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.
Matsuda S1, Durney AR2, He L3, Mukaibo H4.Show Affiliations
- 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.
- 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.
- 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]
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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