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The effect of magnetic nanoparticles on Microcystis aeruginosa removal by a composite coagulant
Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 260–267
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
The effect of magnetic nanoparticles on Microcystis aeruginosa removal by a composite coagulant Chen Jiang, Ren Wang, Wei Ma ∗ Research Centre of Seawater Desalination and Multipurpose Utilization, Department of Chemistry, Dalian University of Technology, Linggong 2, Dalian 116023, PR China
a r t i c l e
i n f o
Article history: Received 25 April 2010 Received in revised form 12 August 2010 Accepted 14 August 2010 Available online 21 August 2010 Keywords: Magnetic nanoparticles Composite coagulant Microcystis aeruginosa removal Polyferric chloride Adsorption
a b s t r a c t A composite coagulant was prepared by magnetic nanoparticles and polyferric chloride (PFC) for Microcystis aeruginosa removal. The magnetic nanoparticles and composite coagulant were characterized in terms of typical properties, structure and morphological analysis (TEM, XRD and FTIR). The coagulation performances of magnetic nanoparticles/PFC (MPFC) and PFC were compared under different pH conditions and coagulant dosages. Natural water spiked with M. aeruginosa cells was also investigated to study the effect of natural organic matter (NOM) on the function of magnetic nanoparticles. The results show that the composite coagulant exhibits improved coagulation efficiency with higher removal values and slighter pH dependence. The better performance could be attributed to the co-effect of PFC and magnetic nanoparticles. The newly added magnetite acts as an adsorber, which favors the M. aeruginosa removal by facilitating the formation of settleable flocs and reducing the negative influence of NOM at optimal dosages. In addition, settling kinetic studies present accelerated settling velocity of MPFC under external magnetic field, emphasising the role plays by magnetic nanoparticles in promoting the coagulation efficiency. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The presence of cyanobacteria in source water poses a significant risk to drinking water treatment. Cyanobacterial Microcysits aeruginosa and their metabolites can produce several kinds of toxins as well as unpleasant tastes and odors, which are considered severe water quality problems. Furthermore, cyanobacteria are known to interfere with the water treatment process, causing short filter runs and microbial regrowth in distribution systems [1–3]. To minimize the threat, the removal of cyanobacteria has been conducted by applying physical, chemical and biological methods such as coagulation, flotation and oxidation technologies [4,5]. Coagulation is believed the key step in conventional water treatment processes. To date, various coagulants such as polymeric and monomeric forms Al and Fe salts have been used to remove cyanobacteria from drinking water [6–8]. Since questions regarding the potential toxicity of residual aluminium in treated water have been arisen [9], increasing attention has been paid on ferric coagulants, especially polyferric coagulants which own advantages such as wider working pH range, lower sensitivity to water temperature and reduced amounts of coagulant [10,11].
∗ Corresponding author. Tel.: +86 411 8470 6303; fax: +86 411 8470 7416. E-mail addresses: [email protected], [email protected] (W. Ma). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.08.033
However, algal cells removal by conventional coagulation is more difficult than inorganic pollutants, due to their low specific density, motility, morphological characteristics and negative surface charge [12]. What’s more, the presence of natural organic matter (NOM) in surface water also has a negative influence on coagulation effectiveness [13–15]. In order to maximize the coagulation efficiency, researches are recently focused on the preparation of composite coagulants, by combining two or more different coagulants together [16,17]. Other than adding coagulants separately which requires two reagent addition systems [18], the new composite coagulant is added to water in one step. With the rapid development of nanotechnology, magnetic nanoparticles are currently being widely studied in water treatment field. It is believed that magnetic nanoparticles (Fe3 O4 ) show the finite-size effect and high ratio of surface-to-volume, resulting in a higher adsorption capacity [19]. In addition, easy separation of loaded magnetic nanoparticles from solution and fast settling velocity can be achieved using an external magnetic field. Many studies have been reported on magnetic Fe3 O4 particles coating with organic materials as adsorbent for metal removal, such as chitosan and polyacrylamide [20,21]. However, few researches have been conducted on the composition of magnetic nanoparticles and coagulant solution to enhance coagulation by the adsorption and magnetic effect of Fe3 O4 nanoparticles. In this paper, a novel composite coagulant for M. aeruginosa removal was prepared by adding magnetic Fe3 O4 nanoparticles to
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polyferric chloride (PFC). The magnetic Fe3 O4 , PFC and magnetic nanoparticles/PFC (MPFC) were characterized in terms of some major properties, structure and morphology analysis. The coagulation behaviors of MPFC and PFC were studied under different initial pH conditions and coagulant dosages. Additionally, effect of nanoparticles on settling rate of flocs and M. aeruginosa removal in the presence of NOM were investigated in comparison with PFC. Particular emphasis was paid to elucidate the potential mechanisms and the effect of magnetic nanoparticles on the enhancement of coagulation efficiency. 2. Materials and methods 2.1. Microcystis aeruginosa culturing M. aeruginosa was obtained from the FACHB (Freshwater Algal Culture Collection of Institute of Hydrobiology) of the Chinese Academy of Sciences. It was incubated in sterilized 1 L glass flasks containing 500 mL of BG11 nutrient solution at 25 ± 1 ◦ C under fluorescent light (2000 lx, 12 h light/12 h dark). The nutrient solution was controlled to pH 7.1 before autoclaving by adding either 0.1 mol/L NaOH or 0.1 mol/L HCl solutions. 2.2. Preparation of magnetic nanoparticles Magnetic Fe3 O4 nanoparticles were synthesized by the coprecipitation method. FeCl3 ·6H2 O (8 g) and FeCl2 ·4H2 O (4 g) were mixed and dissolved in deionized water (80 mL) at 40 ◦ C. NaOH solution (5 mol/L, 30 mL) and PEG-6000 (polyethylene glycol) solution (100 g/L, 10 mL) were successively added to the mixture under vigorous stirring. Then the resultants were aged for 30 min at 80 ◦ C. Finally, the mixture was cooled to room temperature and washed with deionized water and ethanol for several times. 2.3. Preparation of PFC, MPFC and magnetite suspensions PFC was synthesized by slowly adding a NaOH solution (1 mol/L) to a FeCl3 solution (0.1 mol/L) with a stirring speed of 350 rpm at room temperature. The volume of NaOH added to FeCl3 solution was pre-determined to reach the final [OH− ]/[Fe] ratio (B) to 1.0, which was based on comprehensive consideration of coagulant stability and coagulation performance. Subsequently, NaH2 PO4 was added to the PFC solution as stabilizer ([NaH2 PO4 ]/[Fe] = 0.05). The mixture was then diluted to 0.01 mol/L (as Fe) with deionized water. A measured amount of magnetic fluid was injected into PFC solution while stirring thoroughly until it was absolutely mixed with the solution to prepare the composite coagulant MPFC. The mass ratio of magnetic nanoparticles and Fe in PFC was set to 5:1 in this study. Magnetite suspensions were synthesized using the same method, but PFC solution was replaced by equal amount of deionized water that with the same pH as PFC. 2.4. Analytical methods The pH, turbidity, zeta potential and conductivity measurements were performed by using a PHS-2C pH-Meter, a HACH RATIO/XR Turbimeter, a Nano-ZS90 ZETA SIZER and a DDS-11A conductivity meter, respectively. M. aeruginosa cell concentration and NOM content was determined by measuring absorbance at 684 nm (OD684 ) and 254 nm (UV254 ) with an UV1000 spectrophotometer, respectively. Prior to UV254 and zeta potential analysis, collected water samples were filtered through 0.45 m membrane.
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Point of zero charge (PZC) of the magnetite suspensions was determined from the relationship between zeta potential and pH. NaCl was selected as an inert electrolyte and the ionic strength was kept constant in all experiments. Portions of magnetite suspensions were introduced into 0.1 and 0.01 mol/L NaCl solution, respectively. pH values of the solutions were adjusted from about 3 to 10 by addition of 0.1 mol/L HCl or NaOH. Suspensions of the same magnetic nanoparticles to solution ratio (1 g/2 L) were allowed to equilibrate for 1 h before zeta potential was measured by the Nano-ZS90 ZETA SIZER. The magnetic fluid and liquid products of PFC and MPFC were dried for 24 h under 65 ◦ C. The average particle size, size distribution and morphology of the magnetic nanoparticles were studied using a transmission electron microscope (TEM) (JEOL 2200FS). Selected samples of magnetic nanoparticles, solid PFC and MPFC were characterized by X-ray diffraction (XRD) for the determination of crystalline phases, using a Shimadazu XRD-6000 X-Ray diffractometer with Cu K␣ radiation source at 35 kV and 25 mA in the range of 20–70◦ 2Â at a scan rate of 0.06◦ /min. Fourier transform infrared spectroscopy (FTIR) spectra of magnetic Fe3 O4 , PFC and MPFC were recorded with the Avatar-360 Nicolet FTIR Spectrophotometer using KBr pellets in the range 4000–400 cm−1 . Floc samples for XRD determination were first filtered through 0.45 m membranes and then rinsed with deionized water. After that, the samples were mounted on the glass slide and dried by critical point drying. The specimens were characterized by X-ray diffraction in the range of 20–70◦ 2Â at a scan rate of 0.06◦ /min.
2.5. Coagulation jar-test experiments Pure M. aeruginosa cell suspensions were used to evaluate coagulation performances of the coagulants. M. aeruginosa cells at late exponential growth stage were harvested by centrifugation at 3500 rpm for 10 min, and then suspended in 0.5% NaCl solution in order to simulate the ionic strength and keep the cells alive. Natural water was used to investigate the effect of NOM on M. aeruginosa removal. The natural water was collected in Xishan Reservoir, Dalian, China, and was spiked with cultured M. aeruginosa cells after filtration by 0.45 m membrane. The initial cell concentration for all coagulation experiments here was set to OD684 = 0.085 cm−1 . Table 1 shows the properties of the natural water used in the experiments. The coagulation experiments were carried out by using a jar-test apparatus with six paddles at room temperature. 300 mL M. aeruginosa suspensions were transferred into a 500 mL beaker; under rapid stirring of 400 rpm, pre-determined amount of coagulant was dosed into the suspensions; after 2 min, the stirring speed was changed to 50 rpm with a duration of 10 min; subsequently, after 60 min of quiescent settling under an external magnetic field, samples were collected at 2 cm beneath the surface for water quality analysis. For the cultured solution, coagulant dosages ranged from 0.5 to 5 mg/L (as Fe), and three initial pH conditions (pHs 4, 7 and 10) were conducted to evaluate the coagulation efficiency. For the natural water experiments, investigated coagulant dosages were from 2 to 14 mg/L (as Fe) and the initial pH was 7.98.
Table 1 Properties of the natural water samples used in the experiments. Water type
pH
Conductivity (S/cm)
UV254 (cm−1 )
OD684 (cm−1 )
Before spiking After spiking
7.97 7.98
339 378
0.069 0.071
0.001 0.085
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Fig. 1. TEM micrograph for the magnetic nanoparticles.
3. Results and discussion 3.1. Characterization of magnetic nanoparticles, PFC and MPFC The typical TEM micrograph for the magnetic nanoparticles is shown in Fig. 1. It is clear that the magnetic nanoparticles are essentially monodisperse and have a mean diameter of 8–12 nm, signifying that the magnetic Fe3 O4 with nanometer sized particles is successfully prepared. Table 2 presents the major physical–chemical properties of composite reagents. These measurements were conducted after 24 h aging at room temperature. It can be seen that there is an obvious rise in zeta potential with the addition of magnetic nanoparticles. It is worth noting that magnetite is an amphoteric solid, which can develop charges in the protonation and deprotonation reactions of Fe–OH surface sites [22]. The zeta potential/pH curves of magnetite suspensions at different NaCl concentrations are given in Fig. 2. The point of zero charge seems to be at pH 8.9. Therefore, in the presence of magnetic nanoparticles, the composite coagulant is more positively charged at pHs lower than the PZC of the magnetite. Fig. 3 illustrates the XRD patterns of magnetic nanoparticles, solid MPFC and solid PFC samples. It is more than evident that curve (a) exhibits a typical XRD pattern of magnetic Fe3 O4 with spinel structure [23]. Since the PFC sample is amorphous with rather obscure traces of crystallinity, as is shown in curve (c), its peaks are covered up by five characteristic peaks of NaCl (27.3◦ (1 1 1), 31.6◦ (2 0 0), 45.5◦ (2 2 0), 56.5◦ (2 2 2), 66.2◦ (4 0 0) 2Â) (Joint Committee on Powder Diffraction System, JCPDS no.780751). The peaks in curve (b) match well with both curve (a) and curve (c), thus indicating that MPFC is a combination of magnetic nanoparticles and PFC. It is believed that the composition process did not result in the phase change and structure change.
Fig. 2. Zeta potential of magnetite suspensions as a function of pH in the presence of two concentrations of NaCl.
Fig. 4 presents the FTIR spectra of MPFC sample as well as of the pure magnetic nanoparticles and PFC solid sample. The peaks around 3420 and 1610 cm−1 observed in three curves relate to the stretching vibration –OH and to the vibration of water absorbed or complexed in the coagulant [24]. For the magnetic Fe3 O4 nanoparticles (Fig. 4(I)), the characteristic absorbance bands appear at 580 cm−1 which can be assigned to Fe–O bond of Fe3 O4 [25]. For PFC (Fig. 4(II)), the peaks at 679 and 1051 cm−1 attribute to the asymmetric stretch of Fe–O and the bending vibration of the hydroxyl group (Fe–OH), respectively [26,27]. It is evident that the spectrum of MPFC (Fig. 4(III)) is similar to that of PFC, apart from peaks at 679 and 579 cm−1 , indicating that the vibration peak of Fe–O bond of PFC is overlapped up by the absorption peak of Fe3 O4 . Results
Table 2 Main properties of PFC and MPFC. mM /mFe a
pH
Turbidity (NTU)
Zeta potential (mV)
Conductivity (S/cm)
0 5
2.60 2.62
32 4233
25.35 31.1
2600 3180
a
The mass ratio of magnetic nanoparticles and Fe in PFC.
Fig. 3. X-ray diffractometer scan of (a) magnetic nanoparticles, (b) solid MPFC and (c) solid PFC.
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Fig. 4. FTIR spectra of (I) magnetic nanoparticles, (II) PFC and (III) MPFC.
demonstrate that no new chemical bond is created when magnetic nanoparticles compose with PFC, which means that the MPFC is a physical mixture. 3.2. Coagulation behaviors of PFC and MPFC as a function of pH and coagulant dosage The M. aeruginosa removal efficiency of PFC and MPFC and zeta potential of coagulated materials were compared under different initial pH conditions and coagulant dosages. Fig. 5 shows the variation of M. aeruginosa removal efficiency as a function of pH and the coagulant dosage. In the case of PFC, the removal efficiency firstly increases with increasing coagulant dosage, then a sudden drop appears as the dosage further increases. It is noted that the M. aeruginosa removal by PFC is significantly influenced by pH. In acid conditions, PFC performs almost the uniform low coagulation efficiency throughout the dosage range investigated; with the increase of pH, higher removal value was achieved.
Fig. 6. Zeta potentials of (a) coagulated material as a function of coagulant dosage and pH; (b) M. aeruginosa cells.
Fig. 5. Removal of M. aeruginosa by coagulation as a function of coagulant dosage and pH.
As it is suggested in previous studies [28], PFC contains polynuclear complex ions, such as Fe2 (OH)2 4+ , Fe3 (OH)4 5+ , formed by OH bridges and large number of inorganic macromolecular compounds with high positive charge, while M. aeruginosa cells are negatively charged, charge neutralization is proposed to be the main reason for cell destabilization and coagulation. From Figs. 5 and 6a, it is noted that the removal efficiencies of PFC reach the optimal values when zeta potentials of the flocs are close to zero. The increase of coagulant dosage is accompanying with a respective improvement of zeta potential. Moreover, the dosage of maximal removal efficiency increases when pH rises from 4 to 10, mainly due to the fact that zeta potential of M. aeruginosa cells decreases with the increasing pH (Fig. 6b) and more positive charges are needed. Decrease of removal efficiency by high PFC dosage can be attributed to the restabilization that caused by electrostatic repulsion between PFC and charge reversed PFC–cell complex [29]. Particularly, in acid region, owing to the less negative zeta potential of M. aeruginosa cells, less coagulant is required to achieve the equivalent zeta potential, so dosages investigated in this study are rather high for pH 4, causing the low removal efficiency of PFC. These facts suggest that charge neutralization may be the predominant mechanism for PFC.
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Fig. 7. M. aeruginosa removal efficiency of magnetite suspensions as a function of dosage and initial pH.
Fig. 8. Final pH of water samples treated with magnetite suspensions as a function of dosage and initial pH.
Compared with PFC, the M. aeruginosa removal curves of MPFC show a steeper increase at dosages less than 2 mg/L, and then present a plateau at higher dosages without distinct drop. The disparity between PFC and MPFC indicates that the presence of magnetic Fe3 O4 nanoparticles influences the coagulation behavior and, in a sense, changes the dominant coagulation mechanisms. The detailed M. aeruginosa removal mechanisms of MPFC are discussed in Section 3.4.
the positive charges of magnetic nanoparticles. It can be concluded that the main effect of magnetic nanoparticles on M. aeruginosa removal is adsorption that induced by electrostatic attraction.
3.3. Adsorption behavior of magnetic nanoparticles on Microcystis aeruginosa removal In order to investigate the role that magnetic Fe3 O4 nanoparticles plays in coagulation, parallel experiments were conducted by replacing the coagulant with magnetite suspensions. Dosages of magnetic Fe3 O4 nanoparticles were on equal terms with MPFC. The data in Fig. 7 show that, at dosages 10 mg/L and below, the M. aeruginosa removal efficiency generally increases with a decrease in pH and an increase in dosage of magnetic nanoparticles. As mentioned previously, the surface charge of magnetic nanoparticles is sensitive to pH, adsorption that induced by electrostatic attraction between M. aeruginosa cells and nanoparticles could occur at pHs lower than the PZC of magnetite. With the decrease of pH and the increase of magnetic nanoparticles, intensely activated surface of Fe3 O4 nanoparticles substantially possesses more adsorption sites available for M. aeruginosa cells uptake from the solution [16]. With the dosage further increases, slight decrease can be seen in acid and neutral conditions at dosage 25 mg/L. The explanation is that the cell surface sites are saturated by the positively charged nanoparticles and the electrostatic repulsion caused by the superfluous nanoparticles leads to decreasing removal value [30]. Particularly, at initial pH 10, magnetic nanoparticles were supposed to be negatively charged and adsorption behavior should be barely observed. However, it is worth noting that soaring removal efficiency appears at dosages above 5 mg/L. Considering that the magnetite suspensions are acidic (pH 2.6), the increasing addition of the suspensions would greatly affect the final pH of the solution. Fig. 8 illustrates the final pH of water samples treated by magnetite suspensions as a function of initial pH and dosage. It is clear that at initial pH 10, the final pH of the water samples decreases from 9.8 to 7.4 with the increased amount of magnetite suspensions, thus the adsorption capacity is promoted distinctly at higher dosages because of
3.4. Mechanisms of MPFC for Microcystis aeruginosa removal For the case of MPFC, it can be seen that the composite coagulant is effective under broad range of pH and dosages for M. aeruginosa removal, the removal efficiency seems to be the sum of PFC and magnetic nanoparticles. In acid and neutral conditions, the higher removal values of MPFC than that of PFC mainly attribute to the adsorption effect of magnetic nanoparticles. It is supposed that after the addition of MPFC, M. aeruginosa cells first react with positively charged Fe3 O4 nanoparticles to form large complexes, then unoccupied cell surface sites react with PFC. Owing to the high specific gravity and magnetism of Fe3 O4 –cell complexes, high dosages do not result in obviously decreasing removal efficiency. The plateau of zeta potential with the increasing dosage also supports our speculation that adsorption mechanism plays an important role in the M. aeruginosa removal by MPFC. In alkaline condition, the electrostatic repulsion between M. aeruginosa cells and magnetic nanoparticles hinders the adsorption behavior, in the meantime, insufficient positively charged iron polymers for M. aeruginosa cells and nanoparticles result in the relatively low removal values at low dosages. As the dosage increases, the charge of magnetic nanoparticles reverses to positive side and adsorption effect facilitates the removal efficiency again. It is noted that the highest removal values (100%) are achieved at dosages 2 mg/L (as Fe) and above at initial pH 10, while zeta potential shows no more increase. This could be explained as follows. pH not only determines the surface charge of M. aeruginosa cells and magnetic nanoparticles, but also significantly affects the hydrolysis of the PFC [31]. At high dosages with high initial pH, when MPFC is dosed, adsorption effect of magnetic nanoparticles first takes place to form Fe3 O4 –cell complexes, and the overdosed Fe hydrolyzes to form iron hydroxide flocs, then the complexes adsorb on the hydroxide surface and settle down, resulting in the better removal efficiency [32]. To further prove the speculation, Fig. 9 demonstrates the Xray diffraction of the flocs coagulated by PFC and MPFC. Flocs formed by PFC (curve (a)) exhibit amorphous structure with no dis-
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3.5. Effect of magnetic nanoparticles on settling rate
Fig. 9. X-ray diffraction of (a) flocs coagulated by PFC and (b) flocs coagulated by MPFC.
Assessment of a coagulant for coagulation efficiency also includes the settling time, the time taken by the flocs to reorient themselves and separate from the solution [33]. Experiments of the effect of settling time on M. aeruginosa removal efficiency under different dosages and pH conditions were also performed to investigate the settling rate of these two coagulants. The settling rates can also act as an aid for elucidating the applied coagulation mechanisms. As shown in Fig. 10, in acid conditions, other than the low settling rate of the small and scattered flocs that formed by PFC, gradually increased removal efficiency and settling rate are accompanying with an increasing MPFC dosage, signifying the critical influence of magnetic nanoparticles on the settling velocity of the flocs. While in alkaline conditions, the superiority of MPFC in settling rate is more distinct at high dosages. As discussed above, the formation of Fe3 O4 –cell complex and its adsorption to iron hydroxide favored the formation of flocs with high specific gravity and settling rate. Besides, the M. aeruginosa removal efficiency of MPFC can reach at least 80% within 120 min under most of the conditions investigated. The data emphatically show that adding magnetic nanoparticles enhances the coagulation efficiency through accelerated settling rate. 3.6. Effect of magnetic nanoparticles on Microcystis aeruginosa removal in the presence of NOM
tinct peaks. Nevertheless, flocs formed by MPFC (curve (b)) present three characteristic peaks of magnetic nanoparticles (Fig. 3, curve (a)), indicating that magnetic nanoparticles are combined with M. aeruginosa cells and flocs with magnetic properties could be formed. Overall, for the case of MPFC, the removal of M. aeruginosa is the co-effect of PFC and magnetic nanoparticles. The presence of magnetic nanoparticles enhances the coagulation efficiency by facilitating the formation of large and better settleable flocs, through the adsorption mechanism.
As is shown in Fig. 11a, the presence of NOM decreases the M. aeruginosa removal efficiency and increases the coagulant demand (optimal dosage increases from 2 mg/L (as Fe) for cultured solution to 14 mg/L (as Fe) for natural water). In the case of PFC, M. aeruginosa removal efficiency increases gradually with the increasing coagulant dosage and levels off at the higher dosages applied. In the meantime, MPFC gives an unexpected low M. aeruginosa removal efficiency at dosages lower than 10 mg/L (as Fe), but reaches almost 100% removal efficiency at optimal dosage (14 mg/L (as Fe)). The
Fig. 10. Settling kinetic curves of PFC and MPFC as a function of dosage and pH (a) for low dosages and (b) for high dosages.
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with them. Superfluous nanoparticles and iron polymers then act on the M. aeruginosa cells and result in better removal efficiency. It should be underlined that the newly prepared MPFC can achieve higher UV254 removal values, when compared to PFC, conforming the conclusions we have drawn above, regarding the effect of magnetic nanoparticles on NOM removal and the different applied coagulation mechanisms. 4. Conclusions The magnetic nanoparticles/PFC composite coagulant (MPFC) was successfully synthesized by combining prepared magnetic Fe3 O4 nanoparticles and PFC. The structure and morphological analysis of magnetic nanoparticles and MPFC illustrates that the magnetic nanoparticles are well dispersed and the presence of them has no effect on the structure of PFC, that is to say, the new coagulant MPFC is a physical mixture. Coagulation experiments on M. aeruginosa removal under different initial pH and coagulant dosages reveal that MPFC gives a synergistic improvement in the removal efficiency when the dosage exceeds a certain value, as compared to PFC. The predominant M. aeruginosa removal mechanism of PFC and magnetite suspensions is charge neutralization and adsorption, respectively. The improved coagulation efficiency of MPFC owes to the co-effect of PFC and magnetic nanoparticles, and the addition of the latter enhances the coagulation by facilitating the formation of large and magnetic flocs with fast settling velocity. Investigations on the influence of the water background NOM on the removal of M. aeruginosa by two coagulants show higher maximal M. aeruginosa removal value and better NOM removal efficiency of MPFC. The added magnetic nanoparticles react with NOM preferentially and promote the M. aeruginosa removal by reducing the negative influence of NOM at optimal dosages. References
Fig. 11. Coagulation efficiency of natural water spiked with M. aeruginosa cells: (a) M. aeruginosa removal efficiency and (b) UV254 removal efficiency.
removal of NOM (expressed as UV254 absorbance removal) was also investigated and the results are shown in Fig. 11b. Compared to PFC, uniform higher UV254 removal efficiencies are achieved and the maximal removal value increases from 36% to 48% with the addition of magnetic nanoparticles. It is no doubt that the presence of NOM would greatly increase the coagulant demand because of the negative charges they carried and the interaction between organic matter and M. aeruginosa cells [14]. As Illés and Tombácz [34] reported, NOM has a strong influence on the surface charge of the magnetic nanoparticles, the adsorption of NOM on magnetite surfaces leads to an enhanced electrostatic and steric stabilization of particles due to the absorbed layer of highly charged, macromolecular organic material. This could explain the low removal efficiencies of MPFC at low dosages. On addition of the composite coagulant, magnetite nanoparticles first react with the free natural organic matter [35], and the negatively charged NOM layer on the nanoparticle surface makes it hard to remove as well as increases the amount of coagulant required, thus has a negative influence on the removal of M. aeruginosa cells. With the increase of coagulant dosage, NOM would be adsorbed and surrounded by sufficient magnetic nanoparticles and settles down
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
The effect of magnetic nanoparticles on Microcystis aeruginosa removal by a composite coagulant Chen Jiang, Ren Wang, Wei Ma ∗ Research Centre of Seawater Desalination and Multipurpose Utilization, Department of Chemistry, Dalian University of Technology, Linggong 2, Dalian 116023, PR China
a r t i c l e
i n f o
Article history: Received 25 April 2010 Received in revised form 12 August 2010 Accepted 14 August 2010 Available online 21 August 2010 Keywords: Magnetic nanoparticles Composite coagulant Microcystis aeruginosa removal Polyferric chloride Adsorption
a b s t r a c t A composite coagulant was prepared by magnetic nanoparticles and polyferric chloride (PFC) for Microcystis aeruginosa removal. The magnetic nanoparticles and composite coagulant were characterized in terms of typical properties, structure and morphological analysis (TEM, XRD and FTIR). The coagulation performances of magnetic nanoparticles/PFC (MPFC) and PFC were compared under different pH conditions and coagulant dosages. Natural water spiked with M. aeruginosa cells was also investigated to study the effect of natural organic matter (NOM) on the function of magnetic nanoparticles. The results show that the composite coagulant exhibits improved coagulation efficiency with higher removal values and slighter pH dependence. The better performance could be attributed to the co-effect of PFC and magnetic nanoparticles. The newly added magnetite acts as an adsorber, which favors the M. aeruginosa removal by facilitating the formation of settleable flocs and reducing the negative influence of NOM at optimal dosages. In addition, settling kinetic studies present accelerated settling velocity of MPFC under external magnetic field, emphasising the role plays by magnetic nanoparticles in promoting the coagulation efficiency. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The presence of cyanobacteria in source water poses a significant risk to drinking water treatment. Cyanobacterial Microcysits aeruginosa and their metabolites can produce several kinds of toxins as well as unpleasant tastes and odors, which are considered severe water quality problems. Furthermore, cyanobacteria are known to interfere with the water treatment process, causing short filter runs and microbial regrowth in distribution systems [1–3]. To minimize the threat, the removal of cyanobacteria has been conducted by applying physical, chemical and biological methods such as coagulation, flotation and oxidation technologies [4,5]. Coagulation is believed the key step in conventional water treatment processes. To date, various coagulants such as polymeric and monomeric forms Al and Fe salts have been used to remove cyanobacteria from drinking water [6–8]. Since questions regarding the potential toxicity of residual aluminium in treated water have been arisen [9], increasing attention has been paid on ferric coagulants, especially polyferric coagulants which own advantages such as wider working pH range, lower sensitivity to water temperature and reduced amounts of coagulant [10,11].
∗ Corresponding author. Tel.: +86 411 8470 6303; fax: +86 411 8470 7416. E-mail addresses: [email protected], [email protected] (W. Ma). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.08.033
However, algal cells removal by conventional coagulation is more difficult than inorganic pollutants, due to their low specific density, motility, morphological characteristics and negative surface charge [12]. What’s more, the presence of natural organic matter (NOM) in surface water also has a negative influence on coagulation effectiveness [13–15]. In order to maximize the coagulation efficiency, researches are recently focused on the preparation of composite coagulants, by combining two or more different coagulants together [16,17]. Other than adding coagulants separately which requires two reagent addition systems [18], the new composite coagulant is added to water in one step. With the rapid development of nanotechnology, magnetic nanoparticles are currently being widely studied in water treatment field. It is believed that magnetic nanoparticles (Fe3 O4 ) show the finite-size effect and high ratio of surface-to-volume, resulting in a higher adsorption capacity [19]. In addition, easy separation of loaded magnetic nanoparticles from solution and fast settling velocity can be achieved using an external magnetic field. Many studies have been reported on magnetic Fe3 O4 particles coating with organic materials as adsorbent for metal removal, such as chitosan and polyacrylamide [20,21]. However, few researches have been conducted on the composition of magnetic nanoparticles and coagulant solution to enhance coagulation by the adsorption and magnetic effect of Fe3 O4 nanoparticles. In this paper, a novel composite coagulant for M. aeruginosa removal was prepared by adding magnetic Fe3 O4 nanoparticles to
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polyferric chloride (PFC). The magnetic Fe3 O4 , PFC and magnetic nanoparticles/PFC (MPFC) were characterized in terms of some major properties, structure and morphology analysis. The coagulation behaviors of MPFC and PFC were studied under different initial pH conditions and coagulant dosages. Additionally, effect of nanoparticles on settling rate of flocs and M. aeruginosa removal in the presence of NOM were investigated in comparison with PFC. Particular emphasis was paid to elucidate the potential mechanisms and the effect of magnetic nanoparticles on the enhancement of coagulation efficiency. 2. Materials and methods 2.1. Microcystis aeruginosa culturing M. aeruginosa was obtained from the FACHB (Freshwater Algal Culture Collection of Institute of Hydrobiology) of the Chinese Academy of Sciences. It was incubated in sterilized 1 L glass flasks containing 500 mL of BG11 nutrient solution at 25 ± 1 ◦ C under fluorescent light (2000 lx, 12 h light/12 h dark). The nutrient solution was controlled to pH 7.1 before autoclaving by adding either 0.1 mol/L NaOH or 0.1 mol/L HCl solutions. 2.2. Preparation of magnetic nanoparticles Magnetic Fe3 O4 nanoparticles were synthesized by the coprecipitation method. FeCl3 ·6H2 O (8 g) and FeCl2 ·4H2 O (4 g) were mixed and dissolved in deionized water (80 mL) at 40 ◦ C. NaOH solution (5 mol/L, 30 mL) and PEG-6000 (polyethylene glycol) solution (100 g/L, 10 mL) were successively added to the mixture under vigorous stirring. Then the resultants were aged for 30 min at 80 ◦ C. Finally, the mixture was cooled to room temperature and washed with deionized water and ethanol for several times. 2.3. Preparation of PFC, MPFC and magnetite suspensions PFC was synthesized by slowly adding a NaOH solution (1 mol/L) to a FeCl3 solution (0.1 mol/L) with a stirring speed of 350 rpm at room temperature. The volume of NaOH added to FeCl3 solution was pre-determined to reach the final [OH− ]/[Fe] ratio (B) to 1.0, which was based on comprehensive consideration of coagulant stability and coagulation performance. Subsequently, NaH2 PO4 was added to the PFC solution as stabilizer ([NaH2 PO4 ]/[Fe] = 0.05). The mixture was then diluted to 0.01 mol/L (as Fe) with deionized water. A measured amount of magnetic fluid was injected into PFC solution while stirring thoroughly until it was absolutely mixed with the solution to prepare the composite coagulant MPFC. The mass ratio of magnetic nanoparticles and Fe in PFC was set to 5:1 in this study. Magnetite suspensions were synthesized using the same method, but PFC solution was replaced by equal amount of deionized water that with the same pH as PFC. 2.4. Analytical methods The pH, turbidity, zeta potential and conductivity measurements were performed by using a PHS-2C pH-Meter, a HACH RATIO/XR Turbimeter, a Nano-ZS90 ZETA SIZER and a DDS-11A conductivity meter, respectively. M. aeruginosa cell concentration and NOM content was determined by measuring absorbance at 684 nm (OD684 ) and 254 nm (UV254 ) with an UV1000 spectrophotometer, respectively. Prior to UV254 and zeta potential analysis, collected water samples were filtered through 0.45 m membrane.
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Point of zero charge (PZC) of the magnetite suspensions was determined from the relationship between zeta potential and pH. NaCl was selected as an inert electrolyte and the ionic strength was kept constant in all experiments. Portions of magnetite suspensions were introduced into 0.1 and 0.01 mol/L NaCl solution, respectively. pH values of the solutions were adjusted from about 3 to 10 by addition of 0.1 mol/L HCl or NaOH. Suspensions of the same magnetic nanoparticles to solution ratio (1 g/2 L) were allowed to equilibrate for 1 h before zeta potential was measured by the Nano-ZS90 ZETA SIZER. The magnetic fluid and liquid products of PFC and MPFC were dried for 24 h under 65 ◦ C. The average particle size, size distribution and morphology of the magnetic nanoparticles were studied using a transmission electron microscope (TEM) (JEOL 2200FS). Selected samples of magnetic nanoparticles, solid PFC and MPFC were characterized by X-ray diffraction (XRD) for the determination of crystalline phases, using a Shimadazu XRD-6000 X-Ray diffractometer with Cu K␣ radiation source at 35 kV and 25 mA in the range of 20–70◦ 2Â at a scan rate of 0.06◦ /min. Fourier transform infrared spectroscopy (FTIR) spectra of magnetic Fe3 O4 , PFC and MPFC were recorded with the Avatar-360 Nicolet FTIR Spectrophotometer using KBr pellets in the range 4000–400 cm−1 . Floc samples for XRD determination were first filtered through 0.45 m membranes and then rinsed with deionized water. After that, the samples were mounted on the glass slide and dried by critical point drying. The specimens were characterized by X-ray diffraction in the range of 20–70◦ 2Â at a scan rate of 0.06◦ /min.
2.5. Coagulation jar-test experiments Pure M. aeruginosa cell suspensions were used to evaluate coagulation performances of the coagulants. M. aeruginosa cells at late exponential growth stage were harvested by centrifugation at 3500 rpm for 10 min, and then suspended in 0.5% NaCl solution in order to simulate the ionic strength and keep the cells alive. Natural water was used to investigate the effect of NOM on M. aeruginosa removal. The natural water was collected in Xishan Reservoir, Dalian, China, and was spiked with cultured M. aeruginosa cells after filtration by 0.45 m membrane. The initial cell concentration for all coagulation experiments here was set to OD684 = 0.085 cm−1 . Table 1 shows the properties of the natural water used in the experiments. The coagulation experiments were carried out by using a jar-test apparatus with six paddles at room temperature. 300 mL M. aeruginosa suspensions were transferred into a 500 mL beaker; under rapid stirring of 400 rpm, pre-determined amount of coagulant was dosed into the suspensions; after 2 min, the stirring speed was changed to 50 rpm with a duration of 10 min; subsequently, after 60 min of quiescent settling under an external magnetic field, samples were collected at 2 cm beneath the surface for water quality analysis. For the cultured solution, coagulant dosages ranged from 0.5 to 5 mg/L (as Fe), and three initial pH conditions (pHs 4, 7 and 10) were conducted to evaluate the coagulation efficiency. For the natural water experiments, investigated coagulant dosages were from 2 to 14 mg/L (as Fe) and the initial pH was 7.98.
Table 1 Properties of the natural water samples used in the experiments. Water type
pH
Conductivity (S/cm)
UV254 (cm−1 )
OD684 (cm−1 )
Before spiking After spiking
7.97 7.98
339 378
0.069 0.071
0.001 0.085
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Fig. 1. TEM micrograph for the magnetic nanoparticles.
3. Results and discussion 3.1. Characterization of magnetic nanoparticles, PFC and MPFC The typical TEM micrograph for the magnetic nanoparticles is shown in Fig. 1. It is clear that the magnetic nanoparticles are essentially monodisperse and have a mean diameter of 8–12 nm, signifying that the magnetic Fe3 O4 with nanometer sized particles is successfully prepared. Table 2 presents the major physical–chemical properties of composite reagents. These measurements were conducted after 24 h aging at room temperature. It can be seen that there is an obvious rise in zeta potential with the addition of magnetic nanoparticles. It is worth noting that magnetite is an amphoteric solid, which can develop charges in the protonation and deprotonation reactions of Fe–OH surface sites [22]. The zeta potential/pH curves of magnetite suspensions at different NaCl concentrations are given in Fig. 2. The point of zero charge seems to be at pH 8.9. Therefore, in the presence of magnetic nanoparticles, the composite coagulant is more positively charged at pHs lower than the PZC of the magnetite. Fig. 3 illustrates the XRD patterns of magnetic nanoparticles, solid MPFC and solid PFC samples. It is more than evident that curve (a) exhibits a typical XRD pattern of magnetic Fe3 O4 with spinel structure [23]. Since the PFC sample is amorphous with rather obscure traces of crystallinity, as is shown in curve (c), its peaks are covered up by five characteristic peaks of NaCl (27.3◦ (1 1 1), 31.6◦ (2 0 0), 45.5◦ (2 2 0), 56.5◦ (2 2 2), 66.2◦ (4 0 0) 2Â) (Joint Committee on Powder Diffraction System, JCPDS no.780751). The peaks in curve (b) match well with both curve (a) and curve (c), thus indicating that MPFC is a combination of magnetic nanoparticles and PFC. It is believed that the composition process did not result in the phase change and structure change.
Fig. 2. Zeta potential of magnetite suspensions as a function of pH in the presence of two concentrations of NaCl.
Fig. 4 presents the FTIR spectra of MPFC sample as well as of the pure magnetic nanoparticles and PFC solid sample. The peaks around 3420 and 1610 cm−1 observed in three curves relate to the stretching vibration –OH and to the vibration of water absorbed or complexed in the coagulant [24]. For the magnetic Fe3 O4 nanoparticles (Fig. 4(I)), the characteristic absorbance bands appear at 580 cm−1 which can be assigned to Fe–O bond of Fe3 O4 [25]. For PFC (Fig. 4(II)), the peaks at 679 and 1051 cm−1 attribute to the asymmetric stretch of Fe–O and the bending vibration of the hydroxyl group (Fe–OH), respectively [26,27]. It is evident that the spectrum of MPFC (Fig. 4(III)) is similar to that of PFC, apart from peaks at 679 and 579 cm−1 , indicating that the vibration peak of Fe–O bond of PFC is overlapped up by the absorption peak of Fe3 O4 . Results
Table 2 Main properties of PFC and MPFC. mM /mFe a
pH
Turbidity (NTU)
Zeta potential (mV)
Conductivity (S/cm)
0 5
2.60 2.62
32 4233
25.35 31.1
2600 3180
a
The mass ratio of magnetic nanoparticles and Fe in PFC.
Fig. 3. X-ray diffractometer scan of (a) magnetic nanoparticles, (b) solid MPFC and (c) solid PFC.
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Fig. 4. FTIR spectra of (I) magnetic nanoparticles, (II) PFC and (III) MPFC.
demonstrate that no new chemical bond is created when magnetic nanoparticles compose with PFC, which means that the MPFC is a physical mixture. 3.2. Coagulation behaviors of PFC and MPFC as a function of pH and coagulant dosage The M. aeruginosa removal efficiency of PFC and MPFC and zeta potential of coagulated materials were compared under different initial pH conditions and coagulant dosages. Fig. 5 shows the variation of M. aeruginosa removal efficiency as a function of pH and the coagulant dosage. In the case of PFC, the removal efficiency firstly increases with increasing coagulant dosage, then a sudden drop appears as the dosage further increases. It is noted that the M. aeruginosa removal by PFC is significantly influenced by pH. In acid conditions, PFC performs almost the uniform low coagulation efficiency throughout the dosage range investigated; with the increase of pH, higher removal value was achieved.
Fig. 6. Zeta potentials of (a) coagulated material as a function of coagulant dosage and pH; (b) M. aeruginosa cells.
Fig. 5. Removal of M. aeruginosa by coagulation as a function of coagulant dosage and pH.
As it is suggested in previous studies [28], PFC contains polynuclear complex ions, such as Fe2 (OH)2 4+ , Fe3 (OH)4 5+ , formed by OH bridges and large number of inorganic macromolecular compounds with high positive charge, while M. aeruginosa cells are negatively charged, charge neutralization is proposed to be the main reason for cell destabilization and coagulation. From Figs. 5 and 6a, it is noted that the removal efficiencies of PFC reach the optimal values when zeta potentials of the flocs are close to zero. The increase of coagulant dosage is accompanying with a respective improvement of zeta potential. Moreover, the dosage of maximal removal efficiency increases when pH rises from 4 to 10, mainly due to the fact that zeta potential of M. aeruginosa cells decreases with the increasing pH (Fig. 6b) and more positive charges are needed. Decrease of removal efficiency by high PFC dosage can be attributed to the restabilization that caused by electrostatic repulsion between PFC and charge reversed PFC–cell complex [29]. Particularly, in acid region, owing to the less negative zeta potential of M. aeruginosa cells, less coagulant is required to achieve the equivalent zeta potential, so dosages investigated in this study are rather high for pH 4, causing the low removal efficiency of PFC. These facts suggest that charge neutralization may be the predominant mechanism for PFC.
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Fig. 7. M. aeruginosa removal efficiency of magnetite suspensions as a function of dosage and initial pH.
Fig. 8. Final pH of water samples treated with magnetite suspensions as a function of dosage and initial pH.
Compared with PFC, the M. aeruginosa removal curves of MPFC show a steeper increase at dosages less than 2 mg/L, and then present a plateau at higher dosages without distinct drop. The disparity between PFC and MPFC indicates that the presence of magnetic Fe3 O4 nanoparticles influences the coagulation behavior and, in a sense, changes the dominant coagulation mechanisms. The detailed M. aeruginosa removal mechanisms of MPFC are discussed in Section 3.4.
the positive charges of magnetic nanoparticles. It can be concluded that the main effect of magnetic nanoparticles on M. aeruginosa removal is adsorption that induced by electrostatic attraction.
3.3. Adsorption behavior of magnetic nanoparticles on Microcystis aeruginosa removal In order to investigate the role that magnetic Fe3 O4 nanoparticles plays in coagulation, parallel experiments were conducted by replacing the coagulant with magnetite suspensions. Dosages of magnetic Fe3 O4 nanoparticles were on equal terms with MPFC. The data in Fig. 7 show that, at dosages 10 mg/L and below, the M. aeruginosa removal efficiency generally increases with a decrease in pH and an increase in dosage of magnetic nanoparticles. As mentioned previously, the surface charge of magnetic nanoparticles is sensitive to pH, adsorption that induced by electrostatic attraction between M. aeruginosa cells and nanoparticles could occur at pHs lower than the PZC of magnetite. With the decrease of pH and the increase of magnetic nanoparticles, intensely activated surface of Fe3 O4 nanoparticles substantially possesses more adsorption sites available for M. aeruginosa cells uptake from the solution [16]. With the dosage further increases, slight decrease can be seen in acid and neutral conditions at dosage 25 mg/L. The explanation is that the cell surface sites are saturated by the positively charged nanoparticles and the electrostatic repulsion caused by the superfluous nanoparticles leads to decreasing removal value [30]. Particularly, at initial pH 10, magnetic nanoparticles were supposed to be negatively charged and adsorption behavior should be barely observed. However, it is worth noting that soaring removal efficiency appears at dosages above 5 mg/L. Considering that the magnetite suspensions are acidic (pH 2.6), the increasing addition of the suspensions would greatly affect the final pH of the solution. Fig. 8 illustrates the final pH of water samples treated by magnetite suspensions as a function of initial pH and dosage. It is clear that at initial pH 10, the final pH of the water samples decreases from 9.8 to 7.4 with the increased amount of magnetite suspensions, thus the adsorption capacity is promoted distinctly at higher dosages because of
3.4. Mechanisms of MPFC for Microcystis aeruginosa removal For the case of MPFC, it can be seen that the composite coagulant is effective under broad range of pH and dosages for M. aeruginosa removal, the removal efficiency seems to be the sum of PFC and magnetic nanoparticles. In acid and neutral conditions, the higher removal values of MPFC than that of PFC mainly attribute to the adsorption effect of magnetic nanoparticles. It is supposed that after the addition of MPFC, M. aeruginosa cells first react with positively charged Fe3 O4 nanoparticles to form large complexes, then unoccupied cell surface sites react with PFC. Owing to the high specific gravity and magnetism of Fe3 O4 –cell complexes, high dosages do not result in obviously decreasing removal efficiency. The plateau of zeta potential with the increasing dosage also supports our speculation that adsorption mechanism plays an important role in the M. aeruginosa removal by MPFC. In alkaline condition, the electrostatic repulsion between M. aeruginosa cells and magnetic nanoparticles hinders the adsorption behavior, in the meantime, insufficient positively charged iron polymers for M. aeruginosa cells and nanoparticles result in the relatively low removal values at low dosages. As the dosage increases, the charge of magnetic nanoparticles reverses to positive side and adsorption effect facilitates the removal efficiency again. It is noted that the highest removal values (100%) are achieved at dosages 2 mg/L (as Fe) and above at initial pH 10, while zeta potential shows no more increase. This could be explained as follows. pH not only determines the surface charge of M. aeruginosa cells and magnetic nanoparticles, but also significantly affects the hydrolysis of the PFC [31]. At high dosages with high initial pH, when MPFC is dosed, adsorption effect of magnetic nanoparticles first takes place to form Fe3 O4 –cell complexes, and the overdosed Fe hydrolyzes to form iron hydroxide flocs, then the complexes adsorb on the hydroxide surface and settle down, resulting in the better removal efficiency [32]. To further prove the speculation, Fig. 9 demonstrates the Xray diffraction of the flocs coagulated by PFC and MPFC. Flocs formed by PFC (curve (a)) exhibit amorphous structure with no dis-
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3.5. Effect of magnetic nanoparticles on settling rate
Fig. 9. X-ray diffraction of (a) flocs coagulated by PFC and (b) flocs coagulated by MPFC.
Assessment of a coagulant for coagulation efficiency also includes the settling time, the time taken by the flocs to reorient themselves and separate from the solution [33]. Experiments of the effect of settling time on M. aeruginosa removal efficiency under different dosages and pH conditions were also performed to investigate the settling rate of these two coagulants. The settling rates can also act as an aid for elucidating the applied coagulation mechanisms. As shown in Fig. 10, in acid conditions, other than the low settling rate of the small and scattered flocs that formed by PFC, gradually increased removal efficiency and settling rate are accompanying with an increasing MPFC dosage, signifying the critical influence of magnetic nanoparticles on the settling velocity of the flocs. While in alkaline conditions, the superiority of MPFC in settling rate is more distinct at high dosages. As discussed above, the formation of Fe3 O4 –cell complex and its adsorption to iron hydroxide favored the formation of flocs with high specific gravity and settling rate. Besides, the M. aeruginosa removal efficiency of MPFC can reach at least 80% within 120 min under most of the conditions investigated. The data emphatically show that adding magnetic nanoparticles enhances the coagulation efficiency through accelerated settling rate. 3.6. Effect of magnetic nanoparticles on Microcystis aeruginosa removal in the presence of NOM
tinct peaks. Nevertheless, flocs formed by MPFC (curve (b)) present three characteristic peaks of magnetic nanoparticles (Fig. 3, curve (a)), indicating that magnetic nanoparticles are combined with M. aeruginosa cells and flocs with magnetic properties could be formed. Overall, for the case of MPFC, the removal of M. aeruginosa is the co-effect of PFC and magnetic nanoparticles. The presence of magnetic nanoparticles enhances the coagulation efficiency by facilitating the formation of large and better settleable flocs, through the adsorption mechanism.
As is shown in Fig. 11a, the presence of NOM decreases the M. aeruginosa removal efficiency and increases the coagulant demand (optimal dosage increases from 2 mg/L (as Fe) for cultured solution to 14 mg/L (as Fe) for natural water). In the case of PFC, M. aeruginosa removal efficiency increases gradually with the increasing coagulant dosage and levels off at the higher dosages applied. In the meantime, MPFC gives an unexpected low M. aeruginosa removal efficiency at dosages lower than 10 mg/L (as Fe), but reaches almost 100% removal efficiency at optimal dosage (14 mg/L (as Fe)). The
Fig. 10. Settling kinetic curves of PFC and MPFC as a function of dosage and pH (a) for low dosages and (b) for high dosages.
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with them. Superfluous nanoparticles and iron polymers then act on the M. aeruginosa cells and result in better removal efficiency. It should be underlined that the newly prepared MPFC can achieve higher UV254 removal values, when compared to PFC, conforming the conclusions we have drawn above, regarding the effect of magnetic nanoparticles on NOM removal and the different applied coagulation mechanisms. 4. Conclusions The magnetic nanoparticles/PFC composite coagulant (MPFC) was successfully synthesized by combining prepared magnetic Fe3 O4 nanoparticles and PFC. The structure and morphological analysis of magnetic nanoparticles and MPFC illustrates that the magnetic nanoparticles are well dispersed and the presence of them has no effect on the structure of PFC, that is to say, the new coagulant MPFC is a physical mixture. Coagulation experiments on M. aeruginosa removal under different initial pH and coagulant dosages reveal that MPFC gives a synergistic improvement in the removal efficiency when the dosage exceeds a certain value, as compared to PFC. The predominant M. aeruginosa removal mechanism of PFC and magnetite suspensions is charge neutralization and adsorption, respectively. The improved coagulation efficiency of MPFC owes to the co-effect of PFC and magnetic nanoparticles, and the addition of the latter enhances the coagulation by facilitating the formation of large and magnetic flocs with fast settling velocity. Investigations on the influence of the water background NOM on the removal of M. aeruginosa by two coagulants show higher maximal M. aeruginosa removal value and better NOM removal efficiency of MPFC. The added magnetic nanoparticles react with NOM preferentially and promote the M. aeruginosa removal by reducing the negative influence of NOM at optimal dosages. References
Fig. 11. Coagulation efficiency of natural water spiked with M. aeruginosa cells: (a) M. aeruginosa removal efficiency and (b) UV254 removal efficiency.
removal of NOM (expressed as UV254 absorbance removal) was also investigated and the results are shown in Fig. 11b. Compared to PFC, uniform higher UV254 removal efficiencies are achieved and the maximal removal value increases from 36% to 48% with the addition of magnetic nanoparticles. It is no doubt that the presence of NOM would greatly increase the coagulant demand because of the negative charges they carried and the interaction between organic matter and M. aeruginosa cells [14]. As Illés and Tombácz [34] reported, NOM has a strong influence on the surface charge of the magnetic nanoparticles, the adsorption of NOM on magnetite surfaces leads to an enhanced electrostatic and steric stabilization of particles due to the absorbed layer of highly charged, macromolecular organic material. This could explain the low removal efficiencies of MPFC at low dosages. On addition of the composite coagulant, magnetite nanoparticles first react with the free natural organic matter [35], and the negatively charged NOM layer on the nanoparticle surface makes it hard to remove as well as increases the amount of coagulant required, thus has a negative influence on the removal of M. aeruginosa cells. With the increase of coagulant dosage, NOM would be adsorbed and surrounded by sufficient magnetic nanoparticles and settles down
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