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 100 μg g(-1). The concentrations of 16 US EPA PAHs determined by 36 h toluene extractions were 6.09 μg g(-1) for the low-VOC biochar. For high-VOC biochar the total concentrations were 53.42 μg g(-1) and 27.89 μg g(-1), while concentrations of water-soluble PAHs ranged from 1.5 to 2 μg g(-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.

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Carrot green, the algae Chlorella pyrenoidosa, and straw as representatives for different types of biomass are converted by hydrothermal conversion. The amount of nitrogen remaining in the hydrochar as well as in the aqueous phase is determined, and the amount of nitrate, nitrite, and ammonia in the process water is analyzed. The nitrogen content of hydrochar has an significant impact on the properties of hydrochar; therefore, a control of the nitrogen content would be useful to design hydrochar for different applications. With regard to the fate of nitrogen, the different biomass feedstocks show significant differences as a result of the different chemical nature of nitrogen compounds in the feedstock. A complete removal of nitrogen from the hydrochar could not be achieved. In contrast, wood incorporates nitrogen when impregnated with the amino acid cysteine during hydrothermal carbonization.

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