Reversible Flocculation of Microalgae using Magnesium Hydroxide

Abstract

23Flocculation of microalgae is a promising low-cost strategy to harvest microalgae for bulk 24 biomass production. However, residual flocculants can interfere in further downstream 25 processes or influence biomass quality. In this study, a new concept is demonstrated based on 26 reversible magnesium hydroxide flocculation, using Chlorella vulgaris and 27Phaeodactylum tricornutum as respectively a freshwater and a marine model species. We 28 show that flocculation was induced by precipitation of magnesium hydroxide at high pH (10 29 to 10.5). This resulted in a magnesium content of the microalgal biomass of 5% for Chlorella 30 and 18% for Phaeodactylum. After pre-concentration of the microalgal biomass by gravity 31 sedimentation, 95% of the precipitated magnesium hydroxide could be removed from the 32 biomass by mild acidification (pH 7 to 8). The pH fluctuations experienced by the microalgae 33 during flocculation/de-flocculation had no influence on biomass composition (FAME, total N 34 and P, carbohydrates, proteins, mineral content) and on the viability of microalgal cells. 35Magnesium can thus be used as pH-dependent reversible flocculant for harvesting microalgae 36 in both marine and freshwater medium. 37 38 39 3

Harvesting of Microalgae by Means of Flocculation

“…Repeated recycling of the culture medium after flocculation may result in depletion of magnesium from the medium. However, Vandamme et al 2014a demonstrated that 95 % of the precipitated magnesium hydroxide could be recovered by mild acidification, allowing it to be recycled back into the culture medium. The repeated addition of base to induce flocculation and of acid to neutralize the medium can result in accumulation of salts (Castrillo et al 2013).…”

Harvesting microalgal biomass using magnesium coagulation-dissolved air flotation

“…Magnesium coagulation followed by gravity sedimentation or flotation has been used for harvesting both marine and freshwater microalgae [12,13]. However, the interaction between Mg 2þ and microalgal cells and the effects of HPO 4 2À , CO 3 2À are still not well understood.…”

“…Vandamme et al [12] showed that 95% of precipitated magnesium hydroxide could be removed from the biomass with 5 M HCl (pH 7e8). In the present study, HCl was very effective in removing Al 3þ and Mg 2þ from the harvested biomass (Fig.…”

Recyclable polyampholyte flocculants for the cost-effective dewatering of microalgae and cyanobacteria

“…Recently the pH-dependent reversibility of inorganic flocculants such as magnesium upon acidification has even been considered [35].…”