Physical disintegration of biochar: An overlooked process

Authors: Spokas, Kurt; Novak, Jeffrey; MASIELLO, CARRIE - Rice University; JOHNSON, MARK - Environmental Protection Agency (EPA); COLOSKY, EDWARD - University Of Minnesota; Ippolito, James; TRIGO, CARMEN - University Of Minnesota

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2014
Publication Date: 7/11/2014
Publication URL: https://handle.nal.usda.gov/10113/59596
Citation: Spokas, K.A., Novak, J.M., Masiello, C.A., Johnson, M., Colosky, E., Ippolito, J.A., Trigo, C. 2014. Physical disintegration of biochar: An overlooked process. Journal of Environmental Science and Technology. 1(8):326-332.

Interpretive Summary: The name biochar is given to biomass that through chemical/thermal processes is transformed into a form that has been shown to be more resistant to microbial degradation than the original biomass, thereby an agent for carbon sequestration. However, biochar undergoes chemical and physical alterations even under ambient storage conditions. Subsequently, once biochar is placed in the soil system it is further subjected to physical, chemical, and microbial degradation pathways. The main focus of current biochar mineralization/degradation research has been on microbial mineralization rates. Recent data collected from both artificially and naturally weathered biochars suggest that a potential significant pathway of biochar disappearance is through physical breakdown of the biochar structure. Through scanning electron microscopy (SEM) we characterized this physical weathering which increased the spacing between the graphic sheets due to the expansion accompanying water sorption, freeze-thaw, as well as desiccation and rewetting. As these sheets expand (exfoliate) this further accelerates physical break-down of the biochar. Soil texture plays an important role in this process, since clay particles were observed to seal pores and the spaces between the graphic sheets limiting the physical disintegration processes. In addition, these soil particles are virtually impossible to remove quantitatively, thereby skewing the resulting chemical analyses of the weathered biochar making it appear like chemical alteration rather than physical alteration was occurring. Lastly, the micro and nano-scale biochar particles resulting from this physical disintegration are still carbon-rich particles with no detectable alteration in the O:C ratio of the carbon structure, but are now easily suspended and moved by infiltration. There is a need to understand how to produce a biochar that is resistant to physical degradation in order to maximize the long-term C-sequestration potential of biochar in the soil system. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved biochars based on properties to improve soil carbon management.

Technical Abstract: Data collected from both artificially and field (naturally) weathered biochar suggest that a potentially significant pathway of biochar disappearance is through physical breakdown of the biochar structure. Through scanning electron microscopy (SEM) we characterized this physical weathering which increased structural fractures and possessed higher numbers of liberated biochar fragments. This was hypothesized to be due to the graphitic sheet expansion accompanying water sorption coupled with comminution. These fragments can be on the micro and nano-scale, but are still carbon-rich particles with no detectable alteration in the oxygen to carbon ratio of the original biochar. However, these particles are now easily dissolved and could be moved by infiltration. There is a need to understand how to produce biochars that are resistant to physical degradation in order to maximize long-term biochar C-sequestration potential within soil systems.