Rice husk biochar activation and its effects on the characteristics of the final products to be used as sorbents or catalysts support

Annually, large amounts of rice husk (>80 million tonnes) are produced around the world and high quantities of them are produced by developing countries. Determination of chemical and physical properties of new materials produced from the activation of rice husk biochars such as density, ash, surface area, surface topography, surface functional groups and suspension pH could affect sorption capacity and catalyst properties. In this study, biochars (RH), produced from raw rice husk (RAW-RH) at 850^oC through pyrolysis process, were activated with H₂O (W-RH), H₂SO₄ (RH-S), H₃PO₄ (RH-P), and NaOH (RH-ALK) and their physicochemical parameters were analysed.  This study aims to characterize the surface area and the porosity, the suspension pH, and the functional groups observed on the surface of RAW-RH, RH, W-RH, RH-S, RH-P, and RH-ALK. The determination of the surface area, the pore volume and the average pore size was performed by using gas (N₂) adsorption−desorption with the Micromeritics TriStar 3000 Analyzer system using the Brunauer, Emmett, and Teller (BET) equation. Before analysis, the RAW-RH was degassed at 60^oC under mild nitrogen flow for 1 h and RHs were degassed at 300^oC under mild nitrogen flow for 1 h. The results were recorded by Micromeritics TriStar 3000 software. The t-plot for the RHs disclosed that the activation with NaOH leads to high specific surface (938 m²/g) and micropore area (588 m²/g) compared to other RH samples (367-386 m²/g). Simultaneously, the activation of RH increases the pore size of biochar and the highest pore size was observed for the RH-ALK (51 Å) compared to activated RH-S and RH-P (48 Å). This can be attributed to the silica removal from RAW-RH by alkaline solution that creates a mesoporous material which is more suitable as a catalyst support for bulky reactants. RAW-RH presents various peaks on its surface corresponding to different functional groups. During the high pyrolysis temperature (850^oC) most of these peaks either disappear or begin to diminish and only few new peaks appear. Furthermore, RAW-RH has more acidic behavior than the other samples (pH 6.0). For the activated RH, RH-S and RH-P are slightly acidic materials (pH~6.2) and RH-ALK and W-RH are both slightly alkaline (pH~8.5). To conclude, activation with alkali solution creates a more desirable sorbent material since it increases the surface area by 260% compared to activation with acidic solutions which increases the surface area by ~5%.

 

Acknowledgment

We acknowledge support of this work by the project “Research Infrastructure on Food Bioprocessing Development and Innovation Exploitation – Food Innovation RI” (MIS 5027222), which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).