Assessment of polycyclic aromatic hydrocarbons in biochar and biochar-amended agricultural soil from Southern Spain

PURPOSE: The goal of this study was to determine if the application of four biochars produced from different feedstock leads to soil contamination with the 16 US Environmental Protection Agency (EPA) polycyclic aromatic hydrocarbons (PAHs) in biochar-amended pots incubated under greenhouse conditions. MATERIALS AND METHODS: The PAHs were extracted by Soxhlet and quantified by gas chromatography mass spectrometry in the biochars and in biochar-amended Calcic Cambisols on which Lolium perenne was grown under greenhouse conditions during 79 days. Three of the used biochars were produced by pyrolysis at 620 °C from wood chips, paper sludge, and sewage sludge, respectively. The fourth biochar was produced from old grapevine wood by the traditional kiln method. RESULTS AND DISCUSSION: The Soxhlet extraction of the Σ¹⁶PAHs of pure biochars yielded between 959 ± 62 (sewage sludge biochar) and 2613 ± 1380 μg kg⁻¹ (pine wood biochar) dry mass. The lowest abundance of PAHs of sewage sludge biochar is consistent with its very low content of organic carbon and the abundance of ashes. The PAH concentration of the wood biochar produced by kiln was six times higher than the wood biochar produced by fast pyrolysis, indicating that the biochar production process significantly affected the PAH levels. The unamended soil showed PAH levels typically for non-polluted soils (Σ¹⁶PAHs = 59 ± 4 μg kg⁻¹). Addition of 10 t biochar ha⁻¹ soil resulted in a lower increase of the 16 PAHs than expected for wood and paper sludge biochars, whereas for the soils amended with kiln wood biochar, the content of PAHs increased up to a factor of 10. Amendments of 20 and 40 t ha⁻¹ increased PAH concentrations to values on the same order of magnitude than the predicted ones, except for pine wood biochar, where the concentrations were ten times greater than expected. The major divergences were found for high-molecular-weight PAHs. CONCLUSIONS: The production process of biochar affected significantly their PAH levels. The material carbonized in traditional kilns contained the greatest amounts of PAHs. Sampling procedures for biochar-amended soils should be carefully described for future legislation guidelines to guarantee quality biochar application.