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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOGEOCHEMICAL PROCESSES INFLUENCING FORMATION AND STABILIZATION OF SOIL ORGANIC MATTER AND SOIL STRUCTURE Title: Black Carbon, The Pyrogenic Clay Mineral?

Author
item Laird, David

Submitted to: Clay Minerals Society Meeting
Publication Type: Abstract Only
Publication Acceptance Date: June 11, 2009
Publication Date: June 11, 2009
Citation: Laird, D.A. 2009. Black Carbon, The Pyrogenic Clay Mineral? [abstract]. Clay Minerals Society. Paper No. 666-5.

Technical Abstract: Most soils contain significant amounts of black carbon, much of which is present as discrete particles admixed with the coarse clay fraction (0.2–2.0 µm e.s.d.) and can be physically separated from the more abundant diffuse biogenic humic materials. Recent evidence has shown that naturally occurring black carbon particles contain ~60% by mass aromatic carbon (C), with the remainder being a mixture of aliphatic, anomeric and carboxylic C. Black carbon particles have rounded morphologies with both porous and glassy internal structures. Radiocarbon dates of black carbon rich fractions physically isolated from soils indicate that black carbon is old and hence black carbon is assumed to be very stable in soil environments. By contrast, biogenic humic materials isolated from the same soils have modern radiocarbon dates and are more readily metabolized by soil microorganisms during incubations. Such evidence strongly suggests that most black carbon particles were originally charcoal formed during vegetation fires. Fresh charcoal has a low surface charge density and is hydrophobic; however, as charcoal ages in soil environments the surfaces oxidize forming carboxylic groups. Thus explaining why naturally occurring black carbon particles have a high density of variable surface charge and are hydrophilic. Clay size black carbon particles exhibit colloidal behavior in aqueous systems. X-ray diffraction analysis of freshly prepared charcoal reveals short-range order indicating the presence of disordered graphene sheets. Although not traditionally considered clay minerals, the properties and occurrence of black carbon are consistent with the broader definition of a clay mineral.

Last Modified: 8/22/2014
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