mitigate cyanobacterial problems without increase in dissolved toxins, which is a significant advantage over other technologies (
Teixeira and Rosa, 2006;
Wang et al., 2018). For instance, it has been reported that static ultrasonic radiation would induce damage to
M. aeruginosa cells, resulting in increased concentrations of released MC-LR as ultrasonic radiation time increased (
Peng et al., 2023). Similarly, chemical options such as oxidants have the potential to lyse cyanobacterial cells, and improper oxidant dosage would also induce the release of MC-LR, deteriorating water quality (
Ou et al., 2012;
Zhang et al., 2017).
5 Conclusions
This study demonstrates that Fe0@Mg(OH)2 can effectively remove M. aeruginosa with high cell densities in natural water. The negligible release of MCs and AOM during the Fe0@Mg(OH)2 treatment can avoid worsening water quality and reduce the burden on subsequent treatment processes. The effectiveness of Fe0@Mg(OH)2 may be influenced by the water matrix at the start of treatment, while it could be enhanced subsequently without increasing its dosage. Given its high efficiency, Fe0@Mg(OH)2 has the potential for broad applications in immediate control of M. aeruginosa blooms across various water bodies, including ponds, reservoirs, lakes and landscape waters.
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