Particle size dependence of the physicochemical properties of biochar.

The inadequate definition of different particle sizes of biochar impedes the establishment of guidelines concerning the selection of suitable particle sizes. In this study, various properties of a series of ten biochar samples of varying particle size (5-5000 μm) from the same origin have been investigated and compared. Cluster analysis and principal component analysis based on all of the parameters were executed to evaluate the dissimilarity of different particle sizes of biochar, and to classify the size ranges determining the relevant properties. The results showed that the series of ten biochar samples with different particle sizes could be separated into four groups, indicating that there were significant characteristic differences between different particle sizes of biochar. The greater the difference in particle size, the more disparate the properties. Moreover, 75-150 μm has been identified as a turning point along the continuous particle size range. A deviation in the properties of biochar particles smaller than 5 μm confirmed that different preparation methods would lead to highly significant differences between different particle sizes of biochar, even if the pristine materials were the same. The need for definition of the characteristics of biochar required for a specific environmental application should be emphasized.