Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures

Abstract Biochar is considered a promising material for sequestering CO2 from the atmosphere, thus helping to alleviate climate change when returned to the soil. Biochar stability is the most decisive factor determining its C sequestration potential. Mineral modification may improve biochar characteristics, but systematic research on the effect of mineral modification on the C retention and stability of biochar and the associated mechanisms is limited. Therefore, in this study, rice straw was used to produce biochar at various temperatures (300, 400, 500, 600, and 700 °C), with vermiculite as a modified mineral material. Several methods including thermogravimetric analysis, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (NMR) spectroscopy were used to evaluate the effect of vermiculite modification and carbonization temperature on biochar stability. Biochar yield and C retention ratio decreased with increasing temperatures but increased by 13.5–38.8% and 5.2–22.1%, respectively, after vermiculite modification. The ratios of C thermal weight loss, atomic H/C, and C oxidation loss in the biochar were reduced with increasing carbonization temperature, indicating improved thermal, aromatization, and chemical oxidation stability. A trade-off that did not compromise C sequestration potential was optimized at 700 and 600 °C for the unmodified and modified biochar, respectively. Furthermore, the total mineral content of the biochar, particularly Fe, Al, Mg, and Si, were increased by vermiculite modification. FTIR results showed that chemical bonds, such as Si–O–C and Fe–O, were formed or enhanced on the biochar surface after vermiculite modification. This was further certified by the XPS survey spectra. NMR results indicated that biochar stability was enhanced by increasing the aromatization rate during carbonization, that is, by the conversion of C from alkyl and carbonyl C to aromatic C. This study provides a basis for research into and the development of functional biochar and its application in C sequestration.