Mechanisms of degradation of polycyclic aromatic hydrocarbons by biochar and biochar-mineral composites in the dry system

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed in arid/semi-arid soils, and their hydrophobicity and bioaccumulation pose significant threats to ecosystems and human health. Conventional remediation techniques such as advanced oxidation, are limited by high costs and secondary contamination risks, driving the need for efficient and eco-friendly in-situ methods. In this study, the activity of biochar derived from invasive plant Wedelia trilobata (pyrolyzed at 400, 600, and 800 °C; BC400/600/800) were assessed by BET, XPS, CV, and EPR. The efficacy of biochar-mineral (goethite, hematite, magnetite, birnessite, cryptomelane, montmorillonite, and kaolinite) composites for PAHs degradation performance and the mechanisms were studied. BC800 was optimal for benzo( a )pyrene (BaP) degradation (rate constant: 0.0360 min −1 ), attributable to its high specific surface area (301.44 m 2 /g), abundant oxygen-containing functional groups (C O, C O), and electron transfer capacity. The degradation rates were positively correlated with the highest occupied molecular orbital (HOMO) levels of the PAHs (R 2 = 0.6850), confirming dominant electron transfer. Among seven mineral composites, the Bir-biochar combination enhanced BaP degradation by 2.3-fold through the “electron shuttle” function of the Mn(IV)/Mn(III) redox couples. In a dry system, as the moisture increased, the degradation efficiency also increased. This study elucidates the adsorption-electron transfer mechanism of biochar-mineral composites for PAHs degradation in arid environments, providing a theoretical foundation and technical support for utilizing invasive plants utilization and remediating dryland soils polluted with organic species. • BC800 shows highest BaP degradation due to surface area and electron transfer. • PAH degradation correlates with HOMO and IP, confirming electron transfer dominance. • Birnessite-biochar boosts BaP degradation 2.3-fold via electron shuttle. • Invasive plant biochar and minerals offer eco-friendly PAH remediation.