Sorption of Polycyclic Aromatic Hydrocarbons (PAHs) to Biochar and Estimates of PAH Bioavailability

Biochars were produced by slow pyrolysis of corn stover under a nitrogen atmosphere at 450°, 550°, and 750°C. The chars were subjected to artificial aging, i.e., repeatedly freezing and thawing or incubating moist char at 60° and 110°C. A total of 12 materials was produced and characterized. The total polycyclic aromatic hydrocarbon (PAH) contents of 450° and 550° chars were 1.4 and 0.2 mg kg-1, respectively. Extraction and analysis of PAHs in the 750° char was performed, however the quality control associated with this assay indicated that PAHs could not be quantitatively extracted from this material by standard methods. Sorption and mild extraction experiments were performed. Mild extraction with 2- hydroxypropyl-β-cyclodextrin (HPCD), mimics bioaccessibility. Pyrene was used as a probe compound. The sorption data were well described by Freundlich isotherms. Pyrene sorption was strong for all chars, with the amount sorbed at 1μg L-1 dissolved pyrene ranging from 10 to the 6th to over 10 to the 7th μ g kg-1. The pyrene content of the chars was too low to be detected in the HPCD extracts, so the chars were spiked with pyrene and allowed to equilibrate before extraction. Only 10 to 15% of added pyrene was HPCD-extractable from the 450° char and 1 to 5% from the 550° and 750° chars. Aging had small but measureable effects on both sorption and HPCD extraction of pyrene.