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

Pretreatment of biomass with phosphoric acid (HPO) 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 HPO simultaneously captured C and created micropores. Sawdust was soaked in diluted HPO and dried for pyrolytic biochar generation at 350, 500, and 650°C. Results showed that HPO 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 HPO-pretreated biochar were 930 m g and 0.558 cm g, respectively, compared with <51.0 m g and 0.046 cm g in the untreated biochar. The volume of micropores (<10 nm) increased from 59.0% to 78.4-81.9%. The presence of HPO 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 HPO in biomass pyrolysis.