Biogenic nanoporous oxides recovery from by-products of bioenergy production: Rice husks and corncob biochars

The bioenergy production and their by-product valorization are valuable strategies to the sustainability enhanced targets. The present work integrates valorization of thermochemical conversion of residual biomass rice husk and corn cob, focusing on biogenic oxide production from biochar. The bioenergy analysis and biochar properties allowed to establish a specific energy potential (SEP) up to 7.47 kWh_e⁻kg_Oxi⁻¹ and preliminary gas emission factor as greenhouse gas emission (GHG) down to 0.562 kgCO_2−eqkg_Oxid⁻¹ for the production of biochar oxides. A comparative study of the biogenic nanoporous recovery from biochar was done; four biochar samples come from different thermochemical conversions of rice husk, and corncob was treated by hydrochloric acid concentration between 0 and 10% w/w and subsequently oxidized at a temperature between 350 and 750 °C. The biochar and biochar oxides' physicochemical characteristics were carried out by ultimate analysis, Brunauer-Emmett-Telle specific surface area (A_BET), pore size distribution, X-ray fluorescence (XRF), and Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The biochar treatment allowed 55.5% and 41.2% of raw biogenic nanostructure recovery with a specific surface area up to 132 m² g⁻¹ and 25 m² g⁻¹ from the rice husk and corncob biochar, respectively. The biochar oxidation temperature was the most relevant factor for controlling oxides' nanoporosity from biochar, reducing mesopores volume in both cases. However, the hydrochloric acid concentration in leaching favors residual carbon removal and ensures nanostructure preservation of biogenic nanopores.