Effect of barium ferrite particle size on detachment efficiency in magnetophoretic harvesting of oleaginous Chlorella sp

Advancements in Nano-Enhanced microalgae bioprocessing

Microalgae are promising sources of valuable compounds: carotenoids, polyunsaturated fatty acids, lipids, etc. To overcome the feasibility challenge due to low yield and attain commercial potential, researchers merge technologies to enhance algal bioprocess. In this context, nanomaterials are attractive for enhancing microalgal bioprocessing, from cultivation to downstream extraction. Nanomaterials enhance biomass and product yields (mainly lipid and carotenoids) through improved nutrient uptake and stress tolerance during cultivation. They also provide mechanistic insights from recent studies. They also revolutionize harvesting via nano-induced sedimentation, flocculation, and flotation. Downstream processing benefits from nanomaterials, improving extraction and purification. Special attention is given to cost-effective extraction, showcasing nanomaterial integration, and providing a comparative account. The review also profiles nanomaterial types, including metallic nanoparticles, magnetic nanomaterials, carbon-based nanomaterials, silica nanoparticles, polymers, and functionalized nanomaterials. Challenges and future trends are discussed, emphasizing nanomaterials' role in advancing sustainable and efficient microalgal bioprocessing, unlocking their potential for bio-based industries.

Technologies for harvesting the microalgae for industrial applications: Current trends and perspectives

Microalgae are emerging as a promising source for augmenting the supply of essential products to meet global demands in an environmentally sustainable manner. Despite the potential benefits of microalgae in industry, the high energy consumption for harvesting remains a significant obstacle. This review offers a comprehensive overview of microalgae harvesting technologies and their industrial applications, with particular emphasis on the latest advances in flocculation techniques. These cutting-edge methods have been applied to biodiesel production, food and nutraceutical processing, and wastewater treatment. Large-scale harvesting is still severely impeded by the high cost despite progress has been made in laboratory studies. In the future, cost-effective microalgal harvesting will rely on efficient resource utilization, including the use of waste materials and the reuse of media and flocculants. Additionally, precise regulation of biological metabolism will be necessary to overcome algal species-related limitations through the development of extracellular polymeric substance-induced flocculation technology.

Nanomagnetic approach applied to microalgae biomass harvesting: advances, gaps, and perspectives

Microalgae biomass is a versatile option for a myriad of purposes, as it does not require farmable land for cultivation and due of its high CO₂ fixation efficiency during growth. However, biomass harvesting is considered a bottleneck in the process because of its high cost. Magnetic harvesting is a promising method on account of its low cost, high harvesting speed, and efficiency, which can be used to improve the results of other harvesting methods. Here, we present the state of the art of the magnetic harvesting method. Detailed approaches involving different nanomaterials are described, including types, route of synthesis, and functionalization, variables that interfere with harvesting, and recycling methods of nanoparticles and medium. In addition to discussing the overall perspectives of the method, we provide a guideline for future research.

Algal cells harvesting using cost-effective magnetic nano-particles

Innovative iron-based nanoparticles were synthesized, characterized and tested for the first time for harvesting single and mixed algal culture from real wastewater. The tailor-made magnetic nanoparticles (MNPs; Fe-MNP-I and Fe-MNP-II) achieved a percentage algae harvesting efficiency (%AHE) higher than 95% using a concentration of MNPs (C_MNP) of 25 ± 0.3 (std. dev = 0.08) mg.L^-1, mixing speed (M_speed) of 120 ± 2 (std. dev = 0.10) rpm, short contact time (C_t) of 7 ± 0.1 (std. dev = 0.05) min and separation time (SP_t) of 3 ± 0.1 (std. dev = 0.09) min. The optimum operational conditions for harvesting of Chlorella vulgaris (C.v) were determined at (C_MNP = 40 ± 0.4 (std. dev = 0.5) g_MNPs.L^-1, SP_t = 2.5 ± 0.4 (std. dev = 0.1) min, M_speed = 145 ± 3 (std. dev = 1.50) rpm and C_t = 5 ± 0.3 (std. dev = 0.10) min using surface response methodology. Langmuir model describes better the adsorption behavior of algae-Fe-MNP-I system, while both Langmuir and Freundlich fit well the adsorption behavior of algae-Fe-MNP-II. The maximum adsorption capacity of Spirulina platensis (SP.PL) (18.27 ± 0.07 (std. dev = 0.19) mg_DWC.mg_particles^-1) was higher than that for Chlorella vulgaris (C.v) (11.52 ± 0.01 (std. dev = 0.34) mg_DWC.mg_particles^-1) and mixed algal culture (M.X) (17.20 ± 0.07 (std. dev = 0.54) mg_DWC.mg_particles^-1) over Fe-MNP-I. Zeta potential measurements revealed that the adsorption mechanism between MNPs and algal strains is controlled by electrostatic interaction. The synthesized MNPs were recycled 10 times using alkaline-ultrasonic regeneration procedure.

Efficient harvesting of Chlorella pyrenoidosa and Scenedesmus obliquus cultivated in urban sewage by magnetic flocculation using nano-Fe3O4 coated with polyethyleneimine

In this work, a novel flocculation process by using nano-Fe₃O₄ coated with polyethyleneimine (Fe₃O₄@PEI) as magnetic seeds was developed to harvest the microalgae cultivated in urban sewage. Experiment results indicated that the harvest efficiency of Chlorella pyrenoidosa (0.5 g/L) was 98.92 ± 0.41% under the optimal conditions of Fe₃O₄@PEI dose of 20 mL/L, flocculation time of 20 min, and stirring speed of 800 rpm (3 min), while that of Scenedesmus obliquus (0.4 g/L) was 98.45 ± 0.35% under a Fe₃O₄@PEI dose of 16 mL/L, flocculation time of 15 min, and stirring speed of 730 rpm (3 min). Moreover, the process did not reduce the lipid content of microalgae and quality of biodiesel. After microalgae harvest, Fe₃O₄@PEI could be recovered by ultrasonication, re-wrapped with polyethyleneimine and reused to reduce operational cost.

The use of magnetic iron oxide based nanoparticles to improve microalgae harvesting in real wastewater

A novel approach for harvesting Scenedesmus sp. microalgae from real wastewater by using adsorbents of magnetite-based nanoparticles (Fe₃O₄ NPs) was tested in this study for the first time for this microalgae. Using these NPs, the harvesting efficiency was even higher than 95%. The optimal conditions (0.14 gNPs/L, a short magnetic separation time of only 8 min and 27 min of contact time) were found using the response surface methodology. The best fitting of the adsorption equilibrium results was achieved by the Langmuir isotherm model, and the maximum adsorption capacity for Scenedesmus sp. reached 3.49 g dry cell weight (DCW)/g Fe₃O₄ NPs. Zeta potential measurements and the Dubinin-Radushkevich isotherm model analysis pointed out that the main adsorption mechanism between Scenedesmus sp. cells and Fe₃O₄ NPs was electrostatic interaction. Finally, Fe₃O₄ NPs were six times successfully reused by combining an alkaline treatment with an ultrasonication process, which implies microalgae lysis. The results herein obtained highlight the potential for magnetic separation of microalgae from wastewater, which is capable of reaching a high harvesting efficiency in a very short time.

Flocculation Harvesting Techniques for Microalgae: A Review

Microalgae have been considered as one of the most promising biomass feedstocks for various industrial applications such as biofuels, animal/aquaculture feeds, food supplements, nutraceuticals, and pharmaceuticals. Several biotechnological challenges associated with algae cultivation, including the small size and negative surface charge of algal cells as well as the dilution of its cultures, need to be circumvented, which increases the cost and labor. Therefore, efficient biomass recovery or harvesting of diverse algal species represents a critical bottleneck for large-scale algal biorefinery process. Among different algae harvesting techniques (e.g., centrifugation, gravity sedimentation, screening, filtration, and air flotation), the flocculation-based processes have acquired much attention due to their promising efficiency and scalability. This review covers the basics and recent research trends of various flocculation techniques, such as auto-flocculation, bio-flocculation, chemical flocculation, particle-based flocculation, and electrochemical flocculation, and also discusses their advantages and disadvantages. The challenges and prospects for the development of eco-friendly and economical algae harvesting processes have also been outlined here.

Using magnetic materials to harvest microalgal biomass: evaluation of harvesting and detachment efficiency

Using naked iron oxide (Fe₃O₄) and yttrium iron oxide (Y₃Fe₅O₁₂) nanoparticles as flocculants, the harvesting efficiency of Chlorella vulgaris biomass was investigated. The harvesting process includes two steps, which are the separation of microalgae from the culture solution with the magnetic nanoparticles and then the separation of the algae from the magnetic nanoparticles. The optimal dosages and pH values for the magnetic harvesting of microalgal biomass were determined. Results showed that Y₃Fe₅O₁₂ nanoparticles were more efficient in microalgal biomass harvesting than Fe₃O₄ nanoparticles. In an effort to achieve more than 90% of harvesting efficiency, optimal dosages for Fe₃O₄ and Y₃Fe₅O₁₂ to harvest microalgal biomass were 10 and 2.5 g/L, while the appropriate pH values were 6.2 and 7.3, respectively. The harvesting efficiency of Fe₃O₄ and Y₃Fe₅O₁₂ nanoparticles increased as the pH value decreased. The experimental results also showed that under a higher pH value Fe₃O₄ nanoparticles were much easier to be separated from the flocs than Y₃Fe₅O₁₂. 62.9% of Fe₃O₄ nanoparticles could be de-attached from the aggregates, when the floc pH value reached 12.3.

Harvesting of Chlorella vulgaris using Fe3O4 coated with modified plant polyphenol

The Chlorella vulgaris harvesting was explored by magnetic separation using Fe₃O₄ particles coated with the plant polyphenol chemically modified by a Mannich reaction followed by quaternization (Fe₃O₄@Q-PP). The -N(R)₄⁺ and Cl-N⁺-C perssad of the Q-PP were linked to the Fe₃O₄ particles by N-O bonds, as suggested by the X-ray photoelectron spectroscopy spectra. The thermogravimetric analysis displayed the mass percentage of the Q-PP coated on the Fe₃O₄ surface was close to ~ 5%. Compared with the naked Fe₃O₄ particles, zeta potentials of the Fe₃O₄@Q-PP particles were improved from the range of - 17.5~- 25.6 mV to 1.9~36.3 mV at pH 2.1~13.1. A 70.2 G coercive force was obtained for the Fe₃O₄@Q-PP composite, which demonstrated its ferromagnetic behavior. The use of Fe₃O₄@Q-PP resulted in a harvesting efficiency of 90.9% of C. vulgaris cells (3.06 g/L). The Fe₃O₄ particles could be detached from the cell flocs by ultrasonication leading to a recovery efficiency of 96.1% after 10 cycles. The recovered Fe₃O₄ could be re-coated with Q-PP and led to a harvesting efficiency of 80.2% after 10 cycles. The magnetic separation using Fe₃O₄@Q-PP included charge neutralization followed by bridging and then colloid entrapment.

Bare Iron Oxide Nanoparticles for Magnetic Harvesting of Microalgae: From Interaction Behavior to Process Realization

Microalgae continue to gain in importance as a bioresource, while their harvesting remains a major challenge at the moment. This study presents findings on microalgae separation using low-cost, easy-to-process bare iron oxide nanoparticles with the additional contribution of the upscaling demonstration of this simple, adhesion-based process. The high affinity of the cell wall for the inorganic surface enables harvesting efficiencies greater than 95% for Scenedesmus ovalternus and Chlorella vulgaris. Successful separation is possible in a broad range of environmental conditions and primarily depends on the nanoparticle-to-microalgae mass ratio, whereas the effect of pH and ionic strength are less significant when the mass ratio is chosen properly. The weakening of ionic concentration profiles at the interphase due to the successive addition of deionized water leads the microalgae to detach from the nanoparticles. The process works efficiently at the liter scale, enabling complete separation of the microalgae from their medium and the separate recovery of all materials (algae, salts, and nanoparticles). The current lack of profitable harvesting processes for microalgae demands innovative approaches to encourage further development. This application of magnetic nanoparticles is an example of the prospects that nanobiotechnology offers for biomass exploitation.

The effects of extracellular polymeric substances on magnetic iron oxide nanoparticles stability and the removal of microcystin-LR in aqueous environments

The behaviors of nanoparticles rely on the aqueous condition such as natural organic matter (NOM). Therefore the presence of NOM would influence the interaction of nanoparticles with other substances possibly. Here, microcystin-LR (MC-LR) adsorption on iron oxide nanoparticles (IONPs) was studied in an aqueous solution with different types of NOM, including extracellular polymeric substances (EPS) from cyanobacteria and alginic acid sodium salt (AASS) from brown algae. Results revealed that EPS played an important role in stabilizing IONPs and in the toxin adsorption efficiency. The stability of IONPs was heavily depended on the concentration and type of NOM, which can affect the surface charge of IONPs significantly in solution. The enhanced stability of IONPs was due to the electrostatic interactions. Adsorption kinetics and isotherm studies confirmed that NOM can affect the IONPs' adsorption efficiency, and pseudo-second-order kinetics better explained this process. The removal efficiency for MC-LR decreased in the presence of NOM (Control > EPS-M1 > AASS > EPS-M9), indicating that NOM and MC-LR compete for limited adsorption sites. The presence of NOM in a eutrophic environment stabilized the IONPs while inhibiting the MC-LR removal efficiency. This investigation emphasized the negative effect of cyanobacterial EPS on the removal of microcystins when using magnetic separation technology. And this results could also be used to model the transportation of iron minerals carrying toxic substances in aqueous environment.

Sedimentation-induced detachment of magnetite nanoparticles from microalgal flocs

The objective of this study is to develop a simple, one-step approach to separate adsorbed Fe3O4 nanoparticles from microalgal flocs for further downstream processing. Using the wild-type strain of fresh-water algae Chlamydomonas reinhardtii, effective removal of nanoparticles from microalgal flocs by both centrifugal sedimentation (at 1500 or 2000g) and magnetic sedimentation (at 1500 Oe) is demonstrated. At the physiological pH of the solution (i.e., pH 7.0), where the electrostatic force between the nanoparticles and the microalgal cells is strongly attractive, larger separation force was achieved by simply increasing the density and viscosity of the solution to 1.065g/mL and 1.244cP, respectively. The method described here offers significant opportunity for purifying microalgal biomass after nanoparticle-flocculation-based harvesting and decreasing the cost of microalgal biotechnology. This may also find avenues in other applications that apply flocculation, such as algal biofilm formation in photobioreactors and wastewater treatment.

Microfluidic Design of Magnetoresponsive Photonic Microcylinders with Multicompartments

Colloidal photonic structures have been designed to have granular format to use them for paint pigments, encoded carriers, and display pixels. However, conventional approaches only provide spherical or discoid shapes, restricting their applications. Cylindrical granules with fan-shaped compartments in the cross section are appealing for microcarriers with abundant optical codes and active display pigments for color switching. In this work, a stratified laminar flow of concentrated silica particles is employed, formed in a cylindrical microchannel, to produce cylindrical photonic microparticles with multiple compartments. To accomplish this, a microfluidic device is designed to have one cylindrical main channel connected with four branch channels. Four different photocurable suspensions are independently injected through the branches to form quarter-cylindrically compartmentalized streams in the main channel. Local ultraviolet irradiation on the main channel polymerizes the suspension, thereby forming cylindrical microparticles with four compartments. In each compartment, silica particles form ordered array which develops particle size-dependent structural color. Therefore, different colors can be incorporated into single microcylinder by employing different sizes of silica particles. Moreover, one of the compartments can be rendered to be magnetoresponsive by embedding aligned magnetic particles, which enables the remote control of microcylinder orientation and therefore the switching of structural colors.

Magnetic-Nanoflocculant-Assisted Water-Nonpolar Solvent Interface Sieve for Microalgae Harvesting

Exploitation of magnetic flocculants is regarded as a very promising energy-saving approach to microalgae harvesting. However, its practical applicability remains limited, mainly because of the problem of the postharvest separation of magnetic flocculants from microalgal flocs, which is crucial both for magnetic-flocculant recycling and high-purity microalgal biomasses, but which is also a very challenging and energy-consuming step. In the present study, we designed magnetic nanoflocculants dually functionalizable by two different organosilane compounds, (3-aminopropyl)triethoxysilane (APTES) and octyltriethoxysilane (OTES), which flocculate negatively charged microalgae and are readily detachable at the water-nonpolar organic solvent (NOS) interface only by application of an external magnetic field. APTES functionalization imparts a positive zeta potential charge (29.6 mV) to magnetic nanoflocculants, thereby enabling microalgae flocculation with 98.5% harvesting efficiency (with a dosage of 1.6 g of dMNF/g of cells). OTES functionalization imparts lipophilicity to magnetic nanoflocculants to make them compatible with NOS, thus effecting efficient separation of magnetic flocculants passing through the water-NOS interface sieve from hydrophilic microalgae. Our new energy-saving approach to microalgae harvesting concentrates microalgal cultures (∼1.5 g/L) up to 60 g/L, which can be directly connected to the following process of NOS-assisted wet lipid extraction or biodiesel production, and therefore provides, by simplifying multiple downstream processes, a great potential cost reduction in microalgae-based biorefinement.

Application and reactivation of magnetic nanoparticles in Microcystis aeruginosa harvesting

This study developed a magnetic nanoparticles (MNPs) harvesting and reactivation technique for rapid cyanobacteria Microcystis aeruginosa separation. The harvesting of raw MNPs achieved high efficiency of 99.6% with the MNPs dosage of 0.58g MNPs/g dry-biomass, but gradually decreased to 59.1% when directly reused 5 times. With extra ultrasonic chloroform:methanol solvent treatment, the MNPs can be effectively reactivated for M. aeruginosa harvesting with 60% efficiency after 5 times reactivation and the separation efficiency kept above 93% with 0.20g MNPs/g dry-biomass dosage. The cyanobacteria-MNPs complex can be effectively disrupted by ultrasonic chloroform:methanol solvent treatment and the zeta potential was recovered for MNPs electrostatic attraction. The MNPs adsorption followed the Langmuir isotherm, and the maximum adsorption capacity and Langmuir constant was 3.74g dry-biomass/g and 311.64L/g respectively. This MNPs reactivation technique can achieve low energy separation and reduce MNPs consumption by 67%, providing potential engineering implementation for cyanobacterial biomass harvesting.

Recovering Magnetic Fe3O4-ZnO Nanocomposites from Algal Biomass Based on Hydrophobicity Shift under UV Irradiation

Magnetic separation, one of the promising bioseparation technologies, faces the challenges in recovery and reuse of magnetic agents during algal harvesting for biofuel extraction. This study synthesized a steric acid (SA)-coated Fe3O4-ZnO nanocomposite that could shift hydrophobicity under UV365 irradiation. Our results showed that with the transition of surface hydrophobicity under UV365 irradiation, magnetic nanocomposites detached from the concentrated algal biomass. The detachment was partially induced by the oxidation of SA coating layers due to the generation of radicals (e.g., •OH) by ZnO under UV365 illumination. Consequently, the nanocomposite surface shifted from hydrophobic to hydrophilic, which significantly reduced the adhesion between magnetic particles and algae as predicted by the extended Derjaguin and Landau, Verwey, and Overbeek (EDLVO) theory. Such unique hydrophobicity shift may also find many other potential applications that require recovery, recycle, and reuse of valuable nanomaterials to increase sustainability and economically viability.

Recent nanoparticle engineering advances in microalgal cultivation and harvesting processes of biodiesel production: a review

Among the various steps entailed in the production of biodiesel from microalgae, the efficiency and cost-reduction of the cultivation and harvesting steps remain key obstacles to its practical commercialization. Recently, in order to overcome the technical bottlenecks and limitations with regard to both steps, nanoparticle engineering based on particles' unique physico-chemical and mechanical properties has been extensively applied as a powerful analytical and practical tool. These applications include the enhancement of cell growth and/or pigments by light back-scattering, the induction of intracellular lipid accumulation by nutritional competition and/or stress environment, the improvement of cell separation efficiency and processing time from culture broth, the multiple reuse of magnetic nanoparticle flocculant, and integrated one-pot harvesting/cell-disruption. This review presents and discusses the recent nanoparticle-engineering-based developments in the implementation of practical microalgal cultivation and harvesting processes.

Heteroaggregation between PEI-coated magnetic nanoparticles and algae: effect of particle size on algal harvesting efficiency

Colloidal interactions between magnetic nanoparticles (NPs) and algal cells are of paramount significance to magnetophoretic separation of algal biomass from water. This study evaluated the size effect of magnetic NPs (MNPs) coated with polyethylenimine (PEI) on the separation efficiency of Scenedesmus dimorphus as well as on the recovery efficiency of MNPs from algal biomass. Results showed that algal harvesting efficiency (HE) increased from ca. 60% to 85% as the diameter of PEI-coated MNPs increased from 9 to 53 nm. Likewise, algal recovery capacity (algae/MNPs, w/w) also showed the same size dependence. But a large size (247 nm) led to a decline of algal HE, which was correctly interpreted by a settling model that predicts large sizes of MNPs could eventually reduce the settling velocity under magnetophoresis. The extended Derjaguin-Landau-Verwey-Overbeek theory revealed that the particle size and PEI coating both influenced the interaction energies (e.g., energy barrier) between MNPs and algae. Particularly, PEI coating significantly reduced the energy barrier between MNPs and algae and thereby increased their heteroaggregation and algal HE. Moreover, PEI-coated MNPs were recovered from the harvested algae biomass through a chemical-free ultrasonic method, and the recovery efficiency appeared to be higher for larger MNPs. Overall, the synthesized sizes of applied MNPs will not only affect algal HE but also have economic implications on magnetophoretic algal separation technologies.

Influences of surface coating, UV irradiation and magnetic field on the algae removal using magnetite nanoparticles

Magnetophoretic separation is a promising and sustainable technology for rapid algal separation or removal from water. This work demonstrated the application of magnetic magnetite nanoparticles (MNPs) coated with a cationic polymer, polyethylenimine (PEI), toward the separation of Scenedesmus dimorphus from the medium broth. The influences of surface coating, UV irradiation, and magnetic field on the magnetophoretic separation were systematically examined. After PEI coating, zeta potential of MNPs shifted from −7.9 ± 2.0 to +39.0 ± 3.1 mV at a pH of 7.0, which improved MNPs-algae interaction and helped reduce the dose demand of MNPs (e.g., from 0.2 to 0.1 g·g(–1) while the harvesting efficiency (HE) of over 80% remained unchanged). The extended Derjaguin–Landau–Verwey–Overbeek theory predicted a strong attractive force between PEI-coated MNPs and algae, which supported the improved algal harvesting. Moreover, the HE was greater under the UV365 irradiation than that under the UV254, and increased with the irradiation intensity. Continuous application of the external magnetic field at high strength remarkably improved the algal harvesting. Finally, the reuse of MNPs for multiple cycles of algal harvesting was studied, which aimed at increasing the sustainability and lowering the cost.

Magnetoresponsive discoidal photonic crystals toward active color pigments

Photonic microdisks with a multilayered structure are designed from photocurable suspensions by step-by-step photolithography. In each step of photolithography, either a colloidal photonic crystal or a magnetic-particle-laden layer is stacked over the windows of a photomask. Sequential photolithography enables the creation of multilayered photonic microdisks that have brilliant structural colors that can be switched by an external magnetic field.

Repeated use of stable magnetic flocculant for efficient harvest of oleaginous Chlorella sp

In the present study, a simple magnetic-particle recycling strategy was developed for harvest of the oleaginous microalga Chlorella sp. KR-1. The method entails the flocculation of microalgal cells and bare-Fe3O4 magnetic particles (bMP) by electrostatic attraction and the subsequent recovery of the bMP from the harvested flocs by electrostatic repulsion below and above the isoelectric points (IEP), respectively. For 10 recycles, the bMP showed 94-99% and 90-97% harvest and recovery efficiencies, respectively. Furthermore, neither the use of bMP nor pH adjustment showed any adverse effect on the microalgal cell growth or the co-existing bacterial species, as confirmed from the subsequent medium-recycling test and denaturing gradient gel electrophoresis (DGGE) analysis.