<p>This study investigated the ability of dry microalgae <i>Chlorella sorokiniana</i> biomass and its composite as biosorbents for removing arsenic (III) from water. The composite contains different proportions of microalgae, kaolin, and FeCl₃. The study was carried out to determine the optimum conditions for the biological removal of arsenic, including biomass, arsenic concentration, and pH. The highest adsorption capacity of dried biomass was obtained at pH 6. The greatest arsenic adsorption was achieved by 0.5&#xa0;g L⁻¹ composite containing 60% microalgae + 12% kaolin + 28% FeCl₃ at an arsenic concentration of 10&#xa0;mg L⁻¹, with an adsorption capacity (q_e) of 17.8 mgg⁻¹ (89%). Increasing the biosorbent amount to 1&#xa0;g L⁻¹, and expanding the metal concentration to 50&#xa0;mg L⁻¹, reduced arsenic removal efficiency. Kaolin adsorbent (1&#xa0;g L⁻¹) alone showed only 34% adsorption, which was the lowest efficiency compared to other biosorbents at a concentration of 50&#xa0;mg L⁻¹. The incorporation of iron into the clay and microalgae enhanced the efficiency of adsorption. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Fourier Transform Infrared Spectroscopy (FTIR) were employed to examine the morphological characteristics and to characterize the functional groups present in the material. A novel biocomposite (60% Chlorella sorokiniana, 28% FeCl₃, 12% kaolin) achieved 89% arsenite removal at pH 6 (17.8&#xa0;mg&#xa0;g⁻¹ capacity), outperforming individual components. FTIR/SEM revealed binding via hydroxyl/carboxyl groups. This low-cost, scalable composite is a promising candidate for sustainable water treatment applications.</p>