# Flocculation Harvesting Techniques for Microalgae: A Review
## 5 Particle-Based Flocculation
### Aminoclay-Based Nanoparticle  
Various ACs manufactured with cationic metals (Al\({}^{3+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), etc.) and organo-functional materials such as 3-aminopropyltriethoxysilane (APTES) have been applied to several biological and environmental applications. Such biological applications include catalysis, bio-inorganic composites, DNA transformation, gene expression, antimicrobial performance, cytotoxicity, and bio-imaging. Environmental applications of ACs include algal bio-refinement, colorimetric sensing, control of algal blooms, and water/air purification [95]. Some ACs (e.g., magnesium aminoclay (Mg-AC)) cannot prevent light penetration due to their transparency and solubility in aqueous solutions [96]. They also have high CO\({}_{2}\) availability at primary amine groups via CO\({}_{2}\) conversion into carbonate ions [97].  
The ability of ACs to cause flocculation of negatively charged algal cells is attributed to the abundance of amine groups on their surfaces. In one study, Mg-AC was reported to flocculate the freshwater alga _C. vulgaris_ within 5 min at a dosage of 1.0 g/L, with a harvesting efficiency of more than 90% in a wide range of pH from 5 to 12. The same study also showed that the harvesting efficiency using ACs largely depends on the algal species, biomass concentration, and dosage. At low biomass concentrations, flocculation occurs mainly due to charge neutralization and electrostatic bridging between algal cells and AC particles. On the other hand, the sweep flocculation mechanism plays an important role in algae harvesting at high biomass concentrations [98].  
The harvesting efficiency obtained by using ACs is reported to be almost proportional to the oxidation number of the cations. [PERSON] et al. [99] studied the harvesting efficiencies of different ACs and compared them with alum for the flocculation of _Chlorella_ sp. KR-1. They found that aluminum aminoclay (Al-AC) and Mg-AC had superior harvesting efficiencies even at low concentrations. Almost 100% of the algal cells (1.7 g cell/L) could be harvested within 30 min using 0.6 g/L Al-AC and Mg-AC. The order of harvesting efficiency was Al-AC > Mg-AC > Ca-AC (calcium aminoclay) > alum. ACs can also be applied to marine media but with lower efficiency. For example, the marine alga _N. oculata_ could be harvested within 120 min using 0.25 g/L Fe-AC (iron aminoclay) and Mg-AC, while only 5 min were required to harvest freshwater alga _C. vulgaris_[98].  
The AC-based flocculation processes have been improved by forming composites with other materials. Mixing Mg-AC and cerium aminoclay (Ce-AC) resulted in 100% efficiency for cyanobacteria harvesting after 1 h with only 10% of the required concentration for the individual AC [105]. The introduction of nZVI (nanoscale zero-valent iron) nanoparticles as the core of the AC-nZVI composites can create synergy in terms of time and efficiency. The AC-nZVI composite can remarkably reduce the harvesting time (3 min) for _Chlorella_ sp. compared with that (30 min) using original Mg-AC. The former's harvesting efficiency (around 100%) was also higher than that of the latter (70%) [102]. Humic acid and nZVI composites with AC also showed fast floc formation and very high harvesting efficiency (~100%) relative to only AC [101]. However, a bottleneck for using ACs for algae harvestingapplications is its expensive manufacturing, which can limit their utility in scaled-up processes [99]. A second bottleneck is the uncontrollable size and water dispersibility of synthesized ACs [99; 114], which could be overcome by material engineering efforts such as a template-assisted AC synthesis process [114].