Nanoscale Zero-Valent Iron Coated with Magnesium Hydroxide for

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Nanoscale zerovalent iron coated with magnesium hydroxide for effective removal of cyanobacteria from water Jiajia Fan, Yi-bo Hu, and Xiao-yan Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03593 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Sustainable Chemistry & Engineering

Nanoscale zerovalent iron coated with magnesium hydroxide for effective removal of cyanobacteria from water Jiajia Fan † 1, Yi-bo Hu ‡ 1, Xiao-yan Li * ‡,§ †

Ocean College, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, Zhejiang, China

‡

Environmental Engineering Research Centre, Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China

§

Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen 518055, China *Corresponding author: Tel.: +852 28592659; fax: +852 25595337; email: [email protected] 1

Jiajia Fan and Yi-bo Hu contributed equally to this work.

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Abstract The effects of nanoscale zero-valent iron (NZVI, as a benchmark) and a novel core-shell structured nanoparticle, magnesium hydroxide–coated NZVI (NZVI@Mg(OH)2), on the removal and viability of a cyanobacterium, Microcystis aeruginosa, in water were investigated. Dosing of these two nanoparticles at concentrations between 20 and 100 mg/L led to various degrees of sedimentation of cyanobacterial cells from the water column. NZVI@Mg(OH)2 exhibited much better ability to remove cyanobacteria than NZVI. A cell removal efficiency of 86.6% was achieved after 0.5 h with the lowest dosage (20 mg/L) of NZVI@Mg(OH)2, whereas the highest dosing concentration of NZVI (100 mg/L) resulted in the removal of only 57.5% of cells. This improved effectiveness can be attributed to the reduction in electrostatic repulsion between the cyanobacterial cells and NZVI@Mg(OH)2. The membrane integrity of the settled cyanobacterial cells in the sediment was significantly disrupted after the addition of a 100 mg/L dose of NZVI, which may lead to the release of unwanted intracellular metabolites. In contrast, the viability of the settled cyanobacteria was nearly unaffected after treatment with NZVI@Mg(OH)2. It can be concluded that NZVI@Mg(OH)2 is highly effective in the removal of cyanobacteria from the water column. The Mg(OH)2 coating not only protects the NZVI cores from corrosion in water, it also reduces the toxicity of NZVI toward cyanobacterial cells when NZVI technology is applied for cyanobacteria control and removal in water supply and treatment.

Keywords：：Cyanobacteria; cell removal; nanoscale zero-valent iron; Mg(OH)2 coating; water purification


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Introduction The increasing occurrence of cyanobacteria (blue-green algae) blooms associated with eutrophication has presented great challenges to drinking water supplies. In particular, taste and odor problems and toxic metabolites (named as cyanotoxins) from cyanobacteria are frequently reported [1,2]. Efforts have been made in recent years to develop effective technologies to control cyanobacteria problems both in water resources and in the treatment processes. Cyanobacteria generally remain suspended in water due to their small size and similar density to water, thus sedimentation by gravity would enable the removal of cells from the water column. For drinking water treatment plants (DWTPs), the conventional water treatment process, which consists of coagulation, sedimentation, and filtration, can easily be overwhelmed by cyanobacterial blooms [3,4]. Coagulation is a key step to facilitate cyanobacteria sedimentation, but it has shown poor efficiency for this purpose [5,6]. Thus, it is common to add a pretreatment (e.g., chlorination and ozonation) before the conventional treatment process. Oxidation can alter the surface characteristics of cyanobacteria cells, which would enhance cell coagulation to facilitate their subsequent removal [7,8]. However, many studies have shown that pre-oxidation may cause significant lysis of cyanobacterial cells and the concomitant release of undesirable intracellular metabolites [9-12]. Moreover, pretreatment with chlorine and ozone may lead to the formation of toxic disinfection byproducts [13,14]. In addition, the combination of pre-oxidation and coagulation has its inherent complexity in the operation of DWTPs. Therefore, there is still a need for the development of alternative technologies that are not only simple and effective in removing cyanobacteria but also environmentally benign and safe for application.



Nanoparticles have been increasingly used for environmental decontamination via surface-mediated reactions and adsorption [15]. It has been reported that magnetic nanoparticles, such as nanoscale zero-valent iron (NZVI) and magnetite (Fe3O4), can be used to remove algal cells from water and inhibit the cell regrowth for several days [16-18]. Compared to the process that combines pre-oxidation, coagulation, and sedimentation, algal cells attached to magnetic nanoparticles can be more easily removed from the water column via sedimentation or magnetic separation. However, because both magnetic iron particles and algal cells have the same negatively charged surface, electrostatic repulsion would actually hinder their attachment. Therefore, various polyelectrolytes with cationic groups, such as polyethylenimine [19] and amino-rich polyamidoamine [20], have been tested to modify the surface of magnetic particles. However, few studies have been conducted to use more environmentally friendly inorganic materials for the modification of magnetic iron nanoparticles. In addition, one common problem in NZVI application is that iron particles would be rapidly oxidized to form an oxide layer on the NZVI surface [21,22]. The iron oxide layer is unable to protect the underneath iron layer from corrosion. Moreover, the iron oxide can be reactive in producing reactive oxygen species (ROS) [21], harmful to algal cells [23]. Mg(OH)2 nanoparticles have been used as a safe and environmentally friendly adsorbent for pollutant removals [24,25]. Recently, Mg(OH)2 is used to encapsulate NZVI for the protection and controlled release of the reactivity of the NZVI core [26]. The Mg(OH)2 shell can greatly reduce the aqueous corrosion of NZVI and limit the dissolution of Fe(II) from the Mg(OH)2-coated NZVI (NZVI@Mg(OH)2). In addition, the Mg(OH)2 coating layer changes NZVI from a negatively charged surface to a positively charged surface [26], which would favor their attachment to cyanobacterial cells. However, few studies have examined the application potential of NZVI technologies, including the novel NZVI@Mg(OH)2, for 4 ACS Paragon Plus Environment

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removing cyanobacterial cells from water. The main objective of this study was to investigate the removal efficiency of cyanobacterial cells by the NZVI (as a benchmark) and NZVI@Mg(OH)2 treatment. The specific aims were to (1) evaluate the effectiveness of NZVI@Mg(OH)2 and NZVI for removing cyanobacteria via sedimentation, (2) assess the viability (membrane integrity) of cyanobacterial cells after the

treatment with

NZVI@Mg(OH)2 and NZVI, and (3) analyze the mechanisms by the NZVI@Mg(OH)2 or NZVI treatment for removal of cyanobacterial cells.

Material and Methods Materials and reagents A strain of Microcystis aeruginosa (strain FACHB-905) was selected as a model strain of cyanobacteria for the experiments, obtained from the Institute of Aquatic Sciences, Chinese Academy of Sciences, Wuhan, China. The culture was grown in sterilized BG-11 media [27], and sub-cultured routinely to maintain its growth in the logarithmic phase. Cultures were incubated under a constant cool-fluorescent light flux (1500 lux) on a 12-h:12-h light-dark cycle at a constant temperature of 25 ± 1°C to achieve healthy cyanobacterial cultures. The cultures remained mostly unicellular under these laboratory conditions, and cultures with an initial cell concentration of about 2.5×106 cells/mL were used in the subsequent experimental tests. Before the tests, the cyanobacterial suspension was adjusted to pH 7.5 ± 0.1 with either 0.1 M filtered and sterilized hydrochloric acid or sodium hydroxide. All experiments were performed at room temperature (25 ± 1 °C). All chemicals and reagents used in this study were of analytical grade. Synthesis and characterization of NZVI and NZVI@Mg(OH)2 particles NZVI was synthesized by chemical reduction of ferric ions with sodium borohydride in 5 ACS Paragon Plus Environment


aqueous solution following a method previously reported [21, 26]. More details about the synthesis of NZVI are provided in the Supporting Information. The NZVI was coated with the Mg(OH)2 shell using a rate-controlled precipitation method developed in a previous study [26]. Briefly, the NZVI particles were suspended in 40 mL of absolute ethanol in a 100-mL flask reactor under the protection of an N2 atmosphere, and a NZVI suspension containing 0.5 g-Fe/L was placed in a sonication bath at 25 ± 2 °C. An ethanol solution of MgCl2 was then added to the NZVI suspension to attain an Mg-to-Fe mass ratio of 0.5. Last, an ethanol solution of NaOH was added to the suspension with a syringe pump (Longer Pump TJ-3A) at an injection rate of 0.1 mol-OH/(mol-Mg·min) until the molar ratio between the total injected NaOH and Mg reached 2.2. After injection, the suspension was sonicated for another hour. During the process, dosed Mg ions precipitated in the form of Mg(OH)2 and fully recovered in the solid products [26]. The NZVI particles after Mg(OH)2 coating, or NZVI@Mg(OH)2, were thoroughly washed with methanol and ethanol assisted by magnetic separation and dried by blowing N2 gas. Pure Mg(OH)2 nanoparticles were synthesized for comparison by the method for coating except adding NZVI particles. The prepared NZVI and NZVI@Mg(OH)2 nanoparticles were characterized by a vibrating sample magnetometer (Lake Shore 7037) for the magnetic saturation values (Ms), by a surface area analyzer (Micromeritics TriStar II plus 3030) for the BET specific surface area, by X-ray diffraction (XRD, D8 Advanced Diffractometer) for the crystalline features, by X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI 5600) for the surface composition of the materials and by scanning electron microscope (SEM; Hitachi S4800 FEG) and transmission electron microscope (TEM; FEI Tecnai G2 20 S-TWIN) for morphology. The reactive content (Fe0/Fe) of iron nanoparticles was determined by an acid digestion method previously reported [22]. Tests of removal of cyanobacterial cells by NZVI and NZVI@Mg(OH)2 The experimental tests on the removal of cyanobacteria from water were conducted in 6 ACS Paragon Plus Environment

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120-mL glass bottles. All of the iron dosing and cell sedimentation tests were conducted in duplicates, and the cyanobacterial samples without the NZVI or NZVI@Mg(OH)2 addition were run as controls. A suspension of NZVI particles, either NZVI or NZVI@Mg(OH)2, was prepared by adding an appropriate amount of dried NZVI particles in deoxygenated deionized water followed by 5 min of sonication under an N2 atmosphere. Five mL of the NZVI suspension was then mixed into 95 mL of the cyanobacterial cell suspension in the glass bottle to initiate the cyanobacteria removal test. The final Fe dosing concentrations in the cyanobacterial suspensions varied between 20 and 100 mg/L. The glass bottles were placed inside the incubator (25 ± 1°C) without mixing for the treatment and sedimentation tests. During the tests, water samples were collected from the suspensions 1 cm below the water surface at predetermined time intervals up to 6 h for analysis. Analyses Each cyanobacterial suspension sample was analyzed to calculate the cell concentration, evaluate cell lysis, and determine the ξ-potential. The samples for cell counts were treated with Lugol’s iodine, and the cells were then enumerated under a microscope (EVOS FL Auto, Life Technologies, USA) at 400× magnification using a Sedgewick-Rafter counting chamber (S52, PYSER-SGI, UK) [28]. The removal efficiency (Re) of cyanobacterial cells from the water suspension was calculated as follows: Equation (1)

ܴ݁ = (‫ܥ‬଴ − ‫ܥ‬௧ )/‫ܥ‬଴

Where C0 is the cell concentration of cyanobacteria in the control sample, and Ct is the cell concentration of cyanobacteria in the treated sample after time t. The suspensions and sediments were both sampled to analyze the viability of the cyanobacterial cells. The membrane integrity of the suspended cyanobacterial cells was determined with a flow cytometer (CytoFLEX, Beckman Coulter) and SYTOX green staining 7 ACS Paragon Plus Environment


[9,28]. Upon completion of the 6-h sedimentation test, the sediment was collected from each bottle to analyze changes in the settled cyanobacterial cells after NZVI or NZVI@Mg(OH)2 treatment. Each sediment sample was well resuspended with BG-11 media, and the NZVI particles were removed via magnetic separation, before analysis by the flow cytometer. In addition, the morphology and elemental mapping of cyanobacterial cells in the sediment were examined with an SEM equipped with an energy dispersive spectroscope, and the ξ-potential of the suspended cells and NZVI particles was measured with a Delsa Nano C Particle Analyzer (Beckman Coulter). The oxidation-reduction potential (ORP) of the BG-11 media before and after the addition of NZVI-based nanoparticles was measured with an ORP meter (HANNA HI 991002). The cell suspension samples were filtrated through a 0.45 µm membrane and the filtrates were analyzed for dissolved iron and other cations using an inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7900).

Results and Discussion Removal of cyanobacterial cells by NZVI and NZVI@Mg(OH)2 Dosing the NZVI particles effectively facilitated the removal of cyanobacterial cells from water via sedimentation. A series of photographs was obtained for visual evaluation of the results of cyanobacterial sedimentation induced by the addition of NZVI or NZVI@Mg(OH)2 (Figure 1). Dosing of NZVI at 20 and 50 mg/L caused progressive cell sedimentation, and an obvious green cyanobacterial suspension (corresponding to cyanobacterial chlorophyll) remained after 3 h. When the NZVI dosing concentration was increased to 100 mg/L, a black suspension formed in the first 0.5 h, followed by gradual settlement. The water column became much clearer after 3 h as the sediment accumulated on the bottom. The NZVI@Mg(OH)2 particles were much more effective than NZVI in bringing 8 ACS Paragon Plus Environment

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about the removal and sedimentation of cyanobacterial cells. At the lowest dosage of NZVI@Mg(OH)2 (20 mg/L), the water column became clear with few suspended cells after only 0.5 h. The regrowth of cyanobacteria was not observed in water after the NZVI@Mg(OH)2 treatment, as shown by the treated samples after 48 h (Figure 1). In addition to qualitative observation of the removal of cyanobacteria from water via sedimentation, changes in the concentration of the suspended cyanobacterial cells after the addition of NZVI or NZVI@Mg(OH)2 were also quantified under microscopy. The initial cell concentration of cyanobacteria was maintained at 2.5×106 cell/mL. At the low dosage of NZVI (20 mg/L), the cell concentration of cyanobacteria in the suspension remained fairly constant (Figure 2), and the cell removal efficiency via sedimentation was only 9.2% after 3 h (Table 1). At a higher dosage of NZVI (50 mg/L), the cell removal efficiency increased to 39.5% after 3 h. At the highest NZVI dosing concentration (100 mg/L), the effectiveness of cyanobacteria removal via sedimentation greatly increased (Figure 1), to 57.5% after 0.5 h and to 91.7% after 3 h (Table 1). NZVI@Mg(OH)2 exhibited much better ability than NZVI to remove cyanobacterial cells from the water column (Figure 2). Efficient cell removal was achieved at 96.9% with 20 mg/L of NZVI@Mg(OH)2 after 3 h (Table 1). With the addition of 50 or 100 mg/L of NZVI@Mg(OH)2, more than 98% of the cells were removed from the suspension via sedimentation within 0.5 h. After 3 h, hardly any cells could be detected in the water column. The two NZVI particles applied in this study—NZVI and NZVI@Mg(OH)2—were able to remove cyanobacterial cells from water with various degrees of efficiency. This finding is in agreement with Marsalek et al. [16] that NZVI (Nanofer25, a commercial product) could remove cyanobacterial cells from the water column via sedimentation, and the effective NZVI dosing concentration that led to a 50% decrease in the cyanobacterial biomass (EC50) was

50

mg/L

after

24

h.

The

novel

NZVI


particle

applied

in

this


study—NZVI@Mg(OH)2—was

shown

to

be

much

more

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effective

in

removing

cyanobacterial cells from the water column. Even at the lowest NZVI@Mg(OH)2 dose (20 mg/L), the cell removal efficiency reached 86.6% after only 0.5 h (Table 1). NZVI@Mg(OH)2 removes cyanobacteria more efficiently than commercially available magnetic products, such as Nanofer25 [16]. These core-shell structured NZVI@Mg(OH)2 particles provide a new and effective method for the control and removal of cyanobacteria from water. No documentation is available for a direct comparison of efficiency between NZVI@Mg(OH)2 and similar or other materials for removing cyanobacteria. Hence, the effectiveness of NZVI@Mg(OH)2 application is compared to that of existing technologies such as pre-oxidation, coagulation, and sedimentation and their combinations. The reaction time of pre-oxidation generally varies between a few minutes and tens of minutes, and coagulation and sedimentation requires at least 60 min. The dosage of the pre-oxidant or coagulant ranges from a few to tens of milligrams per liter, depending on the type of oxidant and coagulant, the cell concentration, and

water

quality

parameters

such

as

the

pH

[29,30].

An

evaluation

of

pre-oxidation-coagulation showed that its efficiency in cyanobacteria removal can hardly exceed 95% [3,5,31]. A recent study reported a more efficient approach in which 20 µM KMnO4 and 60 µM Fe(II) were applied for preoxidation, followed by 20 µM Al2(SO4)3 for coagulation and sedimentation, and the observed removal efficiency of M. aeruginosa reached 91% [8]. The present study shows that 20 mg/L of NZVI@Mg(OH)2 alone could remove nearly 90% of M. aeruginosa cells within 0.5 h. Thus, the efficiency of NZVI@Mg(OH)2 in the removal of cyanobacterial cells is comparable to or better than that of the KMnO4-Fe(II)-Al2(SO4)3 combination. Interactions between NZVI particles and cyanobacteria for cell removal Because the surface charge influences the suspension stability of algal cells [19], the 10 ACS Paragon Plus Environment

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ζ-potential of the suspended cyanobacteria was measured after the addition of NZVI. The cyanobacterial cells initially had a ζ-potential of -30.2 mV in the BG-11 media, which led to strong electrostatic repulsion between the cells that kept them well suspended in water. The ζ-potential of the control suspension remained nearly constant throughout the 3-h test (Figure 3). In comparison, with the addition of NZVI, the magnitude of ζ-potential of the suspension ζ-potential decreased from -30.2 to -21.4 mV with the addition of NZVI. The apparent decrease in the surface charge might have resulted from the adsorption of Fe(II)/Fe(III) ions on the cell surface. The decreased surface charge would allow easier attachment between the suspended cells and lead to more rapid cyanobacterial sedimentation. Treatment with NZVI@Mg(OH)2 changed the ζ-potential of suspension more significantly, from -30.2 to -13.4 mV (Figure 3). This further decrease in the surface charge effectively promoted cyanobacterial aggregation and sedimentation. In the treatment with pure Mg(OH)2 nanoparticles, the ζ-potential of the cell suspension changed from -30.2 to -22.3 mV after 0.5 h. The change of the ζ-potential of the cells treated with pure Mg(OH)2 and NZVI@Mg(OH)2 was partially due to the dissolution of Mg(OH)2, with Mg ion concentrations of 18.3 and 17.5 mg/L, respectively. There were studies showing that flocculation of microalgae could be achieved by the Mg(OH)2 treatment [32,33], owing likely to the change of ζ-potentials. However, in the present study, the efficiency of cyanobacteria removal by pure Mg(OH)2 nanoparticles was far worse than that by NZVI or NZVI@Mg(OH)2 (Figure S1). Therefore, the effective cyanobacteria sedimentation and removal achieved with NZVI@Mg(OH)2 was not attributed mainly to the ζ-potential changes. NZVI particles also interacted with cyanobacterial cells via attachment. Figure 4a shows the presence of primary NZVI particles in necklace-like chains or clusters with a smooth surface and a diameter of 50 to 100 nm. Sonication was able to break up the NZVI chains into individual particles or short clusters with an average diameter of 282.7 nm in the 11 ACS Paragon Plus Environment


suspension [26]. However, with a high Ms value of 138.9 emu/g for magnetic attraction (Figure S2), the NZVI particles dosed into the aqueous solution joined to form large agglomerates and then settled [34]. During the NZVI aggregation and sedimentation, NZVI particles also attached to suspended cells, thus driving the cyanobacterial sedimentation. According to the SEM image of the sediment, cyanobacterial cells were attached with the NZVI agglomerates (Figure 5b). NZVI-cyanobacteria attachment was also observed in the sediment dosed with NZVI@Mg(OH)2 (Figure 5c). Compared with NZVI particles, The NZVI@Mg(OH)2 particles were surrounded by the Mg(OH)2 coating shell and had a rough surface (Figure 4b), with the BET surface area decreased slightly from 19.3 to 16.3 m2/g. The Mg(OH)2 coating shell was indexed as brucit with the XRD patterns (Figure S3). According to the XPS analysis, the atomic Fe concentration decreased from 42.0% to 1.6% (Figure S4a) after the coating with Mg(OH)2. Meanwhile, a peak of O1s centered at 531.3 eV was detected for NZVI@Mg(OH)2 (Figure S4b), which indicated that metal hydroxide was dominant on the particle surface [25]. These features are different from those of NZVI particles that were formed and supported on porous materials [25,35,36], for which different Fe species and metal oxides could be clearly identified by the XPS survey of Fe 2p and O 1s regions, respectively. Therefore, NZVI@Mg(OH)2 particles have an apparent core@shell structure with the NZVI core covered by the Mg(OH)2 coating shell. The nonmagnetic Mg(OH)2 shell gave NZVI@Mg(OH)2 particles a lower bulk Ms value of 65.6 emu/g compared to that of 138.9 emu/g for NZVI without coating (Figure S2). The decreased magnetic attraction reduced the attachment between the NZVI@Mg(OH)2 particles and prevented them from forming large agglomerates. Moreover, the ζ-potential changed from -15.1 mV for NZVI particles to -1.3 mV for NZVI@Mg(OH)2 in the BG-11 media. In other words, the Mg(OH)2 shell made NZVI@Mg(OH)2 less negative than that of NZVI. As a result, the 12 ACS Paragon Plus Environment

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NZVI@Mg(OH)2 particles attached to the cyanobacterial cells and decreased the apparent ζ-potential of the suspension more effectively than NZVI particles (Figure 3). The reduced electrostatic repulsion between suspended cyanobacterial cells and the mild magnetic attraction between the NZVI@Mg(OH)2 particles attached to the cell surface greatly facilitated cyanobacterial aggregation and led to rapid cell sedimentation. The elemental mapping results shown in Figure 5 reveal the Fe distribution in relation to the cyanobacterial cells in the sediment. Little Fe signal was detected for the cyanobacterial sediment in the control sample (Figure 5a). With the use of NZVI, large NZVI agglomerates were found in the sediment and the cyanobacteria cells were apparently entrapped by the NZVI aggregates (Figure 5b). With the NZVI@Mg(OH)2 treatment, smaller clusters were observed in the sediment, and two phenomena of Fe distribution could be identified. First, the settled cells were also attached to NZVI@Mg(OH)2 aggregates (Figure 5c). In the second phenomenon, which was not found in the sediment treated with NZVI, the settled cells were covered with abundant Fe, whereas the cells were not attached to NZVI@Mg(OH)2 clusters (Figure 5d). The abundant Fe signals were apparently due to the dosing of NZVI@Mg(OH)2 particles, which led to attachment of iron-based nanoparticles or adsorption of iron ions onto the surface of cyanobacterial cells. It has been found that coating Mg(OH)2 onto NZVI particles can effectively keep the particles separated and protect NZVI from corrosion in water [26]. With NZVI treatment, iron ions produced from the aqueous corrosion of Fe0 would be favorably bound or precipitate on the surface of NZVI particles, thus thickening the iron oxide shell [37]. As a result, a limited amount of iron ions, at a concentration of 10-1 mg/L, could be released into the aqueous phase [26]. According to the analysis of dissolved Fe ions (Figure S5), the addition of NZVI particles quickly decreased the Fe ion concentration from 2.9 to 0.49 mg/L after 0.5 h, and to below 0.1 mg/L after 3h. In addition, 13 ACS Paragon Plus Environment


more than half of the NZVI particles were oxidized (Figure S6) and less Fe signals were detected on the surface of the cells treated by NZVI. The results of these analyses suggest that the produced iron ions precipitated on the surface of NZVI particles. In other words, the iron oxide shell of the NZVI surface with a negative charge would hinder the release of iron ions and their adsorption on the cell surface (Figure 5b). Overall, cyanobacterial cells treated with NZVI were mainly removed by the attachment and sedimentation of large NZVI clusters. And the much-enhanced cell removal seen with the NZVI@Mg(OH)2 treatment was attributed to the reduction in electrostatic repulsion between the NZVI particles and the cells and to the reduced stability of the cyanobacterial cells after the attachment of NZVI@Mg(OH)2 particles and cations onto the cell surface.

Viability of cyanobacteria after NZVI and NZVI@Mg(OH)2 treatment The membrane integrity of cyanobacterial cells that remained in the water suspension were relatively unaffected by the NZVI treatment (Table 2). The percentage of intact cells remained nearly constant at around 96% 3 h after NZVI doses of 20 and 50 mg/L. For the highest BZNVI dosage (100 mg/L), approximately 14% of the cells lost their membrane integrity after 3 h. For the cyanobacterial cells treated with the addition of 20 mg/L of NZVI@Mg(OH)2, a slight decrease of 7.5% in intact cells was observed after 0.5 h, and the percentage of intact cells remained fairly constant throughout the subsequent contact time (Table 2). No data were obtained to accurately calculate the percentage of intact cells for NZVI@Mg(OH)2 dosages of 50 and 100 mg/L because too few countable cells remained in the water column. In general, according microscopic observations, nearly all cyanobacterial cells that remained in the water column after the addition of NZVI were intact and healthy. Therefore, there seems little risk of the release of intracellular toxins from the suspended cells after NZVI treatment. 14 ACS Paragon Plus Environment

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The membrane integrity of the cyanobacterial cells in the sediment was also determined. The addition of NZVI at 20 mg/L did not damage the cells’ integrity, but the 50 mg/L dosage caused lysis of cyanobacterial cells (Table 2). The loss of cell integrity increased as the NZVI dosing concentration increased, and with the NZVI dosage of 100 mg/L, only 18.6% of the cyanobacterial cells retained membrane integrity in the sediment after 6 h. In addition, visible alterations in cell morphology could be observed with the SEM (Figure 6). This finding is consistent with that of a previous study that showed the destruction of cyanobacterial cells by NZVI treatment [16]. It has been reported that NZVI exhibits selective toxicity and that it can be especially toxic against cyanobacteria [16]. Other studies have shown that NZVI may cause disruption of microbial cells such as gram-negative bacteria [38,39]. The toxicity of NZVI towards cyanobacterial cells might be contributed to the generation of ROS on the surfaces of the NZVI particles. The surface Fe0 and Fe2+ would be oxidized by dissolved oxygen and produce H2O2 and ·OH free radicals [40], which could bring about oxidative stress to microorganisms [23,41]. The BG-11 media initially had an ORP of 218 mV, and adding NZVI particles significantly changed the ORP to -189 mV within 30 min. The ORP change showed that Fe0 was highly reductive in the BG-11 media. As a result, the Fe0 content of NZVI decreased significantly from 94.6% to 40.1% after 6 h (Figure S6). The sediment treated with 100 mg/L NZVI turned brownish after 24 h (Figure 1), which also evidenced the oxidation NZVI to iron oxides. Although a high dose of NZVI (100 mg/L) removed cyanobacterial cells efficiently from the water within 3 h, most of the settled cells suffered from membrane disruption and even cell breakage. This may lead to a considerable release of intracellular substances, such as odors and toxins, that would require further action to ensure the safety of the treated water for consumption by humans and animals.



For the NZVI@Mg(OH)2 treatment at 20 mg/L, about 90% of the cell population remained intact in the sediment after 6 h (Table 2). The results of cyanobacterial cell viability in the sediment after treatment with 50 and 100 mg/L NZVI@Mg(OH)2 were not obtained because the amount of cells separated from the sediment were insufficient for analysis by the flow cytometer. The limited separation of individual cells actually indicates the firm aggregation and attachment of cyanobacterial cells with the NZVI@Mg(OH)2 particles in the flocs and sediment. The integrity of the settled cyanobacterial cells can be seen in the SEM photographs of the cells (Figure 6). The cyanobacterial cells generally appeared to be intact and healthy in the sediment after treatment with 20 to 100 mg/L NZVI@Mg(OH)2 (Figure 6e to 7g). The Fe0 content of NZVI@Mg(OH)2 after the 6-h test was more than 69% (Figure S6), which was much higher than that of NZVI. The ORP value of BG-11 media decreased marginally from 218 to 177 mV after adding the NZVI@Mg(OH)2 particles within 30 min. Those results confirmed that the Mg(OH)2 coating shell is able to protect the reactivity of the NZVI core and prevent iron from rapid corrosion in water. According to the photographs in Figure 1, the sediment treated with NZVI@Mg(OH)2 remained black after 48 h. Those showed that, unlike NZVI, the NZVI cores were protected to a great extent by the Mg(OH)2 coating shell against corrosion in water. This might also have contributed to the reduced toxicity of the Mg(OH)2-coated NZVI toward the cyanobacterial cells. These comparisons suggest that NZVI@Mg(OH)2 can be more effective and better suited than NZVI for the treatment of cyanobacterial blooms and removal of cells from the water supply.

Conclusions This study shows that these novel nanoparticles—NZVI@Mg(OH)2—have outstanding capability to remove cyanobacteria from the water column. Meanwhile, the effects of


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NZVI@Mg(OH)2 on the membrane integrity and cell lysis of cyanobacteria are rather minimal, which suggests that its risk in application is negligible. Therefore, NZVI@Mg(OH)2 can be developed as an innovative technology for water purification in DWTPs that suffer from cyanobacterial blooms in raw water. Regrowth of cyanobacteria was not observed after the NZVI@Mg(OH)2 treatment. Thus, in addition to the treatment application in DWTPs, NZVI@Mg(OH)2 may find further use as an emergency measure for the control of cyanobacterial blooms in natural water bodies. To validate the applicability and effectiveness of this technology, this research needs to be extended to actual cyanobacterial blooms in real natural waters. Although NZVI@Mg(OH)2 nanoparticles were tested on Microcystis aeruginosa as the model bloom species, the method should also be effective for the treatment and control of blooms of other cyanobacterial populations, such as Cylindrospermopsis, Anabaena, and Oscillatoria.

Acknowledgments This study was supported by project 51708490 from National Natural Science Foundation of China (NSFC) and by grants 17261916 and T21-711/16R from the Research Grants Council (RGC) of the Hong Kong SAR government. The technical assistance of Mr. Keith C.H. Wong is greatly appreciated.

Supporting Information Section 1: Synthesis process of bare NZVI. Section 2: Photographs of M. aeruginosa suspensions after addition of 50 mg/L Mg(OH)2 for 6 h; Magnetic field-dependent magnetization values, XRD patterns, XPS spectra of (a) Fe 2p and (b) O 1s of NZVI and



NZVI@Mg(OH)2 particles; Concentration of dissolved Mg and Fe ions during the cyanobacteria removal tests of NZVI and NZVI@Mg(OH)2 with a NZVI dose of 100 mg/L.

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Figure Captions Figure 1. Photographs of M. aeruginosa suspensions after addition of NZVI or NZVI@Mg(OH)2 at various NZVI dosing concentrations after 0.5, 3, 24, and 48 h. (Initial cell concentration, 2.5×106 cell/mL.) Figure 2. Change in cell density of M. aeruginosa in suspension after dosing with NZVI or NZVI@Mg(OH)2 at initial concentrations of 20, 50, and 100 mg/L, measured up to 3 h. (Initial cell concentration, 2.5×106 cell/mL.) Figure 3. ζ-potentials of particulate matter in suspensions collected from various testing systems: control algal culture and algal suspensions dosed with NZVI and NZVI@Mg(OH)2 nanoparticles. (Initial cell concentration, 2.5×106 cell/mL; NZVI dosing concentration, 100 mg/L.) Figure 4. SEM (left) and TEM (right) images of (a) NZVI and (b) NZVI@Mg(OH)2. Figure 5. SEM images (left) and elemental mapping of Fe (right) of sediment samples after algae sedimentation tests: (a) control algal culture, (b) algal suspension dosed with NZVI and (c) and (d) algal suspension dosed with NZVI@Mg(OH)2. (Initial cell concentration, 2.5×106 cell/mL; NZVI concentration, 100 mg/L, sediment collection time, after 6 h.) Figure 6. Morphology of M. aeruginosa cells observed via SEM (×25,000) from (a) control algal culture and sediments from algal suspensions after NZVI treatment for 6 h at (b) 20, (c) 50, and (d) 100 mg/L, and sediment samples from algal suspensions after NZVI@Mg(OH)2 treatment for 6 h at (e) 20, (f) 50, and (g) 100 mg/L.


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Table 1. Removal efficiencies of Microcystis aeruginosa cells from suspension by sedimentation after addition of NZVI or NZVI@Mg(OH)2 nanoparticles as a function of dosing concentration and contact time.

Time (h)

Cell removal efficiency (%) NZVI treatment

NZVI@Mg(OH)2 treatment

20 mg/L

50 mg/L

100 mg/L

20 mg/L

50 mg/L

100 mg/L

0.5

0.0

19.7

57.5

86.6

98.4

99.7

1.5

3.9

23.9

75.5

94.9

99.6

99.9

3.0

9.2

39.5

91.7

96.9

99.9

99.9

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Table 2. Cell integrity results for Microcystis aeruginosa from different suspensions and sediment samples, including control algal culture and those with addition of NZVI and NZVI@Mg(OH)2 at various concentrations and after various contact periods. Intact cells (%) (control) Time (h) 0 0.5 1.5 3.0 6.0 (sediment)

96.0 96.0 97.2 97.0 97.1

Intact cells (%) (NZVI)

Intact cells (%) (NZVI@Mg(OH)2)

20 mg/L

50 mg/L

100 mg/L

20 mg/L

50 mg/L

100 mg/L

96.0 96.9 97.4 96.8 95.2

96.0 97.5 97.6 97.6 89.5

96.0 87.3 83.6 82.2 18.6

96.0 88.5 89.7 88.2 89.5

96.0 ND ND ND ND

96.0 ND ND ND ND

Note: ND, below the analytical detection threshold.

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Figure 1. Photographs of M. aeruginosa suspensions after addition of NZVI or NZVI@Mg(OH)2 at various NZVI dosing concentrations after 0.5, 3, 24, and 48 h. (Initial cell concentration = 2.5×106 cell/mL.)



Figure 2. Change in cell density of M. aeruginosa in suspension after dosing with NZVI or NZVI@Mg(OH)2 at initial concentrations of 20, 50, and 100 mg/L, measured up to 3 h. (Initial cell concentration, 2.5×106 cell/mL.)


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Figure 3. ζ-potentials of particulate matter in suspensions collected from various testing systems: control algal culture and algal suspensions dosed with NZVI and NZVI@Mg(OH)2 nanoparticles. (Initial cell concentration, 2.5×106 cell/mL; NZVI dosing concentration, 100 mg/L.)



Figure 4. SEM (left) and TEM (right) images of (a) NZVI and (b) NZVI@Mg(OH)2.


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Figure 5. SEM images (left) and elemental mapping of Fe (right) of sediment samples after algae sedimentation tests: (a) control algal culture, (b) algal suspension dosed with NZVI and (c) and (d) algal suspension dosed with NZVI@Mg(OH)2. (Initial cell concentration, 2.5×106 cell/mL; NZVI concentration, 100 mg/L; sediment collection time, after 6 h.) 29 ACS Paragon Plus Environment


Figure 6. Morphology of M. aeruginosa cells observed via SEM (×25,000) from (a) control algal culture and sediments from algal suspensions after NZVI treatment for 6 h at (b) 20, (c) 50, and (d) 100 mg/L, and sediment samples from algal suspensions after NZVI@Mg(OH)2 treatment for 6 h at (e) 20, (f) 50, and (g) 100 mg/L.


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Table of Contents

Nanoscale zero-valent iron coated with Mg(OH)2 shell could be utilized to efficiently and sustainably remove the cyanobacteria cells from water.


Nanoscale Zero-Valent Iron Coated with Magnesium Hydroxide for

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