Reconsideration of heterostructures of biochars: Morphology, particle size, elemental composition, reactivity and toxicity.

Great attention has been paid on biochar due to potential application as soil amendment. The majority of research concerning the structural evolution of biochar commonly considered biochar as a whole. However, the knowledge of structural evolution of biochar resulting from physicochemical disintegration is rarely known. Biochars in this study were categorized into sedimented particles, suspended coarse particles and soluble components and ultrafine particles according to their suspension property. It was found out that these categories were significantly different in morphology, particle size, and elemental composition, demonstrating the presence of heterostructures in biochar. Additionally, the oxidizability of these heterogeneous particles was tested by Starch potassium iodide method and it presented that the oxidizability of the sedimented particles from high-temperature biochar was the highest. Based on the analysis of Luminescent bacteria test, the toxicity of the soluble components and ultrafine particles of low-temperature biochar was higher than that of high-temperature biochar. The heterogeneous structure of biochar and its effect proposed in this study is beneficial to individualize design of biochar sustainable application and to understand disintegration process and environmental risk of biochar in biochar-amended soil.