Recovery of Chlorella biomass using Fe3O4 and Fe3O4@SiO2 magnetic nanoparticles: Isotherm and thermodynamic characteristics of adsorption

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Harvesting microalgae by centrifugation and filtration is very costly, affecting technical and economical feasibility of downstream processing of algal biomas to bioproducts. Flocculation is a potential technique for recovery of algae biomass. Particularly, flocculation using magnetic nanoparticles offers high separation efficiency due to their high surface area and magnetic properties. I

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Nội dung Text: Recovery of Chlorella biomass using Fe3O4 and Fe3O4@SiO2 magnetic nanoparticles: Isotherm and thermodynamic characteristics of adsorption

Cite this paper: Vietnam J. Chem., 2023, 61(S3), 116-125 Research Article DOI: 10.1002/vjch.202300068 Recovery of Chlorella biomass using Fe3O4 and Fe3O4@SiO2 magnetic nanoparticles: isotherm and thermodynamic characteristics of adsorption Do Thi Cam Van1, Lam Van Toan2, Nguyen Thi Phuong Dung2,3, Tran Dang Thuan4*, Dinh Thi Cuc4, Dang Thi Mai4, Pham Thi Mai Huong5, Pham Thi Thanh Yen5 1 HaUI Institute of Technology, Hanoi University of Industry (HaUI), 298 Cau Dien, Bac Tu Liem, Hanoi 11900, Viet Nam 2 Graduate University of Science and Technology, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 3 Department of Applied Science, University of Transport Technology, 54 Trieu Khuc, Thanh Xuan, Hanoi, Viet Nam 4 Institute of Chemistry, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 5 Faculty of Chemical Technology, Hanoi University of Industry (HaUI), 298 Cau Dien, Bac Tu Liem, Hanoi 11900, Viet Nam Submitted February 19, 2023; Revised May 2, 2023; Accepted July 21, 2023 Abstract Harvesting microalgae by centrifugation and filtration is very costly, affecting technical and economical feasibility of downstream processing of algal biomas to bioproducts. Flocculation is a potential technique for recovery of algae biomass. Particularly, flocculation using magnetic nanoparticles offers high separation efficiency due to their high surface area and magnetic properties. In this work, the synthesized magnetic nanoparticles including Fe 3O4 and Fe3O4@SiO2 were used to harvest C. sorokiniana TH01 from aqueous suspension. The effects of pH, adsorbent dosage, algal biomass concentration, temperature and reaction time on harvesting performance were investigated. It was revealed that the optimal C. sorokiniana TH01 harvesting conditions for Fe3O4 were pH of 5, adsorbent dosage of 0.5 g/L, biomass concentration of 2-2.5 g/L, temperature 25oC and reaction time 20-30 min. For Fe3O4@SiO2, the optimal conditions were pH of 7, adsorbent dosage of 0.6 g/L, with the same biomass concentration, temperature and reaction time as Fe3O4. Under the optimal conditions, Fe3O4 and Fe3O4@SiO2 can be regenerated up to three cycles with harvesting efficiency till remained over 55%. Adsorption of C. sorokiniana TH01 on Fe3O4 and Fe3O4@SiO2 exhibited the best fitness with Langmuir isortherm with R2 and maximal adsorption capacity estimated of 0.995 and 6.55 g/g and 0.9971 and 9.53 g/g, respectively. Thermodynamics study revealed that an adsorption of C. sorokiniana TH01 on Fe3O4 and Fe3O4@SiO2 was exothermic process. Furthermore, XRD, EDS, SEM, TGA and FT-IR data confirmed the successful binding of microalgae cells on surfaces of Fe3O4 and Fe3O4@SiO2. The super-magnetization intensities (> 22 emu/g) of the synthesized materials demonstrated an excellent separation capability with external magnetic devices. Keywords. Chlorella, magnetic particles, flocculation, isotherm, thermodynamics . 1. INTRODUCTION lipids, carbohydrates, chlorophylls, carotenoids and other bioactive compounds (astaxanthin, lutein, etc.) Microalgae are regarded as the third generation of which are promising precursors for production of biomass feedstock for biofuels production.[1,2] pharmaceutical, cosmesceutical, food/feed and Microalgae can be grew in different aqueous energy.[4,5] Most of microalgae are phototrophic environments such as artificial, industrial, domestic strains which need light as an energy source for their and agricultural wastewaters in nonarable lands.[3] growth. Due to limitation of photon transfer in Particularly, microalgae can accumulate high content microalgal culture, phototrophic microalgae generally of multiple secondary metabolites such as proteins, achieve maximal dry cell weight of bellow 5-10 g/L,[6] 116 Wiley Online Library © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Tran Dang Thuan et al. leading to dewatering and concentration of biomass polyaluminium chloride and polyacrylamide.[35] become challenging and costly. Various harvesting Fraga-García et al. (2018) studied separation of technologies for recovery of microalgae biomass have Scenedesmus ovalternus and Chlorella vulgaris using been developed and applied such as centrifugation,[7] bare iron oxide nanoparticles.[36] Fu et al. (2021) used filtration (membrane-based technologies),[8-10] magnetic particles synthesized by co-precipitation of [11] coagulation/flocculation, and adsorption.[12] FeCl2 and FeCl3 under alkaline pH (NH3) in Centrifugation and filtration are physics-based investigation of harvesting Nannochloropsis methods which are energy-intensive process when maritime.[37] Microwave-synthesized naked magnetite large volume of microalgae employed for harvesting. (Fe3O4) particles were employed for harvesting of Coagulation/flocculation are physicochemical-based Chlorella vulgaris.[38] Markeb et al. (2019) used approaches which offer a great saving cost as magnetite-based nanoparticles (Fe3O4 NPs) for processing technologies are simpler and harvesting Scenedesmus sp.[39] coagulant/flocculants are commercially available Despite numerous utilizations of magnetic (e.g., cationic starch, poly(aluminum chloride), particles for harvesting microalgae, very few studies chitosan, etc.).[13] However, coagulation/flocculation carried out simultaneously adsorption isotherm and causes coagulants/flocculants-contaminated biomass thermodynamics investigation of microalgae- which may considerably affect biorefinery of later nanoparticle interactions. Moreover, very few studies products.[14,15] Recently, nanomaterials have been used magnetic nanocomposites for harvesting received considerable attention by worldwide microalgae. Therefore, the objectives of this study researchers due to their novel characteristics such as were to (i) synthesize and characterize nanoparticles high surface area[16-18] and easy surface i.e., Fe3O4 and Fe3O4@SiO2, (ii) to investigate the modification,[19-23] thus owning a outstanding effects of pH, nanoparticles dosage, microalgae adsorptive performance for various adsorbates[16-27] concentration, temperature, reaction time and such as microalgae,[28] dye,[16-18,20,22,26], and enzyme.[24] regeneration of nanoparticles on harvesting Although different methods have different efficiency, (iii) to investigate adsorption isotherm and disadvantages and advantages, adsorption of algal thermodynamics of algal biomass adsorption process biomass using nanomaterials in an interesting research of nanoparticles and (iv) to investigate stability of area.[29] Particularly, magnetic nanoparticles own adsorbents during adsorption-regeneration cycles. magnetic property.[30-32] Thus, mixture of algae biomass-bound magnetic nanoparticles can be 2. MATERIALS AND METHODS harvested simply by applying an external magnetic device. Furthermore, desorption of algal biomass from 2.1. Materials. All chemicals in this study were biomass-bound magnetic nanoparticles can be obtained from commercial sources of analytical grade. accomplished easily to separately obtain pure algal biomass and adsorbents. However, harvesting 2.2. Microalgae and seed culture preparation microalgae from aqueous suspensions requires large Refer to supporting material part. amount of adsorbents and their stably adsorptive characteristics. Therefore, magnetic particles should 2.3. Biomass production be synthesized in the form of core-shell composites Refer to supporting material part. which offer two great advantages, (i) shell serves as a protective layer for magnetic core and helps magnetic 2.4. Preparation of Fe3O4 and Fe3O4@SiO2 nanoparticles are stable during reaction and (ii) core- Refer to supporting material part. shell composites help reducing amount of magnetic core used for adsorption, leading to cost reduction of 2.5. Method of harvesting C. sorokiniana TH01 adsorbent synthesis. Numerous studies have used using nano Fe3O4 and Fe3O4@SiO2 magnetic nanoparticles for harvesting various microalgae, however most of studies used pure Fe3O4 2.5.1. Optimization of harvesting C. sorokiniana which is very costly at large-scale harvesting. For TH01 using Fe3O4 and Fe3O4@SiO2 instance, Zhu et al. (2017) used iron oxide (Fe3O4) and yttrium iron oxide (Y3Fe5O12) nanoparticles as Factors affecting harvesting performance of Fe3O4 and flocculants for harvesting of Chlorella vulgaris.[33] Hu Fe3O4@SiO2 for microalgal biomass C. sorokiniana et al. (2013) investigated harvesting performance of TH01 including pH, adsorbent dosage, initial algal Fe3O4 for Nannochloropsis maritime culture.[34] Zhao biomass concentration, temperature and contacting et al. (2015) harvested Chlorella vulgaris with time were investigated to optimize adsorption magnetic flocculation using Fe3O4 coating with conditions. The studied pHs of algal culture were 4, 5, © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 117
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Recovery of Chlorella biomass using … 6, 7, 8, 9 and 10, which were adjusted by addition of 2.7. Adsorption isotherm and thermodynamics either 1 M NaOH or 1 M H2SO4. The adsorbent Refer to supporting material part. dosages were studied at levels of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 g/L. The initial algal concentrations were 2.8. Stability of Fe3O4 and Fe3O4@SiO2 examined at levels of 0.5, 1, 2, 2.5 and 3.0 g/L. The Refer to supporting material part. examined temperatures were set at three levels of 25, 30 and 40 oC. The contacting time was studied from 2.9. Analysis 10 to 60 min. All experiments used algal biomass Refer to supporting material part. volume of 20 mL. Moreover, all algal-adsorbent suspensions were mechanically stirred at 100 rpm for 2.10. Statistical analysis 30 min and stabilized for 5 min. Algal-adsorbents were decanted with an external magnetic device and Experiments were carried out in triplicate (n = 3) and the up-layers were sampled for measuring optical data was presented as mean ± standard deviation (SD). density to evaluate the harvesting efficiency. Statistical analysis was done using one-way ANOVA followed by post hoc Tukey’s test and a p-value of < 0.05 was declared as significant. The statistical 2.5.2. Regeneration of Fe3O4 and Fe3O4@SiO2 analysis was conducted using software package Microsoft Excel Software 2016 (Microsoft, The algal biomass-adsorbed Fe3O4 and Fe3O4@SiO2 Washington, USA). after the 1st treatment under the optimal conditions obtained from Section 2.1.1 was regenerated by gently 3. RESULT AND DISCUSSION mixing with 5 mL deionized water followed by ultrasonication for 30 min to detach algal biomass. The 3.1. Optimization of C. sorokiniana TH01 Fe3O4 and Fe3O4@SiO2 were obtained by an external harvesting efficiency by Fe3O4 and Fe3O4@SiO2 magnetic device and algal culture diluted in deionized water was removed. The obtained Fe3O4 and Variation of pH of algal culture results in changing of Fe3O4@SiO2 was further washed with 70% ethanol to zeta potential of algal suspension and electrical complexly remove algal biomass bound. The second surface of nanoparticles, affecting flocculation treatment was continued by adding 20 mL algal culture efficiency of algal biomass by the nanoparticles.[39] with initial concentration of 2.5 g/L, followed by pH Data shown in figure 1a indicates that the increase of adjustment to 5.0 for Fe3O4 and 7.0 for Fe3O4@SiO2 pH from 4 to 5 resulted in microalgal harvesting and the same procedure as applied for the first efficiency by Fe3O4 increasing from 18.5 to 29.0%. treatment. The regeneration was stopped at the cycle This trend was slightly declined to 25.1% when pH when the capability of nanoparticles resulted in algal was further increased to 6 and considerably reduced to biomass harvesting efficiency of lower than 20%. 4.7-2.1% for the pH values further decreased to levels of 7-10. This implies that the optimal pH value for 2.5.3. Stability of Fe3O4 and Fe3O4@SiO2 harvesting C. sorokiniana TH01 by Fe3O4 was 5. For Fe3O4@SiO2, it was observed that pH 7 was optimal Structural stability of Fe3O4 and Fe3O4@SiO2 after level for recovery of C. sorokiniana TH01 with every regenerated cycle was studied by XRD which harvesting efficiency of 24.0%. The pHs adjusted at 4- was described in Section 2.8. Moreover, ions such as 6 and 8-9 resulted in reduction of C. sorokiniana iron leaching into aqueous suspensions during TH01 harvesting efficiency by Fe3O4@SiO2 to 11.1- adsorption process was measured by an Inductively 23.3% and 12.0 – 15.9%, respectively. Thus, the Coupled Plasma-Atomic Emission Spectrometry (ICP optimal pH levels of 5 and 7 were used for harvesting OES-PQ 9000/Serial 13-5850D-AQ125, Analytik C. sorokiniana TH01 with Fe3O4 and Fe3O4@SiO2 in Jena GmbH+Co. KG, Jena, Germany) according to following experiments, respectively. It was widely US EPA Method 200.7. Moreover, silicon was discussed that flocculation of microalgae- assayed by colorimetric method according to the nanoparticles is controlled by electrostatic method of SMEWW 4500-SiO2.D:2017 using an UV- attraction.[39,40] Moreover, the influence of pH on vis (U2900, Hitachi, Japan). harvesting performance was reported to inter-related to type of nanoparticles and microalgal strain. For instance, recovery of Nannochloropsis by Fe3O4- 2.6. Determination of adsorptive performance of based nanoparticles was less dependent on pH in the nanomaterials on algal biomass. Refer to supporting range of 5-9 with stable harvesting efficiencies of material part. © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 118
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Tran Dang Thuan et al. 97.3-98.1%.[37] The harvesting efficiency of C. sorokiniana TH01 by The effect of adsorbent dosage on C. sorokiniana Fe3O4 and Fe3O4@SiO2 was investigated in adsorption TH01 harvesting performance of Fe3O4 and time range of 0 to 60 min. Data illustrated in figure 1e Fe3O4@SiO2 is displayed in Figure 1b. It is observed reveals that the optimal time to achieve the highest that increasing dosage of Fe3O4 from 0.1 to 0.5 g/L harvesting efficiency of 91.5 and 99.5% by Fe3O4 and significantly enhanced C. sorokiniana TH01 Fe3O4@SiO2 were 20 and 30 min, respectively. The harvesting efficiency from 29.0 to 71.9%. However, extending more contacting time resulted in reduction further addition of Fe3O4 at dosage of 0.6 and 0.7 g/L of C. sorokiniana TH01 harvesting efficiency by both only resulted in a slight improvement of C. Fe3O4 and Fe3O4@SiO2. This data demonstrates that sorokiniana TH01 harvesting efficiency to 72.1 and adsorption of C. sorokiniana TH01 on Fe3O4 and 72.3%, respectively. The Fe3O4@SiO2 exhibited a Fe3O4@SiO2 reaches the equilibrium between 20-30 better capability with the highest harvesting efficiency min. Abo Markeb et al. (2019) reported that of 99.5% was achieved at its dosage of 0.6 g/L. The Scenedesmus sp. can be recovered of 95.68% with dosage of Fe3O4@SiO2 used at 0.1-0.5 g/L resulted in 0.14 g/L Fe3O4 in total 35 min.[39] In another study, lower algal harvesting efficiency of 24.1-69.0%, while Nannochloropsis maritime can be harvested with 0.12 further increased Fe3O4@SiO2 dosage to 0.7 g/L only g/L Fe3O4 achieving over 95% removal efficiency in achieved algal harvesting efficiency of 99.8%. This total 12 min.[34] data is attributed to saturation phenomena of Overall, the optimal conditions for harvesting C. adsorbent in reaction system. Thus, the optimal sorokiniana TH01 by Fe3O4 were pH of 5, adsorbent adsorbent dosages of 0.5 and 0.6 g/L were used for dosage of 0.5 g/L, initial biomass concentration of 2- following harvesting experiments with Fe3O4 and 2.5 g/L, temperature 25oC and reaction time 20-30 Fe3O4@SiO2, respectively. It was evaluated that the min. For Fe3O4@SiO2, the optimal conditions were optimal adsorbent dosages to achieve over 99% of C. pH of 7, adsorbent dosage of 0.6 g/L, initial biomass sorokiniana TH01 harvesting efficiency were 0.33- concentration of 2-2.5 g/L, temperature 25oC and 0.39 g nanoparticles/g algal biomss. This data is reaction time 20-30 min. similar to the result (99.5% at 0.33 mass ratio) obtained from harvesting experiment with Nannochloropsis by Fe3O4-based nanoparticles.[37] The effect of initial algal biomass concentration on C. sorokiniana TH01 harvesting efficiency of Fe3O4 and Fe3O4@SiO2 is depicted in figure 1c. It was observed that the increasing of C. sorokiniana TH01 concentration from 0.5 to 3 g/L resulted in reduction of harvesting efficiency to 97.6 to 83.8% and 99.0 to 86.1% determined for Fe3O4 and Fe3O4@SiO2, respectively. Notably, harvesting performance of Fe3O4 and Fe3O4@SiO2 still achieved over 90-95% when C. sorokiniana TH01 biomass concentration used of 2-2.5 g/L. Therefore, initial biomass concentration recommended for harvesting with Fe3O4 and Fe3O4@SiO2 was 2-2.5 g/L in following experiments. The effect of temperature on C. sorokiniana TH01 harvesting efficiency of Fe3O4 and Fe3O4@SiO2 is Figure 1: Harvesting performance of Fe3O4 and depicted in figure 1d. It was revealed that the Fe3O4@SiO2 for C. sorokiniana TH01 biomass with increasing of temperature from 25 to 40oC caused a variation of adsorptive parameters. pH (a), adsorbent decrease in algal harvesting efficiency by Fe3O4 and dosage (b), biomass concentration (OD740) (c), Fe3O4@SiO2 from 98.2 to 83.7% and 99.1 to 87.3%, temperature (d), adsorption time (e) and regeneration respectively. This data implies that harvesting of C. of adsorbent (f) sorokiniana TH01 is better to carry out under room temperature condition. This is an advantageous point By applying these optimal conditions, the as the algal culture does not need to be heated up, thus regeneration of Fe3O4 and Fe3O4@SiO2 for sequential saving energy in harvesting work. harvesting of C. sorokiniana TH01 was evaluated. Time is also an important factor affecting Data shown in figure 1d displays that increasing harvesting performance of Fe3O4 and Fe3O4@SiO2. number of regeneration from 1 to 5 resulted in © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 119
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Recovery of Chlorella biomass using … reduction of C. sorokiniana TH01 harvesting 28.26 L/g for Fe3O4 and 6.99 L/g for Fe3O4@SiO2) performance from 91.6 to 17% and 99.7 to 11% from Langmuir equation. The Freundlich regression determined for Fe3O4 and Fe3O4@SiO2, respectively. gave the parameters, e.g., n is indicated as the bond This decreasing trend of harvesting efficiency is energy between algal biomass and the adsorbent (n = agreed well with reduction of zeta potentials measured 1.66 for Fe3O4 and 1.37 for Fe3O4@SiO2) and K is for Fe3O4 and Fe3O4@SiO2 after every cycle of related to bond strength (K = 21.65 g0.40 L0.60/g for regeneration (figure S1). The declining trends of zeta Fe3O4 and 20.55 g0.27 L0.73/g for Fe3O4@SiO2). potentials is attributed to adsorbed microalgae lysis by Moreover, R2 was also determined to evaluate the ultrasonication and phosphate adsorption on adsorbent fitness of experimental data with regression curves of surfaces, leading to electrostatic repulsion between the two isotherm models. Parameters and R2 algal biomass and Fe3O4 and Fe3O4@SiO2. There are coefficients are listed in table 1. Data shown in table 1 two possible mechanisms of adsorbent deactivation by reveals that R2 determined for Langmuir model were ultrasound included: (i) disruption of strong bonding 0.9950 and 0.9971 which were higher than 0.9830 and of microalgae and adsorbents; and (ii) refreshment and 0.9807 estimated for Freundlich model for Fe3O4 and dispersion of adsorbents in aqueous suspension. The Fe3O4@SiO2, respectively, indicating that Freundlich first mechanism is the required step before separation isotherm was poorer fit to the experimental data when and recycling of the adsorbents for the next cycles. compared to the Langmuir isotherm. However, R2- Notably, the harvesting capability of Fe3O4 and values determined for Fe3O4 and Fe3O4@SiO2 on the Fe3O4@SiO2 for C. sorokiniana TH01 biomass was Langmuir model were above 0.995, thus Langmuir still remained at 51 and 66% at the third regeneration model is good applicability for studying adsorption of cycle, respectively. It was distinguishable that C. sorokiniana TH01 biomass on Fe3O4 and Fe3O4@SiO2 exhibited a slightly better harvesting Fe3O4@SiO2. Interestingly, the close level of R2 to 1 performance during the first three cycles when for Langmuir model indicating that microalgae compared to those of Fe3O4 with mostly the same harvesting could work as monolayer coverage on both adsorbent dosage of 0.5-0.6 g/L, temperature of 25 oC by Fe3O4 and Fe3O4@SiO2 particles. This is and contact time of 20-30 min. Moreover, the use of agreement with observation of Markeb et al.[27] and Fu Fe3O4@SiO2 can save countable amount of naked et al.[25] reporting that Scenedesmus sp. and Fe3O4 which requires more chemicals for synthesis, Nannochloropsis maritime adsorption on Fe3O4 leading to saving cost of input materials. Interestingly, nanoparticles fitted well with Langmuir model, the use of Fe3O4@SiO2 can prevent the core Fe3O4 respectively. from oxidation by chemicals during reaction, as it is protected by the inert shell SiO2. Therefore, although Table 1: Parameters determined from Langmuir and regeneration is no longer achievement of the same Freundlich adsorption isotherms high harvesting efficiency, the nanomaterial Langmuir Freundlich Fe3O4@SiO2 is a promising flocculants for recovery Parameter Fe3O4 Fe3O4@SiO2 Fe3O4 Fe3O4@SiO2 of C. sorokiniana TH01 from algal culture. Qm (g/g) 6.55 9.53 ND ND b (L/g) 28.26 6.99 ND ND 3.2. Adsorption isotherm and thermodynamics n ND ND 1.66 1.37 K (g(1 – ND ND 21.65 20.55 The adsorption of C. sorokiniana TH01 on Fe3O4 and (1/n)) 1/n L /g) Fe3O4@SiO2 materials was studied by using two R2 0.9950 0.9971 0.9380 0.9807 isotherm models of Langmuir and Freundlich equations. The Langmuir models assumes that Adsorption capacity is dependent on microalgal microalgae harvesting occurs on a homogeneous strain and nanoparticle properties. Fu et al. (2021) surface by monolayer, while the Freundlich model reported that a synthesized Fe3O4 can reach the largest assumes that it occurs on a heterogeneous surface of adsorption capacity Qm of 13.46 g/g for the nanoparticles. Model fitting of experimental data Nannochloropsis.[37] Several surface-modified using Langmuir and Freundlich isotherms are shown nanoparticles achieved higher adsorption capacities in figure S2-a and figure S2-b, respectively. The on different microalgal strains such as 55.9-114.8 g/g derivation and application of equilibrium data for Botryococcus braunii,[40] 21.4-86.21 g/g for obtained from experiments with differently initial Chlorella ellipsoidea,[40] 16.16 g/g for biomass concentrations (e.g., 0.5-3 g/L) yielded two Nannochloropsis maritime.[34] Other studies obtained paramaters, e.g., maximum adsorptive capacity (Qm = lower adsorption capacities such as 3.74 g/g for 6.55 g/g for Fe3O4 and 9.53 g/g for Fe3O4@SiO2) and Microcystis aeruginosa,[41] and 5.83 g/g for Chlorella a constant related to the energy of adsorption (b = ellipsoidea.[40] © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 120
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Tran Dang Thuan et al. Data shown in table 2 indicates that ΔHo Fe3O4@SiO2-C materials were Fe3O4 and measured for C. sorokiniana TH01 adsorption by Fe3O4@SiO2 bound with algal biomass which is Fe3O4 and Fe3O4@SiO2 were -112.61 and -136.84 organic matter with the molecular formula kJ/mol, respectively, demonstrating the exothermic CxHyOzSmPnCop.[45] Thus, EDS pattern of Fe3O4-C and process. The high value of ΔHo reflects the strong Fe3O4@SiO2-C also exhibited organic elements of C, interactions between the C. sorokiniana TH01 N, S (Figure 4-c,d and Table S1). Moreover, Co biomass and the nanomaterials. Moreover, ΔSo were detected in Fe3O4-C material was attributed to determined for Fe3O4 and Fe3O4@SiO2 as -0.35 and Co(NO3)2·6H2O presented in BG-11 medium which -0.42 kJ/mol/K, respectively, indicating the decrease was used for cultivation of the microalgal.[46] of disorder reaction, while ΔGo determined between SEM images indicated that Fe3O4 (Figure S4-e) -10.66 to -3.73 kJ/mol for both two nanomaterials and Fe3O4@SiO2 (Figure S4-f) were spherical shapes. exhibiting a spontaneous process in the low The coverage of SiO2 on the surface of Fe3O4 temperature range.[42] nanoparticles shown in SEM images displayed that the particle morphologies were almost uniform. This Table 2: Thermodynamics parameters for C. result was similar to the observation of Hariani et al. sorokiniana TH01 adsorption by Fe3O4 and (2013).[47] The dimension of Fe3O4 and core-shell Fe3O4@SiO2 structure of Fe3O4@SiO2 are clearly characterized by T ΔHo ΔSo ΔGo TEM and illustrated in figure S5-a and figure S5-b, Materials KC respectively. The adsorption of microalgae cells on (K) (kJ/mol) (kJ/mol/K) (kJ/mol) 298 54.56 -8.95 the surface of Fe3O4 and Fe3O4@SiO2 is displayed as Fe3O4 303 9.64 -112.61 -0.35 -7.21 pieces of matter debris with different shapes and sizes 313 5.13 -3.73 on figure S4-g and figure S4-h, respectively. 298 124.0 -10.66 Fe3O4@SiO2 303 13.49 -136.84 -0.42 -8.55 Magnetic properties of Fe3O4, Fe3O4@SiO2, 313 6.87 -4.31 Fe3O4-C and Fe3O4@SiO2-C are shown in figure S6. Data illustrated that the material Fe3O4 had a 3.3. Characteristics of Fe3O4, Fe3O4@SiO2, Fe3O4- magnetization of up to 54 emu/g saturated in the C and Fe3O4@SiO2-C external magnetic field region from -15000 Oe - 15000 Oe. This level was lower than that of a super- X-ray diffraction results of Fe3O4, Fe3O4@SiO2, paramagnetic cubic Fe3O4 material with a maximum Fe3O4-C and Fe3O4@SiO2-C are presented in figure magnetization of 92 emu/g.[48] In addition, Xu et al. S3. The results show that peaks appeared at the 2theta (2020) have shown that the magnetization of spherical angles of 30.09°, 35.42°, 43.05°, 53.39°, 56.94° and Fe3O4 nanoparticles is in the range of 68.43-75.86 62.51° are attributed to Fe3O4 phases, which are emu/g, while octagonal Fe3O4 nanoparticles will have respectively corresponding to plans of (220), (311), a higher magnetization (88.137 emu/g).[49] After (400), (422), (511) and (440) reported in Sharma et al. coating with SiO2, magnetization intensity of (2014).[43] This data is agreement with XRD spectrum Fe3O4@SiO2 was reduced to 23 emu/g. After using of Fe3O4-01-071-6336 and observation of Wei et al. Fe3O4 to harvest microalgae, Fe3O4-C material was (2012) for a synthesized Fe3O4.[44] As experiments created and the magnetization intensity was reduced were carried out under nitrogen aeration, peaks to 50 emu/g. Similarly, Fe3O4@SiO2-C material had a representing the formation of Fe2O3 was not detected. magnetization strength of 22 emu/g (figure S6). This This is an advantageous aspect of co-precipitation data is agreement with reduction of magnetization method, which could obtain single-phase materials. intensity of magnetic nanoparticles (MNPs) from The X-ray diffraction spectra figure S3 also show 49.51 emu/g to 19.32 emu/g when the materials were that Fe3O4@SiO2, Fe3O4-C, and Fe3O4@SiO2-C still used for harvesting Nannochloropsis maritime.[37] retain the same crystal structure as Fe3O4. It was The decomposition of the materials with heat noteworthy that X-ray diffraction spectra of increased from room temperature to 900oC is shown Fe3O4@SiO2 and Fe3O4@SiO2-C dis not display the in Figure S7. The results revealed that the Fe3O4 appearance of SiO2 crystals. However, Si were material lost 6% of its mass after increasing heat to detected and measured in Fe3O4@SiO2 and 400 oC and maintained a constant mass of 96% of its Fe3O4@SiO2-C by EDS as percentage of 25.49 and original mass until temperature increased to 900oC. 16.34%, respectively (figure S4-b, figure S4-d and The Fe3O4-C material exhibited the same table S1). The data from EDS measurement of Fe3O4 decomposition spectrum as Fe3O4. However, it lost material was agreed with the X-ray diffraction more than 10% of the mass when the temperature rised spectrum, displaying that the purity of Fe3O4 material to 400oC and only maintained a mass below 90% was monophase (figure S4-a). Fe3O4-C and when heating temperature reached 900oC. The © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 121
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Recovery of Chlorella biomass using … obtained data is reasonable because the Fe3O4-C Figure S9 indicates that plane (422) was disappeared composite contains microalgae biomass, which is from XRD spectrum of Fe3O4 regenerated from 5th organic, so it will be decomposed easily at cycle (figure S9-a). Whereas, crystal structure of temperatures above 200 oC. Fe3O4@SiO2 was remained the same as Fe3O4 over Similarly, the material Fe3O4@SiO2 contained five recycling rounds (figure S9-b), demonstrating SiO2 layer, so the mass lost up to 10% when the that Fe3O4@SiO2 was a durable adsorbent for temperature rised to 150oC and remained the constant harvesting microalgae. Ion leaching analysis for Fe3O4 mass until 900oC. The Fe3O4@SiO2-C material revealed that iron concentration was detected in adsorbed algae biomass, so the mass of decomposed aqueous suspension from the third regenerated cycle matter increased to more than 12% when the heat level at 2.14 mg/L, which was increased to 6.32 and 15.99 rised to 180oC, and only maintained a mass of less than mg/L at the fourth and fifth cycles (table S2), 88% of its initial mass when the heat level rised to respectively. Similarity, Silicon was found to be 900oC. The obtained results were completely leached from Fe3O4@SiO2 to aqueous suspension at consistent with the algae harvesting data, which was 0.41 mg/L measured for the third cycle, followed by further confirmed microalgal cells successfully increasing to 1.25 and 3.45 mg/L when regeneration adsorbed to the surface of the nanomaterials. increased to fourth and fifth rounds (table S2), Figure S8 shows the FT-IR spectra of Fe3O4, respectively. Notably, iron was not detected in all Fe3O4@SiO2, Fe3O4-C, and Fe3O4@SiO2-C samples. suspensions of five cycling generations, confirming Bare Fe3O4 nanoparticles with a peak appeared at 447 that core Fe3O4 was stably protected by SiO2 shell. cm–1 belonged to the elongation- oscillation zone of This data further demonstrates that the use of the Fe–O bond, which is characteristic of the Fe3O4 Fe3O4@SiO2 for harvesting microalgal C. sorokiniana nanomaterial. In addition, there is no occurrence of the TH01 is more advantageous when compared to that of characteristic 632 cm–1 peak of Fe2O3 material,[50] Fe3O4. demonstrating the formation of monophase Fe3O4 and has high stability in the synthesis process. Strong 4. CONCLUSION peaks appeared at 1399 cm-1 on Fe3O4 and Fe3O4@SiO2 were attributed to O–H bending. The Magnetic nanoparticles Fe3O4 and Fe3O4@SiO2 were absorption peaks at 583 cm-1 correspond to the successfully synthesized and applied for harvesting C. bending oscillations of Fe–O (Figure S8-a,d) and the sorokiniana TH01 biomass. The optimal C. elongated oscillations of Si–OH.[51] At the same time sorokiniana TH01 harvesting conditions for Fe3O4 the emergence of new peaks at wavelengths 1095 cm– were pH of 5, adsorbent dosage of 0.5 g/L, initial 1 is characteristic of asymmetric elastic oscillations of biomass concentration of 2-2.5 g/L, temperature 25 oC Si–O–Si bonds (figure S8-b and figure S8-d).[52] The and reaction time 20-30 min. For Fe3O4@SiO2, the presence of Si–O–Si and Si–OH bonds is an evidence optimal conditions were pH of 7, adsorbent dosage of to SiO2 being successfully coated on Fe3O4 0.6 g/L, initial biomass concentration of 2-2.5 g/L, nanoparticles by chemical bonds. The absorption temperature 25 oC and reaction time 20-30 min. Under bands at 3381 cm-1 were associated with the optimal conditions, Fe3O4 and Fe3O4@SiO2 can be corresponding –OH expansion and deformation regenerated up to three cycles with harvesting oscillations. These bands indicated the existence of efficiency till remained over 55%. Adsorption of C. hydroxyl groups connected to the surfaces of Fe3O4 sorokiniana TH01 on Fe3O4 and Fe3O4@SiO2 and Fe3O4@SiO2 nanoparticles.[53] Absorption bands exhibited the best fitness with Langmuir isotherm at 1622 cm-1 were associated with stretching and yielding R2 and maximal adsorption capacities of deformation of –OH groups.[54] In addition, the 800 0.995 and 6.55 g/g and 0.9971 and 9.53 g/g, cm-1 centered adsorption bands were related to respectively. Fe3O4, Fe3O4@SiO2, Fe3O4-C and bending of the C–H bond in microalgae biomass Fe3O4@SiO2-C exhibited super-magnetization attached to Fe3O4 nanoparticles and Fe3O4@SiO2 in intensities (> 22 emu/g), demonstrating an excellent Fe3O4-C (figure S8-b) and Fe3O4@SiO2-C materials separation capability with external magnetic devices. (figure S8-d). C-H binding is characteristic of organic Moreover, Fe3O4@SiO2 was demonstrated as a stable matter in algae biomass that can be derived from fats, and durable adsorbent during five regenerated cycles. proteins, carbohydrates.[55] Overall, harvesting microalgae with the synthesized magnetic nanoparticle Fe3O4@SiO2 is promising. 3.4. Stability of adsorbents Supplemental Material. Supplemental material for The stability of the adsorbents after the adsorption this article is available online. process was investigated by XRD. Data shown in © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 122
25728288, 2023, S3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202300068 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Tran Dang Thuan et al. REFERENCES High-quality Chlorella vulgaris biomass harvesting through chitosan and polyacrylamide, Environ. Sci. 1. M. Mofijur, M. G. Rasul, N. M. S. Hassan, M. N. Nabi. Pollut. Res., 2022, 29, 34651-34658. Recent development in the production of third 15. L. Zhu, Z. Li, E. Hiltunen. Microalgae Chlorella generation biodiesel from microalgae, Energy vulgaris biomass harvesting by natural flocculant: Procedia, 2019, 156, 53-58. effects on biomass sedimentation, spent medium 2. H. Chowdhury, B. Loganathan. Third-generation recycling and lipid extraction, Biotechnol. 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