Bio-compatible flotation of Chlorella vulgaris: Study of zeta potential and flotation efficiency

The energy-intensive dewatering of algae biomass, the first step of most downstream processes, remains one of the big challenges for economically relevant photoautotrophic bioprocesses. Due to its scalability and easy construction, froth flotation using the interactions between cells and bubbles shows considerable potential for this type of cost-efficient initial dewatering step. Comprehensive knowledge on both the physico-chemical conditions and the cellular surface properties are an important precondition to harvest cells by flotation. This study investigates the impact of changing the medium composition, specifically varying the pH and adding (bio-) collectors, on the zeta potential of Chlorella vulgaris SAG 211-1b. Decreasing the pH value from physiological to acidic conditions (pH 1–1.5) resulted in a strongly reduced cellular zeta potential. As validated by dispersed-air flotation, this yields a significantly enhanced cell recovery R > 95 %. The impact of the synthetic collector cetyltrimethylammonium bromide and the biopolymer chitosan on the cellular zeta potential and flotation performance was studied, resulting in a 3.3-fold decrease in the surfactant dose when chitosan was used . The basic mechanisms of cell-chitosan interaction were analysed in terms of particle size distribution and surface tension measurements, revealing interactions between flocculation and adsorption during the dispersed-air flotation of C.vulgarisSAG 211-1b.