A simple and rapid harvesting method for microalgae by in situ magnetic separation

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.

Perspectives on cultivation and harvesting technologies of microalgae, towards environmental sustainability and life cycle analysis

The development of algae is seen as a potential and ecologically sound approach to address the increasing demands in multiple sectors. However, successful implementation of processes is highly dependent on effective growing and harvesting methods. The present study provides a complete examination of contemporary techniques employed in the production and harvesting of algae, with a particular emphasis on their sustainability. The review begins by examining several culture strategies, encompassing open ponds, closed photobioreactors, and raceway ponds. The analysis of each method is conducted in a systematic manner, with a particular focus on highlighting their advantages, limitations, and potential for expansion. This approach ensures that the conversation is in line with the objectives of sustainability. Moreover, this study explores essential elements of algae harvesting, including the processes of cell separation, dewatering, and biomass extraction. Traditional methods such as centrifugation, filtration, and sedimentation are examined in conjunction with novel, environmentally concerned strategies including flocculation, electro-coagulation, and membrane filtration. It evaluates the impacts on the environment that are caused by the cultivation process, including the usage of water and land, the use of energy, the production of carbon dioxide, and the runoff of nutrients. Furthermore, this study presents a thorough examination of the current body of research pertaining to Life Cycle Analysis (LCA) studies, presenting a perspective that emphasizes sustainability in the context of algae harvesting systems. In conclusion, the analysis ends up with an examination ahead at potential areas for future study in the cultivation and harvesting of algae. This review is an essential guide for scientists, policymakers, and industry experts associated with the advancement and implementation of algae-based technologies.

Coagulation as an effective method for cyanobacterial bloom control: A review

Eutrophication, the over-enrichment with nutrients, for example, nitrogen and phosphorus, of ponds, reservoirs and lakes, is an urgent water quality issue. The most notorious symptom of eutrophication is a massive proliferation of cyanobacteria, which cause aquatic organism death, impair ecosystem and harm human health. The method considered to be most effective to counteract eutrophication is to reduce external nutrient inputs. However, merely controlling external nutrient load is insufficient to mitigate eutrophication. Consequently, a rapid diminishing of cyanobacterial blooms is relied on in-lake intervention, which may encompass a great variety of different approaches. Coagulation/flocculation is the most used and important water purification unit. Since cyanobacterial cells generally carry negative charges, coagulants are added to water to neutralize the negative charges on the surface of cyanobacteria, causing them to destabilize and precipitate. Most of cyanobacteria and their metabolites can be removed simultaneously. However, when cyanobacterial density is high, sticky secretions distribute outside cells because of the small size of cyanobacteria. The sticky secretions are easily to form complex colloids with coagulants, making it difficult for cyanobacteria to destabilize and resulting in unsatisfactory treatment effects of coagulation on cyanobacteria. Therefore, various coagulants and coagulation methods were developed. In this paper, the focus is on the coagulation of cyanobacteria as a promising tool to manage eutrophication. Basic principles, applications, pros and cons of chemical, physical and biological coagulation are reviewed. In addition, the application of coagulation in water treatment is discussed. It is the aim of this review article to provide a significant reference for large-scale governance of cyanobacterial blooms. PRACTITIONER POINTS: Flocculation was a promising tool for controlling cyanobacteria blooms. Basic principles of four kinds of flocculation methods were elucidated. Flocculant was important in the flocculation process.

Advances in magnetic materials for microplastic separation and degradation

The widespread use of plastics in modern human society has led to severe environmental pollution with microplastics (MP/MPs). The rising consumption of plastics raises the omnipresence of microplastics in aquatic environments, which carry toxic organic matter, transport toxic chemicals, and spread through the food chain, seriously threatening marine life and human health. In this context, several advanced strategies for separating and degrading MPs from water have been developed recently, and magnetic materials and their nanostructures have emerged as promising materials for targeting, adsorbing, transporting, and degrading MPs. However, a comprehensive review of MP remediation using magnetic materials and their nanostructures is currently lacking. The present work provides a critical review of the recent advances in MP removal/degradation using magnetic materials. The focus is on the comparison and analysis of the MP's removal efficiencies of different magnetic materials, including iron/ferrite nanoparticles, magnetic nanocomposites, and micromotors, aiming to unravel the underlying roles of magnetic materials in different types of MP degradation and present the general strategies for designing them with optimal performance. Finally, the review outlines the forthcoming challenges and perspectives in the development of magnetic nanomaterials for MP remediation.

Synergies of pH-induced calcium phosphate precipitation and magnetic separation for energy-efficient harvesting of freshwater microalgae

Energy- and time-consuming concentration steps currently limit the industrial application of microalgae. Compared to state-of-the-art technologies, magnetic separation shows a high potential for efficient harvesting of microalgae. This study presents a novel approach to combine pH-induced calcium phosphate precipitation with cheap natural magnetite microparticles for magnetic separation of the freshwater microalgae Chlorella vulgaris. Harvesting efficiencies up to 98% were achieved at moderate pH and low particle and calcium phosphate concentrations in a model medium. However, cultivation-dependent high loads of algogenic organic matter can severely inhibit flocculation and particle/algae interactions, requiring higher salt concentrations or pH. Harvesting efficiencies above 90% were still attainable at moderate pH with increased calcium phosphate concentrations of 10mM. Acidification of the suspension to pH 5 allows for simple and reversible particle recycling. The presented process provides a promising path to universal and cost-effective harvesting, advancing the utilization of microalgae as a sustainable bioresource.

Prospects of microalgae in nutraceuticals production with nanotechnology applications

Nutraceutical supplements provide health benefits, such as fulfilling the lack of nutrients in the human body or being utilized to treat or cure certain diseases. As the world population is growing, certain countries are experiencing food crisis challenges, causing natural foods are not sustainable to be used for nutraceutical production because it will require large-scale of food supply to produce enriched nutraceutics. The high demand for abundant nutritional compounds has made microalgae a reliable source as they can synthesize high-value molecules through photosynthetic activities. However, some microalgae species are limited in growth and unable to accumulate a significant amount of biomass due to several factors related to environmental conditions. Therefore, adding nanoparticles (NPs) as a photocatalyst is considered to enhance the yield rate of microalgae in an energy-saving and economical way. This review focuses on the composition of microalgal biomass for nutraceutical production, the health perspectives of nutritional compounds on humans, and the application of nanotechnology on microalgae for improved production and harvesting. The results obtained show that microalgal-based compounds indeed have better nutrients content than natural foods. However, nanotechnology must be further comprehended to make them non-hazardous and sustainable.

Fe3O4-PEI Nanocomposites for Magnetic Harvesting of Chlorella vulgaris, Chlorella ellipsoidea, Microcystis aeruginosa, and Auxenochlorella protothecoides

Magnetic separation of microalgae using magnetite is a promising harvesting method as it is fast, reliable, low cost, energy-efficient, and environmentally friendly. In the present work, magnetic harvesting of three green algae (Chlorella vulgaris, Chlorella ellipsoidea, and Auxenochlorella protothecoides) and one cyanobacteria (Microcystis aeruginosa) has been studied. The biomass was flushed with clean air using a 0.22 μm filter and fed CO2 for accelerated growth and faster reach of the exponential growth phase. The microalgae were harvested with magnetite nanoparticles. The nanoparticles were prepared by controlled co-precipitation of Fe2+ and Fe3+ cations in ammonia at room temperature. Subsequently, the prepared Fe3O4 nanoparticles were coated with polyethyleneimine (PEI). The prepared materials were characterized by high-resolution transmission electron microscopy, X-ray diffraction, magnetometry, and zeta potential measurements. The prepared nanomaterials were used for magnetic harvesting of microalgae. The highest harvesting efficiencies were found for PEI-coated Fe3O4. The efficiency was pH-dependent. Higher harvesting efficiencies, up to 99%, were obtained in acidic solutions. The results show that magnetic harvesting can be significantly enhanced by PEI coating, as it increases the positive electrical charge of the nanoparticles. Most importantly, the flocculants can be prepared at room temperature, thereby reducing the production costs.

Latest Expansions in Lipid Enhancement of Microalgae for Biodiesel Production: An Update

Research progress on sustainable and renewable biofuel has gained motion over the years, not just due to the rapid reduction of dwindling fossil fuel supplies but also due to environmental and potential energy security issues as well. Intense interest in microalgae (photosynthetic microbes) as a promising feedstock for third-generation biofuels has grown over recent years. Fuels derived from algae are now considered sustainable biofuels that are promising, renewable, and clean. Therefore, selecting the robust species of microalgae with substantial features for quality biodiesel production is the first step in the way of biofuel production. A contemporary investigation is more focused on several strategies and techniques to achieve higher biomass and triglycerides in microalgae. The improvement in lipid enhancement in microalgae species by genetic manipulation approaches, such as metabolic or genetic alteration, and the use of nanotechnology are the most recent ways of improving the production of biomass and lipids. Hence, the current review collects up-to-date approaches for microalgae lipid increase and biodiesel generation. The strategies for high biomass and high lipid yield are discussed. Additionally, various pretreatment procedures that may aid in lipid harvesting efficiency and improve lipid recovery rate are described.

Recent advances in microbial capture of hydrogen sulfide from sour gas via sulfur-oxidizing bacteria

Biological desulfurization offers several remarkably environmental advantages of operation at ambient temperature and atmospheric pressure, no demand of toxic chemicals as well as the formation of biologically re-usable sulfur (S0), which has attracted increasing attention compared to conventionally physicochemical approaches in removing hydrogen sulfide from sour gas. However, the low biomass of SOB, the acidification of process solution, the recovery of SOB, and the selectivity of bio-S0 limit its industrial application. Therefore, more efforts should be made in the improvement of the BDS process for its industrial application via different research perspectives. This review summarized the recent research advances in the microbial capture of hydrogen sulfide from sour gas based on strain modification, absorption enhancement, and bioreactor modification. Several efficient solutions to limitations for the BDS process were proposed, which paved the way for the future development of BDS industrialization.

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.

Optimization of Microalga Chlorella vulgaris Magnetic Harvesting

Harvesting of microalgae is a crucial step in microalgae-based mass production of different high value-added products. In the present work, magnetic harvesting of Chlorella vulgaris was investigated using microwave-synthesized naked magnetite (Fe₃O₄) particles with an average crystallite diameter of 20 nm. Optimization of the most important parameters of the magnetic harvesting process, namely pH, mass ratio (m_r) of magnetite particles to biomass (g/g), and agitation speed (rpm) of the C. vulgaris biomass-Fe₃O₄ particles mixture, was performed using the response surface methodology (RSM) statistical tool. Harvesting efficiencies higher than 99% were obtained for pH 3.0 and mixing speed greater or equal to 350 rpm. Recovery of magnetic particles via detachment was shown to be feasible and the recovery particles could be reused at least five times with high harvesting efficiency. Consequently, the described harvesting approach of C. vulgaris cells leads to an efficient, simple, and quick process, that does not impair the quality of the harvested biomass.

Microalgal bio-flocculation: present scenario and prospects for commercialization

The need for sustainable production of renewable biofuel has been a global concern in the recent times. Overcoming the tailbacks of the first- and second-generation biofuels, third-generation biofuel using microalgae as feedstock has emerged as a plausible alternative. It has an added advantage of preventing any greenhouse gas (GHG) emissions with simultaneous carbon dioxide sequestration. Dewatering of microalgal culture is one of the many concerns regarding industrial-scale biofuel production. The small size of microalgae and dilute nature of its growth cultures creates huge operational cost during biomass separation, limiting economic feasibility of algae-based fuels. Considering the recovery efficiency, operation economics, technological feasibility and cost-effectiveness, bio-flocculation is a promising method of harvesting. Moreover, advantage of bio-flocculation over other conventional methods is that it does not incur the addition of any external chemical flocculants. This article reviews the current status of bio-flocculation technique for harvesting microalgae at industrial scale. The various microbial strains that can be prospective bioflocculants have been reviewed along with its application and advantages over chemical flocculants. Also, this article proposes that the primary focus of an appropriate harvesting technique should depend on the final utilization of the harvested biomass. This review article attempts to bring forth the beneficial aspects of microbial aided microalgal harvesting with a special attention on genetically modified self-flocculation microalgae.

Methods for separating microplastics from complex solid matrices: Comparative analysis

Microplastics (MPs) are widely found in complex solid matrices such as soil, sediments and sludge. The separation procedure is crucial for effective analysis of MPs, but existing methods varied among studies. Here, we systematically summarize and compare separation methods including density, oil, electrostatic, magnetic, and solvent extraction separation. Density separation is the most commonly used approach, but time-consuming and discharging hazardous materials dependent on extraction solutions. In contrast, oil, electrostatic, magnetic separation and solvent extraction separation are emerging approaches with advantages of low-cost, quick, or environmentally-friendly, but with high request of instruments. Despite variation among these approaches, the separation efficiency is closely related to characteristics of MPs including polymer types, sizes and shapes. The treatment of digestion and fluorescence staining can facilitate the detection of MPs. This analysis suggests that further optimization and improvement of existing approaches can facilitate the development of new separation technology for assaying MPs in complex environmental matrices.

Magnetic seeding coagulation: Effect of Al species and magnetic particles on coagulation efficiency, residual Al, and floc properties

Magnetic seeding coagulation (MSC) process has been used to accelerate flocs sedimentation with an applied magnetic field, offering large handling capacity and low energy consumption. The interactions of three typical Al species, aluminum chloride (AlCl₃), Al₁₃O₄(OH)₂₄⁷⁺ polymer (Al₁₃), and (AlO₄)₂Al₂₈(OH)₅₆¹⁸⁺ polymer (Al₃₀), with magnetic particles (MPs) were examined to clarify the MSC process. In traditional coagulation (TC) process, the aggregation of primary Al_a-dissolved organic matter (DOM) complexes with in-situ-formed polynuclear species generated a large average floc size (226 μm), which was proved to be efficient for DOC removal (52.6%). The weak connections between dissolved Al_a-DOM complexes and MPs led to the negligible changes of dissolved Al after seeding with MPs in AlCl₃. A significant interaction between MPs and Al₁₃ was observed, in which the MPs-Al₁₃-DOM complexes were proposed to be responsible for the significant improvement of DOC removal (from 47% to 52%) and residual total Al reduction (from 1.05 to 0.27 mg Al L^-1) with MPs addition. Al₃₀ produced a lower floc fractal dimension (D_f = 1.88) than AlCl₃ (2.08) and Al₁₃ (1.99) in the TC process, whereas its floc strength (70.9%) and floc recovery (38.5%) were higher than the others. Although more detached fragments were produced with MPs addition, the effective sedimentation of these fragments with the applied magnetic field led to the decrease of residual turbidity and colloidal Al in Al₃₀. The dependence of coagulation behavior to MPs and different Al species can be applied to guide the application of an effective MSC process.

Wastewater based microalgal biorefinery for bioenergy production: Progress and challenges

Treatment of industrial and domestic wastewater is very important to protect downstream users from health risks and meet the freshwater demand of the ever-increasing world population. Different types of wastewater (textile, dairy, pharmaceutical, swine, municipal, etc.) vary in composition and require different treatment strategies. Wastewater management and treatment is an expensive process; hence, it is important to integrate relevant technology into this process to make it more feasible and cost-effective. Wastewater treatment using microalgae-based technology could be a global solution for resource recovery from wastewater and to provide affordable feedstock for bioenergy (biodiesel, biohydrogen, bio-alcohol, methane, and bioelectricity) production. Various microalgal cultivation systems (open or closed photobioreactors), turf scrubber, and hybrid systems have been developed. Although many algal biomass harvesting methods (physical, chemical, biological, and electromagnetic) have been reported, it is still an expensive process. In this review article, resource recovery from wastewater using algal cultivation, biomass harvesting, and various technologies applied in converting algal biomass into bioenergy, along with the various challenges that are encountered are discussed in brief.

Cationic cassava starch and its composite as flocculants for microalgal biomass separation

Commercial- and laboratory modified- cationic cassava starches and their composites with magnetic particles were examined for characteristics and separation efficiency. Scanning electron micrographs showed that cationic starch with an increasing degree of substitution (DS) value (0.0180 to 0.91) showed greater clumped polyhedral granules and became markedly enlarged with disintegrated boundaries. Zeta potential analysis revealed that the increase in the DS value in cationic starches resulted in an increase in positive charge. The maximum harvesting efficiency of 92.86 ± 0.46% was achieved when commercial cationic starch with DS 0.040 at 1.0 g L^-1 was added to the Chlorella sp. solution. The maximum recovery capacity (10.20 ± 0.16 g DCW g starch^-1) was recorded by using commercial cationic starch with DS 0.040 at a lower dosage of 0.1 g L^-1. Their composites showed lower separation efficiency than the commercial cationic starches. The results suggest that the commercial cationic cassava starch with 0.040 DS shows great potential as a flocculant for algal separation. This first report of using commercial cationic cassava starch as a flocculant provides a low cost and convenient process to separate algal cells from the culture medium. Moreover, uncontaminated magnetic particle biomass allows for wide range of algal utilization in food and pharmaceutical biotechnologies.

Interaction energy and detachment of magnetic nanoparticles-algae

Magnetic separation, a promising bioseparation technology, is confronted with the challenges in recovery and recycle of magnetic matters during algae harvesting for biofuel extraction. The thermodynamic method was used to characterize the surface interactions between MNPs and algae cells. Three methods were adopted to detach magnetic nanoparticles-algae (Microcystis aeruginosa, Synechocystis sp., Nannochloropsis maritima and Stigeoclonium sp.) and recover magnetic nanoparticles (MNPs) in this study. The thermodynamic method indicated that the greatest adhesion strength was expected for Stigeoclonium sp. on MNPs. High detachment efficiency of MNP-algae was achieved by ultrasonic-extracting, which got above 90% after 5 recycles. Moreover, the harvesting efficiencies of these four algae cells could remain more than 90% after 5 recycles using a mixture of the regenerated and the raw MNPs.

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.

A comprehensive review on cultivation and harvesting of microalgae for biodiesel production: Environmental pollution control and future directions

Biodiesel is one of the best promising candidates in response to the energy crisis, since it has the capability to minimize most of the environmental problems. Microalgae, as the feedstock of third-generation biodiesel, are considered as one of the most sustainable resources. However, microalgae production for biodiesel feedstock on a large scale is still limited, because of the influences of lipid contents, biomass productivities, lipid extraction technologies, the water used in microalgae cultivation and processes of biomass harvesting. This paper firstly reviews the recent advances in microalgae cultivation and growth processes. Subsequently, current microalgae harvesting technologies are summarized and flocculation mechanisms are analyzed, while the characteristics that the ideal harvesting methods should have are summarized. This review also summarizes the environmental pollution control performances and the key challenges in future. The key suggestions and conclusions in the paper can offer a promising roadmap for the cost-effective biodiesel production.

Ultrasound-assisted coagulation for Microcystis aeruginosa removal using Fe3O4-loaded carbon nanotubes

Harmful cyanobacterial blooms are increasing environmental issues and require novel removal technology since the required doses of algaecides may cause further environmental pollution or treatment facility damage. Herein, we firstly introduce the combination of ultrasound and Fe₃O₄/CNTs as an alternative strategy to enhance coagulation for the removal of Microcystis aeruginosa cells in water. It remarkably enhanced cyanobacterial cell removal and microcystins control, compared with sonication alone (40 kHz ultrasonic bath, 4.2 mJ mL⁻¹). 94.4% cyanobacterial cells were removed using 20 second sonication with 20 mg L⁻¹ Fe₃O₄/CNTs, Al₂(SO₄)₃ coagulation (20 μM). Both sonication time and catalyst dose significantly influenced the cyanobacterial removal. Ultrasound with Fe₃O₄/CNTs only induced a slight increase of cell permeability, which may contribute to the effective control of DOC and microcystins' release in water. The enhanced settlement of the cyanobacterial cells may result from the moderate oxidation on the cell surface. This study suggested a novel ultrasound-Fe₃O₄/CNT process to promote cyanobacteria removal with efficient DOC and microcystin release control, which is a green and safe technology for drinking water treatment.

The combination of sonication and Fe₃O₄/CNTs were applied on Microcystis aeruginosa removal for the first time.

Bare Iron Oxide Nanoparticles: Surface Tunability for Biomedical, Sensing and Environmental Applications

Surface modification is widely assumed as a mandatory prerequisite for the real applicability of iron oxide nanoparticles. This is aimed to endow prolonged stability, electrolyte and pH tolerance as well as a desired specific surface chemistry for further functionalization to these materials. Nevertheless, coating processes have negative consequences on the sustainability of nanomaterial production contributing to high costs, heavy environmental impact and difficult scalability. In this view, bare iron oxide nanoparticles (BIONs) are arousing an increasing interest and the properties and advantages of pristine surface chemistry of iron oxide are becoming popular among the scientific community. In the authors’ knowledge, rare efforts were dedicated to the use of BIONs in biomedicine, biotechnology, food industry and environmental remediation. Furthermore, literature lacks examples highlighting the potential of BIONs as platforms for the creation of more complex nanostructured architectures, and emerging properties achievable by the direct manipulation of pristine iron oxide surfaces have been little studied. Based on authors’ background on BIONs, the present review is aimed at providing hints on the future expansion of these nanomaterials emphasizing the opportunities achievable by tuning their pristine surfaces.

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.

Removal of radioactive cesium from an aqueous solution via bioaccumulation by microalgae and magnetic separation

We evaluated the potential sequestration of cesium (Cs⁺) by microalgae under heterotrophic growth conditions in an attempt to ultimately develop a system for treatment of radioactive wastewater. Thus, we examined the effects of initial Cs⁺ concentration (100–500 μM), pH (5–9), K⁺ and Na⁺ concentrations (0–20 mg/L), and different organic carbon sources (acetate, glycerol, glucose) on Cs⁺ removal. Our initial comparison of nine microalgae indicated that Desmodesmus armatus SCK had removed the most Cs⁺ under various environmental conditions. Addition of organic substrates significantly enhanced Cs⁺ uptake by D. armatus, even in the presence of a competitive cation (K⁺). We also applied magnetic nanoparticles coated with a cationic polymer (polyethylenimine) to separate ¹³⁷Cs-containing microalgal biomass under a magnetic field. Our technique of combining bioaccumulation and magnetic separation successfully removed more than 90% of the radioactive ¹³⁷Cs from an aqueous medium. These results clearly demonstrate that the method described here is a promising bioremediation technique for treatment of radioactive liquid waste.

Potential role of N-acetyl glucosamine in Aspergillus fumigatus-assisted Chlorella pyrenoidosa harvesting

BACKGROUND

Algal harvesting is a major cost which increases biofuel production cost. Algal biofuels are widely studied as third-generation biofuel. However, they are yet not viable because of its high production cost which is majorly contributed by energy-intensive biomass harvesting techniques. Biological harvesting method like fungal-assisted harvesting of microalgae is highly efficient but poses a challenge due to its slow kinetics and poorly understood mechanism.

RESULTS

In this study, we investigate Aspergillus fumigatus-Chlorella pyrenoidosa attachment resulting in a harvesting efficiency of 90% within 4 h. To pinpoint the role of extracellular metabolite, several experiments were performed by eliminating the C. pyrenoidosa or A. fumigatus spent medium from the C. pyrenoidosa-A. fumigatus mixture. In the absence of A. fumigatus spent medium, the harvesting efficiency dropped to 20% compared to > 90% in the control, which was regained after addition of A. fumigatus spent medium. Different treatments of A. fumigatus spent medium showed drop in harvesting efficiency after periodate treatment (≤ 20%) and methanol-chloroform extraction (≤ 20%), indicating the role of sugar-like moiety. HR-LC-MS (high-resolution liquid chromatography-mass spectrometry) results confirmed the presence of N-acetyl-d-glucosamine (GlcNAc) and glucose in the spent medium. When GlcNAc was used as a replacement of A. fumigatus spent medium for harvesting studies, the harvesting process was significantly faster (p < 0.05) till 4 h compared to that with glucose. Further experiments indicated that metabolically active A. fumigatus produced GlcNAc from glucose. Concanavalin A staining and FTIR (Fourier transform infrared spectroscopy) analysis of A. fumigatus spent medium- as well as GlcNAc-incubated C. pyrenoidosa cells suggested the presence of GlcNAc on its cell surface indicated by dark red dots and GlcNAc-specific peaks, while no such characteristic dots or peaks were observed in normal C. pyrenoidosa cells. HR-TEM (High-resolution Transmission electron microscopy) showed the formation of serrated edges on the C. pyrenoidosa cell surface after treatment with A. fumigatus spent medium or GlcNAc, while Atomic force microscopy (AFM) showed an increase in roughness of the C. pyrenoidosa cells surface upon incubation with A. fumigatus spent medium.

CONCLUSIONS

Results strongly suggest that GlcNAc present in A. fumigatus spent medium induces surface changes in C. pyrenoidosa cells that mediate the attachment to A. fumigatus hyphae. Thus, this study provides a better understanding of the A. fumigatus-assisted C. pyrenoidosa harvesting process.

Synthesis and Characterization of Flower-like Carbon-encapsulated Fe-C Nanoparticles for Application as Adsorbing Material

Carbon-encapsulated Fe-C (Fe-C@C) nanoparticles with a divergently flower-like morphology were successfully synthesized for application as an adsorbing material by using freeze-drying and chemical vapor deposition (CVD) methods. The Fe metallic source was first loaded onto a sodium chloride (NaCl) supporter via freeze-drying to obtain the Fe/NaCl composite powder. Then, Fe-C@C nanoparticles were synthesized in the temperature range of 300–450 °C via CVD of acetylene in the Fe/NaCl composite powder using Fe nanoparticles as catalysts and NaCl as supporters. Because the NaCl supporter is water-soluble, the synthesized Fe-C@C nanoparticles were easy to purify, and a high purity was obtained by simple washing and centrifugation. The optimal Fe-C@C nanoparticles, synthesized at 400 °C, possessed a unique divergently flower-like structure and a high specific surface area of 169.4 m²/g that can provide more adsorption sites for contaminants. Adsorption experiments showed that the flower-like Fe-C@C adsorbent exhibited high adsorption capacity (90.14 mg/g) and fast removal of methylene blue (MB). Moreover, the magnetic properties of the nanoparticles, with saturation magnetization of 36.544 emu/g, facilitated their magnetic separation from wastewater. Therefore, the novel flower-like Fe-C@C nanoparticles with integrated adsorptive and magnetic properties have the potential to be an effective adsorbent in dye wastewater treatment.

Biodegradable branched cationic starch with high C/N ratio for Chlorella vulgaris cells concentration: Regulating microalgae flocculation performance by pH

To improve the carbon to nitrogen (C/N) ratio of harvested microalgae biomass for better producing biogas by fermentation, biodegradable cationic starch with high C/N ratio were synthesized to harvest Chlorella vulgaris. The impact of pH was also studied as the zeta potential of both microalgae and cationic starch would change with pH. Results indicated the cationic starch can harvest above 99% of the microalgae and the C/N ratio can rise from 7.50 to 7.90. The zeta potential of microalgae always kept negative and presented a trend of descending firstly and then upgrade. The maximum microalgae biomass flocculation capacity of 1 g cationic starch was 8.62 g with the help of self-flocculation at pH 3. The concentration of flocs formed at pH 11 was 25.74 g L^-1 and the diameter was 0.553 mm which was much larger than the flocs formed at pH 3 (0.208 mm).

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.