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

摘要

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 degrees C using two different carrier gases (N-2 or CO2). Addition of willow and the change of carrier gas from N-2 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 N-2 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 N-2 to CO2 increased the affinity of the biochars to PAHs as confirmed by the distribution coefficients log K-TOC. (C) 2019 Elsevier Ltd. All rights reserved.