Roles of Phosphoric Acid in Biochar Formation: Synchronously Improving Carbon Retention and Sorption Capacity

Abstract

Pretreatment of biomass with phosphoric acid (H 3 PO 4 ) for biochar production was expected to improve carbon (C) retention, porosity structure, and the sorption ability of biochar. This study investigated the interaction of phosphorus with the C structure to elucidate the mechanisms by which H 3 PO 4 simultaneously captured C and created micropores. Sawdust was soaked in diluted H 3 PO 4 and dried for pyrolytic biochar generation at 350, 500, and 650°C. Results showed that H 3 PO 4 pretreatment resulted in 70 to 80% of biomass C retention in biochar, compared with only about 50% remaining without pretreatment. The specific surface area and total pore volume of the H 3 PO 4 –pretreated biochar were 930 m 2 g −1 and 0.558 cm 3 g −1 , respectively, compared with <51.0 m 2 g −1 and 0.046 cm 3 g −1 in the untreated biochar. The volume of micropores (<10 nm) increased from 59.0% to 78.4–81.9%. The presence of H 3 PO 4 shifted the decomposition temperature to a lower value and decreased the energy required for biomass decomposition. Micropore formation was via the insertion of P‐O‐P into the C lattice, leading to swelling and amplification of amorphous form and lattice defect of the C structure, as evidenced by Raman spectrum and small‐angle X‐ray scattering analysis. The crosslinking of P‐O‐P and C bonds resulted in greater biomass C retention in biochar. This biochar‐phosphorus composite had a much higher sorption ability for Pb than the unmodified biochar, which was possibly dominated by phosphate precipitation and surface adsorption. This study provided a simple method to improve biochar properties and explored the multiple benefits of H 3 PO 4 in biomass pyrolysis. Core Ideas Multiple benefits were obtained in biochar formation with H 3 PO 4 pretreatment. H 3 PO 4 increased pore creation, carbon retention, and sorption ability of biochars. H 3 PO 4 decreased the demanded energy for fibrous cells decomposition. H 3 PO 4 promoted an amorphous form and lattice defect of the carbon structure. Insertion of P‐O‐P into carbon lattice leading to carbon dilation and crosslinking.