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

Nano-enabled microalgae bioremediation: Advances in sustainable pollutant removal and value-addition

Microalgae-assisted bioremediation, enriched by nanomaterial integration, offers a sustainable approach to environmental pollution mitigation while harnessing microalgae's potential as a biocatalyst and biorefinery resource. This strategy explores the interaction between microalgae, nanomaterials, and bioremediation, advancing sustainability objectives. The potent combination of microalgae and nanomaterials highlights the biorefinery's promise in effective pollutant removal and valuable algal byproduct production. Various nanomaterials, including metallic nanoparticles and semiconductor quantum dots, are reviewed for their roles in inorganic and organic pollutant removal and enhancement of microalgae growth. Limited studies have been conducted to establish nanomaterial's (CeO2, ZnO, Fe3O4, Al2O3, etc.) role on microalgae in pollution remediation; most studies cover inorganic pollutants (heavy metals and nutrients) remediation, exhibited 50-300% bioremediation efficiency improvement; however, some studies cover antibiotics and toxic dyes removal efficiency with 19-95% improvement. These aspects unveil the complex mechanisms underlying nanomaterial-pollutant-microalgae interactions, focusing on adsorption, photocatalysis, and quantum dot properties. Strategies to enhance bioremediation efficiency are discussed, including pollutant uptake improvement, real-time control, tailored nanomaterial design, and nutrient recovery. The review assesses recent advancements, navigates challenges, and envisions a sustainable future for bioremediation, underlining the transformative capacity of nanomaterial-driven microalgae-assisted bioremediation. This work aligns with Sustainable Development Goals 6 (Clean Water and Sanitation) and 12 (Responsible Consumption and Production) by exploring nanomaterial-enhanced microalgae bioremediation for sustainable pollution management and resource utilization.

New insights into Chlorella vulgaris applications

Environmental pollution is a big challenge that has been faced by humans in contemporary life. In this context, fossil fuel, cement production, and plastic waste pose a direct threat to the environment and biodiversity. One of the prominent solutions is the use of renewable sources, and different organisms to valorize wastes into green energy and bioplastics such as polylactic acid. Chlorella vulgaris, a microalgae, is a promising candidate to resolve these issues due to its ease of cultivation, fast growth, carbon dioxide uptake, and oxygen production during its growth on wastewater along with biofuels, and other productions. Thus, in this article, we focused on the potential of Chlorella vulgaris to be used in wastewater treatment, biohydrogen, biocement, biopolymer, food additives, and preservation, biodiesel which is seen to be the most promising for industrial scale, and related biorefineries with the most recent applications with a brief review of Chlorella and polylactic acid market size to realize the technical/nontechnical reasons behind the cost and obstacles that hinder the industrial production for the mentioned applications. We believe that our findings are important for those who are interested in scientific/financial research about microalgae.

Chemotactic movement and zeta potential dominate Chlamydomonas microsphaera attachment and biocathode development

Microalgal cell attaching and biofilm formation are critical in the application of microalgal biocathode, which severs as one of the hopeful candidates to an original cathode in bioelectrochemical systems. Many efforts have been put in biofilm formation and bioelectrochemical systems for years, but the predominant factors shaping microalgal biocathode formation are sketchy. We launched a pair of researches to investigate microalgal attachment and biofilm formation in the presence/absence of applied voltages using Chlamydomonas microsphaera as a model unicellular motile microalga. In this study, we presented how microalga attached and biofilm formed on a carbon felt surface without applied voltages and try to manifest the most important aspects in this process. Results showed that while nutrient sources did not directly regulate cell attachment onto the carbon felt, limited initial nutrient concentration nevertheless promoted cell attachment. Specifically, nutrient availability did not influence the early stage (20-60 min) of microalgal cell attachment but did significantly impact cell attachment during later stages (240-720 min). Further analysis revealed that nutrient availability-mediated chemotactic movements and zeta potential are crucial to facilitate the initial attachment and subsequent biofilm formation of C. microsphaera onto the surfaces, serving as an important factor controlling microalgal surface attachment. Our results demonstrate that nutrient availability is a dominant factor controlling microalgal surface attachment and subsequent biofilm formation processes. This study provides a mechanistic understanding of microalgal surface attachment and biofilm formation processes on carbon felts surfaces in the absence of applied voltages.

Nano magnetite assisted flocculation for efficient harvesting of lutein and lipid producing microalgae biomass

For commercial scale algal biorefining, harvesting cost is a major bottleneck. Thus, a cost-effective, less-energy intensive, and efficient harvesting method is being investigated. Among several harvesting methods, magnetic flocculation offers the benefits of modest operation, energy savings and quick separation. This study aims to develop novel magnetite-(Fe₃O₄) nanoparticles (MNPs) of 20 nm average size and their high reusability potential to reduce the harvesting cost of microalgae biomass. The MNPs were synthesized and characterized using FTIR, Zeta analyzer, and SEM before performing on Chlorella sorokiniana Kh12 and Tu5. For maximum harvesting efficiency >99%, the optimal culture pH, MNPs concentration, and agitation speed were 3, 200 mg/L, and 450 rpm, respectively. Subsequently, MNPs were recovered via alkaline treatment and reused up to 5 cycles as they retained their reactivity and harvesting efficiency. The studied MNPs-based harvesting method could be adopted at a commercial scale for cost-effective algae biorefinery in the future.

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.

Rare earth element sequestration by Aspergillus oryzae biomass

The fungus Aspergillus oryzae could be shown to be a viable alternative for biosorption of valuable metals from solution. Fungal biomass can be obtained easily in high quantities as a waste of biofermentation processes, and used in a complex, multi-phase solution mimicking naturally occurring, mining-affected water samples. With test solution formulated after natural conditions, formation of secondary Al and Fe phases co-precipitating Ce was recorded in addition to specific biosorption of rare earth elements. Remarkably, the latter were removed from the solution despite the presence of high concentrations of interfering Fe and Al. The biomass was viable even after prolonged incubation in the metal solution, and minimal inhibitory concentrations for single metals were higher than those in the test solution. While precipitation/biosorption of Ce (maximal biosorption efficiency was 58.0 ± 22.3% after 6 h of incubation) coincided with the gross removal of Fe from the metal solution, Y (81.5 ± 11.3% efficiency, 24 h incubation) and Nd (87.4 ± 9.1% efficiency, 24 h incubation) were sequestered later, similarly to Ni and Zn. The biphasic binding pattern specific to single metals could be connected to dynamically changing pH and NH₄⁺ concentrations, which were attributed to the physiological changes taking place in starving A. oryzae biomass. The metals were found extracellularly in minerals associated with the cell wall, and intracellularly precipitated in the vacuoles. The latter process was explained with intracellular metal detoxification resulting in metal resistance.

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.

Single-step dynamic dewatering of microalgae from dilute suspensions using flocculant assisted filtration

Background

Dewatering constitutes a major challenge to the production of microalgae, accounting for 20–30% of the product cost. This presents a setback for the applicability of microalgae in the development of several sustainable products. This study presents an investigation into the dynamic dewatering of microalgae in a combined flocculation-filtration process. The effect of process conditions on the performance of 12 flocculants and their mixtures was assessed.

Results

The mechanism of flocculation via the electrostatic path was dominated by charge neutralization and subsequently followed bridging in a ‘sweep flocculation’ process. Cationic polyacrylamide (CPAM) based flocculants recorded the highest biomass retention with PAM1 and PAM2 attaining 99 and 98% retention with flocculant dosages of 10 and 15 mg/L respectively. Polyvinylamine (PVAM) was also found to improve system stability across the pH range 4–10. Alum was observed to be only effective in charge neutralization, bringing the system close to its isoelectric point (IEP). Chemometric analysis using the multi-criteria decision methods, PROMETHEE and GAIA, was applied to provide a sequential performance ranking based on the net outranking flow (ф) from 207 observations. A graphical exploration of the flocculant performance pattern, grouping the observations into clusters in relation to the decision axis (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi$$\end{document}π), which indicated the weighted resultant of most favorable performance for all criteria was explored.

Conclusion

CPAM based flocculants and their mixtures demonstrated superior performance due to their viscoelastic behaviour under turbulence. The use of PVAM or alum in mixtures with CPAM reduced the required doses of both flocculants, which will provide beneficial financial impact for largescale microalgae dewatering in a flocculant assisted dynamic filtration process. Chemometric analysis based on the physico-chemical properties of the system provides a time saving assessment of performance across several criteria. The study findings provide an important foundation for flocculant assisted dynamic filtration processes.

Unified View of Magnetic Nanoparticle Separation under Magnetophoresis

The migration process of magnetic nanoparticles and colloids in solution under the influence of magnetic field gradients, which is also known as magnetophoresis, is an essential step in the separation technology used in various biomedical and engineering applications. Many works have demonstrated that in specific situations, separation can be performed easily with the weak magnetic field gradients created by permanent magnets, a process known as low-gradient magnetic separation (LGMS). Due to the level of complexity involved, it is not possible to understand the observed kinetics of LGMS within the classical view of magnetophoresis. Our experimental and theoretical investigations in the last years unravelled the existence of two novel physical effects that speed up the magnetophoresis kinetics and explain the observed feasibility of LGMS. Those two effects are (i) cooperative magnetophoresis (due to the cooperative motion of strongly interacting particles) and (ii) magnetophoresis-induced convection (fluid dynamics instability originating from inhomogeneous magnetic gradients). In this feature article, we present a unified view of magnetophoresis based on the extensive research done on these effects. We present the physical basis of each effect and also propose a classification of magnetophoresis into four distinct regimes. This classification is based on the range of values of two dimensionless quantities, namely, aggregation parameter N* and magnetic Grashof number Gr_m, which include all of the dependency of LGMS on various physical parameters (such as particle properties, thermodynamic parameters, fluid properties, and magnetic field properties). This analysis provides a holistic view of the classification of transport mechanisms in LGMS, which could be particularly useful in the design of magnetic separators for engineering applications.

Microalgae harvesting by magnetic flocculation for biodiesel production: current status and potential

With the increasing demand for energy, microalgae, as one of the promising feedstocks of biodiesel, has raised great awareness. Because of its small size, similar density to water and electrical stability, harvesting methods of microalgae that have low energy consumption and that are highly efficient, easy to large-scale and environmentally friendly have become a bottleneck restricting development of the whole process. Among the numerous possible harvesting methods, magnetic flocculation has the advantages of simple operation, fast separation and energy saving and thus is considered as a promising novel harvesting method. In this review, we have summarized the updated status and application potential of magnetic flocculation, including the principle of magnetic flocculation, magnetic flocculating materials, flocculating efficiency and its effect on downstream process. The major challenges such as magnetic materials recovery, large-scale magnetic flocculation device design, and magnetic flocculation costs are also discussed.

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.

Temporal variation of vegetation at two operating landfills and its implications for landfill phytoremediation

Landfilling is a popular method to dispose of solid waste around the world, but this practice is associated with inevitable negative consequences such as air and water pollution and potential health hazards. Phytoremediation is an economical and promising technology to address these problems. To select suitable species for restoring landfills in China, the vegetation characteristics and their physiological activities were investigated in two different landfills. The results showed that the vegetation coverage of the two landfills both increased with the closure time. The species diversity decreased with the closure time in the Chongming landfill, whereas it showed the opposite trend in the Jiangcungou landfill. Five dominant species (Conyza canadensis, Setaria viridis, Sesbania cannabina, Suaeda glauca, and Phragmites australis) and four dominant species (Festuca elata Keng, Amaranthus retroflexus, Lolium multiflorum, and Kochia scoparia) were identified in the Chongming landfill and Jiangcungou landfill, respectively, and these dominant species were mainly annual plants. In addition, the net photosynthetic rate of P. australis and K. scoparia was the highest in the Chongming landfill and Jiangcungou landfill, respectively. Their photosynthetic rates were influenced significantly by stomatal conductance. Based on the overall results, among the dominant species identified, P. australis and K. scoparia represent potentially well suited plants to use in the remediation of landfills in China, because of their strong adaptability to landfill environments and natural occurrence in landfills.

Magnetophoretic Harvesting of Nannochloropsis oculata Using Iron Oxide Immobilized Beads

In this work, the harvesting of Nannochloropsis oculata microalgae through the use of nanosized Fe3O4 immobilized in polyvinyl alcohol (PVA)/sodium alginate (SA) as a flocculant (Fe3O4/PS) is investigated. Using the Fe3O4/PS immobilized beads could reduce the amount of soluble ferrous ions (Fe2+) released from naked Fe3O4 in acid treatment, leading to easy recovery. The characterization was performed under different dosages and pH values of Fe3O4/PS. The results show that the Fe3O4/PS, when applied to the algae culture (500 mg dry cell weight/L), achieves a 96% harvesting efficiency under conditions of a pH of 4 with 200 mT magnetic field intensity. Fe3O4/PS can be directly reused without adjusting the pH value. The recycled Fe3O4/PS shows stability in terms of its surface properties, maintaining more than 80% harvesting efficiency after five recycles. Magnetophoretic harvesting, using immobilized magnetic iron oxide as a particle-based flocculant, is a potential method to reduce challenges related to the cost-effective microalgae-harvesting method.

Factors Affecting Microalgae Production for Biofuels and the Potentials of Chemometric Methods in Assessing and Optimizing Productivity

Microalgae are swift replicating photosynthetic microorganisms with several applications for food, chemicals, medicine and fuel. Microalgae have been identified to be suitable for biofuels production, due to their high lipid contents. Microalgae-based biofuels have the potential to meet the increasing energy demands and reduce greenhouse gas (GHG) emissions. However, the present state of technology does not economically support sustainable large-scale production. The biofuel production process comprises the upstream and downstream processing phases, with several uncertainties involved. This review examines the various production and processing stages, and considers the use of chemometric methods in identifying and understanding relationships from measured study parameters via statistical methods, across microalgae production stages. This approach enables collection of relevant information for system performance assessment. The principal benefit of such analysis is the identification of the key contributing factors, useful for decision makers to improve system design, operation and process economics. Chemometrics proffers options for time saving in data analysis, as well as efficient process optimization, which could be relevant for the continuous growth of the microalgae industry.

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.

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.

Microalgae Chlorella vulgaris biomass harvesting by natural flocculant: effects on biomass sedimentation, spent medium recycling and lipid extraction

BACKGROUND

Microalgal biomass harvesting using traditional chemicals is costly for the production of biofuels, hindering the scale-up process of the technology. Thus, the search for a cost-effective microalgal harvesting method is extremely important. Using chitosan as a natural flocculant to harvest microalgal biomass seems to be an efficient and convenient solution. Although microalgal biomass flocculation by chitosan has been reported in some previous studies, literature on the harvesting of microalgae C. vulgaris biomass using such polymer is scanty. In addition, there is limited information available on whether the usage of chitosan during the harvesting will affect downstream lipid extraction. Still, whether microalgae can be re-grown with the spent medium after chitosan flocculation is still unknown.

RESULTS

In this study, microalgal biomass harvesting using chitosan as a natural flocculant and aluminum sulfate as a traditional flocculant was compared and evaluated. Optimal doses and effects on biomass sedimentation, spent medium recycling and lipid extraction were investigated. The results showed that the optimal doses for chitosan and aluminum sulfate to achieve more than 90% biomass recovery were 0.25 and 2.5 g/L, respectively. The sedimentation time of 10 min was found to be the most appropriate to remove over 90% biomass from culture. The spent medium after chitosan flocculation could be potentially recycled for the re-cultivation of microalgae, which demonstrated robust growth in comparison with those grown in the recycled medium from aluminum sulfate flocculation. The lipid content of microalgae harvested by chitosan reached 32.9, 4.6% higher than that of those harvested by aluminum sulfate, indicating that the application of the natural flocculant would not impact the downstream extraction of microalgal lipids.

CONCLUSION

The results herein presented, demonstrated that chitosan is applicable for microalgal harvesting during the upscaling process. Flocculation method developed by using chitosan as a natural flocculant is a worthy microalgal harvesting method for microalgae-based biofuel production. There is hope that chitosan can be reasonably and technically realistically applied in a full-scale process for the harvesting of microalgal biomass.