The Effect of Low-Temperature Conversion of Plant Materials on the Chemical Composition and Ecotoxicity of Biochars

The chemical composition of biochars varies considerably depending on the chemical composition of biomass used for its production and conditions of the thermal conversion process. In the context of chemical composition, biochars are different from other types of organic matter in that they contain many more aromatic carbon compounds. The aims of this study were as follows: (1) to investigate the effect of pyrolysis process of plant material on the concentrations of macroelements, trace elements and polycyclic aromatic hydrocarbons (PAHs) in the biochars; (2) to evaluate the ecotoxicity of the biochars; (3) to integrate chemical and toxicity properties of biochars in order to assess their safe utilisation as fertiliser materials. It was found that the elemental composition, the contents of macronutrients and trace elements in biochars were determined by the type of converted biomass. In the case of contents of volatile elements, such as nitrogen and sulphur, the process conditions were of great importance. Among the analysed trace elements, only the cadmium content exceeded the limit value for premium class biochars. The process of thermal conversion of organic materials did not cause mobilisation of available forms of most of the studied trace elements. In the course of the study, it was found that the extracts from wheat straw biochar (WSB) were low-toxic, while the extracts from biochars derived from rape straw (RSB), Miscanthus straw (MSB), sawdust (SB), bark (BB) and leaves of trees (LTB) were highly toxic to V. fischeri. Based on the response of the test organism, the analysed biochars were arranged in the following order: WSB < BB < RSB < LTB < MSB < SB. The highest total content of PAHs (Sigma 16 PAHs) was determined in rape straw biochar, and the lowest, in sawdust biochar. The studies revealed a negative correlation between the content of most of PAHs and inhibition of Vibrio fischeri luminescence.