# Flocculation Harvesting Techniques for Microalgae: A Review
## 5 Particle-Based Flocculation
### Magnetic Particle
The magnetic particles most commonly utilized for algae separation are iron oxide (Fe\({}_{3}\)O\({}_{4}\)) and yttrium iron oxide (Y\({}_{3}\)Fe\({}_{5}\)O\({}_{12}\)) nanoparticles. It was reported that 120 mg/L of Fe\({}_{3}\)O\({}_{4}\) nanoparticles could be applied to harvest marine alga _N. maritima_ within 4 min with 95% efficiency [107]. Interestingly, Y\({}_{3}\)Fe\({}_{5}\)O\({}_{12}\) is superior in terms of reuse after detachment from algal aggregates via pH control. However, it should be noted that, at high pH values, Fe\({}_{3}\)O\({}_{4}\) was more easily separated from the flocs than Y\({}_{3}\)Fe\({}_{5}\)O\({}_{12}\)[119]. Harvesting efficiency can be significantly enhanced using \"hybrid magnetic nanoparticles\", which are magnetic particles coated with positively charged materials. These can be applied easily and quickly, to harvest negatively charged algal cells in relatively low flocculant doses [110; 120]. [PERSON] et al. [112] reported the successful use of a chitosan-Fe\({}_{3}\)O\({}_{4}\) nanoparticle composite to harvest _Chlorella_ without the need for pH adjustment. They obtained better than 99% harvesting efficiency using a 1.4 g/L flocculant dosage within 2-5 min with the aid of a permanent magnet. This harvesting method strongly facilitates the recycling of the spent medium for further culture cycles after harvesting. Similarly, Fe\({}_{3}\)O\({}_{4}\) coated with polyethyleneimine (PEI-Fe\({}_{3}\)O\({}_{4}\)) nanoparticles was successfully utilized for harvesting algae. In a harvesting study for C. _ellipsoidea_, the required nanoparticle concentration was only 20 mg/L, which was sufficient to harvest around 97% of the algal biomass within 2 min [110]. Another study attributed the superiority of PEI-coated Fe\({}_{3}\)O\({}_{4}\) over non-coated Fe\({}_{3}\)O\({}_{4}\) in significantly increasing the zeta potential from \(-\)7.9 to +39 mV, respectively. They also found that the algal biomass had been successfully detached from the PEI-Fe\({}_{3}\)O\({}_{4}\)-algae slurry by only sonication for 5 min (500 W), without the addition of HCl or NaOH. In addition, the recovered PEI-Fe\({}_{3}\)O\({}_{4}\) nanoparticles (after detachment) could maintain their harvesting efficiency. In the same study, UV irradiation was used to enhance the recovery and harvesting performance of the magnetic flocculant, which was attributed to changes in the nature of the surface of the algal cells [111]. In the magnetic-flocculation-based algae harvesting process, flocculant recyclability is one of the most important challenges which needs to be resolved in order to reduce operating costs [103; 111]. Acid treatment, sonication, UV irradiation, and hydrophobicity shifting are usually used to facilitate separation of flocculants from harvested algal biomass [121]. Sonication has been proven to be an  
Figure 5: Schematic illustration of magnetic algae harvesting and sequential detaching of BaFe\({}_{12}\)O\({}_{19}\) particles. Reprinted from Reference [103] with the permission of Elsevier. efficient treatment method, as mentioned above [111]. Acid treatment is also considered to be a successful flocculant-floc detaching method. An earlier study [106] demonstrated the ability of Fe\({}_{3}\)O\({}_{4}\) nanoparticles to harvest both \(B\). _braunii_ and _C. ellipsoidae_ with a harvesting efficiency above 98% within 1 min. The harvested cells could be detached from the algae-Fe\({}_{3}\)O\({}_{4}\) flocs using HCl treatment, and the magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles could be reused 5 times with almost the same harvesting efficiency. Hydrophobicity shifting was reported as a tool for flocculant-biomass detachment using the photocatalytic reaction. To shift the particle hydrophobicity, Fe\({}_{3}\)O\({}_{4}\) nanoparticles were coated with steric acid (SA) as a target molecule that could be altered by photocatalytic reaction.