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[en] Highlights: • A low-cost adsorbent made from rice bran was synthesized to remove RB4 from aqueous solutions. • The central composite design of the RSM was applied to optimize the adsorption independent variables. • The ANOVA study demonstrated that the quadratic model was highly significant. • Modified rice bran is a potential adsorbent with high adsorption capacity and facile magnetic separation. - Abstract: Rice bran is a major by-product of the rice milling industry and is abundant in Taiwan. This study proposed a simple method for modifying rice bran to make it a low-cost adsorbent to remove reactive blue 4 (RB4) from aqueous solutions. The effects of independent variables such as dye concentration (100–500 ppm), adsorbent dosage (20–120 mg) and temperature (30–60 °C) on the dye adsorption capacity of the modified rice bran adsorbent were investigated by using the response surface methodology (RSM). The results showed that the dye maximum adsorption capacity of the modified rice bran adsorbent was 151.3 mg g−1 with respect to a dye concentration of 500 ppm, adsorbent dosage of 65.36 mg, and temperature of 60 °C. The adsorption kinetics data followed the pseudo-second-order kinetic model, and the isotherm data fit the Langmuir isotherm model well. The maximum monolayer adsorption capacity was 178.57–185.19 mg g−1, which was comparable to that of other agricultural waste adsorbents used to remove RB4 from aqueous solutions in the literature. The thermodynamics analysis results indicated that the adsorption of RB4 onto the modified rice bran adsorbent is an endothermic, spontaneous monolayer adsorption that occurs through a physical process.