Several recent studies have investigated the potential of magnetite nanoparticles for flocculating microalgae. The nanoparticles can induce flocculation of microalgae and separation of the microalgae from the medium in a magnetic field. This tech­nology thus combines flocculation and separation of flocs in a single process step. In some studies, the magnetite nanoparticles are used as such, without func­tionalization (Xu et al. 2011; Prochazkova et al. 2012). In other studies, the nanoparticles are functionalized by coating the surface with cationic functional groups (Lim et al. 2012; Seo et al. 2014). By using functional groups that have a pH-dependent charge, it is possible to remove the nanoparticles after flocculation by adjusting pH and to reuse them in a second round of harvesting (Cerff et al. 2012; Seo et al. 2014)

In estuaries, flocculation of natural populations of microalgae is sometimes observed in the presence of high concentrations of sediment particles. This floc­culation has been ascribed to interaction between microalgae and clay minerals (Avnimelech et al. 1982). Clays have been used successfully to induce flocculation of natural blooms of microalgae in coastal waters (Sengco and Anderson 2004; Padilla et al. 2010). So far, however, only a few studies have explored the potential of clays for harvesting microalgae. One study explored the use of synthetic orga — noclays with specific surface properties for flocculating Chlorella (Lee et al. 2013b). Farooq et al. (2013) successfully demonstrated aminoclay harvesting of Chlorella vulgaris and Nannochloris oculata. Aminoclays were used recently as a template for nanoscale zerovalent iron synthesis and were shown to be efficient for harvesting Chlorella sp (Lee et al. 2014).