Microbubble-microalgae attachment in ionic liquids (ILs) for efficient microalgae harvesting via foam flotation

Foam flotation has proven to be a promising method for harvesting microalgae on a commercial scale. However, high cost and low harvesting efficiency significantly impact the feasibility of this approach. Much of the harvesting performance hinges on the mechanics of bubble-microalgae attachment and the surfactants that facilitate this. This study investigates the microbubble-microalgae interaction (attachment) in ionic liquid of various carbon chain lengths. The dominance of hydrophobic attachment was evident when microalgae was most hydrophobic in ionic liquid 1,3-didecyl-2-methylimidazolium chloride ([C10C10MIM][CI]) of 90 ppm concentration. This coincided with the highest microbubble contact angle measurement of 40 degrees, while a lower angle of 31 degrees was observed during electrostatic attachment dominant conditions, at [[C10C10MIM][CI] concentration of 25 ppm. Gibb's Free energy was used to correlate the probability of attachment to contact angle measurements, indicating that the spontaneity of attachment occurring was higher at 90 ppm, with-7.61 mN/m, while at 25 ppm, the Gibb's free energy was only-4.98 mN/m. Furthermore, bench-scaled laboratory flotation results showed that at 90 ppm, the imidazolium-based ionic liquid [C10C10MIM][CI] achieved a maximum harvesting efficiency of 99 %, outperforming commercial surfactant cetyltrimethylammonium bromide (CTAB) by 15.2 %, further concurring the dominance of hydrophobic attachment of microbubble-microalgae. These results display the viability of ionic liquids as promising alternatives to commercial chemical surfactants.