Effects of chemical oxidation on phenanthrene sorption by grass- and manure-derived biochars

The oxidation of biochar in the natural environment has been widely observed. However, its influence on the sorption of hydrophobic organic compounds (HOCs) by biochars, especially biochars with high contents of minerals, remains poorly understood. In this study, sorption of phenanthrene (PHE) by grass straw-based biochars (GRABs) and animal waste-based biochars (ANIBs) produced at 450°C before and after oxidation with HNO₃ was investigated. The biochar samples were characterized using elemental analysis, X-ray photoelectron spectroscopy, ¹³C nuclear magnetic resonance, and CO₂ adsorption. Characterization results demonstrate that HNO₃ treatment of biochars caused O enrichment, loss of alkyl C, and rise of aromaticity. The organic C-normalized surface area (CO₂-SA/OC) of both GRABs and ANIBs generally increased after oxidation. The sorption nonlinearity of PHE by the biochars was weakened after HNO₃ treatment. The sorption capacity of PHE by oxidized GRABs was consistently elevated compared with the untreated samples, indicating that the high sorption capacity of PHE by GRABs may be maintained for a long time after being added into soils. By contrast, PHE sorption by ANIBs was unchanged or attenuated after oxidation. Polar groups facilitated the sorption of PHE by GRABs, while inhibited that by ANIBs. Pore-filling and π-π electron donor-acceptor interactions regulated PHE sorption by GRABs. Our results imply that GRABs are promising sorbents for environmental applications in view of their long-lasting sorption capacity.