## 1 Introduction
Biocoagulants comprise polysaccharides, proteins, and other functional groups that may be cationic, anionic, poly-ionic, or non-ionic [15]. The natural coagulants act by chemical adsorption and neutralization. The formation of hydrolyzed species of positive charge in the compound; then, the adsorption occurs at the surface of this particle suspension, and destabilization arises [13; 16]. Biocoagulants/bioflocculants are obtained from living organisms, such as animals, plants, fungi, and bacteria [12]. According to [PERSON] et al. [12], fungi such as _Penicillium_ cells or spores can be used to flocculate and recover _Chlorella vulgaris_ biomass, resulting in a biomass recovery of over 98%. The aggregation resulting from the association between fungi and algae is called algal-fungal flocculation [12]. Biopolymers can be classified as anionic, which generally exhibit poor activation with microalgae, or cationic, that can neutralize or reduce the ionic surface charge of microalgae, resulting in a more efficient harvesting process [12]. The use of biocogulants of plant origin, such as _Margaritaria discoidea_, _Moringa oleifera_, _Cocos uncifera_, _Jatropha curcas_, and _Luffa cylindrica_, and species from the genre Opuntia such as _Opuntia focus-india_, have been shown to be efficient for biomass harvesting of microalgae [14; 15]. Most fruit seed extracts have strong gel formation, which is able to remove the turbidity of water. As an example, the active coagulating ingredient in the _M. discoidea_ and _M. oleifera_ extracts was identified as water-soluble macromolecular hydrocolloidal polysaccharides [14]. They have a polymeric material with the hydroxyl group that easily forms hydrogen bonding in the aqueous medium. The seed of _M. oleifera_ consists of water-soluble flocculating with a protein of cationic nature that acts as an active agent in the process of coagulation [17]. These biocoagulant/bioflocculant are hydrolyzed after dissolution in water and interact with microalgae (negative charge). The interaction destabilizes the surface charge of microalgae, allowing the microalgae cells to agglomerate and settle. According to [PERSON] et al. [18], nucilage extracted from cactus was shown to increase sedimentation rates of microalgae. For this reason, this work received an interest in evaluating microalgae harvesting by nucilage of cacti. The crop of _Opuntia_ has a high potential to harvest microalgae, although little research has been carried out for this purpose. _Opuntia_ has macromolecular polymers, which are flocculating agents. Some factors, such as media composition and environmental conditions, including temperature and pH, affect flocculant production by these cacti. Generally, species from the genre _Opuntia_ have high flocculation efficiency and are cheap, sustainable, and environmentally friendly [19]. The infrared spectra for _Opuntia_ sp., determined by [PERSON] and [PERSON] [20], indicate the presence of carboxylic acid groups that are negatively charged. The high-coagulation capability of the cactus is most likely attributed to the presence of nucilage, which is a viscous complex of carbohydrates. The same authors [20] also reported that galacturonic acid is another possible active ingredient that provides _Opuntia_'s coagulation/flocculation ability to remove the turbidity of wastewater. The mechanism relies on forming a bridge between the particles and the polymer, at which the particles in the solution establish linkages with the polymer. Considering that _Opuntias_ sp. presented very satisfactory results in wastewater treatment and few studies were performed to harvest microalgae, this work aims to optimize the harvesting process of the microalga _Chlorella vulgaris_ using a biocoagulant extracted from _Opuntia cochenillifera_. As far as it is known, _O. cochenillifera_ has not yet been applied for this purpose. The specific objectives of the present study are to (i) evaluate the use of acidic and alkaline _O.