Inherent organic compounds in biochar-Their content, composition and potential toxic effects

Pyrolysis liquids consist of thermal degradation products of biomass in various stages of its decomposition. Therefore, if biochar gets affected by re-condensed pyrolysis liquids it is likely to contain a huge variety of organic compounds. In this study the chemical composition of such compounds associated with two contaminated, high-volatile organic compound (VOC) biochars were investigated and compared with those for a low-VOC biochar. The water-soluble organic compounds with the highest concentrations in the two high-VOC biochars were acetic, formic, butyric and propionic acids; methanol, phenol, o-, m- and p-cresol, and 2,4-dimethylphenol, all with concentrations over 100μgg^-1. The concentrations of 16 US EPA PAHs determined by 36h toluene extractions were 6.09μgg^-1 for the low-VOC biochar. For high-VOC biochar the total concentrations were 53.42μgg^-1 and 27.89μgg^-1, while concentrations of water-soluble PAHs ranged from 1.5 to 2μgg^-1. Despite the concentrations of PAHs exceeding biochar guideline values, it was concluded that, for these particular biochars, the biggest concern for application to soil would be the co-occurrence of VOCs such as low molecular weight (LMW) organic acids and phenols, as these can be highly mobile and have a high potential to cause phytotoxic effects. Therefore, based on results of this study we strongly suggest for VOCs to be included among criteria for assessment of biochar quality.