Influence of biochar particle size on ¹⁴C-phenanthrene extractability and mineralisation in soil

Biochar can influence the bioaccessibility and biodegradation of polycyclic aromatic hydrocarbons (PAHs) in soil, but the role of biochar particle size in this process remains largely underexplored. In this study, soil was spiked with ¹²C- & ¹⁴C-phenanthrene and subsequently amended with biochar of <0.6 mm and 2-4 mm particle size at 0.0 %, 0.1 %, 1.0 %, and 10.0 %, respectively. The amended soils were aged for 60 d and ¹⁴C-phenanthrene extractability and mineralisation were monitored at 1 d, 15 d, 30 d, 45 d, and 60 d. The total residual ¹⁴C-activity and extractable fractions reduced over time with increasing biochar amounts irrespective of biochar particle size. Similarly, longer lag phases, slower rates, and lower extents of mineralisation were observed over time with increasing biochar amounts. Solvent extractability and reduction in residual ¹⁴C-activity were higher in <0.6 mm amended soils, which were attributed to a higher surface area and shorter diffusion pathway. Hydroxypropyl-β-cyclodextrin (HP-β-CD) extracted ¹⁴C-phenanthrene correlated with the extents of mineralisation with stronger agreement in <0.6 mm amended soils (R² = 0.77-0.86) than in 2-4 mm amended soils (R² = 0.57 - 0.82). The weakest correlation was observed at 10.0 % of 2-4 mm. This study demonstrated HP-β-CD's potential for predicting phenanthrene microbial degradation in biochar-amended soils and highlighted the influence of biochar particle size on phenanthrene bioaccessibility and biodegradability. These findings are important for mitigating phenanthrene risks in soil and optimising its sorption stability using biochar.