Methylene Blue Adsorption by Standard Rice Husk Biochars: Evaluation of Process Variables Through Experimental Design

Rice husk-derived biochars produced at 550 °C and 700 °C were evaluated for methylene blue adsorption from aqueous solutions using a full factorial experimental design. The effects of pH (7–9), initial dye concentration (25–75 mg L−1), and adsorbent mass (0.05–0.15 g) and their interactions were systematically analyzed. Initial concentration and adsorbent mass were the most significant factors, followed by the pH–concentration interaction. Equilibrium time depended strongly on operating conditions and biochar type. For RH550, equilibrium times were 6 h at the central point and 8 h under conditions maximizing adsorption capacity. For RH700, equilibrium was reached in 6 h at the central point and reduced to 2 h under optimal conditions, indicating improved kinetics at higher pyrolysis temperature. Isotherm modeling showed that the Toth and Redlich–Peterson models provided the best statistical fit to the equilibrium data, indicating adsorption on a heterogeneous surface. Nevertheless, the Toth model predicted a maximum adsorption capacity (70.8 mg g−1) considerably higher than that supported by the experimental data, whereas the Langmuir model yielded a more physically realistic capacity (28.1 mg g−1), despite its slightly lower statistical performance. Overall, rice husk biochars represent promising candidates for dye removal in aqueous systems.