Efficient microalgae harvesting by organo-building blocks of nanoclays

The synthesis of aminoclays with Mg²⁺ or Fe³⁺, placed in metal centers by sol-gel reaction with 3-aminopropyltriethoxysilane (APTES) as a precursor, is demonstrated, producing -(CH₂)₃NH ₂ organo-functional pendants which are covalent-bonding onto cationic metals. The protonated amine groups in aqueous solution lead the efficient sedimentation (harvesting) of microalgae biomass within approximately 5 min and 120 min for fresh and marine species, respectively. To our surprise, the aminoclays did not depend on microalgae species or media for microalgae harvesting. In particular, the harvesting efficiency (%) of microalgae was not decreased in a wide pH region. The harvesting mechanism can be explained by the sweep flocculation of microalgae, which is confirmed by measurement of zeta potential of aminoclay in aqueous solution where aminoclay shows a positively charged surface in a wide pH region. In order to reduce the cost of aminoclays and to make the harvesting procedures simple, the membrane process using aminoclay-coated cotton filter is introduced for the treatment of 1 L-scale microalgae stocks. It is successfully performed with three recycles using the same aminoclay-coated cotton filter after removing the harvested microalgae biomass. Conclusively, the aminoclay-based microalgae harvesting systems are a promising means of reducing the cost of downstream processes in microalgae-based biorefinery.