Effect mechanism of phosphorous-containing additives on carbon structure evolution and biochar stability enhancement

Abstract

Abstract The regulation of the pyrolysis process is a key step in increasing the carbon sequestration capacity of biochar. The effect of K3PO4 addition on the yield, chemical composition, characteristic functional groups, macromolecular skeleton, graphite crystallites, and stability of biochar was studied in this paper using two-dimensional infrared correlation spectroscopy (2D-PCIS), X-ray photoelectron spectroscopy, Raman spectrum, and other characterization methods combined with thermal/chemical oxidation analysis. It is discovered that adding K3PO4 may effectively minimize the graphitization temperature range and increase biochar's yield, aromaticity, H/C ratio, and proportion of refractory/recalcitrant organic carbon. The 2D-PCIS and Raman analysis revealed that K3PO4 mostly promoted the dehydrogenation and polycondensation process of the aromatic rings in the char precursor, transforming the amorphous carbon structure of biochar into an ordered turbostratic microcrystalline structure. K3PO4 enhanced biochar stability mostly at medium-high temperatures (350 ~ 750℃) by stimulating the transformation of unstable structures of biochar to stable carbon-containing structures or by inhibiting the interaction of its active sites with oxidants through the mineralization process. A 20% phosphorus addition increased biochar's refractory index (R50) by roughly 11%, and it also boosted biochar's oxidation resistance (H2O2 or K2CrO4) efficiency, reducing carbon oxidation loss by up to 7.31%. However, at higher temperatures (> 750 ℃), the doping of phosphorus atoms into the carbon skeleton degraded the biochar structure's stability. The results of this study suggest that using exogenous phosphorus-containing additives is an efficient way to improve the stability of biochar. Graphical abstract

Keywords

• Biochar stability
• Microwave pyrolysis
• K3PO4
• Carbon sequestration
• 2D-PCIS