Assessment of polycyclic aromatic hydrocarbons (PAHs) in biochars produced from crop residues: impact of pyrolysis conditions on their potential hazard

Abstract

Biochar, an emerging highly aromatic porous material produced by pyrolysis of organic residues1 is considered a good amendment for degraded soils. Thus, the interest in using biochar as soil or manure conditioner is continuously increasing during the last decade. Nevertheless, during the pyrolysis process persistent organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) are formed due to incomplete combustion2. These PAHs may enter the environment when the biochar is applied as soil conditioner. The aim of this study was to discern how the pyrolysis conditions affect the risk of PAHs exposure from biochars, thus we examined the total and individual contents of the 16 US EPA PAHs in biochars produced using three different pyrolytic reactors from biomass of rice husks, wood, wheat and sewage sludge at temperatures ranging from 400 to 600 ºC. Results of this study show that pyrolysis conditions are the key factor in the total amount of PAHs present in biochars. The maximum amount of PAHs was observed for the biochar produced in the batch reactor at 400 ºC and decreased with increasing temperature. Increasing the residence times had not significant effect on the PAHs. Looking for a more reliable risk assessment of potential exposure to PAHs in biochar than the thresholds solely based on the ∑PAHs, the total toxic equivalent concentrations (TEC) of the biochars were calculated. TEC values confirmed the need of separating the syngas and bio-oil from the solid phase3. Results of this study constitute valuable information in the development of strategies for producing biochars with minimum risk of PAHs contamination.