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United States Department of Agriculture

Agricultural Research Service

Title: Pyrogenic Carbon-Clay Mineral Associations in Soil Smectites

Authors
item Chappell, Mark - EPA
item Laird, David

Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: November 10, 2005
Publication Date: November 10, 2005
Citation: Chappell, M., Laird, D.A. 2005. Pyrogenic Carbon-Clay Mineral Associations in Soil Smectites [CD-ROM]. In: ASA-CSSA-SSSA Annual Meeting Abstracts, Nov. 6-10, 2005, Salt Lake City, UT.

Technical Abstract: Bioorganic carbon is “sequestered” in soil as humic substances (HS). HS is believed to form strong complexes with clays by a number of bonding mechanisms, including ligand exchange, cation-bridging with metals, and H-bonding to siloxane sheets. However, the recent discovery of pyrogenic or black carbon (BC), originating from pyrolyzed plant materials or industrial soots, further complicates classical models for clay-humic interactions. While certain biopolymers (e.g., proteins, fatty acids) form strong complexes with soil mineral surfaces, BC may exist as discrete particles weakly associated with soil clays. Thus, there is a possibility that BC may be physically separated from clays. For this study, we employed physical, non-chemical separations to evaluate BC association with smectitic soil clays. Samples of a Webster soil coarse (0.2 – 2.0 µm) clay fraction (CF), rich in BC, were separated using sodium polytungsate (SPT). Separations were performed sequentially beginning with a SPT density of 2.6 g cm-3, and then 2.1 and finally 1.5 g cm-3. Black (presumably organic) material floating on top of the liquid was collected with a spatula, washed with distilled water, and filtered onto polycarbonate filter paper. A brown precipitate, presumably the denser inorganic mineral fraction, was also collected and dried on polycarbonate filter paper. While SPT did not separate soil clay minerals, the “organic” fraction in the 1.5 g cm-3 liquid was highly enriched with BC. SEM was used on HF-treated coarse CF to image BC in soil. Also, SEM investigations of non-fractionated soil clay as well as medium (0.02 – 0.2 µm) and fine (< 0.02 µm) soil CF illustrate the loose association of HS with soil smectites.

Last Modified: 10/21/2014
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