PEER-REVIEWED ARTICLE bioresources.com Nguyen et al. (2021). “Nopal cactus & algae removal,” BioResources 16(2), 3469-3493. 3469 Magnetic Nanocomposite Derived from Nopal Cactus Biopolymer and Magnetic Nanoparticles Used for the Microalgae Flocculation of Aqueous Solution Lan Huong Nguyen, a Huu Tap Van, b, * Duong Hong Quan, c and Phuong Thuy Thi Pham d A magnetic nanocomposite, using a Nopal cactus-derived biopolymer in combination with NH4OH-modified cobalt superparamagnetic (CoFe2O4) nanoparticles, was developed as a green flocculant system for recovery of microalgae from aqueous solutions. The obtained magnetic nanomaterials were subsequently dispersed in the biopolymer matrix with the support of ultrasonic waves. The effects of various factors on pectin extraction, fabrication of nanocomposites, and the flocculation process of microalgae were also studied. The characteristics of the obtained materials (pectin, modified magnetic nanoparticles, and nanocomposites) were evaluated via X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometry, thermogravimetric- differential scanning calorimetry, Fourier transform infrared spectroscopy, and zeta potential analysis. The optimal conditions for pectin extraction from Nopal cactus, as well as the fabrication of magnetic nanoparticles, modified magnetic nanoparticles, and nanocomposite were reported. The characteristic data of the fabricated materials showed heat resistance and abundant surface functional groups with high magnetization. The observed flocculation was attributed to the aggregation of unstable and small particles through surface charge neutralization, electrostatic patching, and/or bridging after addition of flocculants. The results showed that the nanocomposites could be a potential green flocculant for recovering microalgae with low cost and high efficiency. Keywords: Flocculation; Magnetic; Microalgae; Nanocomposite; Nopal cactus Contact information: a: Faculty of Environment-Natural Resources and Climate Change, Ho Chi Minh City University of Food Industry (HUFI), Ho Chi Minh City 70580 Vietnam; b: Faculty of Natural Resources and Environment, TNU - University of Sciences (TNUS), Tan Thinh Ward, Thai Nguyen City 250000 Vietnam; c: Ho Chi Minh City University of Food Industry (HUFI), Ho Chi Minh City 70580 Vietnam; d: Faculty of Biological Technology, Ho Chi Minh City University of Food Industry (HUFI), Ho Chi Minh City 70580 Vietnam; * Corresponding authors: Huu Tap Van (tapvh@tnus.edu.vn) INTRODUCTION In recent years, rapid industrialization and population growth in developing countries has continuously put pressure on the quality of the living environments, especially in terms of water resources. More and more rivers have become polluted due to the effluents produced by manufacturing factories. Water pollutants have appeared as threats to entire environments as well as human health; hence it is essential to remove them from the water. Specifically, the water contamination by various nutrients, e.g., nitrogen and phosphorus discharged from domestic, agriculture and industrial wastewater (piggery effluent, food processing factories, leachate, etc.), are the primary causes of the