Determination of polycyclic aromatic hydrocarbons in biochar and biochar amended soil

A method for the determination of the 16 USEPA polycyclic aromatic hydrocarbons (PAHs) in biochar and biochar amended soil was developed. Samples were Soxhlet extracted with an acetone/cyclohexane (1:1) solvent mixture, and PAHs were analyzed by GC–MS after silica gel clean-up. In a comparative study based on reflux extraction, the Soxhlet solvent system acetone/cyclohexane exhibited a higher extraction efficiency of low molecular weight PAHs (e.g. naphthalene) than toluene or dichloromethane. Utilizing a reference biochar, this Soxhlet method possessed a 67–88% recovery of spiked deuterated PAHs (acenaphthene, phenenthrene, and chrysene), analytical precision (as assessed by relative standard deviations) between 5 and 18%, and a limit of detection in the 0.01–0.4 ng g−1 range. The method was successfully validated through the analysis of a certified soil material, and was capable to quantify total PAHs following biochar addition at 1% (w/w). The concentration of the 16 USEPA-PAHs along with the 15 EU-PAHs (priority hazardous substances in food) was determined in a suite of currently available biochars for agricultural field applications, which were derived from a variety of parent materials and pyrolysis conditions. The total PAH levels ranged between 1.2–19 μg g−1 and 0.2–5 μg g−1 interval for USEPA and EU PAHs, respectively. Specifically, benzo[a]pyrene ranged between 0.01 and 0.67 μg g−1 across these various biochars. Considering an application of 20–60 t biochar ha−1, the degree of PAH contamination will be dependent on both the presence of background PAHs in soil and the sorbed concentrations of PAHs on the biochar. Our data, along with PAH levels determined in other studies, suggest that biochars produced by slow pyrolysis from woody biomass possess the lowest level of sorbed PAHs (<10 μg g−1).