Optimizing preparation of activated carbons from rice husk by K2CO3 chemical activation

Abstract: The purpose of the present study was to realize the resource utilization of agricultural byproduct. In this paper, we used rice husk as materials for producing activated carbon. Five factors which affected the yield and iodine absorption performance of rice husk-based activated carbon were sifted and optimized by Plackett-Burman (P-B) and Central Composite Design (CCD). Based on those, the prediction models of yield and iodine adsorption rate were determined and validated. The measurement of iodine absorption rate was based on the method of the GB/T 12,496.1-12,496.22-1999. The results showed that the determination coefficients (R2) of regression equations which established for the yield and iodine adsorption rate of activated carbon samples were 0.90 and 0.85, respectively. The first main factor which influenced the yield of activated carbon was activation temperature followed by activation time and concentration of K2CO3. For the iodine adsorption rate of activated carbon, activation temperature still played an important role in the preparation process, and the next were the concentration of K2CO3 and activation time. The impregnation volume ratio and impregnation time had little influence on the performance of rice husk-based activated carbon. The prediction models which were established by the CCD had a highly significant correlation (P<0.0001) and the determination coefficients (R2) reached 0.92 and 0.90, respectively. There was a strong interaction between the activation temperature and activation time. The response surface analysis indicated that when the activation time was at the center value of 0.92 h and the concentration of K2CO3 remained a constant, the iodine adsorption rate increased with the rise of activation temperature. It can be well illustrated that high activation temperature can promote the chemical reaction. However, the iodine adsorption rate decreased with the extension of activation time when the concentration of K2CO3 was at the center value of 1.5 mol/L and activation temperature remained a constant. This was because that long activation time made the micropore of samples sintered, thus it affected the adsorption performance of rice husk-based activated carbon. With the increased concentration of K2CO3, the iodine adsorption rate showed a gradually increasing trend when the activation temperature was at the center value of 1098 K and activation time remained a constant. It was the reason that more active agent can increase the contact area, which accelerated the reaction distinctively. The optimal conditions were that the rice husk was infused in the K2CO3 solution with concentration and impregnation volume ratio (K2CO3/rice husk) of 1.95 mol/L and 3, respectively after impregnating for 11 h, they were heated for 1.17 h at 1173 K. The prediction value of yield and iodine adsorption rate were 13.61%, 1058.83 mg/g, and the adequacy of the model equation for predicting the optimum response values was verified effectively by the experiment, and the experimental values (14.53%, 1021.30 mg/g) agreed with the predicted values of the model equation with 6.33%, 3.67% deviation, respectively. Our results indicated that optimizing the preparation of rice husk-based activated carbon by using CCD was reliable. This study can provide an important reference for the preparation of activated carbons from rice husk in the industrial production.