Sedimentation-induced detachment of magnetite nanoparticles from microalgal flocs

Significance Statement

Microalgae are single-cell, photosynthetic organisms that can produce such products as biofuels, feedstocks, drugs, and chemicals. Once the products have been synthesized, the microalgae are harvested for further processing downstream. Since the microalgae are too small to be removed as they are, a mechanism known as flocculation is commonly employed. Flocculation can be achieved by mixing positively-charged magnetic nanoparticles (NPs) with negatively-charged microalgae, causing nanoparticles to adhere to microalgae and form flocs. Such flocs are easily separated from solution due to their larger size and attraction to external magnetic field. A problem with such separation process is that complex steps and/or toxic chemicals are used to recover the microalgae and the nanoparticles after flocculation. This paper discusses a simple one-step process that avoids toxic chemicals, and effectively separates the microalgae from nanoparticles so that both can be used again.

Dr. Hitomi Mukaibo and her team focused on the different forces that affect the nanoparticles and the microalgae as it is sinks down through a fluid (i.e., sedimentation force, flotation force, and viscous force). Since the microalgae are much larger than the nanoparticles, they experience larger flotation force and viscous force. Therefore, in a fluid with high density and viscosity, the microalgae experience a larger resistance for sinking than the smaller nanoparticles, and the faster sinking nanoparticles are pulled off from the microalgae surface.

In this paper, the process was conducted using wild-type Chlamydomonas Reinhardtii microalgae (CC124 cells), magnetite (Fe3O4) NPs, either a centrifugal or magnetic force as the sedimentation force, and Percoll® as a commercially available solution with high density and viscosity. Polyhydric alcohol and polysaccharide solution were suggested as alternatives for practical applications. Cell viability was confirmed with a fluorescence-based assay after each procedure. When a centrifugal force was applied to the NP-CC124 flocs, which were placed in a cuvette containing an upper layer of algal culture media and a lower layer of Percoll®, the smaller nanoparticles separated from the CC124 cells and sank to the bottom Percoll® layer in the cuvette, while the CC124 cells remained at the interface of the two layers.

In the case of magnetic sedimentation, the cuvette containing the NP-CC124 flocs in the algal culture media and Percoll® was placed on a ferrite magnet. Once again, the CC124 cells remained at the interface, while the nanoparticles were drawn into the lower Percoll®, towards the magnet, confirming that a magnetically induced tensile force is also effective at recovering ferromagnetic nanoparticles from microalgal flocs.

It was noted that the pH of the algal culture media during the NP-CC124 floc formation makes a difference in whether the nanoparticles could be successfully separated from the microalgal cells during magnetic sedimentation. When the pH is at 7.0 or higher, the binding force increases, resulting in NP-CC124 flocs that sink through the Percoll® layer in the cuvettes without separating. As discussed in the paper, the nanoparticles within the flocs formed under such circumstances could be separated by simply increasing the viscosity and the density of the lower solution layer.

Sedimentation-induced detachment of magnetite nanoparticles from microalgal flocs. Renewable Energy Global Innovations

About the author

Shofu Matsuda received a B.E. degree in the Department of Applied Chemistry from Waseda University, Tokyo Japan, in 2012. He received a M.E. in the Department of Nanoscience and Nanoengineering from Waseda University, in 2014. He is now a Ph.D. student in the Leading Graduate Program in Science and Engineering at Waseda University. He was also a visiting student for 3 months in the Development of Chemical Engineering at University of Rochester, NY USA, in 2015. He was awarded a prize for his presentation in the 11th International Conference on Ferrites. His research interests focus on design and evaluation of ferrite nanoparticles for their application to magnetic hyperthermia and biotechnology. 

About the author

Andrew Durney is a Ph.D. Candidate in the Department of Chemical Engineering at the University of Rochester. His research interests include nanofabrication, electrochemistry, and materials characterization with electron microscopy and x-ray spectroscopy techniques. For his thesis project he is developing an array of conical, hollow microneedles as a platform to deliver dyes, quantum dots, and DNA to microalgae cells. Andrew participated in the Integrative Graduate Education and Research Traineeship (IGERT), a University program funded by the National Science Foundation. He was awarded Honorable Mention by the NSF Graduate Research Fellowships Program. 

About the author

Lijie He graduated from Sun Yat-sen University with a degree in Physics. His research interest is in fundamental theories and simulations. Lijie is really fond of finite element analysis, molecular dynamic and Monte Carlos simulation. Thus he joined Prof. Mukaibo’s group and started modelling delivery process through micro needle array. He is recently admitted to the PhD program of materials science in University of Rochester. His future plan include pursuing the possibility of coupling the results from different scale of simulations and apply them in the field of chemical engineering and mechanical engineering. 

About the author

Hitomi Mukaibo joined the department of Chemical Engineering at University of Rochester in July, 2011. She earned her degrees in Engineering from Waseda University (Ph.D., 2006, M.E., 2004 and B.S., 2002). Her graduate research focus was in assessing the battery performance of Sn-based Li-ion battery anodes. During her successive postdoctoral period with Dr. Charles R. Martin at University of Florida, she changed her research focus from batteries to biosensors and bio/nanoscience. She studied polymeric thin films with nanometric through-holes as a platform to capture and assess biological molecules. These porous films were also used as templates to prepare an array of vertical nanoneedles for gene delivery into microalgal cells. Her current research interests at the University of Rochester include materials science, electrochemistry, biosensors, bio/nanoscience, bioanalytical chemistry, energy and chemistry at interfaces. 

 

Journal Reference

Bioresour Technol. 2016;200:914-20. 

Matsuda S1, Durney AR2, He L3, Mukaibo H4.

Show Affiliations
  1. Department of Chemical Engineering, University of Rochester, P.O. Box 270166, Rochester, NY 14627, USA; Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.
  2. Department of Chemical Engineering, University of Rochester, P.O. Box 270166, Rochester, NY 14627, USA.
  3. Materials Science Program, University of Rochester, P.O. Box 270166, Rochester, NY 14627, USA.
  4. Department of Chemical Engineering, University of Rochester, P.O. Box 270166, Rochester, NY 14627, USA; Materials Science Program, University of Rochester, P.O. Box 270166, Rochester, NY 14627, USA. Electronic address: [email protected]

Abstract

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.

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