Self-assembly of Fe3O4 with natural tannin as composites for microalgal harvesting

Anisotropic structures and size control are attracting extensive attention owing to the effect on magnetic behavior and microalgal harvesting performance. Herein, a natural polymeric tannin was chosen as the shaping ligand to control the self-assembly of tannin-coated Fe3O4 (Fe3O4@tannin) for microalgal magnetic harvesting. Compared to naked Fe3O4, the Fe3O4@tannin presented a smaller hydrodynamic diameter of 287 nm, a larger magnetic saturation of 45.16 emu g(-1), and a higher uptake capacity of 589.99 mg biomass per g of Fe3O4@-tannin. Kinetic experimental data revealed that the magnetic harvesting followed a pseudo-second-order isotherm and the adsorption isotherm fitted the non-linear Freundlich model. The Fe3O4@tannin showed a good recycling potential with only a 7% decrease in harvesting efficiency after 5 cycles. The modified Deryaguin-Landau-Verwey-Overbeek (DLVO) interaction energies among the particles were used to evaluate the algal aggregation mechanism. Furthermore, the results indicated that the magnetic force contributed to the total interaction energies across a wide range of interaction distances. In situ microscopic observation also confirmed the formation of Fe3O4@tannin-to-algae aggregates, which further combined to form chain clusters and collided with free algal cells in the magnetophoretic process.

Automated summary

By using natural polymeric tannin as the shaping ligand, the self-assembly of tannin-coated Fe3O4 was controlled for microalgal magnetic harvesting. The tannin-coated Fe3O4 showed smaller size, higher magnetic saturation, and higher uptake capacity compared to bare Fe3O4. It also exhibited good recycling potential and the magnetic force played a critical role in the overall interaction energies.