Carbon dioxide as a carrier gas and biomass addition decrease the total and bioavailable polycyclic aromatic hydrocarbons in biochar produced from sewage sludge

Abstract

Organic-solvent extractable (C_tot) and freely dissolved (C_free) polycyclic aromatic hydrocarbons (PAHs) (US EPA 16 PAHs) were determined in biochars produced from the mixture of sewage sludge and sewage sludge and willow (8:2 or 6:4, w/w). The pyrolysis was carried out at 500, 600, and 700 °C using two different carrier gases (N₂ or CO₂). Addition of willow and the change of carrier gas from N₂ to CO₂ reduced C_tot PAHs (from 7.0 to 52%) and C_free PAHs (15-29%) content. Co-application of willow and SSL and the use of CO₂ as a carrier gas also beneficially affected the PAHs profile. The biochars produced with willow addition and/or in a CO₂ atmosphere were characterized by a lower (from 9.0 to 62.8%) percentage of 3-6-ring PAHs (C_tot) than the biochars derived from sewage sludge alone in N₂ atmosphere. The contribution of individual C_free PAHs did not differ significantly between biochars. The presence of willow during pyrolysis influenced the direction of the changes in the C_tot PAH content depending on the pyrolysis temperature. For SSL alone, regardless of the carrier gas used, the content of C_tot PAHs was observed to decrease with increasing temperature, whereas in the presence of willow temperature did not affect significantly (P ≥ 0.05) the C_tot PAHs content. The change of carrier gas from N₂ to CO₂ increased the affinity of the biochars to PAHs as confirmed by the distribution coefficients log K_TOC.

Keywords: Biochar; Biomass; Carrier gas; PAHs; Sewage sludge.