CO2 and H2O as sweep gases elevated carbon stability and decreased phytotoxicity of biochars

As a crucial pyrolysis factor for biochar production, the mechanistically effect of different sweep gases on the inherent multi-level chemical structure of biochars is poorly understood, impeding the widespread use of biochars as sustainable carbon-negative materials. This study investigated the effect of CO2 and H2O as sweep gases relative to N2 on the physic-chemical properties of biochars derived from sawdust (SBC-N/C/H) and pig manure (PBC-N/C/H) at 300–700 °C. Meanwhile, carbon sequestration potential and potential environmental risks of SBC-N/C/H and PBC-N/C/H were comprehensively evaluated. Unlike N2, CO2 showed a superior effect on biochar (especially SBCs with more carbonaceous substances), decreased the oxygenated-functional groups content in SBC-C (14.6%–28.1%) relative to SBC-N (25.5%–37.4%) via cracking reaction. Besides, CO2 stimulated the formation of pore structure in biochar (surface area increased from 15.2 m2/g in SBC700-N to 24.1 m2/g in SBC700-C) by boudouard reaction. H2O exhibited a similar but less pronounced effect with CO2, leading to a lower H/C ratio in biochar by aggravated demethylation of methoxyl groups and facilitating a more developed pore structure with water–gas reaction. Both CO2 and H2O enhanced the carbon stability of biochar and carbon sequestration potential in SBC500-C (30.1%) and SBC500-H (28.7%) was higher than SBC500-N (25.6%). Moreover, CO2 and H2O exhibited excellent performance in mitigating phytotoxicity of biochars (especially for PBCs) to radish and cucumber seeds via the reduced release of polycyclic aromatic hydrocarbons and heavy metals. These results proposed an effective approach to improve long-term carbon sequestration and environmental safety of biochars.