Zero waste realization method of rice husk: Silica extraction and methylene blue adsorption

This study determined the optimal conditions for extracting Si from rice husk (RH) for efficient resource utilization of RH and evaluated the adsorption characteristics of methylene blue (MB) by Si-extracted RH (Si-RH). The optimal conditions for extracting silica from RH were mixing 0.6 M NaOH (1:10 ratio) with RH (>2 mm) and reacting at 70 °C for 2 h. The maximum adsorption amount of MB by Si-RH with expanded surface area and reinforced cellulose-hemicellulose-lignin matrix was 99.01 mg/g, which was three times higher than that of RH (34.36 mg/g), and better fit the Langmuir isotherm and Pseudo-second order models. It was found that the adsorption of MB by Si-RH was greatly affected by environmental changes such as reaction time, pH, dosage, and reaction temperature. The surface properties by SEM-EDS and FTIR and the adsorption model equation showed that the adsorption of MB by Si-RH was dominated by complex mechanisms rather than a single mechanism, and these greatly influenced the initial adsorption-desorption process of MB by Si-RH. In continuous adsorption-desorption experiments, Si-RH showed excellent adsorption efficiency for MB. In conclusion, the extraction of silica from RH and utilization of the residue as a dye adsorbent proposed in this study is considered to be the optimal method for realizing zero waste of RH.