Very interesting question. Whether recalcitrance of biochar is related to it's hardness or it is dependent on the presence of different functional groups based temperature of controlled pyrolysis..

Dear Steve, that is indeed a nice question. To my knowledge hardness of biochar has not been assessed yet. However, you may have a look into following references to get an overview of indices used to describe hardness and mechanic stability of coals and charcoal...

B. Kwiecińska, H.I. Petersen, Graphite, semi-graphite, natural coke, and natural char classification—ICCP system, Int. J. Coal Geol. 57 (2004) 99–116. doi:10.1016/j.coal.2003.09.003. Look for keyword "hardness" .

M.R. Assis, L. Brancheriau, A. Napoli, P.F. Trugilho, Factors affecting the mechanics of carbonized wood: literature review, Wood Sci. Technol. 50 (2016) 519–536. doi:10.1007/s00226-016-0812-6. Look for keywords "compression strength" and "friability".

A.C. Scott, The Pre-Quaternary history of fire, Palaeogeogr. Palaeoclimatol. Palaeoecol. 164 (2000) 281–329. doi:http://dx.doi.org/10.1016/S0031-0182(00)00192-9. Look for keyword "friability".

Beside chemical properties (e.g. aromaticity) hardness and/or mechanic stability may also explain C-sequestration, would be good to assess. Please consider also soil texture as a factor affecting stability of charcoal particles - stability may increase depending on texture: sand

Steven, Moh's hardness of scale is related to hardness (in which could also be related to stability towards degradation/decomposition). I have been studying biochar since 2010. Biochars are not equally made. I mean, Biochars that are made (through pyrolysis) above 650 are more recalcitrant than those process in 350-500 degree celcius. It also also important to note that plant-based biochar may or may not decomposed faster than animal-based biochar. Actually, there is a huge gap and a lot of research interests in biochar. For reference you may read these articles:

http://www.biochar-international.org/sites/default/files/IBI-RS-recalcitrance-2-Apr-2010..pdf

Characterization of slow pyrolysis biochars: effects of feedstocks and pyrolysis temperature on biochar properties. by Kloss S1, Zehetner F, Dellantonio A, Hamid R, Ottner F, Liedtke V, Schwanninger M, Gerzabek MH, Soja G.

I actually had tonnes of articles related to biochar. You may email me. Thanks.

Sincerely,

Malou

Thanks Malou, that was quite helpful.

So, my refined question then is: "Does anyone know of any study examining the relationship between durability (as measured by Mohs hardness) and lability (as measured by a low O:C molar ratio)?" In other words, as hardness increases on the X-axis, is there any observed effect on O:C ratio?

Regards,

-Steve

Dear Steven,

This article (from International Biochar Initiative) will surely help. Please keep in touch. Thanks.