Water containing Cr(VI) causes health effects, which makes their treatment necessary. In this work, biochar from sunflower seeds waste was used to remove Cr(VI) from water. The biochar was modified by magnesium and optimized. The Mg-biochar was characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy, and ASAP methods. The SEM images of Mg-biochar showed magnesium oxide spherical particles with nanoflakes. The Mg-biochar specific surface area and pore volume were 67.85 m2 /g and 0.061 m3 /g, respectively. Cr(VI) adsorption decreased from 87.16% to 50.76% with an increase in pH from 2 to 10. The isotherm of the Cr(VI) adsorption by Mg-biochar followed the Langmuir model. Thermodynamic studies showed the endothermic nature of the adsorption. Kinetics of Cr(VI) adsorption by Mg-biochar was in agreement with the pseudo-second-order model and its rate con stant was 2.1 g/mg·min which was large in comparison with other works showing advantage of fast removal kinetics of Mg-biochar. The Mg-biochar adsorbent had the removal of 74.5% under the oper ating conditions of Cr(VI) initial concentration of 10 mg/L, adsorbent dose of 15 g/L, pH of 5, and time of 15 min. Keywords: Biochar; Mg modification; Cr(VI) adsorption; Wastewater treatment; Sunflower seeds waste
