Highly effective and green method to improve the adsorption performance of rice husk biochar

Rice husk biochar is a low-cost carbonaceous adsorbent; however, it has low adsorption capacity. To increase its potential application, biochar should be modified to enhance its porous structure and surface adsorptive sites for better adsorption performance. In this study, a hydrothermal process has been proposed to modify rice husk biochar and evaluate its effect on the structure and rhodamine B adsorption performance. Upon the hydrothermal treatment, surface area and porosity increased significantly from 54.1 m²/g and 0.04 cm³/g to 104.7 m²/g and 0.08 cm³/g, respectively and as result, its adsorption capacity improved by 98.6%. Biochar showed a good adsorption performance in a wide range of pH from 2 to 10 with maximum adsorption at pH = 2.7. Rhodamine B adsorption fits well the Langmuir isotherm model and pseudo-second order kinetic model, that underlines chemical interaction as a major adsorption mechanism. Additionally, hydrothermal treatment improved the recyclability of biochar with only ~ 5.5% loss in the adsorption capacity of modified biochar compared with 33.7% loss of unmodified sample. The addition of ZnCl₂ as catalyst in hydrothermal process does not improve the adsorption performance of modified biochar. This work demonstrated that hydrothermal modification is not only an effective but also green method to improve the adsorption performance of rice husk biochar.