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Larry Murrell1 Yun-feng Chang2 and Greg Thiele3
1. Independent Consultant, 2. Sigma Innova LLC, 3. Micromeritics
Analytical Services

Abstract

Complex mixtures of porous materials have been the subject of intense study for the past seventy years in heterogeneous catalysis due to their utility in converting petroleum fractions to useful products including, most notably, gasoline.  A technique which emerged almost 30 years ago, based on controlled pulses of volumetric doses of nitrogen gas to porous solids at liquid nitrogen temperature, produced details of the pore distribution and the pore volume of these different components of complex mixtures that was not possible from previous instruments.  Thus, this method is described as a high resolution pore distribution analysis (HRPDA) technique. This technique provides an accuracy of 1-2 Å, and was successfully used to measure the thickness of a single molecular layer of tungsten oxide bound to the surface of a transitional alumina.  In this paper we have applied this HRPDA method to the characterization of mixtures of a state-of-the-art, and apparently unique, Cabot carbon black (developed for fuel cell application), mixed with Mississippi loess to prove that the HRPDA approach would be applicable for biochar amended soils at low contents of biochar mixed with soil.  This particular Cabot carbon black, Black Pearl (BP) 2000, is probably at the pinnacle of potential biochar-like materials with a surface area of 1500 square-meters per gram (m2/g) and a pore volume of over 3 cubic centimeters per gram (cc/g).  It is demonstrated by the characterization work from Micromeritics Analytical Services that quite accurate information can be obtain for the individual components as well as mixtures of the carbon black with the loess soil in concentrations of 1-5 wt%.  This work was intended to provide the foundation for characterization of real biochar materials in soil, and to provide details of the transformations of biochar materials in soil over time.  We also utilized a combination of Hg porosimetry and HRPDA to the characterization of a charcoal on the opposite end of the spectrum of BP 2000, a typical charcoal used for briquettes which had a low surface area.  Despite this low surface area there was considerable pore volume at > 1 micrometer size range in this particular charcoal of about 2 cc/g water uptake based on the un-ground briquettes.  This material was characterized and demonstrated to have a quite low surface area of about 2-3 m2/g.  This low surface area charcoal had the pores filled with water, and was subjected to eight cycles of freeze-thaw-freeze (FTF) conditions to simulate conditions in soils subjected to freezing conditions.  The modification of the carbon structure after these limited FTF-cycles was substantial, and suggests great care must be exercised in translation of biochar materials developed in the Amazon Basin to colder climates where the soil is subject to FTF-cycles.  In subsequent experiments BP 2000 was found to undergo major changes when subjected to FTF-cycles, also.  This work suggests that there may be an opportunity for nano-technologists to approach biochar synthesis that can tolerate soils subjected to FTF-conditions.

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Biochar Characterization in Complex Soil Mixtures
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