Optimizing the preparation process and characterize of the performance of Xanthoceras sorbifolium Bunge shell-based activated carbon

Abstract: Xanthoceras sorbifolium Bunge is one type of high-valued oilseed crop in recent years. However, a large amount of shell waste can be generated during oil production. It is a high demand to explore the feasibility of converting crop shells into activated carbon, in order to reduce the environmental waste. Taking the shells of Xanthoceras sorbifolium Bunge as the experimental raw material, this study aims to optimize the H3PO4 activation preparation of activated carbon. The activated carbon with better adsorption performance was obtained after process optimization. This activated carbon was then characterized for the reuse of the shells of Xanthoceras sorbifolium Bunge. A single-factor test was conducted to investigate the effects of the four factors on the yield and iodine adsorption of activated carbon with the solid-liquid ratio, H3PO4 concentration, activation time, and activation temperature as independent variables. A response surface optimization test was then conducted to obtain the finished activated carbon with the best adsorption performance. The interaction effects were also clarified among the above four factors. The moisture, ash, and element were characterized by scanning electron microscopy (SEM), N2 adsorption-desorption isotherm, and Fourier Transform infrared spectroscopy(FT-IR). The results showed that the optimal process conditions were achieved: the solid-liquid ratio of 1:1, H3PO4 concentration of 71%, activation time of 158 min, activation temperature of 540 ℃, and iodine adsorption value of 1 127.377±2.406 mg/g. The influence of each factor on the iodine adsorption was ranked from the activation temperature, activation time, H3PO4 concentration, and solid-liquid ratio in the descending order, in which the interaction between the solid-liquid ratio and H3PO4 concentration was the most significant. The characterization showed that the moisture and ash content of activated carbon decreased significantly, compared with the raw material. The element C increased significantly from 45.124% to 69.702%, whereas, the proportion of H and O decreased, and the element N increased slightly. Moreover, there was no element S in the raw material and activated carbon. SEM images show that there was a rougher surface of activated carbon, where the pore structure was more developed than that of the raw material, even these irregular pores were in the appearance of a sponge. The specific surface area of activated carbon was 841.438 m2/g, the total pore volume was 0.593 cm3/g, and the average pore diameter was 4.361 nm. Furthermore, the pore structure was well developed with the mesoporous, as measured by the fully automatic specific surface area and porosity analyzer. FT-IR spectrograms demonstrate that the functional groups of activated carbon were significantly reduced, compared with the raw material. Specifically, the functional groups were mainly hydrophilic groups C=O favorable for adsorption. Therefore, the activated carbon can be expected to be prepared with a better adsorption capacity of raw material. The finding can also provide a strong reference to reuse the Xanthoceras sorbifolium Bunge shells.