Development of microalgal flocculation stimulated by interaction between metal cations and extracellular polymeric substances

Abstract

In this work, the multiple potentials of seawater was firstly explored to address key issues relating to the cultivation and harvesting of EPS-producing microalgae, Scenedesmus obliquus and Ettlia sp. Seawater could suf？ciently replace some of the key elements in BG11 and TAP, the two most common media for freshwater algal cultivation. The use of seawater had a significantly positive effect on cell aggregation making the harvesting process easy. The aggregation of both species was found to be stimulated by the minerals in the seawater with the help of extracellular polymer substances (EPS). When sea salt, another form of seawater, was tested for the flocculation of EPS producing Ettlia sp. and EPS non-producing Chlorella vulgaris, only Ettlia sp. readily formed cell aggregates whereas C. vulgaris showed negligible floc formation. The key element to cause such floc formation with the help of EPS was found to be calcium ion in the sea salt. Calcium ion can act as an effective flocculant only if EPS is sufficient on the cell wall. When EPS is insufficient, no aggregation occurred due to lack of EPS-calcium ligands on the cell surface. The flocculation was less efficient when EPS level is too high due to adsorption of calcium ions by free-EPS. Thus, optimal level of EPS exists to maximize the flocculation. Although calcium ion is essential to cause flocculation of EPS-producing microalgae, not all calcium-rich compounds are an effective flocculant. Among various calcium sources examined, activated calcium oxide was identified to be the most effective flocculant. Its inherent basicity and high ionization capability were considered to be the main factors for its high performance in flocculation. With low toxicity to environment and low price, calcium can be a good alternative to replace commonly used flocculants.