Simulated-Sunlight-Driven Cell Lysis of Magnetophoretically

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Simulated-sunlight-driven cell lysis of magnetophoretically separated microalgae using ZnFeO octahedrons 2

4

Jung Yoon Seo, Hwan-Jin Jeon, Jeong Won Kim, Jiye Lee, You-Kwan Oh, Chi Won Ahn, and Jae W Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04445 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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Industrial & Engineering Chemistry Research

Simulated-sunlight-driven cell lysis of magnetophoretically separated microalgae using ZnFe2O4 octahedrons Jung Yoon Seo,‡a,b Hwan-Jin Jeon,‡c Jeong Won Kim,a Jiye Lee,d You-Kwan Oh,*d Chi Won Ahn,*a Jae W. Lee*e a

Global Nanotechnology Development Team, National NanoFab Center (NNFC), Daejeon

34141, Republic of Korea b

Climate Technology Strategy Center, Korea Institute of Energy Research (KIER),

Daejeon, 34129, Republic of Korea c

Department of Chemical Engineering and Biotechnology, Korea Polytechnic University

(KPU), Siheung-si, Gyeonggi-do 15073, Republic of Korea d

School of Chemical and Biomolecular Engineering, Pusan National University (PNU),

Busan 46241, Republic of Korea e

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of

Science and Technology (KAIST), Daejeon, 34141, Republic of Korea

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*Corresponding author: Prof. You-Kwan Oh, Tel.: +82-42-860-3697, Fax: +82-42-860-3495, Email address: [email protected] (Y.-K. Oh) Dr. Chi Won Ahn, Tel.: +82-42-366-1525, Fax: +82-42-366-1990, Email address: [email protected] (C. W. Ahn) Prof. Jae W. Lee, Tel.: +82-42-350-3940, Fax: +82-42-350-3910, E-mail address: [email protected] (J. W. Lee) ‡ The scientific contributions of J. Y. S and H.-J. J were equivalent, and their relative order in authorship is arbitrary.

KEYWORDS: Microalgae, ZnFe2O4 octahedrons, Cell disruption, Magnetic particles, Photocatalysts.


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Abstract

In an effort to help meet the demand for promising renewable sources of energy, research into innovative downstream processing for microalgae biorefineries is actively underway. In the current work, we used octahedrally shaped ZnFe2O4 nanoparticles for both harvesting and disrupting the cells of microalgae. We were able to use ZnFe2O4 octahedrons as magnetic flocculants and cell-disruption agents because ZnFe2O4 nanoparticles have both magnetic and photocatalytic properties. The ZnFe2O4 octahedrons, when simply functionalized with the aminosilane N-[3-(trimethoxysilyl)propyl] ethylenediamine, enabled a rapid and energyefficient harvesting of microalgae. Furthermore, the ZnFe2O4 octahedrons, well known for having photocatalytic properties superior to those of ZnFe2O4 nanoparticles with other morphologies, were used to lyse the algal cell wall with the aid of H2O2 under simulated sunlight irradiation. We expect microalgae whose cells can be both magnetophoretically separated and lysed by the same ZnFe2O4 nanoparticles to be utilized as bioenergy resources for more efficient downstream processing than is currently available.



1. Introduction After the Paris Agreement, developing the use of renewable bioenergy to slow down the current rate of climate change has become an appealing pursuit for many scientists and engineers. Microalgae are regarded as the most realizable biomass for biofuels in that they have very fast growth rates (compared to terrestrial energy crops), growth adaptability, and high contents of energy-rich compounds, and they do no compete with arable energy crops.1, 2

In addition, microalgae are capable of sequestrating carbon dioxide during photosynthesis.1

Microalgae-derived biodiesel also benefits from its usability with existing infrastructure.3

Although the microalgal-derived biofuel has huge potential as a candidate to replace petroleum fuels, there are still several challenges that need to be overcome before it can be disseminated as widely as are petroleum fuels. First, it is difficult to separate and concentrate microalgal biomass due to their inherent properties: microalgae are very small, they are stably dispersed in culture media, and the concentration of microalgal culture at a stationary phase is very low.4 In addition, the rigid cell walls and residual water in concentrated microalgal slurry lead to cost increases in successive downstream processing due to the need to carry out cell disruption and lipid extraction.5 For these reasons, the entire process from cultivation to biofuel processing is still very complicated and inefficient from a commercialization perspective.

Recently, magnetophoretic separation has been considered as a promising approach to enhance the energy efficiency of the microalgae harvesting step.6, 7 As the idea of making use of magnetic particles has been introduced into this field, it has become increasingly 4 ACS Paragon Plus Environment

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important to consider various aspects of these particles, such as the cost of synthesizing them, their recyclability, and the ability to use them for both the the harvesting and cell disruption of microalgae.8, 9 Attempts have been made to increase the utilization efficiency of magnetic particles by coating them, functionalizing them, and having them form composites with functionalizing materials.5, 8, 9 For example, Zhang et al. reported the synthesis of stearic acidcoated Fe3O4-ZnO nanocomposites, where ZnO was found to act as a photocatalytic material to change the surface hydrophobicity through photo-oxidization of stearic acid.10 Despite the recent progress, the studied magnetic particles have been mostly limited to iron oxide compounds (i.e., Fe3O4 nanoparticles, Fe3O4 composites).6, 8-11

In this study, we synthesized photocatalytic-responsive magnetic octahedral ZnFe2O4 nanoparticles via a hydrothermal method in order to achieve microalgae separation and cell disruption. The ZnFe2O4 photocatalyst has been shown to effectively absorb visible light due to its narrow bandgap (~ 1.9 eV).12 In particular, the octahedral morphology was found to be beneficial for the photocatalytic chemical reaction owing to the exposure of Fe-rich {111} facets.13-15 In the case of iron-based photocatalysts, H2O2 was also used, in order to prevent a recombination of photo-generated electrons and holes and hence enhance the photocatalytic activity.16 After magnetophoretic separation of the microalgae by the ZnFe2O4 octahedrons, a small quantity of H2O2 was added as a solvent for the microalgae and as an accelerant for photocatalytic lysis of their cell walls. The prepared octahedral ZnFe2O4 particles functionalized with positively charged groups enabled the disruption of the cell-wall surface as well as a rapid separation of the microalgae while sticking to the microalgae.



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2. Experimental 2.1. Preparation and functionalization of ZnFe2O4 octahedrons ZnFe2O4 octahedrons were synthesized using the typical hydrothermal method.15 A mass of 0.7 g of zinc acetate dihydrate (Sigma-Aldrich, 98%) and 2 g of iron sulfate heptahydrate (Sigma-Aldrich, 99%) were dissolved together in 80 ml of distilled water, and into the resulting solution was added 3.205 g of a hydrazine hydrate solution (Sigma-Aldrich, ~80%). The mixed suspension was then transferred into a stainless-steel autoclave lined with Teflon (capacity 100 mL). The autoclave was tightly sealed and kept at 180 °C for 14 h. After the autoclave was cooled to room temperature, the resultant precipitate was centrifuged and washed repeatedly with ethanol and distilled water. Finally, the washed precipitate was frozen by using liquid nitrogen and lyophilized using a freeze-dryer. To functionalize ZnFe2O4 octahedrons with a positive charge, the dried ZnFe2O4 octahedrons (0.3 g) were dispersed in a mixture containing 50 ml of anhydrous toluene and 5 ml of N-[3(trimethoxysilyl)propyl]ethylenediamine (EDA-silane, Sigma-Aldrich, 97%), where the mixture was preheated at 70 °C. The dispersion was stirred for 12 h at 70 °C and then functionalized

ZnFe2O4

octahedrons

(hereafter,

NH2-terminated

ZnFe2O4)

were

magnetically separated from the EDA-silane solution. The NH2-terminated ZnFe2O4 was then washed several times with toluene and ethanol, and then dried in a vacuum oven at 70 °C.

2.2. Cultivation of microalgae The microalgal cells used were Chlorella sp. KR-1, whose seed inoculum was the same as that of our previous reports.17 The microalgal cells were cultivated in 6 L of modified N8 6 ACS Paragon Plus Environment

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medium (7 L Pyrex photobioreactor) that contained 3 mmolL-1 KNO3, 5.44 mmolL-1 KH2PO4, 1.83 mmolL-1 Na2HPO4, 0.20 mmolL-1 MgSO4·7H2O, 0.12 mmolL-1 CaCl2·2H2O, 0.03 mmolL-1 FeNaEDTA·3H2O, 0.01 mmolL-1 ZnSO4·7H2O, 0.07 mmolL-1 MnCl2·4H2O, 0.07 mmolL-1 CuSO4·5H2O and 0.01 mmolL-1 Al2(SO4)3·18H2O. The pH of the resultant medium was 6.5~7. The feed rate of 10% CO2/air to photobioreactor was 0.6 L/min and the light illumination intensity of the fluorescent lamps was 170 μmol/m2·s.

2.3. Characterizations To visualize the morphology of our products, their field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images were obtained using an SU8230 (Hitachi) and Tecnai G2 F30 S-Twin (FEI company), respectively. The crystal structure of the prepared ZnFe2O4 octahedrons was confirmed by using a highresolution powder X-ray diffractometer (XRD, SmartLab, Rigaku). A Zetasizer Nano ZS90 (Malvern) was employed to measure the zeta potentials of the prepared ZnFe2O4 and NH2terminated ZnFe2O4. The solution for measuring these zeta potentials was distilled water at a pH of 6.5. The magnetic hysteresis loop of the ZnFe2O4 octahedrons was evaluated by using a magnetometer at 300 K (MPMS3-Evercool, Quantum Design Inc.).

2.4. Harvesting We evaluated the efficiency of magnetophoretically harvesting the microalgae species Chlorella sp. with NH2-terminated ZnFe2O4 as a function of particle dosage. The cell 7 ACS Paragon Plus Environment


concentration of the initial culture was ~1.9 g cells L-1. NH2-terminated ZnFe2O4 aliquots were added to the culture samples, which were then mixed by manually shaking and vortexing them. The microalgal flocculates (flocs) were separated from this mixture by using an external NdFeB magnet for 0.5 ~ 1 min. The optical density (OD) values of the supernatant liquid were determined by using a UV–Vis spectrophotometer (Optizen 2120 UV, Mecasys Co., Korea). The harvesting efficiency was evaluated as Harvesting efficiency (%) = (1 −

𝑂𝐷𝑎𝑓 𝑂𝐷𝑖

) ×100

(eq. 1),

where ODi and ODaf are the ODs of the culture medium at the initial state and the supernatant liquid after microalgae harvest, respectively. Optical microscopic images were obtained by using a microscope equipped with an AxioCam HRc CCD camera in differential interference contrast (DIC) and fluorescence modes. The optical images were processed by using AxioVision software (Zeiss).

2.5 Photocatalytic cell lysis After magnetophoretic harvesting of microalgae using NH2-terminated ZnFe2O4, the original culture medium was discarded from the vial as much as possible. To the remaining microalgal flocs, we added 1 mL of H2O2 solution (Sigma-Aldrich, 30 wt.%), which is one of the accelerants to disrupt cell walls through photo-Fenton reaction.22 The concentrated slurry was stirred for 4 h under an illumination of simulated solar light (ABET Technologies 10500 with AM 1.5G filter, irradiation intensity = 100 mW/cm2). After the photo-irradiation, the slurry was kept in the dark until it was analyzed with an optical microscope. The flocs were stained with Nile Red and then observed by using fluorescence optical microscopy to


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confirm the cell lysis. Fluorescence images were obtained using a band-pass emission filter set 10 (excitation: BP 450-490; beam splitter: FT 510; emission: BP 515-565) for only the Nile Red observation and a long-pass emission filter set 09 (excitation: BP 450-490; beam splitter: FT 510; emission: LP 515) for simultaneous observation of chlorophyll autofluorescence and Nile Red-stained lipid.

3. Results and discussion 3.1. Integrated downstream process by ZnFe2O4 octahedrons The overall microalgae biorefinery, from microalgae to biofuel, is accompanied by a quite complex downstream process.7 Therefore, if magnetophoretic separation is used as a harvesting method, it is desirable for the magnetic flocculants to have a synergistic effect as well as no adverse effect on the subsequent biorefinery steps (i.e., cell disruption, lipid extraction and conversion).5, 8, 10, 17-19 Figure 1 shows a schematic of how ZnFe2O4 magnetic particles were used in the current study. The Chlorella sp. microalgae separation was conducted in the manner similar to that of the reported magnetophoretic microalgae harvesting.20 NH2-terminated ZnFe2O4 particles, i.e., the ZnFe2O4 magnetic particles functionalized with the positively charged EDA-silane, were introduced into a dilute microalgal culture (~ 1.9 g cells L-1). The NH2-terminated ZnFe2O4 flocculated the microalgal cells, and the resulting flocs were easily separated from the rest of the mixture by using an external magnet. Note especially that, in contrast to previously reported flocculants10, 11, the ZnFe2O4 was both magnetic and had photocatalytic ability even in simulated sunlight irradiation. H2O2 was employed to allow homogenous stirring to be carried out and further to allow the photo-Fenton reaction to take place and hence boost cell 9 ACS Paragon Plus Environment


lysis As illustrated in the enlarged schematic marked as “(2)” in Figure 1, the interaction of ZnFe2O4 particles with H2O2 under simulated sunlight irradiation resulted in a photo-Fenton reaction taking place and hence the formation reactive oxygen species, specifically OH radicals These OH radicals contributed to disrupting the cell walls of flocculated Chlorella sp.; such disruption is a prerequisite for effective lipid extraction.21, 22

3.2. Preparation and functionalization of ZnFe2O4 octahedrons The synthesized ZnFe2O4 particles were observed using SEM and TEM to be octahedrons in shape with widths of about 300 ~ 500 nm, as shown in Figure 2. The prepared particles showed a broad size distribution and aggregated structures. Some truncated octahedral ZnFe2O4 particles were also observed. Figure S1 represents the hydrodynamic diameter of the bare ZnFe2O4 and EDA-silane-functionalized ZnFe2O4 octahedrons, having average sizes of 848.2 nm and 884.7 nm, respectively. The EDA-silane-functionalized ZnFe2O4 octahedrons have the slightly increased average diameter with unimodal distribution, which means that ZnFe2O4 octahedrons were well dispersed in the aqueous solution even after EDA-silane functionalization. The shapes of ZnFe2O4 particles have been previously shown to change from cubic to octahedral during the course of their hydrothermal synthesis. 13 Morphological analyses indicated that the reaction time in the autoclave reactor we used was sufficient to prepare octahedrally shaped ZnFe2O4 particles. A few recent reports indicated nanosized ZnFe2O4 octahedrons enclosed by their {111} facets to be advantageous for producing Fe-rich surfaces and having their Fe ions coordinated by Li ions, and hence for improving the electrochemical characteristics of Li-ion batteries.13, 15, 23, 24 From these studies, we expected that the octahedron morphology would play a beneficial role in the 10 ACS Paragon Plus Environment

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photocatalytic cell-lysis reaction since cell-wall-disrupting hydroxyl and hydroperoxyl radicals have been shown to be generated via the reaction of exposed Fe ions with H2O2.22, 25 A high-resolution TEM image of a synthesized particle (Figure 2c) showed a lattice fringe of 0.486 nm, which well corresponded to the (111) plane of ZnFe2O4. The synthesized ZnFe2O4 octahedrons were also characterized by using XRD, as shown in Figure 3a, by carrying out a hysteresis characterization, as shown in Figure 3b, and by determining zeta potentials, as shown in Figure 4. All of the diffraction peaks were consistent with those of franklinite ZnFe2O4 having the cubic spinel structure (JCPDS card # 221012).16 For applications in magnetophoretic separation, saturation magnetization (MS) is one of the important values, since the magnetic driving force (Fm) is proportional to the magnetization of a material.6 According to a hysteresis characterization, the ZnFe2O4 octahedrons prepared by the hydrothermal method showed superparamagnetic behavior and showed an MS value close to 33.25 emu/g (Figure 3b). This value was similar to or superior to the magnetization values of ZnFe2O4 nanoparticles prepared by a hydrothermal method or other methods such as sol-gel and co-precipitation in other groups, respectively.23, 26 Thus, the hydrothermal method we employed was advantageous not only in its ability to control the morphology of the synthesized ZnFe2O4 particles but also in synthesizing a ZnFe2O4 product with a high MS value.

3.3 Harvesting and cell disruption by ZnFe2O4 octahedrons Figure 4 shows the zeta potentials of the EDA-silane-functionalized as well as nonfunctionalized ZnFe2O4 octahedrons. These zeta potentials were measured to be +28.0 mV and +16.5 mV, respectively. Despite the ZnFe2O4 octahedrons having a positive zeta 11 ACS Paragon Plus Environment


potential of 16.5 mV as described above, they could not separate the negatively charged microalgae, as shown in Figure S2. This result implied a larger electrostatic attraction (namely, a larger difference in the zeta potential between particles and cells) or another attractive force (van der Waals interaction, bridging effect, or Lewis acid-base interactions) to be required. After EDA-silane-functionalization, the NH2-terminated ZnFe2O4 particles resulting from EDA-silane-functionalization did indeed effectively separate the microalgae as the increasing particle dosage (Figure 5). The difference in the effectiveness of the NH2terminated ZnFe2O4 and nonfunctionalized particles was attributed to the above-described approximately 10 mV greater zeta potential of the NH2-terminated ZnFe2O4 particles resulting from coating with EDA-silane. This difference was apparently enough to cause electrostatic attraction between the particles and the cells. Such electrostatic interaction plays a crucial role, especially for freshwater microalgae, in determining the effectiveness of magnetophoretic separation.27 Figures 5b and 5c show optical and SEM images of flocculated algae taken from the red box in Figure 5a. The microalgal flocs were observed to be closely aggregated along ZnFe2O4 aggregates. Several reports showed that cationic macromolecules such as poly(diallyldimethylammonium chloride) (PDDA), polyethylenimine, and chitosan can promote magnetophoretic separation of microalgae through bridging flocculation as well as electrostatic attraction.27-29 Bridging flocculation has been shown to be particularly pronounced for polymers with higher molecular weights.30 Compared to (3aminopropyl)triethoxysilane (APTES), EDA-silane has a longer chain and a secondary amine group in the chain, and is therefore prone to form a crosslinked silane layer and further might be easy to cause a minor bridging flocculation.


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The NH2-terminated ZnFe2O4 octahedrons were also used to help disrupt the cell walls by effecting the photocatalytic Fenton-like reaction. After using the ZnFe2O4 octahedrons to harvest Chlorella sp., the octahedrons and Chlorella sp were mixed with a small amount (1 ml/g cell) of H2O2 solvent, and the resulting mixture was subjected to uniform light irradiation. Fluorescence microscopic analysis showed that lipid droplets stained with Nile Red emitted orange-yellow light when observed with the long-pass emission filter but was cloudy green when observed with the band-pass emission filter. The brilliant red fluorescence observed, shown in Figure 6b, was assigned to the autofluorescence peak of chlorophyll. Compared to the images of intact cells (Figures 6a-c), a few isolated oil droplets (with dimensions of less than 1 μm) and large oil globules were observed outside of the cells (Figures 6d-f), as indicated as white arrows in Figure 6e-f. This result showed that the combination of simulated sunlight irradiation and H2O2 treatment was sufficient to disrupt the cell walls and release oil droplets from the cells. From an empirical point of view, the cell lysis efficiency in this condition was higher than about 85 % in that over 85% of the cells did not contain intracellular lipids, most of them were spreading or forming large and small oil droplets. A potential mechanism for photo-Fenton cell-disruption by using this ZnFe2O4/H2O2/simulated sunlight combination could be explained by the following equations (eq. 2-5). The generated OH radicals could decompose extracellular polymeric matrices of microalgae into cellulose and hemicelluloses, and then into simple sugars such as cellulose oligomers or monomers.21, 31, 32 ZnFe2O4 + hv → ZnFe2O4 (h+ + e-) Fe(III) + e- → Fe(II)

(eq. 2),

(eq. 3),

Fe(II) + H2O2 → Fe(III) + ·OH + OH-

(eq. 4),



·OH + cell wall → degraded cell wall

(eq. 5),

In general, an inevitable stage, cell-disruption, for effective lipid extraction is carried out with a help of by mechanical, chemical, or enzymatic treatment.33 Mechanical disruption methods such as high-pressure/speed homogenization, ultrasonication, and microwave treatment are practically available but require high energy consumption.33 Although chemical treatments might save energy consumptions, they sometimes need to use toxic organic solvents or highly acidic solvents to extract intracellular lipids.34, 35 In this regard, the use as cell-disruption agents of the same materials which are utilized as magnetic harvester in the previous stage, and further the use of renewable solar energy could be a very promising method in terms of both economic and environmental aspects. Compared to broad-bandgap TiO2-based photocatalysts, ZnFe2O4 photocatalysts operated efficiently under simulated sunlight illumination.11, 24 Therefore these ZnFe2O4 photocatalysts were indicated to be more appropriate for photocatalytic cell disruption when using solar light. Moreover, photocatalytic-responsive magnetic octahedral ZnFe2O4 nanoparticles we fabricated can function as a photocatalyst and magnetophoretic separator at the same time without any additional functional-materials, which facilitates highly efficient subsequent downstream processing.

4. Conclusions This study verified the feasibility of using ZnFe2O4 octahedrons as magnetic flocculants and cell-disruption agents. ZnFe2O4 octahedrons were prepared via a hydrothermal method and then functionalized with aminosilane to facilitate its adhesion with algae. A microalgal


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culture was quickly flocculated by injected ZnFe2O4 and then separated by using an external magnetic field. With assistance of H2O2 solvent, the ZnFe2O4 magnetic flocculant activated the algal cell-wall lysis by effecting the photocatalytic Fenton reaction under simulated sunlight irradiation. A meaningful advance made in thus study was the use of ZnFe2O4 as both a magnetic flocculant and, with it intentionally designed to be octahedral, an agent boosting photocatalytic activity. This method would certainly benefit from and may even require additional optimization studies to be carried out, such as optimizing the amount of H2O2 and the light irradiation dose, as well as improving the method for separating the extracted lipid. Nevertheless, the method already holds out the promise of harvesting, in a very cost-effective and environmentally friendly way, the promising future energy source that is microalgae, due to it not only utilizing renewable solar energy but also integrating otherwise complicated downstream processing.

Acknowledgements This research was supported in part by the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (NRF-2015K1A4A3047100). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea

government

(Ministry

of

Science,

ICT

and

Future

Planning)

(NRF-

2017R1C1B1006807). Supporting Information Available: Additional information of a photograph of microalgal



culture not separated by the bare ZnFe2O4 octahedrons is available free of charge via the Internet at http://pubs.acs.org.

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Figure Captions Figure 1. Schematic illustration of the integrated downstream process involving using ZnFe2O4 octahedrons to harvest microalgae and disrupt their cells. Figure 2. (a) SEM and (b, c) TEM images of ZnFe2O4 functionalized with N-[3(trimethoxysilyl)propyl]ethylenediamine. Figure 3. (a) XRD pattern and (b) magnetic hysteresis loop of ZnFe2O4 prepared via a hydrothermal method. Figure 4. Zeta potentials of ZnFe2O4 and NH2-ZnFe2O4 octahedrons in distilled water at 25 °C. Figure 5. (a) Harvesting efficiency of microalgae as a function of dosage of amineterminated ZnFe2O4 octahedrons. (b) Optical microscopic images and (c) SEM image of microalgal flocs resulting from the action of amine-terminated ZnFe2O4 octahedrons (red box in Fig. 5a). Figure 6. Cell disruption resulting from the photocatalytic and Fenton reactions of ZnFe2O4 octahedrons under simulated sunlight irradiation and in the presence of H2O2. (a-c) Optical microscopic images of Nile Red-stained intact Chlorella sp. (d-f) Harvested Chlorella sp. after being subjected to the photocatalytic and Fenton reaction (white arrows: oil droplets).



Figure 1. Schematic illustration of the integrated downstream process involving using ZnFe2O4 octahedrons to harvest microalgae and disrupt their cells.


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Figure 2. (a) SEM and (b, c) TEM images of ZnFe2O4 functionalized with N-[3(trimethoxysilyl)propyl]ethylenediamine.



Figure 3. (a) XRD pattern and (b) magnetic hysteresis loop of ZnFe2O4 prepared via a hydrothermal method.


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Zeta potential (mV)

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20 15 10 5 0

ZnFe2O4

NH2-ZnFe2O4

Figure 4. Zeta potentials of ZnFe2O4 and NH2-ZnFe2O4 octahedrons in distilled water at 25 °C.



Figure 5. (a) Harvesting efficiency of microalgae as a function of dosage of amineterminated ZnFe2O4 octahedrons. (b) Optical microscopic images and (c) SEM image of microalgal flocs resulting from the action of amine-terminated ZnFe2O4 octahedrons (red box in Fig. 5a).


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Figure 6. Cell disruption resulting from the photocatalytic and Fenton reactions of ZnFe2O4 octahedrons under simulated sunlight irradiation and in the presence of H2O2. (a-c) Optical microscopic images of Nile Red-stained intact Chlorella sp. (d-f) Harvested Chlorella sp. after being subjected to the photocatalytic and Fenton reaction (white arrows: oil droplets).



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Simulated-Sunlight-Driven Cell Lysis of Magnetophoretically

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