Effects of Algal Hydrophobicity and Bubble Size on Flotation Separation of Microalgae from Aqueous Medium

Microalgae have great potential to be a feedstock for producing biofuel. One of the major impediments towards the industrial-scale manufacturing of biofuel from microalgae is the ineffective separation of microalgae from water. In this communication, we first carried out froth flotation tests to harvest freshwater microalgae (Chlorella sp. BR2) and marine microalgae (Tetraselmis sp. M8) from dilute culture with and without a collector, tetradecyl trimethylammonium bromide (C14TAB). The surface hydrophobicity of microalgae was measured by using a modified adherence-to-hydrocarbon method. If no collector was added, BR2 showed high hydrophobicity, and its flotation tests in a mechanically agitated cell produced an algal concentrate with an enrichment ratio of 13.5 and 90.3% algae recovery. The natural hydrophobicity of M8 was low, so was its flotation recovery (6.4%). Addition of C14TAB improved M8 recovery to 81.7% with a low enrichment ratio of 2.0. Overall, the flotation performance correlated well with algal hydrophobicity. In search of more effective collectors for marine algae flotation, we measured the hydrophobicity of M8 in aqueous solutions of dodecylammonium hydrochloride (DAH) at different pHs and concentrations. It was found that addition of dodecylammonium hydrochloride (DAH) at 25 ppm and pH 6 significantly enhanced the hydrophobicity of M8. Subsequent mechanical flotation results confirmed that at this chemical condition, M8 enrichment ratio was increased to 5.8 with 86.8% algae recovery. Further improvement was achieved by using a Jameson cell with relatively small air bubbles. The Jameson flotation for M8 gave an enrichment ratio of 11.4 with 97.4% algae recovery.