Polycyclic Aromatic Hydrocarbons (PAHs) in Biochar

Biochar may be an effective soil amendment capable of improving the ion exchange capacity and nutrient and water retention properties of soils. Also, because it is stable for long periods, biochar may be suitable for carbon sequestration. However, biochar can contain various concentrations of polycyclic aromatic hydrocarbons (PAHs), which are known carcinogens (Figure 1). The PAH content of biochar may therefore restrict its beneficial uses or even result in its being classified as a hazardous waste. On the other hand, PAHs sorb strongly to black carbon, which limits their solubility in sediments and soils. Because biochar is a form of black carbon, the PAHs in biochar may have limited mobility and bioavailability.

phenanthrene chemical sturcutre: three carbon rings aligned in a cemi-circle shapeanthracene chemical structure: three carbon rings aligned in a straight linefluoranthene chemical structure: two joined carbon rings are connected to a third carbon ring via a carbon pentagontwo rows of two carbon rings are packed tightly together for a total of four rings with no gaps between the rings
Figure 1: Chemical structure of selected PAH (source image for PAH structures)
bar graph of PAHs in biochar made at different temperatures
PAH recovery from biochar.

Biochar properties depend on the starting material and production conditions. It is likely that PAH sorption and bioavailability also depend on these factors. John Scott (Senior Analytical Chemist at ISTC) collaborated with Drs. Michael Machesky and Thomas Holm (both geochemists at the Illinois State Water Survey) to characterize the binding of a representative PAH compound to a set of biochars and the effects of these biochars on PAH bioavailability.

Results showed that less PAHs are found in the biochar after pyrolysis as the pyrolysis temperature increases. A surrogate PAH (typically not found in nature) was added to biochars that were pyrolyzed at different temperatures (450°, 550°, and 750° Celsius). Nearly 90% of the PAH remained on the biochar produced at 450° and about 85% remained at 550°. However, for biochar produced at 750° the remaining surrogate PAH was approximately 5% or less than the known added amount.

This research was funded by the Illinois Hazardous Waste Research Fund (Grant No. HWR12221).