Chemical stability assessment of biochar has been universally used to indicate its potential of long-term carbon sequestration. The comparative study on oxidation resistance of biochars from diverse series of feedstock is relatively limited, as well as the effects of endogenous minerals on biochar stability. Herein, oxidation resistance of biochars from peanut shell, bamboo, saw dust, reed stalk, furfural residues, seaweed degumming residues and Enteromorpha prolifera at 500 °C (PS500, BB500, SD500, RS500, FR500, SR500 and EP500) was examined by the treatments of H2O2, K2Cr2O7 and thermogravimetric analysis (TGA). Under H2O2 or K2Cr2O7 condition, C loss of algae-derived biochars (SR500 and EP500) was extremely greater than that of other biochars due to higher content of labile carbon components. PS500, BB500, SD500, RS500 and FR500 characterized with similar properties in carbon fraction, but they exhibited different ability to resist oxidation. The mineral fraction of biochars (e.g., content and species) varied with the feedstock, which played complex effects on the oxidation resistance. The mineral decomposition (e.g., CaCO3) in EP500 and SR500 above 500 °C influenced the analysis of biochar stability by TGA. After acid-washing, EP500 and SR500 showed weaker thermal oxidation resistance, agreed with the results of H2O2 and K2Cr2O7 oxidation. The oxidation resistance of biochars was correlated better with O/C ratio, implying that O/C ratio was more robust indicator than other indexes (e.g., H/C ratio and the ratio of D band to G band of Raman). The FTIR, Raman and XPS results further demonstrated the elimination of aliphatics and amorphous aromatics and/or the carboxylation/carbonylation of aromatic structures by H2O2 and K2Cr2O7. These findings are useful for better understanding the impacts of feedstock and inherent minerals on the oxidation resistance of biochars.
Keywords: Acid-washing; Biochar; Carbon chemical structure; Feedstock; Mineral; Stability.
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