Absence of Mobile Carbohydrate Domains in Dry Humic Substances Proven by NMR, and Implications for Organic-Contaminant Sorption Models.
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| Title: | Absence of Mobile Carbohydrate Domains in Dry Humic Substances Proven by NMR, and Implications for Organic-Contaminant Sorption Models. |
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| Authors: | Mao, J.-D.1,2, Schmidt-Rohr, K.1 srohr@iastate.edu |
| Source: | Environmental Science & Technology. 3/15/2006, Vol. 40 Issue 6, p1751-1756. 6p. 8 Diagrams. |
| Subjects: | Humic acid, Fulvic acids, Organic acids, Nuclear magnetic resonance, Absorption, Peat, Energy minerals, Fuel |
| Abstract: | The mobility and domain structure of various standard humic substances have been investigated by ¹H and ⊗H-13C solid-state nuclear magnetic resonance (NMR) experiments. In four dry humic acids, a fulvic acid, a natural organic matter sample, and a whole peat sample, segments that undergo fast, large-amplitude motions account for <9% of the sample. This disproves a previous suggestion, based on ¹H NMR data, that flexible domains, presumably carbohydrates, make up >40% of various humic acids; these putative mobile domains were also linked to dual-mode sorption. In particular, neither the polar alkyl (carbohydrate) nor the aromatic components show any fast, large-amplitude mobility. A small fraction of mobile nonpolar alkyl segments identified by us before is the only component undergoing large-amplitude motions, apart from absorbed water that we observe in humic acids exposed to ambient air. ¹H-13C wide-line separation NMR shows that, contrary to previous suggestions, the dipolar couplings in the aromatic regions are smaller than in the polar alkyl segments, most likely due to differences in local ¹H densities. Series of ¹H-13C heteronuclear correlation experiments with ¹H spin diffusion reveal close proximity of aromatic and polar alkyl segments in several humic acids, precluding carbohydrate domains on a scale of >1-nm diameter. In the standard peat humic acid, nonpolar aromatic segments also do not form sorption domains of significant size, while nonpolar aliphatic domains, which we had previously shown to correlate with sorption capacity, have been confirmed. [ABSTRACT FROM AUTHOR] |
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| Database: | Engineering Source |
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